Channel electron multiplier (CEM) and microchannel plate (MCP) detectors are routinely used in space instrumentation for measurement of space plasmas. Our goal is to understand the relative sensitivities of these detectors to penetrating radiation in space, which can generate background counts and shorten detector lifetime. We use 662 keV γ-rays as a proxy for penetrating radiation such as γ-rays, cosmic rays, and high-energy electrons and protons that are ubiquitous in the space environment. We find that MCP detectors are ~ 20 times more sensitive to 662 keV γ-rays than CEM detectors. This is attributed to the larger total area of multiplication channels in an MCP detector that is sensitive to electronic excitation and ionization resulting from the interaction of penetrating radiation with the detector material. In contrast to the CEM detector, whose quantum efficiency εγ for 662 keV γ-rays is found to be 0.00175 and largely independent of detector bias, the quantum efficiency of the MCP detector is strongly dependent on the detector bias, with a power law index of 5.5. Background counts in MCP detectors from penetrating radiation can be reduced using MCP geometries with higher pitch and smaller channel diameter.

Recent advances in solid-state detector (SSD) technology have demonstrated the detection of ions and electrons down to 1 keV. However, ions at keV energies lose a substantial amount of energy ΔN in a SSD through Coulombic interactions with target nuclei rather than through interactions that contribute to the SSD output pulse, whose magnitude is a measure of the ion’s incident energy. Because ΔN depends on the ion species, detector material, and interaction physics, it represents a fundamental limitation of the output pulse magnitude of the detector. Using 100% quantum collection efficiency silicon photodiodes with a thin (40–60 Å) SiO2 passivation layer, we accurately quantify ΔN for incident 1–120 keV ions and, therefore, evaluate the detection limits of keV ions using silicon detectors.

Using silicon photodiodes with an ultrathin passivation layer, the average total energy lost to silicon target electrons (electronic stopping) by incident low energy ions and the recoil target atoms they generate is directly measured. We find that the total electronic energy deposition and the ratio of the total nuclear to electronic stopping powers for the incident ions and their recoils each follow a simple, universal representation, thus enabling systematic prediction of ion-induced effects in silicon. We also observe a velocity threshold at 0.05 a.u. for the onset of electronic stopping.

We measure the response of silicon photodiodes to irradiation by H+, He+, C+, N+, O+, Ne+, and Ar+ ions with energies up to 60 keV. The unique properties of these photodiodes, including an ultrathin SiO2 dead layer and 100% internal carrier collection efficiency, allow direct measurement of the total energy lost to nuclear (nonionizing) and electronic (ionizing) energy loss processes, which are important for quantifying effects such as damage and charge deposition. When plotted as a function of E/mZ(1/2), where E, m, and Z are the incident ion energy, mass, and atomic number, respectively, the responsivity is found to follow a single curve that represents all ion species and energies used in this study. This enables rapid, accurate estimation of damage and charge deposition by an ion as a function of penetration depth in silicon. A comparison of the measurements with the stopping and range of ions in matter (SRIM) Monte Carlo simulation code shows that SRIM significantly overestimates the fraction of the incident energy lost to electronic stopping processes for E/mZ(1/2) < 2 keV/amu.

He- is observed in the exit charge state distributions of He transmitted through a thin carbon foil over an energy range of 8-80 keV. The observed exit fraction of He- must be formed in the bulk foil or within several Angstroms of the exit surface and reaches a maximum of 2.6 x 10(-4) at 0.7 upsilon (0) where upsilon (0) is the Bohr velocity. The probability of forming the observed He- from its source population of He+ by two sequential electron-capture events decreases exponentially with increasing velocity. This behavior is similar to that of He transiting a Cs vapor, for which this dependence is the result of individual cross sections that collectively drive the charge-state distribution toward higher positive-charge states with increasing projectile velocity. No isotope effect in the exit charge state distributions of He-3 and He-4 is observed within experimental error.

