Q14.

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:

the apparatus you would use and any safety precautions you would take
the measurements you would make
how the measurements would be used to reach a final decision about the emitted radiation.

(Total 6 marks)

The description expected in a competent answer should include a coherent selection of the following points:

Apparatus to be used:

• source,
• tongs for handling the source,
• lead screen,
• ruler,
• γ-ray and α-particle detector such as a Geiger Muller tube, rate-meter or counter
• stopwatch,
• named absorber of varying thicknesses (may be used).

Safety considerations - Safety references must appropriate to a school setting – not to recommend using a . Suitable lead lined gown for example. Suitable examples include:

• do not have source out of storage longer than necessary,
• use long tongs,
• use a lead screen between source and experimenter.

Measurements to be taken:

with no source present:

• switch on the counter
• for a fixed period measured by the stopwatch
• record the number of counts or record the rate-meter reading

with the source present:

• measure and record the distance between the source and detector (or thickness of absorber)
• switch on the counter for a fixed period measured by the stopwatch
• record the number of counts or record the rate-meter reading
• repeat the readings for different distances (or thicknesses of absorber) (longer recording times may be used at longer distances (or thickness of absorber) to improve results, as the count may be very near to background).

Display of data

• Results should be displayed in a table : data taken and calculations from that data
• if a rate-meter was not used for each count you have recorded you have to find the rate by dividing by the count by the time
• subtract the background count-rate from each measured count-rate to obtain the corrected count-rate
• plot a graph of (corrected) count-rate against distance (or thickness of absorber)
• plot a graph of (corrected) count-rate against reciprocal of distance squared or equivalent linear graph to show inverse square relationship in air

 

Analysis of results taken (this is a harder mark to achieve)

This may involve establishing an inverse square fall in intensity in air, or an exponential fall using thicknesses of lead. If a continuous distribution is not used, an absorber or distance in air that would just eliminate γ (30-50cm air / 3-6mm Al) must be used, with and without the source being present or compared to background

• the presence of an α source is shown by a rapid fall in the (corrected) 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 (or at a range beyond that of beta particles in any other absorber, e.g. 3 mm in Al)
• the presence of a γ source is best shown by the graph of (corrected) count-rate against reciprocal of distance squared being a straight line through the origin

High Level (Good to excellent) Answer (5 to 6 marks range)

The candidate refers to all the necessary apparatus and records the count-rate at various distances (or thicknesses of absorber).

The background is accounted for and a safety precaution is taken.

The presence of an α-source is deduced from the rapid fall in the count rate at 2 – 5 cm in air.

The presence of a γ-source is deduced from the existence of a count-rate above background beyond 30 -50 cm in air (or a range in any absorber greater than that of beta particles, e.g. 3 – 6 mm in Al) or from the intensity in air falling as an inverse square of distance or from an exponential fall with the thickness of a material e.g. lead.

The information should be well organised using appropriate specialist vocabulary. There should only be one or two spelling or grammatical errors for this high mark.

If more than one source is used, or a different experiment is described than the question set is answered,the mark is limited to 4 maximum marks.

Intermediate Level (Modest to adequate) Answer (4 to 3 mark range)

The candidate refers to all the necessary apparatus and records the count-rate at different distances (or thicknesses of absorber). The recording of data must be with a named instrument

A safety precaution is stated. Background must be used in some way either for a comparison or subtracted appropriately

The presence of an α-source is deduced from the rapid fall in the count rate at 2 – 5 cm in air .

The presence of the γ-source is deduced from the existence of a count-rate beyond 30 -50 cm in air (or appropriate range in any absorber, e.g. 3 -6 mm in Al).

Some safety aspect is described. One other aspect of the experiment is given such as the background.

The grammar and spelling may have a few shortcomings but the ideas must be clear.

Low Level (Poor to limited) Answer (1-2 mark range)

The candidate describes:

Recording some results at different distances (or thicknesses of absorber) and gives some indication of how the presence of α or γ may be deduced from their range.

Some attempt is made to cover another aspect of the experiment, which might be safety or background.

There may be many grammatical and spelling errors and the information may be poorly organised.