Novel concept for neutron detection: proportional counter filled with 10B nanoparticle aerosol

Abstract

The high neutron detection efficiency, good gamma-ray discrimination and non-toxicity of ³He made of proportional counters filled with this gas the obvious choice for neutron detection, particularly in radiation portal monitors (RPM), used to control the illicit transport of nuclear material, of which neutron detectors are key components. ³He is very rare and during the last decade this gas has become increasingly difficult to acquire. With the exception of BF₃, which is toxic, no other gas can be used for neutron detection in proportional counters. We present an alternative where the ³He atoms are replaced by nanoparticles made of another neutron sensitive material, ¹⁰B. The particles are dispersed in a gaseous volume, forming an aerosol with neutron sensitive properties. A proportional counter filled with such aerosol was exposed to a thermal neutron beam and the recorded response indicates that the neutrons have interacted with the particles in the aerosol. This original technique, which transforms a standard proportional gas mixture into a neutron sensitive aerosol, is a breakthrough in the field of radiation detection and has the potential to become an alternative to the use of ³He in proportional counters.

Figure 1. Left: Neutron interaction in ³He proportional counters: an incoming neutron interacts with a ³He atom, producing a proton (573 keV) and a tritium (191 keV).

In large detectors most of the interactions are of type c) and result in full energy (764 keV) collection. For interactions near the walls there is a probability that one of the products is emitted onto the wall of the detector and its energy is not collected, resulting in the low energy continuum with 2 discontinuities at 191 and 573 keV. Right: pulse-heigh distribution recorded with a ³He proportional counter, adapted from.

Figure 2. Left: Neutron interaction in boron-10 lined proportional counter.

Possible outcomes of a neutron interaction with a boron-10 atom of the layer: (a) α-particle is emitted in the direction of the proportional gas, with energy loss in the boron-10 layer; (b) ⁷Li is emitted in the proportional gas, with energy loss in the boron-10 layer. Right: Pulse-height distribution recorded with a boron-10 linned proportional counter, adapted from. Interactions of type (a) produce events with energies up to the α-particle threshold. Super-imposed on these are the type (b) events, with maximum energy corresponding to the ⁷Li energy.

Figure 3. Results of the Monte Carlo simulation for a single B₄C particle inside a proportional counter, for different particle radii.

Top panel: left - B₄C particle not in contact with the detector walls, energy deposited in the proportional gas by the ⁷Li and the α-particle; right - visualization of a neutron (not depicted) interacting in a 2-micron diameter B₄C particle (green) with production of an α-particle (red) and ⁷Li (blue). Bottom panel: left - particle in contact with the walls of the detector, either the ⁷Li or the α-particle deposit energy in the proportional gas; right - visualization of a neutron (not depicted) interacting in a 2-micron diameter B₄C particle (green) attached to the chamber walls (the ⁷Li is absorbed by the wall).

Figure 4. Pulse-height distributions of neutron induced pulses recorded with B₄C nanoparticles, for different gate biasing voltages.

Pulse-height distributions are normalized to the full energy peak. Red: V_GATE = 150 V. Blue: V_GATE = 50 V. Green: V_GATE = −50 V. Grey: V_GATE = 50 V, neutron beam off (background). Gate blocking efficiency for −50 V was 0.62. V_ANODE = 1850 V. Acquisition time = 400 sec. Neutron flux was 7 × 10³ n/sec.

Figure 5. Left: Schematics of the proportional counter used in this work.

Three ceramic pillars, glued to the top flange of the detector, serve as support for the top and bottom fixing rings. The anode wire was soldered to the SHV connector on the top flange, stretched and fixed to the centre of the bottom ring. Only one of the 18 field cage wires is depicted. Right: cross section view. The positions of the 18 field cage wires are depicted, as well as the bottom fixing ring, where the anode wire is glued to.