Total number of nucleons: mass number 238 Number of protons: atomic number 92U

Transcription

1 9 The Nucleus Protos Neutros Nucleos Total umber of ucleos: mass umber 8 Number of protos: atomic umber 9U Isotopes: idetical atomic umbers differet mass umbers same chemical properties differet uclear properties Nuclear stability ad radioactive decay Radioactivity: reflects kietic stability Types of radioactive decay Frequetly accompaied by -ray emissio high eergy electromagetic radiatio -particle productio U He particle productio 4 9 Th particle 4 Th 9Pa e ( p e) particle 4 9

2 Positro productio positro Na Ne e ( p e) Electro capture ( e e ) 8 Hg e 79 Au ier-orbital electro Spotaeous fissio (usually slow) 54 Cf 98 lighter uclides eutros Empirical rule of uclear stability (proe to decay) With 84 protos: less stable lighter uclides: /p closer to more stable p ratio zoe of stability Emit -particle p Positro emissio p /p = p

3 With eve umbers of ps ad s more stable stable isotopes p 68 eve eve 57 eve odd 5 odd eve 4 odd odd With magic umbers of ps or s, 8,, 5, 8, 6 more stable stable isotopes p (Ca) Decay series emissio i a series 4 4 9U 9Th 9Pa Pb

4 The kietics Decay is a uimolecular process Rate dn dt kn N l( ) kt N ( N for umber of uclides).69 t/ k A characteristic value Uaffected by T, P or chemical form No way to stop!! 8 9 Ex. U half - life : 4.5 yr 9 Datig C-4 method 4 7N 6C 4 H C N e t / = 57 yr I atmosphere: reachig a equilibrium 4 C/ 6 6C remais costat *CO *Orgaic molecule i plats *aimals From 4 C/ C to determie age decay died (equilibrium stops) 4

5 Ex. If ratio is half that of atmosphere 57 yr old Limitatio: ca ot be older tha yr radioactivity too low to be accurate Checked with tree growth: accurate withi % Oldest rock: 9 yr Coolig time for earth surface:.5 9 yr Age of earth at about: yr Ex. A rock with.5 mg 6 Pb/. mg 8 U Assumig 6 Pb is comig from 8 U The origial amout of 8 U was.5 ( 8).. mg 6 The amout decayed For 8 U t / = yr =.69/k k =.69/4.5 9 yr N..69 l kt l( ) ( ) t 9 N. 4.5 yr o t = 8. 8 yr 5

6 Nuclear trasformatios The chage of oe elemet ito aother 99 Rutherford N He O H 7 8 From Ra with high velocity Particle accelerator cyclotro, sychrotro, liear accelerator to accelerate charged particle i order to overcome electrostatic repulsio Cyclotro High frequecy alteratig voltage i a vacuum chamber With vertical magetic field target Neutro bombardmet o acceleratio is ecessary more commo for isotopes sythesis eutral (geerated from uclear reactor) U U Np e t / = mi Elemets with atomic # > 9 are sythesized via artificial trasmutatios trasuraium elemets 6

7 Detectio of radioactivity Geiger couter amplifier ad couter Detect ioizatio by radiatio + _ Ar Widow that ca be peetrated by, or rays Scitillatio couter ( 閃爍計數器 ) high-e radiatio ZS flurorescece (caused by electroic excitatio) measuremet Thermodyamic stability Ex. 8 8H 6 8 O Mass of reactats = 8( ) + 8( ) g =.6784 g Mass of product =.6555 g Smaller by.66 g/mol Eistei: mass eergy coversio E = mc (c: speed of light =. 8 m/s) E = mc = (.66 4 kg/mol)(. 8 m/s) =. J/mol or.4 J per ucleus =.8 J per ucleo = 8. MeV/ucleo ( MeV =.6 J) The reverse is the bidig eergy per ucleo H 7

8 4 Ex. The bidig E per ucleo for the He ucleus AW(He) = 4.6 amu Mass of Mass of 4 He H Mass icludig electros AW(H) =.78 amu ucleus = AW(He) m e ucleus = AW(H) m e Mass of e m = (4.6 m e ) [(.78 m e ) + m ] Mass of eutro = 4.6 (.78) (.87) =.4 amu E = mc = (.4 amu)(.66 7 Kg/amu)(. 8 m/s) = 4.54 J/ucleus.4 J/ucleo Nuclear fissio ad fusio Bidig eergy per ucleo (MeV) Fusio (release more E) H-bomb Fissio (uclear power plat) atomic bomb 5 5 Mass umber Most stable: 56 6 Fe 8

9 Fissio Ex. U Ba Kr Further fissio Cover isotopes, 5 elemets Produce.4 eutro i average More eutros are produced may be explosive Size of the sample Too small: eutro escapes before strikig a ucleus subcritical Too large: eutros are completely cosumed super critical I betwee: chai reactios keep at a costat rate (oe for oe) critical Nuclear reactor keep a self-sustaiig chai reactio 5 U eriched to % (atural abudace:.74) UO pellets i Zr or stailess steel tubes Cotrol rods: made of Cd or B to absorb eutros B Li He Water used as moderator (slow dow eutros but ot reactig) hot coolat out cotrol rods fuel cyliders coolat i 9

10 Nuclear power thermal E steam steam egie biproducts reprocess reusable fuel electricity geeratio waste such as 9 Sr t / = 8.8 yr 9 Pu t / = 4 yr Fusio I the su H H H e H H He 4 He He He 4 He H He Products are geerally ot radioactive H High E ecessary : to overcome repulsios occurs at high T e

11 Effect of radiatio Somatic damage: Geetic damage: damage to the orgaism itself damage to the geetic machiery Biological effects The eergy measured i rads (radiatio absorbed dose) rad = J/kg tissue The peetratig ability stops at ski dow cm highly peetratig (the most dagerous) The ioizig ability very damagig The chemical property of the source Overall effect: rem = rad RBE