Nuclear And Particle Physics

Question 1

What are X-ray tubes simply put

Answer 1

X-ray tubes are an electrical circuit, with a cathode (where electrons are emitted) and an anode (the target metal).

Question 2

Explain how X-rays a produced with tungsten

Answer 2

1) At the cathode, electrons are emitted (boiled off) by the hot filament.
2) This filament is heated by passing a current through it. This current is not the same as the current going through the entire X-ray tube.

3) The cathode is usually in a cup shape, to focus the beam of electrons onto the target metal.
4) The target metal (tungsten) acts as the anode of the circuit, and the high potential difference across the tube (tube voltage) causes the electrons to accelerate towards it. When the electrons smash into the tungsten anode, they decelerate and some of their kinetic energy is converted into electromagnetic energy, in the form of X-ray photons. The tungsten anode emits a continuous spectrum of X-ray radiation.

Question 3

What is the maximum energy that an X ray photon produced from accelerated electrons can have?

Answer 3

The maximum energy of the X-ray photons is equal to the potential difference of the X-ray tube multiplied by the charge of an electron. So, if a potential difference of 50 kV is used in the tube, the maximum X-ray energy will be 50 keV.

Question 4

Why is the tungsten anode (used to produce X-rays rotating)

Answer 4

About 1% of the electrons’ kinetic energy is converted into X-rays.
The rest is converted into heat, so, to avoid overheating, the tungsten anode is rotated at about 3000pm. It’s also mounted on copper — this conducts the heat away effectively.

Question 5

How else are X-rays produced other than by the kenetic energy of the electrons themselves

Answer 5

• X-rays are also produced when beam electrons knock out other electrons from the inner shells of the tungsten atoms.
• Electrons in the atoms’ outer shells move into the vacancies in the lower energy levels, and release energy in the form of X-ray photons.

Question 6

Label diagram in notes

Answer 6

.

Question 7

Why is the tube evacuated

Answer 7

So electrons pass through without interacting with atoms

Question 8

Do CGP example Q pg 180

Answer 8

.

Question 9

What is the intensity of an X-ray beam

Answer 9

The intensity of the X-ray beam is the power (energy per second) per unit area passing through a surface (at right angles).

Question 10

What are the two way to increase the intensity of the x ray beam?

Answer 10

Increase the tube voltage. This gives the electrons more kinetic energy. Higher energy electrons can knock out electrons from shells deeper within the tungsten atoms.

Increase the current supplied to the filament. As the current increases, the filament temperature rises. This liberates more electrons per second (with the same final energy per electron as before), which then produce more X-ray photons per second.

Question 11

What happen to the intensity of a X-ray as it passes through matter

What is the term for this

Answer 11

The intensity (I) of the X-ray beam decreases (attenuates) exponentially with distance from the surface (x), according to the material’s attenuation (absorption) coefficient (p), as the equation on the right shows.

The term attenuation is used to describe the decrease in the intensity of an electromagnetic radiation as it passes through matter.

Question 12

How can the decrease in intensity be shown as an equation

Answer 12

I = I(0) e^-μ x

I(o) is the initial intensity
x is distance from surface
μ material’s attenuation absorption coefficient

Question 13

What are the ways in which X-rays are attenuated?

Answer 13

1) The photoelectric effect — a photon with around 30 keV of energy is absorbed by an electron, which is ejected from its atom. The gap in the electron shell is filled by another electron, which emits a photon.
2) Compton scattering — a photon with around 0.5-5 MeV of energy knocks an electron out of an atom, which causes the photon to lose energy and be scattered.
3) Pair production — a high (> 1.1 MeV) energy photon decays into an electron and a positron upon interaction with the nucleus

Question 14

How do you get contrast in X-ray imaging

Answer 14

How much energy is absorbed by a material depends on its atomic number — so tissues containing atoms with different atomic numbers (e.g. soft tissue and bone) will contrast in the X-ray image.
If the tissues in the region of interest have similar attenuation coefficients then artificial contrast media can be used — e.g. barium meal or iodine. These have high atomic numbers, so they show up clearly in X-ray images and can be followed as they move through a patient’s body.

Question 15

Are there situations where the X-ray is scattered but no energy is lost

Answer 15

Yes, this is known as simple scatte:

The X-ray photon interacts with an electron in the atom, but has less energy than the energy required to remove the electron, so the X-ray photon simply bounces off (is scattered) without any change to its energy.

