Neutron Capture Rates Research Articles

Neutronic behavior of reactor moderated by mixtures of light and heavy waters at different ratios

A Monte-Carlo parametric study was carried out to investigate some nuclear properties of LR-0 reactor when it is moderated by mixtures of light and heavy water at molecular ratios ranging from 0% up to 100% D2O at increments of +10% in D2O. Neutron energy spectrum, fission rates, non-fission neutron capture rates, neutron absorption in poison material, and average fission energy yield from 235U and 238U components of the UO2 fuel were tallied at different regions in the reactor at the 11 heavy water percentage moderators being studied. It was found that the changes in all the tallied parameters with heavy water percentage in moderator were not monotonic, and the nuclear properties of the reactor changed dramatically between 90% and 100% heavy water moderators. The most prominent changes were in neutron energy spectrum, neutrons leaking out from the reactor, neutrons absorption in moderator, and non-fission neutron capture in 238U. These changes were not the same in all regions of the reactor.

Measurement and simulation of the muon-induced neutron yield in lead

A measurement is presented of the neutron production rate in lead by high energy cosmic-ray muons at a depth of 2850m water equivalent (w.e.) and a mean muon energy of 260GeV. The measurement exploits the delayed coincidences between muons and the radiative capture of induced neutrons in a highly segmented tonne scale plastic scintillator detector. Detailed Monte Carlo simulations reproduce well the measured capture times and multiplicities and, within the dynamic range of the instrumentation, the spectrum of energy deposits. By comparing measurements with simulations of neutron capture rates a neutron yield in lead of (5.78-0.28+0.21) ×10-3neutrons/muon/(g/cm2) has been obtained. Absolute agreement between simulation and data is of order 25%. Consequences for deep underground rare event searches are discussed.

General properties of astrophysical reaction rates in explosive nucleosynthesis

Fundamental differences in the prediction of reaction rates with intermediate and heavy target nuclei compared to the ones with light nuclei are discussed, with special emphasis on stellar modifications of the rates. Ground and excited state contributions to the stellar rates are quantified, deriving a linear weighting of excited state contributions despite of a Boltzmann population of the nuclear states. A Coulomb suppression effect of the excited state contributions is identified, acting against the usual Q-value rule in some reactions. The proper inclusion of experimental data in revised stellar rates is shown, containing revised uncertainties. An application to the s-process shows that the actual uncertainties in the neutron capture rates are larger than would be expected from the experimental errors alone. Sensitivities of reaction rates and cross sections are defined and their application in reaction studies is discussed. The conclusion provides a guide to experiment as well as theory on how to best improve the rates used in astrophysical simulations and how to assess their uncertainties.

Experimental study of the 18O(d, p)19O reaction and the ANC Method

The neutron capture rate 18O(n, γ)19O is important for analysis of nucleosynthesis in inhomogeneous Big Bang models and also for models of helium burning in massive red giant stars and AGB stars. Angular distributions of the 18O(d, p)19O reaction were measured at a deuteron energy of 16.3 MeV in NPI in Rez, Czech Republic, with the aim to determine Asymptotic Normalization Coefficients which can then be used for indirect determination of the direct contribution to the 18O(n, γ)19O process. In the experiment, the gas target with 18O isotope of high purity 99.9% was used thus eliminating any contaminating reactions. Reaction products were measured by the set of 8 ΔE-E telescopes consisting of thin and thick silicon surface-barrier detectors. Angular distributions of proton transfers corresponding to 6 levels of 19O up to the 4.1093 MeV excitation energy were determined. The analysis of angular distributions in the angular range from 6 to 64 degree including also the angular distribution of elastically scattered deuterons was carried out by means of ECIS and DWUCK codes. From the determined ANCs the direct contribution to the radiative capture 18O(n, γ)19O was deduced.

Nuclear uncertainties in the s-process simulation

The heavy elements in the Universe are formed during the s- and r-processes mainly in AGB stars and supernovae, respectively. Simulation of s- and r-nucleosynthesis critically depends on the neutron capture and weak decay rates for all the nuclei on the reaction chain. The present work analyzes systematically the neutron capture rates (cross sections) for the s-process nuclei, including ∼3000 rates on ∼200 nuclei. The network calculations for the constant temperature s-process have been performed using the different data sets selected as the nuclear inputs to investigate the uncertainties in the predicted s-abundances. We show that the available cross sections of neutron capture on many s-process nuclei still carry large uncertainties, which lead to low accuracy in the determination of s-process isotope abundances. We analyze the neutron capture cross section data for the same unique isobar nucleus accorded by year from previous work. Such an analysis indicates that the s-process has been studied for more than fifty years and there exist two research stages around 1976 and 2002, respectively. The needs and opportunities for future experiments and theoretical tools are highlighted to remove the existing shortcomings in the neutron capture rates.

