Total body chlorine: Calibration of the in vivo neutron activation measurement

Total body chlorine (TBCl), used to estimate the extracellular space, is measured by delayed-gamma neutron activation (DGNA) using the reaction 37Cl(n,γ)38Cl, at Brookhaven National Laboratory. During the calibration process, we noticed that different values were obtained when different amounts of Cl were placed in the phantom. This non-linear relationship is due to the thermal neutron flux suppression by the thermal neutron capture reaction 35Cl(n,γ)36Cl. Monte Carlo simulations confirm the results of phantom measurements showing an inverse relationship between the Cl content in the phantom and the γ-ray yield per gram Cl. Thus, it is important to calibrate the DGNA system for TBCl using phantom standards containing an amount of Cl close to that expected in the individual undergoing measurement.