bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of the “Critical Velocity” Phenomenon on Chromium-Plated Gun Barrels
Authors: Ho¨lzle Ju¨rg
Published: August 2003

A experimental 120 mm L55 tank gun barrel showed heavy loss of the chromium plating at the muzzle region. Experimental and theoretical studies led to the conclusion that high frequency oscillations, caused by the traveling pressure wave, are responsible for this effect. This so-called “critical velocity” phenomenon must be a design criterion for modern gun barrels.
Effect of NH3 thermal treatment on an atomic layer deposited on tungsten films and formation of W–B–N
Authors: Byun Jeong Soo , Mak Alfred , Zhang Amy , Yoon Alex , Zhang Tong , Gelatos Avgerinos , Jackson Robert , Thakur Randhir , Lee Sang-Yun , Kim Hyoungsub
Published: August 2003

The effect of ammonia (NH3) ambient annealing on a tungsten (W) film deposited by atomic layer deposition at temperatures ranging from 400–700 °C is discussed. The as-deposited film contains approximately 20 at. % of boron which is chemical bound to W (W–B) having a resistivity of 128 μΩ cm. The film has an amorphous structure, which does not transform into crystalline phase during annealing. As a result of annealing in NH3 ambient, a tungsten ternary phase (W–B–N) forms at the surface; its binding configuration depends on the annealing temperature. Below 500 °C, nitrogen is chemically bonded to tungsten (W–N) while maintaining a W–B bond. Above 600 °C, nitrogen-rich W–B–N forms, in which nitrogen atoms have chemical binding with boron (B–N) and tungsten (W–N). It was found that a film annealed at higher temperatures has a resistivity of 107 μΩ cm, and thermal desorption of boron and nitrogen containing species is not observed during the thermal process. In addition, tungsten oxide formed at the surface during exposure to air is reduced during the NH3 annealing process. X-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, and thermal desorption spectroscopy were used for film characterization.
Field emission from nonaligned carbon nanotube–polymer matrix cathodes
Authors: Poa C. H. P. , Smith R. C. , Silva S. R. P. , Watts P. C. P. , Hsu W. K. , Kroto H. W. , Walton D. R. M.
Published: August 2003

We compare the field emission characteristics of multiwalled carbon nanotubes (MWNTs) and boron-doped MWNTs mixed in a polystyrene matrix. The concentration of the nanotubes was varied by weight ratio to that of the polystyrene (PS). Results indicate that the threshold fields are mainly controlled by the electric-field screening effect on the surface due to the high emission current densities measured. The lowest threshold field of 1.6 V/μm was observed from a MWNT-PS ratio of 11.1 wt %. Emission current densities, together with the threshold fields for these films, show that the emission properties are comparable to those of vertically aligned CNTs as well as arrays of carbon fibers. As the procedure to prepare these nonaligned CNT-PS composites is simple, it is proposed that the methodology could be a means of producing large area cathodes suitable for use in flat panel displays.
Electron-beam lithography for thick refractive optical elements in SU-8
Authors: Shields Eric A. , Williamson Fred , Leger James R.
Published: August 2003

We have fabricated refractive optical elements in SU-8 resist using electron-beam lithography. Exposing an 8-μm-thick layer of SU-8 resist with a 30 kV electron beam, we demonstrate fabrication of an f/9 spherical lens, an f/2 spherical lens, and a prism with a 3° tilt. The elements all perform near the diffraction limit, with rms surface variations between 1/10 wave and 1/7 wave. The electron scattering from the thick resist is measured and shown to limit the technique to optical elements with relatively smooth surface figures.
Bonding and structure of ultrathin yttrium oxide films for Si field effect transistor gate dielectric applications
Authors: Ulrich M. D. , Rowe J. E. , Niu D. , Parsons G. N.
Published: August 2003

Soft x-ray photoelectron spectroscopy using synchrotron radiation has been employed to study the interface between Y2O3 films and Si(100). Y2O3 films of ∼8, ∼15, and 65 Å were formed by plasma assisted chemical vapor deposition on HF-last Si(100). With this deposition technique, SiO2 forms at the interface and a kinetically limited silicate layer forms between the resulting SiO2 deposited Y2O3. For 65 Å films, the Y 3d5/2 binding energy was between 158.8 and 159.0 eV, 2.2–2.4 eV higher than the reported value of 156.6 eV for Y2O3. For 8 and 15 Å films, the Y 3d5/2 binding energies were 159.6 and 158.9 eV, respectively. The relatively high binding energies are attributed to hydroxide incorporation in the film. For the ultrathin films, ∼10 Å of SiO2 was formed at the interface during or after the deposition. For the 8 Å film, no silicate is detectable whereas for the 15 Å film, an estimated 4 Å of silicate is present between the interfacial SiO2 and Y2O3 overlayer. Because this transition layer does not form in the 8 Å film, it is concluded that the mixing is kinetically limited. For the 8 Å film, the Si 2p3/2 [SiO2] binding energy was 3.65 eV relative to the substrate peak. For the 15 Å deposition, the Si 2p3/2 [SiO2] binding energy was 3.44 eV and the Si 2p3/2 [silicate] binding energy was 2.65 eV relative to the substrate peak.
Electron paramagnetic resonance biophysical radiation dosimetry with tooth enamel
Authors: Khan Rao F. H.
Published: August 2003

