Improved precision on the experimental E0 decay branching ratio of the Hoyle state

Stellar carbon synthesis occurs exclusively via the $3\alpha$ process, in which three $\alpha$ particles fuse to form $^{12}$C in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the $\alpha$ threshold, and the rate of stellar carbon production depends on the radiative width of this state. The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the $E0$ decay branching ratio of the Hoyle state, and the current $10$\% uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The $E0$ branching ratio was deduced from a series of pair conversion measurements of the $E0$ and $E2$ transitions depopulating the $0^+_2$ Hoyle state and $2^+_1$ state in $^{12}$C, respectively. The excited states were populated by the $^{12}$C$(p,p^\prime)$ reaction at 10.5 MeV beam energy, and the pairs were detected with the electron-positron pair spectrometer, Super-e, at the Australian National University. The deduced branching ratio required knowledge of the proton population of the two states, as well as the alignment of the $2^+_1$ state in the reaction. For this purpose, proton scattering and $\gamma$-ray angular distribution experiments were also performed. An $E0$ branching ratio of $\Gamma^{E0}_{\pi}/\Gamma=8.2(5)\times10^{-6}$ was deduced in the current work, and an adopted value of $\Gamma^{E0}_{\pi}/\Gamma=7.6(4)\times10^{-6}$ is recommended based on a weighted average of previous literature values and the new result. The new recommended value for the $E0$ branching ratio is about 14% larger than the previous adopted value of $\Gamma^{E0}_{\pi}/\Gamma=6.7(6)\times10^{-6}$, while the uncertainty has been reduced from 9% to 5%.Comment: Accepted for publication as a Regular Article in Phys. Rev. C on July 29 202

Energy dependence of the prompt γ -ray emission from the (d,p) -induced fission of U ∗ 234 and Pu ∗ 240

Prompt-fission γ rays are responsible for approximately 5% of the total energy released in fission, and therefore important to understand when modeling nuclear reactors. In this work we present prompt γ-ray emission characteristics in fission as a function of the nuclear excitation energy of the fissioning system. Emitted γ-ray spectra were measured, and γ-ray multiplicities and average and total γ energies per fission were determined for the U(d,pf)233 reaction for excitation energies between 4.8 and 10 MeV, and for the Pu(d,pf)239 reaction between 4.5 and 9 MeV. The spectral characteristics show no significant change as a function of excitation energy above the fission barrier, despite the fact that an extra ∼5 MeV of energy is potentially available in the excited fragments for γ decay. The measured results are compared with model calculations made for prompt γ-ray emission with the fission model code gef. Further comparison with previously obtained results from thermal neutron induced fission is made to characterize possible differences arising from using the surrogate (d,p) reaction

Energy dependence of the prompt γ -ray emission from the (d,p) -induced fission of U ∗ 234 and Pu ∗ 240

Prompt-fission γ rays are responsible for approximately 5% of the total energy released in fission, and therefore important to understand when modeling nuclear reactors. In this work we present prompt γ-ray emission characteristics in fission as a function of the nuclear excitation energy of the fissioning system. Emitted γ-ray spectra were measured, and γ-ray multiplicities and average and total γ energies per fission were determined for the U(d,pf)233 reaction for excitation energies between 4.8 and 10 MeV, and for the Pu(d,pf)239 reaction between 4.5 and 9 MeV. The spectral characteristics show no significant change as a function of excitation energy above the fission barrier, despite the fact that an extra ∼5 MeV of energy is potentially available in the excited fragments for γ decay. The measured results are compared with model calculations made for prompt γ-ray emission with the fission model code gef. Further comparison with previously obtained results from thermal neutron induced fission is made to characterize possible differences arising from using the surrogate (d,p) reaction

