Reduction of the gamma-ray component from Cf-252 fission neutron source - optimization for biological irradiations and comparison with MCNP code

Gamma-rays contribute 33% of the absorbed dose from an unfiltered Cf-252 fi ssion neutron source. To reduce this gamma-lay component and to enable radi obiological experiments at as high a dose rate as possible, Monte Carlo cal culations for several filter materials (Al, Fe, Pb and concrete) have been made using MCNP neutron and photon transport code version 4a. A lead filter of thickness 4 cm was found to reduce the gamma-ray component to 6.7% of t he total dose whilst reducing the neutron dose by only about 10%. Such a fi lter was installed at the MRC Cf-252 neutron irradiation facility and dosim etric measurements were made using a TE-TE chamber and a (LiF)-Li-7(Mg, Cu, P) TLD. Monte Carlo simulations agree with experimental measurements of ne utron and gamma-ray doses within 6%. V79-4 Chinese hamster cells were irradiated with lead-filtered and unfilter ed neutrons and also with Co-60 gamma-rays at two dose rates. The survival fraction obtained for each radiation was consistent with the reduced gamma- ray dose. The relative biological effectiveness for neutrons alone, correct ed for gamma-ray effects, was found to be 9.2 +/- 3.4 from the initial slop es and 3.1 +/- 0.5 at 10% survival, both relative to the acute gamma-rays.