Primary Rays Research Articles

Coherent scatter in diagnostic radiology.

Coherent scatter is often ignored in diagnostic radiology because its cross section is relatively small, and because it is assumed to be indistinguishable from primary radiation. Single-scatter calculations, however, show that coherently scattered photons diverge sufficiently from the primary ray to degrade image contrast, and that they account for a significant fraction of the total scattered energy fluence at the image receptor. Grids and large air gaps are less effective in reducing coherent single scatter than incoherent and multiple scatter. For radiography of the abdomen, coherent first scatter comprises 10% of total scatter and 26% of the primary fluence before a grid, and on the order of 22% and 7.5%, respectively, behind a grid. Coherent first scatter comprises a higher fraction of the total amount of scatter for lower energy examinations such as mammography.

A comparison of the generalized ray and reflectivity methods of waveform synthesis

Summary The generalized ray and reflectivity methods of waveform synthesis have been compared for realistic models of the upper mantle and of the oceanic crust. Both methods entail some approximations, but the one which requires the most careful testing is the truncation of the generalized ray sum. It has been shown in this study that for the types of time windows typically used in waveform modelling the generalized ray sum converges very rapidly to the reflectivity result. In most cases, it is only necessary to include the primary rays in the sum. Additional internal multiples are sometimes necessary if the model contains thin high gradient zones. The primary rays alone do not always correctly predict the amplitude decay of the supercritical reflection from the gradient. Multiples are also required to avoid long-period errors and truncation arrivals if very shallow travelling energy arrives within the time window of interest. It has also been shown that a number of fast approximate methods provide a very cost efficient alternative to the two more exact methods. The best approach to trial and error modelling studies appears to be to begin with a fast approximate method, to advance to generalized ray theory for the final iterations and to use reflectivity to test and to store the results.

THE LARVAL INSTARS OF THE BLACK FLIES STEGOPTERNA MUTATA AND SIMULIUM VITTATUM (DIPTERA: SIMULIIDAE)

AbstractThe six larval instars of Stegopterna mutata (Malloch) and seven larval instars of Simulium vittatum Zetterstedt can be definitively separated by differences in postgenal length, body length, and number of primary cephalic fan rays. Counts of the number of rows of hooks and hooks/row in the anal circlet were inconclusive in separating all instars of either species.

Morphology, Anatomy, and Taxonomic Relationships of Otopappus australis (Asteraceae)

A collection of Otopappus australis from Cerro Campana represents the first report for the genus from Panama. Aspects of the morphology at variance with the type description include longer and narrower leaves and narrower flowering heads. Details of stem and leaf anatomy are reported; exceptionally tall primary wood rays probably relate to the plant's vine-like habit. A variety of primitive and specialized features are found in the floral anatomy. Unusual topographic sites for trichomes include pappus awns, inside the corolla tube, anther appendages, and style branches. The junior authors conclude that this species is more properly a Zexmenia rather than an Otopappus. Geographical and morphological data are uti- lized to demonstrate that 0. australis, Z. columbiana, and Z. mikanjoides are conspecific. The latter two are considered Zexmenia mikanioides var. mikanioides, and the nomen- clatural combination is made to make 0. australis a variety of Z. mikanioides. The genus Otopappus Benth. belongs to the subtribe Verbesininae of the Heliantheae (Asteraceae) and contains about 15 species as presently understood (Hartman and Stuessy, unpubl.). Most taxa are found from western Mexico to Costa Rica. The closest generic relatives are Notoptera Urb. and Salmea DC. (Blake, 1915a; Stuessy, 1977). McVaugh (1972) suggested that the generic distinctiveness of Otopap- pus and Notoptera may be specious and recent revisionary studies (Hart- man and Stuessy, in prep.) have shown that the taxa are congeneric. The genus is unified by its viny habit and winged awns of the achenes. How- ever, one species, 0. australis, is isolated morphologically and geograph- ically. This taxon was originally described by Blake (1924) as a relative of 0. verbesinioides Benth. and as the first representative of the genus from South America. Features of the achenes of 0. australis differ from other species of the genus and suggest possible affinities with other gen- era. Geographically, 0. australis is known from Venezuela and Ecuador, which are outside the known range of all other species of the genus (0. simplex Badillo was also described from Venezuela, but it does not belong in Otopappus; details of that relationship will be described elsewhere). Interest in Otopappus australis has developed independently with the senior author finding material new to Panama, and the junior authors determining specific and generic relationships. Discovery of each other's work suggested that the observations and interpretations be published

The age distribution of galactic cosmic rays

The general solution for the distribution of ages for primary galactic cosmic rays is given for a class of steady-state, bounded models of cosmic-ray diffusion in the Galaxy. Both one-and threedimensional models are considered, with point sources and distributed sources. In all models, the age distribution, is approximately exponential for ages longer than the average age, although for shorter ages the distribution depends on the details of the model. The leaky-box model, with an exponential age distribution, is thus a good approximation to most diffusive models. It is shown that one-dimensional (‘disk’) models are consistent with both age and anisotropy data for galactic cosmic rays regardless of whether production takes place near the galactic center or throughout the disk.

