Radiation: Effects and Safety A.K. Bhati Centre of Advanced Study in Physics Panjab University, Chandigarh
What is Radiation? Radiation is the process of emitting energy through a medium or space in the form of waves or particles
Radiation Ionizing Non-ionizing Directly ionizing (Charged particles) electrons, protons,alpha particles etc. Indirectly ionizing (neutral particles) photons, neutrons etc.
Decay of Radioactive Nuclei (b-, b+, a, g-rays) Production of Radiation Beams in Accelerators, Nuclear Reactors etc. Source of Radiations X-ray equipments Natural Radiations (Cosmic, Terrestrial and internal radiations) No. of disintegrations per unit time [Becquerel (Bq), Curie (Ci)] Strength of Source
Interaction with Matter • Uncharged Radiation Characterized by • Long distance without interactions • Major modifications during the stochastic interactions and therefore by exponential attenuation. • 2. Charged Radiations Characterized by • Many small interactions by continuous showing down • A finite range
Two Types of Events in Materials • Disordering of the crystalline structure due to atomic displacements, thermal spike effects or nuclear transformation. • 2. Ionization and electron transport which can cause changes in chemical or biological properties, increases conductivity, free radical formation and polarization or space change effect.
Action of Radiation at the Biological Level • Absorbed dose (D): Energy deposited by any type of radiation per • unit mass in any target material. • Unit: Joule per kilogram (J/Kg) or 1 gray (Gy) • Old unit: rad (0.01 Gy) 2. Equivalent dose (H): used to quantify the biological effect of dose to individual organs H = D x WR WR – Radiation weighting factor – associated with the energy distribution at the cellular level – 1 for x & g-rays and electrons, and 20 for a-rays 3. Effective dose (E): Takes into account the varying sensitivity of different tissues to radiation E = = WT – Tissue weighting factor – 0.12 for bone marrow – 0.05 for liver – 0.01 for skin
Effects of Radiation on Cells At the cellular level the radiation interacts with the DNA of cell. Excitations - consumes the most energy Ionization - Leads to biological effects via the disruption of chemical bonds. DNA damage can occur in two different ways: • Direct interaction – single and double stand breaks, base • damages, cross-links between different strands of DNA, protein cross-links and intercalations etc • Indirect interaction- Through radical formation, mostly in water , (H+ and OH-) OH- radical is highly reactive. There are many different chemical reactions involving the products of radiation interaction with water, e.g., • OH-+OH- H2O2
These changes can induce- • A correct repair of the DNA by the protection mechanisms of the cell. • An erroneous repair of the DNA leading to viable cell. One of the important mechanism for the induction of the cancer. • Effects of the irradiation on the organism - • Deterministic effect- • - Associated with the destruction of the great number of cells with in the organ in question. These are characterized by. • - The presence of the dose threshold • - Dysfunction of the organ • - A severity of the effect that is function of the dose and generally an immediate occurrence (few hours to few weeks). • 2. Stochastic effect- characterized by a modification at the cellular • level and • - the absence of a demonstrated threshold • - a probabilities of occurrence depending on the dose and a major • latency period.
Risk of cancer induction (based on survivors of the bombing of Hiroshima and Nagasaki): • Probability of cancer induction is linear without threshold. • The risk factor for fetal cancer induction is estimated to • 4.0 X 10-5 mSv-1 • The minimal latency period for cancer induction is of 2 years • for leukemia and 10 years for solid tumors. Hereditary effect of the radiation: No evidence in human population is observed. The risk factor for hereditary disorders induction is estimated to 1.0 X 10-5 mSv-1 (through the extrapolation of results on animals).
Principles and Methods of Radiation Protection • Aim of radiation protection- • to protect man and his descendants • the environment against the harmful effects of ionizing • radiation • Objective of radiation protection- • to prevent every deterministic effect of the radiation • to limit the probability of stochastic effects to an • acceptable level • Objective are achieved through- • Justification • Optimization • limitation • - of the individual doses
Accessibility to areas- • Supervised area: Work sector where a person can receive an annual effective dose higher than 1 mSv, but lower than 6 mSv. • Controlled area: Working sector where a person can receive an annual dose higher than 6 Sv • Protection methods- • External exposure- • - limitation of the duration of the stay in the radiation • field • - the holding of the greatest distance to the source • - the use of shielding between the source and the person • Exposure due to incorporation • - isolation of the radioactive substance (use of ventilation • hoods, glove boxes and recovery containers etc.) • - isolation of the person (use of laboratory cloths, gloves and • masks) isolate the substance-isolate the person.
Operational dosimetric quantities: representative of the limited quantities and easy measurable. • Ambient equivalent dose H*(10) – for the monitoring of external radiation 2. Individual equivalent doses Hp(10) and Hp(0,07) – for the personal dosimetry based on the wearing of a personal passive dosimeter or an active dosimeter. The monitoring of incorporation is based on the measurement of the radiation emitted by the incorporated radioactive substances or on the measurement of the urine activity (determination of the E50). • Occupational exposures – lead to an average individual dose of • the order of 0.1 mSv (1 mSv for nuclear power plants).
Legislation (radioactive protection): Comprises – - International recommendations - National laws and internal regulations of the company Objective of the legislation: - Protection of man and the environment - the principles of authorization for the use of ionizing radiations and of the surveillance by the authorities - Management of radioactive waste - Management of the organization in case of a radiological emergency
By an operation in supervise or controlled area the following practical rules are applicable: - to limit the access to these area to the work necessities; - to reduce the number persons in the controlled area; - to prepare the operation before entering the regulated area; - to wear the individual dosimeter and, if necessary, an operational dosimeter; - to use the radioactive waste container for small quantities of waste as mentioned on the containers; - to let control by the radiation protection group the contamination of material before removing it out of the area; - to obtain a special authorization for work in the accelerator tunnels; - not to drink or eat in the regulated area; - not to remove or displace warning panels.
Particular rules must applied in the following situations: - intervention with the risk of contamination; - use of radioactive source; - industrial radiography in the sector; - management of radioactive waste; - transport of radioactive material from the experimental site.