# Development of a novel, windowless, amorphous selenium based photodetector
for use in liquid noble detectors

M. Rooks 11footnotetext: Corresponding author. S. Abbaszadeh  J. Asaadi  M.
Febbraro  R.W. Gladen22footnotetext: also at Fermi National Accelerator
Laboratory. E. Gramellini  K. Hellier  F. Maria Blaszczyk  A.D. McDonald

###### Abstract

Detection of the vacuum ultraviolet (VUV) scintillation light produced by
liquid noble elements is a central challenge in order to fully exploit the
available timing, topological, and calorimetric information in detectors
leveraging these media. In this paper, we characterize a novel, windowless
amorphous selenium based photodetector with direct sensitivity to VUV light.
We present here the manufacturing and experimental setup used to operate this
detector at low transport electric fields (2.7-5.2 V/$\mu$m) and across a wide
range of temperatures (77K-290K). This work shows that the first proof-of-
principle device windowless amorphous selenium is robust under cryogenic
conditions, responsive to VUV light at cryogenic temperatures, and preserves
argon purity. These findings motivate a continued exploration of amorphous
selenium devices for simultaneous detection of scintillation light and
ionization charge in noble element detectors.

##  1 Introduction

The application and ubiquity of noble liquid detectors in the fields of high
energy physics [1, 2, 3, 4, 5, 6, 7, 8], medical imaging [9, 10, 11, 12], and
rare event searches [13, 14, 15, 16, 17, 18, 19] is due to the many attractive
properties these media provide. Charged particles traversing noble liquids
deposit energy in the form of scintillation light and the ionization charge.
Depending on the application, an experiment may choose to apply an external
electric field and collect ionization electrons. Given the anti-correlation
between the collected ionization charge and the light yield, this comes with a
loss in the overall detected scintillation light.

The collection of the scintillation light is a central tool in noble element
detectors as it provides a number of useful experimental handles. Firstly, the
scintillation light provides a prompt signal (commonly referred to as $t_{0}$)
which allows to record an accurate time associated with the activity observed.
This plays a central role in Time Projection Chambers (TPC’s) [20] which
collect both charge and light as it allows the inference of the position of
the event along the drift dimension from the difference between $t_{0}$ and
the time the charge signal is registered. Secondly, the combination of the
amount of light and charge collected provides a robust estimate of energy
deposited in the noble element detector [21]. Thirdly, techniques in pulse
shape discrimination based on the scintillation light allow to distinguish
recoils due to electrons from recoils due to nuclear interactions [22, 23].
This provides a powerful tool in rare event searches (such as dark matter
applications) to separate signal from background.

Two of the most common liquid nobles, argon (Ar) and xenon (Xe), have very
good scintillation light yields with excellent optical transmission
properties. Thus, even detectors as big as several meter cubed preserve a high
flux of photons observed at the photosensor. One key challenge is that both
elements scintillate in the vacuum ultraviolet (VUV). The typical wavelengths
are 128 nm for liquid argon (LAr) and 178 nm for liquid xenon (LXe). Common
photosensors used to detect low levels of light, e.g. Multi-Pixel Photon
Counters (MPPCs), Silicon photomultipliers (SiPMs), and photo multiplier tubes
(PMTs), are largely insensitive to this wavelength of light due to their
construction and fabrication. More recently, devices custom made to be more
sensitive to VUV wavelengths have started to emerge [24, 25, 26], albeit with
relatively low efficiencies, reaching at most 15-20%. A standard solution to
the mismatch between photosensors’ readout and VUV scintillation light is to
deploy a wavelength shifting (WLS) material that absorbs the VUV light and re-
emits it via fluorescence at a much longer wavelength (typically in the ‘blue’
wavelength). The past years have seen substantial R&D in the field of
wavelength shifters and their application to liquid noble detectors [27]. Two
of the most common WLS materials include 1,1,4,4-tetraphenyl-1,3-butadiene
(TPB) and polyethylene naphthalate (PEN). Despite their ubiquity in
application, these WLS materials have a number of drawbacks including their
deterioration due to environmental effects [28, 29], a complicated delayed
emission time [30, 31], and a relatively low efficiency for the observation of
the re-emmitted photon [32].

The difficulties associated with the detection of the VUV photons has inspired
research into alternative materials which could potentially be sensitive
directly to VUV light. In this paper, we explore an amorphous selenium (aSe)
based detector. Ample literature on aSe based direct conversion active matrix
flat panel imagers (AMFPI) [33] and digital breast tomosynthesis [34] has
taken place in the field of X-ray imaging. The recently developed ability to
perform single-photon-counting (SPC) X-ray experiments using CMOS technology
[35] makes this material an attractive candidate to explore for different
applications. The optical absorption properties of aSe [36] suggest that the
material has excellent efficiency for converting the VUV photons into
electron/hole pairs at shallow depths (nm), thus overcoming potential depth-
dependent effects observed for X-rays [37]. Moreover, the transport properties
of aSe suggest that with sufficiently small distances between the electrodes,
the overall mobilities and lifetimes of the charge carriers should be
sufficiently high to be viable for low photon flux applications.

This paper explores the viability of a windowless aSe based device for
collecting UV light in liquid noble element detectors. As such, the response
of the device is characterized as a function of temperature in the range
relevant for noble element detectors using UV light. The initial exploration
is done at relatively low applied electric fields ($\leq 5$ V/$\mu$m), where
SPC is not expected because of the limited charge yield. Future work is
planned to explore significantly higher electric fields where the holes in aSe
undergo impact ionization and thus liberate additional electron-hole pairs.
This process has been shown to cause amplification of the initial signal and
can result in avalanche gain [38]. The first commercial device utilizing
impact ionization in aSe, referred to as high-gain avalanche rushing
photoconductor (HARP) tubes [39] were initially commercialized in the late
1980s for the broadcast industry. More recently, novel designs in the
electrodes has shown that avalanche multiplication with sensitivity down to
SPC levels is possible [40]. Thus, the thrust of this work is to perform a
characterization of a simple, but novel, aSe based photon detector with its
deployment in a cryogenic environment to understand the feasibility and
limitations of this device.

Section 2 describes the aSe device and the testing apparati used to
characterize the boards behavior as a function of temperature. Section 3
describes the observations and behavior of the aSe based detector. Finally,
Section 4 offers some closing thoughts and conclusions.

##  2 Experimental Setup

In this section, we describe the aSe device under test. Section 2.1 describes
the devices fabrication and characterization. Section 2.2 presents the
experimental apparatus used to test the aSe boards at cryogenic temperatures
and under UV light exposure. Finally, Section 2.3 describes the custom readout
electronics and high voltage supply needed to collect both holes and electrons
at various electric fields.

