Uniform Radiation Field Research Articles

Gas stripping by radiation drag from an interstellar cloud

We perform two-dimensional hydrodynamic simulation of gas stripping by radiation drag from interstellar clouds moving in uniform radiation fields. To include relativistic radiation drag properly, the radiation hydrodynamic equation is solved taking into account the dilution of radiation fields by the optical depth of the cloud. As a result, it is found that the optically thin surface layers are effectively stripped by radiation drag from an optically thick gas cloud, and, simultaneously, the stripped gas loses momentum. The momentum loss time-scale is found to be on the order of 10 8 yr under the intensive radiation fields expected in the early phase of galaxy evolution. The present results show that the radiation drag is an effective mechanism for extracting angular momentum from the interstellar medium and allows it to accrete on to the galactic centre. Mass accretion driven by radiation drag may lead to the formation of a central supermassive black hole.

Therapeutic purpose 9C beams produced in the secondary beam line at HIMAC

Due to the favorable characteristics of heavy-ion beams such as dose localization as well as high relative biological effectiveness, heavy-ion cancer therapy is attracting growing interests all over the world. The application of β-delayed particle decay beams like 9C ions in cancer therapy is expected to further improve the treatment efficacy of the modality of heavy-ions. For the first time 9C beams were produced in the secondary beam line (SBL) of the HIMAC facility with a primary 12C beam of 430 MeV/n. The production conditions providing a 9C beam with higher production rate and purity were optimized using different thick production targets and degraders. The results show setting the momentum acceptance to the full value of 5% the production rate is 9.07 × 10 −6 and the purity is 82.88% for the produced 9C beam under the optimal condition. Because of the large momentum spread for a secondary beam, the momentum distribution of the produced 9C beam at the optimal condition was measured. The momentum among the beam had a nearly Gaussian profile with a standard deviation of 26.1 MeV/c/n and the largest component of the momenta appeared around 838.9 MeV/c/n. In order to form a relatively large uniform irradiation field being appropriate to further biological experiments for the produced 9C beam, the scanning magnets in the irradiation system of the SBL, operating in a wobbling mode, was used. A uniform irradiation field of 20 mm in diameter could be achieved with homogeneity of 93.8%. The work presented in this paper provides a substantial basis for the further studies with 9C beams for the therapeutic purpose.

Simulations of Early Structure Formation: Primordial Gas Clouds

We use cosmological simulations to study the origin of primordial star-forming in a ΛCDM universe, by following the formation of dark matter halos and the cooling of within them. To model the physics of chemically pristine gas, we employ a nonequilibrium treatment of the chemistry of nine species (e−, H, H+, He, He+, He++, H2, H, H−) and include cooling by molecular hydrogen. By considering cosmological volumes, we are able to study the statistical properties of primordial halos, and the high resolution of our simulations enables us to examine these objects in detail. In particular, we explore the hierarchical growth of bound structures forming at redshifts z ≈ 25-30 with total masses in the range ≈105-106 M☉. We find that when the amount of molecular hydrogen in these objects reaches a critical level, cooling by rotational line emission is efficient, and dense clumps of cold form. We identify these gas clouds as sites for primordial star formation. In our simulations, the threshold for cloud formation by molecular cooling corresponds to a critical halo mass of ≈5 × 105 h-1 M☉, in agreement with earlier estimates, but with a weak dependence on redshift in the range z > 16. The complex interplay between the gravitational formation of dark halos and the thermodynamic and chemical evolution of the compromises analytic estimates of the critical H2 fraction. Dynamical heating from mass accretion and mergers opposes relatively inefficient cooling by molecular hydrogen, delaying the production of star-forming in rapidly growing halos. We also investigate the effect of photodissociating ultraviolet radiation on the formation of primordial clouds. We consider two extreme cases, first by including a uniform radiation field in the optically thin limit and second by accounting for the maximum effect of self-shielding in virialized regions. For radiation with Lyman-Werner band flux J > 10-23 ergs s-1 cm-2 Hz-1 sr-1, hydrogen molecules are rapidly dissociated, rendering cooling inefficient. In both the cases we consider, the overall effect can be described by computing an equilibrium H2 abundance for the radiation flux and defining an effective shielding factor. Based on our numerical results, we develop a semianalytic model of the formation of the first stars and demonstrate how it can be coupled with large N-body simulations to predict the star formation rate in the early universe.

