Characterizing gamma fields using isomeric activation ratios

Isomeric activities were induced in indium by gamma irradiation in three different gamma fields, through the reactions ¹¹⁵In(γ, γ′)^115mIn and ¹¹³In(γ, γ′)^113mIn. The irradiation fields were (i) the 15 kCi ⁶⁰Co source available in the University, (ii) the spent fuel gamma irradiator in the pool of the University's Ford Nuclear Reactor (FNR) and (iii) south face of the core of the FNR during routine shut downs. Isomeric activation ratios can serve to characterize gamma fields, provided the response functions of the two (γ, γ′) reactions sample different energy regimes of the gamma spectrum present in the irradiation fields. The response of an isomeric activation detector, 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_In113m was measured in the three gamma fields. The measured ratios were (i) 1.210 ± 0.011 in the ⁶⁰Co source, (ii) 1.314 ± 0.060 in the spent fuel gamma irradiator and (iii) 1.298 ± 0.039 in a location alongside the FNR core during routine shut downs. The measured reaction rate ratios are not only close to each other, but close to unity as well. This indicates that the excitation functions for the reactions ¹¹⁵In(γ, γ′)^115mIn and ¹¹³In(γ, γ′)^113mIn have similar shapes and that for the nuclides ¹¹⁵In and ¹¹³In, the number of activation 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 of gamma spectrum in the field of measurement. However by choosing (γ, γ′) reactions with different shapes for the excitation functions one can measure a set of isomeric activation ratios that characterize a given gamma field.