Novel ΔJ=1 Sequence in ^{78}Ge: Possible Evidence for Triaxiality

A sequence of low-energy levels in _{32}^{78}Ge_{46} has been identified with spins and parity of 2^{+}, 3^{+}, 4^{+}, 5^{+}, and 6^{+}. Decays within this band proceed strictly through ΔJ=1 transitions, unlike similar sequences in neighboring Ge and Se nuclei. Above the 2^{+} level, members of this sequence do not decay into the ground-state band. Moreover, the energy staggering of this sequence has the phase that would be expected for a γ-rigid structure. The energies and branching ratios of many of the levels are described well by shell-model calculations. However, the calculated reduced transition probabilities for the ΔJ=2 in-band transitions imply that they should have been observed, in contradiction with the experiment. Within the calculations of Davydov, Filippov, and Rostovsky for rigid-triaxial rotors with γ=30°, there are sequences of higher-spin levels connected by strong ΔJ=1 transitions which decay in the same manner as those observed experimentally, yet are calculated at too high an excitation energy.

Analysis of the Daya Bay Reactor Antineutrino Flux Changes with Fuel Burnup

We investigate the recent Daya Bay results on the changes in the antineutrino flux and spectrum with the burnup of the reactor fuel. We find that the discrepancy between current model predictions and the Daya Bay results can be traced to the original measured ^{235}U/^{239}Pu ratio of the fission β spectra that were used as a base for the expected antineutrino fluxes. An analysis of the antineutrino spectra that is based on a summation over all fission fragment β decays, using nuclear database input, explains all of the features seen in the Daya Bay evolution data. However, this summation method still allows for an anomaly. We conclude that there is currently not enough information to use the antineutrino flux changes to rule out the possible existence of sterile neutrinos.

Dissecting Reactor Antineutrino Flux Calculations

Current predictions for the antineutrino yield and spectra from a nuclear reactor rely on the experimental electron spectra from ^{235}U, ^{239}Pu, ^{241}Pu and a numerical method to convert these aggregate electron spectra into their corresponding antineutrino ones. In the present work we investigate quantitatively some of the basic assumptions and approximations used in the conversion method, studying first the compatibility between two recent approaches for calculating electron and antineutrino spectra. We then explore different possibilities for the disagreement between the measured Daya Bay and the Huber-Mueller antineutrino spectra, including the ^{238}U contribution as well as the effective charge and the allowed shape assumption used in the conversion method. We observe that including a shape correction of about +6%  MeV^{-1} in conversion calculations can better describe the Daya Bay spectrum. Because of a lack of experimental data, this correction cannot be ruled out, concluding that in order to confirm the existence of the reactor neutrino anomaly, or even quantify it, precisely measured electron spectra for about 50 relevant fission products are needed. With the advent of new rare ion facilities, the measurement of shape factors for these nuclides, for many of which precise beta intensity data from TAGS experiments already exist, would be highly desirable.

Effects of Fission Yield Data in the Calculation of Antineutrino Spectra for ^{235}U(n,fission) at Thermal and Fast Neutron Energies

Fission yields form an integral part of the prediction of antineutrino spectra generated by nuclear reactors, but little attention has been paid to the quality and reliability of the data used in current calculations. Following a critical review of the thermal and fast ENDF/B-VII.1 ^{235}U fission yields, deficiencies are identified and improved yields are obtained, based on corrections of erroneous yields, consistency between decay and fission yield data, and updated isomeric ratios. These corrected yields are used to calculate antineutrino spectra using the summation method. An anomalous value for the thermal fission yield of ^{86}Ge generates an excess of antineutrinos at 5-7 MeV, a feature which is no longer present when the corrected yields are used. Thermal spectra calculated with two distinct fission yield libraries (corrected ENDF/B and JEFF) differ by up to 6% in the 0-7 MeV energy window, allowing for a basic estimate of the uncertainty involved in the fission yield component of summation calculations. Finally, the fast neutron antineutrino spectrum is calculated, which at the moment can only be obtained with the summation method and may be relevant for short baseline reactor experiments using highly enriched uranium fuel.

