Download - Nuclear Class3 Radioactivity
PHY20004NuclearPhysicsClass3:Radioactivity
Inthisclasswewilldescribethebasictypesofnuclearradioactivedecay,thatproduce𝛼,𝛽 and
𝛾 particles
Introducingradioactivity
• Radioactivityisthespontaneoustransformationofanunstablenucleus,involvingtheemissionofradiation
• Radioactivedecaycanoccurifthenucleonsarearrange-ableinalowerenergystatethantheircurrentconfiguration
Wewilldiscussthethreecommontypesofnuclearradioactivedecay,whicharelabelled𝛼, 𝛽, 𝛾 decays
𝛼 decays
Heliumnucleusconsistsof2protonsand2neutrons
(+energy𝑄)
• 𝛼-decayoccurswhenanucleusistoolargetobestable,anddisintegratestoalower-energystatebyejectingaheliumnucleus,whichisalsoknownasan𝛼-particle
• 𝛼-decayrequirestheparenttohavemassnumber𝑨 ≳ 𝟏𝟓𝟎 inorderthatthedecayisspontaneous(i.e.energy𝑄 > 0)
𝛼 decays↵ Decay
↵ decay is due to the emission of a 42He nucleus.
42He is doubly magic and very tightly bound.
↵ decay is energetically favourable for almostall with A�190 and for many A�150.
Why ↵ rather than any other nucleus?Consider energy release (Q) in various possible decays of 232U
n p 2H 3H 3He 4He 5He 6Li 7Li
Q/MeV -7.26 -6.12 -10.70 -10.24 -9.92 +5.41 -2.59 -3.79 -1.94
↵ is easy to form inside a nucleus 2p "# + 2n "#(though the extent to which ↵ particles really exist inside a nucleus is stilldebatable)
Dr. Tina Potter 15. Nuclear Decay 9
(+energy𝑄)
• Althoughothertypesoffissionarepossible,𝛼-decayisdominantbecause He/
0 isverytightlybound(doublymagic!)
• Considertheenergyreleasedfromthefollowingdecaysof𝑈/2/ : theonlyspontaneousdecay(with𝑄 > 0)is 𝐻𝑒0
𝛼 decays
• Theescapeofan𝛼-particlefromthenucleusisanexampleofquantumtunneling
• Tunnelingisaquantumphenomenonwhichallowsthe𝛼-particletoviolateenergyconservationforabriefperiodoftime,owingtothequantumuncertaintyprinciple
• Havingtunneledoutofthenucleus,the𝛼-particleisejectedbyCoulombrepulsion
• Themeanlifetimeoftheunstablenucleusishighlysensitivetotheheightofthepotentialbarrier andcanvarybetween1067s and10:;<yr!(see:theGeiger-Nuttalllaw)
Credit:https://www.nuclear-power.net/
Calculatingtheenergyreleased
• Asthenucleusdisintegratestoalowerenergyconfiguration,𝛼-decayreleasesenergy𝑄 whichtakestheformofthekineticenergyofthedecayproducts
• 𝑄 canbecalculatedasthedifferenceinthebindingenergy𝐵 betweenthedecayproductsandtheinitialnucleus
𝑄 = 𝐵 𝛼 + 𝐵 𝐴 − 4, 𝑍 − 2 − 𝐵(𝐴, 𝑍)
Bindingenergyof𝛼-particleis
𝐵 𝛼 = 28.3𝑀𝑒𝑉
𝛽 decays
• 𝛽-decayisthetransformationofaneutron-rich(orproton-rich)nucleusbythedirectconversionofaneutronintoaproton (orviceversa)
Neutrontransformsintoproton
Protontransformsintoneutron
Credit:https://education.jlab.org
𝛽 decays
• 𝛽-decaytransformsthenumberofprotons𝑍 andneutrons𝑁 inanucleusclosertothelineofstability𝑍~𝑁
• Thereare3formsof𝛽-decay:§ 𝜷6 decay:𝑁 istoolargeforstability,andaneutronbecomesa
protoninvolvingtheemissionofanelectronandanti-neutrino
§ 𝜷: decay:𝑍 istoolargeforstability,andaprotonbecomesaneutroninvolvingtheemissionofapositronandaneutrino
§ Electroncapture:𝑍 istoolargeforstability,andanatomicelectronstraystooclosetothenucleusandreactswithaproton,producinganeutron(withinthenucleus)andaneutrino
𝑛 → 𝑝 + 𝑒6 + �̅�V 𝑋XY → 𝑌:X:;Y + 𝑒66;
< + �̅�V
𝑝 → 𝑛 + 𝑒: + 𝜈V 𝑋XY → 𝑌6X6;Y + 𝑒::;
< + 𝜈V
𝑝 + 𝑒6 → 𝑛 + 𝜈V 𝑋XY + 𝑒66;< → 𝑌6X6;
Y + 𝜈V
Neutrinos
• Theproductionofnewparticlescalledneutrinos isaninterestingfeatureof𝛽-decay
• Neutrinosareweakly-interacting,electrically-neutralparticleswithnear-zeromass
• Theirexistenceisrequiredtoensureenergyandmomentumconservationduring𝛽-decay
• We’llhearmuchmoreaboutneutrinoslaterinthecourse!
TheSuper-Kamiokande detectorinJapan,anunderground
neutrinodetector
Note:aprotoncannotdecayinfreespace(sinceit’sthelightestbaryon),butinsideanucleusitcandecaybyborrowingsomeenergyfromthebindingenergy
𝛾 decays
• 𝛾-decayoccurswhenanucleusisinanexcitedstate–oftenfollowing𝛼- or𝛽-decay– andrevertstothegroundstate,emittingaphoton(alsoknownasa𝛾-ray)
• 𝛾-decayissimilartoatomicde-excitation,butoccursatmuchhigherenergy(~MeV vs.~eV)
𝑋∗XY
Radioactivedecaychains
• Theproductsofradioactivedecaymaythemselvesbeunstabletodecay– creatingadecaychain
• Here’sthedecaychainforUranium-235 – noticetheverydifferenthalf-livesofdifferentsteps!
• We’lldiscussthisinmoredetailinthenextclass
Credit:http://hyperphysics.phy-astr.gsu.edu/
Keytake-aways
• Radioactivedecayisaspontaneousprocesswhichoccurswhennucleonsre-arrangethemselvesintoalower-energyconfiguration
• Wecandistinguishthreemaincases:
• 𝜶-decay:transformationofalargenucleus(𝐴 ≳ 150)bytheemissionofa 𝐻𝑒/
0 particle,viaquantumtunneling
• 𝜷-decay:transformationofaneutronintoaproton(orviceversa),emittinganelectron(orpositron)andneutrino
• 𝜸-decay:de-excitationofanucleus,involvingtheemissionofaphoton