A Cerenkov fiber-optic dosimeter (CFOD) is fabricated using plastic optical fibers to measure Cerenkov radiation induced by a therapeutic photon beam. We measured the Cerenkov radiation generated in optical fibers in various irradiation conditions to evaluate the usability of Cerenkov radiation for a photon beam therapy dosimetry. As a results, the spectral peak of Cerenkov radiation was measured at a wavelength of 515 nm, and the intensity of Cerenkov radiation increased linearly with increasing irradiated length of the optical fiber. Also, the intensity peak of Cerenkov radiation was measured in the irradiation angle range of 30 to 40 deg. In the results of Monte Carlo N-particle transport code simulations, the relationship between fluxes of electrons over Cerenkov threshold energy and energy deposition of a 6 MV photon beam had a nearly linear trend. Finally, percentage depth doses for the 6 MV photon beam could be obtained using the CFOD and the results were compared with those of an ionization chamber. Here, the mean dose difference was about 0.6%. It is anticipated that the novel and simple CFOD can be effectively used for measuring depth doses in radiotherapy dosimetry.

A series of preliminary experiments on an accelerator-driven subcritical reactor (ADSR) with 14 MeV neutrons were conducted at Kyoto University Critical Assembly (KUCA) with the prospect of establishing a new neutron source for research. A critical assembly of a solid-moderated and-reflected core was combined with a Cockcroft-Walton-type accelerator. A neutron shield and a beam duct were installed in the reflector region for directing as large a number as possible of the high-energy 14 MeV neutrons generated by deuteron-tritium (D-T) reactions to the fuel region, since the tritium target is located outside the core. And then, neutrons (14 MeV) were injected into a subcritical system through a polyethylene reflector. The objectives of this paper are to investigate the neutron design accuracy of the ADSR with 14 MeV neutrons and to examine experimentally the neutronic properties of the ADSR with 14 MeV neutrons at KUCA. The reaction rate distribution and the neutron spectrum were measured by the foil activation method for investigating the neutronic properties of the ADSR with 14 MeV neutrons. The eigenvalue and fixed-source calculations were executed using a continuous-energy Monte Carlo calculation code MCNP-4C3 with ENDF/B-VI.2 for the subcriticality and the reaction rate distribution, respectively; the unfolding calculation was done using the SAND-II code coupled with JENDL Activation Cross Section File 96 for the neutron spectrum. The values of the calculated subcriticality and the reaction rate distribution were in good agreement with those of the experiments. The results of the experiments and the calculations demonstrated that the installation of the neutron shield and the beam duct was experimentally valid and that the MCNP-4C3 calculations were accurately carried out for analyzing the neutronic properties of the ADSR with 14 MeV neutrons at KUCA.

In preparation for connecting a Fixed Field Alternating Gradient (FFAG) accelerator at the Kyoto University Critical Assembly (KUCA), static and kinetic experimental and numerical analyses of AcceleratorDriven System (ADS) are envisaged. Improvement of source multiplication by the implementation of a neutron guide is expected to affect the characteristics of the ADS core. The effects on reactivity and neutron flux distribution are investigated by measurements of the subcriticality and the reaction rate distributions of indium wire, respectively, using the KUCA core and the current 14 MeV pulsed neutron generator. A comparison of the results of static measurements with those of calculations provides an estimation of MCNP-4C3 precision with JENDL-3.3 and ENDF/B-VI.2 for the basic research on ADS with 14 MeV neutrons at KUCA. The neutron guide has proved to be experimentally effective in improving external source when it is extended into the fuel region. Since the reactivity range of the actual ADS operation is expected to be around 3%Ák=k, the validity of both measurements and calculations has been demonstrated up to 6%Ák=k for the current KUCA cores. The prompt neutron decay constant is obtained by the pulsed neutron method, and subcriticality by the area ratio method, using an optical fiber detection system developed at KUCA. The optical fiber detector has been assessed as a promising original tool for the measurements of the subcriticality and prompt neutron decay constant, through ADS kinetic experiments with 14 MeV neutrons.

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