This thesis deals with the advancements made in the field of Electron Paramagnetic Resonance (EPR) for biophysical dosimetry with tooth enamel for accident, emergency, and retrospective radiation dose reconstruction. A methodology has been developed to measure retrospective radiation exposures in human tooth enamel. This entails novel sample preparation procedures with minimum mechanical treatment to reduce the preparation induced uncertainties, establish optimum measurement conditions inside the EPR cavity, post‐process the measured spectrum with functional simulation of dosimetric and other interfering signals, and reconstruct dose. By using this technique, retrospective gamma exposures as low as have been successfully deciphered. The notion of dose modifier was introduced in EPR biodosimetry for low dose measurements. It has been demonstrated that by using the modified zero added dose (MZAD) technique for low radiation exposures, doses in 100 mGy ranges can be easily reconstructed in teeth that were previously thought useless for EPR dosimetry. Also, the use of a dose modifier makes robust dose reconstruction possible for higher radiation exposures. The EPR dosimetry technique was also developed for tooth samples extracted from rodents, which represent small tooth sizing. EPR doses in the molars, extracted from the mice irradiated with whole body exposures, were reassessed and shown to be correct within the experimental uncertainty. The sensitivity of human tooth enamel for neutron irradiation, obtained from the 3 MV McMaster K.N. Van de Graaff accelerator, was also studied. For the first time this work has shown that the neutron sensitivity of the tooth enamel is approximately 1/10th of the equivalent gamma sensitivity. Parametric studies for neutron dose rate and neutron energy within the available range of the accelerator, showed no impact on the sensitivity of the tooth enamel. Therefore, tooth enamel can be used as a dosimeter for both neutrons as well as gamma radiation. We will continue experiments to develop this end point as a sensitive accident or emergency tool for our response capabilities.
Electrostatic fields in ionography
Authors: Plewes Don , Johns H. E.
Published: August 2003

There is renewed interest in diagnostic radiology in electrostatic methods of imaging, such as xeroradiography and ionography. This is due to the fact that edge contrast can be achieved, aiding in the visualization of soft tissue tumors. In analyzing the image forming properties of these systems, we chose to solve the electrostatic problems by the method of images. We present methods and recursion formulas for calculating electrostatic fields due to (a) any charge distribution on a slab of dielectric (the solution involves a single infinite series) or (b) the same as the above with the introduction of an additional ground plane parallel to the dielectric surface (the solution now involves double infinite series). Analysis of these fields suggests new methods of controlling edge contrast and development configurations where the field which penetrates through the foil is used to produce the final image rather than the field above the charged surface.
Simple method for the generation of organ and vessel contours from roentgenographic or fluoroscopic images
Authors: Newell John D. , Keller Robert A. , Baily Norman A.
Published: August 2003

A simple method is described for outlining or contouring any area defined by a change in film density or fluorscopic screen intensity. The entire process, except for the positioning of an electronic window, is accomplished using a small computer having appropriate software. The electronic window is operator positioned over the area to be processed. The only requirement is that the window be large enough to encompass the total area to be considered.
Loss of field flatness in AECL teletherapy units from misadjustment of the collimator
Authors: Heintz Philip H.
Published: August 2003

Atomic Energy of Canada Limited 60Co teletherapy units have adjustable collimator hinges to adapt to source diameters from 0.75 to 2.50 cm. Adjustment of the collimator hinge to the manufacturer's settings can cause a loss of radiation field flatness as high as 20% for large fields. The recommended settings appear to clip the top corners of tall source capsules. Opening the hinge adjustment restores field flatness, without changing penumbra or the inverse square law behavior of the teletherapy unit.
Importance of the calibration of all of the parameters affecting radiation exposure measurement
Authors: McCarthy W. A.
Published: August 2003

A radiation calibration error at Victoreen is reported and the background history and immediate cause is discussed.
Mathematical method for determining kVp from x‐ray attenuation measurements
Authors: Joseph Peter M.
Published: August 2003

A mathematical analysis of the shape of x‐ray attenuation curves is presented that suggests a new method of determining the kVp of bremsstrahlung beams. The main result is a simple analytic formula which gives an excellent approximation to the attenuation curve for large thickness of high‐Z absorber. From this, a graphical procedure is derived which yields a straight line whose slope is related to the mass attenuation coefficient evaluated at the peak energy. Included are corrections for aluminum filtration, ion‐chamber energy dependence, and coherent scattering. The method is limited to beams generated by a constant potential and with negligible characteristic photons in the filtered spectrum. Numerical data are given for using copper absorber below 70 kVcp and tin above 90 kVcp.
Absolute fluence measurement for a prototype neutron radiotherapy source
Authors: Beach J. L. , McCullough E. C.
Published: August 2003

