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Gamma ray sources in space their detection and measurement by Scott James Timothy Cleary A dissertation submitted to the Department of Physics, University of Surrey, in partial fulfilment of the degree of Master of Science in Radiation and Environmental Protection Department of Physics Faculty of Engineering & Physical Sciences University of Surrey September 2009 © Scott James Timothy Cleary 2009 1 ABSTRACT Gamma rays were detected from space as early as 1958, however, it is only in the last thirty five years that the use of gamma ray astronomy has come to the fore, and is now one of the most powerful techniques we have for learning about our galaxy and other parts of the universe. All this was through the chance discovery of an extremely powerful source far off in space. As a result we now have a better understanding of the distribution of not only celestial events due to gamma rays emitted in burst, but we also know the distribution of elements throughout the Milky Way from the isotopes produced through the deaths of massive stars. Although due to undetected isotopes or part of an isotopes decay chain we do not yet have a complete understanding of the process. 2 CONTENTS ABSTRACT ........................................................................................................................ 1 CONTENTS ........................................................................................................................ 2 NOMENCLATURE ........................................................................................................... 3 INTRODUCTION .............................................................................................................. 4 BACKGROUND ................................................................................................................ 5 Production of Gamma Rays ............................................................................................ 5 Space Borne Sources of Gamma Rays................................................................................ 9 Sensor Platforms ............................................................................................................... 16 Detectors ........................................................................................................................... 20 Sources of Error ................................................................................................................ 26 Gamma Ray Line Astronomy ........................................................................................... 29 Future Developments. ....................................................................................................... 36 Discussion and Conclusion ............................................................................................... 40 Acknowledgements ........................................................................................................... 42 References ......................................................................................................................... 43 Appendices ........................................................................................................................ 50 Isotope chart .................................................................................................................. 50 List of satellites ............................................................................................................. 52 Hersprung-Russel diagram ........................................................................................... 55 Ultimate 26Al Diagram….. 56 3 NOMENCLATURE SN Thermonuclear Supernovae SNIa Type Ia Supernovae, No Hydrogen fuel SNIb Type Ib Supernovae SNIc Type IC Supernovae SNIIP Type II “Plateau” Supernovae Hydrogen fuelled SNIIL Type II “Linear” Supernovae Hydrogen fuelled SNIIb Type IIb hybrid Supernovae staring as Type II becoming Type Ib AGB stars Asymptotic Giant Branch star Expl.H Hydrostatic(Explosive) NSE Nuclear statistical equilibrium α-NSE α-rich (freeze-out) n-capt Neutron capture WR Wolf-Rayet stars GRB Gamma Ray Burts/Bursters β+-decay Positron emission Main Sequence Star/s A classification for stars that is determined by size and temperature 4 INTRODUCTION Gamma rays were first discovered in 1900 by Villiard, although they become known as gamma rays due to Rutherford. It was not until 1948 when Freeberg and Primakoff theorised that gamma rays are produced in space. The first actual gamma rays detected from space were detected by sensors mounted on a high altitude balloon by the scientists Peterson and Winckler in 1958. This was in the same year the first few satellites to have gamma ray detectors were launched. Gamma rays from outer space are now one of the best ways we have of studying our galaxy and beyond, able to penetrate matter where optical radiation cannot, and more easy to study than any other form of radiation or particle over great distances. They can give us a unique insight into stars, supernovae, pulsars and even black holes. As a result there is a far better understanding of how stars burn their fuel, the final stages of a star’s life and may, in theory, even have an indirect method of detecting dark matter as a result. 5 BACKGROUND Production of Gamma Rays The presence of gamma rays in space was known before they were actually detected. Studies done by Feenberg and Primakoff in 1948, Hayakawa and Hutchinson in 1952 and also by Morrison in 1958 were responsible for the ideas of cosmic ray and dust interactions, supernova explosions and the interactions of electrons with magnetic fields X-rays. Gamma rays are in the very short wavelength region of the electromagnetic spectrum however the main definition of a gamma ray is that its source has to be from the nucleus of an atom and not from one of the electrons surrounding the nucleus. This means X-rays which are generated from electrons can have same energies as gamma rays, but it is important to make a distinction between the two. Gamma rays also have certain attributes which makes them useful for astronomy. Firstly gamma rays have a very low crosssection of interaction with matter, making it easy for them to penetrate interstellar matter. The drawback to this is that our own atmosphere absorbs this radiation before it can reach us on the ground, making direct observations on the ground impossible. Another attribute is that gamma rays are produced by the most powerful celestial events making them useful in the studying the nature of these events. There are two main groups of gamma rays that astrophysicists look at. The first are the continuous emitters, these sources continually produce gamma rays that can be located easily and studied over an extended period of time. The second are what are known as gamma ray bursts or gamma ray bursters (GRBs), these sources give an intense and powerful burst of gamma radiation in a very short period of time in the region of seconds. There are also many types of sources of gamma rays but there are multifarious ways in which they can be produced. Particle-particle collisions, particle-anti-particle annihilation, radioactive decay, fusion, fission or when a charged particle is accelerated. Particle-particle gamma generation occurs when a particle with a large amount of energy collides with another. Examples of this are cosmic rays or protons striking another proton 6 or nucleus. The resultant high energy collisions produce exotic particles called pions. Three types of pions are produced pion 0, pion minus and pion plus. These pions are unstable and decay into other forms. The plus and minus pions decay into muons, the equations for this reaction are given below. The pion 0 decays into gamma rays. As the proton or cosmic ray was moving at high speed at the time of the collision the gamma rays are emitted with only a small angle separating them. As a result of this a spectrum of gamma energies higher than 72Mev is usually seen. p → e+ + π0 [] p + p → π+ + π- π0 → 2γ (98.8%) [7] π+ →μ+ + νμ [7] μ+ → e+ + νe + ν π0 → γ + e+ + e- (1.2%) π- → + μ- + ν μ μ- → e- + ν e μ +νμ The second production method is the particle-anti-particle method, the main example being the pair production method. When a positron and electron collide they annihilate one another and emit gamma rays to conserve total charge energy and momentum. When this happens at rest two annihilation photons of 511keV each are emitted which is the rest mass energy of a positron/electron. It also works the other way so that if a gamma ray of 1.022MeV, twice the rest mass of an electron/positron can become a positron and an electron. These are shown by the equations below. e+ + e- → picosec → γγ (each gamma is 511keV). (Gamma ray has an energy of 1.022MeV) γ → picosec → e+ + e-. Fusion is another way of generating gamma rays; this involves the process of fusing two or more elements together to form a new element along with the release of energy. Some of this energy is in the form of gamma rays. Fusion is explained in a more depth in chapter(?) along with the process called neucleosysthesis. The following equation shows 7 how some gamma rays are produced via fusion. This is the process that occurs in all young stars. 