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All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Robust Avalanche (1.5 kV, 2 kA/cm²) in Vertical GaN Diodes on Patterned Sapphire Substrate

The lack of avalanche capability is a key limitation of current lateral GaN devices. Despite the report of avalanche in vertical GaN-on-GaN devices, the high wafer cost hinders device commercialization. Here, in this work, we demonstrate a circuit-level avalanche in vertical GaN diodes on low-cost patterned sapphire substrate (PSS), with the avalanche voltage (1.57 kV) and avalanche current density (>2 kA/cm 2 ) both being the highest reported in GaN devices on foreign substrates. The PSS enables a lower dislocation density than conventional sapphire substrate and is employed in high-voltage GaN devices for the first time. The avalanche voltage in the circuit test reaches 98% of the parallel-plane limit, further affirming that near-ideal avalanche breakdown can be realized on GaN devices on foreign substrates. These results show the promise of the GaN-on-PSS platform for low-cost, robust power devices.

42 ENGINEERING↗
DOE OSTI2023

Implanted Guard Ring Edge Termination With Avalanche Capability for Vertical GaN Devices

Edge termination is the key building block in power devices to enable near-ideal, avalanche breakdown voltage (BV). Here, this work presents the design, fabrication, and physics of a GaN guard ring (GR) edge termination formed by selective-area nitrogen implantation through an epitaxial p-GaN layer. The fabrication of this termination only includes a single implantation step that does not require precise control of implant depth, rendering a large process latitude. The selective-area implantation produces p-GaN rings that are separated by the implanted, semi-insulating regions. The number and spacing of the p-type rings are found to determine the BV of the vertical GaN p-n diode. The 16-ring structure enables a BV of 1800 V, being 88% of the theoretical 1-D parallel-plane limit. Avalanche characteristics are observed in devices with a large variety of GR designs. Finally, we present a comprehensive survey on the efficiency, fabrication complexity, real estate, and avalanche capability of various edge termination techniques that have been reported in vertical GaN devices. The high efficiency (among the highest reported in avalanche-capable GaN terminations), simple and robust fabrication process, and uniform avalanche capability make this implanted GR a promising edge termination for high-voltage GaN devices.

42 ENGINEERING↗
DOE OSTI2020

Impact of pellet preparation on reflectance measurements of ammonium sulfate by single-angle reflectance spectroscopy

The optical constants, n and k, are required to model the reflection, refraction and transmission of light at the first surface interface of a material, as well as its propagation through the material; here n corresponds to the real component and k the imaginary component of the refractive index. Several different spectroscopic methods have been used to determine the n and k values for different materials, including single angle reflectance spectroscopy and ellipsometry. The single-angle reflectance method quantitatively records the specular reflectance R(v) from a plane parallel face of the material and uses the Kramers-Kronig transform to extract the n and k values. For most compounds, however, it is difficult to obtain a single crystal or high-quality window of sufficient planarity and of the appropriate dimensions (several mm) to make the measurement. For this reason, we further investigate the use of pressed pellets of neat powdered substances to measure optical constants of these substances using the single-angle reflectance method. We have found that surface roughness can significantly influence the measured quantitative reflectance spectrum R(v) and, consequently, the derived n and k parameters. A collaborative study between Defence Research and Development Canada (DRDC) - Valcartier Research Center and Pacific Northwest National Laboratory (PNNL) has been carried out using different pellets of neat ammonium sulfate [(NH4)2SO4] to show how parameters such as the relative fraction of particle sizes for the component powder along with the grinding process of the pure compound all can affect the reflectance spectrum used to derive the optical constants. All pressed pellets were characterized by single-angle reflectance spectroscopy.

