Neutron Threshold Research Articles

Electric and magnetic dipole modes in high-resolution inelastic proton scattering at 0°

Inelastic proton scattering under extreme forward angles including $0^\circ$ and at energies of a few hundred MeV has been established as a new spectroscopic tool for the study of complete dipole strength distributions in nuclei. Such data allow an extraction of the electric dipole polarizability which provides important constraints parameters of the symmetry energy, which determine the neutron skin thickness and the equation of state (EOS) of neutron-rich matter. Also new insight into the much-debated nature of the pygmy dipole resonance (PDR) is obtained. Additionally, the isovector spin-M1 resonance can be studied in heavy nuclei, where only limited experimental information exists so far. Together with much improved results on the isoscalar spin-M1 strength distributions in $N = Z$ nuclei, these data shed new light on the phenomenon of quenching of the nuclear spin response. Using dispersion matching techniques, high energy resolution ($\Delta E/E \leq 10^{-4} \,\,$ full width at half maximum, FWHM) can be achieved in the experiments. In spherical-vibrational nuclei considerable fine structure is observed in the energy region of the isovector giant dipole resonance (IVGDR). A quantitative analysis of the fine structure with wavelet methods provides information on the role of different damping mechanisms contributing to the width of the IVGDR. Furthermore, level densities can be extracted from a fluctuation analysis at excitation energies well above neutron threshold, a region hardly accessible by other means. The combination of the gamma strength function (GSF) extracted from the E1 and M1 strength distributions with the independently derived level density permits novel tests of the Brink-Axel hypothesis underlying all calculations of statistical model reaction cross sections in astrophysical applications in the energy region of the PDR.

РАСЧЕТ ПАРАМЕТРОВ СТОХАСТИЧЕСКОЙ КИНЕТИКИ НЕЙТРОНОВ В ЯДЕРНОМ РЕАКТОРЕ НУЛЕВОЙ МОЩНОСТИ

The majority of neutron-physical problems of nuclear power plants design can be solved on the basis of various approximations to the Boltzmann transport equation in terms of the averaged characteristics of the reactor: the effective multiplication factor, neutron flux, average neutron lifetime, etc. However, the neutron chain reaction itself is always stochastic. There are situations in which the stochastic nature of the chain reaction cannot be ignored. This is the so-called “blind” start-up problem with a weak external neutron source, the work of physical assemblies of “zero” power, the analysis of the reactivity noise of such assemblies, etc. Despite the well-developed theoretical basis for the stochastic description of the behavior of neutrons in a nuclear reactor, there are still not enough calculation algorithms and programs for stochastic kinetics analysis. The paper presents two computational algorithms for point reactor model, which are developed on the basis of the theory of Markov branching random processes. The first one is based on the balance equation for the probabilities of death and birth of prompt and delayed neutrons in a reactor, the second one is based on calculating the first and second moments of neutron number distributions and using the assumption that these distributions can be approximated by gamma distributions with sufficient accuracy. On the basis of these algorithms, programs have been created that allow one to calculate various scenarios for introducing reactivity and an external source into the system. The calculation of pulsed experiments on prompt and delayed neutrons to determine the waiting time of the threshold neutron pulse at the Godiva II installation showed a qualitative agreement of the results for both the two computational algorithms and the experimental data.

Re-estimation of 180Ta nucleosynthesis in light of newly constrained reaction rates

Recent measurements of the nuclear level densities and γ-ray strength functions below the neutron thresholds in 180,181,182Ta are used as input in the nuclear reaction code TALYS. These experimental average quantities are utilized in the calculations of the 179,180,181Ta radiative neutron capture cross sections. From the latter, astrophysical Maxwellian-averaged (n,γ) cross sections (MACS) and reaction rates are extracted, which in turn are used in large astrophysical network calculations to probe the production mechanism of 180Ta. These calculations are performed for two scenarios, the s-process production of 180,181Ta in Asymptotic Giant Branch (AGB) stars and the p-process nucleosynthesis of Tam180 in Type-II supernovae. Based on the results from this work, the s-process in stellar evolution is considered negligible in the production of Tam180 whereas 181Ta is partially produced by AGB stars. The new measurements strongly constrain the production and destruction rates of Tam180 at p-process temperatures and confirm significant production of nature's rarest stable isotope Tam180 by the p-process.

