Theory, Design, and Applications of a Short Magnetic Lens Electron Spectrometer

Abstract

A beta-ray spectrometer has been constructed utilizing the chromatic aberration of a ``thin'' magnetic lens. The lens is wound of 300 pounds of copper wire and does not utilize iron. Its focal length for transmitted electrons can be varied from 25 to 50 cm and the magnification from 0.5 to 2. Circular sources varying in diameter from 0.4 to 1.6 cm are used and the solid angle subtended at the source is varied between 0.025 and 0.1 steradian. The half-maximum width of the transmitted distribution is 0.017 to 0.06 of the momentum of the focused electrons. Larger solid angles or better resolution could probably be obtained. The focusing properties of the instrument are derived from the calculated field distribution of the lens by the method of Busch. The spectrometer has been applied to studies of radioactive disintegration schemes. Appropriate techniques are described for the measurement of discrete and continuous beta-ray spectra, of coincidences of focused electrons with other radiations, and of gamma-ray energies through secondary electron spectra. A special baffle system allows the separation of negatrons and positrons. The performance of the short lens spectrometer is compared with that of other types of beta-ray spectrometer and it is concluded that it is a particularly useful tool for the investigation of disintegration schemes because of its flexibility, the use of large circular sources, and the accessibility of source and counter which are located outside the magnetic field at a distance of 100 cm from each other.