0 +1e Radionuclides - can spontaneously emit particles and radiation which can be expressed by a nuclear equation.

Transcription

1 Radioactivity Radionuclids - can spontanously mit particls and radiation which can b xprssd by a nuclar quation. Spontanous Emission: Mass and charg ar consrvd. 4 2α -β Alpha mission Bta mission U 234 9Th H 3 53I 3 54 X + - n p + - γ Gamma mission γ Positron mission 6C 5 B + p n + - Elctron captur p + - n 2 Radioactivity

2 Particl Charg Mass (g) Nomnclatur alpha H 4 α 2 2 bta β - - gamma γ proton H p nutron n lctron position Radioactivity Nutrons (N) A plot of nutrons vs. protons for th stabl nuclids. A plot of N vs. Z for all stabl nuclids givs ris to a narrow band that vrs abov N/Z= shortly byond Z=. Th N/Z valus for svral stabl nuclids ar givn. Th most common mods of dcay for unstabl nuclids in a particular rgion ar shown: nuclids with a high N/Z ratio oftn undrgo β dcay; thos with a low ratio undrgo - captur or position mission; havy nucli byond th stabl band (and a fw lightr ons) undrgo α dcay. Th blu box in th largr diagram is xpandd to show th stabl and many of th unstabl nuclids in that ara. Not th mods of dcay; α dcay dcrass both N and Z by two; b dcay dcrass N and incrass Z by on; position mission and - captur incras N and dcras Z by on. 4 Radioactivity

3 Prdicting Nuclar Stability - Radionuclids somtims go through a sris of mission (Radioactiv sris) bfor bcoming a stabl nucli. Nuclar disintgration sris for U-238 undr gos α- mission (blu arrows) and β- mission (rd arrows) until it forms stabl Pb-26 (which is an isotop within th blt of stability. 5 Radioactivity Transmutation - Chang of nuclar idntity by artificially striking nuclus with a particl. Many mdical chmothrapy isotops ar formd by transmutation. Many nw lmnts ar discovrd by transmutation Nomnclatur: Targt (bombard, jctd) product 7 O + H 4 N + 4 H In this xampl, Th targt is 7 O, th product is 4 N, th bombarding particl is a proton H (or p) and th jctd particl is th alpha particl 4H (or α). Th nomnclatur is thrfor 7 O (p,α) 4 N Exampl: solv th following. Th answr is in th nxt slid. i) 238 U + n 239 Np + β ( s nxt slid for answr) ii) 238 U (n,γ) 239 U ( s nxt slid for answr) iii) 8 O (n,β) 9 F ( s nxt slid for answr) 6 Radioactivity

4 t 2 t 2 = ln2 k = 573 yr k = ln2 t 2 2 C 4 C = 2 [ 4 C]= 5. ppt basd on 2 C [ 4 C]= 46.4 ppt today ln2 = 573 yr = yr =.2-4 yr Th Shroud of Turin is a linn cloth ovr 4 m long. It bars a faint, strawcolord imag of an adult mal of avrag build who had apparntly bn crucifid. Rliabl rcords of th shroud dat to about 35, but for ths past 6 yars it has bn allgd to b th burial shroud of Jsus Christ. Numrous chmical and othr tsts hav bn don on tiny fragmnts of th shroud in rcnt yars. Th gnral conclusion has bn that th imag was not paintd on th cloth by any traditional mthod, but no on could say xactly how th imag had bn cratd. R-cnt advancs in radiochmical dating mthods, howvr, ld to a nw ffort in to stimat th ag of th cloth. Using radioactiv 4 C, th flax from which th linn was mad was shown to hav bn grown btwn 26 and 39 A.D. Thr is no chanc that th cloth was mad at th tim of Christ. A o = 5. A = 46.4 =ln Ao A = kt 5. ln yr = t t = yr = 32 ±5 AD 7 Radioactivity Radioactiv substanc possss spcial rats of dcay and half livs, t /2. Radioactiv dcay is a first-ordr kintic procss. ln {N/N o } = -k t t /2 =.693/ k Half-lif - Th tim in which a substanc will dcay to onhalf its original mass. 8 Radioactivity

5 Each isotop has its charactristic half-lif unaffctd by xtrnal conditions, i.., tmp, prssur, chmical natur. Radioactivity Radiation can b usd as a diagnostic tool to noninvasivly prob th body Radioactivity

6 Pntrating powr of radiation is a function of mass. γ -radiation pntrats furthr than β-radiation which pntrats furthr than α-radiation. Radiation absorbd by tissu causs xcitation (nonionizing radiation) or ionization (ionizing radiation). Ionizing radiation is much mor harmful than nonionizing radiation Most radiation rsult in radical formation. Fr radicals undrgos a chain raction to form othr radicals. 2 Radioactivity A graph of th sourcs of th avrag annual xposur of th US population to high-nrgy radiation. On avrag ~3mrm/yr sa lvl, but 4 mrm/yr Dnvr. Radioactivity masurmnt: 3 Radioactivity

7 4 Radioactivity