Reactions along the astrophysical s-process path and prospects for neutron radiotherapy with the Liquid-Lithium Target (LiLiT) at the Soreq Applied Research Accelerator Facility (SARAF)

Neutrons play a dominant role in the stellar nucleosynthesis of heavy elements and the quest for accurate experimental determinations of neutron-induced reaction cross sections becomes more stringent with the refinement of nuclear and astrophysical models. We review here an experimental nuclear-astrophysics program using a high-intensity neutron source based on the 7Li(p, n)7Be reaction with a Liquid-Lithium Target (LiLiT) at the Soreq Applied Research Accelerator Facility (SARAF) Phase I. The quasi-Maxwellian neutron spectrum with effective thermal energy $ kT \approx 30$ keV, characteristic of the thick-target 7Li(p, n) yield at proton energy $ E_p \approx 1.92$ MeV close to its neutron threshold, is well suited for laboratory measurements of neutron capture reactions along the astrophysical s -process path. The high-intensity proton beam (in the mA range) of SARAF and the high power (few kW) dissipation of LiLiT result in the most intense source of neutrons available today at stellar-like energies. The principle, design and properties of the LiLiT device and recent measurements of Maxwellian Averaged Cross Sections (MACS) based on activation of targets of astrophysical interest are described. Decay counting or atom counting methods (accelerator mass spectrometry, atom-trap trace analysis) are used for the detection of short-lived or long-lived activation products, respectively. In a different realm of applications, the 7Li(p, n) reaction is a leading candidate as an accelerator-based neutron source for Boron Neutron Capture Therapy (BNCT). The high neutron yield achievable from a liquid-lithium target, its sustainability of operation under kW-power incident beams and the recent availability of small-size high-intensity accelerators are compatible with a hospital-based clinical facility. An effort towards the characterization and realization of a liquid-lithium target for BNCT is reviewed. Perspectives of pending and future developments towards SARAF Phase II, based on a 40MeV, 5mA CW proton/deuteron superconducting linear accelerator, are summarized.

Photoneutron reaction cross section measurements on Mo94 and Zr90 relevant to the p -process nucleosynthesis

The photodisintegration cross sections for the 94Mo({\gamma},n) and 90Zr({\gamma},n) reactions have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity {\gamma}-ray Source (HI{\gamma}S) facility of the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the photoneutron reaction cross sections was measured with high precision from the respective neutron emission thresholds up to 13.5 MeV. These measurements contribute to a broader investigation of nuclear reactions relevant to the understanding of the p-process nucleosynthesis. The results are compared with the predictions of Hauser-Feshbach statistical model calculations using two different models for the dipole {\gamma}-ray strength function. The resulting 94Mo({\gamma},n) and 90Zr({\gamma},n) photoneutron stellar reaction rates as a function of temperature in the typical range of interest for the p-process nucleosynthesis show how sensitive the photoneutron stellar reaction rate can be to the experimental data in the vicinity of the neutron threshold.

The Effect of Super-Radiance on the C-A Transition of the Excimer Molecule XeCl* at 352 nm Excited Only by <sup>235</sup>U Fission Fragments

