Radiological Protection

Radiological Protection For practical purposes of assessing and regulating the hazards of ionizing radiation to workers and the general population, weighting factors are used. A radiation weighting factor is an estimate of the effectiveness per unit dose of the given radiation relative a to low-LET standard. Gy (joule/kg) can be used for any type of radiation. Gy does not describe the biological effects of the different radiations. Weighting factors are dimensionless multiplicative factors used to convert physical dose (Gy) to equivalent dose (Sv) ; i.e., to place biological effects from exposure to different types of radiation on a common scale. A weighting factor is not an RBE. Weighting factors represent a conservative judgment of the envelope of experimental RBEs of practical relevance to low-level human exposure. Radiation Weighting factors Radiation Type and Energy Range Radiation Weighting Factor, WR X and γ rays, all energies 1 Electrons positrons and muons, all energies Neutrons: < 10 keV 10 keV to 100 keV > 100 keV to 2 MeV > 2 MeV to 20 MeV > 20 MeV Protons, (other than recoil protons) and energy > 2 MeV, α particles, fission fragments, heavy nuclei 1 [ICRU 60, 1991] 5 10 20 10 5 2-5 20 For radiation types and energies not listed in the Table above, the following relationships are used to calculate a weighting factor. [Image removed due to copyright considerations] [ICRP, 1991] Q = 1.0 L < 10 keV/µm Q = 0.32 L – 2.2 10 ≤ L ≤ 100 keV/µm Q = 300/(L)1/2 L ≥ 100 keV/µm L = unrestricted LET in water (keV/ µm ) Radiation Typical LET values 1.2 MeV 60Co gamma 250 kVp x rays 10 MeV protons 150 MeV protons 14 MeV neutrons Heavy charged particles 2.5 MeV alpha particles 2 GeV Fe ions 0.3 keV/µm 2 keV/µm 4.7 keV/µm 0.5 keV/µm 12 keV/µm 100-2000 keV/µm 166 keV/µm 1,000 keV/µm Tissue weighting factors Tissue Gonads Red bone marrow Colon Lung Stomach Bladder Breast Liver Esophagus Thyroid Skin Bone surfaces Remainder (ICRU 60, 1991; NCRP 116, 1993) Tissue Weighting Factor, WT 0.20 0.12 0.12 0.12 0.12 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.05 Dosimetric Quantities Quantity Exposure Absorbed dose to tissue T from radiation of type R DT,R Equivalent dose to tissue T HT Effective Dose E Definition Charge per unit mass of air 1 R = 2.58 x 10-4 C/kg Energy of radiation R absorbed per unit mass of tissue T 1 rad = 100 ergs/g 1 Gy = 1 joule/kg 1 Gy = 100 rads Sum of contributions of dose to T from different radiation types, each multiplied by the radiation weighting factor (wR) HT = ΣR wR DT,R Sum of equivalent doses to organs and tissues exposed, each multiplied by the appropriate tissue weighting factor (wT) New Units --- Old Units Roentgen (R) gray (Gy) Radiation absorbed dose (rad) Sievert (Sv) Roentgen equivalent man (rem) Sievert (Sv) rem E = ΣT wT HT Committed Equivalent Dose: for radionuclides incorporated into the body, the integrated dose over time. 50 years for occupational exposure, 70 years for members of the general public. Committed Effective Dose: effective dose integrated over 50 or 70 years. Sources of Radiation Exposure Annual average in U.S. 3.6 mSv Other 1%: Occupational; Fallout; Nuclear Fuel Cycle; Miscellaneous Annual estimated average effective dose equivalent received by a member of the population of the United States. Source Average annual effective dose (µSv) (mrem) Inhaled (Radon and Decay Products) 2000 200 Other Internally Deposited 390 39 Radionuclides Terrestrial Radiation 280 28 Cosmic Radiation 270 27 Cosmogenic Radioactivity 10 1 Rounded total from natural source 3000 300 Rounded total from artificial Sources 600 60 Total 3600 360 Radioactivity in Nature Our world is radioactive and has been since it was created. Over 60 radionuclides can be found in nature, and they can be placed in three general categories: Primordial - been around since the creation of the Earth Singly-occurring Chain or series Cosmogenic - formed as a result of cosmic ray interactions Primordial radionuclides When the earth was first formed a relatively large number of isotopes would have been radioactive. Those with half-lives of less than about 108 years would by now have decayed into stable nuclides. The progeny or decay products of the long-lived radionuclides are also in this heading. Primordial nuclide examples Half-life Nuclide Natural Activity (years) Uranium 7.04 x 108 0.72 % of all natural uranium 235 99.27 % of all natural uranium; 0.5 to 4.7 ppm total Uranium 4.47 x 109 uranium in the common rock types 