Further author information: (Send correspondence to H.R. Stueber)
H.R. Stueber: E-mail<EMAIL_ADDRESS>Telephone: 1 920 252 5189

# The XOC X-ray Beamline: Probing Colder, Quieter, and Softer

Haley R. Stueber  Kavli Institute for Particle Astrophysics and Cosmology,
Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA  Department of
Physics, Stanford University, 382 Via Pueblo Mall, Stanford CA 94305, USA
Tanmoy Chattopadhyay  Kavli Institute for Particle Astrophysics and Cosmology,
Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA  Sven C.
Herrmann  Kavli Institute for Particle Astrophysics and Cosmology, Stanford
University, 452 Lomita Mall, Stanford, CA 94305, USA  Peter Orel  Kavli
Institute for Particle Astrophysics and Cosmology, Stanford University, 452
Lomita Mall, Stanford, CA 94305, USA  Tsion Gebre  Kavli Institute for
Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall,
Stanford, CA 94305, USA  Department of Physics, Stanford University, 382 Via
Pueblo Mall, Stanford CA 94305, USA  Aanand Joshi  Kavli Institute for
Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall,
Stanford, CA 94305, USA  Department of Physics, Stanford University, 382 Via
Pueblo Mall, Stanford CA 94305, USA  Steven W. Allen  Kavli Institute for
Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall,
Stanford, CA 94305, USA  Department of Physics, Stanford University, 382 Via
Pueblo Mall, Stanford CA 94305, USA  SLAC National Accelerator Laboratory,
2575 Sand Hill Road, Menlo Park, CA 94025, USA  Glenn Morris  Kavli Institute
for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall,
Stanford, CA 94305, USA  SLAC National Accelerator Laboratory, 2575 Sand Hill
Road, Menlo Park, CA 94025, USA  Artem Poliszczuk  Kavli Institute for
Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall,
Stanford, CA 94305, USA

###### Abstract

Future strategic X-ray satellite telescopes, such as the probe-class Advanced
X-ray Imaging Satellite (AXIS), will require excellent soft energy response in
their imaging detectors to enable maximum discovery potential. In order to
characterize Charge-Coupled Device (CCD) and Single Electron Sensitive Read
Output (SiSeRO) detectors in the soft X-ray region, the X-ray Astronomy and
Observational Cosmology (XOC) group at Stanford has developed, assembled, and
commissioned a 2.5-meter-long X-ray beamline test system. The beamline is
designed to efficiently produce monoenergetic X-ray fluorescence lines in the
0.3-10 keV energy range and achieve detector temperatures as low as 173 K. We
present design and simulation details of the beamline, and discuss the vacuum,
cooling, and X-ray fluorescence performance achieved. As a workhorse for
future detector characterization at Stanford, the XOC beamline will support
detector development for a broad range of X-ray astronomy instruments.

###### keywords:

X-ray, CCD, SiSeRO, low noise, readout, test system, vacuum chamber, AXIS

##  1 INTRODUCTION

Providing simultaneous imaging and moderate spectroscopic capabilities for
X-ray energies from $\sim 0.3-10$ keV, charge-coupled device (CCD) detectors
revolutionized the field of X-ray astronomy [1, 2, 3]. The CCD detectors
onboard the two current flagship X-ray observatories, the _Chandra X-ray
Observatory_ and the _X-ray Multi-Mirror Mission_ (XMM-Newton), have enabled
groundbreaking insights across a myriad of astrophysical phenomena by
providing high resolution imaging of X-ray sources. Although Chandra and XMM-
Newton have accomplished much throughout their lifetimes, they were deployed
nearly 25 years ago and there is an ever-widening gap between the capabilities
of these operating X-ray observatories and their newer infrared/optical
counterparts. In order to close this gap, the 2020 Decadal Survey of Astronomy
and Astrophysics [4] prioritized a new great X-ray observatory flagship, as
well as a nearer-term X-ray probe-class mission that would build upon the
successes of Chandra and XMM-Newton, providing order of magnitude improvements
in performance. For example, among the X-ray probe-class mission candidates,
the _Advanced X-ray Imaging Satellite_ (AXIS) would uniquely provide arcsecond
spatial resolution imaging across a wide field of view with a mirror
collecting area 10 times that of Chandra. These characteristics will enable
transformative measurements of faint sources, providing a natural complement
to optical/infrared telescopes such as _Roman_ and the _James Webb Space
Telescope_ (JWST). However, for brighter sources, the narrow point-spread
function (PSF) and large collecting area of AXIS increase the risk of photon
pileup, impeding accurate flux and energy measurements [5, 6, 7]. Minimizing
pileup and saturation effects for the small pixel, large focal plane detectors
required of next generation X-ray telescopes necessitates improvements in
detector technology and fast, low-noise, low-power readout methods.

