Design, implementation, and performance of the Astro-H SXS calorimeter array and anticoincidence detector

Abstract

The calorimeter array of the JAXA Astro-H (renamed Hitomi) soft x-ray spectrometer (SXS) was designed to provide unprecedented spectral resolution of spatially extended cosmic x-ray sources and of all cosmic x-ray sources in the Fe-K band around 6 keV, enabling essential plasma diagnostics. The SXS had a square array of 36 x-ray calorimeters at the focal plane. These calorimeters consisted of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers. These devices demonstrated a resolution of better than 4.5 eV at 6 keV when operated at a heat-sink temperature of 50 mK. We will discuss the basic physical parameters of this array, including the array layout, thermal conductance of the link to the heat sink, resistance function, absorber details, and means of attaching the absorber to the thermistor-bearing element. We will also present the thermal characterization of the whole array, including thermal conductance and crosstalk measurements and the results of pulsing the frame temperature via alpha particles, heat pulses, and the environmental background. A silicon ionization detector was located behind the calorimeter array and served to reject events due to cosmic rays. We will briefly describe this anticoincidence detector and its performance. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Ground calibration of the Astro-H (Hitomi) soft x-ray spectrometer

“…The absorber thickness and area were chosen to provide near-unity stopping power to energies of ≈6 keV and as high a filling fraction as was practical given the assembly constraints. 6 The QE of each absorber as a function of incident x-ray energy can be described using QEðEÞ ¼ 1 − exp½−μ HgTe ðEÞΣ, where μ HgTe ðEÞ is the mass absorption coefficient for HgTe and Σ is the areal density. The absorption coefficients for Hg and Te are known; assuming nominal stoichiometry of 1:1 we in turn know μ HgTe ðEÞ for the absorbers, using μ HgTe ðEÞ ¼ ½m Hg μ Hg ðEÞ þ m Te μ Te ðEÞ∕ðm Hg þ m Te Þ, where m Hg and m Te are the atomic masses and μ Hg ðEÞ and μ Te ðEÞ are the mass absorption coefficients from the literature.…”

“…An overview of the anti-coincidence detector appears in the Astro-E2 instrument paper 46 and the changes made to the anti-co readout for Astro-H are reported in Ref. 6. Additional detail about the SXS anti-co, including a schematic of the detector array and anti-co plus the events observed in both due to the in-orbit particle background, may be found in Ref.…”

“…The SXS was commissioned shortly after launch and began making astronomical measurements 4 just days into the mission. 5 The instrument performed as expected until the untimely loss of the mission on March 26, 2016. The SXS microcalorimeter array 6 consisted of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers that operate at a heat-sink temperature of 50 mK. Because of this low operating temperature, the optical path of the SXS included a set of five thin-film aluminized polyimide filters anchored to nested temperature stages.…”

Design and on-orbit operation of the adiabatic demagnetization refrigerator on the Hitomi Soft X-ray Spectrometer instrument

“…X-ray events were then recorded both from external sources and an on-board Fe-55 calibration source. The detector resolution for all pixels was eventually verified to be the same as before launch, ~4.9 eV 17,18,19 .…”

Thermal Simulations of Temperature Excursions on the Athena X-IFU Detector Wafer from Impacts by Cosmic Rays

“…The number of wirebonds is undecided, and this simulation assumes N bonds = 100. Their thermal conductance and geometrical parameters are set from measurements of a single gold wirebond performed at NASA Goddard [5]. Finally, the model makes use of a so-called 'muntin approximation'; in the COMSOL simulation, we do not account for the full complexity of the muntin structure, which is computationally prohibitive.…”