An inexpensive recoil‐proton counter telescope has been designed for absolute fluence measurements of a gas‐target neutron source for radiation therapy. The detector has an absolute efficiency of 1.1×10−9 at 20 cm from an isotropic source and is useful for production rates of 109–1013 neutrons per second. The telescope consists of a thin hydrogenous irradiator foil and a surface‐barrier detector to count recoil protons within a defined solid angle. The telescope provides n‐γ discrimination as well as discrimination against scattered neutrons. Initial tests of the counter telescope were performed using the DD reaction employed in development stages of the gas target. A clear separation of full‐energy recoil protons from background and scattered neutron events was evident in the pulse‐height spectra without the use of coincidence gating techniques.
Saturation curve in gases of high atomic number at pressures up to 8 atm. I. Krypton and xenon
Authors: Boag J. W. , Barish R. J. , Seelentag W. W.
Published: August 2003

The saturation curve has been studied in xenon and in krypton up to a pressure of 8 atm. An empirical formula has been found that describes the fraction of current collected over a wide range of voltages, pressures, ionization intensities, and electrode spacings. This is of practical value in the design of ionography chambers. For krypton the collection fraction fKr= (1+0.25η−1.74)−1, and for xenon fXe= (1+0.16η−1.88)−1, where η=Fp−0.7Vd−2q−1/2 with F=3.61×10−7 and 2.50×10−7 for krypton and xenon, respectively. The ranges of the variables covered in the experiments were p=1–8 atm, V=5–25 000 V, d=0.3–1.3 cm, and q=4×10−9–6×10−8 A/cm3.
Maintenance of condenser‐ion chamber calibration
Authors: Rozenfeld M.
Published: August 2003

Readings made with condenser‐ionization chamber systems are frequently suspect because significant changes in calibration can occur without being evident in the normal use of the instruments. These changes can be traced to physical causes that alter the voltage sensitivity or electrical capacitance. Thus, measurement of these electrical parameters will indicate whether or not a change in calibration has occurred. Formulas are presented for calculating such changes and determining which parameter is responsible. Good agreement was obtained between the results of calculations with these formulas and the results of radiation measurements made with a Victoreen Condenser R‐Meter after several components had been deliberately maladjusted.
Exposure values around an x‐ray scanning transaxial tomograph (EMI scanner)
Authors: Gross G. , McCullough E. C.
Published: August 2003

Measurements of exposure accumulated in a one‐month period in and around a scanning x‐ray transaxial tomograph are reported. For the unit studied (the EMI neurological scanner) values measured indicate that the shielding required is “minimal.”
Specification of electron beam quality from the central‐axis depth absorbed‐dose distribution
Authors: Brahme Anders , Svensson Hans
Published: August 2003

The distribution of absorbed dose in a medium irradiated by broad electron beams has been analyzed from physical and therapeutic points of view. A number of parameters which describe the beam quality and the shape of the absorbed‐dose distribution along the central axis are defined. Several comparisons are made of the values of these parameters obtained from measurements made on different therapy accelerators and from theoretically calculated dose distributions for monoenergetic and monodirectional beams. It is found that the major part of the observed differences between experimental and theoretical results can be attributed to the energy distribution of the accelerator beam and to processes in the scattering material, such as energy straggling and the production of secondary electrons and photons. A simple expression is derived relating the slower falloff in dose found for most accelerator beams to the large energy spread found in these beams. Further, a semiempirical relationship is found which makes it possible to obtain the mean electron energy at the phantom surface from the depth where the absorbed dose has decreased to 50% of its maximum value along the central axis.
Variation in output with a fixed dose setting on a 4‐MV linear accelerator
Authors: Gillin Michael , Quillin Robert
Published: August 2003

Variations in the output with a fixed monitor unit setting as a function of magnetron power of a 4‐MV linear accelerator has been studied. The relationship between radiation output and monitor units was found to vary by as much as 10%. The central‐axis depth dose was found to remain constant within experimental error. Field flatness measurements indicated that for lower power outputs, the radiation field was less flat than for higher power outputs. A mechanism for the change in calibration is suggested based upon field flatness measurements.
Size distribution of human albumin microspheres by forward scattering of monochromatic light
Authors: Padikal Thomas N. , Kereiakes James G. , Robbins Philip J.
Published: August 2003

Optical diffraction is used to study the structure of the Fourier transform of a thin suspension of human albumin microspheres. The intensity profile in the transform plane is then used to derive information about the particle size distribution.
Calculations of two‐dimensional electric field and potential distributions around electrodes in conducting media
Authors: Hutson Richard L.
Published: August 2003

Tumor cells can be killed by heating to 42.5°C. It is convenient to localize heating by passing electric current at 500 kHz through tissue between a pair of electrodes. It is desirable to determine, for a given electrode–tissue configuration, what the resulting field and, therefore, current distribution will be. A computer program FFEARS was used to calculate electric field and potential distributions in two dimensions for three simple cases.
Evaluation of nuclear‐reactor‐produced Iodine‐123
Authors: Blue J. W. , Sodd V. J.
Published: August 2003

Iodine‐123 has such great potential to nuclear medicine that all possible production methods should be considered. In this report an experimental study related to 123l production at a high‐intensity fast flux reactor using the reaction 124Xe(n,2n)123Xe is considered. The conclusion is that 123l could be made in small quantities and the cost would be higher than the cyclotron methods presently used.
A 5‐Year Research and Development Agendum for Ultrasonic Imaging Diagnostic Instrumentation
Authors: Busser John H.
Published: August 2003

