EGS4 Calculations Research Articles

Design, fabrication and characterization of neutron irradiation facility using accelerator-based photoneutron source

In this paper we describe the design of a neutron generator assembly using an electron accelerator. We have carried out design calculations for this assembly by optimizing neutron source strength and maximizing output neutron flux at a reasonable L/D ratio keeping in view the radiography applications. For this purpose, detailed Monte Carlo calculation using EGS-4 code was carried out to estimate photon flux. The results of EGS4 calculations are used in source definition card of MCNP simulation to compute neutron source strength from Beryllium in order to arrive at an optimum design. The parameters such as neutron source strength; collimation ratio etc. were optimized. The thermal neutron flux at each position of this assembly including those at the end of collimator, Cd ratio, neutron-to-gamma ratio has been estimated. Experimental measurements of these parameters are in good agreement with simulation results.

DÜŞÜK VE YÜKSEK ENERJİLİ GAMA IŞINLARININ MARUZ KALMA YIĞILMA FAKTÖRLERİNİN ETKİN ATOM NUMARASINA BAĞIMLILIĞI

The Zeff dependence of exposure buildup factors for plane incident source gamma rays of 200keV, 500keV, 1MeV, 5MeV, 10MeV and 50MeV have been investigated by using electron gamma shower Monte Carlo code, EGS4. To prepare materials with different Zeff, homogen mixtures composed of Li, Cu, Fe and Pb were used in different ratios. All the EGS4 calculations were performed in two dimensional plane geometry for each mixture. To check the accuracy of EGS4 calculations, an extra work has been performed for the exposure buildup factors of Pb, Fe and H2O. Results of these calculations are agreed well with the published data. In the EGS4 calculations, the exposure was obtained from the photon flux and the energy absorption coefficient of air. The effect of Rayleigh scattering was included but effects of bremstrahlung and fluorescent radiation were not taken into account. All calculations were performed with the exponential transform. The calculated data showed that buildup factors vary smoothly with increasing Z. At deep penetration depths, variation of buildup factors has more rapid variation. The value of the buildup factor for an arbitrary atomic number can be obtained by using data resulted from this calculations

The replacement correction factor for the BIPM flat cavity ion chamber and the value of

A graphite flat cavity ionization chamber is used at the BIPM in France to determine the absorbed dose to graphite in a 60Co photon beam and thereby used to determine the product of the value of W/e, the average energy required to produce an ion pair in dry air, and the value of (LDelta/p)a(C), the mean restricted mass collision stopping-power ratio for graphite to air in a 60Co beam. The accuracy of the (W/e) (LDelta/p)a(C) value thus determined depends upon the accuracy of the perturbation correction factors adopted for this chamber. The perturbation effect of this chamber was accounted for by the replacement correction factor whose value was calculated by an analytical method and confirmed by an EGS4 Monte Carlo calculation. The purpose of this study is to investigate the validity of the analytical and the EGS4 calculations by using recently established methods and the EGSnrc Monte Carlo code, a much improved version of EGS4, to calculate the replacement correction factors for the graphite chamber. It is found that the replacement correction factors used for the BIPM chamber are not correct: the values used are smaller than they should be by about 1%. This leads to a 1% overestimation of the (W/e) (LDelta/p)a(C) value determined by using this chamber. This implies that 60Co air kerma standards that are directly proportional to this product need to be reduced by 1%. Based on the values of the replacement correction factors calculated in this study, and on the value of (LDelta/p)a(C) evaluated from ICRU Report No. 37 stopping power for graphite, the value of W/e determined by using the BIPM chamber should be 33.61 +/- 0.08 J/C. If a more recent value of mean excitation energy for graphite (86.8 eV) and grain density are used to evaluate the graphite stopping power, then the value obtained for W/e is 34.15 +/- 0.08 J/C.