###  2.1 Amorphous Selenium Boards

A typical aSe device, as has been used for x-ray and gamma-ray detection [37],
uses a geometric layout which can be described as a “vertical geometry”. This
geometry has the amorphous selenium sandwiched between two horizontal
electrodes, as shown schematically on the left of Figure 1. The electrodes
provide an electric field needed to achieve transport of the charge carriers
created when a photon interacts with the selenium. The vertical geometry can
be used in x-ray and gamma-ray applications because the electrodes are largely
transparent to these photons and thus provides a simplified fabrication
process. However, for use with UV light this configuration is unfavorable
since even a thin amount of material typically used as an upper electrode
(e.g. ITO, gold, copper, etc) will result in a large fraction of all the UV
light being absorbed. To circumvent this problem and allow for feasibility
testing of aSe, we consider a “horizontal geometry” such that in Figure 1.

This geometry consists of a bare printed circuit board (PCB) constructed with
interdigitated electrodes to provide the electric field needed to achieve
transport of the charge carriers. This configuration thus creates a
“windowless” device where the selenium is thermally evaporated directly onto
the board. The selenium is thus exposed directly to the UV source. This
device, as will be shown in this paper, represents a low cost, simple to
manufacture, and scalable solution to a large area VUV sensitive photosensor.
The PCB manufacturing process for areas as large as 2000 cm2 is commercially
ready and low cost [41], the process of uniform and repeatable thermal
evaporation techniques over these areas is well demonstrated [42], and the
ability to scale together large area tiles into one uniform collection plane
is commonly done in experiments [43, 44]. Moreover, the ability for the device
to respond to VUV light using this windowless approach simplifies the
characterization and testing of the device. A study of the electric field
present within the amorphous selenium given the interdigitated electrodes used
in the device tested here is presented in the Appendix B. This study shows
that for the device tested here, the electric field is uniform both across the
electrodes as well as throughout the selenium and follows the geometric
properties one would intuit.

Figure 1: Schematic of the various geometries which an aSe device may be used.
Left: “Vertical Geometry” which utilizes an electrode on the top most layer
which is transparent to the radiation to be detected. Right: “Horizontal
Geometry” which uses interdigitated electrodes to achieve a horizontal
electric field in the aSe and resolves the problem of most electrodes being
non-transparent to VUV light.

The horizontal geometry does present some design challenges. The most readily
available commercial spacing between PCB produced interdigitated electrodes is
limited by the PCB manufacturing process. This results in a limit to the
electric field (in units of Volts/micrometer) which can be applied in such a
configuration. For the experiment presented here, a small commercial board of
20mm$\times$22.5mm was produced with the smallest electrode spacing of $\sim
127\mu$m from a low-cost commercial vendor [41]. The board is shown in Figure
2 before the addition of selenium. In order to obtain avalanche gain in the
aSe, it is necessary to apply higher fields to the prototypes than the case
presented here. Follow-up work on these results will utilize a high density
PCB manufacturing process to explore trace separations down to $25\mu$m.
However, as the applied field increases and the sensitivity to lower incident
photon flux augments, the corresponding increase in dark current will need to
be addressed via the application of electron/hole blocking layers. The details
of which materials provide the best performance has been extensively studied
for x-ray based aSe devices [37], and will need to be explored in this
application. No charge blocking layers were applied for the device under
consideration.

For the boards used in this experiment, the characteristic spacing was
confirmed by obtaining high resolution images using a Nikon Eclipse ME600
microscope paired with a Nikon DXM 1200 digital camera and image editing
software Paint.NET. The typical trace width and spaces were found to be 105.04
$\pm$ 1.94 $\mu$m and 146.57 $\pm$ 1.94 $\mu$m respectively. These values are
used when evaluating the applied electric field in the subsequent measurements
described in Section 3. The trace heights are set by the manufacturing process
and are 35 $\mu$m $\pm$ 5 $\mu$m.

Figure 2: Images of the interdigitated boards used during the experiments
described below at various magnifications. The board is FR4-Standard Tg
130-140C and the electrodes are Electroless nickel immersion gold following
the Restriction of Hazardous Substances guidance (ENIG-RoHS).

Thermal evaporation deposition of selenium onto the boards was performed at
Oak Ridge National Laboratory. An NRC/Varian 3117 E-Beam Vacuum Evaporator was
retrofitted with a molybdenum boat to hold 722 mg of selenium pellets. The
selenium pellets are purchased from Sigma-Aldrich [45] and have a particle
size $<$ 5 mm with a purity of selenium rated for $\geq$ 99.999 %. The PCBs
were placed in a 3D printed mask 10 cm above the boat and the selenium was
heated under high vacuum. The selenium coating was actively measured using a
quartz monitor crystal with an Inficon XTM/2 deposition monitor to $\pm$1 nm
precision to produce a 1.2 $\mu$m aSe layer.

###  2.2 Cryogenic Temperature Test Stand

Testing the viability of the devices for noble elements detectors requires
bringing the boards at liquid elements temperatures ($\sim 80$K). We achieve
this task with the cryogenic test stand shown in Figure 3. The test stand is
housed in a standard 8 in Conflat Flange (CF) cross (Lesker C-0800). The inner
volume is evacuated via a turbo-molecular vacuum pump (Pfeiffer HiCube 80). A
custom heat exchanger is fabricated from two 0.5 inch 304 stainless steel
tubes with 0.125 inch walls which penetrate the top of the flange to allow the
sample under test to be cooled. A block of 304 stainless steel allows the
cryogenic fluid to circulate between the two tubes. The heat exchanger is
cooled via a low pressure liquid nitrogen dewar, where the liquid is allowed
to flow through the heat exchanger. In order to maintain flexibility with the
setup, the sample holder is independent of the heat exchanger and mounts to
the bottom of the heat exchanger. The sample holder used during the tests
described here is machined from 101 copper and is bolted to the heat exchanger
with a sheet of Indium (McMaster 8898N18) placed in between the heat exchanger
and sample holder to aid in the thermal transfer.

The test samples are mounted to the copper sample holder via a custom carrier
PCB’s which is outfitted with a standard M.2 connector, shown on the right of
Figure 3. This connector is used for ease of testing various samples without
interfering/moving the electronics. The sample PCB plugs into the M.2
connector on a carrier PCB which manages the connections for the readout
electronics (shown schematically in Figure 3 in green).

Figure 3: CAD model of the cryogenic teststand, custom carrier board, and test
samples which were fabricated for use in characterization of the aSe devices.

