Higher Physics Radiation Dosimetry

Revision Activity, A, of a radioactive sample, is no of decays per second, measured in Becquerels (Bq) Activity reduces over time, rate of radioactive decay is dependent on the half-life of the radio-isotope

Effects of Radiation Many early researchers into radiations (e.g. Henri Becquerel, Willhelm Roentgen, Marie Curie) died from cancers caused by exposure to radiation Most deaths caused by the nuclear weapons dropped on Japan at the end of WWII were caused by radiation sickness People in Ukraine continue to become ill due to radiation after the Chernobyl Nuclear Power Plant disaster in 1986

Effects of Radiation - Factors What factors affect the harm caused by radiation? Type of radiation Activity of radiation Time of exposure to radiation Type of tissue exposed to radiation

Absorbed Dose The absorbed dose, D, depends on the amount of energy absorbed per unit mass of tissue. The unit for absorbed dose is the Gray (Gy) 1 Gy = 1 J kg-1

Example A 70 kg man absorbs 2.1 J of energy from a radioactive source. Find the absorbed dose. D = E / m = 2.1 ÷ 70 = 0.03 Gy

Dose Equivalent 1 This measure gives a better measure of the biological harm done by exposure to different types of radiation. The absorbed dose is multiplied by the ‘radiation weighting factor’, wR , depending on the type of radiation

Dose Equivalent 2 Equation for Dose Equivalent - The unit for dose equivalent is the Sievert (Sv)

Radiation weighting factor (wR) Dose Equivalent 3 Radiation Weighting Factors, wR , for different radiations are shown below - Radiation Radiation weighting factor (wR) alpha particle 20 beta particle 1 gamma ray X-ray slow (thermal) neutrons ~3 high energy neutrons 10 high energy protons

Example A 70 kg man absorbs 2.1 J of energy from a alpha particle source. Find the equivalent dose. (D = 0.03 Gy , as before) H = D wR = 0.03 x 20 = 0.6 Sv

Dose Equivalent Rate 1 This takes into account the time of exposure Typical dose equivalent rates are measured in micro- or milli-Sieverts per year (mSv y-1 or µSv y-1)

Dose Equivalent Rate 2 Typical dose equivalent rate from background radiation = 2 mSv y-1 Typical dose equivalent rate in Aberdeen from background radiation = 12 mSv y-1 Maximum allowed dose equivalent rate from nuclear industry workers = 10 mSv y-1

Effective Dose In reality the effect also depends on the type of tissue exposed The parts of the body most susceptible to damage from radiation are - Eyes Reproductive organs Consideration of these factors leads to the ‘effective dose’