Search

We report the WIMP dark matter search results using the first physics-run data of the PandaX-II 500 kg liquid xenon dual-phase time-projection chamber, operating at the China JinPing Underground Laboratory. No dark matter candidate is identified above background. In combination with the data set during the commissioning run, with a total exposure of 3.3$\times10^4$ kg-day,the most stringent limit to the spin-independent interaction between the ordinary and WIMP dark matter is set for a range of dark matter mass between 3.5 and 1000 GeV/c$^2$. The best upper limit on the scattering cross section is found $2.5\times 10^{-46}$ cm$^2$ for the WIMP mass 40 GeV/c$^2$ at 90% confidence level., version accepted by PRL, 12 pages, 5+11 figures (including supplementary materials), 4 tables

The angular distributions of elastic scattering for the 6Li +208Pb system have been measured at several energies around the Coulomb barrier. The parameters of optical potential are extracted by means of a phenomenological optical model analysis. It is found that the real and imaginal potentials show a pronounced energy dependence. The behaviour of the potential at the nearly especially sub-barrier energies in the 6Li+208Pb system is quite different from the results of some previous reports observed in other systems, such as 19F+208Pb and 16O+208Pb. This unusual threshold phenomenon indicates that breakup channel is strongly coupled with the elastic channel and has obvious effects on optical potential.

We have measured the fusion cross sections for 48Ca+90,96Zr around the Coulomb barrier and presented them along with the experimental data of 40Ca+90,96Zr. The experimental results are compared with the improved quantum molecular dynamics model calculations. It is shown in comparison that the dynamical effects play an important role in the sub-barrier fusion reactions.

Available data regarding the nuclear halo candidates are systematically analysed in terms of the asymptotic normalization coefficients measured experimentally. On the other hand, we have put forth conditions for nuclear halo formation, and compared with the experimental data. Based on the analytical expressions of the expectation value for the operator r2 in a finite square well potential, we have presented the improved scaling laws for the dimensionless quantity r2/R2 of nuclear halo, which seem to be in reasonable agreement with the experimental data.

The cross section of the direct neutron capture reaction C-12(n,gamma)C-13(1/2(+)) is calculated with the asymptotic normalization coefficient method. The result is in good agreement with a recent experiment at low energy. An enormous enhancement of cross section is found for this direct neutron capture in which a p-wave neutron is captured into an 2s(1/2) orbit with neutron halo. The possible effect of the neutron halo structure presented in this reaction on the s-process in astrophysics is. discussed in general.

The quadrant silicon detector, a kind of passivated implanted planar silicon detector with quadrant structure on the junction side, gained its wide application in charged particle detection. In this paper, the manufacturing procedure, performance test and results of the quadrant silicon detector developed recently at the China Institute of Atomic Energy are presented. The detector is about 300 $\mu$m thick with a 48$\times$48 mm$^{2}$ active area. The leakage current under full depletion bias voltage of -16 V is about 2.5 nA, and the raising time is better than 160 ns. The energy resolution for 5.157 MeV $\alpha$-particle is around the level of $1\%$. Charge sharing effects between the neighboring quads, leading to complicated correlations between two quads, were observed when $\alpha$ particles illuminated on the junction side. It is explained as a result of distortion of electric field of inter-quad region. Such events is only about $0.6\%$ of all events and can be neglected in an actual application.

Elastic scattering angular distributions of the 14N+16O system and the angular distributions of transfer reaction 16O(14N,13 C)17 F at ELab = 76.2 MeV and 57MeV have been measured and calculated by means of the exact finite-range distorted-wave Born approximation with the PTOLEMY code. The optical potential parameters for the weakly bound nuclear system 17F+13 C have been deduced and applied to analyse the elastic scattering angular distributions of the similar systems 17F+12C and 17F+14N which are taken from literature. The result shows that the transfer reaction with stable projectile and target combination can be used as an alternative method to extract the optical potential parameters for the weakly bound nuclear system.

The quasi-elastic scattering excitation function of the doubly magic 16O+208Pb system at a backward angle is measured at sub-barrier energies with high precision. The diffuseness parameters extracted from both the single-channel and the coupled-channels calculations give almost the same value a = 0.76 ± 0.04 fm. The results show that the coupling effect is negligible for the spherical system. The obtained value is smaller than the extracted value from the fusion excitation function, but larger than the value of a = 0.63 fm, which is from the systematic analysis of elastic scattering data.

