Gamma Ray Interactions

When a single gamma ray interacts with the gas in the chamber, it produces a rapidly moving electron which produces secondary electrons. About 10,000 electrons may be formed depending on the gas used in the chamber. The applied voltage can be increased until the amount of recombination is very low. However, further increases do not appreciably increase the number of electrons collected. This region in which all 10,000 electrons are collected is the ionization region. 

The Kinds of Nuclear Reactions. Many different kinds of nuclear reactions have now been studied. Spontaneous radioactivity is a nuclear reaction in which the reactant is a single nucleus. Other known nuclear reactions involve a proton, a deuteron, an alpha particle, a neutron, or a photon (usually a gamma ray) interacting with the nucleus of an atom. The products of a nuclear reaction may be a heavy nucleus and a proton, an electron, a deuteron, an alpha particle, a neutron, two or more neutrons, or a gamma ray. In addition, there occurs the very important type of nuclear reaction in which a very heavy nucleus, made unstable by the addition of a neutron, breaks up into two pans of comparable size, plus several neutrons. This process of fission has been mentioned in Chapter 25 and, is described in a later section of the present chapter. 

Figure 2.8. Gamma-ray interaction coefficients as fnnction of energy in Nal. (From Evans 1955, p. 716.)...

The third mechanism of gamma-ray interaction is pair formation. The gammaray transforms itself near a nucleus into a positron and an electron. For pair formation to occur, the energy of the gamma ray must be at least twice that of the electron rest energy, mc, of 0.511 MeV, so that the energy must be equal to or greater than 1.022 MeV. 

The G-M counter is a simple and relatively inexpensive gas-filled tube with a count-rate meter an amplifier may also be present. The G-M detector counts alpha particles, beta particles, and gamma rays with a very thin window, counts beta particles and gamma rays with a thicker window, and counts gamma rays only with a thick shield. Alpha and beta particles interact in the gas gamma rays interact mostly in the walls, from which electrons enter the gas. The intrinsic efficiency for counting gamma rays relative to beta particles depends on the amount and type of solids surrounding the detection gas. 

Compton scattering Occurs when gamma rays interact with electrons in a material. The photon donates some of its energy to the electron. The electron is ejected from its atom and the reduction in energy of the photon results in an increase in its wavelength. The photon with the remaining energy is deflected. 

---