Development of the microcalorimeter and anticoincidence detector for the Line Emission Mapper x-ray probe

Abstract

.The Line Emission Mapper (LEM) is an x-ray probe mission concept that is designed to provide unprecedented insight into the physics of galaxy formation, including stellar and black-hole feedback and flows of baryonic matter into and out of galaxies. LEM incorporates a light-weight x-ray optic with a large-format microcalorimeter array. The LEM detector utilizes a 14k pixel array of transition-edge sensors (TESs) that will provide <2.5 eV spectral resolution over the energy range 0.2 to 2 keV, along with a field-of-view of 30 arcmin. The microcalorimeter array and readout builds upon the technology developed for the European Space Agency’s (ESA’s) Athena/x-ray Integral Field Unit. Here, we present a detailed overview of the baseline microcalorimeter design, its performance characteristics, including a detailed energy resolution budget and the expected count-rate capability. In addition, we outline the current status and plan for continued technology maturation. Behind the LEM array sits a high-efficiency TES-based anticoincidence (antico) detector that will reject cosmic-ray background events. We will briefly describe the design of the antico and plan for continued development.

Line emission mapper microcalorimeter spectrometer

“…38 Hydras consist of four x-ray absorbers, each with a different thermal link to a single TES. 19 The link conductances are tuned to produce a unique pulse shape for x-rays absorbed in each pixel. The pulse rise-time is used to determine which pixel absorbed the photon.…”

“…The pixels are arranged on a 290 μm pitch and incorporate 0.54 μm thick Au x-ray absorbers that provide unity stopping power at an energy of 1 keV. The detailed design of the LMS microcalorimeter is described by Smith et al 19 The LMS array was assessed at TRL-5 in April 2023 in a NASA/GSFC Technology Readiness Assessment at TRL-5 and benefits from the technical maturity of the Athena/ X-IFU microcalorimeter, developed by the same team at GSFC, and shares many common design elements and interfaces. 22 Much of the TRL-5/6 development work completed for the X-IFU, 39,40 including studies on radiation hardness, 41,42 lifetime, 43,44 susceptibility to vibrations, cosmic-ray mitigation, and energy scale calibration has been incorporated into the LMS design.…”

“…Positioned 1 mm directly behind the LEM sensor array, the anti-co flags background events that could be mistaken for a scientific photon within LEM's signal band. 19 The dominant cosmic rays are minimum ionizing particles (MIPs) in the form of protons and alpha particles, which deposit energy continuously as they traverse the main detector array and the anti-co beneath it. Our 12-channel anti-co is based upon large-area detectors that have been developed by the dark matter detection community.…”

“…The central 7′ region of the array consists of ∼800 transition-edge sensors (TESs), optimized for the 0.3 to 2 keV bandpass, with an energy resolution of 1.3 eV at 1 keV. The rest of the FOV is covered by 4-pixel TES hydras with 2.5 eV energy resolution at 1 keV, 19 as depicted in Fig. 1.…”

“…6(c), the shielding effectiveness, expressed as the magnitude of the lateral external field to the vertical component of the field at the detector array, has a minimum of ∼2200, with an average over the detector surface of ∼4000, which should be more than sufficient assuming AC fields at a level of 1 μT rms or less and given the low measured susceptibility of the high-aspect ratio TES thermistors planned for LEM, which require almost 300 times less magnetic shielding than Athena X-IFU detectors prior to reformulation. 19 Figure 6(d) shows how the modeled shielding factor varies over the extent of the LEM focal plane array. Because the shielding factor is limited by effectiveness of the bending into the x-ray entrance aperture, the effective shielding for localized sources (non-uniform fields) can be higher or lower than this, depending upon the location of the source and the resulting position dependence of the field at the FPA.…”