Neutron Emission Spectra Research Articles

The investigation of the neutron emission spectra of 232Th and 238U for neutron incident energy from 2 to 18 MeV

In this study, experimental neutron-emission spectra produced by (n, xn) reactions on nuclei 232Th have been compared with experimental 238U(n, xn) neutron-emission spectra from 2 to 18 MeV. Angle-integrated cross-sections in neutron induced reactions on targets 238U have been calculated at the bombarding energies from 2 to 18 MeV. In the calculations, the geometry dependent hybrid model and the cascade exciton model including the effects of pre-equilibrium have been used. In addition, we have described how multiple pre-equilibrium emissions can be included in the Feshbach–Kerman-Koonin (FKK) fully quantum-mechanical theory. By analyzing (n, xn) reaction on 238U with the incident energy from 2 to 18 MeV, the importance of multiple pre-equilibrium emission can be seen clearly. All calculated results have been compared with experimental data. The obtained results have been discussed and compared with the available experimental data and found agreement with each other.

Measurement of Neutron Emission Spectra in Li(d,xn) Reaction with Thick and Thin Targets for 40-MeV Deuterons

To improve the data accuracy of the neutron emission spectra of the natLi(d,xn) reaction that will be used as the neutron source in the International Fusion Materials Irradiation Facility, the authors have measured the neutron emission spectra from thick and thin lithium targets bombarded by 40-MeV deuterons at the AVF cyclotron (K = 110) facility of Tohoku University. The neutron spectra were measured at nine laboratory angles between 0 and 110 deg with the time-of-flight method using a beam-swinger system and a well-collimated neutron flight channel, obtained over almost the entire energy range of secondary neutrons using a two-gain method. The data obtained revealed the shape and angular dependence of emission spectra up to the high-energy region called a “high-energy tail.” The experimental results are compared with other experimental data and calculations.

Excitation of octupole, beta- and gamma-vibration band levels of 238U by inelastic neutron scattering

Experimental data on inelastic neutron scattering are analyzed in a Hauser–Feshbach–Moldauer approach with a coupled-channel estimates of the direct reaction contribution. For the direct excitation of the ground state rotational band levels with J π = 0 + , 2 + , 4 + , 6 + , 8 + the rigid rotator model was used, whereas for the direct excitation of members of the β-, γ- ( K π = 0 2 + , 0 3 + , 2 + ) and first octupole band ( K π = 0 − ) a soft-deformable rotator model was used. Quadrupole, octupole, hexadecapole and gamma-deformation parameters are defined by consistent analysis of excited rotational–vibrational band structures and excitation cross sections of the relevant levels. Structures evident in measured neutron emission spectra for E n ∼ 1 – 6 MeV are correlated with excitation of levels of K π = 0 − and K π = 0 2 + , 0 3 + , 2 + bands.

Systematic spectral features in the neutron emission from NB heated JET DT plasmas

Neutron emission spectroscopy (NES) has been used at JET to study deuterium–tritium discharges heated with neutral beam (NB) injection of tritium and deuterium at energies of 150 keV and 75 keV, respectively. The measurements were performed using the magnetic proton recoil neutron spectrometer and the data analysed with several contributing spectral components. The analysis model considers neutrons from reactions involving high-energy NB ions in (co and counter) passing and trapped orbits and NB ions slowed down to epithermal energies besides ion reactions within the thermal bulk. This study was undertaken to explore the level of fuel ion kinetic information that can be obtained for different NB heating scenarios. Time dependent information was extracted to study how the plasma response to NB injection is manifested in the neutron emission for apparent quiescent periods, during spontaneous plasma instabilities (such as sawteeth) and as a result of intentional changes of the plasma conditions such as the NB injection. The discharges for this study were selected to explore the systematic features that neutron emission spectra display for NB heated discharges and the possibility to interpret them in terms of the underlying kinetic state of the fuel ions. The results obtained on the ion temperature, the toroidal rotation and the supra-thermal fraction of the total neutron yield are compared with those of charge exchange recombination spectroscopy and, in some cases, with plasma equilibrium model calculations. A phenomenological analysis of the results is performed from studies of systematics with the object of exploring the uses of the NES diagnostic and their dependence on data quality.

Comparison of neutron emission spectra for D and DT plasmas with auxiliary heating

The DT experimental campaign on JET (1997) represents a major step forward for neutron emission spectroscopy (NES) diagnostic through the high quality data collected by the Magnetic Proton Recoil (MPR) spectrometer. These data for different DT plasma heating scenarios were analyzed here to determine the underlying fuel ion populations which in turn were used to project the 2.5-MeV neutron emission spectra for deuterium plasmas. The results on neutron spectra for DT and D plasmas in the same conditions were compared in order to determine the plasma information that could be expected from NES diagnosis of D plasmas and the instrumental characteristics that would be required. Future NES experiments would make dual sight line observations possible and the added diagnostic value is also assessed based on the present results.

