P. Feffer

Simulations of pulse shape discrimination (PSD) techniques for background reduction in germanium detectors

For modern, actively shielded, narrow aperture germanium detector (GeD) spectrometers at balloon or spacecraft altitudes, the continuum background in the 0.2 to 2 MeV energy range is dominated by the β− decay of radioactive nuclei produced by the interaction of cosmic rays and secondary protons and neutrons within the detector. PSD techniques distinguish single-site (primarily β−-decay) from multiple-site (primarily photon) events by analyzing the shape of the current pulse formed when the electrons and holes resulting from these energy depositions propagate through the detector. Through numerical simulation of the charge collection process, we demonstrate that the effectiveness of PSD can be enhanced by optimizing detector geometry and external electronics. In particular, PSD results for closed-end coaxial detectors with standard (1.2 cm effective diameter) and narrow (0.6 cm diameter) inner bore, and for a true coaxial narrow-bore detector, are presented. With a newly developed PSD algorithm, improvements in sensitivity at 1 MeV of factors of 2.8 and 3.6 for the standard-bore and narrow-bore closed-end coaxial detectors, respectively, and 4.0 for the true coaxial detector, are predicted.

Compton-backscattered annihilation radiation from the Galactic Center region

On 1989 May 22, the High Energy X-ray and Gamma-ray Observatory for Nuclear Emissions, a balloon-borne high-resolution germanium spectrometer with an 18-deg FOV, observed the Galactic Center (GC) from 25 to 2500 keV. The GC photon spectrum is obtained from the count spectrum by a model-independent method which accounts for the effects of passive material in the instrument and scattering in the atmosphere. Besides a positron annihilation line with a flux of (10.0 +/- 2.4) x 10 exp -4 photons/sq cm s and a full width at half-maximum (FWHM) of (2.9 + 1.0, -1.1) keV, the spectrum shows a peak centered at (163.7 +/- 3.4) keV with a flux of (1.55 +/- 0.47) x 10 exp -3 photons/sq cm s and a FWHM of (24.4 +/- 9.2) keV. The energy range 450-507 keV shows no positronium continuum associated with the annihilation line, with a 2-sigma upper limit of 0.90 on the positronium fraction. The 164 keV feature is interpreted as Compton backscatter of broadened and redshifted annihilation radiation, possibly from the source 1E 1740.7-2942.

A High Resolution Gamma-ray and Hard X-ray Spectrometer (HIREGS) for Long Duration Balloon Flights

Abstract The HIREGS gamma-ray spectrometer made a 23-day Long Duration Balloon Flight (LDBF) from Antarctica in January 1995 to observe Galactic sources of gamma-ray and hard X-ray line and continuum emission. The scientific instrument itself is discussed, followed by a more extensive discussion of the characteristics of the instrument unique to LDBFs. The flight performance and preliminary results are briefly summarized.

Pulse Shape Discrimination For Background Rejection In Germanium Gamma-Ray Detectors

We report on the development of a pulse shape discrimination (PSD) technique to reject the B-decay background resulting from activation of germanium (Ge) gamma-ray detectors by cosmic ray secondaries. These B-decays are a major source of background at 0.2-2 MeV energies in well shielded Ge detector systems. The technique exploits the difference between the detected current pulse shapes of single- and multiple-site energy depositions within the detector: B-decays are primarily single-site events while photons at these energies typically Compton scatter before being photoelectrically absorbed to produce multiple-site events. Algorithms have been developed to distinguish between single- and multiple-site pulse shapes. Depending upon the amount of background due to sources other than B-decay, PSD can more than double the detector sensitivity. In addition, we report on tests of PSD by laboratory activation of a detector with a fast neutron source, and on the first direct measurement of the B-decay background at balloon float altitude using a Ge detector with PSD.

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

High-resolution gamma-ray and hard x-ray spectrometer for long- duration balloon flights

The elements of a high resolution gamma-ray spectrometer, developed for observations of solar flares, are described. Emphasis is given to those aspects of the system that relate to its operation on a long duration balloon platform. The performance of the system observed in its first flight, launched from McMurdo Station, Antarctica on 10 January, 1992, is discussed. Background characteristics of the antarctic balloon environment are compared with those observed in conventional mid-latitude balloon flights and the general advantages of long duration ballooning are discussed.

New constraints on the positron annihilation media from the direction of the Galactic center based on the 511 KeV line profiles

We present here an analysis of the profiles and intensities of the 511 keV annihilation line observed in the direction of the Galactic center by high energy resolution detectors. We first investigate the HEXAGONE 1989 May data, where the point source was in a rather low state. We find that a warm medium (temperature of 8000 K) can describe the annihilation of the positrons from the diffuse component of the line. We compare these results with the Bell/Sandia 1977, and GRIS 1988 October flights during high states of the central source, and we then show that the discrepancy in the line shape and width between these two sets of data is explained if the time-variable component of the line coming form the annihilation of the positrons emitted by the central source annihilates in a cold medium (temperature around 80 K)

An observation of annihilation radiation from the Galactic Center region

The Galactic Center region was observed on 22 May 1989 with a high resolution gamma-ray spectrometer flown from Alice Springs, Australia. The instrument contained an array of 12 cooled Ge detectors, each 5.5 cm dia x 5.5 cm long, with an energy resolution of 2.2 keV at 0.5 MeV, which were collimated to 18° FWHM. The observation was made at approximately 4 gm/cm2 atmosphere depth for 6 hours in a series of target and background pointings lasting 20 minutes each. Results from multiparameter Gaussian fits to the data are: 511 keV flux = (8.9 ± 2.7) x 10−4 ph/cm2-sec, line width of 1.1 −1.1+1.4 keV, and < 3.2 keV FWHM at 95% confidence. If the flux is interpreted as diffuse galactic emission, the 24° effective aperture for a uniform source gives a flux of (2.12 ± 0.64) x10−3 ph/cm2-s-rad, consistent with the SMM observations of diffuse galactic flux. The flux and width are both less than the October 1988 measurements with a 17° FWHM instrument which were obtained when the compact source at the galactic center was known to be “on”. Thus the variable compact source in the Galactic Center region may be responsible for the wider 511 keV line emission. The line width limit is consistent with annihilation in the warm (104 K) phase of the interstellar medium.

Observation of the galactic 1809 keV gamma‐ray line with the HEXAGONE spectrometer

We report an observation of the galactic 1809 keV gamma-ray line produced by radioactive {sup 26}Al in the interstellar medium. The measurement was performed with our high resolution germanium spectrometer HEXAGONE on a balloon flight in May 1989 from Alice Springs, Australia. Our differential spectrum of the Galactic Center region shows a narrow line at 1809 keV corresponding to a flux of (1.9 +/{minus} 0.9) {center dot} 10{sup {minus}4} photons {center dot} cm{sup {minus}2} s{sup {minus}1} assuming a source at the Galactic Center. We discuss the available observations of the 1809 keV line in the context of models that have been proposed for the origin of the galactic {sup 26}Al.

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.