Speaker
Description
Purpose
To fully utilize the capabilities of proton therapy, it is crucial to conduct effective quality assurance procedures [1], which employ radiation-hard, dosimetry devices with good spatial resolution. The MATRIX project is dedicated to developing a novel diode array that will facilitate proton imaging and beam monitoring [2]. The current study focused on understanding the response characteristics of individual diode pixels in terms of time. The specific goal was to determine the temporal resolution of individual spots in Pencil Beam Scanning (PBS) mode and to quantify afterglow effects and ghosting for sequentially irradiated spots [3].
Material & Methods
The detector sample consisted of 18 GaN pin diodes mounted on a Sapphire wafer with each diode having a size of 500×500 µm^2. The diodes were arranged in two rows. The time characteristic measurement was conducted on individual diodes operated at a reverse bias of 1.5 V using an oscilloscope. The irradiation was performed in a treatment room at the West German Proton Therapy Centre Essen (WPE) using the PBS mode to deliver individual spots and various sequences of two spots with varying time intervals between them. Experimental data were evaluated according to the IEEE standard [4].
Results
The signal of a single spot (Fig. 1) is characterized by rise times of about 3 ms and fall times in the range of 6 ms (between 90% and 10% levels). This is higher than the turn-off time of the proton machine of about 0.1 ms. An example of a temporally correlated sequence of spots is shown in Fig. 2. In general, when comparing single spot responses with subsequent spots, a slight reduction in the response of the diodes of approximately 5% can be found due to the pre-irradiation.
Conclusion
The results presented offer initial insights into the time behavior of pin-type compound semiconductor sensors under proton irradiation. It has been successfully demonstrated that individual PBS spots can be measured using GaN diodes. By further improving the measurement setup, we will evaluate whether the observed rise and fall times are limited by the GaN diodes. These characterizations will provide valuable insights into the proton signal conversion mechanisms within our detector.
Acknowledgements
“GaN diode array for proton monitoring and imaging”, Project number DFG-505408069/ANR-22-CE92-0047
References
[1] B. Arjomandy et al., “AAPM task group 224: Comprehensive proton therapy machine quality assurance,” Medical Physics, vol. 46, no. 8, Aug. 2019, doi: 10.1002/mp.13622.
[2] J.-Y. Duboz et al., “GaN Schottky diodes for proton beam monitoring,” Biomed. Phys. Eng. Express, vol. 5, no. 2, p. 025015, Jan. 2019, doi: 10.1088/2057-1976/aaf9b4.
[3] W. Zhao, G. DeCrescenzo, S. O. Kasap, and J. A. Rowlands, “Ghosting caused by bulk charge trapping in direct conversion flat-panel detectors using amorphous selenium: Ghosting in amorphous selenium flat-panel detectors,” Med. Phys., vol. 32, no. 2, pp. 488–500, Jan. 2005, doi: 10.1118/1.1843353.
[4] "IEEE Standard for Transitions, Pulses, and Related Waveforms", IEEE Std 181-2011 (Revision of IEEE Std 181-2003), Sep. 06, 2011, doi: 10.1109/IEEESTD.2011.6016198.
| Workshop topics | Detector systems |
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