RADIOACTIVE DECAY

Becquerel’s accidental discovery:

Becquerel laid a sample of uranium on a photographic plate, and when he came in the next morning the plate had been “exposed”. He assumed, correctly, the exposing agent came from the uranium, and he called it X-ray (X = unknown). The cross in the bottom was from a medallion he placed between the uranium and the photographic plate.

There are several main types of natural radioactive decay, and the type it undergoes, depends on the energy of the nucleus and the number of neutrons and protons. 1. Alpha 2. Beta 3. Positron 4. Electron capture 5. Gamma

1. Alpha - α

Consists of 2 neutrons and 2 protons Has a 2+ charge 4 2 He – essentially the nucleus of a helium atom This is a large particle, easily stopped by a piece of paper All of its energy is lost over a very short distance, does not travel very far Generally occurs in very large isotopes because it reduces the mass by 4 and the atomic number by U  4 2 He Th α

2. Beta – β How can an electron come from the nucleus?

2. Beta – β

Consists of one electron which comes from a neutron splitting into an electron and a proton A neutrino (very small particle with no charge and a very small mass) is also released Has a -1 charge Is a very small particle, stopped by thick cement The mass remains the same, but the atomic number increases by Th  0 -1 e Pa β

3. Positron – 0 +1 e Where does this particle come from?

3. Positron – 0 +1 e

3. Positron – 0 +1 e Has the same mass as an electron (very tiny), but the opposite charge 1 proton decays into a neutron (mass) and a positron (charge) Mass remains the same, but atomic number decreases by C  0 +1 e B

4. Electron capture – 0 -1 e

An electron is captured from the surrounding cloud and pulled into the nucleus where it combines with a proton to form a neutron. The mass stays the same, but the atomic number decreases by Rb e (orbital)  Kr

5. Gamma radiation – γ Not a particle, but high energy waves. They are a part of the electromagnetic spectrum, so their speed is the speed of light. It is thought that when the nucleons change energy levels within the nucleus, the energy is given off as gamma waves. These travel extremely long distances and can only be stopped by lead which absorbs the waves (energy).

The penetrating power: Which one(s) cause the most damage immediately? Which one(s) cause the most damage years later?

1. The graph at right shows the decay of U-238 into Pb-206 and all the daughter nuclides in between. Explain why this decay occurs naturally.

I  Xe + ______ Bk  4 2 He + ______ 20 8 O  _______ e Kr e  _______ Ag  0 -1 e + ________

Ra  Rn + ______ ______  14 7 N e Po  Pb + _____ 3 1 H  ______ e K  Ar + ______

Write a balanced nuclear reaction for the following: Bismuth-214 undergoes beta decay

Write a balanced nuclear reaction for the following: gold-195 undergoes electron capture

Write a balanced nuclear reaction for the following: plutonium-242 emits alpha radiation

Write a balanced nuclear reaction for the following: xenon-118 undergoes positron emission

What is a decay series?

A decay series is a series of radioactive decays which continue until a stable nuclide is produced. p. 690 Each arrow represents some type of radioactive decay. The parent is the starting nuclide and the daughter is the resulting nuclide. Gamma radiation may or may not be produced as well. What are the times in each circle?

A half life is the time it takes for approximately half the sample of radioactive nuclides (parents) to decay (become daughters). The more stable the nuclide, usually the longer the half life.

1. How much of a x 10 3 mg sample of Po-214 will remain after 1637 μs? (p. 688)

2. How much of a 50.0g sample of tritium (H-3) will remain after 37 years?

3. If a sample of carbon-14 today contains 0.060mg of carbon-14, how much carbon-14 must have been present in the sample 11,430 years ago?

4. Copper-64 is used to study brain tumors. Assume that the original mass of a sample of Cu-64 is 26.00g. After 64 hours, all that remains is g of Cu-64. What is the half-life of Cu-64?

Radioactivity can also be produced artificially by bombarding nuclei with particles. Neutrons are often used for this purpose. p. 691 shows a typical accelerator which is used. Small particles get going very fast, and then are slammed into a sample. The products are studied.

The biggest one is in Switzerland and France. It is 17 miles in circumference and 574 feet below the earth’s surface. The Hadron Collider.

Why is this done?

This is how elements can be created. p Many of these elements have extremely short half lives.