The interaction potential for spherical-deformed reaction partners is calculated. The shape, separation and orientation dependence of the interaction potential and fusion cross section of the system 32S+154Sm are investigated within the double-folding model of the deformed nuclei. The effective nucleon-nucleon interaction is taken to be the M3Y-Reid potential. The density is considered for three terms of the expansion using the truncated multipole expansion method, which is a deformed Fermi shape with quadrupole and hexadecapole for the density distribution of 154Sm. It is found for the interaction potential that the height and the position of barrier strongly depend on the deformations, the orientation angle of the deformed nucleus, and hence produce great effects on fusion cross section. The integrated fusion cross section is in good agreement with the experimental data.

Dynamical structures of collective excitation in continuum are studied by calculating the isoscalar and isovector strength as well as transition density of nuclei near the drip-line such as 28O and 34Ca. It is found that for some excited states in continuum the proton and neutron transition density calculated from isoscalar and isovector excitation at some given energies may be different, which will affect the calculation of the polarization for nuclei with N≠Z.

The total reaction cross sections (TRCSs) of 29S+28Si have been measured at intermediate energies. An obvious enhancement in TRCS of 29S is observed as compared with its neighbouring nuclei. The TRCSs of 29S+28Si are calculated with the modified Glauber theory in the optical limit and few-body approaches. The different factor d as a possible measure of halo appearance is deduced from the experimental and theoretical data. It is well accepted that 27P is a proton halo nucleus. Although not as anomalous as 27P, the different factor d of 29S is obviously larger than that of its neighbouring isotones of N = 13. This result indicates that a moderate proton halo may exist in 29S nucleus. We calculate the total reaction cross sections for 29S with the modified Glauber theory as a function of incident energy and compare the results with those for 27Si which is a core nucleus of 29S. The measured TRCSs of 27Si+28Si can be described to be satisfactory by the modified Glauber theory of the optical limit approach. Although a diffused nuclear density distribution is used, the theories still inadequately predict the experimental TRCSs of 29S+28Si, which further indicates the possibility of proton halo in 29S.

The analytic expressions of radial matrix elements lc = 0|r|lh = 1, lc = 1|r|lh = 0 and lc = 1|r|lh = 2 in a finite square-well potential are derived. Based on these analytic expressions of radial matrix elements, the neutron direct radiative capture (DRC) processes leading to bound p-orbit from incident s-wave, and leading to s- and d-orbits from incident p-wave are discussed. For the DRC processes leading to loosely bound orbits, the dominant contributions to the radial matrix elements come from the outer region of nuclear potential radius R.

The asymptotic normalization coefficients (ANCs) for three bound states of 13C (12C+n) are extracted from the angular distributions of 12C(d,p) reactions at Elab = 11.8 MeV. With these ANCs, the calculated root-mean-square (rms) radii of these three states agree with our previous results. The ANCs are 1.93±0.17, 1.84±0.16 and 0.149±0.012 fm-1/2, and the rms radii r21/2 are 3.39±0.31, 5.04±0.75 and 3.68±0.40 fm for the ground state, the first and third excitation states, respectively. Further analyses show that the first excitation state with D1=50.3% is a true neutron halo state, while the third excitation state with D1=25.2% is a neutron skin state.

Angular distributions for the 11B(d,p)12B transfer reactions have been measured at Elab = 11.8 MeV. The asymptotic normalization coefficients (ANC) of the ground state, the second and third excitation states for 12B (11B+n) are extracted from the differential cross sections at three forward angles. With these ANCs, the root-mean-square radii are calculated for these three states. The results show that the second and third excitation states of 12B are neutron halo states.

