Angular Distribution of Photoneutrons from Copper and Tungsten Targets Bombarded by 18, 28, and 38 MeV Electrons

Following the angular distribution measurement of bremsstrahlung, we measured the angular distribution of photoneutrons between 0 and 120 degrees from copper and tungsten targets bombarded by 18, 28, and 38MeV electrons at the electron linac of Hokkaido University. The activation detectors and neutron dosimeters were applied to the measurement. The measured results were compared with the results calculated with the Monte Carlo code MCNP5 to verify the calculated results. The calculated results of both the reaction and ambient dose equivalent rates were in good agreement with the measured ones between 20 and 120 degrees in the cases of 28 and 38MeV electrons, but the calculation underestimated the measured data by a factor of 1.5 to 2.5 for 18MeV electrons. A large disagreement was observed at small forward angles of around 0 degrees. It was clarified that this disagreement was due to the contributions of photonuclear reactions in the detectors and dosimeters themselves, and of (e,n) reactions in the target.     

Shielding study on iron and concrete assemblies of bremsstrahlung photons and photoneutrons from copper target bombarded by 18, 28 and 38 MeV electrons

Following the angular distribution measurements of bremsstrahlung photons and photoneutrons, we measured the distributions of photon and neutron dose rates in the iron and concrete assemblies using a copper target bombarded by 18, 28 and 38 MeV electrons at the electron linear accelerator (linac) of Hokkaido University. In this experiment, seven types of shielding assemblies of iron and concrete layers were used and the photon and neutron dosemeters were inserted into the assemblies to get the depth–dose distribution. The measured results were compared with the results calculated using the Monte Carlo code MCNP5 to verify the calculated results. The calculated results of the ambient dose equivalent rates were in agreement with the measured results within 30% accuracy. Since no work on the radiation behavior in the shielding wall of medical linac room has ever been reported, this work gives valuable benchmark data for the detailed shielding design with high accuracy.     

Spatial Distribution of Bremsstrahlung in Water and Water-Iron by 22-MeV Electron Bombardment Measured with Activation Detectors

The spatial distributions of bremsstrahlung in water (1 m thick) and water (60 cm thick)-iron (6.3 cm thick) bombarded by 22-MeV electrons were measured by using a new activation method which we developed. These informations are useful for studying shielding, residual activity and radiation damage of accelerator and target materials.

From the measured activities, the bremsstrahlung spectra in water were evaluated with the LYRA and the SAND-II unfolding codes. The evaluated spectra were in good agreement with the analytical calculation by the DIBRE code, except for the higher energy ends. The longitudinal and the lateral distributions of bremsstrahlung flux in water were obtained by integrating the evaluated spectra above 8 MeV. The agreement of the experimental and the calculated flux distributions was very good except for a large angle to beam direction. The total photon number crossing a plane normal to the beam axis attenuates exponentially along the axial depth. The iso-flux contour of bremsstrahlung flux was given by interpolating the flux distribution curves.

Only the saturated activities of gold detectors were obtained for water-iron in good experimental accuracy. The spatial distribution of gold saturated activities in water-iron clearly shows the attenuating effect due to strong absorption in iron.     

Assessment of Neutron Cross Sections for Titanium with Neutron Spectrum in Titanium Pile

The measurement of angular neutron spectrum in a quasi-spherical pile of Ti was carried out by the linac time-of-flight method for the assessment of neutron cross sections for Ti in the energy range from a few keV to a few MeV. The measured spectrum in the pile is generally in good agreement with the calculated one from ENDF/B-IV (MAT = 1,286 for Ti) except in the energy range from about 60 keV to a few 100 keV, where the calculation gives considerably lower neutron flux than the measurement.

In order to investigate the cause of this discrepancy between the measured and calculated spectra, the total cross sections for Ti were measured by the transmission method. The results give larger values of total cross sections for Ti by about 30% than ENDF/B-IV below 200 keV, and smaller values by about 10% above 200 keV. These results were ensured at 55 and 147 keV by the measurement using a Si-filtered neutron beam. The calculation based on the measured cross sections shows better agreement with the measured spectrum than that based on ENDF/B-IV. The discrepancy is still observed around 100 keV.

The sensitivity analysis shows the importance of cross sections above 1 MeV and elastic cross sections in the resonance energy region to solve the disagreement between the measured spectrum and the calculated one.     

