Beta and Gamma Decay

Contents:

Beta•What it is and the Neutrino Hypothesis•Reverse decay: electron capture•Enrico Fermi, Wolfgang Pauli, and the “little neutral one”

Gamma•What it is•Being Discrete•Whiteboards

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6027Co 60

28Ni + - + e

Beta Decay:

137N 13

6C + + + e

Conservation of chargeBeta minus - electron

“As if” neutron -> proton + electronBeta plus - positron

“As if” proton -> neutron + positronParticles are “of the nucleus” (not orbital) - Neutrino, (anti neutrino) – fudge (Conserve lepton number)

Energy is continuous (i.e. neutrino gets random share)

Pauli, Fermi, and the little neutral one

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Beta decay products were missing energyPauli proposes a particle is carrying away energyFermi names it Neutrino - “Little neutral one” - It.Neutrinos confirmed in 1956, no surprise

Enrico Fermi1901-1954

Wolfgang Pauli1900-1958

e

e

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23994Pu 239

94Pu + Gamma Decay:

Nucleus has energy levelsEnergy of transition emitted as a high energy photon ( ≈ 5 - .05 nm)Usually after a beta or alpha decayMany energies possible Stopped by meters of leadUsed for food irradiation

Gamma ray energies associated with alpha and beta decays – so Alpha and Gamma energies are discrete. (Like spectral lines we saw)

Example: Tl-208 emits a 0.6210 MeV gamma and the neutral (un -excited) atom has a mass of 207.9820047 u. What was the mass before the gamma was emitted?

Example: Tl-208 emits a 0.6210 MeV gamma and the neutral atom has a mass of 207.9820047 u. What was the mass before the gamma was emitted?

Well, 0.6210 MeV has a mass of (0.6210 MeV)/(931.5 MeV) = 0.000666667 uSo the parent is this much more massive: 207.9820047 + 0.000666667 = 207.9826714 u

Whiteboards: Gamma Decay

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W

Parent mass = 239.052464 uProduct mass = 239.052157 uMass defect = parent - products = 0.000307 uEnergy of radiation = (0.000307 u)(931.5 MeV/u) = 0.2860 MeV

23994Pu --> 239

94Pu +

239.052464 --> 239.052157What is the energy of the gamma emitted?

0.2860 MeV

W

Mass of 0.2957 MeV = (0.2957 MeV)/(931.5 MeV/u) = 0.000317445 uExcited parent is more massive than this by that amountParent mass = 245.065484 + 0.000317445 u = 245.0658034 u

24596Cm --> 245

96Cm +

???.?????? --> 245.065486What is the excited mass of the parent, if a 0.2957 MeV gamma photon is emitted?

245.0658034 u