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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. This prospect
offers a bright future for gamma-ray line detection.
31
REFERENCES
Arnett, D., 1996. In Supernovae and Nucleosynthesis. Chichester: Princeton University Press.
p.275.
ASI, 2009. AGILE. [Online]. Available at: http://agile.asdc.asi.it/ [accessed 03 September
2009]
Baum, E.M., Knox, H.D. & Miller, T.R., 2002. Nuclides and Isotopes 16th Edition. 16th ed.
KAPL, Inc.
Chupp, E.L. et al., 1973. Solar Gamma Ray Lines observed during the Solar Activity of
August 2 to August 11, 1972. Nature, 241(DOI:10.1038/241333a0), pp.333-35.
Clayton, D., 2003. Handbook of Isotopes in the Cosmos. Cambridge: Cambridge University
Press.
Diehl, R. et al., 1995. COMPTEL observations of Galactic 26Al emission. Astronomy and
Astrophysics, 298, p.445.
Diehl, R., Lang, M., Kretschmer, K. & Wang, W., 2008. 26Al emission throughout the
Galaxy. New Astronomy Reviews, 52(DOI:10.1016/j.newar.2008.06.024), pp.440-44.
Diehl, R., Prantzos, N. & von Ballmoos, P., 2006. Astrophysical constraints from gamma-ray
spectroscopy. Nuclear Physics A, 777, pp.70-97.
Dielh, R., 2006. Measuring 26Al and 60Fe in the Galaxy. New Astronmy Reviews, 50,
pp.534-39.
ESA, 2009. INTEGRAL. [Online]. Available at: http://sci.esa.int/sciencee/www/object/index.cfm?fobjectid=31175&fbodylongid=719 [accessed 31 August 2009]
ESA, 2009. INTEGRAL Status Report - January 2009. [Online]. Available at:
http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=44115 [accessed 03
SEPTEMBER 2009]
iv
Gallagher, J.S. & Starrfield, S., 1978. Theory and observations of classical novae. Annual
review of astronomy and astrophysics, 16(DOI:10.1146/annurev.aa.16.090178.001131),
pp.171-214.
Gribbin, J. & Rees, M., 1991. Taylor-made for Man. In Cosmic Coincidences. Reading:
Black Swan. pp.244-47.
Harris, M.J. et al., 2005. Detection of γ-ray lines from interstellar 60Fe by the high resolution
spectrometer SPI. Astronomy and Astrophysics, 433(DOI:10.1051/0004-6361:200500093),
pp.L49-52.
Haymes, R.C. et al., 1975. Detection of Nuclear Gamma Rays from the Galactic Centre
Region. The Astrophysical Journal, 201, pp.593-602.
Hudson, H.S. et al., 1980. HEAO 1 observations of gamma-ray lines from a solar flare.
Astrophysical Journal, 236, pp.L91-95.
Imaginova, 2002. Space.com. [Online]. Available at:
http://www.space.com/php/multimedia/imagedisplay/img_display.php?pic=050920_androme
da_galaxy_02.jpg&cap=The+Andromeda+Galaxy+photographed+with+a+12.5inch+telescope+by+amateur+astronomer+Robert+Gendler.+Credit%3A+%A9+2002+R.+Ge
ndler%2C+Photo+by+R.+Gendler [accessed 11 August 2009]
Iocco, F. et al., 2009. Primordial nucleosynthesis: From precision cosmology to fundamental
physics. Physics Reports, 472, pp.1-76.
Iyudin, A.F. et al., 1995. COMPTEL search for ^22^Na line emission from recent novae.
Astronomy and Astrophysics, 300, pp.422-28.
Iyudin, A.F. et al., 2005. Global Galactic distribution of the 1.275 MeV γ-ray line emission.
Astronomy and Astrophysics, 443(DOI:10.1051/0004-6361:20041594.), pp.477-83.
