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Radioactive Dating

Radioactive Dating. Determining Absolute Time. Definitions. Absolute Time: numerical time using a specific units like years Isotopes: Form of an element with more or fewer neutrons than other forms of the same element Decay product: Element produced by the decay of a radioactive isotope

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Radioactive Dating

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  1. Radioactive Dating Determining Absolute Time

  2. Definitions • Absolute Time: numerical time using a specific units like years • Isotopes: Form of an element with more or fewer neutrons than other forms of the same element • Decay product: Element produced by the decay of a radioactive isotope • Decay-product ratio: Ratio between the mass of a radioactive element and the mass of the decay product

  3. How does radioactive decay work? • If an isotope is radioactive, it will break down naturally into a lighter element called a decay product. • This process occurs at a predictable rate and can be used to determine how old an object is.

  4. What is a Half-life? • Half-life: Time required for half of an element's atoms in a sample to change to the decay product. • In each half-life only half of the remaining radioactive atoms decay, no matter how large the sample is.

  5. Look at the diagram below which represents the radioactive decay of uranium-238. The shaded area represents the decay product which is lead-206. The half-life of uranuim-238 is 4.5 billion years, since this object has gone through two half-lives it is 9 billion years old.

  6. How can we determine the number of half-lives that have occurred? • Look at the decay-product ratio • This will help calculate the number of half-lives that have occurred since the sample was formed. • Once you know that you can find the age of the sample.

  7. Decay Product Examples • If an object is made up of 50 % decay product then the object has been through 1 half-life. • 75% decay product equals 2 half-lives • 87.5% decay product equals 3 half-lives • 93.76% decay product equals 4 half-lives, and so on. • Once you know the number of half-lives you can determine the age by using the half-life in years listed on the back of the Reference Tables.

  8. Make a chart to help clear things up • The chart on the next page gives information about the radioactive decay of carbon-14. • You could make a chart like this one for any radioactive decay question. • It will show how the half-lives, radioactive element (or decay product) and the age of the sample are related. • Try doing this to see if it will help you. • You will want to have the front page of your ERST handy

  9. Choosing the Best Radioactive Element for a sample • The sample to be dated must contain a measurable amount of a radioactive element and its decay product • A sample with the remains of living things will likely contain carbon-14 • A geologist must choose the radioactive isotope whose half-life best measure the age of the sample • Example: Using carbon-14 can only get you back 50,000 years, which is relatively very recent • More recent rocks wouldn’t have enough uranium-238 which can measure the oldest rocks on Earth

  10. Confused? • These notes are based on pages 132-133 in the RRB

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