Excitation Energy Region Research Articles

Bose-Einstein condensation of α clusters and new soft mode in C12 – Fe52 4N nuclei in a field-theoretical superfluid cluster model

Bose-Einstein condensation of alpha clusters in light and medium-heavy nuclei is studied in the frame of the field theoretical superfluid cluster model. The order parameter of the phase transition from the Wigner phase to the Nambu-Goldstone phase is a superfluid amplitude, square of the moduli of which is the superfluid density distribution. The zero mode operators due to the spontaneous symmetry breaking of the global phase in the finite number of alpha clusters are rigorously treated. The theory is systematically applied to N alpha nuclei from12C-52Fe at various condensation rates. In 12C it is found that the energy levels of the gas-like well-developed alpha cluster states above the Hoyle state are reproduced well in agreement with experiment for realistic condensation rates of alpha clusters. The electric E2 and E0 transitions are calculated and found to be sensitive to the condensation rates. The profound raison d'etre of the alpha cluster gas-like states above the Hoyle state, whose structure has been interpreted geometrically in the nuclear models without the order parameter such as the cluster models or ab initio calculations, is revealed. It is found that in addition to the Bogoliubov-de Gennes vibrational mode states collective states of the zero mode operators appear systematically at low excitation energies from the N alpha threshold energy. These collective states, new-type soft modes in nuclei due to the Bose-Einstein condensation of the alpha clusters, emerge systematically in light and medium-heavy mass regions and are also located at high excitation energies from the ground state in contrast to the traditional concept of soft mode in the low excitation energy region.

The valence and Rydberg states of difluoromethane: A combined experimental vacuum ultraviolet spectrum absorption and theoretical study by ab initio configuration interaction and density functional computations.

The vacuum ultraviolet (VUV) spectrum for CH2F2 from a new synchrotron study has been combined with earlier data and subjected to detailed scrutiny. The onset of absorption, band I and also band IV, is resolved into broad vibrational peaks, which contrast with the continuous absorption previously claimed. A new theoretical analysis, using a combination of time dependent density functional theory (TDDFT) calculations and complete active space self-consistent field, leads to a major new interpretation. Adiabatic excitation energies (AEEs) and vertical excitation energies, evaluated by these methods, are used to interpret the spectra in unprecedented detail using theoretical vibronic analysis. This includes both Franck-Condon (FC) and Herzberg-Teller (HT) effects on cold and hot bands. These results lead to the re-assignment of several known excited states and the identification of new ones. The lowest calculated AEE sequence for singlet states is 11B1 ∼ 11A2 < 21B1 < 11A1 < 21A1 < 11B2 < 31A1 < 31B1. These, together with calculated higher energy states, give a satisfactory account of the principal maxima observed in the VUV spectrum. Basis sets up to quadruple zeta valence with extensive polarization are used. The diffuse functions within this type of basis generate both valence and low-lying Rydberg excited states. The optimum position for the site of further diffuse functions in the calculations of Rydberg states is shown to lie on the H-atoms. The routine choice on the F-atoms is shown to be inadequate for both CHF3 and CH2F2. The lowest excitation energy region has mixed valence and Rydberg character. TDDFT calculations show that the unusual structure of the onset arises from the near degeneracy of 11B1 and 11A2 valence states, which mix in symmetric and antisymmetric combinations. The absence of fluorescence in the 10.8-11 eV region contrasts with strong absorption. This is interpreted by the 21B1 and 11A1 states where no fluorescence is calculated for these two states, which are only active in absorption. The nature of the two states, 11B1 and 21B1, is fundamentally different, but both are complex owing to the presence of FC and HT effects occurring in different ways. The two most intense bands, close to 12.5 and 15.5 eV, contain valence states as expected; the onset of the 15.5 eV band shows a set of vibrational peaks, but the vibration frequency does not correspond to any of the photoelectron spectral (PES) structure and is clearly valence in nature. The routine use of PES footprints to detect Rydberg states in VUV spectra is shown to be inadequate. The combined effects of FC and HT in the VUV spectral bands lead to additional vibrations when compared with the PES.

Effect of exact thermal pairing on nuclear level density

Nuclear level density is studied within a microscopic approach, which is derived based on the exact thermal pairing of the pairing Hamiltonian for the truncated single-particle levels around the Fermi surface in combination with the finite-temperature independent-particle model for the single-particle levels outside the truncated space. The numerical calculations are carried out for 170,171,172Yb isotopes, whose experimental data are available. The results obtained show that the exact thermal pairing is indeed very important for the valid description of nuclear level density in the low and intermediate regions of excitation energy.