Calculation of Response Function of NE-213 Organic Liquid Scintillator for Gamma-Rays

A Monte Carlo code was newly developed to examine response functions of a 2 in. dia. by 2 in. long NE-213 liquid scintillator for γ-ray. This code included the electron transport and the electron-photon cascade calculation to handle the wall and end effect induced by high energy electrons. Results of the Monte Carlo calculation were compared with measured values for standard sources, reaction γ-ray and thermal neutron capture γ-ray, and fairly good agreement was obtained.

Response functions for monoenergetic γ-ray of energies 1~10 MeV were calculated at 1 MeV intervals. The calculations were made in parallel incident geometry without the peripheral material components surrounding the scintillator. The results are tabulated in this paper. A response matrix calculated from the above data was applied to the unfolding of NE-213 pulse height spectra using the FERDO code and fairly good results were obtained in absolute values.     

Calculation of the electron-electron bremsstrahlung cross-section in the field of atomic electrons

An accurate expression for the free electron-electron bremsstrahlung (ee-brems) cross-section was derived by Haug in 1975, but unfortunately it must be combined with a rather prosaic correction to account for electron binding in atoms. We report on a numerical calculation of the ee-brems cross-section which include the effect of binding explicitly from the outset. Electron-electron bremsstrahlung occurs when a photon is emitted during the collision of two electrons, and an accurate treatment of ee-brems is well-advised in light elements, where it is significant compared to the nuclear bremsstrahlung process dominating in heavier elements. Our calculation allows us to verify and refine the ee-brems cross-section calculation recommended as part of ICRU Report 37. The results are auspicious in that they confirm the validity of the recommended approximations for electrons with high incident energies and over most of the emitted photon energy range. For incident energies on the order of 1 MeV or lower, however, we find a considerable discrepancy between the ee-brems radiative cross-section obtained with the ICRU calculation and with our more accurate approach. Accordingly, we correct existing cross-section data tables for the EGSnrc radiation transport simulation software.     

Ep≤200 MeV能区p+58Ni反应的计算与分析

以现有质子诱发⁵⁸Ni的各种核反应截面、能谱、双微分截面、弹性散射角分布等实验数据为基础，利用自行研制的大型核模型计算程序MEND计算质子能量在200MeV能区内⁵⁸Ni(p,x)反应的截面、能谱、角分布和n、p、α、d、t、³He6种出射轻粒子的双微分截面。MEND程序的理论框架基于球形光学模型、核子的核内级联发射模型、以激子模型为基础的预平衡发射理论、蒸发模型和带宽度涨落修正的Hauser Feshbach统计理论。光学模型中的势参数由APMN程序通过符合p+⁵⁸Ni反应的去弹截面和弹性散射角分布获得。出射粒子的双微分截面则利用MEND程序输出的能谱再通过Kalbach系统学公式计算。将计算结果与实验数据及ENDF/B6评价库进行了比较，计算结果与实验数据基本一致，与ENDF/B6相比，增加了³He的计算，且将能区上推至200MeV。      

Neutron distribution and yield produced by 50MeV／u ^18O—ion on thick targets

Neutron energy,fluence rate,angular distributions and dose equivalent rate distributions around the thick Be,Cu,Au targets bombarded by 50MeV/u ^18O-ion were measured using a threshold detector activation method.At the same time,the neutron yields of ^18O-ion and the neutron emission rates in the forward direction were obtained approximately.     

Comparison of Two Types of Ideal Cascades for Asymmetric Separation Processes

The energy spectra of γ-rays in water, concrete, iron and lead are calculated by PALLAS- PL, SP-Br code for the plane perpendicular incidence. The calculation takes into account both annihilation and bremsstrahlung photons as well as Compton scattered ones as the secondary source.

Comparisons of energy spectra calculated by PALLAS were made with those of the moments method in water, iron and lead. In the PALLAS calculation the bremsstrahlung is neglected for the source energy of 6 MeV in iron because the influence of the bremsstrahlung on the energy spectrum is considered to increase for high energy photons. While for the case of inclusion of the bremsstrahlung source, a comparison of PALLAS calculations with experiments is made in transmitted dose through lead for a plane perpendicular 8-MeV source, resulting in good agreement.

