Nothing in life is to be feared. It is only to be understood. -Marie Curie Segre Chart (Table of Nuclides) Z N 1
Segre Chart (Table of Nuclides) Radioac8ve Decay Antoine Henri Becquerel Marie Curie, née Sklodowska Pierre Curie Nobel Prize in 1903 2
Ac8vity: Laws of Radioac8ve Decay N = number of nuclei Units: R = dn dt 1 Becquerel (Bq) = 1 decay/s 1 Curie (Ci) = 3.7 x 10 10 decays/s Laws of Radioac8ve Decay Radia8on from a radioac8ve sample is due to decay of many independent nuclei. Let each nuclei have a probability of decay per unit 8me given by λ. The ac8vity at 8me t is given by the probability 8mes the number of nuclei at 8me t. R = λn(t) 3
Laws of Radioac8ve Decay The number of nuclei that decay in dt is given by dn ( t) = Rdt = λn( t) dt N( t) = N 0 e R = 0 R e λt λt Laws of Radioac8ve Decay Half life: 8me in which the number of nuclei drops to half its original value. ln 2 0.693 t 1 / 2 = = λ λ 4
Daughter Products As one nuclide decays it creates another. Branching Ra8os 5
Half- lives Decay Energe8cs All Decays: Energy, Momentum, Angular Momentum Strong Interac8on and Electromagne8c decays: N and Z conserved, Parity Conserved Weak Interac8on decays: Atomic number, A, conserved A Nuclide is unstable if There is another nucleon configura8on with a lower total energy. There is a process that can reach the configura8on while obeying the conserva8on rules. 6
General Criteria for Stability All isotopes with Z > 83 are unstable. Pairing: Most even- even nuclei are stable Many odd- even or even- odd nuclei are stable. Only 4 odd- odd nuclei are stable. ( 2 H, 6 Li, 10 B, 14 N) N/Z ra8os For A < 40: N Z For A > 40: N > Z Q- value: Disintegra8on Energy The q- value is the energy difference between the parent nuclide and the daughter products. A ( ) ( ) 2 Q = [ M Z X M products ] c If Q > 0 then the nucleus is unstable and will decay into the daughter products. 7
Decay Processes Comparing Rays β γ α Cardboard Aluminum Lead 8
Types of Decay Alpha par8cles are helium nuclei. Alpha Decay A Z A Z 4 2 X Y +α 9
Alpha Decay Alphas formed inside the nucleus tunnel out of the coulomb barrier. 10
Alpha Decay and Smoke Detectors Smoke detectors are ioniza8on chambers. The alpha s emiied by the Americium- 241 ionize the air between the plates allowing a current to flow between the plates. When smoke enters the detector the par8cles combine with the ions and neutralize the air. The current reduces causing the detector alarm to sound. 11
Beta- minus Decay Beta- minus Decay Beta- minus par8cles are electrons. One of a class of weak interac8on decays. 12
Beta Decay Weak interac8on can transform protons neutrons Beta- minus: Beta- plus: Electron capture: n p + p n + e + e +ν +ν p + e n +ν Beta- minus: Beta decay Beta- plus: A A Z X Z 1 + Y + e +ν Electron capture: A A Z X Z 1 Y + e + +ν A A Z X 1 + e Z Y +ν 13
Valley of Stability Z N Odd A Isobar Plot 14
Even A Isobar Plot The Neutrino Hypothesis 15
Neutrino Mass Neutrino Mass 16
Gamma- Decay Rearrangement of nucleons in nucleus with the emission of a photon. A Z X * A Z X + γ Gamma- ray energies are ~ 1 MeV and higher. Internal Conversion Rather than emit a gamma- ray the nucleus can transfer energy to one of the atomic electrons. The characteris8c x- ray and electron spectra associated with IC dis8nguish it from beta decay. 17
Gamma Decay Gamma decay omen follows either alpha or beta decay. The parent nucleus decays to an excited state in the daughter which de- excites via gamma emission or internal conversion. Quiz 1 18
Quiz 2 Nucleon Emission 19
Decay Processes on the Segre Chart Spontaneous Fission 20
Radioac8ve Series Four decay series contain most of the common radionuclides Each series consists of a succession of daughter products ul8mately derived from a single parent. Thorium (A = 4n) Neptunium (A = 4n+1) Radioac8ve Series (Decay Chains) 232 208 90Th 82 Pb Uranium (A = 4n+2) Ac8nium (A = 4n+3) 237 209 93 Np 83Bi 238 206 92U 82 Pb 235 207 92U 82 Pb 21
Radioac8ve Da8ng 204 Pb stable and not a daughter of any nuclide 206 Pb and 207 Pb are the end of decay chains. Ra8o of 207 Pb/ 204 Pb is roughly constant as the half- life of the Ac8nium chain is short ~7 x 10 8 yrs. Ra8o of 206 Pb/ 204 Pb is s8ll changing as the Uranium chain has t half ~ 4.5 x 10 9 yrs. Comparison of the two ra8os can give the age of a sample. 22
Shroud of Turin Very small samples from the Shroud of Turin have been dated by accelerator mass spectrometry in three independent laboratories. As Controls, three samples whose ages had been determined independently were also dated. The results of radiocarbon measurements at Arizona, Oxford and Zurich yield a calibrated calendar age range with at least 95% confidence for the linen of the Shroud of Turin of AD 1260-1390 (rounded down/up to nearest 10 yr). These results therefore provide conclusive evidence that the linen of the Shroud of Turin is mediaeval. (Nature, Vol. 337, No. 6208, pp. 611-615, 16th February, 1989 ) 23