Experimental investigation of exotic clustering in 13B and 14C using the resonance scattering method

In order to investigate the existence of molecular and/or exotic cluster configurations in Boron and Carbon n-rich isotopes we undertook two experiments: the first experimental study of exotic 9Li+α cluster states in 13B using the resonance scattering method at TRIUMF (Canada), and, with the same technique, the measurement of 10Be+α scattering at LNS in Catania, where a 10Be radioactive beam was produced for the first time. In order to measure the excitation function in a wide energy range, the beams were stopped in a Helium-flooded chamber. In the case of 13B, the elastic excitation function shows the presence of various peaks in an excitation energy region never explored before. In the case of 14C, our exclusive measurement of elastic scattering data with a high intensity beam, sheds some light on the contradictory previously published results [1, 2].

Correlated mass, energy, and angular distributions from bremsstrahlung-induced fission of U234 and Th232 in the energy region of the fission barrier

The bremstrahlung-induced fission of $^{234}\mathrm{U}$ and $^{232}\mathrm{Th}$ has been studied at the superconducting Darmstadt linear accelerator (S-DALINAC) in the excitation energy region close to the fission barrier. Fission-fragment mass and total kinetic energy (TKE) distributions from $^{234}\mathrm{U}$ were studied for the first time in this energy region. The results have been analysed in terms of fission modes, and a dominant yield of the mass-asymmetric standard-2 mode was found in both nuclei. No strong dependence of the fission-mode weights on the excitation energy of the compound nucleus was found. Correlations among mass, TKE, and angular distributions have also been investigated. A correlation in the form of an increased anisotropy for far-asymmetric masses and low TKE were found in both fissioning systems. A possible interpretation of this correlation in terms of fission modes is discussed.

The resonance-like structure observed in the 25Mg(p, γ)26Al reaction

The gamma decay of resonance-like structure (RLS) observed in the 25Mg(p, γ)26Al reaction in the region of excitation energies of 7–9 MeV is studied, and the excitation function of this reaction is measured. The resonance transition strengths of states in the 1.4–2.0 MeV range of accelerated proton energies are determined. The resulting strength distributions of M1 transitions between the resonance states and the low-lying bound states in 26Al are of a resonance nature. The position of the center of gravity of magnetic dipole resonance for the ground state in 26Al is 7.92 MeV, and the total MDR transition strength is 5.7 MeV × μ 2 .

Search for weakM1transitions inCa48with inelastic proton scattering

The spinflip M1 resonance in the doubly magic nucleus $^{48}$Ca, dominated by a single transition, serves as a reference case for the quenching of spin-isospin modes in nuclei. The aim of the present work is a search for weak M1 transitions in $^{48}$Ca with a high-resolution (p,p') experiment at 295 MeV and forward angles including 0 degree and a comparison to results from a similar study using backward-angle electron scattering at low momentum transfers in order to estimate their contribution to the total B(M1) strength. M1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis. The corresponding reduced B(M1) transition strengths are extracted following the approach outlined in J. Birkhan et al., Phys. Rev. C 93, 041302(R) (2016). In total, 29 peaks containing a M1 contribution are found in the excitation energy region 7 - 13 MeV. The resulting B(M1) strength distribution compares well to the electron scattering results considering different factors limiting the sensitivity in both experiments and the enhanced importance of mechanisms breaking the proportionality of nuclear cross sections and electromagnetic matrix elements for weak transitions as studied here. The total strength of 1.19(6) $\mu_N^2$ deduced assuming a non-quenched isoscalar part of the (p,p') cross sections agrees with the (e,e') result of 1.21(13) $\mu_N^2$. A binwise analysis above 10 MeV provides an upper limit of 1.62(23) $\mu_N^2$. The present results confirm that weak transitions contribute about 25% to the total B(M1) strength in $^{48}$Ca and the quenching factors of GT and spin-M1 strength are comparable in fp-shell nuclei. Thus, the role of of meson exchange currents seems to be neglible, in contrast to sd-shell nuclei.

