PDF 3.1 Radiation is All Around Us

7/28/2019 PDF 3.1 Radiation is All Around Us

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Nuclear Reactionsand Radiation

3.1 Radiation is all around us

L. R. Foulke


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Overview Previous Module

Atomic structure, binding energy

Nuclear stability, nuclear decay

Types of radiation This Module

Types of radiation interactions Calculation of reaction rates Fission


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Radiation What is radiation? Transmitted Energy

Types of radiation Electromagnetic (radio, visible, x-rays, rays) Charged particles (electrons, protons, particles) Other (neutrons, neutrinos, other exotic beasts)

Categorized as either ionizing or non-ionizing Depending on whether they can ionize other particles

(i.e., rip off electrons from the atom)


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Effects of Radiation

Interactions The local effects of radiation on a given material is

characterized by two quantities:

Range The average total distance traveled by a single particle of

radiation (cm).

Linear Energy Transfer (LET) The total energy transferred from a particle of radiation to its

surroundings, per unit distance travelled (keV/m).

Range and LET values are unique to the type andenergy of the radiation as well as the target material.


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Charged Particle Interactions

Charged particles (by ascending charge) Electrons, positrons, protons, alpha particles,

recoil nuclei, fission fragments.

As particles travel through a sea ofnegatively charged electrons, long rangeelectrostatic forces act as a drag force.

Charged particle LET is directly proportionalto charge.

Charged particle range is directly proportionalto velocity.

Charge: -1

Charge: +1

Beta particle (electron)

Positron

Proton

Charge: +2

-particle

Charge: > +2

Ionized nucleus(positive ion)

Image Source: See Note 1 for Ionized nucleus


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Charged Particle Interactions

Before After

proton

neutron

electron

Example of Ionization Interaction

Image Source: See Note 2


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Electromagnetic Interactions X and rays interact with free and bound electrons inthe material.

Photoelectric Effect Photon energy is transferred to bound electron, causing it to beejected from electron cloud.

Compton Scattering Photon scatters off of an electron, changing the wavelength of

the photon and giving kinetic energy to the electron.

Pair production Photon with energy > 1.022 MeV spontaneously turns into an

electron and a positron.


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Electromagnetic InteractionsPre-Collision

Photoelectric Effect

protonneutron

electron

Compton Scattering

ray

Pair Production

positron

Annihilation



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Ionizing Radiation Radiation that contains enough energy to remove oneor more electrons from an atom or molecule.

All charged particles are ionizing. Only photons with an energy greater than the

ionization energy of a given atom or molecule are

considered ionizing.

Some molecules are affected by photons in thevisible or UV range, but typically only x-rays and

gamma rays are considered ionizing.


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All three radiation types (charged particles,electromagnetic radiation, and neutrons) are capable of

ionizing target atoms in materials.

Ionization events are the root cause behind ALLobservable effects of radiation.

Ionization reactions damage materials by breaking chemicalbonds and disrupting normal chemical processes (material

embrittlement, biological damage, etc.)

The rate of ionization (damage) depends on the typeand energy of the radiation, as well as the constituentatoms in the target material.

Effects of Radiation


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The number of ionization events that a single particle ofradiation can produce is determined by the energy of

the radiation.

The ionization density is determined by the LET----------------------------------------------------------

Relative Relative

Radiation Range LET----------------------------------------------------------Alpha 1 10,000Beta 100 100Gamma 10,000 1----------------------------------------------------------

Ionization Density


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Ionization DensityAverage Human Cell

neutron

gamma ray

x-ray

alpha particle

Separation of ion clusters relative to size of a human cell (conceptualized)



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Penetrating Properties of Radiation


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Neutron Interactions Neutrons can only interact with atomic nuclei. Neutron Elastic Scattering

Occurs when a neutron strikes a nucleus and transfers kineticenergy, creating a charged recoil nucleus.

Conserves two-body kinetic energy. Only fast neutrons(>1keV) striking light nuclei (H to C) can transfer enough

energy to cause a significant recoil.

Neutron Inelastic Scattering Occurs when a neutron strikes a nucleus and causes excitation

in the nucleus. Nuclear de-excitation releases a ray.

Does not conserve kinetic energy. Only neutrons above anuclide-dependent threshold energy can cause reactions.


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Neutron Interactions Neutron Absorption (Capture + Fission)

Capture occurs when a neutron strikes a nucleus and isabsorbed, increasing the mass number of the isotope by 1.

Addition of the extra neutron leaves the neutron in anexcited state, with too much energy.

Nuclear de-excitation releases rays. Certain combinations of neutrons and protons are

fundamentally unstable. Isotopes with these combinationsundergo further stabilization by emitting a particle:

- decay, + decay, decay, proton emission, neutronemission, internal conversion, electron capture

This process is Radioactive Decay


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Neutron InteractionsPre-Collision

Elastic Collision

proton

neutron

electron

Inelastic Collision

ray

Neutron Capture


neutronbecomes

a protonZ=+1

-


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1. Creative Commons:http://commons.wikimedia.org/wiki/File:Nucleus_drawing.png

2 Reprinted with permission from David Greisheimer,University of Pittsburgh.

3. Public domain:http://commons.wikimedia.org/wiki/File:Diagram_human_cell_nucleus_no_text.png

Image Source Notes

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PDF 3.1 Radiation is All Around Us