Boron-coated straw detector technology as an alternative to helium-3 and boron trifluoride based proportional counters for ground level neutron monitoring: a design study.

<p>The global network of neutron monitors comprises predominantly of the monitor standardised by Carmichael in 1964, the NM-64.&#160; The design of these existing monitors and their instrumentation have changed very little over the last sixty years. &#160;For example, their neutron detectors rely on gas filled proportional counters that are either filled with highly toxic boron trifluoride (BF₃) or helium-3 (³He).&#160; Almost the entire global supply of ³He is derived from a waste product of nuclear weapons programmes and, with the termination of such programmes and reducing nuclear weapons stockpile, the supply of ³He has become limited.&#160; Consequently, ³He supply became strictly controlled in 2008 and its price has fluctuated since.&#160; In some cases, new neutron monitors have reverted to BF₃ filled counter tubes when the price of ³He has been at a premium.&#160; Helium-3 filled proportional counters are also used extensively in radiation portal monitors deployed for homeland security and non-proliferation; objectives which have increased significantly over the last two decades.&#160; The reduced production and increased demand for ³He has led to concerns over its supply and provided the research motivation for alternative neutron detection methods which are viable in terms of sensitivity, stability and gamma-rejection for certain applications.&#160; One of these alternative technologies is based on boron-coated straws (BCS) manufactured and supplied by Proportional Technologies, Inc (PTI). &#160;The technology is built on a patented low-cost technology that enables long copper tubes, known as &#8216;straws&#8217;, to be coated on the inside with a thin layer of ¹⁰B-enriched boron carbide (¹⁰B₄C).&#160; Thermal neutrons captured in the ¹⁰B are converted into secondary particles, through the ¹⁰B(n, &#945;) reaction.&#160; The straws can be of various diameter (circa 4 mm to 15 mm), length (up to 2 m) and shape (round, star or pie) to increase the surface area of ¹⁰B.&#160; Multiple straws can be packed inside a 1&#8221; diameter aluminium tube acting as a single drop-in replacement for traditional ³He detectors or individually distributed directly throughout the moderating medium, thus increasing efficiency by detecting the thermal neutrons at the point that they are created.&#160; BCS-based detectors are widely used in systems for homeland security, safeguards and neutron imaging in direct exchange for ³He tubes.&#160; This study aims to design a neutron monitor utilising BCS technology that is cheaper, more compact and produces comparable results to the existing network of NM-64 monitors.&#160; Monte Carlo simulations using the MCNP radiation transport code to model several BCS-based solutions and an NM-64 computational benchmark are reported.&#160; These models are validated experimentally using a standard PTI portal monitor (PTI-110-NDME) to determine its efficiency, dieaway, deadtime and gamma rejection using a combination of bare ²⁵²Cf, AmLi and ¹³⁷Cs sources.&#160; The PTI-110-NDME consists of a 12&#8221; x 5&#8221; x 1 m high density polyethylene (HDPE) slab with thirty ~15-mm diameter straws, 93 cm active length, embedded uniformly throughout the moderator. &#160;Funded by UK Research & Innovation (UKRI), this research is part of the Space Weather Instrumentation, Measurement, Modelling and Risk (SWIMMR) programme.</p>