Ronnie W. Harper

Energy loss of 1–50keV H, He, C, N, O, Ne, and Ar ions transmitted through thin carbon foils

Thin foils are used extensively in space plasma spectrometers, for example, for generation of a coincidence measurement via secondary electron emission in time-of-flight mass spectrometers and charge conversion of energetic neutral atoms (ENAs) in ENA imagers. An ion or neutral atom passing through the carbon foil experiences energy loss and straggling that can degrade the energy resolution or mass resolution. These effects need to be quantified in order to permit efficient instrument development and modeling. We present measurements of energy loss and energy loss straggling for H, He, C, N, O, Ne, and Ar ions between 1 and 50keV passing through carbon foils of different thicknesses (nominally between 0.5 and 3.0μg∕cm2). We derive empirical relationships for the energy loss and energy loss straggling as a function of foil thickness, ion species, and ion energy. In particular, below ∼10keV the energy loss of hydrogen seems to vary with foil thickness and the energy loss straggling seems to vary with the sq...

Fundamental limits to detection of low-energy ions using silicon solid-state detectors

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.

Absolute detection efficiency of space-based ion mass spectrometers and neutral atom imagers

Space-based ion mass spectrometers and neutral atom imagers often utilize a thin foil for generation of secondary electrons and employ a coincidence measurement between detection of the secondary electrons and detection of the primary ion or neutral atom, allowing unambiguous detection of the particle in a large noise background and determination of properties of the particle using time-of-flight measurement. We demonstrate a simple and straightforward method for laboratory calibration and in situ quantification and monitoring of the absolute detection probabilities of the detectors and the absolute detection efficiency of the detector subsystem without knowledge of the incident particle flux.

Response of 100% internal carrier collection efficiency silicon photodiodes to low-energy ions

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/<2 keV/amu.

Calculation of IMAGE/MENA geometric factors and conversion of images to units of integral and differential flux

The Medium Energy Neutral Atom (MENA) instrument flown on the NASA IMAGE spacecraft is a time-of-flight neutral particle imager designed to image energetic neutral atom emissions from the Earth’s inner magnetosphere over an energy per mass range of 1–60keV∕amu. Images are generated by combining data from three separate heads and have a nominal angular resolution of 4°×4°. Here, we present a first-principles calculation of the geometric factors for each of the start-byte/stop-byte combinations for each of the three heads in the IMAGE/MENA instrument based on a detailed understanding of the its physical construction. The geometric factors are used to compute combined integral flux images and it is demonstrated that they result in head-to-head matching of the data that are both continuous and physically reasonable. We also discuss several issues associated with energy binning as a means for constructing differential flux images and present a powerful and robust approach that solves several critical problems ...

Damage induced in 100% internal carrier collection efficiency silicon photodiodes by 10-60 keV ion irradiation

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.

A wide field of view plasma spectrometer

Here we present a fundamentally new type of space plasma spectrometer, the wide field of view plasma spectrometer, whose field of view is >1.25π ster using fewer resources than traditional methods. The enabling component is analogous to a pinhole camera with an electrostatic energy-angle filter at the image plane. Particle energy-per-charge is selected with a tunable bias voltage applied to the filter plate relative to the pinhole aperture plate. For a given bias voltage, charged particles from different directions are focused by different angles to different locations. Particles with appropriate locations and angles can transit the filter plate and are measured using a microchannel plate detector with a position-sensitive anode. Full energy and angle coverage are obtained using a single high-voltage power supply, resulting in considerable resource savings and allowing measurements at fast timescales. Lastly, we present laboratory prototype measurements and simulations demonstrating the instrument concept and discuss optimizations of the instrument design for application to space measurements.

Comparative Response of Microchannel Plate and Channel Electron Multiplier Detectors to Penetrating Radiation in Space

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.

New Magnetospheric Ion Composition Measurement Techniques

This paper provides a brief overview of three new instruments designed to measure ion composition in the magnetosphere at energies from a few eV/q to a maximum of 200 keV/q. Ion composition is not currently monitored at geosynchronous orbit and is an important ingredient to understand the dynamic near‐Earth space environment. The unique ion composition measurement techniques in the detector sections following an electrostatic analyzer are the focus of this paper. The first instrument for geosynchronous orbit, the Advanced Miniaturized Plasma Spectrometer (AMPS), exploits the properties of a solid‐state detector to measure ion composition by an E/q x E technique. The Z Plasma Spectrometer (ZPS) design, also for geosynchronous orbit, uses selective filtering by different thicknesses of carbon foil to separate H+ from O+. The third instrument, the Helium Oxygen Proton Electron spectrometer (HOPE), was selected for the upcoming Radiation Belt Storm Probes mission (RBSP) and uses a gated time‐of‐flight (TOF) t...