Low-energy Charged Particles Research Articles

Frictional cooling: Latest experimental results and first application

Frictional cooling — that is cooling a beam of very low-energy charged particles by moderation in matter and simultaneous acceleration in an electrostatic field — has been shown to be feasible during our experiments in 1994/1995 at PSI. As indicated by our previous closed-form and Monte-Carlo calculations we found a significant increase in spectral density and a decrease in the angular spread for a beam of negative muons. With the help of the cooled muons we were able to measure the diffusion times for p μ and d μ atoms in low-pressure (0.25 to 12 mbar) hydrogen gas. From this the mean kinetic energy of p μ atoms at the end of the cascade in 1 mbar hydrogen gas was determined to be (2.6 ± 0.6) eV (preliminary value).

A three‐dimensional azimuthally symmetric model atmosphere for Io: 2. Plasma effect on the surface

Because SO2 gas in Io's atmosphere is transported poleward where it condenses, the absence of bright polar caps on Io is intriguing. In this paper, we show that the energetic plasma ions measured by the low energy charged particles (LECP) instrument on Voyager carry most of the plasma energy and they deposit sufficient energy in Io's surface to alter the molecular composition and hence the “color” of the surface. We use an extrapolation of the LECP energy flux measured by Voyager, together with the calculated surface condensation rate, to obtain the energy deposited per molecule condensed on the surface in the region above 50° latitude. Because vapor deposited SO2 is spectrally flat above ∼0.4 μm, a small concentration of absorbers can efficiently darken the surface. Based on the laboratory data of Moore [1984], a dose ∼0.5–5 eV per molecule at 88 K produces significant color change in vapor‐deposited SO2. At the poles the energy deposited is ∼10 eV per molecule condensed and at 50° latitude it is ∼1 eV per molecule condensed so that these regions will not appear as a bright frost. In contrast, the fractional deposition rate in this region of photodissociation products is small, and the sputter removal rate of SO2 is slower than the chemical alteration rate.

Comprehensive analysis of electron observations at Saturn: Voyager 1 and 2

We present a comprehensive analysis of Voyager 1 and 2 electron observations within Saturn's magnetosphere. This analysis entails the merging of electron observations from the Plasma Science (PLS) experiment, the Low Energy Charged Particle (LECP) experiment and the Cosmic Ray System (CRS) experiment. For each encounter, the three instruments combined allow us to compute the electron energy spectra over a wide range of energies from 10 eV to ∼ 2MeV between the closest approaches and L = 18.5. The instruments use different technologies, different sensitivities, and different fields of view; however, we observe a surprisingly good matching of the data sets on a 15‐min timescale. The PLS‐LECP‐CRS spectra include the low‐energy thermal component of the magnetospheric plasma, the keV suprathermal electrons, and the high‐energy tail extending into the MeV energy range. From the combined spectra, we compute a comprehensive set of macroscopic parameters (electron density, pressure, beta factor, and electron current at the spacecraft): the analysis reveals a variety of radial gradients for these quantities and the corresponding electron populations. We also compute phase space densities over a wide range in energy and radial distances, analyzing local time symmetries, electron source distributions, and temporal variations of Saturn's magnetosphere. The ultimate goal of this study is to provide a comprehensive empirical model of the charged particle population within Saturn's magnetosphere. It will be used to support the development of the Cassini mission and to allow detailed planning of the tour design with regard to charged particle science and radiation hazards.

Hot plasma parameters of Jupiter's inner magnetosphere

The bulk parameters of the hot (>20 keV) plasmas of Jupiter's inner magnetosphere, including the vicinity of the Io plasma torus, are presented for the first time (L = 5 to 20 RJ). The low‐energy charged particle (LECP) instrument on Voyager 1 that obtained the data presented here was severely overdriven within the inner regions of Jupiter's magnetosphere. On the basis of laboratory calibrations using a flight spare instrument, a Monte Carlo computer algorithm has been constructed that simulates the response of the LECP instrument to very high particle intensities. This algorithm has allowed for the extraction of the hot plasma parameters in the Jovian regions of interest. The hot plasma components discussed here dominate over other components with respect to such high‐order moments as the plasma pressures and energy intensities. Our findings include the following items. (1) Radial pressure gradients change from positive (antiplanetward) to negative as one moves outward past about 7.3 RJ. While the observed hot plasma distributions will impede the radial transport, via centrifugal interchange, of iogenic plasmas throughout the Io plasma torus regions out to 8 RJ, the plasma impoundment concept of Siscoe et al. [1981] for explaining the so‐called “ramp” in the flux shell content profile of iogenic plasmas (7.4–7.8 RJ [Bagenal, 1994]) is not supported. (2) We predict a radial ordering for the generation of the aurora, which translates into a latitudinal structure for auroral emissions. Planetward of about 12 RJ, intense aurora (10 ergs/(cm2 s) precipitation) can only be caused by ion precipitation, whereas outside of about 12 RJ such intense aurora can only be caused by electron precipitation. Uncertainties concerning the causes of Jovian aurora may stem in part from failures of some observations to resolve the latitudinal structure that is anticipated here and possibly from changes in the auroral configuration and/or charged particle spectral properties since the Voyager epoch.

