A new experimental set-up for measuring beta spectra with metrological accuracy is under development. In its current version, it has been designed using a 0.5 mm thick and 450 mm² surface area PIPS-type silicon semiconductor detector. Preliminary Monte-Carlo simulations of the detection chamber determined the geometry and materials to be used in order to minimize distortion in the measured spectrum.
These deformations are due to physical phenomena (diffusion and backscattering of electrons, self-absorption in the source) and detection (resolution and linearity). The detector is cooled to liquid nitrogen temperature and maintained under secondary vacuum.
Since the quality of radioactive sources is crucial for obtaining quality spectra, a study has been carried out on the preparation of thin sources. The technique used is to manufacture the sources by electro-pulverization and then freeze-drying the radioactive deposition on a 500 nm thick Vyns film.
Calibration was performed using 109Cd and 133Ba conversion electron sources. A resolution of 4.4 keV is obtained at 60 keV and the detection threshold is 15 keV. Different types of beta spectra (14C, 60Co, 151Sm, 99Tc) have been measured and corrected for the remaining deformations. A response function can be determined for each measuring geometry using Monte-Carlo simulations. These simulations include the very precise geometric information of the detector obtained by X-ray radiography. They were validated by the excellent agreement between the simulated and measured spectra.
Analysis of the detection system’s simulated impulse response is the most promising way to define this response function precisely. Initial results show that deconvolution of the measured spectrum with the detection system’s response allows the real shape of the beta spectrum to be obtained with great precision.
Bisch et al., Evaluation of the beta spectral shapes, 16th International Congress of Metrology, (Paris, France, 7-10 October 2013,) EDPSciences, Web of Conferences, 07004 (2013)
DOI : 10.1051/ metrology/201307004
Mougeot X., Systematic comparison of beta spectra calculations using improved analytical screening correction with experimental shape factors, Applied Radiations and Isotopes 109,177-182 (2016).
DOI : 10.1016/j.apradiso.2015.11.030
Our job: metrology
Implemented methods for the establishment of national references must be adapted to the radiation type and its intensity. They are based on measurement technics such as calorimetry, ionometry and chemical dosimetry.
The variety of the emitted radiation and physical forms of the sources oblige to adapt the measurement process in order to establish national references: methods with defined geometries, or 4 π countings geometries, coincidence countings, etc.