Radiation Detectors and Their Properties

Draw and label an ion chamber

Generally describe how an ion chamber works and what it measures

1. A cavity filled with air is surrounded by two electrodes charged to around 300 V.

2. When ionizing radiation interacts within the active volume it creates ion pairs (+/-).

3. 34 eV is required on average to create an ion pair in air (and is the work function in air).

4. The ions are swept away by the potential between the plates.  This charge or current is read by the electrometer.

5. By using a calibrated chamber, one can extract dose from the ionization reading.

6. Can be pressurized to remove dependence on atmospheric temperature and pressure and to increase the density of the gas (increases response).

Measures charge

What are the pros and cons of ion chambers and what are they useful for

Advantages

1. Good energy independence for Farmer chambers (40 kV - 50 MV).

2. Accurate within 1% with proper care.

Disadvantages

1. Some dose rate dependence (changes recombination).

2. Requires somewhat large size (inappropriate for small fields).

Useful for

• Linac outputs

• PDD

Useful range

• Above about 10 cGy but depends on the electrometer

Radiographic film operational theory

1. Consists of a transparent base film coated with an emulsion of silver bromide (Ag+ and Br- ions).

2. When radiation (or light) hits the film, it frees loosely bound electrons.

3. These electrons aggregate around impurities forming a negative charge which attracts the Ag+ toward it forming neutral silver atoms (Ag).  This is called the latent image.

4. “Developing”:

1. First, the film encounters the developer (base) which amplifies the amount of silver found on any latent image grains.

2. The fixer (acid) is next which removes any remaining silver bromide that has not interacted (makes the film resistant to light).

3. The film is then washed in water to remove the fixer.

What is the quantity measured for radiographic film

Optical density (OD) which is the log of the ratio of the original amount of light over transmitted

Pros and cons of radiographic film, what its useful for, and useful range

Pros

1. Extremely high resolution (film grain is on the order of 1 micrometer).

2. Large measuring area.

Cons

1. Images are very dependent on the developer maintaining exact conditions for all developed films (chemical composition, temperature, etc.).

2. Only accurate to within 3-5% with careful quality control.

3. Sensitive to visible light.

4. Rather strong energy dependence below 400 keV (Ag, Z = 45).

Useful for

1. Profiles (flatness and symmetry).

2. IMRT fields.

3. Light versus radiation fields.

4. Star shots.

5. Lots and lots of different things.

Useful range

1. Strongly dependent on film type (speed).

2. Somewhere in the range of 0 - 6 Gy.

radiochromic film operational theory

1. Film consists of an active layer of radiation-sensitive polymers that change their chemical structure to a blue shade as a function of radiation exposure.

2. These polymers sit on an inert polyester substrate (usually mylar).

what does radiochromic film measure

Degree of shading from background turning blue with increasing exposure (yellow-marker dye used for EBT2 GafChromic film).

Pros and cons of radiochromic film, what its useful for, and useful range

Advantages

1. Wonderfully high spatial resolution.

2. Very close to tissue equivalent (Z_eff =  7.5).

3. No chemical processing needed (accurate within 2-3%).

4. Energy independent from the keV to MeV range.

5. Not sensitive to incandescent light (sensitive to UV in fluorescent light though).

Disadvantages

1. Calibration needed to go from relative to absolute dosimetry.

2. Even though ‘wet’ processing materials are not needed, you still need a fairly sophisticated transmission scanner for reliable results.

3. Wait 24 hours before reading due to continuing reactions.

Useful for

1. Is useful for cases where a lot of measurements are needed such as for a dose plane.

2. Radiosurgery cases, such as IMRS, where spatial features (in addition to absolute values) of dose distribution need to be well-characterized.

3. Quick profiles (e.g. wedge profiles for flatness and symmetry measurements) can be quickly done using a simple setup.

Useful range

1. 0.1 Gy - 50 Gy (some film products tout reliability when exceeding 100 Gy for radiation therapy applications, e.g. cone measurements for Trigeminal Neuralgia cases).  TG-55 (radiochromic film dosimetry) states that this range can be extended to 10⁶ Gy.

Diodes operational theory

1. Made of semiconducting materials (usually silicon or germanium). Density is about 1800 times that of air.

2. The band gap (ionization energy) is about 3.5 eV. Compared to air (34 eV) we get about 10 times more charge carriers for a given energy.

