Incident X-ray Energy Research Articles

Soft X-ray spectroscopy with a scintillation detector

Spectroscopy, especially EXAFS, in the soft X-ray region has been limited by many factors. Among them the lack of adequate detectors is an important one. Recently scintillation detectors have been shown to provide high signal to noise ratio and smooth spectral response in this energy range [1,2]. P31 outperforms many other scintillators in the energy range 300–1000 eV. In this work, the basic properties of P31 are studied: its response is linear in the range of 10 7–10 10 photons/(s mm 2), its optical spectrum is independent of the incident X-ray energy. A gas phase and a solid state experiment employing these detectors are presented. The oxygen K-edge spectra thus obtained enabled EXAFS analysis in this energy range.

A semi-empirical model for the X-ray Si(Li) detector response function

The entire response function model for a Si(Li) detector over the range from about 4 to 20 keV has been developed. The values of the model parameters obtained by the least-squares fitting of the pulse-height spectra from a number of pure element samples excited by a 109Cd X-ray source are fitted to simple functional forms of the incident X-ray energy to produce the generalized response function for this Si(Li) detector. An interactive computer program has been devised and implemented on a VAX 11/750 minicomputer for determining the parameter values from a pulse-height spectrum of the X rays from a pure element sample. An unexplained additional feature between the K α and K β X-ray peaks is observed, which necessitates using different sets of parameter values to describe some of the features that are associated with the K α and K β X-ray peaks.

Physical characteristics of scattered radiation in diagnostic radiology: Monte Carlo simulation studies.

We applied Monte Carlo methods for the simulation of x-ray scattering in water phantoms. The phantom thickness was varied from 5 to 20 cm, and the monoenergetic incident x rays were varied from 15 to 100 keV. Eight screen pairs and a total absorption system were used as x-ray receptors. We determined the angular, spectral, and spatial distributions of the scattered radiation and the scatter fractions recorded in the image plane. The dependence of these properties on the incident x-ray energy, the phantom thickness, and the energy response of the recording system was examined. The results of this study provide useful information for the development of antiscatter techniques and for the evaluation of radiographic procedures.

Investigation of Energy Responses of Germanium Detectors and Correction of Measured Spectra by Means of Monte Carlo Simulation

A Monte Carlo program was developed which determines the energy response of X-ray detectors of various "nested" sizes simultaneously. Twenty-one planar germanium detectors with cylindrical crystals were chosen, which are commercially available for X-ray spectrometry. The photopeak efficiency, K alpha and K beta escape fractions. Compton fraction, elastic fraction, penetration fraction, and the Compton continuum were tabulated as a function of incident X-ray energies from 12 to 300 keV in 2-keV increments. Coefficients of polynomials fitted to the Compton continua were also calculated to facilitate computer implementation of spectrum corrections. Spectrum correction methods which use the Compton continua determined by Monte Carlo calculation were compared with those approximated by rectangles. Comparison of the residual root-mean-square (RRMS) values between the incident and corrected spectra showed that the former method yielded lower RRMS values. However, the differences in these values are small, indicating that the simple rectangular approximation may be adequate for most spectral measurements.

Relative efficiencies and physical characteristics for a selected group of X-ray phosphors

We have measured the energy dependent conversion factors and quantum efficiencies for a number of selected phosphors in response to 0.7 to 6.0 keV X-rays. We have investigated the effects of deposition techniques, optical attenuation, dead layer material and front surface aluminization upon the light yield of the phosphor. From these measurements a simple model was derived that enables us to predict the light yield of a phosphor over a wide range of incident X-ray energies.

Measurement of the low energy tail spectra adjacent to the x-ray photopeak in Si(Li) x-ray detectors

Low energy tail spectra, which are inherently observed adjacent to the X-ray photopeak in the use of a Si(Li) X-ray detector, have been measured for the 1.3–3.7 keV X-ray energy range. Three different Si(Li) detectors have been tested. The tail/peak count ratios as a function of incident X-ray energy have exhibited a rapid change at the boundary energy of the Si K absorption edge. From the observed tail/peak count ratios, the thickness of the Si layer which produces the tail spectrum has been estimated; the thicknesses for the three tested detectors are about 0.05, 0.1 and 0.2 μm. In addition to the carrier diffusion effect, it is proposed in the case of a Si(Li) detector that the effect of the escape (penetration) of Auger and phtoelectrons from (into) the intrinsic region plays an important role in the production of the tail spectra.

