Detector subsystems

Plastic scintillator anticoincidence The EGRET detection system includes a plastic scintillator which forms a dome surrounding the upper portions of the system. Its purpose is to help discriminate against the energetic cosmic ray particles (mostly protons) that are continuously traversing the detector; Fig. 5a. When such a particle traverses the scintillator, it creates a track of ionization that results in a faint flash of light.

The scintillator consists of a clear plastic (polystyrene) whose atoms emit ultraviolet light as a result of having been ionized by the pulse of electric field from a passing charged particle. An additive to the plastic causes the UV light to be

Figure 6.5. (a) Simplified view of the Energetic Gamma-Ray Experiment Telescope (EGRET) carried on the Compton gamma-ray observatory. It made use of scintillators and 36 spark-chamber modules (of which only 9 are shown) to detect and measure gamma rays from 20 MeV to 30 GeV while at the same time rejecting the much more numerous charged cosmic ray particles. Measurement of the tracks and total energy of the converted electron-positron pair yield the energy and arrival direction of the gamma ray. (b) Top view of wires in a portion of a spark chamber module. The spark is indicated by the filled circle, and the flow of current by the dark lines. Only one spark is shown.

Figure 6.5. (a) Simplified view of the Energetic Gamma-Ray Experiment Telescope (EGRET) carried on the Compton gamma-ray observatory. It made use of scintillators and 36 spark-chamber modules (of which only 9 are shown) to detect and measure gamma rays from 20 MeV to 30 GeV while at the same time rejecting the much more numerous charged cosmic ray particles. Measurement of the tracks and total energy of the converted electron-positron pair yield the energy and arrival direction of the gamma ray. (b) Top view of wires in a portion of a spark chamber module. The spark is indicated by the filled circle, and the flow of current by the dark lines. Only one spark is shown.

absorbed and re-emitted as optical light. The scintillator is totally enclosed in a light-tight covering to exclude stray light. The inside of the covering is highly reflective so that the light rays reflect back and forth in the plastic with small absorption until they happen to strike one or more of the photomultipliers at the bottom edges of the scintillator (Fig. 5a). These convert the light to a voltage pulse which informs the digital logic that a cosmic ray particle traversed the experiment.

In contrast, a gamma ray is not charged and normally would not disturb the atoms of the plastic; it will be absorbed catastrophically in material of higher atomic number within the detector. One can thus distinguish a charged cosmic ray particle from a gamma ray by the presence or absence of a signal from the plastic scintillator. The absence of a such a pulse at the same time as other indications of radiation in the detector is called an anticoincidence event. It could well be a desired gamma ray.

Spark chamber detection of electron-positron pair

The gamma rays are detected in a bank of 36 spark-chamber modules stacked vertically on one another (Fig. 5a), They are interspersed with foils of tantalum, a material of high atomic number (Z = 73). The foils have a high cross section for the conversion of a gamma ray to an electron-positron pair, often called simply an electron pair. This interaction occurs when the gamma ray interacts with the Coulomb field of the nucleus of a high- Z atom in a process called pair production. The electrons created in this interaction take up most of the gamma-ray energy. Thus they have substantial kinetic energies and tend to travel in the same direction as the gamma ray as shown in the figure. The role of the spark chambers is to record the tracks of these particles.

Each spark chamber module is a large flat enclosure containing gas and two parallel planes of parallel and closely spaced wires (wire separation —1 mm). The two planes are separated by about 3 mm in the vertical (z) direction, and the planes are rotated such that the wires in one plane run at right angles to the wires in the other plane; the wires in the two planes run in the x and y directions respectively (Fig. 5a,b). It is like a window screen, except that the two planes (x and y) are separated vertically by about 3 mm.

The passage of the electron creates a track of ionization in the gas of each chamber it traverses. If a high voltage pulse is immediately applied between the top (x) and bottom (y) planes, a spark will occur at the x,y location of the ionization track. The high voltage must be applied soon but it can be delayed briefly until the detection logic indicates the desired type of event has occurred. One wishes to avoid unnecessary firings because it takes some time for the system to recover from the application of the high voltage.