Two PT-100 thin film resistive thermal devices (RTD’s) (P0K1.232.6W.Y.010
[46]) are mounted to monitor the temperature of the heat exchanger and the
device under test. The rate of cooldown is determined by the flow of cyrogenic
fluid through the heat exchanger. For the tests performed here, a manual valve
was adjusted to maintain a cool down rate between 1.1 and 1.7 Kelvin/minute
using liquid nitrogen. Figure 4 shows the typical cool-down curves over eight
data runs compared to an uncontrolled cool-down where liquid nitrogen was
allowed to flow at its maximum rate (resulting in achieving $<80$K in under 30
mins). The samples can be kept at $\sim$80 K for extended periods of time by
continuously flowing liquid nitrogen at a slow, but fixed rate.

The typical warm-up time varies slightly depending on the conditions in the
lab, and is largely driven by the ambient temperature. Samples regularly reach
room temperature within $\sim$10 hours after the nitrogen is shut off and the
total data taking period per experiment lasting $\sim$40 hours in total.

Figure 4: The data recorded from the PT-100 thin film resistive thermal
devices during the cooldown of the experiment for the eight main data taking
campaigns described below as well as one “uncontrolled” cooldown where the
device was allowed to cool as fast as possible . The rate of temperature
change was targeted to be between 1-2 Kelvin / minute during experimental
operations. The relevant temperatures for various liquid cryogens (xenon,
argon, and nitrogen) are noted on the plot for reference.

###  2.3 Data Acquisition and Readout Electronics

The data acquisition system and readout electronics are shown schematically in
Figure 5. The system is driven by a Raspberry Pi 3 Model B [47] running a
Python script which controls two Arduino’s [48]. Two (P0K1.232.3K.B.010.M.U)
RTD Platinum Sensors inside the cryogenic test stand are readout using the
Arduino UNO coupled with a ARD-LTC2499 24-bit ADC data acquisition shield
allowing the measured resistance to be converted to a temperature with milli-
Kelvin precision. The temperatures are recorded to an external solid state
drive via a USB port on the Raspberry Pi. The RTD’s have a dedicated Rigol
DP832 power supply set at 2.048 V to match the ADC threshold. The Arduino Nano
serves as a trigger for a 5 Watt Hamamatsu L11316-11 Xenon flashlamp [49] and
LeCroy 6050 WaveRunner oscilloscope by providing a 5V signal with a rate
configurable by the Raspberry Pi. The flashlamp has a dedicated PS-305D power
supply set to 24 VDC. The oscilloscope triggers on the input from the
flashlamp signal. Data files from the oscilloscope are stored on an external
solid state drive. The temperature data and recorded waveforms are merged
offline via the file number and the timestamp.

Figure 5: Schematic of the data acquisition system and readout electronics
utilized in the characterization of the aSe device.

In order to provide a high voltage (HV) bias between $\pm 750$ volts to the
aSe board and readout the subsequent signals generated from its exposure to UV
light, a custom HV/readout setup was implemented. The HV is generated via an
ENCO DCDC converter which is powered by a 12V lead acid battery. The HV is
filtered and applied to the sample. The charge is read out off the HV line via
a decoupling capacitor and the signal is amplified with an Amptec A250 charge
sensitive amplifier with an intrafet IFN152 as the input jfet [50]. The output
of the A250 is then sent to the oscilloscope for data collection. All
electronics are housed in a shielded enclosure to further reduce noise.

The high voltage supply was tested for stability over a 12 hour period and
found to be stable to less than $0.1\%$ of the target value. The batteries on
the readout box were regularly recharged to ensure no unexpected variation in
the applied voltage. The xenon flashlamp output power was also tested for
stability and repeatability by directly coupling the fiber optic to a THORLABS
DET10A2 photodiode [51] and found to have a ‘shot-to-shot’ variation of $\sim
3\%$ (consistent with the lamps design document) and to have a consistent
light output over a period of 12+ hours to within $1\%$.

##  3 Results and Discussion

Table 1 summarizes the nine data taking campaigns. The first campaign
quantifies the effect of bulk trapping at various temperatures, referred to as
ghosting. This phenomenon and the data used to understand its impact are
described in Section 3.1. Six of the data campaigns were designed to test the
response of the aSe device as a function of temperature at different applied
electric fields. These results are discussed in Section 3.3 and detailed
numerical results are summarized in Appendix A. Two data campaigns were taken
to verify the repeatability of the results as a function of temperature and
are described in the appendix Section C. Variability in the results found
during the repeat measurements is treated as a systematic on the results. The
results in section 3.4.1 report the robustness tests against cryo-cycling.

Data Campaign | Applied Voltage | Electric Field | Charge Carrier
---|---|---|---
| (Volts) | (Volts / $\mu$m) |
Ghosting | +400 | +2.73 | Electrons
| +400 | +2.73 | Electrons
| -400 | -2.73 | Holes
Temperature | +530 | +3.62 | Electrons
Characterization | -530 | -3.62 | Holes
| +750 | +5.16 | Electrons
| -750 | -5.16 | Holes
Repeatability | +400 | +2.73 | Electrons
| -400 | -2.73 | Holes
Table 1: Summary of the data taking campaigns.

###  3.1 Exposure Dependent Signal Reduction in aSe

The phenomenon of a change in the sensitivity of aSe based x-ray imaging
detectors as a result of previous exposure to radiation is referred to as
‘ghosting’ [52]. This phenomenon, which typically results in a decrease in
sensitivity with subsequent exposures, has been determined to have the
dominant mechanism due to bulk trapping of electrons which recombine
subsequently with x-ray generated holes [53]. Holes may also become trapped in
the aSe, affecting the response in either charge collection polarity. The
typical lifetime, $\tau$, for a charge carrier to be released from a trap has
the form [54]

$\tau=\frac{\exp(E_{T}/kT)}{\nu}$ (3.1)

where $E_{T}$ is the energy depth of the trap (eestimated to be 0.9 eV above
the valence band for holes and 1.2 eV below the conduction band edge for
electrons [55]), $k$ is Boltzmann’s constant, $T$ is the absolute temperature,
and $\nu$ is the phonon frequency (taken as $10^{11}$ s-1 [53]). For room
temperature operation $\tau$ has been found to be on the order of minutes for
holes and hours for electrons.

The overall impact that ghosting has on the performance of the aSe based
detector is found to depend on both the applied electric field (reducing the
effect of ghosting with increased field) and the time interval between
exposures (with the effect of ghosting decreasing with longer time between
exposures). This phenomenon has been observed in aSe detectors when exposed to
x-rays and when the detector is in a vertical geometry [56].

