Descripton of the COMPTEL Instrument
The Compton Telescope: capable of imaging 1 steradian of the sky in the 0.8-30 MeV range.A detailed description of the COMPTEL experiment and its operating characteristics can be found in the report "Instrument Description and Performance of the Imaging Gamma-Ray Telescope COMPTEL aboard NASA's Compton Gamma Ray Observatory" by V. Schönfelder et al., 1993, ApJ Suppl., 86, 657. We provide in the sections below a summary description of the COMPTEL instrument and its properties.
1. Description
COMPTEL consists of two detector arrays. In the upper one, a liquid scintillator, NE 213A, is used and, in the lower, NaI crystals. Gamma rays are detected by two successive interactions: an incident cosmic gamma-ray is first Compton scattered in the upper detector, then totally absorbed in the lower. The locations of the interactions and energy losses in both detectors are measured. The accuracy in the measurement of these parameters determines the overall energy and angular resolution of the telescope. In some ways, COMPTEL is similar to an optical camera: the upper detector, analogous to a camera's lens, directs the light onto the second detector, comparable to the film, in which the scattered photon is absorbed. Although the photons are not focused, as in the case of an optical camera, data obtained by COMPTEL can be used to reconstruct sky images over a wide field of view with a resolution of a few degrees.
A schematic drawing of the telescope is shown in Figure III-1. The two detectors are separated by a distance of 1.5 m. Each detector is entirely surrounded by a thin anticoincidence shield of plastic scintillator which rejects charged particles. On the sides of the telescope structure, between both detectors, are two small plastic scintillator detectors containing weak 60Co sources, used for in-flight calibration of the instrument. Each calibration source consists of a cylindrical piece of 60Co-doped plastic scintillator of 3 mm thickness and 1.2 cm diameter viewed by two 1.9 cm-diameter photomultiplier tubes (PMTs).
The Upper Detector (D1) consists of 7 cylindrical modules of liquid scintillator NE 213A. Each module is 27.6 cm in diameter, 8.5 cm thick, and viewed by eight photomultiplier tubes. The total area of the upper detector is approximately 4188 cm^2. The Lower Detector (D2) consists of 14 cylindrical NaI (Tl) blocks of 7.5 cm thickness and 28 cm diameter, which are mounted on a supporting baseplate. Each block of NaI is viewed from below by seven photomultiplier tubes. The total geometrical area of the lower detector is 8620 cm^2. Each anticoincidence shield consists of two 1.5 cm-thick domes of plastic scintillator NE 110, with each dome viewed by 24 photomultiplier tubes. With the exception of the front-end electronics, all detector electronics are mounted on a platform outside the detector assembly.
2. Instrument Modes for Data Acquisition
The Instrument Ground Support Equipment (IGSE) for COMPTEL is located at the University of New Hampshire, USA. All commands sent to the instrument originate at the University of New Hampshire, which also receives daily "quick-look" data during real-time passes of the spacecraft. These data are inspected immediately upon receipt in order to verify the satisfactory health, safety, and performance of the instrument.
A. Primary Modes for the Acquisition of Scientific Data
1) Double-Scatter Telescope Mode
COMPTEL operates primarily as a double-scatter gamma-ray telescope. In this mode an incident cosmic gamma ray is electronically identified by a delayed coincidence satisfying time-of-flight criteria between the upper and the lower detectors, combined with the absence of a signal from all charged particle shields and from the calibration detectors. Gamma-ray events are initially screened on board the spacecraft according to coarse pulse-height, time-of-flight, and charged-particle veto limits, which are applied to reject background events. The data are transmitted in telemetry packets where events satisfying all selection criteria (referred to as "Gamma 1" events) have first priority for transmission in the telemetry stream, followed by events selected according to a secondary set of onboard criteria ("Gamma 2" events). The telemetry rate of 6125 bits per second permits the transmission of approximately 20 events per second from the spacecraft.
The quantities measured for each selected gamma-ray event are:
- The energy of the recoil electron of the Compton scattered gamma ray in the upper detector Ee' (Ee' > 50 keV), determined from the summed pulse heights of the eight PMTs associated with the triggered D1 module;
- The location of the interaction in the upper detector, determined from the relative pulse heights of the eight PMTs associated with the triggered D1 module;
- The pulse shape of the scintillation in the upper detector, provided by the output of a pulse-shape discriminator circuit;
- The energy loss Ee" in the lower detector ( Ee" > 500 keV), determined from the summed pulse heights of the seven PMTs associated with the triggered D2 module;
- The location of the interaction in the lower detector, determined from the relative pulse heights of the seven PMTs associated with the triggered D2 module;
- The time-of-flight of the scattered gamma ray from the upper to the lower detector; and
- The absolute time of the event.
The energy of an incident cosmic gamma ray is estimated by summing the energies deposited in the upper and lower detectors, assuming total absorption of the scattered photon. From the energy losses and the interaction locations recorded for both the upper and lower detectors the arrival direction of an incident gamma ray is calculated by application of the Compton scattering formula. For a true Compton scatter event the arrival direction of a cosmic gamma ray is known to lie on a so-called "event circle" on the sky. The center of the circle is the direction of the scattered gamma-ray within the telescope, and its angular radius is derived from the energy losses in both detectors. In the ideal case of totally-absorbed Compton-scattered photons, the intersection of many such event circles on the sky yields the location of the gamma-ray source. In actual practice a detailed understanding of the properties of the scintillation detectors, and of the response of the telescope to background, multiple interactions, and partial absorption events, must be exploited to determine accurate source locations.
