Promising plastics and neutron detection

Posted by () on Feb 16 2012
VERTIC Blog >> Arms Control and Disarmament

Hugh Chalmers, London

For those attempting to detect sensitive fissile materials, the nature of their quarry creates significant obstacles to their hunt. Issues relating to safety, security, secrecy and size all work against nuclear inspectors, and are all unavoidably the result of the items of concern. The radiation produced by some materials, and the potentially devastating uses of others, requires such a high level of material isolation that direct interaction by nuclear inspectors is highly unlikely. This makes finding these materials somewhat like looking for a needle in a haystack, without the ability to search through the haystack. And the consequences of missing the needle can be huge. This problem was not over-stated by the former International Atomic Energy Agency (IAEA) chief Mohamed ElBaradei when he said: ‘Either we begin finding creative, outside-the-box solutions or the international nuclear safeguards regime will become obsolete.’ Thankfully, recent advances in radiation detection technology show that this call has not gone unanswered.

Scientists in the US have recently developed a material that can help sift through the large quantity of radioactive materials presented during nuclear safeguards inspections and border screenings, identifying any that might be of concern. The team, from Lawrence Livermore National Laboratory (LLNL), have designed a new plastic which emits a characteristic glow when exposed to atomic particles known as neutrons. These miniscule, electrically-neutral particles are radiated from materials by the same fission process which, when forced into a chain reaction, creates a nuclear explosion.

Unlike other forms of radiation, such as gamma rays, significant quantities of neutrons are a convincing indication of the presence of fissile, rather than simply radioactive, materials. According to their published results, the scientists have been able to design the plastic so that these characteristic particles create a recognisably different glow to that of other, less specific, forms of radiation. While there are a number of other detectors capable of similar discrimination, all have their drawbacks. What then could this new plastic do for the international nuclear safeguards regime?

Safeguarding the atom
The international safeguards regime in its role verifying the peaceful uses of nuclear energy focuses most of its concern on two materials; uranium and plutonium. As the two principle materials involved in creating nuclear explosions, these materials also enjoy the attention of nuclear arms control inspectors and border control authorities. These nuclear investigators are assisted in their search by the very characteristics that make these materials so dangerous; both contain a relatively large number of neutrons. For example, while a hydrogen atom contains no neutrons, and only one proton and one electron, the most common isotope of uranium contains 146 neutrons, and 92 protons and electrons.

These neutrons act as a type of glue, keeping all the positively-charged protons together in the atom’s nucleus when they would normally violently repel each other. However, this glue is not particularly stable. Over time, or if more neutrons are added, this ‘heavy’ atom can split into two lighter, more stable atoms. This process of fission releases a large amount of energy in the form of both gamma rays and spare neutrons. Usefully, detecting these two products and measuring their rate of emission can indicate the presence of fissile material and, given a knowledge of the material, a guess at its quantity.

Nuclear investigators therefore rely heavily on detecting these emissions when inferring the presence of fissile materials. Neutron detectors in particular are used extensively throughout the international nuclear safeguards regime, the verification protocol of the New START agreement and at large ports and border crossings. These detectors can vary enormously in size, shape and operating technique depending on the task at hand. For example the IAEA manual on safeguards techniques and equipment lists over 30 different neutron detectors for measuring both fresh and irradiated nuclear fuel in its various forms.

From principle to practice
This variety however indicates just how difficult it can be to make reliable and safe inferences regarding the presence of fissile materials with neutron detectors. Although neutrons can penetrate through relatively thick shielding, their range is not infinite. Establishing just how these detectors should be used, at what distance, and for how long requires a huge amount of forethought. Placing the detector too close to materials could present a safety hazard to the operators, or even a security hazard to the owners. Placing it too far away could prevent the operators from detecting the material at all. This is why the IAEA has developed such a range of detectors, including remote detectors which can operate under water and close to fuel stocks without risking dangerous exposure. This is also why the New START verification protocol annex dedicates over 20 pages, or nearly one-third of the entire document, to establishing in minute detail exactly how these detectors should be used.

Compounding these procedural issues are two important technical problems. Firstly, the neutron ‘glue’ holding atoms of uranium together is far stronger than that which holds atoms of plutonium together. Uranium atoms do not spontaneously split often enough to produce a detectable number of neutrons. This process has to be carefully induced by adding a destabilising number of neutrons to the atom. For New START inspections this is particularly problematic. Inducing fission in warhead materials, while not as disastrous as you might think, still presents an unacceptable level of risk for hosts. As such this practice is currently not allowed in the New START agreement.

