Search

A precision measurement by the Alpha Magnetic Spectrometer on the International Space Station of the positron fraction in primary cosmic rays in the energy range from 0.5 to 350 GeV based on $6.8\times {10}^6$ positron and electron events is presented. The very accurate data show that the positron fraction is steadily increasing from 10 to $\sim 250\mathrm{GeV}$, but, from 20 to 250 GeV, the slope decreases by an order of magnitude. The positron fraction spectrum shows no fine structure, and the positron to electron ratio shows no observable anisotropy. Together, these features show the existence of new physical phenomena.

Excited states in ${}^{99}\mathrm{Tc}$ with energies up to 6 MeV have been populated using the ${}^{96}\mathrm{Zr}({}^7\mathrm{Li},4n){}^{99}\mathrm{Tc}$ reaction with a laboratory beam energy of 35 MeV. Coincident $\gamma$ rays from excited nuclei produced in the reactions were detected using an array of coaxial, planar, and clover-type high-purity germanium detectors. A total of 60 new $\gamma$-ray transitions and 21 new levels are identified and placed into a new level scheme. Two collective bands assigned to be built on the $\pi g_{9/2}\left[422\right]5/2{}^{+}$ and $\pi p_{1/2}\left[301\right]1/2{}^{-}$ Nilsson configurations have been extended with spins up to 35/2 and $33/2 \hslash$, respectively. Backbending and signature inversion have been observed in the yrast band. The large signature splitting of the positive-parity band in ${}^{99}\mathrm{Tc}$ may be caused by a triaxial deformation, which agrees well with the electromagnetic properties, theoretical calculations based on total Routhian surface, and triaxial particle-rotor model calculations.

A precise measurement of the proton flux in primary cosmic rays with rigidity (momentum/charge) from 1 GV to 1.8 TV is presented based on 300 million events. Knowledge of the rigidity dependence of the proton flux is important in understanding the origin, acceleration, and propagation of cosmic rays. We present the detailed variation with rigidity of the flux spectral index for the first time. The spectral index progressively hardens at high rigidities.

A precision measurement by AMS of the positron fraction in primary cosmic rays in the energy range from 0.5 to 500 GeV based on 10.9 million positron and electron events is presented. This measurement extends the energy range of our previous observation and increases its precision. The new results show, for the first time, that above $\sim 200\mathrm{GeV}$ the positron fraction no longer exhibits an increase with energy.

We present a measurement of the cosmic ray $(e^{+}+e^{-})$ flux in the range 0.5 GeV to 1 TeV based on the analysis of 10.6 million $(e^{+}+e^{-})$ events collected by AMS. The statistics and the resolution of AMS provide a precision measurement of the flux. The flux is smooth and reveals new and distinct information. Above 30.2 GeV, the flux can be described by a single power law with a spectral index $\gamma =-3.170\pm 0.008(\text{stat}+\text{syst})\pm 0.008(\text{energy}\text{scale})$.

High-spin states in the ${}^{129}\mathrm{Xe}$ nucleus are studied with the reaction ${}^{124}\mathrm{Sn}\left({}^9\mathrm{Be},4n\right)$ at a beam energy of 36 MeV. The level scheme is extended significantly. For the positive-parity band, the $\alpha =+1/2$ and the $\alpha =-1/2$ signature components are combined to form a complete band structure based on the $3/2^{+}$ state with spin and parity up to $21/2^{+}$. For the negative-parity band based on the $11/2^{-}$ state, the $\alpha =+1/2$ signature component is newly established and both the $\alpha =+1/2$ and the $\alpha =-1/2$ signature components also form a complete band structure up to the $35/2^{-}$ state. The positive- and negative-parity bands are proposed to originate from $\nu d_{3/2}3/2^{+}\left[402\right]$ and $\nu h_{11/2}11/2^{-}\left[505\right]$ Nilsson configurations, respectively. A backbending is observed in the negative-parity band, which originates from the alignments of two $h_{11/2}$ protons according to crank shell model calculations. Based on the total Routhian surface and quasiparticle triaxial rotor model calculations, the negative-parity band is interpreted as a triaxially deformed shape with $\gamma \approx -{30}^{\circ }$, while the positive-parity band is associated with $\gamma$ softness, in accordance with previous studies. In the high-spin states, three decoupled bands and one oblate band with $\gamma \approx -{60}^{\circ }$ are newly identified. The systematics and other characteristics of these bands are discussed.

