Radiation Protection Fundamentals and Biological Effects: Session 1

Transcription

1 Radiation Protection Fundamentals and Biological Effects: Session 1 Reading assignment: LLE Radiological Controls Manual (LLEINST 6610): Part 1 UR Radiation Safety Training Manual and Resource Book: Parts 5 and 6

2 Atomic structure and types of radiation Three atomic particles are of concern to health physics electrons: 1, ~1/2000 amu neutrons: 0, 1 amu protons: +1, 1 amu Long waves MHz VHF FM 100 MHz 10 8 VHF AM Radio, TV 500 MHz Radar UHF Microwaves 1000 MHz Far IR Thermal IR Electromagnetic spectrum Infrared Near IR Visible Frequency (Hz) Damage increases as the particle matter interaction increases mass charge 1000 m 100 m 10 m 1 m 10 cm 1 cm 1000 nm 1 mm 100 nm 10 nm 100 nm 10 nm Wavelength 700 nm 600 nm 500 nm 400 nm Ultraviolet X rays Gamma rays The gamma and x-ray fractions of the electromagnetic spectrum are relevant to health physics 1000 nm 1 nm 1 nm 1 Å 0.1 nm 0.1 Å E17008

3 Neutron excess or deficit in the nucleus is responsible for radioactive decay Heavy atoms (U) break up by releasing alpha particles (two protons and two neutrons) spontaneous fission (usually stimulated by n bombardment) gamma photon release Light atoms (T, Co-60) break up by beta emission electron capture gamma photon release E17025

4 Electrons interact with valence electrons and nuclei in one of two ways X-ray Electron 1 0 b Recoil electron Ionization occurs when the incident electron knocks loose a valence electron, leaving behind a charged atom and a mobile secondary electron Electron Target nucleus Tungsten X-ray Bremsstrahlung (x-ray emission) occurs when the incident electron is deflected by a nucleus E17009 The number of ionizations due to beta radiation increases with beta energy and the mass of the ablator

5 Neutrons can interact with matter only by direct collision Partial energy loss occurs with each collision Many collisions are required and several ion pairs are formed before the neutron loses its energy The largest energy loss per collision occurs when neutrons pass a medium of roughly equal mass (i.e., hydrogen-containing matter) water plastic body tissue The best shield for neutron radiation is hydrogenous matter E17026

6 Protons and alpha particles interact electrically with matter Protons do not penetrate matter to any extent because of their charge; these particles are not of concern to Health Physics Alpha particles are helium atoms that have lost their valence charge: +2 mass: 4 amu Cause more damage than any other form of radiation because alpha particles are massive Alpha particles do not penetrate matter: paper and skin stop these particles Alpha particles represent only an internal hazard E17027

7 Gamma radiation is released when excited nuclei relax 1 0 b MeV Parent nucleus Cobalt 60 Daughter nucleus Ni MeV Gamma rays 60 Co 27 0 b 1 60 Co Ni c Ni 0 b c 2.5 c Gamma photons have no mass and no charge Gamma photons deliver a whole body dose because they penetrate the entire body E17004

8 Gamma photon can be absorbed or scatter by matter c photon 1 0 b X-ray photon Photoelectric absorption direct collision knocks valence electrons out of orbit dominant below 0.5 MeV c photon Recoil electron c photon E17001 Compton scattering scattered, highly directional, lower-energy gamma photon depends on the number of electrons in absorber recoil electron emission dominant between 0.5 and 10 MeV

9 The energy of the gamma photon determines how it will interact with valence electrons and nuclei 1 0 b c photon Electron positron pair production +1 0 b 120 Z of absorber 100 Photoelectric 80 effect dominant v = x Compton effect dominant Pair production dominant v = l E ho in MeV

10 Radioactive materials decay at a characteristic rate Radioactive decay rate is exponential A = A 0 exp ( m t) where: m is the decay constant for a given radioactive material A is the activity (number of disintegrations/s) t is time Activity (A) = m * N where: N is the number of particles Half life is a convenient method to characterize a radioisotope A/A 0 = 1/2 and t 1/2 = ln(2)/m t 1/2 is the time required to lose half the original activity E17028

11 Terms and units used to describe radiation Radioactive nuclide unstable nuclide that tries to achieve a more-stable configuration by emitting energy (particles or e/m radiation) Ionizing radiation radiation with enough energy to ionize matter Half-life the time required for half of a given amount of radionuclide to decay Activity the rate at which radioactive nuclides disintegrate (dps, DPM) - 1 disintegration/s = 1 Becquerel (Bq) - 1 Ci = disintegrations/s (Bq) E16007

12 Ionizing radiation has both cellular and genetic effects in living organisms Cellular effects radiation ionizes water in cells to form free radicals (H +, OH ) free radicals attack proteins or DNA strands free radicals can recombine to make H 2 O 2 and poison the cell under normal circumstances the cell repairs itself Genetic effects damage to cells in the reproductive systems high radiation dose is required for mutations to occur No evidence for radiation-induced mutations in Hiroshima and Nagasaki survivors and their children Chronic exposure is less damaging than acute exposure because the body has a chance to repair the damage E17029

13 Radiation exposure limits and natural sources Radiation exposure is limited by law in the workplace to the following: Radiation worker exposure limit 5,000 mrem/year General public exposure limit 100 mrem/year Natural radiation sources and exposure levels to individuals are as follows: Cosmic rays (outer space) 45 mrem/year Terrestrial (natural minerals) 65 mrem/year Internal (elements in the body) 25 mrem/year G6704a

14 Effects of large acute exposures Dose (rem) Effect 5 No clinical effects 50 Minor blood chemistry changes 100 Minor radiation sickness in about 10% of population 150 Minor radiation sickness in about 25% of population 200 Radiation sickness in about 50% of population 300 Radiation sickness in all exposed, about 20% death rate within one month 450 About 50% death rate without medical treatment 500 Radiation sickness within 4 h, over 50% death rate G6708a

15 Typical ionizing radiation from other sources Dose (mr/yr) Source 4 Reading glossy magazines for 1 h/d from U and K 20 One chest x ray Wearing enameled jewelry 10 h/week from U Radon gas in tightly insulated home Foods and fertilizers from K and U 100 Flying 5,000 miles per month Dental x rays 1,000 2,400 Uranium-glazed dishes (Fiesta, orange) 2,000 5,000 Smoking 1 pack of cigarettes/day From Po and Pb produced by U decay G6707a

16 Terms and units used to describe radiation safety (Absorbed) dose (D) the amount of energy imparted by ionizing radiation to matter rad is the unit of absorbed dose Quality factor (QF) a weighting factor to account for the biological effectiveness of differing radiation QF = 1 for beta, x rays, and gamma rays = 20 for alpha particles = 3 to 10 for neutrons Dose equivalent (H) the amount of biological damage caused by ionizing radiation rem is the unit of dose equivalent H = D QF E16008

17 The aim of radiation protection is to reduce exposure to: As Low As Resonably Achievable (the ALARA principle) H = Dose rate time At LLE, time = number of target shots Closed access during shots E17006

18 The type of radiation determines which shielding to use Paper Plastic Lead Concrete ALPHA BETA X-RAYS GAMMA NEUTRONS < Thickness (cm) E17007