Radiation - physics

Question 1

Photons

Answer 1

Photon is a ‘packet’ of energy, can be thought of as single particle of light or x-ray
An x-ray beam is made up of a great many photons

Question 2

Atomic physics

Answer 2

-vely charged electrons orbit a +vely charged nucleus with each orbit having a fixed energy energy level
An electron in an orbit has an energy equal to that energy level
Energies dependent on particular atom
Transition of an electron to higher energy orbit will require input of energy, in this case a photon
Transition of electron to a lower energy orbit will cause a loss of energy, in this case a photon will be emitted

Question 3

If an electron in orbit 1 (so has E=10), and a photon with at least E=20 hits it

Answer 3

It can go up to orbit 2 (as E=30)

Question 4

If space becomes available in orbit 1

Answer 4

An electron from orbit 2 will drop into it and give off a photo of E=2-

Question 5

If we give an electron enough energy

Answer 5

We can completley free it from pull of nucleus and we have a +ve ion (the atom minus 1 electron) and one free electron
This process is ionisation

Question 6

What are first and second shells called

Answer 6

K-shell and L-shell

Question 7

Electromagnetic radiation includes

Answer 7

Light, UV, microwave, radio waves, X-rays and gamma rays

Question 8

Electromagnetic radiation

Answer 8

Not charged
Beams consist of many single photons
Travels at speed of light
Energy determines frequency and wavelength; E=hv
h= Planck constant
v=frequency

Question 9

Particle radiation includes

Answer 9

Alpha and beta (electrons) particles from radioactive decay

Question 10

Particle radiation

Answer 10

Many particles are charged
Beams consist of many individual particles
Speed and energy are dependent on initial emission conditions

Question 11

Why can electromagnetic radiation be bad

Answer 11

Free radicals produced

Question 12

Alpha particles

Answer 12

Unstable nucleus can emit 2 neutrons and 2 protons to reduce proton number
Emitted particle identical to H nucleus, having charge of +2
Relatively high mass and charge make alpha particle highly ionising
Alpha particle is stopped by ~10mm air or less than a mm of matter

Question 13

Beta particles

Answer 13

An electron. Name merely designates its origin
Unstable nucleus ejects an electron by internal conversion or when a neutron converts into a proton and an electron (n->p+e- +y)
Emitted beta particle has charge of -1
Also highly ionising
More penetrating than alpha particle but still stopped by few mm of aluminium

Question 14

As frequency increases

Answer 14

Energy of individual photon gets higher

Wavelength get smaller

Question 15

X-rays and gamma rays

Answer 15

Exactly the same, just denotes where they have come from

Question 16

Gamma rays

Answer 16

Excited nucleus may emit y-ray in order to return to its ground state
Alpha and beta emitters also emit y-rays because the daughter (Y) nuclei produced are often in excited states as result of their decay
Emitted y-ray has no charge
y-ray less ionising than particulate radiations
y-ray can be very penetrating, tens of mm Pb may be required to reduce their intensity appreciably
y-rays identical to x-rays, except for manner of production

Question 17

X-rays discovered in

Answer 17

Nov 1985 by Roentgen, medical X-ray departments running by 1896

Question 18

Production of x-rays

Answer 18

produced by accelerating electrons towards metal target, in high-voltage, evacualted tubes
Two distinct processes by which this creates x-rays which in turn shapes the spectrum of photons emitted
1) Bremsstrahlung
2) Characteristic x-ray formation

Question 19

Diagnostic X-ray tube

Answer 19

Needs to produce x-rays as efficiently as possible
Creating high x-ray intensities generates a lot of heat
Anode needs high Z to absorb electrons and high melting point of 3410degrees C and atomic number of 74
Glass envelope makes it vacuum
Copper block takes away heat
Lead shielding around whole tube to stop x-rays going all directions
Sloped target to direct electrons towards thin window
Cathode on filament side, anode on copper block side (Look at lecture)

Question 20

Bremsstrahlung

Answer 20

Braking radiation

Continuous spectrum produced by rapid deceleration of electrons passing close to target nuclei (look at slides)

Question 21

Characteristic X-ray formation

Answer 21

An electron is knocked out of K shell, leaving gap
L shell electron falls into gap and emits energy as x-ray photon
Energy of photon emitted = (energy of electron in L shell) - (energy electron now has in K shell)
Which is determined by material ionised, hence ‘characteristic’
(look at slides)

