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Detectors Flashcards

1
Q

What type of modes do detectors have?

A

pulse mode, current mode, or mean square voltage (MSV) mode.

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2
Q

Describe pulse mode.

A
  • Pulse mode is the most common, and the only mode capable of measuring the energy of the radiation. Pulsed mode records each individual quantum of interaction-this means we can actually know the individual energy of each event.
  • count the number of pulses, or even set some energy threshold below which pulses aren’t counted
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3
Q

Describe current mode.

A

If event rates are high enough, the number of charges being collected can produce an outright current. This will vary depending on the response time of the detector, T. Longer response time slows it, but reduces statistical fluctuations.

Current mode assumes all of the interaction events are similar

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4
Q

How is Mean square voltage (MSV) different from current mode?

A

Current mode assumes all of the interaction events are similar, and can be counted as roughly the same. If this is not the case, MSV mode can be used to specifically weight larger or smaller events in certain ways. Like with neutron interactions, where we have little photon signal relative to neutron measurements.

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5
Q

What are Resistors and Capacitors used for in a circuit?

A

A resistor and capacitor are added in with the detector output to form a preamp. By adjusting the values of R and C, we can alter the time constant tau. Tau, relative to collection time, can dictate how briefly the pulses produced last

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6
Q

How is the magnitude of the pulse measured?

A

Values of C dictate the magnitude of the produced pulses via:

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7
Q

How do we characterize energy resolution?

A

We use FWHM to characterize resolution
We can characterize this with the Full Width Half Max. In a Gaussian Distribution (common), this is found as 2.35*standard deviations.

In a process with multiple factors (noise, signal, drift, etc.) the FWHMs actually add in quadrature.

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8
Q

What are two types of detection efficiency?

A
  1. Absolute efficiency- The number of recorded pulses divided by the total number of quanta emitted by the source.
  2. Intrinsic Efficiency- The number of recorded pulses divided by the total number of quanta incident on the detector. We can relate them via solid angle:
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9
Q

How do we measure source emission quanta?

A

individual peak efficiencies, related by some ratio of peak to total: r= Epeak/Etotal

We can use intrinsic peak efficiency to estimate source emission quanta:

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10
Q

What are dosimeters?

A

Dosimeters measure dose, usually through some other, related variable.

There are many types, each reads off a different variable/reaction

Functions over a different range of radiation levels.

Some types are absolute- giving direct measurement. Others only measure dose relative to some other medium.

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11
Q

What is dosimeter energy dependence?

A

Different dosimeter types react differently to different interaction types, leading to some energy dependence over the ranges for which different interaction effects dominate.

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12
Q

Describe an ion chamber.

A
  • Most basic kind of detector,
  • ion chambers have some gas filled volume in which ion pairs are formed by the incidence of radiation.
  • Applied potentials cause these ion pairs to be collected by electrodes, thus producing charge signals for us to read out.
  • We use Bragg-Gray Cavity theory to determine dose to the wall surrounding the gas (chamber material) from exposure in the gas cavity.
  • Ionization generally happens around 10-25eV
  • most suited for lower-energy radiation
  • Ion chambers can be absolute or relative dosimetry. They are more often relative
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13
Q

How is dose calculated different from actual dose (in humans etc)?

A
  • One, that the number of ion pairs produced in a gas doesn’t exactly match up to how energy would be deposited in a human.
  • Two, we do not include any ionization events caused by bremsstrahlung production in the medium.
    • That said, it’s a decent enough approximation, generally.
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14
Q

How do we modify K to get KC?

A

Kerma can be modifed to only describ ecollisional Kerma Kc. (Bremsstrahlung component removed)

The value g is often very small (~0.0003), and gets smaller with lesser incident energy.

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15
Q

List free electron interactions inside an ion chamber.

A
  1. Charge transfer
  2. Electron attachment
  3. Recombination
  4. Diffusion
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16
Q

Explain what happens during Charge transfer in the ion chamber.

A

Charge transfer: Positive ion encounters a neutral gas molecule and takes an electron from it. The new molecule becomes an ion, while the old becomes neutral.

