Computers: Virtual Dating – C14

Go to: http://www.sciencecourseware.com/VirtualDating/files/RC0/RC_0.html

The radiocarbon dating method was developed in the 1940's by Willard F. Libby and a team of scientists at the University of Chicago. It subsequently evolved into the most powerful method of dating late Pleistocene and Holocene artifacts and geologic events up to about 50,000 years in age. The radiocarbon method is applied in many different scientific fields, including archeology, geology, oceanography, hydrology, atmospheric science, and paleoclimatology. For his leadership, Libby received the Nobel Prize in Chemistry in 1960.

1. The atomic mass number (the sum of protons and neutrons in the nucleus of an atom) for N-14 and C-14 is the same. How many protons does N-14 have and how many does C-14 have? a. 14 of each b. 7 & 7 c. 6 & 8 d. 7 & 6 e. 8 & 6

2. C-14 forms from the interaction of Cosmic rays with N-14 by____ and C-14 reverts back to N-14 by____ a. beta decay and neutron capture b electron loss and electron capture c. neutron capture and beta decay d. proton capture both ways e. no way to know

3. Of the three isotopes of carbon, which is most abundant and which is radioactive? a. C-12 & C-13 b. C-13 & C-14 c. C-14 & C-12 d. C-12 & C-14 e. no way to know

4. Generated in the upper atmosphere, C-14 makes up only 0.00000000010% of all carbon atoms. This is the same as 1 atom of C-14 per every____ atoms of Carbon. a. 1 thousand b. 1 million c. 1 Billion d. 1000 Billion or 1 Trillion e. no way to know

5. Because of interaction and exchange with atmosphere and oceans, all living tissue maintains a fixed proportion of C-14. After death, the amount of C-14 begins to decrease. a. True b. False

6. A C-14 date estimates the time since the death of once-living things, up to about 50,000 years. Which of the following materials CANNOT be dated by this method? a. bones b. wood c. soil d. pottery with imbedded charcoal e. volcanic ash from Pompeii

After the death of an organism, C-14 decays to N-14. The rate, as for all radioactive decay, is related to the amount of C- 14 present at any given time. In principle, any material that still has a sufficient amount of C-14 can be dated, with the date representing the time since the material stopped exchanging carbon with the atmosphere.

The illustration on the computer demonstrates how the amount of radioactive C-14 decreases with time. Use the numerical spinner below the illustration to change the illustrations through time. Each number is one half-life different in time than the preceding or following one. The daughter product, N-14, is not retained by a specimen because it is a gas and escapes as it forms. 7. What % of C-14 remains in a carbon-bearing material after 3 half-lives of time have gone by? a. 50% b. 25 % c. 12.5% d. 6.25% e. 0.39%

8. How many half-lives of time have passed when only a little more than 3% of a sample original C-14 still remains? a. 1 b. 3 c. 5 d. 7 e. unknown

9. How much daughter N-14 can be found in a buried ancient tree stump that is 4 half-lives old? a. 6.25% b. 93.75% c. 4% d. 96% e. Very little since N-14 is a gas and escapes

10. What fraction of original C-14 can be found in a sample after 10 HL? a. over 1/10 b. between 1/10 and 1/100 c. between 1/100 and 1/1000 d. less than 1/1000 e. none

The half-life of C-14 is 5730 years. EX: A sample that has decayed for three half-life’s 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 (disintegrations 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.

Directions on computer: Drag your mouse pointer on the red dot of C-14 decay curve below to see how % C-14 remaining and half-life are related. With 60% parent remaining, 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.

11. About what percent of modern C-14 remains in a sample that is two half-lives old? a. 2% b. 3% c. 10% d. 12% e. 25%

12. What percent of modern C-14 remains in a sample that is five half-lives old? a. 2% b. 3% c. 5% d. 25% e. 50%

13. What is the approximate age, in years, of a sample that contains 90 % of its original C-14? a. 900 b. 1950 c. 3000 d. 5000 e. 27500

14. What is the approximate age, in years, of a sample that contains 30 % of its original C-14? a. 4500 b. 6200 c. 7303 d. 9700 e. 15500

15. What is the approximate age, in years, of a sample that contains 3 % of its original C-14? a. 850 b. 1750 c. 3000 d. 5000 e. 28000

16. 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? a. 52,000 b. 57,300 c. 69,000 d. 74,000 e. 80,000