THE APPLICATION OF LIQUID SCINTILLATION COUNTING METHODS AND MULTICHANNEL ANALYZER TECHNIQUES IN LYOLUMINESCENCE DOSIMETRY

RICHARD EVAN HANIG, Purdue University

Abstract

Lyoluminescence, the emission of light during the dissolution of previously irradiated solids, has shown potential as a radiation dosimeter. In order to improve the sensitivity and lower the detection limits of a lyoluminescence (LL) dosimetry system, liquid scintillation counting methods and multichannel analyzer techniques were employed. Initial studies to determine the feasibility of using liquid scintillation counting methods for LL readout were directed toward establishing suitable concentrations of solvent (luminol) and phosphor (trehalose). Based on reports in the literature and preliminary trials with various phosphor masses and solvent concentrations, it was found that the disaccharide trehalose in 10 mg amounts was suitable for the dose range of interest. The trehalose samples were subjected to absorbed doses of 0.23, 1.15, 5.75, and 28.75 rads from a Co-60 gamma ray source at a dose rate of 3.7 rads/min. The solvent used for LL readout was a solution of luminol at a concentration of 7.0 x 10('-5) M with a pH of 11.75. As part of the initial feasibility study, a special sample holder was developed for the read-out process. The sample holder was a double cylinder vial that kept the solvent and phosphor isolated until the sample was in position in the liquid scintillation counter, at which time the sample and solvent were brought together for dissolution to occur. The resulting light emission was detected by the photomultiplier tubes of the liquid scintillation counting system. Three different parameters were measured as indicators of dose: (1) the total net counts, (2) the maximum count rate recorded on the liquid scintillation counter's count rate meter, and (3) the channels ratio (from the counts in designated energy channels of the liquid scintillation counter). All of the parameters was found to be linear with dose over the dose range studied. The correlation coefficients for the linear models were all above 0.90; however, the standard deviations were as high as 40% and it was felt that additional refinements of the methodology were needed. A second study was conducted involving the coupling of the liquid scintillation counter to two multichannel analyzers in order to evaluate the pulse height spectrum and the intensity vs. time characteristics of the output from the scintillation counter. Some modifications were also made in the phosphor masses, the solvent concentrations, and the techniques for mixing phosphor and solvent in the counting vial. For this study, samples of trehalose in 76 mg amounts were subjected, in parallel experiments, to five exposures from either a Cs-137 gamma source or a PuBe neutron source. Irradiation times were selected to give 0.05, 0.10, 0.20, 0.40, and 0.80 rads (kerma), respectively. The kerma rate involved for both sources was 0.01('9) rads/hr. The solvent was a luminol solution at a concentration of 3.5 x 10('-4) M and a pH of 10.7. For the intensity-time characteristics of the system, a type of signal-to-noise ratio was measured. The correlation coefficients for linear models were greater than 0.90 for both gamma and neutron responses; however, the relative standard deviation ranged as high as 34%. For the pulse height spectra, two parameters were measured as indicators of kerma. The first was ratio of total counts during dissolution of the sugar divided by total counts before mixing or dissolution takes place. The correlation coefficients for linear models in this case were 0.65 and 0.88 for gamma and neutron, respectively; the relative standard deviations were as high as 40%. The second parameter measured was channels ratio; for the range of doses used, this paramter was a very poor indicator of kerma with the kerma vs. response data showing a correlation coefficient of 0.73 for gamma and only 0.30 for neutron.

Degree

Ph.D.

Subject Area

Radiation

COinS