Multi Wire Proportional Counters for Neutron Detection

Introduction: For Home-Land Security (HLS) applications, neutron detection plays a prominent role in the interdiction of illegal traffic of special nuclear material (SNM). To detect neutrons emitted by SNM, high intrinsic efficiency sensors with large sensitive area are required at reasonable prices. Generally, neutrons are detected through reactions in converter materials. The required characteristics of these materials are high interaction cross-section and efficient gamma discrimination. Gas proportional detectors based on ³He converters possess these qualities, but due to low production and excessive usage of this isotope in the last two decades, the global reserves have decreased extremely, and his price has sharply raised. As a practical alternative to ³He, in this research we study the use of neutron converters based on coating materials, such as Gadolinium (Gd) or Boron-Carbide (¹⁰B4C), ¹⁰B enriched.

Methods: In this study, we are developing a Multi Wire Proportional Counter (MWPC) for neutron detection. The detector housing is a flat aluminum box that serves as a cathode. The positive electrode consists on a grid of 25 µm stainless-steel wires located in the median plane of the detector. These wires are thin enough to allow effective Townshend avalanche amplification. By physical Vapor Deposition (PVD) methods, a thin layer of a converter materials is deposited in the inner volume of the detector. Except some resonances above 100 keV, these converters demonstrate a 1/v cross-section dependence. A polyethylene moderator in front of the detector increases the intrinsic efficiency. The reaction products of the neutrons absorbed in the reactive coating are charged particles. These particles, such as ⁷Li or ⁴He nuclei from ¹⁰B (n, α)⁷Li reactions or electrons from natGd (n, γ + e)Gd reactions ionize a sealed gas, producing fast charge pulses in the order of ~10-13 Coulomb. The Pulse processing circuitry includes a comparator with selectable threshold to discriminate gamma background from neutron pulses.

Results: The large self-energy absorption of the reaction products in the coating material drastically limits the effective thickness of the converter. Using MCNP and SRIM codes for neutron and reaction products interaction in the converter materials we optimize the converter thickness of each type of material. Simulation results show optimal thickness of nearly 1 um for Boron-Carbide converters and 5 um for Gadolinium. The calculated efficiencies to thermal neutrons are nearly 5% for B4C and 10% for Gd. Since Gd reaction products are low energy electrons, poor gamma discrimination restricts the range of applications of these types of converters.

Conclusions: Performed simulations and preliminary experimental set-up models, shows that MWPC’s that include several layers of converter materials, can be an effective alternative to ³He for portable neutron detection instrumentation.