Development of a novel and low-cost anthropomorphic pelvis phantom for 3D dosimetry in radiotherapy.

Purpose: The aim of this study was to construct a low-cost, anthropomorphic, and 3D-printed pelvis phantom and evaluate the feasibility of its use to perform 3D dosimetry with commercially available bead thermoluminescent dosimeters (TLDs). Material and methods: A novel anthropomorphic female phantom was developed with all relevant pelvic organs to position the bead TLDs. Organs were 3D-printed using acrylonitrile butadiene styrene. Phantom components were confirmed to have mass density and computed tomography (CT) numbers similar to relevant tissues. To find out clinically required spatial resolution of beads to cause no perturbation effect, TLDs were positioned with 2.5, 5, and 7.5 mm spacing on the surface of syringe. After taking a CT scan and creating a 4-field conformal radiotherapy plan, 3 dose planes were extracted from the treatment planning system (TPS) at different depths. By using a 2D-gamma analysis, the TPS reports were compared with and without the presence of beads. Moreover, the bead TLDs were placed on the organs' surfaces of the pelvis phantom and exposed to high-dose-rate (HDR) Co source. TLDs' readouts were compared with the TPS calculated doses, and dose surface histograms (DSHs) of organs were plotted.Results: 3D-printed phantom organs agreed well with body tissues regarding both their design and radiation properties. Furthermore, the 2D-gamma analysis on the syringe showed more than 99% points passed 3%- and 3-mm criteria at different depths. By calculating the integral dose of DSHs, the percentage differences were -1.5%, 2%, 5%, and 10% for uterus, rectum, bladder, and sigmoid, respectively. Also, combined standard uncertainty was estimated as 3.5% (k= 1). Conclusions: A customized pelvis phantom was successfully built and assessed to confirm properties similar to body tissues. Additionally, no significant perturbation effect with different bead resolutions was presented by the external TPS, with 0.1 mm dose grid resolution

Temperature Dependent Charge Transport Studies in Polycrystalline and Single Crystal CVD Diamond Detectors.

Diamond has been regarded as a promising radiation detector material for the use as a solid state ionising chamber for decades. To improve detection performance, the parameters affecting the charge transport need to be understood, especially defects leading to degradation from the expected behaviour of an “ideal” crystal. Recently, chemical vapour deposited (CVD) single crystal diamond has become available, offering the opportunity to study the properties of this synthesised material independent of grain boundaries. This work focuses on particle induced charge pulses studied as a function of temperature between 200 K and 296 K in polycrystalline and single crystal material. Thermally activated re-emission of holes out of shallow trap levels is clearly observed in this temperature range. The electrical characterisation is complemented by other techniques, such as polarising microscopy and luminescence studies. Time resolved ion beam induced charge (IBIC) imaging on a single crystal was also performed. The charge collection efficiency of the devices hardly shows a temperature dependence in the investigated range, but polarisation and priming affect the detection performance. The pattern in the IBIC images of the single crystal reflects the spatial distribution of the luminescence signature of nitrogen impurities and dislocations, giving direct evidence of their degrading effect on the charge carrier mobility-lifetime product (μt). Additionally, there is some structure in the μt images evolving with the growth direction of the CVD material. The spatial distribution of internal electric fields will be discussed together with the effects of polarisation/priming processes. The recent progress in high purity single crystal synthesis of CVD diamond is a major step for its application as a radiation detector with spectroscopic properties. The study of these single crystals will improve the understanding of the relation between defects and the bulk charge transport properties of diamond

Temperature Dependent Charge Transport Studies in Polycrystalline and Single Crystal CVD Diamond Detectors.

Diamond has been regarded as a promising radiation detector material for the use as a solid state ionising chamber for decades. To improve detection performance, the parameters affecting the charge transport need to be understood, especially defects leading to degradation from the expected behaviour of an “ideal” crystal. Recently, chemical vapour deposited (CVD) single crystal diamond has become available, offering the opportunity to study the properties of this synthesised material independent of grain boundaries. This work focuses on particle induced charge pulses studied as a function of temperature between 200 K and 296 K in polycrystalline and single crystal material. Thermally activated re-emission of holes out of shallow trap levels is clearly observed in this temperature range. The electrical characterisation is complemented by other techniques, such as polarising microscopy and luminescence studies. Time resolved ion beam induced charge (IBIC) imaging on a single crystal was also performed. The charge collection efficiency of the devices hardly shows a temperature dependence in the investigated range, but polarisation and priming affect the detection performance. The pattern in the IBIC images of the single crystal reflects the spatial distribution of the luminescence signature of nitrogen impurities and dislocations, giving direct evidence of their degrading effect on the charge carrier mobility-lifetime product (μt). Additionally, there is some structure in the μt images evolving with the growth direction of the CVD material. The spatial distribution of internal electric fields will be discussed together with the effects of polarisation/priming processes. The recent progress in high purity single crystal synthesis of CVD diamond is a major step for its application as a radiation detector with spectroscopic properties. The study of these single crystals will improve the understanding of the relation between defects and the bulk charge transport properties of diamond

