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# AstroPix: Novel monolithic active pixel silicon sensors for future gamma-ray
telescopes

Amanda L. Steinhebel  NASA Postdoctoral Program Fellow  NASA Goddard Space
Flight Center, Greenbelt, MD, USA  Henrike Fleischhack  Catholic University of
America, Washington, DC, USA  NASA Goddard Space Flight Center, Greenbelt, MD,
USA  Center for Research and Exploration in Space Science and Technology,
NASA/GSFC, Greenbelt, MD, USA  Nicolas Striebig  Karlsruhe Institute of
Technology, Karlsruhe, Germany  Manoj Jadhav  Argonne National Laboratory,
Lemont, IL, USA  Yusuke Suda  Hiroshima University, Higashi-Hiroshima City,
Hiroshima, Japan  Ricardo Luz  Argonne National Laboratory, Lemont, IL, USA
Carolyn Kierans  NASA Goddard Space Flight Center, Greenbelt, MD, USA  Regina
Caputo  NASA Goddard Space Flight Center, Greenbelt, MD, USA  Hiroyasu Tajima
Institute for Space-Environment Research, Nagoya University, Nagoya, Aichi,
Japan  Richard Leys  Karlsruhe Institute of Technology, Karlsruhe, Germany
Ivan Peric  Karlsruhe Institute of Technology, Karlsruhe, Germany  Jessica
Metcalfe  Argonne National Laboratory, Lemont, IL, USA  Jeremy S. Perkins
NASA Goddard Space Flight Center, Greenbelt, MD, USA

###### Abstract

Space-based $\gamma$-ray telescopes such as the $Fermi$ Large Area Telescope
have used single sided silicon strip detectors to track secondary charged
particles produced by primary $\gamma$-rays with high resolution. At the lower
energies targeted by keV-MeV telescopes, two dimensional position information
within a single detector is required for event reconstruction - especially in
the Compton regime. This work describes the development of monolithic CMOS
active pixel silicon sensors – AstroPix – as a novel technology for use in
future $\gamma$-ray telescopes. Based upon sensors (ATLASPix) designed for use
in the ATLAS detector at the Large Hadron Collider, AstroPix has the potential
to maintain high performance while reducing noise with low power consumption.
This is achieved with the dual detection and readout capabilities in each CMOS
pixel. The status of AstroPix development and testing, as well as outlook for
future testing and application, will be presented.

###### keywords:

AstroPix, silicon pixel detectors, Compton telescopes, MAPS, high energy
astrophysics

##  1 INTRODUCTION

The modern era of multi-messenger astronomy requires that many cosmic
messengers be monitored, including the full electromagnetic spectrum. The ‘MeV
Gap’ in instrument sensitivity to $\gamma$-rays in the keV-MeV range must be
addressed to allow input to multi-messenger investigations in this regime. In
order to ensure high precision and sensitivity, a next-generation Pair/Compton
telescope targeting the MeV $\gamma$-ray sky requires detectors with high
energy and position resolution in three-dimensions, as well as a low energy
threshold. The technology used to track the paths of secondary charged
particles resultant of incident $\gamma$-rays in currently flying $\gamma$-ray
telescopes such as single- and double-sided silicon strips cannot easily meet
these challenges - the long strips introduce noise which impacts the
measurement (see Ref 1). Instead, Complementary Metal-Oxide-Semiconductor
(CMOS) monolithic active pixel silicon (MAPS) sensors can be used.

Individual pixels on these sensors can a range in size from 10 $\mu$m2 \- 1
mm2, allowing for great control over position resolution (though this must be
optimized with respect to power draw and data rate). The use of in-pixel CMOS
circuitry (see Ref. 2) eliminates the necessity for an external ASIC to record
and digitize data as the charge collection and digitization is all done on-
chip. A CMOS chip (see Fig. 1) incorporates charge collection, signal
amplification, and a comparator for signal acceptance in a shared substrate
embedded directly into the pixel matrix in shallow wells. The wells are
isolated from the bulk substrate by a deep n-well. A charge-depleting bias
voltage is applied individually to every pixel allowing for faster and more
complete charge collection as compared to sensors that rely on diffusion. This
makes AstroPix an HVMAPS sensor - MAPS that are depleted with high-voltage
(HV) ($\mathcal{O}(100$V)) at a pixel level.