α clustering in Si28 probed through the identification of high-lying 0+ states

Background: Aspects of the nuclear structure of light $\ensuremath{\alpha}$-conjugate nuclei have long been associated with nuclear clustering based on $\ensuremath{\alpha}$ particles and heavier $\ensuremath{\alpha}$-conjugate systems such as $^{12}\mathrm{C}$ and $^{16}\mathrm{O}$. Such structures are associated with strong deformation corresponding to superdeformed or even hyperdeformed bands. Superdeformed bands have been identified in $^{40}\mathrm{Ca}$ and neighboring nuclei and find good description within shell model, mean-field, and $\ensuremath{\alpha}$-cluster models. The utility of the $\ensuremath{\alpha}$-cluster description may be probed further by extending such studies to more challenging cases comprising lighter $\ensuremath{\alpha}$-conjugate nuclei such as $^{24}\mathrm{Mg}, ^{28}\mathrm{Si}$, and $^{32}\mathrm{S}$.Purpose: The purpose of this study is to look for the number and energy of isoscalar ${0}^{+}$ states in $^{28}\mathrm{Si}$. These states are the potential bandheads for superdeformed bands in $^{28}\mathrm{Si}$ corresponding to the exotic structures of $^{28}\mathrm{Si}$. Of particular interest is locating the ${0}^{+}$ bandhead of the previously identified superdeformed band in $^{28}\mathrm{Si}$.Methods: $\ensuremath{\alpha}$-particle inelastic scattering from a $^{\mathrm{nat}}\mathrm{Si}$ target at very forward angles including ${0}^{\ensuremath{\circ}}$ has been performed at the iThemba Laboratory for Accelerator-Based Sciences in South Africa. Scattered particles corresponding to the excitation energy region of 6 to 14 MeV were momentum-analysed in the K600 magnetic spectrometer and detected at the focal plane using two multiwire drift chambers and two plastic scintillators.Results: Several ${0}^{+}$ states have been identified above 9 MeV in $^{28}\mathrm{Si}$. A newly identified 9.71 MeV ${0}^{+}$ state is a strong candidate for the bandhead of the previously discussed superdeformed band. The multichannel dynamical symmetry of the semimicroscopic algebraic model predicts the spectrum of the excited ${0}^{+}$ states. The theoretical prediction is in good agreement with the experimental finding, supporting the assignment of the 9.71-MeV state as the bandhead of a superdeformed band.Conclusion: Excited isoscalar ${0}^{+}$ states in $^{28}\mathrm{Si}$ have been identified. The number of states observed in the present experiment shows good agreement with the prediction of the multichannel dynamical symmetry.

Shape coexistence in the N = 19 neutron-rich nucleus 31 Mg explored by β – γ spectroscopy of spin-polarized 31 Na

Abstract The structure of excited states in the neutron-rich nucleus 31 Mg, which is in the region of the “island of inversion” associated with the neutron magic number N = 20 , is studied by β – γ spectroscopy of spin-polarized 31 Na. Among the 31 Mg levels below the one neutron separation energy of 2.3 MeV, the spin values of all five positive-parity levels are unambiguously determined by observing the anisotropic β decay. Two rotational bands with K π = 1 / 2 + and 1 / 2 − are proposed based on the spins and energies of the levels. Comparison on a level-by-level basis is performed between the experimental results and theoretical calculations by the antisymmetrized molecular dynamics (AMD) plus generator coordinate method (GCM). It is found that various nuclear structures coexist in the low excitation energy region in 31 Mg.

Investigation of the ^{10}Li shell inversion by neutron continuum transfer reaction.

This Letter reports a study of the highly debated ^{10}Li structure through the d(^{9}Li,p)^{10}Li one-neutron transfer reaction at 100MeV. The ^{10}Li energy spectrum is measured up to 4.6MeV and angular distributions corresponding to different excitation energy regions are reported for the first time. The comparison between data and theoretical predictions, including pairing correlation effects, shows the existence of a p_{1/2} resonance at 0.45±0.03 MeV excitation energy, while no evidence for a significant s-wave contribution close to the threshold energy is observed. Moreover, two high-lying structures are populated at 1.5 and 2.9MeV. The corresponding angular distributions suggest a significant s_{1/2} partial-wave contribution for the 1.5MeV structure and a mixing of configurations at higher energy, with the d_{5/2} partial-wave contributing the most to the cross section.

Spectral dependence of photoemission in multiphoton ionization of NO2 by femtosecond pulses in the 375-430 nm range.

We investigate the multiphoton ionization of NO2 using tunable (430-375 nm) femtosecond pulses and photoelectron-photoion coincidence momentum spectroscopy. In order to understand the complex electronic and nuclear photodynamics at play following absorption of three to five photons, we also report extended photoionization calculations using correlated targets and coupled channels. Exploring the multiphoton dissociative ionization (MPDI) and multiphoton ionization (MPI) processes over such a broad energy range enables us to lend further support to our work carried out around 400 nm of a femtosecond laser [S. Marggi Poullain et al., J. Phys. B: At., Mol. Opt. Phys., 2014, 47, 124024]. Two excitation energy regions are identified and discussed in terms of the proposed reaction pathways, highlighting the significant role of Rydberg states, such as the [R*(6a1)-1, 3pσ] Rydberg state, in the NO2 multiphoton excitation and photoionization. These new results support our previous assumption that different bent and linear geometries of the NO2+(X1Σg) ionic state contribute to the MPDI and MPI, consistent with the reported calculations which reveal an important vibronic coupling characterizing the photoemission. Remarkably, the strong anisotropy of the recoil frame photoelectron angular distribution (RFPAD) previously observed at 400 nm appears as a fingerprint across the whole explored photon energy range.