The work is devoted to the study of spectral, temporal and generation characteristics of plasma generated in dense gas mixtures by the products of neutron nuclear reaction 235U(n, f). This plasma differs in its properties from the discharge plasma, as has the track structure and low temperature of electrons, and the presence in gas mixtures of electronegative gas can become without electrons. On the basis of such nuclear-excited plasma excimer gas lasers with the nuclear pumping, carrying out direct transformation of nuclear energy to the laser radiation UV range of lengths of waves, can be created. We found out high efficiency of formation of excimer molecules of XeCl* in dense (~ 760 Торр) Ar - Xe - CCl 4 of gas mixture with low (~ 1015 mol/cм3) concentration of CCl4 at the nuclear pumping. Here we report the first clear observation of super-radiance at 352 nm on C-A transition of excimer molecule XeCl* excited solely by nuclear pumping Ar-Xe-CCl4 gas mixture at a pressure of 760 Torr with a low concentration of CCl4 (0.15Torr). The mixture was excited by products of neutron nuclear reaction 235U(n,f) pumped at a rate of 2kW/cm3. A laser cell with a length of 100cm was placed inside an optical resonator tuned at 352 nm. Super-radiance had a very sharp increase and low neutron threshold (about 2÷4.1014 neutron/cm2. s) when we used the tuning resonator at 352 nm wavelength with 10.8% reflection of output and 62.1% reflection of back spherical mirrors, and super-radiance disappeared if the laser output mirror was replaced by a quartz window.

Noncommutative dispersion relation and mass-radius relation of white dwarfs

The equation of state of the electron degenerate gas in a white dwarf is usually treated by employing the ideal dispersion relation. However, the effect of quantum gravity is expected to be inevitably present and when this effect is considered through a non-commutative formulation, the dispersion relation undergoes a substantial modification. In this paper, we take such a modified dispersion relation and find the corresponding equation of state for the degenerate electron gas in white dwarfs. Hence we solve the equation of hydrostatic equilibrium and find that this leads to the possibility of the existence of excessively high values of masses exceeding the Chandrasekhar limit, although the quantum gravity effect is taken to be very small. It is only when we impose the additional effect of neutronization that we obtain white dwarfs with masses close to the Chandrasekhar limit with nonzero radii at the neutronization threshold. We demonstrate these results by giving numerical estimates for the masses and radii of helium, carbon and oxygen white dwarfs.

Photoneutron cross sections for Ni isotopes: Toward understanding (n,γ) cross sections relevant to weak s -process nucleosynthesis

Photoneutron cross sections were measured for $^{58}$Ni, $^{60}$Ni, $^{61}$Ni, and $^{64}$Ni at energies between the one-neutron and two-neutron thresholds using quasi-monochromatic $\gamma$-ray beams produced in laser Compton-scattering at the NewSUBARU synchrotron radiation facility. The new photoneutron data are used to extract the $\gamma$-ray strength function above the neutron threshold complementing the information obtained by the Oslo method below the threshold. We discuss radiative neutron capture cross sections and the Maxwellian-averaged cross sections for Ni isotopes including $^{63}$Ni, a branching point nucleus along the weak s-process path. The cross sections are calculated with the experimentally constrained $\gamma$-ray strength functions from the Hartree-Fock-Bogolyubov plus quasi-particle-random phase approximation based on the Gogny D1M interaction for both $E1$ and $M1$ components and supplemented with the $M1$ upbend.

Verification of detailed balance for γ absorption and emission in Dy isotopes

The photo-neutron cross sections of $^{162,163}\rm{Dy}$ have been measured for the first time in an energy region from the neutron threshold ($S_n$) up to $\approx$ $13$~MeV. The ($\gamma$,n) reaction was induced with quasi-monochromatic laser Compton-scattered $\gamma$ rays, produced at the NewSUBARU laboratory. The corresponding $\gamma$-ray strength functions ($\gamma$SF) have been calculated from the photo-neutron cross sections. The data are compared to reanalyzed $\gamma$SFs of $^{160-164}\rm{Dy}$, which are measured below $S_n$. The excellent agreement with the photo-neutron data at $S_n$ confirms the principle of detailed balance. Thus, a complete $\gamma$SF is established covering in total the energy region of 1 MeV $\leq$ E$_{\gamma}$ $\leq$ 13 MeV. These mid-shell well-deformed dysprosium isotopes all show scissors resonances with very similar structures. We find that our data predict the same integrated scissors strength as ($\gamma,\gamma^\prime$) data when integrated over the same energy range, which shows that the scissors mode very likely is consistent with the generalized Brink hypothesis. Finally, using the $\gamma$SFs as input in the reaction code TALYS, we have deduced radiative neutron-capture cross sections and compared them to direct measurements. We find a very good agreement within the uncertainties, which gives further support to the experimentally determined $\gamma$SFs.

