11institutetext: Sebastian Diebold 22institutetext: Institut für Astronomie
und Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen,
22email<EMAIL_ADDRESS>
# Proportional counters and microchannel plates

Sebastian Diebold corresponding author

###### Abstract

Developed right at the beginning of the space age in the 1940s, the
proportional counter was the first detector used in X-ray astronomy and stayed
its workhorse for almost four decades. Although the principle of such a
detector seems to be rather simple, over time it underwent considerable
performance improvements and the lifetime under orbital conditions was
extended tremendously. Particularly the invention of position-sensitive
proportional counters provided new and sophisticated methods to discriminate
background and thus enabled observations of much weaker sources.
A leap forward in position resolution was achieved with the advent of
microchannel plate (MCP) detectors in the 1970s. In contrary to gas filled
detectors, they provide no considerable energy resolution but feature spatial
resolutions reaching down to a few tens of micrometers, fitting ideally the
angular resolution of the novel grazing incidence imaging X-ray telescopes
upcoming at that time.
Even today, both types of detectors are still relevant in space-based
astronomy. However, in case of MCPs new developments focus on the far and
extreme ultraviolet wavelength range, while the Chandra X-ray observatory is
most likely the last mission applying this technology for X-rays. In contrast,
compact detectors with gas electron multiplier (GEM) foils and micropattern
readout are currently under heavy development for the soft X-ray range, since
they allow for the first time to measure polarization in X-rays over a broad
energy range.
This chapter presents the principles of proportional counters and MCP
detectors, highlights the respective performance characteristics, and
summarizes their most important applications in X-ray astronomy.

## Keywords

X-ray astronomy, instrumentation, radiation detectors, proportional counters,
imaging proportional counters, microchannel plate detectors

##  1 Introduction

The history of gas-filled detectors dates back to the onset of nuclear physics
at the beginning of the twentieth century with the studies on gas ionization
by Thomson1899 (58) and the first detector implementations by Rutherford1908
(49). Shortly after, in 1912, such detectors led to the discovery of cosmic
rays when Hess1912 (29) used them on several balloon flights to investigate
the origin of natural radiation.

The proportional counter was introduced in the late 1940s, followed by years
of intensive further development mainly driven by its applications in particle
physics Knoll2010 (33). This phase proceeded until the introduction of NaI
scintillation detectors in the 1950s and the first semiconductor detectors in
the 1960s. However, the developments on proportional counters continued and
lead to the position-sensitive single wire proportional counter (SWPC) and to
multi-wire detectors that feature two dimensional imaging capabilities
Fraser2009 (20).

From the first rocket missions investigating the X-ray radiation from the sun
in 1948 Keller1995 (31), gas-filled detectors and particularly proportional
counters were for almost four decades the workhorse in imaging as in non-
imaging soft X-ray astronomy Pfeffermann2008a (41). In 1962, the first
celestial X-ray source outside the solar system was discovered with a
proportional counter on a sounding rocket with a remarkable energy resolution
of about 20 % at 6 keV. This success hold on and for two decades all main
advantages in the field were due to missions applying proportional counters
Bulgarelli2020 (8). A milestone in the field that has to be mentioned was the
first satellite mission fully dedicated to X-ray astronomy: the Uhuru
satellite launched in 1970 Giacconi1971 (27). It featured a proportional
counter sensitive in the band 2–20 keV with a relatively large sensitive area
of $0.084\,\mathrm{m}^{2}$ and an angular resolution of $1^{\circ}\times
10^{\circ}$ FWHM constrained by a collimator.

According to Fraser2009 (20), in X-ray astronomy the main aspects of
proportional counters compared to other detectors can be summarized in these
three points:

  1. 1.

relatively large active effective area,

  2. 2.

moderate energy and spatial resolution, and

  3. 3.

high sensitivity.

Microchannel plates (MCP) – the second detector technology that is covered in
this chapter – started as a military development for night vision devices, but
was declassified and MCPs became commercially available. The main application
for the MCP technology is the position-resolved detection of charged particles
with a very low energy threshold, but it is also an excellent option for
photon detection when an appropriate photocathode for the wavelength range of
interest is used. Since the 1980s, MCPs were for many years the main
technology for astronomical instruments over the complete ultraviolet (UV)
spectral range and they are still competitive in this waveband to silicon-
based detectors like CCDs or CMOS. In fact, the best technology for a certain
application depends strongly on the individual instrumental requirements and
the parameters of the satellite platform. Not only in the UV, but also in the
visible and the X-ray bands MCP detectors were successfully applied. However,
new instruments and missions with MCP detectors are only proposed in the far
(FUV) and extreme UV (EUV, sometimes also XUV).

Considering X-ray astronomy, the key feature of MCP detectors is the
unprecedented position resolution. Therefore, it is no wonder that with HRI
(High Resolution Imager) Kellogg1976 (32) flown on the Einstein (HEAO-2)
observatory Giacconi1979 (26) the first modern imaging X-ray telescope
employed an MCP detector. The same instrument design was reused after
significant further developments for the HRI on ROSAT (ROentgen SATellite)
Pfeffermann1987 (42) and the HRC (High Resolution Camera) of the Chandra X-ray
observatory Weisskopf2002 (66) that is still operable today 23 years after its
launch. Furthermore, an MCP detector was also used in the WFC (Wide Field
Camera) of ROSAT Barstow1985 (2).

Both detector types discussed here work in counting mode, meaning that they
both register individual photon events. While the proportional counter has an
intrinsic (medium) energy resolution, MCP-based detectors usually only resolve
energy when used with a dispersive element like a grating and then exploiting
their superior position resolution. A second distinguishing feature is the
need for a window for the proportional counter to separate the gas volume
while an MCP detector can be operated open face. However, the gain in quantum
detection efficiency by not having transmission losses in a window for MCP
detectors is always over-compensated by the lower intrinsic quantum efficiency
compared to a proportional counter.

Meanwhile, proportional counters as well as MCP detectors were almost
completely replaced by their principal competitors in X-ray astronomy, namely
silicon-based detectors Knoll2010 (33). However, there are a few niches for
which these technologies are still developed: while MCP detectors are further
optimized for the UV and EUV where they can still be competitive to silicon
technology depending on the application and the requirements, position
sensitive proportional counters are applied recently to measure polarization
in X-rays – a longstanding and scientifically highly interesting topic that is
now tackled by several missions.

This chapter presents in Section 2 the general concept of proportional
counters, their basic parameters, and some considerations on applying them in
X-ray space missions. In Section 3 the principle of imaging proportional
counters and their application in X-ray astronomy are discussed, including the
relevance of micropattern gas detectors for X-ray polarimetry. Section 4
explains the functionality of MCP detectors and highlights applications in
X-ray as well as UV and EUV astronomy. Section 5 concludes with an outlook on
the future prospects of both discussed detector technologies.

##  2 Proportional counters

The basic principle of a proportional counter is best introduced by looking at
the simple radial tube geometry as sketched in Figure 1. A thin wire is
mounted coaxially in a cylindrical conductive tube, which is filled with the
counting gas and hermetically sealed. While the tube is on ground potential
acting as cathode, the insulated wire is connected to a positive high voltage
via a load resistance $R_{\mathrm{l}}$ and forms the anode. When ionizing
radiation enters the tube and generates ion pairs in the gas, the high voltage
accelerates the electrons towards the anode wire. For the detection of soft
X-rays a thin window is needed to allow transmission into the sensitive
detection volume.

Figure 1:  Sketch of a proportional counter in the simple radial tube
geometry. An X-ray photon interacting with the counting gas generates a
photoelectron that loses its energy by ionizing gas atoms. The generated
electrons are drifted towards the central anode wire by an externally applied
high voltage. When the electrons enter the multiplication region around the
anode wire they gain enough energy to ionize neutral gas atoms and initiate a
charge avalanche. The signal from the anode wire is read out via a
capacitively coupled charge-sensitive preamplifier.

The field strength is increasing towards the anode wire, subdividing the gas
volume in weak and strong electric field regions, usually called drift region
and multiplication or avalanche region, respectively. When the electrons gain
enough kinetic energy on a mean free path length, collisions with gas atoms
can generate secondary electrons in an avalanche process called gas
multiplication Knoll2010 (33). For reading out, the anode wire is capacitively
coupled to a charge-sensitive preamplifier that converts the collected charge
to a voltage signal. If the applied high voltage and thus the field strength
are set properly, the amplitude of the output signal is proportional to the
energy deposited by the incident ionizing particle or photon, allowing
counting and spectroscopy.

The complete detection process for soft X-rays in proportional counters is
summarized in a schematic overview in Figure 2. It should be noted that the
photoelectron is not necessarily carrying away most of the energy. A typical
example is given in Fraser2009 (20): a 6 keV X-ray photon is absorbed in a
mixture of 90 % xenon and 10 % $\mathrm{CH}_{4}$. In this configuration 99.83
% of the primary interactions are with the xenon L-shell (mean energy 5.0
keV). These lead to the production of a 1.0 keV photoelectron and a 4.2 keV
L-fluorescence photon ($14\,\%$ probability) or a $3.4\,\mathrm{keV}$ Auger
electron (86 %) since the mean M-shell binding energy in xenon is 0.8 keV.

Figure 2:  Schematic overview of the interaction process of soft X-rays in a
proportional counter. If the photon energy $E$ exceeds the binding energy
$E_{ij}$ of the $j$th shell of the $i$th component of the counting gas
mixture, $p_{ij}$ is the probability that a photoelectron is released from
this shell. $a_{i}$ is the fraction of the $i$th atomic component in the
mixture and $\sigma_{ij}$ are the respective photoelectric cross-sections. The
excited ion can relax either via the emission of an Auger electron or by
emitting a fluorescent photon with the probability $\omega_{ij}$. The
probability that the fluorescent photon escapes the sensitve counter volume is
$e_{ij}$.

The individual steps of event detection in proportional counters are discussed
in more detail in the following, together with the relevant performance
parameters.

###  2.1 Photon interaction via the photoelectric effect

The dominating interaction process of soft X-rays below $50\,\mathrm{keV}$ is
the photoelectric effect. In this process, the photon is absorbed by an
electron of an inner atomic shell, usually the K-shell ($n=1$) if the photon
energy is sufficient, and this so-called photoelectron carries away the
initial photon energy minus the binding energy. The photoelectric cross
section is proportional to $Z^{n}E^{-8/3}$ with the atomic number $Z$ of the
counting gas, the exponent $n$ in the range 4–5, and the photon energy $E$
Fraser2009 (20). This implies that for the detection of higher photon energies
the mean atomic number $Z$ of the counting gas should be increased to
compensate for the enlarged mean free path. Secondly, particularly for the
softest part of the spectrum, window materials should be as light weight as
possible and built from low-$Z$ material to avoid absorption in the window.

