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Successful use of PTB water calorimeter in 280 MeV/u 12C beam at GSI


The form of radiotherapy carried out at the Gesellschaft für Schwerionenforschung, GSI (Society for Heavy-Ion Research), uses a high-energy carbon ion beam to destroy the tumour. The ion beam is thereby directed very precisely onto the whole tumour volume by means of the so-called grid-scanning method. To date, there exists no suitable primary standard for the determination of the absorbed dose to water for these irradiation conditions. In a first experiment, PTB’s portable water calorimeter was therefore used at GSI in a 280 MeV/u carbon ion beam in order to demonstrate the suitability of the calorimetric measurement procedure.

For the measurements, a 12C beam with a width of 9 mm was used to irradiate a field of 50 mm x 50 mm with 169 discrete points which were distributed over 13 lines and 13 rows. This type of scanning was repeated 12 times in order to obtain an almost homogeneous dose distribution over the whole size of the field by superimposing the Gaussian-shaped intensity distribution of the single beam positions. A total irradiation took approx. 120 s.

The figure shows the time dependency of the resistance changes of the thermistors during an irradiation measured by means of the calorimetric detector. The steps caused by the 12-fold repeat of the scanning are clearly to be seen.

Figure : Time dependency of the resistance changes measured by means of the calorimetric detector during an irradiation of approx. 120 s in the 280 MeV/u carbon ion beam at GSI. The steps are caused by the 12-fold repeat of the scanning procedure.

The measurements were carried out at a water depth of 5 cm, i.e. in the plateau region of the depth-dose curve of the 280 MeV/u beam. The Bragg peak of the beam is located at approx. 15 cm water depth.

The heat transport effects occurring during and after the irradiations in the calorimeter and which are caused by the steep gradients of the initial temperature distribution have been simulated by means of the finite element method. Thereby, the real time dependency of the whole scanning procedure, including the spill structure of the ion beam, must be taken into account. Due to the technique of acceleration, the ion beam is available in 2 s spills, whereby a 2 s break occurs between two consecutive spills. As a result of the time-consuming heat conduction calculations, correction factors of approx. 0.8% occur which must be taken into account when determining the absorbed dose to water. Similar simulations must be carried out to determine heat conduction effects which are caused by the irradiation of the materials the calorimetric detector is made of.

On the basis of the measurements which have been carried out up to now and of the preliminary analysis of the data, it is to be expected that the absorbed dose to water in the 12C beam can be determined with the aid of the calorimeter with a standard measurement uncertainty of less than 1%.