Fission ntawm ib lub nucleus yog kev sib cais ntawm ib qho hnyav atom rau hauv ob feem ntawm kwv yees li qhov sib npaug, nrog rau kev tso tawm ntau lub zog.
Qhov kev tshawb pom ntawm nuclear fission pib lub sijhawm tshiab - "lub hnub nyoog atomic". Lub peev xwm ntawm nws txoj kev siv tau thiab qhov piv ntawm kev pheej hmoo kom tau txais txiaj ntsig los ntawm nws txoj kev siv tsis tau tsuas yog ua rau muaj ntau yam kev sib raug zoo, kev nom kev tswv, kev lag luam thiab kev tshawb fawb, tab sis kuj muaj teeb meem loj. Txawm hais tias los ntawm qhov kev xav ntawm kev tshawb fawb nkaus xwb, cov txheej txheem ntawm nuclear fission tau tsim ntau qhov kev sib tw thiab cov teeb meem, thiab cov lus piav qhia ua tiav ntawm nws yog qhov teeb meem ntawm lub neej yav tom ntej.
Sib faib tau txiaj ntsig
Lub zog khi (ib nucleon) sib txawv rau cov nuclei sib txawv. Cov hnyav dua muaj lub zog khi qis dua li cov nyob nruab nrab ntawm lub rooj zaum.
Qhov no txhais tau hais tias hnyav nuclei nrog tus lej atomic ntau dua 100 tau txais txiaj ntsig los ntawm kev faib ua ob qho me me, yog li tso tawm lub zog uashloov mus rau hauv kinetic zog ntawm fragments. Cov txheej txheem no hu ua kev sib cais ntawm lub atomic nucleus.
Raws li txoj kev ruaj ntseg nkhaus, uas qhia tau hais tias qhov kev cia siab ntawm tus naj npawb ntawm cov protons ntawm cov xov tooj ntawm cov neutrons rau cov nuclides ruaj khov, hnyav dua nuclei nyiam ntau dua neutrons (piv rau cov xov tooj ntawm protons) dua li cov sib zog. Qhov no qhia tau hais tias nrog rau kev sib cais, qee qhov "spare" neutrons yuav raug tso tawm. Tsis tas li ntawd, lawv tseem yuav siv qee lub zog tso tawm. Kev tshawb fawb ntawm nuclear fission ntawm lub uranium atom qhia tias 3-4 neutrons raug tso tawm: 238 U → 145 La + 90 Br + 3n.
Tus lej atomic (thiab atomic mass) ntawm ib feem tsis sib npaug rau ib nrab ntawm cov atomic loj ntawm niam txiv. Qhov sib txawv ntawm qhov loj ntawm atoms tsim los ntawm kev sib cais feem ntau yog li 50. Txawm li cas los xij, qhov laj thawj ntawm qhov no tseem tsis tau nkag siab tag nrho.
Lub zog khi ntawm 238U, 145La thiab 90Br yog 1803, 1198 thiab 763 MeV, feem. Qhov no txhais tau hais tias raws li qhov tshwm sim ntawm cov tshuaj tiv thaiv no, lub zog fission ntawm uranium nucleus raug tso tawm, sib npaug ntawm 1198 + 763-1803=158 MeV.
Txoj kev ntawm kev sib cais ntawm tus kheej tau paub hauv qhov xwm txheej, tab sis lawv tsawg heev. Qhov nruab nrab lub neej ntawm cov txheej txheem no yog kwv yees li 10 17xyoo, thiab, piv txwv li, lub neej nruab nrab ntawm alpha lwj ntawm tib radionuclide yog li 10 11 xyoo.
Yog vim li cas qhov no yog txhawm rau faib ua ob ntu, cov ntsiav yuav tsumthawj zaug deformation (stretch) rau hauv cov duab ellipsoidal, thiab tom qab ntawd, ua ntej qhov kawg faib ua ob ntu, tsim "caj dab" hauv nruab nrab.
