The (He, tf) as a surrogate reaction to determine (n, f) cross sections in the 10-20 MeV energy range

The surrogate reaction ²³⁸U(³He, tf) is used to determine the ²³⁷Np(n, f) cross section indirectly over an equivalent neutron energy range from 10 to 20 MeV. A self-supporting ∼761 μg/cm² metallic ²³⁸U foil was bombarded with a 42 MeV ³He²⁺ beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL). Outgoing charged particles and fission fragments were identified using the Silicon Telescope Array for Reaction Studies (STARS) consisted of two 140 μm and one 1000 μm Micron S2 type silicon detectors. The ²³⁷Np(n, f) cross sections, determined indirectly, were compared with the ²³⁷Np(n, f) cross section data from direct measurements, the Evaluated Nuclear Data File (ENDF/B-VII.0), and the Japanese Evaluated Nuclear Data Library (JENDL 3.3) and found to closely follow those datasets. Use of the (³He, tf) reaction as a surrogate to extract (n, f) cross sections in the 10-20 MeV equivalent neutron energy range is found to be suitable.     

Non-dipole second order parameters of the photoelectron angular distribution for elements Z = 1-100 in the photoelectron energy range 1-10 keV

Presented here are the photoelectron angular distribution non-dipole parameters associated with the terms of the second order O [(kr)²] (k is the photon energy and r is the radius of the ionized atomic shell) for both unpolarized and linearly polarized radiation. The parameters are given for atomic shells with binding energies lower than 2 keV of all elements 1 ? Z ? 100 for four values of photoelectron energy in the range 1−10 keV. In this range, the second-order terms are shown to make a significant contribution (up to ∼30%) to the angular differential cross section. The inclusion of these terms becomes all the more important in calculations of the differential cross section ratio for the fixed geometry of angles which is measured experimentally in the case of linearly polarized radiation. The Dirac-Fock-Slater potential is used in the calculations. The hole left by the emitted electron is taken into account in the frozen orbital approximation.     

Some cross sections of Bromine isotopes induced by 14 MeV neutrons

Activation cross sections of Bromine isotopes induced by 14 MeV neutrons were measured. The cross sections for three (n, 2n), two (n, p) and one (n, α) reactions are reported in this work. The cross sections for ⁸¹Br (n, 2n) ^80gBr, ⁸¹Br (n, 2n) ^80mBr and ⁸¹Br (n, p) ^81gSe reactions at neutron energy of 13.5 MeV; and ⁸¹Br (n, p) ^81gSe at 14.1 MeV are given for the first time. The measured results are discussed and compared with the previous works.     

He+ (np) cross sections for 2, 3, 4 and 5 MeV C4+ ions colliding with He atoms

The cross sections of ionization plus excitation of He are measured by the C⁴⁺ + He prototype reaction with energies ranging from 2 to 5 MeV. Theoretically the independent electron approximation is used to calculate the HeII (np) ionization plus excitation cross sections. The results of the calculations are compared with our experimental data.     

The dissociation of CH and CH2 molecules in He and N2 at beam energies of 80-250 keV and possible implications for radiocarbon mass spectrometry

Isotopic ratios of ¹⁴C at natural levels can be efficiently measured with accelerator mass spectrometry (AMS). In compact AMS systems, ¹³CH and ¹²CH₂ molecular interferences are destroyed in collisions with the stripper gas, a process which can be described by dissociation cross sections. These dissociation cross sections determine the gas areal density required for sufficient attenuation of the interfering molecular beams, and are therefore key parameters in the effort to further reduce the terminal voltage and thus the size of the AMS system. We measured the dissociation cross sections of ¹³CH and ¹²CH₂ in N₂ and He in the energy range of 80-250 keV. In N₂, cross sections were constant for energies above 100 keV with average values per molecule of (8.1 ± 0.4) × 10⁻¹⁶ cm² for ¹³CH and (9.5 ± 0.5) × 10⁻¹⁶ cm² for ¹²CH₂. In He, cross sections were constant over the full measured range of 80-150 keV with average values of (4.2 ± 0.3) × 10^{− 16} cm² and (4.8 ± 0.4) × 10⁻¹⁶ cm², respectively. A considerable reduction of the terminal voltage from the currently used 200 kV while using N₂ for ¹³CH and ¹²CH₂ molecule dissociation is not possible: the required N₂ areal densities of ∼1.4 μg/cm², consequential angular straggling and a decreasing 1+ charge state fraction would reduce the ion beam transmission too much. This is not the case for He: sufficient molecule dissociation can be obtained with gas densities of ∼0.4 μg/cm², for which angular straggling is relatively small. In addition, the 1+ charge state fraction still increases at lower stripping energies. Thus, the usage of He for stripping and molecule dissociation might allow the development of even smaller ¹⁴C-AMS systems than available today.     