Inelastic X-ray scattering cross sections of Ne

Inelastic X-ray differential scattering cross sections of Ne have been computed to compare the effect of three non-relativistic approximations: Waller-Hartree, impulse and excited state. The same Hartree-Slater wavefunctions were employed throughout, and momentum transfer 0.1<or=K/4 pi <or=1.5 AA-1 was included. For momentum transfer below 0.5 AA-1 the Waller-Hartree and excited-state results were found to agree within 4%, whereas the impulse approximation results are up to 300% higher. The Compton profiles of the individual atomic shells were computed in the excited-state approximation for three values of incident X-ray energy and are found to vary in height and shape with energy and scattering angle, in contrast with the constant profiles predicted by the impulse approximation. Results of theoretical ratios of X-ray inelastic and elastic scattering cross sections are compared with data from an experiment performed with synchrotron radiation. Waller-Hartree and excited-state results agree fairly well with experiment.

Effect of collimators on the measurement of diagnostic x-ray spectra

One of the difficulties in the measurement of diagnostic x-ray spectra is the high fluence rate of the photons, which may exceed the capability of the spectrometer system. Pinhole collimators are often used which reduce the count rate to an acceptable level. The measured spectrum, however, may be distorted due to the penetration of photons through the collimator material. In this study, the effects of collimators on the measured spectra were evaluated. As an indicator of collimator performance, the authors determined the transmission equivalent aperture (TEA), through which a portion of the incident photons impinge on the detector. The dependence of the TEA on the geometry of collimation, pinhole area, collimator material and thickness, detector area and incident x-ray energy was investigated. The penetration correction factors (PCF) for the measured spectra were also derived. The authors obtained good correspondence between the theoretical spectra and the measured spectra which were corrected for collimator penetration by means of the calculated PCFs. These results indicate that diagnostic x-ray spectra generated at high tube current can be measured if one uses a collimator with a small aperture together with its PCF.

Wiener spectral effects of spatial correlation between the sites of characteristic x-ray emission and reabsorption in radiographic screen-film systems

When characteristic x-rays are generated and reabsorbed in the phosphor of a radiographic screen-film system, the positions at which light is emitted from the initial and secondary interactions are correlated. A simple statistical model is developed to account for the effect of this correlation on the Wiener spectrum of quantum mottle. Unlike previous models, which ignore spatial correlation, the new model predicts that not only noise magnitude but also noise texture is changed as the incident x-ray energy exceeds the phosphor K-edge.

Monte Carlo modeling of electrophotographic x-ray detectors

A Monte Carlo model for an electroradiographic detector is presented. It takes into consideration recombination as a loss factor inherent in the physics of the x-ray photon attenuation, but one whose importance decreases with increasing photon energy, owing to the increasing spatial dispersity of the hole-electron pairs, therewith. This treatment reasonably simulates both the applied field and the x-ray photon energy dependences observed experimentally for the apparent carrier pair production energy in amorphous Se and particulate, tetragonal PbO devices. An activation energy for free carrier generation in Se, despite the excess thermal energy with which the carriers are initially produced, is also required to make the model correspond with experiment in the Se case. Bonding theory suggests that this is an inevitable consequence of the disordered state of the amorphous radioconductor. Detective Quantum Efficiency estimates can be extracted from the Monte Carlo simulation. These data indicate that the primary loss factor in xeroradiographic detectors, from the standpoint of signal-to-noise ratio, is incomplete attenuation of the incident x-ray energy.

