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

CaHeAr

ArHeS

SHeSi

4020

42

3618

3618

42

3216

3216

42

2814

NiHeS

FeHeCr

CrHeCa

5628

42

5226

5226

42

4824

4422

42

4020

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 occurThe shock wave stalls because of photodisintegration and copious

neutrino losses

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Core-collapseTwo 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 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.

1010

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R-process The r-process is a succession of rapid neutron

captures on iron seed nuclei, hence the name r-process.

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

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.

EB(Z,N) = ZMp+NMn - M(Z,N)

Fusion and Fission 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 .

Fusion Reactions

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 PhysicsStability: see sheet detailing stable isotopesRadiations: 1) , 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 () decayexample: 92U238 90Th234 + 24 +

(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 + 22. superscripts must be conserved(conservation of mass) 238 = 234 + 4

eta minus-) decayexample: 6C14

7N14 + -10 + 00

(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 [0] to balance everything!

Positron (+) decayexample: 6C11

5B11 + +10 + 00

(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 [0] to balance everything!

Electron CaptureAn 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 PhysicsGeneral Rules:1) emitted to reduce mass, only emitted if

mass number above 2092) emitted to change neutron into proton,

happens when have too many neutrons3) emitted (or electron captured) to change

proton into neutron, happens when have too few neutrons

4) emitted to conserve energy in reaction, may accompany or .

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 productions-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 feature near 615 nm.

Type Ic

Weak or no helium lines and no strong silicon absorption feature near 615 nm.

Type IIP

Reaches a "plateau" in its light curve

Type IIL

Displays a "linear" decrease in its light curve.

Core-Collapse

The supernova's spectrum do not contain a line of hydrogen

The supernova's spectrum contains a line of hydrogen

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Proton unstable

Neutron unstable eepn

eenp

supernova