High Radiation Fields Research Articles

Study of polymer gel for dose imaging in radiotherapy

Gel dosimeters in the form of layers are a promising tool for in-phantom absorbed dose imaging in radiotherapy, either with high gradient X-ray or electron radiation fields, as in conformal radiotherapies, or with neutron beams, as in boron neutron capture therapy (BNCT). A normoxic polymer gel has been studied, and the results are compared with those obtained with Fricke gel dosimeters exposed to the same fields and in the same phantom configuration. Although promising, polymer gel layers need further improvements in order to obtain higher reliability.

Pseudo-Capacitor Structure for Direct Nuclear Energy Conversion

AbstractThe advanced space missions need for more power opened the way for advanced nuclear reactors and for alternative power conversion procedures. The most advanced power systems available in space are the fuel cells and nuclear reactors. Both systems manifest low efficiencies for converting the primary energy into electricity and as consequence are requiring high heat dump into space mainly by infrared radiation. The thermo-nuclear power generator also requires a high temperature gas turbine and a mechano-electric generator, finally driving to low conversion efficiencies. The new nano-materials offer the possibility of creating direct energy conversion devices able of achieving high conversion efficiencies up to 99% in the cryogenic versions. The interest for direct conversion of the nuclear energy into electricity appeared in early 1940th, by the invention of the thermo-ionic fission device by Linder. Then a series of patents and scientific papers improved gradually the designs and performances of the devices, up to the actual concepts of beta-voltaic and liquid-electronics. The most intuitive direct conversion device looks mainly like a super mirror- or a heterogeneous super-capacitor. The issues on its operation are related to global conversion efficiencies and the stable operation life-time in high radiation field. There are combinations of nano-structures and actinides assuring both the neutron flux stability, by meeting criticality conditions and the direct conversion or the nuclear energy into electricity. Achieving a high efficiency internal conversion of the nuclear energy into electricity is not enough if it is not completed by a high efficiency power extraction system from the nuclear reactor core into the outside load. The development of the new MEMS devices and micro electronics in the 40 nm technologies provides an excellent background for the production of the electric power harvesting and conversion devices embedded in the fuel. The new nano-structured materials may be produced as radiation energy harvesting tiles that are free of actinides, using them for harvesting the energy of radioactive sources and controlled fusion devices, or may include actinides in their structure achieving critical or sub-critical accelerator driven nuclear reactor assemblies. Another predictable advantage of the nano-structure is the property of self-repairing and self-organizing to compensate the radiation damage and improve the lifetime. Due to direct conversion the power density of the new materials may increase from the actual average of 0.2 kw/cm3 to about 1 kw/mm3 driving to miniaturization of nuclear power sources and reductions of the shield weight. At these dimensions and power densities of few thousands horse power per liter the nuclear power source becomes suitable for mobile applications as powering trains, strategic airplanes, etc. These new developments may drive to the production of high power solid-state compact nuclear battery for space applications, leading to a new development stage.

Dynamical behavior of electron transport in AlGaAs/GaAs double-barrier structures under a high-frequency radiation field

We investigate the time-dependent dynamical behavior of electron transport in AlGaAs/GaAs double-barrier structures under a high-frequency radiation field. The effects of the radiation field with different amplitude and frequency on the real-time and mean current-voltage curves are taken into account. We find that the amplitude and frequency of the radiation field affect the final stable state current-voltage (I-V) behaviors, which leads to the switching between different current states at a smaller bias than that of the absence of the radiation field, and both current hysteresis and resonant peaks are suppressed by the external radiation field. The high radiation field strength can make the resonant peak of current split and the hysteresis of current disappear. This effect provides the potential to use double-barrier structure as a THz photoelectric switch.

