Prog Med Phys.  2014 Mar;25(1):46-52. 10.14316/pmp.2014.25.1.46.

Determining Ion Collection Efficiency in a Liquid Ionization Chamber in Co-60 Beam

Affiliations
  • 1Department of Nuclear Engineering, Hanyang University, Seoul, Korea. chkim@hanyang.ac.kr
  • 2Research Center for Radiotherapy, Korea Institute of Radiological and Medical Sciences, Seoul, Korea.

Abstract

Liquid ionization chamber is filled with liquid equivalent material unlike air filled ionization chamber. The high density material allow very small-volume chamber to be constructed that still have a sufficiently high sensitivity. However liquid ionization chamber should be considered for both initial recombination and general recombination. We, therefore, studied using the Co-60 beam as the continuous beam and the microLion chamber (PTW) for comparing the ion collection efficiency by Greening theory, two-dose rate method and our experiment method. The measurements were carried out using Theratron 780 as the cobalt machine and water phantom and 0.6 cc Farmer type ionization chamber was used with microLion chamber in same condition for measuring the charge of microLion chamber according to the dose rates. Dose rate was in 0.125~0.746 Gy/min and voltages applied to the microLion chamber were +400, +600 and +800 V. As the result, the collection efficiency by three method was generally less than 1%. In particular, our experimental collection efficiency was in good agreement within 0.3% with Greening theory except the lowest two dose rates. The collection efficiency by two-dose rate method also agreed with Greening theory generally less than 1%, but the difference was about 4% when the difference of two dose rates were lower. The ion recombination correction factors by Greening theory, two-dose rate method and our experiment were 1.0233, 1.0239 and 1.0316, respectively, in SSD 80 cm, depth 5 cm recommended by TRS-398 protocol. Therefore we confirmed that the loss by ion recombination was about 3% in this condition. We think that our experiment method for ion recombination correction will be useful tool for radiation dosimetry in continuous beam.

Keyword

Liquid ionization chamber; Collection efficiency; Ion recombination

MeSH Terms

Cobalt
Radiometry
Recombination, Genetic
Silver Sulfadiazine
Water
Cobalt
Silver Sulfadiazine
Water

Figure

  • Fig. 1. Diagram showing the setup of the 0.6 cc ionization chamber and microLion liquid ionization chamber in water phantom.

  • Fig. 2. The ionization current of air filled ionization chamber (left) and microLion liquid ionization chamber (right) versus applied voltage for Co-60 beam.

  • Fig. 3. General correction efficiency relative to the dose rates (Gy/min) of the microLion chamber in different voltages (+800, +600 and +400) determined by Greening theory.

  • Fig. 4. Comparison of correction efficiency obtained by the Greening theory, two dose rate method and our experiment method.


Reference

1. Wickman G. A liquid ionization chamber with high spatial resolution. Phys Med Biol. 19:66–72. 1974.
Article
2. Wickman G, Nyström H. The use of liquids in ionization chambers for high precision radiotherapy dosimetry. Phys Med Biol. 37:1789–1812. 1992.
Article
3. Daşu A, Löfroth PO, Wickman G. Liquid ionization chamber measurements of dose distributions in small 6 MV photon beams. Phys Med Biol. 43:21–36. 1998.
4. Pardo J, Franco L, Gómez F, et al. Development and operation of a pixel segmented liquid-filled linear array for radiotherapy quality assurance. Phys Med Biol. 50:1703–1716. 2005.
Article
5. Choi SH, Kim CH, Huh HD, Kim SH, Kim KB. Determination of the beam quality correction factor kQ,Q0 for the microLion chamber in a clinical photon beam. J Kor Phys Soc. 62(1):152–158. 2013.
6. Eberle K, Engler J, Hartmann G, Hofmann R, Hörandel JR. First tests of a liquid ionization chamber to monitor intensity modulated radiation beams. Phys Med Biol. 48:3555–3564. 2003.
Article
7. Chung E, Soisson E, Seuntjens J. Dose homogeneity specification for reference dosimetry of nonstandard fields. Med Phys. 39:407–414. 2012.
Article
8. Johansson B, Wickman G, Bahar-Gogani J. General collection efficiency for liquid isooctane and tetramethylsilane in pulsed radiation. Phys Med Biol. 42:1929–1938. 1997.
Article
9. Pardo-Montero J, Gómez F. Determining charge collection efficiency in parallel-plate liquid ionization chambers. Phys Med Biol. 54:3677–3689. 2009.
Article
10. Tölli H, Sjögren R, Wendelsten M. A two-dose-rate method for general recombination correction for liquid ionization chambers in pulsed beams. Phys Med Biol. 55:4247–4260. 2010.
Article
11. Andersson J, Tölli H. Application of the two-dose-rate method for general recombination correction for liquid ionization chambers in continuous beams. Phys Med Biol. 56:299–314. 2010.
Article
12. Greening J. Saturation characteristics of parallel-plate ionization chambers. Phys Med Biol. 9:143–154. 1964.
Article
13. PTW, Ionization chamber Type 31018 (microLion), User Manual. 2007.
14. International Atomic Energy Agency (IAEA). Absorbed dose determination in external beam radiotherapy: An international code of practice for dosimetry based on standards of absorbed dose to water IAEA Technical Report Series (TRS) 398 (Vienna: IAEA). 2000.
Full Text Links
  • PMP
Actions
Cited
CITED
export Copy
Close
Share
  • Twitter
  • Facebook
Similar articles
Copyright © 2024 by Korean Association of Medical Journal Editors. All rights reserved.     E-mail: koreamed@kamje.or.kr