Download The presence of gamma rays in space was known before they were

Gamma ray sources in space their detection and
measurement
by
Scott James Timothy Cleary
A dissertation submitted to the Department of Physics,
University of Surrey, in partial fulfilment of the degree of
Master of Science in Radiation and Environmental Protection
Department of Physics
Faculty of Engineering & Physical Sciences
University of Surrey
September 2009
© Scott James Timothy Cleary 2009
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ABSTRACT
Gamma rays were detected from space as early as 1958, however, it is only in the last
thirty five years that the use of gamma ray astronomy has come to the fore, and is now
one of the most powerful techniques we have for learning about our galaxy and other
parts of the universe. All this was through the chance discovery of an extremely powerful
source far off in space. As a result we now have a better understanding of the distribution
of not only celestial events due to gamma rays emitted in burst, but we also know the
distribution of elements throughout the Milky Way from the isotopes produced through
the deaths of massive stars. Although due to undetected isotopes or part of an isotopes
decay chain we do not yet have a complete understanding of the process.
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CONTENTS
ABSTRACT ........................................................................................................................ 1
CONTENTS ........................................................................................................................ 2
NOMENCLATURE ........................................................................................................... 3
INTRODUCTION .............................................................................................................. 4
BACKGROUND ................................................................................................................ 5
Production of Gamma Rays ............................................................................................ 5
Space Borne Sources of Gamma Rays................................................................................ 9
Sensor Platforms ............................................................................................................... 16
Detectors ........................................................................................................................... 20
Sources of Error ................................................................................................................ 26
Gamma Ray Line Astronomy ........................................................................................... 29
Future Developments. ....................................................................................................... 36
Discussion and Conclusion ............................................................................................... 40
Acknowledgements ........................................................................................................... 42
References ......................................................................................................................... 43
Appendices ........................................................................................................................ 50
Isotope chart .................................................................................................................. 50
List of satellites ............................................................................................................. 52
Hersprung-Russel diagram ........................................................................................... 55
Ultimate 26Al Diagram…..
56
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NOMENCLATURE
SN
Thermonuclear Supernovae
SNIa
Type Ia Supernovae, No Hydrogen fuel
SNIb
Type Ib Supernovae
SNIc
Type IC Supernovae
SNIIP
Type II “Plateau” Supernovae Hydrogen fuelled
SNIIL
Type II “Linear” Supernovae Hydrogen fuelled
SNIIb
Type IIb hybrid Supernovae staring as Type II becoming
Type Ib
AGB stars
Asymptotic Giant Branch star
Expl.H
Hydrostatic(Explosive)
NSE
Nuclear statistical equilibrium
α-NSE
α-rich (freeze-out)
n-capt
Neutron capture
WR
Wolf-Rayet stars
GRB
Gamma Ray Burts/Bursters
β+-decay
Positron emission
Main Sequence Star/s
A classification for stars that is determined by size and
temperature
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INTRODUCTION
Gamma rays were first discovered in 1900 by Villiard, although they become known as
gamma rays due to Rutherford. It was not until 1948 when Freeberg and Primakoff
theorised that gamma rays are produced in space. The first actual gamma rays detected
from space were detected by sensors mounted on a high altitude balloon by the scientists
Peterson and Winckler in 1958. This was in the same year the first few satellites to have
gamma ray detectors were launched. Gamma rays from outer space are now one of the
best ways we have of studying our galaxy and beyond, able to penetrate matter where
optical radiation cannot, and more easy to study than any other form of radiation or
particle over great distances. They can give us a unique insight into stars, supernovae,
pulsars and even black holes. As a result there is a far better understanding of how stars
burn their fuel, the final stages of a star’s life and may, in theory, even have an indirect
method of detecting dark matter as a result.
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BACKGROUND
Production of Gamma Rays
The presence of gamma rays in space was known before they were actually detected.
Studies done by Feenberg and Primakoff in 1948, Hayakawa and Hutchinson in 1952 and
also by Morrison in 1958 were responsible for the ideas of cosmic ray and dust
interactions, supernova explosions and the interactions of electrons with magnetic fields
X-rays.
Gamma rays are in the very short wavelength region of the electromagnetic spectrum
however the main definition of a gamma ray is that its source has to be from the nucleus
of an atom and not from one of the electrons surrounding the nucleus. This means X-rays
which are generated from electrons can have same energies as gamma rays, but it is
important to make a distinction between the two. Gamma rays also have certain attributes
which makes them useful for astronomy. Firstly gamma rays have a very low crosssection of interaction with matter, making it easy for them to penetrate interstellar matter.
The drawback to this is that our own atmosphere absorbs this radiation before it can reach
us on the ground, making direct observations on the ground impossible. Another attribute
is that gamma rays are produced by the most powerful celestial events making them
useful in the studying the nature of these events.
There are two main groups of gamma rays that astrophysicists look at. The first are the
continuous emitters, these sources continually produce gamma rays that can be located
easily and studied over an extended period of time. The second are what are known as
gamma ray bursts or gamma ray bursters (GRBs), these sources give an intense and
powerful burst of gamma radiation in a very short period of time in the region of seconds.
There are also many types of sources of gamma rays but there are multifarious ways in
which they can be produced. Particle-particle collisions, particle-anti-particle
annihilation, radioactive decay, fusion, fission or when a charged particle is accelerated.
Particle-particle gamma generation occurs when a particle with a large amount of energy
collides with another. Examples of this are cosmic rays or protons striking another proton
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or nucleus. The resultant high energy collisions produce exotic particles called pions.
Three types of pions are produced pion 0, pion minus and pion plus. These pions are
unstable and decay into other forms. The plus and minus pions decay into muons, the
equations for this reaction are given below. The pion 0 decays into gamma rays. As the
proton or cosmic ray was moving at high speed at the time of the collision the gamma
rays are emitted with only a small angle separating them. As a result of this a spectrum of
gamma energies higher than 72Mev is usually seen.
p → e+ + π0 []
p + p → π+ + π-
π0 → 2γ (98.8%) [7]
π+ →μ+ + νμ [7]
μ+ → e+ + νe + ν
π0 → γ + e+ + e- (1.2%)
π- → + μ- + ν
μ
μ- → e- + ν
e
μ
+νμ
The second production method is the particle-anti-particle method, the main example
being the pair production method. When a positron and electron collide they annihilate
one another and emit gamma rays to conserve total charge energy and momentum. When
this happens at rest two annihilation photons of 511keV each are emitted which is the rest
mass energy of a positron/electron. It also works the other way so that if a gamma ray of
1.022MeV, twice the rest mass of an electron/positron can become a positron and an
electron. These are shown by the equations below.
e+ + e- → picosec → γγ (each gamma is 511keV).
(Gamma ray has an energy of 1.022MeV) γ → picosec → e+ + e-.
Fusion is another way of generating gamma rays; this involves the process of fusing two
or more elements together to form a new element along with the release of energy. Some
of this energy is in the form of gamma rays. Fusion is explained in a more depth in
chapter(?) along with the process called neucleosysthesis. The following equation shows
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how some gamma rays are produced via fusion. This is the process that occurs in all
young stars.
1H
+ 1 H → 2 H + e+ + ν
2H
3He
+ 1H → 3He + γ
+ 3He → 4He + 1H + 1H
This next process is connected to the previous one, fusion during the life of stars. Gamma
decay is where an excited nucleus of an atom de-excites further by emitting a photon of
gamma radiation. This process is responsible for providing information about the source
via gamma ray emission. Gamma rays emitted in this fashion give a distinct line
spectrum that identifies the type of source nuclei. This process is associated with
nucleosynthesis. This process is described in more detail (in the space borne sources of
gamma rays chapter). An example of radioactive decay is given below, this equation
shows that 137Cs gives off a gamma ray with a specific energy.
137Cs
→ 137Ba(meta) + e- → 137Ba + γ (662keV)
Fission is another method of producing gamma rays. Fission occurs when a heavy
unstable element breaks up into fragments rather than by the other decay methods. When
this happens in addition to neutrons, gamma rays are also emitted. An example of fission
is given below [1].
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Acceleration of charged particles is the final method for the production of gamma rays.
Fast moving charged particles such as natural alpha particles or accelerated ions can
induce nuclear reactions giving rise to excited nuclei which de-excite almost instantly by
gamma emission.
