Radiation Detector Types

Subgroup I: Radiation Physics and Detection

All detectors work on the principle that

radiation can ionize and excite matter

Radiation interacts with matter and the interactions lead to a

transfer of energy

types of clinical detectors

• scintillation

• semiconductors

types of health physics detectors

• gas-filled

• scintillation

• photographic emulsion (film)

• optically stimulated luminescent (OSL)

• thermo luminescent (TLD)

gas filled detectors

• radiation ionizes the gas molecules contained in the detector

• ion pairs are created

• the detector contains two electrodes, positive and negative

• a voltage difference exists between the two electrodes that causes the ion pairs to feel an electrostatic pull

• the detector uses the electric field to collect and measure the number of ion pairs that are produced in the gas volume

• the behavior of the ion pairs is dependent upon the applied voltage across the gas volume

In a gas-filled detector, a voltage difference exists between the two electrodes that causes the ion pairs to feel ___

an electrostatic pull

How does a gas-filled detector measure radiation?

The detector uses the electric field to collect and measure the number of ion pairs that are produced in the gas volume.

What are the two gas-filled detector operation modes?

current mode and pulse mode

current mode (gas-filled detector)

• electric field causes ions to flow

• detector can measure the flow of charges

• measures rate of ionization

current mode (gas-filled detector) equation

Current (I) = charge (Q) / time

In current mode (gas-filled detector), the amount of current produced in the detector is ___ to the radiation exposure rate.

directly proportional

pulse mode

• measures each individual quantum that interacts

• output is pulses per unit time (counts per minute)

• high count rates will result in loss of counts

In pulse mode (gas-filled detector), the measured cpm in the detector is ___ to the radiation exposure rate.

directly proportional

In pulse mode (gas-filled detector), a high count rate will result in

a loss of counts

gas-filled detectors in a nuclear medicine department

Geiger-Mueller survey meters, ionization chambers, cutie pies, dose calibrators

ionization chamber region of response

• applied voltage sufficient to collect all primary ion paris, no recombination

• current measuring devices

• current proportional to number of ion pairs

• can distinguish between alphas and betas

ionization devices in a nuclear medicine department

dose calibrators, cutie pies, pocket dosimeters

dose calibrator

• gas-filled well ionization chamber

• gas is pressurized to increase detection sensitivity

• current measured by electrometer

• pushbutton selector used to adjust the electrometer readout in activity

cutie pie (ionization chamber)

• low sensitivity, high range; 0-500 R/hr

• slow response

• low energy dependence

• filling gas is air

• can directly measure exposure (mR, R) or exposure rate (mR/hr, R/h)

exposure

amount of charge produced in mass of air

Geiger Mueller region of response

• gas amplificatoin on the order of 10⁶ to 10⁸

• pulse size is NOT proportional to the initial ionization

• usually operated in pulse mode

• GM discharge must be quenched

quenching a GM meter can be accomplished two ways:

electronically or chemically

electronically quenching a GM meter

lower the anode voltage until all positive charges have been collected (aka external quenching)

chemically quenching a GM meter

incorporating a quenching agent in the detector filling gas (aka self-quenching)

self-quenching agents

organics and halogens

Geiger-Mueller devices

• GM survey and area meters

• very sensitive, can measure background

• 0-200 mrem/hr

• fast response

• CPM or mR/hr display

• energy dependent

What are GM devices used for?

surveying and detection

What device is best for accurate dose measurement?

ionization chamber

What device is best for detecting low levels of radiation?

Geiger Mueller survey meter

thin end window (for GM survey meter)

pancake (for GM survey meter)

scintillation

a material that converts kinetic energy of ionizing radiation into a flash of light

luminescence

emission of light following absorption of energy

two types of luminescence

fluorescence and phosphorescence

fluorescence

prompt emission of visible light

phosphorescence

delayed emission of visible light

If using a luminescent detector, it should convert energy through ___ rather than ___.

fluorescence, phosphorescence

classes of scintillators

• organic

  • plastics

  • liquids

• inorganic

  • NaI(Tl) - Thallium activated Sodium Iodide

  • CsI(Tl) - Thallium activated Cesium Iodide

  • BGO - Bismuth Germinate

  • GSO - Gadolinium Oxyorthosilicate

  • LSO - Lutetium Oxyorthosilicate

clinical scintillators

gamma cameras, uptake probes, PET scanners

health physics scintillators

well counter, uptake probe

how inorganic scintillators function

• electrons are available in discrete bands of energy in a crystal lattice

• radiation deposits energy in the scintillator, resulting in many ionizations

• electrons from ionizations will be elevated to the conduction band

• return to valence band results in emission of a photon

• visible light photons are easily detected

inorganic scintillator valence band

represents electrons bound at lattice sites

inorganic scintillator conduction band

represents electrons that will never be found in a pure state

inorganic scintillator forbidden band

represents electrons that will never be found in a pure state

To enhance the probability of visible photons being emitted during de-excitation in an inorganic scintillator, ___ are added to the scintillator.

small amounts of impurities

activators (inorganic scintillator)

impurities in the scintillator that create special sites in the lattice

scintillators are characterized by ___

decay time, the time it takes for the scintillation light to emerge

Shorter decay times in inorganic scintillators are ___

desirable as they allow for separation of events

semiconductor detector characteristics

• generate a lot of information (10x more than air-filled detectors)

• takes little energy to form an electron-hole pair

• excellent energy resolution due to limited statistical fluctuations

types of semiconductors

• HPGe - high purity geranium

• SiLi - lithium silicon

• CZT - cadmium zinc telluride

how semiconductors function

• electrons are available in descrete bands of energy in a crystal lattice

• valence bands represent electrons bound at lattice sites

• ionizing radiation interacts with lattice and deposits energy and releases electrons

• electrons leave behind holes

• an electron-hole pair is created

• by applying an electrical potential to the semiconductor, a depletion area is created

• charges are collected in the depletion areas

• the charge or current generated in the semiconductor are proportional to the energy deposited in the depletion area

semiconductors are also known as

solid state detectors

semiconductors conduction band

represents electrons that freely migrate, these electrons leave behind holes

A semiconductor operates essentially as a ___

solid state ionization detector

In a semiconductor, the charge or current generated are ___ to the energy deposited in the depletion area.

proportional