[1] Hai Jin

1]Department of Astronomy, Tsinghua University, 100084, Beijing, China

# Preliminary Design of Detector Assembly for DIXE

Jiejia Liu<EMAIL_ADDRESS>Sifan Wang
<EMAIL_ADDRESS><EMAIL_ADDRESS>Qian Wang
<EMAIL_ADDRESS>Wei Cui<EMAIL_ADDRESS>[

###### Abstract

Diffuse X-ray Explorer (DIXE) is a proposed X-ray spectroscopic survey
experiment for the China Space Station. Its detector assembly (DA) contains
the transition edge sensor (TES) microcalorimeter and readout electronics
based on the superconducting quantum interference device (SQUID) on the cold
stage. The cold stage is thermally connected to the ADR stage, and a Kevlar
suspension is used to stabilize and isolate it from the 4 K environment. TES
and SQUID are both sensitive to the magnetic field, so a hybrid shielding
structure consisting of an outer Cryoperm shield and an inner niobium shield
is used to attenuate the magnetic field. In addition, IR/optical/UV photons
can produce shot noise and thus degrade the energy resolution of the TES
microcalorimeter. A blocking filter assembly is designed to minimize the
effects. In it, five filters are mounted at different temperature stages,
reducing the probability of IR/optical/UV photons reaching the detector
through multiple reflections between filters and absorption. This paper will
describe the preliminary design of the detector assembly and its optimization.

###### keywords:

X-ray Astronomy, DIXE, magnetic shielding, blocking filter

##  1 Introduction

Diffuse X-ray Explorer (DIXE) is an X-ray spectroscopic survey experiment
proposed for the China Space Station (CSS) [1]. DIXE is going to conduct a
three-year high-resolution X-ray spectroscopic all-sky survey, probing the
physical properties of the hot baryons in the Milky Way, and tracing the
Galactic feedback processes. The primary scientific objective of DIXE is to
gain insights into the origins of large-scale structures of hot gas in the
Milky Way, including the Local Bubble, the eROSITA Bubble, Cygnus Superbubble
(and other superbubbles), as well as the Galactic halo [1].

Fig 1 shows a preliminary design of the detector assembly (DA) of DIXE. The DA
consists principally of the TES microcalorimeter array and the associated
SQUID readout electronics, magnetic shields and a set of optical blocking
filters. The design of the X-ray detectors involves a TES microcalorimeter
array with 10$\times$10 pixels at the mK stage, each with a power consumption
of about 1 pW. These microcalorimeters are required to have an energy
resolution of 6 eV or better over an energy range of 0.1-10 keV. The readout
system adopts a frequency-division multiplexing (FDM) scheme. For DIXE, the
design calls for a multiplexing factor of 25, so four FDM modules are needed
to readout 100 pixels. Each module employs a two-stage SQUID system: the first
stage at the cold stage with 2 nW power dissipation and the second stage at
the 4K stage with a heat load under 200 nW. The detector (and SQUID
electronics as well as Inductor-Capacitor (LC) resonators) is surrounded by
the magnetic shields, and looks through the blocking filters. Further details
on the development of the TES microcalorimeters are described in [1] and [2].
In this paper, we’ll be focusing on the design of the DA.

The available space for the DA and the adiabatic demagnetization refrigerator
(ADR) cooling system is constrained, due to the limited volume of the
compartments (up to $720\rm\,mm\times 570\,mm\times 720\,mm$) in the
transporting vehicle (to the CSS), which presents the most severe challenge in
the design and optimization of the DA. Other constraints include the total
mass of the payload ($<$200 kg) and the electrical power ($<$1000 W). The
lifetime of DIXE is projected to be three years. The aims of optimizing the DA
design are: 1) To provide mechanical support for the TES microcalorimeter
detector array and multiplexing readout electronics, ensuring that they can
survive the launch; 2) To reduce magnetic field near the detector array
through effective shielding, minimizing its effects on the detector and
associated superconducting readout electronics; and 3) To block radiation at
UV/optical/IR wavelengths through the use of filters, reducing the degradation
in the energy resolution of the detector caused by shot noises. The
optimization should balance structural integrity, magnetic shielding, and
shot-noise minimization to achieve the desired energy resolution.

