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Determination of the water absorbed dose in neutron beams and ion beams with ionization chambers and a transportable water calorimeter

  • Fundamentals of Metrology

A transportable and compact water calorimeter was used in hospitals and research institutes which use different types of particle beams for cancer therapy: in a collimated neutron beam (Clinical Centre, Essen), in proton beams (HMI in Berlin and iTHEMBA LABS in South Africa) and in a 12C heavy ion therapy beam (GSI in Darmstadt). The water absorbed dose was determined with uncertainties of less than 1.8 % and confirmed measurements with standard ionization chambers.

For the realization of the unit of the water absorbed dose in therapy fields, PTB uses a water calorimeter as primary standard. Working Group 6.41 operates a special water calorimeter as standard measuring instrument for the water absorbed dose in collimated neutron fields and in ion beams. It is well suited for radiation fields of different radiation types and energies. Measurements performed with this water calorimeter in the 60Co reference photon field of PTB were compared with ionization chamber measurements. The results of both measuring procedures agreed within 0.3 %.

The calorimeter is compact and transportable and was developed for use in clinical radiation fields. In the last few years, it has been used for the precise determination of the water absorbed dose at PTB and at the Clinical Centre of Essen University (mean neutron energy ), at the Eye Therapy Facility of the HMI in Berlin (mean proton energy ), at iThemba LABS in Cape Town, South Africa (proton energy Ep = 182 MeV), as well as in the 12C heavy ion therapy beam of the GSI in Darmstadt (ion energy E(12C) = 430 MeV/u) [1].

In a water calorimeter, the energy delivered by radiation is measured as temperature rise (0.24 mK/Gy) using very sensitive sensors. In addition to the measurements performed with the calorimeter, the dose was determined in the above-mentioned radiation fields by the usual standard procedure, i.e. by ionization chamber dosimetry. When calorimeter and ionization chamber measurements are compared in different radiation fields (Table), one recognizes, on the one hand, the small relative standard uncertainty of less than 1.8 % in the case of calorimetry compared to 3.3 % obtained by ionometry and, on the other hand, the good agreement between the two procedures. The caloric defect for the water used in the calorimeter was determined in independent test series [2]. This correction for the energy dose takes into accountthe fraction of the energy absorbed in the water which does not result in a temperature change of the absorber (water).

Table: Comparison of calibration factors for the respective beam monitors with regard to the water absorbed dose for different radiation types, determined with the calorimeter or with ionization chambers.

Radiation type (Institute)
Calibration factor for the monitor
(in Gy / monitor unit)
relative deviation of the calibration factors %
Protons (HMI)
1.36 ± 0.02 1.36 ± 0.04 ± 0.0
Protons (TLABS)
Ep = 182 MeV
2.95 ± 0.04 2.97 ± 0.09 + 0.7
Neutrons (PTB)
306 ± 5 303 ± 10 - 1.0
12C- Ions (GSI)
E(12C) = 430 MeV/u
2.77 ± 0.05 2.69 ± 0.08 - 3.0


  1. H.J. Brede, K.-D. Greif, P. Heeg, J. Heese, O. Hecker, D.T.L. Jones, H. Kluge, and D. Schardt: Absorbed Dose to Water Determination with Ionization Chamber Dosimetry and Calorimetry in Restricted Neutron, Photon, Proton and Heavy Ion Radiation Fields. Submitted to Physics in Medicine and Biology
  2. H.J. Brede: Darstellung der Wasserenergiedosis im kollimierten, gemischten Neutronen-Photonenfeld der PTB. Report PTB-N-45 (2004, in German)


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