Question 16

What does CAT scan stand for

Answer 16

Computerised axial tomography

Question 17

What do cat scans produce

Answer 17

scans produce an image of a two-dimensional slice through the body.

Question 18

How do CAT scans work

Answer 18

The patient lies on a table, which slides in and out of a ring.
This ring is made up of
detectors and a rotating X-ray beam.
3)
The X-ray beam fans out and rotates around the body.
It is picked up by the detectors. A computer works out how much attenuation has been caused by each part of the body and produces a high quality image.
4)
CAT scans produce more detailed images than regular X-rays, especially for soft tissue. The data can also be manipulated to generate a 3D image.

Question 19

What are CAT scan benefits

Answer 19

4)
CAT scans produce more detailed images than regular X-rays, especially for soft tissue. The data can also be manipulated to generate a 3D image.

Question 20

What are medical tracers

Answer 20

Medical tracers are radioactive substances that are used to show tissue or organ function.

Question 21

How do medical tracers differ to other imaging types?

Answer 21

Other types of imaging, e.g. X-rays, only show the structure of organs — medical tracers show structure and function.

Question 22

What are the parts that make up a medical tracer

Answer 22

Medical tracers usually consist of a radioactive isotope — e.g. technetium-99m or fluorine-18 — bound to a substance that is used by the body - e.g. glucose or water.

Question 23

How does a medical tracer enter the body?
What will a medical tracer go when it first enters and body?

Answer 23

The tracer is injected into or swallowed by the patient and then moves through the body to the region of interest. Where the tracer goes depends on the substance the isotope is bound to - i.e. it goes anywhere that the substance would normally go, and is used how that substance is normally used.

Question 24

How is the radiation released by a medical tracer recorded

Answer 24

The radiation emitted is recorded (e.g. by a gamma camera or PET scanner
- see below and next page) and an image of inside the patient produced.

Question 25

What can medical tracers be used to detect?

Answer 25

Tracers can show areas of damaged tissue in the heart by detecting areas of decreased blood flow.
This can reveal coronary artery disease and damaged or dead heart muscle caused by heart attacks.

They can identify active cancer tumours by showing metabolic activity in tissue. Cancer cells have a much higher metabolism than healthy cells because they’re growing fast, so take up more tracer

• Tracers can show blood flow and activity in the brain. This helps research and treat neurological conditions like Parkinson’s, Alzheimer’s, epilepsy, depression, etc.

Question 26

Give example of two isotopes used a medical tracers and how they are used

Answer 26

Technetium-99m is widely used in medical tracers because it emits y-radiation, has a half-life of 6 hours (long enough for data to be recorded, but short enough to limit the radiation to an acceptable level) and decays to a much more stable isotope.
Fluorine-18 is used in PET scans as it usually undergoes beta plus decay.
It has a half-life of 110 minutes, meaning the patient is exposed to radioactivity for a much shorter amount of time than with technetium-99m.

Question 27

List the 5 main parts of a gamma camera and their uses

Answer 27

1) Lead shield — stops radiation from other sources entering the camera.

2) Lead collimator — a piece of lead with thousands of vertical holes in it — only y-rays parallel to the holes can pass through.

3) Sodium iodide crystal — emits a flash of light (scintillates) whenever a y-ray hits it.
4)Photomultiplier tubes — detect the flashes of light from the crystal and turn them into pulses of electricity.

5)Electronic circuit — collects the signals from the photomultiplier tubes and sends them to a
computer for processing into an image which is used to help the doctor diagnose the patient.

Question 28

What are positive of a gamma camera?

Answer 28

Pros
Gamma cameras are useful in helping to diagnose patients without the need for surgery.
They are cheaper than a PET scanner but are still fairly expensive

Con:
They also use ionising radiation which is bad for you

Question 29

How does a PET scan work

Answer 29

1) In a PET (positron emission tomography) scan, the patient is injected with a substance used by the body, e.g. glucose, containing a positron-emitting radiotracer with a short half-life, e.g. 13N, 15O, 18F.
2) The patient is left for a time to allow the radiotracer to move through the body to the organs.
3) Positrons emitted by the radioisotope collide with electrons in the organs, causing them to annihilate, emitting high-energy gamma rays in the process.

4) Detectors around the body record these gamma rays, and a computer builds up a map of the radioactivity in the body.

The distribution of radioactivity matches up with metabolic activity. This is because more of the radioactive glucose (or whatever) injected into the patient is taken up and used by cells that are doing more work (cells with an increased metabolism, in other words).