Figure 1: The preliminary design of the detector assembly. The left panel
shows the internal layout of the payload. The detector assembly (DA,
highlighted in yellow) and the ADR cooling system are enclosed in the cyan
box. ①: A supporting frame is affixed to the 4 K stage of the cooling
apparatus. ②: The FAA stage of the adiabatic demagnetization refrigerator
(ADR). ③: The GGG stage of the ADR. The FAA stage is thermally connected to
the cold plate on which the detector array is situated. ④: The filter system
with different colors delineating filters at various temperature stages. The
top-right panel shows the sizes of the 4 K supporting frame, and the bottom-
right panel displays the interior structure of the DA.

##  2 Mechanical and thermal design

The space constraints limit the size and configuration of the sub-4K unit of
the payload, which consists of the DA and the ADR cooling stages. Fig 1 shows
a preliminary design. The sub-4K unit is arranged in an ‘L’-shaped
configuration, and the sizes are shown in the top right panel in Fig 1. The
ADR has a cooling power of $2~{}\rm\mu W$ at $100~{}\rm mK$, corresponding to
an expected hold time of $9~{}\rm h$ (for a duty cycle around 90%). This
necessitates a design that minimizes the heat load on the cold stage while
maintaining mechanical robustness.

The mechanical structure of the DA is shown in Fig 1. The total mass is about
2.4$~{}\rm kg$. The detector array and the superconducting readout electronics
are mounted on the cold plate, which is supported by six Kevlar bundles of 50
mm in length of the support pillar (see Fig. 1 right panel). The mass of the
cold plate needs to be minimized to avoid resonance vibration at low
frequencies. So the nonuniform thickness of the cold plate is designed to
reduce the mass, with thick layers $4~{}\rm mm$ and thin layers $1~{}\rm mm$.
Six ribs are adopted to stiffen the plate. The niobium (Nb) shield is
connected to and supported by the pillar through interface flanges. The
support pillar is screwed onto the flanges, which are made of high-
permeability mu-metals for the purpose of the magnetic shield (see § 3). The
bottom mu-metal flange is mounted on the supporting frame which is cooled by
the cryocoolers to 4 K, along with the hybrid magnetic shields, and the
support pillars. The cold plate is thermally isolated from the 4 K stage and
sits nominally at $50-100$ mK.

A copper strip is thermally connected with the cold end of the second stage
ADR, which cools the cold plate to the transition temperature of TES. The heat
load due to thermal conduction through the Kevlar support is expected to
dominate other environmental heat input to the cold plate, and the total heat
load is required to be less than 1 $\mu\rm W$, in order to stay within the
limited cooling power of the ADR. The heat load per Kevlar bundle is given by:

$q=\frac{A}{L}\int_{T_{\rm cold}}^{T_{\rm warm}}k(T)\ \mathrm{d}T,$ (1)

where $A$ and $L$ are the cross-section and length of the Kevlar bundle,
respectively. Taking the thermal conductivity of Kevlar ($k(T)$) from the
literature [3], we use the equation to compute the heat conducted from the
warm end (at $T_{\rm warm}=4\ \rm K$) to the cold end (at $T_{\rm cold}=0.05\
\rm K$), and arrive at a heat load of $\sim 0.4\ \rm\mu W$ for six Kevlars
with 480 threads per bundle.

Mechanically, the challenge is for the detector assembly to survive launch
vibration. We consider an acceleration level of $\sim 13\ \rm g$ (rms) [4]
(although it is expected to be lower for DIXE). For a cold plate of mass $\sim
76\ \rm g$, it corresponds to a launch load of about 9.8 N (rms). We
conservatively multiply it by a factor of 8 to arrive at 78 N, which is to be
compared with the breaking strength of the Kevlar bundles. For a bundle of 480
threads, it has been shown that the breaking point is at $\sim 163\ \rm
N$111https://www.matweb.com/index.aspx, which leaves plenty of margin in the
design.

