The BATSE Instrument
Click image for larger view
The Burst and Transient Spectrometer Experiment: an all sky monitor sensitive from about 20-600 keV.
One of the eight identically configured detector modules of BATSE is shown schematically in Figure V-1. Each detector module contains two NaI(Tl) scintillation detectors: a Large Area Detector (LAD) optimized for sensitivity and directional response, and a Spectroscopy Detector (SD) optimized for energy coverage and energy resolution. The eight planes of the LADS are parallel to the eight faces of a regular octahedron, with the orthogonal primary axes of the octahedron aligned with the coordinate axes of the Compton spacecraft.
The LAD detector is a disk of NaI scintillation crystal 20 inches in diameter and one-half inch thick, mounted on a three-quarters inch layer of quartz. The large diameter-to-thickness ratio of the scintillation crystal produces a detector response similar to that of a cosine function at low energies where the crystal is opaque to incident radiation. At energies above 300 keV, the angular response is flatter than a cosine. A light collector housing on each detector brings the scintillation light into three 5-inch diameter photomultiplier tubes. The signals from the three tubes are summed at the detector. A quarter-inch plastic scintillation detector in front of the LAD is used as an anticoincidence shield to reduce the background due to charged particles. A thin lead and tin shield inside the light collector housing reduces the amount of background and scattered radiation entering the back side.
The spectroscopy detector is an uncollimated NaI(Tl) scintillation detector 5 inches in diameter and 3 inches thick. A single 5 inch photomultiplier tube is directly coupled to the scintillation detector window. The housing of the PMT has a passive lead/tin shield similar to that of the LADs. The crystal housing has a 3-inch diameter 50 mils thickness beryllium window on its front face in order to provide high efficiency down to 10 keV. The axis of symmetry of an SD is offset by 19 degrees from the LAD axis for mechanical reasons.
Scintillation pulses from the detectors are processed by a gated baseline restoration circuit in order to minimize spectral distortion at high counting rates. Pulses are processed in parallel by a high-speed, four-channel discriminator circuit and by a slower pulse height analyzer system. The nominal equivalent energies of the upper three discriminators for the LADs are 60, 110, and 325 keV. The lower-level discriminators are programmable, and currently set near 20 keV. Two of the fast discriminators for the SDs are set at energies above the energies analyzed by the pulse height system. The gain of each detector system is determined by the high voltage applied to the PMTs. The SDs have been operated at different gains in order to span from 10 keV to greater than 100 MeV.
Each of the eight BATSE detector modules sends data to the Central Electronics Unit (CEU). The CEU contains hardware and software that accumulates the data into several large RAM memory buffers. Extensive use of commandable parameters, plus the capability to reprogram the flight software, insures that BATSE will have the flexibility to respond to unforeseen conditions in orbit or newly discovered gamma-ray phenomena. Signals from the pulse-height converters are used to construct 128-channel spectra from the LADs and 256-channel spectra from the SDs. Each of the spectra are subdivided into ranges with different dispersions to increase the dynamic range and to efficiently use the available telemetry space. These energy channels are also mapped into 16 coarse energy channels using programmable lookup tables, one for the LADs and one for the SDs. This permits a tradeoff of time resolution for energy resolution in several of the data types. Discriminator events are accumulated in the hardware every 64 ms. The CEU hardware constructs various data types from the discriminators, 16-channel, 128-channel, and 256-channel spectral data.
The CEU produces data for telemetry at a uniform rate of one packet every 2.048 seconds. Each of these packets contains LAD and SD discriminator rates, 16-channel LAD spectra, and housekeeping information. In addition there is a variable portion of the packet which normally contains either high resolution spectral data or epoch folded data for pulsar observations. The CEU contains hardware for folding 16-channel data on board at the approximate period of a candidate pulsed source from selected detectors. Two pulsars of different periods can be simultaneously acquired, however the same set of detectors must be selected for both. The accumulation of pulsar data and high resolution LAD and SD spectra is driven by an on-board schedule that is synchronized with the Compton orbit. This schedule is updated biweekly with the change in Compton spacecraft orientation.
BATSE detects gamma-ray bursts on-board by examining the count rates of each of the eight LADs for statistically significant increases above background on each of three time scales: 64 ms, 256 ms, and 1024 ms. The discriminator rates in channels 2 and 3 (approximately 60 to 325 keV) are used. The background rate is determined for each detector over a commandable time interval currently set at 17.4 seconds. The statistical significance required for a burst trigger is set separately for each of the three time scales, with a quantization of 0.0625 sigma. These thresholds are currently set at 5.5 sigma. At least two detectors must exceed threshold for a burst trigger to occur. An additional requirement for burst triggering is that the detector with the greatest increase in count rate must have an increase in the charged particle rate that is less than a specified fraction of the increase in the neutral rate. This is done in order to avoid triggering on charged-particle event encounters, such as those produced by spacecraft containing nuclear reactor power sources.
When a gamma-ray burst is detected, the CEU enters a fast data acquisition mode and rapidly stores a variety of data types into memory. Over a period of about 80 minutes for weak bursts and 105 minutes for strong bursts the normally scheduled output of pulsar and high resolution spectra is suspended, and the collected burst data is read out in the variable portion of the data packets. The normal output schedule then resumes, in proper synchronization with the Compton orbit. While the burst memories are being telemetered, the trigger thresholds are temporarily revised to values corresponding to the maximum rates detected during the burst. Thus, a stronger burst will terminate the readout of a weaker burst and overwrite the burst memories. The data available from a overwritten burst is timing dependent, but includes at a minimum the 64 ms resolution discriminator data (DISCSC).
The failure of the Compton tape recorders has led to a loss of some burst data. New flight software now corrects this problem. With the new software burst readouts are suspended during telemetry gaps. A new datatype, DISCLB, is collected to compensate for the loss of DISCSC and DISCLA data during telemetry gaps. In addition, three channels of the DISCLA data are collected during gaps in the normal telemetry, using a 1-kbyte/sec telemetry channel.
Whenever a burst trigger occurs BATSE provides the other Compton instruments a signal that may be used to initiate special burst data processing modes. BATSE also provides a signal to the other Compton instruments if a triggered burst appears to be a solar flare. This signal can be used by COMPTEL to enter the neutron detection mode and by OSSE to point to the sun or enter a more appropriate observation mode. The main criterion for generating the solar flare trigger signal is that the relative count rates are consistent within limits for a burst coming from the direction of the sun. A lower limit is also placed on the largest count rate occurring within the first two seconds after the burst trigger, and a constraint can be placed upon the burst's spectral hardness.
If you have a question about CGRO, please contact us via the Feedback form.