Generators for Gamma Rays and Neutrons and Radiotherapeutic Possibilities

Abstract

THE technic of high voltage generators and high voltage discharge tubes for the production of penetrating x-rays has been successfully applied to nuclear physics. In fact, generators for one million volts and more, designed to produce x-rays by accelerating electrons (the usual way), can be used to accelerate positive particles merely by reversing the tension. Positive particles may be produced by voltages of the order of 50 kv. in tubes not essentially different from the good old gas tubes, the positive ions emerging through a hole in the cathode. These positive ions (canal rays) are protons if the gas in the discharge tube is hydrogen, deutons in the case of heavy hydrogen, and α-particles in the case of helium. The acceleration of the positive particles takes place in a tube which shows, as we shall see in more detail later on, a great analogy with a high voltage x-ray tube. At the other hand, the results of nuclear physics have stimulated further development of high voltage technic. Generators for voltages of many millions of volts have been designed. The first interesting nuclear reaction by means of high voltage was performed by Cockcroft and Walton in 1932 (1). The lithium nucleus was split up into two helium nuclei or α-particles by means of protons accelerated with a voltage of some hundreds of kilovolts. It was shown later that this reaction is possible with voltages as low as 10 kv. (2, 3); but the output increases rapidly with the voltage, as is the case for most nuclear reactions. Moreover, many reactions cannot be expected to take place below certain minimum voltages. So, for instance, may a beryllium nucleus be split up by x-rays or γ-rays equivalent to one and a half million volts as a minimum value. At voltages of little over one million volts (1 mv.) a very interesting phenomenon appears: a gamma quantum is transformed into a pair of particles, a positron and an electron. The production of x-rays (γ-rays) of one million volts becomes extremely efficient, the output at one million volts being equivalent to more than 1 kg. of radium at 1 ma. tube current. A short description of a high voltage generator developed by the author, given at the American Congress of Radiology in 1933, has been published (4). This generator has been further developed up to a voltage of four million volts. A full description has been published recently (5). The article contains also a brief critical survey of the other methods to produce fast particles with and without high voltages. Figure 1 shows a generator of two million volts to earth, being the negative half of a four-million-volts set at the Eindhoven laboratories. Figure 2 shows a one and a quarter million volts generator delivered to, and in use for many months now at, the Cavendish Laboratory, Cambridge. For further details we refer to the paper mentioned above (4) and proceed to describe—(a) A million-volt x-ray or γ-ray tube of the sealed-off type; (b) A high voltage neutron tube.