##  3 Magnetic shielding

###  3.1 Requirement

As superconductor components, the performance of TES microcalorimeter and
SQUID readout electronics are susceptible to magnetic fields [5]. The
background magnetic field arises mainly from the residual field of the ADR
magnet and the Earth field. We have adopted a $\sim 4\times 10^{4}~{}\rm
Gauss$ as a starting point for the magnetic shielding design. It is a
conservative value as the maximum field wouldn’t surpass $4\times 10^{4}~{}\rm
Gauss$ under our ADR operation requirement [6]. In this case, the residual
magnetic field attenuated by the ADR magnetic shielding is $\lesssim 0.1~{}\rm
Gauss$. In contrast, for the optimal performance of the detectors, the field
should be reduced to less than $0.01~{}\rm Gauss$ [7].

Since the space is limited, multiple configurations of the two-stage ADR and
the magnetic shielding system are considered, and a side-by-side design is
adopted to minimize the background magnetic field asserted on the magnetic
shields (shown in left panel of Fig 1). The initial design of magnetic
shielding is shown in Fig. 2. It combines a high-permeability outer shield and
a superconductor inner shield. Cryoperm, which is used as the material for the
outer shield, is a high-permeability Fe/Ni alloy optimized for cryogenic
applications222http://www.cryopermshielding.com/cryoperm-shielding.php. The
high-permeability shield is divided into four parts to enable assembly. The
middle two parts have a thickness of $2~{}\mbox{mm}$, separated by a
$1~{}\mbox{mm}$ gap. The top and bottom parts are thicker ($4-6~{}\mbox{mm}$)
to ensure mechanical robustness, as they also support the pillars (Fig. 1).
The inner shield is made of Nb. Note that the bottom of the Nb shield is
sealed to enhance magnetic field isolation. As a result, an opening has been
made on the right side of both the inner and outer shields. This opening is
designed for establishing a thermal connection to the ADR, as detailed and
demonstrated in § 1. A hexagonal structure is adopted for easier fixation and
connection. The thickness of the Nb shield is $1\ \rm mm$.

In this design, it is the outermost blocking filter (at 300 K) that limits the
field of view (FoV) of the payload (see § 4). We define the collimator
response as the ratio of the effective area $S_{\rm eff}$ to the total area of
the detector array $S_{\rm TES}$, and compute it as a function of the incident
angle ($\theta$) of incoming X-rays. The results are also shown in Fig. 2. For
a radius of $12~{}\rm mm$ and a height of 130 mm above the detector plane, the
offset angle is about $5^{\circ}$ as the ratio decreases to 0.5. This leads to
a full-width half maximum (FWHM) of $10^{\circ}$ (shown as the solid white
contour on the right panel of Fig 2), which meets the requirement of DIXE. The
neck portion of the inner shield must not block the FoV.

Figure 2: Left: Initial design of the magnetic shielding for DIXE. Right:
Collimator response (see the main text for a definition). The color map in
polar coordinates shows the variation of the response over the field of view.
The fiducial values of the response and the corresponding off-axis angle are
indicated in the map. The dashed and solid curves show the contour with the
ratio values of 0.8 and 0.5.

###  3.2 Static Modeling

We use COMSOL Multiphysics with the AC/DC module for magnetic shielding
simulations. In the static simulation, A zero-field cooling (ZFC) process is
assumed, and a constant relative permeability $\mu_{r}=20000$ for the Cryoperm
shield [8] and ADR shield is adopted. The lower critical magnetic field of Nb
at 4 K is $H_{\rm c1}\sim 1444\ \rm Oe$ ($\sim 18\ \rm Gauss$) [9]. We assume
a background magnetic field of $B_{\rm res}\lesssim 1\ \rm Gauss$, and ignore
the magnetic flux pinning effect in type II superconductor (Nb).

We start the modeling by applying a constant magnetic field $\mathbf{B}_{\rm
res}$ along the axial ($z$) and radial $(x,y)$ direction. The $x$ direction
goes across the center of the hole for thermal connections. The underlying
physics is described by Maxwell’s equations:

$\displaystyle\nabla\times\mathbf{H}=\mathbf{J}$ (2)
$\displaystyle\mathbf{B}=\nabla\times\mathbf{A}+\mathbf{B}_{\rm res}$
$\displaystyle\mathbf{J}=\sigma\mathbf{E}$
$\displaystyle\mathbf{B}=\mu_{0}\mu_{r}\mathbf{H}$

The effectiveness of the system can be quantified by the shielding factor,
defined as:

$S=\frac{B_{\rm TES}}{B_{\rm res}},$ (3)

where $B_{\rm TES}$ is the magnetic field at the position of the TES detector
array and $B_{\rm res}$ is the residual background magnetic field.

