Search

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 report the observation of new properties of primary cosmic rays He, C, and O measured in the rigidity (momentum/charge) range 2 GV to 3 TV with $90\times {10}^6$ helium, $8.4\times {10}^6$ carbon, and $7.0\times {10}^6$ oxygen nuclei collected by the Alpha Magnetic Spectrometer (AMS) during the first five years of operation. Above 60 GV, these three spectra have identical rigidity dependence. They all deviate from a single power law above 200 GV and harden in an identical way.

We report the observation of new properties of primary cosmic rays, neon (Ne), magnesium (Mg), and silicon (Si), measured in the rigidity range 2.15 GV to 3.0 TV with $1.8\times {10}^6\mathrm{Ne}$, $2.2\times {10}^6\mathrm{Mg}$, and $1.6\times {10}^6\mathrm{Si}$ nuclei collected by the Alpha Magnetic Spectrometer experiment on the International Space Station. The Ne and Mg spectra have identical rigidity dependence above 3.65 GV. The three spectra have identical rigidity dependence above 86.5 GV, deviate from a single power law above 200 GV, and harden in an identical way. Unexpectedly, above 86.5 GV the rigidity dependence of primary cosmic rays Ne, Mg, and Si spectra is different from the rigidity dependence of primary cosmic rays He, C, and O. This shows that the Ne, Mg, and Si and He, C, and O are two different classes of primary cosmic rays.

We present the precision measurement of 2824 daily helium fluxes in cosmic rays from May 20, 2011 to October 29, 2019 in the rigidity interval from 1.71 to 100 GV based on $7.6\times {10}^8$ helium nuclei collected with the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station. The helium flux and the helium to proton flux ratio exhibit variations on multiple timescales. In nearly all the time intervals from 2014 to 2018, we observed recurrent helium flux variations with a period of 27 days. Shorter periods of 9 days and 13.5 days are observed in 2016. The strength of all three periodicities changes with time and rigidity. In the entire time period, we found that below $\sim 7\mathrm{GV}$ the helium flux exhibits larger time variations than the proton flux, and above $\sim 7\mathrm{GV}$ the helium to proton flux ratio is time independent. Remarkably, below 2.4 GV a hysteresis between the helium to proton flux ratio and the helium flux was observed at greater than the $7\sigma$ level. This shows that at low rigidity the modulation of the helium to proton flux ratio is different before and after the solar maximum in 2014.

We report the properties of primary cosmic-ray sulfur (S) in the rigidity range 2.15 GV to 3.0 TV based on $0.38\times {10}^6$ sulfur nuclei collected by the Alpha Magnetic Spectrometer experiment (AMS). We observed that above 90 GV the rigidity dependence of the S flux is identical to the rigidity dependence of Ne-Mg-Si fluxes, which is different from the rigidity dependence of the He-C-O-Fe fluxes. We found that, similar to N, Na, and Al cosmic rays, over the entire rigidity range, the traditional primary cosmic rays S, Ne, Mg, and C all have sizeable secondary components, and the S, Ne, and Mg fluxes are well described by the weighted sum of the primary silicon flux and the secondary fluorine flux, and the C flux is well described by the weighted sum of the primary oxygen flux and the secondary boron flux. The primary and secondary contributions of the traditional primary cosmic-ray fluxes of C, Ne, Mg, and S (even $Z$ elements) are distinctly different from the primary and secondary contributions of the N, Na, and Al (odd $Z$ elements) fluxes. The abundance ratio at the source for $\mathrm{S}/\mathrm{Si}$ is $0.167\pm 0.006$, for $\mathrm{Ne}/\mathrm{Si}$ is $0.833\pm 0.025$, for $\mathrm{Mg}/\mathrm{Si}$ is $0.994\pm 0.029$, and for $\mathrm{C}/\mathrm{O}$ is $0.836\pm 0.025$. These values are determined independent of cosmic-ray propagation.

