Magnetic beta-ray spectrometers of several types have been used extensively to obtain information by the detection of low energy gamma-rays. The photoelectric effect, Compton scattering, and atomic internal conversion have yielded information regarding nuclei energy level spacings, transition schemes, relative and absolute intensities of gamma radiation, and the multipole character of nuclear radiation. However, these processes have low cross sections for high energy gamma rays, so that some other process is necessary for the study of the high energy region.
Pair production, unlike the secondary processes mentioned, is increasingly probable at higher gamma ray energies. Useful information can be gained by the study of the pairs produced in the external, internal, and nuclear pair formation processes. Most generally, the total energy and number of the pairs detected are used to determine energy level spacings in nuclei, and to find what gamma ray transitions occur as well as the probability for their occurrence.
A potentially fruitful field of study is opened by studying internal and nuclear pairs. Information concerning the multipole character of the radiation and consequently the angular momentum and parity changes in nuclear transitions can possibly be gained by the study of internal pair formation. Nuclear pairs immediately indicate the angular momenta of the excited levels from which they arise.
This paper describes the development and use of a magnetic lens spectrometer for the detection of pairs. The spectrometer has been used as a medium resolution spectrometer for high energy gamma rays. It is capable of detecting either external, internal, or nuclear pairs. Measurements made using these three processes will be described.