Much progress has been made in developing technology that has the potential to
fulfill these requirements, including recent work done with Active Pixel
Sensors (APS) such as Hybrid CMOS detectors (HCDs) [8, 9] and Depleted Field
Effect Transistor (DEPFET) detectors [10]. However, current HCDs suffer from
relatively high readout noise[11], while DEPFET X-ray imaging detectors focus
on relatively large pixel sizes[12], making them less-than-ideal candidates
for missions targeting high spatial resolution. X-ray CCD technology, however,
can deliver the speed and noise performance required for missions like AXIS.
To this end, in collaboration with MIT and MIT Lincoln Laboratory, Stanford
University is developing integrated electronics to rapidly read out these
detectors with high speed, low noise, and low power consumption. To operate
these detectors at high speeds and characterize their spectroscopic and noise
performance in a way that reflects the conditions and requirements for future
missions, we needed a test system that could probe the full range of necessary
X-ray energies and operating temperatures, under representative vacuum
conditions. To accomplish this, we have constructed and commissioned the XOC
X-ray Beamline at Stanford. In this manuscript, we detail its design and
demonstrated performance, and present some initial X-ray spectra gathered
using the beamline.

##  2 The XOC X-ray Beamline Experimental Test System

The XOC X-ray beamline fundamentally consists of an X-ray fluorescence source
and a detector chamber, under vacuum and separated by roughly two meters of
standard tubular vacuum flange components. The detector is mounted vertically
to the inside door of the detector chamber, which also contains the cooling
system, a retractable ${}^{55}\mathrm{Fe}$ radioisotope source, flux monitor,
and detector readout electronics. A SolidWorks111https://www.solidworks.com/
model for the XOC X-ray beamline is shown in Fig. 1, and the fully assembled
and commissioned beamline is pictured in Fig. 2. The process for detector
readout in the beamline is shown in the block diagram of Fig. 3. The readout
module consists of a fast, low-noise pre-amplifier board that receives the
output signals from the detector and serves them to an Archon
controller222http://www.sta-inc.net/archon/ outside the beamline, via a potted
flex cable. The Archon both digitizes the output signal and supplies clock and
bias signals to the CCD. The beamline detector chamber and source-side
assemblies are discussed further in the following sections. We also discuss
the cooling and system monitoring modules, and give a general description of
the detectors and the readout electronics.

---
Figure 1:  SolidWorks model images of the XOC X-ray Beamline. The model
consists of an X-ray source and fluorescence targets, and a large chamber-side
containing the detector and readout electronics, as well as an auxiliary
${}^{55}\mathrm{Fe}$ source. The source-side and detector chamber are
separated by roughly 2 meters of standard vacuum flange components. The model
view in the upper right indicates the locations of the vacuum pumps and
pressure gauges on the beamline. The model view in the lower right indicates
the location of the valve used for venting the beamline.
---
Figure 2:  Fully assembled XOC X-ray beamline on top of an optical table. In
the image, the far left side of the beamline is the detector chamber-side,
while on the far right is the source-side.
---
Figure 3:  Basic readout process block diagram for XOC X-ray Beamline[13]. The
readout module consists of an amplifier board and the Archon controller, which
also sources the clock and bias signals.

###  2.1 Beamline Source-Side

The source-end of the beamline (Fig. 4a) contains a broadband bremsstrahlung
continuum X-ray source. X-rays from this source impact on a target wheel,
angled at 45 degrees to the primary X-ray beam. Fluorescent X-ray photons
emitted from the target travel down the length of the beamline to the
detector, which is mounted on the chamber door. The process is shown in Fig.
4b. A range of target materials are accessible on two rotatable,
interchangeable target wheels, enabling the beamline to produce mono-energetic
lines spanning the entire energy range of interest. The sections below detail
the X-ray source specifications and performance as well as the source end
model and methods.

