Half-life and radioactive decay
In a nutshell
Radioactive decay is completely random. The amount of nuclei in a radioactive substance will slowly decrease as it decays and the time it takes for the substance to reach half the original number of nuclei is called its half-life. This lesson will discuss half-life and radioactive decay, how to calculate the activity of an isotope and how to calculate the half-life of a substance.
Equations
Description | Equation |
Activity and number of nuclei relation | |
Exponential decay of nuclei | N=N0e−λt |
Exponential decay of activity | A=A0e−λt |
Half-life and decay constant relation | λt1/2=ln(2) |
Definitions
Keyword | Definition |
Half-life | The time it takes for the amount of nuclei in a radioactive substance to decrease by half. |
Activity | The rate of radioactive decay. |
Isotope | The names given to a group of nuclei with the same number of protons but differing numbers of neutrons. |
Variable definitions
Quantity name | symbol | derived unit | si unit |
half life | | | |
activity | | | |
decay constant | | | |
number of nuclei | | | |
Decay and half-life
On average, radioactive decay is completely random. There is no way to predict which nucleus in a sample will decay next and every nucleus has the same chance of decaying per unit time. The decay of the nuclei is also not affected by any external factors, nor does it depend on other nuclei.
The half-life t1/2 of a substance is the time it takes for the number of nuclei in a particular sample to decrease by half. A sample decays exponentially. After one half-life, the amount of nuclei will be halved and after two, the amount of nuclei will be halved again. So, after two half-lives, the amount of nuclei will be a quarter of the original.
A substance with a very short half-life will decay quickly, releasing lots of radiation to begin with and quickly giving off less and less as the sample decays. A substance with a long half-life will release radiation over a longer period of time, during which the number of nuclei will decrease slowly.
Half-life from a graph
The graph below shows the activity of a sample decreasing with time.
The half-life t1/2 can be determined by finding the point on the y-axis where activity has dropped to half of its original value 2A0. A line can be drawn to the curve, and then from the curve down to the x-axis to determine at what time this value of activity occurs. This time is the half-life t1/2.
Activity
Activity A is also called the rate of radioactive decay and is measured in Becquerels, given the symbol Bq. 1Bq indicates that one nucleus decays every second. The activity of a source will exponentially decay over time, and the half-life also refers to the time it takes for the activity to halve.
Example
Carbon-14 has a half-life of 5700 years. What percentage of carbon-14 will remain after 17100 years?
17100 years corresponds to three half-lives of carbon-14, indicating that the amount of carbon-14 will have been halved three times in that time.
12.5% of the original carbon-14 will remain after 17100 years.
The decay constant
For a sample with a number of undecayed nuclei N, the number of nuclei that are decaying ΔN is proportional to both N and the time elapsed Δt. This is the case because a sample with less nuclei will have a lower activity than one with more nuclei.
ΔtΔN∝−N
This value ΔtΔN is the activity of the source and can be written as a constant for a known isotope. This is known as the decay constant, given the symbol λ. This equation can now be rewritten in terms of activity and decay constant, which has units s−1.
A=λN
Example
There are 2.0×1016 undecayed nuclei in a source with a decay constant of 1.0×10−9s−1. What is the activity of the source?
First, write down the known values:
N=2.0×1016
λ=1.0×10−9
Next, write down the equation for activity:
A=λN
Substitute in the values and calculate the final answer:
A=(1.0×10−9)×(2.0×1016)
A=2.0×107Bq
The sample has an activity of 2.0×107Bq.
Exponential decay
The solution to the decay equation relating time and number of undecayed nuclei is given by the following equation.
N=N0e−λt
where N0 is the initial number of undecayed nuclei, e is the natural logarithmic base (with a value of 2.718) and t is the time elapsed. Since activity is directly proportional to number of undecayed nuclei, A can be rewritten in an exponential form as well.
A=A0e−λt
where A0 is the sample's initial activity.
Decay constant and half-life are linked together by these logarithmic equations. The first equation shows that one half-life has passed when N is equal to 2N0, so t must equal t1/2 at that time. Substitute these values back into the equation.
2N0=N0e−λt1/2
21=e−λt1/2
This equation can then be rewritten without the negative power.
2=eλt1/2
Finally, take the natural logarithm of both sides to arrive at the final equation.
λt1/2=ln(2)
Example
What is the half-life of a sample with a decay constant of 1.0×10−9s−1?
First, write down the known values:
λ=1.0×10−9
Next, write down the equation and rearrange for half-life:
λt1/2=ln(2)
t1/2=λln(2)
Finally, substitute in the values and calculate the answer:
t1/2=(1.0×10−9)ln(2)
t1/2=22.0 years
The half-life of the sample is 22.0 years.