   Doping:

1. A pure semiconductor is not very useful.

2. For an n type semiconductor pentavalent (electron donor) materials are added.

3. For a p type semiconductor trivalent (electron acceptor “hole”) materials are added.

4. Junctions of p-type and n-type materials (p-n junctions):

1. When an n type and p type material are joined, a depletion region forms (devoid of charge carriers):

1. Electrons from the n side hop over to the p side.

2. Holes from the p side hop over to the n side.

3. This continues until the electric field between the n and p sides balances the diffusing force

4. This area is called the depletion region and is the active volume of the detector.

5. The inherent electric field sweeps any electrons created by ionizing interactions to the n side and similarly any holes to the p side (this is a current and can be measured).

   Reverse biasing:

1. A negative voltage applied to p side and a positive voltage to the n side.

2. Increases the size of depletion region (more sensitive).

3. Increases the electric field (less recombination).

Diodes quantity measured, pros and cons, useful for, and useful range

measure current or charge

Advantages

1. Can be made very small.

2. Can directly measure %DD for electrons (stopping power varies similarly to water with energy).

Disadvantages

1. Subject to reduced sensitivity with radiation damage.

2. Temperature dependence (up to 0.5% per ^oC).

3. Angular dependence.

4. Field size dependent.

5. Requires frequent recalibration.

6. Strong energy dependence.

Useful for

1. Small field sizes (output factor or %DD).

2. Electron %DD.

3. In vivo dose measurements.

Useful Range: above 5 cGy

sketch a MOSFET

MOSFET operational theory

1. onsists of a p-type semiconductor (gate) sandwiched between two n-type semiconductors (source and drain) (or the other way p-n-p).

2. MOSFETs are usually employed as a current “valve” in electronics where a small change in voltage applied to the gate results in a large change in current flowing from the source to the drain (an amplifier).

3. For a n-p-n type a positive voltage is applied to the gate, when ionizing radiation interacts in a mosfet, electrons are attracted to the gate and holes are attracted to the silicon substrate.

4. Ordinarily, there is a minimal amount of voltage required to be applied to the gate to start current flowing from the source to the drain.  This “threshold voltage” is increased by the ionizing radiation.

5. The increase in threshold voltage is linearly related to the received dose.

MOSFET quantity measured, advantages and disadvantages, useful for and useful range

Measures threshold voltage

Advantages

1. Extremely small (smaller than diodes).

2. Non destructive reading (permanent threshold voltage change).

3. Dose rate independent.

Disadvantages

1. Large temperature dependence of threshold voltage.

1. This can be removed through the use of 2 MOSFET’s on the same chip biased at different levels

2. Finite lifetime up to about 100 Gy

Useful for

1. Penumbra.

2. Very small field sizes (cones).

3. Surface dose.

Useful range: above 5 cGy

TLD operational theory

1. Made of a material that forms a crystal lattice (LiF for tissue equivalence and CaF₂ for high sensitivity are common).

2. Electrons exist in two planes: The conduction band (high energy) and The valence band (low energy).

3. Impurities are added to the crystal to form trapping centers (Mg or Mn).

4. The “traps” impurities allow electrons to become trapped in between the conduction band and the valence band.

5. Heat is applied to free the electron from the trap which falls to the valence band and releases visible light.

6. The amount of light released is proportional to the radiation dose delivered.

7. A plot of the quantity of light released from the crystal versus the temperature of the crystal is called the “glow curve”.

8. Given a long time at high heat a crystal will become “annealed”, that is, have no more trapped electrons and can be used again

TLD quantity measured

light emission from the crystal

Advantages of TLDs

1. No wires.

2. Can be very small.

3. LiF is relatively energy independent down to around 100 kV.

4. Can be used as a neutron detector when prepared with Lithium-6

   Use two different TLD chips Li-6 responds to neutron and gamma radiation (TLD-600)

   Li-7 (naturally occurring lithium) responds only to gamma radiation (TLD-700)

Disadvantages of TLDs

1. Accurate to within about 3%.

2. Spontaneous decay of traps due to thermal effects results in fading over time.

Useful for and useful range of TLD

1. Ir-192 doses such as in HDR applications.

2. TBI, TSE applications.

3. Personnel Dosimeters

Useful range is mGy to 100Gy - at very high doses becomes supralinear

OSLD operational theory

1. Nearly identical theory to TLD except instead of using heat to read the chip we use optical light from a laser or LED to discharge the traps.

