Radioactive decay Learning Objectives Understand what happens to the nuclei of radioactive atoms during alpha, beta and gamma decay. Know the definitions of “half-life” and how to perform simple calculations using it. Understand the basic principles of nuclear power generation.

What we used to think… It was believed that atoms were: 1.Spheres of positive charge. 2.With negative charges spread through it. This resembled a plum-pudding, so it was called the ‘Plum –pudding’ model. This was wrong! How did we discover current ideas about the structure of the atom?

Rutherford’s team: Ernest Rutherford and his team of scientists performed a famous experiment in Manchester: They fired some alpha particles at a piece of thin gold foil (only a few atoms thick): If the ‘Plum Pudding’ model of the atom was correct, the alpha particles should pass straight through and only be slightly deflected. This did not happen.

What Rutherford’s team observed…….. 1.Most of the alpha particles went straight through the foil. 2.Some alpha particles were deflected through large angles. 3. A very few alpha particles were reflected straight back.

Rutherford’s conclusions ObservationConclusion Most alpha particles went straight through the foil. A few were deflected through large angles. A very few were reflected straight back. Atoms are mostly space. The nucleus is very small compared to the size of the atom and it contains most of the mass and all the positive charge.

Radioactive Decay Why is it that there are different types of radiation? What is going on inside the nucleus? The three types of decay are……… Alpha Beta Gamma

Alpha decay Type of decay: What is emitted? Description of decay: Example of decay: Effect on A and Z: Alpha particle (helium nuclei) U  Th +  + energy neutrons and 2 protons are emitted from the nucleus. A decreases by 4, Z decreases by 2 (A-4, Z-2)

Beta decay Type of decay: What is emitted? Description of decay: Example of decay: Effect on A and Z: High energy electron C  N +  + energy A neutron in the nucleus decays into a proton and a high energy electron which is emitted. A stays the same, Z increases by 1 (A=, Z+1)

Gamma decay Type of decay: What is emitted? Description of decay: Effect on A and Z: High energy electromagnetic radiation. Nucleus changes shape into a more stable shape. Gamma radiation emitted as a result. A stays the same, Z stays the same (A=, Z=)

Half life The time it takes the number of radioactive nuclei in a sample to decrease by 50%. The time it takes the count rate from a radioisotope to decrease by 50%. There are two definitions of half life: You must learn both of these definitions!

Graphical representation of half life Decay rate (counts/min) Time (min) What is the half life of the radioisotope represented by the following graph? The time it takes the count rate to decrease from 80 per min to 40 per min is what? 2 mins Double check, the time it takes the count rate to decrease from 40 per min to 20 per min is? 2 mins The half life of the radioisotope is 2 mins.

Half life questions 1.What are the two definitions of half life? 2.If 1/64 th of an original radioisotope is left after 1 hour, what is the half life of the sample? 3.A radioisotope has a half life of 12 minutes. What fraction of the radioisotope will be left after 2 hours? 4.The background radiation in a laboratory is 13 counts per minute. The count rate from a radioisotope is measured and it has a reading of 119 counts per minute. If the half life of the radioisotope is 10 minutes, what will be the reading 20 minutes later?

Carbon Dating All living things take in a little radioactive carbon-14 in photosynthesis, as well as the normal carbon-12. When living things die, they stop taking in carbon-14 and so the carbon-14 present at death slowly decays to carbon-12 (half-life is years). The radioactivity due to the decay of carbon-14 can be used to date bones, wood, paper and cloth. All living things take in a little radioactive carbon-14 in photosynthesis, as well as the normal carbon-12. When living things die, they stop taking in carbon-14 and so the carbon-14 present at death slowly decays to carbon-12 (half-life is years). The radioactivity due to the decay of carbon-14 can be used to date bones, wood, paper and cloth.

Example A fresh bone gives a radioactive count of 170 counts per minute. Another ancient bone of the same mass gives a count rate of 50 counts per minute. The background count is 10 counts per minute. How old is the bone? Counts due to bones are = 160 (fresh) and =40 (ancient) The count rate of the carbon-14 has fallen to one quarter of its original value, i.e. 160/2 = 80, 80/2=40. This is two half lives, So the bone is 5600 x 2 =11200 years old. Counts due to bones are = 160 (fresh) and =40 (ancient) The count rate of the carbon-14 has fallen to one quarter of its original value, i.e. 160/2 = 80, 80/2=40. This is two half lives, So the bone is 5600 x 2 =11200 years old. click

Nuclear Power When a nucleus decays it gives out heat energy. In a nuclear power station, the uranium-235 atoms decay and give out energy and neutrons. Each time a uranium atom splits it produces 2 or 3 neutrons (depending on the reaction). These go on to hit other uranium atoms, which causes them to decay. A chain reaction is set up where more and more energy is released. In a nuclear reactor the process is carefully controlled so that neutrons are absorbed harmlessly and the energy released is controlled. In a nuclear bomb the reaction is not controlled, and the bomb explodes! When a nucleus decays it gives out heat energy. In a nuclear power station, the uranium-235 atoms decay and give out energy and neutrons. Each time a uranium atom splits it produces 2 or 3 neutrons (depending on the reaction). These go on to hit other uranium atoms, which causes them to decay. A chain reaction is set up where more and more energy is released. In a nuclear reactor the process is carefully controlled so that neutrons are absorbed harmlessly and the energy released is controlled. In a nuclear bomb the reaction is not controlled, and the bomb explodes!

Nuclear Power - fission Fast neutron from previous decay cause the Uranium nucleus to split.

Nuclear Power Kr Ba n n n n Fission Uranium In the reaction above a neutron from a previous decay can lead to more and more decays. This is called a chain reaction. More decays

Radioactive waste Radioactive waste from nuclear power stations is an environmental concern. The problem is the waste stays radioactive for thousands of years. The current solutions are: 1. Store it at the nuclear power station until is filled up. 2. Dump it far out at sea. 3. Store it deep underground in non-permeable rock.

Radiation questions 1.What are the three types of radiation? 2.Which type of radiation is the most penetrating? 3.Why is radioactive waste not stored in permeable rock? 4.Why should nuclear power stations not be situated in geologically active regions? Alpha, beta and gamma Gamma It could contaminate water that seeps through the rock. Earthquakes could cause radioactive spills.

During alpha decay, which of the following is true? A.Relative atomic mass increases by 2 B.Relative atomic mass decreases by 2 C.Relative atomic mass increases by 4 D.Relative atomic mass decreases by 4

During beta decay, which of the following is true? A.Atomic number increases by 1 B.Atomic number decreases by 1 C.Atomic number increases by 2 D.Atomic number decreases by 2

What fraction of a radioactive sample is left after 4 half lives? A.1/2 B.1/4 C.1/8 D.1/16