Embed Size (px)
Citation preview
Syllabus points Each species of radionuclide has a half-life which indicates the rate of decay
This includes applying the relationship:
𝑁 = 𝑁01
2
𝑛
Learning goalsDefine:
half-life
activity
background radiation
Identify that a stable isotope does not have a half-life
Identify that each radioisotope has a unique half-life
Describe how a detection meters (e.g. Geiger counters) work
Identify that an activity measurement from a radioisotope needs to be corrected by subtracting background radiation
Use and rearrange the equations for activity 𝑁 = 𝑁0(1
2)𝑛 to
solve problems
Give the SI units for: Activity
Half-life Different radioisotopes decay at different rates
Radioactive decay is a spontaneous process that can’t be controlled
Each radioisotope has its own characteristic decay rate (called half-life)
The decay rate is unaffected by physical and chemical conditions (e.g. pressure, temperature)
Half-life (𝑡1/2) = the time it takes for half the atoms in a sample to
decay
Parent atoms remaining after certain number of half-livesFrom: http://www.kgs.ku.edu/Extension/geotopics/earth_age.html
ActivityActivity = number of atoms in a sample that
decay per second
Activity is measured in becquerels (Bq)
1 Bq = 1 decay per second
Activity will decrease over time, over one half-life the activity will halve
Detecting Radiation One instrument used to detect radiation is
a Geiger counter
It detects radiation such as alpha particles, beta particles and gamma rays
It consists of a pair of electrodes surrounded by a gas. The gas used is usually Helium or Argon. When radiation enters the tube it can ionize the gas. The ions (and electrons) are attracted to the electrodes and an electric current is produced. A scaler counts the current pulses, and one obtains a "count" whenever radiation ionizes the gas.
Example 1 A radioactive element has a half-life of 10 minutes.
Originally there are 8.0 * 1020 atoms in the sample of the element.a) How many atoms decay in 10 minutes?
b) How many atoms of the radioactive element remain after one hour?
Example 1 A radioactive element has a half-life of 10
minutes. Originally there are 8.0 * 1020 atoms in the sample of the element.a) How many atoms decay in 10 minutes?
Example 1 A radioactive element has a half-life of 10 minutes.
Originally there are 8.0 x 1020 atoms in the sample of the element.b) How many atoms of the radioactive element remain after one hour?
Half-life Equation
𝑁 = 𝑁01
2
𝑛
𝑁 = 𝑁01
2
𝑛
Original number of atoms
Number of atoms remaining
Number of half-lives
Half-life Equation
The activity is directly proportional to the number of radioisotopes. So you can also use activity or mass in the same equation, so long as the units of N and N0 are the same as one another.
𝑁 = 𝑁01
2
𝑛
Original number of atoms
Number of atoms remaining
Half-life Equation
Example 1 A radioactive element has a half-life of 10 minutes. Originally
there are 8.0x1020 atoms in the sample of the element.b) How many atoms of the radioactive element remain after one hour?
Example 2
The half-life of Zn-71 is 2.4 minutes. If one had 100 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?
Example 2The half-life of Zn-71 is 2.4 minutes. If one had 100 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?
Example 3
Os-182 has a half-life of 21.5 hours. How many grams of a 10 gram sample would have decayed after exactly two half-lives?
Example 3Os-182 has a half-life of 21.5 hours. How many grams of a 10 gram sample would have decayed after exactly two half-lives?
ResourcesFurther Reading
Award Ceremony speech about carbon dating by Willard Libby, winner of the Nobel Prize in Chemistry in 1960
Buy a Geiger counter kit for only US$125
AV
Tyler DeWitt - Nuclear Half-Life: Demo and Explanation (5:53)
Tyler DeWitt - Nuclear Half-life calculations (8:03)
How did you go? Define:
half-life
activity
background radiation
Identify that a stable isotope does not have a half-life
Identify that each radioisotope has a unique half-life
Describe how a detection meters (e.g. Geiger counters) work
Identify that an activity measurement from a radioisotope needs to be corrected by subtracting background radiation
Use and rearrange the equations for activity 𝑁 = 𝑁0(1
2)𝑛 to solve problems
Give the SI units for: activity
