Lecture 13 - Intro to Dosimetry & Gas Detectors

ONCOL 243 - Radiation Safety. University of Alberta

why do we need radiation detectors

because radiation is invisible, we need it for dosimetry, calibrating machines, monitoring ourselves, and leakage surveys

What are the 4 types of detectors most frequently used

1. Gas filled detectors

2. scintillators

3. TLD & OSL

4. Semiconductors

how do gas detectors work

radiation ionizes gas, ions move in electric field, charge is collected and measured

how to scintillators work

radiation creates UV or visible light, light is detected

how do TLD and OSL detectors work

Thermoluminescent or optically stimulated luminescent materials store energy from radiation exposure, which is released as light when heated or stimulated, allowing for measurement.

how do semiconductor detectors work

use diodes to create electrical pulses when radiation interacts with semiconductor material, generating electron-hole pairs that are collected to produce a measurable signal.

table of radiation detectors

what are the two detection modes

pulsed mode and current mode

pulsed mode

A detection method where radiation events are registered as discrete pulses, allowing for the measurement of individual interactions with the detector.

issue with pulsed mode

It can lead to dead time, where the detector is unable to register subsequent events immediately after an initial detection, potentially causing losses in measurement accuracy.

current (continous/integrating) mode

A detection method where the output signal is proportional to the radiation intensity over a period of time, allowing for continuous measurement and integration of radiation events.

what are the three types of detectors (by type of information)

1. dosimeters

   • indictate net amount of energy deposited

2. counters

   • indicate number of interactions (GM counter)

3. spectrometers

   • provide information on energy distribution (NAI scintillators)

what 8 items describe what makes a good detector?

1. accuracy

2. precision

3. detection limit

4. measurement range

5. dose response

6. dose rate dependence

7. radiation quality response

8. spatial resolution

Accuracy

ability to indicate dose correctly

ABc: Accuracy = being correct

Precision

reproducibility of results under similar conditions

PQR: precision = question of reproducibility

Detection Limit

lowest dose detectable

measurement range

the range between lowest and highest detectable dose

dose response

should be linearly proportional to dose

• or reproducible and predictable

dose rate dependence

should be independent of the dose rate applied

• important for linacs

Radiation Quality Response Variation

the variability in detector response due to different types of radiation, affecting measurements and dosimetry.

Spatial resolution

should determine the dose in small volume

Formula for detection efficiency

ideal detection efficiency

1.0

what is intrinsic efficiency, and what is it’s formula

what percent of particles that reach detector are detected

what does intrinsic efficiency depend on?

type of detector

thickness

density

atomic number of material

does film have good or bad intrinsic efficiency

Film has bad intrinsic efficiency due to its limited ability to detect particles compared to other types of detectors.

what is geometric efficiency, and it’s formula

the amount of particles reaching the detector compared to the number of particles emitted

what radiation meters have the best geometric efficiency?

well detectors

what factor effect geometric efficiency

• detector size and geometry (large more sensitive)

• distance between radioactive material and detector

• back-scatter towards detector

  • absorption of radiaiton before it enters detector

what is the formula for detection efficiency derive from geometric and intrinsic efficiency

Radiation emission detection 1

direct incident on detector

radiation emission detection 2

radiation back scatterd from surface onto detector

• some energy lost

radiation emission miss 3

radiation absorbed by detector covering

radiaiton emission miss 4

not incident on detector

radiation emission miss 5

if detector were closer, this would fall on the detected volume

simply, how do gas filled detectors work

volume of gas between two electrodes is ionized by radiation creating ion pairs

• electrons attracted to anode

• postive ions attracted to cathode

current generated

three types of ionization gas filled detectors

1. ionization chamber

2. geiger-muller counter

3. proportional counter

what is the main difference between the three types of detectors

the applied voltage between the electrodes

• the more voltage applied, the more sensitive the detector

what is the shape of the electrode in ionization chambers

any shape, usually 2 parallel plates or cylinders

what is the shape of the electrode in GM and proportional counters

the anode is a thin wire

the behaviour of the gas detector depends on the ______

voltage applied

what happens when more voltage is applied

more ion pairs are generated, creating more current

describe ion chambers in terms of the their applied voltage and current output

Ion chambers operate at a relatively low voltage, producing a current that is directly proportional to the amount of ionization in the gas, allowing for accurate measurement of radiation.

ion chambers are not very sensitive to radiation, but are good at measuring dose

describe geiger-muller counters in terms of their applied voltage and current output

Geiger-Muller counters operate at a higher voltage compared to ion chambers, resulting in a larger current output for each ionizing event. They are highly sensitive to radiation and can detect low levels of ionizing radiation effectively.

