National Committee for the Certification of Radiation Protection Officer FUNDAMENTALS OF IONIZING...

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National Committee for the Certification of Radiation Protection Officer

FUNDAMENTALS OF IONIZING RADIATION

• Introduction• Atomic Structure• Radioactivity• Types of Ionizing Radiation• Quantity and Radiation Units• Uses of Ionizing Radiations• Radiation Sources• Summary

ContentsContents

• Radiation is a general term use to describe emission and transmission of energy through space in the form of waves, including charged and uncharged particles as well as electromagnetic radiation.

• Radiation consists of two types: ionizing and non-ionizing radiation.

• Ionizing radiation causes ionization whereas non-ionizing radiation does not cause ionization when it interacts with matter.

IntroductionIntroduction

Elements arranged

according to their

atomic weight

Central positive nucleus with

surrounding in the orbits negatively

charged electrons

1870 - Mendeleef 1913 - Danish physicist - Niels Bohr

Atomic StructureAtomic Structure

• In Nucleus Neutrons

(neutral) and Protons (+)

• Elementary Electrons (-)

Atomic Structure (cont.)Atomic Structure (cont.)

Atomic Number (Z)

• Neutral atom: number of electrons is equal to protons

where:

X - chemical symbol of element A - mass number (= number of protons + number of neutrons) Z - atomic number (= number of protons)

Isotopes and Nuclides

• Isotopes

Atoms of the same atomic number but different mass number.

Equal number of protons but different number of neutrons.

E.g. 1H, 2H, 3H.o Isotopes refer to the same element. o Isotopes of any element may also be called

nuclides.

Stable

• Nuclear stability - neutron-proton ratio (N/P) is crucial.

• Stable nuclide - correct balance in the number of neutrons and protons in the nucleus.

• Ratio too low or too high: Nucleus unstable. Rearrange itself into a more stable configuration through transformation

or spontaneous decays and the emission of radiation. Nucleus radioactive.

• Stable elements in the low atomic number range have an almost equal number of neutrons, N and protons, Z.

RadioactivityRadioactivity

Stability Curve

Radioactivity (cont.)Radioactivity (cont.)

Unstable Nuclides

• As Z increases beyond about 20: N/P ratio becomes greater than 1. Keeps on increasing with Z.

• Beta radiation: Emission of energetic electrons. Results when an N/P ratio is too high for stability.

• Positron emission or electron capture occurs when it is too low for stability.

Line of Stability

• Close to line Return by beta

• Far from line Alpha decay Fission Neutron decay

Neutron

Fission

Definition and Unit of Radioactivity

• Unstable nucleus: Transforms or decays spontaneously into nuclides of other elements. Emits radiation. Radioactive.

• This property is called radioactivity.

• Transformation is termed radioactive decay.

• Nuclide called radionuclide or radioisotope for the same element.

• Radionuclide activity - a measure of the number of nuclear transformations taking place every second.

• Unit - Becquerel (Bq) = one nuclear transformation per second.

Half-Life and Decay Law

• Half-life (T1/2) - time required for nuclei to decay one half its original value.

• Activity (A) or transformation rate of a radionuclide is proportional to the number of unstable nuclei and an exponent function of time as follows:

A = Ao e -t

where A is activity at time t, Ao is the initial activity and is the decay constant

λ = 0.693/ T1/2

What is radiation?

• Energy that moves from the source to another object where it is finally absorbed.

• Comprise ionizing and non-ionizing.

neutron

-(negatron)

+(positron)

cosmic

X-rays

Electron

Types of Ionizing RadiationTypes of Ionizing Radiation

Types of Ionizing Radiation (cont.)Types of Ionizing Radiation (cont.)

Alpha Particle

• Helium nucleus: 2 protons and 2 neutrons tightly bound together.

• Emitted from naturally occurring radio nuclides such as uranium and thorium.

• Lose energy by collisions with atomic electrons - cause ionizations to occur.

• Directly ionizing particles - cause ionization of an atom without any intermediate interaction taking place.