Iyudin, A.F. et al., 1994. COMPTEL observations of 44Ti gamma-ray line emission from Cas
A. Astronomy and Astrophysics, 284, pp.L1-L4.
Iyudin, A.F. et al., 1998. Emission from 44Ti associated with a previously unknown Galactic
supernova. Nature, 396(DOI:10.1038/24106), pp.142-44.
v
Janka, H.-T. et al., 2007. Theory of core-collapse supernovae. Physics Reports,
442(10.1016/j.physrep.2007.02.002), p.39.
Johnson, W.N.I., Harnden, F.R.J. & Haymes, R.C., 1972. The Spectrum of Low-Energy
Gamma Radiation from the Galactic-Center Region. Astrophysical Journal,
172(DOI:10.1086/180878), pp.L1-L7.
Keenan, P.C., 1954. Classification of the S-Type Stars. Astrophysical Journal,
120(DOI:10.1086/145937), p.484.
Knödlseder, J., 2000. Constraints on stellar yields and SNe from gamma-ray line
observations. New Astronomy Reviews, 44(DOI:10.1016/S1387-6473(00)00046-4), pp.31520.
Krane, K.S., 1989. Introductory Nuclear Physics. Holboken, USA: John Wiley & Sons.
Kurfess, J.D. et al., 1992. Oriented Scintillation Spectrometer Experiment observations of
Co-57 in SN 1987A. Astrophysical Journal, 399(DOI:10.1086/186626), pp.L137-40.
Leising, M.D. & Share, G.H., 1990. The gamma-ray light curves of SN 1987A. Astrophysical
Journal, 357(DOI:10.1086/168952), pp.638-48.
Leventhal, M., MacCallum, C.J. & Stang, P.D., 1978. Detection of 511 keV positron
annihilation radiation from the galactic center direction. Astrophysical Journal,
225(DOI:10.1086/182782), pp.L11-14.
Lodders, K., 2003. Solar System Abundances and Condensation Temperatures of the
Elements. Astrophysical Journal, 591, p.1220–1247.
Mahoney, W.A., Ling, J.C., Jacobson, A.S. & Lingenfelter, R.E., 1982. Diffuse galactic
gamma-ray line emission from nucleosynthetic Fe-60, Al-26, and Na-22 - Preliminary limits
from HEAO 3. Astrophysical Journal, 262(DOI:10.1086/160469), pp.742, 748.
Mahoney, W.A., Ling, J.C., Jacobson, A.S. & Tapphorn, R.M., 1980. THE HEAO 3
GAMMA-RAY SPECTROMETER. Nuclear Instruments and Methods, 178, pp.363-81.
Mahoney, W.A., Ling, J.C., Wheaton, W.A. & Jacobson, A.S., 1984. HEAO 3 discovery of
Al-26 in the interstellar medium. Astrophysical Journal, 286( DOI:10.1086/162632), pp.57885.
vi
Matz, S.M. et al., 1988. Gamma-ray line emission from SN1987A. Nature,
331(DOI:10.1038/331416a0), pp.416-18.
Merrill, P., 1952. Spectroscopic Observations of Stars of Class S. Astrophysical Journal, 116,
pp.21-26.
Moore, C.E., 1956. Atomic Spectra--Their Role in Astrophysics. Vistas in Astronomy, 2,
pp.1209-22.
Morris, D.J. et al., 1997. Reassessment of the 56Co emission from SN 1991T. The fourth
compton symposium. AIP Conference Proceedings, 410(DOI:10.1063/1.54174), pp.1084-88.
NASA, 2007. HEASARC. [Online]. Available at:
http://heasarc.gsfc.nasa.gov/Images/oso/oso7_ucsd.gif [accessed 16 August 2009]
NNDC, 2003. NuDat 2.5. [Online]. Available at:
http://www.nndc.bnl.gov/nudat2/chartNuc.jsp [accessed 7 August 2009]
Pagel, B., 2009. Nucleosynthesis and Chemical Evolution of Galaxies. 2nd ed. Cambridge:
Cambridge university Press.
Pbroks13, 2008. Doppler Effect. [Online]. Available at:
http://en.wikipedia.org/wiki/File:Doppler_effect.svg [accessed 8 September 2009]
Peebles, P., 1966. Primordial Helium Abundance and Big Bang Fireball. Astrophysical
Journal, 146, pp.542-52.
