australian national university, canberra department of ... dasgupta department of nuclear physics...

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