Electrical and Computer Engineering ETDs

Publication Date

4-29-1964

Abstract

This dissertation is an experimental and theoretical treatise on the design criteria of a phoswich transducer. The major emphasis is on the application in a gamma-ray spectrometer. The conversion efficiency, gamma-ray cross sections, response to charged particles, luminescence decay time, spectral response and physical qualities are discussed, in considerable detail, in making the optimum choices for the inorganic crystal and the organic phosphor which will act as the charged particle anticoincident shield. The effect of the thickness of the anticoincident shield on resolution, the percentage of gamma-rays absorbed, the percentage of gamma-rays that are atten­uated in energy and the lower threshold for charged particle re­jection is analyzed. The principle involved, which causes a difference in the gamma-ray spectra when the anticoincident shield is operating, is determined experimentally. The signal limitations that influence the dynamic range of the phoswich transducer for the energy analysis of the gamma-rays are discussed, The statistical factors that affect the resolution and the dynamic range are deter­mined. The optimum choice of; integrating time constants for both signal paths, the input to tli1e analyzer and the input to the anticoincident circuit are determined mathematically, The cause of the adverse effect of operating a phoswich transducer in a high flux of highly-energized charged particles is determined experimentally.

The best choice for an inorganic crystal is Csl(Tl). A new figure of 9.25% has been obtained for the conversion efficiency by using a photomultiplier tube with a S-20 spectral response. The complete luminescence decay curve for Csl (Tl) is plotted in a graph. The long-term metastable phosphorescence decays to the noise level of the photomultiplier in 2 .5 milliseconds.

The combination of a plastic scintillator with Csl(Tl) gives a dynamic range ratio of 222. For the range from 56 keV to 12.5 MeV, the optimum thickness of plastic is one-sixteenth inch. This thickness causes the resolution of a Cs-137 photopeak to broaden by 6%. The percentage of gamma-rays that have their energies attenuated outside the photopeak is less than one percent.

When the anticoincide1ce shield is operating, 62.5% of the difference in gamma-ray spectra is due to a Compton scattered electron escaping from the crystal and interacting in the plastic anticoincidence shield, causing that event to be rejected from analysis.

The frequency distributions of the amplitude variations of the voltage pulses, for energy analysis, and the fluctuations on a single voltage pulse, from ai:1. inorganic crystal for discrimination, are both Gaussian. The upper limit of the dynamic range was not limited by the voltage fluctuations, but by the amplitude of the inorganic crystal pulse.

The theoretical maximum ratio of pulse heights from the fast and slow scintillators is degraded by the transit time spread of the photomultiplier and the integrating time constant of the output circuit. A mathematical treatment, to obtain the ratio of the pulse heights from general equations, is presented. The results of this analysis show, that for a given phoswich transducer, there is an optimum integrating tie constant. Graphs are included which show the relative pulse heights and the time spread between the fast and slow voltage pulses as a function of the integrating time constant.

Operating the phoswich transducer in a high flux of highly-energized charged particles as the effect of increasing the photo­multiplier gain variance and only slightly increasing the crystal and photocathode transfer variance. The photomultiplier gain variance can be decreased by operating the tube with a higher dynode bleeder current. An increase in gain was noted. This gain shift is dependent upon the flux rate and this necessitates means of calibration.

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Ahmed Erteza

Second Committee Member

William Jackson Byatt

Third Committee Member

Donald Childress Thorn

Fourth Committee Member

Arnold Herman Koschmann

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