# Count rate vs pd across a gm tube

#### Fairydust07

3 radioactive sources each emit alpha, beta, and gamma radiation but have half-lives of seconds, minutes, and years respectively. I need to investigate how the count rate on a GM tube due to gamma radiation alone, depends on the potential difference across the GM tube. - given 3 sources, GM tube, standard radioactive apparatus and other apparatus available in a school lab. Also, show how the source was chosen and reasons for choice of half life. Ensure the count rates due to alpha and beta do not affect that due to gamma. (note: not supplied with equipment to make magnetic or electric fields)......can someone please help and explain?....a diagram would also aid....thanks (Worried)

#### physicsquest

PHF Helper
Choose the source with a half-life of secs .With this you will get more readings and sooner ; unless you want to wait for years to finish this! With higher no of readings, your accuracy will be greater.
Set up the GM counter with a suitable arrangement to vary the P.D. across it. Set the P.D. at some suitable value.
Set up the source at some suitable distance away. All the 3 types of radiation will strike the counter. Then use powerful magnets to set up a strong magnetic field perpendicular to the source- G.M direction. The alpha and beta radiation will be deflected away as they have +ve and –ve charges respectively. Gamma radiation being photons will not be affected. Continue adding magnets till there is no further change in reading. This implies that only Gamma rays are striking the counter now.
Now vary the voltage across the counter and take your readings. Don’t get exposed to radiation and turn into superman or something!

Fairydust07

#### Fairydust07

Could i use an absorber to get rid of alpha and beta instead?...probably a thick sheet of Al metal?..what about ensuring that the count rates due to alpha and beta do not affect that of gamma?..their count rates wont show up right? This confuses me because the question says that i need to show the control of other variables-ie how i ensure that the count rates due to alpha and beta do not affect that of gamma..this should not apply once i have gotten rid of alpha and beta should it?

#### physicsquest

PHF Helper
The problem with using a sheet is that you cannot know whether gamma also is absorbed and the increased reading using a thinner sheet could aslo be due to alpha and beta.
Please go thru my previous post carefully. When you use magnetic fields the alpha and beta are deflected away while gamma is not affected. When you increase the field a time will come when further increase in the field does not lead to a drop in reading. This demonstrates that you have got rid of them and thus taken care of the

"need to show the control of other variables-ie how i ensure that the count rates due to alpha and beta do not affect that of gamma".

Try increasing the distance also. alpha and beta particles don't travel as far as gamma in a medium (air here).

If permanaent magnets are inconvenient, try setting up an electro magnet and you can get the desired effect by increasing the current thru' its coil.

#### Fairydust07

I don't want to bring any more trouble but I'm not supplied with equipment to make magnetic or electric fields...that was especially noted..

#### physicsquest

PHF Helper
Well then you can use the earth's magnetic field can't you? But that would be too weak . You said no equipment for electric/ magnetic fields does that mean no magnets also?

In that case just increase the distance between the source and detector and use a thin sheet of suitable absorbing material ( you will have to experiment here though) and show that the reading doesn't change irrespective of whether the sheet is inserted in the path or not. Alpha and beta radiation are absorbed within short distances in air and the sheet will help further. If the reading changes we know that the sheet does affect it but cant guarantee that it absorbs all alpha and beta. If it doesn't change at all, we know there is no more of those two.
Alpha paricles can be stopped by thin paper sheets, and Beta by an Aluminium sheet a few mm thick.
You could first insert the thin paper sheet and show control of alpha then the Al sheet and show control of beta . Introduce more sheets till the reading does not fall further. Only this guarantees full elimination of those two.

#### monti

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