Gamma ray bursts/bursters, these sources are not looked at for specific line spectra. In
addition to this they only emit gamma radiation for only a brief period of time. However
knowing and trying to learn from these sources is still useful.
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Space Borne Sources of Gamma Rays
In addition to distant suns there are a number of other important sources of gamma rays.
These include Black Holes, Pulsars, Supernovae, and Neutron stars. In order to fully
explain these objects we have to cover the present state of theory on the creation and
evolution of stars.
All of the above celestial events have a common factor that is important to their creation
or reason for producing GRBs. This is called solar mass; a solar mass is the weight of the
object with respect to our own Sun. So one solar mass equal to the weight of our sun. In
addition to this it should be noted that during the lifetime of a sun it will lose mass, via
energy expelled in light and the rest of the EM spectrum, and also via solar flares, solar
wind, etc. So when reference to solar mass is made, this is the solar mass at that particular
stage in the star’s life.
Stars form when enough dust and gas, predominantly hydrogen, collects in one area of
space due to gravity. The stage where a star is just forming and gathering matter is called
the proto-star stage, it does not yet have enough mass to undergo fusion and become a hot
star. As they coalesce they begin to heat up, this causes the collected matter to expand.
This is important for two reasons if it did not happen it would just collapse in upon itself.
The second reason is this heat that is generated is responsible for the ignition of nuclear
fusion. The combination of these two forces is called hydrostatic equilibrium. Should a
proto-star fail to achieve enough mass to become a star, less than 0.08 solar masses, it
will instead become a brown dwarf, essentially a failed star. When a proto-star has
enough mass the core will ignite hydrogen-hydrogen fusion, (PP I Chain), this is the first
stage for all stars.
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The first picture shows the early stages of a proto-star formation, a) shows stellar dust and gas gathering in
a cloud, b) shows the gas is beginning to coalesce and the core is beginning to heat up, c) at this stage a
proto-star is forming at the centre of the cloud as it continues to contract [2]. The Second picture is an
artist’s impression of a Brown Dwarf, in this particular instance the brown dwarf is surrounded by the
remnants of the dust cloud that formed it as it has failed to ignite [3].
The lifetime of the stars is dependant primarily on one thing: the mass of the star. The
larger the star the hotter it burns, this has two effects on the stars lifetime, firstly as it is
burning hotter it burns its fuel at a faster rate than a smaller star. A chart of the type of
stars there are called main sequence stars is added in the appendices. It does also means
that the star is hot enough to go through several stages of fusion fuel burning. As stated
earlier all stars that become heavy enough to undergo fusion start off by burning
hydrogen to form helium. This process is called neucleosynthesis, the first part being the
PP I Chain. This follows the reaction below.
1
H + 1H → 2H + e+ + ν
2
3
H + 1H → 3He + γ
He + 3He → 4He + 1H + 1H
The PP II chain. This reaction shown below is a multi stage reaction.
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1
H + 1H → 2H + e+ + ν
1
3
H + 2H → 3He + γ
He + 4He → 7Be + γ
7
7
Be + e- → 7Li + ν
Li + 1H → 2(4He)
This leads on to the final stage, the PP III chain.
1
8
H + 7Be → 8B +γ
B → 8Be + e+ + ν
8
Be → 2(4He)
After this, if the star is hot enough, it will go through what is known as the CNO cycle.
C + 1H → 13N +γ
12
13
N → 13C + e+ + ν
C + 1H → 14N +γ
13
14
15
15
N + 1H → 15O +γ
O → 15N + e+ + ν
N + 1H → 12C + 4He
This process will continue through various stages until it gets to Iron. At this point the
amount of energy released in fusing iron nuclei together is not as much as the energy put
into fusing it. At this point the star will begin to collapse.
Neutron stars are the remnants of a dead star, one that was not heavy enough to become a
black hole but sufficiently heavy to partially collapse its remaining core, somewhere
between 1.35 and 2.1 solar masses. In this case the electrons and protons of the core of
the dead star have been pulled together under gravity to form neutrons.
p + e- → n + ve
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The above picture is an artist impression of a neutron star [4].
Black holes are created when especially large suns, around 100 solar masses, collapse in
on themselves when they have burnt up all their fuel through fusion. The remains of the
core of the sun is so heavy that it collapses beyond the neutron star stage until it becomes
an incredibly heavy point, known as a quantum singularity, which is effectively
dimensionless and bends time and space around it. Anything that approaches too close to
this point, into an area called the event horizon, is pulled into it with no chance of escape.
This includes light and the rest of the electromagnetic spectrum. The reason why these
Black Holes are a source of gamma rays is that as the matter approaches a Black hole it
speeds up and heats up and then emits energy before it is absorbed. This energy release
takes the form of electromagnetic waves including gamma rays. These are often emitted
in high energy bursts rather than continuous transmission.
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The above picture is an artist impression of a black hole as part of a binary system. In this instance this the
black hole is pulling in material from the other star, as a result of this the matter is accelerated toward the
singularity at the centre of the black hole getting heated up as it goes. Before it enters the event horizon it
gives off a burst of energy, in this case shown by the two beams above and below the black hole [5].
Pulsars, or pulsating stars, are rotating neutron stars that emit regular electromagnetic
pulses caused by the rotation of the star. The two beams of energy are emitted from its
magnetic poles. However the magnetic poles are not in the same axis as the axis of
rotation. Effectively the pulsar acts like a lighthouse, with the two beams; one of the
beams is angled up the other beam angled down.
The picture on the left shows an artists impression of a pulsar [6]. The Diagram on the right shows the
features of a pulsar. The pulsar has an axis of rotation of axis as labelled on the diagram. It also has two
magnetic poles; it is at the poles where the beams of radiation are released. These two beams act like search
lights rotating around a central point, as they rotate they cross the path of the earth and we see them as
pulsed flashes. [7].
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Supernovae are immense stellar explosions; these are caused in one of two ways. The
first way is when a star has burnt up all its fusionable fuel and the core collapses,
explosively expelling the entire outer layer.
The second process is when a white dwarf star accumulates enough matter to cause a
brief critical fusion explosion. This material is accumulated from a nearby companion.
White dwarfs are the core remnant of a small sun. When a sun of a certain size, 1.44 solar
masses or less, burns up the last of its fusionable material, instead of becoming a black
hole or neutron star it becomes a white dwarf. The outer layers of the star are discharged
as a planetary nebula. It is thought that our own sun will undergo this process.
Either process of generating a supernova gives off an enormous amount of energy, the
equivalent amount in an instant that our sun could produce in its lifetime. In addition to
this the explosion that results is also responsible for the production of elements that are
heavier than iron. This effectively is the final stage in nucleosynthesis. As mentioned
earlier a star stops undergoing fusion when it reaches iron as the amount of energy
released in fusing iron is less than what is put in. However due to the explosion itself
having so much energy released in a small period of time means that neutrons that are
present in the star can be absorbed by the heavier elements in one of two ways via the sprocess or the r-process. The s-process is the slow process of neutron capture over time a
time period shorter than the beta decay process. The r-process or rapid process is captures
neutrons much quicker than the beta decay. This produces neutron rich elements.
It should however be noted that there are differing types of supernova. The two core
types are type I and Type II, Type I, simply put the difference between these two is if the
Balmer lines for hydrogen are present. For Type I there are no Hydrogen lines present,
for Type II they are present. However each type is sub divided into further groups. Type I
sub groups are Ia, Ib and Ic, whereas Type II are subdivided into IIP and IIL.
Type Ia supernovas are thought to occur in binary systems and are due to matter from a
companion star igniting on a companion white dwarf as mentioned earlier. They are also
are the most luminous of all the supernovae and are one of the celestial events that are
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considered standard candles. They also show the presence of an ionised silicon line. Ib
and Ic do not show this line. Ib shows the strong presence of a helium line whilst Ic show
little or no presence of this line.
Type II as mentioned earlier are split into two groups, Type II sub groups are
differentiated depending on how they emit light. Both types first start emitting light in a
large flash as do all supernovae, however, IIP supernovae light will plateau after the
initial subsidence after the flash and then some time later decline linearly. IIL supernovae
have a linear subsidence in the amount of light after the initial flash.
