Imaging through Compton scattering and pair creation

11 Imaging through Compton scattering and pair creation Volker Schönfelder I and Gottfried Kanbach I Abstract Compton telescopes and pair-creation telescopes are the most successful instruments used in space-based γ-ray astronomy in the energy range from 0.3 MeV to 30 GeV. The principles of measurement of both kinds of telescopes are explained and an overview of early Compton and pair telescopes is given. The properties and capabilities of COMPTEL and EGRET aboard NASA s CGRO are described. These two instruments have performed the first-ever all-sky survey in γ-ray astronomy above 1 MeV. The other two CGRO instruments OSSE and BATSE have complemented these surveys towards lower energies (for this purpose, the omnidirectionally sensitive BATSE instrument used its capability to monitor hard X-ray sources >20 kev by means of Earth occultation). Finally, the outlook for future Compton and pair creation telescopes is given. Compton telescopes Principle In the 1960s and early 1970s the observation of photons in the energy range from about 1 MeV to 30 MeV was considered an impossible task in astronomy. Instruments relying on photo- and pair production are too inefficient at these energies. The only physical process with reasonable interaction probability is the Compton effect. However, it is impossible to derive energy and arrival direction from a single Compton interaction. This is only possible, if at least two Compton interactions occur. The classical Compton telescope consists of two detector planes, the first scattering plane D1, and the second scattering plane D2 (Figure 11.1, left side). From the locations of both interactions the direction of the scattered γ-ray is determined, and from the energy losses in both detectors, E 1 and E 2, the scattering angle ϕ can be derived. Hence, the arrival direction of the primary γ-ray is known to lie on a circle defined by a cone with opening angle ϕ around the direction of the scattered γ-ray. It is best to choose a low-z-material for D1 in order to have a high I MPE Max-Planck-Institut für extraterrestrische Physik, Garching, Germany 207

208 11. Imaging through Compton scattering and pair creation Figure 11.1: Left: principle of Compton telescope. Right: data space of Compton telescope data. Compton scattering probability; D2 should consist of a high-z-material to achieve a high absorption probability of the scattered γ-ray. If the scattered γ-ray energy E 2 is totally absorbed in D2, the energy E γ of the primary gamma-ray is determined by and the scattering angle ϕ by E γ = E 1 + E 2, (11.1) cosϕ = 1 m e c 2 0 E 2 + m e c 2 0 E 1 + E 2. (11.2) If the scattered γ-ray is not totally absorbed in D2, then E 1 +E 2 < E γ and the reconstructed scattering angle, called ϕ, is larger than the true scattering angle ϕ: cosϕ > 1 m ec 2 0 E 2 + m ec 2 0 E 1 + E 2 = cos ϕ. (11.3) By accepting only doubly scattered events with small scattering angles ( ϕ < ϕ max, where ϕ max can be chosen arbitrarily), the field of view of the telescope can be modified with software. The energy resolution of a Compton telescope depends on the accuracy of the energy measurements in D1 and D2; the angular resolution depends on two factors: first on the accuracy of measurement of the scattering locations in D1 and D2, and second on the accuracy of the determination of ϕ (which again depends on the accuracy of the energy measurement). As in case of the pair telescope, where the recoil energy of the nucleus limits the angular resolution, there is also a fundamental

