Calculation of the pulse height distribution induced by fast neutrons in a scintillation detector

The probability distribution of light pulses produced in an organic scintillator by impinging fast neutrons is calculated. It is assumed that the slowing down takes place on a mixture of hydrogen and carbon atoms, and that the light produced is a non-linear function of the energy transferred to the H atoms and a linear function of the transferred energy to C atoms in each individual collision. The light pulses from all collisions of one neutron add up. Due to the non-linear relationship between energy transfer and light production, neutrons with the same total transferred energy (after several collisions) but with a different collision history generate different amounts of light. The pulse height distribution of n-times collided neutrons is calculated for the few first values, i.e. n = 1 - 4. The results are compared with Monte-Carlo simulations and an excellent agreement is observed. Due to the limitations in transferred energy, the results for collisions with only carbon are of less interest, hence the main emphasis is put on the light generation in collisions with H atoms. The treatment for slowing down in a mixture of H and C atoms is also outlined and solved analytically. (authors)