1H + 1 H → 2 H + e+ + ν 2H 3He + 1H → 3He + γ + 3He → 4He + 1H + 1H This next process is connected to the previous one, fusion during the life of stars. Gamma decay is where an excited nucleus of an atom de-excites further by emitting a photon of gamma radiation. This process is responsible for providing information about the source via gamma ray emission. Gamma rays emitted in this fashion give a distinct line spectrum that identifies the type of source nuclei. This process is associated with nucleosynthesis. This process is described in more detail (in the space borne sources of gamma rays chapter). An example of radioactive decay is given below, this equation shows that 137Cs gives off a gamma ray with a specific energy. 137Cs → 137Ba(meta) + e- → 137Ba + γ (662keV) Fission is another method of producing gamma rays. Fission occurs when a heavy unstable element breaks up into fragments rather than by the other decay methods. When this happens in addition to neutrons, gamma rays are also emitted. An example of fission is given below [1]. 8 Acceleration of charged particles is the final method for the production of gamma rays. Fast moving charged particles such as natural alpha particles or accelerated ions can induce nuclear reactions giving rise to excited nuclei which de-excite almost instantly by gamma emission. Gamma ray bursts/bursters, these sources are not looked at for specific line spectra. In addition to this they only emit gamma radiation for only a brief period of time. However knowing and trying to learn from these sources is still useful. 9 Space Borne Sources of Gamma Rays In addition to distant suns there are a number of other important sources of gamma rays. These include Black Holes, Pulsars, Supernovae, and Neutron stars. In order to fully explain these objects we have to cover the present state of theory on the creation and evolution of stars. All of the above celestial events have a common factor that is important to their creation or reason for producing GRBs. This is called solar mass; a solar mass is the weight of the object with respect to our own Sun. So one solar mass equal to the weight of our sun. In addition to this it should be noted that during the lifetime of a sun it will lose mass, via energy expelled in light and the rest of the EM spectrum, and also via solar flares, solar wind, etc. So when reference to solar mass is made, this is the solar mass at that particular stage in the star’s life. Stars form when enough dust and gas, predominantly hydrogen, collects in one area of space due to gravity. The stage where a star is just forming and gathering matter is called the proto-star stage, it does not yet have enough mass to undergo fusion and become a hot star. As they coalesce they begin to heat up, this causes the collected matter to expand. This is important for two reasons if it did not happen it would just collapse in upon itself. The second reason is this heat that is generated is responsible for the ignition of nuclear fusion. The combination of these two forces is called hydrostatic equilibrium. Should a proto-star fail to achieve enough mass to become a star, less than 0.08 solar masses, it will instead become a brown dwarf, essentially a failed star. When a proto-star has enough mass the core will ignite hydrogen-hydrogen fusion, (PP I Chain), this is the first stage for all stars. 10 The first picture shows the early stages of a proto-star formation, a) shows stellar dust and gas gathering in a cloud, b) shows the gas is beginning to coalesce and the core is beginning to heat up, c) at this stage a proto-star is forming at the centre of the cloud as it continues to contract [2]. The Second picture is an artist’s impression of a Brown Dwarf, in this particular instance the brown dwarf is surrounded by the remnants of the dust cloud that formed it as it has failed to ignite [3]. The lifetime of the stars is dependant primarily on one thing: the mass of the star. The larger the star the hotter it burns, this has two effects on the stars lifetime, firstly as it is burning hotter it burns its fuel at a faster rate than a smaller star. A chart of the type of stars there are called main sequence stars is added in the appendices. It does also means that the star is hot enough to go through several stages of fusion fuel burning. As stated earlier all stars that become heavy enough to undergo fusion start off by burning hydrogen to form helium. This process is called neucleosynthesis, the first part being the PP I Chain. This follows the reaction below. 1 H + 1H → 2H + e+ + ν 2 3 H + 1H → 3He + γ He + 3He → 4He + 1H + 1H The PP II chain. This reaction shown below is a multi stage reaction. 11 1 H + 1H → 2H + e+ + ν 1 3 H + 2H → 3He + γ He + 4He → 7Be + γ 7 7 Be + e- → 7Li + ν Li + 1H → 2(4He) This leads on to the final stage, the PP III chain. 1 8 H + 7Be → 8B +γ B → 8Be + e+ + ν 8 Be → 2(4He) After this, if the star is hot enough, it will go through what is known as the CNO cycle. C + 1H → 13N +γ 12 13 N → 13C + e+ + ν C + 1H → 14N +γ 13 14 15 15 N + 1H → 15O +γ O → 15N + e+ + ν N + 1H → 12C + 4He This process will continue through various stages until it gets to Iron. At this point the amount of energy released in fusing iron nuclei together is not as much as the energy put into fusing it. At this point the star will begin to collapse. Neutron stars are the remnants of a dead star, one that was not heavy enough to become a black hole but sufficiently heavy to partially collapse its remaining core, somewhere between 1.35 and 2.1 solar masses. In this case the electrons and protons of the core of the dead star have been pulled together under gravity to form neutrons. p + e- → n + ve 12 The above picture is an artist impression of a neutron star [4]. Black holes are created when especially large suns, around 100 solar masses, collapse in on themselves when they have burnt up all their fuel through fusion. The remains of the core of the sun is so heavy that it collapses beyond the neutron star stage until it becomes an incredibly heavy point, known as a quantum singularity, which is effectively dimensionless and bends time and space around it. Anything that approaches too close to this point, into an area called the event horizon, is pulled into it with no chance of escape. This includes light and the rest of the electromagnetic spectrum. The reason why these Black Holes are a source of gamma rays is that as the matter approaches a Black hole it speeds up and heats up and then emits energy before it is absorbed. This energy release takes the form of electromagnetic waves including gamma rays. These are often emitted in high energy bursts rather than continuous transmission. 