Level densities and thermodynamical properties of Pt and Au isotopes

The nuclear level densities of $^{194-196}$Pt and $^{197,198}$Au below the neutron separation energy have been measured using transfer and scattering reactions. All the level density distributions follow the constant-temperature description. Each group of isotopes is characterized by the same temperature above the energy threshold corresponding to the breaking of the first Cooper pair. A constant entropy excess $\Delta S=1.9$ and $1.1$ $k_B$ is observed in $^{195}$Pt and $^{198}$Au with respect to $^{196}$Pt and $^{197}$Au, respectively, giving information on the available single-particle level space for the last unpaired valence neutron. The breaking of nucleon Cooper pairs is revealed by sequential peaks in the microcanonical caloric curve

137,138,139^{137,138,139}La(nn, γ\gamma) cross sections constrained with statistical decay properties of 138,139,140^{138,139,140}La nuclei

The nuclear level densities and $\gamma$-ray strength functions of $^{138,139,140}$La were measured using the $^{139}$La($^{3}$He, $\alpha$), $^{139}$La($^{3}$He, $^{3}$He$^\prime$) and $^{139}$La(d, p) reactions. The particle-$\gamma$ coincidences were recorded with the silicon particle telescope (SiRi) and NaI(Tl) (CACTUS) arrays. In the context of these experimental results, the low-energy enhancement in the A$\sim$140 region is discussed. The $^{137,138,139}$La($n, \gamma)$ cross sections were calculated at $s$- and $p$-process temperatures using the experimentally measured nuclear level densities and $\gamma$-ray strength functions. Good agreement is found between $^{139}$La($n, \gamma)$ calculated cross sections and previous measurements

Enhanced low-energy γ\gamma-decay strength of 70^{70}Ni and its robustness within the shell model

Neutron-capture reactions on very neutron-rich nuclei are essential for heavy-element nucleosynthesis through the rapid neutron-capture process, now shown to take place in neutron-star merger events. For these exotic nuclei, radiative neutron capture is extremely sensitive to their $\gamma$-emission probability at very low $\gamma$ energies. In this work, we present measurements of the $\gamma$-decay strength of $^{70}$Ni over the wide range $1.3 \leq E_{\gamma} \leq 8$ MeV. A significant enhancement is found in the $\gamma$-decay strength for transitions with $E_\gamma < 3$ MeV. At present, this is the most neutron-rich nucleus displaying this feature, proving that this phenomenon is not restricted to stable nuclei. We have performed $E1$-strength calculations within the quasiparticle time-blocking approximation, which describe our data above $E_\gamma \simeq 5$ MeV very well. Moreover, large-scale shell-model calculations indicate an $M1$ nature of the low-energy $\gamma$ strength. This turns out to be remarkably robust with respect to the choice of interaction, truncation and model space, and we predict its presence in the whole isotopic chain, in particular the neutron-rich $^{72,74,76}\mathrm{Ni}$.Comment: 9 pages, 9 figure

Report on 240Am(n,x) surrogate cross section test measurement

The main goal of the test measurement was to determine the feasibility of the {sup 243}Am(p,t) reaction as a surrogate for {sup 240}Am(n,f). No data cross section data exists for neutron induced reactions on {sup 240}Am; the half-life of this isotope is only 2.1 days making direct measurements difficult, if not impossible. The 48-hour experiment was conducted using the STARS/LIBERACE experimental facility located at the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory in August 2011. A description of the experiment and results is given. The beam energy was initially chosen to be 39 MeV in order to measure an equivalent neutron energy range from 0 to 20 MeV. However, the proton beam was not stopped in the farady cup and the beam was deposited in the surrounding shielding material. The shielding material was not conductive, and a beam current, needed for proper tuning of the beam as well as experimental monitoring, could not be read. If the {sup 240}Am(n,f) surrogate experiment is to be run at LBNL, simple modifications to the beam collection site will need to be made. The beam energy was reduced to 29 MeV, which was within an energy regime of prior experiments and tuning conditions at STARS/LIBERACE. At this energy, the beam current was successfully tuned and measured. At 29 MeV, data was collected with both the {sup 243}Am and {sup 238}U targets. An example particle identification plot is shown in Fig. 1. The triton-fission coincidence rate for the {sup 243}Am target and {sup 238}U target were measured. Coincidence rates of 0.0233(1) cps and 0.150(6) cps were observed for the {sup 243}Am and {sup 238}U targets, respectively. The difference in count rate is largely attributed to the available target material - the {sup 238}U target contains approximately 7 times more atoms than the {sup 243}Am. A proton beam current of {approx}0.7 nA was used for measurements on both targets. Assuming a full experimental run under similar conditions, an estimate for the run time needed was made. Figure 2 shows the number of days needed as a function of acceptable uncertainty for a measurement of 1-20 MeV equivalent neutron energy, binned into 200 keV increments. A 5% measurement will take 3 days for U, but 20 days for Am. It may be difficult to be the sole user of the LBNL cyclotron, or another facility, for such an extended period. However, a 10% measurement will take 19 hours for U, and 5 days for Am. Such a run period is more reasonable and will allow for the first ever measurement of the {sup 240}Am(n,f) cross section. We also anticipate 40% more beam time being available at Texas A&amp;M Cyclotron Institute compared to LBNL in FY2012. The increased amount of beam time will allow us to accumulate better statistics then what would have been available at LBNL

Magnetization-driven Lifshitz transition and charge-spin coupling in the kagome metal YMn6Sn6

The Fermi surface (FS) is essential for understanding the properties of metals. It can change under both conventional symmetry-breaking phase transitions and Lifshitz transitions (LTs), where the FS, but not the crystal symmetry, changes abruptly. Magnetic phase transitions involving uniformly rotating spin textures are conventional in nature, requiring strong spin-orbit coupling (SOC) to influence the FS topology and generate measurable properties. LTs driven by a continuously varying magnetization are rarely discussed. Here we present two such manifestations in the magnetotransport of the kagome magnet YMn6Sn6: one caused by changes in the magnetic structure and another by a magnetization-driven LT. The former yields a 10% magnetoresistance enhancement without a strong SOC, while the latter a 45% reduction in the resistivity. These phenomena offer a unique view into the interplay of magnetism and electronic topology, and for understanding the rare-earth counterparts, such as TbMn6Sn6, recently shown to harbor correlated topological physics

Completing the nuclear reaction puzzle of the nucleosynthesis of 92Mo

One of the greatest questions for modern physics to address is how elements heavier than iron are created in extreme, astrophysical environments. A particularly challenging part of that question is the creation of the so-called p-nuclei, which are believed to be mainly produced in some types of supernovae. The lack of needed nuclear data presents an obstacle in nailing down the precise site and astrophysical conditions. In this work, we present for the first time measurements on the nuclear level density and average strength function of $^{92}$Mo. State-of-the-art p-process calculations systematically underestimate the observed solar abundance of this isotope. Our data provide stringent constraints on the $^{91}$Nb$(p,{\gamma})^{92}$Mo reaction rate, which is the last unmeasured reaction in the nucleosynthesis puzzle of $^{92}$Mo. Based on our results, we conclude that the $^{92}$Mo abundance anomaly is not due to the nuclear physics input to astrophysical model calculations.Comment: Submitted to PR