CASSPERR: A γ–γ cascade detector for resonant nuclear reaction analysis

Resonant nuclear reaction analysis (RNRA) is sometimes the only technique able to quantify elements in a matrix containing other elements. Background due to cosmic rays and natural radioactivity has limited traditional RNRA to samples with relatively high concentrations of the measured element, or to facilities with large amounts of passive shielding. Many nuclear reactions of interest in RNRA produce excited states in the resulting nuclei that then de-excite by product-specific sequences of photon emissions. The CAScade SPEctrometer for Resonant Reactions (CASSPERR) selects only events that match the desired combination of photon emissions. Rejection of other events greatly reduces the background, thus improving the signal to noise ratio (SNR). Since it is constructed from available commercial components, CASSPERR opens RNRA to typical ion beam analysis facilities. The design, operation and evaluation of CASSPERR with applications to materials science are currently under investigation.     

Effect of γ-ray irradiation on Nd–Fe–B alloys

We report on how γ-ray irradiation affects the magnetic properties of a powder sample of Nd–Fe–B, which was irradiated at room temperature with doses up to 700 kGy. Both the magnetic properties and surface morphology were changed by the effects of the γ-ray irradiation. The unirradiated and irradiated samples were then characterized using the VSM, XRD and SEM techniques.     

Analysis of structural modifications in γ-irradiated PbO–B2O3–SiO2 glasses by FTIR spectroscopy

The effect of γ-irradiation on the structure of lead borosilicate glasses of varying composition has been probed by FTIR spectroscopy, before and immediately after γ-irradiation. The glasses were irradiated at Calliope ⁶⁰Co plant (RC ENEA Casaccia, Rome), and the spectra were recorded after absorbed doses of 50 Gy, 500 Gy, and 4 kGy. The structural analysis have been made considering both the effect of composition and of irradiation. The experimental results clearly indicate that after irradiation a significant change in structure of borosilicate glass network is observed.     

The effect of ageing temperature and time on the mechanical properties of Fe-NiCrMo alloys with different contents of δ ferrite

Laboratory cast alloys with 2–27% of δ ferrite were aged for up to 17,520 h in the temperature range 290–350 °C. Tensile and Charpy tests were performed at 22 and 290 °C on specimens aged for different times, and the microhardnesses of both constituents of the microstructure were determined for the alloy with 27% of δ ferrite. The effects of the content of δ ferrite, the ageing and testing temperature, and the ageing time on mechanical properties and notch toughness are presented and discussed.     

N(α,p)O nuclear reaction cross-section at 4.9–6.1 MeV

The ¹⁴N(α,p)¹⁷O nuclear reaction cross-section was measured in the energy range 4.9–6.1 MeV at the laboratory angle θ = 172° in order to apply ion beam analysis to the detection of N in metal oxynitride layers. Comparison between the new calculation and data in literature was performed, highlighting important discrepancies between the new results and old cross-section data. Application of the new calculations to a standard TiN film suggests improved accuracy for the data in the present work.     

Activity level of gross α and gross β in airborne aerosol samples around the Qinshan NPP

The monitoring results of gross α and gross β activity from 2001 to 2005 for environmental airborne aerosol samples around the Qinshan NPP base are presented in this paper. A total of 170 aerosol samples were collected from monitoring sites of Caichenmen village, Qinlian village, Xiajiawan village and Yangliucun village around the Qinshan NPP base. The measured specific activity of gross α and gross β are in the range of 0.02 - 0.38 mBq/m^3 and 0.10 - 1.81 mBq/m^3, respectively, with an average of 0.11 mBq/m^3 and 0.45mBq/m^3, respectively. They are lower than the average of 0.15 mBq/m^3 and 0.52mBq/m^3, of reference site at Hangzhou City. It is indicated that the specific activity of gross α and gross β for environmental aerosol samples around the Qinshan NPP base had not been increased in normal operating conditions of the NPP.     

Proton-induced γ-ray analysis of lithium in thick samples

A method for proton-induced γ-ray analysis (PIGE) of Li (either elemental or isotopic composition) in thick samples is presented in this work, which is based on a code that integrates the nuclear reaction excitation function, measured at the same experimental conditions, along the depth of the sample. The energy steps needed to define accurately the excitation function are used as energy intervals for the integration procedure. The excitation function and cross-sections of the reaction ⁷Li(p,p^′γ)⁷Li, for protons in the energy range 0.60–2.5 MeV, were obtained and used as input to calculate lithium concentrations in thick and intermediate samples. The calculated thick target yields of samples with several amounts of lithium were compared with the respective measured yields, showing that this is a reliable and accurate method for PIGE analysis of Li in thick samples.     

