Speaker
Description
For several decades, beta decay has been studied by researchers around the world in order to measure the characteristic parameters: Q-value, half-life, branching ratio, etc. While these studies have correctly characterized some of these parameters, the energy spectra of the emitted beta particles are less well known, in particular in the case of forbidden transitions. Over the past ten years, many research topics have identified a strong need for better constraints on these spectra, both experimental and theoretical. This is particularly the case in dark matter studies to constrain the background noise of some experiments; in neutrino physics to try to describe the anomaly of the antineutrino flux emitted by nuclear reactors; or in radionuclide metrology to improve the definition of the becquerel (the SI derived unit of radioactivity) by reducing the uncertainty of activity measurements carried out by liquid scintillation counting. High-precision measurements of beta spectra would also allow certain predictions of the Standard Model to be tested and could highlight new physics.
On the theory side, the French national metrology laboratory for ionizing radiation (the Laboratoire National Henri Becquerel, LNE-LNHB) has been involved in recent improvements made in the calculation of beta spectra, in particular in the treatment of the forbidden non-unique transitions. However, there still exists to this day a lack of high-precision measurements in order to test and constrain the theoretical predictions, particularly as such measurements are notoriously very challenging.
In this context, the LNE-LNHB has been developing for the past few years a 4π detection system for high-precision beta decay energy spectra. The apparatus is based on two silicon detectors in a compact geometry optimized to cover more than 98% of the solid angle. The detectors are identical and of PIPS (Passivated Implanted Planar Silicon) type, with a thickness of 1 mm that allows a maximum beta particle energy of 850 keV to be measured. To achieve such a compactness and minimize the radiation energy attenuation, a new procedure to produce ultra-thin radioactive sources has been established, based on the use of 0.7 μm Mylar films. Finally, a deconvolution method based on high-precision Monte Carlo simulations has been developed to account for the various distortion effects in the reconstructed energy spectra. Very promising results have been obtained with this apparatus for 14C and 99Tc sources.
In the present contribution, we will discuss the recent developments performed on the detection system. Most notably, two 5mm-thick Si(Li) detectors have replaced the PIPS detectors, allowing the measurement of beta spectra with a maximum energy up to 3 MeV and with a better energy resolution. The procedure for the preparation of the sources will be briefly described. Characterization of the system response to electron emissions from calibration sources (207Bi, 109Cd…) will be presented. A new numerical data acquisition system to treat the two detector signals in coincidence has also been implemented and will be discussed. Finally, new measurements for medium-energy beta decay radionuclides will be presented, e.g. 90Sr/90Y, 89Sr and 32P.