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Engineering analyses on Surveyor lunar dust particle detector instrumentation, and ground support equipment
Apollo 12 suprathermal ion detector, and mass and energy spectra of positive ions near lunar surface
Observations from Apollo 14 suprathermal ion detector including ionospheric ion density, mass and energy spectra during venting in LM cabin, large ion cloud, and ion resulting from impacts
The techniques and procedures for producing very-large-format pulse-counting array detector systems for use in forthcoming high-energy astrophysics facilities were defined, and the structures and performance characteristics of high-sensitivity photocathodes for use at soft X-ray wavelengths between 100 and 1 A were determined. The progress made to date in each of these areas are described and the tasks that will be undertaken when the program is continued are summarized.
The development and testing of a detector to be used in the Energetic X-ray Imaging Telescope Experiment (EXITE) are reported. It consists of a 34 cm diameter NaI(Tl) crystal coupled directly to a single large image intensifier tube with associated silicon PIN diode readout. The measured spatial and energy resolutions at 122 keV are 6mm (FWHM) and 9 percent (FWHM), respectively. This energy resolution is about 50 percent better than that of any previously flown hard X-ray experiment. These resolutions decrease with the square root of the energy of the incident X-ray, indicating that they are determined by the number of photons emitted in the NaI(Tl) scintillator light flash.
The Lunar Reconnaissance Orbiter's (LRO), Lunar Exploration Neutron Detector (LEND) was developed to refine the lunar surface hydrogen (H) measurements generated by the Lunar Prospector Neutron Spectrometer. LPNS measurements indicated a approx.4,6% decrease in polar epithermal fluxes equivalent to (1.5+/-0,8)% H concentration and are direct geochemical evidence indicating water /high H at the poles. Given the similar operational and instrumental objectives of the LEND and LPNS systems, an important science analysis step for LEND is to test correlation with existing research including LPNS measurements. In this analysis, we compare corrected low altitude epithermal rate data from LPNS available via NASA's Planetary Data System (PDS) with calibrated LEND epithermal maps using a cross-correlation technique
A HgCdTe avalanche photodiode (APD) focal plane array assembly with linear mode photon-counting capability has been developed for space lidar applications. An integrated detector cooler assembly (IDCA) has been built using a miniature Stirling cooler. A microlens array has been included to improve the fill factor. The HgCdTe APD has a spectral response from 0.9- to 4.3-μm wavelengths, a photon detection efficiency as high as 70%, and a dark count rate of <250 kHz at 110 K. The mass of the IDCA is 0.8 kg and the total electrical power consumption is about 7 W. The HgCdTe APD arrays have been characterized at NASA Goddard Space Flight Center. A series of environmental tests have been conducted for the IDCAs, including vibration, thermal cycling, and thermal vacuum tests. A description of the device and the test results at NASA are given in this paper.
The highly directional flow of energetic ions down the magnetosheath is described using three different look directions of the three Apollo 15 SIDE instruments. The effects of the interaction of the LM ascent-engine exhaust with the magnetosheath ions observed at the Apollo 15 site are discussed, and a preliminary analysis of the SIDE data for Apollo 12 and 14 is included.
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A design concept for the receiver portion of a proposed free-space optical-communication terminal calls for integration of its communication and pointing detectors. As explained below, this would entail a departure from prior designs, in which pointing and communication detectors have been separate. As used here, communication detector denotes a single high-speed photodetector used for reception of a laser beam that has been modulated to convey information, while pointing detector denotes an array of photodetectors (typically, a quad-cell detector or a charge-coupled device) used in sensing the pointing error (the error in the aim of a receiver telescope, relative to the laser-beam axis). The pointing detector of this or any free-space optical-communication receiver is necessary for proper acquisition and tracking of the received laser beam. The suitably processed output of the pointing detector is fed back to a fine-steering mirror to reduce any pointing error and thereby maintain optimum reception. Heretofore, it has been common practice to pass the incoming laser beam through a beam splitter that sends about 10 percent of the beam power to a pointing detector and the rest to a separate communication detector, as illustrated in the upper part of the figure. One disadvantage of this is that because only 10 percent of the received signal power is available for use by the pointing detector, the signal-to-noise ratio (SNR) at the pointing detector is lower than it otherwise would be. The performance of the pointing detector is correspondingly limited. Another disadvantage is that the alignment between the communication and pointing detectors is critical and must be ensured by means of a calibration procedure. According to the proposal, there would be no beam splitter. The communication and pointing detectors would be positioned coaxially in the same focal plane, as shown in the lower part of the figure: the communication detector would occupy the central part of the focal plane, while the pointing detector would occupy the surrounding area. This arrangement would inherently ensure the proper alignment of the detectors with each other.
