MetroMMC: Electron-Capture Spectrometry with Cryogenic Calorimeters for Science and Technology

Abstract

Accurate decay data of radionuclides are necessary for many fields of science and technology, ranging from medicine and particle physics to metrology. However, data that are in use today are mostly based on measurements or theoretical calculation methods that are rather old. Recent measurements with cryogenic detectors and other methods show significant discrepancies to both older experimental data and theory in some cases. Moreover, the old results often suffer from large or underestimated uncertainties. This is in particular the case for electron-capture (EC) decays, where only a few selected radionuclides have ever been measured. To systematically address these shortcomings, the European metrology project MetroMMC aims at investigating six radionuclides decaying by EC. The nuclides are chosen to cover a wide range of atomic numbers Z , which results in a wide range of decay energies and includes different decay modes, such as pure EC or EC accompanied by γ-and/or β +-transitions. These will be measured using metallic magnetic calorimeters (MMCs), cryogenic energy-dispersive detectors with high-energy resolution, low-energy threshold and high, adjustable stopping power that are well suited for measurements of the total decay energy and X-ray spectrometry. Within the MetroMMC project, these detectors are used to obtain X-ray emission intensities of external sources as well as fractional EC probabilities of sources embedded in a 4π absorber. Experimentally determined nuclear and atomic data will be compared to state-of-the-art theoretical calculations which will be further developed within the project. This contribution introduces the MetroMMC project and in particular its experimental approach. The challenges in EC spectrometry are to adapt the detectors and the source preparation to the different decay channels and the wide energy range involved, while keeping the good resolution and especially the low-energy threshold to measure the EC from outer shells. Keywords Electron-capture decay • Metallic magnetic calorimeter • Radionuclide metrology project has received funding from the EMPIR program co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation program.

Low Temperature Microcalorimeters for Decay Energy Spectroscopy

“…At Los Alamos National Laboratory, measurements of 193 Pt were done to understand the atomic effects on electron capture [44]. The European collaboration, MetroMMC was created to explore electron capture [45]. At Lawrence Livermore National Laboratory, the 7 Be measurements first performed in 2002 with Si:P:B thermistors [17] were revisited in 2020 with Superconducting Tunnel Junctions [46,47], adding to the body of literature on atomic effects on nuclear weak decay.…”

Superconducting detectors for rare event searches in experimental astroparticle physics

“…Superconducting detectors also provide superior performance in gamma-ray and alpha radionuclide analysis [10][11][12]. In particular, superconducting detectors equipped with metal 4π absorbers enable decay energy spectroscopy for alpha-and beta-emitting radionuclides, a new spectroscopic method for the accurate measurement of spectral shapes and activities [13][14][15]. This new method has also been adopted to study important properties of neutrinos such as the rest mass through the end-point measurement of beta-decay spectra [16,17].…”

Determination of Fractional Electron Capture Probabilities of 125I Using Metallic Magnetic Calorimeters

“…Apart from this nuclide important in neutrino physics [1,2], EC nuclides play an increasingly important role in nuclear medicine [3,4], and precise decay data on EC are also needed in radionuclide metrology [5] and to assess the low energy background in various nuclear physics experiments [6]. However, precise and rather complete experimental data sets on EC are scarce (the available data sets like 7 Be, 22 Na, 59 Ni, 54 Mn, 55 Fe, 57 Co, 68 Ga, 109 Cd, 125 I, 129 I, 204 Tl, 207 Bi can be found in Monographie BIPM 5 tables [7]), and accurate theoretical predictions encounter similar difficulties due to the precision of the atomic modeling, nuclear structure effects or radiative corrections, as in beta decays [7][8][9].…”

“…Fractional EC probability refers to the capture probabilities on the individual electron shells, i.e., PL/PK, PL/PM, and so on. The project chose six radionuclides, 65 Zn, 109 Cd, 125 I, 54 Mn, 59 Ni, and 41 Ca, to cover a broad range of atomic numbers and different degrees of forbiddenness, an essential feature of weak interaction transitions that strongly impacts the decay probabilities [9]. MMCs were optimized for each of these in 4π geometry.…”