Analysis of epithermal neutron production by near-threshold (p,n) reactions

A recent advance in portable accelerator neutron source development was research on production of epithermal neutrons by near-threshold charged-particle reactions. When the projectile energy is accurately controlled at an energy close to the reaction threshold, the neutrons produced will have energies less than or around 100 keV and can be used with little or no moderation or filtration in neutron capture therapy. Although the total neutron yield is lower than at higher proton energies, the epithermal neutron flux may be sufficiently intense because of the softer energy spectrum and the requirement for less neutron moderation. This paper presents an analysis of the main characteristics of epithermal neutron production by this method using the Li (p,n) reaction as an example. The energy, yield and angular characteristics of neutron emission are discussed. The achievable epithermal fluxes are computed from experimental data. The results are used to assess the feasibility of near-threshold production of epithermal neutrons for neutron capture therapy with compact accelerators such as a RFQ proton acceelerator. The results indicated that, using a Li₃N target, 1 mA of 2 MeV protons will produce 10⁹ n/cm²/s with an average energy of 83 keV while 5.6 mA of 1.91 MeV protons can produce 10⁹ n/cm²/s with an average energy of 45 keV.