High sensitivity is a basic requirement for a new generation of thermal detectors. To meet the requirement, close-packed, two-dimensional silicon detector arrays have been developed in NASA Goddard Space Flight Center. The goal of the task is to fabricate detector arrays configured with thermal detectors such as infrared bolometers and x-ray calorimeters to use in space fliGht missions. This paper focuses on the fabrication and the mechanical testing of detector arrays in a 0.2 mm pixel size, the smallest pop-up detectors being developed so far. These array structures, nicknamed "PUDS" for "Pop-Up Detectors", are fabricated on I pm thick, single-crystal, silicon membranes. Their designs have been refined so we can utilize the flexibility of thin silicon films by actually folding the silicon membranes to 90 degrees in order to obtain close-packed two-dimensional arrays. The PUD elements consist of a detector platform and two legs for mechanical support while also serving as electrical and thermal paths. Torsion bars and cantilevers connecting the detector platform to the legs provide additional flexures for strain relief. Using micro-electromechanical structure (MEMS) fabrication techniques, including photolithography, anisotropic chemical etching, reactive-ion etching, and laser dicing, we have fabricated PLTD detector arrays of fourteen designs with a variation of four parameters including cantilever length, torsion bar length and width, and leg length. Folding tests were conducted to test mechanical stress distribution for the array structures. We obtained folding yields and selected optimum design parameters to reach minimal stress levels. Computer simulation was also employed to verify mechanical behaviors of PUDs in the folding process. In addition, scanning electron microscopy was utilized to examine the flatness of detectors and the alignment of detector pixels in arrays. The fabrication of thermistors and heaters on the pop-up detectors is under way, preparing us for the next step of the experiment, the thermal test.
Investigators have used infrared devices to accurately measure heated surface temperatures. Several of these applications have been for turbine heat transfer studies involving film cooling and surface roughness, typically, these measurements use an infrared camera positioned externally to the test section. In cascade studies, where several blades are used to ensure periodic flow, adjacent blades block the externally positioned camera's views of the test blade. To obtain a more complete mapping of the surface temperatures, researchers at the NASA Glenn Research Center fabricated a probe with an infrared detector to sense the blade temperatures. The probe size was kept small to minimize the flow disturbance. By traversing and rotating the probe, using the same approach as for total pressure surveys, one can find the blade surface temperatures. Probe mounted infrared detectors are appropriate for measuring surface temperatures where an externally positioned infrared camera is unable to completely view the test object. This probe consists of a 8-mm gallium arsenide (GaAs) lens mounted in front of a mercury-cadmium-zinc-tellurium (HgCdZnTe) detector. This type of photovoltaic detector was chosen because of its high sensitivity to temperature when the detector is uncooled. The particular application is for relatively low surface temperatures, typically ambient to 100 C. This requires a detector sensitive at long wavelengths. The detector is a commercial product enclosed in a 9-mm-diameter package. The GaAs lens material was chosen because of its glass-like hardness and its good long-wavelength transmission characteristics. When assembled, the 6.4-mm probe stem is held in the traversing actuator. Since the entire probe is above the measurement plane, the flow field disturbance in the measurement plane is minimized. This particular probe body is somewhat wider than necessary, because it was designed to have replaceable detectors and lenses. The signal for the detector is fed through the hollow probe body. The detector's signal goes to an externally mounted preamplifier. The detector assembly, along with a preamplifier, is calibrated as a function of the surface temperature for various detector temperatures. The output voltage is a function of both the detector and object temperatures.
For satellite sensor, the striping observed in images is typically associated with the relative multiple detector gain difference derived from the calibration. A method using deep convective cloud (DCC) measurements to assess the difference among detectors after calibration is proposed and demonstrated for select reflective solar bands (RSBs) of the Moderate Resolution Imaging Spectroradiometer (MODIS). Each detector of MODIS RSB is calibrated independently using a solar diffuser (SD). Although the SD is expected to accurately characterize detector response, the uncertainties associated with the SD degradation and characterization result in inadequacies in the estimation of each detector's gain. This work takes advantage of the DCC technique to assess detector uniformity and scan mirror side difference for RSB. The detector differences for Terra MODIS Collection 6 are less than 1% for bands 1, 3-5, and 18 and up to 2% for bands 6, 19, and 26. The largest difference is up to 4% for band 7. Most Aqua bands have detector differences less than 0.5% except bands 19 and 26 with up to 1.5%. Normally, large differences occur for edge detectors. The long-term trending shows seasonal oscillations in detector differences for some bands, which are correlated with the instrument temperature. The detector uniformities were evaluated for both unaggregated and aggregated detectors for MODIS band 1 and bands 3-7, and their consistencies are verified. The assessment results were validated by applying a direct correction to reflectance images. These assessments can lead to improvements to the calibration algorithm and therefore a reduction in striping observed in the calibrated imagery.