The comparator output for every pixel is sent to the synthesized digital logic
which is located on the periphery of the chip. Here, the signal is digitized
and can be read out for analysis. In this way, there is no need to directly
bump-bond to each pixel to achieve high position resolution (as done with
hybrid technology) as this strategy would introduce passive material into the
active area of the detector. Large detectors designed with silicon strips
require daisy-chaining the strips together, but the lack of this daisy-
chaining with a HVMAPS sensor reduces capacitance and therefore noise in the
signal. The AstroPix data collection strategy aims to reduce per-pixel power
and overall data bandwidth. To this end, the comparator outputs for each row
and column are OR’ed together and these two channels are sent to the digital
periphery. In this way, the output signal is similar to the output of a
silicon strip-based detector and row/column pairing algorithms will be
employed to determine which signals correspond to which interaction if there
are multiple triggers in a chip. Currently, AstroPix utilizes SPI readout from
a DAQ which interfaces with an FPGA.

CMOS technology is familiar to industry and commercial endeavors, but novel in
this application for space-based telescopes. AstroPix is a CMOS HVMAPS under
design for this explicit purpose. The design of AstroPix is based upon work
previously conducted by the ATLAS Collaboration at the Large Hadron Collider
(LHC) for the development of an upgrade to their inner tracker subdetector
system (see Ref. 3). This custom MAPS detector, ATLASPix, was designed for the
LHC environment - optimized for minimum ionizing particles with nanosecond
timing capability.

The AstroPix design philosophy utilizes ATLASPix as a starting point, and
involves a sequence of AstroPix versions in order to advance the design in
steps from ATLASPix to a flight-ready AstroPix. These versions are denoted
with an underscored `_v`. This strategy ensures functional chips at each stage
of testing. With continued optimization and characterization studies such as
those outlined in this work, AstroPix can serve as a baseline detector for
future telescope concepts - especially those that require high-resolution and
low-noise/low-power electronics or those targeting the keV-MeV $\gamma$-ray
regime. For more background information about detection strategies of keV-MeV
$\gamma$-rays and its impact on telescope design, see Ref. 1.

Figure 1: Illustration of an HVMAPS CMOS pixel with HV bias voltage (left)
containing charge collection and amplification/comparator architecture
embedded in the pixel matrix (figure from 4). This matrix reads out to a
control unit on the digital periphery (right) for digitization and chip
readout.

Section 2 outlines the first changes made to ATLASPix to produce a $bona-fide$
AstroPix chip, AstroPix_v1. This section overviews the characterization effort
of this sensor, including analysis of analog data, background estimation, and
the creation of a calibration curve. Following the design strategy of
incremental changes, AstroPix_v1 was superseded by AstroPix_v2 - the current
design under testing. Section 3 mirrors the structure of Section 2 with the
updated sensor. AstroPix_v2 was exposed to multiple particle beams and
underwent preliminary radiation testing which is discussed in Section 4.
Section 5 outlines plans for the next version of AstroPix, AstroPix_v3, and
environmental testing plans. It also contains a general outlook and summary.

##  2 ASTROPIX$\\_$V1

The feasibility of using ATLASPix as a technological starting point for
AstroPix was directly tested at NASA Goddard Space Flight Center (GSFC) in
Ref. 1 where the measured energy resolution was found to be 7.69$\pm 0.13\%$
at 5.89 keV and $7.27\pm 1.18\%$ at 30.1 keV. Analog recorded data (response
pulses as measured from one pixel) was used for this measurement. Digital data
(bitstreams with encoded hit information from the full array) was also
considered, but individual pixel responses were untuned resulting in large
spreads in the Gaussian-fit response distribution (up to $\sigma/\mu=60$%)
when considering the summed response of the array as a whole.

Testing and characterization of the first AstroPix version, AstroPix_v1, has
been completed and is presented in this section. Like ATLASPix, both analog
data and digital data will be collected from AstroPix. Although digital data
is intended to be utilized in flight, preliminary analog data is the focus of
AstroPix_v1 studies as digital data readout was not available in AstroPix_v1
due to a flaw in the chip design. Missing shielding allowed parasitic
capacitance to induce feedback and cause oscillations in the comparators.
These oscillations were reproducible in simulation and are considered
understood. All subsequent versions of the chip correct this defect.

Unless otherwise specified, the energy resolution is defined as the ratio of
full width at half maximum (FWHM) of the reconstructed energy distribution
over the mean of the reconstructed energy, expressed as a percent. The
reconstructed energy spectrum for a mono-energetic input is assumed to follow
a Gaussian shape, with mean $\mu$ and width $\sigma$. Thus,
$E_{res}=2.355*|\sigma|/\mu~{}*100\%$.

