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Detecting Radiation

Detecting Radiation. A Geiger counter is the most familiar tool for detecting radiation. The probe of this device contains argon gas. When radiation hits the gas it ionizes it, or knocks and electron off. Ar → Ar + + e -

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Detecting Radiation

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  1. Detecting Radiation • A Geiger counter is the most familiar tool for detecting radiation. • The probe of this device contains argon gas. When radiation hits the gas it ionizes it, or knocks and electron off. • Ar→ Ar+ + e- • These electrons falling off creates a weak electric pulse, which makes a speaker click.

  2. Half life • Half life is the time it will take for half the material to decay into radiation. • Unstable isotopes have a short half life (3.8 days for Radon-222) • (Carbon-10 has a half life of 19.2 s) • More stable ones have a longer half life (5715 years for Carbon-14) • Stable isotopes have no half life since they do NOT decay. (Carbon-12)

  3. More Half life • If two half lives pass… • the material is not gone • you actually have ¼ remaining • half is left after the first half life, then half of that after the second half life. • Not as much radiation is coming out (since there is less mass) but it is still there.

  4. Graphing half life mass of isotope 0 1 2 3 4 number of half lives

  5. Carbon dating • Dates of very old materials are determined using carbon-14 or C-14 dating. • It can only be used on things once alive. • This is done by measuring the number of radioactive C-14 isotopes.

  6. How it works • Radiation on this planet causes radioactive isotopes to form. • A known percentage of the carbon dioxide in the air contains the radioactive C-14 isotope. • This carbon dioxide is used to “build” all living things (plants use it for food, animals eat the plants etc.)

  7. Finding an age • The amount of C-14 in an object can be measured. • This amount is compared to the amount assumed to be there when it died. • You count the half lives to determine its age.

  8. Disputing Carbon Dating • Some people have stated the amount of radiation hitting the planet (causing the known percentage of C-14) has changed over time. • This would cause serious errors in these calculations.

  9. Potassium-40 dating • Rocks (never living) can also be dated if they have other certain isotopes. • K-40 decays into Ar-40. • When a rock is formed we can assume all gases would escape, so all argon in a rock should be the product of K-40 decay. • measure the K-40 and compare it to the Ar-40 and you can determine its age.

  10. Uranium-238 dating • U-238 decays into Pb-206 which is extremely rare. • If you have a rock with U-238 and Pb-206 present, you can assume the Pb-206 came from the decay of U-238. • Scientists have come up with the 4.5 billion year age of the planet using these methods.

  11. For example • If you measure 15 g of C-14 and you assume you started with 60 g, then the object is… • 11,430 years old • 60g30g15g (2 half lives) • 5715 years x 2 = 11,430 years

  12. Math • The equation is difficult to use, so instead we will read it off a graph. • Here is equation mf/mi = 1/ 2hl • Percentage left is current mass/initial mass x100 % = mf/mi x 100 • Multiply the number of half lives by the value of one half life to get an age.

  13. Problems • If you have 32% of a material left, how many half lives have passed? • 1.64 half lives • If you have 17% of Ra-223 left, how old is it? • 2.55 half lives x 11 days = • 28 days

  14. Problems • If original sample had 78 g of Indium-115, and you now have 13 g left; how old is the sample? • 2.6 half lives x 4.41x1014yrs • 1.1 x1015 years • A sample of Radon-222 is 9.4 days old. There are .27 g present, how much was originally present? • 1.5 g

  15. Problems • If you have 15 g of Tc-96, and you assume you started with 88 g, how old is the object? • If you have an 8.0 day old sample of Radon-222 and there are 25 g present, how much was there to start?

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