Download SL2 IIIC Carter 280911 - Particle Solids Interactions group

The Nucleus, almost the final frontier
IIIC Specialist Lecture 2
28 September 2011
Prof Carter
School of Physics, Wits
Geiger, Marsden, Rutherford expt.
– (Geiger, Marsden, 1906 - 1911) (interpreted by Rutherford, 1911)
– get α particles from radioactive source
– make “beam” of particles using “collimators”
(lead plates with holes in them, holes aligned in straight line)
– bombard foils of gold, silver, copper with beam
– measure scattering angles of particles with scintillating screen (ZnS)
Geiger Marsden experiment:
result
– most particles only slightly deflected (i.e. by small
angles), but some by large angles - even backward
– measured angular distribution of scattered particles
did not agree with expectations from Thomson model
(only small angles expected)
– but did agree with that expected from scattering on
small, dense positively charged nucleus with diameter
< 10-14 m, surrounded by electrons at ≈10-10 m
Ernest
Rutherford
1871-1937
Proton
• “Canal rays”
– 1898: Wilhelm Wien:
opposite of “cathode rays”
• Positive charge in
nucleus (1900 – 1920)
– Atoms are neutral
• positive charge needed to cancel electron’s negative charge
• Rutherford atom: positive charge in nucleus
– periodic table ⇒ realized that the positive charge of any
nucleus could be accounted for by an integer number of
hydrogen nuclei -- protons
Neutron
•
Walther Bothe 1930
– bombard light elements (e.g.
neutral radiation emitted
9
4 Be)
with alpha particles ⇒
• Irène and Frederic Joliot-Curie (1931)
– pass radiation released from Be target through paraffin wax ⇒
protons with energies up to 5.7 MeV released
– if neutral radiation = photons, their energy would have to be 50 MeV
-- puzzle
•
puzzle solved by James Chadwick (1932)
– “assume that radiation is not quantum radiation, but a neutral
particle with mass approximately equal to that of the proton”
– identified with the “neutron” suggested by Rutherford in 1920
– observed reaction was
4He + 9Be → 13C* → 12C + n
4
6
6
Structure of nucleus
• size (Rutherford 1910, Hofstadter 1950s)
R = r0 A1/3, r0 = 1.2 x 10-15 m = 1.2 fm;
i.e. ≈ 0.15 nucleons / fm3
• generally spherical shape, almost uniform density
• made up of protons and neutrons -- “nucleons”
fermions (spin ½), have magnetic moment
• nucleons confined to small region (“potential well”)
occupy discrete energy levels
two distinct sets of energy levels, one for protons, one for neutrons
proton energy levels slightly higher than those of neutrons –
due to electrostatic repulsion
spin ½ ⇒ Pauli principle
Nuclear Sizes - examples
1
3
r = ro (A )
ro = 1.2 x 10-15 m
Find the ratio of the radii for the following nuclei:
1H, 12C, 56Fe, 208Pb, 238U
1
3
1
3
1
3
1
3
1 : 12 : 56 : 208 : 238
1
3
1 : 2.89 : 3.83 : 5.92 : 6.20
A, N, Z
• for natural nuclei
–
Z range 1 (hydrogen) to
92 (Uranium)
–
A range from 1 (hydrogen) to
238 (Uranium)
• N = neutron number = A-Z
• N – Z = “neutron excess”
increases with Z
• nomenclature
EB/A vs A
“Fireflies or great balls fire
in the sky: stars and stardust”
Bioluminescence to attract mates or prey,
with a lighting efficiency of up to 96%.
“Fireflies or great balls fire
in the sky: stars and stardust”
Star gazing from Lion King.
Pumbaa: Timon?
Timon: Yeah?
Pumbaa: Ever wonder what those sparkly
dots are up there?
Timon: Pumbaa. I don't wonder; I know.
Pumbaa: Oh. What are they?
Timon: They're fireflies. Fireflies that uh... got
stuck up on that big... bluish-black... thing.
Pumbaa: Oh. Gee. I always thought that they
were great balls of gas burning billions of
miles away.
Timon: Pumbaa, wit' you, everything's gas.
Amenhotep IV reigned 1379 to 1362 BC.
Suppressed ancient religion of Egypt and
instituted a monotheistic worship of the Sun God.
Succeeded by his queen, Nefertiti.
The old gods were reinstated.
Nefertiti
Akhenaton, Nefertiti
and Sun God
The Sun is a normal G2 star, one of more than
100 billion stars in our galaxy.
Diameter: 1,390,000 km
Solar mass: 1.989x1030 kg
Temperature:
5,800 K (surface)
15,600,000 K (core)
Electromagnetic Spectrum
Soft X-Ray Telescope
Extreme UV Imaging Telescope Fe IX. X 17.1 nm
Extreme UV Imaging telescope Fe XII 19.5 nm
Extreme UV Imaging Telescope Fe XV 28.4 nm
Extreme UV Imaging Telescope He II 30.4 nm
Hydrogen alpha 656.3 nm
Spectroheliogram He I 1083 nm
Infant stars in the Milky Way
Sun Facts
Solar age = 4.57x109 y
Solar radius = 695,990 km
Surface density = 2.07x10-7 g/cm3
Surface temperature = 5770 K
Surface composition =
70% H, 28% He, 2% (C, N, O)
by mass
Solar mass = 1.989x1030 kg
Central density = 150 g/cm3
Central temperature =
15,600,000 K
Central composition =
35% H, 63% He, 2% (C, N, O)
by mass
The Proton-Proton Cycle
+ 1H → 2H + e+ + ν
e+ + e- → γ + γ
2H + 1H → 3He + γ
1H
1 pp collision in 1022 → fusion!