###  2.1 Design Optimization for AstroPix_v1

The sensitivity of future $\gamma$-ray telescopes in the keV-GeV range is
driven by the effective area, angular resolution, and energy resolution of the
instrument. By using a thick wafer of at least 500 $\mu$m, AstroPix must
measure energy resolution at FWHM of 5 keV from 25-122 keV and 5.6 keV at 622
keV with a pixel dynamic range of 25-700 keV. AstroPix pixel size is a
compromise between position resolution and power consumption. Smaller pixels
provide higher position resolution however an increased number of pixels
causes increased data rates and higher overall power consumption.

ATLASPix provided a promising starting point, but itself does not meet
telescope design criteria (see Table 1). From here, AstroPix_v1 was designed
with 18 rows and columns of $175\times 175$ $\mu$m pixels on an un-thinned 725
$\mu$m thick wafer. The wafer thickness enables the larger dynamic range
desired by AstroPix, and larger pixels do not sacrifice energy resolution.
Loosening the timing requirement from ATLASPix allows for a simpler readout
system. In turn, noise-reducing components could be added on-chip. For
example, the addition of a low pass filter after the charge amplifier allowed
for improved energy resolution.

Table 1: Parameters of ATLASPix and AstroPix. | ATLASPix | AstroPix
---|---|---
| (Measured) | (Required)
Sampling rate [ns] | 16 |  $\sim$ 600
$E_{res.}$ (FWHM) | 7% at 30.1 keV | 9.7% at 122 keV
Pixel size [$\mu$m] | 150$\times$50 | $1000\times 1000$
Thickness [$\mu$m] | 100 | 500
Dynamic range [keV] | 5-32 | 25-700
Power consumption [mW/cm2] | 150 | 1.5

###  2.2 AstroPix_v1 Testing Setup

AstroPix_v1 is controlled and configured by a Nexys Video FPGA and readout
system adaptor (GECCO) board. Three extension boards are required, which
enable 1) configuration, 2) voltage setting, and 3) the injection of a square
analog transient signal for chip testing. AstroPix_v1 is mounted to a carrier
board (see Fig. 2) using conductive pressure sensitive tape and then wire-
bonded. This carrier board connects directly to the GECCO board through an
PCIe slot. High-voltage reverse bias is supplied for charge depletion on a
pixel-by-pixel basis.

Figure 2: AstroPix_v1 mounted on a carrier board. A 3D-printed shield covers
the chip with a layer of kapton to protect the wireboards.

Analog signals are displayed on a mixed domain oscilloscope. SMA ports on the
injection card and output pixels are connected directly to the oscilloscope.
Two AstroPix_v1 chips mounted to carrier boards were tested at GSFC and are
considered in this work - Chip003 and Chip004. Each chip has 18 pixels
connected to pads and capable of analog readout. Two pixels were chosen for
the studies presented in this work - (1,18) or ‘amp1’ and (18,1) or ‘amp2’
indicates pixel (18,1). This provides two pixels in the corners of the array
to avoid crosstalk. Known chip defects prohibit the use of the digital readout
system, so all studies performed in the work will focus only on the analog
signal detected in these two spatially separated pixels.

Response signals are read with a supplied bias voltage of -60V. This value is
not optimized for full depletion, and as such it is not anticipated that
AstroPix_v1 meets the full AstroPix design requirement of 500 $\mu$m
depletion. With the $250\pm 50~{}\Omega*$cm resistivity wafers used for
AstroPix_v1, ithe depletion depth was estimated to 3-100 $\mu$m.

###  2.3 AstroPix_v1 Characterization Studies

####  2.3.1 Charge Injection Studies

AstroPix is designed along with the capability to receive an injected charge
signal. This artificial signal is used as an initial test to confirm that
correct configuration is successfully being passed to the AstroPix chip, and
that analog and digital responses can be triggered upon and read out. The
injected signal itself, a square wave with programmable frequency and duration
can be observed with a digital oscilloscope. This injected pulse is used to
trigger data collection upon the falling edge.

Using a bias voltage of -60 V, both pixels respond but with different pulse
shapes. For chip003, amp1 reliably has lower pulse height and longer pulse
duration than amp2. This in part is due to the shaping seen in amp1 with a
more linear pulse decay rather than the more exponential nature seen in amp2.
This behavior is opposite for chip004 (see Fig. 3) where amp1 showed much
higher pulse heights than amp2. The linear or exponential nature of the pulse
decay are consistent between amp1 and amp2 of the different chips. This leads
to a pulse duration for amp1 of both chips of around 600 $\mu$s compared to
300 $\mu$s for amp2. As this analog signal is readout from the chip prior to
digitization, the differences in pulse shape are related to the variability of
amplification and shaping electronics present in each pixel.