Precision measurements of cosmic ray positrons are presented up to 1 TeV based on 1.9 million positrons collected by the Alpha Magnetic Spectrometer on the International Space Station. The positron flux exhibits complex energy dependence. Its distinctive properties are (a) a significant excess starting from $25.2\pm 1.8\mathrm{GeV}$ compared to the lower-energy, power-law trend, (b) a sharp dropoff above $284_{-64}^{+91}\mathrm{GeV}$, (c) in the entire energy range the positron flux is well described by the sum of a term associated with the positrons produced in the collision of cosmic rays, which dominates at low energies, and a new source term of positrons, which dominates at high energies, and (d) a finite energy cutoff of the source term of $E_s=810_{-180}^{+310}\mathrm{GeV}$ is established with a significance of more than $4\sigma$. These experimental data on cosmic ray positrons show that, at high energies, they predominantly originate either from dark matter annihilation or from other astrophysical sources.

We present the precision measurements of 11 years of daily cosmic positron fluxes in the rigidity range from 1.00 to 41.9 GV based on $3.4\times {10}^6$ positrons collected with the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station. The positron fluxes show distinctly different time variations from the electron fluxes at short and long timescales. A hysteresis between the electron fluxes and the positron fluxes is observed with a significance greater than $5\sigma$ at rigidities below 8.5 GV. On the contrary, the positron fluxes and the proton fluxes show similar time variation. Remarkably, we found that positron fluxes are modulated more than proton fluxes with a significance greater than $5\sigma$ for rigidities below 7 GV. These continuous daily positron fluxes, together with AMS daily electron, proton, and helium fluxes over an 11-year solar cycle, provide unique input to the understanding of both the charge-sign and mass dependencies of cosmic rays in the heliosphere.

We have investigated atomic arrangements and their electronic properties of the well-ordered thallium overlayer structures formed on the Si(111)-$7\times 7$ surface. As for other trivalent atoms, Tl is found to form a well-defined $\sqrt{3}\times \sqrt{3}$ surface, indicating the absence of a so-called “inert pair effect” considered only for Tl. Another well ordered $1\times 1$ surface at 1.0 monolayer appears to be semiconducting in our angle-resolved photoemission spectra dominated by a unique dispersive surface band near the Fermi level. Our theoretical calculations using density-functional theory show that Tl adatoms occupy the $T_4$ sites and saturate all the dangling bonds of surface Si atoms to make the surface semiconducting with a band gap of 0.34 eV. The filled surface band observed has been well reproduced in our band calculations.

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.

A precision measurement by AMS of the antiproton flux and the antiproton-to-proton flux ratio in primary cosmic rays in the absolute rigidity range from 1 to 450 GV is presented based on $3.49\times 10^5$ antiproton events and $2.42\times 10^9$ proton events. The fluxes and flux ratios of charged elementary particles in cosmic rays are also presented. In the absolute rigidity range $\sim 60$ to $\sim 500\mathrm{GV}$, the antiproton $\overline{p}$, proton $p$, and positron $e^{+}$ fluxes are found to have nearly identical rigidity dependence and the electron $e^{-}$ flux exhibits a different rigidity dependence. Below 60 GV, the ($\overline{p}/p$), ($\overline{p}/e^{+}$), and ($p/e^{+}$) flux ratios each reaches a maximum. From $\sim 60$ to $\sim 500\mathrm{GV}$, the ($\overline{p}/p$), ($\overline{p}/e^{+}$), and ($p/e^{+}$) flux ratios show no rigidity dependence. These are new observations of the properties of elementary particles in the cosmos.

Resonant tunneling between fractional quantum Hall edge states is studied in the Luttinger liquid picture. For the ν=1/3 Laughlin parent state, the resonance line shape is a universal function whose width scales to zero at zero temperature. Extensive quantum Monte Carlo simulations are presented for ν=1/3 which confirm this picture and provide a parameter-free prediction for the line shape.

