Neutron Detection Technology

The detection of atomic and subatomic particles such as neutrons is usually carried out using the signature produced by these particles while they interact with the surroundings.

The fundamental characteristics of the particles influence the nature of their interaction. The fundamentals of a neutron detector are illustrated in figure 1.
 

Figure 1. Neutron detection technology. Image Credit: dawn.jpl.nasa.gov.

Neutron detection is defined as the detection of neutrons using an aptly positioned detector. There are two main aspects, software, and hardware which help in effective and efficient neutron detection. Therefore, the type of neutron detector and parameters such as detector shielding, source-detector distance, and solid angle as well as the graphical analysis tools involved in the set-up of the detection system are the key aspects for neutron detection to be effective.

Neutrons are neutral in charge and resist direct ionization, and as a result, are more difficult to detect. They react with materials by way of elastic or inelastic scattering giving rise to various products.

The different approaches to neutron detection are set out below:

• Absorptive reactions – used to detect low energy neutrons which react by emitting ionized particles of high energy.
• Activation processes – Involve radiative capture or spallation in which neutrons react with absorbers and produce reaction products, which emit beta or gamma particles on decay.
• Elastic scattering reactions – used to indirectly detect high energy neutrons that transfer energy on collision with the atomic nuclei in the detector thus producing ions.

Research

In 2008, Clark C.W. detected individual neutrons using an optical method. The scientists also recorded the neutrons across a broad range of intensities. This neutron detector was based on light emission from hydrogen atoms during the absorption of neutrons by helium-3 atoms.

The tritium and hydrogen atoms produced on neutron absorption by a helium-3 atom can be excited using collisions with helium gas, which causes them to emit Lyman alpha light. This light emission is recorded by the new Lyman alpha neutron detector (LAND). Experts from the team claimed that this novel detector can detect both single and multiple neutrons, which the existing neutron detectors were not capable of doing, as they relied upon electrical discharges. Thus, the optical detection method was a huge success as it responds more rapidly compared to electronic detectors.

In 2011, Kansas State University researchers came up with an electro-optic neutron detector, which featured multi-wire gas tubes. The team from the university's Semiconductor Materials and Radiological Technologies Laboratory (SMART Lab) created the detector using thin, lithium fluoride saturated foam sections and boron-10 aerogels. Both lithium-6 and boron-10 are excellent neutron absorbers. By spacing out the placement of the two, the scientists created an efficient neutron detector.

Nuclear engineering expert, Steven Bellinger has worked in the SMART Lab in the field of neutron detector research since 2005. His work focused on enhancing solid-state micro-structured semiconductor neutron detectors, expanding them into bigger arrays. It also involves the development of more specialized devices such as a neutron spectrometer.

Considerable progress has been achieved in solid-state neutron detectors, which detect charged particles emitted by neutrons during interactions with converter materials. Recent research shows that 3-D solid-state devices that resemble a honeycomb structure with hexagonal holes and thin walls made up of silicon can have up to 45% efficiency in thermal neutron detection.

Current Applications

Neutron detectors are widely used in national security for threat detection, arms control, and nuclear material assay, and directionally sensitive neutron detectors can help detect nuclear threats with good accuracy. Studies show that advanced semiconductor detectors loaded with boron are capable of sensing incoming neutron direction vectors.

Devices like nuclear safeguards and portal monitors can detect fissile materials and they employ gas filled detectors. Helium proportional tubes have been found to be useful in thermal neutron detection for plutonium. Boron trifluoride (BF3)-filled proportional counters are inexpensive and capable of better gamma discrimination than helium detectors.

However, there are environmental and safety concerns about the usage of BF3, lithium or boron loaded plastic scintillators which can be produced in several different shapes for specific applications.

Semiconductor neutron detectors have been shown to have high efficiency in security applications, but their size is limited and they are also costly.

Several other types of neutron detectors find applications in different sectors. For example, radiation detectors are highly useful in medical imaging, oil well logging and also in the automotive industry.

Future Developments

Neutron detectors have had very slow growth over the past few decades. There is a compelling need for enhanced neutron detection caused by a technology gap over the years. Future studies need to recognize this need and exploit the latest advances in technology to build the next generation of super-efficient neutron detectors for various requirements.

Key improvements in neutron detection will be possible only if new detectors are more efficient, fieldable and applicable for a broad range of devices and applications. Future developments in neutron detection will likely include techniques such as DT-neutron detection and neutron spectrometry.

With more developments in this field, solid-state neutron detectors are likely to replace detectors based on gas ionization in many applications.

Future research studies related to neutron detection need to focus largely on improving the efficiency, cost-effectiveness, and eco-friendliness of existing neutron detectors.

Sources and Further Reading

• Massachusetts Institute of Technology – Helium-3 Neutron Proportional Counters.
• C. W. Clark, A. K. Thompson, M .A. Coplan, J. W. Cooper, P. Hughes and R. E. Vest, Observation of the n(3He,t)p reaction by detection of far-ultraviolet radiation. Presented at the 2008 March Meeting of the American Physical Society, New Orleans, La., March 10-14, 2008. paper B14.00010.
• Kansas State University – Smart science: Student research helps develop new radiation detectors.
• Nuclear Security Science and Policy Institute - G. Spence, "Directionally Sensitive Neutron Detector For Homeland Security Applications," M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2012).
• Fredrick Seguin. MIT, for the NIC team – Compact DD-neutron detector for OMEGA and the NIF.
• Nikolic R.J., Cheung C.L., Reinhardt C.E., and Wang T.F. Future of Semiconductor Based Thermal Neutron Cetectors. 2006. Barry Chin Li Cheung Publications. Paper 14.
• Danon Y, Clinton J, Huang K.C., LiCausi N, Dahal R, Lu J.J.Q., and Bhat I. Towards high efficiency solid-state thermal and fast neutron detectors. 2012 JINST 7 C03014 doi:10.1088/1748-0221/7/03/C03014.

This article was updated on 14^th February, 2020.

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