Numerical Simulations of High‐Redshift Star Formation in Dwarf Galaxies

We present first results from three-dimensional hydrodynamic simulations of the high-redshift formation of dwarf galaxies. The simulations use an Eulerian adaptive mesh refinement technique to follow the nonequilibrium chemistry of hydrogen and helium with cosmological initial conditions drawn from a popular Λ-dominated, cold dark matter (CDM) model. We include the effects of reionization using a uniform radiation field, a phenomenological description of the effect of star formation, and, in a separate simulation, the effects of stellar feedback. The results highlight the effects of stellar feedback and photoionization on the baryon content and star formation of galaxies with virial temperatures of approximately 104 K. Dwarf-sized dark matter halos that assemble prior to reionization are able to form stars. Most halos of similar mass that assemble after reionization do not form stars by redshift 3. Dwarf galaxies that form stars show large variations in their gas content because of stellar feedback and photoionization effects. Baryon-to-dark matter mass ratios are found to lie below the cosmic mean as a result of stellar feedback. The supposed substructure problem of CDM is critically assessed on the basis of these results. The star formation histories modulated by radiative and stellar feedbacks are discussed. In addition, metallicities of individual objects are shown to be naturally correlated with their mass-to-light ratios, as is also evident in the properties of local dwarf galaxies.

The calibration of dosimetry instruments used in interventional radiology.

No general agreement about the definition of the patient dose exists. As regards the radiation health risk, the doses to specific organs, Hi, are the ultimate measures for a patient dose. Values of the calibration measured, Hi, are provided only by calculational means. Out of the whole process of patient dose determination, the instruments to measure X ray spectra, FSD, field dimensions and Ka can be calibrated, X ray quality is derived from the total filtration and kV value. The actual dynamic and X ray quality ranges shall be considered when air kerma and DAP meters are calibrated. A DAP meter measurement averages the uniform radiation field specific for the X ray tube assembly used and for the beam shaping technique performed. Therefore, a DAP meter calibrated on site is preferable for patient dosimetry in interventional radiology.

Ionizing radiation in smoothed particle hydrodynamics

A new method for the inclusion of ionizing radiation from uniform radiation fields into 3D Smoothed Particle Hydrodynamics (SPHI) simulations is presented. We calculate the optical depth for the Lyman continuum radiation from the source towards the SPHI particles by ray-tracing integration. The time-dependent ionization rate equation is then solved locally for the particles within the ionizing radiation field. Using test calculations, we explore the numerical behaviour of the code with respect to the implementation of the time-dependent ionization rate equation. We also test the coupling of the heating caused by the ionization to the hydrodynamical part of the SPHI code.

Magnetoplasmon excitations in quantum dot arrays

Motivated by the far-infrared transmission experiments of Demel et al., we have investigated the magnetoplasmon excitations in an array of quantum dots within the Thomas–Fermi–Dirac–von Weizsäcker (TFDW) approximation. Detailed calculations of the magnetic dispersion and power absorption from a uniform radiation field unambiguously demonstrates that the noncircular symmetry of the individual dots is responsible for the anticrossing behaviour observed in the experiments. The interdot Coulomb interaction is unimportant at the interdot separation of the samples studied.

Demonstration of a time-integrating microdosimeter

A tissue-equivalent spherical proportional counter is used with a modified amplifier system to measure specific energy deposited from a uniform radiation field for short periods of time (on the order of microseconds to milliseconds) in order to extrapolate to dose in sub-micron tissue volumes. The signal is integrated over a variable collection time which is adjusted with a square-wave pulse. Charge from partical passages is collected on the anode during the period in which the integrator is triggered, and the signal decays quickly to zero after the integrator feedback switch resets; the process repeats for every “triggering” pulse. Measurements of energy deposited from X-rays are examined. Spectral characteristics as a function of charge collection time are observed and frequency plots of specific energy and collection time-interval are presented.

Radiobiology of α Particles: I. Exposure System and Dosimetry

A 238Pu alpha-particle exposure apparatus was designed and constructed for use in radiobiological studies with cultured cell systems. The system provides a wide dynamic range of absorbed doses and a uniform radiation field. Average dose rate in air was measured with a small-volume ionization chamber. Estimates of dose rate at the cell surface were obtained from measurements taken with a silicon surface barrier detector. Particle fluence uniformity and fluence rate were measured using track etch procedures. The design and dosimetric characterization of the apparatus are discussed.

Lineshape theory of doppler-free coupled closely spaced transitions in the presence of a strong radiation field: The effect of non-resonant interactions

The lineshape of a pair of Doppler-free closely spaced transitions sharing a common level in the presence of a uniform radiation field is studied by solving the coupled optical Bloch equations numerically. It is shown that the non-linear term corresponding to the product of the two transition dipoles and the density matrix element connecting the closely spaced upper levels broadens the lineshape and shifts the peak frequency positions.

