Measurement, in roentgens, of the gamma radiation from radium by the free-air ionization chamber

Abstract

The free-air ionization chamber is the accepted standard for calibrating X-ray dosage measuring instruments. With the advent of supervoltage X-r ays, the need for proved standards for this range has arisen.The present paper describes an investigation of the applicability of the free-air pressure ionization chamber for measuring radiation at the high-frequency end of the X-ray spectrum, that of the gamma radiation from radium.Measurements were made with a narrow collimated beam of gamma rays at pressures up to 10 atm. Because of inability to obtain full current-voltage saturation, application was. made of the extrapolation of reciprocal current-voltage curves in accordance with the Jaffe-Zanstra theory. This was first tested carefully with 350-kv X-rays at 1-, 6-, and lO-atm pressure and found to hold within our experimental limits. It was found that at 10-atm pressure a plate separation of about 35 cm is necessary to utilize the full electron range. Ranges in the forward direction were not effectively greater than 70 cm. Measurements of the gamma-ray emission from radium gave a value of 8.16 roentgens per milligram per hour at a distance of 1 cm, which is in line with several similar detcrminations by other methods. CONTENTS Page

The ionization method of measuring neutron energy

“…The same difficulty was encountered in the measurement of absorbed X-ray energy, when it was found that the parallel plate chamber which effectively measured X-ray ionization in an extended gaseous medium and was satisfactory for voltages up to 200 kV., became too clumsy an instrument when made on a scale adequate for the measurement of high-voltage X-rays or y-rays. The most convenient solution of this difficulty has proved to be by the use of very small ionization chambers which, if constructed of suitable material, have been shown experimentally to yield the same value for the ionization per unit volume of the enclosed gas as would be observed with a parallel plate chamber of appropriate dimensions (52,80). The underlying principle is as follows.…”