Precision measurements by the Alpha Magnetic Spectrometer on the International Space Station of the primary cosmic-ray electron flux in the range 0.5 to 700 GeV and the positron flux in the range 0.5 to 500 GeV are presented. The electron flux and the positron flux each require a description beyond a single power-law spectrum. Both the electron flux and the positron flux change their behavior at $\sim 30\mathrm{GeV}$ but the fluxes are significantly different in their magnitude and energy dependence. Between 20 and 200 GeV the positron spectral index is significantly harder than the electron spectral index. The determination of the differing behavior of the spectral indices versus energy is a new observation and provides important information on the origins of cosmic-ray electrons and positrons.

Knowledge of the precise rigidity dependence of the helium flux is important in understanding the origin, acceleration, and propagation of cosmic rays. A precise measurement of the helium flux in primary cosmic rays with rigidity (momentum/charge) from 1.9 GV to 3 TV based on 50 million events is presented and compared to the proton flux. The detailed variation with rigidity of the helium flux spectral index is presented for the first time. The spectral index progressively hardens at rigidities larger than 100 GV. The rigidity dependence of the helium flux spectral index is similar to that of the proton spectral index though the magnitudes are different. Remarkably, the spectral index of the proton to helium flux ratio increases with rigidity up to 45 GV and then becomes constant; the flux ratio above 45 GV is well described by a single power law.

Fission fragments resulting from the fission of target-like nuclei produced in the ${}^{40}\mathrm{Ar}+{}^{197}\mathrm{Au}$ reaction at 35 MeV/u are measured in coincidence with the emitted light charged particles (LCPs). Comparison of the $N/Z$ composition of the LCPs at middle and large angles in the laboratory frame shows that particles emitted at smaller angles, which contain a larger contribution from dynamical emission, are more neutron rich. A moving-source model is used to fit the energy spectra of the hydrogen isotopes. A hierarchy from proton to deuteron and triton is observed in the multiplicity ratio between the intermediate velocity source and the compound nucleus source. This ratio is sensitive to the dynamical emission at early stages of the reaction and to statistical emission lasting up to the scission point. Calculations with the improved quantum molecular dynamics (ImQMD) transport-model qualitatively support the picture that more free and bound neutrons are emitted during the early stage, showing a clear dependence of $N/Z$ on the parametrization of the symmetry energy. The time-dependent isospin composition of the emitted particles thus may be used to probe the symmetry energy at subsaturation densities.

Background: ${}^{139}$Pr is located in a transitional region of neutron number close to the $N=82$ shell. The study of its high-spin states and collective bands is important for systematically understanding the nuclear structural characteristics in this region.

Purpose: To investigate the high-spin levels and to search for oblate bands in ${}^{139}$Pr.

Methods: The high-spin states of ${}^{139}$Pr have been studied via the reaction ${}^{124}$Sn(${}^{19}$F,4$n$) at a beam energy of 80 MeV. The experiment was carried out at the HI-13 Tandem Accelerator at the China Institute of Atomic Energy (CIAE). The data analysis was done by using the $\gamma$-$\gamma$ coincidence method.

Results: The level scheme of ${}^{139}$Pr has been expanded with spin up to 45/2$\hslash$. A total of 39 new levels and 45 new transitions are identified. Four collective band structures at high-spin states have been newly established. From systematic analysis, one of the bands is proposed as a double decoupled band; two bands are proposed as oblate bands with $\gamma \sim -{60}^{\circ }$; another band is suggested as an oblate-triaxial band with $\gamma \sim -{90}^{\circ }$. The other characteristics for these bands are discussed.

Conclusions: A new level scheme in ${}^{139}$Pr has been established and the collective bands at high spin have been identified. The result shows that the strong oblate shape-driving effect is caused by neutrons at the high-spin states in ${}^{139}$Pr.

The low spin states of ${}^{138}$Nd have been reinvestigated via the ${}^{124}$Te(${}^{19}$F,$4n1p$) reaction at a beam energy of 103 MeV. The quasi-one-phonon $\gamma$-vibrational band based on the 1013.7 keV level has been expanded and the quasi-two-phonon $\gamma$-vibrational band built on the 1842.7 keV level has been proposed. A systematic comparison supports our assignments for multiphonon $\gamma$-vibrational bands in ${}^{138}$Nd. The characteristics of $\gamma$ softness in ${}^{138}$Nd have been discussed. Triaxial projected shell model calculations can well describe the rotational feature of the bands, but they fail to reproduce simultaneously the bandhead energies of the two $\gamma$-vibrational bands if a fixed triaxial deformation is assumed.