We observe a reduction of the signal peak amplitude when exposing our
windowless horizontal geometry detector to repeated pulses of light from the
xenon lamp. This phenomenon suggests the effect is likely due to ghosting. The
top of Figure 6 shows an example of the pulse amplitude recorded when the
board is exposed to the Xenon flashlamp at a pulse rate of 0.1 Hz over a
period of 12 hours. After a period of $\sim 6$ hours at room temperature, the
system reaches an equilibrium state where the pulse amplitude is no longer
noticeably changes. We attribute this to reaching a balance of the clearing of
electron/hole traps at the given field and the creation of new traps with new
electron/hole pairs created from the VUV light.

Given the application for the device under test, we explore this behavior in
cold. Two dedicated runs were taken, the first at room temperature ($\sim
270$K) and the second at cryogenic temperature ($\sim 80$K). For both these
runs the board was allowed to be at the designated temperature for a period of
hours before being exposed to the Xenon flash lamp. The pulse amplitude,
defined in Section 3.2, was then recorded over a six hour period. This data is
shown in the bottom of Figure 6. As anticipated, the time it takes to reach
equilibrium at room temperature is longer than in cold. This is because the
lifetime of the traps depends on the the temperature of the sample and becomes
larger at lower temperatures, as shown in equation 3.1 The relative
equilibrium pulse amplitude is different between the “warm” and “cold” data,
but the stability of this equilibrium is similar. This is consistent with the
model that thermal motion is what leads to the clearing of traps and thus at
cryogenic temperatures becomes more pronounced.

Figure 6: Top: The peak amplitude, defined in Section 3.2, at room temperature
in the cryogenic temperature stand under vacuum and biased to +400 volts over
a 12 hour data taking period when exposed to the xenon flashlamp every 10.8
seconds. The reponse of the aSe board can be seen to degrade over time until
eventually reaching an equilibrium state after $\sim 6$ hours. Bottom: The
recorded peak amplitude over a 6 hour period when the board is held at ambient
temperature (red $\sim 290$ K) and when held at cryogenic temperature (blue
$\sim 80$ K) in the cryogenic temperature stand under vacuum and biased to
+400 volts and exposed to the xenon flash lamp every 10.8 seconds. The pulse
amplitude drops much more quickly to the equilibrium state for the cryogenic
temperature and can be interpreted as the longer lifetime for the charge traps
associated with the ghosting effect.

To mitigate the impact of the ghosting effect across the measurements
described below, we expose the system to UV light at a fixed frequency of 2 Hz
for $\sim 6$ hours before beginning the cool down process. Moreover, we make
the cool down process as slow as possible, typically ranging between two and
three hours to allow for the system to stably transition. Finally, we remain
at our lowest temperature we can achieve ($\sim 80$K) for a period of $\sim 7$
hours before allowing the system to warm up. We continue to take data until
the system returns to the previous peak amplitude equilibrium state when at
room temperature before cooling down. The various stages described above are
shown in Figure 7.

Figure 7: Peak amplitudes with experimental phases marked for the mobility
extraction region, ghosting mitigation (I), cooling down (II), lowest
temperature (III) & warming up (IV).

With the mitigation strategy in place to account for the effects of ghosting,
Section 3.2 describes the data quality cleanup and analysis procedures used.

###  3.2 Data Quality and Analysis Procedure

The data recorded using the setup described in Section 2.3 has two files
recorded. The first file is the waveform captured from the oscilloscope and
the second is the temperature data recorded from the RTD’s and saved on the
Raspberry Pi. These two files are indexed such that they are matched in time
and thus the data files are combined to provide a single file with both the
recorded waveform and temperature.

Data quality was performed for all campaigns following the procedure described
in this section. Applying positive polarity voltage resulted in “positive
waves”, while applying negative polarity voltage resulted in “negative waves”.
Figure 8 shows typical example waveforms in both cases. The illustrations are
annotated to highlight the important features used to calculate the peak
amplitude and area.

To define the start time of each wave ($t_{0}$), we account for the known
delay between the trigger pulse sent to the flashlamp and the actual formation
of a light pulse. According to the flashlamp data sheet [49], the delay
relative to the input pulse is $\sim 4.0-4.5~{}\mu$s. The data show a
characteristic “pick-up” due to inductive coupling between the flashlamp and
the signal line for the aSe board. This peak (which is labeled ‘Amigo’, as it
provides a friendly reference point) appears reliably at $4.36~{}\mu$s after
the trigger signal, thus defining our $t_{0}$.

Once the start of the waveform is defined, the waves are fitted using a
LOcally WEighted Scatterplot Smoothing (LOWESS) [57, 58] statistical package.
The fit ranges between $t_{0}$ and $t=600~{}\mu$s providing a smoothed
function of the waveform. The peak amplitude of the wave is found by sampling
between $100<t<600~{}\mu$s and locating the minimum for negative waves or
maximum for positive waves. The area under the fit (integrated voltage) was
calculated using the trapezoidal rule. Bounds of the integral were set by
fixing the lower limit to 237 ns after the start of the fit and the upper
limit to the fit and abscissa intersection following the peak amplitude. The
lower limit was chosen to ensure integration occurs after any inductive noise
seen in “the Amigo” has died out. The lower limit accounts for a documented
delay and jitter time in the xenon flash lamp.

Waveforms of particularly small negative amplitudes fail to return to
baseline, and thus no area can be calculated. An alternate baseline correction
is attempted in these cases by taking the baseline mean sufficiently far from
where an intersection would occur ($700<t<1100~{}\mu$s) and correcting that
baseline. When the alternate baseline correction failed, the wave was removed
from the data set. When the correction succeeded, the peak amplitude is
recalculated, and the area found. Additionally, waves that produced integrated
areas with an incorrect sign were removed from the data set. An illustration
of the methods described above is given in Figure 8.

Figure 8: Example waveforms when collecting holes at ambient temperature
(top), electrons at ambient temperature (middle), and electrons at cryogenic
temperature (bottom). The figure highlights the methods described above of
locating the start of the waveform via the characteristic inductive noise
pickup (labeled “Amigo”) from the start of the flashlamp, the effect of the
smoothing algorithm are shown as the solid red line, the end of the pulse is
identified as the “intersection”, the peak amplitude, and integrated area are
all illustrated. The bottom plot also illustrates the region of the waveform
used when the alternative baseline correction is needed.

Values for peak amplitude and integrated areas are accumulated, averaged and
the standard deviation of the set calculated. If an individual waveform is
found to have a peak amplitude or integrated area greater than one standard
deviation this waveform is removed as analysis shows these waveforms are
typically saturated with external noise and thus shouldn’t be considered. A
new list index is then created keeping only waveforms which pass this filter.

With the data cleanup completed, we extract the relevant physics from the
remaining waveforms.

