Radiation Physics and Dosimetry PDF

Radiation Physics and Dosimetry MPHY360 7. Prime quantities & their units in medical radiation dosimetry Prof. Azza Helal Dr Azza Helal 1) Exposure dose: amount of ionization per mass of air due to x and gamma rays. The SI unit of radiation exposure is the coulomb per...

Radiation Physics and Dosimetry MPHY360 7. Prime quantities & their units in medical radiation dosimetry Prof. Azza Helal Dr Azza Helal 1) Exposure dose: amount of ionization per mass of air due to x and gamma rays. The SI unit of radiation exposure is the coulomb per kilogram (C/kg), but in practice, the roentgen is used. Roentgen: traditional unit of exposure dose Amount of x or gamma rays which is required to produce ionisation of 1cc of dry air and librate positive and negative charge, each carries one esu (electrostatic unit) Dr Azza Helal 2) Absorbed dose: the amount of energy deposited by ionizing radiation in a substance. Units of Absorbed Dose; Gray & RAD Gray: Is the deposition of one joule of energy in one Kg of any medium by any type of ionizing radiation due to excitation & ionization (joules/ kg). (SI unit) Gy = 100 RAD. RAD: Is the deposition of one hundred ergs of energy in one gram of any material. (traditional unit) Dr Azza Helal  Joule is the amount of energy expanded in moving an object one meter against a force of one Newton. It is SI unit of energy. It equals 107 Erg Erg is the amount of energy expanded in moving an object one cm against a force of one dyne. It is CGS system of unit of energy (cm-gram-second system) Newton = 105 dyne  Biological effect of a given absorbed dose of any type of radiation will not be of same magnitude as the effect of same absorbed dose of anther radiation. It also differ with tissue type. For same dose to organ, alpha or neutron radiation will cause greater harm compared with gamma rays, x rays, or electrons because ionization events produced by alpha or neutron will be much more closely spaced. Dr Azza Helal  The variation in the magnitude of biological effects due to different types of radiation is described by "radiation weighting factor" for the specific radiation type. 3) Equivalent dose: a measure of radiation specific biologic damage in human. It is the absorbed dose to an organ, adjusted to account for the effectiveness of the type of radiation. It equals absorbed dose multiplied by a radiation weighting factor WR. 4) Effective dose: a measure of radiation and organ system specific damage in human. It is the absorbed dose to an organ, adjusted to account for the effectiveness of the type of radiation and the differences in tissue sensitivities. It equals absorbed dose multiplied by radiation weighting factor WR & tissue weighting factor WT. Dr Azza Helal Rem (Roentgen equivalent man): traditional unit for equivalent and effective dose Rem is the amount of any type of radiation which produces same biological effect as one RAD of x or gamma ray. Sievert (Sv)= 100Rem Sievert is SI unit for equivalent & effective dose.  Equivalent dose= absorbed dose X WR Effective dose = absorbed dose X WR X WT Equivalent dose X WT 5) Kerma “kinetic energy released per unit mass” Kerma is defined as the sum of the kinetic energies of all charged particles librated by indirectly ionizing radiation such as photons and neutrons in sample of matter divided by the mass of the sample. Kerma is measured by Gray As both absorbed dose and kerma use same units. Dr Azza Helal  So what is the difference between two quantities? While absorbed dose measures the energy deposited in unit mass of the material by any type of ionising radiation. Kerma measures amount of energy transferred from photons to electrons per unit mass for indirectly ionizing radiation (gamma rays, x rays, or neutrons).  At low energies, kerma approximately equals absorbed dose, since most of initial kinetic energies of all charged particles deposit their energy in sample at certain position (transfer & deposition of energy is virtually equal) However at higher energies, kerma is > absorbed dose because a photon may interact with tissue in one position & create electron that possesses enough energy to deposit energy at location away from interaction point (escape region of interest before deposit their energy) Dr Azza Helal 6) Radioactivity is the spontaneous emission of radiation in the form of particles or high energy photons resulting from a nuclear reaction. It is also known as radioactive decay, nuclear decay, nuclear disintegration, or radioactive disintegration. The amount of radioactivity is not measured by mass or number of the atoms. It is measured in term of number of disintegrations which takes place /second in that material (death rate). Unites of radioactivity Curie Radioactivity of a sample which decays at a rate of 3.7X1010 disintegrations /sec. Becquerel Radioactivity of sample which decays at rate of one dis/sec Ci=37GBq