Technology

The ¹⁰B(n,alpha)  reaction and detector design

The boron-coated straw (BCS) technology is built on a patented low-cost technology that places long copper tubes (straws) of variable diameter, coated on the inside with a thin layer of ¹⁰B-enriched boron carbide (¹⁰B₄C). Thermal neutrons captured in ¹⁰B are converted into secondary particles, through the ¹⁰B(n,α) reaction. The secondary ⁷Li and α (or ⁴He nucleus) particles are emitted isotropically in opposite directions with kinetic energies of 1.47 MeV and 0.84 MeV, respectively (dictated by the conservation of energy and momentum). For a boron carbide layer that is only 1 μm thick, one of the two charged particles will escape the wall 78% of the time, and ionize the gas contained within the straw. Each BCS is operated as a proportional counter, with the tube wall acting as the cathode, and a thin wire tensioned through its center serving as the anode electrode, operated at a high positive potential. Primary electrons liberated in the gas drift to the anode, and in the high electric field close to the anode, avalanche multiplication occurs. This avalanche delivers an amplified charge on the anode wire, which is the detected signal. Standard charge sensitive preamplifier and shaping circuitry are used to produce a low-noise pulse for each neutron event. Gamma interactions in the wall produce near minimum ionizing electrons that deposit a small fraction of the energy of the heavily ionizing alpha and Li products. Gamma signals are effectively discriminated with a simple pulse height threshold.

The detector design, as just discussed, consists primarily of a BCS inside an aluminum tube with a tensioned wire down the shared axis of the tube and BCS. Endcaps are laser welded onto the tube to form a hermetic seal, and the working gas that supports the avalanche multiplication is flushed into the tube using the gas port that is then sealed. In comparison to ³He-based detectors, which require multiple atm of pressure to maintain high sensitivity, 1 atm or less is sufficient for any BCS sensitivity. For cases where greater sensitivity is required in a BCS-based detector, BCS with unique cross sections and/or multiple straws can be placed inside a single aluminum tube, effectively increasing the density of ¹⁰B inside the tube.

The impact of BCS detectors

BCS detectors can be similar in form to ³He detectors, facilitating straightforward 1:1 replacement of ³He tubes, while preserving the existing housing, amplifier and high voltage supply. Yet BCS detectors improve upon ³He in several ways, making them significantly more versatile in their applications:

• Naturally, BCS detectors that rely on ¹⁰B, which is abundantly available on the commercial market, will be significantly less expensive. Not only does BCS technology employ a more abundant resource than ³He, but it is also safer and more reliable in the long term compared to other ³He replacement technologies. BCS are safer to work with than manufacture than ¹⁰BF₃ gaseous detectors while avoiding many of the issues that ¹⁰BF₃ detectors have in common with ³ B₄C enrichment with ¹⁰B is more environmentally friendly than ⁶Li enrichment for 6Li scintillators and doesn’t have the same detector aging issues that ⁶Li scintillators do.
• BCS detectors can reach significantly smaller sizes, because the secondary particles resulting from neutron capture in ³He travel significantly longer distances than for 10B. When those secondary particles collide with the detector wall, they cannot deposit their energy into the counting gas and therefore the signal is not counted for that event.
• BCS detectors have an advantage in multiplicity or coincidence counting for identification of special nuclear materials. Only one of the secondary particles will escape the wall and be counted, as compared to ³He detectors which capture neutrons in the gas volume of the detector, ensuring by definition that both secondary particles will be counted. In this way, BCS detectors have significantly reduced error on the timestamp associated with each neutron event, making it easier to measure neutron multiplicity.
• BCS do not require high-pressure working gas to operate efficiently, which makes them safer, easier to work with, and significantly lighter than their ³He counterparts. BCS detectors are straight forward to transport and, should catastrophic failure occur, they fail safely without explosion. BCS require no maintenance of any high pressure and high purity working gas as in a ³He detector. BCS do not need thick metal walls to maintain pressure and prevent leaks, which makes it possible to group the detectors closer together for improved spatial coverage and sensitivity.

Boron Coated Straw Production

Learn more about PTI's automated production of Boron Coated Straws (BCSs) in the video.