We define an 80 mm thick sphere as the infinite element domain to model the
field boundary conditions at infinity. For the static simulation, we adopt the
Magnetic Insulation condition for the superconductor shield, forcing
$\mathbf{n}\times\mathbf{A}=0$ at the shield boundaries, where $\mathbf{n}$ is
the normal vector of the shield’s surfaces. The result is shown in Fig 3. The
shielding factor is above $10^{5}$ for an axial background field and between
$10^{4}-10^{5}$ for a radial background field. The magnetic field after
attenuation should be less than $1\ \rm\mu T$ with $\sim 1\ \rm Gauss$
background field.

Figure 3: Simulated shielding factors. The solid-blue, dashed-salmon, and dash
dotted-gray curves show the variation of shielding factors along the axial
$z$, radial $x$, and $y$ directions, respectively, for a constant background
field of 1 Gauss. The dotted line indicates the position of the cold plate.

In real cases, the residual magnetic field produced by the ADR magnet is
complicated. During the operation, the magnetic field produced by the
superconducting coil in the ADR is attenuated by another Cryoperm shield
surrounding the ADR. Here, we simulate the shielding effects for the full
field. The model setup is shown in Fig. 4, and the geometric parameters are
summarized in Table 1. The magnetic field in the superconducting coil is
simulated by applying a counterclockwise current (as viewed from the $z$
direction). The current density is assumed to be $J\sim 2\times 10^{8}\ \rm
A/m^{2}$, which leads to a central field of about $4~{}\rm T$ and a background
field of $<0.1\ \rm Gauss$. Taking into account the necessary holes for Kevlar
suspensions and the thermal connection, the magnetic field at the position of
the detector array is $\sim 10^{-7}-10^{-6}\ \rm Gauss$. For an initial
design, we adopt the most conservative approach by adding a 10 mm thick
mumetal shield for the ADR. Cryoperm is not utilized for the ADR shield since
the magnetic field generated by the magnet surpasses the saturated magnetic
field capacity of Cryoperm, which reduces the shielding factor. A thinner
mumetal shield for ADR could be considered to reduce the total mass, which
would result in a higher background field. Even without a shield around the
ADR, the field inside the inner shield would be only $B_{\rm TES}\sim 10^{-4}\
\rm Gauss$ when the magnetic field produced by the superconducting magnet is
$\sim 4\ \rm T$.

Figure 4: Distribution of the simulated magnetic field based on the initial design of the magnetic shielding. Left: Geometric setup for the COMSOL simulation. The magnetic shielding for the detector assembly and for the ADR model are both shown. Right: Distribution of the simulated magnetic field. The colored map shows the value of the magnetic field in Gauss with the field lines in grey.  Table 1: The design parameters (in units of mm) both for the magnetic shielding of the detector assembly and for the ADR. Here, $r_{\rm body}$ refers to the outer radius of the main body of a shield, while $r_{\rm op}$ to the inner radius of the neck of the shield; $d$ indicates the thickness of the main boy of the shield, and $h_{\rm op}$ the height of the neck.  Geometric |  Cryoperm shield |  Niobium shield |  ADR shield
---|---|---|---
parameters |  |  |
$r_{\rm body}$ |  63 |  50 |  57.5
$d$ |  2 |  1 |  10
$r_{\rm op}$ |  22.5 |  12 |  5
$h_{\rm op}$ |  45 |  55 |  123

##  4 Filter system

Blocking filters are used to minimize the shot noise generated by
UV/optical/infrared photons, which degrades the energy resolution of the
detectors, as the microcalorimeters also respond to those photons. Following
the design adopted by the X-ray Quantum Calorimeter (XQC) sounding rocket
experiment [4], we examine the filters based on aluminum-coated polyimide
(PI). For an initial design, we place five filters at different temperature
stages, as summarized in Table 2. The requirement is that the degradation of
energy resolution should be less than $5\%$ (i.e., $0.3\ \rm eV$ for a
detector resolution of 6 eV). In addition, the absorption by the filters needs
to be minimized at the lowest energies, as DIXE is designed to cover an energy
range down to $0.1~{}\rm keV$ (based on its key science objectives [1]). The
requirement is that the transmission exceeds 10% at 0.2 keV. Fig. 5 shows
these constraints and the allowed parameter space (shown as the hatched area)
for the thickness of the Aluminum and Polyimide films.

