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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
Curiosity …
. and a quest for answers
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
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)
Ds
17 Novem
ber 2006, Germ
any
How
were these elem
ents produced in N
ature?
Ds
Rg
What the “big bang” created
Ds
Rg
What the “big bang” created
Discovery supports big bang scenario
John C.
MatherG
eorge F.Sm
oot
2006 Nobel P
rize
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?
What m
akes the Sunshine?
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
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
Energy crisis of the 19th century!
no consistent scientific answer in the
19th century
Solution from
discoveries in the 20th century
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
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
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)
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
++
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
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
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
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
But how
come the S
un shines…
…and the U
niverse exists
Quantum
tunnelling
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
Application to tunnelling – from
studies of radioactivity
George G
amow
Wrote a series of very nicepopular science books
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
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
Nuclear Fusion
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# #
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
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
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
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
from The C
omic P
erspective by Bennett et al
pushing out
(thermal
pressure)
The balancing act……
Gravity
(pushing down)
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
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
•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)
•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
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)
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
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!
Supernova -
the death of a star
From N
ASA/ESA collaboration
Crab nebula –
the expandingrem
nant of asupernovaexplosion
• 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
From Parting the C
osmic Veil, By K. Lang, Springer Verlag
The origin of elements
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
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
•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)
Typical experimental setup:
speedingnuclei
Schem
atic picture
e.g.4H
e
Targete.g. 12C
detector
Set-up at A
ustralian National U
niversity
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
Pirates
Experim
ental challenge:
Find Wally am
ongst Millions to billions of pirates
Reduce the num
ber of pirates
To make it easier to find W
ally
Gran Sasso
underground halls
Cosm
ic Rays
Cosm
ic Rays
Pirates: cosmic rays
(reduce by going underground)
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
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
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
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
precision measurem
ents required to understand thebehaviour of these com
plex quantum system
s
Com
plex quantum system
Instead ofsingle barrier
Variable
barriers
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
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
Synthesis of heavy elements at G
SI, Germ
any
Separates pirates from
Wally
Fusion of very heavy nuclei 48Ca + 243A
m (R
ussia)
Ds
Rg17 N
ovember 2006, G
ermany
Today’s talk
H
Rg
nucleus
fusion
Quantum
world
Supernova
Superheavies
Strong, W
eak forces
A long journey w
ith new encounters…
.
feeling tired ?
But …
I hope you will
now be curious to find
out more
