Disintegration of Light Elements by Fast Protons

J. D. Cockcroft and E. T. S. Walton

Editor’s Note

Here Cockcroft and Walton update their studies of how high-energy protons interact with the nuclei of light elements. Since their report of artificially induced disintegration of lithium, they had improved the sensitivity of their detecting apparatus and extended their study to the element boron. Here they record a huge number of alpha particles created by proton bombardment, suggesting a nuclear reaction in which the boron nucleus boron-11 becomes boron-12 on absorbing a proton, and subsequently disintegrates into three alpha particles.　　中文

SINCE the publication of our paper1 on the disintegration of elements by fast protons, we have examined some of the light elements more carefully, using much thinner mica windows than we had previously employed on the high voltage tube. With the present arrangement, we can count particles which have passed through only 6 mm. air equivalent of absorber on their way from the target to the ionisation chamber.　　中文

In the case of lithium, we have found, in addition to the α-particle group of 8.4 cm. range, another group of particles of much shorter range. The number of these is about equal to that of the long range particles and their maximum range is about 2 cm. The ionisation produced by them indicates that they are α-particles. It will be of interest to examine whether any γ-rays are emitted corresponding to the difference of the energies of the α-particles in the two groups, but on account of the smallness of the effect to be expected, a sensitive method will be necessary.　　中文

In the case of boron, the number of particles observed increases rapidly as the total absorption between the target and the ionisation chamber is reduced. The maximum range of these particles is about 3 cm. and in our earlier experiments we determined the number of particles only after passing through the equivalent of 2.9 cm. of air, so that we were very nearly at the end of the range. Decreasing the absorber to 6 mm. of air gives an enormous increase in the number of particles. In this way about twenty-five times as many particles have been obtained from boron as from lithium under the same conditions. We estimate that there is roughly one particle emitted per two million incident protons at 500 kilovolts. The ionisation produced by the particles suggests that they are α-particles, and the energy of the main group would support the assumption that a proton enters the B11 nucleus and the resulting nucleus breaks up into three α-particles. There also seem to be present a small number of particles with ranges up to about 5 cm.　　中文

(131, 23; 1933)

J. D. Cockcroft and E. T. S. Walton: Cavendish Laboratory, Cambridge, Dec. 22.

Reference:

1. Proc. Roy. Soc., A, 137, 229 (1932).