. Biophysical research methods. Biophysics -- Research. XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 547 neutron radiation. The problem has been discussed in some detail by Zimmer (92), Aebersold and Anslow {91), and Gray (23). In es- sence, the problem is the same as that of measuring the energy ab- sorbed by tissue exposed to X radiation or which is uniformly per- meated by a radioactive substance, and the solution depends on the fact that, if into any solid medium in which ionizing particles are being uniformly generated a small empty cavity is introduced, the number and speed of the partic
. Biophysical research methods. Biophysics -- Research. XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 547 neutron radiation. The problem has been discussed in some detail by Zimmer (92), Aebersold and Anslow {91), and Gray (23). In es- sence, the problem is the same as that of measuring the energy ab- sorbed by tissue exposed to X radiation or which is uniformly per- meated by a radioactive substance, and the solution depends on the fact that, if into any solid medium in which ionizing particles are being uniformly generated a small empty cavity is introduced, the number and speed of the particles crossing any small area of the cavity in any given direction will be the same as for the particles crossing an equal area similarly situated in the solid. This is self-evident in the case of protons for the particles moving in such a direction as to enter the cavity, for it is clear that since there is no appreciable scattering of protons removal of solid material to the right of area A in Figure 17. Figure 17 to form the cavity can have no influence on the protons crossing the boundary into the cavity. It may be shown to be the case, also, for particles moving in the opposite direction. It is, in fact, true gener- ally for all types of particle, including particles that are easily scat- tered through large angles, such as electrons. Suppose the cavity to be filled with gas. A group of particles crossing area A in any given direction will lose a certain amount of energy in the course of ionizing the gas. The same group of particles passing across an equal and similarly shaped volume of solid (Fig. 17) will lose very much more energy. The ratio of the two energy losses may be denoted by p. This ratio is known as the stopping power of the solid relative to the gas for the particles in question. The ratio is to a first approximation equal to the ratio of the densities of the solid and the gas. The same will be true for all groups of par- ticles crossing the cavity and the equi
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