As the photoelectron travels through the gas it loses energy mostly via
inelastic collisions with gas atoms forming ion pairs. The typical range of
the photoelectron in X-ray counters is 0.1 – 1 mm Fraser2009 (20). The number
$N$ of ion pairs that are generated is linked to the energy $E$ of the
photoelectron via the mean energy $w$ for the creation of an ion pair. Without
considering any secondary effects the number $N$ of generated electrons by a
photoelectron with energy $E$ can be estimated by

$N=E/w.$ (1)

$w$ depends on the gas mixture with typical values in the range 25 – 35 eV
Knoll2010 (33). In X-ray counters often noble gases are used, mainly argon
with $w=26.2\,\mathrm{eV}$ for soft X-rays and xenon with
$w=21.5\,\mathrm{eV}$ for harder X-rays Sipila1976 (56). Thus, for low-energy
X-rays below 10 keV the number of secondary ion pairs is of the order of just
a few hundred.

Since the formation of secondary ion pairs by a photoelectron cannot be
described as a series of independent processes, the actual variance
$\sigma^{2}_{N}$ in the number of generated ion pairs is considerably smaller
than expected from Poissonian statistics by a factor $F$ that is called the
Fano factor Fano1947 (18):

$\sigma^{2}_{N}=FN$ (2)

Therefore, the variance $\sigma^{2}_{N}$ is still proportional to $N$ and the
energy resolution $E/\Delta E$ scales with $\sqrt{N}$. Although an analytical
description for $F$ exists, the factor is usually determined experimentally
for a specific material Fraser2009 (20). Empirical values for $F$ group around
0.15 for typical counting gases Edgar2011 (15).

Since the corresponding electrical charge generated for soft X-rays is thus of
the same magnitude than the equivalent noise charge at the input of a low-
noise preamplifier, an internal amplification via gas multiplication is
necessary to perform spectroscopy with a significant signal-to-noise ratio
(SNR).

###  2.2 Gas multiplication and energy resolution

A critical parameter for the operation of a proportional counter is the
applied high voltage. Together with the electrode geometry it defines the
strength of the electric field and thus distinguishes three operational modes
of a gaseous detector:

  1. 1.

ion chamber mode, in which the charges scatter only elastically with gas atoms
and drift slowly to the electrodes;

  2. 2.

proportional chamber mode, in which the electrons gain enough energy between
collisions to ionize neutral gas atoms and thus form a charge avalanche;

  3. 3.

Geiger counter mode, in which the acceleration of the electrons is so high
that the charge avalanche saturates because the applied field is compensated
by the space charge of the generated ions.

Figure 3 shows generically the different voltage regimes and how the recorded
pulse height depends on the applied high voltage for low and high X-ray
energies. The transition region between the proportional and the Geiger-Müller
regions is usually not used in scientific detectors.

Figure 3:  Generic diagram of the different voltage regimes and the according
operational modes of a gas counter. In the ion chamber regime the charges
drift without multiplication. In the proportional regime at intermediate
voltages a linear amplification of the signal by gas multiplication sets in,
which becomes non-linear at higher voltage. In the Geiger-Müller regime the
amplification completely saturates and the pulse amplitude depends only on the
voltage and not on the photon energy any more. At even higher voltages a
continuous discharge can be triggered that leads to degradation and ultimately
destroys the counter.

Only in the proportional chamber mode, the amount of read out charge created
by primary X-rays in the few keV range is large enough to allow a detection
with discrimination of the energy information. The critical field strength for
typical gases at atmospheric pressure to form such a Townsend avalanche is of
the order of $10^{6}\,\mathrm{V/m}$ Knoll2010 (33). In order to achieve a
uniform multiplication that is independent on the exact interaction point in
the counting gas, the multiplication region must be much smaller than the
total gas volume. Also the geometry, e.g. the wire thickness or the wire
distance in a multiwire configuration, is critical to reach a sufficient field
uniformity and larger tolerances lead to a decrease of the achievable energy
resolution Fraser2009 (20).

The derived variance $\sigma^{2}_{e}$ of the charge signal and the typical
noise contribution from charge amplification $\sigma^{2}_{\text{amp}}$ yields
a one-sigma energy resolution of

$E/\Delta E=w\sqrt{\sigma^{2}_{e}+\sigma^{2}_{\text{amp}}}.$ (3)

Therefore, with an ideal detector system an energy resolution of 15 % can be
reached at 6 keV Edgar2011 (15).

At high rates, the energy resolution of a proportional counter can be degraded
due to the rather long drift times of the positive ions that are formed close
to the anode wire. These ions shield the applied high voltage and thus lead to
a local deformation of the electric field, which can affect the gain of
subsequent events. It is almost impossible to account for this effect because
of its highly localized nature. It could be addressed in position sensitive
counters (s. Section 3), but even there it is usually not corrected.

Critical for the stable operation of a proportional counter is the generation
of UV photons by excited gas atoms in the multiplication region as well as
during the neutralization of ions at the cathode. UV photons hitting the
cathode surface can trigger the emission of additional electrons that can
induce subsequent avalanches and eventually a continuous discharge of the
counter. This effect is mitigated by the addition of a few percent of a
polyatomic quench gas, e.g. $\mathrm{CH_{4}}$ or $\mathrm{CO_{2}}$, to the
usually used noble gases. Such an additive firstly absorbs UV photons and
converts the energy into heat, secondly reduces via charge exchange the amount
of noble gas ions reaching the cathode, and thirdly increases the drift
velocity and thus reduces the influence of gas impurities Pfeffermann2008a
(41, 51).

###  2.3 Detection efficiency and response function

For the detection of soft X-rays a thin window of light material is needed to
allow transmission into the counting gas down to keV energies and below. Metal
windows of beryllium or aluminum allow transmission down to 1.5 keV and can be
used for sealed gas cells. For even lower transmission thresholds, micrometer
thick plastics like polypropylene have to be used, but these require a gas
supply system to maintain the pressure Pfeffermann2008a (41). The window
thickness is usually a compromise between lower energy threshold on the one
hand and mechanical stability and gas tightness on the other. Particularly for
large windows that enable large sensitive areas, a careful assessment of the
applicable forces with respect to the accelerations during launch is
necessary. Usually a grid of higher thickness or stronger material is used to
support large windows.

The probability for an X-ray photon to pass through the window and interact in
the fill gas can be expressed in terms of the optical depths
$\tau_{\mathrm{w}}$ and $\tau_{\mathrm{g}}$ of window and gas, respectively:

$P=e^{-\tau_{\mathrm{w}}}(1-e^{-\tau_{\mathrm{g}}})$ (4)

The optical depth of a certain material is linearly depending on the mass
density $\rho$, the energy dependent mass attenuation coefficient $\mu(E)$,
and the material thickness $d$:

$\tau=\rho\mu(E)d$ (5)

The mass attenuation coefficient $\mu(E)$ is simply the interaction cross-
section $\sigma(E)$ per mass $m$. As depicted above, X-ray photons primarily
interact with inner shell electrons, while the valence electrons are
responsible for the chemical bonds. Therefore, the chemistry of the material
plays a minor role for the X-ray cross-section and the cross-section for a
compound material or gas mixture is simply the sum of the individual cross-
sections of the constituents.

While $\sigma(E)$ is in general a smooth function, it shows sudden jumps when
the photon energy is sufficient to ionize a better bound electron level. Since
the $K_{\alpha}$ energies of common counter gases lie within the energy band
of interest ($K_{\alpha}$ energies of argon, krypton, and xenon are 2.97 keV,
12.6 keV, and 29.7 keV, respectively), the response functions can be
complicated Knoll2010 (33). While the cross-section and therefore also the
mass attenuation coefficient is declining with rising X-ray energy, a steep
increase is seen when reaching the K-shell energy (see Figure 4).

Figure 4:  Mass attenuation coefficients for argon, xenon, and krypton as
typical fill gases for proportional counters. A sharp increase of the
interaction probability occurs when the photon energy is sufficient to ionize
the next inner shell. The respective shell is marked in the plot with M, L,
and K. Data obtained from Hubbell2004 (30).

Furthermore, the characteristic X-ray radiation (fluorescence) generated by
the photoelectric absorption carries away a significant portion of the primary
photon energy. Events in which these X-rays are not absorbed in the gas lead
to an additional escape peak in the pulse height spectrum, which is shifted
from the full-energy peak to lower energies by the $K_{\alpha}$ energy.
Additional peaks in the spectrum might originate from the characteristic lines
generated when X-rays are absorbed in the entrance window or in the walls.
Besides their higher X-ray transparency, low Z window materials and surface
covers are thus also favorable to minimize this contribution Knoll2010 (33).

Due to all these effects, the conversion from photon energy to mean pulse
height – often referred to as gain – can be non-linear with jumps and slope
changes, particularly near absorption edges. Even a monochromatic source
produces a complicated pulse height spectrum with several features. Thus, a
direct estimate of the true photon spectrum from measurement data is not
possible and usually a response matrix file (RMF) is used to analyze the data
of scientific observations. Such an RMF contains the measured probabilities
for the different possible pulse heights over the complete photon energy range
in which a detector is sensitive Edgar2011 (15).

###  2.4 Time resolution, dead time, and rate limitation

The time resolution of a proportional counter is dominated by the drift time
between the interaction point of the photon and the avalanche region and thus
depends particularly on the size of the drift region. Since drift velocities
of electrons in gases are high and reach $10^{6}$ – $10^{7}\,\mathrm{cm/s}$
also for moderate electric fields, ultimate time resolutions below
$1\,\mathrm{\mu s}$ are reachable Pfeffermann2008a (41).

However, also in a fast detector system an event can only be recorded
correctly when a sufficient time interval has passed after the previous event
during which the detector system has attained its ground state. All produced
charges need to be collected – for a proportional counter this is dominated by
the drift time of the positive ions to the cathode – so that the electric
field is completely recovered and the output voltage has settled at the
baseline value. Further, the electronic processing of the previous event needs
to have finished setting the analog-to-digital converter (ADC) in a ready
state for the next event.

In case of a second photon event before full recovery we speak of pile-up or
dead time. The probability for such events increases with increasing count
rate. Pile-up means that two or more photon events are so close together in
time that the electronics cannot distinguish between them. The signals of
these events add up and only a single event with about the sum of the energies
is recorded. Due to the distorted waveform, e.g. a longer rise time or a
multiple peak structure, some detector systems can filter at least a fraction
of pile-up events Usman2018 (64).

