Kenneth R. Fuller

The Lunar Prospector gamma-ray and neutron spectrometers

Gamma-ray and neutron spectrometers (GRS and NS, respectively) are included in the payload complement of Lunar Prospector (LP) that is currently orbiting the Moon. Specific objectives of the GRS are to map abundances of O, Si, Fe, Ti, U, Th, K, and perhaps, Mg, Al, and Ca, to depths of about 20 cm. Those of the NS are to search for water ice to depths of about 50 cm near the lunar poles and to map regolith maturity. The designs of both spectrometers are described and their performance in both the laboratory and in lunar orbit are presented. ( 1999 Elsevier Science B.V. All rights reserved.

Gamma-ray and neutron spectrometer for the Dawn mission to 1 Ceres and 4 Vesta

We present the design of the gamma-ray and neutron spectrometer (GR/NS) for Dawn, which is a NASA Discovery-class mission to explore two of the largest main-belt asteroids, 1 Ceres and 4 Vesta, whose accretion is believed to have been interrupted by the early formation of Jupiter. Dawn will determine the composition and structure of these protoplanetary bodies, providing context for a large number of primitive meteorites in our sample collection and a better understanding of processes occurring shortly after the onset of condensation of the solar nebula. The Dawn GR/NS design draws on experience from the successful Lunar Prospector and Mars Odyssey missions to enable accurate mapping of the surface composition and stratigraphy of major elements, radioactive elements, and hydrogen at both asteroids. Here, we describe the overall design of the GR/NS and compare the expected performance of the neutron spectrometer subsystem to the neutron spectrometer on Mars Odyssey. We also describe radiation damage studies carried out on CdZnTe detectors, which will be components of the primary gamma-ray spectrometer on Dawn. We conclude that provisions for annealing at moderate temperatures (40/spl deg/C to 60/spl deg/C) must be made to ensure that the spectrometer will function optimally over the nine-year mission.

CdZnTe gamma ray spectrometer for orbital planetary missions

We present the design and analysis of a new gamma ray spectrometer for planetary science that uses an array of CdZnTe detectors to achieve the detection efficiency needed for orbital measurements. The use of CdZnTe will provide significantly improved pulse height resolution relative to scintillation-based detectors, with commensurate improvement in the accuracy of elemental abundances determined by gamma ray and neutron spectroscopy. The spectrometer can be flown either on the instrument deck of the spacecraft or on a boom. For deck-mounted systems, a BGO anticoincidence shield is included in the design to suppress the response of the CdZnTe detector to gamma rays that originate in the spacecraft. The BGO shield also serves as a backup spectrometer, providing heritage from earlier planetary science missions and reducing the risk associated with the implementation of new technology.

Mapping the elemental composition of Ceres and Vesta: Dawn’s gamma ray and neutron detector

Dawn is a NASA discovery mission that will explore the main belt asteroids (1) Ceres and (4) Vesta. Ceres and Vesta are among the oldest bodies in the solar system and represent very different evolutionary paths. By studying these ancient, complementary asteroids, we will answer fundamental questions about the early solar system and planetary formation processes. The Dawn payload consists of a Framing Camera (FC), a visual and infrared mapping spectrometer (VIR), and a Gamma Ray and Neutron Detector (GRaND). The instruments provide data needed to investigate the structure, geology, mineralogy, and geochemistry of the asteroids. GRaND provides the data for the geochemistry investigation, including maps of most major elements and selected radioactive and trace elements. An updated description of the GRaND instrument is given along with the expected performance of GRaND at Vesta and Ceres. Approaches to combine data from FC, VIR and GRaND are discussed.

A validation payload for space and atmospheric nuclear event detection

We describe an experimental flight validation payload for detecting atmospheric and space nuclear events with a planned launch date in 2004. The five detector subsystems in the payload employ 27 sensors including Si, CdZnTe, gas proportional counter tubes, photomultiplier tubes, channel electron multipliers and photodiodes. Detection of events is based on simultaneous measurements of gamma rays, neutrons, and charged particles with wide dynamic ranges of deposited energy and count rates. The sensors and electronics are housed in one package with approximate mass and power consumption of 27 kg and 50 watts, respectively. The instrument uses sophisticated on-board digital signal processing and multi-layer triggering algorithms to detect and assess the validity of small signals in a large background radiation environment. This paper presents system configuration and preliminary test data from the first of the two units in development.

Analysis and design methodology for the development of optimized direct detection CO2 DIAL receivers

The analysis methodology and corresponding analytical tools for the design of optimized, low-noise, hard target return CO2 differential absorption lidar (DIAL) receiver systems implementing both single element detectors and multi-pixel imaging arrays for passive/active, remote-sensing applications are presented. System parameters and components composing the receiver include: aperture, focal length, field of view, cold shield requirements, image plane dimensions, pixel dimensions, pixel pitch and fill factor, detection quantum efficiency, optical filter requirements, amplifier and temporal sampling parameters. The performance analysis is accomplished by calculating the systems CO2 laser range response, total noise, optical geometric form factor and optical resolution. The noise components include speckle, photon noise due to signal, scene and atmospheric background, cold shield, and electronic noise. System resolution is simulated through cascaded optical transfer functions and incudes effects due to atmosphere, optics, image sampling, and system motion. Experimental results of a developmental single-element detector receiver designed to detect 100 ns wide laser pulses (10 - 100 kHz pulse repetition rates) backscattered from hard- targets at nominal ranges of 10 km are presented. The receiver sensitivity is near-background noise limited, given an 8.5 - 11.5 micrometer radiant optical bandwidth, with the total noise floor spectrally white for maximum pulse averaging efficiency.

Low-noise detector and amplifier design for 100 ns direct detection CO{sub 2} LIDAR receiver

The development and test results of a prototype detector/amplifier design for a background limited, pulsed 100 ns, 10 - 100 kHz repetition rate LIDAR/DIAL receiver system are presented. Design objectives include near-matched filter detection of received pulse amplitude and round trip time-of- flight, and the elimination of excess correlated detector/amplifier noise for optimal pulse averaging. A novel pole-zero cancellation amplifier, coupled with a state-of-the- art SBRC (Santa Barbara Research Center) infrared detector was implemented to meet design objectives. The pole-zero cancellation amplifier utilizes a tunable, pseudo-matched filter technique to match the width of the laser pulse to the shaping time of the filter for optimal SNR performance. Low frequency correlated noise, (1/f and drift noise) is rejected through a second order high gain feedback loop. The amplifier also employs an active detector bias stage minimizing detector drift. Experimental results will be provided that demonstrate near-background limited, 100 ns pulse detection performance given an 8.5 - 11.5 micrometer (300 K B.B.) radiant background, with the total noise floor spectrally white for optimal pulse averaging efficiency.