## Australian Museum

Appendix Radioactive Dating The technique how to explain radioactive dating comparing the abundance ratio of a radioactive isotope to a reference isotope to determine the age of a material is called radioactive dating. Many isotopes have been studied, probing a wide range of time scales. The isotope 14 C, a radioactive form of carbon, is produced in the upper atmosphere by neutrons striking 14 N nuclei. The neutron is captured by the 14 N nucleus and knocks out a proton. Thus, we have a different element, 14 C.

The isotope, 14 C, is transported as 14 CO 2absorbed by plants, and eaten by animals. If we were to measure the ratio of 14 C to 12 C today, we would find a value of about one 14 C atom for each one-trillion 12 C atoms. Once living things die, they no longer can exchange carbon with the environment. The isotope 14 C is radioactive, and beta-decays with a half-life of 5, years. This means that in 5, years, only half of the 14 C will remain, and after 11, years, only one quarter of the 14 C remains.

Thus, the ratio of 14 C to 12 C will change from one in one-trillion at the time of death to one in two trillion 5, years later and one in four-trillion 11, years later. Very accurate measurements of the amount of 14 C remaining, either by observing the beta decay of 14 C or by accelerator mass spectroscopy using a particle accelerator to separate 12 C from 14 C and counting the amount of each allows one to date the death of the once-living things.

Perhaps you have heard of Ice Man, a man living in the Alps who died and was entombed in glacial ice until recently when the ice moved and melted. The man's body was recovered and pieces of tissue were studied for their 14 C content by accelerator mass spectroscopy. The best estimate from this dating technique says the man lived between and BC. The boat of a pharaoh was discovered in a sealed crypt and reassembled in a museum near the pyramids see Fig.

Its wood was dated using 14 C to be about 4, years old. Other methods of dating are used for non-living things. A potassium-argon method of dating, developed inmeasures the amount of 40 Ar arising from the 40 K decay and is compared to the amount of 40 K remaining in the rock. From the ratio, the time since the formation of the rock can be calculated. The age of our galaxy and earth also can be estimated using radioactive dating. Using the decays of uranium and thorium, our galaxy has been how to explain radioactive dating to be between 10 and 20 billion years old and the earth has been found to be 4.

The Universe must be older than our galaxy. Within experimental error, this estimate agrees with the 15 billion year estimate of the age of the Universe. Chapter Head Home Table of Contents Glossary Appendix. The technique of comparing the abundance ratio of a radioactive isotope to a reference how to explain radioactive dating to determine the age of a material is called radioactive dating. Radioactive Dating The technique of comparing the abundance ratio of a radioactive isotope to a reference isotope to determine the age of a material is called radioactive dating.