Astrophysical Reaction Rates and the Low-energy Enhancement in the <span class="cmmi-10">γ</span> Strength

An unexpected enhancement in the γ-strength function for E≤ 3MeV for Ti, Sc, V, Fe, and Mo nuclei close to the valley of stability has been discovered at the Oslo Cyclotron Laboratory. Provided that this enhancement is present also in very neutron-rich nuclei, it could give an increase in the neutron-capture rates up to two orders of magnitude. However, it is still an open question whether this structure persists when approaching the neutron drip line.

New Precision Measurements of theU235(n,γ)Cross Section

The neutron capture cross section of (235)U was measured for the neutron incident energy region between 4 eV and 1 MeV at the DANCE facility at the Los Alamos Neutron Science Center with an unprecedented accuracy of 2-3% at 1 keV. The new methodology combined three independent measurements. In the main experiment, a thick actinide sample was used to determine neutron capture and neutron-induced fission rates simultaneously. In the second measurement, a fission tagging detector was used with a thin actinide sample and detailed characteristics of the prompt-fission gamma rays were obtained. In the third measurement, the neutron scattering background was characterized using a sample of (208)Pb. The relative capture cross section was obtained from the experiment with the thick (235)U sample using a ratio method after the subtraction of the fission and neutron scattering backgrounds. Our result indicates errors that are as large as 30% in the 0.5-2.5 keV region, in the current knowledge of neutron capture as embodied in major nuclear data evaluations. Future modifications of these databases using the improved precision data given herein will have significant impacts in neutronics calculations for a variety of nuclear technologies.

Monte Carlo analysis of the long-lived fission product neutron capture rates at the Transmutation by Adiabatic Resonance Crossing (TARC) experiment

The design of Accelerator Driven Systems (ADS) requires the development of simulation tools that are able to describe in a realistic way their nuclear performance and transmutation rate capability. In this publication, we present an evaluation of state of the art Monte Carlo design tools to assess their performance concerning transmutation of long-lived fission products. This work, performed under the umbrella of the International Atomic Energy Agency, analyses two important aspects for transmutation systems: moderation on Lead and neutron captures of 99Tc, 127I and 129I. The analysis of the results shows how shielding effects due to the resonances at epithermal energies of these nuclides affects strongly their transmutation rate. The results suggest that some research effort should be undertaken to improve the quality of Iodine nuclear data at epithermal and fast neutron energy to obtain a reliable transmutation estimation.

Optimization of HANARO cold neutron induced prompt gamma activation analysis system by using Monte Carlo code

Prompt gamma activation analysis (PGAA) is a nuclear analytical technique for non-destructive determination of elemental and isotopic compositions. The principle of PGAA technique is based on detection of captured gamma-ray emitted from an analytical sample while being irradiated with neutrons. Use of a cold neutron beam guide greatly reduces the gamma-ray background at the analytical sample while maintaining a neutron capture rate is comparable to that of standard thermal neutron PGAA. A new cold neutron induced prompt gamma activation analysis (CN-PGAA) system has been under construction since April of 2009 at the HANARO Cold Neutron Building (KAERI, Republic of KOREA). In this study, the Compton suppression factor of the CN-PGAA system was estimated to be 5.5 using a 60Co radioactive source in conjunction with the MCNPX simulations. Several parameters of the CN-PGAA system were studied to estimate and optimize the performance of the system: scintillation material in the guarded detector of a Compton suppression spectrometer (CSS); the relative positions of the HPGe detector and annular detector; and the distance between the HPGe detector and back catcher BGO detectors of the CSS. In addition, the neutron ray-trace simulation package, McStas, was adopted to predict the neutron flux and wavelength distribution at the end of the cold neutron beam guide. These results served as input for the MCNPX simulation of the CN-PGAA system.

Influence of neutron capture rates in the rare earth region on ther-process abundance pattern

We study the sensitivity of the r-process abundance pattern to neutron capture rates along the rare earth region (A~150 to A~180). We introduce the concepts of large nuclear flow and flow saturation which determine the neutron capture rates that are influential in setting the rare earth abundances. We illustrate the value of the two concepts by considering high entropy conditions favorable for rare earth peak production and identifying important neutron capture rates among the rare earth isotopes. We also show how these rates influence nuclear flow and specific sections of the abundance pattern.