209 limit to the angular resolution of a Compton telescope: it is defined by the orbital energy of bound electrons in the scattering material, which leads to a Doppler broadening of the recoil energy of the Compton scattered electrons (Zoglauer and Kanbach 2003). The point spread function of a Compton telescope can be described in a powerful way in a 3-dimensional data space defined by two orthogonal celestial coordinates (χ/ψ) describing the direction of the scattered γ-ray, and the scattering angle ϕ as z-axis (see Figure 11.1, right hand side). Each recorded event is represented by a single point in this data space. In the ideal case, in which the scattered γ-ray is totally absorbed in D2, the pattern of data points from a γ-ray point source with celestial coordinates (χ 0 /ψ 0 ) lies on a cone mantle in the (χ/ψ/ϕ)-space, where the cone apex is at (χ 0 /ψ 0 ), and the cone semi-angle is 45. The response density along the cone follows the Klein Nishina cross-section. This idealized conemantle response is blurred by measurement inaccuracies in D1 and D2, especially for incompletely absorbed events in D2, which fill the interior of the cone to a certain extent. The minimum detectable flux for a Compton telescope from a point source depends apart from the effective detection area and the observation time mainly on the number of background events (see Equation 11.4). In order to reduce the background, a time-of-flight measurement of the scattered γ-ray from D1 to D2 can be performed. In this way only events are accepted that first scatter in D1 and then in D2. Events that first scatter in D2 and then interact in D1 are rejected. If the material at D1 is chosen properly (e.g., organic liquid scintillator), part of the neutron-induced background can be suppressed by means of pulse shape measurements. Early Compton telescopes The first instrument which was described in the literature and which made use of two subsequent Compton interactions was a spectrometer for accurate energy determination of incident γ-rays, but it was not a telescope (Hofstadter and McIntyre 1950). It consisted of two scintillation detectors in backscatter arrangement. Peterson and Howard (1961) described a real double-scattering Compton telescope, which was designed to achieve directional collimation by using the kinematics of two Compton scattering processes. The directionality was achieved by postulating low energy losses in the first of two NaI(Tl) detectors, and high energy losses in the second one. No time-of-flight measurement was performed. The instrument was flown on OSO-1 on 6 March 1962, but was not successful due to an unexpected very high background (see Greisen 1966). In 1971, a re-birth of the Compton telescope concept took place in Garching at the Max-Planck-Institute for Extraterrestrial Physics (MPE) (Schönfelder et al 1973). The first Compton telescope at MPE was simple. It consisted of two plastic scintillator arrays separated by a distance of 1.5 m. Downward and upward scattered events could be identified and separated by means of a time-of-flight measurement of the scattered γ-ray. This telescope was operated as a directional collimator only. The first balloon flights with this telescope in 1973 and 1974 were very successful.

210 11. Imaging through Compton scattering and pair creation Practically at the same time independent work on Compton telescopes started at the University of California, Riverside (White et al 1973). The group there already had a functioning double-scattering neutron balloon experiment which used the time-of-flight technique to measure the energy of the scattered neutrons. In the first balloon flights with this instrument, γ-rays were rejected as background. But in 1972, the interest at Riverside changed from neutrons to γ-rays. From that time onwards their instrument was mainly operated as a Compton telescope. In the years after 1973 the main activities with Compton telescopes took place at Garching and Riverside. In 1975 the MPE group started building a new balloon telescope which was almost the size of COMPTEL. It was successfully flown three times on balloons. The COMPTEL collaboration started its activities in 1977. It then took nearly 14 years to get COMPTEL into orbit. COMPTEL COMPTEL aboard NASA s CGRO was in orbit for nine years from 1991 to 2000 (Schönfelder et al 1993). It performed the first all-sky survey in the energy range from 1 MeV to 30 MeV. A photomontage of COMPTEL is shown in Figure 11.2. The upper detector D1 consists of seven modules of liquid scintillator NE 213A. Each module of 28 cm diameter and 8.5 cm thickness is viewed from the sides by eight photomultiplier tubes. The total geometrical area of D1 is 4300 cm 2. The lower detector D2, 1.5 m from D1, consists of 14 modules of NaI(Tl) scintillators. Each module of 28 cm diameter and 7.5 cm thickness is viewed from below by seven photomultipliers. The total geometrical area of D2 is 8600 cm 2. From the relative pulse heights of the photomultiplier tubes in each D1- and D2-module, the locations of the interactions are determined to within about 2 cm. The sum of the photomultiplier signals of each module provides the energy losses E 1 and E 2 (see Equations 11.1 to 11.3). Both D1 and D2 are completely surrounded by veto-domes of 1.5 cm thick plastic scintillator to reject charged particles. COMPTEL covers the energy range from 0.8 MeV to 30 MeV. Within its large field of view of about 1 sr, its angular resolution is 1.7 to 4.4 FWHM (depending on energy). The relative energy resolution is in the range 5 % to 8 % FWHM, and the effective detection areas are in the range 20 cm 2 to 30 cm 2 (again depending on energy). The sensitivity of COMPTEL is significantly determined by the instrumental background. A substantial suppression is achieved by the combination of the charge-particle veto domes, the time-of-flight measurement technique, pulseshape discrimination in D1, Earth-horizon angle cuts, and proper event selections in energy- and ϕ-space. The analysis of the remaining background rate has shown a direct correlation with the charged particle veto rate, a dependency on the orbital cut-off rigidity and on the orbit altitude, and an increase with mission-time (due to a build-up of radioactive nuclei). Therefore, it has to be concluded that only a small fraction of the background rate is due to diffuse cosmic γ-radiation; most of it is produced in the telescope itself or in the surrounding material next to COMPTEL. Figure 11.3 illustrates how such processes can simulate double-scatter events in COMPTEL. The first type of instrumental background events are caused by secondary γ-rays which are produced within the COMPTEL field of view either

