arXiv subjects
Publications and source records attributed to P. Zhang.
In the past decades, beam-driven plasma wakefield acceleration (PWFA) experiments have seen remarkable progress by using high-energy particle beams such as electron, positron and proton beams to drive wakes in neutral gas or pre-ionized plasma. This review highlights a few recent experiments in the world to compare experiment parameters and results.
The Smith Purcell radiation as one promising THz radiation source has been studied for decades. Various structure designs and beam configurations have been proposed. In this paper, we report our recent exploration of Smith Purcell radiation driven by electron beam accelerated by plasma wakefield.
Isochronous mass spectrometry has been applied in the storage ring CSRe to measure the masses of the $T_z=-3/2$ nuclei $^{27}$P and $^{29}$S. The new mass excess value $ME$($^{29}$S) $=-3094(13)$~keV is 66(52)~keV larger than the result of the previous $^{32}$S($^3$He,$^{6}$He)$^{29}$S reaction measurement in 1973 and a factor of 3.8 more precise. The new result for $^{29}$S, together with those of the $T=3/2$ isobaric analog states (IAS) in $^{29}$P, $^{29}$Si, and $^{29}$Al, fit well into the quadratic form of the Isobaric Multiplet Mass Equation IMME. The mass excess of $^{27}$P has been remeasured to be $ME(^{27}$P$)=-685(42)$ keV. By analyzing the linear and quadratic coefficients of the IMME in the $T_z=-3/2$ $sd$-shell nuclei, it was found that the ratio of the Coulomb radius parameters is $R\approx0.96$ and is nearly the same for all $T=3/2$ isospin multiplets. Such a nearly constant $R$-value, apparently valid for the entire light mass region with $A>9$, can be used to set stringent constraints on the isovector and isotensor components of the isospin non-conserving forces in theoretical calculations.
The Jiangmen Underground Neutrino Observatory (JUNO), a 20ktons multi-purpose underground liquid scintillator detector, was proposed with the determination of the neutrino mass hierarchy as a primary physics goal. Due to low background requirement of the experiment, a multi-veto system ,which consists of a water Cherenkov detector and a top tracker detector, is required. In order to keep the water quality good and remove the radon in the water, a ultra-pure water system, a radon removal system and radon concentration measurement system have been designed. In this paper, the radon removal equipments and its radon removal limit will be presented.
Using isochronous mass spectrometry at the experimental storage ring CSRe in Lanzhou, the masses of $^{82}$Zr and $^{84}$Nb were measured for the first time with an uncertainty of $\sim 10$ keV, and the masses of $^{79}$Y, $^{81}$Zr, and $^{83}$Nb were re-determined with a higher precision. %The latter differ significantly from their literature values. The latter are significantly less bound than their literature values. Our new and accurate masses remove the irregularities of the mass surface in this region of the nuclear chart. Our results do not support the predicted island of pronounced low $α$ separation energies for neutron-deficient Mo and Tc isotopes, making the formation of Zr-Nb cycle in the $rp$-process unlikely. The new proton separation energy of $^{83}$Nb was determined to be 490(400)~keV smaller than that in the Atomic Mass Evaluation 2012. This partly removes the overproduction of the $p$-nucleus $^{84}$Sr relative to the neutron-deficient molybdenum isotopes in the previous $νp$-process simulations.
In this paper, we propose an adaptive fast solver for a general class of symmetric positive definite (SPD) matrices which include the well-known graph Laplacian. We achieve this by developing an adaptive operator compression scheme and a multiresolution matrix factorization algorithm which achieve nearly optimal performance on both complexity and well-posedness. To develop our adaptive operator compression and multiresolution matrix factorization methods, we first introduce a novel notion of energy decomposition for SPD matrix $A$ using the representation of energy elements. The interaction between these energy elements depicts the underlying topological structure of the operator. This concept of decomposition naturally reflects the hidden geometric structure of the operator which inherits the localities of the structure. By utilizing the intrinsic geometric information under this Energy framework, we propose a systematic operator compression scheme for the inverse operator $A^{-1}$. In particular, with an appropriate partition of the underlying geometric structure, we can construct localized basis by using the concept of interior and closed energy. Meanwhile, two important localized quantities are introduced, namely the error factor and the condition factor. Our error analysis results show that these two factors will be the guidelines for finding the appropriate partition of the basis functions such that prescribed compression error and acceptable condition number can be achieved. By virtue of this insight, we propose the Patch Pairing algorithm to realize our energy partition framework for operator compression with controllable compression error and condition number.
