UTILIZATION OF A BGO DETECTOR AS AN ACTIVE OXYGEN TARGET

The (n, n'yx) cross section for the 6.13 MeV state in oxygen has recen tly become of general interest because of the possibility of using thi s process to assay oxygen as a part of non-intrusive inspections. Loca lized densities of carbon, oxygen, and nitrogen are particularly usefu l;in determining the presence of explosives and/or drugs in containers of all sizes, from suitcases to cargo containers. The presence of oxy gen in EGO (Bi4Ge3O12) scintillator makes this detector suitable for u se as an active target for the measurement of the energy dependence of the excitation, of the first (6.049 MeV 0(+)) and second (6.130 MeV 3 (-)) excited states in O-16 by fast neutron interactions. An active ta rget functions as both a target and an active device such as a detecto r. The de-excitations of the 6.049 and 6.130 states take place by nucl ear pair production and gamma-ray emission respectively. There is a la rge probability of absorbing all of the de-excitation energy in the sc intillator in either of these cases. Since the energies deposited in t he scintillator by these transitions are very close, the de-excitation s are indistinguishable. However, since the cross section for the exci tation of the 6.13 MeV state is believed to be larger than that of the 6.049 MeV, the major measured features of the energy variations are t hose related to the second state. The validity of the technique was in itially tested using (MCNP) calculations. The calculations established that the detected neutron count rate in the crystal was proportional to the cross-sections used as input for the calculations, and that the constant of proportionality did not vary; with neutron energy. Subseq uently, measurements were made with a EGO detector as an active oxygen target. The results clearly show a strong energy dependence including several resonances.