PRE-EQUILIBRIUM EMISSION IN DIFFERENTIAL CROSS-SECTION CALCULATIONS AND ANALYSIS OF EXPERIMENTAL DATA FOR 232Th

In this study, neutron-emission spectra produced by (n,xn) reactions on nuclei 232 Th have been calculated. Angle-integrated cross-sections in neutron induced reactions on targets 232 Th have been calculated at the bombarding energies from 2 MeV to 18 MeV. We have investigated multiple pre-equilibrium matrix element constant from internal transition for 232 Th (n,xn) neutron emission spectra. In the calculations, the geometry dependent hybrid model and the cascade exciton model including the effects of pre-equilibrium have been used. Pre-equilibrium direct effects have been examined by using full exciton model. In addition, we have described how multiple pre-equilibrium emissions can be included in the Feshbach–Kerman–Koonin (FKK) fully quantum-mechanical theory. By analyzing (n,xn) reaction on 232 Th , with the incident energy from 2 MeV to 18 MeV, the importance of multiple pre-equilibrium emission can be seen clearly. All calculated results have been compared with experimental data. The obtained results have been discussed and compared with the available experimental data and found agreement with each other.

Experimental studies on the neutron emission spectrum and activation cross-section for 40 MeV deuterons in IFMIF accelerator structural elements

In order to improve the nuclear data required in the safety design of the International Fusion Materials Irradiation Facility (IFMIF), we have measured the neutron emission spectra and the activation cross-sections of the IFMIF accelerator structural elements, C and Al, for 40 MeV deuterons using the Tohoku University AVF cyclotron. Neutron spectra from thick C and Al targets were measured with the time-of-flight method at ten laboratory angles between 0- and 110-deg. using a beam swinger system and a well collimated neutron flight channel. The data were obtained over almost entire energy range of secondary neutrons using a two-detector method. Activation cross-sections were measured by detecting the γ-rays from C and Al targets with a high-pure Ge detector. The stacked target technique was used to obtain the data from 40 MeV down to the threshold energy.

Measurements of Differential Thick Target Neutron Yields and 7Be Production in the Li, 9Be (d, n) Reactions for 25 MeV Deuterons

Neutron emission spectra and production rates of radioactive nuclide 7Be in the (d, n) reactions on thick lithium and 9Be targets have been measured for 25 MeV deuterons at the Tohoku University AVF cyclotron (K=110) facility to provide basic data for the design of the intense neutron source, International Fusion Reactor Material Irradiation Test Facility (IFMIF). Neutron spectra were measured with the time-of-flight method at ten laboratory angles between 0° and 90° by using a beam swinger system. Data were obtained down to ~1 MeV from the highest energy, using a two-gain NE213 detector system. Induced radioactivity of 7Be was measured by detecting the 478 keV γ-rays from 7Be with a pure Ge detector. The experimental results of neutron spectra revealed clearly the entire shape of the neutron emission spectrum and the angular dependence including so called “high energy tail” extending up to ~40 MeV. The present data at 0° are in reasonable agreement with corresponding value by Lone et al. at 23 MeV above ~5 MeV, but show large discrepancy in the lower energy region. The results of 7Be production for both nuclei were much larger than expected by the calculation with a recent code IRAC.

Ion temperature and plasma rotation profile effects in the neutron emission spectrum

The instrumental factors and measuring conditions affecting neutron emission spectrometry measurements of tokamak plasmas are described and analyzed. The measured energy broadening and shift of the neutron emission is used to deduce ion temperature (Ti) and toroidal plasma rotation velocity (Vt) representing average (effective) values for the nonuniform plasma volume viewed. Analytical expressions are derived for the relationship between the line-volume integrated effective temperature (Teff) and the radial profile Ti(r) for the case of thermal plasmas with isotropic neutron emission; effects on Teff due to spectral broadening from the radial dependence Vt(r) were also considered. The analysis method presented here is applied to high quality data obtained with the magnetic proton recoil neutron spectrometer installed at Joint European Torus for measurements of deuterium–tritium plasmas. Similarly, cases of anisotropic neutron emission were quantitatively assessed.

Experimental studies on the neutron emission spectrum and induced radioactivity of the 7Li(d,n) reaction in the 20–40 MeV region

To improve the data accuracy of the neutron emission spectrum of the 7Li(d,n) reaction and the radioactivity ( 7Be, 3H, etc.) accumulated in the 7Li target in IFMIF, we have measured the neutron emission spectrum and the radioactivity of 7Be induced in the lithium target for 25 MeV deuterons at the Tohoku University AVF cyclotron ( K=110) facility. Neutron spectra were measured with the time-of-flight (TOF) method at four laboratory angles by using a beam swinger system and a well collimated TOF channel. Induced radioactivity was measured by detecting the gamma-rays from 7Be with a pure Ge detector. Experimental results are compared with other experimental data. The present result of neutron emission spectra are in qualitative agreement with other experimental data but that of 7Be production was much larger than expected by the recent codes. Measurement will be extended to several incident energies up to 40 MeV.