Question 30

Sketch a cat gamma camera and pet scanner

Answer 30

.l. CGP

Question 31

What are the pros of a PET scan

Answer 31

By looking at which cells are doing more work, doctors can help diagnose illnesses in patients - like detecting the higher activity of cancer cells. PET scanners allow patients to be diagnosed without having to have surgery and the radiotracers used have a short half-life so the patient is exposed to radiation for only a short time.

Question 32

Cons of pet scanning

Answer 32

Short half life of radio-tracers means only a short time period when a patient can be scanned
- unlike with gamma cameras, where the tracer takes much longer to decay.
PET scanners are also incredibly expensive (due to requiring a particle accelerator to make its medical tracers) , meaning not many hospitals own one. This means some doctors may have to make difficult decisions about whether a patient should be sent for a PET scan.

Question 33

What do gamma cameras do?

Answer 33

The y-rays emitted by radiotracers injected into a patient’s body are detected using a gamma camera.

Question 34

What are examples of ionizing radiation?

Answer 34

X-rays, gamma rays, and alpha and beta particles are all classed as ionizing radiation

Question 35

What does ionizing radiation do

Answer 35

When they interact with matter they ionise atoms or molecules to form ions — usually by removing an electron - and this can damage cells.

Question 36

What is the cell damage caused by ionizing radiation?

Answer 36

1) Cell mutations and cancerous tumours by altering or damaging the cell’s DNA.
2) Cell sterility by stopping the cell from reproducing.
3) Cell death — the cell is destroyed completely.

Question 37

What are the macroscopic effects of ionizing radiation (and cell damage)

Answer 37

The macroscopic effects of ionising radiation (i.e. the large-scale effects) include tumours, skin burns, sterility, radiation sickness, hair loss and death

Question 38

When is the only time radiation is used in medicine

Answer 38

radiation is only used when the benefits to the patient outweigh the risks — i.e. radiation doses are limited and only used when it’s absolutely necessary.

Question 39

What makes gamma photons the ideal source of radiation in the bodŷ

Answer 39

Least Ionizing

Question 40

What is another term for medical tracers

Answer 40

Radio pharmaceuticals

Question 41

What is ultra sound?

Answer 41

Ultrasound waves are longitudinal waves with higher frequencies than humans can hear (>20 000 Hz).

Question 42

What frequencies of ultra sound is used for medical purposes

Answer 42

For medical purposes, frequencies are usually from 1 to 15 MHz.

Question 43

What happens when ultra sound waves meet a boundary

Answer 43

When an ultrasound wave meets a boundary between two different materials, some of it is reflected and some of it passes through (undergoing refraction if the angle of incidence is not 90°

Question 44

How can ultra sound be used to generate an image

Answer 44

The reflected waves (caused when a ultra sound wave meets a boundary) are detected by the ultrasound scanner and are used to generate an image.

Question 45

What is an equation that can be used to find the acoustic impedance, Z, of a medium ?

Answer 45

Z= ρc

Where ρ is materials density
c is speed of sound in that material

Question 46

What are the units of acoustic impedance

Answer 46

Kgm^-2s^-1

Question 47

How does the intensity of a reflected ultrasound wave vary with the difference of acoustic impedance at a boundary between two materials.

How can this be expressed by a equation.

Answer 47

Say an ultrasound wave travels through a material with an impedance Z ₁. It hits the boundary between this material and another with an impedance Z ₂. The incident wave has an intensity of lo:

If the two materials have a large difference in impedance, then most of the energy is reflected (the intensity of the reflected wave I ᵣ will be high). If the impedance of the two materials is the same then there is no reflection.

I ᵣ/I ₀ = (Z ₂ — Z ₁)^2 / (Z ₂ — Z ₁)^2

Question 48

What are the advantages of ultra sound imaging

Answer 48

1) There are no known hazards — in particular, no exposure to ionising radiation.
2) It’s good for imaging soft tissues, since you can obtain real-time images.
3) Ultrasound devices are relatively cheap and portable.
4) The scan is a quick procedure (10-15 minutes) and the patient can move during the scan.

Question 49

What are the disadvantages of ultra sound imaging

Answer 49

1) Ultrasound doesn’t penetrate bone — so it can’t be used to detect fractures or examine the brain.
2)Ultrasound cannot pass through air spaces in the body (due to the mismatch in impedance) — so it can’t produce images from behind the lungs.
3) It can’t give detail on solid masses.
4)Ultrasound can’t give information about any solid masses found.

