Hideo Hirayama

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.

Radiation damage in silicon microstrip detectors

Abstract A radiation damage effect on silicon strip detectors of pn-junction type is investigated using a high energy proton beam. In order to clarify the cause of leakage current increase, several variations of strip detectors with different surface structures were made and tested. No appreciable differences in leakage current increase are observed among these samples. A strong temperature dependence on leakage current is observed. This can be explained by a formation of radiation induced trap energy levels in the bulk silicon. A moderate room temperature annealing is seen. A pulse height degradation of about 10–20% is observed at a few Mrad of radiation. Some implications of the present results are discussed for possible application of silicon semiconductor detectors in future high energy hadron colliders.

Calculation of Gamma-Ray Buildup Factors up to Depths of 100 mfp by the Method of Invariant Embedding, (II) : Improved Treatment of Bremsstrahlung

An improved method to calculate the gamma-ray buildup factors including bremsstrahlung has been developed. The exposure buildup factors with bremsstrahlung were computed by the present method for lead, iron and water at the source energy of 10.0 MeV up to depths of 100 mfp. The accuracy of the present method was checked by comparison with the calculations by use of EGS4. Excellent agreement was obtained between the calculations by both methods about the exposure buildup factors per energy (energy spectrum of transmitted photons) for lead up to depths of 10 mfp and the ratio of the exposure buildup factor with bremsstrahlung to that without bremsstrahlung for lead, iron and water up to depths of 40 mfp. It is confirmed that the present method has an accuracy sufficient to be used to the generation of an improved set of gamma-ray buildup factors including bremsstrahlung.

Calculation of Gamma-ray Exposure Buildup Factors up to 40mfp using the EGS4 Monte Carlo Code with a Particle Splitting

Gamma-ray exposure buildup factors up to 40 mfp have been calculated using the Monte Carlo code EGS4 for water, iron and lead for point isotropic sources. The new algorithm which applies particle-splitting at each preset depth to simulate almost same number of particles from the preset depth is developed in order to obtain reasonable results with EGS4 at deep-penetrations. Comparisons of these results to the standard data calculated with the moments method or the discrete ordinates code PALLAS show good agreements if the same cross section data and energy absorption coefficients are used. The EGS4 calculations with this new algorithm will be a useful tool for future studies of gamma-ray buildup factors including the review of the standard data.

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.

Application of the EGS4 Monte Carlo Code to a Study of Multilayer Gamma-Ray Exposure Buildup Factors of up to 40 mfp

Multilayer γ-ray exposure buildup factors of up to 40 mfp were calculated using an electron-photon cascade Monte Carlo code, EGS4, as a point isotropic source. A kind of splitting technique was used in the EGS4 calculations in order to obtain reasonable results at very deep penetration problems, such as 40 mfp. The double-layer γ-ray exposure buildup factors were calculated for combinations of water, iron and lead for 0.1, 0.3, 0.6, 1.0, 3.0, 6.0 and 10MeV γ-rays. The thickness of the first medium was set to 1, 5, 10 or 20 mfp. As typical triple-layered shields, the calculations were performed for 4 different configurations of water, iron and lead. The behavior of multilayer buildup factors for deep penetration problems was studied using the obtained results. Buildup factors after the bounday show the tendency to vary largely depending on the material combination and the source energy of γ-rays.

Detailed Behavior of Exposure Buildup Factor in Stratified Shields for Plane-Normal and Point Isotropic Sources, Including the Effects of Bremsstrahlung and Fluorescent Radiation

Exposure buildup factors, energy spectra, and angular flux distribution for plane-normal incident and point isotropic source gamma rays of 0.1, 0.5, 1, 3, 6, and 10MeV penetrating two-layer water-lead and lead-water shields are calculated with the point Monte Carlo code EGS4. The effects of bremsstrahlung and fluorescent radiation are included. The value of the buildup factor in the second layer lies between those for infinite media of both materials in two source geometries in the 0.5- to 3-MeV energy range. However, this behavior varies remarkably and is enhanced with a bremsstrahlung contribution, when the source energy is higher than that corresponding to the minimum in the attenuation coefficient of lead. This varies equally with the fluorescent contribution when the source energy is close to the K edge of lead.

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.