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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~\hbox{keV} \, \gamma $ -rays as a proxy for penetrating radiation such as $\gamma $ -rays, cosmic rays, and high-energy electrons and protons that are ubiquitous in the space environment. We find that MCP detectors are $\sim 20$ times more sensitive to $662~\hbox{keV} \, \gamma $ -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 ${\varepsilon _\gamma} $ for $662~\hbox{keV} \, \gamma $ -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.

Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) is a NASA Explorer Mission-of-Opportunity to stereoscopically image the Earth’s magnetosphere for the first time. TWINS extends our understanding of magnetospheric structure and processes by providing simultaneous Energetic Neutral Atom (ENA) imaging from two widely separated locations. TWINS observes ENAs from 1–100 keV with high angular (∼4°×4°) and time (∼1-minute) resolution. The TWINS Ly-α monitor measures the geocoronal hydrogen density to aid in ENA analysis while environmental sensors provide contemporaneous measurements of the local charged particle environments. By imaging ENAs with identical instruments from two widely spaced, high-altitude, high-inclination spacecraft, TWINS enables three-dimensional visualization of the large-scale structures and dynamics within the magnetosphere for the first time. This “instrument paper” documents the TWINS design, construction, calibration, and initial results. Finally, the appendix of this paper describes and documents the Southwest Research Institute (SwRI) instrument calibration facility; this facility was used for all TWINS instrument-level calibrations.

We measure the response of silicon photodiodes to irradiation by H/sup +/, He/sup +/, C/sup +/, N/sup +/, O/sup +/, Ne/sup +/, and Ar/sup +/ ions with energies up to 60 keV. The unique properties of these photodiodes, including an ultrathin SiO/sub 2/ 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/sup 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/sup 1/2/

We have measured energy distributions of electrons emitted from clean aluminum surfaces by 20 keV/amu H + , He + and He 0 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 v th ∼1.3 v Fermi 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 v th . With the aid of a simple model, we suggest that this sub-threshold excitation results from energetic secondary electrons.

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.

We measure the change in the response of 100% internal carrier collection efficiency silicon photodiodes having 60 /spl Aring/ SiO/sub 2/ 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 /spl Phi/ and use this to define a damage constant /spl beta/. The correlation of /spl beta/ with the nuclear stopping power of the incident ion instead of with the total energy lost to nuclear stopping indicates that damage in a channel lying within the n-type silicon near the Si-SiO/sub 2/ 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 /spl beta/ is proportional to the damage cross section. Over the energy range of this study, damage cross sections of N/sup +/, Ne/sup +/, and Ar/sup +/ are 10-100 times that of He/sup +/, and /spl sim/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. By comparing the data to a Monte Carlo simulation of electron interactions with the photodiode over an energy range of 1-40 keV, we derive an average electron-hole pair creation energy of 3.71 eV, in close agreement with other studies. 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. 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.

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 A) 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.

Small-angle neutron scattering was used to investigate the microstructure of the high-temperature Lα phase in the AOT/water/decane system [AOT = sodium bis(2-ethylhexyl) sulfosuccinate]. The system was modeled as randomly oriented lamellar stacks with a one-dimensional paracrystalline distortion. For samples injected into cells at high temperature, the two-dimensional Q maps show anisotropic scattering consistent with partial alignment of the phase, while samples injected near room temperature do not exhibit this feature. From the latter samples, it was found that a stack typically contains about 15 lamellae, about 80 A apart. Each layer is coated by a diffuse surfactant interface with a characteristic thickness of approximately 9 A. The fact that the scattering spectra do not exhibit maxima beyond the first order is explained by the observation that the Hosemann g factor gives about a 20% variation in the interlamellar spacing. The mean spacing is inversely proportional to the surfactant concentration, as expected for a lamellar phase. Over-estimation of the zero-order scattering predicted by the paracrystal model is attributed primarily to improper treatment of the contrast between each layer and the space-filling lamellar matrix.

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.

The Medium Energy Neutral Atom (MENA) imager was developed in response to the Imaging from the Magnetopause to the Aurora for Global Exploration (IMAGE) requirement to produce images of energetic neutral atoms (ENAs) in the energy range from 1 to 30 keV. These images will be used to infer characteristics of magnetospheric ion distributions. The MENA imager is a slit camera that images incident ENAs in the polar angle (based on a conventional spherical coordinate system defined by the spacecraft spin axis) and utilizes the spacecraft spin to image in azimuth. The speed of incident ENAs is determined by measuring the time-of-flight (TOF) from the entrance aperture to the detector. A carbon foil in the entrance aperture yields secondary electrons, which are imaged using a position-sensitive Start detector segment. This provides both the one- dimensional (1D) position at which the ENA passed through the aperture and a Start time for the TOF system. Impact of the incident ENA on the 1D position-sensitive Stop detector segment provides both a Stop-timing signal and the location that the ENA impacts the detector. The ENA incident polar angle is derived from the measured Stop and Start positions. Species identification (H vs. O) is based on variation in secondary electron yield with mass for a fixed ENA speed. The MENA imager is designed to produce images with 8 4 angular resolution over a field of view 140 360, over an energy range from 1 keV to 30 keV. Thus, the MENA imager is well suited to conduct measurements relevant to the Earth's ring current, plasma sheet, and (at times) magnetosheath and cusp.

Ultraviolet (UV) blocking filters are needed for atomic flux imaging in environments where high levels of ultraviolet radiation are present. Freestanding gratings are a promising candidate for UV filtering. They have a high aspect ratio (∼13), narrow (∼40 nm) slots, and effectively block UV radiation. The grating fabrication process makes use of several etching, electroplating, and lithographic steps and includes an optional step to plug pinholes induced by particles during processing. Gratings were successfully manufactured and tested. Measured UV transmissions of ∼10−5 and particle transmissions of ∼10% are in agreement with theoretical predictions.