Indirect Investigation of the D + 6Li Reaction at Low Energies Relevant for Nuclear Astrophysics

The indirect investigation of low-energy charged-particle reactions relevant for nuclear astrophysics is considered, employing statistical tensor analysis. Data from the quasi-free contribution to the {sup 6}Li({sup 6}Li, 2{alpha}){sup 4}He reaction at projectile energies above the Coulomb barrier are analyzed in order to extract the excitation function of the {sup 6}Li({ital d},{alpha}){sup 4}He reaction at very low relative energy. The results are compared with an astrophysical factor extracted from the free-reaction cross section data. The limits of the method are discussed. {copyright} {ital 1996 The American Astronomical Society.}

Low-energy charged particles at near equatorial latitudes according to “MIR” orbital station data

Some date analysis results obtained by means of the “SPRUT-V” device, mounted onboard the orbital station “MIR” in Jan. 1991 are presented. This device measures the energy distribution of E e>35,>75,>300,>600 keV electron fluxes and E p=0.1…8.0 MeV proton fluxes. Special attention is given to: 1. The registration of electron fluxes with energy of tens and hundreds of keV at L<1.1 in the longitude range of 80–110° in the northern hemisphere. 2. The registration of electrons in the stable trapping region and the occurence of >75 keV sporatic electron fluxes at L<1.5, conjugate to the Brazil anomaly area. 3. The registration of proton fluxes at about all longitudes near the geomagnetic equator at L<1.3.

Hot ions in Jupiter's magnetodisc: A model for Voyager 2 low‐energy charged particle measurements

The Low‐Energy Charged Particle (LECP) instrument on the Voyager 2 spacecraft acquired a comprehensive set of directional and energy‐dependent information on the nature of hot ions in the Jovian magnetodisc. The LECP measurements in the energy range 30 keV to 5 MeV, where the ion pressure dominates the total plasma pressure, have been successfully fit to a two‐species convected k distribution function model for hot ions in the Jovian magnetodisc in the vicinity of neutral sheet crossings. The regions where the model could be used ranged from 60 to 30 RJ on the dayside (inbound) and 75 to 125 RJ on the nightside (outbound). With this model, the full angular and spectral information from the lowest‐energy LECP detectors has been deconvolved using a nonlinear least squares technique to reveal the heavy ion pressure, density, and temperature distinct from the corresponding hot proton parameters. The pressure is dominated by heavy ions in the outer magnetosphere. The temperature of protons remains nearly constant at 20 keV (dayside) and 10 keV (nightside), whereas the heavy ion temperature shows a distinct increase with radial distance paralleling the corotation or pickup energy of heavy ions. A neutral wind of heavy atoms, originating in the near‐Io regions and ionized during their flight through the outer magnetosphere by solar radiation, may be the seed population for the heavy ions measured by the LECP. The convection velocity of the plasma is subcorotational, reduced from the rigid value by a factor of ∼2, but increases with increasing distance from 30 to 60 RJ in the dayside region and from 75 to 85 RJ in the nightside region. The trend stops beyond 85 RJ in the nightside region, but there is still a substantial corotational flow that extends from 85 RJ to at least 130 RJ. In all the regions studied, the particle anisotropies in the LECP scan plane below ∼2 MeV are believed to result primarily from the Compton‐Getting effect and not from gradient anisotropies or particles executing nonadiabatic orbits as they encounter the neutral sheet. Gradient anisotropies are not important even in the distant nightside neutral sheet region (&gt;85 RJ) below ∼2 MeV. The large flow velocities and increasing heavy ion temperatures are consistent with a strong corotational electric field and imply that the mass loading due to lower‐energy heavy ion plasma via outward transport from Io is insufficient to disrupt corotation within ∼60 RJ during the Voyager 2 encounter.

