The new generation of x-ray and gamma-ray detectors employ cryogenic detectors known as transition-edge sensors (TES) due to their high energy resolution and photon detection rates. These detectors require a refrigeration module that can operate at the transition temperature of the TES’s superconducting film—usually at mK temperatures. DR-TES consists of a novel mini-dilution refrigerator (DR) from Chase Research Cryogenics that can be used in balloon-borne missions to cool detectors to temperatures between 10 to 100mK. To test the viability of this DR module, we will be cooling down a SLEDGEHAMMER detector fabricated by the National Institute of Standards and Technology quantum sensor group. The SLEDGEHAMMER microcalorimeter uses TESs coupled to superconducting quantum interference devices which are in turn coupled to microwave resonators to detect x-rays and gamma-rays. We plan to fly the SLEDGEHAMMER detector cooled by the mini-DR on a stratospheric balloon flight in August of 2024 at Fort Sumner, NM. As a follow-up mission, 511-CAM will use a modified version of the detector to map the 511keV emission from the galactic center region.
The 511 keV γ-ray emission from the galactic center region may fully or partially originate from the annihilation of positrons from dark matter particles with electrons from the interstellar medium. Alternatively, the positrons could be created by astrophysical sources, involving exclusively standard model physics. We describe here a new concept for a 511 keV mission called 511-CAM (511 keV gamma-ray camera using microcalorimeters) that combines focusing γ-ray optics with a stack of transition edge sensor microcalorimeter arrays in the focal plane. The 511-CAM detector assembly has a projected 511 keV energy resolution of 390 eV full width half maximum or better, and improves by a factor of at least 11 on the performance of state-of-the-art Ge-based Compton telescopes. Combining this unprecedented energy resolution with sub-arcmin angular resolutions afforded by Laue lens or channeling optics could make substantial contributions toward identifying the origin of the 511 keV emission through discovering and characterizing point sources and measuring line-of-sight velocities of the emitting plasmas.
Core collapse supernovae are thought to be one of the main sources in the galaxy of elements heavier than iron. Understanding the origin of the elements is thus tightly linked to our understanding of the explosion mechanism of supernovae and supernova nucleosynthesis. X-ray and gamma-ray observations of young supernova remnants, combined with improved theoretical modeling, have resulted in enormous improvements in our knowledge of these events. The isotope Ti44 is one of the most sensitive probes of the innermost regions of the core collapse engine, and its spatial and velocity distribution are key observables. Hard x-ray imaging spectroscopy with the Nuclear Spectroscopic Telescope Array (NuSTAR) has provided new insights into the structure of the supernova remnant Cassiopeia A (Cas A), establishing the convective nature of the supernova engine. However, many questions about the details of this engine remain. We present here the concept for a balloon-borne follow-up mission called A SuperConducting ENergetic x-ray Telescope (ASCENT). ASCENT uses transition edge sensor gamma-ray microcalorimeter detectors with a demonstrated 55-eV full-width half maximum energy resolution at 97 keV. This 8- to 16-fold improvement in energy resolution over NuSTAR will allow for high-resolution imaging and spectroscopy of the Ti44 emission. This will allow for a detailed reconstruction of gamma-ray line redshifts, widths, and shapes, allowing us to address questions such as, What is the source of the neutron star kicks? What is the dominant production pathway for Ti44? Is the engine of Cas A unique?
NIST has developed microwave multiplexed microcalorimeter arrays for the detection of hard X-rays andγ-rays (Bennett et al. 2012, Mates et al. 2017). The arrays are made of tin or bismuth absorbers that are read-out with arrays of Transition Edge Sensors (TES). Each TES is coupled via a SQUID to a microwave resonator, and a single microwave line is used to sample the response of the resonators of all pixels. The detector arrays achieve an energy resolution of 55 eV FWHM at 97 keV. We report here on the performance of a 34-pixels prototype TES with a collimated 50μm diameter 20-50 keV X-ray beam as well as a Eu(152) source for a future balloon flight. We will furthermore describe a planned stratospheric balloon flight that will be used to demonstrate the performance of a novel mini-dilution refrigerator and the 34-pixels prototype detector in a space environment.