13 The above picture is an artist impression of a black hole as part of a binary system. In this instance this the black hole is pulling in material from the other star, as a result of this the matter is accelerated toward the singularity at the centre of the black hole getting heated up as it goes. Before it enters the event horizon it gives off a burst of energy, in this case shown by the two beams above and below the black hole [5]. Pulsars, or pulsating stars, are rotating neutron stars that emit regular electromagnetic pulses caused by the rotation of the star. The two beams of energy are emitted from its magnetic poles. However the magnetic poles are not in the same axis as the axis of rotation. Effectively the pulsar acts like a lighthouse, with the two beams; one of the beams is angled up the other beam angled down. The picture on the left shows an artists impression of a pulsar [6]. The Diagram on the right shows the features of a pulsar. The pulsar has an axis of rotation of axis as labelled on the diagram. It also has two magnetic poles; it is at the poles where the beams of radiation are released. These two beams act like search lights rotating around a central point, as they rotate they cross the path of the earth and we see them as pulsed flashes. [7]. 14 Supernovae are immense stellar explosions; these are caused in one of two ways. The first way is when a star has burnt up all its fusionable fuel and the core collapses, explosively expelling the entire outer layer. The second process is when a white dwarf star accumulates enough matter to cause a brief critical fusion explosion. This material is accumulated from a nearby companion. White dwarfs are the core remnant of a small sun. When a sun of a certain size, 1.44 solar masses or less, burns up the last of its fusionable material, instead of becoming a black hole or neutron star it becomes a white dwarf. The outer layers of the star are discharged as a planetary nebula. It is thought that our own sun will undergo this process. Either process of generating a supernova gives off an enormous amount of energy, the equivalent amount in an instant that our sun could produce in its lifetime. In addition to this the explosion that results is also responsible for the production of elements that are heavier than iron. This effectively is the final stage in nucleosynthesis. As mentioned earlier a star stops undergoing fusion when it reaches iron as the amount of energy released in fusing iron is less than what is put in. However due to the explosion itself having so much energy released in a small period of time means that neutrons that are present in the star can be absorbed by the heavier elements in one of two ways via the sprocess or the r-process. The s-process is the slow process of neutron capture over time a time period shorter than the beta decay process. The r-process or rapid process is captures neutrons much quicker than the beta decay. This produces neutron rich elements. It should however be noted that there are differing types of supernova. The two core types are type I and Type II, Type I, simply put the difference between these two is if the Balmer lines for hydrogen are present. For Type I there are no Hydrogen lines present, for Type II they are present. However each type is sub divided into further groups. Type I sub groups are Ia, Ib and Ic, whereas Type II are subdivided into IIP and IIL. Type Ia supernovas are thought to occur in binary systems and are due to matter from a companion star igniting on a companion white dwarf as mentioned earlier. They are also are the most luminous of all the supernovae and are one of the celestial events that are 15 considered standard candles. They also show the presence of an ionised silicon line. Ib and Ic do not show this line. Ib shows the strong presence of a helium line whilst Ic show little or no presence of this line. Type II as mentioned earlier are split into two groups, Type II sub groups are differentiated depending on how they emit light. Both types first start emitting light in a large flash as do all supernovae, however, IIP supernovae light will plateau after the initial subsidence after the flash and then some time later decline linearly. IIL supernovae have a linear subsidence in the amount of light after the initial flash. There has been recently reference to supernovae actually changing types. Such Supernovae include SN 1993J and SN2003bg. In this class of supernova, a Type II supernovae meaning it shows the presence of hydrogen at the beginning but as time goes on helium becomes the dominate emission, so it becomes a Type Ib. The result is a new classification of type IIb. The image on the left is an artist’s impression of a star losing matter to its white dwarf binary partner. The matter is being explosively ignited by the white dwarf. The white dwarf being the bright point of light at the centre of the explosion [8]. The image on the right is a picture of the Crab Nebula supernova remnant [9]. 16 Sensor Platforms The first GRB was detected by Peterson and Winckler. Its source was a solar flare and it was detected by a balloon borne experiment in 1958. Later in 1961 saw the launch of the first satellites, The Orbiting Solar Observatory (OSO) and Explorer 11 with equipment that could detect gamma rays. The OSO was designed to look at the sun for high energy gamma rays; it did not detect any such radiations. Explorer 11 looked for gamma rays with energy over 50MeV. This satellite did record events but was not targetable or specific in what it looked for. The first satellites launched with sensors that looked for Gamma rays were launched to look for the presence of illegal nuclear weapons testing both on the ground and in the atmosphere. They were launched by the American military and the satellites were called Vela. Twelve of these satellites were built in all. This was during the mid to late 1960s. However during mid-1969, Ray Klebasabel, whilst going through data for the 2nd of July 1967, found a double humped curve. After checking to confirm these were not nuclear detonations, it was subsequently confirmed that these were the first detected GRBs (gamma ray bursts) from space, by satellite. Vela-5A/B satellites, the satellites are stacked one atop the other, but are released separately [13]. 17 During the early seventies the first satellite to look outward was launched. This was called SAS-2, the second satellite of a group of three. It observed for less than a year but during that time it detected the gamma ray burst from the pulsar, Geminga. Since then there have been an increasing number of satellites launched with gamma ray detectors on them. The picture on the left is of the SAS-2 satellite [14]. The picture on the right is of the most recent Gamma Ray detector Satellite INTERGRAL [15]. A list of satellites involved in gamma missions as well as ground and balloon based systems is added in the appendices that shows gives the names of all the satellites that have been launched with a gamma ray sensor on them for observing celestial events, as well as current ground based Cerenkov experiments and recent balloon based experiments Sensors mounted onto a satellite provide advantages, however they also have some disadvantages. One of the advantages is there is no need to take into account naturally occurring gamma radiation from nearby sources, such as those produced by cosmic rays striking the earths atmosphere. However, effects from the sun now need to be taken into account, such as solar flares and sunspots. In space you do not have to take into account the distortion of the atmosphere. However, any problems with the equipment are major issues to solve once launched. Size and weight of the sensor are also now major issues, as 18 these will increase the cost significantly. One benefit satellites have over ground based systems is they are not limited to which sections of sky they can look at. However, the number one problem for ground based systems is our atmosphere. It is true that the Vela satellites were designed to look for gamma rays originating on or near the surface as well as space but the source of these was a nuclear explosion, which is by no means a small or weak source. Any gamma radiation coming in is going to be attenuated by our atmosphere. To this end there is only really one type of gamma ray detector that can be used on the ground. This is a Cerenkov detector. At present all of the major ground based systems are Cerenkov detectors. The reason for this is that unlike other sensors that are hampered by our atmosphere, contrary to other detectors Cerenkov detectors have to actively utilize the atmosphere as part of it make up in order to detect gamma rays. A Picture of The H.E.S.S Array at night in Namibia near the Gamsberg Mountain [16]. There is also a third option, one which has been used for longer