Muaj peev xwm barrier
Nyob rau hauv lub xeev deformed, ob lub zog ua rau lub hauv paus. Ib qho ntawm lawv yog lub zog saum npoo av (qhov nro ntawm cov kua dej piav qhia nws cov duab kheej kheej), thiab lwm qhov yog Coulomb repulsion ntawm fission fragments. Ua ke lawv tsim muaj peev xwm thaiv tau.
Raws li qhov xwm txheej ntawm alpha lwj, txhawm rau kom muaj kev sib cais ntawm cov uranium atom nucleus tshwm sim, cov tawg yuav tsum kov yeej qhov teeb meem no siv quantum tunneling. Qhov teeb meem yog hais txog 6 MeV, xws li nyob rau hauv cov ntaub ntawv ntawm alpha lwj, tab sis qhov tshwm sim ntawm tunneling ntawm ib tug α particle yog ntau ntau dua li cov khoom hnyav atom fission.
Kev sib cais
Ntau yam yuav ua rau muaj kev cuam tshuam ntawm uranium nucleus. Hauv qhov no, niam txiv nucleus yog irradiated nrog neutrons. Yog tias niam txiv absorbs nws, lawv khi, tso lub zog khi rau hauv daim ntawv ntawm lub zog vibrational uas muaj peev xwm tshaj li 6 MeV yuav tsum tau kov yeej lub peev xwm thaiv.
Qhov twg lub zog ntawm ib qho ntxiv neutron tsis txaus los kov yeej lub peev xwm thaiv, qhov xwm txheej neutron yuav tsum muaj lub zog kinetic yam tsawg kawg nkaus thiaj li muaj peev xwm ua rau muaj kev sib cais ntawm lub atom. Nyob rau hauv rooj plaub ntawm 238 U daim ntawv cog lus zog ntxivneutrons ploj lawm txog 1 MeV. Qhov no txhais tau hais tias fission ntawm uranium nucleus yog induced los ntawm ib tug neutron nrog lub zog kinetic ntau dua 1 MeV. Ntawm qhov tod tes, cov isotope 235 U muaj ib qho tsis sib xws neutron. Thaum lub nucleus absorbs ib qho ntxiv, nws tsim ib khub nrog nws, thiab raws li qhov tshwm sim ntawm qhov kev sib koom ua ke, ntxiv zog khi ntxiv tshwm. Qhov no txaus los tso tawm lub zog tsim nyog rau lub nucleus kom kov yeej lub peev xwm barrier thiab cov isotope fission tshwm sim thaum sib tsoo nrog ib qho neutron.
Beta Decay
Txawm hais tias qhov fission cov tshuaj tiv thaiv tawm peb los yog plaub neutrons, cov fragments tseem muaj ntau dua neutrons dua lawv cov isobars ruaj khov. Qhov no txhais tau hais tias fission fragments feem ntau tsis ruaj khov tiv thaiv beta lwj.
Piv txwv li, thaum uranium fission tshwm sim 238 U, qhov ruaj khov isobar nrog A=145 yog neodymium 145 Nd, uas txhais tau tias cov lanthanum fragment 145 La decays nyob rau hauv peb theem, txhua zaus emitting ib tug electron thiab ib tug antineutrino, kom txog rau thaum ib tug ruaj khov nuclide yog tsim. Qhov ruaj khov isobar nrog A=90 yog zirconium 90 Zr, yog li qhov tawg tawg bromine 90 Br decays nyob rau hauv tsib theem ntawm β-decay saw.
Cov β-decay chains tso tawm lub zog ntxiv, yuav luag tag nrho cov uas tau nqa los ntawm electrons thiab antineutrinos.
