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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

The brighter-fatter effect in the JWST MIRI Si:As IBC detectors: I. Observations, impact on science, and modeling

Context.The Mid-Infrared Instrument (MIRI) on board theJames WebbSpace Telescope (JWST) uses three Si:As impurity band conduction (IBC) detector arrays. The output voltage level of each MIRI detector pixel is digitally recorded by sampling up the ramp. For uniform or low-contrast illumination, the pixel ramps become nonlinear in a predictable way, but in areas of high contrast, the nonlinearity curve becomes much more complex. The origin of the effect is poorly understood and currently not calibrated out of the data. Aims.We provide observational evidence of the brighter-fatter effect (BFE) in MIRI conventional and high-contrast coronagraphic imaging, low-resolution spectroscopy, and medium-resolution spectroscopy data, and we investigate the physical mechanism that gives rise to the effect on the MIRI detector pixel raw voltage integration ramps. Methods.We used public data from the JWST/MIRI commissioning and Cycle 1 phase. We also developed a numerical electrostatic model of the MIRI detectors using a modified version of the publicPoisson_CCDcode. Results.We find that the physical mechanism behind the BFE manifesting in MIRI data is fundamentally different to that of charge-coupled devices and photodiode arrays such as the Hawaii-XRG near-infrared detectors used by the NIRISS, NIRCam, and NIRSpec instruments on board JWST. Observationally, the BFE makes the JWST MIRI data yield 10–25% larger point sources and spectral line profiles as a function of the relative level of de-biasing of neighboring detector pixels. This broadening impacts the MIRI absolute flux calibration, time-series observations of faint companions, and point spread function modeling and subtraction. We also find that the intra-pixel 2D profile of the shrinking Si:As IBC detector depletion region directly impacts the accuracy of the pixel ramp nonlinearity calibration model.

Astronomy & Astrophysics↗
DOE OSTI2016

An online, energy-resolving beam profile detector for laser-driven proton beams

In this paper, a scintillator-based online beam profile detector for the characterization of laser-driven proton beams is presented. Using a pixelated matrix with varying absorber thicknesses, the proton beam is spatially resolved in two dimensions and simultaneously energy-resolved. A thin plastic scintillator placed behind the absorber and read out by a CCD camera is used as the active detector material. The spatial detector resolution reaches down to ∼4 mm and the detector can resolve proton beam profiles for up to 9 proton threshold energies. With these detector design parameters, the spatial characteristics of the proton distribution and its cut-off energy can be analyzed online and on-shot under vacuum conditions. The paper discusses the detector design, its characterization and calibration at a conventional proton source, as well as the first detector application at a laser-driven proton source.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

High sensitivity hexagonal boron nitride lateral neutron detectors

Hexagonal boron nitride (h-BN) thermal neutron detectors have demonstrated the highest detection efficiency among all solid-state detectors (at 58% for a detection area of 1 mm 2 and 53% for a detection area of 9 mm 2 ). However, scaling up the detector size of vertical h-BN detectors is challenging due to increased dark current, capacitance, and surface recombination with the increasing detection area. Here, we report the demonstration of a 29 mm 2 thermal neutron detector fabricated from a freestanding 10 B enriched h-BN epilayer of 90 μm in thickness with a detection efficiency of 50% by employing a lateral device geometry. The lateral detector geometry takes advantage of the unique layered structure of h-BN which naturally provides higher in-plane carrier mobilities than those in the vertical direction. Moreover, due to the reduced area of metals in contact with the h-BN material, the detrimental effects associated with the surface recombination at the metal contacts and device capacitance were reduced, which resulted in improved charge collection efficiency and signal to noise ratios. Furthermore, this work laid the ground work for scaling up to large size neutron detectors based on h-BN.

