M. Pollard

Observation of SN 1987A with the gamma-ray spectrometer HEXAGONE

The HEXAGONE balloon-borne spectrometer was flown from Alice Springs (Australia) on 1989 May 22. HEXAGONE is a high-resolution gamma-ray spectrometer and consists of an array of twelve cooled germanium detectors (field of view 19° at 511 keV). One of the observed targets was the supernova 1987A and it was seen during 9.9 hr, 818 days after the initial optical outburst. No significant hard X-ray or gamma-ray emission is detected in the final spectrum of SN 1987A

Analysis of Carbon Particle Deposition on Teflon Filters Using Fourier Transform Infrared Spectroscopy

Fourier transform infrared absorption measurements of particulate elemental carbon are compared with results obtained using a thermal combustion method of analysis. Ambient air samples were collected on Teflon and quartz filters on each day during the sampling study. The quartz filters were analyzed for elemental and organic carbon using a thermal combustion method of analysis. The Teflon filters were analyzed for elemental carbon by infrared transmission in the region 650 - 666 cm/sup -1/. Good correlation was found between the infrared absorbance on the Teflon filters and the elemental carbon determined from analysis of the quartz filters. Calibration methods for the infrared technique are discussed.

Mercury emissions from a modified in-situ oil shale retort

Commercial oil-shale production has the potential to release significant amounts of mercury to the atmosphere. Two techniques to measure mercury in oil-shale-retort offgas, Zeeman atomic absorption spectroscopy and gold-bead amalgamation collection and analysis, are discussed and compared. A technique for speciating between organic and atomic forms of Hg is also discussed. The measured mercury emission rates and speciation results are presented.

Fourier Transform Infrared and Ion-Chromatographic Sulfate Analysis of Ambient Air Samples

Fourier transform infrared (FTIR) spectrometry has been evaluated as a method for determining the sulfate concentration of ambient aerosol particulate samples. Samples were collected on Teflon filters. The filters were analyzed for sulfate by both FTIR and ion chromatography (IC). There is good agreement between sulfate analysis by IC and analysis of the FTIR transmission spectra of the air filters during the first 5 1/2 days of the Carbonaceous Species Methods Intercomparison Study in which the ambient sulfate concentrations were above the 5.8 μg/cm2 lower limit of detection of the FTIR technique. A method to improve the FTIR lower limit of detection is discussed. The difficulties incurred during background subtraction of the infrared spectra are described.

Germanium detector vacuum encapsulation

The encapulation of germanium detectors has been a long sought after goal. We have begun to develop encapsulation technology that should significantly improve the viability of germanium gamma‐ray detectors for a number of important applications. A specialized vacuum chamber has been constructed in which the detector and the encapsulating module are processed in high vacuum. Very high vacuum conductance is achieved within the valveless encapsulating module. The detector module is then sealed without breaking the chamber vacuum. The details of the vacuum chamber, valveless module, processing, and sealing method are presented in the paper.

An observation of the Galactic center region with the HEXAGONE high resolution gamma‐ray spectrometer

The galactic center region was observed for 6 hours on 22 May 1989 from a high altitude balloon with the HEXAGONE high resolution gamma-ray spectrometer. The instrument had a 285 cm{sup 2} array of cooled germanium detectors with an energy resolution of 2.2 keV at 511 keV and an 18{degree} FWHM field of view. 511 keV gamma-rays from electron-positron annihilation and 1809 keV gamma-rays from the radioactive decay of {sup 26}Al were observed to have fluxes of 8.9{times}10{sup {minus}4} and 1.9{times}10{sup {minus}4} ph/cm{sup 2}-s, respectively. Continuum emission was detected from 20 to 800 keV and preliminary results have been obtained for the spectrum. Below 120 keV this is well described by power law with a slope of {minus}2.6. In the 120--250 keV band the spectrum contains a broad line-like feature with a flux of (2 to 6){times}10{sup {minus}3} ph/cm{sup 2}-s, depending on the assumed underlying continuum. This is interpreted as the result of Compton backscattering of {similar to}511 keV photons from a compact source of electron-positron annihilation radiation.

Observation of the 511 keV annihilation line in the direction of the galactic center with HEXAGONE

The HEXAGONE balloon‐borne spectrometer has flown on 22 May 1989. HEXAGONE is a high resolution gamma‐ray spectrometer and consists of an array of twelve cooled germanium detectors. One of the observed targets was the Galactic Center and its vicinity (field of view 19° at 511 keV) and it was seen during 6.3 hours. The 511 keV annihilation line was observed with a flux of (8.88±2.67)×10−4 γcm−2 s−1, a width 1.09+1.38, −1.09 keV and its centroid at 511.54±0.38 keV. The results are consistent with an upper limit of 8.3×104 K for the temperature of the annihilation medium of the positrons.The HEXAGONE balloon‐borne spectrometer has flown on 22 May 1989. HEXAGONE is a high resolution gamma‐ray spectrometer and consists of an array of twelve cooled germanium detectors. One of the observed targets was the Galactic Center and its vicinity (field of view 19° at 511 keV) and it was seen during 6.3 hours. The 511 keV annihilation line was observed with a flux of (8.88±2.67)×10−4 γcm−2 s−1, a width 1.09+1.38, −1.09 keV and its centroid at 511.54±0.38 keV. The results are consistent with an upper limit of 8.3×104 K for the temperature of the annihilation medium of the positrons.

An Observation of SN1987A with a New High Resolution Gamma-Ray Spectrometer

The discovery (Matz et al. [1]) of gamma-ray line emission at 847 and 1238 keV from radioactive 56Co in the recent supernova SN1987A proved that explosive nucleosynthesis occurred in this supernova. Gamma-ray light curves derived from these and subsequent observations (Cook et al. [2], Mahoney et al. [3], Sandie et al. [4], Rester et al. [5], Teegarden et al. [6]) have a broad plateau from August 1987 to October 1988, with an 847 keV flux of ~ 7×l0-4 ph/cm2-sec (Tueller et al. [7]). The early detection of gamma rays required the inclusion of mixing or clumping in the models, (e.g. Pinto and Woosley [8] and Chan and Lingenfelter [9]). The gamma-ray fluxes are predicted, e.g. Bussard et al. [10], to peak at about day 400 and then decrease, by a factor of ~ 6 at day 800, as the effect of increasing transparency becomes dominated by radioactive decay. Then they should depend primarily on the amount of 56Co produced and little on the degree on mixing since most of the 56Co should be exposed. Thus measurements of the 56Co gamma-ray line fluxes and profiles will continue to be important during the decline of SN1987A.

Electronic considerations for externally segmented germanium detectors

The dominant background source for germanium gamma ray detector spectrometers used for some astrophysics observations is internal β decay. Externally segmented germanium gamma ray coaxial detectors can identify β decay by localizing the event. Energetic gamma rays interact in the germanium detector by multiple Compton interactions while β decay is a local process. In order to recognize the difference between gamma rays and β decay events the external electrode (outside of detector) is electrically partitioned. The instrumentation of these external segments and the consequence with respect to the spectrometer energy signal is examined.