Instruments for Radiation Detection

and Measurement

Lab # 3 (1)

• In nuclear medicine it is important to ascertain the

– Presence– Type– Intensity– Energy of radiations emitted by radionuclides

• Two commonly used devices

– Gas-filled detectors – Scintillation detectors

Gas-Filled Detectors

• The operation of a gas-filled detector is based on the ionization of gas molecules by radiations, followed by collection of the ion pairs as current with the application of a voltage between two electrodes.

• The measured current is primarily proportional to the applied voltage and the amount of radiations.

ionization of gas moleculesby radiations

collection of the ion pairs as current withthe application of a voltage between two electrodes

The measured current isprimarily proportional to the applied voltage and the amount of radiations.

• The two most commonly used gas-filled detectors are

– Ionization chambers• Cutie-Pie counters used for measuring high

intensity radiation sources, such as output from x-ray machines

• Dose calibrators measures the activity of radiopharmaceuticals

– Geiger-Müller (GM) counters.

•At a lower voltages from 50 to 300 V, only the primary ion pairs formed by the initial radiation are collected.

•At higher voltages from 1000 to 1200 V, the current becomes identical regardless of how many ion pairs are produced by the incident radiation.

Dose Calibrators

• one of the most essential instruments for measuring the activity of radionuclides– Cylindrically shaped – Sealed chamber with a central well – Filled with argon and traces of halogen at high

pressure

Geiger-Müller (GM) Counters

• One of the most sensitive detectors

Scintillation Detecting Instruments

• g-ray detecting equipment• Most commonly used:

– well counters– Thyroid probes– g or scintillation

• All these instruments are g-ray detecting devices • Consist of:• Collimator (excluding well counter) • Sodium iodide detector• Photomultiplier tube• Preamplifier• Pulse height analyzer• Display or Storage

• Scintillation detectors consist of scintilator emitting flashes of light after absorbing gamma or x radiation. The light photons produced are then converted to an electrical pulse by means of a photomultiplier tube. The pulse is amplified by a linear amplifier, sorted by a pulse-height analyzer and then registred as a count. Different solid or liquid scintillators are used for different types of radiation. In nuclear medicine, sodium iodide solid crystals with a trace of thallium NaI(Tl) are used for gamma and x ray detection.

g rays from a source interact in the sodium iodide detector and light photons are emitted.

The light photons will strike the photocathode of a photomultiplier(PM) tube and a pulse is generated at the end of the PM tube.

The pulse is first amplified by a preamplifier and then by a linear amplifier