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

High-resolution virtual Frisch grid gamma-ray detectors based on as-grown CdZnTeSe with reduced defects

X- and gamma-ray detectors arequickly becoming essential tools for science and technology in various fields. These detectors offer broad applications such as homeland security, nonproliferation, nuclear security, medical imaging, astrophysics, and high energy physics. All these applications necessitate high-resolution detectors operable at room temperature and available at a reasonable cost. CdZnTe (CZT) is the material of choice for this purpose; however, the material still suffers from intrinsic defects such as highly decorated subgrain boundary networks and a high concentration of secondary phases. These defects not only hinder the charge transport but also create a spatial inhomogeneity in the charge transport properties, subsequently causing substantial degradation in detector response particularly for relatively thick (>1 cm) detectors. Some of the material deficiencies suffered by CZT have been addressed by adding selenium into the CZT matrix. Selenium was found to be very effective in producing material that is principally free from a subgrain boundary network with the occasional appearance of subgrain boundaries with reduced secondary phases decorating these boundaries. The resulting quaternary compound CdZnTeSe (CZTS) reflected excellent material quality mitigating some major deficiencies suffered by CZT. Virtual Frisch grid detectors were fabricated from the as-grown CZTS ingots, and they demonstrated high resolution spectroscopic grade. The excellent CZTS material contained very low defects and was found to potentially increase the yield of high-quality detectors as compared to CZT.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

Cosmic Muon Veto for the mini-ICAL detector at IICHEP, Madurai

A 51-kiloton magnetised Iron Calorimeter (ICAL) detector, using Resistive Plate Chambers (RPCs) as active detector elements, aims to study atmospheric neutrinos. A prototype - 1/600 of the weight of ICAL, called mini-ICAL was installed in the INO transit campus at Madurai. A modest proof-of-principle cosmic muon veto detector of about 1 m × 1 m × 0.3 m dimensions was setup a few years ago, using scintillator paddles. The measured cosmic muon veto efficiency of 99.98% and simulation studies of muon-induced background events in the ICAL detector surrounded by an efficient veto detector were promising. This led to the idea of constructing a bigger cosmic muon veto around the mini-ICAL detector. Details of the design and construction of the detector including the electronics, trigger and DAQ systems planned will be briefly presented.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2021

Development of a 83 mKr source for the calibration of the CENNS-10 liquid argon detector

We report on the preparation of and calibration measurements with a 83 mKr source for the CENNS-10 liquid argon detector. 83 mKr atoms generated in the decay of a 83 Rb source were introduced into the detector via injection into the Ar circulation loop. Scintillation light arising from the 9.4 keV and 32.1 keV conversion electrons in the decay of 83 mKr in the detector volume were then observed. This calibration source allows the characterization of the low-energy response of the CENNS-10 detector and is applicable to other low-energy-threshold detectors. The energy resolution of the detector was measured to be 9% at the total 83 mKr decay energy of 41.5 keV. Lastly, we performed an analysis to separately calibrate the detector using the two conversion electrons at 9.4 keV and 32.1 keV.

47 OTHER INSTRUMENTATION↗
DOE OSTI2026

Characterization of Low-energy Ionization Signals in Silicon Detectors for the Nab Experiment

The Nab (Neutron a b) experiment is designed to measure the beta-antineutrino angular correlation in free neutron 𝛽 decay with an ultimate precision goal of 0.1%, providing input for tests of Cabibbo-Kobayashi-Maskawa matrix unitarity. This measurement is performed via detection of electrons and protons in delayed coincidence using custom large-area segmented silicon detectors. We present the characterization of one such detector system to establish the proton energy and timing response, using a dedicated proton accelerator. The detected proton peak was studied for 25, 30, and 35 keV incident protons on a set of detector segments and multiple cooling cycles over a one-year period. Ionization losses were consistent with models of the detector dead layer with thicknesses less than 100 nm. The detected proton peak was stable within the uncertainty from energy calibration (0.25 keV). The rise times of detector pulses from 109Cd and 113Sn conversion electron sources were used to extract the impurity density profile and establish a precise model for the detector timing response. The observed impurity density profile varied from (2±2)×109/cm3 at the center to (26±2)×109/cm3 at the edge. This impurity density profile was then used to characterize systematic effects in proton time-of-flight measurements due to detector pulse-shape effects; the resultant proton timing systematic uncertainties were below 0.3 ns, which is sufficient for the Nab experiment.

