. The American journal of roentgenology, radium therapy and nuclear medicine . s a filter for x-rays because it absorbsabnormally the shorter wave-lengths, copper is preferable fortherapeutic purposes. Our experiments show that copper andzinc have substantially the same absorbing power as brass. 482 Some Problems of Radiation Therapy sary and sufficient filter to employ withthis type of roentgen radiation. The require-ments of the particular case to be treated,as well as practical considerations andlimitations, must be taken into is probable, however, that mm. ofbrass is a suffi
. The American journal of roentgenology, radium therapy and nuclear medicine . s a filter for x-rays because it absorbsabnormally the shorter wave-lengths, copper is preferable fortherapeutic purposes. Our experiments show that copper andzinc have substantially the same absorbing power as brass. 482 Some Problems of Radiation Therapy sary and sufficient filter to employ withthis type of roentgen radiation. The require-ments of the particular case to be treated,as well as practical considerations andlimitations, must be taken into is probable, however, that mm. ofbrass is a sufficient filter for the deepest . _ ■ , rays include a wide range of wave-lengths,but the distribution of energy in theirspectrum is such that a filter of 2 mm. ofbrass removes all the soft radiation, and thetransmitted radiation is of much shorterwave-length than ordinary .v-rays. Inother words, the range of wave-lengthspresent in x-rays is inconspicuous ingamma rays. Therefore there is a sharpchange in the quality of the transmittedradiation occurring at apoint between Thickness of Tissue—Centimeters Fig. 2. radiation therapy which would beattempted with this voltage. The difficulty experienced in decidingwhat is the most suitable filtration for thex-rays of these experiments does not existin the case of radium rays. The conclusionreached is that 2 mm. of brass is sufficientas a primary filter, and actually this is alittle more than the minimum necessarythickness. The reason for this lies in thecomposition of the heterogeneous beam ofradiation which we wish to filter in the 2cases. The radiation emitted by ordinaryradium containers consists of beta andgamma rays. The beta rays are easilyabsorbed by matter and can be eliminatedby a filter of 2 mm. of brass. The gamma I 2 3 4 Filter Coef. ofAbsorp-tion,Cm. Hall ,Cm. PerCent Time of Metal Mm. Trans. ureby Filter AI 4. Brass o AI 6 AI 10 Brass o AI 15 Brass I 4 08 4 25 5 33
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