There has been recently reference to supernovae actually changing types. Such
Supernovae include SN 1993J and SN2003bg. In this class of supernova, a Type II
supernovae meaning it shows the presence of hydrogen at the beginning but as time goes
on helium becomes the dominate emission, so it becomes a Type Ib. The result is a new
classification of type IIb.
The image on the left is an artist’s impression of a star losing matter to its white dwarf binary partner. The
matter is being explosively ignited by the white dwarf. The white dwarf being the bright point of light at
the centre of the explosion [8]. The image on the right is a picture of the Crab Nebula supernova remnant
[9].
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Sensor Platforms
The first GRB was detected by Peterson and Winckler. Its source was a solar flare and it
was detected by a balloon borne experiment in 1958. Later in 1961 saw the launch of the
first satellites, The Orbiting Solar Observatory (OSO) and Explorer 11 with equipment
that could detect gamma rays. The OSO was designed to look at the sun for high energy
gamma rays; it did not detect any such radiations. Explorer 11 looked for gamma rays
with energy over 50MeV. This satellite did record events but was not targetable or
specific in what it looked for.
The first satellites launched with sensors that looked for Gamma rays were launched to
look for the presence of illegal nuclear weapons testing both on the ground and in the
atmosphere. They were launched by the American military and the satellites were called
Vela. Twelve of these satellites were built in all. This was during the mid to late 1960s.
However during mid-1969, Ray Klebasabel, whilst going through data for the 2nd of July
1967, found a double humped curve. After checking to confirm these were not nuclear
detonations, it was subsequently confirmed that these were the first detected GRBs
(gamma ray bursts) from space, by satellite.
Vela-5A/B satellites, the satellites are stacked one atop the other, but are released separately [13].
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During the early seventies the first satellite to look outward was launched. This was
called SAS-2, the second satellite of a group of three. It observed for less than a year but
during that time it detected the gamma ray burst from the pulsar, Geminga. Since then
there have been an increasing number of satellites launched with gamma ray detectors on
them.
The picture on the left is of the SAS-2 satellite [14]. The picture on the right is of the most recent Gamma
Ray detector Satellite INTERGRAL [15].
A list of satellites involved in gamma missions as well as ground and balloon based
systems is added in the appendices that shows gives the names of all the satellites that
have been launched with a gamma ray sensor on them for observing celestial events, as
well as current ground based Cerenkov experiments and recent balloon based
experiments
Sensors mounted onto a satellite provide advantages, however they also have some
disadvantages. One of the advantages is there is no need to take into account naturally
occurring gamma radiation from nearby sources, such as those produced by cosmic rays
striking the earths atmosphere. However, effects from the sun now need to be taken into
account, such as solar flares and sunspots. In space you do not have to take into account
the distortion of the atmosphere. However, any problems with the equipment are major
issues to solve once launched. Size and weight of the sensor are also now major issues, as
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these will increase the cost significantly. One benefit satellites have over ground based
systems is they are not limited to which sections of sky they can look at.
However, the number one problem for ground based systems is our atmosphere. It is true
that the Vela satellites were designed to look for gamma rays originating on or near the
surface as well as space but the source of these was a nuclear explosion, which is by no
means a small or weak source. Any gamma radiation coming in is going to be attenuated
by our atmosphere. To this end there is only really one type of gamma ray detector that
can be used on the ground. This is a Cerenkov detector. At present all of the major
ground based systems are Cerenkov detectors. The reason for this is that unlike other
sensors that are hampered by our atmosphere, contrary to other detectors Cerenkov
detectors have to actively utilize the atmosphere as part of it make up in order to detect
gamma rays.
A Picture of The H.E.S.S Array at night in Namibia near the Gamsberg Mountain [16].
There is also a third option, one which has been used for longer than satellites and one
which allows for the sensors to be raised above most of the earths atmosphere, balloons.
Although aircraft may seem like a better alternative (indeed organisations like NASA
have access to some of the best aircraft in the world). Balloons are able to do one thing
airplanes can not do for an appreciable amount of time, and that is to loiter. This is what
the sensors need to be able to do and which is a problem even for gliders. Even if a plane
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could loiter the additional cost of fuel required for staying aloft for long periods would be
prohibitive. Using balloons as a method for scientific packages has been used for a long
time and the technology involved has improved. Experiments that used to only carry a
small amount to be held aloft for a few days are now are able to lift up to 2000kg up to
heights of 40km and stay up for up to 3 weeks. With communication technology
improving as well, receiving the information is now easier and cheaper too. There is an
added benefit that it is relatively inexpensive to launch and modify the experiments. It
could be said that the balloon is a test bed for sensors before they are used on satellites.
One drawback is in order to view a section of sky of interest the balloon has to be flown
to that location.
One of the NASA GRIS Balloon mounted Gamma Ray detectors [17].
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Detectors
There are multiple types of gamma ray detectors available; however due to the
improvements with time as well as suitability for use, choosing the right detector for the
job is crucial. As mentioned earlier, the first satellite to detect a GBR was one of the Vela
satellites. These had two gamma detectors mounted on them; the second of these was
composed of 6 CsI gamma ray detectors with a total surface area of 60cm 2. The current
list of types of gamma ray detectors include spark chambers, Cerenkov counters,
proportional counters, scintillators, such as NaI(Tl) or CsI, with photomultipliers or
photodiodes and silicon based counters, such as germanium or CdZnTe. However the
benefits of the actual detector are not the only issues to consider.
A spark chamber was the device used on the first few dedicated space borne gamma ray
detectors. It works is via a multi-layered method of detection. Firstly a gamma ray enters
the upper part of the spark chamber assembly by passing through a plastic scintillator.
The scintillator allows photons through but stops heavy ions and particles. The gamma
ray then passes into a high Z material; at higher energies this produces pair production.
Choosing the material for this part is important as you only want gamma rays to undergo
pair production and not cosmic rays. The positron electron pair then pass into the lower
half of the detector containing the spark chamber, and this consists of multiple thin
electrified parallel metal plates, contained in a noble gas environment. As the particles
travel through the spark chamber, it ionises the gas between the plates creating an electric
breakdown path immediately after the particle, thus allowing the particle paths to be
visible. This can be recorded and information about the particle pair such as their
direction can be learnt. The final stage is for the pair to enter a crystal scintillator. Where
they will then lose the last of their energy in creating scintillation photons to be recorded
by photomultipliers. If it was a particle pair then these should both be detected at a
similar time [18, 19, 20, 22].
21
The above diagram shows the stages involved in detecting a gamma ray using a spark chamber. Starting
with the gamma ray going through the scintillator at the top, producing an electron-positron pair in the high
Z number material, going through to the spark chamber and finally being absorbed in a scintillator and the
product photons being detected by photomultipliers at the end [21].
Cerenkov detectors are predominantly, although not exclusively, used on ground based
telescopes. The reason for this is they bypass the problem of having the atmosphere
attenuating the gamma rays, by making it part of the detector. Thus when a space borne
gamma ray enters the earth’s atmosphere it produces an effect know as an air shower. In
this air shower, pair production occurs, creating a positron-electron pair. As they head
ground ward they interact through Bremsstrahlung and Compton scattering, losing energy
as they go via photons that are created, these in turn produce more and more positronelectron pairs and the process is repeated again and again, until the particles run out of
energy. However the energy these particles have is very large, a good fraction of the
speed of light. As such when they travel through a medium faster than light within the
medium they will produce Cerenkov radiation. It should be noted that this is not faster
than light in a vacuum, as this is theoretically impossible. Light travels at different speeds
depending on what medium it travels through. As the charged particles pass nearby atoms
they polarize them and this producing light in the blue part of the optical spectrum. As
the polarized molecules depolarize back to a ground state. This is the Cerenkov radiation.
It is this radiation that the detectors look for as it signals the arrival of gamma rays [22,
23, 24].
22
To detect Cerenkov radiation an optical dish is required although an array of these dishes
is preferred, but only one is needed in order for it to work. Ideally these should be placed
at the highest altitude possible. This is not surprising as these telescopes suffer from the
same problems as optical telescopes, such as atmospheric distortion and light pollution.
These dishes then focus the Cerenkov light onto an array of photomultipliers which
convert a light signal into an electrical one. The information from the photomultiplier
array is then processed and recorded. Due to the fact that there is an array of
photomultipliers and if there are multiple optical arrays, then an image of the Cerenkov
light can be made.