238 1.6 to 20 ppm in the common rock types with a crustal Thorium 1.41 x 1010 average of 10.7 ppm 232 0.42 pCi/g (16 Bq/kg) in limestone and 1.3 pCi/g Radium 1.60 x 103 (48 Bq/kg) in igneous rock 226 Noble Gas; annual average air concentrations range in Radon 222 3.82 days the US from 0.016 pCi/L (0.6 Bq/m3) to 0.75 pCi/L (28 Bq/m3) Potassium 1.28 x 109 Widespread, e.g., soil ~ 1-30 pCi/g (0.037-1.1 Bq/g) 40 Natural Radioactivity in soil How much natural radioactivity is found in an area 1 square mile, by 1 foot deep (total volume ~ 7.9 x 105 m3)? Activity levels vary greatly depending on soil type, mineral make-up and density (~1.58 g/cm3). This table represents calculations using typical numbers. Natural Radioactivity by the Mile Activity used Nuclide in calculation 0.7 pCi/gm (25 Bq/kg) Uranium 1.1 pCi/g (40 Bq/kg) Thorium Potassium 11 pCi/g (400 Bq/kg) 40 1.3 pCi/g (48 Bq/kg) Radium 0.17 pCi/gm (10 kBq/m3) Radon soil Mass of Nuclide 2,200 kg 12,000 kg 0.8 curies (31 GBq) 1.4 curies (52 GBq) 2000 kg 13 curies (500 GBq) 1.7 g 1.7 curies (63 GBq) 11 µg 0.2 curies (7.4 GBq) Activity “Single” primordial nuclides • At least 22 naturally occurring single or nor-series primordial radionuclides have been identified. • Most of these have such long half-lives, small isotopic and elemental abundances and small biological uptake and concentration that they give little environmental dose. • The most important is potassium-40. Potassium is a metal with 3 natural isotopes, 39, 40 and 41. Only 40K is radioactive and it has a half life of 1.26 x 109 years. Chain or series-decaying primordial radionuclides Radioactive series refers to any of four independent sets of unstable heavy atomic nuclei that decay through a sequence of alpha and beta decays until a stable nucleus is achieved. Three of the sets, the thorium series, uranium series, and actinium series, called natural or classical series, are headed by naturally occurring species of heavy unstable nuclei that have half-lives comparable to the age of the elements. Important points about series-decaying radionuclides • 3 main series • the fourth (neptunium) exists only with man-made isotopes, but probably existed early in the life of the earth • the 3 main series decay schemes all produce radon (but primary radon source, the longest half-life, is the uranium series). Series name Thorium Begins 232 Th T1/2 Ends Gas (T1/2) 1.4 x 1010 yr 208 220 Pb Rn (55.6 sec) thoron Uranium 238 4.5 x 109 yr 206 222 Actinium 235 7.1 x 108 yr 207 219 U U Pb Pb Rn (3.8 days) radon Rn (4.0 sec) actinon Uranium 238 decay scheme. • Branching occurs when the radionuclide is unstable to both alpha and beta decay, for example, 218Po. • Gamma emission occurs in most steps. [Image removed due to copyright considerations] Major characteristics of the radionuclides that comprise the natural decay series for 232Th, 235U, and 238U Natural 232Th decay series Nuclide Half-lifeb Principle mode of decayc 232 1.4E+10 y 5.75 y 6.13 h 1.913 y Th Ra 228 Ac 228 Th 228 Natural 235U decay series Natural 238U decay series Nuclide Half-lifeb Principle mode of decayc α β β α 235 7.0E+08 y 1.06 d 3.3E+04 y 2.2E+01 y α β α α (1.4 %) α α α β α (36%) 227 U Th 231 Pa 227 Ac 231 Nuclide Half-lifeb Principle mode of decayc 238 U Th 234 Pa 234 U 4.5E+09 y 24.10 d 1.17 min 2.5E+05 y α β β α 234 β (98.6 %) 224 Ra Rn 216 Po 212 Pb 212 Bi 220 3.66 d 55.6 s 1.5E–02 s 10.64 h 1.01 h Th Fr 223 Ra 219 At 219 Rn 18.7 d 21.8 min 11.43 d 56 s 3.96 s α β α α α 230 Th Ra 222 Rn 218 Po 218 At 7.5E+04 y 1.6E+03 y 3.85 d 3.1 min 1.5 s α α α α α 215 Bi Po 215 At 7.6 min 1.8E–03 s 1.0E–07 s β α α 214 Pb Bi 214 Po 27 min 19.9 min 1.6E–04 s β β α 211 36.1 min β 210 1.30 min β 211 25.2 s α 210 22.6 y β 211 2.14 min α 210 5.01 d β 207 4.77 min β 210 138.4 d α 207 (stable) (stable) 206 8.2 min β 206 4.20 min β 206 (stable) (stable) 223 226 β (64%) 212 Po Tl 208 Pb 208 3.0E–07 s 3.053 min (stable) α β (stable) 215 Pb Po Bi Tl Pb 214 Tl Pb Bi Po Hg Tl Pb b c y–years; d–days; h–hours; min–minutes; and s–seconds. α–alpha decay; β–negative beta decay; EC–electron capture; and IT–isomeric transition (radioactive transition from one nuclear isomer to another of lower energy). Cosmogenic Radiation Cosmogenic Nuclides Nuclide Half-life C-14 Tritium Be-7 Source Cosmic-ray interactions, 5730 yr 14 N(n,p)14C Cosmic-ray interactions with N and 12.3 yr O; spallation from cosmic-rays, 6 Li(n,alpha)3H 53.28 Cosmic-ray interactions with days N and O Specific Activity ~15 Bq/g 1.2 x 10-3 Bq/kg 0.01 Bq/kg