Figure 3: The average of 100 traces from both AstroPix_v1 chips in response to
an injected pulse of 1.0 V.

From this study, we can conclude that the probed pixel both respond to the
injected charge albeit with different pulse shapes. This builds confidence in
the circuitry and response of the chip, as well as data readout, collection,
and analysis methods. It also illustrates the degree of variation that can
occur in pixel responses. In future AstroPix versions, pixels will be
indiviually tuned to limit the variability in pulse shape.

####  2.3.2 Threshold Measurements

AstroPix requirements define a 25 keV low threshold. This enables the
measurement of low-energy astrophysical photons through photoabsorption into a
single pixel. In order to measure the threshold of AstroPix_v1 and compare the
performance to requirements, a low threshold for data collection must be set
such that the sensor is protected from randomly triggering upon noise at an
overwhelming rate.

Low-energy depositions from ambient sources (such as fluorescent lighting) or
electronics noise are disregarded as background from subsequent analyses with
a hard cut on acceptable pulse heights. This defines a low threshold with
which measurements are recorded. Compton-scattered deposits within this regime
cannot be uniquely identified and are lost. Dedicated runs with no radioactive
source and a threshold low enough to trigger upon background hits are used to
define the threshold level as that which eliminates all activity from the
background runs. This threshold is set on the analog data collecting
oscilloscope with millivolt precision. The shape of the resulting background
distribution of hits is Gaussian. The amplitude of the response of the analog
probed pixels in different chips can differ due to fabrication differences
(see Fig. 3). Minimum thresholds of analog signal pulse height for different
pixels are shown in Table 2. These threshold values can also be interpreted as
energy values after calibration is defined in Section 2.3.3.

Additional background events from Compton-scattered particles from radioactive
sources cannot be removed with a hard cut. These are included in subsequent
analysis.

Table 2: Threshold peak height required for a trace to be considered ‘signal’ (and not background or noise). Version | Chip | Pixel | Threshold [mV] | Chip | Pixel | Threshold [mV]
---|---|---|---|---|---|---
v1 | 003 | amp1 | 20 | 004 | amp1 | 10
v1 | 003 | amp2 | 30 | 004 | amp2 | 20
v2 | 1 | amp1 | 60 | - | - | -

####  2.3.3 Energy Calibration

Charge injection studies of Sec. 2.3.1 validate that AstroPix_v1 responds to
signals and fostered the development of software tools, including data
collection and analysis. From here, a correlation between AstroPix_v1 response
(analog pulse height) and incident particle energy is measured in order to
calibrate response spectra into units of energy. Radioactive isotopes with
known emission lines were used. The sources utilized in these calibration
studies (see Table 3) have emission lines spanning 14 keV - 122 keV. These
sources probe the low end of AstroPix’s sensitivity regime. Statistics in
these studies can be limited due to the radioactivity of the source and the
analog readout scheme. Analog data from only one single pixel can be read out
during a collection run, leading to a very small sensitive area on the chip.
Additionally, the cross section decays with increased photon energy, leading
to lower statistics for higher energy incident particles.

Table 3: Properties of radioactive isotopes used for calibration curve creation. The Compton edge of the Cobalt-57 122 keV photopeak is also considered. Isotope | Radioactivity [mCi] | Emission Line(s) [keV]
---|---|---
Cobalt-57 | 0.9973 | 14.41, 122.06
|  | 39.46
Cadmium-109 | 1.031 | 22.16, 88.03
Barium-133 | 1.05$\times 10^{-4}$ | 30.97
Americium-214 | 0.1065 | 59.54

Analog data was triggered for readout by the AstroPix_v1 response itself. The
height of each response peak is utilized as a proxy for energy deposition. The
collection of these signals create spectra of raw data (see Fig. 4) that
contain peaks associated with anticipated photopeaks but also high rates of
signal recorded between expected peaks. For the remainder of this work, the
30.97 keV photopeak of Barium-133 will be used a particular case study to
illustrate raw and calibrated spectra, performance of calibration software,
and AstroPix response. Like with Cobalt-57, below the Barium-133 photopeak
backscattered signal can also be identified.

Figure 4: Spectrum of the raw data collected over 16 hours with AstroPix_v1
chip003 amp1 of Cobalt-57. Photopeaks associated with 14.41 keV and 122.06 keV
emission can be seen, as well as a Compton edge at 39.46 keV. Between these
features, AstroPix_v1 experiences high background rates from Compton-scattered
interactions.