Simulation of the ELIGANT-GN array performances at ELI-NP for gamma beam energies larger than neutron threshold

Performance of the ELIGANT-GN array being developed at ELI-NP was evaluated using dedicated GEANT4 simulation code. The array is designed to consist of 17 LaBr3:Ce and 17 CeBr3 detectors as well as 33 BC501A and 29 6Li-glass detectors. The energy and time responses of the detectors were studied giving information on gamma and neutron absolute detection efficiencies of the array. A planned day-one experiment regarding the gamma and neutron decay from GDR in 208Pb is discussed and, based on simulations, the data analysis procedure is proposed. The detection efficiencies as well as the count rates estimations are presented.

Correlated Production and Analog Transport of Fission Neutrons and Photons Using Fission Models FREYA, FIFRELIN, and the Monte Carlo Code TRIPOLI-4

Fission modeling in general-purpose Monte Carlo transport codes often relies on average nuclear data provided by the international evaluation libraries. As such, only average fission multiplicities are available and correlations between fission neutrons and photons are missing. Whereas uncorrelated fission physics is usually sufficient for standard reactor core and radiation shielding calculations, correlated fission secondaries are required for specialized nuclear instrumentation and detector modeling. For coincidence counting detector optimization for instance, the precise simulation of fission neutrons and photons that remain correlated in time from birth to detection is essential. New developments were recently integrated into the Monte Carlo transport code TRIPOLI-4 to model fission physics more precisely, the purpose being to access event-by-event fission events from two different fission models: Fission Reaction Event Yield Algorithm (FREYA) and Fission Fragment Evaporation Leading to an Investigation of Nuclear Data (FIFRELIN). TRIPOLI-4 simulations can now be performed, either by connecting via an application programming interface to the Lawrence Livermore National Laboratory fission library including FREYA, or by reading the external fission event data files produced by FIFRELIN beforehand. These new capabilities enable us to easily compare results from the Monte Carlo transport calculations using the two fission models in a nuclear instrumentation application. In the first part of this paper, the broad underlying principles of the two fission models are recalled. We then present the experimental measurements of neutron angular correlations for 252Cf(sf) and 240Pu(sf). The correlations were measured for several neutron kinetic energy thresholds. In the latter part of this paper, simulation results are compared with the experimental data. Spontaneous fissions in 252Cf and 240Pu are modeled by FREYA or FIFRELIN. Emitted neutrons and photons are subsequently transported to an array of scintillators by TRIPOLI-4 in the analog mode to preserve their correlations. Angular correlations between fission neutrons obtained independently from these TRIPOLI-4 simulations, using either FREYA or FIFRELIN, are compared with the experimental results. For 240Pu(sf), the measured correlations were used to tune one of the FREYA model parameters. Similarly, different sets of parameters have been tested in FIFRELIN to try to improve the agreement with the experimental data.

Activation Measurements of Cross Sections for Ground and Isomeric States Production in Neutron Threshold Reactions on Y and Au

The paper describes cross-section measurements consisting of neutron activation using a 7Li(p,n) quasi mono-energetic neutron source and gamma-ray spectrometry performed at the Nuclear Physics Institute of the Czech Academy of Sciences in Řež, Czech Republic. The analysis of the excitation functions of neutron-induced threshold reactions for the energy range from 17 up to 34 MeV is presented. The cross sections for the population of both ground and isomeric states were measured in yttrium and gold. A comprehensive comparison of the measured cross sections with earlier experimental data from the EXFOR database, TALYS calculations, and evaluated nuclear data libraries is presented.

