Transcript Groupmeeting_shshiu_20090803_nuclearx

Introduction to nuclear physics
Hal
Nucleosynthesis
Stable nuclei
Four major types of nucleosynthesis

1. Big Bang nucleosynthesis

2. Stellar nucleosynthesis

3. Explosive nucleosynthesis

4. Cosmic ray spallation
Big Bang
nucleosynthesis

Primordial
nucleosynthesis took
place just a few
minutes after the Big
Bang and is believed
to be responsible for
the formation of light
element like, D, He, Li.

It was widespread,
encompassing the
entire universe.
Stellar nucleosynthesis
Stellar nucleosynthesis occurs in stars during the
process of stellar evolution. It is responsible for the
generation of elements from He to Fe by nuclear fusion
processes.
 The most important reactions in stellar nucleosynthesis:
1. The proton-proton chain
2. The carbon-nitrogen-oxygen cycle
3. The triple-alpha process
4. Carbon burning process
5. Neon burning process
6. Oxygen burning process
7. Silicon burning process

The proton-proton chain


The proton–proton chain
dominates in stars the size
of the Sun or smaller.
P-P chain is a very slow
process.
 1H
+ 1H → 2H + e+ + νe
 2H
+ 1H → 3He + γ
 3He
+3He → 4He + 1H + 1H
The carbon-nitrogen-oxygen cycle

The CNO cycle is the
dominant source of
energy in stars heavier
than about 1.5 times the
mass of the sun.
The triple-alpha process

The triple alpha process
is a set of nuclear fusion
reactions by which three
helium nuclei are
transformed into carbon.
 4He
+ 4He ↔ 8Be
 8Be
+ 4He ↔ 12C + γ

The star which have 3Msun ~8Msun can start this
process.
Burning process

Carbon burning process
12C + 12C → 20Ne + 4He
Neon burning process
20Ne + γ → 16O + 4He
Oxygen burning process
16O + 16O → 28Si + 4He
Silicon burning process
28
14
32
Si  24He 16
S
40
20
32
16
S  He  Ar
48
24
36
18
Ar  He  Ca
52
26






4
2
36
18
4
2
40
20
44
Ca 24He  22
Cr
52
Cr  24He  26
Fe
56
S  24He  28
Ni
The star which have >8Msun can start burning process.
S-process


The s-process is a succession of Slow neutron captures.
The s-process occurs in Asymptotic Giant Branch(agb)
stars.
Explosive nucleosynthesis

The explosive nucleosynthesis produces the elements
heavier than iron by an intense burst of nuclear reactions
that typically last mere seconds during the explosion of
the supernova core.
The general reactions
in Explosive
nucleosynthesis:
1. R-process(core-collapse
supernova)

2.
RP-process(nova)
Nova

Nova

Super nova

Hyper nova
Core-collapse
This does not occur
The shock wave stalls because of
photodisintegration and copious neutrino losses
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Core-collapse
Two processes robs the iron core of the energy it needs
to maintain its pressure and avoid collapse.
1. Electron capture by nuclei: At density above 1010
g cm-3
electrons are squeezed into iron-group nuclei.

2.
Photodisintegraton: At high temperature the radiation
also begins to melt down some of the iron nuclei to
helium.
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R-process

The r-process is a succession of rapid neutron
captures on iron seed nuclei, hence the name rprocess.
RP-process



The rapid proton capture process consists of consecutive
proton captures onto nuclei to produce heavier elements.
The possible sites suggested for the rp-process are binary
systems. One star is a compact object, the other one is
low mass black hole or neutron star.
The rp-process is constrained by alpha decay, which puts
an upper limit on the end point at 105Te.
Cosmic ray spallation

Cosmic ray spallation produces some of the light
elements present in the universe like Li, Be, B.

It refers to the formation of elements from the impact of
cosmic rays with matter.

This process goes on not only in deep space, but in our
upper atmosphere due to the impact of cosmic rays.
conclusion
Backup
Binding energy per nucleon
EB(Z,N) = ZMp+NMn - M(Z,N)

Nuclei with the largest binding
energy per nucleon are the most
stable.

