Low-temperature Performance of $\mathrm{Gd_3(Ga, Al)_5O_{12}}$:Ce Scintillators

TL;DR

This study evaluates the low-temperature performance of two GAGG:Ce scintillators as internal anti-coincidence detectors for cryogenic TES-based X-ray/gamma-ray missions. Using a cryogenic setup with a coincident readout against a plastic scintillator, the authors measure how decay time and light yield of GAGG:Ce crystals vary from room temperature down to cryogenic temperatures, with particular focus at 4 K. They find that at 4 K both light yield and decay time are close to room-temperature values (light yield ~70–80% of room, decay times within ~10–30%), while a sharp transition at around 2 K reduces the decay time to about 60 ns, demonstrating a robust, though partially unexplained, cryogenic response. Overall, the results support the feasibility of integrating GAGG:Ce ACDs inside a cryostat for multi-Kelvin operation, while highlighting an intriguing low-temperature effect that merits further investigation. Data and analysis code are publicly archived.

Abstract

The last years have seen the first cryogenic detectors to be proposed for usage on balloon-borne missions. In such missions, the instrument will be exposed to the high radiation environment of the upper atmosphere. This radiation can induce a significant background to the measurements, something which can be mitigated through the use of an anti-coincidence shield. For hard X-ray and gamma-ray detectors such a shield typically consists photomultiplier tubes or, more recently, silicon photomultipliers coupled to scintillators placed around the detector. When using cryogenic detectors, the shield can be placed around the entire cryostat which will make it large, heavy and expensive. For the ASCENT (A SuperConducting ENergetic x-ray Telescope) mission, which uses Transition Edge Detectors, it was therefore considered to instead place the shield inside. This comes with the challenge of operating it at cryogenic temperatures. For this purpose, we tested the performance of 2 different types of GAGG:Ce scintillators down to 15 mK for the first time. Although significant variations of both the decay time and the light yield were found when varying the temperature, at 4 K its performance was found to be similar to that at room temperature. Furthermore, unexpected behavior around 2 K was found for both types of GAGG:Ce, leading to more in depth studies around these temperatures. Overall, the studies show that the combination of materials will allow to produce a functional anti-coincidence shield at several Kelvin.

Figures (11)

• Figure 1: Schematic of the setup used to measure the performance of GAGG at different temperatures. The GAGG sample is connected to the mixing chamber stage of a BlueFors LD 250 cryostat. Optical windows in the radiation shields and the vacuum vessel allow observing the GAGG from the outside using a photomultiplier tube coupled to the vacuum window with a light-tight seal. The GAGG is illuminated by a ∼ 10.8Bq (∼ 290) $^{22}\text{Na}$ source placed at approximately 0.7 distance in a collimated lead enclosure outside the cryostat. The isotope $^{22}\text{Na}$ is a positron emitter resulting in the emission of two back-to-back 511 gamma-rays. A fast plastic scintillator coupled directly to a PMT is placed equidistantly opposite the GAGG. The signal from both PMTs is used to create a coincidence signal, which triggers the readout of signals from the PMT observing the GAGG as shown in figure \ref{['fig:GAGG_electronics']}.
• Figure 2: Schematic of the readout electronics from figure \ref{['fig:GAGG_setup']} used for measuring the relative light yield and the decay time of the GAGG scintillators. Signals from the two PMTs are sent to amplifiers. The amplified signal is processed by discriminators whose output is passed to a coincidence unit. In case of a coincidence a trigger is sent to an oscilloscope which reads out the non-amplified signals of the two PMTs. The traces from both PMTs are read out using a laptop computer and stored for further analysis.
• Figure 3: An example of the traces from the two PMTs for a valid coincident event. The top figure shows the trace from the PMT connected directly to a plastic scintillator, while the bottom one shows the signal from the PMT observing the GAGG:Ce crystal. The signal from the GAGG:Ce consists of 2 pulses from 2 separate photo-electrons. A consistent reflection from the signal in the cables can be seen for each pulse which is cleaned up in the analysis.
• Figure 4: Top: Example of the reconstructed scintillation pulses produced by summing 2400 traces taken at room temperature from the first GAGG:Ce crystal after aligning them based on the peak position of the pulse in the plastic scintillator. The pulse is fitted with the function given in equation \ref{['eq:2']}. Bottom: Same as the top figure but for 800 traces taken at 50.
• Figure 5: The pulse height spectrum from the PMT acquired through illumination with a pulsed light source. The dynode peak can be seen at around 1 followed by a continuum produced by the sum of the photoelectron peaks.