Energy calibration of a cosmic ray ionization spectrometer

The NASA/GSFC High Energy Cosmic Ray Experiment was calibrated at the AGS at Brookhaven National Laboratory during the summer of 1970 using protons and pions with energies from 9.3 GeV to 17.6 GeV. The best measure found for the energy E of an incoming primary particle is ΣI, the total number of ionizing particles observed in the instrument, summed over the various iron modules. The resolution in the calibration energy range is about ±30% (s.d.) over a wide range of incident angles and positions. The calibration function may be parameterized as E = ∑ I/ K, where K is predominantly a function of the location of the first interaction and the trajectory of the incoming particle. To a fair approximation the geometrical dependence of K can be encompassed by writing K as a function of d, the distance from the first interaction along the primary ray to the edge of the instrument. Empirically, K=5.83 [1−exp(− d/ λ)] cos θ, where λ is a characteristic length which is a slowly varying function of energy. The value of K, and thus the average energy values calculated from the experimental data are accurate to about ±10% under calibration conditions.

New Taxa, Names and Combinations in Cyperaceae for the 'Flora of West Tropical Africa'

Perennial up to 4 m. high. Stems acutely triangular, stout, up to 2 cm. across. Leaves up to 2 m. long, about 2 cm. wide. Inflorescence 40-50 cm. long, bisumbellate, sometimes lacking spikelets; bracts 10-12, lowermost up to 125 cm. long; primary rays 15-35 cm. long, triangular, scabrid above; secondary rays up to Io cm. long. Achene elliptical or narrowly obovate, o-8-I.o x o-3-o-45 mm., surface minutely puncticulate.

Wax retraction as a technique for compensating the effect of surface irregularities in high-energy radiotherapy.

The technique of filling an irregular body contour with tissue-equivalent wax and retracting the wax in the beam direction, without making an allowance for beam divergence, has been examined from the point of view of (a) electron contamination on the skin; (b) errors in depth dose which arise from loss of scatter and also from any change in the path of the primary ray. Skin contamination at a given point has been found to depend on the angle subtended there by the irradiated wax. This leads to the simple working rule that if a retracting distance equal to the average field dimension is chosen, the skin dose will be reduced to 50 per cent of its value before retraction. Depth dose errors, i.e., deviations from the values as read in the normal isodose chart, have been found to depend chiefly on retracting distance and the thickness of the retracted wax. The technique is feasible provided a retracting distance can be found which gives an acceptable skin dose and small depth dose errors. These may be estimated from the appropriate graphs. The conditions are most easily fulfilled for small fields.

A Distortion Camera For Ray Tracing

Ray tracing is a powerful technique for realistic image generation. Typically a simple camera definition is used, whereby a 3D environment is mapped onto an image plane either by an orthographic or perspective projection. The concept of the usual simple camera definition is extended in several ways to achieve distorted views or projections of the object scene. The origins of primary rays are not required to lie on a plane anymore. Thus projections onto curved image surfaces are possible. The directions of primary rays may be chosen according to various nonlinear functions that allow nonstandard projections of the environment. The concept of centers of interest (coins)is introduced which enables to concentrate on especially interesting portions of object space. A center of interest is a 3D position that either distorts (attracts) a portion of the image surface or locally influences the directions of primary rays. The results of a test implementation are presented to show the feasibility of the methods presented. Several applications of distorted images are: creating elemental holographic images and raster omnimax images, virtual reality and arts.

Beugung im inhomogenen Primärstrahlwellenfeld. I. Prinzip einer Phasenmessung von Elektronenbeungungsinterferenzen

In addition to the amplitude information, phase information can also be obtained from a generalized diffraction experiment (in spite of the inability of the detector to observe phases directly) on condition that the primary ray can be controlled in a defined way. A single diffraction pattern suffices for the determination of the phases except in the case of a centre of symmetry in real space, and the combination of at least two patterns removes this ambiguity.

Electron Flux Spectra and Radiation Yields in Solids Irradiated by Beta Rays Invited Paper

During the last few years a program has been under way at Oak Ridge National Laboratory to measure the electron flux in an irradiated medium. A typical source consists of several curies of beta radioactivity uniformly dispersed in a metal, the whole approximating a homogeneous, uniformly and isotropically irradiated medium. The electron flux escaping from a cavity within the medium is measured over the energy range from about 50 keV down to 1 eV. At the higher energies the flux observed is due entirely to the beta rays from the source, whereas below 10 keV the flux rises by as many as five orders of magnitude, this rise being due to the generation of secondary, tertiary, quarternary and higher generations of electrons. The measurements are in approximate agreement with the Spencer-Fano theory of electron slowing down above ten keV, but tend to indicate that the theory underestimates the flux below this energy by as much as a factor of four. Theoretical ionization and plasmon cross sections for aluminum have been combined with our flux data to calculate the probability of occurrence of a given type of event from a beta ray from a copper-64 or gold-198 source. Calculations show that most K-ionizations (~3) are caused by the primary beta ray whereas L-shell ionizations and plasmon excitations tend to come from the secondary electrons (~300 for the former and ~20,000 for the latter).