Quasi-molecular orbital calculation using the DV-Xα method in Ne–Ne collisions

Molecular orbital calculations were performed for Ne–Ne quasi-molecules formed during collisions by changing the internuclear distance between 0.01 and 10 a.u. using the discrete variational (DV)-Xα MO calculational method. The equilibrium number of electrons on the basis atomic orbitals were estimated as a function of internuclear distance from which the critical internuclear distance to form quasi-molecular orbitals were deduced.     

Atomic-scale simulation of displacement cascades and amorphization in β-SiC

Molecular dynamics (MD) methods with a modified Tersoff potential have been used to simulate Si displacement cascades with energies up to 50 keV and to compare clustering behavior for Si and Au recoils in β-SiC (3C). The results show that the lifetime of the thermal spike is very short compared to that in metals, and that the surviving defects are dominated by C interstitials and vacancies for Si displacement cascades. Only 19% of the interstitial population is contained in clusters, with the largest cluster containing only four interstitial atoms for energetic Si recoils. The energy dependence of stable defect formation exhibits a power-law relationship. The high energy Si recoil generates multiple sub-cascades and forms dispersed defect configurations. These results suggest that in-cascade amorphization in SiC does not occur with any high degree of probability during the lifetime of Si cascades. On the other hand, large disordered domains are created in the cascades produced by 10 keV Au recoils. Structure analysis indicates that these highly disordered regions have amorphous characteristics. The data for the cluster spectra have been used to calculate the relative cross-sections for in-cascade amorphization (or clustering) and defect-stimulated amorphization. The ratios of these cross-sections for Si and Au are in excellent agreement with those derived from a fit of the direct-impact/defect-stimulated model to experimental data.     

Effects of secondary electrons emitted from surroundings on defect formation in silica glass under γ-ray irradiation

We have investigated the effects of secondary electrons and photons emitted from surrounding materials on defect formation in silica glass under γ-ray irradiation. SiO₂ (silica) glass plates and those sandwiched in a pair of various material disks (carbon, stainless steel or lead) were irradiated by γ-ray, and the optical absorption spectra (UV–vis spectra) of the silica glass plates before and after the irradiation were examined. UV–vis spectra of the glass plates after the irradiation showed three absorption bands peaked around 2 eV, 4 eV and 5.8 eV being assigned to color centers relate metal impurities (Al and Ge) and oxygen-deficient centers like E′ center, respectively. All three bands were found to grow with γ-ray irradiation dose and saturated at higher doses, and absorbance of the bands at the saturation for the sandwiched glass plates was higher than that for the bare glass plate. Moreover, the saturated absorbance was higher for the glass plate sandwiched with heavier materials. Employing Monte Carlo N-Particle (MCNP) code for the simulation of the photon–electron transport process, enhanced energy deposition and numbers of secondary electrons and photons emitted from sandwiching material disks to a silica glass plate were calculated. The higher deposition energy correlates well to the higher saturated absorbance, indicating that the secondary electrons and photons emitted from the disks clearly enhanced the defect formation in the sandwiched silica glass plates. This suggests the existence of the dose effect above a critical does, i.e. the irradiation with higher dose will result in higher saturated absorbance.     

Measurement of α particle energy loss in biological tissue below 2 MeV

The energy loss of α particles crossing biological tissue at energies between 0.8 and 2.2 MeV has been measured. This energy range is very important for boron neutron capture therapy, based on the ¹⁰B(n,α)⁷Li reaction, which emits α particles with energies of 1.78 and 1.47 MeV. One of the methods used for the measurement of the boron concentration in tissue is based on the deconvolution of the α spectra obtained from neutron irradiation of thin (70 μm) tissue samples. For this technique, a knowledge of the behaviour of the energy loss of the particles in the irradiated tissue is of critical importance. In particular, the curve of the residual energy as a function of the distance travelled in the tissue must be known. In this paper, the results of an experiment carried out with an ²⁴¹Am source and a series of cryostatic sections of rat-lung tissue are presented. The experimental measurements are compared with the results of Monte Carlo calculations performed with the MCNPX code.     

