Forward-peaked Angular Distributions Research Articles

New method for detecting fast neutrino flavor conversions in core-collapse supernova models with two-moment neutrino transport

Fast-pairwise neutrino oscillations potentially affect many aspects of core-collapse supernova (CCSN): the explosion mechanism, neutrino signals, and nucleosynthesis in the ejecta. This particular mode of collective neutrino oscillations has a deep connection to the angular structure of neutrinos in momentum space; for instance, the appearance of electron neutrinos lepton number (ELN) angular crossings in momentum space is a good indicator of occurrences of the flavor conversions. However, many multi-dimensional (multi-D) CCSN simulations are carried out with approximate neutrino transport (such as two-moment methods), which limits the access to the angular distributions of neutrinos, i.e., inhibits ELN-crossing searches. In this paper, we develop a new method of ELN-crossing search in these CCSN simulations. The required data is the zero-th and first angular moments of neutrinos and matter profile, all of which are available in CCSN models with two-moment method. One of the novelties of our new method is to use a ray-tracing neutrino transport to determine ELNs in the direction of the stellar center. It is designed to compensate for shortcomings of the crossing searches only with the two angular moments. We assess the capability of the method by carrying out a detailed comparison to results of full Boltzmann neutrino transport in 1D and 2D CCSN models. We find that the ray-tracing neutrino transport improves the accuracy of crossing searches; indeed, the appearance/disappearance of the crossings is accurately detected even in the region of forward-peaked angular distributions. The new method is computationally cheap and has a benefit of efficient parallelization; hence, it will be useful for ELN-crossing searches in any CCSN models employed two-moment neutrino transport.

Correlations between fission fragment and neutron anisotropies in neutron-induced fission

Several sources of angular anisotropy for fission fragments and prompt neutrons have been studied in neutron-induced fission reactions. These include kinematic recoils of the target from the incident neutron beam and the fragments from the emission of the prompt neutrons, preferential directions of the emission of the fission fragments with respect to the beam axis due to the population of particular transition states at the fission barrier, and forward-peaked angular distributions of pre-equilibrium neutrons which are emitted before the formation of a compound nucleus. In addition, there are several potential sources of angular anisotropies that are more difficult to disentangle: the angular distributions of prompt neutrons from fully accelerated fragments or from scission neutrons, and the emission of neutrons from fission fragments that are not fully accelerated. In this work, we study the effects of the first group of anisotropy sources, particularly exploring the correlations between the fission fragment anisotropy and the resulting neutron anisotropy. While kinematic effects were already accounted for in our Hauser-Feshbach Monte Carlo code, $\mathtt{CGMF}$, anisotropic angular distributions for the fission fragments and pre-equilibrium neutrons resulting from neutron-induced fission on $^{233,234,235,238}$U, $^{239,241}$Pu, and $^{237}$Np have been introduced for the first time. The effects of these sources of anisotropy are examined over a range of incident neutron energies, from thermal to 20 MeV, and compared to experimental data from the Chi-Nu liquid scintillator array. The anisotropy of the fission fragments is reflected in the anisotropy of the prompt neutrons, especially as the outgoing energy of the prompt neutrons increases, allowing for an extraction of the fission fragment anisotropy to be made from a measurement of the neutrons.

Suppression of the multi-azimuthal-angle instability in dense neutrino gas during supernova accretion phase

It has been recently pointed out that by removing the axial symmetry in the ``multi-angle effects'' associated with the neutrino-neutrino interactions for supernova (SN) neutrinos a new multi-azimuthal-angle (MAA) instability would arise. In particular, for a flux ordering ${F}_{{\ensuremath{\nu}}_{e}}>{F}_{{\overline{\ensuremath{\nu}}}_{e}}>{F}_{{\ensuremath{\nu}}_{x}}$, as expected during the SN accretion phase, this instability occurs in the normal neutrino mass hierarchy. However, during this phase, the ordinary matter density can be larger than the neutrino one, suppressing the self-induced conversions. In this regard, we investigate the matter suppression of the MAA effects, performing a linearized stability analysis of the neutrino equations of motion, in the presence of realistic SN density profiles. We compare these results with the numerical solution of the SN neutrino nonlinear evolution equations. Assuming axially symmetric distributions of neutrino momenta, we find that the large matter term strongly inhibits the MAA effects. In particular, the hindrance becomes stronger including realistic forward-peaked neutrino angular distributions. As a result, in our model for a $10.8\text{ }{M}_{\ensuremath{\bigodot}}$ iron-core SNe, MAA instability does not trigger any flavor conversion during the accretion phase. Instead, for a $8.8\text{ }{M}_{\ensuremath{\bigodot}}$ O-Ne-Mg core SN model, with lower matter density profile and less forward-peaked angular distributions, flavor conversions are possible also at early times.

