RadioactivityRadioactivity

UnstableUnstable

Some nuclei are unstable and give off radiation.Some nuclei are unstable and give off radiation.• Radioactivity - Bequerel 1896Radioactivity - Bequerel 1896• Three types observed: alpha, beta, gammaThree types observed: alpha, beta, gamma

Alpha ParticlesAlpha Particles Alpha particles are Alpha particles are 44He nuclei.He nuclei.

• Mass approximately 4 uMass approximately 4 u

• Charge is +2Charge is +2

• Generally from the decay of Generally from the decay of heavy nucleiheavy nuclei

The energy of the alpha The energy of the alpha particle is due to the mass particle is due to the mass difference of the daughter difference of the daughter nuclei.nuclei.

QYX AZ

AZ

He42

42

Alpha decay produces an Alpha decay produces an alpha particle.alpha particle.• Includes excess energyIncludes excess energy

Radon DecayRadon Decay Radon 222 decays into Radon 222 decays into

polonium by alpha decay. Find polonium by alpha decay. Find the energy released.the energy released.

The parent nucleus is The parent nucleus is 222222Rn.Rn. Get the reaction equation.Get the reaction equation.

• Look up mass energiesLook up mass energies

The energy released is The energy released is • QQ = = MMRn222Rn222MMPo218Po218MMHe4He4

• QQ = 12.89 MeV = 12.89 MeV

Most of the energy will go to Most of the energy will go to the alpha.the alpha.

HePoRn 42

21884

22286

Beta ParticlesBeta Particles Electron decayElectron decay

• Nucleus emits an electron and Nucleus emits an electron and antineutrinoantineutrino

• Atomic number increasesAtomic number increases

• Energy goes to Energy goes to ee and and • Some decays include a Some decays include a

photon as wellphoton as well

Positron decayPositron decay• Nucleus emits a positron and Nucleus emits a positron and

a neutrinoa neutrino

• Atomic number decreasesAtomic number decreases

• Kinematics like electron decayKinematics like electron decay

• Same result as electron Same result as electron capture – no beta outcapture – no beta out

00

01

6028

6027 NiCo

e 00

01

2210

2211 NeNa e

00

2210

2211

01 NeNa

e

00

011 eYX A

ZAZ 0

0011 eYX A

ZAZ

NeutrinosNeutrinos Neutrinos are leptonsNeutrinos are leptons

• Neutral partners of e, Neutral partners of e, , , • Very light massVery light mass

• Stable particlesStable particles

Produced with lepton partner.Produced with lepton partner.

Neutrinos rarely interact with Neutrinos rarely interact with matter.matter.• Difficult to measureDifficult to measure

Electron neutrino, Electron neutrino, ee

• Mass < 2.8 eVMass < 2.8 eV

Muon neutrino, Muon neutrino,

• Mass < 0.19 MeVMass < 0.19 MeV

• mm22 = 0.002 eV = 0.002 eV22 (m < 3.5 eV) (m < 3.5 eV)

Tau neutrino, Tau neutrino,

• Mass < 18.2 MeVMass < 18.2 MeV

Gamma ParticlesGamma Particles Nuclei can decay by emitting a Nuclei can decay by emitting a

gamma ray.gamma ray.• High energy photonHigh energy photon

Nuclear energy levels are like Nuclear energy levels are like atomic levels.atomic levels.

TlTl 20881

*20881

Excited StatesExcited States Nuclear decay can leave a Nuclear decay can leave a

nucleus in an excited state.nucleus in an excited state.• Many possible states may be Many possible states may be

reachedreached

• Lifetime typically 10Lifetime typically 10-10-10 s s

Excess energy may be lost as Excess energy may be lost as a photon or electron.a photon or electron.• Single gammaSingle gamma

• Series of gamma emissionsSeries of gamma emissions

• Internal conversion betaInternal conversion beta

Ra22688

4.785 MeV

Rn22286

0.186 MeV

0 MeV

94.4%

5.5%

2.2% 3.3%

Decay ProductsDecay Products

next