Chronology of Milestone Events in Particle Physics - ELLIS 1927
Chronology of Milestone Events in Particle Physics

ELLIS 1927

Ellis, C.D.; Wooster, W.A.;
The Continuous Spectrum of Rays
Nature 119 (1927) 563;

Full Text
The continuous spectrum of the -rays arising from radio-active bodies is a matter of great importance in the study of their disintegration. Two opposite views have been held about the origin of this continuous spectrum. It has been suggested that, as in the -ray case, the nucleus, at each disintegration, emits an electron having a fixed characteristic energy, and that this process is identical for different atoms of the same body. The continuous spectrum given by these disintegration electrons is then explained as being due to secondary effects, into the nature of which we need not enter here. The alternative theory supposes that the process of emission of the electron is not the same for different atoms, and that the continuous spectrum is a fundamental characteristic of the type of atom disintegrating. Discussing of these views has hitherto been concerned with the problem of whether or not certain specified secondary effects could produce the observed heterogenity, and although no satisfactory explanation has yet been given by the assumption of secondary effects, it was most important to clear up the problem by a direct method.
There is a ready means of distinguishing between the two views, since in one case a given quantity of energy would be emitted at each disintegration equal to or greater than the maximum energy observed in the electrons escaping from the atom, whereas in the second case the average energy per disintegration would be expected to equal the average energy of the particles emitted. If we were to measure the total energy given out by a known amount of material, as, for example, by enclosing it in a thick-walled calorimeter, then in the first case the heating effect should lead to an average energy per disintegration equal to or greater than the fastest electron emitted, no matter in what way this energy was afterwards split up by secondary effects. Since on the second hypothesis no secondary effects are presumed to be present, the heating effect should correspond simply to the average kinetic energy of the particles forming the continuous spectrum.
To avoid complications due to -rays or to -rays from parent or successive atoms, we measured the heating effect in a thick-walled calorimeter of a known quantity of radium E. This measurement proved difficult because of the small rate of evolution of heat, but by taking special precautions it has been possible to show that the average energy emitted at each disintegration of radium E is 340,000-30,000 volts. This result is a striking confirmation of the hypothesis that the continuous spectrum is emitted as such from the nucleus, since the average energy of the particles as determined by ionization measurements over the whole spectrum gives a value about 390,000 volts, whereas if the energy emitted per disintegration were equal to that of the fastest -rays, the corresponding value of the heating would be three times as large - in fact, 1,050,000 volts.
Many interesting points are raised by the question of how a nucleus, otherwise quantized, can emit electrons with velocities varying over a wide range, but consideration of these will be deferred until the publication of the full results.

Accelerator SOURCE Detectors IONIZATION, CALO

Related references
More (later) information appears in
C. D. Ellis and W. A. Wooster, Proc. Roy. Soc. A117 (1927) 109;

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Confirmation that the spectrum is continuous.
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