PRINCIPLES OF RADIOLOGICAL PHYSICS 7 



the electrons and are also called "positive electrons" or positive /3 

 particles. They are not a normal constituent of matter because 

 they quickly vanish by combining with one of the ever-present 

 electrons of matter (see the following paragraph and Sect. 2-2b). 

 Positrons may emerge from atomic nuclei as a result of nuclear 

 transformations (see next paragraph) or from matter exposed to 

 high-energy radiations (see Sect. 2-2b). For further data see 

 Richtmyer and Kennard (1947), pp. 598 ff. 



(2) Heavy charged particles, which are atoms of any kind of matter 

 stripped of some or all of their electrons, i.e., turned into positive 

 ions. They are produced and studied conveniently when an electric 

 discharge passes through a rarefied gas or vapor in a suitable arrange- 

 ment (see, for example, Richtmyer and Kennard, 1947, pp. 542 ff.). 

 The following types of ions are widely used as the constituents of 

 radiations : 



(c) The 'proton, a bare nucleus of the common isotope of hydrogen, 



with atomic weight 1.008 and a positive charge ecjual to e. 

 {d) The deuteron, a bare nucleus of the heavy isotope of hydrogen, 



with atomic weight 2.015 and a positive charge equal to e. 

 (e) The alpha particle, a bare nucleus of helium, stripped of both 



electrons, with atomic weight 4.003 and a positive charge ec^ual to 



2e. 



(3) Neutral particles, one of which has great practical importance, 

 namely: 



(/) The neutron, with atomic weight 1.009 and no electric charge. 

 Neutrons are a normal constituent of the nuclei of all kinds of 

 matter except the common isotope of hydrogen. They are 

 released from nuclei only in the course of transformations induced 

 by high-energy radiations (see, for example, Richtmyer and 

 Kennard, 1947, pp. 615 ff.). 



The constituent particles of radiations are by no means strictly unchangeable 

 entities, even though some of them are normal constituents of matter. In fact, 

 none of the particles is strictly unchangeable, and some have only a fleeting 

 existence under ordinary conditions. Thus any positron quickly vanishes, 

 together with an electron, giving rise to high-energy electromagnetic radiation. 

 Similarly, electrons would disappear quickly if the world contained an excess of 

 positrons. 



Isolated neutrons are intrinsically unstable. They can turn into protons, 

 ejecting at the same time a negative electron and, presumably, a neutrino (see 

 notes in the first part of Sect. 1). Neutrons, if left to themselves, undergo this 

 transformation at random, now one neutron and then another, at irregular 

 intervals. On the average, half the neutrons existing at any one time are 

 expected to turn into protons in approximately a quarter of an hour. However, 

 the great majority of free neutrons disappear by capture in some atomic nucleus 

 in a much shorter time, i.e., before changing into protons. 



