Spherical Electrostatic Analyzer for Measurement of Nuclear Reaction Energies

Abstract

A spherical electrostatic analyzer was constructed for use with the Wisconsin cylindrical analyzer in accurate measurements of nuclear reaction energies. The focusing properties of such an instrument have been described by Purcell. It has the unique feature of providing a large solid angle and high resolution for a very small angular spread of the reaction products. Only a portion of the entire sphere is used. This is 90° along the path of the particles with a 60° azimuthal opening. The mean radius of the gap is 18 inches and the gap spacing is $\frac{3}{16}$ inch. One million volt protons may be focused with a gap voltage of 20.2 kv. The analyzer is designed to fit over the cylindrical analyzer and to accept particles at an angle of 135° from the bombarding beam. Alignment, focus, and resolution tests show that the difficult machining of the plates has been done satisfactorily and that the analyzer is properly aligned. A shutter allows small sectors of the total gap to be tested individually. A sharp focus is obtained and all sectors focus particles of the same energy for a given gap voltage. Resolution tests and calibration are made using monoenergetic protons scattered from thin targets of heavy elements. A plot of the number of particles entering the detector vs the voltage across the spherical gap gives the resolution. With 1‐mm object and image apertures the measured energy resolution is 650. The measured effective solid angle with this source size and resolution is 3.2×10⁻³ steradian.