Topic 6. Counting Systems (II)

• Semiconductor systems

• Liquid Scintillation Detectors

• Gas-filled detectors

• In vivo counting systems

Semiconductor Detectors

• Semiconductor detectors are not very much used in nuclear medicine because of their small size and high cost (operated at low temperature etc.). Fluorescence scanning, radiation purity evaluation (purity of radiopharmaceuticals) and tracer studies employing many radionuclides simultaneously) are the main applications in nuclear medicine (where high energy resolution is required)

• Semiconductor detectors are widely used in nuclear physics, chemistry and material science due to its superior energy resolution.

Semiconductor Detector Systems

• Silicon detectors are often used for low energy x-rays and Ge for γ rays (Si has lower detection efficiency than Ge for energy larger than 40 keV due to its lower atomic number and density)

• Semiconductors detectors has 20-80 times better energy resolution than NaI(Tl) detector

CdTe and CZT

• Operate at room temperature.

• High atomic number therefore high stopping power.

• Expensive restricted to small sizes.

Liquid Scintillation Detectors(1)

• Scintillator is dissolved in a solvent material in a vial and the radioactive sample is added to this mixture.

• The vial of mixture is placed in a light enclosure between a pair of PM tubes to detect scintillation events

Liquid Scintillation Detectors(2)

• Liquid scintillation solution comprises of four components: solvent, primary solute, secondary solute and additives

• Solvent: dissolves sample and scintillation material. absorbs radiation from sample;

• Primary solute: absorbs energy from solvent and emits UV or near visible light.

• Secondary solute: shifts the wave length of the photons for optimal coupling with PM tubes.

• Additives: improves performances (efficiency and dissolution etc.)

Liquid Scintillation Detectors(3)

• Liquid scintillation detectors are used primarily to count very low energy particle (β emission) or x and γ rays.

• The main detection problem is the spontaneous thermal emission of electrons from photo-cathode of the PM tube (keep low temperature and use pulse height discrimination -- sacrifice energy resolution)

• The most effective reduction of noise is the coincident circuitry.

• Multiple single channel analyzers are used for counting of multiple radionuclides and quench correction.

Pulse Height Spectrometry

• Pulse height spectrum is mainly used in selection of energy for counting, either to reduce background noise or to distinguish different radionuclides in mixed sample counting

Energy and Efficiency Calibration(1)

• LLD and ULD settings of Single Channel Analyzers can be adjusted for different purposes

• In case of mixed source counting, different SCA has different window to fit different radionuclide (as less crosstalk as possible)

• Three SCAs Liquid Scintillation system is designed for the quench correction in single radionuclide detection

Quench Corrections(1)

• Quenching in LS detection refers to the reduction of light output from the sample

• Quenching could be caused by chemical, colour or dilution.

• Chemical quenching is caused by some non-scintillator substances that absorb energy from the solvent

• Colour quenching is caused by some substances that absorb lights emitted from the primary or secondary solutes

• Dilution occurs when large radioactive volume is added to the solution and the scintillator output efficiency is reduced.

Quench Corrections(2)

• Principle effect of quenching is to cause a shift of the energy spectrum to lower energies and a loss of counting rate due to the LLD setting (reject noise).

• Three methods are used to correct the quenching effect: internal standardisation, channel ratio and automatic external standardisation methods.

Internal Standardisation Method

• Requires two counting measurements and a known quantity of the Standard sample radionuclide (non-quenched standard sample -- STD)

• Measure the sample first, then add the standard to the mixture and measure again.

• The counting efficiency is eff=[cpm(STD+sample)-cpm(sample)]/μCi standard

• The activity of sample is μCi(sample)=cpm(sample)/eff

Channel Ratio Method

• Obtain a quench curve by using standard sample (non-quenched) and gradually adding quenching substances. Channel ratios between channel 2 and channel 1 is then related to the counting efficiency

• Sample is measured only once and the actual activity is obtained by comparison to the quench curve

Automatic External Standardisation

• First, a series of known quenched standard sample and the quenched standard sample plus an external 137Cs γ ray source (STD) are measured. A series of ratios AES = [cpm(sample+STD) - cpm(sample channel 2)]/ [spm(sample+STD)-cpm(sample Channel 1)] is determined and these are correlated to the counting efficiency.

• Sample and sample plus external standard 137Cs γ ray source (STD) are measured and the AES ratio is then determined. The actual activity of the sample is determined from the counting efficiency corresponding AES.

Sample Preparation Techniques

• Non-polar sample can be dissolved into the non-polar scintillator solution directly

• Polar samples can be dissolved into non-polar scintillator solutions by adding some other polar substances.

• Polar sample can also be mixed with scintillator solution as suspension

• Combustion and subsequent dissolution into the scintillator solution is another technique for some organic samples.

Liquid and Gas Flow Counting

• Liquid scintillation counting system can be used to count the activities of flowing gases and liquids. The difference is that the scintillation solution is replaced by finely dispersed scintillation crystals where the gas or liquid can flow through. Other parts of the system is the same as the LS counter

• It is often used for the monitoring of effluent from gas or liquid chromatographs.

Automatic Multiple Sample Liquid Scintillation Counters

• Automatic multiple sample liquid scintillation counters are designed to handle large amount samples or repeated counting.

Multi-Sample Liquid Counter

Gas Filled Detectors

• A number of gas filled detectors are used in nuclear medicine; Ionisation chamber, proportional counter; G-M tubes etc.

• Gas filled detectors are not efficient in detecting γ rays in nuclear medicine but still find some specialised applications

Dose Calibrators (1)

• Dose calibrators are gas filled ionising chambers. The gas is air and sealed to avoid variations in temperature and atmospheric pressure.

• Dose calibrators are used to assay large quantities of activities where it is too large for NaI(Tl) detector (generator, patient preparation, shipment etc).

• The activity is determined by measuring the total amount of ionisations in the chamber with no inherent ability of energy discrimination.

Dose Calibrator

Dose Calibrators (2)

• Sample volume effects should be investigated experimentally when a new dose calibrator is acquired. The volume used should be consistent.

• The linearity of response versus sample activity should also be checked from time to time.

Gas Flow Counters

• Proportional and G-M counters have been used for monitoring β emitting gas flows.

• The flowing gas passes through the detector’s gas chamber.

• These systems have good detection efficiency (intrinsic and geometric) for β emitters.

In Vivo Counting Systems

• In vivo refers to human or animals body

• Probe system is designed to detect single organ or localised parts of the body

• A typical probe system employs 5x5cm NaI(Tl) cylinder crystal plus cylindrical or conical shaped collimator (as well as PM tube etc).

NaI(Tl) Probe System

Surgical Gamma Ray Probes

Gamma Ray Probe System

In Vivo Counting Systems (2)

• Whole body counting system is designed to measure total radioactivity of whole body (not local activity).

• Most whole body counters employ large NaI(Tl) crystal (15-30cm diam x 5-10 cm thick) in order to detect high energy photons and small activities.