Isomer ratios for products of photonuclear reactions on 121Sb

Over the past several years various preequilibrium model approaches for nuclear reactions were developed. Diversified detailed experimental data in the medium excitation energy region for nucleus are needed for reasonable selection among these theoretical models. Lack of experimental data in this energy region does essentially limit the possibilities for analysis and comparison of different preequilibrium theoretical models. For photonuclear reactions this energy region extends between bremsstrahlung energies nearly 30–100 MeV. Experimental measurements and estimations of isomer ratios for products of photonuclear reactions with multiple particle escape on antimony have been performed using bremsstrahlung with end-point energies 38, 43 and 53 MeV. Method of induced activity measurement was applied. For acquisition of gamma spectra we used HPGe spectrometer with 20% efficiency and energy resolution 1.9 keV for 1332 keV gamma line of 60 Co. Linear accelerator of electrons LU-40 was a source of bremsstrahlung. Energy resolution of electron beam was about 1% and mean current was within (3.8–5.3) μA.

Investigation of Cluster States in $^{13}$B Using the $^{9}$Li--$\alpha $ Resonant Elastic Scattering

The excitation function of the resonant reaction 4He(9Li,α) was measured with the aim of investigating the compound nucleus 13B. These measurements were performed in inverse kinematics at center-of-mass scattering angles close to 180° by using a thick 4He gas target and a 9Li beam. The 13B excitation energy region explored was 14-20 MeV where 9Li-αurations of 13B are predicted by Antysimmetrised Molecular Dynamics calculations. The measured excitation function at θcm= 180°s different clear structures in a 13B excitation energy region which was experimentally unknown.

Shell-model study of 31S at excitations relevant to the thermonuclear 30P(p, $\gamma$ )31S reaction rate

Recent experimental data about the 30P(p, $ \gamma$ ) reaction have been reported, where a list of confirmed 31S excited states within 500 keV of the 30P(p, $ \gamma$ )31S threshold at $ E_{x} = 6131$ keV and their properties were studied. Definitive constraints on $ J^{\pi}$ are only given for two levels; the other ones are either tentative (five states) or undefined (six states). In order to confirm the proposed spin/parity assignments and to determine the unknown ones a comparison with shell-model calculations is crucial. We have performed a shell-model calculation using the $ (0+1)\hbar\omega$ PSDPF interaction to calculate the excitation energy spectrum of mass 31 ( 31S - 31P up to 7000 keV. Between 6100 and 6700 keV, an excitation energy region of astrophysical interest, we obtain a set of negative-parity states that are good candidates for the observed ones. A complete spectrum of positive- and negative-parity states can now be proposed based also on a comparison with the 31P spectrum up to 7000 keV.

Evidence for excitation of two resonance states in the isovector two-baryon system with a mass of 2.2 GeV/c2

We report on measurements of the differential cross section $d\ensuremath{\sigma}/d\mathrm{\ensuremath{\Omega}}$ and the first measurement of the analyzing power ${A}_{y}$ in the $\mathrm{\ensuremath{\Delta}}(1232)$ excitation energy region of the reaction $pp\ensuremath{\rightarrow}{{pp}}_{s}{\ensuremath{\pi}}^{0}$ where ${{pp}}_{s}$ is a $^{1}S_{0}$ proton pair. The experiment has been performed with the ANKE spectrometer at COSY-J\ulich. The data reveal a peak in the energy dependence of the forward ${{pp}}_{s}$ differential cross section, a minimum at zero degrees of its angular distribution, and a large analyzing power. The results present a direct manifestation of two two-baryon resonance-like states with ${J}^{P}={2}^{\ensuremath{-}}$ and ${0}^{\ensuremath{-}}$ and an invariant mass of $2.2\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}/{c}^{2}$.

