radiation electromagnetic radiation ionizing radiation –capable of separating molecules into...
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Radiation
• Electromagnetic radiation
• Ionizing radiation– capable of separating molecules into cations
and anions– e.g. X-rays
• Non-ionizing radiation– doesn’t normally disassociate molecules– e.g. radio frequency (RF) rays
Atomic reminders• Nucleus of an atom contains 1 or more
protons, and 0 or more neutrons
• Neutrons and protons have essentially equal mass, but proton has positive charge while neutron is electrically neutral
• Elemental identity of an atom is determined by the number of protons (atomic number)– Why?
Atomic reminders (cont’d)
• Common form of an element has a certain mass number = protons + neutrons
• Other forms (isotopes) have the same number protons, but different numbers of neutrons
• These are often unstable and prone to decay
Radioactive Decay
• Unstable isotopes decay spontaneously, emitting various combinations of particles and energy
• Some of these decay processes change either the isotope or the elemental identity of the atom
• The rate of decay is characteristic of the starting isotope, and described by half-life
Decay Examples
• 6C14 7N14 + e + -– A neutron turns into a
proton, an electron & an antineutrino
• 92U238 2He4 +
90Th234
Radioactive Half-life
• Different nuclides (isotopes) are distinguished by different decay constants ()
• Radioactive Half-life the length of time required for half of the atoms in the sample to decay
• Decay is exponential: Nt = N0e-t
• So 0.5*N0 = N0e-t, t1/2 = -ln(0.5)/
Types of Ionizing Radiation
• Alpha– Nuclei of He atom (2 neutrons & 2 protons)– Heavy (6.642x10-4g) and slow (104 miles/s)– Lots of interactions with energy transfer (30-
100K ion pairs/cm air)– Travel only 1-8 cm in air, less in solids (like
skin)– Most hazardous to humans from inside
Types of Ionizing Radiation
• Beta– Electrons emitted from nucleus, charged
(usually negative)– Light (9.130x10-28g) and fast (SOL: 3x108m/s)– Fewer interactions (< 200 ion pairs/cm air)– Travel several meters in air, several cm through
human skin– Can cause damage under skin, eyes
Types of Ionizing Radiation
• Gamma– Electromagnetic ray (photon), neutral charge– Frequency characteristic of nuclide– Massless and fast (SOL)– Lower production of ion pairs than & – Different penetration for different materials:
• 5 cm in lead• 50 cm in water• 50,000 cm in air
Types of Ionizing Radiation
• X-Ray– Electromagnetic ray (photon), neutral charge– Frequency in X-ray portion of spectrum
(generally lower energy than )– Massless and fast (SOL)– Lower production of ion pairs than & – Penetration varies with energy
Radiation Measurement
• Geiger Counter– tube of readily ionized gas exposed to source– ions produced attracted to charged wire and
produce electrical pulse
• Scintillation Counter:– Detector materials emit light when struck by
radioactive emission– Photo tube detects light
Dosimeters
• Thermoluminescence Detector– Lithium fluoride exposed to radiation results in
electrons raised to higher energy states (where they tend to remain for a period of time)
– “Relax” with heat and measure emitted light
• Film Badge– Photographic film develops with exposure– Assess degree of exposure of film as f(t)
Activity
• The transformation (disintegration) rate of a radioactive substance– Curie (Ci)– Becquerel (Bq) - S.I. Unit
• 1 Bq = 1 disintegration per second (dps)
• 1 Ci = 3.7 x 1010 dps = 3.7 x 1010 Bq
Absorbed Dose
• A physical quantity which represents the energy imparted by radiation onto an absorbing material.– RAD (radiation absorbed dose):
• A unit of dose as energy per unit exposed mass• 1 RAD = 100 ergs/g = 0.01 joules/g• Doesn’t differentiate different types of radiation
– Gray (Gy) - SI Unit
• 1 Gy = 1 joule per kilogram = 100 rads
Dose Equivalent
• An expression of dose in terms of its biological effect.
• Takes account of the fact that, for a given absorbed dose, a radiation of one type and/or energy may give rise to a greater biological effect than a radiation of another type and/or energy.
• REM (Radiation Exposed Man):– REM = RAD * QF – QF = Quality Factor (10 for , 1-1.7 for , & x-ray)
Dose Equivalent (cont’d)
• QF is used to compare the biological damage producing potential of various types of radiation, given equal absorbed doses.
• The effectiveness of radiation in producing damage is related to the energy loss of the radiation per unit path length. – REM– SIEVERT (Sv) - S.I. Unit
• 1 Sv = 100 REMs
Exposure
• A quantity that expresses the ability of radiation to ionize air and thereby create electric charges which can be collected and measured– Roentgen (R)
• 1 R = 2.58 x 10-4 q/kg of air
Exposure Limits
• Regulated by NRC (10 CFR)• Not to exceed 0.1 rem/yr excluding
– background radiation– medical radiation (including voluntary
participation in research studies)
• Not to exceed 0.02 REM/hr (unrestricted)• Can apply for authorization up to 0.5
REM/yr– Demonstration of need
Health Effects
• Long-term, low dose• Short-term, high dose• Various studies available, e.g.
– http://www.doh.wa.gov/Hanford/publications/overview/overview.html
• Hiroshima and Nagasaki survivors widely studied, e.g.– http://www.hiroshima-cdas.or.jp/HICARE/en/index.ht
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• What happened at Hiroshima?
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