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Monte Carlo simulations for setting up a reference field for radiation protection at accelerator facilities

23.12.2020

The use of linacs in medicine, industry and research has been experiencing a constant increase. A gap in metrology for radiation protection has been identified, in particular for pulsed radiation fields with energies in the MeV range. Without a reference field for these radiation fields, traceable dosimetry to check the shielding of accelerator facilities is not possible. A reference field for such a purpose should include radiation energies of up to 20 MeV and high dose rates as are to be expected in the event of weak shielding. Furthermore, the contributions of photons, electrons and neutrons to the dose rate must be evaluated. Monte Carlo (MC) simulations play a key role in designing and characterizing such a reference field.

The Geant4 MC toolkit was used to test various types of shielding. This toolkit allows the transport of photons, electrons and neutrons to be tracked. The tested shields differ in terms of their material composition, dimensions and arrangements. The simulation was carried out using phase‑space files for the beam qualities (6 MV, 10 MV, and 25 MV) of PTB’s Elekta Precise Linac. The radiation field of 10 cm × 10 cm at the isocenter was used.

Geometrical importance biasing was implemented as a variance reduction technique to obtain statistically significant energy depositions in the detector located after the shielding. The detector consisted of two boxes with the following dimensions: 20 × 20 × 2 cm3 and 2 × 2 × 0.5 cm3. The smaller box was embedded in the larger one. During the simulation, the contribution of each particle to the absorbed dose rate was determined, and fluence spectra were measured at various positions behind the shielding. A prior simulation was necessary to obtain a calibration factor that converts relative MC dose values to ambient equivalent dose rates.

The ambient dose equivalent was calculated by convoluting the photon, electron, and neutron fluence spectra by the dose rate conversion coefficient. Four geometric setups with different types of shielding have been simulated to date: (1) a primary wall with a thickness of 2 m; (2) a cage inside the linac room; (3) four and (4) eight high‑density concrete block arrangements inside the linac room. All those configurations were centered on the beam axis. For each of these setups, the elemental composition and the density of the shielding were varied to reproduce experimental conditions. Figure 1 shows the example of the setup of a barite concrete cage with a density of 3.14 g/cm3 as shielding. It also indicates the photon fluence spectra before and after the cage inside the linac room. New configurations are currently being tested – both experimentally and by means of Monte Carlo simulations – to meet the requirements of a pulsed reference field.

Fig. 1: Barite cage with a density of 3.14 g/cm3 as shielding in the linac room (left), and fluence spectra for the 25 MV source 30 cm before and after the cage (right).

Parts of this work have been supported by the German Federal Office for Radiation Protection (BfS) with the project code number 3619S2236. This project is titled “Setting up and characterizing a reference field to ensure radiation protection at accelerator facilities in medicine and research and to test and calibrate the corresponding measuring instruments”.

 

References

[01]    Zutz H.* and Hupe O., Ambient dose and dose rate measurements in the vicinity of Elekta Precise Accelerators for radiation therapy. Radiat. Prot. Dosim. 162, 431–437 (2014).

[02]    Ambrosi, P., Borowski, M. and Iwatschenko, M. Considerations concerning the use of counting active personal dosemeters in pulsed radiation fields of ionising radiation. Radiat. Prot. Dosim. 139, 483–493 (2010).

[03]    Ankerhold, U., Hupe, O. and Ambrosi, P. Deficiencies of active electronic radiation protection dosemeters in pulsed fields. Radiat. Prot. Dosim. 135, 149–153 (2009).

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