An in-beam Compton-suppressed Ge spectrometer for nondestructive neutron activation analysis

A high-efficiency compton background suppressed gamma-ray spectrometer by anti-coincidence counting with a NaI(Tl)-shield around a central HPGe-detector for in-beam prompt gamma-ray neutron activation analysis (AC-PGNAA) using a Cf-252 neutron source has been designed and built to provide simultaneous anti-coincidence spectrometry of natural, industrial and environmental samples. The spectrometer consists of a high-purity germanium detector as the main detector and a large volume cylindrical NaI(Tl) detector as a guard detector. The Ge detector is a closed-end coaxial detector with dimensions of 4.35 cm length, 4.8 cm diameter and a relative photopeak efficiency of 16.3%. The guard counter has a 29 cm diameter by a 29 cm long NaI(Tl) cristal. The anti-compton (AC) array is located in a large cubic (1 m"3) polyethylene irradiation facility with a 1.5*10"8 n/sec Cf-252 source at its axial center where the irradiated sample is in the form of a polyethylene bottle of 50 ml. The irradiation facility contains two steel-lead collimators: One for neutron beam guide with polyethylene plug for neutron thermalization; the second collimator is for gamma-emission detection where a 2.5 cm thick polyehtylene box filled with Li_2CO_3 absorbers enriched in Li-6 is inserted near to the target to remove scattered neutrons within the detected gamma-beam. While the end of the collimator is further shielded by lead (20 mm thick) which had a conically shaped collimating aperture, its smallest opening had a diameter of 16 mm to protect the front part of the NaI(Tl) assembly from direct gamma-radiation from the target. The AC-arrays are further shielded from the surrounding background by housing it in a lead-shield of 5 cm thickness lined with cadmium and copper sheets of 1 mm each. The associated electronics working in conventional fast-slow mode have been designed to operate independently and simultaneously in the anti-coincidence mode as well as in the coincidence or in the normal passive mode. When operating in the anti-coincidence mode the compton edge of Cs-137 is reduced by a factor of 7.9 to provide a peak-to-compton edge ratio of 500/1. The assembly has the ability to measure instantaneously, simultaneously and nondestructively bulk samples up to about 50 cm"3. Major constituent elements in several rocks and minerals such as H, B, N, Na, Mg, Al, Si, Cl, K, Ca, P, S, Ti, Fe, Sm, Nd, Mn and Gd can be determined, while oxygen cannot be measured due to its small capture cross section (0.27 mb). Several important minor and trace elements such as B, Cd and Hg beside the low residual activity, rare earths and short-lived isotopes could be detected. The sensitivity of the AC-PGNAA technique is limited by the available neutron flux at the target matrix and the neutron absorption cross section of the elements of interest. PGNAA has the advantage to estimate the constituent elements which are difficult to be measured through the delayed gamm-ray measurements such as B, Bi, C, H, P, Tl, Be, Cl and S in industrial and reference materials and those elements which are transformed into other stable isotopes when undergoing neutron capture. The design of the spectrometer assembly, its properties and performance are described. (orig.)SIGLEAvailable from TIB Hannover: ZA 5141(5181) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman