Measurements and calculations of thermal neutron fluence rate and neutron energy spectra resulting from moderation of Cf-252 fast neutrons: Applications for neutron capture therapy

Cf-252 is a neutron emitting radioisotope which has promise for both standa rd brachytherapy and neutron capture enhanced brachytherapy. In this study, experimental measurements and calculations were used to determine the ther mal neutron fluence rate, phi(th) [n cm(-2) s(-1) mg(-1)], in the vicinity of Cf-252 applicator tube (AT) type sources. Results of these measurements were confirmed with Monte Carlo calculations performed in a distributed man ner on multiple workstations using MCNP. Three studies were executed: (1) r elative phi(th) as a function of distance from a Cf-252 AT source in an A-1 50 tissue equivalent plastic phantom using thermoluminescent dosimeters (TL Ds) of varying Li-6/Li enrichment, (2) phi(th) measured with gold foils in a 114 liter water phantom 5 cm from two Cf-252 AT sources, and (3) calculat ions of the impact of phantom material composition (e.g., A-150, water, bra in, muscle) on phi(th) from moderated Cf-252 fast neutrons. TLD results and Monte Carlo calculations in A-150 of relative phi(th) typically agreed wit hin 1% and at most differed by 3% for distances from 1 to 6 cm. Foil measur ements followed the ASTM E 262-86e protocol, and the ratio of activated pla in and Cd encased gold foils (7.31) agreed well with the calculated ratio ( 7.26). Measured phi(th) at 5 cm (1.70+/-0.10 X 10(7) n cm(-2) s(-1) mg(-1)) was 10% greater than that determined using MCNP (1.55+/-0.12 X 10(7) n cm( -2) s(-1) mg(-1)), but was within the combined uncertainties. Compared with A-150 at a distance of 1 cm, phi(th) was 20%, 22%, and 32% less for water, brain, and muscle, respectively; these ratios decreased to 16%, 16%, and 2 4% less, respectively, at a distance of 5 cm from the source in a 15 cm dia meter phantom. Comparisons of these results generally agreed with those in the literature for a value of 2 X 10(7) n cm(-2) s(-1) mg(-1) in water at 3 cm. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(0 0)01208-6].