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17
Q

Explain what happens during Electron attachment in the ion chamber.

A

Electron Attachment: Freed electron attaches to a neutral gas molecule. It then becomes a negative ion that moves more slowly than the electron did, due to increased mass.

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18
Q

Explain what happens during recombination.

A

Recombination: Free electron (or negative ion from Charge Transfer) meets positive ion and recombines, effectively reverting the ion-pair creation.

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19
Q

Explain what happens during Diffusion in an ion chamber.

A

Diffusion: Charged particles move from high to low densities. More so for electrons, since they are faster.

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20
Q

What is electron mobility?

A

Electron mobility characterizes how quickly an electron can move through a metal or semiconductor when pulled by an electric field.

  • Mu is the ‘mobility’ of the charges and is higher for electrons than ions (~x1000), making collection time on the order of microseconds.
  • Mobility depends on a material charges are moving in

Charge drift velocity:

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21
Q

Describe electronegativity and how it affects ion chambers.

A

Electronegativity is the ability of an atom to attract electrons.

  • some neutral gasses may collect freed electrons and become negative ions.
  • The charge is still there, but now moves more slowly due to increased mass
  • removes charge from the system
  • electronegative gases (SF6) - more likely to absorb electrons and become negative ions
  • not-electronegative gasses (Argon) will reduce ion pair recombination in a system.
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22
Q

Describe charge multiplication and how it affects ion chambers.

A
  • We use electric fields to accelerate radiation-generated charged particles (secondary quanta) through some gas to a collecting electrode.
  • If we accelerate them fast enough, though, these ion pairs may be able to produce more ion pairs of their own by ionizing the fill gas

In some models of detector, we do this on purpose. For typical ion chambers, we do not want this effect.

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23
Q

What are two types of recombination? Describe them.

A

Initial Recombination: Sometimes the electron and ion may immediately recombine after generation. This effect is independent of dose, and often negligible at typical operating characteristics.

General Recombination: Some time after the initial ionization event, recombination may occur elsewhere in the volume. This is dose dependent.

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24
Q

How do we account for recombination?

A

We can make correction factors to account for recombination by considering the charges collected at various different operating potentials for the ion chamber. (Pion in therapy)

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25
Q

Draw and describe 3 regions of saturation curve (charge vs voltage curve)

A

Three regions:

  1. Recombination region
  2. Near-saturation region
  3. Charge multiplication region
26
Q

What are Op Amps and what are they used for? Sketch it.

A

An Operational Amplifier, or op-amp for short, is fundamentally a voltage amplifying device.

  • 1 output, 2 inputs (inverting (gets feedback from output) and non-inverting)
  • output tries t adjust and make inputs equal
  • R amplifies the input
  • C used to measure charge
27
Q

General structures of Ion chambers? List 4 types.

A

Generally, ion chambers are made of some gas-filled cavity surrounded by a wall of some material, then placed inside some medium (like water, or a phantom).

28
Q

What purpose does the wall serve in an ion chamber?

A

The wall serves to protect our gas cavity from unwanted contaminants, and to provide secondary charged particles to establish (T)CPE (buildup). Walls must be thick enough to cause CPE, but not too thick to notably perturb the beam (<1% particle range).

29
Q

What materials are used for ion chamber walls and why?

A

We want to match the material of our wall as best we can to the local medium. Since the medium is often a tank of water, or a water-equivalent plastic, we want the wall to be of such too. We do this by finding materials with similar Zeff

30
Q

How is Zeff calculated?

What is Zeff of water?

A
  1. The effective Z of a material, Zeff, is determined by the Z of each constituent element, and what fraction of the total electrons they contribute
  2. Zeff of water is approximately 7.5
31
Q

Calculate Zeff of water

A

Water is 2H+1O. Each H was 1 electron, O has 8. So there are 10 total electrons, 2 from H, 8 from O. So we have 2/10 for H and 8/10 for O.

32
Q

Correction factor: PTP

Temperature and pressure

A
  • ambient temperature and pressure can have effects
  • To account for this, we can use another correction factor PTP.
33
Q

Correction factor: Pion

Recombination

A

We can make correction factors to account for recombination by considering the charges collected at various different operating potentials for the ion chamber.

