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Discussion question: If a nucleus undergoes radioactive decay it must be older than the other nuclei in the sample that haven’t decayed.A. TrueB. False
Types of radioactive decay Gamma Beta (+,-, e.c.) Alpha
Neutron emission Fission Fusion
http://ie.lbl.gov/education/isotopes.htm
Gamma DecayNucleus in excited state gives off energy as a photon
𝐶∗6❑
❑14 → 𝐶❑
6❑
❑14 +𝛾
Beta Decay – electron and positron decay
Changes a proton to a neutron or a neutron to a proton
189F + e
β- 146C
e.c.
14162Smβ+
22688Ra
Alpha Decay--42He given off
Alpha particle is a very lowenergy system / very tightly bound
23892U 234
90Th + ? T1/2 = 4.5 Gy
A. alphaB. beta +C. beta –D. e.c.E. gamma
23490Th 234
91Pa + ? T1/2 = 24 days
A. alphaB. beta +C. beta –D. e.c.E. gamma
Why do some things decay, other don’t?
MagicNumbers
Image from http://pms.iitk.ernet.in/ICT/physics_courses/akj/AKJain_IITR_Ch_2.htm
Why do some things decay, other don’t?
MagicNumbers
Image from http://pms.iitk.ernet.in/ICT/physics_courses/akj/AKJain_IITR_Ch_2.htm
Small nuclei – N ~ Z
Large nuclei – N > Z
Radiometric Dating Carbon Dating
– Cosmic rays release neutrons– 14N + n → 14C + 1H
Half life of ~5760 years– Compare carbon-12 to carbon-14 ratio– Limitations
Must be organic Has the ratio of C-12 to C-14 always been
the same?– Calibration
50,000 years limit (not much C-14 left)
2 Nephi 2:26And the Messiah cometh in the fulness of time, that he may redeem the children of men from the fall. And because that they are redeemed from the fall they have become free forever, knowing good from evil; to act for themselves and not to be acted upon, save it be by the punishment of the law at the great and last day, according to the commandments which God hath given.
More Proof
22688Ra eventually becomes 206
82Pb. What is released in this process?
A. only alphasB. only betasC. both but more alphas than betasD. both but the same number of alphas
and betasE. both but more betas than alphas
N
Z
U23892
85 9080
145
140
Th23490
Pa23491
U23492Th230
90
Ra22690
Th23290 Np237
93 U23892 U235
92
http://en.wikipedia.org/wiki/Decay_chain
Penetration Depth Alpha– only a few cm through air Beta—only through air—blocked by
paper or sheet of metal Gamma rays—short wavelength light
—penetrate several cm lead or a meter of concrete. Easily penetrates the skin and interacts with human cells—these are the dangerous ones.
What has to be true for a particular type of nuclear
decay to happen? Conservation of . . .
– Energy, momentum, angular momentum Compare rest energy OK if we go down in energy – turn into
kinetic– Lepton number, Baryon number, …
Force to make it happen
For radioactive decay where No
is the number of nuclei you start with and N is the number of nuclei after a time t
toeNN
How do I solve for λ?
Discussion question: Over the course of 3 hours, 15% of a radioactive material decays. What is its half-life?A. 4.1 hrsB. 12.8 hrsC. 24.0 hrsD. 68.6 hrsE. 84.2 hrs
#decays/sec=λN 1 Ci = 3.7x1010 decays/sec
Fission: Heavy Elements can reduceenergy (i.e. increase bindingenergy per nucleon) bysplitting roughly in half.