Scope of Pb-Sn binary alloys as gamma rays shielding material

An attempt has been made to prepare binary alloys of Pb and Sn in different compositions. The physical properties of the prepared alloys viz. dimensions and density has been measured. Further, different shielding parameters: effective atomic number (Z_eff), electron density (N_e), mean free path (mfp) were computed for the prepared Pb-Sn binary alloys in the wide energy range from 1.0 keV to 100.0 GeV. The variation of effective atomic number, electron density, mean free path for the selected alloys with incident photon energy has been analyzed. The work has been extended to compute exposure buildup factor (EBF) using five parametric geometric progression (GP) fitting method in the energy range 0.015–15.0 MeV and up to the penetration depth of 40 mfp. Further, the variation of exposure buildup factor with incident photon energy as well as penetration depth for the selected alloys has been investigated.     

Photon energy absorption and exposure buildup factors for deep penetration in human tissues

Using photons in therapeutic and diagnostic medicine requires accurate computation of their attenuation coefficients in human tissues.The buildup factor,a multiplicative coefficient quantifying the ratio of scattered to primary photons,measures the degree of violation of the Beer-Lambert law.In this study,the gamma-ray isotropic point source buildup factors,specifically,the energy absorption buildup factor (EABF) and exposure buildup factor,are estimated.The computational methods used include the geometric progression fitting method and simulation using the Geant4 (version 10.4) Monte Carlo simulation toolkit.The buildup factors of 30 human tissues were evaluated in an energy range of 0.015-15 MeV for penetration depths up to 100 mean free paths (mfp).At all penetration depths,it was observed that the EABF seems to be independent of the mfp at a photon energy of 1.5 MeV and also independent of the equivalent atomic number (Zeq)in the photon energy range of 1.5-15 MeV.However,thebuildup factors were inversely proportional to Zeq for energies below 1.5 MeV.Moreover,the Geant4 simulations of the EABF of water were in agreement with the available standard data.(The deviations were less than5%.) The buildup factors evaluated in the present study could be useful for controlling human exposure to radiation.     

Evaluation of gamma ray buildup factor data in water with MCNP4C code

The exposure buildup factors for gamma and X-ray photons in water are computed using the MCNP4C code. The results are obtained for the energy range 0.04–6 MeV and penetration depths up to 10 mfp. The results are compared with the published buildup factor data during 1960–2010. Both agreements and discrepancies are observed between our results and the data appearing in the literature. It is concluded that at higher energies and deep penetration the existing data for photon buildup factor in water are still inadequate.     

Buildup of gamma ray photons in flyash concretes: A study

The gamma ray buildup factors of flyash concretes have been calculated by using Geometrical Progression formula in the energy region of 0.015–15 MeV as well as up to a penetration depth of 40 mean free paths, and have been studied as a function of incident photon energy. From the obtained results it is seen that for a fixed penetration depth the values of buildup factor are very large in the medium energy region and are small in the low and high energy regions. The results have been shown graphically.     

A comprehensive study on energy absorption and exposure buildup factors for some essential amino acids, fatty acids and carbohydrates in the energy range 0.015-15 MeV up to 40 mean free path

The gamma ray energy absorption (EABF) and exposure buildup factors (EBF) have been calculated for some essential amino acids, fatty acids and carbohydrates in the energy region 0.015-15 MeV up to a penetration depth of 40 mfp (mean free path). The five parameter geometric progression (G-P) fitting approximation has been used to calculate both EABF and EBF. Variations of EABF and EBF with incident photon energy, penetration depth and weight fraction of elements have been studied. While the significant variations in EABF and EBF for amino acids and fatty acids have been observed at the intermediate energy region where Compton scattering is the main photon interaction process, the values of EABF and EBF appear to be almost the same for all carbohydrates in the continuous energy region. It has been observed that the fatty acids have the largest EABF and EBF at 0.08 and 0.1 MeV, respectively, whereas the maximum values of EABF and EBF have been observed for aminoacids and carbohydrates at 0.1 MeV. At the fixed energy of 1.5 MeV, the variation of EABF with penetration depth appears to be independent of the variations in chemical composition of the amino acids, fatty acids and carbohydrates. Significant variations were also observed between EABF and EBF which may be due to the variations in chemical composition of the given materials.     