At most scientific meetings, the emphasis is on the results of research, not on the means by which those results were achieved. However, it was the intent of the Alliance for Engineering in Medicine and Biology in the symposium organized at the 1975 AAPM meeting to describe the instrumentation now available to researchers and practicing physicians in the field of ultrasonic diagnostic imaging. We feel that this is an appropriate and useful presentation, especially because it concerns a rapidly changing field. The papers that follow summarize the current state‐of‐the‐art of, first, single‐transducer imaging and, second, two‐dimensional transducer array technology. (A third paper, summarizing the state‐of‐the‐art of ultrasonic tissue signatures, was presented by Dennis H. Le Croissette, Jet Propulsion Laboratory, Pasadena, California 91103, but is not included here because it is to be published this spring in Ultrasonic Tissue Characterization, the proceedings of a seminar held by the National Bureau of Standards 28–30 May 1975.) These papers together offer a very broad picture of where ultrasonic diagnostic imaging instrumentation stands today, and what may be expected in the near future.
Analysis of in vivo volume measurements obtained with diagnostic ultrasound
Authors: Zagzebski James A. , Banjavic Richard A. , Tolbert Donald D.
Published: August 2003

A study was carried out to investigate the errors involved in obtaining volumes by means of ultrasound echograms and a small computer. The work was done on live pigs’ kidneys (130–200 cm3). The errors include those associated with the scanner itself, those involved in the interpretation of the contour boundaries, and those relating to the algorithms used to determine the volumes from the contours. An average deviation of 25.4 cm3 from actual volumes was found, while 58% of the measurements were within ±10%. Computer programs allowing for the display of contours and intercepts of corresponding orthogonal scan planes were used to reanalyze the data resulting in an average deviation of 14.7 cm3 with 78% of the measurements within ±10%.
Dose levels due to neutrons in the vicinity of high‐energy medical accelerators
Authors: McGinley P. H. , Wood M. , Mills M. , Rodriguez R.
Published: August 2003

High‐energy photons are generated for use in radiation therapy by the deceleration of electrons in metal targets. Fast neutrons are also generated as a result of (γ, n) and (e, e′n) interactions in the target, beam compensator filter, and collimator material. In this work the absorbed dose due to neutrons was measured at the center of a 10×10‐cm photon beam and 5 cm outside of the beam edge for a number of treatment units (Varian Clinac‐18, Allis–Chalmers 25‐MeV betatron, and Brown Boveri 45‐MeV betatron). Dose levels due to slow and fast neutrons were also established outside of the treatment rooms and a Bonner sphere neutron spectrometer system was employed to determine the neutron energy spectrum due to stray neutron radiation at each accelerator. For the linac it was found that the neutron dose at the beam center was 0.0039% of the photon dose and values of 0.04% and 0.053% were observed for the Allis–Chalmers betatron and the Brown Boveri betatron. Dose‐equivalent rates in the range 0.3–22.5 mrem were measured for points outside the treatment rooms when the accelerators were operated at a photon dose rate of 100 rad/min at the treatment position.
Direct‐response ultraviolet thermoluminescent dosimeter
Authors: Dhar A. , DeWerd L. A. , Stoebe T. G.
Published: August 2003

An integrating solid‐state dosimeter has been developed to detect ultraviolet radiation using the process of “direct” ultraviolet stimulation. This process yields a thermoluminescent signal after an exposure to ultraviolet radiation, without prior exposure to ionizing radiation. Magnesium oxide crystals were found to show a directly stimulated thermoluminescent glow peak at 145 °C which can be utilized as a linear, energy‐independent ultraviolet dosimeter.
Calibration instrumentation used by the AAPM Radiological Physics Center
Authors: Grant Walter , Cundiff Jackson , Hanson William , Gagnon William , Shalek Robert
Published: August 2003

The Keithley model 602 electrometer–Farmer ionization chamber combination has served as the standard dosimetry system for calibration measurements made by the AAPM Radiological Physics Center at 156 different institutions. The constancy of this system is compared to that for the Baldwin–Farmer Secondary Standard Dosemeter type 2502, and is based on measurements made over a 3‐yr period on a Cobalt‐60 γ‐ray source. A Strontium‐90 check source is also shown to be a reliable standard, and frequent checks with such a source can prevent serious calibration errors.
LiF dosimetric studies of cavity displacement effect and reference depths for electron beams
Authors: Fregene A. O.
Published: August 2003

The total effects of the cavity displacement and the perturbation of the field by the cavity in thimble‐chamber measurements of absorbed dose were investigated for electron beams of energy 15, 20, 30, and 39 MeV and for 60CO γ radiation by LiF dosimeter rods of diameter 1 mm and length 6 mm. The measurements were carried out in a polystyrene phantom at recommended reference depths and at the respective electronic equilibrium depths, for cavities of varying sizes. Owing to the cavity effect, the ratio of absorbed dose of a 5‐mm‐diameter cavity to that of a point cavity, at the reference depth, was 1.028 for 60Co γ radiation; this ratio varied from 1.022 at 15 MeV to 1.033 for 39‐MeV electrons. Although the cavity effect is present over regions of the depth dose where dose is constant with depth, here it was less dependent on cavity size than when the cavity was positioned at ICRU depths. Both the displacement and perturbation of the radiation field effects are automatically corrected through the experimental arrangement in this work.
Unequal weighting of given doses in opposed fields in treatment of cancer of the tonsillar region using 60Co, 4‐, 8‐, 15‐, 24‐MVp photons
Authors: Wollin M. , Gilbert H. A. , Kagan A. R.
Published: August 2003