However, it is important to consider the problem of cosmic rays with this setup. These
can be detected as well as the gamma ray protons produced by Cerenkov radiation in the
atmosphere. Cosmic ray Cerenkov radiation is the main source of noise with these
detectors. The only way around this is to use computer modelling. Using the imaging
taken, it is possible to eliminate the cosmic ray “background”; this is possible due to one
factor that differs between cosmic ray and gamma ray induced Cerenkov radiation.
Cosmic ray Cerenkov radiation tends to have a larger angular distribution, whereas
gamma ray Cerenkov has a much narrower distribution. By eliminating images which do
not fit a gamma rays distribution it is possible to remove a majority of the noise produced
by Cosmic rays.
A computer simulation of the particle interactions and paths in an air shower [25].
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Proportional counters work by the ionisation of a noble gas caused by the passage of a
charged particle which may be induced by energetic photons. For the detection of gamma
rays this gas is either argon or xenon. There is an electrical field present inside the
proportional counter and this, along with the presence of the ionising radiation, causes an
electron avalanche. The electrical pulse produced by the avalanche is proportional to the
energy of the detected photon, thus allowing for spectral information to be retrieved. In
addition to this, if a grid arrangement is used inside of the proportional counter, then it
also allows it to be used as a form of imager. Although primarily used for tracking GRB,
it is from these bursters that we can find new supernovae and other sources to look at
gamma line astronomy [26].
Another method for detecting gamma rays is the use of crystal scintillators such as
NaI(Tl) or CsI(Na). Similar to spark chambers, scintillators do not detect gamma rays
directly. The gamma rays produce charged particles (either electrons or electrons and
positrons) via the three photon interactions with matter. These are the photo-electric
effect, Compton Effect or pair production, in the scintillator. When a charged particle
interacts with the scintillator it produces light. These can be detected and recorded either
with an array of photomultipliers or photodiodes. Adding up all of the energy of these
recorded events for the array, allows us to estimate the energy of the gamma ray [27].
Compton Scattering occurs in the region of 1 MeV to 30MeV, and is the dominant form
of interaction for the three types of matter photon interactions within this energy range,
making it useful for gamma ray detection. A Compton Scatter Telescope usually consists
of two layers of detectors one atop the other. An incident gamma ray passing through the
top layer travels firstly through a scintillator where it is scattered off an electron. It then
travels down to a second scintillator, here it is absorbed and a light pulse emitted and
recorded. Both layers of scintillator material have photomultipliers or photodiodes to
calculate the points of interaction and energy transferred in each layer. There is an added
complication with this type of sensor as the angle of entry does not match up with the
angle of passage through the two detection layers. However, knowing the laws governing
how Compton scattering works and the energy of the interactions allow for the angle to
24
be calculated. However, the directionality of the angle cannot be determined uniquely.
This means that the origin of the gamma ray is on the edge of the circumference of a
cone, with the point of the cone at the first layer of the detector angled so that it lines up
with the direction of travel between the two layers of the detector. As a result this means
determining the source of the gamma ray is problematic and requires careful data analysis
([22, 28, 29, 30].
The two diagrams above show how a Compton telescope works. The first is a diagram on the left, shows
the inner workings of the COMPTEL telescope. The second shows the area (the red circle), also known as
the spread function, that can be worked out to determine the possible direction of an incident gamma ray
(green line) that has been detected [28, 29].
Semiconductor detectors offer many advantages over other detectors. The first is their
size, for the same area as another type of detector it can be remarkably thinner. They also
offer better energy resolution and spatial resolution. In addition to this they have less
noise. They are not without their drawbacks as they are expensive to produce as well as
the problems with growing the crystals to a large size. Two of the most popular gamma
semiconductor detectors are CdZnTe and germanium. INTEGRAL has 19 germanium
detectors on it, this way it has linked many of the crystals together to act as a large single
crystal. The semiconductor is composed of multiple layers of materials. The first layer is
the valence layer or p-type layer, the next is the band gap and the final layer is the
25
conduction band, also known as the n-type layer Semiconductor detectors work by the
photoelectric effect, when an incident gamma ray interacts with the detector it creates
electron/hole pairs. The n-type layer of the material will except holes while the p-type
layer of the material will except electrons, as a result a current is produced the attributes
of this current tell us about the gamma ray that produced it.[30].
One of the characteristics of semi conductor detectors in use out in space is that they
require a coded aperture mask to form an image since it is difficult to focus the gamma
rays [31].
Things can be done to improve the above detectors to make the more accurate or reduce
noise such as anti-coincidence systems that eliminate any false readings due to pair
production, make the data more accurate.
26
Sources of Error
It should be noted that any gamma rays detected will have to be corrected for various
factors that occur due mainly to the expansion of the universe and also due to the
expansion of the celestial events where gamma line spectra are produced. One of these
factors is known as the Doppler shift. We are all familiar with the change in pitch of the
siren of a police car as passes by at speed. A similar process happens with light.
There are two kinds of Doppler shift; these are red shift and blue shift. As the universe is
considered to be expanding since its creation in the Big Bang it makes sense that a
majority of what is observed is seen to be moving away from us. Red shift occurs when a
source of electromagnetic radiation is moving at relativistic speed, away from the
observer. Relativistic speed is considered for an object moving away from an observer at
an appreciable fraction of the speed of light. Thus it follows that blue shift occurs when
an object is moving toward us, again at a relativistic speed. The amount of shift in the
wavelength is proportional to the speed of the emitter relative to the observer. This
process has been used to deduce the speed of objects observed in space.
The diagram above shows the effects of both types of “shift”. The star in the middle of the diagram is the
source of light. If the observer on the left is stationary with the star moving toward them the light is blue
shifted. For the other observer on the right the star is moving away from them and the light is red shifted
[32].
27
This is made more complicated when viewing/imaging events in our own galaxy due to
its rotation. Our galaxy, the Milky Way is a barred spiral galaxy and the solar system is
positioned in one of its arms or spirals. When looking at just one point in our galaxy it is
easy to account for the red shift or blue shift of the object being viewed. However if a
scan of the whole galaxy is made, which allows us to look at the distribution of gamma
line sources such as
26
Al and positrons, throughout our galaxy, then we encounter an
additional problem of Doppler broadening. Doppler broadening occurs when we have to
combine readings that have been red shifted with those that have been blue shifted.
Normally if a reading were to be taken of a stationary isotope the reading would give us a
narrow peak at its gamma line value. With Doppler broadening the width of the peak is
larger due to the combination of both types of shift being added together to give a
resultant peak [36].
The above diagrams show the effect of galactic rotation on the gamma rays received. The diagram on the
left shows the tiny energy shifts (<1keV) measured for the 26Al gamma line arriving from different parts of
the galactic plane. The diagram on the right shows our position in our galaxy with the arrow showing the
direction of rotation. The two highlighted regions show the areas that are blue shifted and redshirted as well
as an area that is reasonably stationary, due to this rotation of the galaxy [33, 34].
An addition problem is there is Thermal Doppler broadening. This is different from
Doppler broadening as this effect occurs when looking at just one event. In thermal
broadening the nucleus of the emitting atom is moving as it emits. Now in a supernovae
there are many emitting nuclei all of them moving in various directions at relativistic
28
speeds. The outcome of this is that some of the signals are again blue and red shifted,
which also results in a broadening of the observed distribution of the received signals.
The above diagram shows the difference is observed spectra between a gas at rest, the top spectrum, and a
gas with particles moving at relativistic speeds, given by the bottom spectrum [35].
29
Gamma Ray Line Astronomy
Gamma ray line astronomy looks for the distinctive lines of gamma ray energies given
off by specific isotopes when they decay. This technique has been developed over the last
30 years. The first device to look out into outer space from orbit for specific lines was
HEAO-C1.
Nucleosynthesis (add chapter No) is responsible for the production of these isotopes. As
such using gamma line astronomy we can learn much about the generation and
distribution of these isotopes in other parts of the universe as well as our own galaxy, the
Milky Way. This is due to the fact that gamma rays themselves can transverse the great
distances involved with little chance of being attenuated along the way. This includes
clouds of interstellar dust in the galactic plane which normal light cannot penetrate. In
addition to this they are not fundamentally affected by changes to physical conditions,
such as temperature or density, (Roland Diehl et al,[]).