High-resolution study of levels in the astrophysically important nucleus Mg26 and resulting updated level assignments

The $^{22}$Ne($\alpha,n$)$^{25}$Mg reaction is an important source of neutrons for the s-process. Due to the difficulty in directly measuring the cross section of this reaction and the competing $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg reaction, indirect methods are frequently used to try to instead calculate the relevant reaction rates. Uncertainty, however, persists in the number and properties of the pertinent excited states of $^{26}$Mg. In this paper, the results of proton and deuteron scattering from $^{26}$Mg are presented, giving a list of levels in $^{26}$Mg. In addition, level assignments in $^{26}$Mg are discussed, including some possible changes to the previously accepted assignments in $^{26}$Mg.

Quasiparticle formulation of a multiphonon method and its application to the 20O nucleus

We outline briefly the quasiparticle formulation of a microscopic multiphonon approach known as equation of motion phonon method and discuss its application to the neutron rich semi-magic 20O. Our calculation emphasizes the crucial role of the two-phonon configurations, responsible for establishing an almost one to one correspondence between theoretical and experimental energy levels and for fragmenting the dipole strength, in qualitative agreement with the data. In particular, the fragmentation yields two dipole levels below the neutron threshold which correspond to the two peaks recently observed.

Gamma Strength Functions and Level Densities from High-Resolution Proton Scattering under 0°

Inelastic proton scattering at energies of a few 100 MeV and forward angles including 0° provides a novel method to measure gamma strength functions (GSF) in nuclei in an energy range of about 5 – 20 MeV. The experiments provide not only the E1 but also the M1 part of the GSF. The latter is poorly known in heavy nuclei. Comparison with gamma decay data (e.g. from the Oslo method) allows to test the generalised Brink-Axel (BA) hypothesis in the energy region of the pygmy dipole resonance (PDR) crucial for the modelling of (n,γ) and (γ,n) reactions in astrophysical reaction networks. From the two test cases studied, 208Pb remains inconclusive in the energy region of the PDR because of large Porter-Thomas fluctuations due to the small level density (LD), while the BA hypothesis seems to hold in case of 96Mo. A fluctuation analysis of the high-resolution data also provides a direct measure of the LD in the energy region of the isovector giant dipole resonance (IVGDR) well above the neutron threshold, where hardly any experimental information is available. This permits an independent test of the decomposition of GSF and LD in Oslo-type experiments.

Correlated Production and Analog Transport of Fission Neutrons and Photons using Fission Models FREYA, FIFRELIN and the Monte Carlo Code TRIPOLI-4® .

Fission modeling in general-purpose Monte Carlo transport codes often relies on average nuclear data provided by international evaluation libraries. As such, only average fission multiplicities are available and correlations between fission neutrons and photons are missing. Whereas uncorrelated fission physics is usually sufficient for standard reactor core and radiation shielding calculations, correlated fission secondaries are required for specialized nuclear instrumentation and detector modeling. For coincidence counting detector optimization for instance, precise simulation of fission neutrons and photons that remain correlated in time from birth to detection is essential. New developments were recently integrated into the Monte Carlo transport code TRIPOLI-4 to model fission physics more precisely, the purpose being to access event-by-event fission events from two different fission models: FREYA and FIFRELIN. TRIPOLI-4 simulations can now be performed, either by connecting via an API to the LLNL fission library including FREYA, or by reading external fission event data files produced by FIFRELIN beforehand. These new capabilities enable us to easily compare results from Monte Carlo transport calculations using the two fission models in a nuclear instrumentation application. In the first part of this paper, broad underlying principles of the two fission models are recalled. We then present experimental measurements of neutron angular correlations for 252Cf(sf) and 240Pu(sf). The correlations were measured for several neutron kinetic energy thresholds. In the latter part of the paper, simulation results are compared to experimental data. Spontaneous fissions in 252Cf and 240Pu are modeled by FREYA or FIFRELIN. Emitted neutrons and photons are subsequently transported to an array of scintillators by TRIPOLI-4 in analog mode to preserve their correlations. Angular correlations between fission neutrons obtained independently from these TRIPOLI-4 simulations, using either FREYA or FIFRELIN, are compared to experimental results. For 240Pu(sf), the measured correlations were used to tune the model parameters.