Fission cross sections and angular distributions have been measured for the reactions of 16 O + 232 Th and 238 U , and 19 F + 208 Pb and 232 Th at near- and sub-barrier energies. The fission excitation functions are rather well reproduced on the basis of Wong model or coupled channels theory. However, the models which reproduce the sub-barrier fusion cross sections fail to account for the experimental anisotropies of fission fragments. It is found that the observed anisotropies are much larger than expected. For the first time it has been observed that the anisotropies as a function of the center-of-mass energy show a peak centered near 4.5 MeV below the fusion barrier for several reaction systems. The present approaches fail to explain these anomalies. For 19 F + 208 Pb systems, our results confirm the prediction of an approximately constant value for the mean square spin of the compound nucleus produced in far sub-barrier fusion reaction.

Fission cross sections and angular distributions have been measured for the 19 F + 208 Pb reaction at bombarding energies from 83 to 105 MeV. The fission excitation function is well reproduced on the basis of the coupled-channels theory. The fission-fragment angular distributions are calculated in terms of the transition-state theory, with the transmission coefficients extracted from the excitation function calculation. It is found that a discrepancy between the observations and the predictions in angular anisotropy of fission fragments exists at near- and sub-barrier energies, except for lower and higher energy regions where the discrepancy tends to disappear. Moreover, the anisotropies as a function of the center-of-mass energy show a shoulder around 82 MeV. Our results clearly indicate the considerable effects of the coupling on the sub-barrier fusion cross section and on the near-barrier compound-nucleus spin distribution, and confirm the prediction of an approximately constant value for the mean square spin of a compound nucleus produced in a far sub-barrier fusion reaction.

An ab initio calculation of the electric-field gradient (EFG) at the site of a phosphorous impurity substituting an Al atom in α-Al2O3 is carried out using the WIEN2k code with the full-potential linearized augmented plane wave plus local orbital method (LAPW+lo) in the frame of density functional theory. The atomic lattice relaxations caused by the implanted impurities were calculated for two different charged states to well describe the electronic structure of the doped system. The EFG at the site of the phosphorous impurity in the charged supercell calculated with the exchange-correlation potential of the Wu-Cohen generalized gradient approximation (WC-GGA) is 0.573 × 1021 V/m2. Then, the nuclear quadrupole moment of the I = 3 state in 28P is deduced to be 137 mb from the quadrupole interaction frequency of 190 kHz measured recently by the β-NQR method.

The charge-exchange spin-dipole (SD) and spin-quadrupole (SQ) strength functions of 90Zr are calculated with and without the tensor terms of the Skyrme interaction in self-consistent HF+RPA approach. It is found that, in SD and SQ transitions, the RPA correlations associated with the tensor terms shin dramatically the strengths of (Ylσ)λ = l−1 and (Ylσ)λ = 1 modes upward and downward, respectively, and also shift the strengths of (Ylσ)λ = l + 1 modes upward. The coupling between (Yl = λ − 1σ)λ and (Yl = λ + 1σ)λ modes arising from the tensor correlation is noticeable. The RPA tensor correlations produce strengths of SD and SQ modes, which are distributed in a much wider energy range, and the (Ylσ)λ = l − 1 modes dominate the high energy part of the strength functions. These energy shifts and coupling effects of different modes can be understood qualitatively by expressing a finite range tensor force in a separable form.

Experiment 18Ne+ 197Au was performed in Heavy Ion Research Facility Radioactive Ion Beam Line at Lanzhou (RIBLL). The nuclear energy levels of 18Ne were built by complete kinematical reconstruction of the decay products and diproton decay were observed from 18Ne excited states. At 6.15 MeV, the experimental relative momentum and angular correlations of the emitted protons are given. The obvious enhancement at |qrel| = 23 MeV/c and θc.m. = 40° shows the experimental criterion of 2He cluster decay from 18Neq.

The optical potential parameters for the halo nucleus system 6He+12C are extracted from fits to the measured angular distributions of 11B(7Li, 6He)12C reaction at energies of 18.3 and 28.3MeV with distorted-wave Born approximation analysis. The characters of the obtained optical potential parameters are basically consistent with the results extracted from the fits to the elastic-scattering angular distributions in the literature.

Using the experimental results, we have calculated the density distributions of valence nucleons in terms of the single-particle potential model. The outside probabilities and contributions of valence nucleons given by these calculations are considered as the criterion of halo states. By studying the variation of root-mean-square radius with the separation energy, conditions for the existence of halo states are pointed out, especially of the proton halo states. These have realistic meanings for judging and searching the halo states.