Shell model study on the astrophysical neutron capture of 8Li

The astrophysical important neutron capture of 8Li is investigated by combining the shell model and potential model. Three effective interactions, SFO, PSDMK2 and PSDWBP are used to calculate the spectroscopic factors and reaction widths. For the resonant capture from 8Li to the first continuum state of 9Li , the three effective interactions give similar neutron partial widths, and they are well compared with the experimental results. However, the calculated photon widths are over 5 times less than the previous estimate. This will make the substantial difference that, at high temperature, the direct capture mechanism still dominates. The calculated capture rates generally agree well with the experimental data. The uncertainty of calculated cross-sections and capture rates mainly results from the different prediction of spectroscopic factors for the three effective interactions. The total neutron capture rates in our calculations are less than 4300 cm3 mole−1 s−1 for T9 < 5 which confirms that the main reaction flow will proceed through the reaction 8Li (α, n) 11B in the stellar environments.

Formation of the rare-earth peak: Gaining insight into late-timer-process dynamics

We study the formation and final structure of the rare-earth peak ($A\ensuremath{\sim}160$) of the $r$-process nucleosynthesis. Under high-entropy conditions ($S&gt;100{k}_{B}$), the rare-earth peak forms at late times in the $r$-process after neutron exhaustion (neutron-to-seed ratio $R=1$) as matter decays back to stability. Since rare-earth peak formation does not occur during $(n,\ensuremath{\gamma})\ensuremath{\rightleftarrows}(\ensuremath{\gamma},n)$ equilibrium it is sensitive to the strong interplay between late-time thermodynamic evolution and nuclear physics input. Depending on the conditions, the peak forms either because of the pattern of the neutron capture rates or because of the pattern of the separation energies. We analyze three nuclear data sets under different thermodynamic conditions. We find that the subtleties of each nuclear data set, including separation energies and neutron capture rates, influence not only the final shape of the peak but also when it forms. We identify the range of nuclei which are influential in rare-earth peak formation.

Gamma-ray strength at low energies using relativistic QRPA with exact coupling to the continuum

Continuum-quasiparticle random-phase Approximation (CQRPA) within the relativistic point-coupling model with density-dependent coupling constants is applied to investigate collective excitations in spherical nuclei. In particular we study the impact of the exact continuum on the giant-dipole and pygmy resonance of several Sn isotopes as well as the radiative neutron capture rates of importance for astrophysical calculations.

Measurement and analysis of neutron capture rate of U-238 in an alternate depleted uranium/polyethylene system

In order to check the conceptual design of subcritical reactor, an alternate depleted uranium/polyethylene-shell simulation device was established, to carry out uranium-238 neutron capture rate experiment using activation technique. The depleted uranium foils were activated at 90o to the incident. By measuring the 277.6 keV -ray emitted from 239Np generated by 238U (n, ) 239U reaction and correcting self-absorption of uranium experimentally, the 238U (n, ) reaction rate with an uncertainty between 3.5% and 3.7% and a total neutron capture of 2.24 0.09 of the system were obtained. The experiment was simulated using MCNP5 code with ENDF libraries. The simulations and measurements accord within 5% for the 238U (n, ) reaction rate and within 1% for the total neutron capture rate of 238U.

Stellar neutron capture rates and thesprocess

Neutron reactions are responsible for the formation of the elements heavier than iron. The corresponding scenarios relate to helium burning in Red Giant stars (s process) and to supernova explosions (r and p processes). The s process, which operates in or near the valley of β -stability, has produced about half of the elemental abundances between Fe and Bi. Accurate (n, γ ) cross sections are the essential input for s process studies, because they determine the abundances produced by that process. Following a brief summary of the neutron capture processes, the focus will be set on the s process in massive stars, where the role of reliable cross section information is particularly important. Eventually, the intriguing aspects of the origin of 60 Fe will be addressed. Attempts to determine the stellar cross section of that isotope are pushing experimental possibilities to their limits and present a pertinent challenge for future facilities.

