Episode 512: Nuclear equations

Transcription

1 Episod 52: Nuclar quations Now that your studnts ar familiar with diffrnt typs of radiation, you can look at th procsss by which thy ar mittd. Summary Discussion: Nuclid notation and N- plot. ( minuts) Studnt Qustions: Practic with notation. ( minuts) Workd Exampls: Equations for alpha, bta and gamma dcay. (2 minuts) Studnt Qustions: Practic with nuclar quations. (3 minuts) Discussion: Nuclid notation Rvis nuclid notation: A X Discuss how A = mass or nuclon numbr, = charg or atomic numbr and N = nutron numbr ar rlatd (A = + N). Discuss isotops (common xampls: H, D and T, U-235 and U-238, C- and C-2). St th task of finding out th nam for nuclids having th sam A but diffrnt (isobars), and th sam N but diffrnt (isotons). Show an N- plot (Sgrè plot). Nutron numbr n p + β - + ν A β - β + B p n + β + + ν Proton numbr

2 Studnt qustions: Practic with notation St som simpl qustions involving nuclid notation. TAP 52-: Nuclid notation Grid showing chang in A and with diffrnt missions Nuclon numbr (A) A, - A, A, + β + β - α A-, -2 Proton numbr () Nutron numbr (N) β + N+, - N, N-2, -2 β - α 2 N-, + Proton numbr ()

3 Workd xampls: Equations for alpha, bta and gamma dcay Nuclar dcay procsss can b rprsntd by nuclar quations. Th word quation implis that th two sids of th quation must balanc in som way. TAP 52-2: Dcay procsss You could giv xampls of quations for th sourcs usd in school and collg labs. α sourcs ar amricium-2, 2 95 Am Am 93 Np H. β - sourcs ar strontium-9, Sr Np Th undrlying procss is: n > p ν 9 38Sr Hr, ν is an antinutrino. Your spcification may rquir you to xplain why this is ndd to balanc th quation. You can translat n > p + - into th A notation: n H γ sourcs ar cobalt-6 27 Co. Th γ radiation coms from th radioactiv daughtr 28 Ni of th 6 6 β dcay of th 27 Co. Th 28 Ni is formd in an xcitd stat and so almost immdiatly loss th nrgy by mitting a γ ray. Thy ar only mittd aftr an α or β dcay, and all such γ rays hav a wll-dfind nrgy. (So a cobalt-6 sourc which is a pur gamma mittr must b dsignd so that btas ar not mittd. How? (by ncasing in mtal which is thick nough to absorb th btas but which still allows gammas to scap.) Studnt qustions: Practic with nuclar quations TAP 52-3: Practic with nuclar quations Th mor unusual dcay procsss (positron mission, nutron mission, lctron captur) could b includd, and studnts challngd to writ thm as nuclar quations. 3

4 TAP 52-: Nuclid notation A priodic tabl may b ndd A Writ down th nuclar notation ( X ) for: (a) an alpha particl (b) a proton (c) a hydrogn nuclus (d) a nutron () a bta particl (f) a positron. A 2 Writ down th nuclar notation ( X ) for (a) carbon 3 (b) nitrogn (c) non 22 (d) tin 8 () iron 5

5 Answrs (a) an alpha particl 2 H (b) a proton H or p (c) a hydrogn nuclus H (d) a nutron n () a bta particl (f) a positron (a) carbon-3 6 C (b) nitrogn- 7 N 22 (c) non-22 N 8 (d) tin-8 5Sn 5 () iron-5 26 F 5

6 TAP 52-2: Dcay procsss Radioactiv dcay procsss α dcay N α 2 N 2 proton numbr 2 fwr protons 2 fwr nutrons β dcay N + N proton numbr β ν mor proton lss nutron β + dcay β + N+ N proton numbr ν lss proton mor nutron γ dcay N γ proton numbr sam protons and nutrons 6