than satellites and one which allows for the sensors to be raised above most of the earths atmosphere, balloons. Although aircraft may seem like a better alternative (indeed organisations like NASA have access to some of the best aircraft in the world). Balloons are able to do one thing airplanes can not do for an appreciable amount of time, and that is to loiter. This is what the sensors need to be able to do and which is a problem even for gliders. Even if a plane 19 could loiter the additional cost of fuel required for staying aloft for long periods would be prohibitive. Using balloons as a method for scientific packages has been used for a long time and the technology involved has improved. Experiments that used to only carry a small amount to be held aloft for a few days are now are able to lift up to 2000kg up to heights of 40km and stay up for up to 3 weeks. With communication technology improving as well, receiving the information is now easier and cheaper too. There is an added benefit that it is relatively inexpensive to launch and modify the experiments. It could be said that the balloon is a test bed for sensors before they are used on satellites. One drawback is in order to view a section of sky of interest the balloon has to be flown to that location. One of the NASA GRIS Balloon mounted Gamma Ray detectors [17]. 20 Detectors There are multiple types of gamma ray detectors available; however due to the improvements with time as well as suitability for use, choosing the right detector for the job is crucial. As mentioned earlier, the first satellite to detect a GBR was one of the Vela satellites. These had two gamma detectors mounted on them; the second of these was composed of 6 CsI gamma ray detectors with a total surface area of 60cm 2. The current list of types of gamma ray detectors include spark chambers, Cerenkov counters, proportional counters, scintillators, such as NaI(Tl) or CsI, with photomultipliers or photodiodes and silicon based counters, such as germanium or CdZnTe. However the benefits of the actual detector are not the only issues to consider. A spark chamber was the device used on the first few dedicated space borne gamma ray detectors. It works is via a multi-layered method of detection. Firstly a gamma ray enters the upper part of the spark chamber assembly by passing through a plastic scintillator. The scintillator allows photons through but stops heavy ions and particles. The gamma ray then passes into a high Z material; at higher energies this produces pair production. Choosing the material for this part is important as you only want gamma rays to undergo pair production and not cosmic rays. The positron electron pair then pass into the lower half of the detector containing the spark chamber, and this consists of multiple thin electrified parallel metal plates, contained in a noble gas environment. As the particles travel through the spark chamber, it ionises the gas between the plates creating an electric breakdown path immediately after the particle, thus allowing the particle paths to be visible. This can be recorded and information about the particle pair such as their direction can be learnt. The final stage is for the pair to enter a crystal scintillator. Where they will then lose the last of their energy in creating scintillation photons to be recorded by photomultipliers. If it was a particle pair then these should both be detected at a similar time [18, 19, 20, 22]. 21 The above diagram shows the stages involved in detecting a gamma ray using a spark chamber. Starting with the gamma ray going through the scintillator at the top, producing an electron-positron pair in the high Z number material, going through to the spark chamber and finally being absorbed in a scintillator and the product photons being detected by photomultipliers at the end [21]. Cerenkov detectors are predominantly, although not exclusively, used on ground based telescopes. The reason for this is they bypass the problem of having the atmosphere attenuating the gamma rays, by making it part of the detector. Thus when a space borne gamma ray enters the earth’s atmosphere it produces an effect know as an air shower. In this air shower, pair production occurs, creating a positron-electron pair. As they head ground ward they interact through Bremsstrahlung and Compton scattering, losing energy as they go via photons that are created, these in turn produce more and more positronelectron pairs and the process is repeated again and again, until the particles run out of energy. However the energy these particles have is very large, a good fraction of the speed of light. As such when they travel through a medium faster than light within the medium they will produce Cerenkov radiation. It should be noted that this is not faster than light in a vacuum, as this is theoretically impossible. Light travels at different speeds depending on what medium it travels through. As the charged particles pass nearby atoms they polarize them and this producing light in the blue part of the optical spectrum. As the polarized molecules depolarize back to a ground state. This is the Cerenkov radiation. It is this radiation that the detectors look for as it signals the arrival of gamma rays [22, 23, 24]. 22 To detect Cerenkov radiation an optical dish is required although an array of these dishes is preferred, but only one is needed in order for it to work. Ideally these should be placed at the highest altitude possible. This is not surprising as these telescopes suffer from the same problems as optical telescopes, such as atmospheric distortion and light pollution. These dishes then focus the Cerenkov light onto an array of photomultipliers which convert a light signal into an electrical one. The information from the photomultiplier array is then processed and recorded. Due to the fact that there is an array of photomultipliers and if there are multiple optical arrays, then an image of the Cerenkov light can be made. However, it is important to consider the problem of cosmic rays with this setup. These can be detected as well as the gamma ray protons produced by Cerenkov radiation in the atmosphere. Cosmic ray Cerenkov radiation is the main source of noise with these detectors. The only way around this is to use computer modelling. Using the imaging taken, it is possible to eliminate the cosmic ray “background”; this is possible due to one factor that differs between cosmic ray and gamma ray induced Cerenkov radiation. Cosmic ray Cerenkov radiation tends to have a larger angular distribution, whereas gamma ray Cerenkov has a much narrower distribution. By eliminating images which do not fit a gamma rays distribution it is possible to remove a majority of the noise produced by Cosmic rays. A computer simulation of the particle interactions and paths in an air shower [25]. 23 Proportional counters work by the ionisation of a noble gas caused by the passage of a charged particle which may be induced by energetic photons. For the detection of gamma rays this gas is either argon or xenon. There is an electrical field present inside the proportional counter and this, along with the presence of the ionising radiation, causes an electron avalanche. The electrical pulse produced by the avalanche is proportional to the energy of the detected photon, thus allowing for spectral information to be retrieved. In addition to this, if a grid arrangement is used inside of the proportional counter, then it also allows it to be used as a form of imager. Although primarily used for tracking GRB, it is from these bursters that we can find new supernovae and other sources to look at gamma line astronomy [26]. Another method for detecting gamma rays is the use of crystal scintillators such as NaI(Tl) or CsI(Na). Similar to spark chambers, scintillators do not detect gamma rays directly. The gamma rays produce charged particles (either electrons or electrons and positrons) via the three photon interactions with matter. These are the photo-electric effect, Compton Effect or pair production, in the scintillator. When a charged particle interacts with the scintillator it produces light. These can be detected and recorded either with an array of photomultipliers or photodiodes. Adding up all of the energy of these recorded