To briefly explain and give some idea of the size of these distances these gamma rays
travel, some explanation is needed. A light-year is the distance light travels in one year in
a vacuum; this is equal to 9.4605284*1015 meters or just under 1013 km. Another unit of
measurement that might be used is the parsec. This is defined as the distance from the
sun, (also known as an astronomical unit or 1 AU), which results in a parallax of one
second arc as seen from earth. An illustration of how this works is shown in the diagram
below. This is equal to 3.26156 light-years or 3.085*1016 metres. The nearest star to our
own sun is Proxima Centauri; this is 4.24 light-years or 1.30 parsecs.
At present the only other means of measuring the reactions going on in a star is via direct
neutrino measurements. Neutrinos are uncharged particles which have a very small mass.
Consequently they are difficult to detect and almost impossible to image which makes it
difficult to be sure of their source But they do have very high relativistic velocities and
can arrive at the earth fairly soon after the optical photons have signalled a related
astronomical event that produced them.
30
It should be noted that although gamma rays allow us to look at these events we are
limited in what isotopes we can look at. The lighter nuclei tend to be shrouded from view
by the rest of the stellar envelope, thereby they never leaving the interior of the star, thus
denying us a chance to look at them. This means that most of the observed gammas tend
to be from the heavier nuclei that are produced and get ejected in the final stages of a
stellar explosion or other such catastrophic event. In addition to this a large amount of the
decaying isotopes needs to decay over a similar time in order for it to be detected. These
two factors mean that most of the stars studied by this method tend to be very large.
Isotope
7
Be
Mean
Lifetime
Ni
111d
57
Ni
390d
Na
3.8y
44
Ti
89y
26
Al
1.04*106y
60
Fe
2.0*106y
e+
γ-ray energy keV
Be →7Li*
Ni → 56Co* → 56Fe* + e+
478 (0.1)
158, 812; 847 (1), 1238
(0.68), 2598 (0.17), 1771
(0.15)
Co → 57Fe*
122 (0.86), 136 (0.11)
Na → 22Ne* + e+
Ti → 44Sc* → 44Ca* + e+
1275 (1)
78 (0.95), 68 (0.078); 1157
(1)
Al → 26Mg* + e+
1809 (1)
Fe → 60Co* → 60Ni*
59 (), 1173 (1), 1332 (1)
7
77d
56
22
Decay Chain
…105y
56
57
22
44
26
60
e+ +e- → Ps → γγ
511 (1)
The above chart shows all the isotopes that are used in gamma line astronomy at present, this is an abridged
version, and the full detailed chart is in the appendices. Underline energy values are one which have been
detected [].
7
Be is slightly different than most of the other nuclei looked at in nucleosynthesis as it is
actually produced during the stars’ life time in addition to being present at its end. The
problem is that during the stars’ burning phase it is present, but not in an area that is
readily detectable. This is because it is not in an area of the star that is opaque to gamma
rays, either the core or one of the inner layers; hence we do not usually seen an emission
31
from it during this stage of a star’s life. With its short mean lifetime of 77 days this might
not be long enough to be noticeable at the end of a star’s life in a supernova. However,
the distinctive 478 keV line has been detected from our own star [36,37] during a solar
flare. So we know it is present in nucleosynthesis. However thus far, other than our own
Sun, it has not been detected elsewhere.
56
Ni production has so far only been seen in two supernovae, SN1987A [37, 38] in the
form of 847 keV and 1.238 MeV gamma lines and SN1991T [39]. This means that this
isotope is produced via nucleosynthesis in a similar fashion to the other isotopes studied
in gamma line astronomy. However, due to its properties such as its short mean lifetime
of just 111 days for its longest lived detectable isotope, it is thought to be linked with
GRB. The link between Type Ic supernovae and GRB is to do with the optical afterglow
from these GBR’s which indicate the presence of a supernova. Type Ic supernovae are
due to the explosion of a carbon-oxygen star. These stars evolve when a massive star
loses its hydrogen and helium envelopes. They eject into space a large amount of
56
Ni
and it is this that gives the supernovae some of its gamma ray luminosity. The best
estimate of the ejected matter is around 0.3-0.7 solar masses.
57
Ni gamma ray lines were observed, both 122 keV and 136 keV with the Gamma-Ray
Spectrometer aboard the SMM (Solar Maximum Mission) in 1988 and also by the
Oriented Scintillation Spectrometer Experiment aboard the Compton Gamma Ray
Observatory in 1992 [37, ,39,], both observations coming from SN1987A supernova.
This means it was detected by the same instruments at the same time as one of the 56Ni
detections. Other than proving that the nucleosynthesis of heavier elements does occur
during a supernova, it has not been detected elsewhere since.
22
Na has been searched for in a way similar to how 7Be was discovered, by looking in the
local area for the biggest source of gamma rays, the Sun. This is in addition to other
sources such as nearby supernovae. However unlike 7Be no signals have been detected.
In this case it was the SMM satellite that was scanning in particular for the
line [42].
22
Na gamma
32
The presence of
44
Ti in the form of all three of its gamma lines, (78 keV, 68 keV and
1.157 MeV), were detected in the young supernova remnant Cas A (Cassiopeia A), this
was in 1997. Due to its characteristics, primarily its mean lifetime, 44Ti it is considered an
ideal element to observe for study of the inner region of a supernova during its early
stages. This idea has been brought about by our knowledge of GRB [43]. We know from
GRB that axial jets form during a core collapse producing an asymmetrical core collapse.
If this proves to be the case then it is possible that the
44
Ti is ejected at high speed.
However so far Cas A and SN1987A are the only times these gamma ray lines have been
detected and the Cas A is the least unambiguous reading of the two. This does pose some
new questions with regards to our current understanding of the way supernovae work.
There is also an idea that 44Ti is produced via other astrophysical means either by the
GRB from neutron star surfaces.
The 26Al 1.809 MeV gamma line is one of the most studied spectral lines in the gamma
region. There are many reasons for this. Particularly its mean lifetime of 1.04 My, this
shows the production of recent (relatively speaking) massive stars that have an average
lifetime measured in tens of millions of years. This also shows us that nucleosynthesis of
stars is an ongoing occurrence. This is supported by the fact that the first readings taken
for
26
Al were in 1978 on the HEAO-C Satellite and there have been many more since
then. One of which is the COMPTEL experiment that mapped out the
26
Al emission in
the galactic plane. This shows that most of the 26Al production is in the galactic plane and
not outside of it; this is in line with predicted models. There are regions where the
production is larger than the rest, such as Cygnus. One of the current instruments looking
at
26
Al production is the SPI on INTEGRAL. As this is a more modern and accurate
satellite with its 19 element Ge camera, it is doing its own sweep of the sky looking at the
26
Al emission line [44, 45, 46, 47].
33
The above diagram shows the distribution of 26Al modelled around our galaxy, the Milky Way, using
information from the COMPTEL and INTEGRAL/SPI systems. As you can see it is concentrated mostly in
the plane of the galactic especially the galactic core; however there are the regions of interest such as
Cygnus, Vela, Orion and Sco-Cen. A more detailed diagram showing readings from each area is included
in the appendices [49].
60
Fe has posed as something of a problem with regard to gamma line astronomy. It has
been recorded by both the RHESSI’s sensors and also by those of the SPI camera on
INTEGRAL in the form of 1.173MeV and 1.332MeV gamma lines. So we know it is
being produced by nucleosynthesis. By comparing the 26Al emissions with 60Fe we can
build up a model of the core collapse nucleosynthesis. Wherever 26Al is detected, we also
expect to see the presence of
My and
60
60
Fe as well. They have similar mean lifetimes;
Fe is 2 My. However, the amount of
60
26
Al 1.04
Fe detected is only a 10th of the
26
Al
signal, with the RFe-Al ratio considered to be 14% +/- 2% which, considering what we
expect for nucleosynthesis is in line with earlier predictions of around 15% [46, 48].
