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Radiation and its detection

Radiation and its detection. Physics 123. Binding energy per nucleon. Fusion. Fission. Too many protons Electrostatic repulsion. Most tightly woven nuclei in the middle of the Mendeleev’s table Energetically most favorable place. Not enough nucleons to build up strong force. Isotopes.

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Radiation and its detection

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  1. Radiation and its detection Physics 123 Lecture XXII

  2. Binding energy per nucleon Fusion Fission Too many protons Electrostatic repulsion Most tightly woven nuclei in the middle of the Mendeleev’s table Energetically most favorable place Not enough nucleons to build up strong force Lecture XXII

  3. Isotopes • Same chemical element (Z=Np) can have different number of neutrons –isotopes: • Carbon Z=66 p, but it can have • 5n: 11C, 6n: 12C, 7n: 13C, 8n: 14C, 9n: 15C, 10n: 16C • Only 12C is stable (98.9% of C on Earth) • Unstable isotopes radioactively decay Lecture XXII

  4. Radiation • Types of radioactive particles: • a – radiation (He nuclei = 2p+2n, charge = +2e); • b – radiation (this is just an electron, charge = -e); • g – radiation (these are photons, charge=0) • Neutrons (charge=0) • Sources of radiation • Naturally radioactive elements (e.g. plutonuium, uranium…), • Induced radiation (by bombarding with energetic particles), • Human activity (nuclear power plants, X-rays) • Cosmic rays (atmosphere shields most of it) • Atomic bomb Lecture XXII

  5. a-radiation • 22688Ra22286Rn+a • a-particle – nucleus of He = 2p+2n • Z decreases by 2 when a-particle is emitted, A decreases by 4 • Why not just p’s and n’s separately? • Because of binding energy m(a)<2m(p)+2m(n) • Why not then larger nuclei? • a-particle is the most “tightly bound” nucleus. Just like atoms nuclei have energy shells • Protons and neutrons are fermions like electron – Pauli exclusion principle works as well • In the ground state we can have 2p (spin up, spin down) and 2n (spin up, spin down) • Protons and neutrons are not identical particles (e.g. different electric charge), so Pauli principle does not apply to a proton and neutron Lecture XXII

  6. a-radiation • If this decay is energetically favored why does not it happen immediately? • Tunneling through a barrier • Probability depends exponentially on the thickness of the barrier • That is why lifetimes of radiation materials differ by many orders of magnitude 1ms-1010 years Lecture XXII

  7. b-decay • 146C147N+e-+ne • In b-decay nucleus charge (Z) changes by +1, while mass (and A) essentially stays the same: me<<mp, mn<< me (though not zero!!! as the book says; the book is old, new discoveries were made) • Electron is not one of the “atomic orbital” electrons, it was created in the neutron decay: np+e+ne • Neutrino has no electric charge and does not participate in strong interactions – its chances of interaction with matter are rather low – was not detected until 1956 • Its existence was predicted by Pauli in 1930 based on energy conservation in b-decays Lecture XXII

  8. b+-decay • 1910Ne199Fe+e++ne • In b+-decay nucleus charge (Z) changes by -1, • e+ - is a positron – antipartner of an electron • Positrons don’t live long, when matter meets antimatter they annihilate. There are plenty of atomic electrons to annihilate with e-+e+ gg • Electron capture (from innermost shell – K-shell): 74Be+e-73Li+n Lecture XXII

  9. g-decay • g-rays are just very energetic photons (keV, MeV) • Because nuclei energy is much larger than atomic energy, the spacing between energy levels is larger as well (though space-wise nucleus is much smaller than atom), hence photons emitted in nuclear transitions are a lot more energetic • Charge and mass stays the same in this transition Lecture XXII

  10. Radioactive decay law • Nuclei decay is a random process, number of particles DN decaying in interval of time Dt is proportional to the total number of particles: DN= -lNDt • Differential equation Lecture XXII

  11. Rate of decay • Number of particles decaying per unit of time: Lecture XXII

  12. Half-life • Half-life – period of time T1/2 in which the number of particles decreases by a factor of 2: N=N0/2 Lecture XXII

  13. Radioactive dating • The majority of carbon atoms are 126C, but small fraction (1.3x10-12) is radioactive isotope 146C (half-life 5730 yr) • This ratio in atmosphere is constant for many thousand years • Carbon is absorbed by living organisms in CO2 during the life process, after death it remains fixed • The age of remains can be determined by the ratio of 146C to 126C Lecture XXII

  14. Example of radioactive dating • An animal bone fragment has carbon mass 200g. It registers an activity of 16 decays/s. What is the age of the bone? Lecture XXII

  15. Decay series Lecture XXII

  16. Detection of radiation • Different types of radiation interact differently with matter (=atomic structures) • Charged particles ionize atoms – ionizing radiation • Photons can also ionize atoms + Compton effect +pair production • Alpha particles and neutrons can displace nuclei from crystal structure + interact with nuclei strongly • Radiation dose = amount of energy deposited per kg of mass • 1 rad = 10-2J/kg Lecture XXII

  17. Magnetic force on moving charge • Charged particles bend in magnetic field, there charge to momentum ratio can be determined from the radius of curvature • Magnetic force = Centripetal force F=qvB • Centripetal acceleration a=v2/R • Newton’s second law F=ma qvB=mv2/R In experiments B is known, q=e most of the time, Measure R- measure mv For a given v measure m – magnetic spectrometer Lecture XXII

  18. Detection of radiation Lecture XXII

  19. p-n diode • The current flows through p-n junction if electrons have vacancies to jump to, it does not flow in the opposite direction • When ionizing particle goes through p-n diode it produces e-hole pairs – current starts flowing – detect the particle with high position precision P-type vacancies +++ +++ +++ P-type +++ +++ +++ current No current + - - --- --- --- Electron flow + --- --- --- n-type electrons n-type Reverse bias Forward bias Lecture XXII

  20. Radiation effect on humans • Effects on humans are different from different types of radiation  quality factor (QF): • QF(g) = QF(b) = 1; QF(a) = 20. • Effective dose (in rem) = dose (in rad) x QF • Levels of radiation • Natural background 0.36 rem /year • Medical X-ray 0.040 rem /year • US recommended upper limit 0.5 rem/year • Radiation workers ~ 5rem /year • Radiation damage in biological organisms – alteration in DNA • Somatic (any part of body, but reproductive) • First affect the blood cells (shortest regeneration time) • Can lead to cancer (by altering the DNA) • Genetic (reproductive organs) • Leads to mutations, smaller dose is harmful • Rate matters • Short dose of 1000rem is fatal • 400 rem over short period of time – 50% fatal • 400 rem over several weeks usually not fatal Lecture XXII

  21. Radiation in medicine • Radiation therapy • Focus radiation on cancer cells (kill the bad guys) • Medical imaging • Use radioactive isotopes to tag molecules • PET (positron emission tomography): • e++e-=2g • Photons travel in a straight line – easy to find their source Lecture XXII

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