How do scientists know the half life of a substance when the half life is longer than the time we have known about the substance?
In: Physics
By studying the radiant particles that are produced as it decays and analyzing the way it decays and the elements that are produced. Basically it’s math and equations.
To make an analogy, imagine that we know that inside of a box are 100 mouse traps with ping pong balls balanced on them, and that each time a mouse trap changes states, a ping pong ball is lost (as it’s tossed off the trap). We can’t see inside the box and so we can’t see how quickly this happens on it’s own. But there is a funnel at the bottom with a hole cut out, and we can count how often the ping pong balls are ejected. By knowing the average of how quickly balls are radiated, and how many traps were in the box to begin with, we can work out how quickly they will decay.
For simplicity sake, imagine that element A becomes element B with no intermediate elements. If you know how many atoms are in your sample, what they decay into and the mass of the decayed atoms, then you can measure the radioactive counts to determine how quickly it’s happening. From that you can estimate on average how long it would take for that sample to convert half of it’s atoms to the new element.
It helps to think about what radiation actually is. It’s the bits that are lost when a heavier element decays into a lighter element. It’s made up of the bits and pieces that don’t fit in the new element and are ejected or radiated out. Alpha radiation for instance is a helium nucleus. Beta radiation is basically an electron (or positron). Gamma is basically really energetic light. There is also neutron radiation which is exactly what it sounds like.
The point is that by knowing the equations for radioactive decay, we know what will be radiated when it occurs, and we can then count how often that radiation is occurring in order to measure the speed it’s decaying. All of that information together gives us the half life.
In real life, there are often several elements that a heavy element will decay into, and those elements may in turn be unstable and also decay. There are also different kinds of radiation depending on what is decaying. There are also neutrons which can actually collide with non radioactive elements, and make them radioactive. So the math can get pretty complex.
[this image](https://qph.fs.quoracdn.net/main-qimg-ad2ea8c669e7138e5ee166947866c71e.webp) shows how Uranium 235 decays into several different elements. Notice that each is linked with the symbols for alpha and beta. This shows what particle is given, and what the new element is, how that element decays and what particles it gives. Some give both alpha and beta particles and there is more than one way they can decay (more than one path from uranium to lead). Notice there are some common problematic elements there like Radon, which can be produced naturally as uranium decays. It’s a gas and collects in pockets under ground and it can seep up to the surface, which is why in areas of the country where there are uranium minerals, radon detectors are so important in places like basements.
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