Deep penetration experiment through concrete and iron for shielding benchmark with 800 MeV proton accelerator

The shielding experiment of deep penetration was performed at an intense spallation neutron source facility, ISIS, of the Rutherford Appleton Laboratory (RAL) in 1998. This shielding benchmark experiment at an intense high-energy accelerator facility will also give useful informations for estimating the accuracy of the transport calculation for deep penetration. ISIS is an 800 MeV, 200x10-6 A-proton accelerator facility. Neutrons are produced at a tantalum target, which is shielded with approximately 3-m iron and 1-m ordinary concrete. The additional shielding blocks were placed upon the top center of the bulk shield in order to get the neutron attenuation length. These are concrete blocks (2.33g/cm3 ) of 119-cm diam by 20-cm thickness, which were piled up to 120-cm thickness, and iron blocks (7.8g/cm3 ) of the same diam by 10-cm thickness, which were piled up to 60-cm thickness. These shielding blocks were surrounded with the iron igloo of 60-cm thickness and 196-cm height to depress the spurious background neutrons. Since the neutrons were produced from the target as the burst pulses corresponding to the 50 Hz synchrotron operation, we used the activation detectors of graphite, aluminum and bismuth using 12C(n, 2n)11C, 27Al(n,alpha)24Na and 209Bi(n,xn)210-x Bi (x = 6 to approx. 10) reactions for neutron measurements. We also used the multimoderator spectrometer including the indium activation detector using 115In(n, gamma)116mIn reaction and the current-type neutron dose-equivalent counter. The gamma rays from the activation detectors were measured with two HPGe detectors after irradiation. In order to get good statistics of photo-peak counts, the repeated irradiations and activity measurements were performed for each shielding material and thickness several times a day. From thus-obtained activation rates, we could get the neutron flux and energy spectrum by unfolding with the SAND-2 code and the activation cross section data of ENDF/B-VI high-energy file. We then obtained that the attenuation lengths of neutron flux of energy above 20 MeV are 125 g/cm2 for concrete and 155 g/cm2 for iron. These deep penetration data are to be compared with the transport calculation. (author)