Taylor, RJ [North Carolina State University]↗
DOE OSTI2021

Radiation-Induced Dark Counts for Silicon Single-Photon Detectors in Space

Single-photon detectors operating on satellites for use in a quantum communications network can incur large dark-count-rate increases from the natural radiation environment of space. Displacement damage to the material lattice of a detector from the ionizing radiation can result in a permanent dark-count increase in the detector. In this work, we analyze the radiation-induced dark-count rate of a silicon single-photon avalanche diode onboard a satellite at different orbiting altitudes, as well as, the additional radiation from a nuclear-disturbed environment caused by a high-altitude nuclear explosion. For detectors on low Earth orbit satellites, protons are the biggest source of radiation damage and are best mitigated by choosing an orbit that minimizes exposure when passing through the South Atlantic Anomaly and Polar Cusps. Detectors on medium Earth orbit and geostationary orbit satellites, if shielded by more than 10 mm of aluminum, provide the best platform in terms of the least amount of radiation damage to the detectors. In the event of a high-altitude nuclear explosion, the artificial radiation belts produced by the explosion will cause too much damage to silicon single-photon detectors on low Earth orbit satellites and render them unfit for quantum communications in less than a day. Higher orbit satellites will only suffer minor dark-count-rate increases from the artificial radiation belts.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

High-dynamic-range coherent diffractive imaging: ptychography using the mixed-mode pixel array detector

Coherent (X-ray) diffractive imaging (CDI) is an increasingly popular form of X-ray microscopy, mainly due to its potential to produce high-resolution images and the lack of an objective lens between the sample and its corresponding imaging detector. One challenge, however, is that very high dynamic range diffraction data must be collected to produce both quantitative and high-resolution images. In this work, hard X-ray ptychographic coherent diffractive imaging has been performed at the P10 beamline of the PETRA III synchrotron to demonstrate the potential of a very wide dynamic range imaging X-ray detector (the Mixed-Mode Pixel Array Detector, or MM-PAD). The detector is capable of single photon detection, detecting fluxes exceeding 1 × 10 8 8-keV photons pixel -1 s -1 , and framing at 1 kHz. A ptychographic reconstruction was performed using a peak focal intensity on the order of 1 × 10 10 photons µm -2 s -1 within an area of approximately 325 nm × 603 nm. This was done without need of a beam stop and with a very modest attenuation, while `still' images of the empty beam far-field intensity were recorded without any attenuation. The treatment of the detector frames and CDI methodology for reconstruction of non-sensitive detector regions, partially also extending the active detector area, are described.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Results From a Prototype Combination TPC Cherenkov Detector With GEM Readout

A combined time projection chamber-Cherenkov (TPCC) prototype detector has been developed as part of the detector research and development program for a future electron-ion collider (E!C). The prototype was tested at the Fermilab test beam facility (FTBF) to provide a proof of the principle to demonstrate the ability to measure particle tracks and provide particle identification (P!D) information within a common detector volume. The time projection chamber (TPC) portion consists of a 10 × 10 × 10 cm 3 field cage, which delivers charge from tracks to a quadruple gas electron multiplier (GEM) with zigzag-shaped charge collection anodes. The Cherenkov portion consists of a photosensitive quadruple GEM detector with a Cs! photocathode. As tracks pass through the drift volume of the TPC, the generated Cherenkov light is able to escape through sparsely arranged wires making up one side of the field cage, facing the Cs! photocathode. The Cherenkov detector is thus operated in a windowless, proximity focused configuration for high efficiency. Pure CF 4 is used as the working gas for both detector components, mainly due to its transparency into the deep UV, as well as its high N0. Results from the beam test, including the position resolution as well as the particle id capabilities of the detector, are discussed in this paper.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2021

Improved Localization Precision and Angular Resolution of a Cylindrical, Time-Encoded Imaging System From Adaptive Detector Movements