Neutron Scattering on 238U and 232Th

Consistent analysis of neutron scattering data of 238U and 232Th with Hauser-Feshbach-Moldauer theory and coupled channel model is carried out. Collective bands structures are reproduced within soft deformable rotator model. The direct excitation of ground state and vibrational band levels is calculated within rigid rotator and soft (deformable) rotator models, respectively. Structures evident in measured neutron emission spectra are correlated with excitation of vibrational levels of Kπ = 0− and Kπ = 0+, 2+ bands. Prompt fission neutron spectra data are described. Contributions of neutrons, emitted in (n, nf) reactions are shown to be strongly dependent on the emissive fission contribution to the observed fission cross section.

Prediction/modelling of the neutron emission from JET discharges

The neutron emission from the JET tokamak is investigated using an extensive set of diagnostics, permitting the instantaneous neutron yield, the radial profile of the neutron emission and neutron energy spectra to be studied. Apart from their importance as an immediate indication of plasma fusion performance, the customary use for neutron measurements is as a test of the internal consistency of the non-neutron diagnostic data, from which the expected neutron production can be predicted. However, because contours of equal neutron emissivity are not necessarily coincident with magnetic flux surfaces, a fully satisfactory numerical analysis requires the application of highly complex transport codes such as TRANSP. In this paper, a far simpler approach is adopted wherein the neutron emission spatial profiles are used to define the plasma geometry. A two-volume model is used, with a core volume that encompasses about (2/3) of the neutron emission and the peripheral volume the remainder. The overall approach provides an interpretation of the measured neutron data, for both deuterium and deuterium–tritium (D–T) plasma discharges, that are as accurate as the basic non-nuclear plasma data warrant. The model includes the empirical assumption that particles, along with their energies and momenta, are transported macroscopically in accordance with classical conservation laws. This first-order estimate of cross-field transport (which, for D–T plasmas, determines the D : T fuel concentration ratio in the plasma core) is fine-tuned to reproduce the experimental ion and electron temperature data. The success of this model demonstrates that the observed plasma rotation rates, temperatures and the resulting neutron emission can be broadly explained in terms of macroscopic transport.

Neutron emission from JET DT plasmas with RF heating on minority hydrogen

The neutron emission spectrum from d+t→α+n reactions has been measured as a means to study the plasma response to radio frequency (RF) power coupled to hydrogen and deuteron minority components (through fundamental and second harmonic, respectively) in a tritium discharge at JET. The spectrum was measured with the magnetic proton recoil spectrometer and was analysed in terms of two spectral components due to thermal (TH) and high-energy (HE) deuterons interacting with the bulk ion population of thermal tritons. The results were used to derive information on the deuteron population in terms of temperatures (TTH and THE) as well as corresponding particle and kinetic energy densities of the plasma; the bulk ion temperature (Ti = TTH) was determined both before (with Ohmic heating only) and during the RF pulse. Similar information on protons was derived from other measurements in order to estimate the different RF effects on protons and deuterons. This paper illustrates qualitatively the type of empirical ion kinetic information that can be obtained from neutron emission spectroscopy; the data serves as a basis for comparison with results of predictive and interpretative models on RF effects in plasmas.

Effect of the preequilibrium process upon fast neutron fission spectra from238U

A preequilibrium process for the prefission neutron which is emitted before scission is calculated with the model of Feshbach, Kerman, and Koonin. A forward-peaked angular distribution of the neutron emission from ${}^{238}\mathrm{U}$ bombarded by 14 and 18 MeV neutrons is expressed with the statistical multistep compound process and the one-step direct process. The fission neutron energy spectra are calculated with the model of Madland and Nix, with some modifications by Ohsawa et al. The calculated total neutron emission spectra and their energy-angle distributions (double-differential cross sections) are compared with the experimental data, and a strength of the residual interaction ${V}_{0}$ is estimated. The comparisons of the calculations with the experimental data show that the 14 MeV data are well reproduced but the 18 MeV data are underestimated. Anisotropy is seen in the angle-differential fission spectra, and this is due to an existence of the prefission neutron.