Interaction of displacement cascade with helium bubbles in α-iron: Computer simulation

Molecular dynamics (MD) method has been performed to study the interaction of displacement cascade with He bubbles with two sets of potentials. The results show that the stability of He bubbles depends much on the initial He–vacancy (He/V) ratio and the recoil energy. For an initial He/V ratio of 3, the cascade leads to the increase in the number of vacancies in the He bubble and the decrease in the He/V ratio. For an initial He/V ratio of 0.5, the interaction of a cascade with the He/V bubble results in the decrease in the number of vacancies and the increase in the He/V ratio. For an initial He/V ratio of 1, the stability of the bubbles slightly depends on the primary knock-on atom (PKA) energy. Furthermore, a large number of self-interstitial atom clusters are formed after cascade collision for the He/V ratio of 3, while large vacancy clusters are observed for the He/V ratio of 0.5. However, some differences of defect production and clustering between the two sets of potentials are observed, which may be associated the formation energies of He–V clusters, the binding energies of vacancies and He atoms to the clusters and the probability of subcascade formation.     

Native defect properties in β-SiC: Ab initio and empirical potential calculations

There is considerable ambiguity regarding the formation of native defects and their clusters in silicon carbide (SiC), since different empirical potentials give different results, particular for the stability of interstitial configurations. Density functional theory (DFT) is used to study the formation and properties of native defects in β-SiC. The DFT results are compared with those calculated by molecular dynamics (MD) simulations using the Tersoff potentials, with modified cut-off distances and parameters obtained from the literature. The formation energy of vacancies and antisite defects obtained by DFT calculations are in good agreement with those given by the Tersoff potential, regardless of the cut-off distances, but for interstitials there is a disparity between the two methods, depending on the cut-off distances used in the Tersoff potential. The present results provide guidelines for evaluating the quality and fit of empirical potentials for large-scale simulations of irradiation damage (displacement cascades) and point defect migration (recombination or annealing) in SiC.     

Influence of chemical effect on the Kβ-to-Kα x-ray intensity ratios of Cr, Mn and Co in CrSe, MnSe, MnS and CoS

Kβ-to-Kα x-ray intensity ratios of Cr, Mn and Co have been measured in pure metals and in CrSe, MnSe, MnS and CoS compounds following excitation by 59.54 keV γ-rays from a 200 mCi ²⁴¹Am point-source. Comparison of the measured Kβ-to-Kα intensity ratios with the results of multiconfiguration Dirac–Fock (MCDF) calculations indicates a significant increase of the 3d electron population (in relation to the pure metals) for Cr in CrSe (by 1.0 ± 0.3) and for Mn in MnS (by 0.9 ± 0.3). Our Kβ-to-Kα intensity ratio for Co in CoS does not show any significant change of 3d electron population. The only observed increase of the Kβ-to-Kα intensity ratio for Mn in MnSe indicates the decrease of the number of 3d electrons by 0.6 ± 0.3. To explain evaluated changes of the 3d electron population for all the compounds, one can consider either rearrangement of electrons between 3d and 4s states of the metal or transfer of electrons from the 3d state of the metal to the ligand atom or vice versa. Although the sulphide compounds (MnS, CoS) show similar behaviour, the selenide compounds (CrSe, MnSe) show opposite behaviour which may be attributed to anomalous behaviour of rearrangement of electrons between the valence states of Cr as it happens in the case of free atom. The d–p hybridization effect which was not taken into account in our model theoretical calculation may be partly responsible for the increased Kβ-to-Kα ratio of Mn in MnSe and a comparison of the results for MnS and MnSe suggests that the nature of covalent bonding in MnSe is stronger than that of MnS.     

Summary on international benchmark experiments for effective delayed neutron fraction (βeff)

To improve the accuracy of prediction of β_eff, an international program of benchmark experiments was planned. This program consisted of two parts; the BERENICE-MASURCA and the FCA XIX series of experiments. The former was carried out in the fast critical facility MASURCA of CEA, FRANCE between 1993 and 1994. The latter one was carried out in the FCA, JAERI between 1995 and 1998. In these benchmark experiments, various experimental techniques were applied to measure the β_eff. Through the synthesis of the different results, a recommended value for each core was provided and the accuracy of the measurements was evaluated to be better than 3%. The calculations showed good agreement of the recommended β_eff values within 3% for JENDL-3.2 and ENDF/B-VI delayed neutron data sets.     