events for the array, allows us to estimate the energy of the gamma ray [27]. Compton Scattering occurs in the region of 1 MeV to 30MeV, and is the dominant form of interaction for the three types of matter photon interactions within this energy range, making it useful for gamma ray detection. A Compton Scatter Telescope usually consists of two layers of detectors one atop the other. An incident gamma ray passing through the top layer travels firstly through a scintillator where it is scattered off an electron. It then travels down to a second scintillator, here it is absorbed and a light pulse emitted and recorded. Both layers of scintillator material have photomultipliers or photodiodes to calculate the points of interaction and energy transferred in each layer. There is an added complication with this type of sensor as the angle of entry does not match up with the angle of passage through the two detection layers. However, knowing the laws governing how Compton scattering works and the energy of the interactions allow for the angle to 24 be calculated. However, the directionality of the angle cannot be determined uniquely. This means that the origin of the gamma ray is on the edge of the circumference of a cone, with the point of the cone at the first layer of the detector angled so that it lines up with the direction of travel between the two layers of the detector. As a result this means determining the source of the gamma ray is problematic and requires careful data analysis ([22, 28, 29, 30]. The two diagrams above show how a Compton telescope works. The first is a diagram on the left, shows the inner workings of the COMPTEL telescope. The second shows the area (the red circle), also known as the spread function, that can be worked out to determine the possible direction of an incident gamma ray (green line) that has been detected [28, 29]. Semiconductor detectors offer many advantages over other detectors. The first is their size, for the same area as another type of detector it can be remarkably thinner. They also offer better energy resolution and spatial resolution. In addition to this they have less noise. They are not without their drawbacks as they are expensive to produce as well as the problems with growing the crystals to a large size. Two of the most popular gamma semiconductor detectors are CdZnTe and germanium. INTEGRAL has 19 germanium detectors on it, this way it has linked many of the crystals together to act as a large single crystal. The semiconductor is composed of multiple layers of materials. The first layer is the valence layer or p-type layer, the next is the band gap and the final layer is the 25 conduction band, also known as the n-type layer Semiconductor detectors work by the photoelectric effect, when an incident gamma ray interacts with the detector it creates electron/hole pairs. The n-type layer of the material will except holes while the p-type layer of the material will except electrons, as a result a current is produced the attributes of this current tell us about the gamma ray that produced it.[30]. One of the characteristics of semi conductor detectors in use out in space is that they require a coded aperture mask to form an image since it is difficult to focus the gamma rays [31]. Things can be done to improve the above detectors to make the more accurate or reduce noise such as anti-coincidence systems that eliminate any false readings due to pair production, make the data more accurate. 26 Sources of Error It should be noted that any gamma rays detected will have to be corrected for various factors that occur due mainly to the expansion of the universe and also due to the expansion of the celestial events where gamma line spectra are produced. One of these factors is known as the Doppler shift. We are all familiar with the change in pitch of the siren of a police car as passes by at speed. A similar process happens with light. There are two kinds of Doppler shift; these are red shift and blue shift. As the universe is considered to be expanding since its creation in the Big Bang it makes sense that a majority of what is observed is seen to be moving away from us. Red shift occurs when a source of electromagnetic radiation is moving at relativistic speed, away from the observer. Relativistic speed is considered for an object moving away from an observer at an appreciable fraction of the speed of light. Thus it follows that blue shift occurs when an object is moving toward us, again at a relativistic speed. The amount of shift in the wavelength is proportional to the speed of the emitter relative to the observer. This process has been used to deduce the speed of objects observed in space. The diagram above shows the effects of both types of “shift”. The star in the middle of the diagram is the source of light. If the observer on the left is stationary with the star moving toward them the light is blue shifted. For the other observer on the right the star is moving away from them and the light is red shifted [32]. 27 This is made more complicated when viewing/imaging events in our own galaxy due to its rotation. Our galaxy, the Milky Way is a barred spiral galaxy and the solar system is positioned in one of its arms or spirals. When looking at just one point in our galaxy it is easy to account for the red shift or blue shift of the object being viewed. However if a scan of the whole galaxy is made, which allows us to look at the distribution of gamma line sources such as 26 Al and positrons, throughout our galaxy, then we encounter an additional problem of Doppler broadening. Doppler broadening occurs when we have to combine readings that have been red shifted with those that have been blue shifted. Normally if a reading were to be taken of a stationary isotope the reading would give us a narrow peak at its gamma line value. With Doppler broadening the width of the peak is larger due to the combination of both types of shift being added together to give a resultant peak [36]. The above diagrams show the effect of galactic rotation on the gamma rays received. The diagram on the left shows the tiny energy shifts (<1keV) measured for the 26Al gamma line arriving from different parts of the galactic plane. The diagram on the right shows our position in our galaxy with the arrow showing the direction of rotation. The two highlighted regions show the areas that are blue shifted and redshirted as well as an area that is reasonably stationary, due to this rotation of the galaxy [33, 34]. An addition problem is there is Thermal Doppler broadening. This is different from Doppler broadening as this effect occurs when looking at just one event. In thermal broadening the nucleus of the emitting atom is moving as it emits. Now in a supernovae there are many emitting nuclei all of them moving in various directions at relativistic 28 speeds. The outcome of this is that some of the signals are again blue and red shifted, which also results in a broadening of the observed distribution of the received signals. The above diagram shows the difference is observed spectra between a gas at rest, the top spectrum, and a gas with particles moving at relativistic speeds, given by the bottom spectrum [35]. 