australian national university, canberra department of ... dasgupta department of nuclear physics...
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Stardust all around us:Fusion and Elem
entform
ation
Mahananda D
asgupta
Departm
ent of Nuclear P
hysics
Australian N
ational University, C
anberra
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Curiosity …
. and a quest for answers
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What is m
atter made of?
What m
akes the Sun shine?
Helios the S
un god in hischariot (G
reek mythology)
Seeking explanations …
.. since ancient times
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600 BC
- World originated from
water
(Greek philosopher - Thales)
450 BC
- Matter m
ade of four “roots”/“elements”
Air, Fire, E
arth, Water (G
reek phil. - Em
pedocles) C
hina (above + wood), India (above + space)
~1660 - hypothesis - corpuscular structure of matter,
more than just four elem
ents (R
obert Boyle – E
ngland/Ireland)
1789 - list of 33 “elements”, including “light”, “caloric”
(Laviosier - France)
1869 - Periodic table – classification of elem
ents(M
endeleev - Russia)
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Ds
17 Novem
ber 2006, Germ
any
How
were these elem
ents produced in N
ature?
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Ds
Rg
What the “big bang” created
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Ds
Rg
What the “big bang” created
Discovery supports big bang scenario
John C.
MatherG
eorge F.Sm
oot
2006 Nobel P
rize
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Ds
Rg
What the “big bang” created
Fields (2002)
What the big bang created
Mass num
ber50
100150
200
1
0.01
10-4
10-6
10-8
10-10
10-12
10-14
Mass fraction
H (m
ass number 1)
He (m
ass number 4)
more than billion tim
es less of
Lithium
Carbon
Nitrogen
What w
e find today
Grevesse & N
oelsM
ass number
50100
150200
1
0.01
10-4
10-6
10-8
10-10
10-12
10-14
Mass fraction
What created the heavier
elements?
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What m
akes the Sunshine?
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energy radiated by the Sun in one m
illionth of a sec
=
energy consumed by m
ankind in one year!
What produces this vast am
ount of energy?
Sun’s enorm
ous energyoutput - supports life on E
arth
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Clash of tw
o scientific giants – Kelvin and D
arwin
Popular theory C
oal fired Sun – show
n to be incorrect in1848 as S
un would last only few
thousand years
1860 - First scientific attempt (H
elmholtz, Lord K
elvin)
Gravitation causes contraction – liberates energy
5th century B
C S
un is a mass of blazing m
etal (Anaxagoras)
•can sustain S
un for 15 million years
•age of E
arth greater than billion years
implied m
inimum
of the age of Sun
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Energy crisis of the 19th century!
no consistent scientific answer in the
19th century
Solution from
discoveries in the 20th century
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Key (unrelated!) discoveries tow
ards a solution
see http://nobelprize.org/nobel_prizes/physics/ and articles therein
Radioactivity (discovery in 1896)
- showed age of earth 4.6 B
illion years
gravitational contraction theory
- led to other cluesB
ecquerel - Nobel prize 1903
Albert E
instein
Equivalence of energy and m
ass
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Aston - N
obel prize 1922
Four hydrogennuclei areheavier than H
e
Equivalent to energy, E
= m c 2
Transform 4 H
nuclei He
+ difference between m
asses( m
)
But is this kind of reaction possible?
Required - understanding of the nucleus and
- Quantum
mechanics
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Nucleus in the cells of plants, anim
als, humans
ATO
MIC
NU
CLEU
S:
human cheek cells
object at the centre of atoms such as oxygen,
carbon etc.
CELL N
UC
LEUS:
Discovered by R
utherford in 1911
(from N
elson, NZ)
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made up of protons &
neutrons
line-up of 5,000,000,000 nuclei =thickness of hum
an hair
incredibly small and dense
positivelycharged
neutral
ATO
MIC
NU
CLEU
S
10-14 m
atom
Nucleus is positively charged
+
Density of lead = 11.3 g/cm
3
Nuclear density = 10
15 g/cm3
10-10 m
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++
SN
NS
Potential
energy
r
++
++
Like charges repel
(Coulom
b force) R
epulsive electrostatic forceQ
2
Q1 Q
2
r 2= K
Q1
r
identicalpolesrepel
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Magnets w
ithsuperglue onends NS
GLUE
SN
Repulsive electrostatic force
++
positive charges repel
SN
NS
++
Strong attraction
at small
separations
Strong A
ttractiveN
uclear Force
acts at short distances
137 times stronger than electrostatic
forces
SU
PE
RG
LUE
bring closer
Bring closer – m
ore repulsion
! can never stick together
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Net force
attractiveN
et forcerepulsive
peak/ barrier
Potential
energy
attractivenuclear
Attractive N
uclear ForceR
epulsive Electrostatic Force
vs.nuclei with high kinetic energy
(velocity) overcome the barrier
and feel the attractive forceleading to fusion
r
fusion-landbarrier
Repulsive
electrostaticr
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For fusion: do H-nuclei in the Sun have
sufficient kinetic energy (velocity)?
