Nuclear Experiment

In the events of peripheral dissociation of relativistic nuclei in the nuclear track emulsion, it is possible to study the emerging ensembles of He and H nuclei, including those from decays of the unstable 8 Be and 9 B nuclei, as well as the Hoyle state. These extremely short-lived states are identified by invariant masses calculated from the opening angles in 2 α -pairs, 2 αp - and 3 α -triplets in the approximation of conservation of momentum per nucleon of the primary nucleus. In the same approach, it is possible to search for more complex states. This paper explores the correlation between the formation of 8 Be nuclei and the multiplicity of accompanying α -particles in the dissociation of relativistic 16 O, 22 Ne, 28 Si, and 197 Au nuclei. On this basis, estimates of such a correlation are presented for the unstable 9 B nucleus and the Hoyle state. An enhancement in the 8 Be contribution to dissociation with the α -particle multiplicity is found. Decays of 9 B nuclei and Hoyle states follow the same trend.

The 2 H (e, e ′ p)n cross sections have been measured at negative 4-momentum transfers of Q 2 =4.5±0.5 (GeV/c) 2 and Q 2 =3.5±0.5 (GeV/c) 2 reaching neutron recoil (missing) momenta up to p r ∼ 1.0 GeV/c. The data have been obtained at fixed neutron recoil angles 5 ∘ ≤ θ nq ≤ 95 ∘ with respect to the 3-momentum transfer q → . The new data agree well with the previous data which reached p r ∼550 MeV/c. At θ nq = 35 ∘ and 45 ∘ , final state interactions (FSI), meson exchange currents (MEC) and isobar configurations (IC) are suppressed and the plane wave impulse approximation (PWIA) provides the dominant cross section contribution. The new data are compared to recent theoretical calculations, and a significant disagreement for recoil momenta p r >700 MeV/c is observed.

The bremsstrahlung flux-averaged cross-sections ?��? E γmax )??and the cross-sections per equivalent photon ?��? E γmax ) Q ??were measured for the photonuclear multiparticle reaction 27 Al(γ,x;x= 3 He+pd+2pn ) 24 Na at bremsstrahlung end-point energies ranging from 40 MeV to 95 MeV. The experiments were performed using the beam from the NSC KIPT electron linear accelerator LUE-40 with the use of the γ -activation technique. The bremsstrahlung quantum flux was calculated with the program GEANT4 and, in addition, was monitored by means of the 100 Mo(γ,n ) 99 Mo reaction. The cross-sections ?(E) were computed using the TALYS1.9 code with the default options. The measured average cross-sections ?��? E γmax )??and ?��? E γmax ) Q ??have appeared to be higher by factors of 2.0 to 2.4 than the theoretical results. The experimental results have been found to be in good agreement with the data of other laboratories. Consideration is given to special features of calculation of ?��? E γmax )??and ?��? E γmax ) Q ??for the 27 Al(γ,x ) 24 Na reaction, with occurrence of three 27 Al photodisintegration channels. The paper also discusses the possibility of using the 27 Al(γ,x ) 24 Na reaction for monitoring the bremsstrahlung γ -quantum flux in the photon energy region above 30 MeV.

We report a systematic measurement of cumulants, C n , for net-proton, proton and antiproton, and correlation functions, κ n , for proton and antiproton multiplicity distributions up to the fourth order in Au+Au collisions at s NN ????????= 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4 and 200 GeV. The C n and κ n are presented as a function of collision energy, centrality and kinematic acceptance in rapidity, y , and transverse momentum, p T . The data were taken during the first phase of the Beam Energy Scan (BES) program (2010 -- 2017) at the Relativistic Heavy Ion Collider (RHIC) facility. The measurements are carried out at midrapidity ( |y|< 0.5) and transverse momentum 0.4 < p T < 2.0 GeV/ c , using the STAR detector at RHIC. We observe a non-monotonic energy dependence ( s NN ????????= 7.7 -- 62.4 GeV) of the net-proton C 4 / C 2 with the significance of 3.1 ? for the 0-5\% central Au+Au collisions. This is consistent with the expectations of critical fluctuations in a QCD-inspired model. Thermal and transport model calculations show a monotonic variation with s NN ????????. For the multiparticle correlation functions, we observe significant negative values for a two-particle correlation function, κ 2 , of protons and antiprotons, which are mainly due to the effects of baryon number conservation. Furthermore, it is found that the four-particle correlation function, κ 4 , of protons plays a role in determining the energy dependence of proton C 4 / C 1 below 19.6 GeV, which cannot be solely understood by the negative values of κ 2 for protons.