Optimization of the recoil-direction-sensitive target for muon capture

The optimal thickness of layers composing nucleus-recoil-sensitive target that enables the measurement of a particular orientation parameter of the final nuclei 12B gs( I = 1 +) and 16N gs( I = 2 −) in the reactions μ − + 12 C(0 +) → v μ + 12 B gs (I = 1 +) and μ − + 16 O(0 +) → v μ + 16 N gs (I = 2 −) is estimated according to the current status of art of the slowing down of low energy (recoil energy of 12B is 376 keV and that of 16N is 300 keV) charged particles in matter. The choice of materials for orientation preserving (Ni, Ag, Rh) and for orientation-destroying (Al) layers which sandwich the production target layer C or BeO is based on the data obtained with polarized 12B and 13N radioactive nuclei beams at the Université Catholique de Louvain.

Over the southern solar pole: low-energy interplanetary charged particles.

The heliosphere instrument for spectrum, composition, and anisotropy (HISCALE) recorded the fluxes of low-energy ions and electrons (> 50 kiloelectron volts) when Ulysses crossed the southern solar polar region and revealed that the large-scale structure of the heliosphere to at least approximately -75 degrees was significantly influenced by the near-equatorial heliospheric current sheet. Electrons in particular were accelerated by the current sheet-produced and poleward-propagating interplanetary reverse shock at helioradii far from the Ulysses location. At heliolatitudes higher than approximately -75 degrees on the Ulysses ascent to the pole and approximately -50 degrees on the descent, small, less regular enhancements of the lowest energy electron fluxes were measured whose relations to the current sheet were less clear. The anomalous component of low-energy (approximately 2 to 5 megaelectron volts per nucleon) oxygen flux at the highest heliolatitudes was found to be approximately 10(-8) [per square centimeter per second per steradian (per kiloelectronvolt per nucleon)]; the anomalous Ne/O ratio was approximately 0.25.

Latitude-associated differences in the Low Energy Charged Particle activity at Voyagers 1 and 2 during 1991 to early 1994

The Low Energy Charged Particle (LECP) instruments on Voyagers 1 (V1) and 2 (V2) measure the differential in energy fluxes and anisotropies of low energy ions≥30 keV and electrons≥20 keV differential in energy ion composition≥200 keV/nuc, and the integral rates of cosmic ray protons>70 MeV (Krimigiset al., 1977). We discuss shock-accelerated ions and latitude-associated differences between V1 and V2 during 1991 to April 1994.

Ionization energy for charged particles in Ge.

The response of a hyperpure Ge detector (volume \ensuremath{\sim}0.7 ${\mathrm{cm}}^{3}$) to ionizing radiation has been studied with regard to the radiation ionization energy ratio, ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{ion}}$/${\mathrm{\ensuremath{\varepsilon}}}_{0}$, using radioactive (\ensuremath{\alpha},\ensuremath{\gamma}) sources and accelerated ion beams ${(}^{1}$H${,}^{4}$He). These data provide evidence that ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{ion}}$ is not identically equal to ${\mathrm{\ensuremath{\varepsilon}}}_{0}$ for low-energy (e.g., few MeV) charged particles, in qualitative accord with data for Si detectors. The apparent thickness of the ion-implanted entrance window of the detector exhibits a marked dependence on temperature in the region 80--183 K. As well, channeling effects have been observed which can be attributed to residual crystalline structure in this inactive layer.

Electron distributions in the inner Jovian magnetosphere: Voyager 1 observations

Using several improvements in the analysis of the observations of the Low Energy Charged Particle (LECP) experiment on Voyager 1, electron phase space densities in the inner Jovian magnetosphere (5‐10 RJ) were first calculated at constant first and second invariants (represented by µ and K, respectively), based on the LECP measurements. The calculated electron phase space density profiles show that in the inner Jovian magnetosphere there exist evident time and longitude variations, energetic electron injections, and present radial transport and distributed losses. To study the radial and pitch angle diffusions of Jovian electrons, we have calculated the phase space densities in the K‐L space. It is found that the electron population in the inner Jovian magnetosphere seems to consist of two components: electrons radially diffusing from a main external source and electrons generated from local sources. The radially diffusing electrons have a relatively time stationary and isotropic distribution, while the locally created electrons mainly concentrate around the equatorial plane and have relatively lower energies, in comparison with the inward diffusing electrons. Consequently, the sources of precipitation losses to the ionosphere must be primarily electrons transported from outer sources, and the major precipitations occur in the inner magnetosphere (L &lt; 7.5 RJ). In the inner Jovian magnetosphere (L = 5 ∼ 10 RJ) it is estimated that for electrons with magnetic moment µ = 300 MeV/G, the diffusion coefficient D is roughly 10−8 ∼ 10−6 RJ2 s−1, and the lifetime against the diffusion losses is of the order of 104 ∼ 106 s.