Nuclear Data Validation and Re-evaluation by Means of Integral Experiments

Integral experiments are a rich source of information with which a wide range of validation and comparison exercises can be made in the activation data field. Having access to a variety of sources allows the energy dependence of the analysis to be more complete. Materials samples were irradiated in a wide range of simulated fusion D-T neutron fields at JAERI FNS, Sergiev Posad SNEG-13, FZK Isochron-cyclotron and ENEAFNG and different responses such as neutron spectra, emission spectra, kerma, radioactivity and decay heat have been measured, allowing direct comparison with the code predictions. The results of the different validation exercises, when correlated with other sources of information, led to a set of corrective measures to be taken on some important reactions and these were implemented on the European Activation File EAF-99. These include cross section renormalisation, profile correction in a given energy range, threshold and Q-value correction, partial cross section adjustment and even new energy-dependent isomeric branching ratios.

Neutron spectrometry of radio-frequency heated deuterium–tritium plasmas

Spectrometry of the neutron emission has been used to probe the nonthermal features of the fuel ion velocity distributions in tokamak plasmas, especially, those arising from auxiliary heating. The first time resolved measurements are reported from the use of the new magnetic proton recoil (MPR) spectrometer in observations of deuterium–tritium plasmas at JET with strong ion cyclotron resonance heating. Results from preliminary analysis data are presented and compared with simulated neutron emission spectra based on the assumptions of a thermal plasma (a single Maxwellian ion distribution) or anisotropic Maxwellians with parallel and perpendicular temperatures, beside their relative amplitudes. The diagnostic results attained are also used to assess the diagnostic capabilities and the potential for use of MPR based diagnostics on burning plasma tokamaks.

Measurements of Double-Differential Neutron Emission Cross Sections of 6Li, 7Li and 9Be for 11.5 MeV and 18.0 MeV Neutrons

Double-differential neutron emission cross sections (DDXs) of 6Li, 7Li and 9Be were measured for 18.0 MeV and 11.5 MeV incident neutrons produced by the T(d, n) and 15N(d, n) reactions respectively, using the Tohoku University Dynamitron time-of-flight (TOF) spectrometer. The data were obtained at 13 laboratory angles, and angular-differential cross sections (ADXs) of elastic and inelastic scattering neutrons were derived from the DDXs. For 11.5 MeV neutrons, we obtained the neutron emission spectra over the secondary neutron energies by newly employing the double TOF method as well as the conventional one. In the measurements at 18.0 MeV, we achieved better energy resolution than in our previous studies by using a neutron detector that has a larger solid angle and a thinner tritium target. The experimental results of DDXs and ADXs were compared with our previous results and the evaluated data given in JENDL-3.2, JENDL Fusion File and ENDF/B-VI. It is found that the JENDL data reproduce the experimental ones very well.

Microscopic two-component multistep direct theory for continuum nuclear reactions

We present a multistep direct reaction theory for analyzing nucleon-induced reactions to the continuum for incident energies up to 200 MeV. Two principal advances in multistep direct theory are studied. (1) A microscopical approach is given for calculating distorted-wave Born approximation (DWBA) transitions to the continuum, where transitions to all accessible 1$p$1$h$ shell model states are explicitly determined. These states, obtained from a simple noninteracting Nilsson model, are assumed to be spread according to Gaussian distributions. In this approach, therefore, state densities are not used. We also provide a link with more conventional methods that utilize particle-hole state densities, and present a more accurate technique for sampling the DWBA strength. (2) A two-component formulation of multistep direct reactions is given, where neutron and proton excitations are explicitly accounted for in the evolution of the reaction, for all orders of scattering. We show that the attractive convolution structure for multistep processes persists within a two-component formalism, and conveniently automatically generates the many reaction pathways that can occur in the Feshbach-Kerman-Koonin expansion of the multistep cross section when neutron and proton excitations are followed. This formalism is particularly important for the simultaneous analyses of neutron and proton emission spectra. The multistep direct theory is applied, along with theories for multistep compound, compound, and collective reactions, to analyze experimental emission spectra for a range of targets and energies. Particular attention is paid to a complete and comprehensive analysis of all important decay channels and reaction mechanisms. We show that the theory correctly accounts for measured neutron and proton emission angle-integrated spectra, as well as angular distributions. Additionally, we note that these microscopic and two-component developments facilitate more fundamental studies into effective nucleon-nucleon interactions in multistep calculations.

α-particle knock-on signature in the neutron emission of DT plasmas

The fast alpha-particle kinetic effects in fusion plasmas of deuterium and tritium are studied in the perspective that they can give rise to minority populations of fast fuel ions. The resulting modification of the neutron emission spectrum is computed fo

Collective reactions to the continuum in neutron scattering by niobium.

The preequilibrium cross sections for the direct excitation of collective states, including giant resonances, in the continuum region, are calculated and combined with the multistep direct, multistep compound, and compound nucleus cross sections to give a complete analysis of neutron scattering by niobium. These collective cross sections we calculated in the framework of DWBA. A good description of neutron emission spectra and angular distributions has been obtained at incident energies 14.1, 20, and 25.7 MeV.