Development of decay energy spectroscopy using low temperature detectors

Low Temperature Microcalorimeters for Decay Energy Spectroscopy

Low Temperature Detectors have been used to measure embedded radioisotopes in a measurement mode known as Decay Energy Spectroscopy (DES) since 1992. DES microcalorimeter measurements have been used for applications ranging from neutrino mass measurements to metrology to measurements for safeguards and medical nuclides. While the low temperature detectors have extremely high intrinsic energy resolution (several times better than semiconductor detectors), the energy resolution achieved in practice is strongly dependent on factors such as sample preparation method. This review seeks to present the literature consensus on what has been learned by looking at the energy resolution as a function of various choices of detector, absorber, and sample preparation methods.

Measurement of 227Ac impurity in 225Ac using decay energy spectroscopy

²²⁵Ac is a valuable medical radionuclide for targeted α therapy, but ²²⁷Ac is an undesirable byproduct of an accelerator-based synthesis method under investigation. Sufficient detector sensitivity is critical for quantifying the trace impurity of ²²⁷Ac, with the ²²⁷Ac/²²⁵Ac activity ratio predicted to be approximately 0.15% by end-of-bombardment (EOB). Superconducting transition edge sensor (TES) microcalorimeters offer high resolution energy spectroscopy using the normal-to-superconducting phase transition to measure small changes in temperature. By embedding ²²⁵Ac production samples in a gold foil thermally coupled to a TES microcalorimeter we can measure the decay energies of the radionuclides embedded with high resolution and 100% detection efficiency. This technique, known as decay energy spectroscopy (DES), collapses several peaks from α decays into single Q-value peaks. In practice there are more complex factors in the interpretation of data using DES, which we will discuss herein. Using this technique we measured the EOB ²²⁷Ac impurity to be (0.142 ± 0.005)% for a single production sample. This demonstration has shown that DES is a useful tool for quantitative measurements of complicated spectra.

Measurement of the electron capture probabilities of 55Fe with a metallic magnetic calorimeter

The ratios of the electron capture probabilities P_K, P_L and P_M of ⁵⁵Fe have been measured with a metallic magnetic calorimeter, a specific type of cryogenic detector. The ⁵⁵Fe source was enclosed in the detector absorber, whose dimensions were chosen such that the detection efficiency for Mn K X-rays was larger than 99.99%. Since all electrons and photons emitted by the source are absorbed in the detector, the detection efficiency is virtually 100% for K, L and M captures. The energy threshold was low enough to allow for clear separation of the M captures (~ 80eV) from noise. The capture probability ratios were translated to capture probabilities using the recommended value for the probability of the undetected N captures. The resulting values are in agreement both with the recommended values of P_K, P_L and P_M and with the experimental data of Pengra and coworkers.

Concept design of a time-of-flight spectrometer for the measurement of the energy of alpha particles

The knowledge of the energies of the alpha particles emitted in the radioactive decay of a nuclide is a key factor in the construction of its decay scheme. Virtually all existing data are based on a few absolute measurements made by magnetic spectrometry (MS), to which most other MS measurements are traced. An alternative solution would be the use of time-of-flight detectors. This paper discusses the main aspects to be considered in the design of such detectors, and the performances that could be reasonably expected. Based on the concepts discussed here, it is estimated that an energy resolution about 2.5keV may be attainable with a good quality source.