Microdosimetric measurements in the thermal neutron irradiation facility of LENA reactor

•A counter with two mini TEPCs, both equipped with electrical-field guard tubes, has been constructed.•The microdosimetric spectrum of the LENA-reactor irradiation vane has been studied.•The radiation-field quality (RBE) assessment confirms that the Dn/Dtot ratio is not an accurate parameter to characterize the BNCT radiation field.

A twin TEPC with electric-field guard tubes has been constructed to be used to characterize the BNCT field of the irradiation facility of LENA reactor. One of the two mini TEPC was doped with 50 ppm of 10B in order to simulate the BNC events occurring in BNCT. By properly processing the two microdosimetric spectra, the gamma, neutron and BNC spectral components can be derived with good precision (~6%). However, direct measurements of 10B in some doped plastic samples, which were used for constructing the cathode walls, point out the scarce accuracy of the nominal 10B concentration value. The influence of the Boral® door, which closes the irradiation channel, has been measured. The gamma dose increases significantly (+51%) when the Boral® door is closed. The crypt-cell-regeneration weighting function has been used to measure the quality, namely the RBEµ value, of the radiation field in different conditions. The measured RBEµ values are only partially consistent with the RBE values of other BNCT facilities.