Predictions of multiple K- and L-shell ionization by alpha-particle impact on atoms with Z = 10 to 100

The peaked binary-encounter approximation is used to compute cross sections for removing one or more electrons from K- and L-shells of atomic targets with Z = 10, 15, 20, 25, 30, 40, 60, 80 and 100 by bombardment with alpha particles. The cross sections, σ_{1K, nL} and σ_{2K, nL}, are plotted over a range of alpha energies in the MeV region near the peaks of these cross sections. The relative magnitudes of the σ_{1K, nL} and σ_{2K, nL} cross sections are similar. Estimates of multiple-ionization cross sections for other projectiles may be determined in some instances by means of the z² scaling law for the ionization probability.     

Calculation and analysis of p + Ca reaction cross sections at incident energies from threshold to 250 MeV

All cross sections, angular distributions and energy spectra of neutron, proton, deuteron, triton, helium, alpha particle emission for p + ^{40,42,43,44,46,48,nat}Ca reactions have been calculated and analyzed at incident proton energies from threshold to 250 MeV by nuclear theoretical models. The theoretical calculated results are in good agreement with existing experimental data.     

Cross sections for ionization of K, L and M shells of atoms by impact of electrons and positrons with energies up to 1 GeV: Analytical formulas

Analytical formulas are presented for the easy calculation of cross sections for ionization of K, L and M shells of neutral atoms by impact of electrons and positrons with kinetic energies up to 1 GeV. Each formula contains a number of parameters that are characteristic of the element, the active electron shell and the projectile particle. The values of these parameters were determined by fitting the cross section values in an extensive database that was calculated recently by means of a composite algorithm that combines the distorted-wave and plane-wave Born approximations. Tables of parameter values are given for all elements, from hydrogen (Z=1) to einsteinium (Z=99). The proposed analytical expressions yield ionization cross sections that agree with those in the numerical database to within about 1%, except for projectiles with near-threshold energies.     

Molybdenum L-shell X-ray production by 350-600 keV Xe (q = 25-30) ions

Molybdenum L-shell X-rays were produced by Xe^q+ (q = 25-30) bombardment at low energies from 2.65 to 4.55 keV/amu (350-600 keV). We observed a kinetic energy threshold of Mo L-shell ionization down to 2.65-3.03 keV/amu (350-400 keV). The charge state effect of the incident ions was not observed which shows that the ions were neutralized, reaching an equilibrium charge state and losing their initial charge state memory before production of L-shell vacancies resulted in X-ray production. The experimental ionization cross sections were compared with those from Binary Encounter Approximation theory. Taking into account projectile deflection in the target nuclear Coulomb field, the ionization cross section of Mo L-shell near the kinetic energy threshold was well described.     

Charge resolution of CR-39 plastic nuclear track detectors for intermediate energy heavy ions

The charge resolution (δZ) for heavy ions (nuclear charge: Z < 40) of 0.1-1 GeV/n energy in CR-39 plastic nuclear track detector (PNTD) and its dependence on etching time, and on projectile Z and energy were investigated and optimized as part of an effort to make precise measurements of projectile charge-changing cross sections. Two types of CR-39 PNTD, HARZLAS TD-1 and BARYOTRAK, were exposed to heavy ion beams with seven values of Z behind thick targets to produce projectile fragments. Following chemical etching (7 N NaOH at 70 °C) for varying etch times, δZ of the projectiles was determined for each detector type. A strong dependence of δZ on the amount of bulk etch (B) was seen. It was also observed that δZ can be remarkably improved with longer etching time as a function of B^−1/2, in accordance with the trend seen in other types of track detector such as glass nuclear track detector. However, for B ? 60 μm (30 h etching), saturation occurs and there is no further improvement in δZ. Analysis of the correlations between projectile Z, energy, detector response, and fluctuation of the response make it possible to develop a model to predict the δZ for projectiles of given Z and energy. The predicted and measured values of δZ show good agreement within 10%. We conclude that 4 ? Z ? 30 at intermediate energy can be identified with good δZ in these detectors. The predictive model will be used in designing future cross section measurement experiments.     

Double differential cross sections of n + Cu reactions

The energy spectra and double differential cross sections of neutron, proton, deuteron, triton, helium and alpha-particle emissions for n + ^63,65,nat.Cu reactions are calculated and analyzed at incident neutron energies below 200 MeV. The optical model, the intra-nuclear cascade model, direct reaction theories, the unified Hauser–Feshbach and exciton model which includes the improved Iwamoto–Harada model are used. Theoretically calculated results are compared with the existing experimental data.     