34
Q

Correction factor: Ppol

Polarity effects

A

We can set up polarity (+ vs - electrodes) however in a chamber. However, due to mechanical complications, there may sometimes be inconsistencies in readings with reversed polarities. We use another correction factor to account for this.

35
Q

What are chamber guards used for?

A

To address fringe field effects we see outside, we use ‘guards’, additional plates that will collect the charges at the edges, but not report them as the main electrodes do. This allows us to make the collecting field only cover what we want.

36
Q

What are Free-Air Chambers? (FAC)

A

Rather than using cavity theory, some chambers are open air, and can measure exposure/KERMA directly.

The free air chamber is an ionization chamber without walls.

  • used at lower energies (<0.5 MeV)
  • need large length to accomplish CPE
  • We can have scatter within the chamber too. Using tubes of attenuating plastic can lessen this effect.
37
Q

What is the ionization energy of free air?

A

(W/e)air

  • Ionization energy of free air (33.97 J/C) is used in damn near all dose calculations.
  • Experimentally measured with a calorimeter
38
Q

How does a calorimeter work?

A

Calorimeter is an apparatus for measuring the amount of heat involved in a chemical reaction or other process.

Calorimeters are able to make more direct measurements of energy deposition in a material by very precisely detecting changed in temperature

39
Q

Describe Geiger-Mueller tubes.

A

G-M tubes purposefully implement many charge multiplications (Townsend avalanche).

After so many avalanches, the effect terminates, so all pulses from a G-M Tube are the same height, regardless of incident radiation type/energy. They can only be used for counting (at relatively low rates, too).

40
Q

What is Townsend avalanche?

A

Accelerated electrons can create more ionizations to excite more electrons.

41
Q

What is Geiger discharge?

A

Uncontrolled spread of avalanches throughout the entire detector is known as a Geiger discharge.

avalanche electrons → excite atoms → and release photons → starting another avalanche→ chain of avalanches

Positive ions created and collected on the anode → wire becoming thicker → weakening the electric field→ slows electrons→ electrons not able to ionize→ stops avalanche

42
Q

What do we use to fill GM Tubes?

What is quench gas?

A

noble gases (not very electronegative) like Argon.

To prevent avalanche creation from neutralization after avalanche termination, we mix in some quench gas with a lower ionization potential.

These quench gas molecules become ions via charge transfer, making it so that when they are collected at the cathode (instead of main gas ions), the energy transfer is less, and won’t accidentally free an electron.

43
Q

What is GM deadtime and efficiency?

A
  • fairly long deadtime in G-M Tubes
  • very high detector efficiency
44
Q

What types of detectors are there?

A
  • ionization chambers
  • solid state (scintillator, semiconductor, TLDs, OSLDs, diamonds, MOSFETs, diodes)
  • cherenkov detectors
  • chemical dosimeters
  • thermal (calorimeters: water, graphite)
45
Q

What is a scintillator?

A

Scintillator is something that takes input ionizing radiation and emits visible light photons with some proportionality

We collect these visible light photons after scintillator emission using photodiodes. We may also use a photon multiplier tube PMT in between for amplification of the light signal.

46
Q

What is fluorescence in scintillators?

A

Fluorescence: Immediate (~ns) emission of visible light photons following the radiation. This is from the transition of a charge to a lower quantum state.

47
Q

What is slow fluorescence?

A

Slow Fluorescence: Also called a Beta process, these emit light at longer wavelengths and over longer periods of time (~10s mus).

48
Q

What is delayed fluorescence?

A

Delayed Fluorescence: Also called phosphorescence or Alpha process, this has the same spectrum as with fluorescence, but only emits the light with some trigger

49
Q

What is a scintillator and phosphor?

A

For the purposes of this study, a scintillator is a substance that fluoresces in response to ionizing radiation. A phosphor is a substance that can emit light at any any timing in response to any radiation.

By these rules, a scintillator is a subtype of phosphors.

50
Q

What are common scintillator materials?