Diaz, Emmanuela↗
DOE OSTI2014

Iso-uncertainty control in an experimental fluoroscopy system

Purpose: X-ray fluoroscopy remains an important imaging modality in a number of image-guided procedures due to its real-time nature and excellent spatial detail. However, the radiation dose delivered raises concerns about its use particularly in lengthy treatment procedures (>0.5 h). The authors have previously presented an algorithm that employs feedback of geometric uncertainty to control dose while maintaining a desired targeting uncertainty during fluoroscopic tracking of fiducials. The method was tested using simulations of motion against controlled noise fields. In this paper, the authors embody the previously reported method in a physical prototype and present changes to the controller required to function in a practical setting. Methods: The metric for feedback used in this study is based on the trace of the covariance of the state of the system, tr(C). The state is defined here as the 2D location of a fiducial on a plane parallel to the detector. A relationship between this metric and the tube current is first developed empirically. This relationship is extended to create a manifold that incorporates a latent variable representing the estimated background attenuation. The manifold is then used within the controller to dynamically adjust the tube current and maintain a specified targeting uncertainty. To evaluate the performance of the proposed method, an acrylic sphere (1.6 mm in diameter) was tracked at tube currents ranging from 0.5 to 0.9 mA (0.033 s) at a fixed energy of 80 kVp. The images were acquired on a Varian Paxscan 4030A (2048 × 1536 pixels, ∼100 cm source-to-axis distance, ∼160 cm source-to-detector distance). The sphere was tracked using a particle filter under two background conditions: (1) uniform sheets of acrylic and (2) an acrylic wedge. The measured tr(C) was used in conjunction with a learned manifold to modulate the tube current in order to maintain a specified uncertainty as the sphere traversed regions of varying thickness corresponding to the acrylic sheets in the background. Results: With feedback engaged, the tracking error was found to correlate well with the specified targeting uncertainty. Tracking of the fiducial was found to be robust to changes in the attenuation presented by the varying background conditions. For a desired uncertainty of 5.0 mm, comparison of the feedback framework with a comparable system employing fixed exposure demonstrated dose savings of 29%. Conclusions: This work presents a relation between a state descriptor, tr(C), the x-ray tube current used, and an estimate of the background attenuation. This relation is leveraged to modulate the tube current in order to maintain a desired geometric uncertainty during fluoroscopy. The authors’ work demonstrates the use of the method in a real x-ray fluoroscopy system with physical motion against varying backgrounds. The method offers potential savings in imaging dose to patients and staff while maintaining tracking uncertainty during fluoroscopy-guided treatment procedures.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Technical Note: Experimental determination of the effective point of measurement of two cylindrical ionization chambers in a clinical proton beam

Purpose: IAEA TRS-398 notes that cylindrical ionization chambers are preferred for reference proton dosimetry. If a cylindrical ionization chamber is used in a phantom to measure the dose as a function of depth, the effective point of measurement (EPOM) must be taken into account. IAEA TRS-398 recommends a displacement of 0.75 times the inner cavity radius (0.75R) for heavy ion beams. Theoretical models by Palmans and by Bhullar and Watchman confirmed this value. However, the experimental results vary from author to author. The purpose of this study is to accurately measure the displacement and explain the past experimental discrepancies. Methods: In this work, we measured the EPOM of cylindrical ionization chambers with high accuracy by comparing the Bragg-peak position obtained with cylindrical ionization chambers (PTW 30013, PTW 31016) to that obtained using a plane-parallel ionization chamber (PTW 34045). Results: The EPOMs of PTW 30013 and 31016 were shifted by 0.92 ± 0.07 R with R = 3.05 mm and 0.90 ± 0.14 R with R = 1.45 mm, respectively, from the reference point toward the source. Conclusions: The EPOMs obtained were greater than the value of 0.75R proposed by the IAEA TRS-398 and the analytical results.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2015

