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Radioactivity question

Why does the detection of alpha particles suddenly drop when it G-M tube is 4 cm from the source? I know the air molecules 'absorbs' the alpha particles, but why the sudden drop as opposed to gradual drop, like in beta particles or gamma rays.

Reply 1

Mehh

8

Mauve

Why does the detection of alpha particles suddenly drop when it G-M tube is 4 cm from the source? I know the air molecules 'absorbs' the alpha particles, but why the sudden drop as opposed to gradual drop, like in beta particles or gamma rays.

Alpha particles have a lot of momentum in them. It takes a number of collisions with air particles to bring them to a stop, before they can finally absorb/steal electrons from the surroundings.
This means that it takes a certain distance for the alpha particles to lose their energy, which is what you measured.
However I must stress that the distance depends on the alpha source.

Reply 2

Mohit_C

10

A beam of alpha particles is a monokinetic beam. This means that all the particles have the same energy in them and so their ionizing abilities is the same for all; this can be seen when tracks of alpha particles are viewed in a bubble/cloud chamber i.e. the length of the tracks are all the same.
Basically as they all have the same energy, the distance they can reach up to is the same therefore when this distance is exceeded, it all stops all of a sudden.

Reply 3

sufiankane

2

Mauve

Why does the detection of alpha particles suddenly drop when it G-M tube is 4 cm from the source? I know the air molecules 'absorbs' the alpha particles, but why the sudden drop as opposed to gradual drop, like in beta particles or gamma rays.

when forming alpha [articles is that AS SOON AS IT HAS ENOUGH energy it just emits an alpha particle hence they all got same energy. Beta is different as loads go out at dfferent energies. since energy is = to distance it travels if they all got same energy then they all go same distance.

Reply 4

Eau

OP

15

sufiankane

when forming alpha [articles is that AS SOON AS IT HAS ENOUGH energy it just emits an alpha particle hence they all got same energy. Beta is different as loads go out at dfferent energies. since energy is = to distance it travels if they all got same energy then they all go same distance.

Why do alpha particles have the same energy while beta particles don't?

Reply 5

F1 fanatic

16

Mauve

Why do alpha particles have the same energy while beta particles don't?

beta decay produces 2 particles, electrons and neutrinos (+ the proton). The distribution of energies between these 2 can vary so that the electron can have a range of energies.

Reply 6

sufiankane

2

Mauve

Why do alpha particles have the same energy while beta particles don't?

aint 100% sure but ma teacher said that alpha particles need loads of energy, so it has to 'build up' and as soon as it has enough it emits it. it aint a decent one but best i cna do.

Reply 7

Mohit_C

10

I seem to have forgotten Radioactivity. Can someone just remind me the changes that occur in the nucleus for the 3 types of decay? For instance, alpha particle decay is when two protons and two neutrons are released. What is it when it's the beta + and beta - decay?

It just saves opening another thread.

Reply 8

F1 fanatic

16

Mohit_C

I seem to have forgotten Radioactivity. Can someone just remind me the changes that occur in the nucleus for the 3 types of decay? For instance, alpha particle decay is when two protons and two neutrons are released. What is it when it's the beta + and beta - decay?

It just saves opening another thread.

beta minus:

n \rightarrow p + e^- + \bar{\nu_e}

beta plus:

p \rightarrow n + e^+ + \nu_e

Reply 9

Mehh

8

Mauve

Why do alpha particles have the same energy while beta particles don't?

I don't like that at all.
The reason is because as F1 says.

First of all you have to understand that all nuclear reactions are like chemical reactions. They conserve energy. That is to say the reaction will release the same amount of energy each and everytime.

Alpha decay is what we call a two body problem. This means there is two 'degrees of freedom'. We can give energy to two places, the atom that spat out the alpha particle, or the alpha particle itself.
That energy must add up to the total energy released in the reaction. This reduces the degrees of freedom down to 1 (we can work out how much energy is in one particle if we know the other one, so you can't "Freely" change the energy of both).
Another constrain exists, its called Newton's 3rd Law, conservation of momentum. This reduces the degrees of freedom down to zero, as we know that momentum must be shared equally between them.

Now when we apply the same reasoning to the beta decay, a three body problem we find a problem. We only have two constrains (which reduce freedoms by 2), but have 3 particles (the Beta particle, the atom AND a neutrino [which we can't detect]).

In fact the existance of the neutrino was infered by a very very clever man called Wolfgang Ernst Pauli, over 25 years before its discovery, by this experiment.

Radioactivity question

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