Figure 5: Degradation in energy resolution $\Delta E_{\rm deg}$ (in solid
lines) in the unit of eV and X-ray transmission (in dashed lines) as a
function of the thickness of the polyimide substrate and of the aluminum
layer. The X-ray transmission is computed at an energy of 0.2 keV. The hatched
area shows the parameter space that satisfies the DIXE requirements, with the
blue cross indicating the values adopted for the preliminary design.

We calculate the reflection and transmission coefficients for the filters
based on the matrix form of wave propagation theories. The refractive index of
the filter material is used in the calculation. Oxidation of aluminum is also
considered. The aluminum oxide layer occurs at both sides of the aluminum film
with a thickness of $\sim 30~{}\rm\AA$ [10]. This is because polyimide is
transparent to oxygen, exposing the bottom side of the aluminum (the side
connected to the PI layer) to oxygen as well. The values of refractive index
and extinction coefficient for Al, PI, and $\rm Al_{2}O_{3}$ used in the
calculation are referred to Hagemann et al. [11], Cavidi et al. [12], and
Querry & Marvin [13].

For the calculation of energy resolution degradation $\Delta E_{\rm deg}$, we
treat each filer as a blackbody source, and $\Delta E_{\rm deg}$ is given by
[14]:

$\displaystyle{\rm NEP}^{2}(T)=\frac{1}{N_{\rm
pix}}\int_{\lambda_{\text{min}}}^{\lambda_{\text{max}}}P_{\lambda}(T)\frac{hc}{\lambda}\,\mathrm{d}\lambda,$
(4) $\displaystyle\Delta E_{\text{deg}}=2.35\times 6.2415\times
10^{18}\times\sqrt{{\rm NEP^{2}}(T)\tau},$ (5)

where ${\rm NEP}$ is the noise equivalent power in the units of $\rm
W/\sqrt{Hz}$, $\Delta E_{\rm deg}$ the corresponding energy resolution in the
units of eV, $P_{\lambda}(T)$ the spatially integrated, wavelength-weighted
radiation power arriving at the TES array in the units of $\rm W/\AA$, $N_{\rm
pix}$ the number of pixels, and $\tau$ the integration time. Fig. 5 shows the
degradation in energy resolution due to the shot noise ($\Delta E_{\rm deg}$)
and the X-ray transmission at $0.2\ \rm keV$, over a range of the thickness of
the aluminum layer and the polyimide substrate. Based on the requirements of
the energy resolution degradation and X-ray transmission, the allowed
parameter space is limited to the hatched area. Here, we adopt $1000\ \rm\AA$
and $200\ \rm\AA$ for the polyimide and aluminum layers, respectively. This
combination is proven to perform well and is less challenging for fabrication
[10]. The transmission curve over a wavelength range covering UV, optical, and
IR for a single filter is shown in Fig. 6 (left panel). Also shown in the
figure (right panel) is the X-ray transmission curve for the case of all five
filters. The radiative power from each filter and the contribution to shot-
noise-caused energy resolution degradation are summarized in Table 2.

