Fast neutron sensitivity for 3He detectors and comparison with Boron-10 based neutron detectors

Abstract

The 3 He-based neutron detectors are no longer the default solution for neutron scattering applications. Both the inability of fulfilling the requirements in performance, needed for the new instruments, and the shortage of 3 He, drove a series of research programs aiming to find new technologies for neutron detection. The characteristics of the new detector technologies have been extensively tested to prove their effectiveness with respect to the state-of-the-art technology.Among these, the background rejection capability is crucial to determine. The signal-to-background ratio is strongly related to the performance figure-of-merit for most instruments. These are designed to exploit the high flux expected from the new high intensity neutron sources. Therefore, an inadequate background rejection could significantly affect the measurements, leading to detector saturation and misleading events. This is of particular importance for the kind of techniques in which the signals are rather weak.For the first time, the sensitivity of 3 He detectors to fast neutrons, up to E n = 10 MeV, has been estimated. Two independent measurements are presented: a direct calculation based on a subtraction method used to disentangle the thermal and the fast neutron contribution, while a further evidence is calculated indirectly through a comparison with the recently published data from a 10 B-based detector. Both investigations give a characterization on the order of magnitude for the sensitivity. A set of simulations is presented as well in order to support and to validate the results of the measurements. A sensitivity of 4 · 10 −3 is observed from the data. This is two orders of magnitude higher than that previously observed in 10 B-based detectors.

Verification of He-3 proportional counters’ fast neutron sensitivity through a comparison with He-4 detectors

“…The Helium-4 detector at 10 bar indicates the sensitivity of the Helium-3 counter for the same gas pressure, and it is of the order of −3 . This result is in agreement with both the simulations and experimental results carried out in [57].…”

“…On the other hand, in Helium-3 detectors the detection of thermal and cold neutrons cannot be disentangled from the detection of fast or epi-thermal neutrons. Simulations and indirect measurements have been performed on this matter [57].…”

“…The gamma-ray source was used in order to have a comparison with the previous work on gamma-ray sensitivity of Boron-10 and Helium-3 detectors [32,[55][56][57]. Co-60 mainly emits gamma rays at 1.173 MeV and 1.332 MeV with unitary intensity; any other emitted gamma ray can be neglected for our purposes because of their weak intensity.…”

“…It has been shown that Boron-10-based detectors have a similar gamma-ray sensitivity [32,55] of Helium-3-based detectors, but about 100 times lower fast neutron sensitivity (10 −5 ) [56,57]. Thus, for a single thermal neutron detected they can tolerate a fast neutron flux of 10 3 while keeping the same background rejection level (below 1 %).…”

“…An indirect method based on a series of measurements with different absorbers and simulations has indicated that Helium-3 has a fast neutron sensitivity of the order of 10 −3 [57].…”