Several examples of the calculated energy spectra in each material are presented at penetration distances of 0.5-, 1.0-, 2.0-, 4.0-, 7.0-, 10.0-, 20.0- and 40.0-mfp in 43 mfp depth for source energies of 0.1-     

Comparison of photoneutron yield from beryllium irradiated with bremsstrahlung radiation of different peak energy

In an effort to set up microtron based photoneutron source and in order to optimize the neutron yield, photoneutron production from beryllium has been studied for different volumes of beryllium irradiated by different peak energy bremsstrahlung radiation. The theoretical estimation of neutron yield has been carried out using the MCNP simulation for 8.75, 8.15 and 7.58 MeV peak energy of bremsstrahlung radiation. The experimental measurements were carried out using two types of detectors: SSNTD CR-39 and custom designed Silver wrapped GM detector. The neutron yield corresponding to beryllium of volume 381.70 cm³ are found to be 2.13E+09, 2.00E+09 and 1.74E09 n/s (MCNP calculation values) for electrons of energy 8.75, 8.17 and 7.48 MeV, respectively. The experimental results are compared with the MCNP simulated results and are good agreement.     

Soft Upsets in 16K Dynamic RAMs Induced by Single High Energy Photons

Soft upsets have been observed in dynamic random access memories (RAMs) that can be attributed to single high energy photon interactions. In the experiments, bremsstrahlung produced by the interaction of 40 MeV electrons with a thin tungsten converter has been found to produce soft upsets at flux levels well below those where photocurrent generation of upsets dominates. The number of upsets observed at low photon fluxes depends on the total number of photons which have been incident on the device but is independent of the dose rate. This behavior is consistent with preliminary calculations which assume that the upsets are caused by alpha particles Produced in the silicon chip by the nuclear reaction 28Si(?,?)24Mg. In these calculations the bremsstrahlung spectrum and the reaction cross section were integrated over the range from 15 to 22 MeV.     

p+~(107,109)Ag核反应数据的计算和分析

本工作通过理论计算的方法获得了一套适合入射能量为从阈值到200 MeV的p+107,109 Ag核反应全套微观数据。首先,使用光学模型理论进行调参计算,得到了一套适合入射能量为从阈值到340MeV的p+107,109 Ag核反应Becchetti-Greenlees光学势参数,这套参数与实验数据符合很好。其次,在这套光学势参数的基础上用扭曲波玻恩近似对入射能量从阈值到200MeV的p+107,109 Ag直接非弹性散射截面进行了计算。最后,使用核反应统计理论计算了入射能量从阈值到200MeV的p+107,109 Ag核反应各反应道的截面和出射粒子能谱,得到了该能区p+107,109 Ag核反应全套微观数据。将所有计算值与实验数据进行比较,结果表明,所得到的全套微观数据与实验数据符合很好。     

蒙特卡罗程序EGS4在γ照射量积累因子计算中的应用

使用蒙特卡罗程序EGS4计算了入射光子能量范围从0.015 MeV到15 MeV、屏蔽厚度达到40个平均自由程的普通混凝土的γ照射量积累因子,程序计算中使用的是PHOTX光子截面库、各向同性点源和无限均匀介质模型,针对深穿透问题采用了粒子分裂技巧,并考虑了轫致辐射、荧光效应和相干(瑞利)散射对积累因子的影响。并将本文的计算结果与相关文献进行了比较分析,表明本文使用EGS4计算的积累因子数据是可靠的,可以更广泛的使用蒙特卡罗方法来计算γ照射量积累因子。     

Double-Differential Neutron Emission Cross Sections of 6Li and 7Li at Incident Neutron Energies of 4.2, 5.4, 6.0 and 14.2 MeV

Energy-angle double-differential neutron emission cross sections of lithium isotopes were measured at incident neutron energies of 4.2, 5.4 and 14.2 MeV for ⁶Li and of 5.4, 6.0 and 14.2 MeV for ⁷Li using a time-of-flight spectrometer. Care was taken in background subtraction and in data correction for sample-size effects. Detailed comparison of the present results was made with the evaluated data in JENDL-3PR1. A spectrum fitting method was used to extract the ^6,7Li(n, n'x)α and (n, 2n) reaction cross sections. Neutrons emitted from the (n, 2n) reactions were well described by the conventional evaporation model. A simple calculation with a final-state Coulomb interaction was effectively applied for the ^6,7Li(n, n'x)α reactions. Angle-integrated cross sections of the ⁷Li(n, n't)α reaction were in good agreement with the JENDL-3PR1 data except the data measured at 6.0MeV. The angular distributions of elastically and inelastically scattered neutrons were successfully analyzed with the coupled-channel method at the incident neutron energy of 14.2 MeV.     

