A-level Physics (Advancing Physics)/Risks, Doses and Dose Equivalents

Risk Radioactivity results in risk. This could be a risk of death, or a risk of developing cancer. In physics, risk is what you expect, on average, to happen: risk = probability × consequence risk So, if there is a 1 in 500 chance that someone gets run over by a car when crossing the road, the risk involved in allowing 500 people to cross the road is one accident. Absorbed Dose Absorbed dose is measured in grays, commonly denoted Gy. One gray is defined as one joule absorbed per kilogram. The equation for absorbed dose is: D = E m AD where E is the energy given off by the source, and m is the mass of the affected thing. To calculate the absorbed dose in terms of numbers of particles with a given frequency, wavelength or energy (in eV electronvolts, you may be expected to use the equations E = h ⋅ f = h ⋅ c0 λ E(γ) where h is the Planck constant, f the frequency of the photon, c0 the speed of light in free space, and λ the wavelength of the photon, or E = n ⋅ e Q where n is the number of particles, and e is the magnitude of the charge of one electron. If someone is exposed to a certain activity (particles per second) over a period of time, the absorbed dose accumulates. Dose Equivalent right|thumb|200px|Worlwide hourly dose equivalent due to cosmic rays per hour (in sievert). Absorbed dose does not give a full picture of the potential harm radioactivity can do to you. Different types of radiation do different amounts of damage. Absorbed dose equivalent, measured in sieverts (denoted Sv) attempts to compensate. To calculate the dose equivalent, multiply the dose in grays by the quality factor of the particles absorbed. These quality factors are given in the table below. Radiation Type Quality Factor photons and leptons electrons among other particles 1 neutrons with an energy > 10 MeV or < 10 keV, protons 5 neutrons with an energy between 10 and 100 keV, or between 2 and 20 MeV 10 neutrons with an energy between 100 keV and 2 MeV and atomic nuclei 20 The equation for the effective dose (ED) received is: effective absorbed = absorbed dose × quality factor H Questions A mobile phone emits electromagnetic radiation. 1.2 W watts of power are absorbed per kilogram. Assuming that the radiation is absorbed uniformly across a 5 kg kilogramme head, what dose of radiation would be delivered to the head when making a 10‐minute telephone call? What dose equivalent does this correspond to? How many nuclei are there in 1 mg milligramme of americium‑241? A ham sandwich becomes contaminated with 1 μg microgramme of americium‑241, and is eaten by an 80 kg kilogramme person. The half‐life of americium‑241 is 432 years. Given that americium‑241 gives off 5.638 MeV megaelectronevoltes alpha particles, how long would it be before a dose equivalent of 6 Sv sievert is absorbed, making death certain? What assumptions have you made? Worked solutions.