To the first order, the localization precision and angular resolution of a cylindrical, time-encoded imaging (c-TEI) system is governed by the geometry of the system. Improving either measure requires increasing the mask radius or decreasing the detector diameter, both of which are undesirable. Here, we propose an alternative option of repositioning the detector within the mask to increase the detector-to-mask distance in the direction of a source, thereby improving the localization precision and angular resolution in that direction. Since the detector-to-mask distance only increases for a small portion of the field of view (FOV), we propose implementing adaptive imaging where one leverages data collected during the measurement to optimize the system configuration. This article utilizes both simulations and experiments to set upper bounds on the potential gain from adaptive detector movements for one and two sources in the FOV. When only one source is present, adaptive detector movements can improve the localization precision and angular resolution by 20% for a source at 90 cm and by 32% for a far-field source. When two sources are present, adaptive detector movements can improve localization precision and angular resolution by up to 50% for sources that are ~10° apart (90 cm from the system). We experimentally verify these results through maximum likelihood estimation of the source position(s) and image reconstruction of point sources that are close together. As a demonstration of an adaptive imaging algorithm, we image a complex arrangement of special nuclear material at the Zero Power Physics Reactor facility at Idaho National Laboratory.

46 - INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AN↗
DOE OSTI2012

Dosimetric characterization of a synthetic single crystal diamond detector in clinical radiation therapy small photon beams

Purpose: To determine the potentialities of synthetic single crystal diamond Schottky diodes for accurate dose measurements in radiation therapy small photon beams. Methods: The dosimetric properties of a diamond-based detector were assessed by comparison with a reference microionization chamber. The diamond device was operated at zero bias voltage under irradiation with high-energy radiotherapic photon beams. The stability of the detector response and its dose and dose rate dependence were measured. Different square field sizes ranging from 1 Multiplication-Sign 1 cm{sup 2} to 10 Multiplication-Sign 10 cm{sup 2} were used during comparative dose distribution measurements by means of percentage depth dose curves (PDDs), lateral beam profiles, and output factors. The angular and temperature dependence of the diamond detector response were also studied. Results: The detector response shows a deviation from linearity of less than {+-}0.5% in the 0.01-7 Gy range and dose rate dependence below {+-}0.5% in the 1-6 Gy/min range. PDDs and output factors are in good agreement with those measured by the reference ionization chamber within 1%. No angular dependence is observed by rotating the detector along its axis, while {approx}3.5% maximum difference is measured by varying the radiation incidence angle in the polar direction. The temperature dependence was investigated as well and a {+-}0.2% variation of the detector response is found in the 18-40 Degree-Sign C range. Conclusions: The obtained results indicate the investigated synthetic diamond-based detector as a candidate for small field clinical radiation dosimetry in advanced radiation therapy techniques.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Cascaded-systems analyses and the detective quantum efficiency of single-Z x-ray detectors including photoelectric, coherent and incoherent interactions

Purpose: Theoretical models of the detective quantum efficiency (DQE) of x-ray detectors are an important step in new detector development by providing an understanding of performance limitations and benchmarks. Previous cascaded-systems analysis (CSA) models accounted for photoelectric interactions only. This paper describes an extension of the CSA approach to incorporate coherent and incoherent interactions, important for low-Z detectors such as silicon and selenium. Methods: A parallel-cascade approach is used to describe the three types of x-ray interactions. The description of incoherent scatter required developing expressions for signal and noise transfer through an 'energy-labeled reabsorption' process where the parameters describing reabsorption are random functions of the scatter photon energy. The description of coherent scatter requires the use of scatter form factors that may not be accurate for some crystalline detector materials. The model includes the effects of scatter reabsorption and escape, charge collection, secondary quantum sinks, noise aliasing, and additive noise. Model results are validated by Monte Carlo calculations for Si and Se detectors assuming free-atom atomic form factors. Results: The new signal and noise transfer expressions were validated by showing agreement with Monte Carlo results. Coherent and incoherent scatter can degrade the DQE of Si and sometimes Se detectors depending on detector thickness and incident-photon energy. Incoherent scatter can produce a substantial low-frequency drop in the modulation transfer function and DQE. Conclusions: A generally useful CSA model of the DQE is described that is believed valid for any single-Z material up to 10 cycles/mm at both mammographic and radiographic energies within the limitations of Fourier-based linear-systems models and the use of coherent-scatter form factors. The model describes a substantial low-frequency drop in the DQE of Si systems due to incoherent scatter above 20-40 keV.

