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Gamma-ray Line Sources in Space By Richard Henry Congdon 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 Electronics and Physical Sciences University of Surrey September 2009 ACKNOWLEDGEMENTS I wish to thank my supervisor Professor Philip Walker for the encouragement and intellectual stimulus he provided me with during the preparation this thesis. I thank my wife Ina for her encouragement over the two years of this part-time MSc, and for walking to work once a week so I could attend this course. I extend my gratitude to Michelle and Ian at the Royal Mail delivery office in Stowmarket for their help in assuring that my day off coincided with my attendance day at the University of Surrey, over the last two years. i ABSTRACT This thesis reviews the progress that has been made in detecting gamma-ray lines, produced by the decay of radionuclides, which fingerprint the continuing nucleosynthesis in the universe. 26Al with gamma-ray emission energy of 1806.65 keV is the most widely detected throughout the galactic plane. The other reported detections are: the 847 keV and 1238 keV lines from 56Co, the 122 keV line from 56Co, the 44Sc line energy 1157 keV reported as the detection of 44Ti and later confirmed with the detection the actual 44Ti energy lines of 67.9 keV and 78.4 keV, the undetected 60Fe 58.6 keV line which in the meantime has been reported with the observation of the 1173 keV and 1333 keV from its daughter decay product 60 Co, detection of positrons has been reported and although linked to the β+ decay of 26Al and 44 Ti has recently been linked to hard low mass X-ray binaries. 22Na has been mapped but not reported as detected. Other short lived candidates await detection with the arrival of the next supernova. ii CONTENTS ACKNOWLEDGEMENTS ........................................................................................................ i ABSTRACT ...............................................................................................................................ii 1 INTRODUCTION .............................................................................................................. 1 2 MECHANISMS OF NUCLEOSYNTHESIS ..................................................................... 4 2.1 BIG BANG PRIMORDIAL NUCLEOSYNTHESIS ..................................................... 4 2.2 STELLAR NUCLEOSYNTHESIS A<60 ...................................................................... 6 2.3 STELLAR NUCLEOSYNTHESIS A>60 ...................................................................... 8 3 HISTORY OF GAMMA-RAY LINE DETECTION ....................................................... 10 4 GAMMA-RAY LINE SOURCES.................................................................................... 16 4.1 DETECTIONS .............................................................................................................. 17 4.1.1 ALUMINIUM-26 (26Al) ........................................................................................... 17 4.1.2 COBALT ................................................................................................................... 20 4.1.2.1 COBALT – 56 (56Co) and NICKEL – 56 (56Ni) ................................................... 20 COLBALT-57 (57Co) ................................................................................................ 21 4.1.3 4.1.2.2 COBALT – 57 (57Co) and NICKEL – 57 (57Ni) ................................................... 21 4.1.2.3 COBALT – 60 (60Co) and IRON – 60 (60Fe) ........................................................ 23 4.1.3 TITANIUM-44 (44Ti) ................................................................................................ 24 4.1.4 POSITRONS (e+, β+) ................................................................................................ 25 4.2 MARGINAL DETECTIONS ....................................................................................... 26 4.2.1 SODIUM – 22 (22Na) ................................................................................................ 26 5 DOPPLER EFFECTS ....................................................................................................... 28 6 CONCLUSION ................................................................................................................ 31 REFERENCES ......................................................................................................................... iv APPENDIX A: NUCLEAR DECAY SCHEMES..................................................................... x iii 1 INTRODUCTION Only a few of the elements in the universe were created at its beginning during the Big Bang, namely: hydrogen, deuterium, helium-3 and lithium (1H, 2H, 3He, 7Li). No further stable elements were created at this time because the rapid expansion and cooling, as the Universe expanded into of the void. The rest of the elements were created and continue to be created in the stars (Krane, 1989). The evidence for nucleosynthesis in the stars is found from the identification of radionuclides (RNs) with a mean life time τ of less than the age of the universe. The first of the RNs to be detected was technetium identified from visible spectrographs from S class stars, using 200inch optical telescope, including R Andromedae illustrated in Figure 1. A spectral class S designates a red star with a high concentration of zirconium oxide (ZrO) (Keenan, 1954). Figure 1 (a) Spectrum of technetium in R Andromedae an s class star, absorption lines are labelled with their wavelength in Angstroms (1 Angstrom = 1 x 10-10m) (Moore, 1956). Figure 1(b) shows The Andromeda Galaxy photographed with a 12.5-inch telescope by amateur astronomer Robert Gendler (Imaginova, 2002). Technetium is an element with no stable isotope (Merrill, 1952) and a mean lifetime much less than the age of the star. Therefore its synthesis must be on-going. Optical spectroscopy is limited to identifying elements and unable to distinguish between isotopes of an element. 