1. Higher Physics – Unit 3 3.5Dosimetry and Safety

2. Activity of Radiation The activity of a radioactive source is the average number of nuclei decaying per unit time. activity (Bq) number of nuclei decaying (no unit) time (s) 1 becquerel (Bq) = 1 decay per second

3. Absorbed Dose When tissue is exposed to radiation, the absorbed dose is the energy absorbed per unit mass of tissue. absorbed dose (Gray, Gy) energy (J) mass (kg) 1 gray (Gy) = 1 joule per kilogram

4. The risk of biological harm to tissue depends on absorbed dose type of radiation body organ / tissue exposed. Weighting Factor The radiation weighting factor (W R ) is a measure of the biological effect of the radiation. RadiationWeighting Factor α particle20 fast neutrons10 gamma rays1 Weighting factor has no unit.

5. Equivalent Dose The equivalent dose is a measure of the biological damage caused by radiation on living tissue. Equivalent dose is the product of absorbed dose and radiation weighting factor. equivalent dose (Sievert, Sv) weighting factor absorbed dose (Gray, Gy)

6. Equivalent Dose Rate Equivalent dose rate is the dose per unit time. equivalent dose rate (Sv s -1, Sv h -1 ) equivalent dose (Sievert, Sv) time (s, mins, h)

7. Background Radiation Every day we are exposed to small amounts of radiation known as background radiation. The average annual effective dose for people in the UK is 2 mSv. Background RadiationEffective Dose Cosmic rays from space.0.3 mSv Radioactivity from rocks & soil0.3 mSv Radioactivity from human body0.4 mSv Inhaled radon gas1.0 mSv

8. Exposure Limits For the general public, the limit is 5 mSv per year in addition to background radiation. Workers who are exposed to radiation during their employment, the limit is 50 mSv per year in addition to background radiation.

9. Half-Value Thickness Experiment to measure half-value thickness of an absorber lead discs G-M tube counter γ source

10. Method The count rate is measured with: no radiation source (background radiation) radiation source no absorber radiation source, one lead disc radiation source, two lead discs etc. The count rate per second is found by measuring the count over two minutes and calculating an average. The corrected count rate is the count rate less the background count.

11. Graph of Results The corrected count rate is plotted against absorber thickness. absorber thickness corrected count rate Conclusion Half value thickness of an absorber is the thickness of absorber which reduces the intensity of radiation by half.

12. Half-Value Thickness Calculations Example 1 The dose equivalent rate from a radioisotope is 64 μ Sv h -1. A barrier which is 120 cm thick is placed in front of the radioisotope. The dose equivalent rate falls to 8 μ Sv h -1. Calculate the half value thickness of the barrier.

13. 3 half value thicknesses

14. Example 2 The half thickness of lead for a particular gamma source is 11 mm. A 33 mm thick piece of lead is placed in front of the gamma source which has an unshielded dose equivalent rate of 60 μ Sv h -1. Calculate the dose equivalent rate of the shielded gamma source.

15. Questions 1.A gamma source has a corrected count rate of 256 counts per second. When 30 cm of material is placed between the source and detector the count rate falls to 64 c.p.s. Calculate the half value thickness of the material. 2.The half value thickness of water is 200 mm. A used fuel element has a dose equivalent of 40 Sv h -1. Calculate the dose equivalent rate when 1 metre of water is placed between the fuel rod and the detector. 15 cm 1.25 Sv h -1

16. Reducing Exposure The equivalent dose rate from a radioactive source is reduced by shielding (placing an absorber in the path of radiation) increasing distance from the source. The reduction in equivalent dose rate depends on the material and thickness of absorber used.