High Dose Rate Environment Research Articles

Development of contamination detection system combined with various remote devices

The detection of alpha and beta contamination locations is important for decontaminating nuclear facilities. In the high radiation dose rate environment at the decommissioning sites, the contamination measurement by the workers is not effective. Thus, we developed a remote automatic contamination measurement system using a new scintillator-based detector. A 50 mmφ × 100-μm-thick YAlO3(Ce)(YAP:Ce) scintillator was coupled with a flat panel-type multianode photomultiplier tube. The detector was installed downwards at the bottom of a robot. It has an energy measurement capability, and the energy measurement could discriminate the alpha particles from the beta and 222Rn alpha particles. With the energy information, alpha and beta particles could be identified and mapped simultaneously. In addition, a slow-moving robot could be used to obtain statistically sufficient counts in a single run measurement, allowing the evaluation of surface contamination density using only alpha particles. The remote automatic contamination measurement system will be useful in visualizing the contamination distribution in environments that are inaccessible to workers.

Hydrogenated amorphous silicon high flux x-ray detectors for synchrotron microbeam radiation therapy

Objective. Microbeam radiation therapy (MRT) is an alternative emerging radiotherapy treatment modality which has demonstrated effective radioresistant tumour control while sparing surrounding healthy tissue in preclinical trials. This apparent selectivity is achieved through MRT combining ultra-high dose rates with micron-scale spatial fractionation of the delivered x-ray treatment field. Quality assurance dosimetry for MRT must therefore overcome a significant challenge, as detectors require both a high dynamic range and a high spatial resolution to perform accurately. Approach. In this work, a series of radiation hard a-Si:H diodes, with different thicknesses and carrier selective contact configurations, have been characterised for x-ray dosimetry and real-time beam monitoring applications in extremely high flux beamlines utilised for MRT at the Australian Synchrotron. Results. These devices displayed superior radiation hardness under constant high dose-rate irradiations on the order of 6000 Gy s−1, with a variation in response of 10% over a delivered dose range of approximately 600 kGy. Dose linearity of each detector to x-rays with a peak energy of 117 keV is reported, with sensitivities ranging from (2.74 ± 0.02) nC/Gy to (4.96 ± 0.02) nC/Gy. For detectors with 0.8 μm thick active a-Si:H layer, their operation in an edge-on orientation allows for the reconstruction of micron-size beam profiles (microbeams). The microbeams, with a nominal full-width-half-max of 50 μm and a peak-to-peak separation of 400 μm, were reconstructed with extreme accuracy. The full-width-half-max was observed as 55 ± 1 μm. Evaluation of the peak-to-valley dose ratio and dose-rate dependence of the devices, as well as an x-ray induced charge (XBIC) map of a single pixel is also reported. Significance. These devices based on novel a-Si:H technology possess a unique combination of accurate dosimetric performance and radiation resistance, making them an ideal candidate for x-ray dosimetry in high dose-rate environments such as FLASH and MRT.

Dosimetry for FLASH Radiotherapy: A Review of Tools and the Role of Radioluminescence and Cherenkov Emission

While spatial dose conformity delivered to a target volume has been pushed to its practical limits with advanced treatment planning and delivery, investigations in novel temporal dose delivery are unfolding new mechanisms. Recent advances in ultra-high dose radiotherapy, abbreviated as FLASH, indicate the potential for reduction in healthy tissue damage while preserving tumor control. FLASH therapy relies on very high dose rate of > 40Gy/sec with sub-second temporal beam modulation, taking a seemingly opposite direction from the conventional paradigm of fractionated therapy. FLASH brings unique challenges to dosimetry, beam control, and verification, as well as complexity of radiobiological effective dose through altered tissue response. In this review, we compare the dosimetric methods capable of operating under high dose rate environments. Due to excellent dose-rate independence, superior spatial (~<1 mm) and temporal (~ns) resolution achievable with Cherenkov and scintillation-based detectors, we show that luminescent detectors have a key role to play in the development of FLASH-RT, as the field rapidly progresses towards clinical adaptation. Additionally, we show that the unique ability of certain luminescence-based methods to provide tumor oxygenation maps in real-time with submillimeter resolution can elucidate the radiobiological mechanisms behind the FLASH effect. In particular, such techniques will be crucial for understanding the role of oxygen in mediating the FLASH effect.

