Nuclear Experiment

Photon branching ratios are critical input data for activities such as nuclear materials protection and accounting because they allow material compositions to be extracted from measurements of gamma-ray intensities. Uncertainties in these branching ratios are often a limiting source of uncertainty in composition determination. Here, we use high statistics, high resolution (~60-70eV full-width-at-half-maximum at 100 keV) gamma-ray spectra acquired using microcalorimeter sensors to substantially reduce the uncertainties for 11 plutonium (238Pu,239Pu,241Pu) and 241Am branching ratios important for material control and accountability and nuclear forensics in the energy range of 125 keV to 208 keV. We show a reduction in uncertainty of over a factor of three for one branching ratio and a factor of 2{3 for four branching ratios.

New limits on β + EC and ECEC processes in 74 Se have been obtained using a 600 cm 3 HPGe detector and an external source consisting of 1600 g of a natural selenium powder. For different β + EC and ECEC transitions (to the ground and excited states) obtained limits are on the level ∼(0.2−4.8)× 10 19 yr at 90\% C.L. In particular, for the potentially resonant transition into the 1204.2 keV excited state of 74 Ge a lower half-life limit of 1.1× 10 19 yr at 90\% C.L. has been obtained. Possibility to increase the sensitivity of such measurements is discussed.

Stellar carbon synthesis occurs exclusively via the 3α process, in which three α 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 α 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 ′ ) 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 γ -ray angular distribution experiments were also performed. An E0 branching ratio of Γ E0 π /Γ=8.2(5)× 10 −6 was deduced in the current work, and an adopted value of Γ E0 π /Γ=7.6(4)× 10 −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 Γ E0 π /Γ=6.7(6)× 10 −6 , while the uncertainty has been reduced from 9% to 5%.

The first evidence of jet quenching was observed at RHIC via suppression of single high p T hadron R AA and the disappearance of the away-side jet peak in two-particle correlations. Since then, hadron R AA and two-particle correlations continue to be useful probes of the QGP in heavy ion collisions, since the particles involved are fragments of the jets produced in the initial hard scattering. PHENIX recently improved the width measurements extracted from π 0 -hadron correlations after removing the higher order flow terms in the underlying event subtraction. Measurements of the away-side jet correlated with high p T neutral pions show an increase in low momentum particle production at wide angles consistent with theoretical expectations for energy loss. The system size dependence of energy loss is further investigated at PHENIX by measuring the absolute yield and R AA for various hadron species at high p T in several collision systems including U+U and Cu+Au . These proceedings will present the newest PHENIX R AA and two-particle correlation measurements and their role in our understanding of jet quenching and medium response in heavy ion collisions

We report on the first in-beam γ -ray spectroscopy of the proton-dripline nucleus 40 Sc using two-nucleon pickup onto an intermediate-energy rare-isotope beam of 38 Ca. The 9 Be( 38 Ca, 40 Sc +γ )X reaction at 60.9 MeV/nucleon mid-target energy selectively populates states in 40 Sc for which the transferred proton and neutron couple to high orbital angular momentum. In turn, due to angular-momentum selection rules in proton emission and the nuclear structure and energetics of 39 Ca, such states in 40 Sc then exhibit γ -decay branches although they are well above the proton separation energy. This work uniquely complements results from particle spectroscopy following charge-exchange reactions on 40 Ca as well as 40 Ti EC/ β + decay which both display very different selectivities. The population and γ -ray decay of the previously known first ( 5 − ) state at 892 keV and the observation of a new level at 2744 keV are discussed in comparison to the mirror nucleus and shell-model calculations. On the experimental side, this work shows that high-resolution in-beam γ -ray spectroscopy is possible with new generation Ge arrays for reactions induced by rare-isotope beams on the level of a few μ b of cross section.

The odd- Z 251 Md nucleus was studied using combined γ -ray and conversion-electron in-beam spectroscopy. Besides the previously observed rotational band based on the [521]1/ 2 − configuration, another rotational structure has been identified using γ - γ coincidences. The use of electron spectroscopy allowed the rotational bands to be observed over a larger rotational frequency range. Using the transition intensities that depend on the gyromagnetic factor, a [514]7/ 2 − single-particle configuration has been inferred for this band, i.e., the ground-state band. A physical background that dominates the electron spectrum with an intensity of ≃ 60% was well reproduced by simulating a set of unresolved excited bands. Moreover, a detailed analysis of the intensity profile as a function of the angular momentum provided a method for deriving the orbital gyromagnetic factor, namely g K = 0.69 +0.19 −0.16 for the ground-state band. The odd- Z 249 Md was studied using γ -ray in-beam spectroscopy. Evidence for octupole correlations resulting from the mixing of the Δl=Δj=3 [521]3/ 2 − and [633]7/ 2 + Nilsson orbitals were found in both 249,251 Md. A surprising similarity of the 251 Md ground-state band transition energies with those of the excited band of 255 Lr has been discussed in terms of identical bands. Skyrme-Hartree-Fock-Bogoliubov calculations were performed to investigate the origin of the similarities between these bands.

These proceedings report on measurements of the jet spectrum and nuclear modification factor for inclusive full jets (containing both charged and neutral constituents) in Pb-Pb and pp collisions at s NN − − − √ =5.02 TeV recorded with the ALICE detector. These measurements use a machine learning based background correction, which reduces residual fluctuations. This method allows for measurements to lower transverse momenta and larger jet resolution parameter (R) than previously possible in ALICE. In this method, machine learning techniques are used to correct the jet transverse momentum on a jet-by-jet basis using jet parameters such as information about the constituents of the jet. Studies that investigate the effect of the potential fragmentation bias introduced by learning from constituents will also be discussed. With these studies in mind, the results are compared to theoretical predictions.

The intensities of the ten strongest γ -ray transitions following the 111 Sn ( T 1/2 =35.3 m) decay have been determined via comparison of the two sets of the experimental photonucleon reaction yields driven using the traditional activation equation and the activation equation for the genetically coupled radioactive nuclei. The found absolute intensities of the γ -ray transitions in question were happened to be noticeably different from the currently recommended values.

Jets lose energy as they propagate through the Quark-Gluon Plasma, modifying their parton shower. Jet substructure, which provides access to the evolution of jet splittings, is expected to be sensitive to interactions between the medium and the jet, providing the opportunity to further constrain both jet and medium properties. By utilizing grooming techniques, we can focus on the most pertinent hard splittings. Of particular interest is the search for large transverse momentum kicks which may indicate the presence of point-like scatters within the Quark-Gluon Plasma. We explore the jet substructure of inclusive jets in pp and Pb-Pb collisions at s NN − − − √ =5.02 TeV, utilizing Soft Drop and other grooming methods, as well as the Lund Plane, in order to access the hardest jet splitting, with a particular focus on the hardest k T splitting.

The dynamics present in the fusion of neutron-rich nuclei is explored through the comparison of experimental cross-sections at above-barrier energies with measurements of the interaction cross-section at relativistic energies. The increase of fusion dynamics with increasing neutron excess is clearly demonstrated. Experimental cross-sections are compared with the predictions of a Sao Paulo model using relativistic mean field density distributions and the impact of different interactions is explored.