36 MATERIALS SCIENCE↗
DOE OSTI2014

SU-E-I-40: New Method for Measurement of Task-Specific, High-Resolution Detector System Performance

Purpose: Although generalized linear system analytic metrics such as GMTF and GDQE can evaluate performance of the whole imaging system including detector, scatter and focal-spot, a simplified task-specific measured metric may help to better compare detector systems. Methods: Low quantum-noise images of a neuro-vascular stent with a modified ANSI head phantom were obtained from the average of many exposures taken with the high-resolution Micro-Angiographic Fluoroscope (MAF) and with a Flat Panel Detector (FPD). The square of the Fourier Transform of each averaged image, equivalent to the measured product of the system GMTF and the object function in spatial-frequency space, was then divided by the normalized noise power spectra (NNPS) for each respective system to obtain a task-specific generalized signal-to-noise ratio. A generalized measured relative object detectability (GM-ROD) was obtained by taking the ratio of the integral of the resulting expressions for each detector system to give an overall metric that enables a realistic systems comparison for the given detection task. Results: The GM-ROD provides comparison of relative performance of detector systems from actual measurements of the object function as imaged by those detector systems. This metric includes noise correlations and spatial frequencies relevant to the specific object. Additionally, the integration bounds for the GM-ROD can be selected to emphasis the higher frequency band of each detector if high-resolution image details are to be evaluated. Examples of this new metric are discussed with a comparison of the MAF to the FPD for neuro-vascular interventional imaging. Conclusion: The GM-ROD is a new direct-measured task-specific metric that can provide clinically relevant comparison of the relative performance of imaging systems. Supported by NIH Grant: 2R01EB002873 and an equipment grant from Toshiba Medical Systems Corporation.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Development of a novel depth of interaction PET detector using highly multiplexed G-APD cross-strip encoding

Purpose: The aim of this study was to develop a prototype PET detector module for a combined small animal positron emission tomography and magnetic resonance imaging (PET/MRI) system. The most important factor for small animal imaging applications is the detection sensitivity of the PET camera, which can be optimized by utilizing longer scintillation crystals. At the same time, small animal PET systems must yield a high spatial resolution. The measured object is very close to the PET detector because the bore diameter of a high field animal MR scanner is limited. When used in combination with long scintillation crystals, these small-bore PET systems generate parallax errors that ultimately lead to a decreased spatial resolution. Thus, we developed a depth of interaction (DoI) encoding PET detector module that has a uniform spatial resolution across the whole field of view (FOV), high detection sensitivity, compactness, and insensitivity to magnetic fields. Methods: The approach was based on Geiger mode avalanche photodiode (G-APD) detectors with cross-strip encoding. The number of readout channels was reduced by a factor of 36 for the chosen block elements. Two 12 × 2 G-APD strip arrays (25μm cells) were placed perpendicular on each face of a 12 × 12 lutetium oxyorthosilicate crystal block with a crystal size of 1.55 × 1.55 × 20 mm. The strip arrays were multiplexed into two channels and used to calculate the x, y coordinates for each array and the deposited energy. The DoI was measured in step sizes of 1.8 mm by a collimated {sup 18}F source. The coincident resolved time (CRT) was analyzed at all DoI positions by acquiring the waveform for each event and applying a digital leading edge discriminator. Results: All 144 crystals were well resolved in the crystal flood map. The average full width half maximum (FWHM) energy resolution of the detector was 12.8% ± 1.5% with a FWHM CRT of 1.14 ± 0.02 ns. The average FWHM DoI resolution over 12 crystals was 2.90 ± 0.15 mm. Conclusions: The novel DoI PET detector, which is based on strip G-APD arrays, yielded a DoI resolution of 2.9 mm and excellent timing and energy resolution. Its high multiplexing factor reduces the number of electronic channels. Thus, this cross-strip approach enables low-cost, high-performance PET detectors for dedicated small animal PET and PET/MRI and potentially clinical PET/MRI systems.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