(a)

(b)

(c)

Figure 4: (a) Beamline source end with the bremsstrahlung X-ray source
attached. (b) SolidWorks model image of the source end. The white dashed
arrows indicate the path of photons from the X-ray source to the target wheel,
and the subsequent fluorescence photons from the target wheel into the
beamline. (c) One of the target wheels with titanium (upper left), teflon
(upper right), copper (lower right), and aluminum (lower left) targets
attached.

####  2.1.1 X-ray Source

The primary X-ray source used in the XOC X-ray Beamline is a Micro X-ray
Seeray Integrated Water-cooled X-ray
Tube333https://microxray.com/products/seeray-water-cooled-x-ray-tube/, which
is pictured in Fig. 5. This source is powered by a Spellman uXHP 50 kV 100 W
high voltage power supply444https://www.spellmanhv.com/en/high-voltage-power-
supplies/uXHP, which features an analog interface connector and corresponding
graphical user interface (GUI) that enables remote control and monitoring of
the X-ray source tube voltage, emission current, and filament current. The
power supply has been set up with external fail-safe interlocking, ensuring
X-rays will not be generated unless the interlock switch is closed.

The X-ray tube generates X-rays by accelerating electrons from a hot cathode
filament towards a tungsten anode using high voltage. The rapid deceleration
of the electrons at the anode stimulates bremsstrahlung X-ray emission. The
upper limit of the energy of the photons emitted by the source depends on the
high voltage potential, which is adjustable from 4-60 kV. The maximum anode
current of the source is 2.0 mA, which limits the rate of X-ray photons
emitted from the source. The source is protected by a 0.005 inch thick
beryllium window, which is mostly transparent to X-rays excepting some
attenuation at soft energies[14].

---
Figure 5:  Micro X-ray Seeray Integrated Water-Cooled X-ray Tube X-ray source.
The high voltage range is 4-60 kV, with a maximum tube current of 2.0 mA.

####  2.1.2 X-ray Fluorescence Methods

To produce mono-energetic X-ray photons across a broad range of energies, we
use fluorescence methods. Bremsstrahlung photons from the X-ray source
irradiate a target, mounted on a wheel within the source end, exciting
electrons in the target material. As those electrons decay, they emit
characteristic K-alpha and K-beta photons whose energy depends on the target
material. Two rotatable and interchangeable target wheels are available, each
carrying four separate targets (Fig. 4c). The wheel is designed and located
such that only one target is illuminated by the X-ray source at a time. In
total, the two target wheels can produce 11 distinguishable fluorescence
emission lines spanning the energy range 0.3 - 10 keV. The purities,
characteristic energies, and thicknesses of the targets are summarized in
Table 1.

Table 1:  Target Materials and their Purities, Characteristic Fluorescence Energies, Thicknesses, and Fano Limit Full Width Half Maximums Target Material | Purity | Thickness | Characteristic Energies | Fano FWHM
---|---|---|---|---
Teflon | N/A | 200 $\mu$m | 0.67 keV (Fluorine K$\alpha$) | 39 eV
Magnesium | 99.9% | 200 $\mu$m | 1.25 keV K$\alpha$ | 54 eV
Aluminum | 99.999% | 200 $\mu$m | 1.49 keV K$\alpha$ | 59 eV
Potassium Chloride | 99.8% | 1000 $\mu$m |  2.62 keV (Chlorine K$\alpha$) 3.31 keV (Potassium K$\alpha$) |  78 eV 87 eV
Titanium | 99.99% | 100 $\mu$m |  4.51 keV K$\alpha$ 4.93 keV K$\beta$ |  102 eV 107 eV
Iron | 99.99% | 125 $\mu$m |  6.40 keV K$\alpha$ 7.06 keV K$\beta$ |  121 eV 128 eV
Copper | 99.99% | 100 $\mu$m |  8.05 keV K$\alpha$ 8.91 keV K$\beta$ |  136 eV 143 eV

The last column of the table lists approximate theoretical full width half
maximums (FWHM) at the fano limit in silicon for each target material. These
were determined assuming an average ionization energy for silicon of 3.6 eV,
and the fano factor was taken to be 0.116 for silicon[15]. Simulations of two
representative target spectra are given in section 3.3 and the measured flux
and contamination for those targets are presented in section 4.

