Radiation Detectors & Instrumentation Flashcards
Why do we need radiation detectors?
No human sense can accurately detect radiation, minimize dose, calculate dose in therapy/imaging
What are the 5 interactions of photons with matter?
Photoelectric effect, compton scattering, pair production, coherent (Rayleigh) scattering, photonuclear effect
Describe photoelectric effect…
Photon is absorbed, and a photoelectron is emitted.
Describe compton scattering…
Part of photon energy is absorbed by and emitted electron, and a scattered photon continues with lower energy
Describe pair production…
A positron/electron pair is created during nuclear interaction. It requires an incoming photon energy greater than or equal to 1.022 MeV
Describe coherent (Rayleigh) scattering…
A photon interacts with nucleus and is slightly deflected with very little to no energy loss. It is only significant at low energies (<50 keV), effect is usually ignored for dose calculations. It’s the reason the sky is blue!
Describe photonuclear effect…
Photons interact with and excite the nucleus. The nucleus then emits a proton or a neutron. (Note, protons can contribute to dose but the amount is minimal, and neutrons can escape vault or activate other materials.) These are prevalent when photon energies are greater than 10MV. High energy Linacs are shielded against neutrons.
What is the probability of interaction dependant upon?
the energy of the photon, and the compostion of the target.
How do interactions allow for detection?
When photons interact with matter, ions/electrons are created, or neutrons or excited states are generated which supplies a chain of secondary ionizations. Basically, they produce detectable stuff.
Describe the basic principles of detection…
All detectors of ionizing radiation requires interaction of radiation with matter. Ionizing radiation deposites energy in matter by ionization and excitation. Both of these processes (ionization and excitation) are involved in detection of radiation events.
Define ionization…
When the energy transferred is sufficient to cause and orbital electron to be stripped away from its parent atom completely
Creates ion pair (One +, one -)
Define Excitation…
When an orbital electron gains enough energy to jump shells but not enough to remove it from the atom.
Excited electrons jump shell will fall down to original shell, releasing characteristic radiation (x-ray)
x-ray may produce additional ionizations
What are the 4 types of detectors?
Gaseous detectors, solid-state (Semi-conductor) detectors, radiographic film, scintillation detectors
What are some examples of gaseous detectors?
Farmer chamber, survey meter, dose calibrators, proportional counters, geiger-mueller detectors
What is an example of solid-state detectors?
Ge, Si
What are the 2 subgroups or scintillation detectors, and what are some examples of those?
Solid crystals: Nal(TI), Csl(Na), BGO
Luminescent powder: TLD, OSLD
Describe the basic principle of gas filled detectors…
The volume of gas between electrodes at a high potential difference (gas acts as an insulator)
The negatiobe electrode is the cathode and the positive electrode is the anode
No radiation = no ions = no current flowing between electrodes
Radiation = ionization of gas molecules
Electrons produced (-) are attracted toward the anode (+), while the positive (+) ions are attracted towards the cathode (-).
Current is proportional to amount of ionization produced in gas
Output current obserbed as a electric signal in the form of pulses.
Describe the recombination region…
Recombination of ions occurs at low voltages, region is not suitable for radiation detection i.e. at this low voltage region, ion pairs created recombine. The actual charge produced greater than charge measured.
Describe the ionization chamber region…
Voltage is sufficient to cause complete collection (100%) of all the charges produced (curve becomes flat and reaches saturation, Farmer chamber operates in this region, 50-300 Volts for saturation
All gaseous ion chambers operate at a coltage that produces a saturation current, it levels off because all chages are already collected and rate of formation is constant.
Describe the proportional region…
Voltage beyond the saturation region, amount of electric charge measured due to radiation is proportional to the energy and type of radiation
Describe the Geiger-Mueller region…
Voltage beyond the propotionality region, amount of charge collected in each ionizing event is same regardless of how much additional voltage is applied
How much energy is needed to produce one ion pair in air?
33.97 eV/i.p = 33.97 J/C
How many ions are produced if the energy of one 1MeV photon is completely absorbed in air?
1*10^6 eV * (1 i.p. / 33.97 eV) = 29,437 ions
About how much eV does it take to produce an ion pair in tissue?
25 eV
What is a gas ionization chan=mber, and what are some examples?
Ionization chambers measure the total charge produced from radiation interactions. They are sometimes filled with gases, or left open to the air.
- Farmer ion chamber
- Survey meters
- Nuclear medicine dose calibrators
What gases may be found in a gas ionization chamber?
methane, propane, or a tissue equivalen gas
What is found between the electrodes in most gas ionization chambers?
Air
Gas filled chambers may or may not be sealed from the atmosphere. If it is not sealed, what needs to be accounted for?
The measured value should be corrected for air pressure deviations.
(pV = nRT)
How does a gas filled detector work?
- Radiation causes the formation of ions in the chamber
- The central electrode collects the electrons
- The voltage applied between the electrodes should be sufficient enough to collect all ions created
- If voltage is too low we get a hifh probability that the ions will recombine and the signal will be lost
What is a farmer chamber?
AKA thible chamber
Voltage applied is ~300 V
Unsealed volume must correct for air pressure
Standard chamber used for absolute dosimetry in LINAC output measurements
The chamber volume is 0.6 cm^3
The central collecting electrode is made of a low Z material such as aluminum.
What is a survey meter?
Survey meters are used to monitor radiation levels for radiation protection purposes.
It is battery operated and portable.
The ionization chamber is contained inside the survey meter.
Describe dose calibrators
Used in nuclear medicine department to assay patient doses in a syringe or vial prior to injection.
Sealed chamber eliminates the need to adjust output due to atmospheric pressure variation
Reads in units of activity (Ci or Bq)
Ionization chamber is of a well??
Advantages and disadvantages of ionization chambers?
Advantages
- Good uniform response to gamma/x-ray radiation and give an accurate overall dose reading
- Can measure high radiation output
- Sustained high radiation levels, does not degrade gas
Disadvantages
– Very low electronic output requiring sophisticated
electrometer circuit
– Operation and accuracy easily affected by moisture
– Cannot measure energy of radiation - no energy
spectrum information
Describe proportional counters…
Detector voltage is increased so liberated electrons
will gain velocity
– Electrons produce secondary ionizations proportional to
the number of electrons created
* This allows us to distinguish between different
ionization energies and types
– Alpha ionizations
– Beta ionizations
– Gamma (x-ray) ionizations
- Sealed and filled with noble gases
– Argon or Xenon - Commonly used in research applications for
measuring radiation from alpha and beta particles
Advantages and disadvantages of proportional counters?
Advantages
– Can measure energy of radiation and provide energy
spectrum information
– Can discriminate between particles
– Large area detectors can be constructed
Disadvantages
– Anode wires delicate and can lose efficiency
– Efficiency and operation affected by infiltration of
oxygen into gas
– Measurement windows easily damaged in large area
detectors