1 Gamma-ray spectroscopy is able to distinguish unambiguously between radioactive isotopes of elements, with the abundance being proportional to the line intensity, without being affected by physical conditions around the source such as temperature (except electron capture ). The distance from detector to source is limited, by the low number of gamma-ray photons detected per second requiring count times of weeks to months, to sources within our Galaxy and up to 10 Mpc from the detector (Diehl et al, 2006). When most radioactive nuclei decay, the daughter nucleus is left in an excited state and returns to the ground state by emitting gamma-rays of a characteristic energy difference between the nuclear states (Krane, 1989). Figure 2 shows the decay scheme for 60Co with the characteristic energy gamma-rays emitted by the excited daughter nucleus 60Ni, as an example of the process that produces gamma-ray lines. Figure 2 shows the nuclear decay scheme for 60Co (NNDC, 2003) and shows the gammaray intensity as a percentage which is high for the two characteristic de-excitation lines 1173.2 and 1332.5 keV of the daughter nucleus 60Ni. The parent decays to the daughter with a half-life of 1925.25 days (5.27 yr). 2 These characteristic energies form peaks above the background and are referred to as lines (because of their similarity with optical spectrum lines). An example of a gamma-ray line spectrum is shown in Figure 3. Figure 3 shows a gamma-ray line spectrum for a 60Co source, measured by the author, with the 1173 keV and 1332 keV lines. Each radionuclide has a characteristic fingerprint of one or more energy lines specific to that radionuclide. The gamma-rays are measured using a suitable detector outside the Earth’s atmosphere; because gamma-rays entering the atmosphere interact with the atmosphere and are absorbed making their detection almost impossible at the surface of the Earth (Pinkau, 2009). The gamma-ray line spectra produced by the galactic sources provide additional information, via the Doppler Effect, about the speed of ejection from the star and the direction of rotation of the arm of the galaxy in which the source is located. Gamma-ray de-excitation lines are also produced following accelerated particle interactions from sources such as solar flares. The lines are typically 4.44 MeV from 12C and 6.13 MeV from 16O which are not the result of nucleosynthesis simply because there cannot be long lived radioactive nuclei that decay into the excited states of these nuclei (Ramaty & Lingenfelter, 1995). This thesis is a review of the progress made in finding gamma-ray line sources in the universe and the understanding of the nucleosynthesis producing the sources. 3 2 MECHANISMS OF NUCLEOSYNTHESIS The starting point for an explanation of the processes of nucleosynthesis of elements is the observed abundances of the elements. The observed values have been measured from solar photosphere abundances and meteorite abundances. The theory of nucleosynthesis explains the observed abundances illustrated in Figure 4. Figure 4, shows the observed elemental abundances of the proto-sun (solar system abundances) for the first 42 elements in the periodic table with vertical error bars. Graph drawn by author adapted from data by (Lodders, 2003). 2.1 BIG BANG PRIMORDIAL NUCLEOSYNTHESIS Only a few of the elements in the universe were created at its beginning during the Big Bang. In this instant the temperature of the photons was so high that any particle could come into existence, with the possible exception of free quarks. Within the first 250 s the first stable light elemental nuclei where formed (1H, 2H, 3He, 7Li ) appearing in their atomic form about 7 x105 yr later, when the photon energy had fallen below their ionisation energies. No further stable elements were created at this time because the rapid expansion and cooling, as the universe expanded into of the void. The rest of the elements were created and continue to be 4 created in the stars (Krane, 1989). The standard Big Bang nucleosynthesis theory (SBBN) developed by Peebles (Peebles, 1966) and others accounts for astrophysical measurements of primordial abundances of 1H, 2H, 3He, 7Li. The theoretical abundance values for these nuclei are matched against the observed abundances shown in Figure 4. The synthesis of nuclei, larger than the hydrogen nucleus, must begin with the reaction: Equation 1 The reverse reaction takes place more often than the forward reaction because of the high concentration of photons. The average energy of the photons is below the photo dissociation energy of deuterium of 2.225 MeV (1.1125 MeV per nucleon shown in Figure 5) equal to a temperature of 2.58 x 1010 K. There are 109 times more photons