###  2.2 Detector Chamber

The main chamber of the XOC X-ray Beamline, pictured in Fig. 6a, was
manufactured by Ideal Vacuum Products555https://www.idealvac.com/en-us/ based
on a SolidWorks model that we provided. The chamber is made of stainless steel
and has dimensions of roughly 70 cm in each direction. The door of the chamber
hosts standard vacuum flange components used for the vacuum pump, pressure
sensor, temperature sensor, silicon drift detector (SDD), and cryocooler
connections. The chamber also houses the CCD detector and its readout
electronics, the cooling module, as well as an ${}^{55}\mathrm{Fe}$ X-ray
source, which can be retracted and rotated to change the direction of
radiation via a vacuum-compatible wobble stick. In this section, we outline
the mechanical design of the detector chamber and detector mounting scheme,
and highlight the various regulatory systems and fail-safes employed,
including vacuum, venting, cooling, and flux monitoring.

(a)

(b)

Figure 6: (a) Detector chamber door with the Archon controller, detector
temperature sensor connector flange, cryo-cooler flange, and SDD monitor
connector flange. (b) Detector mounting scheme on the inside of the beamline
door.

####  2.2.1 Cooling and Temperature Control Module

A vital aspect of the beamline is its ability to efficiently cool the detector
and maintain stable temperatures during testing. Efficient cooling is achieved
with an Edwards Polycold PCC Compact Cryo-
cooler666https://www.edwardsvacuum.com/en-us/our-markets/semiconductor-and-
electronics/cryogenics/cryochillers/pcc-compact-coolers which, by itself, is
able to achieve temperatures as low as 70 K. The cold-end of the cooler is
mounted to the detector chamber door, and it is thermally coupled to an
aluminum block atop which the detector housing sits. This thermal connection
is achieved by clamping a stack of thin copper film straps and indium foils to
the cold head with a copper block. The other end of the straps is clamped to
the bottom of the aluminum block under the detector, which is supported using
thermally insulated peek plastic standoffs. A layer of Tflex-500 thermally
conductive gap filler adhesive
padding777https://www.laird.com/sites/default/files/2019-10/Tflex%20500%20Series.pdf
is sandwiched between the aluminum block and the back of the detector package
to ensure uniform thermal contact. This setup and its components are shown in
Fig. 7.

(a)

(b)

(c)

Figure 7: (a) The detector cooling system with the aluminum cold block, copper
straps, and copper block that thermally couples the system to the cold head of
cryo-cooler. Resistance temperature detector (RTD) sensors are connected to
the aluminum block and copper plate. (b) Heater connection inside of aluminum
block. (c) Cooling system with readout PCB mounted and RTD temperature sensors
connected to chamber door flange. An additional RTD sensor is soldered to the
detector socket pins on the PCB.

It is important during operation that the CCD parameters do not have any
temperature dependent fluctuations. We use RTD sensors coupled with a
proportional-integral-derivative (PID) temperature control loop to monitor and
stabilize the detector temperature[16]. There are two RTDs in the system; one
is adhered via a thermally conductive but electrically insulating epoxy to the
copper plate on the cold-head, and the other is soldered onto pins of the
detector socket, which is mounted to the readout preamplifier board. Such a
configuration makes it possible to monitor both the detector and the cryo-
cooler cold-end temperatures. These 2-wire PT 1000 Tewa Sensor LLC
RTDs888https://www.digikey.com/en/products/detail/tewa-sensors-llc/TT-
PT-1000B-2050-11-AUNI/9817197 are wired to their respective Adafruit MAX31865
RTD temperature sensor amplifier
boards999https://www.adafruit.com/product/3648, which are operated by a
Raspberry Pi111https://www.raspberrypi.com/. As such, the Raspberry Pi both
controls the bias and the clocking conditions of the temperature sensors and
runs the PID temperature control loop. The PID regulates the detector
temperature by reading out the adjacent RTD sensor and using it as the input
for the PID feedback loop along with the reference temperature setpoint given
by the user. The PID actuator is a Birk Manufacturing cartridge
heater222https://www.digikey.com/en/products/detail/birk-
manufacturing/DC1101/15790977 embedded in the aluminum block. With these
cooling methods we are able to achieve stable operation down to 173 K with a
temperature stability of roughly 0.1 K.