MOSFET detectors in quality assurance of tomotherapy treatments

Background and purpose The purpose of this work was to characterize metal oxide semiconductor field-effect transistors (MOSFETs) in a 6 MV conventional linac and investigate their use for quality assurance of radiotherapy treatments with a tomotherapy Hi-Art unit. Materials and methods High sensitivity and standard sensitivity MOSFETs were first calibrated and then tested for reproducibility, field size dependence, and accuracy of measuring surface dose in a 6 MV beam as well as in a tomotherapy Hi-Art unit. In vivo measurements were performed on both a RANDO phantom and several head and neck cancer patients treated with tomotherapy and compared to TLD measurements and treatment plan doses to evaluate the performance of MOSFETs in a high gradient radiation field. Results The average calibration factor found was 0.345 ± 2.5% cGy/mV for the high sensitivity MOSFETs tested and 0.901 ± 2.4% cGy/mV for the standard sensitivity MOSFETs. MOSFET measured surface doses had an average agreement with ion chamber measurements of 1.55% for the high sensitivity MOSFET and 5.23% for the standard sensitivity MOSFET when averaged over all trials and field sizes tested. No significant dependence on field size was found for the standard sensitivity MOSFETs, however a maximum difference of 5.34% was found for the high sensitivity MOSFET calibration factors in the field sizes tested. Measurements made with MOSFETS on head and neck patients treated on a tomotherapy Hi-Art unit had an average agreement of (3.26 ± 0.03)% with TLD measurements, however the average of the absolute difference between the MOSFET measurements and the treatment plan skin doses was (12.2 ± 7.5)%. The MOSFET measured patient skin doses also had good reproducibility, with inter-fraction deviations ranging from 1.4% to 6.6%. Similar results were found from trials using a RANDO phantom. Conclusions The MOSFETs performed well when used in the tomotherapy Hi-Art unit and did not increase the overall treatment set-up time when used for patient measurements. It was found that MOSFETs are suitable detectors for surface dose measurements in both conventional beam and tomotherapy treatments and they can provide valuable skin dose information in areas where the treatment planning system may not be accurate.

Development of One-dimensional Fiber-Optic Radiation Sensor for Measuring Dose Distributions of High Energy Photon Beams

The purpose of this study was to develop a method for measuring the one-dimensional dose distribution of a high-energy photon beam using a miniaturized high-resolution fiber-optic radiation sensor array. The measurements were made by thin plastic optical fibers with organic scintillating fiber sensor probes that emit the visible wavelength of light. The scintillating light is guided to a silicon photodiode array by plastic optical fibers in order to convert light output to an electrical signal. The one-dimensional spatial dependence of photon beam is measured by a one-dimensional fiberoptic sensor array in a poly(methyl methacrylate) (PMMA) phantom. It is shown that this fiber-optic radiation sensor has better spatial resolution than a conventional ionization chamber and much less time is required to measure one-dimensional dose distribution in the high radiation fields. The real-time and the high spatial resolution measurements due to the small detector volume make this system suitable for dosimetry in radiation therapy.

Real-Time Measurement of Material Elastic Properties in a High Gamma Irradiation Environment

This paper describes the first noncontact elastic vibration measurements of an object in a high gamma radiation field. Using a laser-coupled resonant ultrasound technique, the vibration modes of an Inconel hollow capped cylinder were measured as the gamma radiation field was increased to 104 Gy/h. This measurement technique allowed shifts in the resonant frequency of the sample’s vibration modes to be tracked over a 170-h period. The vibration mode frequencies changed in a manner consistent with the temperature dependence of the elastic stiffness coefficients of the material. These results demonstrate the efficacy of the laser approach for real-time resonant ultrasound measurements in this severely hostile nuclear environment.

Study of the current–voltage characteristics of a SiC radiation detector irradiated by Co-60 gamma-rays

The SiC semiconductor has recently emerged as an attractive material for an ionization radiation detection. A wide bandgap (3.03 eV) and high radiation damage resistance allow for the fabrication of detectors capable of operating at a high-temperature and in high radiation fields. The major aim of our study is to develop a robust detector which will be applied to harsh radiation environments. In this study, we fabricated the SiC radiation detectors and measured the current–voltage characteristics of SiC detectors irradiated by Co-60 gamma-ray source. The I– V curves showed a decrease of the leakage current with increasing dose rate of Co-60 gamma-ray in the 0–100 V bias voltage range.

A method to eliminate the effect of radiation on thermocouple performance

In-core temperature measurements are pivotal in maintaining nuclear reactors in a safe state of operation. Thermocouples serve as the liaison in ensuring this safe state. The realization of the thermocouple's full potential is hindered by the fact that thermocouples cannot be situated in areas with high radiation fields. Radiation has the potential of generating voltages in the thermocouple wires, hence producing an error in the temperature transmitter output. In this paper, a mathematical model is developed to quantify the effect that radiation from the Canada Deuterium Uranium (CANDU) Nuclear Power Plants (NPPs) has on the thermocouple temperature reading. Subsequently, a method to offset the effect of radiation on the thermocouple is proposed. Simulation is performed to verify the effectiveness of the proposed system.