Background correction method for portable thyroid dose monitor using gamma-ray spectrometer developed at JAEA in high dose rate environment

A portable thyroid dose monitoring system has been developed at the Japan Atomic Energy Agency (JAEA) to assess the thyroid equivalent dose for workers and members of the public in a high dose rate environment. The background (B.G.) photon correction is required for an accurate measurement in a high dose rate environment at an early stage after a nuclear accident. The responses of the system to the B.G. photons were calculated with the simplified cylindrical phantoms made of PMMA or the voxel phantoms using the PHITS code. Then they were compared each other among the respective age groups. It was found that the cylindrical phantom with an appropriate size and properly arrangement can mimic the human body in the high dose rate caused by 131I sources. As such we successfully developed the B.G. photon correction method using the cylindrical phantoms; one of those is used as an effective way to calibrate the monitoring system by inserting a133Ba point source into it. As an additional phantom is not required, this method has a great advantage in the practical operation of the B.G. photon correction.

Application of InGaP space solar cells for a radiation dosimetry at high dose rates environment of Fukushima Daiichi nuclear power plant

ABSTRACT Decommissioning the Fukushima Daiichi nuclear power plant (1F) after the accident caused by a tsunami in 2011 requires characterization of the fuel debris by dose distribution measurement. This paper describes the experimental and theoretical behavior of a radiation detector applied with InGaP solar cells is investigated and allow the localization and characterization of the fuel debris. In the irradiation test, it was observed that the radiation-induced current output of the InGaP solar cells increases linearly with increasing dose rates of 60Co γ-rays. For measurements at low dose rates, it becomes clear that the minimum detectable dose rate and resolution can be determined by analyzing the noise characterization. The maximum detectable level of radiation dosimetry for the InGaP solar cell was found to be higher than the highest γ-ray dose rate observable at the reactor core for 1F plants. Additionally, as an analysis of the radiation-induced current, it is attempted to express a relational expression between the absorbed dose rate and the creation of radiation-induced current pairs in the solar cells. The experimental and simulation results suggest that solar cells can be powerful tools for radiation dosimetry in high dose rate environments near the debris of the 1F plant.

Development of a new pinhole camera for imaging in high dose-rate environments

After Fukushima’s nuclear disaster in Japan, the decontamination operation is successfully ongoing, and restrictions from some areas were lifted in April 2017. However, the radiation dose rate in the Fukushima Daiichi Plant is still so high (e.g., from a few mSv/h up to 530 Sv/h) that the decommissioning operation of the reactor remains a serious problem. Visualization of radioactive materials would help address this, but no gamma camera is available at this moment that can take images in such a high dose-rate environment. In this study, we developed a new gamma camera featuring a wide dynamic range from sub-mSv/h to more than 680 Sv/h, for a quick and accurate visualization of radioactive materials. The camera consists of a pinhole collimator, a Gd2O2S:Tb (GOS) scintillator sheet, and an electron multiplying charge-coupled device (EM-CCD). Gamma rays passing through the pinhole collimator hit the GOS scintillator sheet, which emits scintillation light. The luminescence of the GOS scintillator sheet is monitored in real-time with an EM-CCD with an internal multiplication gain of up to 20. By changing the exposure time, electron multiplication gain, and aperture of the EM-CCD, a wider dynamic range covering five orders of magnitude in the radiation dose can be monitored for the first time. We show that the positions of a 137Cs source (662 keV) and 60Co source (1173, 1333 keV) are identified correctly with a typical angular resolution of 10° full width at half maximum (FWHM). We also confirmed a linear relation between the absorbed dose and the luminescence of the GOS scintillator sheet. Finally, we propose a new concept of ”color imaging”, by using multi-layered scintillators consisting of a Ga3Al2Gd3O12 (GAGG) scintillator, fluorescent glass, and a plastic scintillator.

21pCA-12 シンチレータを用いた高線量場測定用コンプトンカメラの開発

21pCA-12 シンチレータを用いた高線量場測定用コンプトンカメラの開発

Radiation Imaging System using a Compact Gamma-ray Imager mounted on a Remotely Operated Machine

The development of remote and quick radiation imaging methods for high dose-rate environments is important to accelerate decommissioning of the Fukushima Daiichi Nuclear Power Station (FDNPS). We have developed a remote radiation imaging system consisting of a Compton camera and a multicopter-type drone to remotely measure the distribution of radioactive contaminations inside FDNPS. In outdoor performance evaluation tests in the coastal areas of Fukushima, Japan, the drone system succeeded in observing several hotspots from the sky. In addition, we are developing technology for simultaneous localization and mapping of remotely operated machines such as drones to complete the remote radiation imaging system, which can fly autonomously.

Limits of Inorganic Scintillating Materials to Operate in a High Dose Rate Environment at Future Collider Experiments

This report gives a review of the radiation damages induced by gammas and energetic hadrons in inorganic scintillating materials. We discuss the creation and recovery of color centers in scintillating materials under gamma and high energy proton irradiation, the two main causes of irradiation in High Luminosity LHC. The induced phosphorescence and the production of radioisotopes in heavy and light inorganic scintillators and in surrounding construction materials, as well as their influence on the possible design of the detector working in a high dose rate environment, are also described.

