Neutron resonance spectroscopy (NRS) is a technique where neutrons are used as a probe for materials that are inaccessible to other techniques, for example samples within containers or behind shielding.
This technique capitalises on the weak interaction of neutrons with high-Z materials, commonly those that are opaque to X-rays. NRS requires a beam of neutrons with energies of 1-100 eV. Higher energy neutrons produced by laser interactions are moderated down to this energy. It has been shown that the directed neutron fluxes from laser driven sources (circa 109 n/shot in the forward direction) is sufficient for NRS.
Each isotope has a set of preferential absorptions of neutrons with certain energies – these are described as resonances. NRS exploits the characteristic neutron resonance energies to provide qualitative data on the elemental composition of samples.
NRS is a powerful technique for characterisation of nuclear fuel (fresh and spent) and other materials in the nuclear industry. Examination of potential fuel formulations for nuclear power is expensive and usually examined destructively, meaning that key failure modes could be missed. In addition, non-destructive test methods have the potential to reduce the requirement for ‘hot cells’ and remote manipulators used to handle radioactive materials safely.
Many neutron sources traditionally used for NRS (research fission reactors) are shutting down without being replaced, meaning that new technologies are essential. Lasers (including EPAC) are expected to be capable of NRS in the next decade. EPAC will help to drive the development of these techniques with its high repetition rate, providing higher average flux of neutrons compared to existing lasers.
Laser driven sources offer several advantages over spallation sources and other compact neutron sources for NRS techniques. In particular, they can produce extremely short pulses thereby reducing the flight path length required for time-of-flight analysis.