Calibration of radiotherapy grade cylindrical and plane-parallel ionization chambers

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

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

• Secondary standard calibrations in terms of absorbed dose to water in Co-60 beam (N_Dw) 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 (N_k) 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. As such dose rates down to about 2 Gy/h (as in 2021) can be reached with this irradiator. Collimator opening can be opened up to 35 cm x 35 cm at 100 cm to accommodate very large samples, of course with a slightly 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.