####  2.2.2 System Monitoring and Fail-Safes

To ensure safe and reliable operation of the beamline, we have implemented a
variety of fail-safes and monitoring systems. Namely, these include a pressure
monitoring and alarm system, venting and purging with clean dry air and a
pressure relief valve, back-up power supplies for the vacuum pumps, and the
X-ray source interlocking fail-safe outlined in section 2.1.1. Two vacuum
pumps are used to pump out the beamline to pressures as low as 5e-8 mbar. An
Agilent IDP-7 Scroll
Pump333https://www.agilent.com/store/productDetail.jsp?catalogId=X3807-64000&catId=SubCat2ECS_1465486
and Varian TV 551 Navigator Turbo
Pump444https://www.ajvs.com/product_info.php?products_id=17586&1808 are
attached to the chamber-side of the beamline, and an Agilent TPS-flexy Turbo
Pumping
System555https://www.agilent.com/store/productDetail.jsp?catalogId=X1699-64087&catId=SubCat2ECS_1466018
pumps out the source-side. Gate valves at different positions throughout the
beamline make it possible to bring the chamber to atmosphere while the main
body and source end of the beamline can remain under vacuum. A venting valve
positioned near the detector chamber (Fig. 1) can be opened to purge the
chamber with clean dry air. For general safety and the safety of the detector
and SDD monitor, we have a pressure relief valve which will open in the event
of pressures exceeding 1.2 atmospheres in the chamber during purging. For
monitoring the pressure, four pressure gauges are mounted at different points
in the beamline, indicated in the aerial view of the SolidWorks beamline model
in Fig. 1. The first gauge monitors the pressure in the detector chamber, the
second monitors the pressure near the main chamber pump, the third monitors
pressure in the middle of the beamline, and the last monitors the pressure
near in source end of the beamline. The pressure monitors used are Agilent
FRG-700 Full Range Pirani Inverted Magnetron
Gauges666https://www.agilent.com/store/productDetail.jsp?catalogId=FRG700KF25&catId=SubCat2ECS_1465644.
The pressure in each sector of the beamline is recorded continuously via a
Python program, and if it rises above a critical pressure of 5e-5 mbar,
Slack777https://slack.com/ alerts are sent to all lab members every five
minutes until sub-critical pressure is restored. The vacuum pumps are attached
to uninterruptible power supplies (UPS) to ensure vacuum is maintained even in
the event of a power outage. In this way, it is possible to prevent a sudden
loss of vacuum, which could damage the source and lead to condensation buildup
on the detector while cold.

####  2.2.3 Silicon Drift Detector Flux Monitor

To characterize detector performance, it is essential to know line fluxes of
fluorescent photons within the detector chamber. To make these measurements, a
silicon drift detector (SDD) is mounted on the chamber door, near the plane of
the detector, as pictured in Fig. 6b. Particle interaction simulations that
were carried out for the beamline (discussed in 3.3) suggest that the photon
distribution on the detector chamber door appears uniform (Fig. 8). Therefore,
by being in the field of view, the SDD provides accurate and simultaneous
measurements of incident flux and can be used for fast and easy detector
quantum efficiency estimates. SDD detectors generally consist of a window, a
fully depleted silicon bulk, and a collection anode, the small size of the
which leads to a small device capacitance. Because of this, these devices
typically demonstrate very low noise and high resolution at high count
rates[17]. The beamline SDD model is an Amptek 25 $mm^{2}$ Fast Silicon Drift
Detector888https://www.amptek.com/products/x-ray-detectors/fastsdd-x-ray-
detectors-for-xrf-eds/fastsdd-silicon-drift-detector with a C2 (silicon
nitride) X-ray window, which enables detection of photon energies down to 0.3
keV. The SDD comes with a built-in thermo-electric cooler (TEC) that cools the
detector to 210K. The SDD is heat sunk to the detector chamber door. While the
CCD is collecting data, the flux monitor continuously builds a live spectrum.
This spectrum is extremely useful for aligning the targets in the source beam,
making it easy to see the relative change in monochromatic line flux as the
target wheel is being rotated. In this way, the SDD is also used to ensure
that only one target is fully illuminated at a time.

---
Figure 8:  Simulated photon counts distribution on beamline detector chamber
door.

###  2.3 CCID-93 Detector and Readout Module

In this section, we give a brief overview of the detectors currently being
tested in the beamline together with their corresponding readout electronics.
Details of the detectors and readout electronics can be found in [18], [19],
[20], [21], [22], [23], and [24], while latest updates are given in
proceedings [25], [26], [27], and [28].

The CCID-93 detectors fabricated by our collaborators at MIT Lincoln
Laboratory (MIT-LL) have two variants of on-chip output amplifier stages:

  * •

A source follower p-channel junction field effect transistor (p-JFET)[18, 19,
20, 21, 22]. This is a high impedance, voltage based readout method.

  * •

A Single Electron Sensitive ReadOut (SiSeRO) sensor consisting of a p-channel
metal-oxide semiconductor field effect transistory (p-MOSFET), which is based
on a drain current readout technique[29, 30, 31].

The detector output signal is amplified in a preamplifier board, designed
specifically to accommodate both the p-JFET and the SiSeRO output stages. The
preamplifier board has the option of using either a discrete electronics
implementation or an application specific integrated circuit (ASIC) for the
readout. The ASIC used is the multi-channel readout chip (MCRC)[23, 32, 24]
developed by Stanford University. Its core advantages include a small physical
footprint and low-power consumption, while providing fast, low noise readout.
The preamplifier board output signal is transmitted from inside the detector
chamber through a 200-pin flex cable potted in a custom-made flange (Fig. 9)
to a Semiconductor Technology Associates, Inc Archon controller. The Archon is
an FPGA-based instrument that provides control signals like clocks and biases
to the detector and simultaneously digitizes the signal of interest. In
addition, the Archon controller features an internal digital signal processing
pipeline that extracts the relevant information from the acquired detector
waveform to reconstruct a 2-dimensional image. A more detailed description of
the Archon controller modules is given in [13].

---
Figure 9:  200-pin flex cable potted in custom flange on detector chamber
door.

##  3 GEANT4 Simulations of Beamline Performance

Before fabrication, to optimize the performance of the XOC beamline, including
line fluxes, spectral contamination and radiation leakage, we carried out
Monte Carlo simulations of the particle interactions that would take place
during operation of the source. To model these complex particle-matter
interactions, the open source GEANT4 toolkit999https://geant4.web.cern.ch/[33]
was used, which includes a comprehensive set of physics processes for
electromagnetic, strong, and weak interactions of particles in matter over a
broad energy range. GEANT4 contains an extensive database of materials to
assign to a number of pre-defined geometries as well as the option to import
computer aided design (CAD) models. In the sections below, we describe the
model geometries and material properties assigned in the GEANT4 model to each
of the beamline components, and define the interaction physics included in the
model. We also present simulation results for some representative targets.

###  3.1 Geometries and Materials

In order to build our model geometries and define their material compositions,
we utilize a modified version of the GEANT4 detector construction class
G4VUserDetectorConstruction. Geometries can be defined in GEANT4 either by
using built-in shape functions such as G4Box or G4Sphere, or by importing CAD
models. For example, the beamline SolidWorks models for the detector chamber,
main body, and source end were imported as .stl files into GEANT4 using the
CADmesh2 library[34]. These components were assumed to be made of 304L
stainless steel (69% iron, 18.5% chromium, 9% nickel, 2% manganese, and 1%
silicon with less than 1% of carbon, phosphorous, sulfur, and nitrogen). The
chemical compositions defined in the simulations for 304L stainless steel and
all other materials are detailed in Appendix B.

The targets and target wheel were defined as G4Box geometries. The targets
were assumed to be pure with the exception of the teflon target, which was
defined as 24% carbon and 76% fluorine. The material of the wheel itself was
defined as an aluminum-6061 alloy. The X-ray source contains a beryllium
window which causes some attenuation of soft X-rays, so this window was also
included in the simulations. The detector is defined as an active volume made
of silicon, which means that during simulation, particle interaction types and
positions as well as energy depositions in the detector volume are recorded.
Dimensions of the detector box geometry were given as 200mm x 200mm with
thickness 0.1mm. To reduce simulation times and to accumulate enough detector
photon counts to produce a simulated spectrum, the detector area used in
simulation was scaled up by a factor of about 2500 from that of a real CCID-93
detector. Since the actual expected number of counts scales directly with
detector area, the actual expected number of detected photons would be 2500
times less than is achieved in simulation.

