Unit 4 - Lesson 13: Half-Life Flashcards
When radioactive decay occurs, what happens to a radioactive nucleus?
It disappears.
Over time, what happens to the quantity of ionising radiation emitted by a radioactive substance?
Over time, a radioactive substance emits less ionising radiation. Although, it never reaches zero.
Because the ionising radiation emitted by a radioactive substance never reaches zero, what idea do we use to measure how quickly the activity drops off?
We use the idea of a half-life to measure how quickly the activity drops off.
What is Half-life?
Half-life is the time taken for half the radioactive atoms now present to decay.
OR
The time taken for the activity (or count rate) to fall by half.
If a substance has a short half-life, what does that mean for the rate of release of radiation?
The substance will disappear quickly, but radiation is released rapidly.
If a substance has a long half-life, what does that mean for the rate of release of radiation?
The substance will release radiation more slowly, but remains for a long time.
What do we use to measure how old something is?
Radioactive decay half-life.
What does radioactive decay half-life use to measure how old something is?
It uses the stable isotope of carbon, carbon-12, and the unstable isotope, carbon-14. All living things contain carbon-12 and carbon-14. Carbon-12 is the most abundant carbon isotope on Earth. We can compare the amount of carbon-12 and carbon-14 in a sample to see how old it is.
Where does carbon-14 come from?
High energy cosmic rays frequently strikes our atmosphere. Some come from our sun. Others, we don’t know where they come from. When cosmic rays hit atoms at the top of the atmosphere, they break apart nuclei, releasing neutrons. If the neutron hits other nitrogen atoms, it can cause nuclear transformations (element changes). This reaction produces carbon-14. The C^-14 bonds to oxygen in the atmosphere, creating CO2. Plants take in this CO2 for photosynthesis, animals eat the plants and then, suddenly, all plants and animals contain C^-14.
Describe the reaction between neutron and nitrogen.
If the neutron hits other nitrogen atoms, it can cause nuclear transformation (element changes). This reaction produces carbon-14.
How does C^-14 manage to spread everywhere?
C^-14 bonds to oxygen in the atmosphere, creating CO2. Plants take in this CO2 for photosynthesis. Animals eat the plants and then, suddenly, all plants and animals contain C^-14.
In living organisms, what is the carbon-14 continuously renewed by? When an organism dies, is the carbon-14 still renewed?
Cosmic rays and photosynthesis. When an organism dies, it isn’t renewed.
What is the half-life of carbon-14 (approximately)?
6000 years.
What is the half-life of carbon-12?
There is not one. It won’t break apart.
What are the problems with radiocarbon dating?
It assumes cosmic rays are hitting the atmosphere at a constant rate and amplitude.
It can’t measure things older than 50,000-60,000 years because after 10 half-lives, the amount remaining radioactive nuclei in the sample is too small to measure accurately.
QUESTION: The activity of a radioisotope is 800 Bq (the unit of radioactivity is Becquerels). Four hours later, it has dropped to 50 Bq. What is the half-life?
ANSWER:
800 / 2 = 400 (1 Half-Life)
800 / 2 / 2 = 200 (2 Half-Lives)
800 / 2 / 2 / 2 = 100 (3 Half-Lives)
800 / 2 / 2 / 2 / 2 = 50 (4 Half-Lives)
It takes 4 half-lives for the activity to fall from 800 to 50. As this happens over 4 hours, the half-life is divided by 4 (1 hour).
The half-life is 1 hour.
What happens to the amount of radioactive activity in the first half-life, then the 2nd half-life, then the 3rd half-life, and then the 4th half-life?
The amount of radioactive activity will reduce by half in the first half-life.
The amount of radioactive activity will reduce by a quarter with 2 half-lives.
The amount of radioactive activity will reduce by an eighth with 3 half-lives.
The amount of radioactive activity will reduce by a sixteenth with 4-half-lives.
What is the unit of radiation and who is it named after and why?
The unit of radiation is Becquerel (Bq). It’s named after the scientist Henri Becquerel, who discovered radioactivity. He left a chunk of uranium rock on a photographic plate in a closed drawer and discovered that it produced an image on the plate. This was incredible to him because light is needed to create an image. The rock had created its own source of radiation.
What can we use to measure the ionizing radiation produced from radioactive isotopes?
A Geiger-muller tube attached to a rate meter.
What is a Geiger-Muller tube? Describe its properties and features.
This tube is hand-held. It consists of a casing that conducts electricity. Normally this is glass with a conductive inner lining. Inside the tube (but not touching the casing) is an electrode that obviously also conducts electricity. On the front end of the tube is a window made of mica.
There is an electrode inside the Geiger-Muller tube (but not touching the casing). What is this electrode powered by?
A high-voltage supply.
In a Geiger-Muller tube, how is the case and electrode prevented from meeting?
The case and electrode are prevented from meeting by very low pressure, inert gas i.e. the two are insulated from each other.
On the front end of the tube is a window made of mica. If ionising radiation from a radioactive source enters through the mica window, what can it do to the inert gas in the tube? What is the result of this?
It can ionise the inert gas in the tube. This releases electrons that will allow a small amount of current to go from the electrode to the case, which is detected by a circuit.
What apparatus can be used to count the number of ionising particles/waves entering the Geiger-Muller tube?
A counter, stopwatch or ratemeter.
Is background radiation natural or man-made?
It can be either.
Cosmic rays can result in some background radiation. How do cosmic rays cause the production of carbon-14?
Cosmic rays have enough energy to break apart nuclei in the upper atmosphere. The atoms that are normally broken are nitrogen. They release neutrons and become carbon-14. Because of this, living organisms contain radioactive isotopes (C-14) also (as we eat plants that absorb C-14).
What are radioactive materials used in?
Medicine and to produce power so radioactivity results from this.
What apparatus can be used to demonstrate that different ionising radiation can penetrate through different materials?
A Geiger counter.
How can we investigate the penetrating powers of different kinds of radiation?
What results do we expect from this investigation?
- Measure the background radiation. Write down the number of counts after 5 minutes. Repeat 3 times and take an average.
- Place each material between the source of radiation and the Geiger window and measure counts for 5 minutes.
We expect the counter to produce less counts per second when a material is placed between the source and the Geiger-Muller tube. The gamma rays should produce the highest counts per second and the alpha particles the lowest.
When investigating the penetrating powers of different kinds of radiation, what should we do to be safe?
Handle with tongs away from the body.
Never point the material at someone.
Store the material in a lead lined box.
How can we measure the half-life of a radioisotope using a graph?
We can use a Geiger-Muller tube and rate meter to create an activity versus time graph. This can then be used to find the half-life of the substance.
METHOD:
1. Measure the background radiation using a Geiger-Muller tube.
2. Measure the activity from your sample. Do this by recording the count rate at regular time intervals over 5 minutes.
3. Subtract the background radiation measurement from the sample measurement.
4. Record your results in a table with three sections (the first one is Time t/min, the second one is Count Rate / Bq, and the third one is Corrected Count Rate C/Bq) /
5. Plot your results in a graph. Make sure your plots show the background radiation subtracted.
6. The half-life of the substance can be measured by halving the activity from the top plot point (y axis), moving across to the plot line and down to the time.