###  3.3 Characterization across temperatures

To characterize the response of the aSe device, the peak amplitude (mV) and
integrated area of the pulse (mV$\cdot\mu$s) are calculated for different
temperature ranges. Within a given temperature range, 20 independent waveforms
are averaged. The same procedure described above is used to calculate the peak
amplitude, area and the standard deviation.

Figures 9, 10, and 11 show the results for three different applied fields on
the peak amplitude as a function of temperature. These results are summarized
in Tables 2, 3, 4 in the appendix. A few general trends are observed from this
data:

  1. 1.

While the magnitude of the peak amplitude is noticeably reduced at the lowest
temperatures, it is definitively non-zero and has a pulse shape consistent
with a response due to signal from the flashlamp

  2. 2.

The magnitude of the peak amplitude scales approximately with the size of the
applied field, as would be expected. As an example ratio of the fields
$\frac{3.62\text{V}/\mu\text{m}}{2.73\text{V}/\mu\text{m}}=1.32$ and the ratio
of the peak amplitudes at 265K-285K for those fields is 1.96 and between
75K-85K the ratio of the peak amplitudes is 1.42. Similar trends can be seen
when looking at fields between
$\frac{5.16\text{V}/\mu\text{m}}{3.62\text{V}/\mu\text{m}}=1.43$ and the ratio
of the peak amplitudes at 265K-285K for those fields is 1.42 and between
75K-85K the ratio of the peak amplitudes is 1.1. The data is summarized in
Tables 2, 3, 4.

  3. 3.

The peak amplitude at the lowest temperatures is consistently higher when
collecting electrons rather than holes. This trend holds true within the
uncertainties of the measurement as the samples were warmed up.

It is worth noting that as the sample warms up, the effects due to ghosting
dominate near 280 - 290 K. As the exposure to light in warm continues, the
samples return to a similar equilibrium state to the start of the data taking,
prior to cooling.

Figure 9: Left: The peak amplitude of the pulse recorded as over time during
cooldown and warm up. Right: The average waveform as a function of
temperature. The top row represent data for an applied voltage of -400 V
(-2.73 V/$\mu$m electric field) and the bottom row represent data for an
applied voltage of +400 V (+2.73 V/$\mu$ m electric field). Figure 10: Left:
The peak amplitude of the pulse recorded as over time during cooldown and warm
up. Right: The average waveform as a function of temperature. The top row
represent data for an applied voltage of -530 V (-3.62 V/$\mu$m electric
field) and the bottom row represent data for an applied voltage of +530 V
(+3.62 V/$\mu$ m electric field). Figure 11: Left: The peak amplitude of the
pulse recorded as over time during cooldown and warm up. Right: The average
waveform as a function of temperature. The top row represent data for an
applied voltage of -750 V (-5.16 V/$\mu$m electric field) and the bottom row
represent data for an applied voltage of +750 V (+5.16 V/$\mu$ m electric
field).

Figure 12 shows the integrated pulse area as a function of temperature and
applied voltage. These results are also summarized in Tables 2, 3, 4 in the
appendix. A similar set of observations can be seen in the amplitudes as was
seen in the pulse areas. This consistency gives confidence that the same
physics driving the pulse amplitude is present in the overall shape of the
pulse, thus confirming that the signal is due to the response of the aSe
detector.

Figure 12: The pulse integral for as a function of temperature for all applied
voltages plotted in linear (top) and log (bottom) scale. The bands represent
the standard deviation of the measured integral when averaging together the
waveforms within a given temperature bin.

###  3.4 Usability in liquid noble element detectors

This section describes the additional tests performed to ensure usability of
amorphous selenium based devices in liquid noble elements time projection
chambers. First, tests described in Section 3.4.1 demonstrate robustness of
the prototypes against cryogenic cycling, checking if the deposited selenium
remains on the board even after the exposure to extreme temperatures. Second,
tests described in Section 3.4.2 explore whether the introduction of aSe
degrades argon purity.

####  3.4.1 Robustness against cryogenic cycling

In addition to the repeated thermal cycling of these boards during the data
taking campaigns – after which no noticeable damage was observed – the aSe
coated boards were imaged using Scanning Electron Microscope (SEM) before and
after submersion in a liquid nitrogen (LN2) bath. The boards were lowered into
the LN2 bath from room temperature over a period of 10 mins with care taken to
ensure no condensation formed on the board during the submersion. The LN2 bath
was then allowed to evaporate over a period of $>8$ hours and then imaged
again afterwards. Examples of the before and after SEM images can be seen in
Figure 13. No apparent damage or cracking of the aSe layer can be seen in the
SEM images and none could be seen during visual inspection. Taken together
with the repeatable behavior of the aSe setup after multiple thermocycles
provides confidence that such a windowless aSe detector is robust at the
cryogenic temperatures of a liquid noble environment.

Figure 13: SEM images taken at 30x and 100x of the same region of an aSe board
before and after cryocycling in LN2. A scan of the board showed no noticeable
defects of the aSe layer following cryocycling.

####  3.4.2 Electronegative contaminants test

A key feature of noble elements for particle detectors is the dual response to
the passage of charged particles in the active volume in the form of
correlated ionization charge and scintillation light. When developing new
concepts for detectors intending to use both mechanisms, it is important to
test that the light detection system does not suppress charge collection. The
presence of electronegative contaminants in the liquid element, such as oxygen
and water, is particularly pernicious since these molecules quench the charge
produced by the ionizing radiation. While noble element TPCs use hermetically
sealed and leak-checked vessels to abate the leakage of external contaminants
into the system, a sizable source of impurities can be introduced from the
outgassing of internal surfaces.

We tested whether the outgassing of the aSe boards reaches levels harmful to
charge collection by performing a measurement of the electron lifetime and
water content at the Fermilab Material Test Stand (MTS)[59] located at the
Liquid Noble Test Facility (PAB). The MTS is a 250 l liquid argon cryostat
which allows to monitor the level of electronegative contaminants introduced
by the material under examination by positioning the material in the argon gas
vapor (ullage) and submerging the material in the liquid. The MTS is equipped
with an internal filtration system for oxygen and water contamination, which
can be turn on or off as needed, and with a purity monitor to directly measure
the effect of any material on the electron lifetime in the liquid argon.

We examined a PCB board coated with 35 $\mu$m selenium on a surface area of
2$\times$2 cm2. The board was cleaned by wiping the uncoated surface of the
board with alcohol. After the insertion of the board in the MTS, the sample
chamber was purged with argon gas and evacuated several times to eliminate the
contaminants acquired during insertion before the introduction in the active
volume. Three runs were performed in the MTS. The first had no sample in the
vessel and serves as a control to understand the behavior of the system when
when the filters are on and off. The second has the sample suspended in the
ullage where the effects of degradation to purity due to outgassing should be
the most pronounced. Finally, the sample is lowered into liquid argon and left
submerged.

