Different radioactive elements have different half-lives. Why are the half-lives different for different elements? How is the half-life of a particular element measured?

315 views

[ad_1]

Different radioactive elements have different half-lives. Why are the half-lives different for different elements? How is the half-life of a particular element measured?

In: Chemistry
[ad_2]

Think of the half life as a measure of how stable the material is.

Imagine I have a row of bottles on a really narrow wall. I might expect that over time some will fall off. The half life would be the average time taken for half the bottles to fall off. If I have an even narrower wall (a less stable material) then I would expect half of the bottles to fall off in less time on average – a shorter half life.

Measuring the half life is just a matter of measuring the number of bottles on the wall at time A, then coming back some time later and measuring the number of bottles at time B. In actual fact it might be easier to watch the wall and record each time a bottle falls off and smashes – a decay event.

So half lives are different based on the stability of the atom in question. The stability is determined by size and ratio of protons to neutrons. Obviously for every atom and isotope these numbers are different. This causes the difference in stability and thus different half lives.

As for how half lives are found we take a sample and measure the radiation coming off a known amount. Knowing how much a single decomposition of the atom releases we can then calculate half life.

Or if you are cool just hold a lump of metal and count how long till half of it is gone/s

I’ll start by explaining that the nucleus is a tiny ball containing protons and neutrons; there’s forces that want to hold them together, and other forces that want to tear them apart, and that depends a lot on the nucleus’s size. There’s a very narrow window of nucleus size for stable atoms, and if you have too many or too few neutrons, the nucleus will want to split. That’s why two isotopes (nuclei of the same element with different numbers of neutrons) will have different half-lives. When that split happens, it typically releases energy in the form of radiation, and becomes one or more smaller atoms of different elements.

The rate at which that split happens is measured in half-lives, or the time it would take for a given sample to have half of its atoms decay into different elements. So say you’ve got 1000 atoms of Carbon14. Under normal circumstances, it would take 5,730 years for 500 of those to decay. The half-life is 5,730 years.

Some isotopes like xenon-124 are technically radioactive, but their half-life is 10^22 years. It’s quite stable. That can be measured by having a large quantity and counting the number of atoms that decay using fancy instruments that can pick up radioactive decay. Others only exist for fractions of a second in particle colliders. Those are measured similarly, by estimating what you’re putting in and what you’re getting out in terms of radiation and end isotopes.