Energy and Angular Responses of the Criticality Accident Detector using a Plastic Scintillator

The Japan Atomic Energy Agency (JAEA), Nuclear Fuel Cycle Engineering Laboratories, operates a spent fuel reprocessing plant and MOX (Plutonium-Uranium Mixed Oxide) fuel fabrication plants. Criticality accident detectors have been installed in these facilities. The detector, the Toshiba RD120, is composed of a plastic scintillator coupled to a photomultiplier tube, and an operational amplifier. The alarm triggering point is set to 1.0–3.6 mGy.h−1 in photon dose rate to detect the minimum accident of concern. However, a plastic scintillator is principally sensitive not only to primary photons but also to neutrons by secondary photons and heavy charged particles produced in the detector itself. The authors calculated energy and angular responses of the RD120 criticality accident detector to photons and neutrons using Monte Carlo computer codes. The response to primary photons was evaluated with the MCNP-4B and EGS4 calculations, and photon and X-ray irradiation experiments. The response to neutrons that produce secondary photons and heavy charged particles from neutron interactions was computed using the MCNP-4B and SCINFUL, respectively. As a result, reliable response functions were obtained. These results will be a great help in reassessing the coverage area and in determining the appropriate triggering dose rate level in criticality accidents.

Dose backscatter factors for selected beta sources as a function of source, calcified plaque and contrast agent using Monte Carlo calculations

Electron backscattering is a prominent secondary effect in beta dosimetry, and is closely correlated with factors such as the geometries of the source and the scattering material, as well as the composition of the scattering material. Previous results were varied depending on the experimental setup and detector resolution, and were generally performed for monoenergetic electron beams, which makes direct application of these factors to beta sources difficult. In this work, backscattering factors were calculated with MCNP 4C for selected beta sources currently in use (32P and 90Sr/90Y) as well as for sources of potential therapeutic use (45Ca, 142Pr and 185W). Specifically, the calculations used beta spectra generated by the SADDE MOD2 code. The factors were calculated as a function of the distance from the interface between water and scatterers. The scatterers include source surroundings, source supporting materials and contrast agents commonly used for imaging purposes in brachytherapy. The results were fit by a simple function for future incorporation into a dose point kernel code. Due to the high-Z material content (iodine, Z = 53) in the contrast agent, a significant dose backscatter was observed near the water interface. Different cross-section algorithms in the MCNP code (inherent and ITS 3.0) affect the factor calculations. The results generated by the ITS 3.0 algorithm closely matched the EGS4 calculations for 32P. The dependence of backscatter factors on log(Z + 1) (Baily 1980 Med. Phys. 7 514–9) was observed for all the beta sources with a high correlation coefficient, R (>0.95). The overall results indicate that the backscattering effect is significant at short distances from the surface of the interface between water and the scattering material. This model can also aid in choosing a source support or mixing materials for beta brachytherapy sources.

Systematic Comparisons of Measurements and EGS4 Regarding 20-40 keV Photons

To investigate the validity of the EGS4 code regarding photon transport in the keV region, we performed systematic comparisons of measurements and EGS4. The scattered-energy spectra of monochronized synchrotron-radiation photons toward 90° by samples (C, Cu, Pb) were measured using high-purity Ge detectors. The incident photons were in a linearly polarized beam of 20, 30 and 40 keV. EGS4 calculations were performed in default status and with improvements of low-energy photon transport (linearly polarized photon scattering, Doppler broadening, L-X and K-X). The measurement and calculations were compared in absolute values. Both the intensity and the shape of the peaks were compared. EGS4 calculations with improvements in low-energy photon transport agreed well with the measurements.

Monte Carlo calculations of the dose backscatter factor for monoenergetic electrons

Recently, there has been growing interest in beta emitters for therapeutic uses, especially in connection with so-called endovascular (or intravascular) brachytherapy. Since accurate dose estimation is necessary for the success of such applications, some problems in beta-ray dosimetry need further study. Among these problems, we have investigated the effect of electron backscattering on dose, which has significance not only for accurate dose estimation but also for new source design. In this study, an empirical measure of electron backscattering, known as the dose backscatter factor, was calculated using EGS4 Monte Carlo calculations for monoenergetic electrons and various scattering materials. Electron energies were 0.1, 0.5, 1.0, 2.0 and 3.0 MeV in combination with Al (Z = 13), Ti (Z = 22), Sr (Z = 38), Ag (Z = 47) and Pt (Z = 78) scatterers. The dose backscatter factor ranged from 10% to 60%, depending on electron energy and material, and was found to increase with the atomic number Z by a log(Z + 1) relationship. A method is presented for calculating the beta-ray dose backscatter factors using the results of this study. To demonstrate the efficacy of this method, a dose backscatter factor depth profile for near a water/aluminium interface was calculated and these calculated results were found to generally reproduce the depth profile obtained from direct EGS4 calculations using the spectrum. The data presented in this study can be used to calculate dose backscatter factors for any combination of beta emitter/scatterer whose atomic number ranges from 13 to 78.

