Physics experimentsGRADE 9-10+ 138430-EN | Radioactivity Alpha particles, the spark detector Objective In this experiment we study the alpha radiation from a radioactive source. We will investigate what it takes to stop the radiation and we will find the range of alpha particles in air. Principle The spark detector has two electrodes carrying a high voltage – just below the threshold for spontaneous sparking. When an alpha particle pass through the air between the electrodes, some of the air molecules are ionized. The ions and the free electrons will trigger a spark. Different materials can be placed between the source and the detector and the distance can be varied. 44 Sample results The graph shows how the range in air is reduced when a number of cling film layers are inserted in front of the detector. One layer of this particular film reduces the range with 6.7 mm. This can be compared with the thickness of the foil as declared on the package or even better with a measu- red thickness. It turns out that the plastic foil in this case is equivalent to an air layer about 900 times thicker. (This ratio is not far from that between the density of the two materials.) Equipment list The complete list below may conveniently be refer- red to as one of the following item numbers: 138430A (option A: For Risø sources) 138430B (option B: For disc sources). 138430C (option C: For cylindrical sources). The photo shows Option A. Item no. 514100 514120 514110 512110 367060 105740 105741 118530 118540 # Note Text (1) a Experiment bench, f. Risø sources, including absorbers Option A (1) a Experiment bench, f. disc sources, including absorbers Option B (1) a Exp. bench, f. cylindrical sources, including absorbers Option C 1 Spark detector 1 High voltage supply (0-6 kV) 1 Safety cable 100 cm, black 1 Safety cable 100 cm, red 1 Aluminium foil, 20 m roll 1 Cling film, 60 m roll Notes a) Please select one option. Regarding the three ty- pes of sources mentioned, please see p. 45 Also required — Am-241 source. Please see p. 45 for details — Paper (normal copier paper, 80 g/m 2 ) frederiksen-scientific.comPage 1Page 3

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