Wobbler facility for biomedical experiments.

A system for spreading relativistic heavy ion beams into large uniform radiation fields has been developed. The charged particles passing through the system are deflected into azimuthally symmetric distributions which can then be superimposed to produce dose distributions as large as 30 cm in diameter with less than +/- 3.5% variation in uniformity.

Microdosimetry of Pulsed Radiation Fields Employing the Variance Method

The relationship between dose mean lineal energy and relative variance has been exploited previously to derive yD from the calculated variance in current measurements in steady and uniform radiation fields. Recently Kellerer and Rossi made the observation that utilization of two detectors can make the variance technique practicable in time-varying fields. We report here the first measurements of yD for 10 MeV X rays and 9 and 18 MeV electrons from a pulsed linear accelerator using the variance method. Two independent analog-to-digital converters were used to obtain data from two spherical proportional counters in synchrony with the beam pulse. The method is described in detail and results are reported for site diameters of 1/2, 1, and 2 microns. Data for an accurate determination of yD can be obtained with this technique in less than 1 min, making possible an essentially "on line" determination of yD or zD in a clinical situation.

Irradiation facility and physical characteristics of a 18.5 MeV/n alpha beam for cell killing experiments

A simple irradiation facility for biological experiments using a 18.5 MeV/n alpha beam has been developed. This compact irradiation facility provides a sufficiently uniform irradiation field. Physical characteristics of the alpha beam, such as a dose distribution and LET (linear energy transfer), were measured in this field. The results were consistent with theoretically calculated results.

Treatment of equine cutaneous neoplasia by radiotherapy using iridium 192 linear sources.

The treatment of equine cutaneous tumours by conventional or cryosurgical techniques can be limited where the position of the tumour makes radical excision or freezing impractical or dangerous. Radiotherapy provides an effective and practical alternative. The use of iridium pins with guide needles allows accurate positioning of sources and uniform radiation fields to be achieved. The subsequent removal of the pins reduces the period of incapacity and reduces the radiation risk when compared to permanently implanted sources. Twenty-seven tumours on 26 horses were treated by this method with a 100 per cent success rate after a single irradiation. The technique of implantation is described and the criteria used to select cases and to assess the efficacy of this treatment are discussed.

Multidimensional spectroscopy

The quantum states produced by non-linear interaction of a coherent uniform radiation field with dipolar matter are evaluated by a time-dependent perturbation expansion of the density matrix. The non-linear terms of the expansion are Fourier transformed to yield multi-dimensional spectra which indicate the connectivities in the underlying energy level diagrams. The spectra can be measured in experiments with multiple resonance, multiple pulse or stochastic excitation. Although the theory presented is of general validity in coherent spectroscopy, emphasis is placed on its application in non-linear N.M.R. spectroscopy. It illustrates particularly well the common basis of double, 2D correlated and multi-dimensional stochastic magnetic resonance.

Track structure theory in radiobiology and in radiation detection

The response of biological cells, and many physical radiation and track detectors to ionizing radiations and to energetic heavily ionizing particles, results from the secondary and higher generation electrons ejected from the atoms and molecules of the detector by the incident primary radiation. The theory uses a calculation of the radial distribution of local dose deposited by secondary electrons (delta-rays) from an energetic heavy ion as a transfer function, relating the dose-response relation measured (or postulated) for a particular detector in a uniform radiation field (gamma-rays) to obtain the radial distribution in response about the ion's path, and thus the structure of the track of a particle. Subsequent calculations yield the response of the detector to radiation fields of arbitrary quality. The models which have been used for detector response arise from target theory, and are of the form of statistical models called multi-hit or multi-target detectors, in which it is assumed that there are sensitive elements (emulsion grains, or biological cell nuclei) which may require many hits (emulsion grains) or single hits in different targets (say, cellular chromosomes) in order to produce the observed end-point. Physically, a hit is interpreted as a ‘registered event’ caused by an electron passing through the sensitive site, with an efficiency which depends on the electron's speed. Some knowledge of size of the sensitive volume and of the sensitive target is required to make the transition from gamma-ray response to heavy ion response. Critical differences in the pattern of response of biological systems and physical detectors to radiations of different quality arise from the number of electrons which must pass through the sensitive volume to produce the recorded end-point. For biological cells this is typically 2 or more. This characteristic multi-hittedness results in survival curves with shoulders, or supralinear dose-response relations for gamma-irradiation, and for an ‘RBE’ which can exceed 1 at appropriate values of the ‘LET’. One-hit detectors cannot mimic the response of biological cells to radiations of different quality. From the beginning it has been clear that SSNTD's (etchable plastics) are not 1-hit detectors. But even now, we do not know their characteristic response to gamma-rays. We are not able to produce a satisfactory theory of track structure in these detectors. There is only a hint, that etching rate is nominally proportional to the quantity z 4 β 4 of the incident ion, suggesting the possibility of a ‘2-or-more’ hit detector. Recent work has demonstrated that many-hit physical detectors do exist. From both emulsion sensitometry and from the structure of tracks of heavy ions, we are able to show that emulsion-developer combinations exist which yield many-hit response. There is also some evidence that the supralinearity in thermoluminescent dosimeters arises from a mixture of 1-hit and 2-hit response, perhaps of different trap structures within the same TLD crystal. These detectors can be expected to mimic the response of biological cells to radiations of different quality. Their patterns of response may help us to understand better the structure of particle tracks in SSNTD's.