SU-E-T-104: An Examination of Dose in the Buildup and Build-Down Regions

Purpose: To examine dose in the buildup and build-down regions and compare measurements made with various models and dosimeters Methods: Dose was examined in a 30×30cm {sup 2} phantom of water-equivalent plastic with 10cm of backscatter for various field sizes. Examination was performed with radiochromic film and optically-stimulated-luminescent-dosimeter (OSLD) chips, and compared against a plane-parallel chamber with a correction factor applied to approximate the response of an extrapolation chamber. For the build-down region, a correction factor to account for table absorption and chamber orientation in the posterior-anterior direction was applied. The measurement depths used for the film were halfway through their sensitive volumes, and a polynomial best fit curve was used to determine the dose to their surfaces. This chamber was also compared with the dose expected in a clinical kernel-based computer model, and a clinical Boltzmann-transport-equation-based (BTE) computer model. The two models were also compared against each other for cases with air gaps in the buildup region. Results: Within 3mm, all dosimeters and models agreed with the chamber within 10% for all field sizes. At the entrance surface, film differed in comparison with the chamber from +90% to +15%, the BTE-model by +140 to +3%, and the kernel-based model by +20% to −25%, decreasing with increasing field size. At the exit surface, film differed in comparison with the chamber from −10% to −15%, the BTE-model by −53% to −50%, the kernel-based model by −55% to −57%, mostly independent of field size. Conclusion: The largest differences compared with the chamber were found at the surface for all field sizes. Differences decreased with increasing field size and increasing depth in phantom. Air gaps in the buildup region cause dose buildup to occur again post-gap, but the effect decreases with increasing phantom thickness prior to the gap.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

SU-E-T-198: Comparison Between a PTW MicroDiamond Dosimeter and a Markus Chamber in a 62 MeV/n Carbon Ion Beam

Purpose: To investigate the linear energy transfer (LET) dependence of a PTW Freiburg microDiamond dosimeter, we compared its response to the response of a plane-parallel Markus chamber in a 62 MeV/n mono-energetic carbon ion beam. Methods: The response of both detectors has been studied as a function of depth in graphite by adding or removing graphite plates in front of the detectors. To account for fluctuations of the beam, we used two setups with different monitor chambers. The depth of the effective point of measurement of both detectors has been converted into a graphite equivalent depth using ICRU Report 73 data. As recommended by IAEA TRS-398, the response of the Markus chamber has been corrected for temperature, pressure, polarity effects and ion recombination. The latter required an additional experiment; to quantify the effect of volume recombination and initial recombination, measurements have been performed at different voltages and different dose rates. Results: As expected, the dominant process leading to ion recombination for carbon ion beam is the initial recombination. At the entrance, the ion recombination correction equals 1.1% and the value is approximately constant in the plateau region. Due to the increase of the LET in the Bragg peak region, we observe a strong increase of the ion recombination correction, up to 6.1% at the distal edge. Comparison between the microDiamond response and the Markus chamber response shows good agreement in the plateau region. However, we observe a 13.6% under response of the microDiamond in the Bragg peak. Conclusion: Increasing between 1% and 6%, the depth dependent ion recombination correction has to be applied to the Markus response. The comparison between the microDiamond and the Markus chamber indicates that there is an under-response of the microDiamond in the vicinity of the Bragg peak due to the increased LET.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

SU-F-BRD-15: Quality Correction Factors in Scanned Or Broad Proton Therapy Beams Are Indistinguishable

Purpose: The IAEA TRS-398 code of practice details the reference conditions for reference dosimetry of proton beams using ionization chambers and the required beam quality correction factors (kQ). Pencil beam scanning (PBS) requires multiple spots to reproduce the reference conditions. The objective is to demonstrate, using Monte Carlo (MC) calculations, that kQ factors for broad beams can be used for scanned beams under the same reference conditions with no significant additional uncertainty. We consider hereafter the general Alfonso formalism (Alfonso et al, 2008) for non-standard beam. Methods: To approach the reference conditions and the associated dose distributions, PBS must combine many pencil beams with range modulation and shaping techniques different than those used in passive systems (broad beams). This might lead to a different energy spectrum at the measurement point. In order to evaluate the impact of these differences on kQ factors, ion chamber responses are computed with MC (Geant4 9.6) in a dedicated scanned pencil beam (Q-pcsr) producing a 10×10cm2 composite field with a flat dose distribution from 10 to 16 cm depth. Ion chamber responses are also computed by MC in a broad beam with quality Q-ds (double scattering). The dose distribution of Q -pcsr matches the dose distribution of Q-ds. k-(Q-pcsr,Q-ds) is computed for a 2×2×0.2cm{sup 3} idealized air cavity and a realistic plane-parallel ion chamber (IC). Results: Under reference conditions, quality correction factors for a scanned composite field versus a broad beam are the same for air cavity dose response, k-(Q-pcsr,Q-ds) =1.001±0.001 and for a Roos IC, k-(Q-pcsr,Q-ds) =0.999±0.005. Conclusion: Quality correction factors for ion chamber response in scanned and broad proton therapy beams are identical under reference conditions within the calculation uncertainties. The results indicate that quality correction factors published in IAEA TRS-398 can be used for scanned beams in the SOBP of a high-energy proton beam. Jefferson Sorriaux is financed by the Walloon Region under the convention 1217662. Jefferson Sorriaux is sponsored by a public-private partnership IBA - Walloon Region.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Comparison and uncertainty evaluation of different calibration protocols and ionization chambers for low-energy surface brachytherapy dosimetry