The largest binding energy per
nucleon is 8.7 MeV, for mass
number A = 60.

Beyond bismuth, A = 209, nuclei
are unstable.
Fusion and Fission Reactions
Fusion Reactions
To obtain a fusion reaction, we must
bring two nuclei sufficiently close
together for them to repel each other,
as they are both charged positively.
A certain amount of energy is therefore vital to
cross this barrier and arrive in the zone,
extremely close to the nucleus, where there are
the nuclear forces capable of getting the better
of electrostatic repulsion. The probability of
crossing this barrier may be quantified by the "
effective cross section". The variation against
interaction energy expressed in keV of effective
cross sections of several fusion reactions is
shown on the graph .
Fission Chain Reaction
At each step energy is released !
Nuclear fusion chain in the Sun
The energy radiated from solar
surface is produced in the
interior of the Sun by fusion of
light nuclei to heavier, more
strongly bound nuclei.
Homework: Calculate the released energy.
Nuclear Fission
Homework: Calculate
the released energy
Nuclear Physics
Stability: see sheet detailing stable isotopes
Radiations:
1) a, b-, b, g are all emitted;
2) protons and neutrons are NOT emitted, except in the
case of mass numbers 5 and 9;
3) alphas are emitted only for mass numbers greater than
209, except in the case of mass number 8.
Alpha (a) decay
234 + a4 + g
example: 92U238
90Th
2
(it is not obvious whether there is a gamma emitted; this
must be looked up in each case) Mass is reduced!
NOTE: 1. subscripts must be conserved (conservation of
charge) 92 = 90 + 2
2. superscripts must be conserved
(conservation of mass) 238 = 234 + 4
Beta minus (b-) decay
14 + b0 + u0
example: 6C14
7N
-1
0
(a neutron turned into a proton by emitting an electron;
however, one particle [the neutron] turned into two
[the proton and the electron].
Charge and mass numbers are conserved, but
since all three are fermions [spin 1/2 particles],
angular momentum, particle number, and
energy are not! Need the
anti-neutrino [0u0] to balance everything!
Positron (b+) decay
11 +
0
0
example: 6C11
5B
+1b + 0u
(a proton turned into a neutron by emitting a positron;
however, one particle [the proton] turned into two
[the neutron and the positron].
Charge and mass numbers are conserved, but
since all three are fermions [spin 1/2 particles],
angular momentum, particle number, and
energy are not! Need the
neutrino [0u0] to balance everything!
Electron Capture
An alternative to positron emission is “Electron Capture”.
Instead of emitting a positron, some nuclei appear to
absorb an electron and emit a gamma ray. The net result
is the same: a proton is changed into a neutron and
energy is released in the process.
Nuclear Physics
General Rules:
1) a emitted to reduce mass, only emitted if mass
number above 209
2) b- emitted to change neutron into proton, happens
when have too many neutrons
3) b emitted (or electron captured) to change proton
into neutron, happens when have too few neutrons
4) g emitted to conserve energy in reaction, may
accompany a or b.
r-process nucleosynthesis
R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08
2/25
Core-collapse

The starting point is a star heavier than about 8 solar
masses.
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Element production

s-process: The s-process or slow-neutron-capture-process is a
nucleosynthesis process that occurs at relatively low neutron
density and intermediate temperature conditions in stars

r-process: The r-process is a nucleosynthesis process
occurring in core-collapse supernova.
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The classification of supernova
Type Ia Lacks hydrogen and presents a singly-ionized silicon line at 615.0 nm, near
peak light.
Type Ib Non-ionized helium line at 587.6 nm and no strong silicon absorption
The supernova's
spectrum do not contain a line of hydrogen
feature near
615 nm.
Type Ic Weak or no helium lines and no strong silicon absorption feature near
Core-Collapse
615 nm.
Type IIP Reaches a "plateau" in its light curve
The supernova's spectrum contains a line of hydrogen
Type IIL Displays a "linear" decrease in its light curve.
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p  n  e   e
Proton unstable
Neutron unstable
n  p  e -  e
supernova