ON THE ANALYSIS OF GAMMA RAY PULSE HEIGHT SPECTRA

A method for determining the gamma ray energy spectrum from the pulse height spectrum measured using a NaI(Tl) scintillation detector was developed. The polyenergetic gamma pulse height spectrum was synthesized by using a series of normalized pulse height distributions characteristic of the various monoenergetic components in the beam. The contribution of each of the monconergetic components to the pulse height spectrum was determined using least square fitting techniques. The calculations are programmed for use on an IBM 704 computer. From a knowledge of the contribution of each monoenergetic component and crystal efficiency for each given energy, the energy spectrum incident upon the crystal can be determined. The method was extended to the analysis of continuous gamma ray energy spectra. A general technique for pulse height analysis, the so-called dynamic least squares peeling off process, is also proposed. The shape of the pulse height spectra and also the crystal efffciencies are dependent upon source-detector geometry for cylindrical NaI(Tl) crystals. Experimental and theoretical results indicate that this geometrical dependence can be greatly reduced if spherical crystals are used. Thus in certain cases where source-detector geometry cannot be specified, gamma energy spectra can still be determined from the measurement of pulse height spectra.more » The techniques were applied to a number of problems, the determination of thermal fluxes, the determination of complex decay schemes, and the determination of linear absorption coefficients and energy build-up factor of 0.66 Mev passing through steel shields. Experimental results were obtained, and compared with theoretical calculations. (auth)« less