Alpha Particle

• Cause large specific ionization (number of ion pairs formed per unit energy) because of their double positive charge and large mass.

• Lose their energy in short distance hence range in media is short (less of external hazard but present an internal hazard).

Beta Particles

• High speed electrons emitted by a radionuclide.

• Either be positive (positron) or negative (electrons).

• Not emitted with discrete energies but show a continuous energy spectrum (unlike alphas).

• Lose energy more frequently through ionization and excitation.

Beta Particles

• Directly ionizing particles - cause ionization of an atom without any intermediate interaction taking place.

• For high speed electron (more than 1 MeV) more energy may be lost in the form of X-rays due to interaction with the nuclear field in dense material (bremsstrahlung).

Beta Particles

• Produce less ionization per unit length than alpha particles because of its smaller mass and charge.

• Have greater range than alpha hence, depending on its energy constitute an external hazard.

• Not as great an internal hazard as alpha particles.

X- and Gamma-Rays

• Electromagnetic waves or photons.

• Ionization produced is almost all secondary.

• Most concern as potential external hazards.

• Great range in air and other media.

• High Z shielding material is needed to prevent or minimize the intensity of radiation hazard.

Interactions of X- and Gamma-rays With

Matter

• Photoelectric effect• Compton scattering• Pair production

Photoelectric Effect

• Interaction of photon has a particle-particle collision with an atomic electron.

• Photon transfers all of its energy to the electron.

• If the energy is sufficient to release the electron from its atomic orbit, the atom is ionized.

Types of Ionizing Radiation Cont.)Types of Ionizing Radiation Cont.)

Compton Scatter

Happens when a photon collides with an atomic electron and transfers only part of its energy to the electron.

Rest of the original photon’s energy is radiated as a lower energy photon.

Secondary photon travels in a different direction from the one creating it.

Pair Production

• Happens when photon, in the presence of a nuclear field, disappears, changing all its energy into matter in the form of an electron and a positive electron (positron).

• Incoming photon must have energy greater than 1.02 MeV.

Pair Production

• Positron loses its extra energy, if any, by ionization.

• Positron has lost all of its energy, it unites with an electron and the two particles disappear, or annihilate giving rise to two characteristic annihilation gamma photons.

Neutrons

• Produced by nuclear reactions

Thermal (when their energies <0.025 eV). Intermediate (when their energies are between 0.5

eV to keV). Fast (when their energies are between 10 keV to

MeV). Relativistic (when their energies are > 10 MeV).

Neutrons

• Interaction with matter is totally secondary in nature. It gives up energy to recoil nucleus which causes ionization in the medium.

• Most concern as external hazard.

• No charge and could therefore travel great distances in air and other media.

• Fast neutron being more hazardous than neutrons of lower energy.

• Capture of thermal neutron in some materials will produce gamma ray.

• ICRU - International Commission on Radiological Units and Measurements.

• There are five physical quantities and 8 types of units (SI and non-SI):

1. Activity 2. Exposure3. Absorbed dose4. Dose equivalent5. Effective dose

Quantity and Radiation UnitsQuantity and Radiation Units

Activity

• Unit – Becquerel (Bq)• 1 Bq is one nuclear disintegration per second• Old unit – Curie (Ci)

Exposure

• Unit – Coulomb per kilogram (C kg-1)• Old unit – Roentgen (R)

Quantity and Radiation Units (cont.)Quantity and Radiation Units (cont.)

• Defined as the absorbed energy per unit of mass.• Measured in Gray.• Always equal to or less than the KERMA.

1 J/kg Incident EnergyScattered Energy

Absorbed Dose

• Unit – Gray (Gy) – after a British Scientist.

• Quantity of energy absorbed per unit mass of the medium.

• 1 Gy is defined as the absorption of 1 joule of energy in one kilogram of material.

• Old unit – rad (radiation absorbed dose).

• Because cells have a great ability to repair DNA damage it generally requires more than one event to cause permanent damage to the DNA.

• High LET radiation has a much greater chance to create the multiple points of damage that are required.