Figure 6: The transmission of UV/Optical/IR photons for a single filter (left) and X-rays (right) with five filters. Each filter is made of a 200 $\rm\AA$ aluminum film coated on a 1000 $\rm\AA$-thick polyimide film. A $30~{}\rm\AA$ thick $\rm Al_{2}O_{3}$ film on both sides of aluminum is also considered. Table 2: The properties of the five filters and the contribution for radiative heat load and energy resolution degradation. The columns are: (1) The index for each filter; (2) The temperature stage for each filter in the unit of K; (3) The distance between the filter and the TES detector array in the unit of mm; (4) The radiative power contributed by each filter in the unit of W; (5) The energy resolution degradation in the unit of eV. Filter index |  $T$ |  $z$ |  $P$ |  $\Delta E_{\rm FWHM}$
---|---|---|---|---
(1) |  (2) |  (3) |  (4) |  (5)
1 |  1 |  80 |  $1.9\times 10^{-17}$ |  0.0043
2 |  4 |  86 |  $7.2\times 10^{-16}$ |  0.031
3 |  20 |  100 |  $8.8\times 10^{-17}$ |  0.020
4 |  77 |  115 |  $2.1\times 10^{-18}$ |  0.0066
5 |  300 |  130 |  $1.0\times 10^{-16}$ |  0.16
Total |  |  |  $9.3\times 10^{-16}$ |  0.16

##  5 Discussion

In the present design, the heat load to the cold plate through the Kevlar
support is still a bit high (see § 1), compared with the cooling power
(estimated to be $\sim 2~{}\mu$W) of the ADR. A reduction by a factor of 2
might be feasible by using fewer Kevlar threads, or by lowering the
temperature gradient along the Kevlar support with a thermal link from the 1 K
stage to the middle of the Kevlar bundle.

The design of the ADR, and notably the size of the salt pill, impacts the
design of the DA magnetic shielding by dictating the necessary maximum
magnetic field. For our ADR, with a target hold time of 9 hours and a duty
cycle of 90%, an increase in the salt pill’s mass reduces the maximum magnetic
field. This reduction allows for smaller and more compact DA magnetic shields.
The trade-off, however, is a considerable increase in the total mass of the
salt pill-magnet assembly, as well as adversely affecting the ADR’s duty
cycle. Future studies will aim to refine the ADR design further.

Not yet considered is the need for anticoincidence detectors. Geant 4
simulations are being carried out to assess the particle background.

##  6 Conclusion

A preliminary design of the DIXE DA is presented and analyzed. It meets the
requirements of mechanical robustness, magnetic shielding, heat load, and
energy resolution degradation.

Acknowledgments

We wish to thank Dr. Dan McCammon and all members of the DIXE collaboration
team for useful discussion. This work was supported in part by the Ministry of
Science and Technology of China through Grant 2022YFC2205100, by China
National Space Administration (CNSA) through a technology development grant,
and by the National Natural Science Foundation of China through Grants
11927805, 12203027, 11803014, and 12220101004.