The discharge thus takes place between the two planes, from one wire in the x plane to one in the y plane (dark marks in Fig. 5a and dark circle in Fig. 5b). A surge of current flows in each of these two wires (dark lines in top view, Fig. 5b). To record the x,y location of the spark, one must record which two wires carried the current. This is done with the aid of discrete magnetic cores, one of which is placed on the end of each wire (Fig. 5b).

Each core is shaped like a small doughnut and the wire passes through the hole. A current passing through a core will force it into a "+" state wherein the north magnetic pole is in the direction of current flow. If, before the spark, all of the cores in the chamber are placed in the opposite "—" state, then only those cores on the current carrying wires will be found in the "+" state after the spark.

After the event, the magnetic states of all the cores are pulsed with a voltage pulse to reset them. Those that had been flipped by the spark respond differently (electrically) from the others. In this manner, the circuitry registers which x wire and which y wire carried the spark current, and this defines the x, y position of the spark in the module in question. The x, y locations from the several modules permit one to follow the track of the electron or positron, or both, through the several detector layers and to locate the track or tracks in three dimensions.

The presence of two electron tracks means that each spark chamber module must produce a spark at two locations simultaneously. This requires a very uniform spacing between the two planes of wires so one spark will not short out the high voltage before the other spark can develop. A fast-rising and high-voltage pulse also helps insure that the two sparks will occur. The two tracks produce two x coordinates and two y coordinates which yields four x -y locations of which only two are correct. The resolution of this ambiguity by means of a simple rearrangement of the modules is left to the reader.

Timing scintillation detectors (up-down discrimination)

The electrons also pass through two plastic "timing" scintillators which are above and below the lower group of spark chambers (Fig. 5a). These detectors operate on the same principle as the anticoincidence scintillator; traversal by the electron pair results in an electrical pulse from the photomultipliers. In this case the times of the pulses are precisely recorded and compared. The sequence of times from the two scintillators indicates whether the particles were moving upward or downward. This allows one to eliminate upward moving cosmic rays. This discrimination requires very fast electronics because the particles are highly relativistic, traveling close to the speed of light, or 0.3 m per ns.

The passage of the electrons through the timing scintillators in the correct direction, together with the absence of any pulse in the anticoincidence scintillator, indicates the likely presence of a downward-moving gamma ray that converted to an electron-positron pair. The electronic logic then immediately directs the spark-chamber circuitry to apply high voltage to the chambers. This reveals the tracks of the electron pair as described above.

Energies and arrival directions The electrons finally pass into a crystal scintillator. This is a high-Z material, sodium iodide (NaI), that causes the electrons to undergo multiple interactions until they have given up their entire kinetic energies to ionization. The recombinations of the ions and electrons cause it to scintillate, to emit light. The light is collected by photomultipliers. The combined output of all the photomultiplier tubes viewing this scintillator is approximately proportional to the total energy lost by the original positron and electron in the crystal scintillator.

The total energy loss of the two particles in their traversals through the chambers, the tantalum, and finally in the NaI should approximate the initial energy of the gamma ray. From this information and from the tracks of the two particles, one can deduce the arrival direction of the gamma ray and its total energy. The arrival direction can be obtained to about 10' and the energy determined to about 15%. Note that the location of the gamma ray in the chamber is not important; it is the arrival direction that indicates from whence on the sky the gamma ray came.

BATSE experiment

The Burst and Transient Source Experiment (BATSE) also flew on CGRO. Its appeal for us here is the power inherent in its simplicity. Its primary objective was to study celestial flashes of gamma rays (gamma-ray bursts, GRB) that last only for a few minutes and that had been known but unexplained since 1967. The GRBs arrive at a rate of about one per day at completely random times with each coming from a different direction in the sky. This made them extremely hard to study; one never knew from where or when the next would suddenly appear; and then it would be gone. It was not even known if they came from our solar neighborhood, farther out in the Galaxy, or from intergalactic space.

The BATSE was designed to observe the entire sky surrounding the spacecraft (except that part blocked by the earth) in order to detect as many GRBs as possible. The objective was to study the intensity and spectral time profiles of many individual GRBs as well as the distribution of arrival directions and brightnesses of the class as a whole. It was sensitive in the energy range 20 keV to 2 MeV, much lower than that of the EGRET experiment (>20 MeV). In its 9 years in space, it detected about 3000 bursts.