than protons or neutrons (nucleons) which mean there are a large number of photons, in the high energy tail of the photon energy distribution, with energies above 2.225 MeV available to reverse the process. The large photon-nucleon density ratio η-1 = 109 stops deuterium synthesis causing a deuterium bottleneck which eventually stops when sufficient expansion has reduced the high energy tail and the average photon energy to 100 keV equal to a temperature of 9 x 109 K (Iocco et al, 2009). With the formation of deuterium, other reactions become possible, all with nuclear binding energies above (more negative than) that of deuterium (see Figure 5), so if the temperature is low enough for deuterium to form then these other products will also form. The primary end product will be helium which will soak up all the neutrons. A few mass 7 nuclei will form but only a small quantity is produced as the temperature is too low, and the low temperature prevents the re-conversion to helium. This is one explanation for the low observed abundance of lithium shown in Figure 4. 5 Figure 5., Shows the variation of the average binding energy per nucleon, for selected elements and isotopes with increasing mass numbers up to 58. Binding energy is always negative and only zero at an infinite distance from the nucleus and represents the work necessary to dissociate the nucleus into separate nucleons. Binding energy values calculated and presented by author from data (Baum et al, 2002). Error bars are atoo small to be displayed. 2.2 STELLAR NUCLEOSYNTHESIS A<60 A star begins life as a cloud of hydrogen and helium gas which collapses under gravity, in doing so the individual atoms gain kinetic energy from their gravitational potential energy and so increase their temperature. When the temperature is high enough fusion occurs between the hydrogen atoms and the outward pressure from the radiation produced halts the gravitational collapse and the star, like our Sun, begins an equilibrium phase. This hydrogen burning phase lasts until all the hydrogen has been consumed, gravitational collapse resumes until a high enough temperature is reached to start helium burning. The increased temperature causes a larger radiation pressure expanding the sun’s surface by a large enough factor to lower the surface temperature causing a red colour – red giant stage (Krane, 1989). Two helium nuclei fuse to produce 8Be which is unstable and breaks apart in 10-17 s. However a third helium nuclei fuses with the beryllium nucleus to form 12C because of a 6 resonant energy level in the 12C nucleus first proposed by Fred Hoyle in 1954 (Gribbin & Rees, 1991). The absence of this resonance in 16O prevents all the 12C from being converted into 16O. When all the helium had been depleted, gravitational collapse resumes until a high enough temperature is reached. Some lower mass stars end their lives here and cool to lumps of matter. If the star is above 4M⨀ then a high enough temperature is reached to allow carbon burning Two carbon nuclei fuse to produce neon (20Ne) plus helium 12C + 12C → 20Ne + 4He. Once the carbon has been consumed, gravitational collapse resumes until a high enough temperature is reached. Once again some lower mass stars end their lives here and cool to lumps of matter. If the star is above 9M⨀ then a high enough temperature is reached to allow neon burning. One 20Ne neon nucleus fuses with a 4He to form 24Mg while ejecting a gamma-ray, another 20 Ne absorbs a gamma-ray and produces 4He and 16O. As the temperature rises, oxygen burning begins which produces silicon. For stars bigger than 20M⨀ the temperature is high enough for a large number of complex nuclear reactions that eventually produce nuclei of mass number 56 (56Ni, 56Co, 56Fe). At this stage massive stars consist of concentric shells (Figure 6) that are the relics of its previous burning phases (Janka et al, 2007). Figure 6, shows an illustration of the concentric shell structure in massive 25M⨀ star. The vertical axis shows the logarithm of the mass fraction, while the horizontal axis shows the logarithm of the temperature/K and density/gcm-1 (Pagel, 2009). 7 The concentric shells or ‘onion ring’ structure model of the star is used to explain the presence of previously synthesised elements that remain after the burning stages. The boundaries are not defined so as to allow mixing of the gases (plasma). At this point energy is no longer released in fusion reactions (see Figure 5). The gravitational forces take over again and the star collapses in on itself forming a ball of neutrons at the centre the star. The collapsing material speeds towards its centre and the shock of hitting the neutron core increases the temperature greatly creating a shock wave and the star explodes outwards in a supernova explosion, ejecting the matter synthesised. 