A study of thickness and penetration depth dependence of specific absorbed fraction of energy in bone

G-P fitting method has been used to compute energy absorption build-up factor of bone compact and bone cortical for wide energy range (0.015–15 MeV). The computed absorption build-up factor is used to estimate specific absorbed fraction of energy. The thickness of the medium up to 10 mm and with penetration depth up to 40 mean free paths considered. The dependence of specific absorbed fraction of energy on incident photon energy, penetration and the thickness of the medium have also been studied. The variations of specific absorbed fraction of energy with incident photon energy, penetration and the thickness of the medium are shown in graphs. The computed specific absorbed fractions of energy values are more accurate than the data available in the literature because the variation of an effective atomic number with energy is also considered in the calculation. The application of the present work is also discussed.     

Photon energy absorption parameters for some polymers

Some photon energy absorption parameters viz. mass energy absorption coefficient (μ/ρ)_en, photon energy absorption effective atomic number (Z_PEA), electron density (N_e) and KERMA relative to air has been computed in the energy range from 1 keV to 20 MeV for some polymers such as nylon, poly-acrylo-nitrile, poly-methyl-acrylate, poly-vinyl-chloride, poly-styrene, synthetic rubber and poly-tetra-fluro-ethylene. The dependence of different parameters on incident photon energy and chemical composition of the selected polymers has been studied .     

Determination of gamma and fast neutron shielding parameters of magnetite concretes

In this study, some gamma shielding parameters such as mass attenuation coefficient (μ_ρ), effective atomic number (Z_eff), electron density (N_el) and buildup factors have been investigated for concretes with and without magnetite aggregate. The measurements have been carried out using 1.25 MeV (mean energy of 1.1732 MeV and 1.3325 MeV photons of a ⁶⁰Co radioactive source) gamma photons. The theoretical values of μ_ρ have been calculated in the energy range from 1 keV to 100 GeV by WinXCom computer code and these values were used in order to calculate the values of Z_eff and N_el. And fast neutron shielding parameter namely effective removal cross-sections (Σ_R, cm⁻¹) have been calculated. In addition, Energy absorption buildup factors (EABF) and exposure buildup factors (EBF) values of concrete samples have been calculated for photon energy 0.015–15 MeV up to 40 mfp (mean free path) penetration depths. The results of this study showed that the magnetite concrete is more efficient than the ordinary concrete for fast neutrons and gamma rays.     

Gamma-Ray Shielding Effectiveness of Some Alloys for Fusion Reactor Design

The gamma-ray shielding effectiveness of some oxide dispersion-strengthen (ODS) alloys by means of mass attenuation coefficients, mean free path, exposure buildup factors and energy absorption buildup factors have been investigated in the present study. The buildup factors were calculated using geometrical progression method for photon energy 0.015–15 MeV up to 40 mfp penetration depth. The mass attenuation coefficients were calculated by using XCOM program and Geant4 simulation methods and found a very good agreement. Our investigation signifies that the low iron content ODS alloys are superior shielding materials with the lower buildup factors. This study should be useful for selection of shielding materials for their applications in fusion reactors design and future nuclear reactor technologies.     

Analysis of Transmitted Gamma-Rays by Multiple Scattering Method, (I)

Dose buildup factors and number spectra of γ-rays transmitted through a homogeneous finite slab have been estimated by the multiple scattering method, taking into account scattering — including back scattering — up to the fourth order.

The calculations were performed for 1.0, 3.0, 6.0, 8.0 and 10.0 MeV γ-rays normally incident on lead, iron and water slabs of thicknesses from 1 to 15 mfp.

The results of the above calculations are in good agreement with those from other calculations, such as by Monte Carlo and response matrix methods, especially for heavy shielding materials of practical importance and γ-rays of high incident energy.

Further, a method is proposed with which the contribution to the total dose buildup factor by the γ-rays of the fifth and higher orders of scattering can be estimated approximately. With this method, good agreement was obtained with the dose buildup factor calculated by the Monte Carlo method, even for light shielding materials and γ-rays of low incident energy.     