Heavy quarks, like charm and beauty, are sensitive probes to investigate the colour-deconfined medium created in high-energy heavy-ion collisions, the Quark-Gluon Plasma (QGP). The ALICE Collaboration measured the production of D 0 , D + , D ∗+ and D + s in Pb-Pb collisions at s NN − − − √ = 5.02 TeV. The measurement of the nuclear modification factor ( R AA ) provides informations on the in-medium parton energy loss. In addition, the comparison between D + s and the non-strange D-meson R AA allows to investigate possible modifications of the charm-quark hadronisation mechanism in the QGP. The most recent results for these observables, which were obtained by analysing the latest 2018 data sample of Pb-Pb collisions, are presented as well as the comparison with theoretical model calculations.

Excited states of 129 In populated following the β -decay of 129 Cd were experimentally studied with the GRIFFIN spectrometer at the ISAC facility of TRIUMF, Canada. A 480-MeV proton beam was impinged on a uranium carbide target and 129 Cd was extracted using the Ion Guide Laser Ion Source (IG-LIS). β - and γ -rays following the decay of 129 Cd were detected with the GRIFFIN spectrometer comprising the plastic scintillator SCEPTAR and 16 high-purity germanium (HPGe) clover-type detectors. %, along with the β -particles were detected with SCEPTAR. From the β - γ - γ coincidence analysis, 32 new transitions and 7 new excited states were established, expanding the previously known level scheme of 129 In. The logft values deduced from the β -feeding intensities suggest that some of the high-lying states were populated by the ν0 g 7/2 →π0 g 9/2 allowed Gamow-Teller (GT) transition, which indicates that the allowed GT transition is more dominant in the 129 Cd decay than previously reported. Observation of fragmented Gamow-Teller strengths is consistent with theoretical calculations.

The astrophysical s -process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding s -process nucleosynthesis is the neutron flux generated by the 22 Ne(α,n) 25 Mg reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing 22 Ne(α,γ) 26 Mg reaction, is not well constrained in the important temperature regime from ∼0.2 -- 0.4 ~GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the 22 Ne( 6 Li,d) 26 Mg reaction in inverse kinematics, detecting the outgoing deuterons and 25,26 Mg recoils in coincidence. We have established a new n/γ decay branching ratio of 1.14(26) for the key E x =11.32 MeV resonance in 26 Mg , which results in a new (α,n) strength for this resonance of 42(11) μ eV when combined with the well-established (α,γ) strength of this resonance. We have also determined new upper limits on the α partial widths of neutron-unbound resonances at E x =11.112, 11.163 , 11.169 , and 11.171 MeV. Monte-Carlo calculations of the stellar 22 Ne(α,n) 25 Mg and 22 Ne(α,γ) 26 Mg rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from ∼0.2 -- 0.4 ~GK.

Investigation of the 50 V electron-capture to the 2 + 1553.8 keV level of 50 Ti and search for β − decay of 50 V to the 2 + 783.3 keV level of 50 Cr (both those decays are fourfold forbidden with Δ J Δπ = 4 + ) have been performed using a vanadium sample of natural isotopic abundance with mass of 955 g. The measurements were conducted with the help of an ultra low-background HPGe-detector system located 225 m underground in the laboratory HADES (Belgium). The measured value of the half-life of 50 V for electron capture was T EC 1/2 =( 2.77 +0.20 −0.19 )× 10 17 yr. The β − -decay branch was not detected and the corresponding lower bound of the half-life was T β 1/2 ≥8.9× 10 18 yr at the 90\% confidence level.

Decay spectroscopy of the long-lived states in 186 Tl has been performed at the ISOLDE Decay Station at ISOLDE, CERN. The α decay from the low-spin ( 2 − ) state in 186 Tl was observed for the first time and a half-life of 3.4 +0.5 −0.4 s was determined. Based on the α -decay energy, the relative positions of the long-lived states were fixed, with the ( 2 − ) state as the ground state, the 7 (+) state at 77(56)~keV and the 10 (−) state at 451(56) keV. The level scheme of the internal decay of the 186 Tl( 10 (−) ) state ( T 1/2 =3.40(9) s), which was known to decay solely through emission of 374 keV γ -ray transition, was extended and a lower-limit for the β -decay branching b β >5.9(3)% was determined. The extracted retardation factors for the γ decay of the 10 (−) state were compared to the available data in neighboring odd-odd thallium isotopes indicating the importance of the π d 3/2 shell in the isomeric decay and significant structure differences between 184 Tl and 186 Tl.

An extended investigation of the low-spin structure of the 65 Ni nucleus was performed at the Institut Laue-Langevin, Grenoble, via the neutron capture reaction 64 Ni(n, γ ) 65 Ni, using the FIPPS HPGe array. The level scheme of 65 Ni was significantly expanded, with 2 new levels and 87 newly found transitions. Angular correlation analyses were also performed, allowing us to assign spins and parities for a number of states, and to determine multipolarity mixing ratios for selected γ transitions. The low-energy part of the experimental level scheme (up to about 1.4 MeV) was compared with Monte Carlo Shell Model calculations, which predict spherical shapes for all states, apart from the 9/2 + and the second excited 1/2 − states of oblate deformation.