Activation cross sections for Os(n,p)Re, Os(n,p)Re and Os(n,n′)Os reactions from 13.5 to 14.8 MeV

Cross sections for (n,p) and (n,n′) reactions have been measured on osmium isotopes at the neutron energies from 13.5 to 14.8 MeV using the activation technique in combination with high-resolution gamma-ray spectroscopy. Neutrons were produced via the ³H(d,n)⁴He reaction using solid TiT. The neutron fluences were determined using the monitor reaction ⁹³Nb(n,2n)^92mNb. Data are reported for the following reactions: ¹⁹⁰Os(n,p)^190mRe, ¹⁹⁰Os(n,p)^190gRe, ¹⁹⁰Os(n,p)¹⁹⁰Re, ¹⁸⁸Os(n,p)¹⁸⁸Re and ¹⁹⁰Os(n,n′)^190mOs. Nuclear model calculations using the code HFTT, which employs the Hauser-Feshbach (statistical model) and exciton model (precompound effects) formalisms, were undertaken to describe the formation of the products. The cross sections were discussed and compared with experimental data found in the literature, with values of model calculations including the pre-equilibrium contribution, and with evaluation data of JEFF-3.1/A.     

Determination of K shell absorption jump factors and jump ratios in the elements between Tm(Z = 69) and Os(Z = 76) by measuring K shell fluorescence parameters

The K shell absorption jump factors and jump ratios have been measured in the elements between Tm (Z = 69) and Os(Z = 76) without having any mass attenuation coefficient at the upper and lower energy branch of the K absorption edge. The jump factors and jump ratios for these elements have been determined by measuring K shell fluorescence parameters such as the total atomic absorption cross-sections, the K_α X-ray production cross-sections, the intensity ratio of the K_β and K_α X-rays and the K shell fluorescence yields. We have performed the measurements for the calculations of these values in attenuation and direct excitation experimental geometry. The K X-ray photons are excited in the target using 123.6 keV gamma-rays from a strong ⁵⁷Co source, and detected with an Ultra-LEGe solid state detector with a resolution 0.15 keV at 5.9 keV. The measured values have been compared with theoretical and others’ experimental values. The results have been plotted versus atomic number.     

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.     

The elastic He+d cross section of relevance for knock-on effects in radio frequency heated (He)D plasmas

The cross section for d+³He elastic scattering has been determined for the angular range 20-180° (CM) for beam energies E_d = 0.05 to 11 MeV through combined use of experimental data, Coulomb scattering and extrapolations. The results are used to study, for instance, how the cross section is affected by nuclear interaction contributions. Implications of these results on the calculation of knock on effects in (³He)D plasmas subjected to RF heating and their manifestations in the spectrum of the d + d fusion neutron emission are discussed.     

Relativistic cross sections for atomic K- and L-shell ionization by protons,calculated from a Dirac-Hartree-Slater model

Relativistic direct ionization cross sections for proton impact have been computed for atomic K and L shells, with Dirac-Hartree-Slater wave functions. Corrections for binding, polarization, and Coulomb deflection are included. Results are tabulated for proton energies from about 0.1 to 3 MeV, for 27 elements with atomic numbers 22 ? Z ? 92.     

Investigations of multiple backscattering and albedos of 1.12 MeV gamma photons in elements and alloys

The energy and intensity distributions of multiple backscattering of 1.12 MeV gamma photons emerging from targets of elements and alloys are observed as a function of thickness and atomic number (Z) of the target. The numbers of these multiply backscattered events show an increase with increase in target thickness, and then saturate for a particular thickness of the target called saturation thickness (depth). The saturation thickness decreases with increasing atomic number and varies as e^−Z. The multiple backscattering, an interfering background noise in Compton profile, has been successfully used to assign the ‘‘effective atomic number’’ to alloys. Monte Carlo calculations also support the present experimental results. The number, energy and dose albedos are also found to be saturating for the same thickness where the numbers of multiply backscattered events saturate.     

Stopping of 0.3-1.2 MeV/u protons and alpha particles in Si

The stopping cross sections ε(E) of silicon for protons and alpha particles have been measured over the velocity range 0.3-1.2 MeV/u from a Si//SiO₂//Si (SIMOX) target using the Rutherford backscattering spectrometry (RBS) with special emphasis put on experimental aspects. A detection geometry coupling simultaneously two solid-state Si detectors placed at 165° and 150° relative to each side of the incident beam direction was used to measure the energies of the scattered ions and determine their energy losses within the stopping medium. In this way, the basic energy parameter, E_x, at the Si/SiO₂ interface for a given incident energy E₀ is the same for ions backscattered in the two directions off both the Si and O target elements, and systematic uncertainties in the ε(E) data mainly originating from the target thickness are significantly minimized. A powerful computer code has been elaborated for extracting the relevant ε(E) experimental data and the associated overall uncertainty that amounts to less than 3%. The measured ε(E) data sets were found to be in fair agreement with Paul’s compilation and with values calculated by the SRIM 06 computer code. In the case of ⁴He⁺ ions, experimental data for the γ effective charge parameter have been deduced by scaling the measured stopping cross sections to those of protons crossing the same target with the same velocity, and compared to the predictions of the SRIM 06 computer code. It is found that the γ-parameter values generated by the latter code slightly deviate from experiment over the velocity region around the stopping cross section maximum where strong charge exchanges usually occur.