Some other cosmogenic radionuclides are 10Be, 26Al, 36Cl, 80Kr, 14C, 32Si, 39Ar, 22 Na, 35S, 37Ar, 33P, 32P, 38Mg, 24Na, 38S, 31Si, 18F, 39Cl, 38Cl, 34mCl. Track structure of a cosmic ray collision in a nuclear emulsion Variations in cosmic ray intensity at the earth’s surface are due to: • Time: sunspot cycles • Latitude: magnetic field lines • Altitude: attenuation in the upper atmosphere Dose at the surface from cosmic rays [Image removed due to copyright considerations] Internal Radiation What makes a radionuclide biologically important? • Abundance (both elemental and isotopic) • Half-life • Decay scheme (emission type and energy) • Chemical state • Chemical behavior in the body • Does it concentrate? • Ultimate location • Rate of excretion How do the series radionuclides contribute to our dose? Inhalation Isotopes of radon (inert gas, but may decay in the lung) Dust; e.g., our main source of uranium is due to resuspension of dust particles from the earth. Uranium is ubiquitous, a natural constituent of all rocks and soil. Externally- gamma emission occurs in most decay steps. Internally-Consumption in food and drinking water Natural Radioactivity in the body Nuclide Total Mass of Nuclide Total Activity of Nuclide Uranium Thorium Potassium 40 Radium Carbon 14 Tritium Polonium 90 µg 30 µg 17 mg 31 pg 95 µg 0.06 pg 0.2 pg 30 pCi (1.1 Bq) 3 pCi (0.11 Bq) 120 nCi (4.4 kBq) 30 pCi (1.1 Bq) 0.4 µCi (15 kBq) 0.6 nCi (23 Bq) 1 nCi (37 Bq) Daily Intake of Nuclides 1.9 µg 3 µg 0.39 mg 2.3 pg 1.8 µg 0.003 pg ~0.6 µg It would be reasonable to assume that all of the radionuclides found in your environment would exist in the body in some small amount. The internally deposited radionuclides contribute about 11% of the total annual dose. Uranium • Present in all rocks and soil, and thus in both our food and in dust. • High concentrations in phosphate rocks (and thus in commercial fertilizers). • Absorbed by the skeleton which receives roughly 3 µSv/year from uranium. Radium • Also present in all rocks and soils. • Food is a more important source of intake • 226Ra and its daughter products (beginning with 222Rn) contribute the major dose components from naturally occurring internal emitters. • Dissolves readily, chemically similar to calcium. • Absorbed from the soil by plants and passed up the food chain to humans • variations in Ra levels in soil lead to variations in Ra levels in food • 80% of the total body Ra is in bone (~7mrem/year). • The rest is uniformly distributed in soft tissue. Thorium • Lots in dust but little is incorporated in food • Thorium is present in the highest concentrations in pulmonary lymph nodes and lung, indicating that the principle source of exposure is due to inhalation of suspended soil particles. • Ultimately a bone seeker with a long residence time • Since it is very slowly removed from bone, concentration increases with age. Lead • Also a bone seeker, half-life in bone is ~ 104 days. Polonium • Unlike other naturally occurring α-emitters, 210Po deposits in soft tissue not bone. • Two groups exist for which the dose from 210Po is apt to be exceptionally high. o Cigarette smokers o Residents of the north who subsist on caribou and reindeer. • Reindeer eat lichens that absorb trace elements in the atmosphere (210Po and 210 Pb). The 210Po content of Lapps living in northern Finland is ~12 times higher than the residents of southern Finland. • Liver dose in the Laplanders is 170 mrem/year compared to 15 mrem/year for those in the south. Doses from Medical Applications Commercial Air Travel Calculated cosmic ray doses to a person flying in subsonic and supersonic aircraft under normal solar conditions Supersonic flight at 62,000 (19 Subsonic flight at 36,000 ft (11 km) km) Route Flight Dose per round trip Dose per round trip Flight duration duration (mrad) (µGy) (mrad) (µGy) (hrs) (hrs) Los Angeles11.1 4.8 48 3.8 3.7 37 Paris 8.3 3.6 36 2.8 2.6 26 Chicago-Paris 3.1 31 2.6 2.4 24 New York-Paris 7.4 New York7.0 2.9 29 2.4 2.2 22 London Los Angeles-New 5.2 1.9 19 1.9 1.3 13 York 4.4 44 6.2 2.1 21 Sydney-Acapulco 17.4 Issues: • Should airline people be considered general public? or radiation workers? • What about corporate aviation? (altitudes almost as high as supersonic Concorde but travel is sub-sonic and thus time in air is high) • Business travelers: frequent fliers have no restriction of # hours per year in flight. • What about pregnant women? • Should the traveling public be alerted to sunspot activity? • Is legal action possible?