Analysis of Transmitted Gammaγ-rays by the Multiple Scattering Method, (II)

Dose buildup factors and number spectra of γ-rays transmitted through stratified slabs have been estimated by the multiple scattering method, taking scattering into consideration up to the fourth order and including back scattering.

Further, the method proposed in the previous paper for estimating the contribution to the total dose buildup factor by γ-rays of the fifth and higher orders of scattering was applied to the present case of stratified slabs made up of different materials.

The calculations covered the range of source energy from 1 to 10 MeV, and various arrangements of strata made up of combinations of two of the three materials water, iron and lead, making up slabs of total thicknesses up to 6 mfp. Calculations were also performed for ⁶⁰Co γ-rays normally incident on stratified slabs of water-lead-water.

The results of the above calculations were found to be in good agreement with those of other calculations, such as by Monte Carlo method and by numerical integration of the photon transport equation. The present results were also found to agree well with the dose buildup factors calculated by our own empirical formula. Comparisons with experimental results too have proved good.     

Estimation of neutron production from accelerator head assembly of 15 MV medical LINAC using FLUKA simulations

For the production of a clinical 15 MeV photon beam, the design of accelerator head assembly has been optimized using Monte Carlo based FLUKA code. The accelerator head assembly consists of e-γ target, flattening filter, primary collimator and an adjustable rectangular secondary collimator. The accelerators used for radiation therapy generate continuous energy gamma rays called Bremsstrahlung (BR) by impinging high energy electrons on high Z materials. The electron accelerators operating above 10 MeV can result in the production of neutrons, mainly due to photo nuclear reaction (γ, n) induced by high energy photons in the accelerator head materials. These neutrons contaminate the therapeutic beam and give a non-negligible contribution to patient dose. The gamma dose and neutron dose equivalent at the patient plane (SSD = 100 cm) were obtained at different field sizes of 0 × 0, 10 × 10, 20 × 20, 30 × 30 and 40 × 40 cm², respectively. The maximum neutron dose equivalent is observed near the central axis of 30 × 30 cm² field size. This is 0.71% of the central axis photon dose rate of 0.34 Gy/min at 1 μA electron beam current.     

Barium-borate-flyash glasses: As radiation shielding materials

The attenuation coefficients of barium-borate-flyash glasses have been measured for γ-ray photon energies of 356, 662, 1173 and 1332 keV using narrow beam transmission geometry. The photon beam was highly collimated and overall scatter acceptance angle was less than 3°. Our results have an uncertainty of less than 3%. These coefficients were then used to obtain the values of mean free path (mfp), effective atomic number and electron density. Good agreements have been observed between experimental and theoretical values of these parameters. From the studies of the obtained results it is reported here that from the shielding point of view the barium-borate-flyash glasses are better shields to γ-radiations in comparison to the standard radiation shielding concretes and also to the ordinary barium-borate glasses.     

Long-term measurements of equilibrium factor with electrochemically etched CR-39 SSNTD

Recently, our group proposed a method (proxy equilibrium factor method) using a bare LR 115 detector for long-term monitoring of the equilibrium factor. Due to the presence of an upper alpha-particle energy threshold for track formation in the LR 115 detector, the partial sensitivities to ²²²Rn, ²¹⁸Po and ²¹⁴Po were the same, which made possible measurements of a proxy equilibrium factor F_p that was well correlated with the equilibrium factor. In the present work, the method is extended to CR-39 detectors which have better-controlled etching properties but do not have an upper energy threshold. An exposed bare CR-39 detector is first pre-etched in 6.25 N NaOH solution at 70 °C for 6 h, and then etched electrochemically in a 6.25 N NaOH solution with ac voltage of 400 V (peak to peak) and 5 kHz applied across the detectors for 1 h at room temperature. Under these conditions, for tracks corresponding to incident angles larger than or equal to 50°, the treeing efficiency is 0% and 100% for incident energies smaller than and larger than 4 MeV, respectively. A simple method is then proposed to obtain the total number of tracks formed below the upper energy threshold of 4 MeV, from which the proxy equilibrium factor method can apply.     

