By measuring the argon content of many rocks that contain potassium, scientists can calculate the age of the rock.Radioactive substances will give out radiation all the time, regardless of what happens to them physically or chemically.They can even use this idea to detect when toothpaste tubes are full of toothpaste! Photographic radiation detectors - these make use of the fact that radiation can change the colour of photographic film.The more radiation that is absorbed by the film the darker the colour it will go when it is developed. As the half-life is very long for Carbon-14, objects that are thousands of years old can be compared to new substances and the change in the amount of Carbon-14 can date the object.If the radiation source is outside of the body it must be able to penetrate to the required depth in the body.(Alpha radiation can't travel through the skin remember! Uses in industry - one of the main uses for radioactivity in industry is to detect the thickness of materials.
Half-life - is the time it takes for half of the radioactive particles to decay.When an animal or plant dies it stops taking in carbon. If we compare the carbon-14 with that from a living thing, and knowing the half-life of carbon-14, the age of animal and plant remains can be calculated. Twelve out of every 1000 potassium atoms is the radioistope potassium-40.Its half life is a staggering twelve thousand years and decays to eventually form the stable argon atom.Alpha particles would not be able to go through metal at all, gamma waves would go straight through regardless of the thickness.Beta particles should be used, as any change in thickness would change the amount of particles that could go through the metal.