If the detector is intrinsically fast like a proportional counter, it is
possible to use a fast trigger technique to detect incoming events and to
close the electronics for a defined time to not process new events until the
detector output has certainly reached its baseline value. In case a second
event arrives during dead time, the dead time window is restarted. This so-
called paralyzing dead time mode leads to the situation that at very high
count rates the detector hardly processes any events and thus sets a hard
limit on the maximum count rate. However, to not excessively degrade a
proportional counter the actual rate during observations should be restricted
to lower values, typically well below $1000\,\text{/s}$.

In order to know the real exposure time – e.g. for the correction of the count
rate or the calculation of the photon flux from a particular source – the dead
time needs to be subtracted from the exposure time. The resulting time is then
called the live time, in which the detector was able to register events. Due
to the different mechanisms creating dead time, the exact determination can be
quite complex and usually only a partial correction is possible Usman2018
(64).

However, various techniques are applied to determine the dead time: a periodic
pulser signal can be injected in an unused part of the spectrum and the
fraction of recorded to sent pulser events yields the fraction of live time to
exposure time. Another method uses the typical event length and cuts extended
events that are likely pile-up. From the number and time structure of such
events also the live time can be deduced.

It is important to note here that also background events induce dead time in a
detector system, even if they can be identified and rejected, thus reduce the
live time of observations and the instrumental throughput. Therefore, a
thorough simulation of the instrument in the orbital radiation environment is
crucial to assess the performance and whether the scientific goals of a
mission can be reached (cf. Section 2.5).

###  2.5 Operation in space: background and lifetime

An X-ray detector in space must withstand the highly variable and partially
quite harsh orbital radiation environment. On the one hand, the exposure to
charged particle and excessive photon fluxes degrades the detector performance
– particularly the energy resolution but also the sensitivity might be
affected – and, on the other hand, external and internal radiation sources
create background in scientific observations. This list covers the most
important sources of radiation in orbit besides the astrophysical target:

  * •

cosmic rays are charged particles with a wide energy spectrum up to the
highest energies;

  * •

soft protons are mostly of solar origin, can reach X-ray detectors thorough
optics and filters, are highly variable, and often deposit energy in the
instrumental X-ray band Diebold2015 (14);

  * •

trapped charged particles in the radiation belts of the Earth;

  * •

fluorescent X-rays emitted from surrounding materials of the detector;

  * •

radioactive impurities in the detector, e.g. not completely radio-pure
counting gas or trace elements in the housing and sealings; and

  * •

the diffuse X-ray background.

Depending on the orbit of the mission the influence of these individual
components on the instrument varies strongly. It is interesting to note that
for most astrophysical X-ray sources and the typical instruments the
background rate exceeds the X-ray event rate Pfeffermann2008a (41). Therefore,
methods for background detection and rejection via event selection are applied
in all modern X-ray instruments.

In case of minimal ionizing particles (MIPs) that deposit about
$1.5\,\text{MeV/cm}$ the size of a proportional counter is usually large
enough so that the deposited energy is above the upper threshold of the
detector and thus the event is rejected. Another internal but more
sophisticated method to suppress background events exploits the difference in
the geometric shape of the charge cloud induced by a particle and by a photon.
While photon events produce a more localized, point-like ionization cloud,
charged particles leave an extended ionized track. This leads to different
rise times of the output signals and thus provides a handle to discriminate
particle events even when the energy is within the band of interest.

A very efficient method to identify background events is the use of an
anticoincidence detector – sometimes also called veto detector – that ideally
completely surrounds the actual detector volume and leaves only the entrance
window for the X-rays uncovered. In case of simultaneous events in both the
actual and the anticoincidence detector, the events are either directly
rejected or marked as possible background, because of the high probability
being triggered by a non-X-ray interaction. If the anticoincidence detector
shares the same gas volume with the main detector, also fluorescent X-rays
generated by charged particles in the walls can be absorbed with high
efficiency in the anticoincidence region and thus do not participate to the
overall background. However, this works only effectively if the gas column
density of the anticoincidence region is sufficiently high.

By combining these methods overall background rejection efficiencies of up to
99.6 % have been realized even for large area proportional counters for X-ray
astronomy Fraser2009 (20). In the imaging proportional counter PSPC (Position
Sensitive Proportional Counter) of ROSAT a rejection of even 99.85 % has been
reached Pfeffermann2003 (43).

But charged particles not only produce background in the detector, they can
even lead to degradation or failure. Two major mechanisms affect proportional
counters in space: the interaction of heavy charged particles and the aging of
the quench gas. Heavy ionizing particles hitting the detector can deposit up
to four orders of magnitude more energy in the sensitive gas volume than
X-rays. Therefore, the counter design and the operation parameters must be
carefully selected that no permanent discharge or spark discharge can be
triggered by such events, which can lead to permanent failure of the detector,
e.g. by destroying the anode wire.

A permanent aging of proportional counters is initiated by the cracking of
quench gas during the normal operation. In this process hydrocarbons like
$\mathrm{CH_{4}}$ lead to the deposition of polymerization products on the
internal electrodes that induce a progressing gain degradation by effectively
increasing the radius of the anode wire and eventually a permanent discharge,
the so-called Malter effect. Gas composition and purity, the strength of the
electric field, as well as the materials used for wires, housing, and sealing
affect the rate of this degradation Pfeffermann2003 (43). $\mathrm{CO}_{2}$
has been proposed as an alternative quench gas that completely avoids
polymerization and produces only less harmful carbon depositions Ramsey1994
(48).

##  3 Imaging proportional counters

Even for the simple tube geometry of a proportional counter, position sensing
of the event in the direction of the anode wire is possible. Since the drift
field is radially, the position of the avalanche is a good indication where
the primary photon interaction happened. The most common technique uses charge
division, for which the anode wire needs a significant resistance and the
charge is read out on both ends. By dividing the amount of charge recorded at
one end by the total charge the position information is determined. The total
charge holds still the information of the primary energy.

An alternative position sensing technique for the linear counter geometry
exploits the rise time of the pulse which is longer when the charge is
collected on a resistive anode wire far from the preamplifier, since the wire
can be regarded as an $RC$ transmission line because of its finite capacitance
to the cathode. By reading out at both ends of the wire, the achievable
position resolution can be improved considerably by comparing the respective
rise times. Although theoretical considerations show that the charge division
technique should yield position resolutions by up to a factor of two higher,
in practical applications the rise time sensing turned out to be superior due
to its simpler realization in analog electronics Fraser1981 (22, 23).
Therefore, this was also the technique of choice for the application in
detectors for X-ray astronomy, first in one dimension and later also in two
dimensions Fraser2009 (20).

While position sensing in linear counter geometries finds a meaningful
application e.g. in the readout of a dispersive spectrometer, two dimensional
designs are needed for imaging applications. A simple form is to use several
anode wires each with a dedicated readout channel, a so-called multiwire
proportional counter. For a two dimensional position determination of the
photon event, either the wires can be read out on both sides following one of
the schemes presented above, or a grid of crossed wires can be used. However,
the position resolution in the two axis will be significantly different with
different systematics.

The concept of an imaging proportional counter that reaches the same position
resolution in both directions is shown in Figure 5. A layer of thin anode
wires is embedded between two layers of cathode wires that are oriented
perpendicular to each other. The negative charge signal read out from one of
the anodes is used to reconstruct energy and timing information of the X-ray
event while the position is determined from the positive signal induced in the
cathode layers. While in principle the same energy resolution as for a single
wire counter can be reached, the tolerance of the anode wire pitch must be of
the order of micrometer to reach a uniform gain over the whole detector
Pfeffermann2008b (40).

Figure 5:  Concept of an imaging proportional counter. The charges produced by
an X-ray event are drifted through a cathode wire grid to the multiplication
region. The charge cloud is read out at one of the anode wires, producing a
negative signal that is used to determine photon energy and arrival time. The
position information is gained from the positive signals induced in the two
layers of cathode wires, which are oriented perpendicular to each other.

###  3.1 Position resolution

The achievable position resolution in an imaging proportional counter depends
on several components that are all independent from each other and hence add
up to the spacial accuracy $\sigma_{\text{x}}$ of the detector system
Pfeffermann2008b (40):

  * •

$\sigma_{\text{a}}$: Focused X-rays from the mirror system hit the detector
not normal to the surface and the actual absorption depth varies from photon
to photon and with energy. This can be partially mitigated by displacing the
focus plane inside the gas volume.

  * •

$\sigma_{\text{r}}$: The created charge cloud has a finite size depending on
the ranges of the photoelectron and the fluorescent photon or Auger electron.

  * •

$\sigma_{\text{d}}$: During the drift the electron cloud widens via lateral
diffusion.

  * •

$\sigma_{\text{e}}$: The detection accuracy of the position of the electron
cloud depends on its geometrical size before multiplication and the number of
primary electrons. Particularly for low photon energies electronics noise
degrades the resolution further.

$\sigma_{\text{x}}^{2}=\sigma_{\text{a}}^{2}+\sigma_{\text{r}}^{2}+\sigma_{\text{d}}^{2}+\sigma_{\text{e}}^{2}$
(6)

In case of soft X-rays, electron statistics dominate the overall position
resolution. Therefore, the position resolution scales as the energy resolution
with $1/\sqrt{E}$.

###  3.2 Imaging proportional counters in X-ray astronomy

Detectors based on this geometry have been flown on several X-ray missions. In
the IPC (Imaging Proportional Counter) of the Einstein (HEAO-2) mission
Giacconi1979 (26) the cathode wires in each layer were connected in a meander-
like pattern to form two $RC$ delay lines. The event position was
reconstructed by comparing the signal shape at both ends of the delay lines.
The longer the signal traveled before the readout the longer the rise time
became. This concept reached in the IPC instrument a position resolution of
about $1\,\mathrm{mm}$ FWHM Gorenstein1981 (28).

For the PSPC (Position Sensitive Proportional Counter) instrument of ROSAT the
readout scheme was refined by using individual channels for strips of four
adjacent cathode wires. Since the charge signal of an X-ray event was
distributed on several strips, a center of gravity calculation allowed a
position resolution smaller than the strip width. Eventually, the PSPC reached
a spatial resolution of $230\,\mathrm{\mu m}$ Pfeffermann2003 (43).
Additionally to the background suppression with a system of five anti-
coincidence detectors, the charge distribution over the strips was also
exploited to reject particle-induced events.