We have measured energy distributions of electrons emitted from clean aluminum surfaces by 20 keV/amu H+, He+ and He0 projectiles. Ratios of energy distributions obtained from two types of projectiles carry information on differences in the electron excitation collisions. We discuss these differences in terms of screening of the projectile by valence electrons, Auger capture and projectile ionization, with the aid of linear response theory. We find the shapes of secondary electron energy spectra are dominated by the energy dependence of the electron escape depth and of the binary ion–electron interaction. The latter must be determined by considering the screened potential of the projectile and its changing charge state as it penetrates the solid.

We have studied the excitation of plasmons produced by 100 eV He+, Ne+ and Ar+and by 5–100 keV H+ and He+ projectiles in Al and Mg through the observation of electrons from plasmon decay, ejected from clean and cesiated surfaces. At low velocities, plasmon excitation occurs only for ions of high potential energy and is independent of velocity. The effect of Cs adsorption on this potential plasmon-excitation mechanism on Al surfaces suggests that the excited plasmons are not bulk plasmons, as was assumed previously, but short-wavelength surface plasmons. For ions moving faster than a threshold velocity vth ∼1.3 vFermi predicted by electron gas theories, kinetic plasmon excitation can occur because the valence electrons cannot respond instantaneously to screen the moving charge. We found that, contrary to theoretical expectations, plasmon excitation by H+ and He+ projectiles occurs below vth. With the aid of a simple model, we suggest that this sub-threshold excitation results from energetic secondary electrons.

Analyses of planetary surfaces have traditionally relied on remote and in-situ studies to provide compositional information. Remote observations from orbiting spacecraft provide spatially wide coverage, but often do not provide the spatial resolution necessary for understanding detailed geological origin and evolution issues. Conversely, in-situ studies can provide detailed information, but typically have limited scope and can suffer from limited access to remote areas. Laser-Induced-Plasma Ion Mass Spectrometry (LIMS) offers an opportunity to perform isotopic and elemental analysis of planetary surfaces at stand-off distances (10-100 m) without requiring sample manipulation arms or sampling chambers. The technique uses an advanced ion mass spectrometer to obtain mass and energy spectra from the ionized plume produced by a high intensity (1x10 W/cm) laser pulse, and can measure the mass of atomic ions independent of the incoming charge state or incident energy. As a stand off technique, LIMS represents a highly unique and complementary tool for surface analysis. With LIMS, each laser pulse can produce an entire mass spectrum, so it is possible to perform rapid analysis at numerous locations over a wide area around the rover. Also, information can be gathered from areas not directly accessible by rovers such as high rocks or sheer cliff faces. Perhaps the most profound advantage of LIMS over stand-off spectroscopies such as infrared absorption and reflectance is the ability of the laser to interrogate pristine surfaces by using multiple laser-shots in the same place, thereby removing outer surfaces that differ from the bulk material due to weathering. With minimal additional resources, LIMS can be used in concert with Laser Induced Breakdown Spectroscopy (LIBS) [1] or standoff laser Raman spectroscopy [2] to provide complementary composition information. In particular, LIMS has the potenital for high mass resolution, enabling precise isotopic measurements while other stand-off instruments give mineralogical or elemental composition. This capability is attained by measuring the ion time of flight through a patented linear electric field segment [3]. This avoids the energy resolution limitations suffered by previous laser ablation, timeof-flight techniques based on the time of arrival of the ion flux after the laser-pulse (e.g. Managadze and Shutyaev, 1993 [4].) Our design has the added advantage of distinguishing ions from potentially interfering molecular isobars. Since such LIMS studies have been exceedingly sparse, we have begun this study by assembling a test system consisting of two vacuum systems joined by a 3 m long ion conduit. One vacuum system contains the laser target, while the other contains an electrostatic energy analyzer (ESA). The preliminary use of a simple ESA allows us to carefully measure the laserinduced plasma properties' dependence on parameters such as target material, matrix effects, pulse duration and focus, and surface roughness. Later analyses wil use modified IMS (Cassini) and/or PEPE (DS-1) spectrometers to obtain mass spectra independent of energy. Initial results will be presented.

We report energy distributions of electrons emitted from Al and Mg surfaces bombarded by 3-100-keV/amu hydrogen and deuterium ions. The energy spectra contain structure consistent with the decay of bulk plasmons, even below the velocity threshold expected from current theories. To explain the results we compare the importance of additional mechanisms involving electron capture, lattice-assisted excitation, and excitation by fast secondary electrons.