Low-energy enhancement in the γ -ray strength functions of Ge 73,74

The γ-ray strength functions and level densities of Ge73,74 have been extracted up to the neutron-separation energy Sn from particle-γ coincidence data using the Oslo method. Moreover, the γ-ray strength function of Ge74 above Sn has been determined from photoneutron measurements; hence these two experiments cover the range of Eγ≈1-13 MeV for Ge74. The obtained data show that both Ge73,74 display an increase in strength at low γ energies. The experimental γ-ray strength functions are compared with M1 strength functions deduced from average B(M1) values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in Ge73,74 are adopted in the calculations of the Ge72,73(n,γ) cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement

La 137,138,139 (n,γ) cross sections constrained with statistical decay properties of la 138,139,140 nuclei

The nuclear level densities and γ-ray strength functions of La138,139,140 were measured using the La139(He3,α), La139(He3,He3′), and La139(d,p) reactions. The particle-γ 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∼140 region is discussed. The La137,138,139(n,γ) cross sections were calculated at s- and p-process temperatures using the experimentally measured nuclear level densities and γ-ray strength functions. Good agreement is found between La139(n,γ) calculated cross sections and previous measurements

Statistical properties of Pu 243, and Pu 242 (n,γ) cross section calculation

The level density and γ-ray strength function (γSF) of Pu243 have been measured in the quasicontinuum using the Oslo method. Excited states in Pu243 were populated using the Pu242(d,p) reaction. The level density closely follows the constant-temperature level density formula for excitation energies above the pairing gap. The γSF displays a double-humped resonance at low energy as also seen in previous investigations of actinide isotopes. The structure is interpreted as the scissors resonance and has a centroid of ωSR=2.42(5) MeV and a total strength of BSR=10.1(15)μN2, which is in excellent agreement with sum-rule estimates. The measured level density and γSF were used to calculate the Pu242(n,γ) cross section in a neutron energy range for which there were previously no measured data

Publisher's Note: Statistical properties of Pu 243, and Pu 242 (n,γ) cross section calculation (Physical Review C (2016) 93 (014323) DOI: 10.1103/PhysRevC.93.014323)

This paper was published online on 29 January 2016 with an error in the author list. The ninth author's name should read as "F. L. Bello Garrote." The author's name has been corrected as of 5 September 2019. The author's name is incorrect in the printed version of the journal

Statistical properties of Pu 243, and Pu 242 (n,γ) cross section calculation

The level density and γ-ray strength function (γSF) of Pu243 have been measured in the quasicontinuum using the Oslo method. Excited states in Pu243 were populated using the Pu242(d,p) reaction. The level density closely follows the constant-temperature level density formula for excitation energies above the pairing gap. The γSF displays a double-humped resonance at low energy as also seen in previous investigations of actinide isotopes. The structure is interpreted as the scissors resonance and has a centroid of ωSR=2.42(5) MeV and a total strength of BSR=10.1(15)μN2, which is in excellent agreement with sum-rule estimates. The measured level density and γSF were used to calculate the Pu242(n,γ) cross section in a neutron energy range for which there were previously no measured data

Structure of low-lying states in Sm-140 studied by Coulomb excitation

The electromagnetic structure of 140Sm was studied in a low-energy Coulomb excitation experiment with a radioactive ion beam from the REX-ISOLDE facility at CERN. The 2+ and 4+ states of the ground-state band and a second 2+ state were populated by multistep excitation. The analysis of the differential Coulomb excitation cross sections yielded reduced transition probabilities between all observed states and the spectroscopic quadrupole moment for the 2+ 1 state. The experimental results are compared to large-scale shell model calculations and beyond-mean-field calculations based on the Gogny D1S interaction with a five-dimensional collective Hamiltonian formalism. Simpler geometric and algebraic models are also employed to interpret the experimental data. The results indicate that 140Sm shows considerable γ softness, but in contrast to earlier speculation no signs of shape coexistence at low excitation energy. This work sheds more light on the onset of deformation and collectivity in this mass region.15 pages, 12 figuresstatus: publishe