We report the first full-dimensional potential energy surface (PES) and quantum mechanical close-coupling calculations for scattering of SiO due to H$_2$. The full-dimensional interaction potential surface was computed using the explicitly correlated coupled-cluster (CCSD(T)-F12b) method and fitted using an invariant polynomial approach. Pure rotational quenching cross sections from initial states $v_1=0$, $j_1$=1-5 of SiO in collision with H$_2$ are calculated for collision energies between 1.0 and 5000 cm$^{-1}$. State-to-state rotational rate coefficients are calculated at temperatures between 5 and 1000 K. The rotational rate coefficients of SiO with para-H$_2$ are compared with previous approximate results which were obtained using SiO-He PESs or scaled from SiO-He rate coefficients. Rovibrational state-to-state and total quenching cross sections and rate coefficients for initially excited SiO($v_1=1, j_1$=0 and 1) in collisions with para-H$_2$($v_2=0,j_2=0$) and ortho-H$_2$($v_2=0,j_2=1$) were also obtained. The application of the current collisional rate coefficients to astrophysics is briefly discussed.
Spatially indirect Type-II band alignment in magnetically-doped quantum dot (QD) structures provides unexplored opportunities to control the magnetic interaction between carrier wavefunction in the QD and magnetic impurities. Unlike the extensively studied, spatially direct, QDs with Type-I band alignment where both electrons and holes are confined in the QD, in ZnTe QDs embedded in a (Zn,Mn)Se matrix only the holes are confined in the QDs. Photoexcitation with photon energy 3.06 eV (2.54 eV) generates electron-hole pairs predominantly in the (Zn,Mn)Se matrix (ZnTe QDs). The photoluminescence (PL) at 7 K in the presence of an external magnetic field exhibits an up to three-fold increase in the saturation red shift with the 2.54 eV excitation compared to the shift observed with 3.06 eV excitation. This unexpected result is attributed to multiple hole occupancy of the QD and the resulting increased penetration of the hole wavefunction tail further into the (Zn,Mn)Se matrix. The proposed model is supported by microscopic calculations which accurately include the role of hole-hole Coulomb interactions as well as the hole-Mn spin exchange interactions.
The radioactive noble gas $^{222}$Rn, which can be dissolved in water, is an important background source for JUNO. In this paper, based on the water system of JUNO prototype, two kinds of high sensitivity radon detectors have been proposed and developed. The sensitivity of Si-PIN Rn detector, which uses a Si-PIN photodiode to detect the $α$ from $^{214}$Po decay, is $\sim$9.0~mBq/m$^3$. The sensitivity of LS Rn detector, which uses liquid scintillator to detect the coincident signals of $β$ from $^{214}$Bi decay and $α$ from $^{214}$Po decay, is $\sim$64.0~mBq/m$^3$. Both of the two kinds of Rn detector have the potential to be developed as an online Rn concentration monitoring equipment for JUNO veto detector.
The inorganic crystals have been widely used for dark matter direct searching for many decades. However, limited by the crystal growth technique, a lot of small crystals have to be used together for large target mass, which results in a degradation of light collection efficiency. An experiment was built up to study the degradation, and the method of soaking crystals into mineral oil to improve the efficiency as well as reduce the interface effect were proposed and validated. Good data and MC agreements were achieved in the experiment.
In heavy-fermion compounds, f electrons show both itinerant and localized behaviour depending on the external conditions, and the hybridization between localized f electrons and itinerant conduction bands gives rise to their exotic properties like heavy-fermions, magnetic orders and unconventional superconductivity. Duo to the risk of handling radioactive actinide materials, the direct experimental evidence of the band structure evolution across the localized-itinerant and magnetic transitions for 5f electrons is lacking. Here, by using angle-resolved photoelectron spectroscopy, we revealed the dual nature (localized vs itinerant) and the development of two different kinds of heavy quasi-particle bands of 5f electrons in antiferromagnetic (AFM) USb2. Partially opened energy gaps were observed on one quasi-particle 5f band cross the AFM transition around 203 K, indicating that the magnetic orders in USb2 are of spin density wave (SDW) type similar to Cr. The localized 5f electrons and itinerant conduction bands hybridize to form another heavy quasi-particle band at about 120 K, and then open hybridization gaps at even lower temperature. Our results provide direct spectral demonstration of the localized-itinerant transition, hybridization and SDW transition of 5f electrons for uranium-based materials.