High Resolution One-Dimensional Dose Measurement Using a Movable Miniature Fiber-Optic Radiation Sensor

The aim of this study is to develop a new method to measure one-dimensional dose distribution of high-energy electron using a miniature fiber-optic radiation sensor. The measurements are made by a thin plastic optical fiber with an organic scintillating sensor tip. The scintillating light in the visible wavelength range is guided to a silicon photodiode by plastic optical fiber in order to convert light output to electrical signal. The one-dimensional spatial dependence of elctron beam is measured by moving the sensor tip with uniform speed. It is shown that this fiber-optic radiation sensor has better spatial resolution than conventional ion chamber and it needs much less time to measure one-dimensional dose distribution in the high radiation fields.

Pulse height defect in pCVD and scCVD diamond based detectors

Polycrystalline (pCVD) and single crystal (scCVD) diamond films grown from Chemical Vapour Deposition (CVD), if sufficiently pure at Raman analysis, are very good materials for beam or flux monitors inside accelerators or nuclear reactors. This is because they are very hard to damage in high radiation fields and very resistant to high temperatures. Films of pCVD diamond are, however, not so good as spectroscopy detectors due to inhomogeneities induced by their growth in grains with the consequent presence of grain boundaries which worsen their energy resolution. The latter can be significantly improved by growing scCVD diamond films onto HPHT synthetic diamond substrates. We have shown that it is possible to measure the density of defects inside diamond specimens using as probes suitable penetrating nuclear radiations. With the preliminary results reported here we'll show that, bombarding CVD diamond films grown at Roma “Tor Vergata” with energetic protons and 4He, 6Li and 12C ions produced in the accelerators of Catania laboratories, the pulse height defects are higher than those in silicon detectors and likewise well described by a power law in the deposited energy. Furthermore, we'll show that pulse heights for the same particles seem to depend on the duration of the measurement, thus exhibiting a sort of depolarization of the insulator when exposed to the electric voltage which makes it a particle detector.

Diffuse Atomic and Molecular Clouds

Diffuse interstellar clouds have long been thought to be relatively devoid of molecules, because of their low densities and high radiation fields. However, in the past ten years or so, a plethora of polyatomic molecules have been observed in diffuse clouds, via their rotational, vibrational, and electronic transitions. In this review, we propose a new systematic classification method for the different types of interstellar clouds: diffuse atomic, diffuse molecular, translucent, and dense. We review the observations of molecules (both diatomic and polyatomic) in diffuse clouds and discuss how molecules can be utilized as indicators of the physical and chemical conditions within these clouds. We review the progress made in the modeling of the chemistry in these clouds, and the (significant) challenges that remain in this endeavor. We also review the evidence for the existence of very large molecules in diffuse clouds, and discuss a few specific clouds of particular interest.

Influence of hydroxyl-water interaction on structural transformations during Cs + exchange in microporous H-CST

Crystalline silicotitanate (CST) molecular sieve is being tested as an ion exchanger for targeted removal of Cs from high level radioactive nuclear waste solutions. CST is resistant to high radiation fields and highly alkaline/acidic solutions which make it an ideal sequestration candidate. The mechanisms and dynamics of the crystallographic structural changes involved are the fundamental properties of selectivity in this material, and have not been studied previously. This study investigates the molecular processes of ion exchange in real time using neutron and X-ray diffraction as well molecular dynamics (MD) simulations. These techniques in unison have provided the most complete description of the Cs exchange process in hydrogen form of CST (H-CST). In the H-CST structure, the elliptical channels cannot accommodate Cs due to unacceptable short Cs–O bond distances. From time-resolved X-ray diffraction studies, the elliptical channels in H-CST transform to circular geometry after 8 wt.% Cs exchange. Although structural transformations were observed and the symmetry changed from space group P42/mbc to P42/mcm, the mechanisms were still unknown. Identifying all atom types and positions in the CST structure will ultimately explain the mechanisms of selectivity for Cs in H-CST. Time-resolved and static neutron diffraction studies added the proton positions on the water molecules and hydroxyl units to the model derived from X-ray scattering. The results show that as Cs diffuses through the structure, water molecules must change orientations and positions to hydrate Cs and maintain the hydrogen bond network. The new configuration of the proton on the water sites are closer to the framework and force close proton–proton repulsion with the framework hydroxyl. This repulsion bends the hydroxyl and drives the transformation of the CST structure. Hydroxyl bending and water dynamics have been confirmed using MD simulations. The short timescales of MD simulations is ideal to study these fast molecular motions and emphasizes the important role hydrogen has in CST toward Cs selectivity.