In addition to doing a survey of 26Al, INTEGRAL is also doing a survey of the 511 keV
gamma line. Although not a radioactive isotope, positron emission lines are still being
looked at as they can also reveal information about our own galaxy we are in. Now we
would expect to see evidence of anti-matter annihilation emissions from areas such as
34
stars and supernovae and novae, as they produce proton rich isotopes that will emit
positrons in order to increase their stability. As with 26Al most of the readings for the 511
keV lines are in the plane of the galactic. Again this makes sense as this is where the stars
and both supernovae and novae are in this area and they both produce positrons from the
decay reactions of
22
Na,
44
Ti,
56
Ni and most importantly due to its abundance
26
Al.
However, the 511keV positron emission line is the brightest gamma ray line seen. This
emission is brightest , in the centre of the galaxy, why this is, is not yet fully understood.
One line of reasoning that takes this into account is dark matter. It is thought that the
large amount of positrons produced is a by-product of the presence of dark matter. This is
only a theory at present and needs further study.
The above diagram shows the distribution of 511 keV gamma rays throughout the galaxy recorded by
INTEGRAL. As is quite clearly the case the largest concentration is centred on the galactic bulge at the
centre of the galaxy [50].
Although already repeatedly and briefly, GRB’s are used for finding out information
about gamma line astronomy. GRB’s in addition to giving us information about black
holes, pulsars and other celestial events, most importantly supernovae, and the simplest
way they do this is by letting us know when and where a new one has formed. When a
supernova forms, (mentioned in space borne sources of gamma rays chapter) it gives off
a GRB. Thus using gamma ray sensors all around the world and by working quickly we
35
can trace the GRB back to its source, if it proves to be a supernovae then this would give
us a chance to study it in its early stages and perhaps study some of the lesser seen
isotopes by observing it from the beginning, whilst they are most abundant.
36
Future Developments.
Although a lot has been learned from gamma line astronomy there is much still to be
learned. Some new ideas are being looked at, that could take gamma line astronomy use
even further. However there are some points that at present won’t change at present the
best way to study gamma line astronomy is in space or by high altitude balloon.
Gamma-ray lenses are a new idea for hardware to help with the collection of gamma line
data. These would also help to create a large area of detection by concentrating the
gamma rays onto a small area detector. This is done by Bragg diffraction in a Laue lens
with a high Z number crystal, copper is a good example. This is being planned for the
MAX space vehicle, but has already been used on a balloon borne experiment. In order
for it to work concentric rings of different material would need to be used with different
diffraction values. The outer rings would diffract the gamma rays more than the inner
rings thus the overall result is like that of an optical lens. One of the major stumbling
blocks is that the focal length is very large. This effectively means that two satellites
would be needed operating at the large distances apart one with the detector and one with
the lens. In addition to this the lens has an additional drawback in that the energy bands it
can focus are very small. This means that each mission could only look at one area such
as the 511 keV energy range for positrons.
37
The above diagrams show aspects of the use of Laue diffraction lens, the first shows the configuration of
two concentric circles of two different diffracting materials acting as a lens. The second picture is an artist
impression of one way of linking the two halves of the detector, the bottom half housing the detector and
solar panels, the second part consisting of the Laue lens being held at a fixed distance in front of the
detector by an expanding structure [51][52].
Multilayer coatings as the name implies, is the use of multiple layers of coatings on atop
the other, alternating between one material and another material. In addition to this they
vary in thickness the outer layers being thicker than the inner layers. This means that
different wavelength photons are reflected giving a distribution of wavelengths. The two
types of materials vary in density so that there is a high density contrast between them.
One layer usually consists of higher density materials such as Tungsten or Platinum. The
second layer is made out of low density materials such as Carbon and Silicon. These
layers reflect lower energy gamma and X-rays. As mentioned earlier the boundary
between gamma rays and X-rays is sometimes a bit blurred, as nuclear spectral lines and
characteristic atomic X-ray lines can end up having similar energies. These mirrors work
in the hard X-ray and gamma area up to 80 keV, beyond this region they start absorbing
the energy rather than reflecting it. The use of these mirrors will allow for large area
collection on a small sensor. NASA is launching a satellite in 2011, called the NuSTAR
with such a mirror mounted on it. This will improve studies of the 68 keV and 78 keV
line emitted by 44Ti.
38
The above diagram shows how the layers or work on a multilayer mirror work the outer layers are larger an
reflect the larger wavelengths, each subsequent layer after the outermost gets progressively thinner each
time the wavelength of the reflected light gets shorter [53].
Another form of lensing for gamma rays is via the use of a Frensel Lens [56]. The lens
works by using both diffraction and refraction. They originally worked by blocking part
of the radiation. As such they are not incredibly efficient at around 10%. However with
variations on the original theme it is possible to increase this. The type of Frensel lens
used is one called a Phase Fresnel Lens (PFL, type c in the diagram). The result of using
this lens would be an increase in the angular resolution and away around the problem of
scaling up gamma ray technologies.
39
The above diagram shows the arrangement of the lens. The first part of the diagram shows how the lens
looks head on, with the a to be representing variants of the lens (a) A Frensel Zone Plate (FZP), (b) A Phase
Zone Plate and (PZP) (c) A Phase Fresnel Lens (PFL)[54].
At the moment Compton telescopes suffer from the fact that it is not possible to
determine the direction to a reasonable degree of each gamma detected. One method that
is currently being looked at involves measuring the direction of the scattered electrons in
the top level of the detector. By combining this with the other information that can be
learned from a gamma ray by a Compton telescope it should become possible to
eliminate the spread function. This would reduce the amount of data analysis involved a
great deal.
There is a line of thought that suggests that gamma line astronomy could be used in order
to find dark matter. Dark matter is a little understood form of matter that is very hard to
detect. At present one of the only ways it has been shown to exist is through the collisions
of galaxies and the effects of gravity due to matter, including dark matter, confirming its
presence. At present estimates dark matter is thought to make up about 22% of the energy
in the universe. The rest is composed of 74% dark energy with only 4% composed of
normal matter. The reasoning behind it is to down to the large amount of positrons in the
galactic bulge. So far most of these studies are computer models that involve WIMP’s
(weakly interacting massive particles). One such model states that that dark matter
particles annihilate at rest with 10 MeV in the galactic bulge giving rise to electronpositron pairs via a process called the t-channel exchange of heavy fermions [55].
40
Discussion and Conclusion
As it stands what information we have currently is limited as we only have a limited
number of Supernovae that have been studied. We can only study the supernovae as they
appear; this is just a question of time. With a supernovae core collapse rate of one ~ 50
years it is a slow process.
However, another part of this is due to the fact that some of the isotopes in question have
a very short lifespan and we need the satellite/detector to take readings as soon after the
event as possible. This is currently exactly what scientists are working toward with
satellites like INTEGRAL, by having a sensor to look for gamma ray bursts so that it is
then able to locate the source and then using its main instrument learn what it can with
little or no delay, combine this with the ground based arrays this improves the coverage
considerably. The other problem is the distance and location of the events. Combining all
of these factors means we can only learn about these events as often as they happen for
information that occurs immediately after term, otherwise we can only learn from the
long term effects which do not give us a complete picture.
Even though INTEGEAL has done an amazing job since its Oct 2002 launch it has
limitations, one of these is the ability to make an image of 60Fe gamma rays []. However
the ability of satellites will only improve with the new developments listed in the Future
Developments chapter. Each new mission will bring more information and ideas as well
as questions.
Gamma-rays are still the most viable option for learning about nucleosynthesis in the
galaxy and beyond. The process will only improve with new developments such as given
in (future developments). At present there is no better alternative for this.
At the time of this dissertation there was no clear relationship between the types of
supernova and the gamma ray lines emitted. I think this would prove a useful, even
though it would be a very long term project. As it may prove to be a correlation between
the unique conditions of each supernova that the distribution of gamma ray lines emitted
41
would vary for each isotope. We know that there are processes involved in supernovae
such as the r and s processes that determine the type of isotopes produced, this may have
something to do with the supernova type.
As for the other elements that such as using gamma radiation from GRB to find celestial
events this is just an ongoing process and is still the best way of finding these events.
42
Acknowledgements
I would like to thank first of all Dr Gilboy, Prof Walker, Prof Regan and Dr Bradley for
taking the time to explain some of the physics that I didn’t understand, give suggestions
on the content and provide encouragement throughout.
I would also like to thank Mrs Jan Walker from the Learning Support group for helping
me with the structuring, planning, organising, presentation and further encouragement for
this dissertation.