To allow for comparison across runs, the same testing procedure is repeated
for each run. First, the filtration system is activated, the electron lifetime
is allowed to stabilize, and data is collected for several hours. Next, the
filter system is switched off, and the decay of electron lifetime is observed.
Figure 14 shows the results of the testing procedure for the three runs. The
average lifetime during active filtration and the shape of the decay following
the shut off of the filters are consistent across all runs. Thus, we conclude
that the presence of the coated board does not suppress charge collection and
does not negatively impact electron lifetime.

Figure 14: Left: average electron lifetime during active filtration period as
read directly from the purity monitor (raw reading). The data is shown for the
three run conditions: no sample (blue), sample in the ullage (orange), sample
in the liquid (green). Right: calibrated lifetime as a function of time during
the period when the filtration system is inactive.

##  4 Conclusions

In this paper, we have presented the response of a novel, windowless amorphous
selenium based photon detector to UV light as a function of applied electric
field and temperature. The device is constructed from low-cost commercially
available printed circuit boards and simple thermal evaporation of selenium
onto the board.

This initial exploration shows that such a device is: i) robust under
cryogenic conditions, with the selenium remaining undamaged under cryogenic
cycling and demonstrating the same performance after repeated thermal cycles,
ii) responsive at cryogenic temperatures consistent with common liquid noble
detectors (e.g. LAr and LXe), iii) the response of the device is consistent
with similar results shown for x-rays and gamma-rays (e.g. the observance of
ghosting effects and the strength of the electron signal compared to the hole
signal), and iv) preserving noble elements purity.

Our finding that the device continues to respond at temperatures relevant to
liquid noble detectors commonly used in high energy physics is particularly
relevant to set future R&D directions. While the flux of photons used in this
experiment is quite high, we have provided a proof-of-principle demonstration
that such a device could be be sensitive to a lower photon flux in a cryogenic
environment, provided that a higher applied electric field is applied.
Exploration into the response of such an aSe based device is ongoing with
additional results expected to follow this work shortly.

A device based on the concept tested here opens the door to the possibility of
making an integrated charge (Q) plus light (L) sensor, referred to as a “Q+L
sensor”. Such a sensor could simultaneously be sensitive to both the VUV
photons produced in a liquid noble detector as well as the ionization charge
created during the interaction of a charged particle with the noble element
medium. A conceptual sketch of such a device using the Q-Pix [60] charge
readout architecture is shown in Figure 15. Such a device, using amorphous
selenium as the photoconductor, could have a large effective surface area,
provide increased sensitivity to low energy physics and greater fidelity in
energy reconstruction.

Figure 15: Conceptual sketch of an integrated charge and light (Q+L) sensor
utilizing a windowless photoconductor, such as the device tested in this work,
to directly detect the VUV photons produced in a noble element TPC. The
conceptual device depicted here would use the same readout architecture used
for the detection of ionization charge to detect the charge from the
photoconductor. In this schematic, this is shown as the Q-Pix charge readout
solution described in Reference [60].

The quantification of the improvement such a device will offer, as well as the
realization of such a device in an experimental setup is the subject of future
ongoing work.

## Acknowledgments

The authors would like to thank Scott Eichmann from the University of Texas
Arlington machine shop for fabricating the cryogenic test stand. The authors
also would like to thank Prof. Shiva Abbaszadeh and Dr. Katie Hellier from the
University of California Santa Cruz for the aSe deposition of the boards used
in the argon purity measurement. Thermal evaporation of the primary devices
was done at Oak Ridge National Labs which is a DOE Office of Science User
Facility. This material is based upon work supported by the U.S. Department of
Energy, Office of Science, Office of High Energy Physics Award Number DE-
SC0020065. This document was prepared by the authors using the resources of
the Fermi National Accelerator Laboratory (Fermilab), a U.S. Department of
Energy, Office of Science, HEP User Facility. Fermilab is managed by Fermi
Research Alliance, LLC (FRA), acting under Contract No. DE-AC02-07CH11359.

##  Appendix A Summary of pulse characterization across temperatures

Here we provide the corresponding data to the plots in Figures 9-12 associated
with the pulse height and area as a function of temperature for the different
applied fields. The errors quoted in these tables reflect the standard
deviation from the averaging techniques described in Section 3.2.

Field | Charge Carrier | Temperature | Peak Amplitude | Integrated Pulse Area
---|---|---|---|---
(V/$\mu$m) |  | (K) | (mV) | (mV$\cdot\mu$s)
+2.73 | Electrons | 75-85 | 4.2 $\pm$ 0.1 | 854.7 $\pm$ 17.5
85-105 | 4.4 $\pm$ 0.0 | 890.1 $\pm$ 14.2
105-125 | 4.7 $\pm$ 0.1 | 953.7 $\pm$ 13.8
125-145 | 5.0 $\pm$ 0.0 | 1051.4 $\pm$ 12.4
145-165 | 5.4 $\pm$ 0.1 | 1191.9 $\pm$ 23.9
165-185 | 6.7 $\pm$ 0.2 | 1574.2 $\pm$ 34.5
185-205 | 10.1 $\pm$ 0.5 | 2296.7 $\pm$ 66.2
205-225 | 22.5 $\pm$ 2.4 | 4996.6 $\pm$ 499.2
225-245 | 72.4 $\pm$ 5.3 | 15204.2 $\pm$ 994.6
|  | 245-265 | 119.2 $\pm$ 3.4 | 23695.2 $\pm$ 622.6
|  | 265-285 | 179.4 $\pm$ 4.0 | 36369.9 $\pm$ 762.2
|  | 285-300 | 75.2 $\pm$ 7.1 | 15389.1 $\pm$ 1547.9
-2.73 | Holes | 75-85 | 1.3 $\pm$ 0.2 | 38.5 $\pm$ 6.7
85-105 | 1.3 $\pm$ 0.1 | 41.3 $\pm$ 4.9
105-125 | 1.6 $\pm$ 0.1 | 71.6 $\pm$ 5.6
125-145 | 1.9 $\pm$ 0.1 | 147.4 $\pm$ 23.8
145-165 | 2.4 $\pm$ 0.1 | 345.7 $\pm$ 33.7
165-185 | 3.4 $\pm$ 0.1 | 715.1 $\pm$ 26.1
185-205 | 6.1 $\pm$ 0.3 | 1394.3 $\pm$ 49.2
205-225 | 16.8 $\pm$ 1.8 | 3682.3 $\pm$ 373.8
225-245 | 51.0 $\pm$ 2.9 | 10505.1 $\pm$ 541.2
|  | 245-265 | 71.6 $\pm$ 1.8 | 14000.7 $\pm$ 263.8
|  | 265-285 | 87.7 $\pm$ 2.2 | 17431.7 $\pm$ 380.7
|  | 285-300 | 46.4 $\pm$ 4.0 | 9486.1 $\pm$ 804.7
Table 2: Summary of the mean peak amplitude and integrated pulse area across
the temperature range probed for an electric field of $\pm$ 2.73 V/$\mu$m.