Spectroscopic gamma camera for use in high dose environments

We developed a pinhole gamma camera to measure distributions of radioactive material contaminants and to identify radionuclides in extraordinarily high dose regions (1000mSv/h). The developed gamma camera is characterized by: (1) tolerance for high dose rate environments; (2) high spatial and spectral resolution for identifying unknown contaminating sources; and (3) good usability for being carried on a robot and remotely controlled. These are achieved by using a compact pixelated detector module with CdTe semiconductors, efficient shielding, and a fine resolution pinhole collimator. The gamma camera weighs less than 100kg, and its field of view is an 8m square in the case of a distance of 10m and its image is divided into 256 (16×16) pixels. From the laboratory test, we found the energy resolution at the 662keV photopeak was 2.3% FWHM, which is enough to identify the radionuclides. We found that the count rate per background dose rate was 220cpsh/mSv and the maximum count rate was 300kcps, so the maximum dose rate of the environment where the gamma camera can be operated was calculated as 1400mSv/h. We investigated the reactor building of Unit 1 at the Fukushima Dai-ichi Nuclear Power Plant using the gamma camera and could identify the unknown contaminating source in the dose rate environment that was as high as 659mSv/h.

SU‐F‐BRE‐11: Neutron Measurements Around the Varian TrueBeam Linac

Purpose:With the emergence of flattening filter free (FFF) photon beams, several authors have noted many advantages to their use. One such advantage is the decrease in neutron production by photonuclear reactions in the linac head. In the present work we investigate the reduction in neutrons from a Varian TrueBeam linac using the Nested Neutron Spectrometer (NNS, Detec). The neutron spectrum, total fluence and source strength were measured and compared for 10 MV with and without flattening filter and the effect of moderation by the room and maze was studied for the 15 MV beam.Methods:The NNS, similar to traditional Bonner sphere detectors but operated in current mode, was used to measure the neutron fluence and spectrum. The NNS was validated for use in high dose rate environments using Monte Carlo simulations and calibrated at NIST and NRC Canada. Measurements were performed at several positions within the treatment room and maze with the linac jaws closed to maximize neutron production.Results:The measurements showed a total fluence reduction between 35‐40% in the room and maze when the flattening filter was removed. The neutron source strength Qn was calculated from in‐room fluence measurements and was found to be 0.042 × 102 n/Gy, 0.026 × 102 n/Gy and 0.59 × 1012 n/Gy for the 10 MV, the 10 MV FFF and 15 MV beams, respectively. We measured ambient equivalent doses of 11 mSv/hr, 7 mSv/hr and 218 mSv/hr for the 10 MV, 10 MV FFF and 15 MV by the head.Conclusion:Our measurements revealed a decrease in total fluence, neutron source strength and equivalent dose of approximately 35‐40% across the treatment room for the FFF compared to FF modes. This demonstrates, as expected, that the flattening filter is a major component of the neutron production for the TrueBeam.The authors greatly acknowledge support form the Canadian Nuclear Commission and the Natural Sciences and Engineering Research Council of Canada through the CREATE program. Co‐authors Dubeau and Witharana are employees of Detec (Gatineau, Quebec), manufacturer of the Nested Neutron Spectrometer.

Simulations and Measurements of Secondary Electron Emission Beam Loss Monitors for LHC

Secondary Emission Monitor (SEM) is a part of the LHC Beam Loss Monitoring system, which is providing the number of particles lost from the primary hadron beam by measuring the radiation field induced by their interaction with matter surrounding the beam pipe. SEM detectors will be used in the high dose rate environments of LHC because of their low sensitivity and excellent linearity. The design of SEM consists of two bias electrodes separating the produced secondary electrons from the Ti signal electrode placed in a vacuum steel cylinder. The response of the chamber was tested with bunched and continuous proton beams and compared with a reference detector and GEANT4 simulations.

Total Ionizing Dose Hardness Assurance Issues for High Dose Rate Environments

Transistors and ICs were irradiated at dose rates from 0.2 to 2times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> rad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s using a wide range of radiation sources. The mechanisms causing parametric IC failure varied with dose rate. At low dose rates from 0.2 to 100 rad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s, parametric IC failure in these devices was dominated by radiation-induced degradation of the gate oxide transistors. At dose rates from 1.8times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> rad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s, parametric IC degradation was dominated by large increases in radiation-induced parasitic field oxide leakage current. At very high dose rates of 2times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> rad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s, no parametric failure was observed due to debiasing effects caused by rail-span collapse. These differences in dose rate response can make hardness assurance testing for high dose rate environments very challenging. Simple ldquoovertestsrdquo at dose rates from 50 to 300 rad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s may greatly underestimate the radiation hardness of ICs in high dose rate environments. Because the failure mechanism may vary with dose rate, circuit design, and/or device technology, the best procedure for ensuring IC radiation hardness in greater than 300 rad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/s environments is to use radiation sources that mimic the system environment.

