Physics Exam 3

What is the unit of a photon

photons/cm^2 - sec

What is the equation of intensity of a photon?

photon flux (energy/photon)

What is the unit for exposure

Roentgens

What is the relationship between exposure and intensity

Proportional

What is photon flux

the rate at which photons pass through a given space or strike a surface, measured as the number of photons per second

What is the area of a sphere

4πr²

How do we find the number of photons passing through the unit area of a sphere?

Divide the decay rate (A) by the area of sphere (4pieRsquared)

What is the inverse square law? What is the equation?

As distance increases from a source, the intensity of photons decreases as a squared function. I2 = I1 x (d2/d1)^2

Doubling the distance does what to exposure

1/4 of exposure

What is attenuation defined as

Removal of radiation from a beam via absorption or scatter

What is the linear attenuation coefficient defined as

The probability of attenuation

What is the equation for attenuation

Same as the decay equation

What is HVL defined as and what is its equation

The thickness of a material that reduces intensity of a beam to 1/2.

Ln(2)/μ

What is TVL defined as and what is its equation

Thickness of a material that reduces the intensity of beam to 10%

Ln(10)/μ

What is the mean free path defined as and what is its equation

Average distance photon travels before interaction

1/μ

What 3 things is HVL dependent on

Photon energy, attenuating material, and geometry

What is the HVL of 140 keV y-rays (Tc99m) in tissue, aluminum, and lead?

44, 18, 0.3

What factors contribute to attenuation?

Energy, atomic number, and material density

Linear attenuation coefficient is a sum of?

Contributions from each interaction type

Higher μ means higher ...

attenuation

μ _________ with increasing energy

decreases

μ __________ with increasing Z -atomic number and increasing material density

increases

Intuitively describe the relationship between attenuation and energy and material

Higher Z materials are more dense so they attenuate more. High energy radiation is harder to stop.

What is the relationship between μ and density

It is proportional to density

What does dividing the linear attenuation coefficient by density (p) do?

It removes any dependence on the material's phase/state of matter.

What is the diagnostic energy range

30 kev to 511 kev

Which interactions fall within the diagnostic energy range that we use

Photoelectric absorption and Compton Scattering

Know this graph

What are the different scatterings that fall under Coherent Scattering?

Rayleigh and Thomson scattering

What happens in coherent scattering?

A photon goes in and out through an atom with the same energy but in a different direction

What is the typical energy level for coherent scattering

Occurs primarily at photon energies less than 20 keV

What is coherent scattering proportional to

Atomic number and energy

What happens to the photon in photoelectric absorption

It disappears

What do we get as a result of photoelectric absorption

We get an ejected electron known as a photoelectron that comes out at an angle and characteristic radiation

The relative probability of photoelectric absorption increases as ...

Photon energy decreases and the atomic number/stopping material increases

What characteristic radiation can occur as a result of photoelectric absorption

Auger e- or x ray

What is ionization defined as

Atom minus one electron

How do characteristic x rays occur

Occurs as the outer shell electrons fill the vacancy of inner shell electrons

How do auger electrons occur

Characteristic x ray interacts with an orbital electron and then an electron is ejected

Auger electron is more probably in what kind of material

Low Z

Graph for photoelectric absorption and energy

What happens in compton scattering

Incident photon is scattered with a lower energy and produces a scatter, lower energy photon and a recoil electron

What major problem do we run into with compton scatter

Mispositioned counts because of the scattered photon

The probability for compton scatter increases as?

Photon energy decreases and physical density decreases

How do we find out the energy of the scattered photon and the energy of the recoiled electron that we get as a result of compton scatter

We use that one complicated equation for compton scattered photon and the energy of the recoil electron is Ere = Ey - Esc

What angle do we use for the compton scatter equation

Angle of scattered photon

Know the symbols for the interaction types

Check the slides please

Compton scatter preferentially interacts with what

Outer shell electrons

Compton scatter _________ with increasing energy

decreases

Compton scatter ________ with increasing electron density

increases

Graph for compton scatter

What is the threshold for pair production

1.022 MeV

Relative probability increases as

Photon energy increases and atomic number material increases

What does the photon in pair production interact with

nuclear electric field

What is produced from pair production

positron and negatron

Pair production on graph

What is the threshold for photodisintegration

5 MeV

What do we get as a result of photodisintegration

Spallation products

What does the photon interact with in photodisintegration

Nucleus

The relative probability increases for photodisintegration as...

Photon energy increases or atomic number/stopping material increases

*Of no consequence in nuclear medicine

Which photon interactions are with orbital electrons and produce low energy x and gamma rays

Photoelectric effect

Compton scatter

Which photon interactions are with the nucleus and produce high energy x and gamma rays

Pair production and photodisintegration

Remember that if something says fraction then use that in the equation as a fraction of transmitted photon over incident photon equals e to the negative mu x

just know that L O L

What is the HVL equation

HVL = ln(2)/μ

What is the HVL in lead (mm) of Tc99m, I-131, and Annihilation?

0.27, 2.3, 4.1

What is the TVL in lead (mm) of Tc99m, I-131, and Annihilation?

0.9, 7.7, 13.5

*Just kinda know that they are a factor of 3.3x of each other

What is Coulomb's law equation?

F=q1q2/r

We get alpha, B-, and B+ particles from? What do we get e- from?

Decay processes. We get e- from photon interactions like photoelectric absorption and compton scattering

What is an example of collisional energy loss? What is the max energy exchange possible?

Ionization. The max energy exchange possible is half of the original energy.

What is an example of radiative energy loss? What does it intercat with?

Bremsstrahlung. It interacts with the electric field of the nuclei. the electron is decelerated rapidly that energy is radiated away.

How much energy is required to produce an ion pair

On average, 34 eV

What is specific ionization

The number of ion pairs produced per distance traveled

What is range

Distance traveled until radiation loses its ionizing capability

What is used to shield Bremsstrahlung radiation

Low Z material and then a lead container to absorb the few Bremsstrahlung photons created