3He
+ 3He → 4He + 1H + 1H
4H → 4He
Deuterium creation 3He creation
4He
creation
Stardust
7.65 MeV above 12C ground state
Sir Fred Hoyle
1915-2001
Stardust – II
7.12 MeV
7.19 MeV
History of Sunspots
1613 – Galileo discovers sunspots
1859 – Heinrich Schwabe announces
discovery of the sunspot cycle
1859 – Richard Carrington
discovers solar Flares
Galileo Galilei (1564–
1642)
Galileo claimed to have
been the first to
discover sunspots,
although he did not
immediately recognise
their significance.
Sunspot
Observations by
Galileo
The division of
sunspots into umbral
and penumbral
regions is clearly
apparent in this
drawing from
Galileo’s Istoia
(1613)
Christopher Scheiner (1575 – 1650)
The frontispiece of Fr. Scheiner’s Rosa
Ursina sive Sol (1630) shows Scheiner
making observations with the help of an
assistant.
An early example of
a photograph of the
Sun taken by L.M.
Rutherford on 21
September 1870.
There are numerous
clearly defined
sunspot groups.
THEORY OF SUNSPOT FORMATION
Early in the 19th century
Sir William Herschel,
discoverer of Uranus, then a leading
observer postulated that sunspots
were holes in the fiery luminous
outer layers of the Sun.
Through these holes it was possible to
see towards the solid surface on which,
he believed, living creatures
almost certainly existed.
THEORY OF SUNSPOT FORMATION
Leighton-Babcock Model (1960’s)
Weak magnetic fields, 100 G, generated in Convective Zone
about 200,000 km below Photosphere by circulating currents.
Differential rotation of the Sun, 25 day period at equator and
28 days towards the poles, winds up the initially weak fields.
• Flux tubes are formed 100’s km
diameter.
• Bundles of flux tangle together
producing fields 2000 – 4000 G.
• Magnetic pressure makes flux bundles
buoyant, so float to surface.
• Flux bundles pierce the Photosphere,
field spreads out to form sunspot.
• Winding up starts spot formation at
about 40 deg. N and S.
Following spot F opposite polarity to pole moves
towards pole to neutralise field resulting in
“unwinding” of previously wound up lines to produce
straight field lines again.
Sunspot maximum every 11 y observed.
Photospheric Magnetogram
Magnetogram Ca II 8542 A
Rudolf Wolf devised the basic formula
for calculating sunspots in 1848:
R = k (10g +s),
where R is the sunspot number,
g is number of sunspot groups,
s is number individual spots all groups and
k is scaling factor (usually <1).
Wavelet Analysis
Wavelet Analysis
Wavelet Analysis
Wavelet Analysis
WAVELET ANALYSIS
11 year Sunspot Cycle
Scale
years
Date years
WHY ARE SUNSPOTS DARK?
Debate has been going on for centuries!
• The intense localised magnetic field in the
umbra reduces the flow of hot material to that
region of the Photosphere.
• Alternatively, strong magnetic fields enhance
flow of heat but converts about 80% into
“hydromagnetic waves” which propagate through
the Photosphere without dissipating significant
amounts of energy. Instead, contributes to
heating up the solar atmosphere higher up above
the sunspot group.
Extended, complex group
of sunspots
Nuclear Giant Resonances
simple collective excitations
Isoscalar
Isovector
∆T = 0
∆S = 0
∆T = 1
∆S = 0
Monopole
∆L = 0
Dipole
∆L = 1
Quadrupole
∆L = 2
Courtesy of P. Adrich
1p-1h
excitations
in 208Pb
Z=82 N=126
GQR Systematics
Bertrand et al., NP A354(1981)129c
Fine Structure of Giant Resonances
High energy resolution is crucial
Possible probes: electron and hadron scattering
Fine Structure of Giant Resonances
High energy resolution is crucial
Possible probes: electron and hadron scattering
Darmstadt 1980’s
∆E = 50 keV
IUCF K600 1991
∆E = 50 keV
iTL K600 2001
∆E < 40 keV
Nucleus as a container that
rotates slowly containing
nucleons travelling at one third
speed of light.
Like a bee hive
containing fast moving bees.
No long range order so
moment of inertia between
that of a solid and
that of shell containing fluid.
Surface Waves on the
surface of the Sun
Bumps on the surface are
due to sound waves
generated and trapped
inside the Sun.
What does an ENGINEER do?
ENGINEERING
What does a BOTANIST do?
BOTANY
What does a CHEMIST do?
CHEMISTRY
What does a PHYSICIST do?
EVERYTHING
Snake of Sizes
PHYSICISTS
do
everything
under the
Sun
Supernova Cycle
Main sequence
Stars
Physicists
can do
everything,
even
ride a
bicycle!