Boron and thermal neutron interactions on borosilica window photomultiplier tubes

The borosilica, a very common PMT window and envelope material, contains 5{\%} Boron (1{\%} $^{10}$B). The high cross section for $^{10}$B capturing thermal neutrons (3980 barn), is a concern for LHC experiments using borosilica window PMTs. This study investigates the rate and the size of the signals generated by thermal neutron boron interaction in borosilica window PMTs; Hamamatsu R7525-HA and R7600U-200-M4. Although virtually all of the thermal neutrons incident on the borosilicate glass are absorbed, probability of generating a PMT signal was measured to be 3$\times $10$^{-4}$ and 3$\times $10$^{-6}$ for R7525-HA and R7600U-200-M4 PMTs, respectively. For these signals the average pulse size was found to be between 20--30 photoelectrons. We also discuss that four anode PMTs allow the elimination of these events with an of?ine algorithm.