CHERENCUBE: Concept definition and implementation challenges of a Cherenkov-based detector block for PET

Purpose: A new concept for a depth-of-interaction (DOI) capable time-of-flight (TOF) PET detector is defined, based only on the detection of Cherenkov photons. The proposed “CHERENCUBE” consists of a cubic Cherenkov radiator with position-sensitive photodetectors covering each crystal face. By means of the spatial distribution of the detected photons and their time of arrival, the point of interaction of the gamma-ray in the crystal can be determined. This study analyzes through theoretical calculations and Monte Carlo simulations the potential advantages of the concept toward reaching a Cherenkov-only detector for TOF-PET with DOI capability. Furthermore, an algorithm for the DOI estimation is presented and the requirements for a practical implementation of the proposed concept are defined. Methods: The Monte Carlo simulations consisted of a cubic crystal with one photodetector coupled to each one of the faces of the cube. The sensitive area of the detector matched exactly the crystal size, which was varied in 1 mm steps between 1 × 1 × 1 mm{sup 3} and 10 × 10 × 10 mm{sup 3}. For each size, five independent simulations of ten thousand 511 keV gamma-rays were triggered at a fixed distance of 10 mm. The crystal chosen was PbWO{sub 4}. Its scintillation properties were simulated, but only Cherenkov photons were analyzed. Photodetectors were simulated having perfect photodetection efficiency and infinite time resolution. For every generated particle, the analysis considered its creation process, parent and daughter particles, energy, origin coordinates, trajectory, and time and position of detection. The DOI determination is based on the distribution of the emission time of all photons per event. These values are calculated as a function of the coordinates of detection and origin for every photon. The common origin is estimated by finding the distribution with the most similar emission time-points. Results: Detection efficiency increases with crystal size from 8.2% (1 × 1 × 1 mm{sup 3}) to 58.6% (10 × 10 × 10 mm{sup 3}) and decreases applying a photon detection threshold of 5/10/20 photons to 6.3%/4.3%/0.7% and 49.3%/30.4%/2.8%, respectively. The detection rate in the six photodetectors is uniform due to the nearly isotropic cone emission. Most cones originated after a photoelectric effect interaction, with two dominating peaks for the kinetic energy of the electron at 422.99 and 441.47 keV. The detection distance between same-event photons defines the spatial resolution of the detector required for individual photon recognition, with 20% of the detected photons having their closest neighbor within a distance of 5% of the length of the cube. Same-event photons are detected within a time window whose width is determined by the crystal size, with values of 30 and 150 ps for a 1 × 1 × 1 mm{sup 3} and a 10 × 10 × 10 mm{sup 3} cube, respectively. The DOI reconstruction has an accuracy of approximately 23% of the length of the cube, with an average value of 2.2 mm for a 10 × 10 × 10 mm{sup 3} CHERENCUBE. Conclusions: The proposed concept requires a detector with high photodetection efficiency. The structure of the sensitive surface of the detector should be a two dimensional array of microcells, able to provide individual detection coordinates and time stamps. The microcell size determines the ability to recognize individual photons, influencing detection efficiency. The 3D DOI recognition relies on the accuracy of the time stamps and detection coordinates, without the need for a recognition of the projected patterns of photons. The refractive index of the material defines a detector intrinsic energy-based rejection of scattered PET events at the cost of reduced sensitivity.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Characterization of a synthetic single crystal diamond detector for dosimetry in spatially fractionated synchrotron x-ray fields