211 Figure 11.2: Schematic view of COMPTEL. inside D1 (type A events) or around D1 (type B events). If the production material is inside the veto domes, prompt secondary γ-ray production can only occur via secondary neutrons produced somewhere within the 15 t heavy spacecraft. If the material is outside COMPTEL s veto domes (e.g., in the neighbouring telescopes OSSE or EGRET), then secondary γ-rays can be produced by cosmic-ray interactions or again by secondary neutrons. The second important category of background events are cascade events. These are produced by nuclear interactions which lead to the simultaneous emission of at least two γ-rays: one of them hits D1, and the other one D2 (type C- and type D-events). A significant fraction of these are suppressed by time-of-flight measurements. Finally, cosmic-ray showers in the neighbouring experiments and materials can produce a number of γ-rays which are spatially uncorrelated, but time correlated (type F-events). Again, time-of-flight measurements can suppress significant fractions of these. The actual total in-flight instrumental background was higher than expected prior to launch by about a factor of four. As a consequence, the sensitivity of COMPTEL was two times lower than expected. Fortunately, however, this loss in sensitivity could be compensated by the four times longer mission lifetime (which originally was planned to be 2.25 years only). The actually achieved point-source sensitivity for a deep observation (t = 6 10 6 s) was 6.3 10 5 cm 2 s 1 for continuum emission between 1 MeV and 30 MeV (corresponding to about 3 % of the Crab-flux) and 1.6 10 5 cm 2 s 1 for line emission at 1.157 MeV and 1.809 MeV ( 44 Ti and 26 Al lines).

212 11. Imaging through Compton scattering and pair creation Figure 11.3: Illustration of masking background events in COMPTEL. Plans for future Compton telescopes Large efforts are presently being undertaken worldwide to develop the next generation of Compton telescopes. COMPTEL was the first successful Compton telescope put into space. It opened the 1 MeV to 30 MeV range as a new window to astronomy. From COMPTEL we have learned that the sky is rich in phenomena and objects that can be studied around 1 MeV. But it is also true that with COMP- TEL we could see only the tip of the iceberg. The sensitivity achieved was still modest. The next telescope would have to be more sensitive by one or two orders of magnitude. Very different concepts of Compton telescopes are presently being studied, investigated and tested. Instead of scintillators other detector materials are also considered and tested, like silicon strip detectors, position-sensitive germanium detectors, Cd-Te detectors, liquid xenon gas detectors, and high-pressure gas detectors. In order to achieve the required improvement in sensitivity, the detection efficiency has to be drastically increased and the background has to be reduced considerably. An increase of the efficiency by a factor of 5 to 10 is possible, if large solid angles (e.g., 2π sr) for the scattered γ-ray are allowed (in the case of COMPTEL the angle was only 0.5 sr). Such telescopes would be necessarily very compact, and it would then no longer be possible to perform a time-of-flight measurement. In these telescopes a rejection of upwards scattered γ-rays could be achieved by the so-called direction-of-motion parameter which determines the sequence of interactions from consistency checks of the Compton kinematics. A very significant background reduction can be achieved by localising the arrival direction of the primary γ-ray on the event circle. This is possible, if the track of the scattered Compton electron is measured as well (like in the proto-type telescope MEGA; Kanbach et al 2003). To improve the detection sensitivity of point sources, it is most effective to choose detector materials which provide the best possible energy resolution (to minimize the angular resolution element of the telescope). The choice of the satellite orbit, especially its inclination and altitude, has another huge effect on the