Differential cross sections for the electron impact excitation of pyrimidine

We report on differential cross section (DCS) measurements for the electron-impact excitation of the electronic states of pyrimidine. The energy range of the present measurements was 15-50 eV with the angular range of the measurements being 10°-90°. All measured DCSs displayed forward-peaked angular distributions, consistent with the relatively large magnitudes for the dipole moment and dipole polarizability of pyrimidine. Excitations to triplet states were found to be particularly important in some energy loss features at the lower incident electron energies. To the best of our knowledge there are no other experimental data or theoretical computations against which we can compare the present results.

Stability analysis of collective neutrino oscillations in the supernova accretion phase with realistic energy and angle distributions

We revisit our previous results on the matter suppression of self-induced neutrino flavor conversions during a supernova (SN) accretion phase, performing a linearized stability analysis of the neutrino equations of motion, in the presence of realistic SN density profiles. In our previous numerical study, we used a simplified model based on an isotropic neutrino emission with a single typical energy. Here, we take into account realistic neutrino energy and angle distributions. We find that multienergy effects have a subleading impact in the flavor stability of the SN neutrino fluxes with respect to our previous single-energy results. Conversely, realistic forward-peaked neutrino angular distributions would enhance the matter suppression of the self-induced oscillations with respect to an isotropic neutrino emission. As a result, in our models for iron-core SNe, collective flavor conversions have a negligible impact on the characterization of the observable neutrino signal during the accretion phase. Instead, for a low-mass O-Ne-Mg core SN model, with lower matter density profile and less forward-peaked angular distributions, collective conversions are possible also at early times.

Equilibrium and pre-equilibrium processes in theMn55(Li6,xp) andFe57(α,xp) reactions

Spectra of outgoing neutrons and protons from the 6Li+55Mn reaction and protons from the a+57Fe reaction have been measured with beams of 15 MeV 6Li ions and 30 MeV alpha-particles. These reactions proceed through the same 61Ni nucleus at the same excitation energy, thus allowing the difference in reaction mechanism to be studied. It is shown that spectra from the first reaction measured at backward angles are due to emission from a traditional compound nucleus reaction, in which the intermediate nucleus has reached statistical equilibrium; the spectra from the second reaction contain a significant fraction of pre-equilibrium emission at all angles. Level density pa- rameters of the residual nucleus 60Co have been obtained from the first reaction. Both emission spectra and angular distributions have been measured for the second reaction. It was found that the pre-equilibrium component exhibits a forward-peaked angular distribution, as expected, but with a steeper slope than predicted and with an unusual slight rise at angles above 120\deg. The backward- angle rise is explained qualitatively by the dominance of the multi-step compound mechanism at backward angles.

Molecular dynamics simulations of matrix-assisted laser desorption—connections to experiment

The molecular dynamics (MD) simulation technique has been applied to investigate fundamental aspects of matrix-assisted laser desorption. In this paper, we focus on direct comparisons of the results from the simulations with experimental data and on establishing links between the measured or calculated parameters and the basic mechanisms of molecular ejection. The results on the fluence dependence of the ablation/desorption yields and composition of the ejected plume are compared with mass spectrometry and trapping plate experiments. Implications of the prediction of a fluence threshold for ablation are discussed. The strongly forward-peaked velocity and angular distributions of matrix and analyte molecules, predicted in the simulations, are related to the experimental distributions. The shapes and amplitudes of the acoustic waves transmitted from the absorption region through the irradiated sample are compared to recent photoacoustic measurements and related to the ejection mechanisms. The conformational changes during plume evolution and the ejection velocities of analyte molecules are studied and the directions for future investigations are discussed. Finally, we demonstrate that the MD simulation technique can be used to model other processes relevant to mass spectrometry applications, such as laser disintegration of aerosol particles and laser ablation in the presence of photochemical reactions.

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.

Hydrogen adsorption and desorption on silicon revisited

Recent molecular beam data on the energy dependence of sticking coefficients for dissociative adsorption of hydrogen molecules on silicon are analyzed in terms of the five dimensional (5D) quantum reaction dynamics used before to describe data on sticking as a function of surface temperature and on state resolved desorption. The sticking coefficient of the 5D model depended strongly on the energy of surface phonons but weakly on the kinetic energy of the incoming H2 molecules. In a recent molecular beam experiment the latter dependence was measured directly and found to be substantially stronger than concluded from the earlier data. These results have important consequences for the energy and angular distribution of desorbing particles: If the parameters of the 5D model are adjusted so as to describe the new data, it predicts a mean normal kinetic energy of desorbing particles which is now about twice the thermal value. This prediction is rather model independent and can be derived almost directly from the measured data by invoking “detailed balance.” This new increased normal mean energy then is closer to ab initio generalized gradient approximation (GGA) slab calculations predicting about three times the thermal value. The increased kinetic energy also leads to a more forward-peaked angular distribution ∝(cos θ)11 to 12 instead of ∝(cos θ)3 to 4.