2.3 STELLAR NUCLEOSYNTHESIS A>60 Production of the heavier nuclei occurs by neutron capture and depends on the neutron flux density and nuclear half lives. There are two processes: The (rapid) r process is thought to occur in core collapse supernovae, which involves a series of rapid neutron capture on iron seed nuclei, three successive captures starting at 56Fe: 56 Fe + n →57Fe + γ, 57Fe + n →58Fe + γ, 58Fe + n → 59Fe + γ 59 Fe is radioactive with a half life of 45 days. If the neutron density is high and the probability of capture is every few seconds then the neutron capture sequence will continue until an isotope that is so high in neutrons that it decays before a neutron can be captured as shown in Figure 7 with 68Fe. Then β decays to an isotope with one higher atomic number. The (slow) s process of neutron capture is thought to occur in asymptotic giant branch stars (AGB) of mass 0.6 – 10 M⨀ and involves the slow capture of neutrons which allows products to decay by β emission to form heavier elements. Starting at 59Fe: with a half life of 45 days. If the neutron density is low and the probability of neutron capture is less than one in 45 days then 59Fe will β decay to stable 59Co, which will undergo neutron capture to radioactive 60Co (Krane, 1989). 8 Figure 7 shows the s (coloured red) β decay process and r (coloured green) neutron capture process adapted from (Krane, 1989). 9 3 HISTORY OF GAMMA-RAY LINE DETECTION The history of gamma-ray line detection parallels the history of high resolution, high efficiency gamma-ray detectors. The main history of gamma-ray line detection is outlined in Table 1. Table 1 shows a time line adapted from material produced by (NASA, 2007) and (Prantzos & Diehl, 1996). Year Event Reference 1971 7th Orbiting Solar Observatory (OSO-7) is launched into orbit. With NaI(Tl) detector Gamma-ray emission lines detected from solar flares in August via an (Chupp et al, 1972 experiment aboard OSO-7. This includes the 511 keV positron 1973) annihilation line, the 2.223 MeV neutron-capture line, and the weak detection of C & O de-excitation lines at 4.438 MeV and 6.129 MeV. 1974 Balloon NaI(Tl) crystal detection of 4.43 MeV gamma-ray de- (Haymes et excitation lines from 12C. al, 1975) The 1st High Energy Astrophysical Observatory (HEAO 1) is launched on 12th 1977 1977 1979 August. It carries several X-ray and gamma-ray experiments. Detection of 511 keV positron annihilation radiation from the galactic centre (Leventhal et direction via a balloon-borne experiment. al, 1978) 3rd High Energy Astrophysical Observatory (HEAO-3) launched into orbit. Figure 8, shows an exploded view of HEAO-3 GRS (Mahoney et al, 1980) 10 The HEAO-3 used a HPGe detector .The cooling system for the HPGe detector is of passing interest because the detector was cooled with solid methane and ammonia (Cf. Figure 8) as part of a gamma-ray spectrometer. The CsI(Na) acted as an anticoincidence shield system (Mahoney et al, 1980). Table 2 shows a time line (continued from Table 1) adapted from material produced by (NASA, 2007) and (Prantzos & Diehl, 1996). Year 1980 Event Reference Solar Maximum Mission (SMM) launched into orbit carrying Gamma-Ray Spectrometer (GRS). Announcement of the detection made via an experiment aboard (Hudson et 1980 HEAO 1, of the 2.223 and 4.43 MeV lines during a large solar flare in al, 1980) July 1978. The SMM gamma-ray spectrometer (cf. Table 2) consisted of seven 7.5 cm x 7.5 cm NaI scintillation detectors with a CsI anticoincidence shield system, giving an energy resolution of 95 keV full width half maximum (FWHM) at 1800 keV with excellent signal to background ratio of >20%. The aperture of 160o does not allow spatial information to be derived directly. Figure 9 shows the high energy half of the 4 detector summed gamma-ray spectrum of the galactic plane measured by the HEAO-3 over a 2 week period in 1979. Energy lines of expected radionuclides are shown (Mahoney et al, 1982). 11 Figure 10 shows the total net diffuse galactic gamma-ray emission near 1809 keV, normalised to the direction of the galactic centre, based on improved analysis of (Figure 9) 1979 data, Channel width = 2 keV. The solid curve is a best fit of the data. Vertical error bars shown. Table 3 shows a time line (continued from Table 2 ) adapted from material produced by (NASA, 2007) and (Prantzos & Diehl, 1996). Year Event Reference The announcement of the detection, made via the GRS aboard SMM, (Share et al, 1985 of the gamma-ray emission line from the Galactic Centre at 1.81 MeV 1985) due to the decay of 26 Al. Provided confirmation of the HEAO-3 discovery of 26Al in 1984. 1988 Detection, made via the GRS aboard SMM, of gamma rays from (Matz et al, SN1987A due to the radioactive decay of 56Co. 1988) Observation of Doppler broadening of 56Co gamma-ray emission lines (Teegarden 1989 via GRIS (Gamma-Ray Imaging Spectrometer), a balloon-borne et al, 1989) experiment. 1991 Compton Gamma-Ray Observatory (CGRO) deployed into space by space shuttle Atlantis. Of four instruments, the Oriented Scintillation Spectrometer Experiment (OSSE) (cf. Table 3) and the Compton telescope (COMPTEL) are relevant to this thesis. COMPTEL is composed 12 of two layers of detectors (Figure 11) the upper layer consists of liquid scintillation tanks filled with NE213A. An incident photon is Compton scattered here and absorbed in the lower layer of 14 x 28 cm diameter NaI(Tl) scintillator crystals viewed with photomultiplier tubes. The angular response is ~3.8o (FWHM) at 1800 keV and an energy resolution of 8% (144 keV). Figure 11 Schematic view of COMPTEL (Schoenfelder et al, 1993) 13 Table 4 shows a time line (continued from Table 3) adapted from material produced by (NASA, 2007) and (Prantzos & Diehl, 1996). Year Discovery Reference based on OSSE observations, the first direct measurement of the mass of (Kurfess 1992 57 Co produced in SN1987A. The ratio of 57 Ni/56Ni is estimated to be et al, slightly larger than, but consistent with, the solar ratio of 57Fe/56Fe. 1994 1992) Detection made via COMPTEL observations, of the radioactive decay of (Iyudin et 44 Ti at 1.16 MeV in the Cassiopeia A (Cas A) supernova remnant (SNR). al, 1998) Diehl et al. release the first map at 1.809 MeV of the entire Galactic 1995 Plane, based on COMPTEL observations, and estimate the total amount (Diehl et al, 1995) 26 of radioactive Al in the Galaxy to be ≤ 1 M⨀. 