The Lynx x-ray microcalorimeter (LXM) is an imaging spectrometer for the Lynx satellite mission, an x-ray telescope being considered by NASA to be a new flagship mission. Lynx will enable unique astrophysical observations into the x-ray universe due to its high angular resolution and large field of view. The LXM consists of an array of over 100,000 pixels and poses a significant technological challenge to achieve the high degree of multiplexing required to read out these sensors. We discuss the details of microwave superconducting quantum interference device (SQUID) multiplexing and describe why it is ideally suited to the needs of the LXM. This case is made by summarizing the current and predicted performance of microwave SQUID multiplexing and describing the steps needed to optimize designs for all the LXM arrays. Finally, we describe our plan to advance the technology readiness level (TRL) of microwave SQUID multiplexing of the LXM microcalorimeters to TRL-5 by 2024.
SQUID Time-Division Multiplexing (TDM) is a technique for the readout of arrays of Transition-Edge Sensors (TESs) for x-ray and gamma-ray science. TDM has been deployed in many recent 250-pixel-scale instruments including at synchrotron light sources and particle-accelerator facilities, as well as in table-top experiments. Two TES spectrometers employing TDM readout will soon be deployed to electron-beam ion-trap facilities.
TDM is also under development as a back-up readout option for the X-ray Integral Field Unit (X-IFU) of the Athena satellite mission. The 3,840 TES pixels of the X-IFU will enable efficient, high resolution spectroscopy (2.5 eV FWHM at 7 keV) of extended astrophysical sources. Multiplexing factors of 40 or more sensors per readout column are planned for the X-IFU. To advance the maturity of TDM readout for Athena, we are creating a focal-plane assembly for the readout of 960 TES pixels in a 24 column by 40 row configuration. We will describe the design and experimental progress on this technology demonstrator.
In a TDM system, each dc-biased TES has its own first-stage SQUID. Rows of these first-stage-SQUIDs are turned on and off sequentially such that the signal from only one TES at a time per readout column is passed to a series-array SQUID, to a room-temperature preamplifier, and to digital-feedback electronics. Recent implementations of TDM have a row period of 160 ns and non-multiplexed amplifier noise of 0.19 micro-Phi_0/sqrt(Hz) referred to the first-stage SQUID.
Some benchmark demonstrations of TDM with x-ray TES sensors include achievement of 2.55 eV FWHM energy resolution at 5.9 keV in a 32-row, 1-column configuration. Here, the fastest slew rates in the TES currents were similar to those of the X-IFU “LPA2” detector model. We have also achieved 2.72 eV FWHM resolution in a 32-row, 6-column configuration that contained 144 high-quality TESs that were similar to the much faster X-IFU “LPA1” pixels. We will describe on-going efforts to read out TDM arrays at the 6x32 scale and larger, as well as efforts to improve the performance of TDM system subcomponents. We will also describe system-level performance metrics such as cross-talk.
SQUID Code-Division Multiplexing (CDM) is closely related to TDM but has important performance advantages. CDM and TDM operation are similar with the main difference being that in CDM, all TESs are observed by the multiplexer at all times, with the polarity of the TES signals switched between rows. Because all TESs are observed by the multiplexer at all times, the sqrt(N_rows) noise-aliasing degradation inherent to TDM is eliminated.
We are developing flux-summing CDM to be drop-in compatible with existing TDM systems. The most recent CDM implementation has a nonmultiplexed noise level of 0.17 micro-Phi_0/sqrt(Hz) referred to the first-stage SQUID and a row period of 160 ns. We have demonstrated 2.77 eV FWM resolution at 5.9 keV in 32-row, 1-column CDM test.
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