Purpose: A surface electronic brachytherapy (EBT) device is in fact an x-ray source collimated with specific applicators. Low-energy (<100 kVp) x-ray beam dosimetry faces several challenges that need to be addressed. A number of calibration protocols have been published for x-ray beam dosimetry. The media in which measurements are performed are the fundamental difference between them. The aim of this study was to evaluate the surface dose rate of a low-energy x-ray source with small field applicators using different calibration standards and different small-volume ionization chambers, comparing the values and uncertainties of each methodology. Methods: The surface dose rate of the EBT unit Esteya (Elekta Brachytherapy, The Netherlands), a 69.5 kVp x-ray source with applicators of 10, 15, 20, 25, and 30 mm diameter, was evaluated using the AAPM TG-61 (based on air kerma) and International Atomic Energy Agency (IAEA) TRS-398 (based on absorbed dose to water) dosimetry protocols for low-energy photon beams. A plane parallel T34013 ionization chamber (PTW Freiburg, Germany) calibrated in terms of both absorbed dose to water and air kerma was used to compare the two dosimetry protocols. Another PTW chamber of the same model was used to evaluate the reproducibility between these chambers. Measurements were also performed with two different Exradin A20 (Standard Imaging, Inc., Middleton, WI) chambers calibrated in terms of air kerma. Results: Differences between surface dose rates measured in air and in water using the T34013 chamber range from 1.6% to 3.3%. No field size dependence has been observed. Differences are below 3.7% when measurements with the A20 and the T34013 chambers calibrated in air are compared. Estimated uncertainty (with coverage factor k = 1) for the T34013 chamber calibrated in water is 2.2%–2.4%, whereas it increases to 2.5% and 2.7% for the A20 and T34013 chambers calibrated in air, respectively. The output factors, measured with the PTW chambers, differ by less than 1.1% for any applicator size when compared to the output factors that were measured with the A20 chamber. Conclusions: Measurements using both dosimetric protocols are consistent, once the overall uncertainties are considered. There is also consistency between measurements performed with both chambers calibrated in air. Both the T34013 and A20 chambers have negligible stem effect. Any x-ray surface brachytherapy system, including Esteya, can be characterized using either one of these calibration protocols and ionization chambers. Having less correction factors, lower uncertainty, and based on measurements, performed in closer to clinical conditions, the TRS-398 protocol seems to be the preferred option.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

Prototypes of self-powered radiation detectors employing intrinsic high-energy current