The anticipated photopeaks (and single Compton edge) from the raw spectra
collected from all the sources in Table 3 are fit with a Gaussian (see Fig. 5
(left) for fitting to Barium-133) where the mean and width are extracted. Due
to high background rates, photopeaks are identified and fit using only the
five most populated bins. The Compton edge for the 122 keV emission line of
Cobalt-57 at 39.46 keV is also considered, and fit with a modified Heaviside
function (derivation in 5). The Gaussian mean is used to create a calibration
curve (see Fig. 6). The response of AstroPix_v1 to increasing charge is
monotonic but not linear and the distribution is best fit with a third-degree
polynomial function.

Figure 5: (Left) Gaussian fit to raw data collected with AstroPix_v1 chip003
amp1 of the Barium-133 30.97 keV photopeak over 65 minutes. The Gaussian mean
$\mu$ is extracted and used to construct a calibration curve. (Right)
Barium-133 spectrum from left panel calibrated with third degree polynomial
fit, with calibration and test data collected from amp1.

Figure 6: AstroPix_v1 calibration curves for chip003 (left) and chip004
(right). Fewer data points were taken for chip004 due to time constraints, but
only four points are required for calibration. Error bars from the error on
the fit parameter fall within the diameter of the markers.

A third degree polynomial describing the relationship between mV and keV the
spectra to be calibrated (see Fig. 5 (right)) with all photopeaks falling
within 7% of the anticipated value. Though this value does not yet meet
AstroPix requirements, it is an impressive start for the first AstroPix design
version. The measurement is limited by the analog readout method of data
collection from a single $175\times 175$ $\mu$m2 pixel. Further improvement
can also be expected as data analysis tools continue to develop.

As the photopeak energy increases the energy resolution after calibration
decreases (see Fig. 7), indicating higher resolution. Since individual pixel
responses differ (as shown in Fig. 3), an individual calibration curve was
constructed from data measured from each pixel. Figure 7 (right) illustrates
the degredation in performance when a calibration curve derived from a
different pixel is used. Figure 5 (right) also illustrates the low energy
threshold that AstroPix_v1 can achieve. With the required noise cuts from
Table 2, AstroPix_v1 measures signal down to 13 keV.

Figure 7: Calibration performance of AstroPix_v1 for all measured photopeaks.
Calibration conducted with a third degree polynomial fit using test data
collected by chip003 amp1 and calibrated by amp1 (left) and amp2 (right).
Calibrating with the same data used to collect test data results in a
calibrated mean energy that is closer to the expected value across the entire
spectrum, and generally lower energy resolution.

##  3 ASTROPIX$\\_$V2

AstroPix_v2 follows AstroPix_v1 part of the planned incremental design
strategy. A key milestone achieved with AstroPix_v2 is the ability to read and
record digital data in addition to the analog data investigated with
AstroPix_v1 after the implementation of shielding against parasitic
capacitance causing comparator oscillations. The results presented in this
section consider only analog data in order to draw direct comparisons between
ATLASPix and AstroPix_v1. Preliminary digital data conclusions will be
presented in Section 5.

###  3.1 Design Optimization and Setup for AstroPix_v2

Pixel size is one of the incremental changes realized in AstroPix_v2, where
the $175\times 175$ $\mu$m2 pixels of AstroPix_v1 are enlarged to $250\times
250$ $\mu$m2. An AstroPix_v2 chip is $1\times 1$ cm2 and contains a $35\times
35$ array of these $250\times 250$ $\mu$m2 pixels.

AstroPix_v2 also features an updated guard ring design around these larger
pixels to help control leakage and allow for a higher depletion voltage
enabling a deeper depletion depth. By removing unused digital to analog
converters (DACs) from the bias block, AstroPix_v2 also sees a reduction of
analog power from 14.7 mW/cm2 to 3.4 mW/cm2. Previously identified issues with
crosstalk from AstroPix_v1 were resolved in AstroPix_v2, and a global
timestamp was enabled such that digital signal can be held in buffer during
readout and read out with the subsequent trigger.

The test setup for AstroPix_v2 is identical to that of AstroPix_v1, where the
chip interfaces with a Nexys FPGA and GECCO board. AstroPix_v2 is also mounted
onto a carrier board, so the AstroPix_v1 setup can be reused. Software updates
have enabled Python wrappers to facilitate chip configuration and data-taking.
Analog data can be collected from 35 pixels (one full row) of AstroPix_v2.
This section focuses on the analog response of a single pixel located in the
corner of the array.