The location of redox centers in biological membranes determined by resonance X-ray diffraction. I. Observation of the resonance effect

We have developed resonance X-ray diffraction methods to locate for the first time intrinsic metal atoms associated with redox centers within biological membrane systems. The study of membranes containing dilute concentrations of resonant scatterers has been made possible by the development of synchrotron radiation sources of X-rays. The technique permits altering the scattering power of a particular atom relative to others by varying the incident X-ray energy. Thus, this method may be used to locate a metal atom within a complex integral protein without chemical modification of the membrane. We present resonance diffraction data taken with synchroton radiation for two different membrane systems: cytochrome oxidase incorporated into lipid vesicles and a photosynthetic reaction center-cytochrome c complex also reincorporated into lipid vesicles.

Excitation energy dependence of the photoemission spectra of metals

The x-ray photoemission spectra (XPS) of simple metals are investigated by using the methods of many-body perturbation theory. Several features of the XPS, such as the infrared catastrophe effect, the effect of intrinsic and extrinsic plasmon production, etc., have previously been considered in different approximations by various authors. In this paper, a unified theory, using a nonseparable core-hole conduction-electron scattering potential, is proposed to describe these features with special attention to the interference effects. The evolution of the shapes of the main line and the first plasmon satellite along with the background is considered as a function of the incident x-ray energy. Numerical results of the intensity and the shape of these lines are presented for Na and A1 for the incident photon energies of $5{\ensuremath{\omega}}_{p}$ and $90{\ensuremath{\omega}}_{p}$.

Relative efficiencies of energy to photographic density conversions in typical screen-film systems.

This study compared the speeds of six typical screen-film systems and the energy absorbed by their screen pairs as a function of incident x-ray energy. Three of the screen-film systems studied had calcium fungstate phosphors, one contained barium strontium sulfate, and two were composed of a mixture of lanthanum oxysulfide with larger amounts of gadolinium oxysulfide. A source of monoenergetic radiation was used for measurement of the relationship between these two quantities as a function of keV. With certain exceptions, the speeds of screen-film systems act in a manner similar to that which would be predicted if only the energy absorption properties of their screens were considered.

Surface analysis by electron spectroscopy at high pressures

Surface analyses are now made by techniques such as Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and appearance potential spectroscopy (APS). These techniques utilize low-energy electrons and have high surface sensitivity but cannot be used at high pressures [≳10−4 Torr(≳10−2 Pa)]. Optical techinques can be used at high pressures but their surface sensitivity is poor. It is proposed to combine these approaches by using the high-pressure gas as a converter. Two situations are considered. First, variable-energy x-rays are used to produce variable-energy photoelectrons from the gas which impinge on the sample; these electrons constitute a suitable source for APs. Second, the variable-energy x-rays produce photoelectrons from the sample (XPS) of variable energy; these electrons can be detected using the gas for APS. In both cases, the derivative of the x-ray yield from the gas-sample cell is measured as a function of incident x-ray energy; features in the derivative spectrum can be correlated with the core levels of surface atoms of the sample. Design calculations based on x-ray intensities from available sources indicate that the proposed method could only be useful for specialized applications.

Spatial resolution in a xenon filled MWPC X-ray imaging detector — A computing physics approach

In order to examine the physical limitations imposed on the imaging capability of a xenon filled (NTP) MWPC X-ray imaging detector, a physical model of the X-ray absorption processes is computed using Monte Carlo methods. Various instrumental (i.e. electronic) characteristics are also included in so far as they affect the spatial resolution. The model shows that the physical processes limit the image quality over the whole range of incident X-ray energy which is of interest in biomedical work. However, we find that a narrow band X-ray beam (∼10 keV fwhm) centred on ∼42 keV can yield spatial resolution comparable with MWPC wire spacings (∼0.5 mm fwhm) and the possibility of good imagins at a reasonable overall detection efficiency (3%).

X-ray scatter background signals in transmission radiography.

