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

massof isotope

number of half lives0 1 2 3 4

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

Types of radiodating There are different types of

radiodating for different scenarios. For all radiodating techniques, the

amount of a radioactive isotope is compared to an amount present in the past.

The half life is used to determine the age

Carbon dating Dates of some materials can

determined using carbon-14 or C-14 dating.

Carbon dating can only be used on things once alive.

Carbon dating is ONLY useful for the recent past (50,000 years maximum)

This is done by measuring the number of radioactive C-14 isotopes.

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.)

Continuing Therefore all living things are made

up of a known percentage of C-14. Once that living thing dies, it stops

taking in new C-14 isotopes. The radioactive isotope begins to

decay at a known rate (half life of 5715 years)