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.
Authors: Aspinall Michael , Binnersley Cory , Bradnam Steve , Croft Stephen , Joyce Malcolm , Packer Lee , Wild Jim 
Published: March 2022

&lt;p&gt;The global network of neutron monitors comprises predominantly of the monitor standardised by Carmichael in 1964, the NM-64.&amp;#160; The design of these existing monitors and their instrumentation have changed very little over the last sixty years. &amp;#160;For example, their neutron detectors rely on gas filled proportional counters that are either filled with highly toxic boron trifluoride (BF&lt;sub&gt;3&lt;/sub&gt;) or helium-3 (&lt;sup&gt;3&lt;/sup&gt;He).&amp;#160; Almost the entire global supply of &lt;sup&gt;3&lt;/sup&gt;He 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 &lt;sup&gt;3&lt;/sup&gt;He has become limited.&amp;#160; Consequently, &lt;sup&gt;3&lt;/sup&gt;He supply became strictly controlled in 2008 and its price has fluctuated since.&amp;#160; In some cases, new neutron monitors have reverted to BF&lt;sub&gt;3&lt;/sub&gt; filled counter tubes when the price of &lt;sup&gt;3&lt;/sup&gt;He has been at a premium.&amp;#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.&amp;#160; The reduced production and increased demand for &lt;sup&gt;3&lt;/sup&gt;He 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.&amp;#160; One of these alternative technologies is based on boron-coated straws (BCS) manufactured and supplied by Proportional Technologies, Inc (PTI). &amp;#160;The technology is built on a patented low-cost technology that enables long copper tubes, known as &amp;#8216;straws&amp;#8217;, to be coated on the inside with a thin layer of &lt;sup&gt;10&lt;/sup&gt;B-enriched boron carbide (&lt;sup&gt;10&lt;/sup&gt;B&lt;sub&gt;4&lt;/sub&gt;C).&amp;#160; Thermal neutrons captured in the &lt;sup&gt;10&lt;/sup&gt;B are converted into secondary particles, through the &lt;sup&gt;10&lt;/sup&gt;B(n, &amp;#945;) reaction.&amp;#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 &lt;sup&gt;10&lt;/sup&gt;B.&amp;#160; Multiple straws can be packed inside a 1&amp;#8221; diameter aluminium tube acting as a single drop-in replacement for traditional &lt;sup&gt;3&lt;/sup&gt;He detectors or individually distributed directly throughout the moderating medium, thus increasing efficiency by detecting the thermal neutrons at the point that they are created.&amp;#160; BCS-based detectors are widely used in systems for homeland security, safeguards and neutron imaging in direct exchange for &lt;sup&gt;3&lt;/sup&gt;He tubes.&amp;#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.&amp;#160; Monte Carlo simulations using the MCNP radiation transport code to model several BCS-based solutions and an NM-64 computational benchmark are reported.&amp;#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 &lt;sup&gt;252&lt;/sup&gt;Cf, AmLi and &lt;sup&gt;137&lt;/sup&gt;Cs sources.&amp;#160; The PTI-110-NDME consists of a 12&amp;#8221; x 5&amp;#8221; x 1 m high density polyethylene (HDPE) slab with thirty ~15-mm diameter straws, 93 cm active length, embedded uniformly throughout the moderator. &amp;#160;Funded by UK Research &amp; Innovation (UKRI), this research is part of the Space Weather Instrumentation, Measurement, Modelling and Risk (SWIMMR) programme.&lt;/p&gt;