Dynamicalr-process studies within the neutrino-driven wind scenario and its sensitivity to the nuclear physics input

We use results from long-time core-collapse supernovae simulations to investigate the impact of the late time evolution of the ejecta and of the nuclear physics input on the calculated r-process abundances. Based on the latest hydrodynamical simulations, heavy r-process elements cannot be synthesized in the neutrino-driven winds that follow the supernova explosion. However, by artificially increasing the wind entropy, elements up to A=195 can be made. In this way one can reproduce the typical behavior of high-entropy ejecta where the r-process is expected to occur. We identify which nuclear physics input is more important depending on the dynamical evolution of the ejecta. When the evolution proceeds at high temperatures (hot r-process), an (n,g)-(g,n) equilibrium is reached. While at low temperature (cold r-process) there is a competition between neutron captures and beta decays. In the first phase of the r-process, while enough neutrons are available, the most relevant nuclear physics input are the nuclear masses for the hot r-process and the neutron capture and beta-decay rates for the cold r-process. At the end of this phase, the abundances follow a steady beta flow for the hot r-process and a steady flow of neutron captures and beta decays for the cold r-process. After neutrons are almost exhausted, matter decays to stability and our results show that in both cases neutron captures are key for determining the final abundances, the position of the r-process peaks, and the formation of the rare-earth peak. In all the cases studied, we find that the freeze out occurs in a timescale of several seconds.

First tomographic image of neutron capture rate in a BNCT facility

This work discusses the development of online dosimetry of the boron dose via Single Photon Emission Computed Tomography (SPECT) during a BNCT treatment irradiation. Such a system will allow the online computation of boron dose maps without the large current uncertainties in the assessment of the boron concentration in different tissues. The first tomographic boron dose image with a SPECT prototype is shown.

Dipole strength inLa139below the neutron-separation energy

The $\ensuremath{\gamma}$-ray strength function is an important input quantity for the determination of the photoreaction rate and the neutron capture rate for astrophysics as well as for nuclear technologies. To test model predictions, the photoabsorption cross section of $^{139}\mathrm{La}$ up to the neutron-separation energy was measured using bremsstrahlung produced at the electron accelerator ELBE of Forschungszentrum Dresden-Rossendorf with an electron beam of $11.5$ MeV kinetic energy. The experimental data were analyzed by applying Monte Carlo simulations of $\ensuremath{\gamma}$-ray cascades to obtain the intensities of the ground-state transitions and their branching ratios. We found a significant enhancement of electric dipole strength in the energy range from $6$ to $10$ MeV that may be related with a pygmy dipole resonance. The present data are combined with photoneutron cross sections for $^{139}\mathrm{La}$ and compared with results of calculations on the basis of a quasiparticle-random-phase approximation using an instantaneous-shape sampling.

Neutron physics of the Re/Os clock. III. Resonance analyses and stellar (n,γ) cross sections ofOs186,187,188

Neutron resonance analyses have been performed for the capture cross sections of Os-186, Os-187, and Os-188 measured at the n_TOF facility at CERN. Resonance parameters have been extracted up to 5, 3, and 8 keV, respectively, using the SAMMY code for a full R-matrix fit of the capture yields. From these results average resonance parameters were derived by a statistical analysis to provide a comprehensive experimental basis for modeling of the stellar neutron capture rates of these isotopes in terms of the Hauser-Feshbach statistical model. Consistent calculations for the capture and inelastic reaction channels are crucial for the evaluation of stellar enhancement factors to correct the Maxwellian averaged cross sections obtained from experimental data for the effect of thermally populated excited states. These factors have been calculated for the full temperature range of current scenarios of s-process nucleosynthesis using the combined information of the experimental data in the region of resolved resonances and in the continuum. The consequences of this analysis for the s-process component of the Os-187 abundance and the related impact on the evaluation of the time duration of galactic nucleosynthesis via the Re/Os cosmochronometer are discussed.

A benchmarked MCNP model of the in vivo detection of gadolinium by prompt gamma neutron activation analysis

Abstract Gadolinium (Gd)-based contrast agents are a valuable diagnostic aid for magnetic resonance imaging (MRI). The amount of free Gd deposited in tissues following contrast enhanced MRI is of toxicological concern. The McMaster University in vivo prompt gamma neutron activation analysis facility has been adapted for the detection of Gd in the kidney, liver, and the leg muscle. A simple model of the HPGe detector used for detection of the prompt γ-rays following Gd neutron capture has been created using Monte Carlo simulation. A separate simulation describing the neutron collimation and shielding apparatus has been modified to determine the neutron capture rate in the Gd phantoms. The MCNP simulation results have been confirmed by experimental measurement. The deviations between MCNP and the experiment were between 1% and 18%, with an average deviation of 3.8 ± 6.7%. The validated MCNP model is to be used to improve the Gd in vivo measurement sensitivity by determining the best neutron moderator/reflector arrangement.