IONIZING RADIATION

RADIOLOGICAL PHYSICS The science of ionizing radiation and its interaction with matter Special interest in the energy absorbed Radiation dosimetry quantitative determination of that energy

HISTORY

Discovery of x rays by Wilhelm Rontgen Radioactivity by Henri Becquerel Radium by the Curies in the 1890s Research on food irradiation began as early as 1905 HOW IT STARTED

FOOD IRRADIATION Year Event 1905 Scientists receive patents for a food preservative process that uses ionizing radiation to kill bacteria in food. 1921 U.S. patent is granted for a process to kill Trichinella spiral is in meat by using X ray technology. 1953 1980 The U.S. government forms the National Food Irradiation Program. Under this program, the U.S. Army and the Atomic Energy Commission sponsor many research projects on food irradiation. 1958 The Food, Drug, and Cosmetic Act is amended and defines sources of radiation intended for use in processing food as a new food additive. Act administered by FDA. 1963 FDA approves irradiation to control insects in wheat and flour. 1964 FDA approves irradiation to inhibit sprouting in white potatoes. 1964 1968 The U.S. Army and the Atomic Energy Commission petition FDA to approve the irradiation of several packaging materials. 1966 The U.S. Army and USDA petition FDA to approve the irradiation of ham. 1971 FDA approves the irradiation of several packaging materials based in the 1964 68 petition by the U.S. Army and the Atomic Energy Commission. 1976 The U.S. Army contracts with commercial companies to study the wholesomeness of irradiated ham, pork, and chicken. 1980 USDA inherits the U.S. Army's food irradiation program. 1985 FDA approves irradiation at specific doses to control Trichinella spiral is in pork. FDA approves irradiation at specific doses to delay maturation, inhibit growth, and disinfect foods, including vegetables and 1986 spices.the Federal Meat Inspection Act is amended to permit gamma radiation to control Trichinella spiral is in fresh or previously frozen pork. Law is administered by USDA. 1990 FDA approves irradiation for poultry to control salmonella and other food borne bacteria. 1992 USDA approves irradiation for poultry to control salmonella and other food borne bacteria. 1997 FDA's regulations are amended to permit ionizing radiation to treat refrigerated or frozen uncooked meat, meat byproducts, and certain food products to control food borne pathogens and to extend shelf life. USDA's regulations are amended to allow the irradiation of refrigerated and frozen uncooked meat, meat byproducts, and certain 2000 other meat food products to reduce the levels of food borne pathogens and to extend shelf life.fda's regulations are amended to permit the irradiation of fresh shell eggs to control salmonella.

PRODUCE On August 22, 2008 the FDA published a final rule that allows the use of irradiation to make fresh iceberg lettuce and fresh spinach safer and last longer without spoiling. This final rule will permit the irradiation of fresh iceberg lettuce and fresh spinach to a maximum absorbed dose of 4.0 kgy, which is effective in reducing microbial pathogens that have been associated with these crops in the past.

TYPES AND SOURCES OF IONIZING RADIATION

IONIZING RADIATIONS Characterized by their ability to excite and ionize atoms of matter with which they interact The energy needed to cause a valence electron to escape an atom is of order of 5 25 ev Radiations must carry kinetic or quantum energies in excess of this magnitude to de called ionizing It includes electromagnetic radiation with wavelength up to about 320 nm

ELECTROMAGNETIC SPECTRUM

GAMMA-RAYS Electromagnetic radiation emitted from a nucleus The quantum energy of any electromagnetic photon is [kev]: hν 1.2398 kev nm Eγ = = λ λ h = Planck's constant = 4.136x10 c = velocity of -18 kev s light 'in vacuo' = 2.998x10 8 m/s

GAMMA-RAYS The quantum energy of a photon of 0.1 nm wavelength is? 12.4 kev Practical range of photon energies emitted by radioactive atoms extends from 2.6 kev K α characteristics x rays from electron capture in 37 18 Ar to 6.1 7.1 MeV gamma rays from N 16 7

X-RAYS Electromagnetic radiation emitted by electrons in changing atomic energy levels (called characteristic or fluorescence x rays) or in slowing down in a Coulomb force field (continuous or bremsstrahlung x rays)

X-RAYS A single electron can emit an X ray photon having an energy upon to its own kinetic energy There are two different atomic processes that can produce X ray photons: Continuous X rays: Bremsstrahlung, meaning "braking radiation ; Discrete X rays: K shell emission; they can both occur in heavy atoms like tungsten

SCHEMATIC OF A X-RAY TUBE Under operation, the filament boils off electrons Electrons are accelerated toward the anode in a strong electric field Electrons are brought to rest emitting continuous X rays in all directions

TYPICAL CONTINUOUS X-RAY λ λ A& 12400 = E ev 12400 50000 A& = = (min) 0.348 o A Peak voltage of X ray

DISCRETE X-RAYS Relative intensity Kβ Kα 100 kev Wave length (A) Electron from higher shells fill the inner shell vacancies Characteristic X ray are superimposed on the continuous spectrum Designated Kα, Kβ, and so forth when the K shell vacancy is filled by an electron for L shell, M shell, and so on

X-RAY AND GAMMA-RAY PHOTON Have identical properties Different mode of origins

FAST ELECTRONS If positive in charge positrons If emitted from a nucleus beta rays (+ or ) If they result from a charged particle collision delta rays

FAST ELECTRON GENERATORS Van de Graaff intense continuous beams of electrons up to 12 MeV Linear accelerators (LINAC) pulsed electron beams of much higher energies

HEAVY CHARGED PARTICLES Hydrogen (stable) Deuterium (stable) Tritium (radioactive) Alpha particle Obtained from acceleration by a Coulomb force field in a Van de Graaff or LINAC. Alpha particles are also emitted by some radioactive nuclei Proton the hydrogen nucleus Deuteron the deuterium nucleus, consisting of a proton and neutron bound together by nuclear force Triton a proton and two neutrons similarly bound Alpha particle the helium nucleus (2 protons and 2 neutrons)

NEUTRONS Neutral particles obtained from nuclear reactions since they cannot be accelerated electronically A chain reaction occurs when neutrons from a fission strike another uranium nucleus and create another fission.

RADIATIONS OF INTEREST Differ in their hardness or ability to penetrate thickness of material Soft radiation Alpha particles Low energy X rays Harder radiation Gamma rays neutrons

ICRU RECOMMENDATIONS International Commission on Radiation Units and Measurements Emphasizes the gross differences between the interactions of charged and uncharged radiation with matter: Directly ionizing radiation Indirectly ionizing radiation

DIRECTLY IONIZING RADIATION Fast charged particles, which deliver their energy to matter directly, through many small Coulomb force interactions along the particle s track

INDIRECTLY IONIZING RADIATION X or γ ray photons or neutron First transfer their energy to charged particles in the matter through which they pass in a relatively few large interations The resulting fast charged particles then in turn deliver the energy to the matter