Excitation and neutron decay of xenon by forward-angle scattering of 250 A MeV 17O ions

A technique to study giant resonances by forward-angle scattering of heavy ions in the energy range 100–470 A MeV, using one quadrant of the CELSIUS storage ring as a magnetic spectrometer, is presented. The reaction nat Xe( 17O, 17O′n) has been studied at E( 17O) =250A MeV . A structure in the excitation energy spectrum, compatible with the isovector dipole resonance and the isoscalar quadrupole resonance, was observed, together with a possible candidate for the isovector quadrupole resonance. In addition to neutrons from statistical decay, a large number of non-statistical neutrons in the range E n =5–30 MeV was observed, with a forward-peaked angular distribution.

Gamow-Teller strength in the 208Pb(p,n)208Bi reaction at 134.3 MeV.

The excitation-energy distribution of transition strength to ${1}^{+}$ states was measured for the $^{208}\mathrm{Pb}$(p,n${)}^{208}$Bi reaction at 134.3 MeV for excitation energies up to 38 MeV. Structures observed in neutron time-of-flight spectra with forward-peaked (\ensuremath{\Delta}L=0) angular distributions were identified as ${1}^{+}$ states, except for the transition to the ${0}^{+}$ isobaric analog state. The ${1}^{+}$ strength in these structures was extracted by normalizing the yield above a fitted polynomial background to the Fermi transition strength localized in the isobaric analog state. The Gamow-Teller strength observed in the ${1}^{+}$ peaks is 56% of the 3(N-Z) sum rule when the strength of the ${\ensuremath{\beta}}^{+}$ transitions is assumed to be zero; 45% of 3(N-Z) is observed in the giant resonance and 10% is observed in structures below the giant resonance. Based on a multipole decomposition analysis, an upper limit on the ${1}^{+}$ strength in the apparent continuum to 38 MeV of excitation energy is estimated to be 37% of 3(N-Z). These results are compared with predictions from a shell model that includes a pairing force and a long-range Gamow-Teller force in both the parent and daughter nuclei.

Calculation of Neutron-Induced Reaction Cross Sections of Manganese-55

Neutron-induced reaction cross sections of 55Mn are calculated for the evaluated nuclear data libraries, ENDF/B-VI and JENDL-3. Simultaneously calculated are the inelastic scattering, (n,2n), (n,p), (n,α), (n,np), (n,nα) and (n,γ) reaction cross sections, the angular distributions of emitted neutrons and protons, and the energy distributions of emitted particles and γ-rays. A unified Hauser-Feshbach code is applied to calculate these quantities. Precompound and direct-reaction processes are taken into consideration, in addition to the compound process. The calculated results reproduce the experimental data very well. Forward-peaked angular distributions of continuous neutrons are obtained from the calculation, and found to be consistent with the measurements at 14MeV.

Low-lying structures in the Gamow-Teller strength functions for the double-beta-decaying nuclei 76Ge, 82Se, 128Te, and 130Te.

Excitation-energy distributions of transition strength to ${1}^{+}$ states excited via the (p,n) reaction at 134.4 MeV on targets of $^{76}\mathrm{Ge}$, $^{82}\mathrm{Se}$, $^{128}\mathrm{Te}$, and $^{130}\mathrm{Te}$ were measured for excitation energies up to 25 MeV. Structures observed in the neutron spectra with forward-peaked (\ensuremath{\Delta}L=0) angular distributions were identified as ${1}^{+}$ states, except for the isobaric analog transitions. The total ${1}^{+}$ strength in these reactions was extracted by normalizing the intensity in the ${1}^{+}$ peaks to the Fermi transition strength observed in the isobaric analog state. The Gamow-Teller strength observed in ${1}^{+}$ peaks above a fitted polynomial background is typically 55% of the sum rule obtained by assuming that the strength of ${\ensuremath{\beta}}^{+}$ transitions is negligible. The portion of this strength found at excitation energies less than that of the Gamow-Teller giant resonance varied from 15% for $^{128}\mathrm{Te}$ to 38% for $^{76}\mathrm{Ge}$. Experimental results are compared with predictions of a shell model that includes a pairing force and a long-range Gamow-Teller force in both parent and daughter nuclei. A comparison of the strength functions of the tellurium isotopes is made; this comparison is relevant in determining whether double-beta decay without neutrino emission (0\ensuremath{\nu} decay) is observed in these isotopes.

Calculation of neutron-induced reaction cross sections of Manganese-55.