Chemical composition fiber gratings in a high mixed gamma neutron radiation field

Fiber Bragg gratings are all-fiber wavelength-selective refractive index structures manufactured through UV exposure of optical fibers. Their applications range from WDM all-fiber filters, dispersion compensators and fiber laser resonators for optical telecommunication applications to different types of point or distributed optical fiber sensors for a large variety of applications. The nuclear industry is considering the use of photonic technology for data communication in the next generation of nuclear power plants. In addition, fiber Bragg grating sensors are being evaluated by the nuclear industry for structural integrity and temperature monitoring. This work aims to study, in harsh radiation environments, a new type of FBG referred to as chemical composition grating. These gratings differ from other types of FBG in that their refractive index structure is attributed to a change in the chemical composition. Chemical composition gratings have shown to be extremely temperature stable surviving temperatures in excess of 1000/spl deg/C. We have experimentally studied the effect of very harsh gamma-neutron radiation on the properties of chemical composition gratings fabricated in a Ge-F doped silica optical fiber.

Shielding solutions at the SNS target/accelerator interface

At the Spallation Neutron Source (SNS), a 1.4 MW proton beam at 1 GeV energy intercepts the liquid mercury target and sets free neutrons and a range of other particles. A fraction of the particles is scattered back into the last section of the accelerator tunnel, upstream from the target, and increases the already present radiation field caused by proton beam losses. The accelerator structures are exposed to high radiation fields limiting their lifetime and causing activation. Much higher activation is expected in the target and surrounding structural materials. These components become highly intense decay gamma sources and shine back into the accelerator tunnel even after beam termination. All these issues were investigated to support material selection, shielding design and maintenance planning.

高放射線環境用の架橋ゴム材料の開発

A new compound was developed to obtain a radiation proof crosslinked rubber which shows the excellent performance than the usual elastomer, i. e., ordinary EPDM (Ethylene-propylene-diene terpolymer) based one.As the first study, examinations were carried out with Co-60 gamma-ray exposure test at the room temperature for various candidate rubbers.Based on the exposure test of various rubbers, EPDM compounds were investigated in order to improve the resistance to deterioration under high radiation field. The high performance rubber was achieved by a blend of EPDM and IIR (Butyl rubber). In addition to the polymer system, antioxidant and aromatic oil were used as admixture.The most excellent new crosslinked rubber showed no bleeding, no brooming, less change of hardness and keep 150% elongation after the gamma-ray irradiation at over 7 MGy.

Gamma-ray induced photoconductivity in pyrex, quartz, and vycor

Results of an experimental investigation are reported on photoconductive response of Pyrex, Quartz and Vycor. This research was conducted as a feasibility study for developing a new gamma dose measurement device based on the principle of photoconductivity. However, data collected in this study are equally valuable for various other applications where these materials are to be used in high radiation field. Our experiments and analyzes indicated that the selected dosimeter materials exhibit photoconductivity and respond to changes in gamma dose rate within a useful range. Pyrex glass suffered radiation damage at a relatively high dose rate of 0.25 Gys-1. Quartz and Vycor exhibit significant advantages over Pyrex as dosimeter material. However, their limit of operability was found to be at a dose rate of approximately 28 mGy s-1. Vycor dosimeter in particular appears promising for gamma measurement applications. From an endurance point of view, Quartz appears to be the most suitable material for applications in high dose rate conditions, in that photo-currents in Quartz were observed to be minimum

High-resolution Compton-suppressed CZT and LaCl/sub 3/ detectors for fission products identification

Room-temperature semiconductor CdZnTe (CZT) detectors are currently limited to total detector volumes of 1-2 cm/sup 3/, which is dictated by poor charge transport characteristics. Because of this size limitation, one of the problems in accurately determining isotope identification is the enormous background from Compton scattering events. Eliminating this background will not only increase the sensitivity and accuracy of measurements, but will also help to resolve peaks buried under the background and peaks in close vicinity to others. We are currently developing a fission products detection system based on Compton-suppressed CZT and LaCl/sub 3/ (Ce) detectors. In this application, the detection system is required to operate in a high radiation field. Therefore, a small 10 mm /spl times/10 mm/spl times/5 mm CZT and 13mm/spl times/15mm LaCl/sub 3/ detector are placed inside the center of a well-shielded 76 mm by 76 mm long NaI detector. So far, we have been able to successfully reduce Compton background by a factor of 3.7 to 4.0 for a /sup 137/Cs spectrum. In this work, we will discuss the performance of this detection system using both CZT and LaCl/sub 3/ detectors. The results are compared with MCNP calculations.

