Published online by Cambridge University Press: 24 October 2008
The object of the present paper is to report upon a case of neutron-produced disintegration of a type which has not previously been observed. The nucleus disintegrated was that of carbon and the peculiarity of the disintegration lies in the fact that three heavy particles resulted from the transformation. Hitherto no example of such disintegration resulting in more than two heavy particles has been obtained, and with carbon, in particular, several experiments agree in showing that this more usual type of disintegration is very rare indeed. In similar circumstances, and with neutrons of less than 12 × 106 electron volts energy, disintegration phenomena occur in oxygen or in nitrogen at least ten times as frequently as in carbon. Thus Harkins, Gans and Newson obtained only two examples of disintegration in 3200 pairs of photographs taken with a source of radiothorium and beryllium and an expansion chamber filled with ethylene, and concluded that in each case an atom of oxygen or some other impurity must have been involved. Likewise, Feather, in 2210 pairs of photographs with the neutrons of polonium-beryllium and an expansion chamber filled with a mixture of acetylene and helium, found only one example of a disintegration which could reasonably be ascribed to the nuclear reaction,
† Harkins, Gans and Newson, have also described (Phys. Rev. 44 (1933), 945CrossRefGoogle Scholar) the results from 3200 pairs of photographs in a mixture of difluor-dichlor methane and helium. Here fluorine disintegrations mask any possible “normal” disintegrations of carbon nuclei, but no example of a group of three associated heavy tracks appears to have been observed.
§ This statement, as also certain others in the present paper, depends upon the assumption that the mass of the neutron is 1.0067, in terms of the neutral atom of oxygen O16 = 16.
‡ B11 + H1 → 2Li6 … (6a), B11 + H1 → Be9 + He3 … (6b).
† Capture disintegrations such as C12 + n 1 → Li7 + He4 + H2 … (10a) are obviously impossible, from energy considerations, when the neutrons of polonium-beryllium are employed.
∥ Note added in proof, 23 June. Recently about 500 pairs of photographs have been taken with the neutron source in the centre of an expansion chamber containing carbon tetrafluoride and helium. No further example of three-particle disintegration has been found.
* In this way the direction cosines of OA, OB, OC are determined when angles BOC, COA, AOB are known. Knowledge of any two of the remaining three angles is then sufficient to fix a, b, c, such that a 2 + b 2 + c 2 = 1. If, however, the values of all three angles SOA, SOB, SOC are employed, a, b, c may be found without using this equation. This latter procedure was in fact adopted in the above calculations and it was found that a 2 + b 2 + c 2 = 1.0025. Thus the mean values of the angles given form a consistent set, well within the limits of accuracy of the measurements.
‡ Initial velocities for C are calculated on the assumption that the deflection is due to collision with the nucleus A40, the momenta of the two particles after collision being roughly equal—as the angle of deflection would suggest. The suggestion that, if the disintegration is in fact disintegration by capture, then C is the track of He5 and the deflection is evidence of the subsequent change He5 → He4 + n 1, does not stand the test of calculation.
∥ The precise figures quoted refer to the capture disintegration in which A is the track of He5. Very similar values would be obtained on any other assumption.