7 Extrnal rfrnc This activity is takn from Advancing Physics chaptr 8, 9O 7

8 TAP 52-3: Practic with nuclar quations Th isotop 235 U dcays into anothr lmnt, mitting an alpha particl. What is th lmnt? This lmnt dcays, and th nxt, and so on until a stabl lmnt is rachd. Th complt list of particls mittd in this chain is: U [ β, β, β, β] X. What is th stabl lmnt X? (You could writ down ach lmnt in th sris, but thr is a quickr way.) 2 Th following fission raction can tak plac in a nuclar ractor: U + n 55 Cs + [ ] Rb + [ ] n. Complt th quation, showing how many nutrons ar producd in th raction. What is th significanc of th numbr of nutrons producd? Why ar th products of th raction, casium-37 and rubidium-95, likly to b radioactiv? What typ of dcay ar ths isotops likly to show? 3 Boron absorbs nutrons with rsults as follows: 7 B + n Li α Why is boron suitabl for us in a control rod? 27 Al Whn th isotop 3 is irradiatd with alpha particls, th products from ach aluminium nuclus ar a nutron, and a nuclid that mits positrons to giv th stabl 3 isotop Si. Writ nuclar quations for ths two procsss. 8

9 5 Complt th following nuclar quations. In ach cas dscrib th dcay procss: 3 53I X Ga + n 6 C B + 99m 3 Tc Tc + γ. 6 Th Manhattan Projct, th dvlopmnt of th atomic bomb, ld to th discovry of th transuranic lmnts (lmnts byond uranium in th priodic tabl). Plutonium, lmnt 9, is formd by th bombardmnt of uranium-238 with nutrons. Th nuclar quations ar: U+ U Np 239 n 92 U Np Pu+ Complt th following nuclar quation for th formation of amricium: 239 9Pu + 2 n Am +. Curium is producd if plutonium-239 is bombardd with alpha particls. If th curium 22 isotop is 96 Cm, complt th quation Pu + 2 α If curium is mad th targt for alpha particl bombardmnt californium is producd. Complt th nuclar quation to find th atomic numbr of californium: Cm + 2α Cf + n. By firing havir particls such as carbon or boron ions at th targt matrials havir lmnts can b synthsisd. Complt th nuclar quation (Lw is lawrncium) 252 Cf B Lw + n. On of th transuranic lmnts is commonly found in th hom. Which is this and whr is it usd? 9

10 Practical advic Ths qustions ar intndd for homwork or practic in class. Answrs and workd solutions. Th complt dcay chain involvs th loss of svn alpha particls ( 2 ) and four bta particls ( ). This rprsnts a loss of 7, i.. 28, in mass numbr and (7 2 ), i.., in atomic numbr. X is thrfor an isotop with mass numbr (235 28), i.. 28 Pb 27, and atomic numbr (92 ), i This is lad, 82. α U + n 55 Cs + 37Rb + 3 n. Th raction producs mor nutrons than it absorbs, this will caus a chain raction. To s why th products of th raction ar likly to b radioactiv you nd to consult th plot of nutron numbr against atomic numbr for th known stabl nucli. For lmnts with atomic numbrs up to about 3 th numbr of nutrons in th nuclus is th sam as th numbr of protons if th nuclus is stabl. For highr atomic numbrs th ratio of nutrons to protons gradually incrass to.5. Look at th position of both Cs and 37Rb on th plot and you will s that thy both hav a considrabl xcss of nutrons. Thy ar thrfor likly to b radioactiv. To bcom mor stabl th nuclids nd to dcras th nutron to proton ratio. Th mission of a bta particl dos this, incrasing th numbr of protons by on and dcrasing th numbr of nutrons by on. Ths isotops ar thrfor likly to dcay by mitting bta particls. 3. Whn boron capturs a nutron it is transformd into a stabl isotop. If th control rods ar pushd into th ractor mor nutrons ar absorbd, causing th chain raction to slow down. If thy ar pulld out th chain raction will procd mor vigorously Al + 2 α = 5P + n 3 3 P = Si I X C 5 B + bta dcay positron mission 67 3Ga + 99m 3Tc 99 3 Tc 67 3 n lctron captur nuclar rarrangm nt Pu + 2 n 95 Am Pu + 2α 96 Cm Cm + 2α 98 Cf Cf + 5B 3 Lw + n n n. Amricium is commonly found in homs whr it is usd in smok dtctors. Extrnal rfrnc

11 This activity is takn from Advancing Physics chaptr 8, 2S