Weighting of given doses from parallel opposed fields is a widespread practice in radiation therapy of cancer in the tonsillar region. In order to determine when weighting is useful, dose distributions on the central axis were calculated for various energies (60Co, 4‐, 8‐, 15‐, and 24‐MVp photons), weighting factors, and field separations. Criteria for judging the usefulness of weighting were (i) that variation in the dose across the tumor volume not exceed 10%, and (ii) that the dose to critical tissues outside the target volume be at least 20% less than target volume dose. The clinical situation that met these criteria was a lateral lesion that extended to less than one‐third of the transverse diameter of the head, treated with either 60Co, 8, or 4 MVp with 4:1 weighting. Under these conditions, weighting was considered to be justified.
Dosimetry of 137Cs sources with the Fletcher–Suit gynecological applicator
Authors: Saylor William L. , Dillard Margaret
Published: August 2003

Measurements were made in water about the tandem and ovoid structures of the Fletcher–Suit applicator loaded with a 137Cs source. The dose rate was found to vary as much as 25% as a result of differential absorption in the ovoid. Empirically based trigonometric expressions are suggested which provide the absorption correction as a function of the two angles of a spherical coordinate system. Computer‐calculated dose rates, with and without the absorption corrections, are compared.
Phase effects in diagnostic radiological images
Authors: Gray Joel E. , Trefler Martin
Published: August 2003

The effect of phase on the quality of diagnostic radiological images is investigated on a theoretical basis and with the use of the computer simulations. Phase effects are divided into two categories: linear phase shift and asymmetric phase shift. Using a model of the x‐ray focal spot distribution composed of two Gaussian functions, it is shown that the phase transfer function (PTF) is generally a continuous nonlinear function of spatial frequency. The linear phase shift serves only to shift the location of the image. The asymmetric phase shift in the model studied tends to accompany an improvement in the modulation transfer function (MTF) in the midfrequency region while introducing “wows” into the image. Contrast reversals are shown to be inherent in line spread functions (LSFs) which do not necessarily include linear or asymmetric phase effects, i.e., they are the result of another phenomenon.
The line spread function and modulation transfer function of a computed tomographic scanner
Authors: Judy P. F.
Published: August 2003

A simple method to measure the line spread function of computed tomographic scanners has been developed. The line spread function of the EMI‐Scanner was measured and the MTF of the scanner was calculated. The reproducibility of the method indicated this technique could be utilized to compare the spatial resolution of various computed tomographic scanners.
Calculated neutron dose rates and flux densities from implantable Californium‐252 point and line sources
Authors: Shapiro A. , Schwartz B. , Windham J. P. , Kereiakes J. G.
Published: August 2003

The results of neutron‐transport flux‐density and dose rate calculations for implantable Californium‐252 point and line sources in essentially infinite tissue‐equivalent material are presented. The point‐source flux densities were obtained from a discrete ordinates calculation, and the point dose rates were established by multiplying the flux densities by their appropriate kerma factors. Line‐source dose rates were evaluated by integrating the point dose rates over the length of the line source. Dose‐rate data are given within a 20×20‐cm region from the source center for source lengths of 1.5, 2, and 3 cm. The dose rates established by these calculations showed good agreement with an independent Monte Carlo calculation. Detailed point‐source flux‐density data as a function of energy and position are also given.
Design of flattening filters for the fast‐neutron beam at TAMVEC by use of decrement lines
Authors: Otte V. A. , Smathers J. B. , Wright R. E. , Smith A. R. , Almond P. R.
Published: August 2003

Isodose distributions in a tissue‐equivalent phantom produced by fast neutrons from 50‐MeV deuterons incident on a thick beryllium target exhibit strong forward peaking, particularly for large fields. The design by use of decrement lines and the construction of polyethylene filters used to “flatten” those distributions are discussed and the results are illustrated. Also, the compromises of central‐axis attenuation versus effective filter width and of off‐axis peaking versus depth of “flattening” are discussed.
Calibration in water versus calibration in air for Cobalt‐60 γ rays
Authors: Grant W. H. , Cundiff J. H. , Gagnon W. F. , Hanson W. F. , Shalek R. J.
Published: August 2003

In the United States it is common practice to calibrate Cobalt‐60 teletherapy machines ’'in air,” despite recommendations by the International Commission on Radiation Units and Measurements (ICRU) and other organizations that calibration be accomplished by measurement at 5‐cm depth in a water phantom. A comparison has been made between the results of ionization measurements in air at 80.5‐cm distance from the source and in water at 80‐cm source–skin distance (SSD) for the determination of absorbed dose at three depth (5, 10, and 15 cm) for each of three field sizes (6×6, 10×10, and 20×20 cm2), for a total of 42 Cobalt‐60 machines. The mean of the ratio, absorbed dose from in‐water measurements to absorbed dose at the same depth calculated from in‐air measurements, ranged from 1.031±0.013 at 15‐cm depth for a 6×6‐cm2 field size to 1.009±0.007 at 5‐cm depth for a 20×20‐cm2 field size. Reasons for the differences are offered, and compliance with ICRU recommendations is suggested.
Effect of asymmetric focal spots in angiography
Authors: Burgess A. E.
Published: August 2003

