Year
2024
Abstract
| Gamma ray detectors based on microcalorimeters are operated at low temperatures (~100mK) and offer a 5 to 10 times better energy resolution than high purity germanium (HPGe) detectors. Applications for recently developed instruments based on microcalorimeters include safeguards, spent nuclear fuel analysis as well as improvement of nuclear data [1,2,3,4]. To reduce the time needed to acquire spectra with sufficient counting statistics, large multi pixel arrays containing several hundreds of microcalorimeters have been developed to improve detector efficiency.When compared with e.g. HPGe detectors, extracting quantitative information such as isotopic ratios from microcalorimeter based detectors can be more challenging due to the complex multi pixel detectors and data processing as well as detector efficiency approximations.To improve the quantitative analysis (i.e. determination of isotopic ratios) of spectra obtained from large multi pixel arrays and make informed decisions about instrument design and sample geometry choices, we model the detector efficiency of the SOFIA (Spectrometer Optimized for Facility Integrated Applications) instrument using the Geant4 toolkit. Specifically, our results can be used to reduce uncertainties in spent nuclear fuel measurements and improve the estimation of the Pu content. Our model considers several layers of thermal and magnetic shielding as well as different sample geometries. Sample geometries investigated range from point sources to swipes and thin films as well as solutions and powders contained in vials typically used in analytical laboratories.We also investigate different shielding options designed to reduce gamma rays being absorbed close to the sensitive superconducting resonators, while minimizing the presence of unwanted X-rays that originate in the shielding material being detected in the detector. Radiation dose deposited close to the superconducting resonators is suspected to lead to shifts of the resonance frequencies and ultimately shortens the effective measurement time. Suppressing this effect is especially important when measuring high activity samples like spent nuclear fuel. [1] M. P. Croce et al. “Improved Nondestructive Isotopic Analysis with Practical Microcalorimeter Gamma Spectrometers”, arXiv:2104.03376 (2021).[2] M. P. Croce et al., “Nuclear Facility Experience with the SOFIA Ultra-High-Resolution Microcalorimeter Gamma Spectrometer”, Proceedings of the INMM Annual Meeting (2022).[3] D. Mercer et al., “Quantification of 242Pu with a Microcalorimeter Gamma Spectrometer”, arXiv:2022.02933 (2022).[4] K. Koehler et al., “New Experimentally Observable Gamma-ray Emissions from 241Am Nuclear Decay”, arXiv:2103.15893 (2021). |