I would also like to thank the MSc support staff particularly Miss Alexia Smith, MSc
Course Secretary for all the questions I had with regard to past dissertations and the rules
and procedures that needed to be followed. In addition I would also like to thank Mrs
Christobel Soares-Smith, Physics Department Secretary for her help with the Physics
department library.
Finally I would like to thank my family for all their help and support, as without it I could
not have done this MSc course.
43
References
[1]
http://www.visionlearning.com/library/modules/mid59/Image/VLObject-785-
021205011204.gif National Science Foundation funded site (US).
[2] http://www.daviddarling.info/images/protoplanetary_collapse.jpg Astronomer David
Darling.
[3] http://www.arcadiastreet.com/cgvistas/dwarfs_030.htm
[4] http://www.science.psu.edu/alert/Fox8-2007.htm Eberly College of Science.
[5}http://www.eso.org/gallery/v/ESOPIA/illustrations/phot-36-08-fullres.jpg.html
European Organisation for Astronomical Research.
[6] http://www.nasa.gov/mission_pages/GLAST/multimedia/pulsar_stills.html NASA
[8] http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/whitedwrf.htm Arizona
State Education, Prof George Rieke
[9] http://www.astronet.ru/db/xware/msg/1214949 Moscow State University.
[10]
https://academics.skidmore.edu/weblogs/students/scheng/
Skidmore
College,
Spencer Cheng.
[11] Zelik M & Gregory S A., Introductory Astronomy & Astrophysics, Fourth
edition, Place of publication Saunders College Publishing 1998.
[12] Malcom S Our Evolving Universe, Longair, 1996, Cambridge University Press
1996.
44
[13] http://abyss.uoregon.edu/~js/lectures/gamma_ray_bursts.html University of Oregon,
Department of Physics, James Schombert.
[14] http://nssdcftp.gsfc.nasa.gov/photo_gallery/image/spacecraft/sas2.gif NASA
[15] http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=30907 ESA
[16]http://www.mpihd.mpg.de/hfm/HESS/pages/about/pictures/HESS_IMG/images/HES
S-dark-full.jpg
[17] http://www.srl.caltech.edu/astro/gripphoto.html California Institute of Technology.
[18] http://heasarc.gsfc.nasa.gov/docs/cgro/egret/egret_doc.html NASA, Goddard Space
Flight Center.
[19] heasarc.gsfc.nasa.gov/docs/cosb/cosb_about.html NASA, Goddard Space Flight
Center.
[20] http://imagine.gsfc.nasa.gov/docs/sats_n_data/satellites/showcase_sas2.html NASA,
Goddard Space Flight Center.
[21]
http://imagine.gsfc.nasa.gov/docs/science/how_l2/pair_telescopes.html
NASA,
Goddard Space Flight Center.
[22]http://books.google.co.uk/books?id=eHIMGIVn24kC&pg=PA46&lpg=PA46&dq=ga
mma+compton+telescope&source=bl&ots=2ciBAEBAG5&sig=7ky8uyVeDe3WVLTb4i
mIt68h2xU&hl=en&ei=8L-OSu-qI4XeQaG0PDyDQ&sa=X&oi=book_result&ct=result&resnum=4#v=onepage&q=gamma%20
compton%20telescope&f=false
[23] http://www.mpi-hd.mpg.de/hfm/HESS/pages/about/telescopes/ H.E.S.S Telescope
website.
[24] http://imagine.gsfc.nasa.gov/docs/science/how_l2/cerenkov.html NASA, Goddard
Space Flight Center.
45
[25] http://www.particle.kth.se/~pearce/shower.jpg KTH Royal Institute of Technology.
[26] http://www.tpub.com/content/doe/h1013v2/css/h1013v2_43.htm
[27] http://imagine.gsfc.nasa.gov/docs/science/how_l2/gamma_scintillators.html NASA
[28]
http://imagine.gsfc.nasa.gov/docs/science/how_l2/compton_scatter.html
NASA,
Goddard Space Flight Center.
[29] http://xweb.nrl.navy.mil/gamma/detector/compton/compton.htm US Naval Research
Laboratory, Gamma and Cosmic Ray Astrophysics Branch.
[30] W Catford, Solid State Detector Lecture notes, University of Surrey, 2009
[31] http://astrophysics.gsfc.nasa.gov/cai/coded_intr.html NASA
[32]http://images.google.co.uk/imgres?imgurl=http://www.cyberphysics.pwp.blueyonder.
co.uk/graphics/diagrams/waves/redshift.gif&imgrefurl=http://www.cyberphysics.pwp.blu
eyonder.co.uk/topics/space/redshift.htm&usg=__hPXZ6PHuVe4RgFeGuaKRI8iwhkU=
&h=425&w=398&sz=21&hl=en&start=130&um=1&tbnid=ua5dzqyjlknfiM:&tbnh=126
&tbnw=118&prev=/images%3Fq%3Dred%2Bshift%26ndsp%3D21%26hl%3Den%26sa
%3DN%26start%3D126%26um%3D1
[33, 34] Diehl R., Halloin H., Krestschemer K., Lichhti G. G., Schonfelder V., Strong A.
W., vom Kienlin A., Wang W., Jean P., Knodlseder J., Roques J-P., Weidenspointner G.,
Schanne S., Winkler C. and Wunderer ,26Al Gamma ray line observations of the galaxy,
6th INTEGRAL Workshop ‘The Obscured Universe’ Moscow 3-7 July 2006 (26Al
Gamma Galaxy.pdf)
[35] http://astronomy.swin.edu.au/cms/astro/cosmos/T/Thermal+Doppler+Broadening
Swinburne University of Technology (Australia), Centre for Astrophysics &
Supercomputing.
46
[36] Share G. H. ,Murphy R. J., Gamma Ray Spectroscopy in the Pre-HESSI Era, ASP
Conference Series, 2000
[37] http://www.mpe.mpg.de/gamma/science/lines/ Max-Planck Institute, Roland Diehl.
[38] Keiichi Maeda Nozomu Tominaga, Nucleosynthesis of 56Ni in wind-driven
Supernova explosions and Constraints on the Central Engine of Gamma-Ray Bursts, 5th
Jan 2009
[39] Roland D., Dieter H. H., Nikkos P., Gamma rays from cosmic radioactivities,
Meteoritics and Planetary Science, 42, 19th March 2007.
[40] Diehl R, Lang M, Kretschemer K, Wang W,
26
Al emission throughout the Galaxy,
New Astronomy Reviews, 52, 440-444 24th June 2008. (26Al Emission Galaxy.pdf)
[] Diehl R. Gamma-ray Observations, , Proceedings of Science, International symposium
on Nuclear Astrophysics- Nuclei in the Cosmos- IX CE, Geneva 25-30 June 2006.
[42] Share M. D, Chupp. G. H., E. L. & Kanbach, G Gamma-ray limits on Na-22
production in novae, Leising, Astrophysical Journal Part 1 vol328, May 15th, 1988,
p.755-762.
[43] http://www.mpe.mpg.de/gamma/science/lines/44Ti/44Ti_science.html Max-Planck
Institute, Roland Diehl
[44] Magkotsios G., Timmes F.X., Wiescher M., Fryer C.L., Hungerford A. Young P.,
Bennett M. E., Diehl S.,Herwig F., Hirschi R., Pignatari M.and Rockefellar G.,
56
44
Ti and
Ni core collapse Supernovae, Proceedings of Science, 28th Nov 2008
[45] Leising M., Gamma Ray line Studies of Nuclei in the Cosmos, Proceedings of
Science, 4th Mar 2009.
47
[46] Diehl R., Meassuring 26Al and 60Fe in the Galaxy, New Astronomy Reviews 50 534539, 4th August 2006
[47]http://www.mpe.mpg.de/gamma/science/lines/26Al/figures/26Al_ComptMap26_spec
_innerGal.jpg Max-Planck Institute, Roland Diehl
[48] http://www.mpe.mpg.de/gamma/science/lines/60Fe/60Fe.html Max-Planck Institute,
Roland Diehl
[49] http://www.mpe.mpg.de/gamma/science/lines/26Al/26Al.html Max-Planck Institute,
Roland Diehl
[50] http://www.nature.com/nature/journal/v451/n7175/full/nature06490.html Nature
[51] MAX: Development of a Laue diffraction Lens for nuclear astrophysics
(Laue
Lens.pdf)
[52] http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36959 ESA
[53][54] http://www.cesr.fr/~g-wave05/focusing.html Centre d’Etude Spatiale des
Rayonnements, Astrophysique Systeme Solaire Experimentation Spatiale.