MEGA-MASERS AND GALAXIES

▪ Abstract In the Galaxy, microwave radiation can be amplified in the interstellar medium in the immediate neighborhood of young stellar objects, or circumstellar envelopes around evolved stars, resulting in cosmic maser emission. Cosmic masers exist because, in contrast to terrestrial conditions, the interstellar gas density is very low so that level population in molecules is typically not in thermal equilibrium, and sometimes inverted. In the nuclear regions of external galaxies, there exist very powerful OH (λ18 cm) and H2O (λ1.35 cm) cosmic masers with line luminosities of ∼ 102−104 L⊙, ≥ 106 times more luminous than typical Galactic maser sources. These are the “mega-masers,” found in high-density molecular gas located within parsecs of active galactic nuclei in the case of H2O mega-masers, or within the central 100 pc of nuclear star-burst regions in the case of OH mega-masers. H2O mega-masers are most frequently found in galactic nuclei with Seyfert2 or LINER spectral characteristics, in spiral and some elliptical galaxies. OH mega-masers are found in ultra-luminous IR galaxies (ULIRG) with the warmest IR colors, and importantly, the OH luminosity is observed to increase with the IR luminosity: LOH ∝ L1.2IR. Because of the extremely high-surface brightness, H2O mega-maser emission can be mapped at sub-milli-arc-second resolution by Very Long Baseline Interferometry (VLBI), providing a powerful tool to probe spatial and kinematic distributions of molecular gas in distant galactic nuclei at scales below one parsec. An excellent example is the active galaxy, NGC 4258, in which mapping of the H2O mega-maser emission has provided the first direct evidence in an active galactic nucleus for the existence of a thin Keplerian accretion disk with turbulence, as well as highly compelling evidence for the existence of a massive black hole. The NGC 4258 mega-maser has also provided a geometric distance determination of extremely high precision. H2O mega-maser emission is also found to arise from postshocked gas from the impact of nuclear jets or outflows on the surrounding molecular clouds. High-resolution observations have shown that OH mega-masers originate from the molecular gas medium in 100-pc scale nuclear star-burst regions. It is proposed that such extreme star-burst regions, with extensive high-density gas bathed in a very high far-IR radiation field, are conducive to the formation of a very large number of OH maser sources that collectively produce the OH mega-maser emission. In the early Universe, galaxies or mergers could go through a very luminous phase, powered by intensive star-bursts and AGN formation, and could have extremely large OH and H2O maser luminosities, possibly producing giga-masers. With the increasing sensitivity of new telescopes and receivers, surveys and high-resolution studies of mega-masers and giga-masers will be very important tracers and high-resolution probes of active galactic nuclei, dust embedded star-bursts in the earliest galaxies and galaxy mergers in the epoch of very active star formation at z ∼ 2 and beyond. Distance determination of giga-masers at z ∼ 1–2 can provide on independent measure of how fast the universe is expanding.

Investigation on the chemical behaviour of a crown ether irradiated in high gamma radiation fields

Summary The performance, after irradiation at high gamma radiation doses, of the material known under the commercial name SrResin (Eichrom Industries, USA), consisting of a crown ether, 4,4′(5′)-bis(t-butyl-cyclohexane)-18-crown-6, in a solution of 1-octanol absorbed in a polymeric support, has been characterised. The matrix, disposed in columns moistened with dilute HNO3 or imbibed with 8 M HNO3, was irradiated with a 60Co radiation field at doses from 2 kGy up to 40 kGy, and the separative properties of the columns were measured through the barium–strontium separation. Although the effects of the irradiation are displacement of the elution volumes and an increase of losses of the elements, quantitative separation of barium and strontium is still feasible.

UCN Production With a Single Crystal of Ortho-Deuterium.

The present paper reports on the preliminary experimental results concerning a new concept of ultracold neutron production with a single crystal converter of ortho-deuterium lying in the ground rotational state at the low temperature of about 10 K, which should make it possible to utilize a guided cold neutron beam instead of irradiating the converter material in the inside of high radiation fields. The successful observation of the clear Bragg scattering pattern from the single crystal converter and the reasonable results from the first experimental trial of the ultracold neutron production with the single crystal are shown.