Thimble Ion chamber

1. IONIZATION CHAMBER
(THIMBLE TYPE)
Moayyad Mazen Al Ssabbagh
Master Medical Physics

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

3. • 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.
Source: Podgorsak, E.B. Radiation Oncology Physics

4. Cylindrical thimble ionization chamber
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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

5. Use of Thimble Chambers
• 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
Acrylic
material
is usually
used for
build-up
caps

6. 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

7. Thank you
References:
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