Measurement of deep penetration of neutrons produced by 800-MeV proton beam through concrete and iron at ISIS

A deep penetration experiment through a thick bulk shield was performed at an intense spallation neutron source facility, ISIS, of the Rutherford Appleton Laboratory (RAL), UK. ISIS is an 800 MeV–200 μA proton accelerator facility. Neutrons are produced from a tantalum target, which is shielded with approximately 3-m thick iron and 1-m thick ordinary concrete in the upward direction. 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 0.6-m thicknesses, respectively, using the activation detectors of graphite, bismuth, aluminum and the multi-moderator spectrometer inserted indium. The attenuation lengths of concrete and iron for high-energy neutrons above 20 MeV produced at 90° to the proton beam were obtained from the 12C(n,2n)11C reaction rates of graphite. The neutron spectra through concrete and iron were obtained by the unfolding analysis of the reaction rates of the 12C(n,2n)11C, 27Al(n,α)24Na, 209Bi(n,xn)210−xBi (x=4–10) and 115In(n,γ)116mIn in the energy range of thermal to 400 MeV.