Although not an imaging detector system, the PCA (Proportional Counter Array)
of the RXTE (Rossi X-ray Timing Explorer) mission Bradt1991 (7) used a similar
geometrical concept of individual anode wires embedded in cells surrounded by
grounded wires. While photon events produce signals only in one of the cells,
an energetic charged particle would traverse several cells and can thus be
discriminated. Additionally, to improve background rejection, a veto layer
around the sensitive detector volume is formed within the same gas volume. In
fact, these possibilities for background detection and rejection is one of the
attractive features in applying proportional counters as well as imaging
proportional counters in X-ray astronomy, particularly for the realization of
large sensitive areas.

###  3.3 Micropattern gas detectors and X-ray polarimetry

With micropattern gas detectors a complete new kind of gaseous detectors came
up at the beginning of the 1990s. Due to considerable advances in
manufacturing techniques like photolithography, selective etching, and laser
machining, the wire structures necessary for the strong field multiplication
region of a proportional counter could be replaced by microscopic structures
on glass or plastics substrates Knoll2010 (33, 45).

JEM-X on Integral Lund2003 (35) was the first instrument dedicated to X-ray
astronomy that applied this concept in its position-sensitive microstrip gas
chamber, a configuration proposed first by Oed1988 (38). Fine anode structures
interleaved with wider cathodes are produced on the front surface of a
partially insulating glass substrate. The backside of the substrate features a
second layer of cathodes that are perpendicular to the front ones. Therefore,
geometry and readout are similar to a classical imaging proportional counter
as described above, but in a miniaturized, highly ruggedized form. One of the
main advantages is a superior gain uniformity due to low tolerances and a high
stability of the anode geometry.

A recent and novel application of micropattern gas pixel detectors (GPD) is
polarimetry in the X-ray domain. The measurement of polarization is a long-
standing topic in X-ray astronomy, but different to other wavebands as radio
or visible are polarimetric instruments in the X-ray domain complex and
difficult to realize. Detecting the degree and direction of polarization
yields the possibility to constrain the geometry of the emission region
without spatially resolving the source. A first approach with a considerable
sensitive area on a satellite was already made with OSO-8 in 1975 by using
Bragg reflection off of crystals for which the cross-section is dependent on
the polarization of the incoming radiation. However, this principle works only
in a narrow, almost monochromatic band, and also due to the small effective
area of the instrument the only detected source with a finite polarization was
the crab pulsar Weisskopf1976 (67).

After this first try, the topic was not touched for decades until GPDs enabled
a quite sophisticated principle to measure polarization: The emission
direction of a photoelectron generated via the photoelectric effect is
correlated with the direction $\phi$ of the electric field of the absorbed
photon. Therefore, for a polarized source the number of detected events varies
with $\cos^{2}\phi$. Gas electron multiplier (GEM) foils Sauli1997 (50, 52) in
combination with micropattern anodes directly attached on a readout ASIC
(application specific integrated circuit) allow to record the ionization track
of the photoelectron. The gas cell with about 1 mm of dimethyl ether and
helium is sealed with a beryllium window transparent to X-rays. More details
on this detector concept can be found in the Chapter Gas pixel detectors for
polarimetry within this handbook.

The first mission to apply a gas pixel detector in a polarimetric instrument
is IXPE (International X-ray Polarization Explorer), launched in 2021 and in
operation at the time of writing Weisskopf2022 (68). With eXTP (enhanced X-ray
Timing and Polarimetry) the next mission with a larger grasp that applies the
same technology is already being prepared Zhang2018 (71). Both missions are
presented in detail within this handbook in the section X-ray Missions.

##  4 Microchannel plate detectors

###  4.1 Channel electron multipliers

In order to explain the principle of microchannel plates we first take a look
at a simpler detector that applies the same principle of charge
multiplication: the (straight) channel electron multiplier (CEM). These
devices are somewhat similar to conventional photomultiplier tubes, but
usually smaller in size and with a continuous dynode structure instead of
discrete gain stages. The principle is shown in Figure 6. Incoming ionizing
radiation that hits the inner wall of a tube releases one or more electrons.
In case the primary was a photon we speak of photoelectrons; for particles the
secondaries are called delta electrons. A high voltage applied along the tube
accelerates the electrons that generate additional secondaries when striking
the wall again. In this avalanche process a charge cloud forms that can be
detected on an anode. The gain – typically defined as the mean number of
electrons that are in total released in one event – is in the range of
$10^{4}$ up to more than $10^{8}$. A semiconducting coating on the tube walls
recharges after each event with a time constant depending on the resistivity.
Since the mean free path under atmospheric conditions would not be sufficient
to trigger secondary electron emission, these devices can only be operated
under vacuum.

Figure 6:  Cross-section of a CEM. An incoming X-ray or charged particle
releases an electron when impacting the channel wall. The electron is
accelerated by the applied high voltage and generates an avalanche of
additional electrons. The exiting charge cloud consisting of up to more than
$10^{8}$ electrons can be detected on an anode.

Typical tube diameters of channel electron multipliers are of the order of
millimeters up to several millimeters. First imaging detectors that apply this
concept of charge multiplication were produced by the formation of two-
dimensional arrays of CEMs. However, this intricate and complex procedure led
only to a course grid with a limited sensitive area. A leap in spatial
resolution of such arrays was achieved when producers were able to form
millions of channels with a diameter on the scale of tens of micrometer: the
microchannel plate.

###  4.2 Microchannel plates

Microchannel plates (MCPs) started as military technology for night vision
devices, but were later declassified and applied in many particle and photon
detectors Lampton1981 (34). The principle of multiplying charge is the same as
in the CEM (s. Section 4.1), but an MCP consists of millions to billions of
microscopic multiplier channels arranged in a regular pattern Wiza1979 (69).
The scales of the channel diameter typically range from a few micrometers to a
few tens of micrometers with a pitch only slightly larger than the diameter.
This channel micro-structure sets the ultimate limit on the achievable spatial
resolution of an MCP detector, which is rather easily reached with decent
readouts. Today, MCPs are available in a wide variety of formats and sizes:
plate diameters range from the centimeter scale up to a few ten centimeter,
and while the most common shape is still round, also squared and hexagonal
formats as well as plates with central holes – e.g. for a primary beam – are
available from a handful of companies. Of particular interest for astronomical
instruments are curved plates that are more difficult in production and
accordingly costly but can match better the shape of the focal surface of an
instrument – most prominent example is a curved linear detector for a Rowland
spectrograph. It should be noted here that although the market share of
astronomical applications is minute compared to other fields, many
developments in the MCP technology were initiated by the demands of
astronomers Fraser2009 (20).

For the fabrication of conventional MCPs, a macroscopic tube of lead glass is
mechanically supported by inserting a rod of soluble glass. The assembly is
drawn through an oven after which the thickness is reduced to about 1 mm. The
fibers are cut and assembled in hexagonal stacks, which are again heated and
drawn, cut, and finally assembled in a hexagonal capsule. The tubes that have
now reached their desired diameter of a few micrometer are fused together
under vacuum. The capsule is then sliced, polished, and the edges are grinded
to the desired final shape, usually to a circle. The inner support is removed
by etching and the channels are treated under a hydrogen atmosphere to produce
a semiconducting surface layer until the desired resistivity and secondary
electron emission yield are reached. Finally, metal electrodes are deposited
on both faces of the plates for electrically contacting and applying a high
voltage across the channels.

A rather new development are borosilicate MCPs that are manufactured by the
hollow-tube technique without the need for a soluble support inside the
channels Ertley2018 (16). These MCPs are functionalized with atomic layer
deposition (ALD) by growing first a resistive layer and on top a layer with
high secondary emission yield, i.e. a low electron work function Gebhard2019
(25). These layers formed with ALD have a high uniformity throughout the
channel and their composition is more stable over time than the surface layer
of conventional plates. While for lead glass MCPs a considerable degradation
of the gain and changes in the resistivity appear typically after an
extraction of $0.1\,\mathrm{C/cm^{2}}$ due to the electron bombardment of the
surface layer of the channels, ALD MCPs have proven to be stable up to an
extracted charge of at least $7\,\mathrm{C/cm^{2}}$ Popecki2016 (46).
Furthermore, since lead glass contains traces of radioactive isotopes –
particularly the beta-instable ${}^{40}\mathrm{K}$ – conventional plates have
a typical dark count rate of $>0.25\,\mathrm{/cm^{2}/s}$, while borosilicate
MCPs with their low level of intrinsic radioactivity show a considerably
reduced dark count rate of $<0.05\,\mathrm{/cm^{2}/s}$ Ertley2018 (16).

Photographs of a conventional lead glass MCP, a borosilicate ALD MCP of
typical size, as well as an exceptionally large format borosilicate ALD MCP
are shown in Figure 7.

(a) Conventional MCP, 50 mm diameter

(b) ALD MCP, 50 mm diameter

(c) ALD MCP, $20\times 20\,\mathrm{cm^{2}}$ square

Figure 7:  Photographs of various commercially available MCPs: (a)
Conventional round lead glass MCP (Photonis, 50 mm diameter, 12° bias angle,
0.6 mm thickness, $10\,\upmu\mathrm{m}$ channel diameter, $l/d$ 60:1,
$12\,\mathrm{\upmu m}$ pitch) (b) Round borosilicate ALD MCP (Incom, 50 mm
diameter, 13° bias angle, 0.6 mm thickness, $10\,\upmu\mathrm{m}$ channel
diameter, $l/d$ 60:1, $12\,\mathrm{\upmu m}$ pitch) (c) Large square
borosilicate ALD MCP (Incom, prototype for Gen I LAPPD, $203\times
203\,\mathrm{mm^{2}}$, 0.6 mm thickness, $20\,\upmu\mathrm{m}$ channel
diameter, $l/d$ 30:1)

###  4.3 Operation of MCPs in detectors

A key parameter of an MCP is the gain $g$, i.e. the number of electrons in the
emitted electron cloud. The fact that the gain of an MCP channel – keeping
voltage and channel surface conditions fixed – is mainly determined by the
dimensionless length-to-diameter ratio $l/d$ allowed for the miniaturization
of CEMs to MCPs. Typical $l/d$ ratios of MCPs are in the range 40/1 – 60/1
because such plates show a minimum spatial variation of the gain. However, for
photon-counting applications a larger $l/d$ is usually an advantage and thus
also thicker plates up to 175/1 are available Fraser2009 (20).