Like with AstroPix_v1, a bias voltage of -60V is supplied for these
measurements. Preliminary studies indicate that maximal depletion of the
sensor may occur around -160V, though this may still result in lower depletion
than the design requirement of 500 $\mu$m. The true depletion depth and
optimal bias voltage is still being studied.

###  3.2 AstroPix_v2 Characterization Studies

####  3.2.1 Charge Injection Studies

As with AstroPix_v1, the response of AstroPix_v2 was checked first using an
injected charge. Updated DAC settings allow for the shaping of a shorter
duration pulse (80 $\mu$s vs 600 $\mu$s) with a taller peak (0.41V vs 0.24V)
than that of Fig. 3. This shape allows for shorter deadtime and less pileup as
compared to AstroPix_v1, but also uncovered a limitation in the analog
AstroPix_v2 design - an on-chip PMOS source follower saturates at 500 mV,
serving as an upper limit to the height of analog pulse that can be collected.
This causes the analog response of AstroPix v2 to saturate at energies of 90
keV - a factor of seven less than the required dynamic range of 25-700 keV.
This will be resolved in future AstroPix versions, but must be considered in
the analysis of AstroPix_v2. This limitation is only present in analog data
and does not impact the performance of the digital output.

####  3.2.2 Energy Calibration

A calibration curve for AstroPix_v2 was made using the response to radioactive
isotopes with known emission lines (see Table 3). An identical analysis
strategy is used, where spectra of AstroPix_v2 responses (the pulse height)
are fit by a Gaussian whose parameters are correlated with anticipated
photopeak energies. A higher threshold is required (see Table 2) due to
increased electronics noise from the heightened capacitance related to the
larger pixels. A spectrum is shown in Fig. 8 (left). Again, note the
corresponding increase in measured mean as compared to AstroPix_v1 (see Fig. 5
(left)), yet smaller Gaussian standard deviation. The high rates of
backscattered signal below the 30.97 keV photopeak noted in AstroPix_v1 is
also decreased in AstroPix_v2 possibly due to a larger distance between the
source and the sensor (reducing background from low-energy scattered
$\gamma$-rays) and/or more complete charge collection due to a larger
depletion region.

Figure 8: (Left) Gaussian fit to raw data collected with AstroPix_v2 chip1
amp1 of Barium-133 over 5.5 hours. The Gaussian mean $\mu$ is extracted and
used to construct a calibration curve. (Right) Barium-133 spectrum from left
panel calibrated with cubic spline fit, with calibration and test data
collected from amp1. The shape of the spectrum changes due to the nonlinearity
of the calibration function.

Figure 9: AstroPix_v2 analog calibration curve for chip1. Most error bars fall
within the diameter of the markers. The impact of PMOS source follower
saturation beginning around 350 mV is evident.

Figure 10: Calibration performance for all tested photopeaks. Calibration
conducted with a cubic spline fit using test data collected by chip1 amp1.

Like AstroPix_v1, the analog response of AstroPix_v2 to increasing charge is
monotonic but not linear. The impact of the PMOS source follower saturation
becomes evident around 60 keV and inhibitive around 90 keV as the response
pulse height rises more slowly and asymptoticly approaches a value around 400
mV. This impact is large in analog data analysis, however the analog studies
presented in this work are intended to serve as a preliminary assessment of
the chip’s capabilities. Digital readout will be utilized by future AstroPix
versions (with a first look presented in Section 5), and it is not impacted by
this saturation.

Due to the source follower saturation, the analog calibration curve (see Fig.
9) is best fit with a cubic spline fit. Resulting calibrated spectra (see Fig.
8 (right)) reproduce photopeaks falling within 15% of the anticipated value.
This includes the outlying 88 keV photopeak from Cadmium-109. A lack of
statistics for the raw spectrum led to poor fitting which propagated to the
nonlinearly scaled calibrated spectrum. Without consideration of this point,
the cubic spline calibrations agree with expected values within 3%. This is a
vast improvement over AstroPix_v1 calibration, and provides a point of
comparison for eventual studies of the digital output. A higher degree of
confidence could be placed on a calibration function spanning only from 14 -
60 keV, in the range that AstroPix_v2 is unaffected by source follower
saturation. Figure 8 (right) also illustrates that the low energy threshold of
13 keV is consistent with that of AstroPix_v1.

As the photopeak energy increases the energy resolution after calibration also
increases (see Fig. 10). Compared to AstroPix_v1 (see Fig. 7), AstroPix_v2
measures a better energy resolution for each calibrated point with
$E_{res}<16\%$ for all points as compared to $E_{res}<25\%$ with AstroPix_v1.