A

Na(TI), CsI(TI)-better spatial resolution; light pipes, CsI(Na), Gadox, BGO (bismuth)

51
Q

Pulse mode and Charge integration modes in scintillators?

A

Pulse Mode: In low-intensity systems, we can look at each individual photon pulse introduced, and correlate it’s height. Problems: we may get pulse overlap, or the signal may be too weak next to noise

Charge Integration Mode: More often, we’ll use this mode, where we integrate over the accumulated charge per pixel over some time window. In this case, the total charge collected will be linearly proportional to the energy received via radiation within the time window.

52
Q

Chernekov detectors?

A

Cherenkov light is emitted when a charged particles travels faster than the speed of light in a medium ( v> c/n )

Generally, these optical photons are tracked via a camera.

53
Q

Solid state detectors

A

Radiation helps lift electrons out of the valence band, or a trap near the conduction band. Reading out one of these involves imparting some energy to return the state to equilibrium, which often causes emission of photons.

54
Q

What are TLDs?

A

A TLD, or Thermo-Luminescent Dosimeter releases this light (from trapped energy in the solid state detector) energy by imparting heat into the material. The emitted light is recorded via photomultiplier tubes.

  • high Z
  • LiF:Mg
  • reusable (reset by heating)
  • Readout linear but becomes supralinear at high doses (>5Gy)
55
Q

What are OSLDs?

A

OSLDs work in much the same way as TLDs (traps, read off photons, etc.) but are Optically-Stimulated Luminescent Dosimeters rather than Thermo-Luminescent.

We use green lasers (other colors work, though) to stimulate the release of photon. Notably, while TLDs release all their stored information at once, multiple readings can be made off of an exposed OSLD (though we can fully reset them with enough light).

These are often made of Al2O3:C, and have the same supralinear behavior above ~5Gy.

56
Q

What are semiconductor detectors?

A

Semiconductor detectors exist in the form of PN-Junctions. Generally, we apply no bias to these (for some reason?).

These are nice because they have high sensitivity per volume, good spatial resolution, good mechanical stability, and require no external bias (?).

However, they are temperature dependent and not very water-equivalent (Si)

57
Q

Diamond detectors

A

Diamond detectors used to be too expensive, but can now be synthesized at an acceptable cost by some jackasses in Germany.

These are sturdy solid state detectors with minimal leakage and decent water equivalence (Z=6). Due to their super high heat resistance, they can also be sterilized for in-vivo applications.

They require a bit of warm up dose before they become really effective though

58
Q

MOSFETs

A

Metal Oxide Semiconductor Field Effect Transistor.

Basically, at a certain voltage, current is able to flow through a MOSFET. Irradiation increases this needed threshold. We use the difference in Voltage threshold as the reading, as the value of its shift is proportional to the dose received by the MOSFET.

MOSFETs are often used for in-vivo dosimetry, but operate under a relatively low dose range, and can only be exposed to so much dose (~20 measurements of 2Gy) before being ‘used up’ and must be discarded.

59
Q

Photomultiplier tubes

A

Photomultiplier tubes take input photon signals (~100s of photons) and magnify them while adding minimal noise. Input photon signals often come from scintillators.

The general structure has a photosensitive cathode receive incident photons and emit low energy electrons (it’s possible from some electrons to escape the cathode by thermal energy- weak noise).

These electrons are then accelerated down the tube and multiplied, resulting in a strong electrical output signal from a weak photon input signal.

60
Q

What is a dynode?

A

A dynode is an electrode in a vacuum tube that serves as an electron multiplier through secondary emission.

61
Q

Photodiods

A

Photodiodes also convert incident photons into electrical signals. They’re basically as the semiconductor detectors, with a reverse bias for wider depletion region.

Photodiodes (and PMTs, for that measure) can never actually improve the SNR of the signal. At best, they can only minimize the noise they introduce to make the incident SNR not get any worse.

Also, many light sensors (photodiodes, PMTs) are only really sensitive to certain ranges of light. Must make sure they’re chosen appropriately with the prior phosphor.

62
Q

How do we get dose from temperature?

A

From measured change in temperature, we may calculate dose, but this requires knowing the heat defect, k. Which is also found experimentally.

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