Beyond One-Thousandth Energy Resolution with an AlMn TES Detector

Abstract

The superconducting Transition-Edge Sensor (TES) is a critical technology for next-generation X-ray spectrometers, known for its exceptional energy resolution. In the last decade, TESs based on AlMn alloy films have been extensively used in several cosmic microwave background (CMB) experiments. The advantages of simple fabrication process and easily tunable critical temperature make them an alternative to bilayer TESs. However, they have rarely been applied to X-ray detection until now. We developed an annular AlMn TES for X-ray detection and tested it in a dilution refrigerator with a Superconducting Quantum Interference Device (SQUID) amplifier, achieving an Full Width at Half Maximum (FWHM) of 12.1 +- 0.3 eV at 17.48 keV. To the best of our knowledge, this is the first demonstration of an AlMn TES achieving an energy resolution below 0.1%, highlighting its potential for high-resolution X-ray detection.

Figures (8)

• Figure 1: (a) Schematic of the TES device comprising two Nb electrodes and an AlMn film on a Si$_3$N$_4$/SiO$_2$ membrane (square area). The gold absorber is not shown for clarity. (b) SEM image of the TES detector fabricated.
• Figure 2: (a) Cross-sectional diagram of the test box. The test box is composed of four components: (1) The Cryoperm 10 cover plate with an incident window; (2) An almost enclosed aluminum box housing the SQUID; (3) A heat sink made of oxygen-free copper material (purple); (4) A Nb bottom plate. (b) Magnetic field distribution inside and outside the test box. The magnetic permeability of Cryoperm 10 at 100 mK was supposed to be 50000 in terms of the material's official dataCryoperm-10. The Al and Nb as superconductor were set to have the same permeability of $10^{-5}$, and copper to 1. Based on the laboratory’s geographical location, We determined the direction and strength of the surrounding geomagnetic field: along the direction parallel to the TES (the X-axis and Z-axis), the magnetic field strength was 4 $\mu$T and 45 $\mu$T, respectively; while perpendicular to the TES (the Y-axis), the magnetic field strength was 21 $\mu$T.
• Figure 3: Magnetic field strength at different heights above the TES plane. The solid lines were results for three different shielding designs: (1) The top cover plate and the bottom plate were Nb metal, (2) Both were Cryoperm 10 alloy. (3) The top cover plate was Cryoperm 10 alloy and the bottom plate was Nb metal. The dashed line was the position of the TES detector and aperture.
• Figure 4: A schematic diagram for the TES detector test system. X-ray tube was used to generate high energy photons, LD250 to provide a low temperature circumstance for the TES detector, and the right circuit to bias the TES. The window on the cryostat housing was made of 1.5 mm thick beryllium (Be), and the windows on the internal 50 K, 4 K, and 1 K shields were made of 20 $\mu$m-thick aluminum foil.
• Figure 5: I-V curves of the TES detector at base temperatures from 40 mK to 98 mK. The red circle indicates the bias point, corresponding to a base temperature of 74 mK and a voltage of 0.385 V. The inset is Joule powers of the TES detector operating at 70 % $R_n$ across various bath temperatures.