Q14.
(Total 6 marks) The description expected in a competent answer should include a coherent selection of the following points: Apparatus to 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:
Measurements to be taken:
Display of data
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
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.
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