Spectrum of 198Au Beta Rays in Gold

The Keplertron, a spherical electrostatic electron spectrometer, was used to obtain he slowing-down spectrum of beta rays and their secondary electrons in gold. Two gold discs 0.82 cm in diameter and 0.025 cm thick were irradiated in the LITR at ORNL to about 25 c. One disc was placed tangent to the inner Keplertron sphere and the other parallel and coaxial at a distance of 0.24 cm. The cylindrical cavity thus formed approximated a “black body” source with the electrons emerging from the open cylindrical sides being representative of flux in an infinite, isotropic, uniformly irradiated medium. Electron flux measurements were made from 1 eV to 63 keV. The spectrum,. corrected for surface barrier transmission and spurious spectrometer response due to scattering, was compared on an absolute basis with the simplified continuous slowing-down model of SPENCER and ATTIX. Flux varies from 2.9 × 10−8 electrons cm−2 eV−1 per primary beta ray cm−3 at 63 keV to 1.2 × 10−4 at 11 eV above the bottom of the conduction band.

Anisotropy of Galactic Cosmic Rays and the Interplanetary Magnetic Field

IT has been demonstrated1,2, using data from high counting rate instruments, that the daily variation of secondary cosmic ray intensity measured on the ground can in general be related on each day to an anisotropy of primary galactic cosmic rays. The anisotropy can vary from day to day in magnitude, in the directions with respect to the Earth–Sun line of maximum intensity Tmax and of minimum intensity Tmin outside the influence of the geomagnetic field, and in the energy spectrum of variation. Tmax and Tmin are usually separated by 8–10 h. This has led Rao and Sarabhai to suggest that the anisotropy appears generally to be due to two principal processes, one a modulation equivalent to a pair of oppositely directed positive and negative sources, and the other a negative virtual source approximately oriented along the garden hose direction towards the Sun.

Line scattering images on crystals of NaCl with nickel. II

Using the sifrin integro-differential equation an expression was found for the intensity of the diffracted radiation of a prism, the conditions were found for the formation of a line scattering image, the shape of the line scattering image of a rod in different positions with respect to the primary beam was determined and it was proved that the form of this scattering image does not depend either on the refractive index of the rod or on the wave-length of light of the primary ray. A comparison with the experimentally measured line images showed that in an NaCl crystal with nickel added there are rod-like formations (probably formed by nickel chloride), their principal dimension being oriented uniformly in all equivalent directions (110) of the lattice of the basic substance. The minimum length of the rods was estimated at above 20 000 a and the maximum transversal dimension of the rod below 2500 a.

Gamma-radiation attenuation for measuring bulk density and transient water flow in porous materials

A laboratory method, based on gamma -radiation attenuation, is described for studying transient water flow in packed soil columns. The gamma rays from a 200-mc source of Cs/sup 137/ are collimated through a narrow slit, and the beam is then focused on a soil column. A radiation analyzer in the circuit allows only primary rays of energy 0.66 plus or minus 0.15 Mev to be recorded. Both bulk density (g/cm/sup 3/) and water content of the soil (g/cm/ sup 3/) can be calculated using an absorption equation. The method makes possible rapid and frequent measurement of the water content of soil as a function of time and position without disturbing the soil-water system. Experimental results for water moving into a uniformly packed air-dry soil are presented. (auth)

X-ray transmission factor in estimating bone density.