Radiation Weighting Factor (WR)

• Correct for damage

• Values are 1 for X-rays, gamma-rays and electrons

• Higher for particles (~10)

• All routine medical radiations have a value of one

Equivalent Dose = Absorbed Dose For all practical purposes

Equivalent Dose

• Dose corrected for biological effects.

• Unit - Sievert (Sv) after Swedish Scientist.

• Different radiation causes different effects.

• Equivalent dose is equal to the absorbed dose multiplied by the quality factor (or radiation weighting factor) and other modifying factors.

• Old unit - rem (radiation equivalent man).

Radiation Weighting FactorsRadiation WR X and Gamma-rays, electrons, positrons 1 Neutrons < 10 keV 5 Neutrons 10 to 100 keV 10 Neutrons 100 keV to 2 MeV 20 Neutron 2 to 20 MeV 10 Protons > 2 MeV 5 Alpha partic les, fission & heavy nucleus 20

The Sievert is the units of biological effect or equivalent dose.

• Equivalent Dose = Absorbed Dose x Radiation Weighting Factor (WR)

• Sievert = Gy x WR

Effective Dose

• Unit –Sievert (Sv)

• A measure of harm (or health detriment) rather than true dose.

• Different tissues show different sensitivities to radiation.

• Effective dose is the product of Equivalent Dose and Tissue Weighting Factor.

• Old unit – rem.

Deff = Dorgan x WR x WTorgan

Tissue Weighting Factors

0.01 0.05 0.12 0.20 Bone Surface Skin

Bladder Breast Esophagus Liver Thyroid Remainder

Bone Marrow Colon Lung Stomach

Gonads

Absorbed Dose

Energy/Mass

Equivalent Dose Effective Dose

Corrects for Biological

Effectiveness of Differing

Radiations

Corrects for Organ Sensitivity and Partial Body

Exposures

• Industries Non-destructive testing Gauging Sterilization

• Agriculture Extending the use of product Expanding production

• Power Nuclear reactors

• Archeology Carbon-dating

• Food Technology Extending the life span of foodstuff Genetic modified food

• Medicine Diagnosis, therapy, sterilization

• Research

Uses of Ionizing RadiationsUses of Ionizing Radiations

Natural Radiation Sources

1. Cosmic Rays

• Extraterrestrial radiation.

• Originates in outer space as primary cosmic rays (such as from stars and the sun) and reaches the atmosphere, with which the incoming energy and particles interact, giving rise to secondary cosmic rays.

Radiation SourcesRadiation Sources

2. Terrestrial Radiation

• Emitted from the radioactive nuclides present in varying amounts in soils and rocks, the atmosphere and hydrosphere, and from those radionuclides that are transferred to man through the food chains or by inhalation.

• Leads to both external and internal exposures.

Radiation Sources (cont.)Radiation Sources (cont.)

Man-Made Radiation Sources

1. Medicine

• X-rays and gamma rays from both diagnostic and therapeutic uses.

2. Radioactive Fallout

• Man-made artificial radioactivity produced as a consequence of nuclear weapon tests and nuclear accidents.

• Radionuclides produced in the atmosphere and carried by winds eventually spread and fall throughout the world.

Radiation Sources

Man-Made Radiation Sources

3. Consumer Goods

• Examples of consumer goods containing man-made radionuclide are: electronic tubes emitting X-rays;self-luminous wrist watches;gas lantern mantle; and color television sets

4.Occupational Exposure

• Individuals may be exposed to radiation in the course of their occupation involving radiation sources.

• Radiation of artificial origin is widely used in the manufacturing industry and quality control, as research tools in universities, research institutions and nuclear industry.

Other Sources

Large scale production of electric power by nuclear fission presupposes a cycle of complex operations, most of which involve some discharges of radioactive material top the environment and corresponding exposure of the population at large.

Summary

Thank YouThank Youfor your attention

Any Questions?

National Committee for the Certification of Radiation Protection Officer FUNDAMENTALS OF IONIZING...

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