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Lesson 8 Alpha Decay

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Alpha decay () Decay by the emission of doubly charged helium nuclei 4 He 2+. 238 U 234 Th + 4 He Z = -2, N=-2, A=-4 All nuclei with Z83 decay by -decay as do some rare earth nuclei. Alpha decay is also known in the 100 Sn region.

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Alpha Decay The emitted -particles are monoenergetic,ranging in energy from 1.8-11.6 MeV (typically 4-9 MeV). They can be stopped by a piece of paper and are thus an internal radiation hazard rather than an external hazard. The naturally occurring -emitters form long series of nuclei that decay to one another. Some of these naturally occurring decays series involve isotopes of Rn, a gas.

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Typical Alpha Spectra

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Important Features of Alpha Decay Generally energy of decay increases with increasing Z, but in any case the energy of the emitted -particle is less than the Coulomb barrier for the -nucleus interaction. For e-e nuclei, decay leads to gs of daughter. For odd A nuclei, decay is not to the gs but a low-lying excited state.

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Energetics of Alpha Decay Q alpha

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Energetics of Alpha Decay (cont.) Q alpha --Generally increases with increasing Z, but is subject to shell effects. N=152-154 Z=82, N=126

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Q , T Difference between detecting emitted alpha particle and detecting alpha from implanted atom.

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Closed decay cycles Used to measure unknown masses or Q values for beta decay

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Understanding natural alpha- decay chains In the U natural decay series, see pattern of mixed alpha and beta decays. Why?

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Understanding Alpha Decay The problem:

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Understanding Alpha Decay (cont.) The Geiger Nuttall Law

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Theory of Alpha Decay One of the first successes of quantum mechanics

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After some algebra and simplifying for the nuclear case Then This is called the one-body theory of alpha decay

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How well does one-body theory work? Role of pre-formation factor

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Effect of angular momentum Selection Rules

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Which states of daughter are populated in alpha decay? Dominant effect is Q Angular momentum effect

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Hindrance Factors Hindrance Factor=t 1/2 (meas)/t 1/2 (one-body)

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Proton Decay Same theory as alpha decay, except no pre- formation factor for protons Where do you expect to see proton decay? (S p =0) proton dripline

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Proton Decay(cont.) Proton energies are low and transmission factors are small.

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Proton Decay(cont.) Alpha decay complicates measurements

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Heavy Particle Decay What about emitting particles other than alphas or protons? Q 12C