The Production and Use of Proton-Induced Ultrasoft X-rays

Abstract

A 700 keV Van de Graaff accelerator was used to accelerate protons onto solid targets of different light elements to produce ultrasoft, characteristic X-rays (< 5 keV). The proton energies were calibrated using the (p, y) resonances at 633 keV in Aluminium and at 340 and 483 keV in Fluorine. The X-ray emission characteristics of Aluminium, Carbon, Gold, Silicon/Carbon, Silicon/Nitrogen and Titanium/Boron were studied as a function of incident proton energy, angle of inclination of the target (30° - 60° to the proton beam) and angle of detection of the X-rays (40° - 130° to the beam). Detection of the X-rays was achieved using a gas-flow proportional counter directly coupled to a low-noise pre-amplifier. The resulting spectra, recorded on a multichannel analyser, were well fitted by linear combinations of single Gaussian curves to give peak position (X-ray energy), width and area (X-ray intensity). Carbon contamination of the target surface was studied in detail for the Aluminium target. A number of low beam currents onto the target (10 - 70 nA) were used for total irradiation times of up to 17 hours in order to establish the degree of overall X-ray energy mixing. The information gained from the study of both the Carbon contamination and the X-ray emission characteristics was used to propose practical optimum conditions for the production of ultrasoft X-rays by proton bombardment in their application to biological and biochemical irradiations. A computer code, capable of following the electron track histories resulting from ultrasoft X-ray interactions has been used to compare the details of such energy deposition with the results of mammalian cell irradiations made at the M. R. C. Radiobiology Unit for a number, of different ultrasoft X-ray energies. Such a-comparison has been used to try to identify the mechanisms of radiation action. Included in this work is the application of the computer code to a variety of. characteristic X-ray photon energies, thus extending the available, calculated data.