Lesson 8 Alpha Decay Alpha decay (α) Decay by the emission of doubly charged helium nuclei 4 He 2+....
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Lesson 8 Alpha Decay
Lesson 8 Alpha Decay Alpha decay (α) Decay by the emission of doubly charged helium nuclei 4 He 2+. 238 U 234 Th + 4 He ΔZ = -2, ΔN=-2, ΔA=-4 All nuclei
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.
Slide 4
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.
Slide 5
Typical Alpha Spectra
Slide 6
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.
Slide 7
Energetics of Alpha Decay Q alpha
Slide 8
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
Slide 9
Q , T Difference between detecting emitted alpha particle and
detecting alpha from implanted atom.
Slide 10
Closed decay cycles Used to measure unknown masses or Q values
for beta decay
Slide 11
Understanding natural alpha- decay chains In the U natural
decay series, see pattern of mixed alpha and beta decays. Why?
Slide 12
Understanding Alpha Decay The problem:
Slide 13
Understanding Alpha Decay (cont.) The Geiger Nuttall Law
Slide 14
Theory of Alpha Decay One of the first successes of quantum
mechanics
Slide 15
Slide 16
After some algebra and simplifying for the nuclear case Then
This is called the one-body theory of alpha decay
Slide 17
How well does one-body theory work? Role of pre-formation
factor
Slide 18
Effect of angular momentum Selection Rules
Slide 19
Which states of daughter are populated in alpha decay? Dominant
effect is Q Angular momentum effect