For computational simplicity, it was assumed that smaller-scale elements such
as screws, rods and wires would not have a large impact on the simulation
results and were therefore not included in the model. However, elements in
direct contact with or near to the detector were defined as simplified block
geometries with their corresponding material properties in order to estimate
their contributions to spectral contamination. Such components included the
copper straps and block which thermally couple the detector to the cryo-cooler
head, the G10 plastic standoffs which support the cold block under the
detector, the PCB board on which the detector is mounted, and the ceramic
detector housing itself.

###  3.2 Simulation Physics

For the particle interaction physics used in our simulations, we have user
defined physics lists from the GEANT4 G4VUserPhysicsList class. The physics
list defines processes involving X-ray photons and secondary electrons
generated from X-ray interactions with the beamline geometries. The processes
we’ve included in our simulations are listed in Appendix A.

During simulation, the G4GeneralParticleSource class is configured to generate
source photons at a given position in the world volume. In this configuration,
the photon source was defined as a circular plane with a momentum vector which
directed X-rays into the beamline at 45 degrees from the target wheel. To
emulate the photons which would be generated by the bremsstrahlung X-ray
source in the beamline setup, the simulated source photons were allowed to
take on energies between 1 and 20 keV according to a power law distribution
with an exponent of -1. A visualization of the photon paths and interactions
inside the beamline during simulation can be seen in Fig. 10. From this
visualization, we note negligible leakage of photons from the beamline.

---
Figure 10:  Visualization of GEANT4 simulation of particle interactions inside
the beamline after 400,000 events have taken place. Green tracks indicate
photons and yellow dots indicate interaction locations.

###  3.3 Simulation Results for Fluorescent Targets

Simulations for each of the seven planned fluorescent target materials were
run for 100 million source photons each. The histograms in Fig. 11a and b
present the number of events per 0.2 keV bin of energy which interacted with
the detector volume for the aluminum and copper targets, respectively. The
characteristic K$\alpha$ and K$\beta$ lines for each target are easily
identifiable, with minimal contamination at other energies observed. Based on
the results of these simulations, we felt confident that the beamline would be
able to produce strong, mono-energetic lines across a broad range of X-ray
energies with minimal radiation leakage.

(a)

(b)

Figure 11: Simulated GEANT4 spectra for (a) A 200 $\mu$m thick pure aluminum
target (b) a 100 $\mu$m thick pure copper target.

##  4 Characterization of MIT-LL CCD X-ray Detectors

After the beamline had been fully assembled and commissioned, a CCID-93
detector was mounted inside. Cooled down to 193 K, the detector recorded
spectra for all seven fluorescent targets. Presented here are two
representative spectra for a relatively low energy line and a high energy
line, respectively. The Gaussian fit to the spectrum for the Aluminum target
shown in Fig. 12a indicates a full width half maximum (FWHM) of 75.1 eV at
1.49 keV. This corresponds to a resolution $R=E/\Delta E$ of $\sim 20$. For
the copper target spectrum shown in Fig. 12b, we obtain an FWHM of 145.5 eV at
the 8.05 keV K$\alpha$ energy, resulting in a resolution of $\sim 56$. In
comparison to the values given in Table 1, the resolution of the aluminum line
is within about $\sim 25$ % of the fano limit, while copper is within $\sim 5$
%.

(a)

(b)

Figure 12: CCD spectra for (a) A 200 $\mu$m thick pure aluminum target (b) a
100 $\mu$m thick pure copper target.