Ion accelerators and electron scattering

Several problems of fundamental importance to physics involving highly correlated and few body systems are being addressed in our laboratories. In particular, the breakup of three low energy charged particles is being studied using crossed beams of electrons and ions. The systems involved can consist of the excitation and dissociation of molecular ions or the resonant excitation-auto-double ionization (READI) of simple atomic ions. The use of particle accelerators in conjunction with electron beams provide a unique combination of tools for studies of this nature. We have constructed an apparatus to measure multiparameter angular correlations between reaction products following electron impact ionization of ionic targets to obtain alignment and orientation information. With molecular targets, ground and excited state potential energy diagrams may also be obtained. We will measure the momentum of all reaction products and provide a stringent experimental test of near threshold three-body theory which could lead to a general theoretical treatment for three particles interacting via a Coulomb field.

On the evolution of phases in polycrystalline Li-doped 2212 BSCCO and enhanced superconducting behaviour via n(Li, alpha )T reactions

The authors report on the evolution of phases upon cooling of partially melted Bi-Sr-Ca-Cu-Li-O powders and tapes and on enhanced magnetic flux-pinning through irradiation. Enhanced resistive transitions have been obtained despite significant amounts of (Sr/sub 1- delta /, Ca/sub delta /)/sub n/CuO/sub x/ phases (n=1 or 2). The evolution of phases is investigated in Li-doped and undoped 2212 via extensive microstructural characterizations. To enhance the flux-pinning, Li-doped and undoped 2212 have been neutron irradiated. In addition to the neutron-collision-induced damage, fission reactions produce a homogeneous distribution of high- and low-energy charged particles. Effects of irradiation on the magnetically measured T/sub c/ and J/sub c.m/(H) are reported. Differences between the effects upon doped and undoped superconducting powders are discussed, highlighting the effects of the charged particle damage. >

ULF turbulence in the Neptunian polar cusp

One of the most fortuitous events occurring during the Voyager 2 Neptune encounter was the passage of the spacecraft through the southern magnetic cusp region prior to entry into the magnetosphere. This region was previously identified by a slow, steady change in the magnetic field direction to a dipolar configuration and a slow, steady decrease in electron and ion fluxes. Such observations are similar to those of the terrestrial cusp. Besides these characteristics, the region also possesses enhanced ULF magnetic turbulence at frequencies less than 0.5 Hz as measured by the Voyager 2 magnetometer (MAG) experiment. The activity is particularly strong at the frontside magnetopause boundary and at the backside cusp/magnetosphere boundary. Specifically, in these regions the fluctuation level, ΔB/B, is as large as 6%. The enhanced ULF turbulence extends from 0.01 Hz continuously up to nearly 0.5 Hz, and is consistent with the whistler mode. Calculations indicate that such turbulence should resonate with electrons having energies in the tens of kiloelectron volts. Observations indicate a very strong correlation of the ULF turbulence with the energetic electrons between 22 and 35 keV measured by Voyager's low‐energy charged particle (LECP) experiment. This result suggests a vigorous interaction between the two. Based on an assumed cusp geometry and measured wave levels, the ULF turbulence could generate as much as 107 W of power that would be absorbed by the surrounding magnetosphere. This power input represents about 1% of the total auroral power required to drive the UV aurora and radio emissions. Thus the ULF wave turbulence in the cusp may represent a significant but not complete power source for the magnetosphere.

