X.Mougeot: Resurgence of beta spectrometry in a metrological context
Resurgence of beta spectrometry in a metrological context
CEA, LIST, Laboratoire National Henri Becquerel, F-91191 Gif-sur-Yvette, France
The National Laboratory Henri Becquerel (LNHB) is the French national standards laboratory for ionizing radiation metrology. Among our duties, we provide reference activity standards with the best possible uncertainty and carefully established nuclear decay data. These data are officially recommended by the BIPM (Bureau International des Poids et Mesures).
Our users have asked over the past few years for a precise knowledge of the shapes of beta energy spectra. Beta spectra are needed in ionizing radiation metrology, in nuclear medicine for radiotherapy and dosimetry, as well as in the nuclear industry for calculations of residual power or post-irradiation fuel management. The measurement of antineutrinos from reactors, directly correlated with beta spectra, is currently developing as a technique for monitoring nuclear facilities. An increasing demand also exists for precision beta spectrometry from several fields of fundamental research, like the precise measurements of neutrino oscillations, search for sterile neutrinos, direct measurement of the neutrino mass and nuclear astrophysics. Regarding nuclear data, beta emission properties are crucial features when evaluating the decay schemes of radionuclides. However, experimental information can be missing and theoretical models are mandatory for filling in the blanks.
Beta spectrometry is experiencing a resurgence of interest, having been little studied since the late 1970s. Studies performed so far were mainly focusing on allowed and first forbidden non-unique transitions. The vast majority of published experimental shape factors were established from measurements using magnetic spectrometers. None of them provide any information below 50 keV and the distorting effects of the instrumentation were not fully corrected. At that time, beta spectra were thought to be known accurately enough. Today, users are asking for smaller uncertainties, as well as good knowledge of the spectra for forbidden transitions of higher orders, both unique and non-unique.
Hence, a study of beta spectra has been initiated at LNHB both experimentally and theoretically, funded through several European projects. A novel experimental technique, metallic magnetic calorimetry, has been adapted to low-energy beta spectrometry. The high precision measurement of 63Ni and 241Pu beta spectra highlights an important discrepancy compared with the usual theoretical predictions for these simple transitions. New and more comprehensive calculations were then developed to accurately account for the atomic screening and exchange effects, leading to excellent agreement with the measurements from 0.5 keV to the endpoint energy. A new calculation code, BetaShape, with much improved modelling has recently been released. This code provides beta emission properties and beta and neutrino spectra, propagates the uncertainties from the input parameters and includes a database of experimental shape factors. These recent achievements and the expected outlooks will be presented in details.