Nuclear Experiment

New experimental data on the neutron single-particle character of the Pygmy Dipole Resonance (PDR) in 208 Pb are presented. They were obtained from (d,p) and resonant proton scattering experiments performed at the Q3D spectrograph of the Maier-Leibnitz Laboratory in Garching, Germany. The new data are compared to the large suite of complementary, experimental data available for 208 Pb and establish (d,p) as an additional, valuable, experimental probe to study the PDR and its collectivity. Besides the single-particle character of the states, different features of the strength distributions are discussed and compared to Large-Scale-Shell-Model (LSSM) and energy-density functional (EDF) plus Quasiparticle-Phonon Model (QPM) theoretical approaches to elucidate the microscopic structure of the PDR in 208 Pb.

We report a precision measurement of the parity-violating asymmetry A PV in the elastic scattering of longitudinally polarized electrons from 208 Pb. We measure A PV =550±16(stat)±8 (syst) parts per billion, leading to an extraction of the neutral weak form factor F W ( Q 2 =0.00616 GeV 2 )=0.368±0.013 . Combined with our previous measurement, the extracted neutron skin thickness is R n ??R p =0.283±0.071 ~fm. The result also yields the first significant direct measurement of the interior weak density of 208 Pb: ? 0 W =??.0796±0.0036 (exp.)±0.0013 (theo.) fm ?? leading to the interior baryon density ? 0 b =0.1480±0.0036 (exp.)±0.0013 (theo.) fm ?? . The measurement accurately constrains the density dependence of the symmetry energy of nuclear matter near saturation density, with implications for the size and composition of neutron stars.

Background: The structure of the Hoyle state, a highly α -clustered state at 7.65 MeV in 12 C , has long been the subject of debate. Understanding if the system comprises of three weakly-interacting α -particles in the 0s orbital, known as an α -condensate state, is possible by studying the decay branches of the Hoyle state. Purpose: The direct decay of the Hoyle state into three α -particles, rather than through the 8 Be ground state, can be identified by studying the energy partition of the 3 α -particles arising from the decay. This paper provides details on the break-up mechanism of the Hoyle stating using a new experimental technique. Method: By using beta-delayed charged-particle spectroscopy of 12 N using the TexAT (Texas Active Target) TPC, a high-sensitivity measurement of the direct 3 α decay ratio can be performed without contributions from pile-up events. Results: A Bayesian approach to understanding the contribution of the direct components via a likelihood function shows that the direct component is <0.043% at the 95\% confidence level (C.L.). This value is in agreement with several other studies and here we can demonstrate that a small non-sequential component with a decay fraction of about 10 −4 is most likely. Conclusion: The measurement of the non-sequential component of the Hoyle state decay is performed in an almost medium-free reaction for the first time. The derived upper-limit is in agreement with previous studies and demonstrates sensitivity to the absolute branching ratio. Further experimental studies would need to be combined with robust microscopic theoretical understanding of the decay dynamics to provide additional insight into the idea of the Hoyle state as an α -condensate.

The Department of Nuclear Engineering, University of California Berkeley built a D-D neutron generator called the High Flux Neutron Generator (HFNG). It operates in the range of 100-125 keV of accelerating voltage. The generator produces neutron current of about 10^8 per second. These neutrons have energies between 2.2-2.8 MeV. We report here the results of a measurement of the scattered vs unscattered neutron fluence on polyethylene determined via neutron activation of multiple natural indium foils from a D-D neutron generator. Both the angle-integrated spectrum and the angle differential results are consistent with the predictions of the Monte Carlo N-Particle Transport (MCNP) code, using the ENDF/B-VII.1. This supports shielding calculations in the fast energy region with high density polyethylene (HDPE). To the best of our knowledge no integral benchmark experiment has been performed on polyethylene using D(D,n)alpha neutron spectrum.

Positron beams, both polarized and unpolarized, are identified as essential ingredients for the experimental programs at the next generation of lepton accelerators. In the context of the hadronic physics program at Jefferson Lab (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of nucleons and nuclei, in both the elastic and deep-inelastic regimes. For instance, elastic scattering of polarized and unpolarized electrons and positrons from the nucleon enables a model independent determination of its electromagnetic form factors. Also, the deeply-virtual scattering of polarized and unpolarized electrons and positrons allows unambiguous separation of the different contributions to the cross section of the lepto-production of photons and of lepton-pairs, enabling an accurate determination of the nucleons and nuclei generalized parton distributions, and providing an access to the gravitational form factors. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model of particle physics through the search of a dark photon, the precise measurement of electroweak couplings, and the investigation of charged lepton flavor violation. This document discusses the perspectives of an experimental program with high duty-cycle positron beams at JLab.

