Navy

Improved helium-3 gas counter for detection of nuclear material

Increased efficiency, reduced consumption of helium-3, smaller size, and reduced costs

Sensors

In the upper depiction of the conventional counter, pressure is difficult to optimize. Low pressure leads to poor detector efficiency. High pressure leads to wall losses. If undermoderated, there will be inefficient neutron capture. The lower proposed counter optimizes helium-3 pressure, reduced wall losses, increases gas density for better detector efficiency and is fully moderated for the capture of thermal neutrons.

Helium-3 is the ideal gas to interact with neutrons to create an ionizing event for detection of these charge neutral particles. As such, large-area, helium-3 gas proportional counters are critically important tools for detecting and monitoring special nuclear materials during operation of nuclear sites, regulated transport of nuclear material, and for detection during nefarious transport by terrorists. While the demand for Helium-3 gas counters continues to increase, the availability of the most critical component, the gas itself, is diminishing. Since 2008, there has been a global shortage of helium-3.

Since it is unlikely that production of this gas (which is a byproduct of the radioactive decay of tritium used in nuclear weapons programs) will be significantly increased, researchers are working on ways to use it more efficiently. In this effort, Navy scientists have developed improvements to the design of existing helium-3 gas counters which significantly lower the amount of gas used in detectors. This has the added benefit of reducing the cost of the counters and broadening their deployment.

In practice, the design provides for replacement of the commonly used cylindrical tubes with a single rectangular gas tube. Cylindrical geometry is not optimum with respect to minimization of the wall effect (when the detection occurs close to the detector wall, one of the products ends up absorbed in the wall) because the cylindrical geometry has a relatively large differential element of volume near the tube surface. Additionally, the rectangular cross-section eliminates the air gaps in the polyethylene moderator which surrounds the gas tube as this material readily mates to the faces of the rectangular structure. This improvement in the moderator reduces losses of diffusing thermalized neutrons and improves the detector efficiency.

Further, this approach allows for the use of significantly lower gas pressures. This is done by the introduction of an additional gas, xenon, that ensures high overall density. The use of a high-density gas mixture improves the efficiency of the detector because it is possible to independently adjust the pressures. The helium-3 pressure can be independently adjusted (lowered) to an optimal value that provides for more uniform neutron capture throughout the entire volume of the tube. Independent optimization of the pressure of the helium-3 and the xenon gases provides significantly improved detector efficiency.

Comparative example: a currently deployed counter system has a hollow cavity with rectangular dimensions of 36″×15″×2″ and uses two tubes, each 2″ diameter by 36″ long with a combined volume of about 3.7 l. The gas pressure is 2 atm.

A rectangular replacement counter of about the same volume would have dimensions of 15″×36″×1 cm. This tube would be in a cavity with no air gaps and optimized polyethylene thickness. Monte Carlo N-Particle calculations show a two-fold or greater reduction in the pressure of the helium-3. Calculations further show that a two-fold reduction in the area of the gas tube (7.5″×36″) is achieved.

Given that currently fielded portal monitors (TSA VM-250AGN) use two counters per pillar and two pillars per system, the total cost savings could be over $30,000 for each portal monitor system, depending on the price of helium-3.

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