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.