Introduction. Planetary analyses have traditionally relied on both remote and in-situ observations. Although remote observations from orbiting spacecraft provide spatially wide coverage, they do not, by themselves, provide sufficient information to understand geologic processes. On the other hand, rover-based in-situ analyses can provide detailed geological information but can be time-consuming and limited in scope. The optimum configuration for detailed analysis with wide spatial coverage may be a stand-off technique.

We have measured yields of electrons emitted in the forward and backward directions from ultrathin carbon foils due to 10–100 keV atomic and molecular projectiles. In general, electron yields are higher in the forward than in the backward direction. Their behavior with projectile type and energy can be explained by a competition of a forward peaked angular distribution of initial ionization events and elastic collisions that tend to randomize electron motion in the foil. Experiments with ions with atomic number Z51 – 10 indicate that heavy projectiles produce less electron emission per amount of deposited energy by the projectile near the surface. This is attributed to a larger fraction of low-energy electrons produced by heavy projectiles in the primary ionization event that cannot surmount the surface barrier. For incident molecules, the backward electron yield is less than the sum of the yields of the constituent atoms. Neutral atoms with relatively low ionization potentials produce higher electron yields in the backward direction than the ions of the same species for incident energies above about 5 keV/amu, which is attributed to electron loss from the projectile. @S0163-1829~98!07629-2#

We measure the change in the response of 100% internal carrier collection efficiency silicon photodiodes having 60 Angstrom SiO2 passivation layers due to the damage induced by bombardment with 10-60 keV ions of H, He, N, Ne, and Ar. We find an initially exponential decrease in responsivity with increasing ion fluence Phi and use this to define a damage constant beta. The correlation of beta with the nuclear stopping power of the incident ion instead of the with the total energy lost to nuclear stopping indicates that damage in a channel lying within the n-type silicon near the Si-SiO2 interface dominates the radiation-induced change in the photodiode response. We use a fluid model of electron transport in the channel to derive a universal curve to describe the damage as a function of ion fluence and to show that the damage constant beta is proportional to the damage cross section. Over the energy range of this study, damage cross sections of N+, Ne+, and Ar+ are 10-100 times that of He+, and similar to 1000 times that of H.

Electron irradiation of 100% internal quantum efficiency silicon photodiodes having a thin (60 /spl Aring/) SiO/sub 2/ dead layer results in measured responsivities ranging from 0.056 A/W at an incident electron energy E/sub 0/=0.2 keV to 0.24 A/W at E/sub 0/=40 keV. A theoretical electron-hole pair creation energy of 3.71 eV, in close agreement with other studies, is derived using a Monte Carlo simulation of electron interactions with the photodiode over an energy range of 1 to 40 keV. Analysis of electron energy lost to processes that do not contribute to electron-hole pair creation shows that the energy lost in the SiO/sub 2/ dead layer is dominant for E/sub 0/<1.5 keV, whereas the energy removed by backscattered electrons is dominant for E/sub 0/>1.5 keV. At E/sub 0/=300 eV, the Monte Carlo simulation results show that the electron projected range is significantly less than the dead layer thickness even though the measured response is 0.082 A/W, indicating that electron-hole pairs generated in the oxide dead layer are collected by the junction.

Silicon photodiodes with only a 60 {angstrom} SiO{sub 2} front window are used in fusion and space research for detection of XUV photons with high quantum efficiency. In these environments, plasma ions and electrons can deposit energy in the active layer of the photodiode and influence the XUV measurement. Here, we characterize the response of these photodiodes to bombardment of 1-20 keV electrons and 30 keV H, H{sub 2}, Ne, and Ar. For electrons, the responsivity is 0.24 A/W, inferring an electron-hole pair creation energy of 4.2 eV. The measured responsivity to 30 keV H{sub 2}{sup +} is approximately 0.2 A/W, corresponding to an electron- hole pair creation energy of 5 eV. Photodiode damage due to ion bombardment is observed though an exponential decrease of the responsivity with ion dose. The decrease in responsivity is more rapid with increasing ion mass. Annealing of damage induced by heavy ions is observed by subsequent bombardment with protons.