Tumorigenicity of small highly radioactive particles.

AbstractA theoretical approach to tumorigenic risk from exposure of skin and lung to particles of high specific activity is described. Present methods of setting guide lines for skin and lung exposures assume uniform radiation fields and are not applicable for small sources where the dose rate falls

A simple facility for wholebody irradiation.

excerpt Recent interest in whole-body irradiation of human beings, for example prior to bone marrow transplantation in cases of acute leukemia, has led to studies of suitable facilities to provide uniform dosage (1). While ideally it would seem best to irradiate the whole body at once, patient comfort, length of time of irradiation, and economy of space and equipment must be considered. Methods that have been used or proposed include treatment at a long source-skin distance (SSD) to obtain a sufficiently large field, with or without the patient curled into a small space (2,3), and the use of as many as eight Co60 sources of comparatively low strengths (4). These methods have the disadvantage of requiring a large space devoted specifically to this purpose and generally involving a long irradiation time. There would appear to be little objection to the use of a moving-field technique as long as the treatment time were not unduly protracted. This paper describes such a technique allowing a uniform high dosag...

Lead Sulfide Photosensitive Resistor

FOR missile instrumentation and sensing applications, there are unique advantages in the simplicity of the Kodak Ektron Detector, a photosensitive resistor. The Kodak Ektron Detector utilizes the peculiar electrical properties of lead sulfide. .Normally formed on a 0.30-in.-thick glass blank, it consists of a rectangular deposit of lead sulfide and gold electrodes deposited a t the edges of the sensitive area. Complex, exact arrays and mosaics of such resistors can be produced. The basic unit is overcoated with plastic for protection. Kodak Ektron Detectors are rugged and can be very small. Although they respond to x-ray radiation down to a few Angstroms in wavelength, their useful photosensitivity is usually considered to range from 0.25 micron (2500 Angstroms) to 3.5 microns (35,000 Angstroms). In the visible region, the detector responds to a 2500 K tungsten light source about the same as a red-sensitive gas-filled phototube of comparable sensitive area under comparable conditions. Though the detector competes with phototubes and photomultipliers when exposed to tungsten light, it reaches maximum sensitivity at a wavelength of about 2 microns in the infrared region. Operating at room temperature at 2 microns, the Kodak Ektron Detector gives hundreds of times the response of a good laboratory bolometer. At —40 C, its sensitivity at 2 microns increases about 25 times. The cell's response to chopped or pulsed light sources is usable from steady illumination to as high as 10,000 radiation pulses per sec. In addition to their exceptional spectra response, Kodak Ektron Detectors are characterized by high signal-to-noise ratio, time constants in the range of 400 to 1000 microsec under normal conditions, dark resistance of 0.2 to 0.8 megohm for any square cell, and a negative coefficient of resistance. In a uniform radiation field, the signal-to-noise ratio at a given voltage varies directly with the square root of the cell area. Since the most interesting properties of these photoresistors are associated with infrared response, investigations have been chiefly in the wavelength region from 1 to 3 microns. Use of geometry is made to obtain a desired dark-resistance value. Choice of cell configurations ranges over an extended scale of resistance values. Because of the permissible variety in cell geometry, high-impedance detectors need not be long, narrow units, but may be folded'' into compact rectangular patterns. Fig. 1 depicts three detectors of the same over-all size but with impedances of (a) 0.002 megohm, (b) 0.2 megohm, and (c) 20 megohms, respecFig. 1