Pinkau, K., 2009. History of gamma-ray telescopes and astronomy. Experimental Astronomy,
25(DOI: 10.1007/s10686-009-9143-z), pp.157-71.
Plüschke, S. et al., 2001. The COMPTEL 1.809 MeV survey. In Battrick, B., Scientific
editors:Gimenez, A., Reglero, V. & Winkler, C. eds. Exploring the gamma-ray universe.
Proceedings of the Fourth INTEGRAL Workshop, 4-8 September 2000, Alicante, Spain.
Noordwijk ESA Publications Division.
Prantzos, N. & Diehl, R., 1996. Radioactive 26Al in the galaxy: observations versus theory.
Physics Reports, 267(DOI:10.1016/0370-1573(95)00055-0), pp.1-69.
Ramaty, R. & Lingenfelter, R.E., 1995. Astrophysical Gamma Ray Lines. [Online]. Available
at: " http://arxiv.org/abs/astro-ph/9503045v1 [accessed 10 August 2009]
vii
Renaud, M. et al., 2006. The Signature of 44Ti in Cassiopeia A Revealed by IBIS/ISGRI on
INTEGRAL. The Astrophysical Journal, Volume, 647(DOI:10.1086/507300), pp.L41-44.
SAO, 2009. Photo Page:Chandra X-ray Observatory. [Online]. Available at:
http://chandra.harvard.edu/photo/2005/casa/ [accessed 5 September 2009]
Schoenfelder, V. et al., 1993. Instrument description and performance of the Imaging
Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory.
Astrophysical Journal, 86(DOI:10.1086/191794), pp.657-92.
Schönfelder, V. et al., 2000. The first COMPTEL source catalogue. Astronomy and
Astrophysics Supplement, 143(DOI:10.1051/aas:2000101), pp.145-79.
Share, G.H. et al., 1985. Detection of galactic Al-26 gamma radiation by the SMM
spectrometer. Astrophysical Journal, 292( DOI:10.1086/184473 ), pp.L61-65.
Smith, D.M., 2004. RHESSI results on γ-ray lines from diffuse radioactivity. New Astronomy
Reviews, 48(DOI:10.1016/j.newar.2003.11.011.), pp.87-91.
Teegarden, B.J. et al., 1989. Resolution of the 1,238-keV gamma-ray line from supernova
1987A. Nature, 339(DOI:10.1038/339122a0), pp.122-23.
Vedrenne, G. et al., 2003. SPI: The spectrometer aboard INTEGRAL. Astronomy and
Astrophysics, 411(DOI:10.1051/0004-6361:20031482), pp.L63-70.
Vink, J., 2005. Gamma-ray observations of explosive nucleosynthesis products. Advances in
Space Research, 35(DOI:10.1016/j.asr.2005.01.097), pp.976-86.
Vink, J. et al., 2001. Detection of the 67.9 and 78.4 keV Lines Associated with the
Radioactive Decay of 44Ti in Cassiopeia A. The Astrophysical Journal,
560(DOI:10.1086/324172), pp.L79-82.
Wang, W. et al., 2007. SPI observations of the diffuse 60Fe emission in the Galaxy.
Astronomy and Astrophysics, 469(DOI:10.1051/0004-6361:20066982), pp.1005-12.
Wang, W., Lang, M., Diehl, R. & Halloin, P., 2009. Spectral and intensity variations of
Galactic 26Al emission. Astronomy and Astrophysics, (arXiv:0902.0211v1), pp.1-12.
Weidenspointner, G. et al., 2008. An asymmetric distribution of positrons in the Galactic disk
revealed by γ-rays. Nature, 451(DOI:10.1038/nature06490), pp.159-62.
viii
Weizmann Institute, 2009. The amazing supernova SNF 20070406-008 (=SN 2007bi).
[Online]. Available at: http://www.weizmann.ac.il/home/galyam/webimages/sn1987A.jpg
[accessed 1 September 2009]
ix
APPENDIX A: NUCLEAR DECAY SCHEMES (NNDC, 2003) and (Arnett, 1996)
x
xi
xii
xiii
xiv
xv