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.

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.

EGS4 calculations of the dose to the electronic equipment in the PEP-II ring

Abstract The radiation doses to the electronic devices placed under the HER (High Energy Ring) in the PEP-II storage ring were calculated using the EGS4 code. The doses result from a 4-GeV positron and a 10-GeV electron beam uniformly lost around the ring. The annual particle loss per beam was 6.1 × 10 17 , including both stored beam loss and injection loss. The vacuum chambers (the targets for beam losses) of the LER (Low Energy Ring) and the HER were 1-cm-thick aluminum and 0.5-cm-thick copper, respectively. The shielding from the C-shaped iron bending magnet and the aluminum coil windings of the HER was considered. Both vacuum chambers were located about 1 m from the left wall of the ring (modeled as a 3.5 m wide, 2.9 m high and 100 m long tunnel and with a concrete wall thickness of 60 cm). The LER was 90 cm above the HER, which was 80 cm above the floor. The calculations show that the electronic devices need to be shielded on the left side by 1-cm-thick iron for the dose to remain below a damage threshold of 10 4 rad, when integrated over a ten-year period. The dose from non-uniform stored beam loss around the ring, due to high vacuum pressure in local regions from the ion avalanche effect, is also analyzed and reported in the Appendix A.

Giant dipole resonance neutron yields produced by electrons as a function of target material and thickness.

This paper characterizes the functional dependence of the giant dipole resonance neutron yield produced by electrons in terms of the atomic number (Z) and thickness (T) of the target. The yields were calculated by integrating, over the photon energy, the product of the differential photon track length and published photoneutron cross sections. The EGS4 Monte Carlo code and analytical formulas were used to calculate the differential photon track length. In thick targets, the Giant Dipole Resonance neutron yield approaches a saturation value as target thickness T increases to 10 radiation lengths. A formula, 8 x 10(-6) x (Z1/2 + 0.12 Z3/2 - 0.001 Z5/2) n electron-1 MeV-1, developed from EGS4 calculations, estimates thick-target neutron yields for incident electron energies Eo above 50 MeV. Giant dipole resonance neutron yields, calculated by several analytic formulas for the differential photon track length, are compared with EGS4 calculations. Modifications to the analytic formulas are suggested. A scaling function is derived to estimate, from the thick-target formula, neutron yields produced in thin targets.

A comparison of dosimetry techniques in stereotactic radiosurgery

Accurate dosimetry of small-field photon beams used in stereotactic radiosurgery (SRS) can be made difficult because of the presence of lateral electronic disequilibrium and steep dose gradients. In the published literature, data acquisition for radiosurgery is mainly based on diode and film dosimetry, and sometimes on small ionization chamber or thermolominescence dosimetry. These techniques generally do not provide the required precision because of their energy dependence and/or poor resolution. In this work PTW diamond detectors and Monte Carlo (EGS4) techniques have been added to the above tools to measure and calculate SRS treatment planning requirements. The validity of the EGS4 generated data has been confirmed by comparing results to those obtained with an ionization chamber, where the field size is large enough for electronic equilibrium to be established at the central axis. Using EGS4 calculations, the beam characteristics under the experimental conditions have also been quantified. It was shown that diamond detectors are potentially ideal for SRS and yield more accurate results than the above traditional modes of dosimetry.

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.

Beta-ray dose distributions from point sources in an infinite water medium.

The ACCEPT Monte Carlo code has been used to calculate radial dose distributions around isotropic point sources of monoenergetic electrons between 0.01 and 10 MeV in an infinite water medium. The results were averaged over beta spectra to derive distributions for 147 beta emitters of which 32 are presented. More extensive tables of distributions will be presented in a report. Distributions for monoenergetic electrons agree with recent ETRAN-code calculations of Berger and Seltzer within 2%, except at very short distances where there are differences up to several percent. Results calculated by the EGS4 code differ by up to a few percent. Distributions for beta emitters are in excellent agreement with both experimental results and ETRAN and EGS4 calculations, except at very short distances.