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First recorded observations of B-10 and Li-6 reactions with neutrons
GOLDHABER M (1986) Introductory remarks. In: Proceedings of a Workshop on Neutron Capture Therapy. Reports BNL-51994.
Edited by R. G. Fairchild and V. P. Bond. Upton, NY: Brookhaven National Laboratory, pp. 1—2
INTRODUCTORY REMARKS
M. Goldhaber
Brookhaven National Laboratory
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
CHADWICK, J., GOLDHABER, M. Disintegration by Slow Neutrons. Nature 135, 65 (1935).
https://doi.org/10.1038/135065a0
**A quote from the paper:**
“With lithium, the kicks observed were of two kinds, one due to doubly charged particles and one to singly charged particles. By covering the lithium target with aluminium foils we found that the singly charged particles had a maximum range of about 5.5 cm. in air, and that the range of the doubly charged particles was less than 1.5 cm. This suggests
that the particles arise from the reaction
3Li6 + 0n1 ==> 2He4 + 1H3.
From the masses of the nuclei concerned, an energy release of about 5 million electron volts is expected, and a range of the H3 particle which agrees well with that observed. In the case of boron, the majority of the particles appear to be doubly charged and to have ranges less than 5 mm. in air. The only reaction which appears to fit the facts is
5B10 + 0n1 ==> 2He4 + 2He4 + 1H3."
TAYLOR, H., GOLDHABER, M. Detection of Nuclear Disintegration in a Photographic Emulsion. Nature 135, 341 (1935). https://doi.org/10.1038/135341a0
**A quote from the paper:**
“It has been shown recently by Chadwick and Goldhaber, and by Fermi and his collaborators, that some light nuclei, particularly lithium and boron, are disintegrated by slow neutrons. In the case of boron, the mass-energy relations seemed best satisfied by assuming a disintegration into three particles. The simplest reaction, namely:
B10 + n1 ==> Li7 + He4
should, according to the accepted masses of the particles, release some two million e. volts more energy than is observed. Unless the existence of new isotopes, He6 or Li8, of improbably low masses, be assumed, no other disintegration into two particles would fit the mass-energy relations.”
The emulsion image of 10B(n,a)7Li reaction found in the NATURE paper immediately above.
SZILAKD, L. Absorption of Residual Neutrons. Nature 136, 950–951 (1935).
https://doi.org/10.1038/136950b0
**A quote from the paper:**
“At present, for various experimental reasons, the best choice for the chemical detection of an artificially-produced element seemed to be helium originating from boron according to the reaction.
B10 + n1 ==> Li7 + He4
In a closed copper vessel we bombarded the methyl ester of boron with neutrons. These were produced near the centre of the spherical vessel by the decay of radon, mixed with beryllium, and were slowed down by the hydrogen atoms of the ester and of the water surrounding the metal flask. In a first experiment, by the decay of 450 mC. of radon, sufficient helium was produced for a spectroscopic observation. During a second experiment, lasting seven weeks, we procured enough radon to allow 2,200 mC. of it to decay in our apparatus. This time we were able not only to observe spectroscopically the helium produced but also to measure it; we found, to an accuracy of about 20 per cent, 1.3 x 10-7 c.c. helium.”
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