|
We describe a new imager suitable for measurements of magnetospheric neutrals with energies from about 100 eV to about 10 keY; an energy range adequate for imaging the plasmasheet neutral atoms out to about 10 RE. The instrument, an outgrowth of a study of atom-surface collisions in support of satellite drag calculations, separates incident photons from neutral atoms by surface scattering and conversion of the neutrals to ions. Subsequently, the. ions formed on the first surface are accelerated through a light rejection section which also disperses the ions according to energy. The dispersed ion beam is then allowed to impact a second surface where a start pulse is generated to obtain ion velocity and energy/charge. The second surface is chosen to give large secondary electron emission without regard to charge state of the particles reflected from it. The reflected particles are detected a second and final time in a position sensitive detector as drift in a field free region. This last detection generates the stop pulse for time-of-flight (velocity) determination. The data supporting the proposed ILENA design is presented in the first part of the paper. INTRODUCTION Global imaging of energetic magnetospheric ions (H and O) has been demonstrated recent1y1'2'. Low energy atom imaging with Carbon foils has been demonstrated in the laboratory for energies down to 1 keY , wherethe sensitivity becomes very small. The question arises whether atom-surface scattering may offer a viable detection mechanism for time-of-flight spectrometry of low energy neutral atoms and whether this might provide a lower energy limit for detection, perhaps near 100 eV, with good energy resolution (small straggling effects). This paper reviews a number of neutral atomsurface collision properties and the design of an imager of low energy neutral atoms (ILENA) that follows from these properties. ILENA differs from other approaches in that it depends on the reflection of the incident atoms from a surface. Hence, it is important to demonstrate (a) that neutral atoms are scattered into a narrow lobe (quasi-specular reflection), (b) that a significant fraction of the reflected particles change charge (positive or negative), and (c) that the secondary electron yield at the surface is sufficiently large to ensure high detection efficiency. We have begun such studies at our laboratory and report here on the quasi-specular character of the scattering at energies below 100 eV. We review older laboratory data which indicate that items (a), (b) and (c) above are favorable for energies from <100eV up to 10 keV or more. The specific application to ILENA is deserving of immediate attention. The data reveals that the main advantage of ILENA is the extension of the energy range of neutral atom imaging to the vicinity of 100 32/SPIE Vol. 1744 Instrumentation for Magnetospheric Imagery (1992) ° 19401 70/92/$4.OO IMAGER OF LOW ENERGY NEUTRAL ATOMS (ILENA): IMAGING NEUTRALS FROM THE MAGNETOSPHERE AT ENERGiES BELOW 20 KEy Federico A. Herrero and Mark F. Smith Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center Greenbelt, Maryland 20771 Mailing address: Code 696, NASA Goddard, Greenbelt, MD 20771 Tel.(301)-286-5660, FAX (301)-286-9240 ABSTRACT We describe a new imager suitable for measurements of magnetospheric neutrals with energies from about 100 eV to about 10 keY; an energy range adequate for imaging the plasmasheet neutral atoms out to about 10 RE. The instrument, an outgrowth of a study of atom-surface collisions in support of satellite drag calculations, separates incident photons from neutral atoms by surface scattering and conversion of the neutrals to ions. Subsequently, the ions formed on the first surface are accelerated through a light rejection section which also disperses the ions according to energy. The dispersed ion beam is then allowed to impact a second surface where a start pulse is generated to obtain ion velocity and energy/charge. The second surface is chosen to give large secondary electron emission without regard to charge state of the particles reflected from it. The reflected particles are detected a second and final time in a position sensitive detector á.s drift in a field free region. This last detection generates the stop pulse for time-of-flight (velocity) determination. The data supporting the proposed ILENA design is presented in the first part of the paper.
|