Purpose: The authors experimentally investigate the effect of direct energy conversion of x-rays via selfpowered Auger- and photocurrent, potentially suitable to practical radiation detection and dosimetry in medical applications. Experimental results are compared to computational predictions. The detector the authors consider is a thin-film multilayer device, composed of alternating disparate electrically conductive and insulating layers. This paper focuses on the experiments while a companion paper introduces the fundamental concepts of high-energy current (HEC) detectors. Methods: The energy of ionizing radiation is directly converted to detector signal via electric current induced by high-energy secondary electrons generated in the detector material by the incident primary radiation. The HEC electrons also ionize the dielectric and the resultant charge carriers are selfcollected due to the contact potential of the disparate electrodes. Thus, an electric current is induced in the conductors in two different ways without the need for externally applied bias voltage or amplification. Thus, generated signal in turn is digitized by a data acquisition system. To determine the fundamental properties of the HEC detector and to demonstrate its feasibility for medical applications, the authors used a planar geometry composed of multilayer microstructures. Various detectors with up to seven conducting layers with different combinations of materials (250 μm Al, 35 μm Cu, 100 μm Pb) and air gaps (100 μm) were exposed to nearly plane-parallel 60–120 kVp x-ray beams. For the experimental design and verification, the authors performed coupled electron–photon radiation transport computations. The detector signal was measured using a commercial data acquisition system with 24 bits dynamic range, 0.4 fC sensitivity, and 0.9 ms sampling time. Results: Measured signals for the prototype detector varied depending on the number of layers, material type, and incident photon energy, and it was in the range of 30–150 nA/cm{sup 2} for unit air kerma (1 Gy), which is viable for practical applications. The experiments had an excellent agreement with the computations. Within the examined range of 60–120 kVp, the energy dependence of the HEC (normalized to the x-ray tube output) was relatively small. Conclusions: Based on the experimental results for 100 ms sampling time, it would be possible to measure the time dependence of x-ray beams for x-ray tube current of 0.1 mA or higher. Significant advantages of the HEC device are that generation of its signal does not require external power supply, it can be made in any size and shape, including flexible curvilinear forms, and it is inexpensive. It remains to be determined, which of the potential applications in medical dosimetry (both in vivo and external), or radiation protection would benefit from such selfpowered detectors.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Experimental and Monte Carlo studies of fluence corrections for graphite calorimetry in low- and high-energy clinical proton beams

Purpose: The aim of this study was to determine fluence corrections necessary to convert absorbed dose to graphite, measured by graphite calorimetry, to absorbed dose to water. Fluence corrections were obtained from experiments and Monte Carlo simulations in low- and high-energy proton beams. Methods: Fluence corrections were calculated to account for the difference in fluence between water and graphite at equivalent depths. Measurements were performed with narrow proton beams. Plane-parallel-plate ionization chambers with a large collecting area compared to the beam diameter were used to intercept the whole beam. High- and low-energy proton beams were provided by a scanning and double scattering delivery system, respectively. A mathematical formalism was established to relate fluence corrections derived from Monte Carlo simulations, using the FLUKA code [A. Ferrari et al., “FLUKA: A multi-particle transport code,” in CERN 2005-10, INFN/TC 05/11, SLAC-R-773 (2005) and T. T. Böhlen et al., “The FLUKA Code: Developments and challenges for high energy and medical applications,” Nucl. Data Sheets 120, 211–214 (2014)], to partial fluence corrections measured experimentally. Results: A good agreement was found between the partial fluence corrections derived by Monte Carlo simulations and those determined experimentally. For a high-energy beam of 180 MeV, the fluence corrections from Monte Carlo simulations were found to increase from 0.99 to 1.04 with depth. In the case of a low-energy beam of 60 MeV, the magnitude of fluence corrections was approximately 0.99 at all depths when calculated in the sensitive area of the chamber used in the experiments. Fluence correction calculations were also performed for a larger area and found to increase from 0.99 at the surface to 1.01 at greater depths. Conclusions: Fluence corrections obtained experimentally are partial fluence corrections because they account for differences in the primary and part of the secondary particle fluence. A correction factor, F(d), has been established to relate fluence corrections defined theoretically to partial fluence corrections derived experimentally. The findings presented here are also relevant to water and tissue-equivalent-plastic materials given their carbon content.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