Precision measurements by the Alpha Magnetic Spectrometer (AMS) on the International Space Station of ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ fluxes are presented. The measurements are based on 100 million ${}^4\mathrm{He}$ nuclei in the rigidity range from 2.1 to 21 GV and 18 million ${}^3\mathrm{He}$ from 1.9 to 15 GV collected from May 2011 to November 2017. We observed that the ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ fluxes exhibit nearly identical variations with time. The relative magnitude of the variations decreases with increasing rigidity. The rigidity dependence of the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio is measured for the first time. Below 4 GV, the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio was found to have a significant long-term time dependence. Above 4 GV, the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio was found to be time independent, and its rigidity dependence is well described by a single power law $\propto R^{\mathrm{\Delta }}$ with $\mathrm{\Delta }=-0.294\pm 0.004$. Unexpectedly, this value is in agreement with the B/O and B/C spectral indices at high energies.

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.

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.

By employing ab initio total-energy calculations we have studied adsorption and diffusion of Ag atoms on a dimer-reconstructed Si(001) surface. For a single Ag adsorption, the twofold-coordinated cave site above the fourth Si layer atom was found to be the most stable, in agreement with previous works. Inspection of the electronic structures at the cave site revealed that the Ag-Si bonds originate from low-lying $4d$ electrons and are covalent. Our calculations also exhibit another stable adsorption at the pedestal site that is slightly higher by 0.03 eV in energy than the cave site. Further potential-energy-surface calculations showed that the surface diffusion of a single Ag adatom is unexpectedly highly isotropic and that the energy barrier is 0.5 eV. When more Ag adatoms are adsorbed on the Si surface, the Ag adatoms are expected to form dimers. Actually, we obtained an energy gain of 0.36 eV/dimer through the dimerization. The diffusion of an Ag dimer was also investigated. Surprisingly, we found a very rapid surface-dimer diffusion with an energy barrier of 0.48 eV slightly lower than that of the single Ag adatom. In contrast to the diffusion of the single Ag adatom, the dimer diffusion is anisotropic and preferably occurs along the valley between Si dimer rows by concerted flipflop processes.

Initial nitridation of the Si(100) surface is investigated using photoemission, ion-scattering and ab initio calculations. After dissociation of NO or $\mathrm{N}{\mathrm{H}}_{\mathrm{3}}$, nitrogen atoms are found to spontaneously form a stable, highly coordinated $\mathrm{N}\mathrm{S}{\mathrm{i}}_{\mathrm{3}}$ species even at room temperature. The majority of the N species is incorporated into the subsurface Si layers occupying an interstitial site, whose atomic structure and unique bonding mechanism is clarified through ab initio calculations. This unusual adsorption behavior elucidates the atomistic mechanism of initial silicon nitride formation on the surface and has important implication on the N-rich layer formation at the ${\mathrm{S}\mathrm{i}\mathrm{O}}_x{\mathrm{N}}_y/\mathrm{S}\mathrm{i}$ interface.

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.

Precision measurements by the Alpha Magnetic Spectrometer (AMS) on the International Space Station of the deuteron ($D$) flux are presented. The measurements are based on $21\times {10}^6$ $D$ nuclei in the rigidity range from 1.9 to 21 GV collected from May 2011 to April 2021. We observe that over the entire rigidity range the $D$ flux exhibits nearly identical time variations with the $p$, ${}^3\mathrm{He}$, and ${}^4\mathrm{He}$ fluxes. Above 4.5 GV, the $D/{}^4\mathrm{He}$ flux ratio is time independent and its rigidity dependence is well described by a single power law $\propto R^{\mathrm{\Delta }}$ with ${\mathrm{\Delta }}_{D/{}^4\mathrm{He}}=-0.108\pm 0.005$. This is in contrast with the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio for which we find ${\mathrm{\Delta }}_{{}^3\mathrm{He}/{}^4\mathrm{He}}=-0.289\pm 0.003$. Above $\sim 13\mathrm{GV}$ we find a nearly identical rigidity dependence of the $D$ and $p$ fluxes with a $D/p$ flux ratio of $0.027\pm 0.001$. These unexpected observations indicate that cosmic deuterons have a sizable primarylike component. With a method independent of cosmic ray propagation, we obtain the primary component of the $D$ flux equal to $9.4\pm 0.5\%$ of the ${}^4\mathrm{He}$ flux and the secondary component of the $D$ flux equal to $58\pm 5\%$ of the ${}^3\mathrm{He}$ flux.