Purpose: Modern radiotherapy modalities often use small or nonstandard fields to ensure highly localized and precise dose delivery, challenging conventional clinical dosimetry protocols. The emergence of preclinical spatially fractionated synchrotron radiotherapies with high dose-rate, sub-millimetric parallel kilovoltage x-ray beams, has pushed clinical dosimetry to its limit. A commercially available synthetic single crystal diamond detector designed for small field dosimetry has been characterized to assess its potential as a dosimeter for synchrotron microbeam and minibeam radiotherapy. Methods: Experiments were carried out using a synthetic diamond detector on the imaging and medical beamline (IMBL) at the Australian Synchrotron. The energy dependence of the detector was characterized by cross-referencing with a calibrated ionization chamber in monoenergetic beams in the energy range 30–120 keV. The dose-rate dependence was measured in the range 1–700 Gy/s. Dosimetric quantities were measured in filtered white beams, with a weighted mean energy of 95 keV, in broadbeam and spatially fractionated geometries, and compared to reference dosimeters. Results: The detector exhibits an energy dependence; however, beam quality correction factors (k{sub Q}) have been measured for energies in the range 30–120 keV. The k{sub Q} factor for the weighted mean energy of the IMBL radiotherapy spectrum, 95 keV, is 1.05 ± 0.09. The detector response is independent of dose-rate in the range 1–700 Gy/s. The percentage depth dose curves measured by the diamond detector were compared to ionization chambers and agreed to within 2%. Profile measurements of microbeam and minibeam arrays were performed. The beams are well resolved and the full width at halfmaximum agrees with the nominal width of the beams. The peak to valley dose ratio (PVDR) calculated from the profiles at various depths in water agrees within experimental error with PVDR calculations from Gafchromic film data. Conclusions: The synthetic diamond detector is now well characterized and will be used to develop an experimental dosimetry protocol for spatially fractionated synchrotron radiotherapy.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

SU-G-TeP2-08: Evaluation of Plastic Scintillator Detector for Small Field Stereotactic Patient-Specific Quality Assurance

Purpose: To evaluate the performance of a commercial plastic scintillator detector (PSD) for small-field stereotactic patient-specific quality assurance using flattening-filter-free (FFF) beams. Methods: A total of ten spherical targets (volume range:[0.03cc–2cc]) were planned using Dynamic Conformal Arc(DCA-10 plans) and Volumetric Modulated Arc Therapy(VMAT-10 plans) techniques in Eclipse(AAA v.11, 1mm dose calculation grid size). Additionally, 15 previously-treated cranial and spine SRS plans were evaluated (6 DCA, 9 VMAT, volume range:[0.04cc–119.02cc]). All measurements were acquired using Varian Edge equipped with HDMLC. Three detectors were used: PinPoint ion chamber (PTW;active volume 0.015cc), Exradin W1 PSD (Standard Imaging;active volume 0.002cc), and Gafchromic EBT3 film (Ashland). PinPoint and PSD were positioned perpendicular to beam axis in a Lucy phantom (Standard Imaging). Films were placed at isocenter in solid water. Calibration films were delivered for absolute dose analysis. Results: For large spherical targets(>1.5cc) with DCA, all detectors agreed within 1% of AAA calculations. As target volume decreased, PSD measured higher doses than AAA (maximum difference: 3.3% at 0.03cc target), while PinPoint chamber measured lower doses (maximum difference:-3.8% at 0.03cc target). Inter-detector differences between pinpoint and PSD increased with decreasing target size; differences>5% were observed for targets<0.09cc. Similar trends for inter-detector behavior were observed for clinical plans. For target sizes<0.08cc, PSD measured>5% higher dose than PinPoint chamber (maximum difference: 9.25% at 0.04cc target). Film demonstrated agreement of −0.19±1.47% with PSD for all spherical targets, and agreement within −0.98±2.25% for all 15 clinical targets. Unlike DCA, VMAT plans did not show improved AAA-to-detector agreements for large targets. Conclusion: For all targets, the PSD measurements agreed with film within 1.0%, on average. For small volume targets (<0.10cc), PSD agreed with film but measured significantly higher doses (>5%) compared with the pin point ion chamber. The plastic scintillator detector appears to be suitable for accurate measurements of small SRS targets.

60 APPLIED LIFE SCIENCES↗