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A deep penetration experiment through a thick bulk shield was performed at an intense spallation neutron source facility, ISIS, of the Rutherford Appleton Laboratory. ISIS is an 800MeV-200 µ A proton accelerator facility. Neutrons are produced from a tantalum target, and are shielded with approximately 3m thick iron and lm thick ordinary concrete. On the top of the shield, we measured the neutron flux attenuation through concrete and iron shields which were additionally placed up to 1.2 m and 0.6m thicknesses, respectively, using activation detectors of carbon, aluminum and bismuth, and also indium-loaded multimoderator spectrometer. The dose attenuation was simultaneously measured with the neutron and photon survey meters. The attenuation lengths of concrete and iron for high energy neutrons above 20MeV were obtained from the 12 C(n,2n) reaction of carbon, and the neutron spectra penetrated through the additional shield and on the target shield top were obtained from the 12 C(n,2n), 27 Al(n,a) and 209 Bi(n, xn) reactions, and multi-moderator spectrometer.We are now analyzing the measured results to compare with the shielding calculation.

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The yields of charged particles that are emitted following the capture of negative pions have been measured. By using various combinations of Si and CsI(T1) detectors, the counter telescope was able to measure yields of protons, deuterons, tritons and helium nuclei down to 2 MeV and heavier nuclei down to 5 MeV. Targets of graphite, water, muscle-equivalent solution and rigid bone substitute were used, and data were taken a t a scattering angle of 90" at both the peak and the plateau of the depth-dose distribution and additionally at 45' in the plateau. The observed thick target yields showed substantial numbers of deuterons and tritons in all targets as well as helium and lithium nuclei. For the graphite target only, beryllium nuclei also were observed. The number ratio of protons to deuterons and tritons was determined from the thick target spectra in carbon to be 1.27. This disagrees with the value 3.1 calculated by Guthrie et al. Our yields show agreement however with the results of Castleberry et al. and Budyashov et al. over the narrower energy range used by these workers. A W-value for a chamber at the peak containing nitrogen is calculated to be 35.8 eV per ion pair, indicating that an ionisation chamber calibrated against low LET radiation will underestimate the peak pion dose by 3.5%. Initial secondary particle spectra and dose : LET distributions are calculated from the measured yields. No significant difference is observed in the LET : dose distributions in muscleequivalent solution and rigid bone substitute. I n muscle-equivalent solution it is found that 46% of the total dose delivered by charged particles a t the peak has an LET > 10 keV pm-l and about 29% with an LET > 30 keV pm-l. The comparable figures for the plateau are 42% and 22% respectively.

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