Imaging and location of fast neutron emissions by real-time time-of-flight

The proposed research comprises a study into a different way in which to use neutrons to characterise nuclear environments. The proposed project is a collaboration led by the Department of Engineering at Lancaster University with expertise from the Culham Centre for Fusion Energy. Neutrons make up approximately half of the subatomic particles that constitute the nuclei of matter around us. They are uncharged and therefore do not interact strongly with the environment in comparison with other common forms of radiation such as gamma rays, protons electrons etc. However, they are emitted as a result of nuclear reactions in reactors and particle accelerators and are thus present in many environments associated with truly global applications of nuclear technology such as nuclear power, propulsion, the prevention of nuclear proliferation and nuclear medicine. Fortunately, because fast neutrons do not travel at the speed of light in the same way that gamma rays do, they are readily separated from gamma rays on the basis of how long they take to travel a set distance. This approach is widely referred to as 'time-of-flight' (ToF).

Exploiting the difference between the speed of fast neutrons and gamma rays on the basis of their time-of-flight is currently limited to rather esoteric applications such as finding the energy of neutrons at standards laboratories and particle accelerator facilities, for example at NPL and nTOF (CERN), respectively. Whilst this application is very important in the specific field of radiation metrology, wider application of the technique to infer the location of radioactive substances and the distribution of radiation emissions in industry and medicine has not yet been achieved.

The focus of this proposal is to invert the widely-accepted ToF approach referred to above (by which the usual objective is to estimate the energy of neutrons) to see if we can locate and image the origin of fast neutrons in environments where they arise. The hypothesis at the focus of this proposal is that, based on our prior knowledge of the energy distribution of neutrons in typical environments of interest (such as power reactors and medical facilities), is to determine whether it is possible to obtain an estimate of the distance from the site of detection to the site of neutron emission. This information might then be used to locate and potentially image a neutron-emitting system or substance; an Engineering use of the time-of-flight method that has not been explored before. This research will thus determine whether time-of-flight, performed digitally and in real-time, can be used to indicate the location and to potentially image a source of neutron radiation. The key objective of this research is to determine whether the ToF approach can be applied as an Engineering capability that will have a range of potential applications in industry and medicine. These include for example: preventing the theft of nuclear materials (nuclear safeguards), nuclear reactor characterisation, nuclear security and the characterisation of neutron fields that result from the use of protons in cancer therapy.

We envisage six areas of impact: nuclear security & safeguards; proton therapy; 3He replacement; nuclear reactor monitoring & accident recovery; nuclear decommissioning and neutron metrology and fusion. These are all of exceptional relevance at the current time for both the UK research community, UK society and UK industry & commerce. They are also areas that are, without exception, of relevance to the global community in this field at the present time.

For example:

- In the nuclear security and safeguards area significant impact is plausible in the area of improved monitoring and characterisation for nuclear materials. With many more power reactors being built in a widening international nuclear power community, mixed with a variety of reactor designs and new concepts such as small modular reactors etc., there is an increasing need to safeguard the international transfer of fuel against the risk of proliferation. This research will advance the characterisation of fast neutrons from fission reactions which are often the sole basis for the detection of these materials via, for example, 240Pu spontaneous fission or stimulated fission in 235U.

- Proton and hadron therapy are highly topical at the present time due to the significant potential they hold for the treatment of cancer, especially in children. However, the high-energy protons used in these therapies can yield a significant neutron flux that, if unchecked, can be a source of dose to both patients and clinicians. This research will advance the spatial characterisation of fast neutrons in these environments.

- Virtually all helium-3 detectors currently in use in the detection of neutrons for security, safeguards and plutonium stockpile monitoring will have an alternative technology identified as a replacement due to supply constraints on 3He; this research will impact this need. This area clearly has significant commercial and societal relevance, especially in the UK as we hold the largest civil plutonium stockpile currently reliant on 3He monitoring.

- Reactor monitoring and accident recovery has been cast into the public eye relatively recently as a result of the effects of the earthquake and subsequent Tsunami on the Fukushima Daiichi reactors in Japan. This research will advance current capabilities in the spatial characterisation of fast neutron fields that is highly relevant to monitoring containment environments in both pressurised water reactors and boiling water reactors. It is also relevant to reactor monitoring during all anticipated phases of operation i.e. steady-state operation, shutdown and post-accident.

- Nuclear decommissioning remains an important and significant requirement for many countries that have legacy facilities from past nuclear activities. This research will benefit decommissioning activities where the presence of nuclear material is suspected; for example where fuel waste residues are present in process systems and aged reactor systems.

- Neutron metrology and fusion are important areas for this research since it builds directly on current neutron metrology capabilities and offers the potential for a new metrology service by which environments associated with these requirements might be characterised in terms of the neutron distribution in space as well as energy.