IB Physics 12

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IB Physics 12

• Nuclear Physics IV
• Mr. Jean

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The plan

• Video clip of the day
• Beta Gamma Decay Models
• Practice Questions
• Time to work on Quest Questions

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Decay Reviews

• https//www.youtube.com/watch?vsqeUtQ83sr4
  (ALPHA) (first 6 minutes)
• https//www.youtube.com/watch?vmD1FK2Cj62Y
  (BETA) (first 10 minutes)
• https//www.youtube.com/watch?v7unp33xJSJE
  (GAMMA) (first 13 minutes)

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Gamma and Beta decays are similar

• Unlike a decay, ß and ? decays are closely
  related (e.g. like cousins).
• They often occur together as in the typical
  decay scheme (i.e. 198Au)
• They just involved changes in nucleon states (p
  n, n p, p p)
• They involve the same basic force (?, W )
  carrier but in different state
• But ß decays are generally much slower
  (100,000) than ? decays (produced by EM force)
  because the Ws are heavy particles (which makes
  force weaker)

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Gamma and Beta decays are very similar
Decay Name of
process Interaction Out Channel
Nucleon Zero Leptons
Gamma Decay
EM
Internal Conversion
EM
Nucleon One Lepton
weak
Electron Capture
Pair Internal Conversion
EM
Nucleon Two Leptons
ß Decay
weak
weak
ß- Decay
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Feynman Diagrams - Similarity
OUT CHANNEL ---- One nucleon 2 leptons
p
n
BETA PLUS DECAY
BETA MINUS DECAY
PAIR INTERNAL CONVERSION
All these decay types are similar in
structure They all have a 4 point vertex They all
have 3 particles in the final state The fact that
the Q of the decay is shared between 3 particles
means that the outgoing observed particle ie.
electron or positron has a spectrum of energies
in the range (0 to Q).
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Feynman Diagrams - Similarity
OUT CHANNEL ---- One nucleon 1 lepton
n
p
p
p
p
p
?
INTERNAL CONVERSION
ELECTRON CAPTURE
GAMMA DECAY
Mono-energetic photons
Mono-energetic electrons
Mono-energetic neutrinos
All these decays have only two particles in their
output state. The Q of the decay is shared
between only 2 particles Conservation of Energy
The emitted particle (? , e-, ?e) is
monoenergetic.
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Quark level Feynman Diagrams - Similarity
BETA PLUS DECAY
BETA MINUS DECAY
PAIR INTERNAL CONVERSION
The proton is made of 3 quarks uud (up, up,
down) The neutron is made also of 3 quarks - udd
(up, down, down) We see the very close similarity
of pattern between reactions through W and ?
particles. NOTE only vertices of 3
particles are now seen (makes sense)
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Quark level Feynman Diagrams - Similarity
?
INTERNAL CONVERSION
ELECTRON CAPTURE
GAMMA DECAY
Again we see that there are ONLY 3 PARTICLE
VERTICES We see the similarity of the decays are
propogated through the intermedicate Force
particles (W and ?). Remember in INTERNAL CONV.
And ELECTRON CAPTURE the electron comes from the
core electron orbitals of THE ATOM.
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Radioactive Materials
The rate of decay for radioactive substances is
expressed in terms of the activity R, given by
N Number of undecayed nuclei
One becquerel (Bq) is an activity equal to one
disintegration per second (1 s-1).
One curie (Ci) is the activity of a radioactive
material that decays at the rate of 3.7 x 1010 Bq
or 3.7 x 1010 disintegrations per second.
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The Half-Life
The half-life T1/2 of an isotope is the time in
which one-half of its unstable nuclei will decay.
Where n is number of half-lives
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Example

• Radium-226 has a half-life of 1620 years.
• This is shown in the graph below. One kg of
  radium-226 begins the thing. After one half-life
  (1620years) only half of the sample remains the
  other half has decayed into some other element.
• After two half-lives only one fourth would remain
  and so on.

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Half-Life (Cont.)
The same reasoning will apply to activity R or to
amount of material. In general, the following
three equations can be applied to radioactivity
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Example 6 A sample of iodine-131 has an initial
activity of 5 mCi. The half-life of I-131 is 8
days. What is the activity of the sample 32 days
later?
First we determine the number of half-lives
n 4 half-lives
R 0.313 mCi
There would also be 1/16 remaining of the mass
and 1/16 of the number of nuclei.
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To do

• Work on Quest Assignment
• Review and Study Decay Models
• Read about Feynman Models