213 instrumental background. At least some if not all of these aspects are taken into account in the new investigations and developments mentioned above. Pair-creation telescopes The direct conversion of energy into matter was first observed when energetic photons interacted in a bubble chamber and showed, on the basis of magnetic deflection, the creation of a pair of oppositely charged particles: a negative electron and its anti-particle, the positively charged positron. This conversion becomes possible in the field of a nucleus or electron above a photon threshold energy of E γ,min = 2 m e c 2 0 (1 + m e/m), where m e is the electron rest mass, c 0 the speed of light, and M is the target particle s mass, i.e., above 1.022 MeV for conversion on a heavy nucleus and above 2.044 MeV for conversion on an electron. After the conversion the pair electrons essentially carry the remaining photon energy as kinetic energy and their summed momentum vectors correspond to the photon momentum minus the recoil momentum of the conversion target nucleus. This recoil momentum is very hard to determine in practice. Its typical value of 0.5 MeV/c 0 has to be taken as one of the physical limitations to the angular resolution of a pair-creation telescope. The second limit derives from multiple small-angle scattering of the propagating pair electrons, which quickly lose their original direction, especially if they are of low energy and have to pass dense detector materials. The original momenta are of course needed to reconstruct the direction of the primary photon, and it is clear that this will be possible with much better precision for high-energy electrons. A third and more technical aspect is the positional accuracy of the tracking detector. The first two effects are most noticeable at energies below 50 MeV, at which energy the recoil uncertainty would amount already to an angular dispersion of 2.4. The tracker resolution eventually limits the angular accuracy at high energies. Basic design of pair-creation telescopes The sequence of detection of high-energy photons in a satellite telescope begins with the conversion of photons into charged particles in a high-z target material. A high-z material is preferred since the cross section for pair-creation (the stopping power ) varies as Z 2. The kinetic energy and momenta of all secondary particles (the (e + /e ) pair and a possibly formed electromagnetic shower) have then to be measured with the best available precision. Finally the detector system must be able to reject background radiation like cosmic rays with very high efficiency. It is evident that the first two requirements pose a conflict: a massive conversion detector, which is good for detection efficiency, would lead to severe small-angle scattering of the pair particles and consequently to degraded angular resolution. Amelioration of that conflict leads to a principle of construction where the conversion material is arranged in thin layers, which are interspersed with tracking detectors. The trajectories of the newly created pair particles can then be determined as soon as they emerge from the conversion layer. Energy measurements of the pair particles can be performed in two ways: either the observed small-angle Coulomb scattering