The resistance over an MCP is the result of the individual channel resistances
connected in parallel. Typical values are $>100\,\mathrm{M\Omega}$ with the
actual value following roughly a scaling with $l/A$, where $A$ is the plate
area. The applied voltage generates a finite current through the
semiconducting channel walls and thus heats the material. Since the resistance
has a non-ohmic behavior and decreases with increased bias voltage, a thermal
runaway can be triggered when the critical current is exceeded, leading to the
destruction of the plate. From this point of view a larger resistance seems
favorable. However, on the other hand, the resistance influences the count
rate capability since at high count rates the positively charged channel walls
might not fully regenerate between individual events and thus decrease the
applied field. This leads to the typical local gain drop of MCPs at high count
rates and operation near saturation, in which the positive space charges in
the walls compensate the applied electrical field.

Most MCPs have straight channels which are inclined with respect to the
surface normal around 10°. This inclination increases the response to photons
incident normal to the surface, but more important it allows to suppress ion
feedback, particularly when more than one plate is operated in a detector.
Typical detector configurations feature either two plates – the so-called
Chevron stack – or three plates – the Z-stack. The direction of the channel
inclination is inverted in each plate, as shown in Figure 8 for a stack of two
MCPs. While a single plate is limited to a gain of about $10^{4}$ before
reaching saturation, a stack of two plates achieves up to $10^{8}$ and a three
plates stack can even exceed $10^{9}$.

Figure 8:  Cross-section of a stack of two MCPs. Photoelectrons, which are
produced when X-rays interact with the top plate, are accelerated inside the
channels by the applied high voltage. When impacting the channel wall an
avalanche of secondary electrons is generated, finally forming a charge cloud
of typically $10^{5}$ – $10^{8}$ electrons.

The necessary gain strongly depends on the application – particularly for
imaging purposes on the type of position-sensitive anode and the required
position resolution. The generic gain-over-voltage characteristics of the
three common plate configurations are shown in Figure 9. A key quantity to
exploit the photon-counting capabilities of an MCP detector is the charge
distribution. For a photon-counting operation without any readout noise the
event threshold has to be set above the electronics noise level of the system
without cutting away photon events and thus sacrificing quantum efficiency.
This is achieved best when a clear separation between the electronic noise
charge (ENC) of the readout system and the charge distribution of real events
is achieved. The inlay plots in Figure 9display the typical charge
distributions under different operation conditions and show that stack of at
least two MCPs is needed to fulfill this requirement, since the charge
distribution of a single MCP is an exponential without a peak structure for
photon induced events.

Figure 9:  Generic gain dependence on the applied voltage for a single-stage,
a stack of two (Chevron stack), and a stack of three MCPs (Z-stack). The
inlayed plots illustrate the typical shape of the actual charge distribution
at the points on the gain curves (probability vs. number of electrons in an
event).

A special case are MCPs with curved or J-like channels that also reduce ion
feedback and allow photon-counting operation in single-stage mode Timothy1977
(59). Such plates were mainly applied in MAMA (Multi Anode Microchannel Array)
detectors that achieve with a rather simple digital-like readout a high
spatial resolution. Such detectors have been applied in the NUV and FUV
channel of STIS (Space Telescope Imaging Spectrograph) as well as in other
instruments of the Hubble space telescope (HST) Woodgate1998 (70, 60).

Similarly to CEMs, MCPs can only be operated under high vacuum conditions for
several reasons:

  1. 1.

Under atmospheric conditions the mean free path for electrons would not be
sufficient to reach and impact the opposite channel wall and thus trigger
secondary electron emission.

  2. 2.

If the pressure is not sufficiently low a large number of positive ions are
created within the channels and are accelerated towards the front face of the
plate. Due to their higher mass they move much slower than electrons and when
they impact the channel wall, they trigger the formation of additional delayed
pulses – the so-called ion feedback.

  3. 3.

Most photocathode materials that are coated on the top face to increase the
sensitivity degrade rapidly under atmospheric conditions. Some are
hygroscopic, others degrade under exposure to oxygen. Depending on the
specific material the time constant for this process varies from seconds to
hours.

  4. 4.

Applying typically $1\,\mathrm{kV}$ to a millimeter thick plate is only
possible under high vacuum without risking corona discharges that would
immediately destroy the MCPs. Since the breakthrough voltage of air and other
gases intially drops when lowering the pressure and reaches a minimum in the
$1\,\mathrm{mbar}$ regime, a pressure below $10^{-3}\,\text{mbar}$ is
necessary for a safe and stable operation of MCPs.

###  4.4 Quantum detection efficiency

The quantum detection efficiency (QDE) is one of the basic benchmark
parameters for a radiation detector. It is defined as the probability that a
single photon hitting the sensitive detector area is registered and processed
by the electronics. Care has to be taken because literature quite often refers
only to quantum efficiency (QE) without clarifying what is meant exactly. In
particular, this can be misleading when comparing photon-integrating detectors
to photon-counting detectors like MCPs.

One of the primary parameters defining the QDE of an MCP detector is the open
area ratio (OAR) – the ratio of open channel surface to the total surface of
the plate. For conventional lead glass MCPs, the lead content in the bulk
material of up to 48 % by weight is mostly responsible for the hard X-ray and
gamma-ray response, while soft X-rays and UV photons interact only with the
roughly 10 nm thick semiconducting surface layer that is depleted from lead
and enriched with lighter elements Fraser2009 (20). Therefore, particular for
the softer part of the X-ray spectrum, an increased OAR should lead to a
higher QDE. With small straight channels of a few micrometer in diameter an
OAR of up to roughly 70 % can be reached. By introducing in the fabrication
process a second etching step that affects mainly the front-side channel
openings, a funnel shape can be created that can increase the OAR to more than
90 %.

Another more common method to increase the QDE is to place an electron
repeller grid – usually a thin wire mesh or metal foil – in front of the
detector to which a negative bias voltage is applied with respect to the front
surface of the first MCP (cf. Figure 11). When a photon hits the bulk material
between the channels and the associated photoelectron is emitted away from the
detector, the electric field bents the electron back towards the MCP where it
may again hit the bulk and be lost or enter a channel and be detected.
Thickness and distance of the grid wires as well as the applied voltage are a
trade-off between optimizing the field and thus the position resolution and
reducing shadowing effects.

Uncoated MCPs have in the EUV and soft X-ray bands a QDE in the range of 1-10
%, typically decreasing with energy Fraser2009 (20). Additionally, a strong
dependance on the angle of incidence is seen, with lower efficiencies at close
to normal as well as grazing incidence and peaking close to the critical angle
of reflection from the MCP glass. This empirical finding is in accordance with
theoretical predictions Bjorkholm1977 (5, 21). The QDE can be enhanced by
coating a photocathode on the front surface that extends into the beginning of
the channels. While the photocathode material of choice for the first soft
X-ray detectors was magnesium fluoride (MgF2) that was mainly chosen for its
stability, cesium iodide (CsI) became later the baseline material and was
commonly used in a large waveband from 180 nm down to 0.2 nm. A drawback is
the hygroscopic nature of CsI leading to degradation within minutes when
handled under atmospheric conditions. Even water molecules gettered in the
bulk of an MCP can induce this degradation process, therefore the plate should
be baked out thoroughly before coating Fraser1982 (21). A more stable option
that reaches even higher efficiencies between 150 nm and 4 nm is potassium
bromide (KBr). An MCP with a KBr photocathode can be handled in air for about
half an hour without considerable negative effects and even longer when
flushed with dry nitrogen. Cesium bromide (CsBr) is another option for the
short wavelengths between 10 nm and 2 nm.

Besides an increased QDE in the detection waveband a photocathode offers the
possibility to avoid background from longer wavelength by choosing a material
with a sufficiently wide band gap. This allows the fabrication of solar-blind
detectors with a vanishing sensitivity for visible light. For EUV and X-ray
detectors the Lyman-Alpha line at 121.6 nm is often a considerable source of
background and even typical wide-bandgap photocathodes have a finite
sensitivity at this wavelength, thus a thin metal filter is usually foreseen
to remove this out of band background together with low energy charged
particles.

Particularly in the NUV and down to about 110 nm in the FUV band,
semitransparent photocathodes coated on the inside of a window are an
interesting alternative to thick, opaque photocathodes directly on the MCP.
With a stable and thick window – MgF2 for the FUV, quartz for the NUV – a
sealed detector can be build that does not need any shutter mechanism to keep
MCPs and photocathode under vacuum or in a protective gas atmosphere before it
can be opened in space Siegmund2021 (54, 11). This is not possible for EUV and
X-ray detectors as no material with sufficiently high transmission for these
wavelengths exist that is leak tight and stable to cope with the pressure
difference. However, semitransparent photocathodes on X-ray transparent layers
were investigated and used for some time in combination with shutter
mechanisms, but it could be proven that these are not more efficient than
conventional opaque photocathodes while being less reproducible Fraser2009
(20). In contrast to opaque photocathodes, the thickness is a critical
parameter in this semitransparent mode, since the layer must be thick enough
to absorb the photon but at the same time thin enough to allow the emission of
the photoelectron.

While channel pitch $p$ and diameter $d$ are not relevant for the sensitivity
for soft X-rays, the detection shifts to the bulk and the photoelectron paths
are longer when the photon energy increases, and thus a thick plate with thin
walls, i.e. large length $l$ and small $p-d$, is optimal for hard X-rays.
However, while around 20 keV up to 10 % peak QDE can be achieved, the
efficiency stays at higher energies up to 1 MeV quite constant below 4 %
Fraser2009 (20). Therefore, technologies with tremendously higher QDE than
MCPs exist for this range.

###  4.5 Position-sensitive readout, spatial and temporal resolution

Historically, MCP detectors were the first detectors for soft X-ray astronomy
featuring a high spatial resolution significantly below $100\,\mathrm{\mu m}$.
The ultimate limit for single events is the channel pitch, and with modern
readouts the question is not how to reach this but instead how low the power
consumption can be – to facilitate the thermal design of the instrument – and
how low the gain can be chosen – to enlarge the MCP lifetime. However, for a
statistical sample of $n$ counts in a particular feature, a continuously
digitally oversampled detector can reach a spatial FWHM that is even better
than the channel pitch by a factor $\sqrt{n}$ Siegmund1986 (53).

