This paper describes an experiment to measure the yield of chromosome aberrations in human lymphocytes produced by a beam of predominantly 24 keV neutrons. The yield of dicentrics is linear with dose with a slope coefficient α of 0.821 ±0.031 dicentrics. cell-1.Gy-1. This coefficient is very close to that observed for fission neutrons of mean energy of 0.9 MeV. The main interaction of 24 keV neutrons with tissue is the production of protons of energy 24 keV and less which have a maximum range of 0.5 μm. Thus the experiment investigates the biological effectiveness of the track ends of higher energy protons. Previously inferred predictions of the relationships between the coefficient α and LET and between α and lineal energy, y, determined in 1 μm spheres, showed that analysis based on LET is much superior to that based on y. The findings are consistent with results using low energy X rays to investigate the biological effects of short range low energy electron tracks. Both experiments indicate that energy deposited in volumes of dimension much smaller than 1μm is relevant for explaining biological effects.