CHARACTERIZING GAMMA-FIELDS USING ISOMERIC ACTIVATION RATIOS

Isomeric activities were induced in indium by gamma irradiation in thr ee different gamma fields, through the reactions In-115(gamma, gamma') In-115m and In-113(gamma, gamma')In-113m. The irradiation fields were (i) the 15 kCi Co-60 source available in the University, (ii) the spen t fuel gamma irradiator in the pool of the University's Ford Nuclear R eactor (FNR) and (iii) south face of the core of the FNR during routin e shut downs. Isomeric activation ratios can serve to characterize gam ma fields, provided the response functions of the two (gamma, gamma') reactions sample different energy regimes of the gamma spectrum presen t in the irradiation fields. The response of an isomeric activation de tector, in turn, depends on the number of activation energy levels of the nuclide and the probabilities with which the activation levels de- populate to the isomeric level. The reaction rate ratio R(In115m)/R(In 113m) was measured in the three gamma fields. The measured ratios were (i) 1.210 +/- 0.011 in the Co-60 source, (ii) 1.314 +/- 0.060 in the spent fuel gamma irradiator and (iii) 1.298 +/- 0.039 in a location al ongside the FNR core during routine shut downs. The measured reaction rate ratios are not only close to each other, but close to unity as we ll. This indicates that the excitation functions for the reactions In- 115(gamma, gamma')In-115m and In-113(gamma, gamma')In-113m have simila r shapes and that for the nuclides In-115 and In-113, the number of ac tivation energy levels and the probabilities with which they populate the isomeric levels are very similar to each other. Thus, the ratio R( In115m)/R(In113m) Will not yield any information regarding the shape o f gamma spectrum in the field of measurement. However by choosing (gam ma, gamma') reactions with different shapes for the excitation functio ns one can measure a set of isomeric activation ratios that characteri ze a given gamma field.