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Beam Scans with High-Resolution Detectors: Impact of Detector Response On Profile Peaks and Valleys

J Duan

J Duan*, E Subashi , Y Yang , F Yin , Duke University Medical Center, Durham, NC


SU-I-GPD-T-454 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: High-resolution detectors such as small volume ion chambers, unshielded diodes and diamond detectors are preferred detectors for beam profile scans to reduce the volume averaging effect, especially in the penumbra region. Published studies show that some high-resolution detectors over respond in small field measurement. The purpose of the present study is to assess the impact of the response of high-resolution detectors on profile scans of regularly sized field with center blocks.

Methods: Five high-resolution detectors (IC: 0.007cc and 0.01cc; Unshielded Diode: 0.95mm diameterx0.5mm and 0.8mm diameterx0.03mm; Diamond: 2.2mm diameterx0.001mm) and a 0.125cc IC were used to measure the beam profile of a field composed of three rectangular openings (1cmX10cm, 4cmx10cm and 4cmx10cm) separated by 2cm and 1cm gaps, respectively. The beam profile appeared in order as 1cm-wide peak, 2cm-wide valley, 4cm-wide peak, 1cm-wide valley and 4cm-wide peak. The profiles were scanned in 0.2mm step at 10cm depth in water, and normalized to central-axis (CAX) which was at the center of the second opening. The profiles were analyzed and peak height, valley depth and valley-to-peak height (VPH) were obtained for each detector.

Results: Variations up to 2.5% in the peak height and up to 25.3 % (4.2% relative to CAX value) in the valley depth were observed. Some profiles showed both higher peaks and deeper valleys than others. The VPH differed by up to 6.3% relative to CAX value. The diamond detector had the greatest VPH while 0.007cc IC and 0.125cc IC had the smallest VPHs. The VPH of the diamond and diodes were higher than those of ICs.

Conclusion: The VPH varies substantially among different high-resolution detectors. Diamond detector and unshielded diodes tend to yield higher VPH than ion chambers. Further study is warranted to investigate the signal over-response in the peak and under-response in the valley.

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