Reproducibility of CVD diamond detectors for radiotherapy dosimetry G.T. Betzel a,n , S.P. Lansley a,b , D. McKay c , J. Meyer a,1 a Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand b The MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Canterbury, Christchurch, New Zealand c St. George’s Cancer Care Centre, Christchurch, New Zealand article info Article history: Received 18 January 2012 Received in revised form 11 June 2012 Accepted 15 June 2012 Available online 29 June 2012 Keywords: Diamond detector Reproducibility Synthetic diamond Radiation therapy dosimetry abstract Three in-house X-ray detectors based on diamond chemical vapor deposition (CVD) from the same manufactured batch of single crystal films were investigated for their reproducibility. Leakage current, priming dose, response dynamics, dose linearity, dependence on dose rate and angular dependence were used to evaluate differences between detectors. Slight differences were seen in leakage currents before ( o1.5 pA) and after ( o12 pA) irradiation. A priming dose of 7 Gy and rise and fall times of 2 s were found for all three detectors. Sensitivities differed by up to 10%. Dependence on dose rate were similar (D ¼0.92–0.94). Angular dependence was minimal (97–102% avg.). Differences in detector performance appeared to be primarily due to film thickness, which can significantly change sensitivities (nC Gy 1 ) and applied fields (V mm 1 ) for detectors with small sensitive volumes. Results suggest that preselection of CVD diamond films according to thickness in addition to material quality would be required to avoid individual calibration, which is performed for commercially available natural diamond detectors. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Modern radiation sources and technologies require dosimeters to meet many of the following requirements: high sensitivity, small dimensions, a fast response, energy and dose rate indepen- dence and tissue equivalence [1,2]. A promising material for dosimetry that may fulfill these requirements is diamond. How- ever, the scarcity and variability of natural diamonds with suitable dosimetric properties make it impractical for widespread use commercially. Progress in manufacturing diamond via che- mical vapor deposition (CVD) poses to eliminate these limitations, where specific properties may be controlled and reproduced at a relatively low cost. Diamond has advantages over other materials that are used as dosimeters due to its near tissue-equivalence, high radiation toler- ance and high sensitivity. Despite the fact that its mass density (3.51 g cm 3 ) is higher than that of, e.g., compact bone (1.85 g cm 3 ), water (1.00 g cm 3 ) or muscle (0.92 g cm 3 ) [3], its mass stopping power ratios and mass-energy absorption coefficients are very similar over a long range of photon and electron energies, which makes it a candidate for small field dosimetry [1, 4]. Diamond has been studied extensively for its use in radiation therapy dosimetry, e.g. Ref. [4], and single-crystal and polycrystal- line CVD diamond have both been featured in recent works [5–13]. Early work focused on the use of natural diamond, and it has been the only type of diamond to be used commercially. However, correction factors are unknown between individual detectors as each one must be fabricated using diamonds that are individually unique. The reproducibility of commercial PTW natural diamond detectors has already been shown to differ slightly due to varia- tion in material properties, which, to some extent, also differed from what was reported in the PTW technical specifications [14]. Although these detectors work under an acceptable range of beam qualities and applications, each detector requires calibration due to differences between dosimetric properties measured locally and those reported by the manufacturer. We have reported on investigations of X-ray detectors based on several types of commercially available CVD diamond for radiotherapy dosimetry [15]. X-ray detectors based upon rela- tively inexpensive single crystal CVD diamond were consequently evaluated for their performance and compared to other types of detectors [6,7,9]. These detectors have the potential to be suitable for routine dosimetry using conventional radiotherapy instru- mentation. Most research in the literature on diamond detectors report on individual detectors where fabrication of diamond was performed in the authors’ laboratories. In this study, three Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.06.034 n Corresponding author. Present address: Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD, USA. Tel.: þ1 410 706 6513; fax: þ1 410 328 2618. E-mail address: gbetzel@umm.edu (G.T. Betzel). 1 Present address: Department of Radiation Oncology, University of Washington Medical Center, Seattle, WA, USA. Nuclear Instruments and Methods in Physics Research A 691 (2012) 34–38