The effect of asymmetric focal spots in blood vessel imaging is investigated by computer simulation. Phase distortion effects are studied in isolation from MTF effects. Calculations are done for a number of symmetric and asymmetric foci with identical standard deviations. It is concluded that blood vessel image degradation due to focus asymmetry per se is small and that the LSF standard deviation is a useful size parameter for both symmetric and asymmetric foci.
Photoactivation ratios for specification of high‐energy x‐ray quality: Part I, 15–40‐MV x rays
Authors: Nath Ravinder , Schulz R. J.
Published: August 2003

A sensitive and relatively simple method for obtaining an index of the spectral quality of high‐energy x rays is presented. The method is based upon the use of photoactivation threshold detectors. Photonuclear reactions produce measurable amounts of radioactivity for radiation exposures of several thousand roentgens. Since the threshold energies for photonuclear reactions vary for different nuclei, the ratio of radioactivity induced in two appropriately selected foils is very sensitive to the x‐ray spectrum. Photoactivation ratios (PAR) have been measured for 20–35‐MV x rays using Co, Cu, Y, Zr, and Au activation foils. It is shown that the PAR method offers a sensitive and practical means for quality control of x‐ray spectra, comparison of high‐energy accelerators, and the measurement of variations of spectral quality control at different points in an irradiated volume.
Fast Fourier digital quantum mottle analysis with application to rare earth intensifying screen systems
Authors: Wagner Robert F.
Published: August 2003

The advent of fast Fourier techniques has greatly facilitated the digital analysis of noise power spectra (Wiener spectra) by circumventing the need for the autocorrelation function. We are now able to Fourier analyze film data at about the same rate the microdensitometer–computer system can collect it (1000 points/sec). The new technique has been applied to the analysis of the quantum mottle of several rare earth intensifying screen systems confirming earlier estimates from our pilot studies that such screens are capable of reducing exposure by a factor of about 2 with imaging parameters comparable to those of conventional calcium tungstate systems.
Response of tissue‐equivalent ionization chamber to l5‐MeV neutrons
Authors: Barschall H. H. , Goldberg E.
Published: August 2003

The calibration (in rad/nC) of an air‐filled tissue‐equivalent ionization chamber for neutrons of l5‐MeV average energy was determined by measuring absolute fluences and calculating the kerma per unit fluence. The neutron calibration determined from the γ‐ray calibration and application of the Bragg–Gray relation was found to be 4% higher than that based on the fluence measurement. Additional data were taken to obtain calibration factors with tissue‐equivalent gas; the same difference betwen the two methods was observed.
Backscatter factors in the mammographic energy range
Authors: Dubuque Gregory L. , Cacak Robert K. , Hendee William R.
Published: August 2003

Backscatter factors for kVps and half‐value layers typical of mammographic beams have been determined as a function of beam diameter and half‐value layer. The method utilized thin TLD‐100 chips with the scattering medium alternately present and absent. Results indicate that the field‐size dependence of the backscatter factor for lower half‐value layers is less than that shown in the British Journal of Radiology Supplement 11.
Dosimeter placement in the Rando phantom
Authors: Archer Benjamin R. , Glaze Sharon , North Luceil B. , Bushong Stewart C.
Published: August 2003

Each section of the Alderson Rando phantom contains a tissue‐equivalent plastic coating layer approximately 2 mm thick, applied to both faces. This compensates for material removed in the sawing process. Conventional use of thermoluminescent dosimeters positions them totally or partially within the coating layer. Analysis shows that, in the lung region, dosimeters placed in this layer received a dose averaging 39% lower than those placed at midsection. Where bony structures interfere, some dosimeters in the coating layer received an 18% higher dose than those at midsection. Therefore, positioning dosimeters at the center of a section is recommended.
Validity of computer simulation of blood vessel imaging in angiography
Authors: Doi Kunio , Loo Leh‐Nien , Rossmann Kurt
Published: August 2003

The computer‐simulation technique for imaging of vessels in angiography is being recognized as a useful tool for the study of the effect of physical parameters on the vessel image. We have examined the validity of this technique by comparing the computed image distribution directly with experimental results. The input x‐ray pattern of a blood vessel phantom, the LSF of the focal spot, and the LSF of a screen–film system were measured. From the data obtained, the final image distributions of the phantom were computed and compared with the experimentally determined distributions. A high‐precision device for the alignment and positioning of the vessel phantom was employed in the experiments. Results indicate that computer simulation can correctly predict the image distribution of the blood vessel phantom.
Corrections to absorbed dose calculations for tissue inhomogeneities
Authors: Sontag Marc R. , Cunningham J. R.
Published: August 2003

Traditional methods for correcting for the presence of tissue inhomogeneities may produce errors as great as 10% at points within or close to the inhomogeneity. A more accurate method is presented which employs tissue–air ratios raised to some power dependent on the relative electron densities of the inhomogeneities involved. Corrections may be made for points that lie within or below an inhomogeneity as well as for multiple inhomogeneities. Measurements were made in phantoms containing aluminum or cork inhomogeneities. Agreement between measured and predicted results was usually within 2%–3%.
Satellite digital display for the Clinac‐18
Authors: Stevens Marcus D. , Rosen Isaac I. , Lane Richard G.
Published: August 2003