The case study of the 30.97 keV photopeak of Barium-133 is considered in Table
4. Analog measurements from AstroPix_v2 perform better than those from
AstroPix_v1, and the planned incremental upgrades to AstroPix design is
delivering results consistently closer to design requirements and with higher
precision. It is assumed that the final AstroPix design will be dominated by
electronics noise from the range of 25-122 keV with a constant energy
resolution of 5 keV RMS. Therefore the required energy resolution performance
at 30.97 keV of 38% is already being met with analog measurements made in the
lab of all tested AstroPixversions. It must be noted, though, that the
presented measurements are the result of analog measurements of a single,
well-characterized pixel and ultimate performance of AstroPix must be judged
on the digital performance of the full array.

The lowest measured energy resolution with ATLASPix is also included in Table
4, although the thinner ATLASPix wafer is easier to fully deplete and was
previously optimized by the ATLASPix team for measurement while AstroPix
characterization and optimization is occurring in tandem.

Table 4: Comparison of AstroPix measurements with ATLAPix performance and AstroPix requirements. AstroPix energy resolution requirement is 5 keV RMS at 122 keV which is assumed to be constant down to 25 keV, or a 38% energy resolution at 30.97 keV. Energy resolution values are stated with respect to the FWHM. The 500 $\mu$m wafer requirement represents the depletion depth requirement. | Energy Resolution at 30.97 keV [%] | Wafer Thickness [$\mu$m]
---|---|---
Requirement | $38\%$ | 500
ATLASPix | $7.3\pm 1.2$ | 100
AstroPix_v1 | $19.9\pm 7.4$ | 700
AstroPix_v2 | $15.4\pm 2.9$ | 700

When considering calibration as a whole, AstroPix_v2 experiences limitations
in its calibration power due largely to the PMOS source follower saturation at
500 mV. The effective range of calibration from 0-60 keV is more limited than
that of AstroPix_v1 which could successfully calibrate up to 122 keV. However,
measured points within this valid range were better calibrated with
AstroPix_v2 as compared to AstroPix_v1. This effect is noted and will improve
with updated hardware in future AstroPix iterations, however a shift from
analog data to digital data will also limit the scope of this setback. Digital
data, not analog data, will be utilized exclusively in the final AstroPix
design so shortcomings in the analog circuitry (used at this stage for
testing) do not necessarily inhibit the overall intended chip performance.

##  4 Radiation Testing

Preliminary testing of AstroPix_v2 has been conducted in two separate beam
environments. The intention of this testing is to raise the technology
readiness level (TRL) of AstroPix and increase confidence in the chip’s
capabilities on orbit. Results presented here are preliminary, and currently
undergoing independent assessment.

AstroPix_v2 participated in two campaigns at the Fermilab Test Beam Facility
(Ref. 6) where it was exposed to a 300 kHz 120 GeV proton beam. Both the
digital and analog performance was assessed in these qualitative tests.
AstroPix_v2 was able to register and record signals in this high-energy and
high-flux environment, though the rate of interaction is much greater than the
environment AstroPix is designed for in orbit. This high rate overwhelmed the
current software designed for debugging and chip characterization, which
optimized for lower rates of $<$5 Hz and is not the final flight-ready
version. None of the monitored power rails increased current draw during beam
running as compared to bench running. These successful tests were the first
radiation tests of any AstroPix chip and serve as an important milestone for
the continued development of the technology.

AstroPix_v2 was also subjected to radiation testing at the 88-Inch Cyclotron
operated in conjunction with the Berkeley Accelerator Space Effects facility
at Lawrence Berkeley National Laboratory (Ref. 7). Here, heavy ions ranging
from Argon to Xenon with an atomic tune of 16 MeV/(atomic mass unit)
illuminated AstroPix_v2. The susceptibility to latchup was first recorded.
This is a state of inactivity in which an incident ion deposits significant
charge in a pixel, inducing a perpetually open transistor and leading to
runaway current draws. The latchup state must be corrected through a chip
reset and reconfiguration. The sensor successfully survived this test,
withstanding a fluence (integrated flux) of $1\times 10^{7}$ particles/cm2 at
an effective linear energy transfer (LET) of 65 MeV*cm2/mg.

Testing for single event functional interrupts (SEFI) was also conducted. This
event may occur when a heavy ion collision interacts with the digital
periphery and flips a bit upon readout causing data degradation or corrupted
configuration. Proper operation can be restored with a chip reset or
reconfiguration as no permanent damage is sustained. Possible evidence of
SEFIs with an onset of LET=22.5 MeV*cm2/mg were noted during the run,
identifying elements of the digital framework of the chip that may be
susceptible. This informed an updated design of future AstroPix versions,
which can limit the onset of SEFIs to higher LET values.