Cross-section measurements of the 94Mo(γ,n) and 90Zr(γ,n) reactions using real photons at the HIγS facility

The photodisintegration reaction cross-sections for 94Mo(γ,n) and 90Zr(γ,n) have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity γ-Ray Source (HIγS) facility, Triangle University Nuclear Laboratory (TUNL). The measurements were focused primarily on studying the energy dependence of the photoneutron cross sections, which is the most direct way of testing statistical models, and were performed close to the respective neutron thresholds and above up to ~ 20 MeV. Neutrons from the (γ,n) reactions were detected using a 4π assembly of 3He proportional counters developed at Los Alamos National Laboratory and presently available at TUNL. While the 94Mo(γ,n) cross section measurement aims to contribute to a broader investigation for understanding the γ-process (the mechanism responsible for the nucleosynthesis of the so-called p-nuclei), the information from the 90Zr(γ,n) data is relevant to constrain QRPA calculations of γ-ray strength functions in this mass region. In this contribution, we will present our preliminary results of the total (γ,n) excitation functions for the two photoneutron reactions on 94Mo and 90Zr.

Electric Dipole Strength Below the Neutron Threshold Energy in <sup>238</sup>U Nucleus

The E1 response of 238U actinide nuclei below the particle threshold was studied using translational and Galilean invariant QRPA. Especially strength properties of the K=0- and K=1- electric dipole excitations are investigated and studied their contribution to the formation of PDR mode below neutron separation energy.

Electric dipole strength and dipole polarizability in 48Ca within a fully self-consistent second random–phase approximation

The second random–phase–approximation model corrected by a subtraction procedure designed to cure double counting, instabilities, and ultraviolet divergences, is employed for the first time to analyze the dipole strength and polarizability in 48Ca. All the terms of the residual interaction are included, leading to a fully self-consistent scheme. Results are illustrated with two Skyrme parametrizations, SGII and SLy4. Those obtained with the SGII interaction are particularly satisfactory. In this case, the low-lying strength below the neutron threshold is well reproduced and the giant dipole resonance is described in a very satisfactory way especially in its spreading and fragmentation. Spreading and fragmentation are produced in a natural way within such a theoretical model by the coupling of 1 particle-1 hole and 2 particle-2 hole configurations. Owing to this feature, we may provide for the electric polarizability as a function of the excitation energy a curve with a similar slope around the centroid energy of the giant resonance compared to the corresponding experimental results. This represents a considerable improvement with respect to previous theoretical predictions obtained with the random–phase approximation or with several ab-initio models. In such cases, the spreading width of the excitation cannot be reproduced and the polarizability as a function of the excitation energy displays a stiff increase around the predicted centroid energy of the giant resonance.

Re-examining the Mg26(α,α′)Mg26 reaction: Probing astrophysically important states in Mg26

Background: The $^{22}$Ne($\alpha,n$)$^{25}$Mg reaction is one of the neutron sources for the $s$-process in massive stars. The properties of levels in $^{26}$Mg above the $\alpha$-particle threshold control the strengths of the $^{22}$Ne($\alpha,n$)$^{25}$Mg and $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg reactions. The strengths of these reactions as functions of temperature are one of the major uncertainties in the $s$-process. Methods: Inelastically scattered $\alpha$ particles from a $^{26}$Mg target were momentum-analysed in the K600 magnetic spectrometer at iThemba LABS, South Africa. The differential cross sections of states were deduced from the focal-plane trajectory of the scattered $\alpha$ particles. Based on the differential cross sections, spin and parity assignments to states are made. Results: A new $0^+$ state was observed in addition to a number of other states, some of which can be associated with states observed in other experiments. Some of the deduced $J^\pi$ values of the states observed in the present study show discrepancies with those assigned in an experiment performed at RCNP Osaka. Conclusion: The high level density at this excitation energy in $^{26}$Mg makes assigning $J^\pi$ values to observed states difficult. Further experimental investigations with superior experimental energy resolution are required to clarify the number of levels in $^{26}$Mg, especially between the $\alpha$-particle threshold at 10.615 MeV and the neutron threshold at 11.319 MeV.