Neutron radiography at Birmingham University
In the Dynamitron accelerator of the Physics Department, University of Birmingham, proton currents in excess of 1 mA have been achieved on a natural lithium target at a proton energy of 2.8 MeV and this results in the generation of 1.37x10exp12 n.s-1 from the 7Li(p,n)7Be reaction.
7Li + p → 7Be + n - 1.646 MeV
The threshold energy of the proton for this reaction in 1.881 MeV. At this energy, neutrons are emitted in the forward direction with an energy of 30 keV.
At a proton energy of 2.8 MeV, the maximum kinetic energy of the neutron (emitted in the forward direction - zero degrees) is 1.2 MeV and it is 0.82 MeV when emitted at 90 degrees.
These results can be compared with the yield from the Oxford Instruments superconducting magnet cyclotron for neutron radiography (developed for a project in the 1990s). This produced 4.2x10exp12 n.s-1 from a 12 MeV proton beam (100μA) on to a beryllium target. However, at this proton energy, the peak neutron energy in the forward direction was ~ 10 MeV and mean neutron energy ~ 5MeV. This very much higher neutron energy compared with that achieved with the 2.8 MeV p, Li reaction results in a longer slowing down distance to thermal energy, lower relative thermal neutron peak flux in the moderator and consequently, a larger thermalisation factor.
This source would be very suitable for thermal neutron radiography. If one assumes a figure of 50 as the thermalisation value (converting source strength n/s into peak thermal neutron flux in a water or polyethylene moderator) this would correspond to ~ 2.7x10exp12 n.s-1 for the peak flux.
Introducing a beam hole collimator incorporating neutron absorber materials into the region of this peak flux will reduce the source strength by another factor that would have to be calculated with MCNP.
The geometry of the collimator required to obtain a thermal neutron beam at the position of the imaging plane would need to be included in a Monte Carlo calculation. This would include:- circular neutron aperture area at the position of peak thermal neutron flux, L/D of collimator, length of collimator and amount of neutron absorber material lining the collimator.