Discussion question: If a nucleus undergoes radioactive decay it must be older than the other nuclei in the sample that haven’t decayed. A. True B. False

Types of radioactive decay Gamma Gamma Beta (+,-, e.c.) Beta (+,-, e.c.) Alpha Alpha Neutron emission Neutron emission Fission Fission Fusion Fusion

Gamma Decay Nucleus in excited state gives off energy as a photon

Beta Decay – electron and positron decay Changes a proton to a neutron or a neutron to a proton 18 9 F + e β-β C e.c Sm β+β+

Ra Alpha Decay He given off Alpha particle is a very low energy system / very tightly bound

U Th + ? T 1/2 = 4.5 Gy A. alpha B. beta + C. beta – D. e.c. E. gamma

Th Pa + ? T 1/2 = 24 days A. alpha B. beta + C. beta – D. e.c. E. gamma

Why do some things decay, other don’t? Magic Numbers Image from

Why do some things decay, other don’t? Magic Numbers Image from

Small nuclei – N ~ Z

Large nuclei – N > Z

Radiometric Dating Carbon Dating Carbon Dating –Cosmic rays release neutrons – 14 N + n → 14 C + 1 H  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 2:26 And 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

Ra eventually becomes Pb. What is released in this process? A. only alphas B. only betas C. both but more alphas than betas D. both but the same number of alphas and betas E. both but more betas than alphas

N Z

Penetration Depth Alpha– only a few cm through air Alpha– only a few cm through air Beta—only through air—blocked by paper or sheet of metal 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. 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... 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 Force to make it happen

For radioactive decay where N o is the number of nuclei you start with and N is the number of nuclei after a time t 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 hrs B hrs C hrs D hrs E hrs

#decays/sec=λN 1 Ci = 3.7x10 10 decays/sec 1 Ci = 3.7x10 10 decays/sec

Fission: Heavy Elements can reduce energy (i.e. increase binding energy per nucleon) by splitting roughly in half.