1998 Discovery, based on COMPTEL observations, of 44Ti emission at 1.16 (Iyudin et MeV from a SNR in the Vela Region. al, 1998) First COMPTEL Source Catalogue. It covers the energy range from 0.75 to 30 MeV. The catalogue contains 32 steady sources, 31 GRBs and 21 solar flares. The steady sources include spin-down pulsars, stellar-mass (Schönfel 2000 black holes, SNRs, interstellar clouds, and Active Galactic Nuclei der et al, (AGNs). Line detections include the 26Al line at 1.809 MeV, the 44Ti line at 1.157 MeV, the 56 2000) Co lines at 0.847 MeV & 1.238 MeV, and the neutron-capture line at 2.223 MeV. Ramaty High Energy Solar Spectrographic Imager (RHESSI) solar observatory 2002 is launched on 5th February. Gamma-rays from solar flares are imaged for the (Smith, first time. 2004) International Gamma-Ray Astrophysics Laboratory (INTEGRAL) gamma-ray 2002 observatory launched into orbit by the European Space Agency (ESA). Although intended to observe gamma-rays from the sun RHESSI has proven to be sensitive to gamma-rays generated from radioactive nuclei in the inner galaxy namely 26Al and preliminary detection of 60Fe lines. The imager uses 9 coaxial germanium detectors with energy resolution of 400 at 1-2 MeV (Smith, 2004). 14 Figure 12, shows (left) cut-away view of SPI spectrometer with anti-coincidence mask to reduce background noise, aboard the INTEGRAL spacecraft (right) (ESA, 2009). INTEGRAL, shown in Figure 12 , uses a HPGe detector cooled using a Stirling cooler as part of SPI spectrometer with an energy resolution of 2.33 keV at 1.33 MeV (Vedrenne et al, 2003). Table 5, shows a time line (continued from Table 4) adapted from material produced by (NASA, 2007) and (Prantzos & Diehl, 1996). Year Event Reference 2005 Marginal detection, made by SPI aboard INTEGRAL, of gamma-ray (Harris et al, emission from the decay of 60 Fe and 60 Co in the Galactic Plane at 2005) 1.173 and 1.333 MeV. The 60Fe/26Al ratio is estimated. 2007 – AGILE (Astro-rivelatore Gamma a Immagini LEggero) is launched on April 23. It carries an instrument that is sensitive to high-energy gamma rays. 2008 – The Fermi Gamma-ray Space Telescope (formerly known as GLAST, the Gamma- ray Large Area Space Telescope) is launched on June 11. It carries an instrument that is exceptionally sensitive to high-energy gamma rays, as well as a gamma-ray blast (GRB) monitor. 2012 INTEGRAL’s on-board consumables have sufficient margin to provide nominal mission operations beyond 31 Dec 2012 (ESA, 2009) AGILE (cf. Table 5) is a small Scientific Mission of the Italian Space Agency (ASI). AGILE is devoted to gamma-ray astrophysics and it is a first and unique combination of a gamma-ray and an hard X-ray imager. It will simultaneously detect and image photons in the 30 MeV 50 GeV and in the 18 - 60 keV energy ranges (ASI, 2009). 15 4 GAMMA-RAY LINE SOURCES There are constraints on the number of radionuclides that can be detected. Gamma-ray lines require three conditions to be observable: 1. Hot, dense medium to allow synthesis of radionuclides such as the interior of a star. The site determines the nuclide that is produced. 2. Fast removal of radionuclides from production site to prevent destruction by reaction or decay, also interiors of stars are optically thick to gamma-ray so removal is required for detection. Ejection by convection followed by explosion or stellar winds requires lifetimes of days if not months. 3. High gamma-ray intensity from a short half life t½ (low mean life τ) or abundant long t½ (high τ). These conditions result in a list of candidate radionuclides, shown in Table 6, that may be detectable with gamma-ray detectors (Knödlseder, 2000). Table 6, shows possible gamma-ray line sources with line energy (NNDC, 2003) and colour coded to indicate the line detection status; yellow: detected, blue: marginal detection and green not detected yet. Table adapted from (Knödlseder, 2000). Isotope 26 Al Co 847 1238 112 days Ni 1378 2.14 days Co 122 392 days 1275 3.76 years Na 44 Ti 67.9 44 Sc 60 ↓ 59 7 Be e+ 78.4 Detection Marginal detection (Mahoney et al, 1984) 8.5 days (Matz et al, 1988) (Kurfess et al, 1992) (Iyudin et al, 1995) 86.56 years (Vink et al, 2001) 1157 5.72 days (Iyudin et al, 1998) Fe 58.6 2.16 x 106 years Co 1173 1333 7.61 years 60 Fe 1.04 x 106 yr 812 57 ↓ τ = t½/ln2 158 57 22 V Ni 56 ↓ Mean Life τ 1809 56 ↓ Energy/ke (Smith, 2004) 1099 1292 64.2 days 478 77 days 511 105-107 (Johnson et al, 1972) 16 Condition 2 gives a possible explanation as to why radionuclides with a mean life of less than 100 days have not been detected: 56Ni, 57Ni, 59Fe and 7Be, and explains why (Diehl et al, 2006) do not include these lifetimes in their candidate list. 4.1 DETECTIONS 4.1.1 ALUMINIUM-26 (26Al) 26 Al has a mean life of 1.04 x 106 yr which is less than the age of the galaxy but longer than the average time between the explosive novae and supernovae thought to produce and eject it. The long life time will result is an extended