Field | Charge Carrier | Temperature | Peak Amplitude | Integrated Pulse Area
---|---|---|---|---
(V/$\mu$m) |  | (K) | (mV) | (mV$\cdot\mu$s)
+3.62 | Electrons | 75-85 | 6.0 $\pm$ 0.1 | 1247.3 $\pm$ 33.7
85-105 | 6.0 $\pm$ 0.1 | 1249.5 $\pm$ 24.5
105-125 | 6.4 $\pm$ 0.1 | 1373.8 $\pm$ 23.4
125-145 | 7.2 $\pm$ 0.1 | 1562.6 $\pm$ 25.9
145-165 | 9.0 $\pm$ 0.2 | 2087.5 $\pm$ 25.8
165-185 | 12.9 $\pm$ 0.5 | 3057.1 $\pm$ 87.1
185-205 | 22.8 $\pm$ 1.1 | 5237.1 $\pm$ 224.5
205-225 | 52.4 $\pm$ 4.3 | 11381.6 $\pm$ 881.1
225-245 | 126.4 $\pm$ 7.8 | 26203.4 $\pm$ 1456.2
|  | 245-265 | 205.2 $\pm$ 9.6 | 41276.3 $\pm$ 1923.2
|  | 265-285 | 351.9 $\pm$ 12.0 | 71610.6 $\pm$ 2387.3
|  | 285-300 | 129.1 $\pm$ 4.4 | 26652.3 $\pm$ 1013.4
-3.62 | Holes | 75-85 | 2.8 $\pm$ 0.1 | 199.7 $\pm$ 15.9
85-105 | 2.7 $\pm$ 0.1 | 188.4 $\pm$ 8.3
105-125 | 3.3 $\pm$ 0.1 | 260.9 $\pm$ 27.8
125-145 | 4.3 $\pm$ 0.1 | 607.7 $\pm$ 39.5
145-165 | 5.7 $\pm$ 0.1 | 1010.8 $\pm$ 33.6
165-185 | 9.0 $\pm$ 0.3 | 1832.7 $\pm$ 49.3
185-205 | 16.2 $\pm$ 0.9 | 3631.7 $\pm$ 174.2
205-225 | 37.7 $\pm$ 3.3 | 8415.9 $\pm$ 721.7
225-245 | 97.7 $\pm$ 5.9 | 20223.3 $\pm$ 1055.2
|  | 245-265 | 158.0 $\pm$ 6.2 | 31014.0 $\pm$ 1284.7
|  | 265-285 | 222.4 $\pm$ 5.6 | 45310.1 $\pm$ 1037.2
|  | 285-300 | 84.0 $\pm$ 4.0 | 17456.3 $\pm$ 787.4

Table 3: Summary of the mean peak amplitude and integrated pulse area across
the temperature range probed for an electric field of $\pm$ 3.62 V/$\mu$m.

Field | Charge Carrier | Temperature | Peak Amplitude | Integrated Pulse Area
---|---|---|---|---
(V/$\mu$m) |  | (K) | (mV) | (mV$\cdot\mu$s)
+5.16 | Electrons | 75-85 | 6.6 $\pm$ 0.1 | 1390.9 $\pm$ 31.8
85-105 | 6.5 $\pm$ 0.1 | 1397.3 $\pm$ 29.6
105-125 | 7.4 $\pm$ 0.1 | 1569.6 $\pm$ 16.8
125-145 | 8.5 $\pm$ 0.1 | 1912.3 $\pm$ 29.4
145-165 | 10.4 $\pm$ 0.2 | 2460.3 $\pm$ 40.9
165-185 | 14.7 $\pm$ 0.5 | 3635.4 $\pm$ 78.8
185-205 | 24.1 $\pm$ 1.3 | 5683.4 $\pm$ 199.6
205-225 | 52.6 $\pm$ 5.0 | 11408.9 $\pm$ 1016.4
225-245 | 146.1 $\pm$ 11.2 | 30377.8 $\pm$ 2204.0
|  | 245-265 | 262.6 $\pm$ 13.5 | 52793.7 $\pm$ 2625.3
|  | 265-285 | 441.5 $\pm$ 11.8 | 90199.4 $\pm$ 2151.5
|  | 285-300 | 205.8 $\pm$ 9.6 | 43187.2 $\pm$ 2028.7
-5.16 | Holes | 75-85 | 2.1 $\pm$ 0.1 | 291.6 $\pm$ 32.1
85-105 | 2.1 $\pm$ 0.1 | 303.4 $\pm$ 34.9
105-125 | 3.0 $\pm$ 0.1 | 505.0 $\pm$ 28.0
125-145 | 4.5 $\pm$ 0.2 | 912.5 $\pm$ 72.2
145-165 | 7.1 $\pm$ 0.3 | 1562.7 $\pm$ 83.7
165-185 | 12.7 $\pm$ 0.6 | 2868.2 $\pm$ 92.2
185-205 | 24.9 $\pm$ 1.9 | 5559.9 $\pm$ 342.2
205-225 | 65.8 $\pm$ 6.9 | 14317.0 $\pm$ 1478.4
225-245 | 186.9 $\pm$ 11.3 | 38594.3 $\pm$ 2098.6
|  | 245-265 | 301.1 $\pm$ 13.2 | 60410.5 $\pm$ 2563.7
|  | 265-285 | 492.5 $\pm$ 13.8 | 103303.9 $\pm$ 3409.4
|  | 285-300 | 158.7 $\pm$ 15.8 | 33897.7 $\pm$ 3268.8

Table 4: Summary of the mean peak amplitude and integrated pulse area across
the temperature range probed for an electric field of $\pm$ 5.16 V/$\mu$m.