As a space-borne detector POLAR is designed to conduct hard X-ray polarization measurements of gamma-ray bursts on the statistically significant sample of events and with an unprecedented accuracy. During its development phase a number of tests, calibrations runs and verification measurements were carried out in order to validate instrument functionality and optimize operational parameters. In this article we present results on gain optimization togeter with verification data obtained in the course of broad laboratory and environmental tests. In particular we focus on exposures to the $^{137}$Cs radioactive source and determination of the gain dependence on the high voltage for all 1600 detection channels of the polarimeter. Performance of the instrument is described in detail with respect to the dynamic range, energy resolution and temperature dependence. Gain optimization algorithms and response non-uniformity studies are also broadly discussed. Results presented below constitute important parts for development of the POLAR calibration and operation database.
In the case of underground experiments for neutrino physics or rare event searches, the background caused by cosmic muons contributes significantly and therefore must be identified and rejected. We proposed and optimized a new detector using liquid scintillator with wavelenghth-shifting fibers which can be employed as a veto detector for cosmic muons background rejection. From the prototype study, it has been found that the detector has good performances and is capable of discriminating between muons induced signals and environmental radiation background. Its muons detection efficiency is greater than 98$\%$, and on average, 58 photo-electrons (p.e.) are collected when a muon passes through the detector. To optimize the design and enhance the collection of light, the reflectivity of the coating materials has been studied in detail. A Monte Carlo simulation of the detector has been developed and compared to the performed measurements showing a good agreement between data and simulation results.
Beryllium (Be) is an important material with wide applications ranging from aerospace components to X-ray equipments. Yet a precise understanding of its phase diagram remains elusive. We have investigated the phase stability of Be using a recently developed hybrid free energy computation method that accounts for anharmonic effects by invoking phonon quasiparticles. We find that the hcp to bcc transition occurs near the melting curve at 0<P<11 GPa with a positive Clapeyron slope of 41 K/GPa. The bcc phase exists in a narrow temperature range that shrinks with increasing pressure, explaining the difficulty in observing this phase experimentally. This work also demonstrates the validity of this theoretical framework based on phonon quasiparticle to study structural stability and phase transitions in strongly anharmonic materials.
We performed an angle-resolved photoemission spectroscopy study of BaMn$_2$As$_2$ and BaMn$_2$Sb$_2$, which are isostructural to the parent compound BaFe$_2$As$_2$ of the 122 family of ferropnictide superconductors. We show the existence of a strongly $k_z$-dependent band gap with a minimum at the Brillouin zone center, in agreement with their semiconducting properties. Despite the half-filling of the electronic 3$d$ shell, we show that the band structure in these materials is almost not renormalized from the Kohn-Sham bands of density functional theory. Our photon energy dependent study provides evidence for Mn-pnictide hybridization, which may play a role in tuning the electronic correlations in these compounds.
Masses of $^{52g,52m}$Co were measured for the first time with an accuracy of $\sim 10$ keV, an unprecedented precision reached for short-lived nuclei in the isochronous mass spectrometry. Combining our results with the previous $β$-$γ$ measurements of $^{52}$Ni, the $T=2$, $J^π=0^+$ isobaric analog state (IAS) in $^{52}$Co was newly assigned, questioning the conventional identification of IASs from the $β$-delayed proton emissions. Using our energy of the IAS in $^{52}$Co, the masses of the $T=2$ multiplet fit well into the Isobaric Multiplet Mass Equation. We find that the IAS in $^{52}$Co decays predominantly via $γ$ transitions while the proton emission is negligibly small. According to our large-scale shell model calculations, this phenomenon has been interpreted to be due to very low isospin mixing in the IAS.
We extend an operator Pólya--Szegö type inequality involving the operator geometric mean to any arbitrary operator mean under some mild conditions. Utilizing the Mond--Pečarić method, we present some other related operator inequalities as well.
We performed a comprehensive angle-resolved photoemission spectroscopy study of the electronic band structure of LaOFeAs single crystals. We found that samples cleaved at low temperature show an unstable and highly complicated band structure, whereas samples cleaved at high temperature exhibit a stable and clearer electronic structure. Using \emph{in-situ} surface doping with K and supported by first-principles calculations, we identify both surface and bulk bands. Our assignments are confirmed by the difference in the temperature dependence of the bulk and surface states.