Characterization of a hybrid array of single and multi-absorber transition-edge sensor microcalorimeters for the Line Emission Mapper

Abstract

The Line Emission Mapper (LEM) is a proposed x-ray probe mission to study the physics of galaxy formation through spectral and spatial measurements of x-rays in the energy band of 0.2 to 2 keV. The LEM Microcalorimeter Spectrometer instrument on LEM will have a hybrid transition-edge sensor (TES) microcalorimeter array made up of an inner array of single-pixels with one x-ray absorber connected to one TES and an outer array of multi-absorber microcalorimeters, or "hydras," with four absorbers connected to a single TES, each with a different thermal conductance. Here, we characterize the first hybrid array of single-pixel and multi-absorber microcalorimeters designed for LEM. We present the fundamental transition, noise, and detector performance properties to demonstrate their suitability for the mission. We also show that the spectral resolution at the Al K α line is 1.92 AE 0.02 eV for the 4-pixel hydra (coadded) and 0.90 AE 0.02 eV for the single-pixels. This is significantly better resolution than the LEM mission level requirement. Finally, we demonstrate that the position discrimination between the four pixels of the hydra can be achieved down to 200 eV when measured with a time-division multiplexed readout using timings representative of the anticipated LEM requirements.

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

“…This contribution is almost entirely dominated by the intrinsic noise of the TES, including the TES phonon noise, Johnson noise, and any additional excess detector noise. These values are supported by the recent measurements on representative LEM pixels 29 . Also included are small contributions from the Johnson noise of the shunt resistor used to voltage bias the TES, thermal noise generated by cosmic ray background events that deposit energy in the detector array and a term that accounts for performance variations due to intrinsic pixel non-uniformities, the spread of bias points in a common biased column, and the effects of non-uniformity from the environment.…”

“…Thus, the lower energy range of the LEM pixels and the reduced pixel width are both beneficial in reducing the gain sensitivity. We have recently measured δE/δB=7 meV/nT at 1.5 keV on LMS pixels of width 15 μm, 29 almost ×300 lower than the 2.0 eV/nT at 7 keV measured for Athena/X-IFU pixels of width 50 μm 21 . The low field sensitivity of the pixels is important to the LMS focal plane assembly design, enabling a reduction in the magnetic shielding requirements, and thus smaller, lower mass magnetic shields.…”

“…are reported in Ref. 29. These results demonstrate that the LMS pixel designs can achieve the desired performance requirements in a large format hybrid array and were key to establishing TRL-5 for the LMS detector.…”

“…The baseline properties of the LMS pixels are listed in Table 2 and are derived from the characterization presented in Ref. 29. Since the TES design is the same for both pixel types, the TES transition shape (parameterized by the transition parameters α=T/R ∂R/∂T, β=I/R ∂R/∂I) and noise properties are also very similar.…”

“…Over timescales shorter than can be tracked using the on-board calibration source, these drifts cannot be corrected and thus can degrade the energy resolution. The detector magnetic field, bath temperature, and TES bias gain sensitivities have been recently measured for both LEM single pixels and hydras 29 and are used to consolidate the budget contributions. Unlike the noise terms, the gain sensitivities do not scale with the resolving power of the detector, thus most of these contributions are the same for both pixel types.…”