Sun’s tem
perature ~ 15 Million K
Temp. scale (0 K
= -273 °C)
honouring Lord Kelvin
Hydrogen nuclei do not have sufficient kinetic
energy
or high temperature E
= kT
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But how
come the S
un shines…
…and the U
niverse exists
Quantum
tunnelling
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Heisenberg in 1927
Key concept of quantum
mechanics
particles behave like waves
HO
W D
OES TH
AT W
OR
K?
Quantum
mechanics
- developed in 1920s and 30s
- one of the greatest ideas of 20th century
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Application to tunnelling – from
studies of radioactivity
George G
amow
Wrote a series of very nicepopular science books
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Particle in the crater of an
extinct volcano- D
oes not have sufficient kinetic energy
- Yet it escapes!
The puzzle
The solution•P
article behaves like wave
•Wave spreads through the
surrounding wall to outside
•Appears outside w
ithouthaving to clim
b over the rim
lower the energy com
pared with barrier –
the lower the tunnelling probability
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Classical m
echanics – big objectsNuclei are m
uch much sm
allerthan balls
Quantum
tunnelling leads tofusion
fusionland
fusionland
Quantum
tunnelling
Quantum
mechanics – sm
all objects, e.g. atoms, nuclei
barrier ball can’t appear on the otherside of the hill
No fusion
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Nuclear Fusion
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major problem
remained: H
+ H !
2He
Energy is released in each step
" e +p
n
+4H
e
- solution due to Hans B
ethe
Nobel prize 1967
energy production in stars
2He is unstable - im
mediately goes to tw
o H
(same as a proton)
How
could we get 4H
e (stable)?
" e +p
n# #
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10 Billion Y
ears6 seconds
! e +
Changes proton "
neutron
Strength very sm
all – billionth of strong force
Reaction occurs very infrequently – if step1 via strong
force then Sun w
ould live for just 10-9 seconds!!!
Weak force
Strong force
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4 1H 4H
e + 2 neutrinos + EN
ER
GY
Using E = m
c2
Mass difference (4 H
ydrogen – Helium
) = 0.0477 x 10
-27 kg (0.7%)
4.3 x 10-12 Joules
1038 reactions per second (using 4.3 M
tons of H/sec)
(Sun has 6000 billion billion tons)
Energy generated travels outw
ards to the surfaceand radiated into space as heat and light
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4 1H 4H
e + 2 neutrinos + EN
ER
GY
Energy released:
Chem
ical reaction: Much sm
aller energy released 10 g of petrol : 4.7 M
J 10 g of H
nuclei : 25 ! 106 M
J
Possible in nuclear reaction - not in a chem
ical reaction
Transformation of one elem
ent into another:
Nuclear and chem
ical reactions – the differences
Petrol burning: C
8 H18 + 12.5 O
2 "8C
O2 + 9H
2 O
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Gravitational force tries to contract the star
(inward pressure)
Thermal pressure from
heat – outward pressure
from The C
omic P
erspective by Bennett et al
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from The C
omic P
erspective by Bennett et al
pushing out
(thermal
pressure)
The balancing act……
Gravity
(pushing down)
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More m
assive the star – greater thegravitational pressure ( )
Sm
aller starLarger star
higher thermal pressure ( ) needed to
counteract gravity - needs higher temperatures
more reactions per second
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Stars sim
ilar to the mass of the S
un
Heavier elem
ents not from S
un-like stars
Hydrogen w
ill last for 10 billion years
! age of the Universe
Hydrogen lasts for few
tens of million years
More m
assive stars –hydrogen gets consum
edm
ore rapidly
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•A
fter Hydrogen is exhausted in core
•N
o fusion to generate energy ! no therm
alpressure to counteract gravity
•C
ore contracts - temperature and pressure
increases- C
ore heats up to 100 million degrees!
•S
ufficient energy for He to fuse
(fusion barrier for He is 4 tim
es that of H)
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•E
nergy from fusion of H
elium stabilizes the
star against further gravitational contraction
•O
nce Helium
is exhausted – gravitationalcontraction starts again – core heats up –fusion w
ith carbon starts
Life story of a star – pushing match
between therm
al pressure and gravity
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William
Fowler
(Nobel P
rize 1983)
Fig from C
SIRO
website
The most im
portant steps in producing heavy elements
Fred Hoyle
(coined the word “big
bang” – as a put-down)
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The fusion processes continue to make heavy
elements:
• 4 1H !
4He H
ydrogen Burning (few
million yrs)
3 4He !
12C H
elium B
urning (5 x 105 years)
• 2 12C !
