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Accurate measurements of the beta spectrum of samarium-151


In cooperation with the Laboratoire National Henri Becquerel in France, the beta spectrum of samarium‑151 was thoroughly investigated. Measurement data obtained by means of a metallic magnetic calorimeter were used to determine the beta end-point energy with unprecedented accuracy. Furthermore, the researchers involved in this project succeeded in determining the decay probabilities of two competing beta branches.

The energy distribution of the electrons emitted by beta minus decay is described by a continuous spectrum between zero and a maximum energy that is characteristic of the decaying radionuclide. This maximum energy is also called the end‑point energy. The exact form of the beta spectrum and the end‑point energy are of pivotal importance for many applications. They play a particular role in areas such as activity measurements using liquid scintillation counting. In neutrino physics, beta spectra are also relevant since the corresponding anti‑neutrino spectra can be derived from them.

Some physics textbooks may suggest that calculating beta spectra has long been a given. Highly accurate measurements performed with metallic magnetic calorimeters (MMCs), however, show that the experimental spectra sometimes considerably differ from spectra calculated by means of older devices, or that were measured decades ago by means of other methods. This finding and the particular importance of beta spectra for radionuclide metrology have led to a revival of this research field. The calculation methods have significantly improved and are now considered quite reliable, in particular for allowed and forbidden unique beta decays [1]. There is, however, still a huge need for more research in the field of what is called forbidden non‑unique beta decays.

The Laboratoire National Henri Becquerel (LNHB) in particular has gone to great lengths over the past few years to measure beta spectra with detectors. These detectors, which are now being used successfully at PTB as well, work at very low temperatures – below 50 mK. Besides a very low detection threshold, they also offer a very high energy resolution. They therefore allow even the low‑energy part of beta spectra to be measured accurately, which had not been possible to date with other detectors such as magnetic spectrometers.

A paper [2] jointly published by researchers from the LNHB and from PTB takes a closer look at the decay of samarium‑151. The raw data obtained in France based on MMC measurements were evaluated by the teams of both institutes separately to identify and rule out possible sources of error. The spectra thus obtained by the two research groups are in very good agreement with each other. They were successfully used to determine the end‑point energy of (76430 ± 68) eV by means of Kurie plots. If we compare this value with the currently assumed value from the atomic mass evaluation [3], namely (76500 ± 500) eV, it becomes clear that the standard measurement uncertainty of the new measurements has decreased by more than a factor of seven.

The measurement data were furthermore used to determine the probability of two competing beta branches. Besides the dominant beta transition to the ground state of stable europium‑151, there is another – weaker – branch to an excited state. If the energy of the immediately following low‑energy gamma transition is absorbed in the MMC detector, this leads to a shift of the energies of the weak beta branch inside the measured spectrum. This effect was used to determine the decay probabilities of both branches [2]. The success of this method is at the same time a milestone for another experiment to come. Within the scope of the EMPIR project entitled PrimA-LTD [4], MMC measurements of the radionuclide iodine‑129 are planned. There again, two competing beta branches exist, but in this case, the dominant branch feeds an excited state of xenon‑129, which also leads to a shift of the beta spectrum. It is planned to use this phenomenon, in particular, to measure the low-energy component of the corresponding second forbidden beta transition with the highest possible accuracy. The measurement results will then represent an important starting point for validating and further developing theoretical models.


[1]        BetaShape Code: http://www.lnhb.fr/rd-activities/spectrum-processing-software/

[2]        Kossert, K., Loidl, M., Mougeot, X., Paulsen, M., Ranitzsch, Ph., Rodrigues, M.: High precision measurement of the 151Sm beta decay by means of a metallic magnetic calorimeter. Applied Radiation and Isotopes 185 (2022) 110237, https://doi.org/10.1016/j.apradiso.2022.110237 (Open Access).

[3]        Wang, M., Huang, W.J., Kondev, F.G., Audi, G., 2021. The AME2020 atomic mass evaluation (II). Tables, graphs and references. Chin. Phys. C 45, 030003.

[4]        Website of EMPIR Project PrimA-LTD “Towards new primary activity measurement standardisation methods based on low-temperature detectors”: https://prima-ltd.net/


Opens local program for sending emailK. Kossert, Department  6.1, Working Group 6.14