Nuclear Experiment

The strength of the N = 28 magic number in neutron-rich argon isotopes is examined through high-precision mass measurements of 46−48 Ar, performed with the ISOLTRAP mass spectrometer at ISOLDE/CERN. The new mass values are up to 90 times more precise than previous measurements. While they suggest the persistence of the N = 28 shell closure for argon, we show that this conclusion has to be nuanced in light of the wealth of spectroscopic data and theoretical investigations performed with the \emph{SDPF-U} phenomenological shell model interaction. Our results are also compared with \emph{ab initio} calculations using the Valence Space In-Medium Similarity Renormalization Group and the Self-Consistent Green's Function approaches. Both calculations provide a very good account of mass systematics at and around Z = 18 and, generally, a consistent description of the physics in this region. This combined analysis indicates that 46 Ar is the transition between the closed-shell 48 Ca and collective 44 S.

The persistently mysterious deviations from unity of the ratio of nuclear target structure functions to those of deuterium as measured in deep inelastic scattering (often termed the "EMC Effect") have become the canonical observable for studies of nuclear medium modifications to free nucleon structure in the valence regime. The structure function of the free proton is well known from numerous experiments spanning decades. The free neutron structure function, however, has remained difficult to access. Recently it has been extracted in a systematic study of the global data within a parton distribution function extraction framework and is available from the CTEQ-Jefferson Lab (CJ) Collaboration. Here, we leverage the latter to introduce a new method to study the EMC Effect in nuclei by re-examining existing data in light of the the magnitude of the medium modifications to the free neutron and proton structure functions independently. From the extraction of the free neutron from world data, it is possible to examine the nuclear effects in deuterium and their contribution to our interpretation of the EMC Effect. In this study, we observe that the ratio of the deuteron to the sum of the free neutron and proton structure functions has some x B and Q 2 dependencies that impact the magnitude of the EMC Effect as typically observed. Specifically, different EMC slopes are obtained when data from different x B and Q 2 values are utilized. While a linear correlation persists between the EMC and short range correlation effects, the slope is modified when deuteron nuclear effects are removed.

The elastic scattering 12 C(p,po) Excitation Function (EF) was measured in the energy range Ep =16 - 19.5 MeV with resolution about 10 keV by means of innovative approach - MSS - the Method of Spectra Superposition at the 14-angle Magnetic Spectrograph (Apelsin) at beam with an energy spread of about 200 keV from U-150 cyclotron of INP Ulugbek (Tashkent) of Science Academy of Uzbekistan [1-8]. The obtained EF has a reach structure of anomalies in precise agreement with thresholds and levels data [2,9-12]. Measurements were done on the 12C self-supporting target with thickness 13 mg/cm2 and an area of 1 cm2 in the center of Apelsin. The 20-step Energy Moderator controlled the proton energy, providing a 3.5 MeV wide Ep interval with no readjustment of cyclotron and all the ionic-optics on the 41-meter-long beam-pipe. Energy resolution of Apelsin for protons is better than 5 keV along all focal plane, where particle-product were detected by specially designed coordinate-sensitive gas mixture MWPC with two particle counters in coincidence. The acquisition system was based on IBM PC online with a fast CAMAC branch (custom designed at St-Petersburg PNPI) with fast TDC modules and other fast electronics providing immediate selection and accumulations of events from MWPCs. The NMR-monitor system stabilized spectrographs magnetic field to 3ppm [1,9]. The EF of 12C (p,po) scattering has many resonances that precisely correspond to g.s. and levels of well-known product-nuclei. But 80 percent of anomalies did not have any explanation. A new concept is proposed to explain all unrecognized anomalies. Two expressions Writing I and Writing II proposed to explain the unknown peaks which corresponds to population of A13 phase volume states. The results of the proposed explanation look very promising (Fig. 13). Concept of the Combinative Isobaric Resonances (CIRs) proposed

Neutron kinetic energy spectra in coincidence with low-energy γ -ray multiplicities have been measured around A≈ 110 in the 16 O, 20 Ne + 93 Nb reactions in a compound nuclear excitation energy range of ≈ 90 - 140 MeV. The excitation energy (temperature) and angular momentum (spin) dependence of the inverse level density parameter k has been investigated by comparing the experimental data with statistical Hauser-Feshbach calculation. In contrast to the available systematic in this mass region, the inverse level density parameter showed an appreciable increase as a function of the excitation energy. The extracted k -values at different angular momentum regions, corresponding to different γ -multiplicities also showed an overall increase with the average nuclear spins. The experimental results have been compared with a microscopic statistical-model calculation and found to be in reasonable agreement with the data. The results provide useful information to understand the variation of nuclear level density at high temperature and spins.

