Selective Shielding of a p-Si Detector for Quality Independence

Rikner, Göran; Grusell, Erik

doi:10.3109/02841868509134367

Cited by 41 publications

(27 citation statements)

References 11 publications

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“…This reduces the number of low-energy photons backscattered from the surrounding water which reach the detector, with the intention of making it less energy dependent. Rikner and Grusell 16 showed the addition of a small amount of tungsten behind the silicon reduced the overestimation of depth dose from 3% to 1% for a 20 cmϫ 20 cm 60 Co field. However, since the amount of scattered radiation is low in small fields, this additional shielding may not be useful under the conditions studied in this paper, and of course locates non-water equivalent material in close proximity to the sensitive volume of the detector.…”

Section: Iia1 Detectors Usedmentioning

confidence: 99%

Using a Monte Carlo model to predict dosimetric properties of small radiotherapy photon fields

2008

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Accurate characterization of small-field dosimetry requires measurements to be made with precisely aligned specialized detectors and is thus time consuming and error prone. This work explores measurement differences between detectors by using a Monte Carlo model matched to large-field data to predict properties of smaller fields. Measurements made with a variety of detectors have been compared with calculated results to assess their validity and explore reasons for differences. Unshielded diodes are expected to produce some of the most useful data, as their small sensitive cross sections give good resolution whilst their energy dependence is shown to vary little with depth in a 15 MV linac beam. Their response is shown to be constant with field size over the range 1-10 cm, with a correction of 3% needed for a field size of 0.5 cm. BEAMnrc has been used to create a 15 MV beam model, matched to dosimetric data for square fields larger than 3 cm, and producing small-field profiles and percentage depth doses (PDDs) that agree well with unshielded diode data for field sizes down to 0.5 cm. For fields sizes of 1.5 cm and above, little detector-to-detector variation exists in measured output factors, however for a 0.5 cm field a relative spread of 18% is seen between output factors measured with different detectors-values measured with the diamond and pinpoint detectors lying below that of the unshielded diode, with the shielded diode value being higher. Relative to the corrected unshielded diode measurement, the Monte Carlo modeled output factor is 4.5% low, a discrepancy that is probably due to the focal spot fluence profile and source occlusion modeling. The large-field Monte Carlo model can, therefore, currently be used to predict small-field profiles and PDDs measured with an unshielded diode. However, determination of output factors for the smallest fields requires a more detailed model of focal spot fluence and source occlusion.

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Section: Iia1 Detectors Usedmentioning

confidence: 99%

Using a Monte Carlo model to predict dosimetric properties of small radiotherapy photon fields

2008

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“…The geometrical and material data are mainly based on the information in the paper by Rikner and Grusell. 21 While this paper was under review we have obtained more detailed data from the diode manufacturer, and the model shown in Fig. 2 more accurately represents what is being sold today.…”

Section: A Simulation Of a Si Diode Detectormentioning

confidence: 99%

Monte Carlo study of Si diode response in electron beams

Wang

Rogers

2007

Medical Physics

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Silicon semiconductor diodes measure almost the same depth-dose distributions in both photon and electron beams as those measured by ion chambers. A recent study in ion chamber dosimetry has suggested that the wall correction factor for a parallel-plate ion chamber in electron beams changes with depth by as much as 6%. To investigate diode detector response with respect to depth, a silicon diode model is constructed and the water/silicon dose ratio at various depths in electron beams is calculated using EGSnrc. The results indicate that, for this particular diode model, the diode response per unit water dose (or water/diode dose ratio) in both 6 and 18 MeV electron beams is flat within 2% versus depth, from near the phantom surface to the depth of R50 (with calculation uncertainty <0.3%). This suggests that there must be some other correction factors for ion chambers that counter-balance the large wall correction factor at depth in electron beams. In addition, the beam quality and field-size dependence of the diode model are also calculated. The results show that the water/diode dose ratio remains constant within 2% over the electron energy range from 6 to 18 MeV. The water/diode dose ratio does not depend on field size as long as the incident electron beam is broad and the electron energy is high. However, for a very small beam size (1 X 1 cm(2)) and low electron energy (6 MeV), the water/diode dose ratio may decrease by more than 2% compared to that of a broad beam.

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“…In addition, diode detectors show directional dependency. 1 They also show an enhanced energy response to low-energy photons due to the preponderance of photoelectric interactions in silicon, 7,8 but in small SRS fields, where lateral electron equilibrium does not exist, the contribution of low energy photons is much less than extended fields. 3,5 TLD can, in general, be made smaller than the smallest active volume of an ion chamber.…”

Section: Introductionmentioning

confidence: 99%

Dosimetric properties of radiophotoluminescent glass rod detector in high‐energy photon beams from a linear accelerator and Cyber‐Knife

et al. 2004

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A fully automatic radiophotoluminescent glass rod dosimeter (GRD) system has recently become commercially available. This article discusses the dosimetric properties of the GRD including uniformity and reproducibility of signal, dose linearity, and energy and directional dependence in high-energy photon beams. In addition, energy response is measured in electron beams. The uniformity and reproducibility of the signal from 50 GRDs using a 60Co beam are both +/- 1.1% (one standard deviation). Good dose linearity of the GRD is maintained for doses ranging from 0.5 to 30 Gy, the lower and upper limits of this study, respectively. The GRD response is found to show little energy dependence in photon energies of a 60Co beam, 4 MV (TPR20(10)=0.617) and 10 MV (TPR(20)10=0.744) x-ray beams. However, the GRD responses for 9 MeV (mean energy, Ez = 3.6 MeV) and 16 MeV (Ez = 10.4 MeV) electron beams are 4%-5% lower than that for a 60Co beam in the beam quality dependence. The measured angular dependence of GRD, ranging from 0 degrees (along the long axis of GRD) to 120 degrees is within 1.5% for a 4 MV x-ray beam. As applications, a linear accelerator-based radiosurgery system and Cyber-Knife output factors are measured by a GRD and compared with those from various detectors including a p-type silicon diode detector, a diamond detector, and an ion chamber. It is found that the GRD is a very useful detector for small field dosimetry, in particular, below 10 mm circular fields.

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Selective Shielding of a p-Si Detector for Quality Independence

Cited by 41 publications

References 11 publications

Using a Monte Carlo model to predict dosimetric properties of small radiotherapy photon fields

Using a Monte Carlo model to predict dosimetric properties of small radiotherapy photon fields

Monte Carlo study of Si diode response in electron beams

Dosimetric properties of radiophotoluminescent glass rod detector in high‐energy photon beams from a linear accelerator and Cyber‐Knife

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