diffuse source along the galactic equatorial plane (viewed edge on from the perspective of Earth) and increased emission in regions where sources have been active recently (Mahoney et al, 1984). The presence of 26Al source is recognised by the detection of the 1809 keV gamma-ray line emitted by the excited nucleus of 26Mg, the daughter produced by the β+ decay of 26Al (Clayton, 2003) and electron capture (Baum et al, 2002) both processes result in a proton being converted to a neutron. The decay scheme is shown in Figure 13 and does not show the β+ decay part of the scheme 17 Figure 13 shows the decay scheme of 26Al (NNDC, 2003), with the emission of the characteristic 1809 keV energy gamma-ray by the 26Mg daughter in its excited state. The half life is shown in green as 7.17x105 yr giving a mean life time of τ = 1.03x106 yr. 26 Al is produced by proton capture by 25Mg, thus it must occur in stellar zones where either or both of the reactants are abundant. This can occur in hydrogen burning cores or shells (cf. Figure 6) where protons are abundant, or in Ne – O shells, where 25Mg nuclei are abundant from neon burning reactions. The 26Al produced is ejected in the final supernovae explosion (Diehl et al, 2006), although if produced in the hydrogen shell at the surface of the star it is ejected through the stellar wind. For various reasons it has been difficult to predict, using computer models, amounts of 26Al ejected from a population of massive stars in the full mass range 10 to 100M⨀. Since the initial report of 26Al in 1984, the COMPTEL mission data has been used to produce an all-sky map of 26Al 1806 keV gamma-ray line emission shown in Figure 14. 18 Figure 14 shows the galactic map of 26Al 1808.65 keV gamma-ray line ejected from sources using 9 years of observations with the COMPTEL telescope. The insert shows the spectrum for 26 Al for the galactic centre region from SPI on INTEGRAL. (Diehl et al, 2008) (Plüschke et al, 2001). 19 4.1.2 COBALT 4.1.2.1 COBALT – 56 (56Co) and NICKEL – 56 (56Ni) 56 Co is produced from the radioactive decay of 56Ni: 56Ni→56Co→56Fe by electron capture. 56 Ni is thought to be produced in the hot stellar core by silicon burning and α-rich freeze out during core collapse supernovae. 56Co has a mean life of 112 days which is a lot less than the age of the galaxy. This short life will mean that the source will be close to explosive supernovae thought to produce and eject it. 56Co is recognised by the 847 keV and 1238 keV gamma-ray emission lines (cf. Appendix A). Figure 15 shows the accumulated gamma-ray line spectrum for 56Co accumulated by the SMM GRS (cf. Table 2) which accumulated data from 1st August 1987 (day 160) until May 1988 (day 460) and include some of the weaker lines from supernovae SN1987A which was first observed in February 1987 (Vink, 2005). It is interesting to note that between day 160 – 460 the fraction of 56Co remaining would have decreased from 0.238 to 0.0161, calculated using: fraction remaining where λ is the decay constant and t is the elapsed time (Krane, 1989). Figure 15 shows the gamma-ray spectrum (upper line) of SN 1987A, as accumulated by the SMM-GRS over the period from August 1987 to May 1988 (Leising & Share, 1990). Also shown (lower line) is a smoothed curve from a Monte Carlo model at day 360. Note that apart from the prominent 847 keV and 1238 keV lines, there is also evidence for the weaker 2599 keV and 3250 keV lines from 56Co (Vink, 2005). 20 SN 1987A is the only source where 56Co has been firmly detected, there has been a marginal detection by COMPTEL of 56Co from SN 1991T but, even after re-assessment (Morris et al, 1997) it is not certain if these results will hold up (Vink, 2005). Figure 16 shows stellar57rings around SN 1987A, with the ejected material from the 4.1.3 COLBALT-57 ( Co) supernova explosion at the centre of the inner ring. (Weizmann Institute, 2009). 4.1.2.2 COBALT – 57 (57Co) and NICKEL – 57 (57Ni) In a similar way to 56Co, 57Co is produced from the radioactive decay of 57Ni: 57 Ni→57Co→57Fe by electron capture. 57Ni decays to 57Co by electron capture, where an electron is captured and changes a proton into a neutron, the mass number remains constant. The decay process emits and gamma-ray of energy 122 keV from the daughter 57Fe. The decay has a mean life of 392 days (cf. Table 6), which means that the source will be located close to the production site. 56 Ni is thought to be produced during core collapse of supernovae (Knödlseder, 2000). 21 57 Co was detected by OSSE instrument aboard COMPTEL (cf. Table 3) from the SN 1987A source, presumed to be part of the ejected material. There were two observations of 131 s each; the first between 25th July – 8th August 1991 (1613 – 1627 days after the explosion) and the second between 27th December 1991 and 10th January 1992 (day 1768 – 1782). The data analysis subtracted an assumed back ground in each case (Kurfess et al, 1992). The resulting spectra are shown in Figure 17. As with 56Co it is interesting to note the fraction of 57Co remaining over the periods the two observations were recorded. Between 1613 – 1627 days the fraction would have decreased from 0.0163 – 0.0157 and for 1768 – 1782 the fraction would have decreased from 0.0110 – 0.0106. The observations were late because OSSE was only launched in 1991. Figure 17, shows the gamma-ray energy spectrum for the two observations in 1991. The 127 keV peak of 57Co is visible from the best fit line which is for an exponential model. The negative probably reflects a component from the Large Magellanic Cloud (LMC) in which the source was located (Kurfess et al, 1992). 