Precision quantameter for high energy X-rays

Abstract

An ionization chamber has been designed to serve as a standard for measurements of the intensity of beams of high energy photons. The chamber consists of twelve plates of copper, each of one cm thickness, so that nearly all the energy of the photons is absorbed in the chamber. The ionization in the shower is sampled at the twelve gaps between plates, and the gap widths are arranged so that a Simpson's rule integration of the shower curve is automatically made. By making the last gap larger and by making an appropriate gap at the sides of the chamber, a compensation is made for any leakage of energy out of the chamber. The total energy content of a photon beam, U, should be accurately proportional to the ionization charge measured, q, independently of the spectral shape of the beam. An absolute calibration of the chamber has been calculated to an accuracy of a few percent. A density effect correction to the stopping power of copper has been included in the calculation.