In principle, the same position-sensitive anode designs can be implemented in
MCP detectors that are also used for imaging proportional counters (s. Section
3). A revolutionary anode for MCP detectors was the wedge-and-strip anode
(WSA) Martin1981 (36) that allowed with the principle of charge sharing
spatial resolutions down to about $40\,\mathrm{\mu m}$ with only four readout
channels. The WSA geometry is sketched in Figure 10(a). The x- and
y-coordinates of the center-of-gravity of the charge cloud can be retrieved
with rather simple calculations:

$x=\frac{x_{2}}{x_{1}+x_{2}}$ (7)

$y=\frac{y_{2}}{y_{1}+y_{2}}$ (8)

(a) Wedge-and-strip anode (WSA)

(b) Crossed delay-line anode (DLA)

(c) Co-planar cross-strip anode (CSA)

Figure 10:  Schematic drawings of typical position-sensitive anodes for MCP
detectors: (a) Wedge-and-strip anode: the position is encoded in the charge
shared between two complementary wedges for the x-direction and two strips
with complementary width in the y-direction. (b) Crossed delay-line anode: the
difference in time of arrival on the different ends of the two delay-lines is
used to determine the position information. (c) Co-planar cross-strip anode:
the charge is recorded with a large number of parallel strips in x- and
y-direction with individual readout channels. The position is determined with
a resolution considerably smaller than the strip pitch by applying a
centroiding algorithm to the digitized data.

In an improved design in which two of the four electrodes are combined the
number of channels even reduces to three by the price of increasing the
capacitance per channel. In general, the signal-to-noise ratio of a charge-
sensitive preamplifier increases with decreasing input capacitance and,
therefore, the number of readout channels is usually a trade-off between noise
on the individual channels and the number of channels.

However, similar to the position-sensitive readout of proportional counters,
also for MCP detectors an alternative concept to charge-sharing uses the time-
of-arrival of signals rather than the charge-spreading information to
determine the position. In later astronomical MCP detectors this principle of
the delay-line anode (DLA) clearly dominated. It is sketched in Figure 10(b).
Two delay-lines are routed meander-like over the anode area and time-to-
digital converters (TDC) on both ends of each line sense the time-of-arrival
of a charge pulse. From the time differences the x- and y- positions can be
calculated. Depending on the geometry, the MCP gain, and the complexity of the
readout electronics a DLA can achieve a position resolution below
$10\,\mathrm{\mu m}$ and thus allows imaging of the MCP pores.

Two other anode concepts under development for MCP detectors promise some
advantages over the established ones but have not yet been applied in an
astronomical instrument: the co-planar cross-strip anode (CSA, s. Figure
10(c), Conti2018 (12)) and the direct readout of the charge cloud with an ASIC
Tremsin2020 (61). Both point in the same direction as was already sketched
above for micropattern readout anodes for proportional counters (s. Section
3.3) and can only be realized with modern high density integrated circuits
that feature a large number of channels and sufficient radiation hardness. The
CSA employs a large number of linear electrodes connected to individual
readout channels and by applying a centroid algorithm on the digitized charge
information a position resolution considerably smaller than the strip-pitch
can be reached Conti2018 (12).

Besides a high spatial resolution, the MCP technology offers the possibility
for astonishing high temporal resolution compared to gas or semiconducting
X-ray detectors, but this feature was never exploited in astronomical
detectors. The signal transit time through an MCP is typically around
$100\,\mathrm{ps}$ with a jitter at least a factor of 10 lower and scaling
with the channel diameter $d$. Therefore, MCP detectors offer an ultimate
temporal resolution of the order of $10\,\mathrm{ps}$. A rather simple method
to reach a high temporal resolution without sacrificing spatial resolution is
to decouple these quantities: a fast timing of the order of $500\,\mathrm{ps}$
can be reached by triggering from the pulse on the MCP backside electrode that
is generated when the charge cloud exits, while the event position is encoded
with one of the slow readouts already discussed.

Usually an MCP detector does not feature any energy resolution besides its
out-of-band rejection. However, a poor but finite energy resolution is
possible for soft X-rays if the first plate of a stack is operated with lower
than saturation voltage. In this configuration slight gain variations
depending on the X-ray energy are seen. This would make a two to three color
photometry possible, e.g. with the HRC on Chandra (s. Section 4.6), but has
not been exploited in any X-ray mission.

###  4.6 Applications in EUV and X-ray astronomy

MCP detectors were for many years the preferred detector technology for UV
instrumentation, particularly in the FUV below $180\,\mathrm{nm}$. They were
used on many missions, e.g. IUE (International Ultraviolet Explorer; launched
in 1978), HST (Hubble Space Telescope; launched in 1991, four service
missions), ORFEUS (Orbiting and Retrievable Far and EUV Satellite; flown in
1993 and 1996 with NASA space shuttles), FUSE (Far Ultraviolet Spectroscopic
Explorer; launched in 1999), and GALEX (Galaxy Evolution Explorer, launched in
2003), to name just the most important ones for stellar astronomy.

In the EUV wavelength band from the Lyman limit at $91.2\,\mathrm{nm}$ down to
about $10\,\mathrm{nm}$, hardly an alternative to MCPs exist. A recommendable
overview over instrumentation and science topics in this waveband is given in
Barstow2003 (4), later complemented to account for more recent developments in
Barstow2014 (3). Two major surveys were carried out in the EUV, both with
instruments based on MCP detectors: the first one with the EUV camera (WFI –
Wide Field Imager) on ROSAT (ROentgen SATellite; launched in 1990), and the
second one with EUVE (Extreme Ultraviolet Explorer; launched in 1992), the
first mission fully dedicated to the EUV waveband Bowyer1991 (6). And even
today, particularly when a photon-counting detector is needed, the baseline
technology in the EUV remains MCPs with a KBr photocathode, as planned for the
proposed ESCAPE France2022 (19) and SIRIUS missions Barstow2012 (1).

There have been several missions in the soft X-ray band that employed MCP-
based detectors, often combined with other instruments and detector
technologies on the same satellite. The following compilation is surely not
complete, but tries to give an overview of missions and instruments with an
outstanding impact on the field.

A cornerstone mission for X-ray astronomy was the Einstein (HEAO-2)
observatory launched in 1978 Giacconi1979 (26). It was not only the first
imaging X-ray telescope in space, but also the first mission to apply an MCP
detector in its High Resolution Imager (HRI) Kellogg1976 (32). Observations
with this instrument remained for many years the highest resolution images of
the X-ray sky. A quite recommendable recount of the development history of
this mission was published in Tucker2013 (63). The HRI instrument has been
tested on several rocket flights and finally after four failures it worked
flawlessly in the fifth test flight only four months prior to launch. In space
it worked without problems and produced images in the 200 eV to 4 keV band.
The detector was enclosed in a vacuum housing that was pumped with an ion pump
before a door was opened in space to expose the MgF2 coated sensitive front.
In fact, HEAO-2 featured three HRIs, identical but for the composition of
their UV/ion shields.

In 1990 the German-led ROSAT mission was launched with one of the instruments
onboard going by the same name HRI. This is not a coincidence since this US
contribution to the mission was almost identical to the HRI design for HEAO-2,
only the MgF2 photocathode was exchanged for CsI. While the spatial resolution
stayed the same, this led to a boost in efficiency by a factor 1.5 to 4 over
the spectral range of the instrument of 0.1–2 keV Pfeffermann1987 (42).

With its Wide Field Camera (WFI) contributed by the UK ROSAT featured a second
instrument read out with an MCP detector. It was sensitive in the EUV range
6–20 nm with a field-of-view (FOV) $None$ in diameter. The MCP detector for
the WFI applied also a CsI photocathode similar to the HRI detector, but with
an additional electron repeller grid in front. A further sensitivity
enhancement of up to a factor of 2.5 at the edges of the FOV was reached by
matching the spherical shape of the focal surface with curved MCPs Barstow1985
(2).

ROSAT was designed for a minimal lifetime of 18 months with a goal of five
years, but actually the mission ran for almost nine years. An incident a few
months before the mission clearly demonstrated the severe damage that an
overexposure can create in an astronomical sensor: due to a reaction wheel
failure the HRI was exposed to sunlight and afterwards hardly usable
Truemper1999 (62). However, disregarding this particular event, it is notable
that the MCP-based detectors were still fully operational when the
proportional counters of ROSAT already became inoperable because of gas loss
and thus insufficient pressure.

Also the High Resolution Camera (HRC) of the Chandra X-ray Observatory is a
direct descendant of the Einstein HRI. The major improvements are the twenty
times larger image area while maintaining the same spatial resolution, a thick
CsI photocathode with further enhanced sensitivity, and a partially saturated
operation of the MCP stack that allowed in principle a finite energy
resolution enabling two to three color photometry Weisskopf2003 (65). A sketch
of the HRC-I imaging detector including its housing is shown in Figure 11.
With a sensitive area of $10\times 10\,\mathrm{cm^{2}}$ it is the largest MCP
detector ever used in X-ray astronomy. In order to reduce the background level
of this particularly large sensitive volume the inner part of the detector is
shielded with the high-Z material tantalum, primarily to capture penetrating
high energy photons. Additionally, the sensitive volume is surrounded by an
active anti-coincidence shield so that penetrating highly energetic charged
particles are vetoed.

Figure 11:  Cross-sectional sketch of the Chandra HRC-I detector. A stack of
two square $10\times 10\,\mathrm{cm^{2}}$ MCPs is used with a thick CsI
photocathode coated on the top plate. The exiting charge is detected with a
crossed wire grid that allows a position resolution well matched to the
angular resolution of the X-ray optics. A thin metallic filter provides
shielding from UV photons and ions. Further, to enhance the sensitivity, this
filter is put on a negative potential with respect to the photocathode to
deflect photoelectrons emitted from the surface back to the MCPs. The inner
part of the detector is shielded on five sides: a passive shielding catches
high energy X-rays and an active anti-coincidence system vetos events induced
by high energy particles.

##  5 Future prospects

Most current developments in the MCP domain focus on applications in the FUV
and EUV bands where MCPs still remains the dominating technology for a wide
range of astronomical applications France2022 (19). In the soft X-ray band
above 200 eV the efforts concentrate on semiconductors, e.g. silicon drift
detectors (SDDs) for large sensitive areas Rachevski2014 (47, 17) or active
pixel sensors like depleted field effect transistor arrays (DepFETs)
MuellerSeidlitz2022 (37), as well as on micro-calorimeter arrays with their
unprecedented energy resolution Pajot2018 (39, 57). On the other wavelength
end, silicon detectors like CCDs or CMOS sensors are improved towards lower
wavelengths by anti-reflective coatings and delta-doping so that they are
superior to MCPs in the NUV for a large number of applications.

