IONIZATION CHAMBER (THIMBLE TYPE)

Moayyad Mazen Al SsabbaghMaster Medical Physics

High voltage between central and outer electrode Air ionized, electrons emitted from the atoms The electrons moves to the positive electrode Current induced An electrometer then count the number of total charges Q Q is proportional with the dose to the air volume

Principle

Source: EQUIPCO : http://www.equipcoservices.com/support/tutorials/introduction-to-radiation-monitors/

• The free air chamber is compacted to a very small size

• The wall is made of air- equivalent material To maintain the electron equilibrium in this small size, (thickness is equivalent to the thickness of air)

• Low Z for the wall material.

Thimble Ion Chamber

Source: Podgorsak, E.B. Radiation Oncology Physics

Cylindrical thimble ionization chamber

• Most popular design• Independent of beam direction• Typical volumes between 0.1 - 1 cm3

• Typical radius 2 – 7 mm• Length 4 -25mm• Thickness less than 0.1 g/cm2

• Used for: electron, Photons, Protons or ion beams

• Without a buildup cap limited to 500 kV X-ray beams.

• With using different caps, it can the energy can be extended up to 20 MV but at the expense of the cap size

Use of Thimble Chambers

Acrylic material is usually used for build-up caps

Thimble Ionization chamber

In general, For calibration of Megavoltage photon beam.

Advantages • Accurate and precise • Recommended for beam calibration • Instant readout

Disadvantages

• Connecting cables required • High voltage supply required • Many corrections required for high energy

dosimetry

Thank youReferences:

1. ATTlX, FRANK HERBERT. Introduction to radiological physics and radiation dosimetry. s.l. : WILEY-VCH Verlag GmbH & Co. KGaA, 2004.

2. Ahmed, Syed Naeem. Physics and Engineering of Radiation Detection. 1st. San Diego : Academic Press - Elsevier, 2007. p. 764. ISBN–13: 978-0-12-045581-2.

3. Podgorsak, E.B. Radiation Oncology Physics: A Handbook for Teachers and Students. Vienna : IAEA, 2005. ISBN 92–0–107304–6.

4. Build-Up Caps for Ionization Chambers. PTW Freiburg GmbH 2012: http://www.ptw.de/build_up_caps0.html