Doppler broadening of the n-p capture γ line

The Doppler broadening of photons emitted during the n-p capture process in a nuclear reactor is considered in view of a new finding concerning the zero-point motion of the H atom in the sample. The kinetic energy of the H atoms of the sample was measured directly at 296 K and found to be very high, corresponding to an effective temperature of 991 K, attributable mainly to the zero-motion of the H atom. The measurement was carried out using the pulsed neutron source (ISIS) at the Rutherford Appleton Laboratory (U.K.). A simple expression for calculating the Doppler broadening of the emitted photons in terms of the kinetic energies of the neutron and the proton is deduced. The expression is applicable to any capturing nucleus and not just to the proton.