Concerning local count rate capabilities the MCP technology has reached its
principle limit since many years. The recharging time constant depends on the
plate resistance that cannot be decreased without risking a thermal run-away.
Concerning sensitivity, there is no prospect of substantial improvements in
the EUV and soft X-ray band, while for NUV and FUV new photocathode materials
like GaN and AlGaN are actively developed Conti2020 (10, 9). An enormous
progress was made in the last years concerning lifetime and gain stability
with the development of borosilicate MCPs that are fabricated with the hollow-
tube technique and functionalized with an ALD process Cremer2020 (13).

Substantial efforts are also invested in compact and low-power readout
electronics for cross-strip anodes that allow high spatial resolution with
lower MCP gain as for the baseline cross delay-line anodes Pfeifer2014 (44,
55). Another readout that is still under active development is the direct
readout with an ASIC. Most advanced are here the efforts to use a
Medipix/Timepix chip that was originally developed for high-energy detector
applications Tremsin2020 (61).

An interesting secondary application of the MCP technology is in so-called
lobster eye X-ray optics, a wide field-of-view X-ray imaging technique
invented already in the 1990s Fraser1992 (24) but realized only now in several
upcoming missions: SMILE, SVOM, and Einstein Probe. All these missions are
presented in the section X-ray Missions within this handbook.

In the field of gaseous detectors for X-ray astronomy, during the last years
the development focused on gas pixel detectors for polarimetric instruments.
Although several attempts with instruments on rockets and satellites have been
made over the last 50 years in this domain, only the recent advancements with
gas pixel detector technology allow to determine polarization degree and
direction of astrophysical sources over a wide energy range and with a high
signal-to-noise ratio. While IXPE is already applying this concept in space,
the upcoming eXTP mission is designed to be able to observe an even larger
number of sources for which the polarimetric information will be of paramount
importance to better constrain the geometry and the underlying physics.

## Cross-References

Several chapters in this handbook are dedicated to missions applying
proportional counters:

  * •

The AstroSat Observatory by Singh, K.

  * •

eXTP by Santangelo, A., Nan Zhang, S.

  * •

MAXI: Monitor of All-sky X-ray Image by Mihara, T., Tsunemi, H., Negoro, H.

  * •

IXPE: The Imaging X-ray Polarimetry Explorer by Weisskopf, M., Soffitta, P.,
Ramsey, B., Baldini, L.

More information on advanced gaseous detectors specifically for measuring
X-ray polarization can be found in these chapters:

  * •

Gas pixel detectors for polarimetry by Soffitta, P., Costa, E.

  * •

Time projection chamber X-ray Polarimeters by Black, K., Zajczyk, A.

Microchannel plate detectors are used on the missions in the follow chapters:

  * •

The AstroSat Observatory by Singh, K.

  * •

The Chandra X-ray Observatory by Wilkes, B., Tananbaum, H.

Microchannel plates as collimators for so-called lobster eye wide field X-ray
telescopes are applied in the missions in the follow chapters:

  * •

The Einstein Probe Mission by Yuan, W., Zhang, C., Chen, Y., Ling, Z.

  * •

The SMILE mission by Branduardi-Raymont, G., Wang, C.

  * •

SVOM by Wei, J.

