There is now strong evidence, from observations of directional correlation between the α-particles and neutrons of fission, for the release of 5He as a neutron-unstable light particle with a frequency of a few per 104 low-energy fission events (Cheifetz et al. 1972).

Because the energy spectrum of the neutrons liberated in the disintegration of 5He particles travelling with a fixed kinetic energy is a rectangular spectrum (assuming isotropy of disintegration directions in the centre-of-mass space), it is in principle possible to deduce the kinetic energy spectrum characterising the 5He particles of ternary fission at the moment of disintegration from the energy spectrum of the liberated neutrons.

Detailed calculations have been made in relation to the limitations of actual experimental arrangements with regard to this possibility. These limitations are shown to be severe—not least in respect of statistical accuracy. Indeed, it is shown that considerable difficulty attaches to the ‘simple’ evaluation of the 5He/4He ratio (which Cheifetz et al. estimate as 1 :(8 ± 2) for 252Cf, for α-particles of energy greater than 9 MeV).

An instrumental cause is suggested for the maximum reported by Cheifetz et al. in the energy spectrum ascribed to the 5He disintegration neutrons.