Energetic ion phase space densities in Neptune's magnetosphere

Ion intensities measured by the Voyager 2 Low Energy Charged Particle (LECP) experiment at Neptune have been analyzed to determine ion phase space densities at fixed values of the first and second adiabatic invariants of charged particle motion. The phase space densities are broadly peaked near L = 10 and generally show a rapid decline going toward Neptune, although there is a gap in data coverage at the Proteus (1989N1) minimum L-shell. These profiles are interpreted as indicating generally inward radial diffusion with an energetic ion source near L = 10. There is excellent agreement between inbound and outbound phase space density profiles at the same values of the invariants, suggesting quasistationary and roughly axisymmetric radiation belts. If absorption by Neptune's moons and rings is an important loss process, then the radial diffusion coefficient D LL is on the order of 10 −7 L 3 sec −1, consistent with the Voyager Plasma Science determination for a Triton-associated proton source of 10 25 sec −1. The LECP ion phase space density profiles are consistent with relatively weak L-dependence of D LL , provided that loss rates increase toward the planet; for an extreme case of a loss rate independent of L, D LL ∝ L 6.03. Weaker L dependence of D LL is found for a loss rate τ −1 increasing toward the planet ( D LL ∝ L 3 when τ −1 ∝ L −3.63), suggesting interchange diffusion. With D LL = 6 × 10 −8 L 3 sec −1, the inward diffusing power carried by energetic ions is estimated as 2 × 10 9 W, which would be adequate to power Neptune's aurora if a substantial portion of the ions is lost into Neptune's atmosphere.

Investigation of the neutron detection properties of the fast liquid scintillator BC-505

The energy resolution, the absolute detection efficiency, and the pulse-shape discrimination capability for fast neutrons by the fast liquid scintillator BC-505 have been investigated. The response to fast neutrons was investigated using the proton-tagged neutrons from the d-d reaction generated by the Colorado School of Mines low-energy charged particle accelerator. Using a collimated source of gamma rays from a 60Co source, the spatial resolution of the scintillator was measured by comparing the arrival times of the scintillation signals at the PMTs at the ends of the scintillator cell. A spatial resolution of 2.7 cm was measured corresponding to a total time resolution of about 150 ps. The results of our investigation will be applied to the conceptual design of a ‘‘pin-hole’’ neutron camera which may be used to provide real-time radial distributions of fast neutron sources in fusion plasmas.

S-wave scattering of charged fermions by a magnetic black hole.

We argue that, classically, $s$-wave electrons incident on a magnetically charged black hole are swallowed with probability one: the reflection coefficient vanishes. However, quantum effects can lead to both electromagnetic and gravitational backscattering. We show that, for the case of extremal, magnetically charged, dilatonic black holes and a single flavor of low-energy charged particles, this backscattering is described by a perturbatively computable and unitary $S$-matrix, and that the Hawking radiation in these modes is suppressed near extremality. The interesting and much more difficult case of several flavors is also discussed.

A convected K distribution model for hot ions in the Jovian magnetodisc

Hot ion angular anisotropies measured by the Low Energy Charged Particle (LECP) instrument during the Voyager 2 encounter with the Jovian dayside outer magnetosphere (60–30 RJ) have been fitted to a 2 species convected K distribution function using a non‐linear least squares technique. The resulting parameters are well constrained by the data. The heavy ion species was assumed to be either sulfur or oxygen of unknown charge. The light species was assumed to be protons. The bulk flow speeds deduced from the model were found, contrary to some theories, to increase with increasing radial distance from Jupiter within the radial region addressed, remaining a substantial fraction (∼0.6) of the rigid corotation speed. Agreement with the averaged Voyager Plasma Science (PLS) results was obtained near 30 RJ. The core Maxwellian temperature of the heavy ion distribution functions (∼30–100 keV) increased with increasing radial distance, following the trend anticipated from the corotation pickup of heavy ions. The proton temperature (∼20 keV) remained nearly constant.

The origin of high-energy cosmic rays

FERMI'S suggestion1 that a population of low-energy charged particles could be accelerated by momentum exchange with 'wandering' galactic magnetic fields has long been a favoured mechanism for the generation of cosmic rays. A difficulty with this idea, however, recognized by Fermi2 and others3–5, is that unless the scattering centres of strong magnetic fields have velocities close to the speed of light, the acceleration mechanism is thought to be too slow to produce the highest-energy (~~ 1020 eV) cosmic rays before particles escape from the Galaxy. We argue here that this problem is not intrinsic to the Fermi theory, but results only from approximating the essentially stochastic acceleration process as a deterministic one, in which particles diffuse upwards in energy at some mean rate. The stochastic treatment we present here suggests that the particles that acquire the highest energies are those on the wings of the statistical energy distribution, which proceed not in a pedestrian manner but are accelerated much more effectively as 'high flyers'.