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Calibration of radiotherapy grade cylindrical and plane-parallel ionization chambers

Calibration of radiotherapy grade cylindrical and plane-parallel ionization chambers in terms of absorbed dose to water NDw and in terms of air kerma Nk in Co-60 beam (TRS-398, NCS-18). LNK aims to minimize the time the ionization chamber is not available to its owner. Flexible schedule and 1-2 days return time are possible for these irradiations (015 CAL BELAC accreditation scope).

Radiotherapy calibrations of ionization chambers are done using the G-100 Hopewell irradiator:

  • Secondary standard calibrations in terms of absorbed dose to water in Co-60 beam (NDw) traceable to the water calorimeter of VSL Delft, The Netherlands. The combined expanded uncertainty is about 1.2 % for k = 2 (confidence level 95.4 %).
  • Primary standard calibrations in terms of kerma in air in Co-60 beam (Nk) traceable to the graphite cavity chamber of LNK (primary standard, Kessler et al. [1]). The combined expanded uncertainty is about 1.1 % for k = 2 (confidence level 95.4 %)

The G-100 irradiator is used also for calibrations at other distances than 100 cm, from 85 cm up to 700 cm. Therefore dose rates down to about 2 Gy/h can be reached with this irradiator. Collimator opening can be opened up to 35 cm x 35 cm at 100 cm to accomodated very large samples, of course at the expense of sligtly worse radiation scattering conditions. Best radiation scattering conditions are achieved at minimum collimator opening of 5.5 cm x 5.5 cm at 100 cm distance.

References

[1] Kessler, C., Burns, D, L. C. Mihailescu & S. Chiriotti. 2017. Key comparison BIPM.RI (I)-K1 of the air-kerma standards of the SCK CEN, Belgium and the BIPM in Co-60 gamma radiation. Metrologia, Volume 54 (2017), Technical Supplement.

Figures: Co-60 radiotherapy dosimetry calibrations irradiator G-100 with a PTW 30 cm x 30 cm x 30 cm water phantom prepared for absorbed doe to water Dw measurements.

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