24Mg
Carbon B
urning (600 years)•
! 56Fe
Neon, O
xygen, Silicon B
urning
then… the star
hits a problem
Fusion barrier for He fusing w
ith Fe high &
Iron is a very stable nucleus
No further energy generating
fusion reactions
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Larger star
•S
tar having lived forM
illions of years -im
plodes in seconds!
•C
ore has limited
compressibility –
bounces back like arubber ball
Massive explosion
No outw
ard thermal pressure
Gravity w
ins!
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Supernova -
the death of a star
From N
ASA/ESA collaboration
Crab nebula –
the expandingrem
nant of asupernovaexplosion
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• elem
ents formed in the star are distributed
through the galaxy
• M
ore elements are m
ade - especially thoseheavier than iron
Heavier elem
ents formed by capture of neutrons
56Fe + 6n ! 62Fe !
62Co !
……
. Au, P
b
charge neutral – so no barrier
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From Parting the C
osmic Veil, By K. Lang, Springer Verlag
The origin of elements
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Our origin
Each heavy atom
in our body was built
and processed through ~100–1000 stargenerations since the beginning of tim
e!
We are m
ade of star stuffC
arl Sagan
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Question 3
How
were the elem
ents fromiron to uranium
made ?
To know
! need to do experim
ents
Elem
ents beyond Fe -created by addition ofneutrons – but specificsnot know
n
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•N
eed nuclei with sufficient kinetic energy to
overcome the barrier
http://nobelprize.org/nobel_prizes/physics/articles/kullander/index.html
Experim
ents to learn about fusion on Earth
Nuclei w
ith high velocities
•A
ccelerators used to produce nucleiw
ith high velocities (Fri talk)
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Typical experimental setup:
speedingnuclei
Schem
atic picture
e.g.4H
e
Targete.g. 12C
detector
Set-up at A
ustralian National U
niversity
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Bull’s
eye
hit the bulls eye
For fusion – need to hit the nucleus at the centre of atom
.
- Size shrunk to less than a dust particle
Blindfolded
+C
hance of hitting the dust speck very very small
fusion probability very small
- every fusion event – there are 109 – 10
19
“non-fusion” events
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Pirates
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Experim
ental challenge:
Find Wally am
ongst Millions to billions of pirates
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Reduce the num
ber of pirates
To make it easier to find W
ally
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Gran Sasso
underground halls
Cosm
ic Rays
Cosm
ic Rays
Pirates: cosmic rays
(reduce by going underground)
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LUN
A @
Gran Sasso
4-50 keV A
ccelerator p-, !-beam
s " 1 mA
Rock as passive shielding
cosmic ray background
Reduction # 10
-4
Study of pp-chainse.g. 3H
e+3H
e
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Piratetrapper
Wally
detectors
6.5 T Superconducting Solenoid(lens)
gas filled region
beam
target
Highly efficient device for detection of fusion
products
Separate W
ally – using electric and/or magnetic fields
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0.91.0
1.1-2
10 -110
010 110 210 310
Energy ÷ barrier energy
AN
Um
easurements
theory
1000
10
100
0.1 1
0.01
Phys. R
ev. Lett.,W
ei et al., 67 , 3368
Related to
probabilityof fusion
Fusioncross-section(m
b)
Fusion of heavy nuclei: experiment vs. expectations
16O + 154S
m
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0.91.0
1.1-2
10 -110
010 110 210 310
Energy ÷ barrier energy
AN
Um
easurements
theory
1000
10
100
0.1 1
0.01
Phys. R
ev. Lett.,W
ei et al., 67 , 3368
Related to
probabilityof fusion
factor of 100 discrepancy
Fusioncross-section(m
b)
16O + 154S
m
Fusion of heavy nuclei: experiment vs. expectations
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precision measurem
ents required to understand thebehaviour of these com
plex quantum system
s
Com
plex quantum system
Instead ofsingle barrier
Variable
barriers
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Even if som
e heavy elements w
ere created insupernova ages ago - none w
ould be left
The heaviest of elements
Heaviest elem
ent naturally found on earth: Uranium
92 positivelycharged protonspacked into a tiny
volume
Enorm
ousC
oulomb
repulsion
+M
ore unstable–live for shorterand shorter tim
eM
ore protons
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A large num
ber of scientists are working tow
ardscreating these superheavy elem
ents
There is a limit to stability – all nuclei unstablebeyond this lim
it?
Proton
number
238U
Island of stability –superheavy nuclei
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Synthesis of heavy elements at G
SI, Germ
any
Separates pirates from
Wally
Fusion of very heavy nuclei 48Ca + 243A
m (R
ussia)
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Ds
Rg17 N
ovember 2006, G
ermany
Today’s talk
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H
Rg
nucleus
fusion
Quantum
world
Supernova
Superheavies
Strong, W
eak forces
A long journey w
ith new encounters…
.
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feeling tired ?
But …
I hope you will
now be curious to find
out more