The extended R (2) Ψ m (Δ S 2 ) correlator is presented and examined for its efficacy to detect and characterize the quadrupole charge separation ( Δ S 2 ) associated with the purported Chiral Magnetic Wave (CMW) produced in heavy-ion collisions. Sensitivity tests involving varying degrees of proxy CMW signals injected into events simulated with the Multi-Phase Transport Model (AMPT), show that the R (2) Ψ m (Δ S 2 ) correlator provides discernible responses for the background- and CMW-driven charge separation. This distinction could aid identification of the CMW via measurements of the R (2) Ψ 2 (Δ S 2 ) and R (2) Ψ 3 (Δ S 2 ) correlators, relative to the second- ( Ψ 2 ) and third-order ( Ψ 3 ) event planes. The tests also indicate a level of sensitivity that would allow for robust experimental characterization of the CMW signal.

Knowledge of the decay rates (or half-lives) of radioisotopes is critical in many fields, including medicine, archeology, and nuclear physics, to name just a few. Central to the many uses of radioisotopes is the belief that decay rates are fundamental constants of nature, just as the masses of the radioisotopes themselves are. Recently, the belief that decay rates are fundamental constants has been called into question following the observation of various reported anomalies in decay rates, such as apparent periodic variations. The purpose of this bibliography is to collect in one place the relevant literature on both sides of this issue in the expectation that doing so will deepen our understanding of the available data.

We reinvestigated the neutron multiplicity yields of Ba-Mo, Ce-Zr, Te-Pd, and Nd-Sr from the spontaneous fission of 252 Cf; by (i) using both γ - γ - γ - γ and γ - γ - γ coincidence data, (ii) using up to date level scheme structures, and (iii) cross-checking analogous energy transitions in multiple isotopes, we have achieved higher precision than previous analyses. Particular attention was given to the Ba-Mo pairs where our results clearly confirm that the Ba-Mo yield data have a second hot fission mode where 8, 9, 10, and now 11 neutron evaporation channels are observed. These are the first observations of the 11 neutron channel. These 8-11 neutron channels are observed for the first time in the Ce-Zr pairs, but are not observed in other fission pairs. The measured intensities of the second mode in Ba-Mo and Ce-Zr pairs are ∼ 1.5(4) % and ∼ 1.0(3) % , respectively. These high neutron number evaporation modes can be an indication of hyperdeformation and/or octupole deformation in 143−145 Ba and in 146,148 Ce at scission to give rise to such high neutron multiplicities.

The future Facility for Antiproton and Ion Research (FAIR) is an accelerator-based international center for fundamental and applied research, which presently is under construction in Darmstadt, Germany. An important part of the program is devoted to questions related to astrophysics, including the origin of elements in the universe and the properties of strongly interacting matter under extreme conditions, which are relevant for our understanding of the structure of neutron stars and the dynamics of supernova explosions and neutron star mergers. The Compressed Baryonic Matter (CBM) experiment at FAIR is designed to measure promising observables in high-energy heavy-ion collisions, which are expected to be sensitive to the high-density equation-of-state (EOS) of nuclear matter and to new phases of QCD matter at high densities. The CBM physics program, the relevant observables and the experimental setup will be discussed.

The Facility for Antiproton and Ion Research (FAIR), an international accelerator centre, is under construction in Darmstadt, Germany. FAIR will provide high-intensity primary beams of protons and heavy-ions, and intense secondary beams of antiprotons and of rare short-lived isotopes. These beams, together with a variety of modern experimental setups, will allow to perform a unique research program on nuclear astrophysics, including the exploration of the nucleosynthesis in the universe, and the exploration of QCD matter at high baryon densities, in order to shed light on the properties of neutron stars, and the dynamics of neutron star mergers. The Compressed Baryonic Matter (CBM) experiment at FAIR will investigate collisions between heavy nuclei, and measure various diagnostic probes, which are sensitive to the high-density equation-of-state (EOS), and to the microscopic degrees-of-freedom of high-density matter. The CBM physics program will be discussed.