2. Most common material is Al₂O₃:C (aluminum doped with carbon).

OSLD quantity measured, pros/cons, useful for, useful range

measure light emission from the crystal

Advantages - no wires, small, non destructive reading (only uses 0.5% per read)

Disadvantages - Stronger energy dependence than TLD due to the higher effective Z of the material (not useful for HDR applications).

1. Accurate to within about 3%.

useful for

1. Megavoltage doses.

2. TBI, TSE applications.

3. Dose from kilovoltage x-ray systems (when carefully calibrated).

Useful range: mR to 100Gy

calorimeter operational theory

1. Absorbed dose is defined as J/kg.

2. When radiation interacts it deposits energy (joules) which shows up as heat.

3. If we could accurately measure this temperature change and knowing the mass of the sample we could directly measure dose.

4. 1 Gy raises the temperature of water by 2.39E^{-4 o}C (extremely little).

5. A thermistor is used to measure the temperature change.

1. A thermistor is a semiconductor whose resistance changes with temperature.

2. Changes about 5% per degree Celsius.

6. By measuring the resistance change in the thermistor, we can calculate the temperature change.

Sketch a Bonner Sphere

operational theory of a bonner sphere

1. Made of a Lithium Iodide scintillating detector at the core surrounded by a sphere of polyethylene.

2. The polyethylene moderates high energy neutrons to low energies, and then the lithium-6 captures the neutron producing an alpha particle detected in the scintillator.

3. The small lithium iodide crystal size and the large energy (Q) released by the lithium capture reaction ensures excellent gamma discrimination (measures just the neutrons).

4. Spheres of different sizes may be used to moderate neutrons of different energies, and using several different sizes allows characterization of the neutron spectrum.

5. By happy circumstance, the energy response of 10 to 12 inch Bonner spheres matches that of the dose equivalent weighting factors.  Therefore, these sizes may be used to directly measure dose equivalent without knowing the neutron spectrum.  Due to this, spheres of these sizes are sometimes referred to as REM balls.

What quantity does a bonner sphere measure

neutron flux

Advantage and disadvantage of Bonner Sphere

Advantages

1. Good gamma ray discrimination (only measure neutrons).

2. Directly measure neutron dose equivalent.

Disadvantages

1. Very large.

2. Requires many correction factors and calibration (expensive software).

Useful for measuring

• neutron dose

• neutron energy

Useful range

• background to Gy

Neutron bubble detector operational theory

1. The Bubble Detector is a clear plastic tube which is filled with a special polymer gel and enclosed with a black screw-top cap.

2. Droplets of superheated detector liquid are suspended within the gel.  Superheated means that the droplets are being held above their normal boiling point but aren’t allowed to change into a gas.

3. To measure neutrons, the detector cap is opened and any neutron coming into contact with a droplet would vaporize the droplet turning it into a gas bubble.

4. The bubbles are visible to the naked eye and due to the design of the detector are directly related to the tissue equivalent dose from the neutrons in some denomination of Sv (i.e. it includes the radiation weighting factors).

5. To reset the detector, the cap is closed which squeezes the bubbles back into their liquid state.

6. Optically based bubble readers can be used to better count the number of bubbles created.

Neutron bubble detector quantity measured, adv/disadv/useful range

Measure neutron ionization events

Adv

1. Bubble Detectors aren’t sensitive to photons.  This allows for isolating neutron measurements in cases where photons may also be present.

2. Bubble Detectors can easily be used as personal dosimeters for neutrons based on their size and convenience of use.

Disadv

1. Bubble Detectors have a strong temperature dependence of about 5% per ^oC. However, this can be corrected to a degree by including a volatile liquid in the chamber whose vapor pressure counteracts the temperature sensitivity.

2. Bubbles begin to overlap during viewing when large numbers are present. This limits the number of bubbles that can be present and can lead to large statistical variations.

3. Bubble activation sites are depleted over time leading to reduced sensitivity until the unit is reset.

Useful for neutrons, 1 microSv to 5.5 mSv, energy 200kev to 15 MeV unless includes a material capable of capturing thermal neutrons (like chlorine)

Polymer gel dosimeters operational theory

1. Consists of a mostly water equivalent solid gel.

2. The gel incorporates a chemical that changes its chemical properties when exposed to ionizing radiation.

1. Could be Ferrous Sulfate (like Fricke dosimetry in solid form).

2. Could be polymers (like radiochromic film in 3D).

3. Post irradiation, the gel response is read using either an MRI scanner or a CT to get the 3D dose response.

Fricke dosimetry operational theory

1. Based on the principle that ionizing radiation may produce a chemical change within a material.

2. Comprised of a solution of: Ferrous sulfate (active ingredient), Sodium chloride (salt) that counteracts organic impurities, Sulfuric acid (separates ferrous sulfate).