Ion recombination correction in carbon ion beams

Purpose: In this work, ion recombination is studied as a function of energy and depth in carbon ion beams. Methods: Measurements were performed in three different passively scattered carbon ion beams with energies of 62 MeV/n, 135 MeV/n, and 290 MeV/n using various types of plane-parallel ionization chambers. Experimental results were compared with two analytical models for initial recombination. One model is generally used for photon beams and the other model, developed by Jaffé, takes into account the ionization density along the ion track. An investigation was carried out to ascertain the effect on the ion recombination correction with varying ionization chamber orientation with respect to the direction of the ion tracks. The variation of the ion recombination correction factors as a function of depth was studied for a Markus ionization chamber in the 62 MeV/n nonmodulated carbon ion beam. This variation can be related to the depth distribution of linear energy transfer. Results: Results show that the theory for photon beams is not applicable to carbon ion beams. On the other hand, by optimizing the value of the ionization density and the initial mean-square radius, good agreement is found between Jaffé’s theory and the experimental results. As predicted by Jaffé’s theory, the results confirm that ion recombination corrections strongly decrease with an increasing angle between the ion tracks and the electric field lines. For the Markus ionization chamber, the variation of the ion recombination correction factor with depth was modeled adequately by a sigmoid function, which is approximately constant in the plateau and strongly increasing in the Bragg peak region to values of up to 1.06. Except in the distal edge region, all experimental results are accurately described by Jaffé’s theory. Conclusions: Experimental results confirm that ion recombination in the investigated carbon ion beams is dominated by initial recombination. Ion recombination corrections are found to be significant and cannot be neglected for reference dosimetry and for the determination of depth dose curves in carbon ion beams.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

SU-C-201-03: Ionization Chamber Collection Efficiency in Pulsed Radiation Fields of High Pulse Dose

Purpose: To investigate the reduction of collection efficiency of ionization chambers (IC) by volume recombination and its correction in pulsed fields of very high pulse dose. Methods: Measurements of the collection efficiency of a plane-parallel advanced Markus IC (PTW 34045, 1mm electrode spacing, 300V nominal voltage) were obtained for collection voltages of 100V and 300V by irradiation with a pulsed electron beam (20MeV) of varied pulse dose up to approximately 600mGy (0.8nC liberated charge). A reference measurement was performed with a Faraday cup behind the chamber. It was calibrated for the liberated charge in the IC by a linear fit of IC measurement to reference measurement at low pulse doses. The results were compared to the commonly used two voltage approximation (TVA) and to established theories for volume recombination, with and without considering a fraction of free electrons. In addition, an equation system describing the charge transport and reactions in the chamber was solved numerically. Results: At 100V collection voltage and moderate pulse doses the established theories accurately predict the observed collection efficiency, but at extreme pulse doses a fraction of free electrons needs to be considered. At 300V the observed collection efficiency deviates distinctly from that predicted by any of the established theories, even at low pulse doses. However, the numeric solution of the equation system is able to reproduce the measured collection efficiency across the entire dose range of both voltages with a single set of parameters. Conclusion: At high electric fields (3000V/cm here) the existing theoretical descriptions of collection efficiency, including the TVA, are inadequate to predict pulse dose dependency. Even at low pulse doses they might underestimate collection efficiency. The presented, more accurate numeric solution, which considers additional effects like electric shielding by the charges, might provide a valuable tool for future investigations. This project was funded by the German ministry of research and education (BMBF) under grant number: 03Z1N511 and by the state of Saxony under grant number: B 209.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

SU-F-T-552: A One-Year Evaluation of the QABeamChecker+ for Use with the CyberKnife System

Purpose: By attaching an adapter plate with fiducial markers to the QA BeamChecker+ (Standard Imaging, Inc., Middleton, WI), the output of the CyberKnife can be accurately, efficiently, and consistently evaluated. The adapter plate, known as the Cutting Board, allows for automated alignment of the QABC+ using the CK’s stereoscopic kV image-based treatment localization system (TLS). Described herein is an evaluation of the system following a year of clinical utilization. Methods: Based on a CT scan of the QABC+ and CB, a treatment plan is generated which delivers a beam to each of the 5 plane-parallel ionization chambers. Following absolute calibration of the CK, the QA plan is delivered, and baseline measurements are acquired (and automatically corrected for temperature and pressure). This test was performed at the beginning of each treatment day for a year. A calibration evaluation (using a water-equivalent slab and short thimble chamber) is performed every four weeks, or whenever the QABC+ detects a deviation of more than 1.0%. Results: During baseline evaluation, repeat measurements (n=10) were performed, with an average output of 0.25% with an SD of 0.11%. As a test of the reposition of the QABC+ and CB, ten additional measurements were performed where between each acquisition, the entire system was removed and re-positioned using the TLS. The average output deviation was 0.30% with a SD of 0.13%. During the course of the year, 187 QABC+ measurements and 13 slab-based measurements were performed. The output measurements of the QABC+ correlated well with slab-based measurements (R2=0.909). Conclusion: By using the QABC+ and CB, daily output was evaluated accurately, efficiently, and consistently. From setup to break-down (including analysis), this test required 5 minutes instead of approximately 15 using traditional techniques (collimator-mounted ionization chambers). Additionally, by automatically saving resultant output deviation to a database, trend analysis was simplified. Spectrum Medical Physics, LLC of Greenville, SC has a consulting contract with Standard Imaging of Middleton, WI.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

