CHEM 106 1nd Edition Lecture 18Outline of Last Lecture I. Belt of StabilityII. Decay (Radioactive, Alpha, Beta)III. Natural Decay SeriesIV. Nuclear Fission & FuelsOutline of Current Lecture II. Measuring RadioactivityIII. Radiometric DatingIV. Primordial GeochronometersV. Nuclear MedicineCurrent LectureChapters 2 & 21: Nuclear and Radiochemistry (continued)21.7 Measuring RadioactivityFig. 21.12, pg. 1016α , β , δ Emissions detected because they cause changes in matter (gas, solid, liquid) (disintegrations)# Decay events/ unit time = radioactivity or activity1 disintegration / second = 1 dps 1 dps = 1 Bq BecquerelBq is the Standard Unit; another unit is the Curie (Ci)1 Ci = 3.7 e + 10 Bq = 3.7 e + 10 dpsObserved rate of decay is proportional to the amount of radioactivitySee example 21.5, pg. 1017Ex: How many atoms, moles, and grams in 106 Bq of 223Ra?t12 = 11.4 days A = k N A = activity k = rate constant (dps) N = # atomsSolve for N: N = A/kUse unit of seconds(11.4 days) (24 hours1 day)(60 min1 hour)(60 sec1min) = 9.85 e5 secondsK = 0.6939.85 e5seconds = 7.04 e-7 s-1A was given: 106 Bq = 106 dpsN = A/k = 106disintegrationssecond7.04 e−7s−1 = 1.42 e12 atoms of 223 RaAtoms → moles: (1.42 e12 atoms) 6.022 e23atoms1 mol¿(¿) = 2.36 e-12 molesMoles → grams: (2.36 e-12 moles)(223 grams1 mole) = 5.26 e-10 grams ≈ 0.53 ngEx: 1.25 Bq/L 90Sr in cow’s milk after Fukushima t12 = 28.8 years = 9.08 e8 secondsHow many atoms of 90Sr are in 200 mL of milk?(Convert t12 into k) → k = 7.63 e-10s-1(200 mL 90Sr)(1.25 Bq1000 mL) = 0.250 BqN = A/k = 0.250disintegrationssecond7.63 e−10s−1 = 3.28 e8 atoms of 90SrRadioactivity Penetrating Powerα , β – Kinetic energyγ – photon; wavelength (gamma symbol)Fig. 21.13 Shieldingα – not very penetratingβ – more penetrating than αγ – very penetratingSkip: Gray, Sievert, RBE UnitsRadium + Radon -Spokane, WAFig. 21.1821.10 Radiometric DatingGeochronometryCosmogenic radionuclides14C, carbon dating14N + no1 → 14C + p1114C incorporated into CO2, 14CO2 via photosynthesis, plants incorporate 14C14C in plants transmitted up food chain (eaten)All living organisms have some form of 14C in themWhen the organism dies, it no longer incorporates 14C, t12 is 5,730 years14C → 14N + β +´v−10t = 0 at time when the organism diesThen measure the amount of 14C decay relative to 12C14C/12CFig. 21.22 pg. 102714C/12C has varied with timeEx: 21.7 in text radiocarbon datingAnother example:Analysis of an ancient basket says that the 14C/12C is 30% of value for a living organism. How old is the basket?N+/N0 = (0.5)tt12 ; given Nt = 0.3 N00.3 = (0.5)t5370 yrs take ln of both sidesln 0.3= ln(0.5)∗(t5730) solve for tt = 9,955 yPrimordial Geochronometers:Isotopes with longer half-lives40Kr → 40Ar + β+10t12 = 1.2 e10 years232Th → → → 208Pbt12 = 1.4 e10 years147Sm → → 143Ndt12 = 1.1 e11 yearsNuclear MedicineUsing radioactivity for diagnostic and/or therapeutic purposes99mTc → 99Tc + γt12 = 6 hours heart imaging and bone cancerChelate 99mTc with organic molecule that can be incorporated into heart muscle or boneBoron Neutron Capture Therapy (BNCT)10B + n01 → “B → Li37+ α24Textbook end of chapter problems: 21.85, pg.