Fission barrier of superheavy nuclei and persistence of shell effects at high spin: cases of 254No and 220Th

We report on the first measurement of the fission barrier height in a heavy shell-stabilized nucleus. The fission barrier height of 254No is measured to be Bf=6.0±0.5  MeV at spin 15ℏ and, by extrapolation, Bf=6.6±0.9  MeV at spin 0ℏ. This information is deduced from the measured distribution of entry points in the excitation energy versus spin plane. The same measurement is performed for 220Th and only a lower limit of the fission barrier height can be determined: Bf(I)>8  MeV. Comparisons with theoretical fission barriers test theories that predict properties of superheavy elements.

SU(3) quasidynamical symmetry underlying the Alhassid-Whelan arc of regularity

The first example of an empirically manifested quasidynamical symmetry trajectory in the interior of the symmetry triangle of the interacting boson approximation model is identified for large boson numbers. Along this curve, extending from SU(3) to near the critical line of the first order phase transition, spectra exhibit nearly the same degeneracies that characterize the low energy levels of SU(3). This trajectory also lies close to the Alhassid-Whelan arc of regularity, the unique interior region of regular behavior connecting the SU(3) and U(5) vertices, thus offering a possible symmetry-based interpretation of that narrow zone of regularity amidst regions of more chaotic spectra.

Precise electromagnetic tests of ab initio calculations of light nuclei: states in 10Be

In order to test ab initio calculations of light nuclei, we have remeasured lifetimes in 10Be using the Doppler shift attenuation method (DSAM) following the 7Li(7Li,alpha)10Be reaction at 8 and 10 MeV. The new experiments significantly reduce systematic uncertainties in the DSAM technique. The J(pi) = 2(1)(+) state at 3.37 MeV has tau = 205 +/- (5)(stat) +/- (7)(sys) fs corresponding to a B(E2 down) of 9.2(3)e(2) fm(4) in broad agreement with many calculations. The J(pi) = 2(2)(+) state at 5.96 MeV was found to have a B(E2 down) of 0.11(2)e(2) fm(4) and provides a more discriminating test of nuclear models. New Green's function Monte Carlo calculations for these states and transitions with a number of Hamiltonians are also reported and compared to experiment.

Unified description of 0+ states in a large class of nuclear collective models

A remarkably simple regularity in the energies of 0+ states in a broad class of collective models is discussed. A single formula for all 0+ states in flat-bottomed infinite potentials that depends only on the number of dimensions and a simpler expression applicable to all three interacting boson approximation symmetries in the large N(B) limit are presented. Finally, a connection between the energy expression for 0+ states given by the X5 model and the predictions of the interacting boson approximation near the critical point of the first order phase transition is explored.

Simple empirical order parameter for a first-order quantum phase transition in atomic nuclei

A simple, empirical, easy-to-measure effective order parameter of a first-order phase transition in atomic nuclei is presented, namely, the ratio of the energies of the first excited 6+ and 0+ states, distinguishing between first- and second-order transitions, and taking on a special value in the critical region, as data in Nd-Dy show. In the large NB limit of the interacting boson approximation model, a repeating degeneracy between alternate yrast and successive 0+ states is found in the critical region around the line of a first-order phase transition, pointing to a possible underlying symmetry.

Alternative interpretation of sharply rising E0 strengths in transitional regions

It is shown that strong 0(+)(2)-->0(+)(1) E0 transitions provide a clear signature of phase transitional behavior in finite nuclei. Calculations using the interacting-boson approximation (IBA) show that these transition strengths exhibit a dramatic and robust increase in spherical-deformed shape transition regions, that this rise matches well the existing data, that the predictions of these E0 transitions remain large in deformed nuclei, that they arise from the specific d-boson coherence in the wave functions, and do not necessarily require the explicit mixing of normal and intruder configurations from different IBA spaces.

Experimental confirmation of the Alhassid-Whelan arc of regularity

Evidence is presented to show that a group of nuclei, spanning a range of structures, corresponds to a previously proposed isolated region of regular behavior between vibrational and rotational structures that was never before observed empirically. Nuclei predicted to show such regular spectra correspond to Hamiltonian parameters that lie amidst those giving more chaotic spectra. We identify a key observable that has a one-to-one correspondence to this arc of regularity and which therefore provides both an empirical signature for it and a clue to its underlying nature.