IMPROVEMENT OF THE PROMPT-GAMMA NEUTRON-ACTIVATION FACILITY AT BROOKHAVEN-NATIONAL-LABORATORY

The prompt-gamma neutron activation facility at Brookhaven National La boratory was upgraded to improve both the precision and accuracy of it s in vivo determinations of total body nitrogen. The upgrade, guided b y Monte Carlo simulations, involved elongating and modifying the sourc e collimator and its shielding, repositioning the system's two NaI(TI) detectors, and improving the neutron and gamma shielding of these det ectors. The new source collimator has a graphite reflector around the (PuBe)-Pu-238 neutron source to enhance the low-energy region of the n eutron spectrum incident on the patient. The gamma detectors have been relocated from positions close to the upward-emerging collimated neut ron beam to positions close to and at the sides of the patient. These modifications substantially reduced spurious counts resulting from the capture of small-angle scattered neutrons in the NaI detectors. The p ile-up background under the 10.8 MeV N-14(n, gamma)N-15 spectral peak has been reduced so that the nitrogen peak-to-background ratio has bee n increased by a factor of 2.8. The resulting reduction in the coeffic ient of variation of the total body nitrogen measurements from 3% to 2 .2% has improved the statistical significance of the results possible for any given number of patient measurements. The new system also has a more uniform composite sensitivity.