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Introduction to nuclear physics Hal. Nucleosynthesis Stable nuclei.

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Presentation on theme: "Introduction to nuclear physics Hal. Nucleosynthesis Stable nuclei."— Presentation transcript:

1 Introduction to nuclear physics Hal

2 Nucleosynthesis Stable nuclei

3 Four major types of nucleosynthesis  1. Big Bang nucleosynthesis  2. Stellar nucleosynthesis  3. Explosive nucleosynthesis  4. Cosmic ray spallation

4 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.

5 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

6 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.  1 H + 1 H → 2 H + e + + ν e  2 H + 1 H → 3 He + γ  3 He + 3 He → 4 He + 1 H + 1 H

7 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.

8 The triple-alpha process  The triple alpha process is a set of nuclear fusion reactions by which three helium nuclei are transformed into carbon.  4 He + 4 He ↔ 8 Be  8 Be + 4 He ↔ 12 C + γ  The star which have 3M sun ~8M sun can start this process.

9 Burning process  Carbon burning process  12 C + 12 C → 20 Ne + 4 He  Neon burning process  20 Ne + γ → 16 O + 4 He  Oxygen burning process  16 O + 16 O → 28 Si + 4 He  Silicon burning process The star which have >8M sun can start burning process.

10 S-process  The s-process is a succession of Slow neutron captures.  The s-process occurs in Asymptotic Giant Branch(agb) stars.

11 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)

12 Nova  Nova  Super nova  Hyper nova

13 Core-collapse This does not occur The shock wave stalls because of photodisintegration and copious neutrino losses 12

14 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 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. 11

15 R-process  The r-process is a succession of rapid neutron captures on iron seed nuclei, hence the name r- process.

16 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 105 Te.

17 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.

18 conclusion


20 Backup

21 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. E B (Z,N) = ZM p +NM n - M(Z,N)

22 Fusion and Fission Reactions

23 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

24 Fission Chain Reaction At each step energy is released !

25 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.

26 Nuclear Fission Homework: Calculate the released energy

27 Nuclear Physics Stability: see sheet detailing stable isotopes Radiations: 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.

28 Alpha (  ) decay example: 92 U 238 90 Th 234 + 2  4 +  (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

29  eta minus  - ) decay example: 6 C 14 7 N 14 + -1  0 + 0  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 [   0 ] to balance everything!

30 Positron (  + ) decay example: 6 C 11 5 B 11 + +1  0 + 0  0 (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!

31 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.

32 Nuclear Physics General Rules: 1)  emitted to reduce mass, only emitted if mass number above 209 2)   emitted to change neutron into proton, happens when have too many neutrons 3)   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 .

33 r-process nucleosynthesis R Surman, Astrophysics and Nuclear Physics of the r process, SNP 08 2/25

34 Core-collapse  The starting point is a star heavier than about 8 solar masses. 9

35 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. 24

36 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 7

37 Proton unstable Neutron unstable

38 supernova

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