29 Gamma Ray Line Astronomy Gamma ray line astronomy looks for the distinctive lines of gamma ray energies given off by specific isotopes when they decay. This technique has been developed over the last 30 years. The first device to look out into outer space from orbit for specific lines was HEAO-C1. Nucleosynthesis (add chapter No) is responsible for the production of these isotopes. As such using gamma line astronomy we can learn much about the generation and distribution of these isotopes in other parts of the universe as well as our own galaxy, the Milky Way. This is due to the fact that gamma rays themselves can transverse the great distances involved with little chance of being attenuated along the way. This includes clouds of interstellar dust in the galactic plane which normal light cannot penetrate. In addition to this they are not fundamentally affected by changes to physical conditions, such as temperature or density, (Roland Diehl et al,[]). To briefly explain and give some idea of the size of these distances these gamma rays travel, some explanation is needed. A light-year is the distance light travels in one year in a vacuum; this is equal to 9.4605284*1015 meters or just under 1013 km. Another unit of measurement that might be used is the parsec. This is defined as the distance from the sun, (also known as an astronomical unit or 1 AU), which results in a parallax of one second arc as seen from earth. An illustration of how this works is shown in the diagram below. This is equal to 3.26156 light-years or 3.085*1016 metres. The nearest star to our own sun is Proxima Centauri; this is 4.24 light-years or 1.30 parsecs. At present the only other means of measuring the reactions going on in a star is via direct neutrino measurements. Neutrinos are uncharged particles which have a very small mass. Consequently they are difficult to detect and almost impossible to image which makes it difficult to be sure of their source But they do have very high relativistic velocities and can arrive at the earth fairly soon after the optical photons have signalled a related astronomical event that produced them. 30 It should be noted that although gamma rays allow us to look at these events we are limited in what isotopes we can look at. The lighter nuclei tend to be shrouded from view by the rest of the stellar envelope, thereby they never leaving the interior of the star, thus denying us a chance to look at them. This means that most of the observed gammas tend to be from the heavier nuclei that are produced and get ejected in the final stages of a stellar explosion or other such catastrophic event. In addition to this a large amount of the decaying isotopes needs to decay over a similar time in order for it to be detected. These two factors mean that most of the stars studied by this method tend to be very large. Isotope 7 Be Mean Lifetime Ni 111d 57 Ni 390d Na 3.8y 44 Ti 89y 26 Al 1.04*106y 60 Fe 2.0*106y e+ γ-ray energy keV Be →7Li* Ni → 56Co* → 56Fe* + e+ 478 (0.1) 158, 812; 847 (1), 1238 (0.68), 2598 (0.17), 1771 (0.15) Co → 57Fe* 122 (0.86), 136 (0.11) Na → 22Ne* + e+ Ti → 44Sc* → 44Ca* + e+ 1275 (1) 78 (0.95), 68 (0.078); 1157 (1) Al → 26Mg* + e+ 1809 (1) Fe → 60Co* → 60Ni* 59 (), 1173 (1), 1332 (1) 7 77d 56 22 Decay Chain …105y 56 57 22 44 26 60 e+ +e- → Ps → γγ 511 (1) The above chart shows all the isotopes that are used in gamma line astronomy at present, this is an abridged version, and the full detailed chart is in the appendices. Underline energy values are one which have been detected []. 7 Be is slightly different than most of the other nuclei looked at in nucleosynthesis as it is actually produced during the stars’ life time in addition to being present at its end. The problem is that during the stars’ burning phase it is present, but not in an area that is readily detectable. This is because it is not in an area of the star that is opaque to gamma rays, either the core or one of the inner layers; hence we do not usually seen an emission 31 from it during this stage of a star’s life. With its short mean lifetime of 77 days this might not be long enough to be noticeable at the end of a star’s life in a supernova. However, the distinctive 478 keV line has been detected from our own star [36,37] during a solar flare. So we know it is present in nucleosynthesis. However thus far, other than our own Sun, it has not been detected elsewhere. 56 Ni production has so far only been seen in two supernovae, SN1987A [37, 38] in the form of 847 keV and 1.238 MeV gamma lines and SN1991T [39]. This means that this isotope is produced via nucleosynthesis in a similar fashion to the other isotopes studied in gamma line astronomy. However, due to its properties such as its short mean lifetime of just 111 days for its longest lived detectable isotope, it is thought to be linked with GRB. The link between Type Ic supernovae and GRB is to do with the optical afterglow from these GBR’s which indicate the presence of a supernova. Type Ic supernovae are due to the explosion of a carbon-oxygen star. These stars evolve when a massive star loses its hydrogen and helium envelopes. They eject into space a large amount of 56 Ni and it is this that gives the supernovae some of its gamma ray luminosity. The best estimate of the ejected matter is around 0.3-0.7 solar masses. 57 Ni gamma ray lines were observed, both 122 keV and 136 keV with the Gamma-Ray Spectrometer aboard the SMM (Solar Maximum Mission) in 1988 and also by the Oriented Scintillation Spectrometer Experiment aboard the Compton Gamma Ray Observatory in 1992 [37, ,39,], both observations coming from SN1987A supernova. This means it was detected by the same instruments at the same time as one of the 56Ni detections. Other than proving that the nucleosynthesis of heavier elements does occur during a supernova, it has not been detected elsewhere since. 22 Na has been searched for in a way similar to how 7Be was discovered, by looking in the local area for the biggest source of gamma rays, the Sun. This is in addition to other sources such as nearby supernovae. However unlike 7Be no signals have been detected. In this case it was the SMM satellite that was scanning in particular for the line [42]. 22 Na gamma 32 The presence of 44 Ti in the form of all three of its gamma lines, (78 keV, 68 keV and 1.157 MeV), were detected in the young supernova remnant Cas A (Cassiopeia A), this was in 1997. Due to its characteristics, primarily its mean lifetime, 44Ti it is considered an ideal element to observe for study of the inner region of a supernova during its early stages. This idea has been brought about by our knowledge of GRB [43]. We know from GRB that axial jets form during a core collapse producing an asymmetrical core collapse. If this proves to be the case then it is possible that the 44 Ti is ejected at high speed. However so far Cas A and SN1987A are the only times these gamma ray lines have been detected and the Cas A is the least unambiguous reading of the two. This does pose some new questions with regards to our current understanding of the way supernovae work. There is also an idea that 44Ti is produced via other astrophysical means either by the GRB from neutron star surfaces. The 26Al 1.809 MeV gamma line is one of the most studied spectral lines in the gamma region. There are many reasons for this. Particularly its mean lifetime of 1.04 My, this shows the production of recent (relatively speaking) massive stars that have an average lifetime measured in tens of millions of years. This also shows us that nucleosynthesis of stars is an ongoing occurrence. This is supported by the fact that the first readings taken for 26 Al were in 1978 on the HEAO-C Satellite and there have been many more since then. One of which is the COMPTEL experiment that mapped out the 26 Al emission in the galactic plane. This shows that most of the 26Al production is in the galactic plane and not outside of it; this is in line with predicted models. There are regions where the production is larger than the rest, such as Cygnus. One of the current instruments looking at 26 Al production is the SPI on INTEGRAL. As this is a more modern and accurate satellite with its 19 element Ge camera, it is doing its own sweep of the sky looking at the 26 Al emission line [44, 45, 46, 47]. 33 The above diagram shows the distribution of 26Al modelled around our galaxy, the Milky Way, using information from the COMPTEL and INTEGRAL/SPI systems. As you can see it is concentrated mostly in the plane of the galactic especially the galactic core; however there are the regions of interest such as Cygnus, Vela, Orion and Sco-Cen. A more detailed diagram showing readings from each area is included in the appendices [49]. 