Microchannel plate based detectors with cross delay line image readout have been rapidly implemented for the SUMER and UVCS instruments aboard the Solar Orbiting Heliospheric Observatory (SOHO) mission to be launched in July 1995. In October 1993 a fast track program to build and characterize detectors and detector control electronics was initiated. We present the detector system design for the SOHO UVCS and SUMER detector programs, and results from the detector test program. Two deliverable detectors have been built at this point, a demonstration model for UVCS, and the flight Ly alpha detector for UVCS, both of which are to be delivered in the next few weeks. Test results have also been obtained with one other demonstration detector system. The detector format is 26mm x 9mm, with 1024 x 360 digitized pixels, using a low resistance Z stack of microchannel plates (MCP's) and a multilayer cross delay line anode (XDL). This configuration provides gains of approximately 2 x 10(exp 7) with good pulse height distributions (less than 50% FWHM) under uniform flood illumination, and background levels typical for this configuration (approximately 0.6 event cm (exp -2)sec(exp -1)). Local counting rates up to about 400 events/pixel/sec have been achieved with no degradation of the MCP gain. The detector and event encoding electronics achieves about 25 millimeter FVHM with good linearity (plus or minus approximately 1 pixel) and is stable to high global counting rates (greater than 4 x 10(exp 5) events sec(exp -1)). Flat field images are dominated by MCP fixed pattern noise and are stable, but the MCP multifiber modulation usually expected is uncharacteristically absent. The detector and electronics have also successfully passed both thermal vacuum and vibration tests.
Weather satellite instruments require detectors with a variety of wavelengths ranging from the visible to VLWIR. One of the remote sensing applications is the geostationary GOES-ABI imager covering wavelengths from the 450 to 490 nm band through the 13.0 to 13.6 micron band. There are a total of 16 spectral bands covered. The Cross-track infrared Sounder (CrIS) is a Polar Orbiting interferometric sensor that measures earth radiances at high spectral resolution, using the data to provide pressure, temperature and moisture profiles of the atmosphere. The pressure, temperature and moisture sounding data are used in weather prediction models that track storms, predict levels of precipitation etc. The CrIS instrument contains SWIR (lamba(sub c) approximately 5 micron at 98K), MWIR (lambda(sub c) approximately 9 micron at 98K) and LWIRs (lamba(sub c) approximately 15.5 micron at 81K) bands in three Focal Plane Array Assemblies (FPAAs). GOES-ABI contains three focal plane modules (FPMs), (i) a visible-near infrared module consisting of three visible and three near infrared channels, (ii) a MWIR module comprised of five channels from 3.9 micron to 8.6 micron and (iii) a 9.6 micron to 13.3 micron, five-channel LWIR module. The VNIR FPM operates at 205 K, and the MWIR and LWIR FPMs operate at 60 K. Each spectral channel has a redundant array built into a single detector chip. Switching is thus permitted from the primary selected array in each channel to the redundant array, given any degradation in performance of the primary array during the course of the mission. Silicon p-i-n detectors are used for the 0.47 micron to 0.86 micron channels. The thirteen channels above 1 micron are fabricated in various compositions of Hg1-xCdxTe, and in this particular case using two different detector architectures. The 1.38 micron to 9.61 micron channels are all fabricated in Hg1-xCdxTe grown by Liquid Phase Epitaxy (LPE) using the HDVIP detector architecture. Molecular beam epitaxy (MBE)-grown Hg1-xCdxTe material are used for the LWIR 10.35 micron to 13.3 micron channels fabricated in Double layer planar heterostructure (DLPH) detectors. This is the same architecture used for the CrIS detectors CrIS detectors are 850 micron diameter detectors with each FPAA consisting of nine photovoltaic detectors arranged in a 3 x 3 pattern. Each detector has an accompanying cold preamplifier. SWIR and MWIR FPAAs operate at 98 K and the LWIR FPAA at 81 K, permitting the use of passive radiators to cool the detectors. D* requirements at peak wavelength are ≥ 5.0E+10 Jones for LWIR, ≥ 9.3E+10 Jones for MWIR and ≥ 3.0E+11 Jones for SWIR. All FPAAs exceeded the D* requirements. Measured mean values for the nine photodiodes in each of the LWIR, MWIR and SWIR FPAAs are D* = 5.3 x 10(exp 10) cm-Hz(exp 1/2)/W at 14.0 micron, 1.0 x 10(exp 11) cm-Hz(exp 1/2)/W at 8.0 micron and 3.1 x 10(exp 11) cm-Hz(exp 1/2)/W at 4.64 micron.
The critical problem in high-resolution cosmic gamma-ray spectroscopy in the energy range from 0.02 to 10 MeV is the limited spectral sensitivity of the detectors used. This results from the small effective area of the detectors and the high background noise due to induced radioactivity and scattering in the detectors' high-energy particle environment. The effective area can be increased by increasing the number of detectors, but this becomes prohibitive because of the size and expense of the resulting instrument. We have taken a new approach: a segmented large-volume germanium gamma-ray detector which can effectively discriminate against internal background yet maintain the high spectral resolution and efficiency of conventional coaxial Ge detectors. To verify this concept, a planar detector divided into two segments has been fabricated and laboratory measurements agree well with Monte Carlo calculations. A large coaxial detector which will be divided into five segments is being built using the techniques developed for the planar detector. Monte Carlo calculations show that the sensitivity (minimum detectable flux) of the segmented coaxial detector is a factor of 2-3 better than conventional detectors because of the reduction in the internal background.