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

Types of Ionizing Radiation

• Neutron– 1/4 mass of – Variable energy– Tissue penetration of 1-10 cm

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)

Radiation Units

• Activity

• Absorbed Dose

• Dose Equivalent

• Exposure

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?