Download Ionizing Radiation

I ONIZING R ADIATION
R ADIOLOGICAL P HYSICS

The science of ionizing radiation and its interaction with matter

Special interest in the energy absorbed

Radiation dosimetry –
~
quantitative determination of that energy
H ISTORY
H OW IT STARTED
 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 F OOD I RRADIATION
Year
1905
1921
1953‐1980
1958
1963
1964
1964‐1968
1966
1971
1976
1980
1985
1986
1990
1992
1997
2000
Event
Scientists receive patents for a food preservative process that uses ionizing radiation to kill bacteria in food.
U.S. patent is granted for a process to kill Trichinella spiral is in meat by using X‐ray technology.
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.
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.
FDA approves irradiation to control insects in wheat and flour.
FDA approves irradiation to inhibit sprouting in white potatoes.
The U.S. Army and the Atomic Energy Commission petition FDA to approve the irradiation of several packaging materials.
The U.S. Army and USDA petition FDA to approve the irradiation of ham.
FDA approves the irradiation of several packaging materials based in the 1964‐68 petition by the U.S. Army and the Atomic Energy
Commission.
The U.S. Army contracts with commercial companies to study the wholesomeness of irradiated ham, pork, and chicken.
USDA inherits the U.S. Army's food irradiation program.
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
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.
FDA approves irradiation for poultry to control salmonella and other food‐borne bacteria.
USDA approves irradiation for poultry to control salmonella and other food‐borne bacteria.
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 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.
P RODUCE

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. T YPES AND S OURCES OF I ONIZING
R ADIATION
•
•
•
•
I ONIZING 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
E LECTROMAGNETIC
SPECTRUM
G AMMA - RAYS
 Electromagnetic radiation emitted from a nucleus
 The quantum energy of any electromagnetic photon is [keV]: Eγ =
hν
λ
=
1.2398 keV − nm
λ
h = Planck's constant = 4.136x10-18 keV s
c = velocity of light ' in vacuo' = 2.998x108 m/s
G AMMA - 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 ‐ Kα characteristics x‐rays from 37
electron capture in 18
Ar
~
to 6.1‐7.1 MeV ‐ gamma‐rays from 167 N
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 S CHEMATIC
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
T YPICAL
CONTINUOUS X - RAY
12400
λ A& =
EeV
λ A& (min)
o
12400
=
= 0.348 A
50000
Peak voltage of X‐ray
D ISCRETE X - RAYS
Relative intensity
Kβ
 Electron from higher Kα
shells fill the inner‐shell vacancies
100 keV
 Characteristic X‐ray are superimposed on the continuous spectrum
 Designated Kα, Kβ, and Wave length (A)
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
H EAVY
Hydrogen
(stable)
Deuterium
(stable)


Obtained from acceleration by a Coulomb force field in a Van de Graaff or LINAC. Alpha particles are also emitted by some radioactive nuclei ~
Tritium
(radioactive)
~
~
~
Alpha
particle
CHARGED PARTICLES
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)
N EUTRONS

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.
R ADIATIONS 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
D IRECTLY
IONIZING
RADIATION

Fast charged particles, which deliver their energy to matter directly, through many small Coulomb‐force interactions along the particle’s track
I NDIRECTLY
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