22 4.1.2.3 COBALT – 60 (60Co) and IRON – 60 (60Fe) 60 Fe is produced in pre-supernova stars by the r – process which involves neutron capture as shown in Figure 7 and then β-decays to 60Co with a mean life of 2.16 x 106 years emitting a 58.6 keV gamma-ray in the process (cf. Appendix A). The 60Co then β-decays to stable 60Ni with a mean life of 7.61 years emitting 1173 keV and 1333 keV characteristic gamma-rays in the process (cf. Figure 2 and Figure 3). The 60Fe has a similar mean life to 26Al and will be found some distance from the source producing a diffuse source. Detection of 60Co will produce a less diffuse source. The first detection of 60Fe was preliminarily reported by (Smith, 2004) who shows a coadded spectrum of the two lines (1173 keV and 1333 keV ) observed with RHESSI which was not direction specific because of no detector shielding and used the earth as an occulter (Smith, 2004). A further detection was reported by (Harris et al, 2005) using observations from SPI aboard INTEGRAL from the galactic plane. (Wang et al, 2007) presents a spectrum shown in Figure 18 from SPI observations from the inner galaxy. Figure 18 shows the spectra of the two gamma-ray lines 1173 keV and 1333 keV of 60Co from the inner galaxy (Wang et al, 2007). The 60Fe detections to date have detected the 1173 keV and 1333 keV as proof of detection, which assumes the 60Co lines are identical to those from cosmic 60Fe (Harris et al, 2005). It would probably be more convincing if the 58.6 KeV line had been detected. 23 4.1.3 TITANIUM-44 (44Ti) 44 Ti is uniquely produced by α-rich freeze-out in core collapse supernovae (Arnett, 1996). 44 Ti decays to 44Sc with a mean life τ, of 86.56 years, emitting two a gamma-rays of energies 67.9 keV and 78.4 keV. 44Sc then quickly decays to stable 44Ca in a mean life of 5.72 days, emitting a gamma-ray of energy 1157 keV (cf. Appendix A). Because the mean life of 44 Sc is so small compared to that of its parent 44Ti some authors associate the 1157 keV energy with 44Ti, e.g. (Iyudin et al, 1998). The first source to be located was announced in 1994, based on observations by COMPTEL in July 1992 and February 1993 (Iyudin et al, 1994). The source was the supernova remnant (SNR) Cassiopeia A (Cas A) Figure 20. Cas A is located at a distance of 3.4 kpc (≈11 light years) and is estimated to have exploded in 1671 or 1680 (Renaud et al, 2006) . The spectrum (cf. Figure 19) for these observations displays the characteristic 1157 keV energy gamma-ray line. In 2006 (Renaud et al, 2006) reported the detection of the 67.9 keV and 78.4 keV gamma-ray lines from 44Ti in Cassiopeia A using the INTEGRAL instruments. Figure 19 shows the sum of the background subtracted 44Sc spectra for the two observation periods with typical error bars (Iyudin et al, 1994). 24 Figure 20 “This stunning picture of the supernova remnant Cassiopeia A (Cas A) is a composite of images taken by three of NASA's Great Observatories. Infrared data from the Spitzer Space Telescope are coloured red; optical data from the Hubble Space Telescope are yellow; and X-ray data from the Chandra X-ray Observatory are green and blue” (SAO, 2009) A second source of 44Ti was reported in 1998 by (Iyudin et al, 1998) using data from observations by COMPTEL covering a time interval from April 1991 to October 1997, coming from the Vela region of the galaxy (cf. Figure 14). The source GRO J0852-4642, a young galactic SNR that was missed by optical and radio surveys. The source was found using the 1156 keV gamma-ray line and is about 680 years old. 4.1.4 POSITRONS (e+, β+) When a positron interacts with an electron (e-) both particles are annihilated producing a characteristic 511 keV gamma-ray line – the brightest gamma-ray line in the galaxy. Some positrons are produced by the β+ decay of 26Al and 44T, but the principle source of positrons is 25 uncertain. However recent accumulated observations from INTEGRAL show an asymmetry in the distribution of the 511 keV line in the all-sky map produced with from data the SPI shown in Figure 21. Figure 21 shows a sky map of the 511 keV electron-positron annihilation line (left) and the sky distribution of hard LMXB (right). In both maps the galactic centre is at the origin (Weidenspointner et al, 2008). When compared with the sky distribution of hard low mass X-ray binaries (LMXB) with INTEGRAL/IBIS imager, with strong emission at photon energies greater than 20 keV, a similarity is found. Both maps display a similar asymmetry, indicating that hard LMXBs may be the source of positrons. 4.2 MARGINAL DETECTIONS 4.2.1 SODIUM – 22 (22Na) 22 Na is thought to be synthesised in classical novae at the interface between the white dwarf star and the accreted envelope of hydrogen from the adjacent star, which ignites in a thermonuclear reaction ejecting material (Gallagher & Starrfield, 1978). The 22Na decays by β+-decay to stable 22Ne which emits a 1275 keV gamma-ray. 22Na has a mean life of 3.76 years and would be detected close to the nova production site (cf. Appendix A). 1275 keV emission in the Milky Way has still to be positively detected. However, (Iyudin et al, 2005) have presented a global distribution (cf. Figure 22) derived from all-sky data accumulated by COMPTEL from 1991 – 1997 (Iyudin et al, 2005). 