A2 Physics Planning Exercise OCR

Over a limited thickness of material, the count rate would be APPROXIMATELY exponential since it is the probability of beta penetration. Think of the particles as bouncing through the metal (coming out at all sorts of angles) and losing energy after each collision, the chance of it penetrating the next 'layer' is reduced according to the energy it has lost.

As for plotting graphs of ln's and 1/'s you are simply required to produce a graph to compare the count rate and thickness. If you plot ln's and 1/'s you are assuming you know the relationship. Also if you recieve your count rate during the manufacturing process, the computer will simply look accross the graph and find the x value, why does it have to be a straight line to do this?

Magnetic shielding is taken care of by putting the experiment (other than a slit for aluminium to pass through) in an iron box, 'absorbing' the magnetic field, thick enough to avoid saturation. If the box is thick enough the background count will only be of the gamma radiation, which will remain approximately constant(remember to take background count over a long period of time since it is NOT constant). If you take a background count with the front of the geiger tube completely open, then the background count will reduce according to the thickness of aluminium placed in front of it. My experiment is in the box the whole time so background count, made entirely of gamma, will remain approximately constant. Otherwise your taking away a background count which is actualy reducing each time.

This website gives average beta energy levels for strontium-90 and the according average penetration of thicknesses of aluminium to these energy levels. http://www.fas.harvard.edu/~scidemos/QuantumRelativity/PenetrationandShielding/PenetrationandShielding.html

Maybe this is a bit far into it but i think it helps. More questions?

Reply 61

bardobeing

yep will work fine

Reply 62

BaBy V

ok, can somone explain to me the difference between the procedure and the callibration, because from what i understand
the callibration is me changing the thickness of alluminium to produce my graph, while taking account of backgrouind
then what does the procedure want... random thickesses that i try to match up with the graph?

Reply 63

M15T3R CH4RL35

The procedure is how you would go about getting the raw data.
Calibration is converting the raw data into useful standardised data that can be applyed to a range of situations (not just the experiment).

E.g. I want to find how the magnetic flux density varies with the distance from a pole of a magnet, Using a hall probe.

The procedure would be measuring several distances (e.g. 0cm, 5cm, 10cm...etc) and taking readings of the current indicated by the hall probe at each distance (e.g. 50mA, 30mA, 10mA...etc). The current readings would be your raw values.

Calibration would be appling correct physics - that current is a function of magnetic field strength.

Thus to convert from mA to T you need a current reading from a hall probe in a flux density that is known. e.g. place it next to the pole of a 0.5T electromagnet and find the reading in mA (e.g. 150mA). Then applying correct physics with this conversion factor you can convert the raw mA values into T values and plot a graph of the data. This not only allows a relationship to be established, but also an interface applied to the hall probe so that it automatically displays flux density for any magnet rather than current using the derived conversion factor.

Reply 64

BaBy V

thanx .. lol so there is no other experiment other than the callibration then right?

Reply 65

M15T3R CH4RL35

Yes, 16 allocated marks for the single experiment. Don't sweat the small stuff though, it is only worth 1/4 of an A2 grade!

Reply 66

BaBy V

hey can you say that background is constant, therefore you will just get a systematic error?

Reply 67

fblade

14

BaBy V

hey can you say that background is constant, therefore you will just get a systematic error?

Why would the background constant count as an error?

Reply 68

BaBy V

i thought that it would be the same for each thickness, as it is detected by the GM counter each time ... but is it constant.
Also how do you callibrate the the apparatus... like the micrometer

Reply 69

Coolraj

5

BaBy V

hey can you say that background is constant, therefore you will just get a systematic error?

background radiation? yes i think so.

btw you guys ready for the OCR practical exam on Monday? Any ideas what experiment its going to be based on?

Reply 70

fblade

14

BaBy V

i thought that it would be the same for each thickness, as it is detected by the GM counter each time ... but is it constant.
Also how do you callibrate the the apparatus... like the micrometer