##  Appendix B Modeling of the electric field

Using the online 3D computer-aided design (CAD) design software Fusion360
[61], a model of the interdigitated electrode found in the ‘cookie’ board was
created with the appropriate spacings and materials. Traces are modeled as 107
$\mu$m (W) $\times 1123\mu$m (L) $\times 35\mu$m (H) with the gaps evenly
distributed at 147 $\mu$m. The electrodes are assumed to be silver. This model
was then exported to a online electric field modeling tool, QuickField [62],
which was used to model the behavior of the electric field and electric
potential in the presence of the aSe coating. The aSe layer is assumed to be
1.2$\mu$m thick and is present between the electrodes as well as on top of the
electrodes. For simplicity, no aSe is found on the vertical walls of the
electrodes. A permittivity of 5.8 and 6.9 is assumed for the aSe and Ag
respecitvely. An example output of the simulation for an applied potential of
400 Volts can be seen in Figure 16.

Figure 16: Top: The output of the QuickField simulation for the interdigitated
PCB board used in this work from an overhead perspective showing the electric
field (left) and electric potential (right) when a potential of 400 volts is
applied. Bottom: The output of the QuickField simulation from a horizontal
propspective where the height of the electrodes compared to the thickness of
the selenium layer can be seen both showing the electric field and potential.

Overall, the electric field (and the corresponding gradient in the electric
potential) across the board and within the selenium is uniform and consistent
with the estimated field calculated using the geometry of the board. This can
be better seen in Figure 17, which shows analytically how simulation predicts
the electric potential and field vary as you traverse the gap between
electrodes (left) as well as picking a single point in the middle of the gap
and moving vertically through the simulated layer of aSe (right). In both
cases, the field and potential are found to be as expected.

Figure 17: Left: The simulated electric potential (top) and electric field
(bottom) for three different transport fields across the 147$\mu$m gap between
electrodes in a region of high uniformity. The field can be seen at the
expected value and uniform across the electrode while the potential changes
linearly, as expected. Right: The simulated electric potential (top) and field
(bottom) for a single position in the middle of the gap between the electrodes
($\sim 73.5\mu$m from the electrodes) and then traversing the thickness of the
aSe layer ($1.2\mu$m). The electric field is found to be uniform as a function
of the thickness of the selenium and the potential is a constant.

The areas with the largest non-uniformity in the electric field occur in the
regions where one electrode terminates in the gap of the opposite pair of
interdigitated electrodes. In this region the edges of the electrodes cause
the field to be more non-linear. Figure 18 shows analytically how the electric
potential and field varies as you traverse the gap between electrodes (left)
as well as picking a single point in the middle of the gap and moving
vertically through the simulated layer of aSe (right). The point chosen here
is an area where the variations can be seen in Figure 16 to be the largest.
Due to the geometric effects of the edges of the electrodes, the electric
field can vary 2-3 times larger then the uniform region between the
electrodes. While this is a large variation in the field, this effect is
ultimately determined to be of little significance to the main analysis
presented here. Since the non-uniform gap region represents a small fraction
of the overall surface area of the board (the non-uniform area represents
$<1.5\%$ of the total surface area), the effect on the reconstructed signal
due to photons creating electron/hole pairs in this region is expected to be
quite small.

Figure 18: Left: The simulated electric potential (top) and electric field
(bottom) for three different transport fields across the 147$\mu$m gap between
electrodes in where the field was evidently non-uniform due to the edge
effects of the electrode. The field can be seen to vary significantly from the
geometrically calculated value due to edge and corner effects Right: The
simulated electric potential (top) and field (bottom) for a single position in
the middle of the gap between the electrodes ($\sim 73.5\mu$m from the
electrodes) and then traversing the thickness of the aSe layer ($1.2\mu$m).
Here too the electric field is found vary as a function of the thickness of
the selenium.

##  Appendix C Repeatability

Table 5 summarizes the values for the measurements of the peak amplitude and
integrated pulse area taken during the main data taking campaign as well as
the repeated measurements made multiple days later at $\pm$2.73 V/$\mu$m. The
same analysis technique described in Section 3.2 was utilized for the repeated
measurement data set. The results are seen to vary between $\sim 10-100\%$
when the charge carriers are electrons and $\sim 10-300\%$ when the charge
carriers are holes. The largest variation is seen in the integrated pulse area
and upon inspection of the various waveforms, can be primarily attributed to a
shift in the baseline and the subsequent calculation of the integrated area.
The room temperature measurements are seen to be generally consistent with one
another, with the variation between tests being less than 25% in both the peak
amplitude and integrated area and the largest variations being seen at in the
temperature bins between 205K and 105K.

Field | Charge Carrier | Temperature | Peak Amplitude | Repeat Measurement | Integrated Pulse Area | Repeat Measurement
---|---|---|---|---|---|---
(V/$\mu$m) |  | (K) | (mV) |  Peak Amplitude (mV) | (mV$\cdot\mu$s) |  Integrated Pulse Area (mV$\cdot\mu$s)
+2.73 | Electrons | 75-85 | 4.2 | 5.8 | 854.7 | 1173.5
85-105 | 4.4 | 5.6 | 890.1 | 1141.1
105-125 | 4.7 | 6.0 | 953.7 | 1319.0
125-145 | 5.0 | 7.1 | 1051.4 | 1586.6
145-165 | 5.4 | 9.2 | 1191.9 | 2162.1
165-185 | 6.7 | 12.9 | 1574.2 | 3046.1
185-205 | 10.1 | 21.1 | 2296.7 | 4630.1
205-225 | 22.5 | 35.1 | 4996.6 | 7360.0
225-245 | 72.4 | 76.8 | 15204.2 | 16572.6
|  | 245-265 | 119.2 | 144.2 | 23695.2 | 28149.9
|  | 265-285 | 179.4 | 246.0 | 36369.9 | 49208.5
|  | 285-300 | 75.2 | 84.4 | 15389.1 | 17107.3
-2.73 | Holes | 75-85 | 1.3 | 2.9 | 38.5 | 117.6
85-105 | 1.3 | 3.1 | 41.3 | 196.7
105-125 | 1.6 | 3.8 | 71.6 | 286.7
125-145 | 1.9 | 4.8 | 147.4 | 442.9
145-165 | 2.4 | 6.6 | 345.7 | 724.9
165-185 | 3.4 | 10.3 | 715.1 | 1521.0
185-205 | 6.1 | 16.9 | 1394.3 | 3622.4
205-225 | 16.8 | 31.2 | 3682.3 | 6698.1
225-245 | 51.0 | 59.5 | 10505.1 | 12175.4
|  | 245-265 | 71.6 | 86.0 | 14000.7 | 17374.9
|  | 265-285 | 87.7 | 103.2 | 17431.7 | 21528.1
|  | 285-300 | 46.4 | 52.7 | 9486.1 | 11136.7

Table 5: Summary of the peak amplitude and integrated pulse area found during
the repeat measurement across the temperature range probed for an electric
field of $\pm$2.73 V/$\mu$m.

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