• have no sensitivity between the types of radiation, therefore they don’t measure dose well

voltages of recomibination, ion chamber, proportional, and GM counters

describe the general idea of an ion chamber

An ion chamber is a radiation detector that measures ionizing radiation by collecting charge produced in a gas when radiation ionizes the gas molecules. It operates at low voltage, allowing for accurate dose measurements.

4 limitations of free-air ion chambers

• 3 MeV maximum due to range of electrons liberated in air

  • need to increase distance between plates to maintain EE

• too large

  • inhomogenous E field and greater ion recombination

• too delicate and bulky

• corrections needed for:

  • air attenuation

  • recombination

  • temp, pressure, humidity (for air density)

  • scatter

what are thimble chambers, and how do they compare and contrast with ion chambers

Thimble chambers are a type of ion chamber designed for precise dose measurements, typically used in radiation therapy. They are smaller and more compact than traditional ion chambers, allowing for better spatial resolution, and they often have a more uniform electric field, resulting in reduced ion recombination effects.

what is the outer air region replaced with in a thimble chamber?

thimble chamber walls whcih mass the known mass, volume, and Z_eff of air

solid wall = air equivalent

how thick do thimble chamber walls have to be

thick enough to ensure electronic equilibrium but not so thick as to attenuate photon beam

what does the optimum thickness of the thimble chamber depend on?

the photon energy

• 100-250 kV, wall = 1 mm

• 1.25 MeV, wall = 5 mm

pros of thimble chambers

• works well over range of energies

• sensitive

• well calibrated

• minimal ion recomination losses (due to high enough voltage)

cons of thimble chambers

we get some leakage

how are farmber chambers different than thimble chambers

replace thimble wall with two layers

• graphite outer layer and alumininum

good for all energies and have minimal leakage

why is graphite used in the outer shell of farmer chambers

to reduce the K-edge effects of high Z-metals

what are well detectors used for?

testing radiopharmaceuticals in nuclear medicine

why are parallel plate ionization chambers used?

they are good to measure steep dose gradients as they are very thin

• useful for LINAC energies (~MV)

• not useful in water phantom

what type of gas filled detector are survey meters: ion chamber, proportional counter, or GM counter

ion chamber

• so sensitive to different radiation types

what are the Geiger-Mueller counter avalanches?

The Geiger-Mueller avalanche occurs when ionizing radiation enters the tube, creating ion pairs. The applied high voltage accelerates electrons, triggering a chain reaction of secondary ionizations. This cascade effect, or avalanche, continues exponentially until the entire tube conducts, producing a large, easily detectable electrical pulse.

can GM counters detect the type of radiation

no, the avalanche in a Geiger-Mueller counter is so intense that it quenches any information about the initial energy of the radiation, making it useful only for detecting radiation presence, not its energy level.

lowest reading GM counter can measure compared to ion chamber

GM: 0.01 uSv/hr

ion: 1 uSv/hr

how do spectrometers work?

Spectrometers work by separating light or other electromagnetic radiation into its component wavelengths. A sample interacts with the radiation, and the spectrometer uses a prism, diffraction grating, or other optical element to disperse the light. Detectors then measure the intensity of each wavelength, producing a spectrum that provides information about the sample’s composition, structure, or properties.

can spectrometers provide information about the energy distribution (type) of radiation

Yes, spectrometers analyze the energy distribution of radiation by measuring wavelengths or particle energies, helping identify composition and radiation type.

what mode do spectrometers work on, pulsed or current mode

pulsed

• as a result their may be dead time

how does the spectrometer tell you what kind of element you are being exposed to?

A spectrometer identifies elements by analyzing the unique spectral lines they emit or absorb. Each element has a distinct atomic structure, causing it to interact with radiation in a characteristic way. When excited (by heat, electricity, or radiation), atoms emit or absorb light at specific wavelengths, creating a unique spectral fingerprint. By comparing the detected spectrum to known reference spectra, the spectrometer determines which elements are present.

what are proportional counters, and how are they different from GM counters?

Proportional counters are gas-filled radiation detectors that measure both the presence and energy of ionizing radiation. When radiation ionizes the gas inside, the resulting electrons are accelerated by an electric field, causing a proportional avalanche—a controlled chain reaction of ionizations. Unlike Geiger-Mueller counters, the size of the resulting electrical pulse is proportional to the initial ionization energy, allowing for radiation energy measurement and particle identification.

what are proportional counters a mix of/

GM counters and ion chambers

• still sensitive, but can also measure dose (but not as well as ion chamber)

cons of proportional counters

• not very precise

  • need to be calibrated to particular x-ray energies

• slow with extremly long dead time

  • run on pulsed mode