In addition to the CCD spectra, in Fig. 13, SDD spectra of all 7 targets
across two target wheels (including an additional copper target that was
placed on the second target wheel) were collected and are plotted against a
background spectrum. SDD collection times varied from 60 seconds (copper) to
1450 seconds (teflon), and count rates of between 10 counts per second
(teflon) and 2800 counts per second (copper) were recorded. It should be noted
that the C2 window on the SDD attenuates low energy X-rays; roughly 50-60 % of
the 0.67 keV fluorine photons from the teflon target are transmitted, and
roughly 60-70 % of the 1.25 keV magnesium and 1.49 keV aluminum photons. It is
therefore assumed that the true fluxes of low energy lines at the detector
locations are $1.4-2\times$ greater than recorded by the SDD. The SDD spectra
also reveal peaks from contamination sources, including the X-ray source
tungsten anode L$\alpha$ line at 8.41 keV, as well as the characteristic
K$\alpha$ and K$\beta$ lines from neighboring targets due to imperfect
alignment of the target wheel. Other possible contamination sources which
appeared in all spectra to varying degrees may be coming from the stainless
steel of the beamline body or the aluminum 6061 alloy target wheel (silicon -
1.74 keV K$\alpha$, chromium - 5.41 keV K$\alpha$, manganese - 5.90 keV
K$\alpha$, zinc - 8.64 keV K$\alpha$). To mitigate this contamination, in
future XRF wheel interations we plan to use a pure aluminum wheel.

---
Figure 13:  Superposed spectra for each of the seven beamline targets (teflon,
magnesium, aluminum, KCl, titanium, iron, and copper) as measured by the SDD.
Collection times varied from 60 seconds (copper) to 1450 seconds (teflon).
Count rates ranged from 10 counts/second (teflon) to 2800 counts/second
(copper).

##  5 Summary and future plans

With the XOC X-ray beamline fully assembled and commissioned, and all
regulatory and monitoring systems and safety fail-safes working reliably, we
are able to carry out X-ray detector characterization across a broad range of
energies in the 0.3 - 10 keV band, under vacuum and down to temperatures as
low as 173 K. The beamline has demonstrated reasonable count rates for teflon,
aluminum, magnesium, potassium chloride, titanium, iron, and copper targets,
with minimal contamination and radiation leakage observed. CCD spectra
indicate that within the beamline, we approach the fano limited resolution of
the detectors, especially for higher energy targets. With this robust new test
system, the XOC group will soon move on to characterize new MIT-LL CCID-93++
AXIS prototype detectors. These detectors will feature both p-JFET outputs and
multiple SiSeRO output variants. With multi-channel ASIC readout also enabled
in the beamline, this test system will allow us to characterize and optimize
the speed and noise performances of these new devices.

###### Acknowledgements.

We acknowledge support from the Kavli Institute for Particle Astrophysics and
Cosmology at Stanford through a KIPAC Decadal Grant. We thank the Stanford
Physics Machine shop for their assistance in modifying the detector chamber
door to enable efficient detector and Archon controller mounting. Scientific
measurements being made with the XOC beamline are supported by NASA APRA
grants 80NSSC19K0499 and 80NSSC22K1921.

##  Appendix A GEANT4 Physics List Particle Interactions

For gamma particles or photons:
G4PhotoElectricEffect, G4LivermorePhotoElectricModel, G4ComptonScattering,
G4LivermorePolarizedComptonModel, G4GammaConversion, G4RayleighScattering,
G4LivermorePolarizedRayleighModel, G4EmProcessOptions, and
G4EmLivermorePhysics

For electrons:
G4eMultipleScattering, G4eIonisation,
G4LivermoreIonisationModel, G4eBremsstrahlung, G4LivermoreBremsstrahlungModel,
G4eplusAnnihilation, G4UAtomicDeexcitation, and G4LossTableManager.

##  Appendix B GEANT4 Material Fractional Compositions

Table 2: 304L Stainless Steel Element | Mass Fraction
---|---
Fe | 0.6931
Cr | 0.185
Ni | 0.09
Mn | 0.02
Si | 0.01
N | 0.001
P | 0.00045
C | 0.0003
S | 0.00015
Table 3: PCB Element | Mass Fraction
---|---
Al | 0.3904
O | 0.3045
Si | 0.1414
Cu | 0.0739
Sn | 0.0488
C | 0.041
Table 4: G10 Element | Mass Fraction
---|---
O | 0.385764
H | 0.275853
Si | 0.151423
Ca | 0.081496
Al | 0.044453
N | 0.027945
B | 0.01864
Mg | 0.010842
C | 0.003583
Table 5: Al-6061 Element | Mass Fraction
---|---
Al | 0.96
Mg | 0.012
Si | 0.008
Fe | 0.007
Cu | 0.004
Cr | 0.0035
Zn | 0.0025
Mn | 0.0015
Ti | 0.0015
Table 6: Ceramic Element | Mass Fraction
---|---
Al | 0.6581
N | 0.3419

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