BATSE consisted of eight simple detectors that looked out from the eight corners of the spacecraft (think of a cube; Fig. 6a). Each consisted of a large (0.2 m2) slab of NaI scintillator, sufficiently thick so that the gamma rays would interact within it, creating an avalanche of ionizing particles. The ionization releases optical photons which are detected with three photomultiplier tubes, giving an electrical pulse whenever a gamma ray is detected. The pulse amplitude represents the energy of the gamma ray. Penetrating cosmic ray protons were eliminated by a large sheet of plastic scintillator in front of the NaI. There was metallic shielding in the back to minimize gamma-ray detections from the rear. There was no collimation so the field of view of each was almost 2^ sr, or 50% of the sky (Fig. 6b), with the eight fields of view overlapping substantially. Together they viewed the entire 4^ sr solid angle surrounding the spacecraft, and typically up to four detectors would detect any given GRB.

(a) Arrangement (b) Detector (side view) (c) Detector (top view)

(a) Arrangement (b) Detector (side view) (c) Detector (top view)

Figure 6.6. The Burst and Transient Source Experiment (BATSE) on the Compton gamma-ray observatory. (a) Eight detectors, each with a sr field of view (much larger than shown), view outward from the eight corners of the spacecraft. (b) Side view of one of the detectors with field of view shown. Gamma rays can enter from any direction in the upper hemisphere. (c) Top view of one of the detectors.

Figure 6.6. The Burst and Transient Source Experiment (BATSE) on the Compton gamma-ray observatory. (a) Eight detectors, each with a sr field of view (much larger than shown), view outward from the eight corners of the spacecraft. (b) Side view of one of the detectors with field of view shown. Gamma rays can enter from any direction in the upper hemisphere. (c) Top view of one of the detectors.

A GRB consists of a flash of gamma rays. The number of gamma rays detected in a given time interval, say 0.1 s, indicates the intensity of the flash during that interval. The arrival direction of the gamma rays is determined by the relative numbers of detected gamma rays in the several detectors. A GRB arriving from a distant point arrives as a swarm of gamma rays in planes normal to the direction of propagation (like a plane wave). This flash of gamma rays is detected with several of the detectors with different signal strengths depending upon its arrival direction.

If the arrival direction is normal to one of the detectors, the effective collecting area is large and many gamma rays are absorbed, giving a large gamma-ray count. If the arrival direction is slanted, at an angle removed from the detector normal, the projected area of the detector is less, and fewer gamma rays are absorbed and hence counted. The signals from the several detectors taken together thus give a rough measure of the arrival direction, often within several degrees. They also give a measure of the number flux density of the gamma-ray burst (gamma rays m-2 s-1). Summed over the duration of the burst, one measures the fluence (gamma rays m-2).

The BATSE demonstrated that the GRB arrival directions are highly isotropic; suggesting they were from extragalactic space. It then played a major supportive role in the identification of afterglows of GRBs. These fading glows of radiation from the location of some GRBs are visible at radio, visible and x-ray wavelengths. They linger for days in the optical and even longer in the radio.

The Italian-Dutch BeppoSAX satellite discovered the afterglow phenomenon. It carried a multi-pinhole x-ray camera with a wide field of view that provided celestial positions of GRBs that were accurate to a few arcminutes. This allowed the sensitive focusing x-ray telescope on the satellite to find and study the fading x-ray afterglow. Ground-based optical and radio telescopes could do likewise, given the original arcmin position. Optical spectra of the afterglows provided redshifts that established the extragalactic origin of the GRB.

The GRBs are thus extremely luminous, liberating more energy than the entire rest mass energy of a neutron star. This makes them the most energetic explosions known in the universe, other than the big bang. They could result from the implosion of a massive stellar core into a black hole (hypernova), or from the final moments of two compact stars in a binary system, such as a black hole and a neutron star. As the two objects spiral into one another, radiating their energy away in gravitational waves, they merge into an even more massive black hole with a cataclysmic release of energy (see Section 12.4).

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