Radioactivity Lecture 7 Dosimetry and Exposure Limits
Radiation Exposure - Radiology The radiation impact on biological and genetic materials requires some protective measures!
Units for scaling the decay Classical Unit: 1 Curie [Ci] 1 dn dt decays s [ ] 10 Ci 3.7 10 Modern Unit: 1 Becquerel [Bq] 1 dn dt decay s [ ] Bq 1 The so-called dosimetry units (rad, rem) determine the amount of damage radioactive radiation can do to the human. They depend on the kind and nature of the incident radiation (X-rays, γ-rays, α-particles, β-particle, or neutrons). It also depends on the energy loss of the particular radiation and the associated ionisation effects in the human material.
Refresher example You have a radioactive sample that emits one alpha particle per second. What is the activity of the sample in Bq and in Ci? dn dt 1 1 decay 1 1 s dn dt 1 3.7 10 [ Bq] decays s [ ] 10 10 Ci 3.7 10 3.7 10 [ Bq] [ ] [ ] 11 Bq Ci 2.7 10 [ Ci] 10 The nature of the radiation doesn t make any difference on the activity!
Long lived radioactivity has a half-life of T 1/2 1.28 10 9 years its natural abundance is 0.0118 % γ β + β - Ar Ca Potassium-Argon dating method Ar
in Human Body The human contains ~ 7 10 20 radioactive particles, T 1/2 1.25 10 9 y. What is the activity in Becquerel and Curie? A( t) λ λ N( t) ln 2 T 1/ 2 0.69 1.25 10 9 0.69 9 7 [ y] 1.25 10 3.14 10 [ s] 1.53 10 17 [ s ] 1 A A 1.53 10 10700 17 [ ] [ ] 1 20 1 s 7 10 10700 s 10700[ Bq] 1.07 10 3.7 10 4 7 [ Bq] [ Ci] 2.9 10 [ Ci] 0.3[ µ Ci] 10
Radiological Units Dose: D E m Amount of energy E deposited by radiation into part of mass m. Equivalent Dose: H Q D Radiation independent dose Q is normalization factor Unit Rem or Sievert unit Rad or Gray: 1Gy 1J/1kg; 1 rad 0.01 Gy 0.01 J/kg
Biological Impact Measures Each kind of radiation has a different impact on material through the kind of physical interaction during the stopping of the radiation and the amount of energy transferred to biological system. Each kind of radiation has a specific weighting factor Q to determine the equivalent dose: Photons: Q1 Neutrons: E<10keV Q5 Neutrons: E>10keV Q15 Protons: Q5 Alphas : Q20 In radiology, in particular for medical applications, this is refined by the introduction of -part specific weighting factors for the different kind of radiation particles or waves!
Dose units in Radiology The Sievert (Gray) is a measure of biological effect. 1 Gray (Gy) 1 Joule/kg (Energy/mass) 1 Sievert (Sv) Gray x Q, where Q is a "quality factor" based on the type of particle. Q for electrons, positrons, and x-rays 1 Q 3 to 10 for neutrons, protons dependent upon the energy transferred by these heavier particles. Q 20 for alpha particles and fission fragments. Converting older units: 1 rad 1 centigray 10 milligrays ( 1 rad 1cGy 10 mgy ) 1 Gy 100 rad 1 rem 1 centisievert 10 millisieverts ( 1 rem 1cSv 10 msv ) 1 Sv 100 rem Nominal background radiation absorbed dose of 100 mrad/year 1 mgy/yr. Nominal background radiation dose biological equivalent of 100 mrem/year 1mSv/yr. Occupational whole limit is 5 rem/yr 50 msv/yr. 2.5 mrem/hr or 25 usv/hr is maximum average working level in industry. Exposure rate from Naturally Occurring Radioactive Material; an empirically derived conversion factor for Ra-226 decay series: 1.82 micror/ hour 1 picocurie/gram.
Annual Average Total Effective Dose Equivalent to the U.S. Population Natural Background, Radon 200 mrems Cosmic and Terrestrial source 56 Medical and Dental X-Rays 54(+~100) Internal Source, Tobacco Smoking 280 Other Consumer Products 10 Total, All Population 6 Total, Non-Smokers 360 2 msv 0.6 0.55 0.4 2.8 0.1 6.4 3.6
Radiation doses and regulatory limits (in mrem 0.001Rem0.00001 Sv 0.01mSv10µSv)
Comparing the Risks: Radiation, Smoking, and Driving Equivalent to Procedure Dose Chance of Death Number of Cigarettes Smoked Number of Highway Miles Driven Bone Marrow From Leukemia 131 I treatments for thyrotoxicosis 15 rems 3 x 10-4 2200 5357 Chest radiograph 10 mrems 2 x 10-7 1.5 3.6 Skull examination 78 mrems 1.6 x 10-6 11.4 28 Barium enema 875 mrems 17.5 x 10-6 128 313 A full set of dental X-rays using a high energy X-ray machine and E-speed film has a relative cancer risk equivalent to that of smoking ~2-3 cigarettes.
Internal γ Glowing from On average, 0.27% of the mass of the human is potassium of which 0.021% is radioactive with a half-life of T 1/2 1.25 10 9 [y]. Each decay releases an average of E avg 0.5 MeV β- and γ-radiation, which is mostly absorbed by the but a small fraction escapes the. Calculate, how many radioactive atoms are in your system!
in your mass of mass of the : m potassium in the : m 0. 0027 m mass of radioactive in the : m 0. 00021 m 5. 67 10 7 m g of 6.023 10 23 atoms m 5.67 10 to calculate N N m 7 m for 80 kg : N [ g] [ particles] 8.54 10 6.023 10 15 23 [ particles / g], you need the mass m 6.83 10 20 5.67 10 7 [ particles] m in gramm. N
Calculate the absorbed dose over an average human lifetime of t 70 y for this source of internal exposure. Dose D E t A E absorbed : ( ) m m Activity: A( ) λ N ln2 T N avg 1 2 ln2 15 1 D 70[ y] 9 ( 48.54 88 10 [ g ] m ) 1.25 10 [y] 11 2 2 D 1.66 10 MeV / kg 2.63 10 J / kg 2.63 10 Gy 10 2 2 D 9. 47 10 MeV kg 15. 10 [ J / kg] 15. 10 [ Gy] [ ] 19 : [ ] 1602. 19 with : 1 ev 1.602 10 [ J ] 05. [ MeV ] m [ ] [ ] [ ] [ ] [ ]
Exposure to other natural or manmade radioactivity Tobacco contains α-emitter 210 Po with T 1/2 138.4 days. Through absorption in bronchial system smoking adds 280 mrem/year to the annual dose of US population.
Medical radioactivity D h 8.33 10 A 1.36 10 N M From 32 to >3000 µrad/h 3000µ R 3600s 9 6 3000 10 10 3600s 15 Gy s 6.11 10 Gy Gy Bq 6.11MBq Gy s 6 18 A A 18 6.11 10 ( F ) T1/ 2 ( ) ( F ) 109.7 18 λ F ln 2 ln 2 18 18 18g 12 ( F ) N( F ) 1.7 10 g 1.7 pg 6.022 10 23 6 2 8.33 10 9 0.511MeV 60kg 60s 5.8 10 A 10 0.511 1.602 10 60kg 13 J A 1.36 10 15 Gy A 18 F Certainly much more taken about 4 hours after infusion and distributed via blood flow over entire.
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