session 3: atomic structure and ionizing radiation (cont’d) lecture 3 clrs 321 nuclear medicine...

Session 3: Atomic Structure and Ionizing Radiation (cont’d)

Lecture 3CLRS 321

Nuclear Medicine Physics and Instrumentation 1

Lecture 2 Objectives(Adapted from your Textbook)

• Describe the interactions of charged particles with matter.• Discuss the processes of excitation and ionization.• Describe the processes of photoelectric effect, Compton scattering,

and pair production.• Discuss the production of characteristic X-rays.• Discuss the process that produces Auger electrons.• Write the general form of the attenuation equation for gamma

photons.• Calculate the reduction of gamma radiation using the general

attenuation equation.• State the relationship between the linear attenuation coefficient and

the half-value layer.

Interactions• Excitation

– Charged particle or electromagnetic radiation supplies energy to outer shell electrons

• The “excited” electron moves to a higher shell or subshell

• Electron spontaneously returns to a less excited state giving up electromagnetic radiation

• Ionization– Charged particle or electromagnetic radiation

completely removes electron from atom• Results in an ion pair

Interactions: Excitation

Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 13.

Interactions: Ionization

If an electron has a binding energy of 70 keV, then it would require 70 keV of energy to kick that electron out of its shell.

. Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 11.

Interactions: Alpha & Beta

• Alpha– Typically have energies between 3 & 8 MeV

• Requires about 34 keV to strip an electron from an atom

– Thus alphas can create hundreds of thousands of ion pairs in less than a mm of tissues

• Beta– Can create Bremsstrahlung radiation when near

high Z materials• With pure beta emitters, plastic is better shielding than

lead to avoid Bremsstrahlung radiation

Interactions: Photons• Represent electromagnetic radiation

– Visible light• Reflected or absorbed

– X-rays and gamma rays• One of three (really, maybe four) possibilities

– No interaction (pass through)– Scatter (partially absorbed)– Completely absorbed

» And also may become matter and thus absorbed

• Rate of absorption increases exponentially with distance travelled through matter

Interactions: Photoelectric Effect

Total absorption of Total absorption of a gamma photon a gamma photon at the expense at the expense of an electronof an electron

Photon energy Photon energy must be equal or must be equal or greater than greater than electron binding electron binding energyenergy

Electron falls from Electron falls from outer shell and outer shell and emits emits characteristic X-characteristic X-ray photonray photon

Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 52. binding kineticE E E

Interactions: Compton ScatteringGamma Photons don’t just Gamma Photons don’t just

disappear when they disappear when they confront matter—their confront matter—their energy has to be energy has to be accounted foraccounted for

Compton is a type of scatter in Compton is a type of scatter in which an electron is ejected which an electron is ejected and the gamma photon and the gamma photon continues at a deflected continues at a deflected angleangle

The amount of energy that the The amount of energy that the photon is reduced is photon is reduced is dependent upon the angle dependent upon the angle at which it is scattered at which it is scattered when it ejects the electronwhen it ejects the electron

The more the photon is The more the photon is deflected (greater angle), deflected (greater angle), the less its energy it retainsthe less its energy it retainsPaul Christian, Donald Bernier, James Langan, Nuclear

Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

Interactions: Compton Scatter

Compton events tend to Compton events tend to increase with higher Z increase with higher Z materialmaterial

Compton events tend to Compton events tend to decrease with higher decrease with higher photon energyphoton energy

The incident photon energy is The incident photon energy is equivalent to the binding equivalent to the binding energy of the electron and energy of the electron and its kinetic energy of its its kinetic energy of its recoil, plus the deflected recoil, plus the deflected energy of the photonenergy of the photon

The deflected energy of the The deflected energy of the photon can be calculated photon can be calculated based on its deflected based on its deflected angle (angle (θθ))

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/imgqua/compton2.gif

1 1 cos0.511

OSc

O

EE

E

MeV


The The minimumminimum amount of amount of energy of a energy of a backscattered backscattered (180(180◦◦) Compton Scatter ) Compton Scatter photon photon can be calculated can be calculated as:as:

Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

The maximum amount of back-scatter energy transferred to the recoil electron in a backscatter event can be calculated as:

0min

0210.511

EE

E

MeV

20

max0( 0.2555 )

EE

E MeV

NOTE: YOUR BOOK IS WRONG!


An example for calculating the minimum amount of energy a Tc-99m backscattered 140 keV photon can have:

An example for calculating the maximum energy a recoil electron can have from a maximum backscattered Tc-99m photon:

min

0.1400.090

2(0.140 )1

0.511

MeVE MeV

MeVMeV

2

max

(0.140 )0.049

(0.140 0.2555 )

MeVE MeV

MeV MeV


• What does all this mean???– The minimal energy of a backscattered

photon will form something called the “Backscattered peak” on the energy spectrum (we’ll cover that later).