Effects of pairing correlations on the inverse level density parameter of hot rotating nuclei

Angular momentum dependence of the inverse level density parameter K in the excitation-energy region of ∼ 30 – 40 MeV is studied within the finite-temperature Bardeen-Cooper-Schrieffer (FTBCS) theory and the FTBCS theory that includes the effect due to quasiparticle-number fluctuations (FTBCS1). The two theories take into account the noncollective rotation of the nucleus at nonzero values of z-projection M of the total angular momentum. The comparison between the results obtained within the FTBCS and FTBCS1 as well as the case without pairing correlations and the experimental data for two medium-mass even-even nuclei 108Cd and 122Te shows that by including the pairing corrections the FTBCS and FTBCS1 reproduces quite well all the experimental data, whereas the non-pairing case always overestimates the data. Due to the effect of quasiparticle-number fluctuations, the FTBCS1 gaps at different M values do not collapse at critical temperature TC as in the FTBCS ones but monotonously decrease with increasing T and being finite even at high T. As the result, the values of K obtained within the FTBCS1 are always closer to the experimental data than those obtained within the FTBCS.

Validity of the Generalized Brink-Axel Hypothesis in (238)Np.

We analyze primary γ-ray spectra of the odd-odd (238)Np nucleus extracted from (237)Np(d,pγ)(238)Np coincidence data measured at the Oslo Cyclotron Laboratory. The primary γ spectra cover an excitation-energy region of 0≤E(I)≤5.4 MeV, and allow us to perform a detailed study of the γ-ray strength as a function of excitation energy. Hence, we can test the validity of the generalized Brink-Axel hypothesis, which, in its strictest form, claims no excitation-energy dependence on the γ strength. In this work, using the available high-quality (238)Np data, we show that the γ-ray strength function is to a very large extent independent of the initial and final states. Thus, for the first time, the generalized Brink-Axel hypothesis is experimentally verified for γ transitions between states in the quasicontinuum region, not only for specific collective resonances, but also for the full strength below the neutron separation energy. Based on our findings, the necessary criteria for the generalized Brink-Axel hypothesis to be fulfilled are outlined.

Lyman-series emission after valence and core excitation of water vapor

We report Lyman-series emission cross sections of neutral hydrogen dissociation fragments after valence (15--34 eV) and inner-shell (533--542 eV) excitation of water vapor with monochromatic synchrotron radiation as functions of the exciting-photon energy. In the valence excitation energy region, the thermodynamical limits of the production of the differently excited hydrogen fragments are directly observed and absolute emission cross sections are determined. For resonant inner-shell excitations, the fluorescing excited hydrogen state is found to be strongly dependent on the molecular or Rydberg-like character of the excitation.

Energy Loss Function of Solids Assessed by Ion Beam Energy-Loss Measurements: Practical Application to Ta2O5

We present a study where the energy loss function of Ta2O5, initially derived in the optical limit for a limited region of excitation energies from reflection electron energy loss spectroscopy (REELS) measurements, was improved and extended to the whole momentum and energy excitation region through a suitable theoretical analysis using the Mermin dielectric function and requiring the fulfillment of physically motivated restrictions, such as the f- and KK-sum rules. The material stopping cross section (SCS) and energy-loss straggling measured for 300–2000 keV proton and 200–6000 keV helium ion beams by means of Rutherford backscattering spectrometry (RBS) were compared to the same quantities calculated in the dielectric framework, showing an excellent agreement, which is used to judge the reliability of the Ta2O5 energy loss function. Based on this assessment, we have also predicted the inelastic mean free path and the SCS of energetic electrons in Ta2O5.

Enhanced γ-Ray Emission from Neutron Unbound States Populated in β Decay.

Total absorption spectroscopy is used to investigate the β-decay intensity to states above the neutron separation energy followed by γ-ray emission in (87,88)Br and (94)Rb. Accurate results are obtained thanks to a careful control of systematic errors. An unexpectedly large γ intensity is observed in all three cases extending well beyond the excitation energy region where neutron penetration is hindered by low neutron energy. The γ branching as a function of excitation energy is compared to Hauser-Feshbach model calculations. For (87)Br and (88)Br the γ branching reaches 57% and 20%, respectively, and could be explained as a nuclear structure effect. Some of the states populated in the daughter can only decay through the emission of a large orbital angular momentum neutron with a strongly reduced barrier penetrability. In the case of neutron-rich (94)Rb the observed 4.5% branching is much larger than the calculations performed with standard nuclear statistical model parameters, even after proper correction for fluctuation effects on individual transition widths. The difference can be reconciled by introducing an enhancement of 1 order of magnitude in the photon strength to neutron strength ratio. An increase in the photon strength function of such magnitude for very neutron-rich nuclei, if it proves to be correct, leads to a similar increase in the (n,γ) cross section that would have an impact on r process abundance calculations.