| The half-life of C-14 is 5730 years. A sample that has decayed for
three half-lifes is 3 x 5730 years or 17,190 years old. |
| To determine how much C-14 is in a sample, radiation detectors count
the number of BETA particles released by radioactive decay. The result
is called the ACTIVITY (disintigrations per minute per gram
of carbon--dpm/g). Modern samples have an activity of about 12.5 dpm/g.
Older samples have values less than the modern value. C-14 measurements
are often expressed as "% modern", which is the ratio
of sample activity to modern activity. For example, a % modern value
of 25% means that the sample contains 25% of its initial C-14. |
| The sample size required for this method depends on the type of material,
but is generally in the tens to hundreds of grams range. Very small
samples just don't have enough C-14 to be reliably measured. The smallest
C-14 activity measurable with most methods is about 0.2 % modern, which
corresponds to about 9 half-lives. |
| A Few Questions to Answer |
| Drag your mouse pointer on the red dot of C-14 decay curve
below to see how % C-14 ramaining and half-life are related. With 60%
parent remaing, the half-life value should be a little over 0.7. Use
the C-14 decay curve and the calculator to help you answer the study
questions. The calculator has the Half-life of C-14 (5730 years) built-in
and allows you to convert fractions of Half-lives to years. For example
a sample with only 90% residual C-14 is about 0.16 HL old, which from
the calculator is over 900 years. |
| 3. |
What is the approximate age, in years, of a sample that contains 90
% of its original C-14? |
| |
900
1950
3000
5000
27500 |
| 4. |
What is the approximate age, in years, of a sample that contains 30
% of its original C-14? |
| |
4500
6200
7303
9700
15500 |
| 5. |
What is the approximate age, in years, of a sample that contains 3
% of its original C-14? |
| |
850
1750
3000
5000
28000 |
| 6. |
0.2% is the smallest activity that a sensitive detector can measure
from the emitted beta particles of C-14 decay. Using this value, what
is the approximate maximum age in years that can be determined with
the C-14 method? |
| |
52,000
57,300
69,000
74,000
80,000 |