– Emin of the backscatter photon and Emax of the recoil electron is energy-dependent and the difference between the two increases with incident photon energy

Interactions: Compton ScatterRadionuclide Photon E Emin of

Backscattered Photon

Emax of Recoil Electron

I-125 27.5 keV 24.8 keV 3.3 keV

Xe-133 81 keV 62 keV 19 keV

Tc-99m 140 keV 91 keV 49 keV

I-131 364 keV 150 keV 214 keV

Annihilation 511 keV 170 keV 341 keV

Co-60 1330 keV 214 keV 1116 keV

-- To infinity 255.5 To infinity

From Table 6-2, p. 78, Physics in Nuclear Medicine, 3rd Ed., by Simon Cherry, James Sorenson, and Michael Phelps, Saunders: Philadelphia, 2003.

Since the energy imparted to the recoil electron must exceed the binding energy of the electron, this means that Compton Scatter is more likely to occur at higher incident photon energies (to a point—we will soon see).

Interactions: Pair Production

Requires gamma photon Requires gamma photon of at least 1.022 MeV of at least 1.022 MeV to pass near a high-to pass near a high-electrical field of a electrical field of a nucleusnucleus

Energy is converted to Energy is converted to matter (m=E/cmatter (m=E/c22))

A positron and electron A positron and electron are created, each are created, each with a mass with a mass equivalent of 511keVequivalent of 511keV


Extra Nuclear Release: Bremsstrahlung

Important consideration when using Important consideration when using beta emittersbeta emitters

German for “breaking radiation”German for “breaking radiation”

Beta decelerating in vicinity of high Beta decelerating in vicinity of high density (high Z) nucleus density (high Z) nucleus dissipates energy in the form of dissipates energy in the form of x-ray photonsx-ray photons

Best to use plastic or lucite syringe Best to use plastic or lucite syringe shields with beta emitters to shields with beta emitters to avoid the Bremsstrahlung effect avoid the Bremsstrahlung effect as the beta particles penetrate as the beta particles penetrate lead shieldinglead shielding

Can get poor quality nuclear Can get poor quality nuclear medicine images using medicine images using Bremsstrahlung (ex: Sr-89)Bremsstrahlung (ex: Sr-89)


Extra Nuclear Release:(Energy States of Electrons)

This picture from the Sodee text represents the electron energy states as different speed limits around the nucleus of an atom.

In order for a car at 70 mph to go down to the 65 mph speed limit, it must lose a “quantum” of 5 mph. For electrons, this quantum is in the form of a specific wavelength of electromagnetic radiation.

Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 11.

Extra Nuclear Release: Characteristic X-rays

This figure from your textbook shows what happens when an electron loses energy to move from the L shell to the K shell.

Again Electromagnetic radiation is emitted, but it is of a higher energy (shorter wavelength/higher frequency) than visible light and is in the form of an X-ray photon.

Such an emission is called a “characteristic X-ray” and its “character” is dependent upon and equal to the specific difference in energy states between the L and K shells of the atom.


Extra Nuclear Release: Auger Electrons

Paul Christian & Kristen M. Waterstram-Rich, Nuclear Medicine and Pet/CT: Technology and Techniques, 6th Ed. (St. Louis: Mosby 2004), Fig 2-25, p 57.

Attenuation and Transmission of Photons

This is how all the gamma radiation eventually succumbs to matterThis is how all the gamma radiation eventually succumbs to matter

It is absorbed or It is absorbed or attenuated. attenuated. This is how it relates to instrumentationThis is how it relates to instrumentation


• Attenuation


Combined effects of attenuation is expressed by the linear attenuation coefficient (μ), which is in the units 1/distance(cm-1).

The attenuation of incident radiation (I) can be expressed as follows:

X is the distance through which the incident radiation travels through the attenuating material.

Paul Christian & Kristen M. Waterstram-Rich, Nuclear Medicine and Pet/CT: Technology and Techniques, 6th Ed. (St. Louis: Mosby 2004), p 57.


• Half-Value Layer (HVL)– Similar concept to T1/2

– Layer of attenuating material that will absorb ½ the incident radiation

– Specific for type of material and energy of incident radiation

– Is related to μ according to the following:

Where have we seen this before???



• Substituting the previous for μ, our attenuation equation now looks like…



• An example (from book)– I-131

• (364 keV principle gamma photon E)

– Lead is the shielding• HVL is 0.3 cm for 364 keV photons• Thickness of the lead is 0.9cm

– Incident radiation field is 5mR/hr


10.6932.31

0.3cm

cm


• Mass Attenuation Coefficient– Based on material density

• Is related to the linear attenuation coefficient

– Physicists can break this down so that they can measure attenuation according to Compton scatter, photoelectric effect, and pair production



Next time:Next time:

Basic Electrical ConceptsBasic Electrical Concepts

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session 3: atomic structure and ionizing radiation (cont’d) lecture 3 clrs 321 nuclear medicine...

Documents

electron photon energy

excited electron

bremsstrahlung radiation

subshell electron

matter slide

interactions excitation

kev of energy

reduction of gamma radiation