214 11. Imaging through Compton scattering and pair creation (Molière scattering), which is approximately inversely proportional to the kinetic energy of the particle, can be used at energies up to 1 GeV; or a massive calorimeter made of scintillators is used to absorb the electromagnetic cascade and measure the energy up to high values. Measures to suppress background events caused by energetic particles are (i) to surround the sensitive detector volume by a plastic scintillator anticoincidence detector and (ii) ensure the sequence of the events by, e.g., measuring the direction of flight of the pair particles by a time-delayed coincidence system or a Čerenkov counter. All the high-energy satellite telescopes reviewed in the following sections are built according to these principles: the core is a layered tracking detector for the conversion of photons and the imaging of secondary electrons. Integrated with the tracking chamber is a trigger telescope that detects the occurrence of a pair-conversion event and initiates the read-out of the detector system. Below the tracker a calorimeter, often made of scintillators and photo-detectors, is used to measure the total energy of particles released in the high-energy interaction. A thin plastic anticoincidence counter surrounds the front and sides of the detector to shield against charged particle background. The sensitivity for an imaging telescope for point sources (also called unresolved sources ) is determined by the point-spread function (PSF) and the amount of background that is present under the image of the point source. Let N s be the number of source photons collected within the PSF of angular size θ. The source with flux F was observed during the time T with a telescope of effective area A eff so that N s = F T A eff. The density and rate of background photons on the image is assumed to be n b so that the total number of background photons within the source image is N b = T π θ 2 n b. The statistical significance of an excess of N s counts above the background can then be written as n σ = N s / N b. The flux required for an n σ -significant detection of a source, which is also called the sensitivity limit, is then F σ = n σ θ π nb. (11.4) A eff T As we see the sensitivity improves, i.e., a smaller flux F σ is detectable, when the PSF becomes small, the density of background counts is smaller, the effective area is enlarged, and the observation is performed for a longer time (although only the square-root of time enters). The main characteristics of past and present pair-creation satellite telescopes are listed in Table 11.1. Early pair-creation telescopes SAS-2 and COS-B SAS-2 was a NASA mission that carried as a single payload a spark chamber pair-creation telescope (Derdeyn et al 1972). After a series of non-imaging detectors (e.g., Explorer-11, Kraushaar et al 1965; and OSO-3, 1968; Clark et al 1968) that also detected photons above several 10 MeV via pair creation in stacks of scintillators, the SAS-2 instrument was the first truly imaging pair-creation telescope. Launched on 15 November 1972 the telescope operated from 19 November 1972 until 8 June 1973, when a technical defect in the power supplies terminated the mission. About 55 % of the sky was surveyed in 27 week-long exposures. The equatorial low-altitude orbit of SAS-2 ensured a low-background environment and good

215 Table 11.1: Characteristics of past and present imaging pair-creation telescopes. Mission SAS-2 (Nov 72 Jun 73) COS-B (Aug 75 Apr 82 EGRET/CGRO (Apr 91 Jun 00) AGILE (launch Apr. 2007) Fermi (GLAST) (launch June 2008) Effective Angular area, resolution A eff /cm 2 at 200 MeV Field of view Energy range, E γ /MeV Sky coverage 90 1 0.4 sr > 35 55 % 45 2 0.4 sr > 50 60 % 1300 1.5 0.6 sr 20 to 30000 600 1.2 3 sr 30 to 50000 9000 at 3.5 at 2.4 sr 20 to 10 GeV 100 MeV 300 000 0.15 at 10 GeV 100 % plan: 100 % plan: 100 % sensitivity although only 8000 celestial photons were recorded. The structured galactic disk, several point sources including two pulsars, and the extragalactic background were the main results of the SAS-2 mission (Fichtel et al 1975). The first ESA scientific mission launched in 1975 was the γ-ray telescope COS-B (Bignami et al 1975). Built and operated by a consortium of six European research groups in four countries, COS-B was one of the first truly European science missions. Although nearly of the same size as SAS-2, COS-B featured an important additional detector in the γ-ray telescope: a calorimeter made of a single CsI(Tl) scintillator. Its depth of 4.7 radiation lengths (r.l., equivalent to a depth of 8.5 cm) allowed the effective measurement of photon energies up to several gigaelectronvolts with a relative energy resolution (FWHM) between 50 % and 100 %. COS-B operated until 1982 and deep observations of the galactic disk and of selected extragalactic targets revealed a wealth of high-energy astrophysical results: details of the diffuse galactic γ-ray emission (Mayer-Hasselwander et al 1982), more than 20 point sources and the first extragalactic source, the blazar 3C273 (Swanenburg et al 1981). EGRET CGRO carried as its high-energy telescope the Energetic Gamma-Ray Experiment Telescope (EGRET, Kanbach et al 1988). As successor of SAS-2 and COS-B, EGRET was built by a US-German consortium and combined successful features of the small instruments, such as a large stack of digital gas-filled wire spark chambers with interleaved tantalum conversion layers, a fast directional trigger telescope, a NaI(Tl)-PMT (photomultiplier tube) calorimeter of 8 r.l. thickness, and a large anticoincidence detector to veto charged particles. EGRET had a relative energy

216 11. Imaging through Compton scattering and pair creation resolution of 25 % FWHM and an angular resolution which improved from 10 at 60 MeV to 0.5 at 10 GeV in a large 0.6 sr field of view. EGRET s on-axis effective area of 1300 cm 2 at several hundred megaelectronvolts afforded a sensitivity limit for γ-rays above 100 MeV of 10 7 cm 2 s 1 after a typical two-week-long observation. This was about an order of magnitude better than the sensitivities obtained with SAS-2 and COS-B. CGRO was operated until June 2000, although the limited reservoir of spark-chamber gas for EGRET had curtailed its observations to high-priority targets since 1996. In 2000 CGRO was deliberately de-orbited because of concerns for a failing attitude system, and crashed into the Pacific Ocean. All instruments on CGRO and especially the second generation telescope EGRET established the role of γ-ray astronomy as an important branch of astronomy and astrophysics. EGRET discovered 271 celestial γ-ray sources covering a variety of objects, ranging from the Sun, to isolated spin-down pulsars, binaries with neutron stars or black holes, supernova remnants, quasars, and γ-ray burst sources. About two thirds of the EGRET sources, however, are still unidentified, which is a fundamental challenge to the next generation of instruments. Figure 11.4 shows scaled schematic views of SAS-2, COS-B, and EGRET. AGILE and Fermi (GLAST) First in the next generation of pair-creation telescopes, which are entirely based on solid-state detector technology, is the small Italian mission AGILE, which was launched from India on 23 April 2007 into a nearly equatorial circular orbit of 2.5 inclination and an altitude of 540 km. Taking into account the very low background in the AGILE orbit, the expected responsivity to an extragalactic photon source with 5 σ significance in 10 6 s of observation time will be 10 8 cm 2 s 1 (at energies > 100 MeV), which is already a considerable improvement over the EGRET sensitivity. The Fermi mission, formerly known as GLAST, was designed to continue the successful observations of the EGRET telescope, albeit with much improved sensitivity. The main instrument on Fermi is the Large Area Telescope (LAT). The upper part of Fermi-LAT consists of 16 (arranged as 4 4) towers of pair-conversion tracking chambers, each made of 19 stacked silicon-strip detector pairs (x y resolution) interleaved with lead converter sheets. Each tower measures (33 33) cm 2, giving a total effective detector area of 9000 cm 2 above 1 GeV. The calorimeter, which is located under the tracker, consists of 80 CsI(Tl) scintillator bars per stack viewed by PIN photodiodes. The bars are approximately (2.3 3 31) cm 3 in size and are stacked in layers with alternating perpendicular directions to provide positional information about the shower. LAT is covered over its sensitive field of view with an anticoincidence charged-particle veto shield made of segmented plastic scintillator tiles. The segmentation will reduce the self-veto effect at higher energies resulting from backsplash of shower particles from the calorimeter. Since its launch into a slightly inclined ( 26 ) low Earth orbit on 11 June 2008, Fermi has been performing an all-sky survey by scanning the celestial sphere continuously with the axis pointed to zenith throughout the orbit.

Figure 11.4: Comparison and schematic layout of the pair-creation telescopes SAS-2, COS-B, and EGRET (to scale). 217