Question 22

Difference between gamma and x-rays on graph

Answer 22

Line spectra for gamma, all same energies

Continuous spectra for x-rays, photons of different energies produced

Question 23

Properties of alpha particles

Answer 23

Fluorescence on a phosphor
Has an effect on photographic plate/ film
10^4 number ion pairs/mm in air
Stopped by up to 10^-2 mm Al
Stopped by up to 10mm air
Deflection by E and B as +ve charge
10^7 m s^-1

Question 24

Properties of beta particles

Answer 24

Less fluorescence on a phosphor than alpha
Has an effect on photographic plate/ film
10^2 number ion pairs/mm in air
Stopped by up to mm Al
Stopped by up to m air
Deflection by E and B as -ve charge
10^8 m s^-1 but variable

Question 25

Properties of gamma particles

Answer 25

Very little fluorescence on a phosphor
Has an effect on photographic plate/ film
1 number ion pairs/mm in air
Stopped by up to 100 mm Pb
No deflection by E and B 
3 x 10^8 m s^-1

Question 26

Properties of x-ray particles

Answer 26

Fluorescence on a phosphor
Has an effect on photographic plate/ film
<1 number ion pairs/mm in air
Stopped by up to 1-2 mm Pb
No deflection by E and B 
3 x 10^8 m s^-1

Question 27

Tube current mA

Answer 27

intensity I∞mA
Quality unaffected
Shape of spectrum unaltered

Question 28

Attenuation (removing energy from beam)

Answer 28

For a monoenergetic beam of x-ray photons with intensity I0
μ = Linear Attenuation Coefficient (Higher for lower energy photons. More easily
attenuated)
t is usually measured in kg/m2
μ/ρ the Mass Attenuation Coefficient is measured in m2kg-1
X-rays beams are polyenergetic i.e. have a spectrum of energies. Therefore, the lower energy component is preferentially absorbed.

Question 29

Attenuation (removing energy from beam)

Answer 29

I (intensity) = I0 (initial intensity) x e^μt (thickness)
For a monoenergetic beam of x-ray photons with intensity I0
μ = Linear Attenuation Coefficient (Higher for lower energy photons. More easily
attenuated)
t is usually measured in kg/m2
μ/ρ the Mass Attenuation Coefficient is measured in m2kg-1
X-rays beams are polyenergetic i.e. have a spectrum of energies. Therefore, the lower energy component is preferentially absorbed.

Question 30

Tube voltage kVp

Answer 30

Intensity I∞kVp^2

Quality increases with kVp

Question 31

Tube voltage kVp

Answer 31

Intensity I∞kVp^2

Quality increases with kVp

Question 32

Compromise between contrast and dose

Answer 32

Most tissues are of similar density and Z; therefore need to choose low kV to optimise contrast
But transmission increases with kV (< px dose)
So, we need to compromise between contrast and dose
Highest absorption at low energies
Transmission increases with kV
Lower energies give greatest relative difference in attenuation between different materials

Question 33

Effect of distance

Answer 33

Inverse square law

Question 34

Absorption in tissue - photoelectric effect

Answer 34

Photon interacts with tightly bound ineer electron (80% K, 20% L)
The photons energy is absorbed by electron. If energy absorbed exceeds
electron binding energy electron emitted from atom, known as Photoelectron
The gap in shell filled by electron from higher shell and energy released is emitted as Characteristic Radiation

Question 35

Absorption in tissue - Compton scatter

Answer 35

If binding energy is small it can be ignored (BE&laquo_space;incident photon E. i.e. outer shell electrons)
Therefore probability of interaction remains same whatever nuclear size
As binding energy increases interaction is with whole atom The nucleus is massive wrt the electron and absorbs impact
No energy lost
Wavelength remains same

Question 36

Target atomic number

Answer 36

Intensity I∞Z (Z is number of protons)

Quality unaffected

Question 37

Rectification

Answer 37

Intensity reduced by ripple
Quality reduced by ripple
Constant potential gives maximum intensity and quality

Question 38

Filtration

Answer 38

Intensity decreases with filtration
Quality increases with filtration
Filtration removes low energy radiation

Question 39

Distance from focus

Answer 39

Intensity I∞ r^-2
Quality unaffected
Known as inverse square law

Question 40

How far alpha particles into tissue

Answer 40

Stopped by air or dead layer of skin

Question 41

How far beta particles into tissue

Answer 41

Go into tissue, scatter at some point

May be harmful, may not be

Question 42

How far gamma or x-rays into tissue

Answer 42

Some will scatter in tissue
Some will be absorbed
Some will go all the way through
We need this ratio to detect an image