2) Burst Mode
In addition to the normal double-scatter telescope mode of operation, two of the NaI crystals in the lower D2 detector assembly of COMPTEL are also operated simultaneously as burst detectors. These two modules are used to measure the time history and energy spectra of cosmic gamma-ray bursts and solar flares. The burst detector modules are equipped with a dedicated electronic subsystem, the Burst Spectrum Analyzer (BSA). One of the burst modules provides energy coverage over a low range (~100 keV to ~1 MeV), and the other over a high range (~1 MeV to ~10 MeV). The burst detectors are sensitive to incident radiation over 4 steradians, though the field of view not obstructed by other portions of the spacecraft totals approximately 2.5 sr.
The burst system operates in three different internal modes. The background mode is the usual operating mode: spectra from both burst detectors are accumulated over a tele-commandable period of time (from 2 s to 512 s) and read out continuously or at a reduced rate (also tele-commandable, e.g., every 5 min). These data are used to establish the celestial and instrumental background before and after a burst event. The BSA switches into burst mode upon receipt of a trigger signal from the BATSE experiment. A total of six spectra per energy range are accumulated with an integration time that can be set from 0.1 s to 25.6 s. After completion of the sixth spectrum, the BSA enters a tail mode where up to 256 spectra per energy range with tele-commandable integration times from 2 s to 512 s are accumulated and stored for transmission. The tail mode is of value for longer-lasting burst events (e.g., solar flares). After completion of the tail mode, the BSA resumes the background mode and is ready to receive the next burst trigger.
3) Solar Modes
- i. Solar Gamma-Rays
- In the event of a solar flare, COMPTEL can observe solar gamma rays in both the double-scatter telescope mode, provided the Sun is within the field of view of the instrument, and in the burst mode. Telescope observations of solar gamma rays have the advantage of a known source position, thus eliminating the need in later analysis to consider events that do not originate from the direction of the Sun. Upon receipt of a burst trigger from BATSE solar gamma rays can also be recorded by the two burst detectors aboard COMPTEL. The two burst modules, due to their rapid accumulation rates, can provide information on the initial fast burst of gamma rays from a solar flare.
- ii. Solar Neutrons
- In addition to observing gamma rays from a solar flare, COMPTEL is also capable of detecting solar neutrons. Neutron interactions within the instrument occur when an incident solar neutron elastically scatters off a hydrogen nucleus in the liquid scintillator of an upper D1 module. The scattered neutron may then interact and deposit all or a portion of its energy in one of the lower D2 modules, providing the internal trigger signal necessary for a double scatter event. The energy of the scattered neutron is deduced from its time of flight from the upper to lower detector, which is summed with the energy measured for the recoil proton in the upper D1 module to obtain the energy of the incident solar neutron. The computed scatter angle of the neutron, as with gamma rays, yields an event circle on the sky, which can be further constrained since the true source of the detected neutrons is assumed to be the Sun.
In practice, neutron observations are conducted in the following manner: within two minutes of an initial burst trigger, BATSE sends a second signal to COMPTEL indicating that the burst originates from the general direction of the Sun. COMPTEL can then automatically be commanded to enter an alternate event selection mode to measure solar neutrons for a period of 90 minutes (or approximately one orbit of the spacecraft). This is achieved by shifting the acceptance window for the time of flight of particles from the upper to the lower detector to allow for the slower-moving neutrons, compared to the speed-of-light gamma rays. Gamma ray events continue to be accumulated simultaneously with the neutrons; the two types of particles are later distinguished by their respective time-of-flight and pulse-shape signatures.
B. Secondary Modes for Data Acquisition
In addition to those instrumental modes intended for the acquisition of scientific data, the following modes of operation can also be specified. These secondary modes are intended for the collection of data relevant to in-flight calibration, the assessment of instrument performance, and for the study of background effects due to instrumental activation and atmospheric albedo.
- SAA Mode - During passages through the region of the South Atlantic Anomaly (SAA) the high voltages are switched off to prevent damage to the photomultiplier tubes. The period of SAA transits can be used to verify the pedestals of the on-board pulse height analysis system and to test the burst analysis system.
- Proton Calibration Mode - In this mode data can be obtained to investigate the instrument response to charged particles, and to evaluate the performance of the anticoincidence veto domes in the suppression of the background due to charged particle interactions.
- Upper Detector (D1) Single Event Calibration Mode - Energy spectra of the interactions occurring in the D1 modules are obtained, in addition to normal gamma-ray telescope events. Spectral lines of background interactions can be used to verify the calibration of individual D1 modules. This calibration mode is typically invoked during earth occultation of the primary scientific viewing target.
- Lower Detector (D2) Single Event Calibration Mode - Similar to (3) above, but for the D2 modules.
- Atmospheric Neutron Mode -Similar to the Solar Neutron Mode, described in the previous section, but on command only; intended to investigate instrumental background due to neutrons from the Earth's atmosphere.
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