Improvement of in vivo Neutron Activation Analysis of Mn using a 4p NaI(Tl) detector array

Manganese contents in hand phantoms were analyzed by in vivo neutron activation analysis using a 4p NaI(Tl) detector array. Solution-type phantoms with varying amount of Mn and fixed amounts of physiological normal elements (Na, Cl, Ca) were prepared in a cylindrical shape to simulate fisted hands and were irradiated with the 7Li(p,n) neutron beam. The proton energy and polyethlyene cavity were set at optimized conditions established before. The g-ray spectra were accumulated under the anticoincidence mode. The calibration was done both for the 847 keV full-energy peak area and for the total area of 56Mn, which is defined as the total number of counts anywhere in the g-ray spectrum from 56Mn. From the analysis of the 847 keV peak, the Mn detection limit has been improved by a factor of 1.6 compared to the previous feasibility test thanks to a significant gain in the g-ray detection efficiency. Further improvement by a factor of 1.2 was identified when the total area of 56Mn was used for the calibration. The final sensitivity and detection limit for a 20 mSv hand dose reached 12,050 counts/mg Mn, and 86 mg, respectively. The overall improvement achieved in this study will be a considerable contribution toward realization of the clinical application.