The Determination of Photon Flux for Energies between 150 and 300 Mev

Abstract

A calorimetric method of determining photon flux is described. Absolute calibration was performed with an improved Pb block calorimeter using thermistors as temperature‐sensitive elements. Five‐minute irradiations were employed with a maximum of 33 milliwatts emerging from a 0.5‐in. collimator placed at 115 cm from the betatron target. A secondary standard was developed for the purpose of determining bremsstrahlung flux. This secondary standard was a 4‐in. diameter, parallel plate ionization chamber consisting of a 0.016‐in. Al center foil separated by 0.052‐in. air gaps from 0.25‐in. Cu front and back plates and adapted for use with a Victoreen M‐70 electrometer. With corrections described, the results, in ergs/esu per cm air (NTP), are 7340 at 300 Mev, 6370 at 250 Mev, 5720 at 200 Mev, and 5370 at 150 Mev. Calibration of Victoreen thimbles with various types of converter were also made.