Syuichi Ban

Implementation of the Doppler broadening of a Compton-scattered photon into the EGS4 code

Abstract A modification to the general-purpose Monte Carlo electron-photon transport code EGS4 [1] was made in order to include Doppler broadening of Compton-scattered photon energy due to electron pre-collision motion. The Compton-scattered photon energy is sampled from a cross section formula based on the Compton profile, and the Compton scattering is sustained if the energy imparted to the electron is less than its binding energy. The electron binding effect modifies the scattered photon energy, angular distribution, and total cross section of the Compton scattering, and affects the photon mean free path used in the calculations. In the improved EGS4 code, all of these electron binding effects in Compton scattering are treated consistently. A simulation of 40 keV photon scattering by C and Cu samples was performed using the improved EGS4 code; the calculated scattered photon spectra agreed well with the measurements.

Implementation of linearly-polarized photon scattering into the EGS4 code

Abstract A modification to the general-purpose Monte Carlo electron-photon transport code EGS4 [Nelson, Hirayama and Rogers, SLAC-265] was made in order to include linear polarization in the simulation of photon scattering. Both the Compton and Rayleigh scattering routines were modified to properly account for the electric-field vector of the photon. This vector is calculated at each photon scatter, and is passed along with the photons in the radiation transport, thereby allowing for a precise simulation of polarized multiple scattering. A simulation of the absorbed-dose distribution in a soft tissue equivalent phantom for a linearly polarized incident photon beam, scattering many times, was successfully performed.

Improvements of low-energy photon transport in EGS4

The treatment of low-energy (≤1 MeV) photon transport in the EGS4 code [Nelson, W.R., Hirayama, H. and Rogers, D.W.O., 1985. The EGS4 Code System SLAC-265 Stanford Linear Accelerator Center] has been improved. This also includes an improvement of the treatment of the electrons generated from low-energy photons and photons generated from low-energy electrons. The phenomena treated are: (i) linearly polarized photon scattering, (ii) Doppler broadening of Compton-scattered photons, (iii) L-X-ray and L-photoelectron production and (iv) electron impact ionization. We made measurements to verify the improved EGS4 code using monochromatized synchrotron radiation.

ESTIMATION OF ABSORBED DOSE DUE TO GAS BREMSSTRAHLUNG FROM ELECTRON STORAGE RINGS

We have studied characteristics of the bremsstrahlung produced in electron storage rings by interactions with residual gas in vacuum chambers. Quantitative estimates of intensity, absorbed dose and shielding requirements are given.

Transformation of a system consisting of plane isotropic source and unit sphere detector into a system consisting of point isotropic source and plane detector in Monte Carlo radiation transport calculation

In a nuclear power plant accident, radioactive nuclides may be released which are distributed uniformly on the ground. If estimation of dose rate from such a source by a Monte Carlo calculation is attempted, some difficulty is encountered because the calculation efficiency is very low. To solve this low efficiency problem, we show that a plane isotropic source can be transformed into a point isotropic source by changing the detector shape from a unit sphere to a plane. We verified the validity of this transformation by the numerical comparison of unscattered photon fluence. As an example of this transformation, the ambient dose rate D i was calculated from the uniform radioactive nuclide distribution on the ground using the EGS5 Monte Carlo code. We also measured the radioactivity and ambient dose rate (M) on the KEK campus within a month after the releases from the Fukushima No. 1 Nuclear Power Plant accident. Using radioactivity data and D i, we calculated the ambient dose rate (C). The calculated and measured ambient dose rates agreed reasonably well; their ratio (C/M) was 0.62 to 1.28.

Measurement of transverse attenuation lengths for paraffin, heavy concrete and iron around an external target for 12 GeV protons

Abstract Measurements of transverse attenuation lengths were made above the external target at KEK 12 GeV proton synchrotron using activation detectors. Evaluated attenuation lengths for high energy neutrons in paraffin, heavy concrete and iron shields are (133 ± 16), (163 ± 7) and (188 ± 12) g/cm 2 , respectively. These values are discussed in relation with the inelastic mean free paths.

Development of a microcalorimeter for measuring absolute intensity of synchrotron radiation

Abstract A total absorption calorimeter was developed to measure the absolute intensity of monoenergetic X-ray beams from 10 keV to 70 keV of synchrotron radiation. Experiments with synchrotron radiation have demonstrated that the heat power above about 1 μW due to monoenergetic synchrotron radiation is measured with an accuracy of about 1%.

Measurement of secondary neutron fluxes around beam stop for 500 MeV protons

Abstract Secondary neutron fluxes were measured around the beam stop bombarded by 500 MeV protons using activation detectors. The attenuation length of concrete was obtained for angles between 0° and 100°. To compare with experimental results, Monte Carlo transport calculations were performed using the NMTC code.

Dose measurements in inhomogeneous bone/tissue and lung/tissue phantoms for angiography using synchrotron radiation.

For angiography using synchrotron radiation we measured the absorbed dose distribution in inhomogeneous phantoms with thin LiF:Mg, Cu, P, LiF:Mg, Ti thermoluminescent dosimeters (TLDs) in tissue and lung substitutes, and with Mg2SiO4:Tb TLDs in bone substitute for 33.32 keV monoenergetic photons from synchrotron radiation. The energy responses of the TLDs were measured in air for 10-40 keV monoenergetic photons. The values at 30 keV became smaller by 30% for LiF:Mg, Cu, P and larger by 22% for Mg2SiO4:Tb than the ratio of the mass energy absorption coefficients of the TLDs to that of air. These values were used to modify the calculated response of the TLDs in each phantom material. The absorbed dose distribution obtained was compared with that calculated using the Monte Carlo transport code EGS4 expanded to a low-energy region, and their agreement was confirmed taking linear polarization into account. In the bone substitute the dose increased by a factor of 3.9, while behind the bone the dose decreased drastically because of photon attenuation. In the lung substitute a slight dose difference from that in soft tissue was observed because of its different density. The LiF:Mg, Cu, P TLDs exhibited a better energy response, higher sensitivity and wider linear regions than did the other tissue-equivalent TLDs in the low-energy region.

Measurement of the neutron spectrum by the irradiation of a 2.04-GeV electron beam into thick targets

Abstract Inclusive neutron spectra produced by the irradiation of a 2.04-GeV electron beam into thick Al, Cu, Sn and Pb targets were measured by the TOF method. For such a high-energy region, it was the first experiment to measure the inclusive neutron spectra. A lead shield was placed at the middle point of the flight path to suppress any strong photon or electron background. The effects of the shield on the neutron spectra were calculated by the LAHET2.7 code with a small modification. The measured neutron energy in the experiments was from 10 to 200 MeV . The measured spectra were used for a benchmark test of our modified PICA95. It was found that the neutron spectra predicted by a combination of EGS4, our modified PICA95 and LAHET2.7 codes, tend to underestimate the measured ones.