A satellite digital display of the gantry and collimator positions has been mounted on the Clinac‐18 console. This module provides simultaneous digital readout of the gantry angle, collimator rotation angle, upper‐jaw position, and lower‐jaw position. Continuous display of these parameters during the treatment is important in minimizing patient treatment errors.
Extended expression for tissue‐maximum ratio fitted to the Varian Clinac‐4 and Clinac‐6 accelerators
Authors: Chiu‐Tsao Sou‐Tung , Lerch Irving A. , Barish Robert J.
Published: August 2003

An expression which generates tissue maximum ratio (TMR) data for the Varian Clinac‐4 and Clinac‐6 accelerators is presented. This semiempirical expression can be programmed easily for radiotherapy computer systems or the new generation of scientific calculators. Our technique leads to good agreement with the published data, as demonstrated by a total average percent deviation of the measured TMR values of −0.115% for the Clinac‐4 and −0.002% for the Clinac‐6.
A simple source of fluorescent x rays for the study of radiographic imaging systems
Authors: Vyborny Carl J. , Doi Kunio , Metz Charles E. , Haus Arthur G.
Published: August 2003

The properties of a simple source of fluorescent x rays were investigated with regard to its suitability for the study of radiographic imaging systems. The source, consisting of a diagnostic x‐ray tube, fluorescent targets, and filters, was found to yield highly monoenergetic x‐ray fluxes with intensity sufficient to allow the speed of medium‐ and high‐speed screen–film systems to be studied as a function of x‐ray energy.
Silicon diode detectors used in radiological physics measurements. Part II. Measurement of dosimetry data for high‐energy photons
Authors: Wright Ann E. , Gager L. David
Published: August 2003

Initial calibration of a linear accelerator requires physics instruments to measure accurately central axis depth‐dose and off‐axis data, both in and out of the beam. These data for an 8‐MeV unit were first measured using film, a Farmer 0.6‐cm3 ion chamber, a 0.3‐cm3 ion chamber, and a 0.1‐cm3 silicon diode. Both small probes and film gave a high response compared to the Farmer probe, which has a uniform energy response. Measurements with the diode interfaced to an X–Y recorder required only a fraction of the time required with the chambers, minimizing error due to change in machine output, and permitted resolution of isodose lines in the penumbra. However, corrections required at points in depth due to nonuniform energy response of the unshielded diode were laborious. Construction of a partially shielded diode which duplicates the response of the Farmer probe eliminated the necessity for corrections, permitting rapid accumulation of a wide range of depth‐dose and off‐axis data.
Extrapolation of linear attenuation coefficients of biological materials from diagnostic‐energy x‐ray levels to the megavoltage range
Authors: Payne William H. , McDavid William D. , Waggener Robert G. , Dennis Michael J. , Sank Victor J.
Published: August 2003

A dual‐energy algorithm is used in determining the effective atomic number, atomic density, and electron density of biological substances. These quantities are then used to calculate linear attenuation coefficients at the megavoltage level. The validity of the method is checked several ways, including a comparison of extrapolated values with experimental data reported by Rao and Gregg where linear attenuation coefficients at 60 and 122 keV are used to extrapolate to coefficients at 662 keV. Except for a few instances, the extrapolated values agree quite well with the reported experimental values. This method is also used to calculate coefficients at the 60Co range, and these are compared with experimental values measured in water and various types of tissue‐equivalent materials. An additional algorithm is developed to extrapolate coefficients in water and bone up to 10 MeV. These quantities are compared with accepted values previously reported in the literature.
Photon energy distribution of some typical diagnostic x‐ray beams
Authors: Fewell Thomas R. , Shuping Ralph E.
Published: August 2003

A high‐purity germanium spectrometer system was used to determine primary x‐ray spectra over the 45–90‐kVp region. Methods were devised for producing and examining spectra simulating diagnostic conditions without operating the x‐ray generator at high current levels. The techniques used to correct the experimental data and produce a photon fluence spectrum are discussed. The results, presented graphically and in tables, have been normalized to yield the relative number of photons per 2‐keV interval. Methods for converting a normalized spectrum into a photon fluence spectrum that will produce an exposure of 1 R are presented. The analytical model and procedures used to calculate the K‐escape fraction are discussed.
Proton–beryllium neutron production at 25–55 MeV
Authors: Johnsen S. W.
Published: August 2003

The production of neutrons by stopping 25‐, 35‐, 45‐, and 55‐MeV protons in beryllium has been studied. The yield of neutrons in the forward direction can be approximated by the relation: yield =−0.032×1016⋅E+0.0027×1016⋅E2 neutrons/C sr. The neutron spectra are characterized by a low‐energy evaporation component which has small angular dependence and a high‐energy component which is peaked in the forward direction. Neutrons produced in the forward direction have an average energy approximately 40% of the incident proton energy.
Scattered radiation from a neutron collimator
Authors: Attix F. H. , August L. S. , Shapiro P.
Published: August 2003