26 P1 Figure 22 shows the maximum entropy gamma-ray line energy 1275 keV flux map for galactic centre region, with the P1 region of the galactic bulge labelled. Figure adapted from (Iyudin et al, 2005). Despite not claiming that 22Na has been positively detected, (Iyudin et al, 2005) present a convincing gamma ray spectrum 1275 keV energy line from the galactic bulge region (cf. Figure 23). Figure 23 shows the smoothed spectrum of the galactic bulge P1 region clearly shows the 1270 keV line emission using observations from COMPTEL up to 1997. Also seen is the suppressed 1800keV line. 27 5 DOPPLER EFFECTS The Doppler Effect is illustrated in (Figure 24). A source of waves, electromagnetic in this case, is moving to the left relative to an observer. The frequency of the waves ahead of the source is higher and the frequency of the waves behind the source is lower than that produced by the source. For a light source this produces an effect: Red shift and Blue shift. Blue light has a higher frequency than red light, the spectral lines of an approaching astronomical light source exhibit a blue shift and those of a receding astronomical light source exhibit a red shift (Pbroks13, 2008). This effect is used to determine the speed and direction of rotation of Figure 24 shows a source of waves moving to the left. The frequency is higher on the left (ahead of the source) than on the right (behind the source). (Pbroks13, 2008) the galaxy. In the context of gamma-ray photons where energy E is proportional to frequency ν, an approaching source produces an increase in energy while a receding source produces a decrease in energy from that expected from the source. This causes the central peak of the gamma-ray line to shift to the left or right. In the case of source material ejected from a nova or supernova the material has a range of speeds which causes a range of energy shifts of the central peak producing a broadening of the gamma-ray line. Figure 25 shows the Doppler blue shift in the increase in the peak energy of the of the Lower 26 Al spectrum as compared to the standard laboratory standard 26Al peak energy. The peak energy of the 26Al gamma-ray line from the whole inner galaxy is slightly higher at 1808.92±0.06 keV, indicating a blue shift due to an increased frequency. A summary of four other spectra in addition to the two shown in Figure 25 is shown in Figure 26. 28 Figure 25 shows 26Al spectra of two segments along the galactic plane (lower) -30o<1<0o and (upper) 0o<1<30o that is either side of the galactic centre. The green line indicates the peak energy 1805.65 keV measured in the laboratory and the blue line indicates the blue shift (Wang et al, 2009). The line shifts shown in Figure 26 seem to show the outer reaches of the galaxy moving in the opposite direction to the inner galaxy. Diehl (2006) presents a graph of the expected signature for Doppler shifts along the plane of the galaxy (cf. Figure 27). Note the opposite sign conventions used for the two graphs. 29 Figure 26 shows the 26Al line energy shifts along the galactic plane (-60o < 1 < 60o) relative to 26Al line peak energy of 1808.65 keV at negative longitudes the blue shift is substantial while at positive longitudes the red shift in energy is minor (Wang et al, 2009). Figure 27 shows the Doppler line position shifts viewed along the inner galaxy (Diehl, 2006). Note Diehl uses the opposite sign convention to Wang in his galactic longitude scale. 30 6 CONCLUSION Having outlined some of the nucleosynthesis processes in the universe, this thesis has listed possible candidate products of nucleosynthesis that can be detected. The detection uses space-borne gamma-ray detectors which detect the gamma-ray line energy “fingerprints” of the radioactive decay products of nucleosynthesis. Over half of the candidates have been detected. These include: the well reported 1808 keV line from 26Al with its long mean life giving a diffuse galactic distribution. The 847 keV and 1238 keV lines from 56Co, the 122 keV line from 56Co are possible evidence of the undetected parents 56Ni and 57Ni in the decay chains. Just as the active daughter 44Sc with line energy 1157 keV was reported as the detection of 44Ti and later confirmed with the detection of its own energy lines of 67.9 keV and 78.4 keV. A similar scenario is waiting to be played out in the future for the undetected 60 Fe 58.6 keV line which in the meantime has been reported with the observation of the 1173 keV and 1333 keV from its daughter decay product 60Co. The long mean life of 60Fe means that a diffuse distribution is expected. Detection of positrons has been reported and although linked to the β+ decay of 26Al and 44Ti has only recently been linked to hard low mass X-ray binaries. 22Na has been mapped but not reported as detected. The remaining short lived candidates await detection. Because of their short mean lives the hope is that there will be a space borne detector available, such as integral which is due to last beyond 2012. There is the Italian AGILE mission also. The hope is that with improved detector efficiency and resolution, the lower cost option of using high altitude balloons will be used once astronomers are alerted to the presence of a nova or supernova. 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