Question 50

What happens when you squash or stretch a piezoelectric crystal

Show this on a diagram

Answer 50

Piezoelectric crystals produce a potential difference when they are deformed (squashed or stretched) — the rearrangement in structure displaces the centres of symmetry of their electric charges.

For diagram see CGP pg 184

Question 51

What happens when you apply a pd across a piezoelectric crystal?

Answer 51

When you apply a p.d. across a piezoelectric crystal, the crystal deforms. If the p.d. is alternating, then the crystal vibrates at the same frequency.

Question 52

Explain the role of piezoelectric crystals in a ultra sound transducer

Answer 52

A piezoelectric crystal can act as a receiver of ultrasound (Any ultrasound incident on the crystal will make it deform/vibrate), by converting sound waves into alternating voltages, and also as a transmitter, converting alternating voltages into sound waves. To generate ultrasound, a high-frequency alternating p.d. is applied across opposite faces of a crystal. This repeatedly compresses and expands the crystal.

Question 53

What is done to PZT crystals to increase the ultrasound transducers resolution

Answer 53

The PZT crystal is heavily damped, to produce short pulses and increase the resolution of the device.

Question 54

Explain what crystal is used in ultra sound transducers and why it has its dimensions

Answer 54

Ultrasound transducers use lead zirconate titanate (PZT) crystals. The thickness of the crystal is half the wavelength of the ultrasound that it produces. The frequency is chosen to be the same as the natural frequency of oscillation of the crystal. Ultrasound of this frequency will make the crystal resonate and produce a large signal.

Question 55

Why is a coupling medium used with a ultra sound transducer?

Answer 55

Soft tissue has a very different acoustic impedance from air, so almost all the ultrasound energy is reflected from the surface of the body if there is air between the transducer and the body.
2) To avoid this, you need a coupling medium (usually a gel) between the transducer and the body
- this displaces the air and has an impedance much closer to that of body tissue.
The use of coupling media is an example of impedance matching.

Question 56

What is an A-scan?

Answer 56

1) The amplitude scan (A-Scan) sends a short pulse of ultrasound into the body simultaneously with an electron beam sweeping across a cathode ray oscilloscope (CRO) screen.

Question 57

What are the peaks in an A-scan caused by?

Answer 57

The scanner receives reflected ultrasound pulses that appear as vertical deflections on the CRO screen. Weaker pulses (that have travelled further in the body and arrive later) are amplified more to avoid the loss of valuable data — this process is called time-gain compensation (TGC).

The horizontal positions of the reflected pulses indicate the time the ‘echo’ took to return, and are used to work out distances between structures in the body (e.g. the diameter of a baby’s head in the uterus).

Question 58

.

Answer 58

.

Question 59

How can you gain a steady imagine on the CRO

Answer 59

A stream of pulses can produce a steady image on the screen, although modern CROs can store a digital image after just one exposure.

Question 60

What happens in a B scan

Answer 60

1) In a brightness scan (B-Scan), the electron beam sweeps down a screen rather than across.
-each dot represents the boundary between two tissues
2) The amplitude (and therefore intensity) of the reflected pulses is displayed as the brightness of the spot.
3) You can use a linear array of transducers to produce a two-dimensional image.

Question 61

How do ultra sound waves experience the Doppler effect

Answer 61

Ultrasound waves reflected at an angle to moving cells undergo a change of frequency (or wavelength). The frequency is increased when the blood is moving towards the transducer and decreased when the blood is receding from the transducer. This is caused by the Doppler effect

Question 62

How can the change in frequency of ultrasound waves caused by the Doppler effect be used by doctors

Answer 62

This change of frequency (beat frequency) can allow doctors to find the speed at which those cells are moving (for example, blood cells in an artery).

Question 63

What is the equation used to find the speed of cells from the Doppler effect

Answer 63

Δ f/f = 2vcos θ / c v = c Δ f / 2f cos θ

where f is initial frequency,
Δf change in frequency,
v is the velocity of the moving cell,
c is the speed of sound in that medium
θ is the angle between the ultrasound receiver and the direction in which the cell is moving.

Question 64

How do the components of a CAT scanner produce a high quality images of a patient

Answer 64

X-ray (tube) moves around the patient
A thin (fan-shaped X-ray) beam is used
(Images / scans of) cross-sections through the patient are taken
Any one from:
- A three-dimensional image is produced
-(Soft) tissues can be identified