Co γ-ray irradiation effects on dielectric characteristics of tin oxide films of different thicknesses on n-type Si(1 1 1) substrates

We have tried to determine the effects of ⁶⁰Co gamma irradiation on properties of Au/SnO₂/n-Si (MOS) structures such as dielectric constant (ε′), dielectric loss (ε″), tangent loss (tan δ) and ac conductivity (σ_ac). Three samples were fabricated with different deposition time. The samples were irradiated using a ⁶⁰Co γ-ray source irradiation with the total dose range of 0–500 kGy at room temperature. Capacitance and conductance (C–G–V) measurements were performed at a frequency of 500 kHz in the dark and at room temperature before and after irradiation. The experimental data were analyzed using complex permittivity and electric modulus. The values of ε′, ε″, tan δ and σ_ac showed a strong dependence on the applied voltage and irradiation dose. The dielectric properties of MOS structures have been found to be strongly influenced by the presence of dominant radiation-induced defects. Experimental results show that the interfacial polarization contributes to the improvement of dielectric properties of Au/SnO₂/n-Si (MOS) Schottky diodes.     

Isotope effects in thermal neutron transmission and backscattering processes for ε-phase zirconium hydrides and deuterides

Distributions of hydrogen isotope concentrations in ε-phase zirconium hydrides and deuterides (ε-ZrH_x and ε-ZrD_x: 1.8 < x < 2.0) were investigated by neutron radiography (NRG). The NRG images of the thermal neutron transmission and backscattering revealed hydrogen concentration dependence and isotope differences. The thermal neutron mass attenuation coefficients in relation to the hydrogen isotope concentrations were determined from the transmission NRG images. The results showed the isotope effects of the thermal neutron mass attenuation coefficients for ε-ZrH_x to be about 6–9 times higher than those for ε-ZrD_x. The neutron scattering processes for transmission and backscattering NRG images of ε-ZrH_x and ε-ZrD_x were also analyzed using a general Monte Carlo neutron-particle transport (MCNP) code.     

In-pile electrochemical measurements on AISI 304 and AISI 306 in PWR conditions – Experimental results

In-pile electrochemical measurements were performed in order to investigate the effect of radiation on the electrochemical corrosion behaviour of AISI 304 and AISI 316 in PWR primary water (400 ppm B and 2 ppm Li) at 300 °C. The corrosion potential was continuously monitored during the whole irradiation period. Polarization resistance measurements and electrochemical impedance spectroscopy were performed at regular intervals. Polarization curves were recorded halfway through and at the end of each reactor cycle. All measurements were performed on both an in-flux and an out-of-flux 3-electrode electrochemical cell, each containing a platinum high-temperature reference electrode and an yttrium-stabilized zirconia (YSZ) reference electrode. The results show a small influence of radiation on the corrosion potential. However, the impedance data show a marked difference between in-flux and out-of-flux. The Nyquist diagram shows one semi-circle and one flattened semi-circle of which a branch leaps off indicating an R-(R//C)-(CPE//RW) type equivalent circuit.     

Detailed analysis of (n,p) and (n,α) cross sections in the EAF-2007 and TALYS-generated libraries

Large cross section libraries, such as EAF-2007 used for activation calculations, need to be validated. A traditional approach employs simple systematic formulae based on experimental data and derived at energies such as 14.5 MeV. Such systematics are also used for renormalization of cross sections during library preparation. This approach is gradually being replaced by using results of sophisticated nuclear model calculations. As the energy range of EAF libraries has expanded from 20 to 60 MeV an additional approach is required and the method of ‘statistical analysis of cross sections’ (SACS) has proved to be useful. SACS analyses are carried out for the important charged particle reactions (n,p) and (n,α) in the EAF-2007 and TALYS-6 libraries. TALYS-6 is a calculational library generated by the TALYS model code Version 0.72. SACS considers various reaction statistics such as σ_max, E_max and Δ_1/2 and plots scatter plots against a parameter such as the asymmetry (s). Trend lines using simple algebraic formulae can be fitted to the data and points discrepant from the trends correspond to reactions that require improvement. A new observation is a ‘step’ in the Δ_1/2(s) plot for (n,p) reactions and this has been interpreted as due to competition between (n,p) and (n,n′p) reaction channels. Equations for the various trend curves for EAF-2007 are given.