A fluence of thermal neutrons is 6.5 X 10¹² n/cm2 in a layer of muscle-equivalent tissue 0.1 g/cm²thick. What is the absorbed dose at the middle of the layer?

Answer: 1.8 Gy

C. Thermal-Neutron Interactions in Tissue There are two important interctions of thermal neutrons with tissue: neutron capture by nitrogen, "N(n, p)"C, and neutron capture by hydrogen, H(n, Y) H. The nitrogen interaction releases a kinetic energy of Et, = 0.62 MeV that is shared by the proton (0.58 Me V) and the recoiling nucleus (0.04 MeV). The interaction cross section in this case is o = 1.84 X 1072 cm"/atom, and the number of nitrogen atoms per gram of muscle tissue is N,/m = 1.50 X 10". Thus the kerma deposited in muscle per unit fluence of thermal neutrons is given by Eq. (16.1) as F. = 1.602 x 10 8 X 1.84 X 10-" x 1.50 X 10" x 0.62 = 2.74 x 10" rad cm'in. Since the range of the secondary proton is ~ 10 um in tissue, CPE exists and K = D even in very small tissue samples. Thermal neutrons have a larger probability of capture by hydrogen atoms in mus- cle, even though OH = 3.32 x 10"" cm /atom