Quick Question on Radioactivity

Hey.

I was just looking over my syllabus for my exam on Wednesday, and it asks to know the link between the range of a radioactive radiation, its penetration and its ionisation ability. I've got an idea as to how they work, but I can't nail it specifically, for the penetration. What would you say?

My thoughts are:

- Range and ionisation ability: If it has a high ionisation ability, then it will have a short range. This is because when ionising, the radiation loses energy. As it has less energy, it can travel less far.

How could I improve this last sentence? I'm guessing the radiation loses kinetic energy on collision. Thus it will have a lower speed, and according to the equations of motion, it will travel a smaller distance.

- Penetration and ionisiation ability: As established above, the greater the ionisation ability, the less energy. But how is it that a high energy means its able to penetrate through a thicker material?

- Penetration and range: More range, more penetration. But that's because if there is more range, then there is less ionisation ability; which leads me back to the point above...

Please help!

Reply 1

teachercol

9

Alpha beta and gamma all have about the same energy

Alpha is strongly ionising so it gives up all its energy quickly in a short distance and is prro at penetrating

Beta is poorer at ionising so it takes longer to lose all its energy and is better at penetrating.

Gamma gives up all its energy in a single collision with an atom - but isnt very likely to do it so it has a long range and is very penetrating. .

Reply 2

Theorist

OP

10

teachercol

Alpha beta and gamma all have about the same energy

Alpha is strongly ionising so it gives up all its energy quickly in a short distance and is prro at penetrating

Beta is poorer at ionising so it takes longer to lose all its energy and is better at penetrating.

Gamma gives up all its energy in a single collision with an atom - but isnt very likely to do it so it has a long range and is very penetrating. .

OK, but why is it that for alpha, it's loss of energy means that it cannot travel that far (could you explain in terms of like kinetic energy etc.)?

Furthermore, what is the specific reason as to why radiations of high energy are very penetrating?

Reply 3

F1 fanatic

16

Dharma

OK, but why is it that for alpha, it's loss of energy means that it cannot travel that far (could you explain in terms of like kinetic energy etc.)?

Furthermore, what is the specific reason as to why radiations of high energy are very penetrating?

An alpha particle is ionising, meaning that it easily gives energy to other atoms when it collides with them. If I take away the alpha particles kinetic energy and give it to something else it isn't going to go very far. As you say KE=1/2mv^2 so that it'll travel slower and lose all it's energy through collisions in a very small space of time. Eventually it's energy will be low enough for it to gain electrons and just become a helium atom like any other.

Same reason for high energy particles really, if they have high energy, they travel faster as they have high kinetic energy and so they travel further. If you fire a bullet into a wall it will travel further if it has higher energy (and is therefore going faster). It's the same principle here. You can even relate it back to the SUVAT equations if you like and think of it as a deceleration from a higher velocity compared to a lower one. This doesn't hold perfectly for atoms etc because of them behaving funny, but it's a good enough way of thinking about it at this level.

Reply 4

phys981

10

This doesn't really answer your question but it's on the same theme.

There's an old (Edexcel) exam question where you calculate the velocity of an alpha particle given its kinetic energy. Then you calculate how long it takes to pass through an atom of diameter 10^-10m or something.

Then you compare this with the speed of a beta particle of the same kinetic energy. Clearly, the beta particle must be going much faster because its mass is much less.

The last part of the question asks you to relate this to ionisation - and the answer they want (though I suspect the truth might be a little more complicated) is that the alpha particle, travelling more slowly, spends more time in the atom and therefore has more chance of causing an ionisation. The beta particle, going much faster, spends far less time in the atom so is less likely to cause ionisation.

Quick Question on Radioactivity

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