3. ionizing radiation changes the Fe²⁺ (ferrous) ions into Fe³⁺ (ferric).

4. Ferric ion concentration is determined by spectrophotometry with absorption peaks at 224 and 304 nm.

5. By knowing the quantity of ferric ions and the number of ions produced for a given energy (15.7 atoms per 100 eV), we can determine the energy deposited (dose).

Fricke dosimetry quantity measured, advantages, disadvantages, useful for, useful range

Measures Ferric ion concentration.

Advantages

1. Absolute dosimeter (no calibration).

2. 96% water (read water equivalent).

3. Extremely accurate to within about 0.25% with proper technique.

Disadvantages

1. Very sensitive to organic impurities (over-response).

2. Requires very careful rigorous technique (difficult).

Useful for absolute output

Useful range: Depending on solution concentrations but in the range of 5-30 Gy.

GM detectors operational theory

1. Basically a large ion chamber operated at high voltages (near 1000 volts)

2. The Townsend Avalanche:

1. After an ionization event, the separated electron and positive ion are separated by the voltage across the chamber.

2. The voltage is so high that each ion is accelerated and begins making new ions...which make new ions...which make new ions (the “avalanche”).

3. The avalanche takes a very small signal and amplifies it to a very large one making GM tubes very sensitive

4. However, the avalanche destroys all information about what interacted (no energy information).

5. The entrance window is usually made of mica

GM detector quantity measured

Pulses/counts

GM detector disadvantages, useful for, and useful range

Disadvantages

1. Large dead time (not useful in measuring large radiation fields, pulsed LINAC beams).

2. Can only tell you that radioactive particles are present (not what kind or how much) (it is very energy dependent but can be calibrated for a specific nuclide to read exposure).

Useful for

1. Finding lost seeds.

2. Finding cracks in shielding.

3. Post HDR surveys.

4. Gammas and betas.

Useful range

1. Background to Gy.

2. Careful in high radiation fields as the dead time may cause a significant loss in counts.  In fact, in a suitably high field your detector could read 0, and you could die not knowing what hit you.

Scintillator sketch

Scintillator operational theory

1. Consists of a crystal doped with another material (typical NaI:Tl)

1. The doping improves the frequency of scintillation (emission of visible light after stimulation by radioactivity, fluorescence).

2. The crystal is usually surrounded by reflective materials with a thin entrance window on one side.

3. The reflective material channels the light up to the photomultiplier tube.

4. The photomultiplier tube converts the light to an electric signal and then amplifies it.

1. Inside a photomultiplier tube

1. First, there is the photocathode which converts visible light to electrons

2. Next, the electrons are attracted to a series of dynodes, each held at a higher positive potential, each dynode amplifies the number of electrons

3. Electrons are collected by the anode and sent to the readout electronics

5. The signal is then fed to a multichannel analyzer or electrometer.

Scintillator quantity measured, advantages, disadvantages, useful for

measure amount scintillation light

Advantages: preserves energy info and very sensitive

Disadvantage: absorbs water and must be sealed

Useful for

1. Energy spectrum.

2. Finding lost seeds.

3. Gammas, betas.

Useful range: background to Gy

Proportional coutners operational theory

1. Gas filled detectors that are operated at voltages that produce a signal proportional to the amount of energy deposited in the cavity.

2. Noble gases are typically used (because electrons separate easily) with a quench gas (reduces the dead time), typically 90% argon and 10% methane.

3. Designed so that samples can be placed inside the chamber (especially useful for alphas that have trouble penetrating even the thinnest of entrance windows).

4. Fill gas is highly customizable depending on the application:

1. BF₃ or He-3 for thermal neutrons (boron capture reaction).

2. Methane, helium or other low Z gasses for fast neutrons.

3. Krypton or xenon for high energy gammas.

Proportional counters quantity measured

pulses/coulombs

Prop counters pros and cons

Pro

1. Can be configured to detect almost any kind of radiation.

2. Preserves energy information (identify the nuclide in question).

3. Can detect alpha particles (one of the few detector types capable of this).

Con

1. Usually large and bulky.

2. Requires high purity of the fill gas (no air contamination).

Useful for alphas betas gammas and neutrons

Useful range background to Gy