73 research outputs found
Activation in the COMPTEL double-scattering gamma-ray telescope
Abstract-The COMPTEL gamma-ray telescope has been operating in low Earth orbit for six years, since the launch of the Compton Gamma-Ray Observatory in April 1991. Comparisons of data for different orbits and epochs show evidence of activation on time scales from minutes (27Mg, q,2=9.5 min) to years C2Na, q&.58 yr). The activation is correlated with both the orbital altitude and solar cosmic-ray modulation. Because it requires coincident measurements in two different detectors, COMPTEL is most susceptible to instrumental background events in which two or more photons are produced simultaneously
Observations of the 1991 June 11 solar flare with COMPTEL
The COMPTEL instrument onboard of the Compton Gamma‐Ray Observatory (CGRO) is sensitive to γ‐rays in the energy range from 0.75 to 30 MeV and to neutrons in the energy range from 10 to 100 MeV.
During the period of unexpectedly high solar activity in June 1991, several flares from active region 6659 were observed by COMPTEL. For the flare on June 11, we have analyzed the COMPTEL telescope data, finding strong 2.223 MeV line emission, that declines with a time constant of 11.8 minutes during the satellite orbit in which the flare occurs. It remains visible for at least 4 hours. We obtained preliminary values for the 2.2 MeV and 4–7 MeV fluences. Neutrons with energies above 20 MeV have been detected and their arrival time at the Earth is consistent with the γ‐ray emission during the impulsive phase
COMPTEL observations of gamma‐ray flares in October 1991
The COMPTEL experiment on GRO images 0.75–30 MeV celestial gamma‐radiation that falls within its 1 steradian field of view. During observation 12 (primary target Cen A) in October 1991 the sun had been in the fov and several solar flares associated with the active region 6891 had been observed. Time profile and energy spectra had been produced, using COMPTEL’s primary mode of operation (the telescope mode). Additionally the number of counts received in the D2‐single burst detector (the secondary mode of operation) are given. We summarize the preliminary results on all of these flares
A time dependent model for the activation of COMPTEL
The structure of the CGRO satellite is irradiated by cosmic rays and trapped particles fromradiation belts. These incident particles produce radioactive nuclei in nuclear reactions with the satellite structure. Most of the radiation dose can be attributed to the passages through the South Atlantic Anomaly. The incident particle flux on the COMPTEL instrument is estimated from the event rate of a plastic scintillation detector. This event rate is modeled with a Neural Network simulation. The increase of the event rate during SAA passages is taken as a measure for the amount of induced radioactivity. A Neural Network Model is used to derive the buildup ofradioactive nuclei in the instrument over the first five years of the mission. Measurements of the internal 22Na - and 24Na-activity are used to estimate the proton flux in the SAA. The result is consistent with earlier measurements and models
COMPTEL’s solar flare catalog
COMPTEL, the imaging gamma‐ray telescope, capable of detecting gamma rays in the range of 0.1–30 MeV, is one of four instruments aboard NASA’s Compton Gamma‐Ray Observatory. The Comptel burst detectors (single Defector Mode) have a field of view of ∼2.5 π sr. These detectors of COMPTEL permit measurements of energy spectra and time histories of solar flare gamma‐ray emission. A search through the Single Detector Mode’s data is being conducted. We summarize the preliminary results of this search
The COMPTEL instrumental line background
The instrumental line background of the Compton telescope COMPTEL onboard the
Compton Gamma-Ray Observatory is due to the activation and/or decay of many
isotopes. The major components of this background can be attributed to eight
individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl.
The identification of instrumental lines with specific isotopes is based on the
line energies as well as on the variation of the event rate with time,
cosmic-ray intensity, and deposited radiation dose during passages through the
South-Atlantic Anomaly. The characteristic variation of the event rate due to a
specific isotope depends on its life-time, orbital parameters such as the
altitude of the satellite above Earth, and the solar cycle. A detailed
understanding of the background contributions from instrumental lines is
crucial at MeV energies for measuring the cosmic diffuse gamma-ray background
and for observing gamma-ray line emission in the interstellar medium or from
supernovae and their remnants. Procedures to determine the event rate from each
background isotope are described, and their average activity in spacecraft
materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure
The Locations of Gamma-Ray Bursts Measured by COMPTEL
The COMPTEL instrument on the Compton Gamma Ray Observatory is used to
measure the locations of gamma-ray bursts through direct imaging of MeV
photons. In a comprehensive search, we have detected and localized 29 bursts
observed between 1991 April 19 and 1995 May 31. The average location accuracy
of these events is 1.25\arcdeg (1), including a systematic error of
\sim0.5\arcdeg, which is verified through comparison with Interplanetary
Network (IPN) timing annuli. The combination of COMPTEL and IPN measurements
results in locations for 26 of the bursts with an average ``error box'' area of
only 0.3 deg (1). We find that the angular distribution of
COMPTEL burst locations is consistent with large-scale isotropy and that there
is no statistically significant evidence of small-angle auto-correlations. We
conclude that there is no compelling evidence for burst repetition since no
more than two of the events (or 7% of the 29 bursts) could possibly have
come from the same source. We also find that there is no significant
correlation between the burst locations and either Abell clusters of galaxies
or radio-quiet quasars. Agreement between individual COMPTEL locations and IPN
annuli places a lower limit of 100~AU (95% confidence) on the distance to
the stronger bursts.Comment: Accepted for publication in the Astrophysical Journal, 1998 Jan. 1,
Vol. 492. 33 pages, 9 figures, 5 table
COMPTEL images locations of gamma‐ray bursts
The γ‐ray telescope COMPTEL onboard GRO has so far located 6 gamma‐ray bursts which occurred in its ∼1 sr field of view. The positions of the sources were derived by the maximum‐entropy method. Systematic and statistical uncertainties for the four strongest bursts are approximately 1° to 2° and can be reduced in future analysis
COMPTEL observations of cosmic gamma‐ray bursts
The imaging γ‐ray telescope COMPTEL on board NASA’s Compton Gamma‐Ray Observatory (GRO) has observed many cosmic gamma‐ray bursts during the early mission phase of GRO. COMPTEL records time‐resolved burst spectra over 0.1 MeV to 10 MeV energies, and, for the first time, produces direct single‐telescope gamma‐ray images (0.8–30 MeV) of cosmic gamma‐ray bursts occurring in its 1 sr field of field
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