A permanent magnet for β-ray spectroscopy

Abstract

1. In a great many experiments it is necessary to use magnetic fields for deflecting particles, and while sometimes no great demand is made on the constancy of the field, in the majority of cases it is essential that the field stays constant to at least one part in a thousand. It is clear that the ideal method of attaining this constancy is by the use of a permanent magnet in place of the usual electromagnet. Moreover, since many types of experiments last for periods of from half-an-hour upwards, a great saving of labour would also be effected. The investigation of β-ray spectra will serve as an illustration of these points. The different homogeneous groups of electrons emitted by the radioactive bodies are separated out into a corpuscular spectrum by means of a magnetic field. These groups are usually detected photographically and with the type of source available it is frequently necessary to give exposures of from half-an-hour up to several hours, during which time it is essential that the magnetic field should remain constant. When using an electromagnet, the method adopted is to control the field current by means of a potentiometer. Even after taking precautions about accumulators and the construction of the electrical circuit, it is generally found necessary to check up the constancy of the field at least once every minute and to make some small adjustment in a series resistance. Even with this continual attention it is clear that the current is not strictly constant but fluctuates about a mean value and the magnetic field goes through a small hysteresis cycle. This latter is a fundamental objection which could be removed by the use of a permanent magnet, while the freedom from attention to a field current would make possible a great number of experiments which at the moment would be too laborious to carry out. With the co-operation of Messrs. Edgar Allen & Co., of Sheffield, we have therefore made experiments to determine whether the construction of a permanent magnet of the requisite size was possible with steels now available. These experiments have been successful and a large magnet for use in β-ray spectroscopy has been built.