Bremsstrahlung energy spectra from electrons with kinetic energy 1 keV–10 GeV incident on screened nuclei and orbital electrons of neutral atoms with Z = 1–100

A comprehensive set of bremsstrahlung cross sections (differential in the energy of the emitted photons) is tabulated. The set includes results for electrons with energies from 1 keV to 10 GeV incident on neutral atoms with atomic numbers Z = 1 to 100. For bremsstrahlung in the Coulomb field of the atomic nucleus, use was made of (a) results of Pratt, Tseng, and collaborators based on numerical phase-shift calculations for the screened Coulomb potential at energies below 2 MeV; and (b) the analytical high-energy theory (with Coulomb corrections) of Davies, Bethe, Maximon, and Olsen at energies above 50 MeV, supplemented by the Elwert Coulomb correction factor and the theory of the high-frequency limit given by Jabbur and Pratt. In the high-energy region, the effect of screening was included by the use of Hartree-Fock atomic form factors. A numerical interpolation scheme, applied to suitably scaled cross sections, was used to bridge the gap between the low-energy and high-energy theoretical results, and thus to obtain improved cross sections in the intermediate-energy region 2 to 50 MeV. Bremsstrahlung in the field of the atomic electrons was calculated according to the theory of Haug, combined with screening corrections derived from Hartree-Fock incoherent scattering factors.     

Angular distributions of reflected MeV light ions from a single crystal surface at glancing incidence

The angular distributions of ions reflected from a SnTe(001) surface are measured for a glancing angle incidence of MeV protons and He⁺ ions, with the glancing angle less than 7 mrad. The observed angular distributions are compared with the calculated distributions derived from the stochastic theory taking account of both the thermal vibration of surface atoms and the scattering by electrons near the surface. From comparison of the experimental and calculated results the surface Debye temperature is determined to be 72 ±6 K.     

A function to provide neutron spectrum produced from the 9Be + p reaction with protons of energy below 12 MeV

A function to give the total neutron production cross section, angular distribution, and energy spectrum via the ⁹Be + p reaction has been created by fitting experimental data to characterize compact neutron sources with thick Be targets bombarded by protons with energy below 12 MeV. To examine the suitability of the function, calculations of the angle-dependent neutron energy spectra produced in thick Be targets with 4- and 12-MeV protons using the function were compared with corresponding experiments and calculations using the nuclear data libraries of ENDF/B-VII.0 and JENDL4.0/HE. The function was in better agreement with the experiments than the calculations using the libraries except for at backward angles. The ¹¹⁵In(n,n’)^115mIn reaction rates calculated using GEANT4 with source neutrons given by both the function and ENDF/B-VII.0 were compared with that measured at the RIKEN Accelerator-Driven Compact Neutron Source to evaluate the neutron spectrum above 1 MeV. The function slightly overestimated the measurement by 14% and the calculation with ENDF/B-VII.0 underestimated by 35%. It was concluded that the function can be applied in compact neutron source designs.     

Angular properties of 6.13 MeV source photons penetrating lead,steel and concrete

Experimentally determined angular flux spectra for 6.13 MeV source photons from disc geometry penetrating shielding slabs of lead, steel and concrete are analysed. For all three shield materials, the proportion of photons scattered in a forward direction increases with increasing penetration thickness. At any given polar angle, the scattered photon properties decrease exponentially with increasing shield thickness. Angular exposure dose build-up factor is defined and angular contributions to the scalar build-up factor are presented. Changes in the distribution of photon energies relating to shield penetration thickness and polar angle are discussed. The influence of bremsstrahlung and positron annihilation photon sources are considered.     

Neutrons produced by 75MeV／u ^12C—ion on thick targets

Fluence rates and angular distributions of the neutron emitted by 75MeV/u ^12C-ion bombardment on thick Be and Au targets have been measured by means of the threshold detector activation method.Based on that,the neutron yields,emission rates in the forward direction and neutron dose equivalent rate distributions were deduced.     

Monte Carlo Simulation of the Electron Transport Through Air Slabs: A Comparative Study of penelope, geant3, geant4 and egsnrc Monte Carlo Codes

The Monte Carlo simulation of the electron transport through air slabs is studied with four codes: PENELOPE, GEANT3, Geant4 and EGSnrc. Monoenergetic electron beams with energies 6, 12 and 18 MeV are considered to impinge on air slabs with thicknesses ranging from 10 to 100 cm. The angular and radial distributions of the transmitted electrons are used to make a comparison between the codes. Non-negligible differences are observed in the radial distributions. These differences produce worth effects in the macroscopical dose distribution absorbed in a water phantom situated behind the slabs.