The invention relates to neutron spectrometers.
One of the most challenging tasks that have been set for radiation detectors in the field of Homeland Security is the reliable detection of nuclear weapons and special nuclear materials which may be concealed in cargo crossing international borders. Whilst roughly 1 to 2% of all cargo vehicles carry innocent radioactive materials, typically one in 104 of these are found to be carrying a neutron source. Neutron sources find applications for example in the oil and gas exploration industry, soil moisture gauges, materials analysis and radiography.
A neutron source may be provided by accelerating deuterium ions towards an electrode that has been loaded with tritium. Whilst such sources of neutron radiation can be controlled, other neutron sources are based on radioactive materials which can provide a significant hazard. For example, intense alpha-particle emitters such as Americium-241 or Plutonium-238 are often combined with Beryllium or Lithium metal to provide a neutron source. These sources can be intense and represent a significant hazard in their own right, particularly if they are combined with explosives to provide a so-called “dirty bomb”.
FIG. 5 of the accompanying drawings schematically shows neutron spectra produced by various neutron sources. The graph shown in FIG. 5 is a plot of the number of neutrons counted in arbitrary units against energy in MeV. The neutron spectra for an intense alpha-particle emitter of Americium-241 combined with Lithium (AmLi) and Plutonium-238 combined with Beryllium (PuBe) are shown in FIG. 5 (lines 80 and 82 respectively). The figure also illustrates a neutron spectrum generated from a neutron source generated through nuclear fission (e.g., Californium-252), as illustrated by line 84 in FIG. 5.
It is evident from FIG. 5 that each of the neutron sources can be differentiated, because the relative number of neutrons at specific energy levels differs for each source. For example the AmLi source has a high count of neutrons at low energy (e.g. less then 0.5 MeV) and no neutrons having an energy greater than 2 MeV. However, PuBe has a low neutron count at low energies (e.g. less than 1 MeV) and a spread of neutrons having an energy between 1 and 11 MeV.