Bone density, which is affected by the dietary regimen and certain pathologic situations, has become an extremely important measurement in nutrition and biologic research. Although bone ashing technics have been in common use for a long time, measurements of bone density in vivo have been practiced only in recent years. Mack (2, 3), who in 1927 began to use a radiographic photometric method for assessment of the mineral content of bone, has made many valuable contributions in this field. Other investigators have used variations of her technic for measuring the changes in mineral content of bones in man and in experimental animals. The photometric method of Mack has been developed to a high degree of precision' and quantitative bone density values are derived through the use of a recently described semiautomatic integrating microphotometer (4). However, since large amounts of film are consumed in that method, an alternate system which would not require the exposing and processing of film would be advantageous. With this in view, a device which is capable of rapid measurements of the x-ray transmission factor of bone without the use of film has been constructed in this laboratory and will be described herein. Since x-ray transmission is related in a known manner to the density of bone, the transmission factor obviously becomes useful in detecting changes in skeletal mineralization status. The device is similar to that used for measuring linear x-ray absorption coefficients of elements. Figure 1 is a generalized diagram illustrating its principle. A very sharp, collimated and filtered beam of x-radiation is made to pass through the bone for which the transmission factor is to be determined. A short metallic tube on the opposite side of the bone conducts the residual primary rays to an x-radiation detector. The ratio of the initial intensity to the transmitted intensity is determined by comparing the detector readings with the bone in place and, again, not in place. The transmission factor is then determined by means of the equation TF = I/I0 (1) where I is the intensity after passing through the bone and I0 is the initial intensity. The lower the ratio of I to I0, the denser or thicker the bone. There are several complicating factors which affect the apparent x-ray absorption by bone. These will be discussed individually in the following section. Theoretical Aspects The x-ray transmission factor I/I0 is affected by the chemical composition and thickness of the absorber. According to its composition, a material will have a specific linear absorption coefficient which is numerically equal to the fractional reduction of beam intensity per centimeter of absorber.

On the cosmic ray penetrating showers underground

In 218.5 h of receptive time, 13 penetrating showers were obtained with a large multiplate cloud chamber, at the depth 50 m w.e. from the top of the atmosphere. Five of these showers were produced in lead plates in the chamber. The rate of the number of penetrating showers produced by secondary rays (probably π-mesons) to that by primary rays (probably μ-mesons) is turned out to be 2/3 ∼ 1/1. Therefore it seems impossible to neglect the contributions of secondary particles to the production of penetrating showers underground. The apparent cross section of μ-mesons for the penetrating shower production is estimated as (0.5±0.3)·10−30 cm2/nucleon and the interaction mean free path of secondaries is (157±90) g/cm2.

<i>Ammi visnaga</i> Lam. の試作栽培に就いて

Ammi visnaga Lam., the source of khellin is an annual or biennial crop, which can be cultivated in Japan. The content of khellin in the fruit, as determined by the present authors by the modified method of Abd El-Aziz Abd El-Rahman, was 0.1%, but none was detected from undergrown fruits, and primary and secondary rays of compound umbels. It seems to be more profitable and desirable to cultivate this plant as a biennial crop by sowing in autumn, in Japan. The cultivation of this plant is practically possible by means of transplantation, too, which should preferably be made by the end of April at the latest. The autumn sowingshould be done by the middle of October and spring sowing by the beginning of April at the latest. The cultivation of this plant is suited in a rich and well-drained soil, containing a large amount of clay.

Directional Measurements of the Diurnal Variation of Cosmic-Ray Intensity

Measurements of the solar daily variation in cosmic-ray intensity have been made for a total of 360 days using two arrays of Geiger-Muller counters inclined at 45° to the vertical in the North and South directions. Both the diurnal and semi-diurnal variations are found to be significantly different for the two directions, and since the particles recorded by the two sets have passed through the same amount of atmosphere under similar conditions, it must be supposed that at least a part of the variation is due to causes which lie outside the earth's atmosphere. It is found that both the amplitude and phase of the daily variation for the South-pointing recorder vary with season, whereas in the North direction the amplitude only shows any seasonal variation. The amplitude is greatest in summer in both directions. The experimental results are compared with those of earlier workers and are discussed in relation to some of the interpretations of the daily variation which have been put forward. The difference between the variations in the two directions is interpreted as evidence for a non-isotropic distribution of the primary cosmic rays in space. A variation with sidereal and not with solar time would be expected under these conditions, but it is pointed out that the magnetic field of the sun may modify the direction of arrival of the primary rays at the earth so as to produce a resultant solar time variation.

The Effect of Cosmic Rays on Meteorites

AbstractAn investigation has been made into the effect upon meteorites in space of the primary rays in the exceedingly strong cosmic radiation that falls upon these bodies before they penetrate the Earth's atmosphere. Specimens of meteorites with terrestrial ages (i.e., intervals since time of fall) ranging from 9 months to 457 years were studied by means of the nuclear‐emulsion‐plate technique.The oldest meteorite studied (Ensisheim, Alsace, France) was found to be inactive, and the next oldest (Pitts, Georgia) was only very slightly active. On the other hand, the relatively young Walters, Oklahoma, meteorite showed considerable activity, and a metallic inclusion and one piece of crust from the very recent Norton‐Furnas Counties, Kansas‐Nebraska, achondritic fall of 1948 February 18 were very active indeed. Another piece of Norton crust showed only slight activity, and two other fragments of this achondrite were inactive.Examination of the nuclear “stars” recorded on the emulsion plates shows that the activity observed in the specimens studied is an induced one and cannot be attributed to the low content of natural radioactive elements present in the meteorites. Tentative explanations of this induced activity are advanced in the paper.