William A. Mahoney

The HEAO 3 gamma-ray spectrometer

Abstract The Third High Energy Astronomy Observatory (HEAO 3), successfully launched into low earth orbit on 20 September 1979, carries a large high resolution gamma-ray spectrometer designed for cosmic nuclear spectroscopy. This Gamma-Ray Spectrometer (the HEAO C-1 experiment) consists of a cluster of four coaxial high purity germanium detectors, each with a volume of approximately 100 cm 3 . Surrounding the germanium detectors is a 6.6 cm thick CsI shield operating in active anticoincidence with the central detectors and defining a field of view of about 30° fwhm. An initial energy resolution of 3 keV fwhm at 1.46 MeV was achieved for each detector. All valid events in the germanium detectors are individually analyzed by an 8192 channel pulse area analyzer and transmitted at a maximum rate of 15.6 events/s for each detector. During a 6 month mission, the experiment will perform a complete sky survey for narrow cosmic gamma-ray line emission to the sensitivity level of about 10 −4 photons/cm 2 s over an operating energy range of 0.05–10 MeV.

High spectral resolution measurement of gamma ray lines from the Earth's atmosphere

A search for gamma ray line features from the Earths atmosphere has been conducted using data from the third High Energy Astronomy Observatory (HEAO 3) high spectral resolution gamma ray spectrometer. In addition to the strong line at 0.511 MeV, other intrinsically broadened line features have been observed at 1.63, 2.31, 3.67, 4.43, 5.09, and 6.13 MeV. Since the spectral resolution of the instrument is much finer than the width of the observed line features, the intrinsic width as well as the energy and intensity of each of these lines are reported. Several other predicted lines have also been observed. The characteristics of the lines seen by HEAO 3 are generally consistent with theoretical predictions as well as with previous measurements.

Radiation damage of the HEAO C-1 germanium detectors

Abstract The effects of radiation damage from proton bombardment of the four HEAO C-1 high purity germanium detectors have been measured and compared to predictions. Because of the presence of numerous gamma-ray lines in the detector background spectra and because of the relatively long exposure time of the HEAO 3 satellite to cosmic-ray and trapped protons, it has been possible to measure both the energy and time dependence of radiation damage. After 100 d in orbit, each of the four detectors had been exposed to approximately 3 × 10 7 protons/cm 2 and the average energy resolution at 1460 keV had degraded from 3.1 keV fwhm to 8.6 keV fwhm. The lines were all broadened to the low energy side although the line profile was different for each of the four detectors. The damage-related contribution to the degradation in energy resolution was found to be linear in energy and proton fluence.

Spitzer warm mission transition and operations

Following the successful dynamic planning and implementation of IRAC Warm Instrument Characterization activities, transition to Spitzer Warm Mission operations has gone smoothly. Operation teams procedures and processes required minimal adaptation and the overall composition of the Mission Operation System retained the same functionality it had during the Cryogenic Mission. While the warm mission scheduling has been simplified because all observations are now being made with a single instrument, several other differences have increased the complexity. The bulk of the observations executed to date have been from ten large Exploration Science programs that, combined, have more complex constraints, more observing requests, and more exo-planet observations with durations of up to 145 hours. Communication with the observatory is also becoming more challenging as the Spitzer DSN antenna allocations have been reduced from two tracking passes per day to a single pass impacting both uplink and downlink activities. While IRAC is now operating with only two channels, the data collection rate is roughly 60% of the four-channel rate leaving a somewhat higher average volume collected between the less frequent passes. Also, the maximum downlink data rate is decreasing as the distance to Spitzer increases requiring longer passes. Nevertheless, with well over 90% of the time spent on science observations, efficiency has equaled or exceeded that achieved during the cryogenic mission.

Spitzer operations: scheduling the out years

Spitzer Warm Mission operations have remained robust and exceptionally efficient since the cryogenic mission ended in mid-2009. The distance to the onow exceeds 1 AU, making telecommunications increasingly difficult; however, analysis has shown that two-way communication could be maintained through at least 2017 with minimal loss in observing efficiency. The science program continues to emphasize the characterization of exoplanets, time domain studies, and deep surveys, all of which can impose interesting scheduling constraints. Recent changes have significantly improved on-board data compression, which both enables certain high volume observations and reduces Spitzers demand for competitive Deep Space Network resources.

Lessons learned in extended-extended Spitzer Space Telescope operations

The Spitzer Space Telescope is executing the ninth year of extended operations beyond its 5.5-year prime mission. The project anticipated a maximum extended mission of about four years when the first mission extension was proposed. The robustness of the observatory hardware and the creativity of the project engineers and scientists in overcoming hurdles to operations has enabled a substantially longer mission lifetime. This has led to more challenges with an aging groundsystem due to resource reductions and decisions made early in the extended mission based on a shorter planned lifetime. We provide an overview of the extended mission phases, challenges met in maintaining and enhancing the science productivity, and what we would have done differently if the extended mission was planned from the start to be nearly twice as long as the prime mission.

Spitzer Mission Operation System Planning for IRAC Warm-Instrument Characterization

‡This paper will describe how the Spitzer Mission Operations System planned and executed the characterization phase between Spitzer’s cryogenic mission and its warm mission. To the largest extend possible, the execution of this phase was done with existing processing and procedures. The modifications that were made were in response to the differences of the characterization phase compared to normal phases before and after. The primary two categories of difference are: unknown date of execution due to uncertainty of knowledge of the date of helium depletion, and the short cycle time for data analysis and re-planning during execution. In addition, all of the planning and design had to be done in parallel with normal operations, and we had to transition smoothly back to normal operations following the transition. This paper will also describe the re-planning we had to do following an anomaly discovered in the first days after helium depletion.

The nuclear astrophysics explorer

The Nuclear Astrophysics Explorer (NAE) is a concept for a possible future NASA Explorer mission which would obtain high resolution, E/ΔE∼500, observations of gamma‐ray lines in order to study many fundamental problems in astrophysics. It operates from 15 keV to 10 MeV with a 3σ sensitivity of ∼3×10−6 ph/cm2‐s in a 106 s observation. This is 100 times below the presently known gamma‐ray line fluxes. The NAE uses a heavily shielded array of 9 cooled Ge detectors in a very low background configuration. Its 10° field of view contains a versatile coded mask system which provides 2‐D imaging with 4° resolution, 1‐D imaging with 2° resolution and efficient measurements of emission from diffuse and point sources. The late 1990’s is the earliest the NAE mission could begin. The scientific motivation, instrument concept, mission concept and expected results, and status and plans for the NAE are presented.

Gamma-ray burst observations by the HEAO-3 high-resolution gamma-ray spectrometer

Observations of cosmic gamma‐ray bursts with the JPL High‐Resolution Gamma‐Ray Spectrometer on HEAO‐3 are discussed. Two bursts seen on 1979 November 16 are of particular interest. The first event occurred at 14:16:41 UT and lasted for eight seconds. This event was detected only by the instrument’s five CsI shield segments. The second event occurred 61 sec later, at 14:17:42 UT, and lasted 18 sec. This event was clearly detected by the high‐resolution germanium detectors. Because of the close temporal coincidence of the two events, we consider the possibility that both originated from one celestial source, and that the scanning motion of HEAO, 3 (nominal spin period 20 min) was such as to exclude the first from the (approximately 30° FWHM) field of view of the germanium detectors and include the second. Directional information from the relative response of the shield pieces and Earth occulation constraint are all consistent with this interpretation and high‐resolution spectral data from the second burst a...