## References

  * (1) M.. Barstow et al.  “Observatory-class science with a low-cost EUV astronomy mission”  In _Space Telescopes and Instrumentation 2012: Ultraviolet to Gamma Ray_ 8443 SPIE, 2012, pp. 10–18  International Society for OpticsPhotonics  DOI: 10.1117/12.924903
  * (2) M.. Barstow, R. Willingale, B.. Kent and A. Wells  “Early performance data from the ROSAT XUV wide field camera.”  In _Opt. Acta_ 32.2, 1985, pp. 197–207
  * (3) M.A. Barstow, S.L. Casewell, J.B. Holberg and M.P. Kowalski  “The status and future of EUV astronomy” Stars, Galaxies and Star Formation History in the UV  In _Advances in Space Research_ 53.6, 2014, pp. 1003–1013  DOI: 10.1016/j.asr.2013.08.007
  * (4) Martin A. Barstow and Jay B. Holberg  “Extreme Ultraviolet Astronomy”, Cambridge Astrophysics  Cambridge University Press, 2003  DOI: 10.1017/CBO9780511536144
  * (5) Paul J. Bjorkholm, Leon P. Speybroeck and Michael Hecht  “X-Ray Quantum Efficiency Of Microchannel Plates”  In _X-Ray Imaging_ SPIE, 1977  DOI: 10.1117/12.955471
  * (6) S. Bowyer and R.F. Malina  “The extreme ultraviolet explorer mission”  In _Advances in Space Research_ 11.11 Elsevier BV, 1991, pp. 205–215  DOI: 10.1016/0273-1177(91)90077-w
  * (7) H.V. Bradt, J.H. Swank and R.E. Rothschild  “The X-ray timing explorer”  In _Advances in Space Research_ 11.8 Elsevier BV, 1991, pp. 243–254  DOI: 10.1016/0273-1177(91)90176-k
  * (8) A. Bulgarelli and M. Guainazzi  “Tutorial Guide to X-ray and Gamma-ray Astronomy”  Springer Nature, Singapore, 2020, pp. 119–184
  * (9) Qing Cai et al.  “Progress on AlGaN-based solar-blind ultraviolet photodetectors and focal plane arrays”  In _Light: Science & Applications_ 10.1 Springer ScienceBusiness Media LLC, 2021  DOI: 10.1038/s41377-021-00527-4
  * (10) Lauro Conti et al.  “MCP detector development for UV space missions”  In _Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray_ SPIE, 2020  DOI: 10.1117/12.2560663
  * (11) Lauro Conti et al.  “A photon counting imaging detector for UV space missions”  In _Space Telescopes and Instrumentation 2022: Ultraviolet to Gamma Ray_ SPIE, 2022  DOI: 10.1117/12.2628735
  * (12) Lauro Conti et al.  “MCP detector development for UV space missions”  In _Astrophysics and Space Science_ 363.4 Springer ScienceBusiness Media LLC, 2018  DOI: 10.1007/s10509-018-3283-4
  * (13) Till Cremer et al.  “High-Performance Large-Area Microchannel Plate Detectors for Particle Identification Applications”  In _2020 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC)_ , 2020, pp. 1–8  DOI: 10.1109/NSS/MIC42677.2020.9507831
  * (14) Sebastian Diebold et al.  “Soft proton scattering efficiency measurements on x-ray mirror shells”  In _Experimental Astronomy_ 39.2 Springer ScienceBusiness Media LLC, 2015, pp. 343–365  DOI: 10.1007/s10686-015-9451-4
  * (15) Richard J. Edgar  “Proportional counters and other detector techniques”  In _Handbook of X-ray Astronomy_ Cambridge University Press, New York, 2011, pp. 23–38
  * (16) C. Ertley et al.  “Performance studies of atomic layer deposited microchannel plate electron multipliers”  In _Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment_ 912 Elsevier BV, 2018, pp. 75–77  DOI: 10.1016/j.nima.2017.10.050
  * (17) Y. Evangelista et al.  “Characterization of a novel pixelated Silicon Drift Detector (PixDD) for high-throughput X-ray astrophysics”  In _Journal of Instrumentation_ 13.09 IOP Publishing, 2018, pp. P09011–P09011  DOI: 10.1088/1748-0221/13/09/p09011
  * (18) U. Fano  “Ionization Yield of Radiations. II. The Fluctuations of the Number of Ions”  In _Phys Rev_ 72.1, 1947, pp. 26–29  DOI: 10.1103/PhysRev.72.26
  * (19) Kevin France et al.  “Extreme-ultraviolet Stellar Characterization for Atmospheric Physics and Evolution mission: motivation and overview”  In _Journal of Astronomical Telescopes, Instruments, and Systems_ 8.1 SPIE, 2022, pp. 1–21  DOI: 10.1117/1.JATIS.8.1.014006
  * (20) G.. Fraser  “X-ray Detectors in Astronomy”  Cambridge University Press, New York, 2009  DOI: 10.1017/cbo9780511735554
  * (21) G.. Fraser, M.. Barstow, M.. Whiteley and A. Wells  “Enhanced soft X-ray detection efficiencies for imaging microchannel plate detectors”  In _Nature_ 300.5892 Springer ScienceBusiness Media LLC, 1982, pp. 509–511  DOI: 10.1038/300509a0
  * (22) G.W. Fraser and E. Mathieson  “Signal location by uniform resistive anodes”  In _Nuclear Instruments and Methods_ 179.3 Elsevier BV, 1981, pp. 591–604  DOI: 10.1016/0029-554x(81)90189-0
  * (23) G.W. Fraser and E. Mathieson  “Signal location by uniform resistive anodes”  In _Nuclear Instruments and Methods_ 184.2-3 Elsevier BV, 1981, pp. 537–542  DOI: 10.1016/0029-554x(81)90760-6
  * (24) George W. Fraser, John E. Lees, James F. Pearson, Mark R. Sims and K. Roxburgh  “X-ray focusing using microchannel plates”  In _SPIE Proceedings_ SPIE, 1992  DOI: 10.1117/12.51224
  * (25) Maximilian Gebhard et al.  “Formation of Unsaturated Hydrocarbons and Hydrogen: Surface Chemistry of Methyltrioxorhenium(VII) in ALD of Mixed-Metal Oxide Structures Comprising Re(III) Units”  In _Chemistry of Materials_ 31.19 American Chemical Society (ACS), 2019, pp. 7821–7832  DOI: 10.1021/acs.chemmater.9b01408
  * (26) R. Giacconi et al.  “The Einstein /HEAO 2/ X-ray Observatory”  In _The Astrophysical Journal_ 230 American Astronomical Society, 1979, pp. 540  DOI: 10.1086/157110
  * (27) R. Giacconi, E. Kellogg, P. Gorenstein, H. Gursky and H. Tananbaum  “An X-Ray Scan of the Galactic Plane from UHURU”  In _The Astrophysical Journal_ 165 American Astronomical Society, 1971, pp. L27  DOI: 10.1086/180711
  * (28) P. Gorenstein, F.. Harnden and D.. Fabricant  “In Orbit Performance of the Einstein Observatory/HEAO-2 Imaging Proportional Counter”  In _IEEE Transactions on Nuclear Science_ 28.1 Institute of ElectricalElectronics Engineers (IEEE), 1981, pp. 869–874  DOI: 10.1109/tns.1981.4331295
  * (29) V.. Hess  “Über Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten”  In _Physikalische Zeitschrift_ 13, 1912, pp. 1084–1091
  * (30) J.H. Hubbell and S.M. Seltzer  “Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients, NIST Standard Reference Database 126”  National Institute of StandardsTechnology, 2004  DOI: 10.18434/T4D01F
  * (31) C.. Keller  “X-rays from the Sun”  In _Experientia_ 51.7 Springer ScienceBusiness Media LLC, 1995, pp. 710–720  DOI: 10.1007/bf01941268
  * (32) E. Kellogg, P. Henry, S. Murray, L. Speybroeck and P. Bjorkholm  “High-resolution imaging x-ray detector”  In _Review of Scientific Instruments_ 47.3 AIP Publishing, 1976, pp. 282–290  DOI: 10.1063/1.1134632
  * (33) G.. Knoll  “Radiation Detection and Measurement”  Wiley, New York, 2010
  * (34) Michael Lampton  “The Microchannel Image Intensifier”  In _Scientific American_ 245.5 Scientific American, a division of Nature America, Inc., 1981, pp. 62–71  URL: http://www.jstor.org/stable/24964601
  * (35) N. Lund et al.  “JEM-X: The X-ray monitor aboard INTEGRAL”  In _Astronomy & Astrophysics_ 411.1 EDP Sciences, 2003, pp. L231–L238  DOI: 10.1051/0004-6361:20031358
  * (36) C. Martin, P. Jelinsky, M. Lampton, R.. Malina and H.. Anger  “Wedge-and-strip anodes for centroid-finding position-sensitive photon and particle detectors”  In _Review of Scientific Instruments_ 52.7 AIP Publishing, 1981, pp. 1067–1074  DOI: 10.1063/1.1136710
  * (37) Johannes Müller-Seidlitz et al.  “Spectroscopic performance of flight-like DEPFET sensors for Athena’s WFI”  In _Space Telescopes and Instrumentation 2022: Ultraviolet to Gamma Ray_ SPIE, 2022  DOI: 10.1117/12.2629474
  * (38) A. Oed  “Position-sensitive detector with microstrip anode for electron multiplication with gases”  In _Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment_ 263.2-3 Elsevier BV, 1988, pp. 351–359  DOI: 10.1016/0168-9002(88)90970-9
  * (39) F. Pajot et al.  “The Athena X-ray Integral Field Unit (X-IFU)”  In _Journal of Low Temperature Physics_ 193.5-6 Springer ScienceBusiness Media LLC, 2018, pp. 901–907  DOI: 10.1007/s10909-018-1904-5
  * (40) E. Pfeffermann  “Imaging Proportional Counters”  In _The Universe in X-Rays_ Berlin, Heidelberg: Springer Berlin Heidelberg, 2008, pp. 21–28  DOI: 10.1007/978-3-540-34412-4˙4
  * (41) E. Pfeffermann  “Proportional Counters”  In _The Universe in X-Rays_ Berlin, Heidelberg: Springer Berlin Heidelberg, 2008, pp. 5–14  DOI: 10.1007/978-3-540-34412-4˙2
  * (42) E. Pfeffermann et al.  “The focal plane instrumentation of the ROSAT Telescope”  In _Proc. SPIE_ 733, 1987, pp. 519
  * (43) Elmar Pfeffermann, Ulrich G. Briel and Michael J. Freyberg  “Design and in-orbit-performance of the position sensitive proportional counter onboard the X-ray astronomy satellite ROSAT”  In _Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment_ 515.1-2 Elsevier BV, 2003, pp. 65–69  DOI: 10.1016/j.nima.2003.08.132
  * (44) M Pfeifer et al.  “Low power readout electronics for a UV MCP detector with cross strip anode”  In _Journal of Instrumentation_ 9.03 IOP Publishing, 2014, pp. C03059–C03059  DOI: 10.1088/1748-0221/9/03/c03059
  * (45) S D Pinto  “Micropattern gas detector technologies and applications the work of the RD51 collaboration”  In _IEEE Nuclear Science Symposuim & Medical Imaging Conference_ IEEE, 2010  DOI: 10.1109/nssmic.2010.5873870
  * (46) M.. Popecki et al.  “Microchannel plate fabrication using glass capillary arrays with Atomic Layer Deposition films for resistance and gain”  In _Journal of Geophysical Research: Space Physics_ 121.8 American Geophysical Union (AGU), 2016, pp. 7449–7460  DOI: 10.1002/2016ja022580
  * (47) A Rachevski et al.  “Large-area linear Silicon Drift Detector design for X-ray experiments”  In _Journal of Instrumentation_ 9.07 IOP Publishing, 2014, pp. P07014–P07014  DOI: 10.1088/1748-0221/9/07/p07014
  * (48) Brian D. Ramsey, Robert A. Austin and Rudolf Decher  “Instrumentation for X-ray astronomy”  In _Space Science Reviews_ 69.1-2 Springer ScienceBusiness Media LLC, 1994, pp. 139–204  DOI: 10.1007/bf00756035
  * (49) E. Rutherford and H. Geiger  “An electrical method of counting the number of $\alpha$-particles from radio-active substances”  In _Proc Roy Soc A_ 81, 1908, pp. 141  DOI: 10.1098/rspa.1908.0065
  * (50) F. Sauli  “GEM: A new concept for electron amplification in gas detectors”  In _Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment_ 386.2-3 Elsevier BV, 1997, pp. 531–534  DOI: 10.1016/s0168-9002(96)01172-2
  * (51) Fabio Sauli  “Principles of operation of multiwire proportional and drift chambers”  CERN, 1977  DOI: 10.5170/CERN-1977-009
  * (52) Fabio Sauli  “The gas electron multiplier (GEM): Operating principles and applications”  In _Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment_ 805 Elsevier BV, 2016, pp. 2–24  DOI: 10.1016/j.nima.2015.07.060
  * (53) O… Siegmund, M. Lampton, J. Bixler, S. Bowyer and R.. Malina  “Operational Characteristics of Wedge and Strip Image Readout Systems”  In _IEEE Transactions on Nuclear Science_ 33.1 Institute of ElectricalElectronics Engineers (IEEE), 1986, pp. 723–727  DOI: 10.1109/tns.1986.4337201
  * (54) Oswald H.. Siegmund, Travis Curtis, Jason McPhate, Nate Darling and Till Cremer  “Development of sealed cross strip readout UV detectors”  In _UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XXII_ SPIE, 2021  DOI: 10.1117/12.2601037
  * (55) Oswald H.. Siegmund et al.  “Development of UV imaging detectors with atomic layer deposited microchannel plates and cross strip readouts”  In _X-Ray, Optical, and Infrared Detectors for Astronomy IX_ SPIE, 2020  DOI: 10.1117/12.2561753
  * (56) H. Sipilä  “Energy resolution of the proportional counter”  In _Nucl Instr Meth_ 133, 1976, pp. 251–252  DOI: 10.1016/0029-554X(76)90616-9
  * (57) E. Taralli et al.  “Small Size Transition-Edge Sensors for Future X-Ray Applications”  In _Journal of Low Temperature Physics_ Springer ScienceBusiness Media LLC, 2022  DOI: 10.1007/s10909-022-02709-9
  * (58) J.. Thomson  “XIX. On the theory of the conduction of electricity through gases by charged ions”  In _The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science_ 47.286 Informa UK Limited, 1899, pp. 253–268  DOI: 10.1080/14786449908621258
  * (59) J.. Timothy and R.. Bybee  “Preliminary results with microchannel array plates employing curved microchannels to inhibit ion feedback”  In _Review of Scientific Instruments_ 48.3 AIP Publishing, 1977, pp. 292–299  DOI: 10.1063/1.1135012
  * (60) J. Timothy  “Review of multianode microchannel array detector systems”  In _Journal of Astronomical Telescopes, Instruments, and Systems_ 2.3 SPIE-Intl Soc Optical Eng, 2016, pp. 030901  DOI: 10.1117/1.jatis.2.3.030901
  * (61) A.S. Tremsin and J.V. Vallerga  “Unique capabilities and applications of Microchannel Plate (MCP) detectors with Medipix/Timepix readout”  In _Radiation Measurements_ 130, 2020, pp. 106228  DOI: 10.1016/j.radmeas.2019.106228
  * (62) Joachim Trümper  “ROSAT und seine Nachfolger: Der Forschungssatellit ROSAT hat die Röntgenastronomie revolutioniert”  In _Physik Journal_ 55.9 Wiley, 1999, pp. 45–49  DOI: 10.1002/phbl.19990550910
  * (63) Wallace Tucker and Riccardo Giacconi  “The X-Ray Universe”  Harvard University Press SN - 9780674594449, 2013  DOI: 10.4159/harvard.9780674594449
  * (64) Shoaib Usman and Amol Patil  “Radiation detector deadtime and pile up: A review of the status of science”  In _Nuclear Engineering and Technology_ 50.7 Elsevier BV, 2018, pp. 1006–1016  DOI: 10.1016/j.net.2018.06.014
  * (65) M.. Weisskopf et al.  “An Overview of the Performance of the Chandra X-ray Observatory”  In _Experimental Astronomy_ 16.1 Springer ScienceBusiness Media LLC, 2003, pp. 1–68  DOI: 10.1023/b:expa.0000038953.49421.54
  * (66) M.. Weisskopf et al.  “An Overview of the Performance and Scientific Results from the Chandra X-Ray Observatory”  In _Publications of the Astronomical Society of the Pacific_ 114.791 IOP Publishing, 2002, pp. 1–24  DOI: 10.1086/338108
  * (67) M.. Weisskopf et al.  “Measurement of the X-ray polarization of the Crab Nebula”  In _The Astrophysical Journal_ 208 American Astronomical Society, 1976, pp. L125  DOI: 10.1086/182247
  * (68) Martin C. Weisskopf et al.  “Imaging X-ray Polarimetry Explorer: prelaunch”  In _Journal of Astronomical Telescopes, Instruments, and Systems_ 8.02 SPIE-Intl Soc Optical Eng, 2022  DOI: 10.1117/1.jatis.8.2.026002
  * (69) Joseph Ladislas Wiza  “Microchannel plate detectors”  In _Nuclear Instruments and Methods_ 162.1-3 Elsevier BV, 1979, pp. 587–601  DOI: 10.1016/0029-554x(79)90734-1
  * (70) B.. Woodgate et al.  “The Space Telescope Imaging Spectrograph Design”  In _Publications of the Astronomical Society of the Pacific_ 110.752 IOP Publishing, 1998, pp. 1183–1204  DOI: 10.1086/316243
  * (71) ShuangNan Zhang et al.  “The enhanced X-ray Timing and Polarimetry mission—eXTP”  In _Science China Physics, Mechanics & Astronomy_ 62.2 Springer ScienceBusiness Media LLC, 2018  DOI: 10.1007/s11433-018-9309-2