With monoenergetic x-ray beams incident on polystyrene phantoms, the spectra of the tramsmitted x rays were measured with a scintillation spectrometer. The scattered and unscattered components of the transmitted x-ray fluence at a point on the beam axis were determined as a function of (i) the incident x-ray energy (18, 22, 32, 49, 58, 69, and 660 keV), (ii) the phantom thickness (5.3, 10, and 21 cm), (iii) the scatter solid angle determined by the exposed area of the phantom and the separation distance of the image plane (0.090, 0.31, 0.66, 1.8, 3.5 4.3, 4.8, and 5.1 sr), and (iv) the beam diameter at the image plane (25, 17, and 10 cm). The results indicate that, as the incident x-ray energy decreases from 660 to 30 keV, the contribution of the scattered component to the transmitted fluence increases from approximately 50% to 90% for the 21-cm phantom and from 21% to 50% for the 5.3-cm phantom. For typical cases, the data show the effect of the scatter component on the ratio of the image to the background signals. In addition, the examples show that optimum conditions for maximizing this signal ratio may be obtained by a careful selection of the incident x-ray energy for low-, medium, and high-contrast objects.

The response of the 59Fe uptake system in rat femoral marrow exposed to various x-ray energies.

Rats were given whole-body exposures to four incident X-ray energies. The influence of dose from each of these energies upon iron uptake in the center of the femoral shaftmarrow was noted. Uptake ratios showed a change with increasing dose which may be expressed by the formula: UT = 0.87 exp [−(0.693/D50) × D] + 0.13. It was found that the D50 was essentially the same (regardless of the incident photon energy) and had a pooled value of 75 ± 9 rad. A comparison made with a previous spinal column study showed a D50 value of 71 ± 7 rad. Not only was there no difference in response with the differing photon energies studied, but it seems likely that femoral marrow may be used as a “calibration” for other marrow sites-particularly those which do not lend themselves to the usual dosimetry techniques.

A Depth Dose Calorimeter for High Intensity, Low Voltage Flash X-Ray Machines

A small tantalum foil depth dose calorimeter has been constructed for use with high intensity flash x-ray sources. Incident x-ray energy is deposited in an array of tantalum foils monitored by 25 μ iron-constantan thermocouples. Readout is made with a high speed scanning digital microvoltmeter. Comparison is made between the measured x-ray energy deposition in tantalum and the deposition calculated with a Monte Carlo computer code which takes account of the current and voltage waveforms on the diode.

The normalization of solar X-ray data from many experiments

A conversion factor is used to convert Geiger (GM) tube count rates or ion chamber currents into units of the incident X-ray energy flux in a specified passband. A method is described which varies the passband to optimize these conversion factors such that they are relatively independent of the spectrum of the incident photons. This method was applied to GM tubes flown on Explorers 33 and 35 and Mariner 5 and to ion chambers flown on OSO 3 and OGO 4. Revised conversion factors and passbands are presented, and the resulting absolute solar X-ray fluxes based on these are shown to improve the agreement between the various experiments. Calculations have shown that, although the GM tubes on Explorer 33 viewed the Sun off-axis, the effective passband did not change appreciably, and the simple normalization of the count rates to the count rates of a similar GM tube on Explorer 35 was justified. It is also shown that for the type 6213 GM tube, the passband is effectively 3 to 12 A although gas pressure and window thickness variations may give rise to numerical conversion factor differences of a factor of 6.

Charge Collection Efficiencies for Lithium-Drifted Silicon and Germanium Detectors in the X-Ray Energy Region

Theoretical calculations showing the charge collection efficiencies of Ge(Li) and Si(Li) planar detectors as a function of trapping lengths and incident X-ray energies are presented. It is demonstrated that there should be an intrinsic energy dependence of the charge collection efficiency for radiation lengths comparable to the detector depletion depth. The nonlinear behavior is generally greater for unequal hole and electron trapping lengths. Experimental results confirm this in part, but surface effects appear to accentuate the effect. It is concluded that, neglecting surface effects, the nonlinearities should be eliminated if the radiation is incident in a direction perpendicular to the field. Highest charge collection efficiency, best resolution and linearity are obtained by injecting the photons through the detector side (perpendicular to the field) along a unique plane which is defined by the electron to hole trapping ratio.