TH-CD-201-03: A Real-Time Method to Simultaneously Measure Linear Energy Transfer and Dose for Proton Therapy Using Organic Scintillators

Purpose: The light generated in organic scintillators depends on both the radiation dose and the linear energy transfer (LET). The LET dependence leads to an under-response of the detector in the Bragg peak of proton beams. This phenomenon, called ionization quenching, must be corrected to obtain accurate dose measurements of proton beams. This work exploits the ionization quenching phenomenon to provide a method of measuring LET and auto correcting quenching. Methods: We exposed simultaneously four different organic scintillators (BCF-12, PMMA, PVT, and LSD; 1mm in diameter) and a plane parallel ionization chamber in passively scattered proton beams to doses between 32 and 43 cGy and fluence averaged LET values from 0.47 to 1.26 keV/µm. The LET values for each irradiation condition were determined using a validated Monte Carlo model of the beam line. We determined the quenching parameter in the Birk’s equation for scintillation in BCF-12 for dose measurements. One set of irradiation conditions was used to correlate the scintillation response ratio to the LET values and plot a scintillation response ratio versus LET calibration curve. Irradiation conditions independent from the calibration ones were used to validate this method. Comparisons to the expected values were made on both the basis of dose and LET. Results: Among all the scintillators investigated, the ratio of PMMA to BCF-12 provided the best correlation to LET values and was used as the LET calibration curve. The expected LET values in the validation set were within 2%±6%, which resulted in dose accuracy of 1.5%±5.8% for the range of LET values investigated in this work. Conclusion: We have demonstrated the feasibility of using the ratio between the light output of two organic scintillators to simultaneously measure LET and dose of therapeutic proton beams. Further studies are needed to verify the response in higher LET values.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Experimental and theoretical investigation of the rocking curves measured for MoK{sub α} X-ray characteristic lines in the double-crystal nondispersive scheme

The rocking curves (RCs) for MoK{sub α1} and MoK{sub α2} characteristic X-ray lines have been experimentally and theoretically studied in the nondispersive scheme of an X-ray double-crystal TPC-K diffractometer. The results of measurements and theoretical calculations of double-crystal RCs for characteristic X-rays from tubes with a molybdenum anode and different widths of slits show that a decrease in the slit width leads to an increase in the relative contribution of the MoK{sub α2}-line RC in comparison with the intensity of the tails of the MoK{sub α1}-line RC. It is shown that the second peak of the MoK{sub α2} line becomes increasingly pronounced in the tail of the MoK{sub α1}-line RC with a decrease in the slit width. Two plane-parallel Si plates (input faces (110), diffraction vector h 〈220〉) were used as a monochromator crystal and a sample. The results of measuring double-crystal RCs are in good agreement with theoretical calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Study of the structure of crystalline tellurium on different substrates

The epitaxial growth of tellurium films by chemical vapor deposition is investigated. It is established that, under thermochemical activation conditions, tellurium films grow from vapor phase in the (10 1̄0) plane, parallel to the (001) muscovite substrate and the (0001) sapphire substrate. It is demonstrated that tellurium films on (001) muscovite substrates are more homogeneous and can be detached from the substrate, which makes them interesting for carrying out experiments on epitaxy of CdTe films. Tellurium films on sapphire are more heterogeneous and mosaic; however, their growth is accompanied by the formation of hexagonal tellurium microtubes with a high aspect ratio: a diameter of ~1–10 μm and a length of ~1–10 mm.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