##  5 Future Plans and Summary

The digital output of AstroPix_v2 is still being tested, and will be included
in a dedicated paper. The previously considered analog data is the signal
pulse returned from a single pixel. For digital output, this pulse is
digitized on chip and a bitstream with encoded hit information is read out. A
particle interaction in a pixel will cause the pixel’s comparator to trigger.
The comparator also calculates a Time over Threshold (ToT) value associated
with the particle interaction. This is a true time in microseconds that the
response pulse exceeded a defined threshold value. This ToT value, rather than
pulse height as seen in the analog case, serves as a proxy for the amount of
charge deposited. A larger ToT value indicates a larger charge deposition. The
threshold value used to calculate the ToT is universal for every pixel across
the chip and in AstroPix_v2 cannot be set on a pixel-by-pixel basis. All
comparator readings from the full array are OR’d into one channel containing
row hit information and another of column hit information. The triggering of
any comparator will impact an interrupt signal which indicates that the full
chip should be read out.

Preliminary work shows that the sensor is able to trigger upon, read out, and
decode data (see Fig. 11). A clear photopeak in response to a Barium-133
source can be seen in this ToT spectrum when data from a single digital pixel
is read out. This early work is intended to serve as an illustration that
digital data can be collected and interpreted, and not yet serve as an
indication of AstroPix_v2 digital performance. Work is currently underway to
further understand the performance of the digital readout system with studies
planned to map noise and gain variations between pixels, record digital
signals from more pixels on the array, and optimize the comparator’s threshold
value.

Figure 11: Preliminary spectrum of digital data recorded over 3 hours from one
AstroPix_v2 pixel with a Barium-133 source.

AstroPix_v3 was submitted for fabrication in Summer 2022. Additional power-
reducing improvements will be incorporated into the design with a target total
draw of $<1$mW per cm2. The pixel size will also continue to increasing in
AstroPix_v3, up to $500\times 500$ $\mu$m2. Within this pixel pitch, the
active area will be $300\times 300$ $\mu$m2 and the total array will increase
to $2\times 2$ cm2. This additional space allows for the removal of conductive
$p$-wells between pixels, limiting noted leakage of the charge cloud into the
array periphery. AstroPix_v3 will also be designed with the possibility to
dice the wafer into a quad-chip - four individual chips cut from the wafer as
one $4\times 4$ cm2 square of identical MAPS chips, quadrupling the number of
pixels. This is the unit that will act as a building block for larger
structures such as a future telescope tracker. Readout capability of the quad
chip involving daisy-chaining the signal from one chip through the others on
SPI readout was implemented in AstroPix_v2, but will be fully tested in
AstroPix_v3. With this full quad-chip, environmental testing of of AstroPix_v3
in a space environment is also planned with the development of a four-layer
hosted payload to be included on a sounding rocket mission. The success of
this testing will further raise the TRL. In AstroPix$\\_$v4, digital state
triplication will be introduced to protect the chip against single event
functional interrupts possibly observed in the radiation testing discussed in
Sec. 4.

In summary, AstroPix is a CMOS MAPS chip designed for the space environment. A
robust design and characterization strategy is in place to advance the
technology from its current stage to flight readiness. The current version
under test, AstroPix_v2, is capable of analog and digital readout and a
comprehensive analysis of the analog response has been presented and compared
to the performance of the previous version, AstroPix_v1. Metrics such as total
power draw do not yet meet AstroPix design requirements, but the AstroPix
design strategy relies upon incremental changes between versions building to
the final design. Therefore, the consistent improvement of performance in
updated versions is a success toward realizing a final AstroPix. The
utilization of AstroPix in future $\gamma$-ray telescopes (such as AMEGO-X, as
described in Ref. 8) will enable high energy, angular, and position resolution
for low power consumption these next-generation detectors.

###### Acknowledgements.

The authors would like to thank the input of the AstroPix team for their help
and support in development, fabrication, firmware/software production, and
testing of AstroPix. This work is funded in part by 18-APRA18-0084. HF and MN
acknowledge support by NASA under award number 80GSFC21M0002. AS was supported
by the National Aeronautics and Space Administration (NASA) through a contract
with ORAU. The views and conclusions contained in this document are those of
the authors and should not be interpreted as representing the official
policies, either expressed or implied, of the National Aeronautics and Space
Administration (NASA) or the U.S. Government. The U.S. Government is
authorized to reproduce and distribute reprints for Government purposes
notwithstanding any copyright notation herein.

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