14-MeV Neutron, $\gamma$-Ray, and Pulsed X-Ray Radiation-Induced Effects on Multimode Silica-Based Optical Fibers

In this paper, we investigate the mechanisms of absorbing defect generation by pulsed X-rays (~1 MeV), gamma-rays (~1.2 MeV) and 14-MeV neutrons in silica-based optical fibers. We measure the spectral and temporal dependence of the radiation-induced attenuation (RIA) from the ultraviolet to the near-infrared part of the spectrum (300-900 nm). The choice of the dopants incorporated in the fiber core and cladding strongly affects the fiber radiation tolerance. The structure and generation mechanisms of the point defects responsible for the fiber degradation are discussed on the basis of Gaussian decomposition of the RIA spectra with previously characterized absorption bands. In this spectral range, the absorption bands at 2.2, 2.5, and 3.1 eV (570, 510, and 400 nm) of phosphorus(P)-oxygen hole centers (POHC) explain the increase of losses in P-codoped fibers. For P-free fibers, our measurements showed that the RIA can be attributed to different germanium-related defects with absorption bands centered at 4.41, 2.61, and 1.97 eV (280, 475, and 630 nm) respectively, named Ge(1), GeX, and Ge-NBOHC centers. In the case of pulsed X-rays, another transient defect has been evidenced with an absorption band at 3.0 eV (400 nm, FWHM=0.60 eV) that has been associated in the literature with the [GeO4]- structure. Gamma-rays and 14-MeV neutrons globally lead to the same defect generation mechanisms and the same class of defects contributes to the induced losses. For a high dose rate environment and high-speed transmission measurements, very unstable defects, like self-trapped charges, have to be considered for the evaluation of the transient vulnerability of optical fibers

On the behaviour of plastic scintillators during an experiment in a high dose rate environment

One generally assumes that the radiation damage observed in plastic scintillators which are exposed to a low dose rate radiation ( D ̇ <10 Gy/h) in air is identical with the so-called permanent damage remaining after the end of the annealing process. This assumption is tested in the present experiment. The transmission damage was measured for two scintillating fibers (BCF-12 and SCSF81(Y7)) during and after irradiation in air. In both cases shortlived absorption centers have been observed which decay within hours. They absorb mainly light with wavelengths λ=500–700 nm. In this range of wavelengths and for the investigated doses ( D ∼ < 3 kGy), the radiation induced absorption is considerably higher than the permanent damage. For radiation hardness tests of plastic scintillators especially those emitting green fluorescence light we propose to measure not only the permanent damage but also to study the behaviour during irradiation at realistic dose rates expected in future experiments.

A model for the bipolar-like response of GaAs MESFETs to a high dose rate environment

A model for bipolar mechanism, initiated in some MESFETs during a radiation transient, is presented. It differs from other models in its postulation that the 'neutral base' is formed during the transient and is not present during normal device operation. The response mechanism is due to the geometry of the device and does not depend on its material properties.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dose Rate Tolerant HEXFET Power Supply

A summary of flash x-ray testing of power HEXFET transistors (IRF 351's) and a power supply employing HEXFETs is presented. HEXFETs are shown to exhibit a dose rate damage sensitivity which is triggered by less than one hundreth of the normal single pulsed energy rating. The power supply testing demonstrates that HEXFETs can be used in high power applications in a high dose rate environment if a current limiting configuration is employed.

Dose-Rate Effects in the Permanent Threshold Voltage Shifts of MOS Transistors

Data has been collected that shows the permanent threshold voltage shift occurring in MOS transistors exposed to the same total dose of gamma radiation can be greater in a high dose-rate environment than in a low dose-rate environment. This dose-rate effect is ascribed to a "Photovoltaic" bias generation in the substrate of a device which results in an effective gate bias change (positive for P-channel and negative for N-channel transistors). The bias change ranges from 0 to ±1 volt during the radiation burst. Thus, in a high dose-rate ionizing environment, the permanent gate threshold voltage shift of an MOS device, which is known to be a function of the gate bias during irradiation, will exhibit an indirect dose-rate dependence which is caused by an internal change in instantaneous gate bias.