$ 4\pi$ NaI(T1) Detector Array for in Vivo Neutron Activation Analysis

A $ 4\pi \gamma$-ray spectrometer has been set up at McMaster Accelerator Laboratory for in vivo neutron activation analysis. To achieve an ideal detection geometry eight long NaI(Tl) detectors with square cross sections were arrayed. A central square hole was left empty for sample access. The pulse processing system comprised a signal sum box, a spectroscopy amplifier and an ADC. Spectrum accumulation in coincidence or anticoincidence mode was also feasible by adding separate shaping and time pick-off electronics to each detector unit. The calibration result showed a fairly high efficiency, a peak efficiency of 0.50 at 1332 keV, due to a sufficient detector thickness of 10.2 cm and a length of 40.6 cm. The spectrometer was used for the aluminum analysis in hand phantoms as the first application. The Al sensitivity and the detection limit achieved for a hand dose of 20 mSv are 1,310 cps/mg, 0.24 mg, respectively. The detection limit has been improved by a factor of 2.5.