A radioactive source used in a school laboratory is thought to emit α particles and
γ radiation. Describe an experiment that may be used to verify the types of radiation
emitted by the source. The experiment described should allow you to determine how
the intensity of radiation varies with distance in air or with the thickness of suitable
absorbers.
Your answer should include:
l the apparatus you would use and any safety precautions you would take l the measurements you would make l how the measurements would be used to reach a final decision about the emitted
radiation.
Apparatus:
Source, lead screen, DM detector, stopwatch
Method:
1) with no source present switch on the counter for a
fixed period measured by the stopwatch and record the number of
counts
2) with the source present measure and record the distance between
the source and detector then switch on the counter for a fixed period measured by the
stopwatch and record the number of counts
for each count find the rate by dividing by the time
(longer recording times may be used at longer distances)
3) plot a graph of (corrected) count-rate against 1 / distance squared to show inverse square relationship
Analyis:
the presence of an α source is shown by a rapid fall in the count-rate when the source detector distance is between 2 – 5 cm in
air
the presence of a γ source is shown if the corrected count-rate is still
present when the source detector distance is greater than 30 cm in
air
Safety:
do not have source out of storage longer
than necessary
use long tongs,
use a lead screen between source
and experimenter.
Describe an experiment to determine the value of the Earth’s gravitational
field strength g using a simple pendulum and any other appropriate apparatus.
In your answer you should:
describe how you would arrange the apparatus
indicate how you would make the measurements
explain how you would calculate the value of g by a graphical method
state the experimental procedures you would use to ensure that your result was
accurate.
You may draw a diagram to help you with your answer.
The quality of your written communication will be assessed in your answer.
- Set up apparatus
- measure length of the string using a ruler
- Starting from t = 0, count the number of occilations in 20 seconds (ensure measuring complete ociliations + repeats and means)
- Repeat for 5 different lengths of wire ensuring they are sufficent length
- plot a graph of l against T^2 and find gradient
- use the formula 4*Pi^2 / g to calculate g
If a detector had been moved further away from a source in this experiment, discuss how this would have affected the results and accuracy.
Increasing the distance causes radiation to spread out causing a smaller count rate per interval due to the inverse square law
As there is a smaller total count rate per time interval, this increases uncertainty which cosnequently makes results less accurate
If a graph of ln(RC / minute–1) against time were drawn for results of the raditation experient, state what features of the graph would be similar and what features would be different to the graph with the decetor being closer to the source
Gradient would be the same;
Y - intercept would be lower;
Points would be more spaced out;