218 11. Imaging through Compton scattering and pair creation Figure 11.5: Schematic of Fermi-LAT. The modular structure of 16 trackercalorimeter towers is indicated and one tower is highlighted. Photon pair creation occurs in the upper Si-tracker chamber and the energy of the resulting particle shower is measured in the lower calorimeter detector. Fermi-LAT detects photons from 20 MeV to 300 GeV and can reach pointsource sensitivities for photons above 100 MeV of better than 4 10 9 cm 2 s 1 for an observation time of one year in scanning mode. A schematic view of one of the 16 Fermi-LAT towers is displayed in Figure 11.5. AGILE may be considered as a mini-version of such a LAT tower. The Great Observatory CGRO The CGRO was one of NASA s four Great Space Observatories. It was in orbit from April 1991 to June 2000. The 15 t payload was put into an orbit of 450 km altitude by the space shuttle Atlantis. Apart from EGRET and COMPTEL the spacecraft platform carried two more instruments, namely OSSE and BATSE. OSSE was a collimated scintillation spectrometer of NaI(Tl)-CsI(Na) phoswich detectors with a field of view of 4 11, which mainly covered energies near the transition between X-ray and γ-ray astronomy from 50 kev to about 1 MeV. BATSE was an omni-directionally sensitive Burst And Transient Source Experiment (20 kev to 1.8 MeV), which consisted of eight large-area NaI(Tl)-scintillation detector assemblies, one at each corner of the spacecraft. By comparing the count rates at each of these detectors, the locations of cosmic γ-ray bursters in the sky could be determined to an accuracy of a few degrees. The four CGRO-instruments together covered the energy range 20 kev to 30 GeV, about six orders of magnitude in photon energy. A schematic view of CGRO is shown on the left side of Figure 11.6 together with a photograph of the observatory in orbit just prior to release from the shuttle on the right side.

219 Figure 11.6: Left: schematic view of CGRO, right: photograph of CGRO in orbit aboard the shuttle Atlantis. The main goal of CGRO was to perform the first ever all-sky survey in γ-ray astronomy, and to perform subsequent deep observations of individual sources. Due to the large fields of view of EGRET and COMPTEL (co-aligned in their viewing directions), it took only 1.5 a to perform the full-sky survey. OSSE s field of view was too small for a full-sky survey, but at the end of the mission the instrument was still able to produce an image of the inner radian of the Galaxy by combining all source observations in that region. Within the mission lifetime of CGRO, BATSE detected nearly 3000 γ-ray bursts, which were distributed isotropically over the sky. In addition to these burst observations BATSE has been serving the entire high-energy astrophysics community with an unprecedented, nearly continuous, omni-directionally sensitive monitor of celestial sources above 20 kev by means of the Earth-occultation technique. This technique consists of measuring the size of step-like features in the γ-ray count-rate of the BATSE detectors, when a point source in the sky alternatively rises above or sets below the Earth limb (Harmon et al 1992). Measuring the size of the step gives nearly continuous sampling of source intensity and spectrum as a function of time, typically 15 to 30 times a day. The one-day sensitivity of this technique was typically 0.1 of the intensity of the Crab in that energy range. The most important parameters of any astronomical telescope are its angular resolution and its sensitivity. Figure 11.7 summarizes the angular resolutions of the CGRO instruments EGRET and COMPTEL together with those of AGILE, Fermi, and the INTEGRAL instruments. The resolutions are more or less all in the range of degrees. This seems modest, but it is sufficient considering the relatively small numbers of detected sources (nearly 300 in the case of EGRET and 32 in the case of COMPTEL). Fermi (GLAST) will probably detect several thousand

220 11. Imaging through Compton scattering and pair creation Figure 11.7: Angular resolution of COMPTEL, EGRET, AGILE and Fermi (formerly GLAST) in comparison with other telescopes. sources. To avoid source confusion, the source identification has to be made above 1 GeV, where the angular resolution is well below 1. The continuum sensitivities of COMPTEL, EGRET, AGILE, Fermi and also those of COS-B, INTEGRAL, and the ground-based telescopes HESS and MAGIC operating around 1 TeV are shown in Figure 11.8. Obviously the highest sensitivities are achieved at low energies below 100 kev and at high energies above 100 MeV. Two lessons can be learned from this diagram: first, from energies of a few 100 kev to about 50 MeV a sensitivity gap exists (this is essentially the range in which COMPTEL operated). Large efforts are therefore presently being undertaken to bridge that gap, and there seems to be a general agreement that this goal can be achieved by some kind of an advanced Compton telescope. Second, with the launch of Fermi in 2008 the energy ranges of space- and ground-based γ-ray astronomy will overlap for the first time. The implications of progress with respect to angular resolutions and sensitivities for past generations of γ-ray telescopes are illustrated in an impressive way by the all-sky maps shown in this book in Chapter 3 (Kanbach et al 2010) on γ-ray astronomy. There the maps of EGRET above 100 MeV, and those obtained by COMPTEL in the range from 1 MeV to 30 MeV and at 1.809 MeV ( 26 Al line) are presented.

