A scintillation subtraction spectrometer for eliminating Compton continua from gamma-ray spectra

Abstract

The gamma-ray scintillation counter, using a phosphor such as NaI(Tl) which contains a heavy element, shows a spectrum for each gamma-ray energy present consisting of: (i) a total-energy peak (or "photopeak"); (ii) a Compton continuum extending up to a cutoff just below the total-energy peak; (iii) a backscatter peak at a low energy; (iv) possibly two pair-production peaks with energies 511 and 1022 KeV below the total-energy peak. The overall spectrum of a source giving a wide variety of gamma-ray energies is complex and difficult to interpret (e.g. to determine the energies and intensities of the gamma-rays present). By using an organic (plastic) phosphor, which only shows the Compton continua, and subtracting its spectrum from that of the NaI, a considerably simpler spectrum results. The backscatter peaks do not extend above 250 KeV, and pair-production peaks are negligible for gamma-ray energies below 2000 KeV. Most of the sources with which this spectrometer is intended to be used, have their gamma-ray energies lying within this range, and will be represented only by a total energy peak for each gamma-ray energy.