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 an electron off. Ar → Ar + + e - These electrons falling off creates a weak electric pulse, which makes a speaker click.
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)
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.
Graphing half life mass of isotope number of half lives
Radiodating The age of materials can be determined if you are capable of comparing the amount of radioactive isotope present now, to the amount of radioactive isotope present at some past date. Since the half life is a constant rate, you can calculate its age.
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.
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 60g30g15g (2 half lives) 5715 years x 2 = 11,430 years
Math The equation is difficult to use, so instead we will read it off a graph. Here is equation m f /m i = 1/ 2 hl Percentage left is current mass/initial mass x100 % = m f /m i x 100 Multiply the number of half lives by the value of one half life to get an age.
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
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 4.41x10 14 yrs 1.1 x10 15 years A sample of Radon-222 is 9.4 days old. There are.27 g present, how much was originally present? 1.5 g
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 day old sample of Radon-222 and there are 25 g present, how much was there to start?
Homework If original sample had 28 g of Polonium-214, and you now have 13 g left; how old is the sample?