Neutron-induced reaction cross sections of 55Mn are calculated for the evaluated nuclear data libraries, ENDF/B-VI and JENDL-3. Simultaneously calculated are the inelastic scattering, (n,2n), (n,p), (n,α), (n,np), (n,nα) and (n,γ) reaction cross sections, the angular distributions of emitted neutrons and protons, and the energy distributions of emitted particles and γ-rays. A unified Hauser-Feshbach code is applied to calculate these quantities. Precompound and direct-reaction processes are taken into consideration, in addition to the compound process. The calculated results reproduce the experimental data very well. Forward-peaked angular distributions of continuous neutrons are obtained from the calculation, and found to be consistent with the measurements at 14MeV.

Forward-peaked angular distributions observed in (π, π′) on light nuclei

Pion inelastic scattering on 12 C and 28 Si at extreme forward angles has revealed a set of states not previously observed. Their characteristics suggest they may be the spin-flip E1 giant resonance.

Fragment mass, energy, and angular distributions for theC12(He4, heavy ion) reaction between 49 and 159 MeV

Inclusive mass, energy, and angular distributions of all fragments with $A\ensuremath{\ge}6$ have been measured for alpha-particle-induced reactions on $^{12}\mathrm{C}$ at five energies between 49 and 159 MeV. From these data the evolution of the dominant reaction mechanisms with energy is characterized. Based on analysis of the mean value of the mass distribution, it appears that linear momentum transfer from projectile to target initially increases with beam energy and reaches a maximum at about 30-40 MeV/nucleon. The low-energy data are consistent with compound nucleus formation and simple transfer processes. At higher energies the growth of forward-peaked angular distributions, continuum energy spectra, and the disappearance of two-body states demonstrate the increasing importance of nucleon-nucleon collisions and multibody breakup mechanisms as the velocity of the projectile exceeds the Fermi velocity. The observed total reaction cross section decreases with increasing bombarding energy. Inclusion of these new data into the excitation functions for $6\ensuremath{\le}A\ensuremath{\le}11$, which are required for nucleosynthesis calculations relevant to the origin of the elements Li, Be, and B, does not significantly alter conclusions based on previously available data.

Excitation of ΔL = 0 spin-flip transitions in the N = 28 isotones by 201 MeV proton inelastic scattering

The excitation of 1 + states by inelastic scattering of 201 Me V protons, studied previously on the N = 28 nuclei 48Ca and 51V, has been pursued on 50Ti, 52Cr and 54Fe; for the three latter nuclei, many sharp states are observed which have a very forward-peaked angular distribution characteristic of a ΔL = 0 transition. The results agree reasonably well with (e, e') measurements. A comparison with ( γ−1, γ) results demonstrated the 1 + nature of these states. The data are compared with DWIA calculations performed using the shell-model wave functions of Metsch and Knüpfer. This comparison indicates that he isoscalar excitations are probably weaker than predicted. The ratio of experimental to predicted cross sections ranges from 0.25 for 50Ti to 0.44 for 54Fe.

Nuclear Fluid Dynamics versus Intranuclear Cascade—Possible Evidence for Collective Flow in Central High-Energy Nuclear Collisions

The predictions of a variety of current theoretical models of high-energy nuclear collisions are compared with recent experimental data for central collisions of /sup 20/Ne on /sup 238/U at E/sub lab/ = 393 MeV/n. The experimental observation of broad sideward maxima in the angular distributions of low- and medium-energy protons is reproduced by a nuclear fluid dynamical calculation with final freezeout of the protons. In contrast, the current intranuclear-cascade and simplified collision models predict forward-peaked angular distributions.

Quantitative explanation of the position and profiles of the (n, α) and (n, p) lines observed in a Si detector irradiated with fast neutrons

The position and profile of (n, p 0.1) and (n, α 0) peaks produced by 14.5 MeV neutrons in a Si surface barrier detector have been measured and compared to theoretical predictions. The pulse height defect due to the nuclear interaction of the alphas and recoil nuclei with silicon have been taken into account as well as the differential cross sections dσ/dω for the respective reactions. Good agreement between data and calculations is obtained for an isotropic angular distribution of protons and a forward-peaked angular distribution for α-particles. The results indicate the limits of the use of Si surface barrier detectors as neutron spectrometers.

Fast-Particle Emission in the Deep-Inelastic Reaction Cu+Ne20at 12.6 MeV/nucleon

Fast protons (1-2.4 times the beam velocity) have been studied. The spectral shapes in both singles and coincidence are similar. The most energetic protons are associated with intermediate-velocity products rather than beam-velocity ones. The observed high energies and the forward-peaked angular distributions could be explained by evaporation calculations which included thermal fluctuations in the division of the excitation energy between the two fragments of the intermediate complex.