Photon interaction and energy absorption in glass: A transparent gamma ray shield

The effective atomic number, Z_eff, the effective electron density, N_e,eff, and the energy dependence, ED, have been calculated at photon energies from 1 keV to 1 GeV for CaO-SrO-B₂O₃, PbO-B₂O₃, Bi₂O₃-B₂O₃, and PbO-Bi₂O₃-B₂O₃ glasses with potential applications as gamma ray shielding materials. For medium-Z glasses, Z_eff is about constant and equal to the mean atomic number in a wide energy range, typically 0.3 < E < 4 MeV, where Compton scattering is the main photon interaction process. In contrast, for high-Z glasses there is no energy region where Compton scattering is truly dominating. Heavy-metal oxide glasses containing PbO and/or Bi₂O₃ are promising gamma ray shielding materials due to their high effective atomic number and strong absorption of gamma rays. They compare well with concrete and other standard shielding materials and have the additional advantage of being transparent to visible light. The single-valued effective atomic number calculated by XMuDat is approximately valid at low energies where photoelectric absorption is dominating.     

Effective atomic numbers for photon energy absorption of essential amino acids in the energy range 1 keV to 20 MeV

Effective atomic numbers for photon energy-absorption (Z_PEAeff) of essential amino acids histidine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine have been calculated by a direct method in the energy region of 1 keV to 20 MeV. The Z_PEAeff values have been found to change with energy and composition of the amino acids. The variations of mass energy-absorption coefficient, effective atomic number for photon interaction (Z_PIeff) and Z_PEAeff with energy are shown graphically. Significant differences exist between Z_PIeff and the Z_PEAeff in the energy region of 8-100 keV for histidine and threonine; 6-100 keV for leucine, lysine, tryptophan, phenylalanine and valine; 15-400 keV for methionine. The effect of absorption edge on effective atomic numbers and the possibility of defining two set values of these parameters at the K-absorption edge of high-Z element present in the amino acids are discussed. The reasons for using Z_PEAeff rather than the commonly used Z_PIeff in medical radiation dosimetry for the calculation of absorbed dose in radiation therapy are also discussed.     

Incident photon energy and Z dependence of L X-ray relative intensities

The intensity ratios, I_Lk/I_Lα1 (k = l, η, α₂, β₁, β_2,15, β₃, β₄, β_5,7, β₆, β_9,10, γ_1,5, γ_6,8, γ_2,3, γ₄), have been evaluated for elements with atomic number 36 ? Z ? 92 at incident photon energies ranging E_L1 < E_inc ? 200 keV using currently considered to be more reliable theoretical data sets of different physical parameters, namely, L_i (i = 1-3) subshell photoionization cross sections based on the relativistic Hartree-Fock-Slater model, the X-ray emission rates based on the Dirac-Fock model, and the fluorescence and Coster-Kronig yields based on the Dirac-Hartree-Slater model. At incident photon energies above the K-shell ionization threshold, the contribution to the production of different L X-ray lines due to the additional L_i (i = 1-3) subshell vacancies created following decay of the primary K-shell vacancies have also been included in the present calculations. The important features pertaining to dependence of the tabulated intensity ratios on the incident photon energy and atomic number have been discussed.     

Monte Carlo study of photoneutron production in U-235 following perturbations in cross section data

Active photon interrogation systems may be employed to detect high-Z isotopes without significant spontaneous fission emissions. These systems induce photonuclear reactions with emissions (such as fission neutrons) that may be detected. However, there are inconsistencies in the literature reporting resonance photonuclear interaction data for many isotopes. Recent publications show variations as large as 20% between various measurements of photonuclear cross section data. A perturbation methodology utilizing the modular nature of the MCNPX/MCNP-PoliMi code system has been implemented and is applied here to highly-enriched uranium. Monoenergetic photon sources between 8 and 18 MeV were simulated; neutron detection was performed using the MCNP-PoliMi liquid scintillator model. At photon energies less than 12 MeV, the number of detected neutrons is approximately 70% sensitive to changes in the (γ, f) cross section and 30% sensitive to changes in the (γ, n) cross section. As gamma-ray energy increases the (γ, f) sensitivity increases and the (γ, n) sensitivity decreases. There is a small (γ, 2n) sensitivity at photon energies between 15 and 17 MeV. The ability of modern simulation tools to predict photonuclear responses is greatly limited in this energy region due to the high sensitivity of the simulated results to observed discrepancies in photonuclear cross section data.