221 Figure 11.8: Continuum sensitivities in 10 6 s of COMPTEL, EGRET, AGILE, and Fermi in comparison with other telescopes. Conclusions and outlook During the last decades γ-ray astronomy has opened a new window to astronomy. Tremendous difficulties had to be overcome to reach the present status. The results obtained so far have fascinated theorists and observers from all branches of astronomy, and in addition they have stimulated plans for new missions in which the angular resolutions and sensitivities of the telescopes will steadily improve. Bibliography Bignami GF, Boella G, Burger, JJ (plus nine authors) (1975) The COS-B experiment for gamma-ray astronomy. Space Sci Instrum 1:245 268 Derdeyn SM, Ehrmann CH, Fichtel CE (plus five authors) (1972) SAS-2 digitized spark chamber gamma ray telescope. Nucl Instrum Meth 98:557 566 Fichtel CE, Hartman RC, Kniffen DA (plus five authors) (1975) High-energy gamma-ray results from the second small astronomy satellite. Astrophys J 198:163 182. Greisen, K (1966) Experimental gamma-ray astronomy. In Perspectives of Modern Physics (RE Marshak, ed), Wiley, New York, pp.355 382 Harmon BA, Finger MH, Rubin B (plus six authors) (1992) Occultation analysis of BATSE data: Operational aspects. NASA CP-3137:69 75 Hofstadter R, McIntyre JA (1950) Measurement of gamma-ray energies with two crystals in coincidence. Phys Rev 78:619 620

222 11. Imaging through Compton scattering and pair creation Kanbach G, Bertsch DL, Fichtel CE (plus seven authors) (1988) The project EGRET (Energetic Gamma-Ray Experiment Telescope) on NASA s Gamma- Ray Observatory (GRO). Space Sci Rev 49:69 84 Kanbach G, Andritschke R, Bloser PF (plus three authors) (2003) Concept study for the next generation medium-energy gamma-ray astronomy mission: MEGA. Proc SPIE 4851:1209 1220 Kanbach G, Schönfelder V, Zehnder A (2010) High-energy astrophysics energies above 100 kev. ISSI SR-009:57 71 Kraushaar W, Clark GW, Garmire G (plus three authors) (1965) Explorer XI experiment on cosmic gamma rays. Astrophys J 141:845 863 Kraushaar WL, Clark GW, Garmire G (1968) Preliminary results of gamma-ray observations from OSO-3. Can J Phys 46:414 418 Mayer-Hasselwander HA, Kanbach G, Bennett K (plus seven authors) (1982) Large-scale distribution of galactic gamma radiation observed by COS-B. Astron Astrophys 105:164 175 Schönfelder V, Hirner A, Schneider K (1973) A telescope for soft gamma ray astronomy. Nucl Instrum Meth 107:385 394 Schönfelder V, Aarts H, Bennett K (plus 27 authors) (1993) Instrument description and performance of the Imaging Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory. Astrophys J Suppl Ser 86:657 692 Swanenburg BN, Bennett K, Bignami GF (plus 11 authors) (1981) Second COS B catalog of high-energy gamma-ray sources. Astrophys J 243:L69 L73 White RS, Koga R, Simnett G (1973) Diffuse galactic and Crab Nebula γ-rays from 1.5 to 10 MeV (Abstract). Proc 13th International Conf on Cosmic Rays 1:7 Zoglauer A, Kanbach G (2003) Doppler broadening as a lower limit to the angular resolution of next-generation Compton telescopes. Proc SPIE 4851:1302 1309