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Na adsorption on a bulk Cs substrate at 20 K has been studied with collisionally excited autoionization electron spectroscopy using 1 keV ${\mathrm{Ne}}^{+}$ ion impact. Our results show a slight decrease in the surface work function and a quite small Ne 2${\mathit{p}}^4$3${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$D-to${\mathrm{-}}^3$P conversion rate at very low coverages, suggesting that, in contrast to the expectation based on the electronegativity consideration, a partial charge transfer occurs from Na adatoms to the more electropositive Cs substrate. We attribute this to the shift and broadening of the valence level of Na adatoms to overlap with the substrate conduction band. © 1996 The American Physical Society.

We present a detailed, angle-resolved Si L-shell Auger study by bombarding a single-crystalline Si sample with 10-keV ${\mathrm{Ar}}^{+}$ ions. We have observed a new atomic line at kinetic energy of ∼99 eV which is tentatively assigned to an Auger transition involving two 2p holes in ${\mathrm{Si}}^{+}$. The existence of two atomic peaks at 61.36 and 91.1 eV has also been clearly confirmed. Our Auger spectra show well-split Doppler peaks for the principal ${\mathrm{Si}}^0$ and ${\mathrm{Si}}^{+}$ atomic lines and a strong dependence of the shift amplitude on both incidence and detection angles. Successful computer fitting of the angular dependence of Doppler shift has been achieved by using a simple binary-collision model with the Molière approximation to the Thomas-Fermi screening potential. These results suggest that the first violent Ar-Si asymmetric collisions contribute remarkably to the Si 2p-vacancy creation process and are responsible for the ejection of energetic ${\mathrm{Si}}^{\left(\mathrm{*}\right)}$ particles which is highly directional. The critical minimum Ar-Si approach distance for Si 2p-hole excitation is 0.355 Å, in very good agreement with the value predicted by molecular-orbital theory.

We report energy distributions of electrons emitted from Al surfaces under impact by $1{\mathrm{keV}\mathrm{Ar}}^{+}$ and $1–5{\mathrm{keV}\mathrm{Ne}}^{+}$ ions. The variation of the energy distributions with the angle of incidence is different for both ions and provides information on the mechanism responsible for electron emission. For ${\mathrm{Ar}}^{+}$ electron emission results mainly from Auger neutralization, while for ${\mathrm{Ne}}^{+}$ an important emission mechanism is the decay of plasmon excitations. We find a transition between surface and bulk plasmon excitations as the energy of the ion is increased.

A precision measurement of the nitrogen flux with rigidity (momentum per unit charge) from 2.2 GV to 3.3 TV based on $2.2\times {10}^6$ events is presented. The detailed rigidity dependence of the nitrogen flux spectral index is presented for the first time. The spectral index rapidly hardens at high rigidities and becomes identical to the spectral indices of primary He, C, and O cosmic rays above $\sim 700\mathrm{GV}$. We observed that the nitrogen flux ${\mathrm{\Phi }}_{\mathrm{N}}$ can be presented as the sum of its primary component ${\mathrm{\Phi }}_{\mathrm{N}}^P$ and secondary component ${\mathrm{\Phi }}_{\mathrm{N}}^S$, ${\mathrm{\Phi }}_{\mathrm{N}}={\mathrm{\Phi }}_{\mathrm{N}}^P+{\mathrm{\Phi }}_{\mathrm{N}}^S$, and we found ${\mathrm{\Phi }}_{\mathrm{N}}$ is well described by the weighted sum of the oxygen flux ${\mathrm{\Phi }}_{\mathrm{O}}$ (primary cosmic rays) and the boron flux ${\mathrm{\Phi }}_{\mathrm{B}}$ (secondary cosmic rays), with ${\mathrm{\Phi }}_{\mathrm{N}}^P=(0.090\pm 0.002)\times {\mathrm{\Phi }}_{\mathrm{O}}$ and ${\mathrm{\Phi }}_{\mathrm{N}}^S=(0.62\pm 0.02)\times {\mathrm{\Phi }}_{\mathrm{B}}$ over the entire rigidity range. This corresponds to a change of the contribution of the secondary cosmic ray component in the nitrogen flux from 70% at a few GV to $<30\%$ above 1 TV.

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.

We report angle-resolved studies of projectile ${\mathit{L}}_{2,3}$-shell autoionization electron emission by bombarding a polycrystalline Al surface with 0.2–5-keV ${\mathrm{Ne}}^{+}$ ions. Several previously unresolved transition lines have been observed and assignment to Ne 2${\mathit{p}}^3$nln’l’n’’l’’ and ${\mathrm{Ne}}^{+}$ 2${\mathit{p}}^3$nln’l’ autoionizing states has been proposed. We found that the ${}_{}{}^3{}_{}{}^{}$P triplet is the dominant configuration for the excited Ne 2${\mathit{p}}^4$3${\mathit{s}}^2$ and its relative population depends sensitively on both the primary ion energy and incidence angle. Analysis of the electron spectra shows that most excited Ne particles are backscattered from the surface and decay in vacuum, rather than inside the solid.

