Transcript Radiation
Radiation Activity and Exposure Radiation: Energy (electromagnetic waves or particulates) Ionization: The removal of electrons from an atom Ionizing Radiation: Particles or rays with sufficient energy to remove electrons from atoms • The ionized atom causes changes • which MAY damage cells, • which MAY cause health effects Radiation dosimetry: quantitative description of the effect of radiation on living tissue Types of Detectors 1) Ionization chambers – Ion pairs are formed in gas as a fast charged particle passes through. Under the influence of an electric field, this generates a current. 2) Proportional counters Gas filled detector – introduced in 1940’s. Operated in pulsed mode- using amplification by avalanche breakdown in the gas. Can be used in very low count situations. 3) Geiger-Mueller counters Gas filled detector – Operated in pulsed mode- but with very high voltage Types of Detectors cont. 4) Scintillators (and Photomultiplier tubes) Converts kinetic energy of charged particles into proportionate amounts of detectable light via luminescence. Work predominantly through Photoelectric Effect. 5) Thermoluminescent detectors (TLD)/Semiconductor Diode detectors TLDs consist of a small crystalline dielectric. Heating the crystal releases the energy. 6) Film Silver bromide (AgBr) grains embedded in gelatin layer. 7) many many miscellaneous detectors • Activity: The change in the number of nuclei over a change in time. • Decay Constant: Proportionality constant relating the activity of a radioactive substance to the number of decaying particles • Half-life: the time interval required for the quantity to decay to half of its initial value •A common, but old unit of activity is the Curie (Ci), which was historically defined as the activity of 1g of pure 226Ra •1 Ci = 3.70 x 1010 decays/s or 3.70x 1010 Bq The SI unit for activity is the Becquerel (Bq), where 1 Bq is equal to one disintegration per second. Laboratory source normally have activities in the range kBq to MBq. Absorbed Dose is the energy absorbed per unit mass of material. Consequently, it varies for different materials. The SI unit is the Gray (Gy), the absorption of an average of one joule of energy per kilogram of mass in the target material. A more common unit is the rad. One gray equals 100 rads. The rate of energy deposition along the particle track is known as the linear energy transfer (LET). Types of radiation with a large linear energy transfer, such as heavy charged particles (eg alpha particles) cause greatest biological damage compared to radiation with a low linear energy transfer (eg electrons). Dose Equivalent measures the biological effect of radiation interacting in material. This is important to quantify since the effects of absorbing equal amounts of energy per unit mass can vary greatly depending on the type of radiation (particle). Sievert (Sv) – SI unit of dose equivalent rem – Roentgen Equivalent in Man; common unit of dose equivalent; 1 sievert = 100 rem Dose equivalent is related directly to the absorbed dose by a quality factor Q that characterizes the biological effect of a particular type of radiation. Dose Equivalent = Dose x Quality Factor Quality factor Q for different types of radiation Radiation type Quality factor Q X-rays, β, γ-rays ∼1 low energy protons and neutrons ∼2-5 high energy protons and neutrons ∼5-10 alpha particles ∼20 a measure of how damaging a given type of particle is when compared to an equivalent dose of x-rays US Average ~ 360 mrem/yr Denver, CO ~ 700 mrem/yr Brazil (beaches) ~ 5,000 mrem/yr General Public Dose Limit = 100 mrem/yr Occupational Dose Limit = 5,000 mrem/yr *Activity, Cause of Death • Smoking 1 cigarette Cancer, Heart Disease • Travel 50 miles by car Fatal Accident • Drinking 30 cans of diet soda Cancer (saccharin) • Eating 100 grilled steaks Cancer (benzopyrene) • Chest X-ray (10 mrem) Cancer • *Performing this activity increases your chance of dying by one in a million (1 x 10 - 6 ) Ionizing Radiation Overview • Can not see it, feel it, or smell it • - we must rely on training and equipment to protect ourselves • Relatively simple to detect and measure • - unlike chemical and biological hazards • - we can quickly assess and take action • Biological effects have been intensely studied • for 50 years http://www.ncnr.nist.gov/summerschool/ss03/ lectures/health_physics.pdf