Why diamond dimensions and electrode geometry are crucial for small photon beam dosimetry
Résumé
Recent use of very small photon beams (down to 4 mm) in stereotactic radiotherapy requires new detectors to accurately determine the delivered dose. Diamond detectors have been presented in the literature as an attractive candidate for this application, due to their small detection volume and the diamond atomic number (Z = 6) which is close to water effective atomic number (Zeff ∼ 7.42). However, diamond exhibits a density 3.51 times greater than that of water and recent studies using Monte Carlo simulations have demonstrated the drawback of a high-density detector on small beam output factors.
The current study focuses on geometrical parameters of diamond detector, namely, the diamond dimensions and the electrode geometry, in order to solve the dosimetric issues still observed in small photon beams with diamond detectors. To give better insights to these open questions, we have used both computational method and experimental analysis. This study highlighted that reducing diamond dimensions is crucial for small beam output factor measurements and to limit the influence of its high density. Furthermore, electrodes covering the whole diamond surface were essential for a dose rate independence of the diamond detector. The optimal dosimeter derived from this work presented small diamond dimensions of approximately 1 × 1 × 0.15 mm$^3$, with diamond-like-carbon electrodes covering the whole diamond surface. A dose rate independence of this diamond detector (better than 0.5% over a wide range of dose rates available on a stereotactic dedicated facility) was obtained due to the electrode geometry. Concerning the output factor measurements, a good agreement (better than 1.1%) was observed between this carbon material detector and two types of passive dosimeters (LiF microcubes and EBT2 radiochromic films) for all beam sizes except the smallest field of 0.6 × 0.6 cm$^2$ with a deviation of 2.6%. This new study showed the high performance of this diamond detector in small photon beams, in comparison with various commercially available passive and active dosimeters.
Mots clés
nuclear instrumentation
instrumentation
Particle beams
Stereotactic radiotherapy
Radiochromic film
Semiconductor detectors
Ionization chamber
Diode
Output factor measurements
Monte Carlo
Photoelectrons
Intelligent systems
Geometry
Experimental analysis
Electrodes
Electrode geometries
Effective atomic number
Dosimetry
Dosimeter
Diamond
Diamond like carbon
Carbon films
Atoms
Passive dosimeter
Photons
Computational methods
Carbon-based materials
I-V characteristics
Chemical elements
Radiography
stereotactic radiotherapy
radiotherapy
sensor
detector
diamond detector
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