## References

  *   * Jin et al. [2024] Jin, H., Mao, J., Chen, L., Chen, N., Cui, W., Gao, B., Li, J., Li, X., Liu, J., Quan, J., Jiang, C., Wang, G., Wang, L., Wang, Q., Wang, S., Xiao, A., Zhang, S., Zhong, Q.: Diffuse X-ray Explorer: a high-resolution X-ray spectroscopic sky surveyor on the China Space Station. Journal of Low Temperature Physics, In LTD Special Issue (2024)
  * Wang et al. [2022] Wang, S., Wang, G., Chen, N., Chen, Y., Cui, W., Ding, J., Li, F., Liang, Y., Wang, Q., Wang, Y.: Development of superconducting microcalorimeters for the hubs mission. Superconductivity 4, 100027 (2022) https://doi.org/10.1016/j.supcon.2022.100027
  * Ventura et al. [2000] Ventura, G., Barucci, M., Gottardi, E., Peroni, I.: Low temperature thermal conductivity of Kevlar. Cryogenics 40(7), 489–491 (2000) https://doi.org/10.1016/S0011-2275(00)00062-X
  * McCammon et al. [2002] McCammon, D., Almy, R., Apodaca, E., Bergmann Tiest, W., Cui, W., Deiker, S., Galeazzi, M., Juda, M., Lesser, A., Mihara, T., Morgenthaler, J.P., Sanders, W.T., Zhang, J., Figueroa-Feliciano, E., Kelley, R.L., Moseley, S.H., Mushotzky, R.F., Porter, F.S., Stahle, C.K., Szymkowiak, A.E.: A High Spectral Resolution Observation of the Soft X-Ray Diffuse Background with Thermal Detectors. ApJ 576(1), 188–203 (2002) https://doi.org/10.1086/341727 arXiv:astro-ph/0205012 [astro-ph]
  * Hijmering et al. [2013] Hijmering, R., Khosropanah, P., Ridder, M., Gao, J.R., Lindeman, M., Hoevers, H., van der Kuur, J., Gottardi, L., Jackson, B., Huiting, R., van Litsenburg, M.: Effects of Magnetic Fields on Highly Sensitive TiAu TES Bolometers. IEEE Transactions on Applied Superconductivity 23(3), 2101505–2101505 (2013) https://doi.org/10.1109/TASC.2013.2255945
  * Jiang et al. [2023] Jiang, C., Li, C., Jin, H., Cui, W.: Development of adiabatic demagnetization refrigerator for the HUBS mission. Science Bulletin 68(22), 2709–2711 (2023) https://doi.org/10.1016/j.scib.2023.09.031
  * Hijmering et al. [2014] Hijmering, R.A., den Hartog, R.H., van der Linden, A.J., Ridder, M., Bruijn, M.P., van der Kuur, J., van Leeuwen, B.J., van Winden, P., Jackson, B.: The 160 TES bolometer read-out using FDM for SAFARI. In: Holland, W.S., Zmuidzinas, J. (eds.) Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII. Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, vol. 9153, p. 91531 (2014). https://doi.org/10.1117/12.2056510
  * Bergen et al. [2016] Bergen, A., van Weers, H.J., Bruineman, C., Dhallé, M.M.J., Krooshoop, H.J.G., ter Brake, H.J.M., Ravensberg, K., Jackson, B.D., Wafelbakker, C.K.: Design and validation of a large-format transition edge sensor array magnetic shielding system for space application. Review of Scientific Instruments 87(10), 105109 (2016) https://doi.org/%****␣main.bbl␣Line␣225␣****10.1063/1.4962157
  * Finnemore et al. [1966] Finnemore, D.K., Stromberg, T.F., Swenson, C.A.: Superconducting Properties of High-Purity Niobium. Physical Review 149(1), 231–243 (1966) https://doi.org/10.1103/PhysRev.149.231
  * McCammon et al. [2008] McCammon, D., Barger, K., Brandl, D.E., Brekosky, R.P., Crowder, S.G., Gygax, J.D., Kelley, R.L., Kilbourne, C.A., Lindeman, M.A., Porter, F.S., Rocks, L.E., Szymkowiak, A.E.: The X-ray Quantum Calorimeter Sounding Rocket Experiment: Improvements for the Next Flight. Journal of Low Temperature Physics 151(3-4), 715–720 (2008) https://doi.org/10.1007/s10909-008-9734-5
  * Hagemann et al. [1975] Hagemann, H.-J., Gudat, W., Kunz, C.: Optical constants from the far infrared to the x-ray region: Mg, al, cu, ag, au, bi, c, and al 2 o 3. JOSA 65(6), 742–744 (1975)
  * Cavadi et al. [1999] Cavadi, A., Artale, M.A., Barbera, M., Collura, A., Powell, F.R., Varisco, S.: Measurement of optical constants n and k of lexan and polyimide. In: Siegmund, O.H., Flanagan, K.A. (eds.) EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy X. Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, vol. 3765, pp. 805–815 (1999). https://doi.org/10.1117/12.366568
  * Querry [1985] Querry, M.R.: Optical constants. Contractor Report, Sep. 1982 - May 1984 Missouri Univ., Kansas City. (1985)
  * Barbera et al. [2015] Barbera, M., Branduardi-Raymont, G., Collura, A., Comastri, A., Eder, J., Kamisiński, T., Lo Cicero, U., Meidinger, N., Mineo, T., Molendi, S., Parodi, G., Pilch, A., Piro, L., Rataj, M., Rauw, G., Sciortino, L., Sciortino, S., Wawer, P.: The optical blocking filter for the ATHENA wide field imager: ongoing activities towards the conceptual design. In: Siegmund, O.H. (ed.) UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XIX. Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, vol. 9601, p. 960109 (2015). https://doi.org/10.1117/12.2189326