Historical Notes Related to NDP
First recorded observations of B-10 and Li-6 reactions with neutrons
Disintegration of boron-10 by slow neutrons was observed more than 50 years ago. In the first experiments, with Chadwick, we observed the disintegration with an ionization chamber (see Figure 1). Because of a discrepancy with the nuclear masses as known at that time, we thought that boron-10 bombarded with slow neutrons would disintegrate into three particles [see note below], because the energy seemed too low for a two-body disintegration. Fermi and collaborators found the same reaction a short time later and, not worrying about masses, suggested that only two particles are emitted. Later, with H.J.Taylor, we used photographic emulsions to decide whether two or three particles are emitted. We found only two particles, confirming Fermi's guess (see Figure 2). It needed fast neutrons to produce three particles. The energy missing in this reaction gave us an important hint, leading to a revision of the nuclear mass scale•
Note about disintegration of 10B into 3 particles. There is indeed a minor reaction cross section (several millibarns) for boron-10 to "disintegrate" into two alphas and a helium-3 particle. [10B+n→3He+4He+4He] For details see the paper: https://journals.aps.org/prc/abstract/10.1103/PhysRevC.39.1633
The emulsion image of 10B(n,a)7Li reaction found in the NATURE paper immediately above.