The Absorption of High Frequency Electromagnetic Radiation

Citation

Read, John (1934) The Absorption of High Frequency Electromagnetic Radiation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qfxc-sb65. https://resolver.caltech.edu/CaltechTHESIS:02292024-162747438

Abstract

[Summary] The different mechanisms of absorption of high frequency electromagnetic radiation are described. Theories of scattering by electrons are outlined, and a survey is given of data on photoelectric absorption, absorption by the production of positive and negative electron pairs, and nuclear absorption.

An experiment for the measurement of µ in carbon, aluminum, iron, copper, and lead, is described.

The results for carbon and aluminum indicate it is unlikely that the Klein-Nishina formula is in error by as much as one percent in the wave-length region 50 to 20 x-u.

The results for lead give values for the photoelectric absorption coefficient in agreement with Gray's empirical law between 100 and 38 x-u. Between 25 and 20 x-u. however, the experimental results are higher than Gray's values. An empirical relation is fitted to the experimental points, the origin, and Allen's value at 100 x-u. by the method of least squares. The predictions of this law are compared with available information on the photoelectric absorption coefficient. It is suggested that the true law should lie midway between Gray's law and the new law, and it is pointed out that such a law would leave a balance of absorption at λ 4.7 x-u. close to that required for the production of the observed positive and negative electron pairs. Some results of Ellis and Aston obtained from the "magnetic spectrum" of photoelectrons ejected from platinum by the γ-rays of radium B+C are discussed, and it is shown that they cannot be explained without assuming photoelectric absorption coefficients lower than Gray's values, and at variance with the experimental results of this thesis. If the data is correct, the whole can scarcely be explained without assuming some absorption in addition to Klein-Nishina and photoelectric absorption, exists at wave-lengths as long as 24 x-u.

The results for T for iron, copper, and lead are in agreement with a law of variation T α Z^2·9 at both 24 and 49 x-u.

A discussion of nuclear absorption and the most likely conclusion to reconcile all the data are given.

Thesis Files