Isobaric charge-exchange reactions induced by beams of 112Sn have been investigated at the GSI facilities using the fragment separator FRS. The high-resolving power of this spectrometer makes it possible to obtain the isobaric charge-exchange cross sections with an accuracy of 3% and to separate quasi-elastic and inelastic contributions in the missing-energy spectra, in which the inelastic component is associated to the in-medium excitation of baryonic resonances such as the Δ resonance. We report on the results obtained for the (p,n) and (n,p) channels excited by using different targets that cover a large range in neutron excess.

Gluon density and its distributions inside nuclei and the parton modification of bounded nucleons inside a nucleus, are some of the main standing problems in nuclear and particle physics. In recent years, ultra-peripheral collisions (UPC) of heavy ions have provided a new way of probing the gluon density, which is based on coherent diffractive vector-meson productions, e.g., J/ψ meson. For heavy ions, e.g., Pb, the gluon density is found to be significantly suppressed through the UPC J/ψ measurement, suggesting a strong gluon shadowing effect in heavy nuclei. In this analysis, we aim to look at a unique set of data taken by the STAR experiment, where J/ψ mesons are photoproduced off the deuteron target with no other particle produced, except for the deuteron or its breakup products. The Zero Degree Calorimeter response with respect to the deuteron dissociation by detecting a beam-rapidity neutron is also investigated and provides additional information about the underlying physics process. The cross section of J/ψ photoproduction in the photon-deuteron system is measured at the photon-nucleon center-of-mass energy W∼25 GeV , as well as the momentum transfer t dependence cross section, dσ/dt . Data suggests a wider gluon density distribution than the Hulthen charge density distribution in deuteron.

The 9.2 keV nuclear transition in 227 Th populated in the β -decay of 227 Ac was studied by means of the internal conversion electron spectroscopy. Its multipolarity was proved to be of mixed character M1+E2 and the spectroscopic admixture parameter δ 2 (E2/M1)=0.695 ± 0.248 (| δ (E2/M1)|=0.834 ± 0.210) was determined. Nonzero value of δ (E2/M1) raises a question about the existing theoretical interpretation of low-lying levels of 227 Th.

We report an experimental search for an exotic spin-spin-velocity-dependent interaction between polarized electrons of Rb atoms and polarized electrons of a solid-state mass, violating both the time-reversal and parity symmetries. This search targets a minute effective magnetic field induced by the interaction. A spin-exchange relaxation-free (SERF) magnetometer based on an optically polarized Rb vapor is the key element for both a source of polarized electrons and a high-sensitivity detector. A dysprosium iron garnet (DyIG) serves as the polarized mass, with an extremely small magnetization at the critical temperature around 240 K and a high spin density. To reduce the magnetization, one of major systematic effects, a home-built cooling system controls the mass temperature. To our knowledge, this is the first search for an exotic spin-dependent interaction using the compensated ferrimagnet DyIG as a polarized mass. The experiment set the most stringent limit on the electron-electron coupling strength in the centimeter interaction range, in particular g e V g e V < 10 4 at λ=2 cm.

Our understanding of the low-lying resonance structure in 12 C remains incomplete. We have used the 11 B(p,3α)γ reaction at proton energies of E p =0.5−2.7 MeV as a selective probe of the excitation region above the 3α threshold in 12 C. Transitions to individual levels in 12 C were identified by measuring the 3 α final state with a compact array of charged-particle detectors. Previously identified transitions to narrow levels were confirmed and new transitions to broader levels were observed for the first time. Here, we report cross sections, deduce partial γ -decay widths and discuss the relative importance of direct and resonant capture mechanisms.

Two-neutron transfer reactions serve as an important tool for nuclear structure studies in the neutron rich part of the nuclear chart. In this article, we report on the first experimental attempt to populate the excited states of 140 Ba employing the 2n-neutron transfer reaction 138 Ba( 18 O, 16 O) 140 Ba. 140 Ba is highly important, as it is placed on the onset of octupole correlations and the lifetimes of its excited states are completely unknown, with the sole exception of the first 2 + state. The experiment was carried out at the Horia Hulubei National Institute for Physics and Nuclear Engineering (IFIN-HH) in Magurele, Romania. Lower limits on the lifetimes of ground state band up to the 8 + state are reported. Furthermore, relative cross sections regarding the 2n-transfer reaction with respect to the fusion and the total inelastic reaction channels have been deduced. Further investigation directions of the nuclear structure of 140 Ba are also discussed.