We present high-statistics, precision measurements of the detailed time and energy dependence of the primary cosmic-ray electron flux and positron flux over 79 Bartels rotations from May 2011 to May 2017 in the energy range from 1 to 50 GeV. For the first time, the charge-sign dependent modulation during solar maximum has been investigated in detail by leptons alone. Based on $23.5\times {10}^6$ events, we report the observation of short-term structures on the timescale of months coincident in both the electron flux and the positron flux. These structures are not visible in the $e^{+}/e^{-}$ flux ratio. The precision measurements across the solar polarity reversal show that the ratio exhibits a smooth transition over $830\pm 30$ days from one value to another. The midpoint of the transition shows an energy dependent delay relative to the reversal and changes by $260\pm 30$ days from 1 to 6 GeV.

We present the precision measurement of the daily proton fluxes in cosmic rays from May 20, 2011 to October 29, 2019 (a total of 2824 days or 114 Bartels rotations) in the rigidity interval from 1 to 100 GV based on $5.5\times {10}^9$ protons collected with the Alpha Magnetic Spectrometer aboard the International Space Station. The proton fluxes exhibit variations on multiple timescales. From 2014 to 2018, we observed recurrent 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. The rigidity dependence of the 27-day periodicity is different from the rigidity dependences of 9-day and 13.5-day periods. Unexpectedly, the strength of 9-day and 13.5-day periodicities increases with increasing rigidities up to $\sim 10\mathrm{GV}$ and $\sim 20\mathrm{GV}$, respectively. Then the strength of the periodicities decreases with increasing rigidity up to 100 GV.

The ${}^{17}\mathrm{O}(n,\alpha ){}^{14}\mathrm{C}$ and ${}^{17}\mathrm{O}(\alpha ,n){}^{20}\mathrm{Ne}$ reactions compete to determine the level of neutron recycling that occurs following neutron absorption by the ${}^{16}\mathrm{O}(n,\gamma ){}^{17}\mathrm{O}$ reaction during $s$-process nucleosynthesis. Yet, because of large inconsistencies in the absolute cross section of the ${}^{17}\mathrm{O}(n,\alpha ){}^{14}\mathrm{C}$ reaction, this process retains a large uncertainty. To resolve the inconsistencies in past experimental data, a multichannel $R$-matrix analysis was used to fit ${}^{14}\mathrm{C}(\alpha ,\alpha ){}^{14}\mathrm{C}$ data, thereby constraining the partial widths of the two lowest energy resonances and thus the resonant ${}^{17}\mathrm{O}(n,\alpha ){}^{14}\mathrm{C}$ cross section. A detailed uncertainty analysis, using Bayesian methods, has been performed resulting in an updated reaction rate. Recommendations are given to further improve the cross section determination using this method.

We report on a systematic study of Ne-projectile and Na-target atom autoionization electron emission during 700-eV ${\mathrm{Ne}}^{+}$ ion bombardment of polycrystalline Cr, Cu, Mo, and Pt surfaces covered with an increasing amount of Na. We show that the collisionally excited Ne 2${\mathit{p}}^4$${(}^1$D) singlet core can be efficiently converted to a 2${\mathit{p}}^4$${(}^3$P) triplet through an Auger rearrangement process involving a solid valence electron and that the conversion is highly localized at the excitation site and its rate is strongly dependent on local electronic properties. We found that the Na 2p electron excitation essentially results from an L-shell vacancy transfer from the Ne 2${\mathit{p}}^4$${(}^1$D) at low Na coverages and from direct excitation in Na-Na symmetric collisions at high coverages. We demonstrate the great potential of the low-energy collisionally excited autoionization electron spectroscopy as a surface-analysis technique in alkali-metal adsorption studies.

The relative intensities of the Ne $2p^4\left({}_{}{}^3{}_{}{}^{}P\right)3\mathrm{}{s}^2{}_{}{}^3{}_{}{}^{}P$ and $2p^4\left({}_{}{}^1{}_{}{}^{}D\right)3\mathrm{}{s}^2{}_{}{}^1{}_{}{}^{}D$ autoionization lines produced by 700 eV ${\mathrm{Ne}}^{+}$ ion impact on Na deposited on polycrystalline Al and Mo show a sensitive dependence on Na coverage. We show that the collisionally excited Ne $2p^4$ singlet core configuration can be efficiently converted to a triplet at the immediate vicinity of the excitation site, and that its rate depends strongly on the local electronic properties around the target atomic site. These findings point to a technique to obtain site-specific information of the electronic structure and potentials outside surfaces.

Target LMM atomic Auger electron emission has been studied by low-energy ion bombardment on light-element surfaces. Our results show that the threshold energies for Mg, Al, and Si 2p core electron excitation are the same for impact of ${\mathrm{Ar}}^{+}$, ${\mathrm{Kr}}^{+}$, and ${\mathrm{Xe}}^{+}$ projectiles but are substantially lower for ${\mathrm{Ne}}^{+}$. A sensitive dependence of the energy onset on the ion incidence angle has also been observed. The intensity behavior in the near-threshold energy range is in excellent agreement with simple kinematic calculations using a binary-collision model and by taking into account only the first target-target symmetric encounters.

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.

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 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.

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.

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.

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.