Fast‐neutron beams are being employed in radiotherapy trials and associated radiobiology studies at numerous centers in the U.S., Europe, and Japan. Since collimated beams of various sizes and shapes are employed, it is desirable to know the composition of the scattered radiation component contributed by the collimator. A simple method is shown for deducing the field composition in terms of a three‐component model, from measurements made with three ionization chambers (tissue‐equivalent, graphite, and magnesium). The dose contributed by the scattered radiation in the present example was found to be predominantly due to fast neutrons indistinguishable from those in the primary spectrum (from 35‐MeV D+ on Be). This method may prove useful for measurements in phantoms as well.
Laser “cross‐hair” sidelight
Authors: Boyer Arthur L.
Published: August 2003

A technique is described for forming an optical laser cross‐hair image for use as a radiotherapy sidelight. The technique is developed from a concept introduced earlier by which a line image is formed from a laser beam using a cylindrical lens. By using two cylindrical lenses and appropriate reflectors, two line images can be made to form a cross.
Linearizing mechanisms in conventional tomographic imaging
Authors: Orphanoudakis Stelios C. , Strohbehn John W. , Metz Charles E.
Published: August 2003

Implicit in the concept of conventional tomography and in any attempt to characterize the tomographic process by a modulation transfer function is the assumption that the tomographic process is linear. A Fourier decomposition approach and an analysis of nonlinear contributions to the integrated tomographic image intensity are used in this paper to establish the validity of this assumption and to determine the mechanisms by which the tomographic process is effectively linearized.
Photon and electron response of silicon‐diode neutron detectors
Authors: McCall Richard C. , Jenkins Theodore M. , Oliver George D.
Published: August 2003

The photon response of silicon‐diode neutron detectors is analyzed theoretically and measured in the 15–25‐MeV region. The main mechanism for producing a response in the diode is shown to be the displacement of silicon atoms by scattering of electrons. If the photon source is an electron accelerator target, the response is mostly due to electrons originating in the target with a smaller contribution from electrons produced in the diode by photons generated at small angles to the beam.
Image information content and patient exposure
Authors: Motz J. W. , Danos M.
Published: August 2003

Presently, patient exposure and x‐ray tube kilovoltage are determined by image visibility requirements on x‐ray film. With the employment of image‐processing techniques, image visibility may be manipulated and the exposure may be determined only by the desired information content, i.e., by the required degree of tissue‐density discrimination and spatial resolution. This work gives quantitative relationships between the image information content and the patient exposure, give estimates of the minimum exposures required for the detection of image signals associated with particular radiological exams. Also, for subject thickness larger than approximately 5 cm, the results show that the maximum information content may be obtained at a single kilovoltage and filtration with the simultaneous employment of image‐enhancement and antiscatter techniques. This optimization may be used either to reduce the patient exposure or to increase the retrieved information.
An automated dosimetry data‐acquisition and analysis system at the LAMPF pion therapy facility
Authors: Rosen I. , Smith A. , Lane R. , Kelsey C. , Lake D. , Hogstrom K. , Somers J. , Helland J. , Kittell R. , Amols H. , Bradbury J. , Richman C.
Published: August 2003

An automated data‐acquisition and analysis system has been developed for dosimetry measurements on the pion therapy beam at the Clinton P. Anderson Meson Physics Facility Biomedical Channel in Los Alamos using a PDP‐11/45 computer and CAMAC interface. Initialization, test, and monitor programs allow the user to set the physical limits of scanner travel, test the data lines, calibrate the analog signals for the scanner position, and monitor the analog versus digital values of the scanner position during operation. Data‐acquisition programs scan beams in one, two, and three dimensions. Many options are available to the user in selecting the scan parameters and in changing some of these parameters during scanning. Data‐analysis programs provide reproduction of stored data, comparison of linear scans, beam profiles along any line of a planar or volume scan, and isodose distributions from any planar scan or from any plane of a volume scan. Other programs summarize stored data files and search for specific data according to the user's instruments.
An evaluation of the performance characteristics of different types of collimators used with the EMI brain scanner (MKI) and their significance in specific clinical applications
Authors: Thomas Stephen R. , Schneider Andrew J. , Kereiakes James G. , Lukin Robert R. , Chambers A. Alan , Tomsick Thomas A.
Published: August 2003

The characteristics of various types of collimators have been studied systematically for the EMI Mark I brain scanner. The degree of collimation ranged from the 13‐mm adjacent A and B slices to 3‐mm separated slices. Multiple phantom studies (including variable density immiscible liquid interfaces) have been performed to evaluate the effect of collimation on (a) the line‐spread function, (b) the change in effective absorption number, and (c) volume averaging. Effects associated with the technique settings involving the kVp, the mA, and the scan time were also investigated. The results of TLD dose measurements as a function of collimation are presented. The significance of collimation in specific clinical studies involving small, fairly well‐circumscribed lesions is discussed.
Dynamic nuclear‐medicine image display system using standard multiformatter images
Authors: Walton Philip W.
Published: August 2003

There is much interest in visualizing dynamic physiological function using nuclear‐medicine techniques. In particular, these methods are used in cardiac studies, but it is likely that many other functions will be better visualized with cinematographic‐type displays. A new instrument is described which takes a sequence of images, records on a single sheet of film by standard formatting devices and, by means of digitally controlled mirrors, projects them in rapid sequence onto a screen to produce the dynamic movie display.