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60 UNDERSTANDING THE ATOM SERIES sidered, and the use of absorption curves to study the energy distribu- tion of beta radiation is introduced. The density thickness expressed in milligrams per square centimeter is introduced as a useful term. The film also considers problems of self-absorption, special activity, and backscattering of radiation. RADIATION AND MATTER (1962). 44 minutes. Produced by the former New York University Television Center under the direction of the USAEC's Division of Isotope Develop- ment. For sale by Byron Motion Pictures at $ 45.95 per print, including shipping case. The film, which considers the interaction of radiation with matter, de- velops the various processes by which alpha, beta, and gamma radia- tion give up energy to their surroundings. The similarities and differ- ences of alpha and beta particles are considered, with emphasis on the methods by which ionization occurs. It is pointed out that, since the in- teraction of radiations in the absorption process takes place essentially only with orbital electrons on the atoms, the density of electrons in matter is the determining factor. The relation between energy of a par- ticle and the number of ion pairs formed is also explained. The lec- turer follows with a discussion of gamma, or electromagnetic radia- tion, which is described as a nonionizing event in terms of the initial interaction between photons and atoms. Four possibilities of gamma- ray absorption (excitation, photoelectric effect, Compton effect, and pair production) are discussed. The viewer, however, is alerted to the fact that there is only a certain probability that one particular process may take place rather than another, depending upon the energy of the gamma ray. This probability, expressed as absorption coefficient, is then related to each of the four absorption processes. RADIATION DETECTION BY IONIZATION (1962). 30 minutes. Produced by the Educational Broadcasting Corporation, New York City, under the direction of the USAEC's Division of Iso- tope Development. For sale by Byron Motion Pictures, at $32.53 per print, including shipping case. The basic principles of ionization detectors are described, particularly in relation to the pulse height as a function of voltage curves. Brief descriptions of ionization chambers, proportional counters, and Geiger counters are included, and examples of instruments operating in these regions are shown. Special consideration is given to Geiger counters, including the mechanism of gas quenching and the determination of a counting-rate plateau. The resolving time of a counter is discussed, as well as various components of a practical instrument, including am- plifiers and sealers.