60 Fe has posed as something of a problem with regard to gamma line astronomy. It has been recorded by both the RHESSI’s sensors and also by those of the SPI camera on INTEGRAL in the form of 1.173MeV and 1.332MeV gamma lines. So we know it is being produced by nucleosynthesis. By comparing the 26Al emissions with 60Fe we can build up a model of the core collapse nucleosynthesis. Wherever 26Al is detected, we also expect to see the presence of My and 60 60 Fe as well. They have similar mean lifetimes; Fe is 2 My. However, the amount of 60 26 Al 1.04 Fe detected is only a 10th of the 26 Al signal, with the RFe-Al ratio considered to be 14% +/- 2% which, considering what we expect for nucleosynthesis is in line with earlier predictions of around 15% [46, 48]. In addition to doing a survey of 26Al, INTEGRAL is also doing a survey of the 511 keV gamma line. Although not a radioactive isotope, positron emission lines are still being looked at as they can also reveal information about our own galaxy we are in. Now we would expect to see evidence of anti-matter annihilation emissions from areas such as 34 stars and supernovae and novae, as they produce proton rich isotopes that will emit positrons in order to increase their stability. As with 26Al most of the readings for the 511 keV lines are in the plane of the galactic. Again this makes sense as this is where the stars and both supernovae and novae are in this area and they both produce positrons from the decay reactions of 22 Na, 44 Ti, 56 Ni and most importantly due to its abundance 26 Al. However, the 511keV positron emission line is the brightest gamma ray line seen. This emission is brightest , in the centre of the galaxy, why this is, is not yet fully understood. One line of reasoning that takes this into account is dark matter. It is thought that the large amount of positrons produced is a by-product of the presence of dark matter. This is only a theory at present and needs further study. The above diagram shows the distribution of 511 keV gamma rays throughout the galaxy recorded by INTEGRAL. As is quite clearly the case the largest concentration is centred on the galactic bulge at the centre of the galaxy [50]. Although already repeatedly and briefly, GRB’s are used for finding out information about gamma line astronomy. GRB’s in addition to giving us information about black holes, pulsars and other celestial events, most importantly supernovae, and the simplest way they do this is by letting us know when and where a new one has formed. When a supernova forms, (mentioned in space borne sources of gamma rays chapter) it gives off a GRB. Thus using gamma ray sensors all around the world and by working quickly we 35 can trace the GRB back to its source, if it proves to be a supernovae then this would give us a chance to study it in its early stages and perhaps study some of the lesser seen isotopes by observing it from the beginning, whilst they are most abundant. 36 Future Developments. Although a lot has been learned from gamma line astronomy there is much still to be learned. Some new ideas are being looked at, that could take gamma line astronomy use even further. However there are some points that at present won’t change at present the best way to study gamma line astronomy is in space or by high altitude balloon. Gamma-ray lenses are a new idea for hardware to help with the collection of gamma line data. These would also help to create a large area of detection by concentrating the gamma rays onto a small area detector. This is done by Bragg diffraction in a Laue lens with a high Z number crystal, copper is a good example. This is being planned for the MAX space vehicle, but has already been used on a balloon borne experiment. In order for it to work concentric rings of different material would need to be used with different diffraction values. The outer rings would diffract the gamma rays more than the inner rings thus the overall result is like that of an optical lens. One of the major stumbling blocks is that the focal length is very large. This effectively means that two satellites would be needed operating at the large distances apart one with the detector and one with the lens. In addition to this the lens has an additional drawback in that the energy bands it can focus are very small. This means that each mission could only look at one area such as the 511 keV energy range for positrons. 37 The above diagrams show aspects of the use of Laue diffraction lens, the first shows the configuration of two concentric circles of two different diffracting materials acting as a lens. The second picture is an artist impression of one way of linking the two halves of the detector, the bottom half housing the detector and solar panels, the second part consisting of the Laue lens being held at a fixed distance in front of the detector by an expanding structure [51][52]. Multilayer coatings as the name implies, is the use of multiple layers of coatings on atop the other, alternating between one material and another material. In addition to this they vary in thickness the outer layers being thicker than the inner layers. This means that different wavelength photons are reflected giving a distribution of wavelengths. The two types of materials vary in density so that there is a high density contrast between them. One layer usually consists of higher density materials such as Tungsten or Platinum. The second layer is made out of low density materials such as Carbon and Silicon. These layers reflect lower energy gamma and X-rays. As mentioned earlier the boundary between gamma rays and X-rays is sometimes a bit blurred, as nuclear spectral lines and characteristic atomic X-ray lines can end up having similar energies. These mirrors work in the hard X-ray and gamma area up to 80 keV, beyond this region they start absorbing the energy rather than reflecting it. The use of these mirrors will allow for large area collection on a small sensor. NASA is launching a satellite in 2011, called the NuSTAR with such a mirror mounted on it. This will improve studies of the 68 keV and 78 keV line emitted by 44Ti. 38 The above diagram shows how the layers or work on a multilayer mirror work the outer layers are larger an reflect the larger wavelengths, each subsequent layer after the outermost gets progressively thinner each time the wavelength of the reflected light gets shorter [53]. Another form of lensing for gamma rays is via the use of a Frensel Lens [56]. The lens works by using both diffraction and refraction. They originally worked by blocking part of the radiation. As such they are not incredibly efficient at around 10%. However with variations on the original theme it is possible to increase this. The type of Frensel lens used is one called a Phase Fresnel Lens (PFL, type c in the diagram). The result of using this lens would be an increase in the angular resolution and away around the problem of scaling up gamma ray technologies. 39 The above diagram shows the arrangement of the lens. The first part of the diagram shows how the lens looks head on, with the a to be representing variants of the lens (a) A Frensel Zone Plate (FZP), (b) A Phase Zone Plate and (PZP) (c) A Phase Fresnel Lens (PFL)[54]. At the moment Compton telescopes suffer from the fact that it is not possible to determine the direction to a reasonable degree of each gamma detected. One method that is currently being looked at involves measuring the direction of the scattered electrons in the top level of the detector. By combining this with the other information that can be learned from a gamma ray by a Compton telescope it should become possible to eliminate the spread function. This would reduce the amount of data analysis involved a great deal. There is a line of thought that suggests that gamma line astronomy could be used in order to find dark matter. Dark matter is a little understood form of matter that is very hard to detect. At present one of the only ways it has been shown to exist is through the collisions of galaxies and the effects of gravity due to matter, including dark matter, confirming its presence. At present estimates dark matter is thought to make up about 22% of the energy in the universe. The rest is composed of 74% dark energy with only 4% composed of normal matter. The reasoning behind it is to down to the large amount of positrons in the galactic bulge. So far most of these studies are computer models that involve WIMP’s (weakly interacting massive particles). One such model states that that dark matter particles annihilate at rest with 10 MeV in the galactic bulge giving rise to electronpositron pairs via a process called the t-channel exchange of heavy fermions [55]. 