[55] Could a γ Line Betray the Mass of Light Dark Matter?, J Orloff, 1-19 July 2007
[56] Gerry Skinner, Peter von Ballmoos, Neil Gehrels and John Krizmanic Fresnel lenses
for X-ray and Gamma-ray Astronomy, Edited by Citterio, Oberto; O'Dell, Stephen L.
Editors Proceedings of the SPIE conference 5168 : Optics for EUV, X-ray and GammaRay Astronomy privately published, 25th August 2003.
[57]
www.mpe.mpg.de/gamma/science/lines/60Fe/60Fe.html
Roland Diehl
Max-Planck
Institute,
48
[58] Gamma rays from cosmic radioactivities, Roland DIEHL, Dieter HARTMANN and
Nikos PRANTZOS, Meteorities and Planetary Science, Received 14th October 2006,
revision accepted 19th March 2007 (Gamma Cosmic Radio.pdf)
[59] Astrophysical constraints for gamma-ray spectroscopy, Roland Diehl, Nikos
Prantzos and Peter von Ballmoos, Nucl.Phys.A Special Volume on Nuclear Physics, 17
Feb 2005
[60] Abundances of Isotopes from nuclear line spectroscopy, Jaina Fiehl, Copied from
Diehl, http://www.mpe.mpg.de/gamma/science/lines/ (titelseitte.pdf)
[61] Supporting Gamma-Ray Spectroscopy in Space, Michael Lang, Technische
Universitat Munchen, 3rd July 2006.
[62] http://www.projectcalico.com/Chapter_2.htm
49
50
Appendices
Isotope chart
51
[58,59 ,60. 61]
52
List of satellites
53
Satellite
Built
Status
SAS-2
15/11/1972
Inactive
Band
20MeV1GeV
Cos-B
09/08/1975
Inactive
2keV5Gev
HEAO 3
20/09/1979
Inactive
50keV10MeV
Sensor Type
32-level wire spark-chamber aligned with satellite spin
axis (20 MeV-1 GeV), eff. area 540 cm 2
Magnetic-core, wire-matrix, spark chamber gammaray detector (~30 MeV-5 GeV), eff. area 50 cm2 at 400
MeV, a 2-12 keV proportional counter mounted on the
side of the gamma-ray detector
High Resolution Gamma Ray Spectrometer (HRGRS):
50 keV - 10 MeV, FOV 30°, effective area 75 cm 2 at
100 keV
Gamma
01/07/1990
Inactive
2keV5MeV,
50MeV6GeV
Compton
(CGRO)
05/04/1991
Inactive
30 keV 30 GeV
Coded-mask X-ray telescope (SIGMA) 0.03-1.3 MeV,
eff. area 800 cm2, FOV 5°x5°, Coded-mask X-ray
telescope (ART-P) 4-60 keV, eff. area 1250 cm 2, FOV
1.8°x1.8°, X-ray proportional counter spectrometer
(ART-S), 3-100 keV, eff. area 2400 cm 2 at 10 keV, FOV
2°x2° All-sky monitor (WATCH) 6-120 keV, eff. area 45
cm2, FOV All-sky Gamma-ray burst experiment
(PHEBUS) 0.1-100 MeV, 6 units of 100 cm 2 each, FOV
All-sky Gamma-ray burst experiment (KONUS-B) 0.028 MeV, 7 units of 315 cm2 each, FOV All-sky Gammaray burst experiment (TOURNESOL) 0.002-20 MeV),
FOV 5°x5°
Gamma-1 Telescope 50MeV-6GeV, 2 Scintillator
counters and a Gas Cerenkov counter, eff.area of
2000cm2 Res @ 100MeV 1.5 degrees. Imaging view
+/- 2.5 degrees. Energy Res 12% at 100MeV. Disk-M
Telescope 20keV-5MeV, NaI scintillators, Res 2arcmin
(stopped shortly after launch), Pulsar X-2, 2-25keV Res
30arcmin.
The Burst and Transient Source Experiment (BATSE)
an all sky monitor 20-1000 keV, The Oriented
Scintillation Spectrometer Experiment (OSSE) 0.05-10
MeV energy range, The Compton Telescope (Comptel)
0.8-30 MeV capable of imaging 1 steradian Energetic,
Gamma Ray Experiment Telescope (EGRET) 30 MeV10 GeV
0.5keV400keV
High Resolution Gamma Ray Spectrometer
(HRGRS): 50 keV - 10 MeV, FOV 30°, effective area 75
cm2 at 100 keV, Heavy Nuclei Experiment, Cosmic Ray
Isotope Experiment
French Gamma-ray Telescope (FREGATE; 6-400 keV).
4 NaI(Tl) gamma-ray detectors. Eff. area 120 cm2. FOV
~3 sr. Spectral resolution ~25% @ 20 keV, ~9% @ 662
keV.
3keV10MeV
2 Gamma-ray instruments Spectrometer (SPI; 20 keV 8 MeV) Coded aperature mask. FOV 16°, detector area.
500 cm2 (Germanium array) spectral resolution (E/dE)
500 @ 1 MeV, spatial resolution 2°. Imager (IBIS; 15
keV - 10 MeV) Coded aperature mask. FOV 9° X 9°,
detector area. 2600 cm2 (CdTe array) & 3100 cm2 (CsI
array), spatial resolution 12´.
Granat
01/12/1989
Inactive
2keV100MeV
LEGRI
19/05/1997
Active
HETE
09/10/2002
Active
INTERGRAL
17/10/2002
Active
54
Swift
20/11/2004
Active
AGILE
23/04/2007
Active
Fermi
11/06/2008
Active
Ground
Built
Status
Veritas
Active
0.2keV150keV
Burst Alert Telescope (BAT; 15-150 keV), Wide fieldof-view coded-aperture imager. Eff. area 5240 cm2,
FOV 1.4 sr half coded, ~4 arcmin position accuracy. XRay Telescope (XRT; 0.2-10.0 keV). CCD Imaging
spectrometer. Eff. area 110 cm2 @ 1.5 keV, FOV 23.6´
X 23.6´, ~5 arcsec position accuracy UV/Optical
Telescope (UVOT; 170-650 nm).Modified RitcheyChrétien telescope with image-intensified CCD
detector. 6 color filters and 2 grism, FOV 17´ X 17´, 0.3
arcsec position accuracy.
18keV60keV,
30MeV50GeV
2keV30Mev
Gamma ray imager tungsten-silicon tracker, 30MeV50GeV. Silicon Based X-ray detector, SuperAGILE
(SA), 18keV-60keV. Mini Calorimeter 300keV-100MeV
CsI(Tl). scintillators with photodiodes at both ends in
two orthogonal layers, eff.area 1400cm 2
Glast: 12 Sodium Iodide (NaI) scintillation detectors and
2 Bismuth Germanate (BGO) scintillation detectors.
Band
50GeV50TeV
50 GeV30 TeV
100GeV1TeV
1TeV10PeV
10GeV100TeV
Air Based Cerenkov Detector
Magic
2008
Active
H.E.S.S
2002
Active
HEGRA
1987
Cherenkov
2010
Inactive
Not
Built
Cactus
2001
Inactive
Air Based Cerenkov Detector
Cangoo
2004
Active
Air Based Cerenkov Detector
Balloon
Caltech
GRIP
Launched
Status
Band
30keV2MeV
GRIS
HIREGS IV
Air Based Cerenkov Detector
Air Based Cerenkov Detector
Air Based Cerenkov Detector
Air Based Cerenkov Detector
Sensor Type
NaI(Tl)/CsI(Na)
7 n-type ultra pure germanium detectors
20keV18MeV
PoGO Lite
TIGRE
MEGA
1MeV100MeV
400keV50MeV
NaI(Tl)/CsI(Na)
55
Hersprung-Russel diagram
56
[62]
57
Ultimate 26Al Diagram
58
[Reference on Diagram]