Secondly, although there are very few processes beside fission that produce energetic neutrons, gamma rays are far more common. These are frequently emitted by materials that, while being radioactive, may not be fissile. An interfering background of gamma rays is often built up in nuclear facilities by day-to-day operation and the presence of other radioactive materials. In large enough quantities, even cat litter contains enough radioactive thorium to emit an interfering number of gamma rays. This is why although the IAEA list over 30 types of neutron detectors, the majority utilise just one technique for detecting neutrons; the neutron-induced creation of charged particles in pressurised helium-3 gas.

Looking into gas detectors
In general, gas detectors operate by transferring the energy of incoming neutrons to atoms of gas contained within an electric field. This field acts on the gas atoms in a manner which strains the bonds which hold the charged electrons and protons together. If an incoming neutron is able to transfer enough energy to this atom, these bonds can break. If this happens, the electrical balance between protons and electrons becomes disturbed enough for the particle it to have an overall electric charge. In this case, the electric field then pulls the charged particle towards the edge of the container. As it does so, it impacts upon any other particles which lie in its path, creating yet more unbalanced particles which are similarly pulled to the edge of the container. This chain reaction creates a detectable electrical signal in the container indicating the initial neutron interaction.

Unfortunately, gamma rays are also capable of disturbing the electrical balance in the gas particles. As most nuclear materials emit ten or more times as many gamma rays as neutrons, this can be a real problem. In the case of measuring highly radioactive spent nuclear fuel, the interaction of gamma rays on the gas can be so frequent that no indication of neutron interaction is discernable in the flood of gamma-induced electrical signals. There are two ways around this. The first is to place shielding around the detector which block the passage of gamma rays while permitting neutrons to pass through. In the presence of gamma rays and neutrons of comparable energy, only 5cm of lead is needed to absorb 90% of the incoming gamma rays and only 0.1% of the neutrons. The second is to carefully choose the gas in the chamber. This is why detector designers turn to the helium-3 isotope. While there is an approximately 70% chance a neutron will interact with a helium-3 atom, there is only a 0.01% chance a gamma ray will interact.

So why are scientists developing novel ways of reliably detecting neutrons? Although helium-3 is extremely effective, it has to be stored in the detectors under high pressure and in the presence of extremely strong electrical fields. While the isotope is thought to be abundant on the surface of the moon, it is rarely found naturally on earth. The main source of helium-3 actually comes from the dismantlement of old nuclear weapons. Tritium, which is used to help trigger nuclear explosions, decays over time into helium-3 which is then extracted at dismantlement. However, the rate of warhead dismantlement is not fast enough to keep up with the demand, which has grown in line with the increased demand for border security after the 9/11 attacks. Consequently the price of helium-3 has soared by approximately 2000% over the last few years. There is clearly a gap in the market for cheap, effective and discriminating neutron detectors.

A winning combination
With funding from the US National Nuclear Security Administration, the scientists from LLNL have taken a significant step towards filling this gap. Working with a different neutron detection technique, known as scintillators, the team have developed a plastic which is cheap, easy to produce and, like helium-3, capable of distinguishing between neutrons and gamma rays.

Unlike gas detectors, scintillators indicate the presence of radiation by absorbing radiation energy and emitting it as light. This light can then be translated into an electrical signal which can be easily interpreted. Importantly, some scintillating materials react differently to gamma rays and neutrons, producing bursts of light that last for different durations. If this difference is pronounced enough, it is possible to discriminate between the components of light caused by gamma rays and neutrons.

A number of scintillating materials currently exist which can do this, but these are either expensive, rare, toxic, or flammable. While cheap and stable plastic scintillators are widely used at border controls, these cannot discriminate between gamma rays and neutrons and are mainly used to detect gamma rays only. After experimenting with various scintillating dyes embedded in molecular structures, the LLNL scientists have hit upon a winning combination. By embedding a compound called polyvinyltoluene (more commonly referred to as PVT) with a scintillating dye named 2,5-diphenyloxazole (again, more commonly referred to as PPO), they were able to achieve an efficient and usable level of neutron-gamma ray discrimination.

Promising plastics
At this early stage of development, it is hard to determine exactly what impact this advance will have on the use of neutron detectors in the international nuclear safeguards regime. However, it is clear that this new material shows a great deal of promise. As a plastic it is cheap and easy to fabricate into any number of shapes and sizes, making it applicable for fissile material detection at all scales, from small portable IAEA safeguards inspections to large fixed cargo monitors.

The latter example is particularly relevant as international efforts to prevent the illicit transfer of nuclear materials grow in strength. The United Nations Security Council Resolution 1540 requires all states to ‘develop and maintain appropriate effective border controls and law enforcement efforts to detect, deter, prevent and combat the illicit trafficking of [nuclear materials]’. Given the global decline in helium-3 stocks, new plastic neutron detectors may become the only realistic way of achieving this. Advances such as this could well be one of the ‘creative, outside-the-box solutions’ needed to keep the international nuclear safeguards regime from becoming obsolete.

Last changed: Feb 16 2012 at 7:20 PM




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