Nuclear cov tshuaj tiv thaiv: fission ntawm uranium nuclei
Direct hluav taws xob ntawm ib tug neutron los ntawm ib tug nuclide nrog ib yam nkaus thiabib tug loj tus naj npawb ntawm lawv los xyuas kom meej stability ntawm lub ntsiav yog tsis zoo li. Lub ntsiab lus ntawm no yog tias tsis muaj Coulomb repulsion, thiab yog li lub zog saum npoo zoo li khaws cov neutron hauv kev sib raug zoo nrog niam txiv. Txawm li cas los xij, qee zaum qhov no tshwm sim. Piv txwv li, cov fission fragment 90 Br nyob rau hauv thawj theem ntawm beta decay tsim krypton-90, uas yuav ua tau nyob rau hauv ib qho kev zoo siab nrog lub zog txaus los kov yeej lub zog saum npoo. Nyob rau hauv rooj plaub no, lub emission ntawm neutrons tuaj yeem tshwm sim ncaj qha nrog kev tsim ntawm krypton-89. Qhov isobar no tseem tsis ruaj khov rau β lwj kom txog thaum nws hloov mus rau ruaj khov yttrium-89, yog li krypton-89 decays nyob rau hauv peb kauj ruam.
Uranium fission: chain reaction
Neutrons tawm hauv cov tshuaj tiv thaiv fission tuaj yeem nqus los ntawm lwm tus niam txiv nucleus, uas tom qab ntawd nws tus kheej raug ntxias kom fission. Nyob rau hauv cov ntaub ntawv ntawm uranium-238, peb neutrons uas tau tsim tawm los nrog lub zog ntawm tsawg dua 1 MeV (lub zog tso tawm thaum lub sij hawm fission ntawm uranium nucleus - 158 MeV - feem ntau yog hloov dua siab tshiab rau hauv kinetic zog ntawm fission fragments.), yog li lawv tsis tuaj yeem ua rau muaj kev sib cais ntxiv ntawm cov nuclide no. Txawm li cas los xij, nrog rau qhov tseem ceeb ntawm cov isotope tsis tshua muaj 235U, cov neutrons dawb no tuaj yeem ntes tau los ntawm nuclei 235U, uas tuaj yeem ua rau fission, txij li qhov no, tsis muaj lub zog pib hauv qab uas fission tsis raug ntxias.
This is the chain reaction principle.
Hom kev tawm tsam nuclear
Cia k yog tus naj npawb ntawm cov neutrons uas tsim nyob rau hauv cov qauv ntawm cov khoom fissile nyob rau theem n ntawm cov saw no, muab faib los ntawm cov xov tooj ntawm cov neutrons uas tsim nyob rau theem n - 1. theem n - 1, yog absorbed los ntawm lub nucleus, uas tej zaum yuav raug yuam fission.
• Yog tias k < yog 1, ces cov tshuaj tiv thaiv kab mob tsuas yog fizzle tawm thiab cov txheej txheem yuav nres sai heev. Qhov no yog raws nraim li cas tshwm sim nyob rau hauv tej yam ntuj tso uranium ore, nyob rau hauv uas lub concentration ntawm235U yog li tsawg uas qhov tshwm sim ntawm kev nqus ntawm ib qho ntawm cov neutrons los ntawm cov isotope no yog qhov tsis txaus ntseeg heev.
• Yog k > 1, ces cov tshuaj tiv thaiv saw yuav loj hlob mus txog rau thaum tag nrho cov fissile khoom siv (atomic bomb). Qhov no yog ua tiav los ntawm enriching ntuj ore kom tau ib tug txaus siab concentration ntawm uranium-235. Rau ib qho qauv kheej kheej, tus nqi ntawm k nce nrog kev nce hauv cov neutron absorption probability, uas nyob ntawm lub vojvoog ntawm tus kheej. Yog li ntawd, qhov loj ntawm U yuav tsum tshaj ib co tseem ceeb loj nyob rau hauv thiaj li yuav fission ntawm uranium nuclei (ib tug saw tshuaj tiv thaiv) tshwm sim.
• Yog tias k=1, ces qhov kev tswj xyuas tshwm sim. Qhov no yog siv nyob rau hauv nuclear reactors. Cov txheej txheem yog tswj los ntawm kev faib cov cadmium los yog boron rods ntawm cov uranium, uas nqus feem ntau ntawm cov neutrons (cov ntsiab lus no muaj peev xwm ntes neutrons). Lub fission ntawm uranium nucleus yog txiav txim los ntawm kev txav cov pas nrig kom tus nqi ntawm k tseem sib npaug rau ib qho.
