The RaMsEs group (Radioprotection et Mesures Environnementales) is developing a new compact device for operational neutron dosimetry. The electronic part of the detector is made of an integrated active pixel sensor, originally designed for tracking in particle physics. This device has useful features for neutrons, such as high detection efficiency for charged particles, good radiation resistance, high readout speed, low power consumption and high rejection of photon background. A good response of the device to fast neutrons has already been demonstrated . In order to test the sensibility of the detector to thermal neutrons, experiments have been carried out with a 512 $\times$ 512 pixel CMOS sensor on a californium source moderated with heavy water $({\rm Cf}+{\rm D}_{2}{\rm O})$ on the Van Gogh irradiator at the LMDN (Laboratoire de Mtrologie des Neutrons), IRSN, Cadarache, France. A thin boron converter is used to benefit from the significant cross section of the $^{10}{\rm B}({\rm n},\alpha)^{7}{\rm Li}$ reaction. Results show a high detection efficiency (around $10^{-3}$) of the device to thermal neutrons. Our measurements are in good agreement with GEANT4 Monte Carlo simulations.

Previously we characterized radiophotoluminescent (RPL) detectors for measuring both fast and thermal neutrons for personal monitoring. The linear response and angular dependence, which satisfies the ISO 21909 standard makes their application possible in neutron dosimetry. The polymer CR-39 track detectors remain one of the most used dosimeters where neutron radiations are to be measured, but the visualization process is time consuming. The difference between results obtained by RPL and CR-39 has been discussed mainly for fast neutrons. The present study has also looked at thermal neutron dosimetry, where we found few results in the literature for CR-39. Our measurements demonstrate that RPL detectors can be advantageously incorporated into a dosimeter to measure thermal neutrons.