IDEA:
When a radioactive isotope decays, it creates a
decay product. By comparing the number of parent and daughter atoms
in a sample, we can estimate the amount of time since the sample was
created. In the animation, the radioactive
isotopes are represented by red circles,
the decay products are the blue circles and the neutral
isotopes are the green circles.
Click here to re-run the animation, click here to re-run the animation at a slower speed.
One of the most important tools in geology is radioactive decay. By
measuring the ratio of parent to daughter atoms in a mineral sample,
we can find the time at which a mineral formed. The amount of time it
takes for half of an parent isotope to turn into its daughter isotope
is called the half-life. If you know the
half-life of an isotope, and the amount of parent and daughter atoms
present in a sample, you can calculate the age, t, of the sample using:
is the decay constant, D is the number of
daughter atoms and N is the remaining number of radioactive
atoms. This age is an actual measurement of elapsed time, instead of a
relative measure (e.g., old, older, oldest);
we therefore call time
scales based on radioactive decay absolute time
scales.
However, it is important to remember that an absolute time scale relates to a measurable physical process, not that there are no errors in the measurement. There are many processes which can make a mineral appear to have a different age than it actually does. If daughter atoms can leave, or parent atoms can be added, then the mineral will appear to have a higher parent-daughter ratio, and so will appear younger than it really is. If parent atoms can leave or daughter atoms can be added, then the mineral will have a lower parent-daughter ratio than it should, and so will appear older than it really is. This can happen when the mineral reacts with other things, such as sea-water or ground water. A geochronologist would say that "the box wasn't closed".
How do we know when a given atom will decay? The half-life of an element measures the mean time it takes for half of the parent atoms to decay into daughters but it says nothing about the behavior of any given atom. Instead, the life-time of any given atom is essentially random; one atom may only last one half-life, whereas another may last several hundred half-lives. The mathematical laws that describe radioactive decay also describe a variety of other natural processes, such as rolling dice or the number of raindrops that hit in a square centimeter. Because of this, sometimes these other processes are used to model the decay process; in the animation shown above, we used a random number generator to determine when each particle would decay.
To do this experiment simulating radioactive decay, you will need:
