Gamma Heating in Thin Heavy-Element Absorbers

Abstract

Hard copy (HC)- Microfiche (MF)_ ABSTRACTThe ratio of gamma heating per gram of tungsten to gamma heating per gram of water was calculated for the case of a thin tungsten detector in a water shield. One-dimensional transport theory calculations were used to obtain response polynomials which predict this ratio as a function of source energy, shield thickness, and detector thickness. Electron transport effects were also examined. Useful results a r e presented in the form of graphs.ii GAMMA HEATING IN THIN HEAVY-ELEMENT ABSORBERSby John H. Lynch, Richard J. Crum, and Harry J. Reilly Lewis Research Center SUMMARY Two effects must be considered when converting the gamma heating rate measured using an aqueous dosimeter to the gamma heating rate in a thin tungsten detector immersed in a water shield. These are, first, the difference in heating due to the difference between the mass energy absorption coefficients of the heavy detector and the aqueous dosimeter, and second, the "skin heating effect. t f l The measured heating rates must be corrected to account for these effects. Calculated thin detector heating rates which are obtained using the buildup factor method also require correction for these effects because the buildup factor does not take into account any physical properties of the detector.One-dimensional photon transport-theory calculations were performed to obtain correction factors for these effects for tungsten detectors in a water shield. The results of these calculations were used to construct polynomial equations which predict the combined magnitude of the effects as a function of gamma source energy (from 0.255 to 4.0 MeV), water-shield thickness (from 4.0 to 44.0 cm), and tungsten-detector thickness (from 0.00254 to 0.762 cm). Polynomials were also obtained for the gamma heating (W/g) in water and in tungsten so that multienergy sources can be analyzed.around the center point (2.128-MeV source, 24.0-cm water shield, and 0.044-cm detector) was examined using these polynomials. This ratio was also investigated for a fission spectrum source. The ratio was less than two for detector thickness greater than 0.2 centimeter. The polynomials f i t the data with an average absolute error of less than 10.2 percent.The behavior of the correction factor (W/g in tungsten divided by W/g in water)'The "skin heating effect" is the increased gamma heating that occurs near the surface of the detector. This heating results from the influx of a large quantity of lowenergy Compton scattered gammas. These gammas are absorbed locally due to the large photoelectric cross sections of the heavy element detector.