40 Discussion and Conclusion As it stands what information we have currently is limited as we only have a limited number of Supernovae that have been studied. We can only study the supernovae as they appear; this is just a question of time. With a supernovae core collapse rate of one ~ 50 years it is a slow process. However, another part of this is due to the fact that some of the isotopes in question have a very short lifespan and we need the satellite/detector to take readings as soon after the event as possible. This is currently exactly what scientists are working toward with satellites like INTEGRAL, by having a sensor to look for gamma ray bursts so that it is then able to locate the source and then using its main instrument learn what it can with little or no delay, combine this with the ground based arrays this improves the coverage considerably. The other problem is the distance and location of the events. Combining all of these factors means we can only learn about these events as often as they happen for information that occurs immediately after term, otherwise we can only learn from the long term effects which do not give us a complete picture. Even though INTEGEAL has done an amazing job since its Oct 2002 launch it has limitations, one of these is the ability to make an image of 60Fe gamma rays []. However the ability of satellites will only improve with the new developments listed in the Future Developments chapter. Each new mission will bring more information and ideas as well as questions. Gamma-rays are still the most viable option for learning about nucleosynthesis in the galaxy and beyond. The process will only improve with new developments such as given in (future developments). At present there is no better alternative for this. At the time of this dissertation there was no clear relationship between the types of supernova and the gamma ray lines emitted. I think this would prove a useful, even though it would be a very long term project. As it may prove to be a correlation between the unique conditions of each supernova that the distribution of gamma ray lines emitted 41 would vary for each isotope. We know that there are processes involved in supernovae such as the r and s processes that determine the type of isotopes produced, this may have something to do with the supernova type. As for the other elements that such as using gamma radiation from GRB to find celestial events this is just an ongoing process and is still the best way of finding these events. 42 Acknowledgements I would like to thank first of all Dr Gilboy, Prof Walker, Prof Regan and Dr Bradley for taking the time to explain some of the physics that I didn’t understand, give suggestions on the content and provide encouragement throughout. I would also like to thank Mrs Jan Walker from the Learning Support group for helping me with the structuring, planning, organising, presentation and further encouragement for this dissertation. I would also like to thank the MSc support staff particularly Miss Alexia Smith, MSc Course Secretary for all the questions I had with regard to past dissertations and the rules and procedures that needed to be followed. In addition I would also like to thank Mrs Christobel Soares-Smith, Physics Department Secretary for her help with the Physics department library. 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[57] www.mpe.mpg.de/gamma/science/lines/60Fe/60Fe.html Roland Diehl Max-Planck Institute, 48 [58] Gamma rays from cosmic radioactivities, Roland DIEHL, Dieter HARTMANN and Nikos PRANTZOS, Meteorities and Planetary Science, Received 14th October 2006, revision accepted 19th March 2007 (Gamma Cosmic Radio.pdf) [59] Astrophysical constraints for gamma-ray spectroscopy, Roland Diehl, Nikos Prantzos and Peter von Ballmoos, Nucl.Phys.A Special Volume on Nuclear Physics, 17 Feb 2005 [60] Abundances of Isotopes from nuclear line spectroscopy, Jaina Fiehl, Copied from Diehl, http://www.mpe.mpg.de/gamma/science/lines/ (titelseitte.pdf) [61] Supporting Gamma-Ray Spectroscopy in Space, Michael Lang, Technische Universitat Munchen, 3rd July 2006. [62] http://www.projectcalico.com/Chapter_2.htm 49 50 Appendices Isotope chart 51 [58,59 ,60. 61] 52 List of satellites 53 Satellite Built Status SAS-2 15/11/1972 Inactive Band 20MeV1GeV Cos-B 09/08/1975 Inactive 2keV5Gev HEAO 3 20/09/1979 Inactive 50keV10MeV Sensor Type 32-level wire spark-chamber aligned with satellite spin axis (20 MeV-1 GeV), eff. area 540 cm 2 Magnetic-core, wire-matrix, spark chamber gammaray detector (~30 MeV-5 GeV), eff. area 50 cm2 at 400 MeV, a 2-12 keV proportional counter mounted on the side of the gamma-ray detector High Resolution Gamma Ray Spectrometer (HRGRS): 50 keV - 10 MeV, FOV 30°, effective area 75 cm 2 at 100 keV Gamma 01/07/1990 Inactive 2keV5MeV, 50MeV6GeV Compton (CGRO) 05/04/1991 Inactive 30 keV 30 GeV Coded-mask X-ray telescope (SIGMA) 0.03-1.3 MeV, eff. area 800 cm2, FOV 5°x5°, Coded-mask X-ray telescope (ART-P) 4-60 keV, eff. area 1250 cm 2, FOV 1.8°x1.8°, X-ray proportional counter spectrometer (ART-S), 3-100 keV, eff. area 2400 cm 2 at 10 keV, FOV 2°x2° All-sky monitor (WATCH) 6-120 keV, eff. area 45 cm2, FOV All-sky Gamma-ray burst experiment (PHEBUS) 0.1-100 MeV, 6 units of 100 cm 2 each, FOV All-sky Gamma-ray burst experiment (KONUS-B) 0.028 MeV, 7 units of 315 cm2 each, FOV All-sky Gammaray burst experiment (TOURNESOL) 0.002-20 MeV), FOV 5°x5° Gamma-1 Telescope 50MeV-6GeV, 2 Scintillator counters and a Gas Cerenkov counter, eff.area of 2000cm2 Res @ 100MeV 1.5 degrees. Imaging view +/- 2.5 degrees. Energy Res 12% at 100MeV. Disk-M Telescope 20keV-5MeV, NaI scintillators, Res 2arcmin (stopped shortly after launch), Pulsar X-2, 2-25keV Res 30arcmin. The Burst and Transient Source Experiment (BATSE) an all sky monitor 20-1000 keV, The Oriented Scintillation Spectrometer Experiment (OSSE) 0.05-10 MeV energy range, The Compton Telescope (Comptel) 0.8-30 MeV capable of imaging 1 steradian Energetic, Gamma Ray Experiment Telescope (EGRET) 30 MeV10 GeV 0.5keV400keV High Resolution Gamma Ray Spectrometer (HRGRS): 50 keV - 10 MeV, FOV 30°, effective area 75 cm2 at 100 keV, Heavy Nuclei Experiment, Cosmic Ray Isotope Experiment French Gamma-ray Telescope (FREGATE; 6-400 keV). 4 NaI(Tl) gamma-ray detectors. Eff. area 120 cm2. FOV ~3 sr. Spectral resolution ~25% @ 20 keV, ~9% @ 662 keV. 3keV10MeV 2 Gamma-ray instruments Spectrometer (SPI; 20 keV 8 MeV) Coded aperature mask. FOV 16°, detector area. 500 cm2 (Germanium array) spectral resolution (E/dE) 500 @ 1 MeV, spatial resolution 2°. Imager (IBIS; 15 keV - 10 MeV) Coded aperature mask. FOV 9° X 9°, detector area. 2600 cm2 (CdTe array) & 3100 cm2 (CsI array), spatial resolution 12´. Granat 01/12/1989 Inactive 2keV100MeV LEGRI 19/05/1997 Active HETE 09/10/2002 Active INTERGRAL 17/10/2002 Active 54 Swift 20/11/2004 Active AGILE 23/04/2007 Active Fermi 11/06/2008 Active Ground Built Status Veritas Active 0.2keV150keV Burst Alert Telescope (BAT; 15-150 keV), Wide fieldof-view coded-aperture imager. Eff. area 5240 cm2, FOV 1.4 sr half coded, ~4 arcmin position accuracy. XRay Telescope (XRT; 0.2-10.0 keV). CCD Imaging spectrometer. Eff. area 110 cm2 @ 1.5 keV, FOV 23.6´ X 23.6´, ~5 arcsec position accuracy UV/Optical Telescope (UVOT; 170-650 nm).Modified RitcheyChrétien telescope with image-intensified CCD detector. 6 color filters and 2 grism, FOV 17´ X 17´, 0.3 arcsec position accuracy. 18keV60keV, 30MeV50GeV 2keV30Mev Gamma ray imager tungsten-silicon tracker, 30MeV50GeV. Silicon Based X-ray detector, SuperAGILE (SA), 18keV-60keV. Mini Calorimeter 300keV-100MeV CsI(Tl). scintillators with photodiodes at both ends in two orthogonal layers, eff.area 1400cm 2 Glast: 12 Sodium Iodide (NaI) scintillation detectors and 2 Bismuth Germanate (BGO) scintillation detectors. Band 50GeV50TeV 50 GeV30 TeV 100GeV1TeV 1TeV10PeV 10GeV100TeV Air Based Cerenkov Detector Magic 2008 Active H.E.S.S 2002 Active HEGRA 1987 Cherenkov 2010 Inactive Not Built Cactus 2001 Inactive Air Based Cerenkov Detector Cangoo 2004 Active Air Based Cerenkov Detector Balloon Caltech GRIP Launched Status Band 30keV2MeV GRIS HIREGS IV Air Based Cerenkov Detector Air Based Cerenkov Detector Air Based Cerenkov Detector Air Based Cerenkov Detector Sensor Type NaI(Tl)/CsI(Na) 7 n-type ultra pure germanium detectors 20keV18MeV PoGO Lite TIGRE MEGA 1MeV100MeV 400keV50MeV NaI(Tl)/CsI(Na) 55 Hersprung-Russel diagram 56 [62] 57 Ultimate 26Al Diagram 58 [Reference on Diagram]