The γ-rays of thorium B and of the thorium C bodies

Abstract

A word of explanation is necessary in presenting this paper. It is unfinished, but designedly unfinished. When β-ray spectra were first discovered, the papers describing the experiments made no claim to give complete lists of the lines, publication was justified by the discovery of even a few strong lines. Then came the period when the technique had reached a sufficient stage of development to detect with fair certainty all except the faintest lines, and further, and this is the important point, the attainable accuracy was more than sufficient for all theoretical purposes, since in reality but little theory existed. From this stage we are just emerging. Recent experiments have proved beyond doubt that the γ-rays are associated with transitions between α-particle stationary states in the nucleus, and it is clear that the β-ray spectra will play the same rôle in giving information about these states as have the optical X-ray spectra for the electronic states. Yet this information will not be obtained in its final form by one experiment. Far more we have to expect a gradual accumulation of knowledge in which step by step the exact energies of the lines, their allocation to the correct body, and their intensities are obtained to a sufficient accuracy. The present paper is a contribution to our knowledge of the thorium (B + C + C′ + C″) spectra, and for the reasons which have been mentioned the treatment is neither exhaustive, nor are the results equally detailed for all parts of the spectrum. It is hoped, however, that the results are presented in a form which will prove accessible, and for this reason only slight reference is made to experimental technique. A detailed study, which proved very laborious, has been made of the intensities of the groups. Beyond mere visual estimation, no attack had previously been made on this problem, which I suggest is of equal importance to that of finding the energies of the groups. In the course of this work many new lines were detected, and some discrepancies with previous workers noted. Careful measurements were therefore made of the energies of all the lines. The final results constitute an entirely independent investigation of the energies and intensities of the groups in these spectra.