Temperature dependent charge transport studies in polycrystalline and single crystal CVD diamond detectors

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The effect of dose enhancement near metal interfaces on synthetic diamond based X-ray dosimeters

This study investigates the effects of dose enhancement on the photocurrent performance at metallic interfaces in synthetic diamond detectors based X-ray dosimeters as a function of bias voltages. Monte Carlo (MC) simulations with the BEAMnrc code were carried out to simulate the dose enhancement factor (DEF) and compared against the equivalent photocurrent ratio from experimental investigations. The MC simulation results show that the sensitive region for the absorbed dose distribution covers a few micrometers distances from the interface. Experimentally, two single crystals (SC) and one polycrystalline (PC) synthetic diamond samples were fabricated into detectors with carbon based electrodes by boron and carbon ion implantation. Subsequently; the samples were each mounted inside a tissue equivalent encapsulation to minimize unintended fluence perturbation. Dose enhancement was generated by placing copper, lead or gold near the active volume of the detectors using 50 kVp and 100 kVp X-rays relevant for medical dosimetry. The results show enhancement in the detectors’ photocurrent performance when different metals are butted up to the diamond bulk as expected. The variation in the photocurrent measurement depends on the type of diamond samples, their electrodes’ fabrication and the applied bias voltages indicating that the dose enhancement near the detector may modify their electronic performance

The effect of dose enhancement near metal interfaces on synthetic diamond based X-ray dosimeters

This study investigates the effects of dose enhancement on the photocurrent performance at metallic interfaces in synthetic diamond detectors based X-ray dosimeters as a function of bias voltages. Monte Carlo (MC) simulations with the BEAMnrc code were carried out to simulate the dose enhancement factor (DEF) and compared against the equivalent photocurrent ratio from experimental investigations. The MC simulation results show that the sensitive region for the absorbed dose distribution covers a few micrometers distances from the interface. Experimentally, two single crystals (SC) and one polycrystalline (PC) synthetic diamond samples were fabricated into detectors with carbon based electrodes by boron and carbon ion implantation. Subsequently; the samples were each mounted inside a tissue equivalent encapsulation to minimize unintended fluence perturbation. Dose enhancement was generated by placing copper, lead or gold near the active volume of the detectors using 50 kVp and 100 kVp X-rays relevant for medical dosimetry. The results show enhancement in the detectors’ photocurrent performance when different metals are butted up to the diamond bulk as expected. The variation in the photocurrent measurement depends on the type of diamond samples, their electrodes’ fabrication and the applied bias voltages indicating that the dose enhancement near the detector may modify their electronic performance

Alpha radiation induced space charge stability effects in semi-insulating silicon carbide semiconductors compared to diamond.

Although the use of semi-insulating silicon carbide material for radiation detection purposes has been previously demonstrated, its use in practical applications has been inhibited by space charge stability issues caused by defect concentrations within the material, the so called polarisation effect, by which the count rate and resultant spectrum changes with irradiation time. This is a result of the charge carriers generated during irradiation filling deep level defects within the material, causing space charge buildup and de-activating that trap level until the trapped charge is re-emitted. Consequently, the time dependence of the polarisation effect has been determined by a combination of parameters that can be influenced during operation, namely the incident radiation intensity, ambient light, temperature and bias. The material properties have also been considered through the use of materials with different defect capture cross sections, concentrations and energy level. A thorough characterisation of the alpha irradiation induced polarisation phenomenon in semi-insulating silicon carbide has been conducted to demonstrate that stable operation detectors are in fact possible with this material. The effects were compared to single crystal diamond and polycrystalline diamond, which are known to exhibit similar polarisation issues. The polarisation rate as an effect of incident flux, bias and temperature was determined, with the depolarisation rate as a function of ambient light and bias also demonstrated. Consequently it has been shown that stable operation can be maintained for detectors made from semi-insulating SiC material of active thickness 350 μm at incident alpha radiation fluxes of ±400 V). Furthermore, polarisation can be suitably managed or reduced through the use of light illumination and elevated temperatures (373 K)