X-ray Fluence Research Articles

P252] Energy calibration of semi-conductor detectors for computed tomography

Purpose Semiconductor detectors are widely used in computed tomography (CT). Their high spatial resolution and sensitivity give them an advantage compared to ionization chambers. Unfortunately, their air kerma response is strongly energy-dependent. The aim of this study is the accurate calibration of semiconductor detectors with respect to CT scanner specific spectra at different values of the tube voltage. Methods The air kerma response of several semiconductor detectors of the same type (Doseprofiler, RTI) has been measured free-in-air using ISO-N X-ray qualities. The machine-specific, kerma-averaged calibration factors for different tube potentials are using the known X-ray fluence spectra of a GE Optima CT 660. The results are validated by comparing dose measurements made with the Doseprofiler, which was placed inside the isocenter of the CT, with measurements made using a calibrated ionization chamber with known energy response. In anthropomorphic phantoms, the beam-hardening characteristics of the phantom and the CT source must be considered. The simulation of X-ray fluence spectra inside the inhomogeneous object is very complex, as the angle-dependent material composition of both the phantom and the X-ray source of the CT must be incorporated. Hence, direct calibration with calibrated ionization chambers inside the phantom has been performed, which is the more direct approach. Results It has been shown that the measured response of the semiconductor detectors varies by ∼ 80% for different ISO-N X-ray modalities. Consequently, the calculated CT-specific calibration factors free-in-air in total change by 24% for tube voltages between 80 kV and 140 kV. Dose measurements with the ionization chamber and Doseprofiler showed good agreement, with the results diverging by less than 2% after correction. The in-phantom calibration with an ionization chamber lead to comparable results with differences of 20%-24% in the calibration factors for values of the tube voltage between 80 kV and 140 kV and for different anthropomorphic phantoms. Conclusions Semiconductor detectors have the advantage of high spatial resolution and sensitivity. However, the air kerma response of these detectors is typically very poor. Hence spectral-corrected calibration factors or cross-calibration with ionization chambers is absolutely necessary in order to achieve the highest accuracy in the absolute dose measurement for all tube potentials.

Perturbing the spin crossover transition activation energies in Fe(H2B(pz)2)2(bipy) with zwitterionic additions

A thermal component to the soft x-ray induced spin crossover transition exists in the switching of a spin crossover compound (complex [Fe{H2B(pz)2}2(bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine) combined with the dipolar molecular additives (zwitterionic p-benzoquinonemonoimine C6H2()2()2). The addition of the zwitterionic molecule locks the Fe(II) complex in a largely low spin state configuration over an unusually broad temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K. It is demonstrated here that the process of exciting the [Fe{H2B(pz)2}2(bipy)] moiety, in the presence of with C6H2()2()2, to an electronic state characteristic of the high spin state though incident x-ray fluences, has a thermal activation energies are determined to 14–18 meV for a range of mixing ratios from 1:2 to 1:10. Those activation energies are also significantly reduced as compared to values of 60–80 meV found for nanometer thin films of [Fe{H2B(pz)2}2(bipy)] on SiO2.

The Temporal Development of Dust Formation and Destruction in Nova Sagittarii 2015#2 (V5668 SGR): A Panchromatic Study

Abstract We present 5–28 μm SOFIA FORECAST spectroscopy complemented by panchromatic X-ray through infrared observations of the CO nova V5668 Sgr documenting the formation and destruction of dust during ∼500 days following outburst. Dust condensation commenced by 82 days after outburst at a temperature of ∼1090 K. The condensation temperature indicates that the condensate was amorphous carbon. There was a gradual decrease of the grain size and dust mass during the recovery phase. Absolute parameter values given here are for an assumed distance of 1.2 kpc. We conclude that the maximum mass of dust produced was 1.2 × 10−7 M ⊙ if the dust was amorphous carbon. The average grain radius grew to a maximum of ∼2.9 μm at a temperature of ∼720 K around day 113 when the shell visual optical depth was τ v ∼ 5.4. Maximum grain growth was followed by a period of grain destruction. X-rays were detected with Swift from day 95 to beyond day 500. The Swift X-ray count rate due to the hot white dwarf peaked around day 220, when its spectrum was that of a kT = 35 eV blackbody. The temperature, together with the supersoft X-ray turn-on and turn-off times, suggests a white dwarf mass of ∼1.1 M ⊙. We show that the X-ray fluence was sufficient to destroy the dust. Our data show that the post-dust event X-ray brightening is not due to dust destruction, which certainly occurred, as the dust is optically thin to X-rays.

On the properties of synchrotron-like X-ray emission from laser wakefield accelerated electron beams

The electric and magnetic fields responsible for electron acceleration in a Laser Wakefield Accelerator (LWFA) also cause electrons to radiate x-ray photons. Such x-ray pulses have several desirable properties including short duration and being well collimated with tunable high energy. We measure the scaling of this x-ray source experimentally up to laser powers greater than 100 TW. An increase in laser power allows electron trapping at a lower density as well as with an increased trapped charge. These effects resulted in an x-ray fluence that was measured to increase non-linearly with laser power. The fluence of x-rays was also compared with that produced from K-α emission resulting from a solid target interaction for the same energy laser pulse. The flux was shown to be comparable, but the LWFA x-rays had a significantly smaller source size. This indicates that such a source may be useful as a backlighter for probing high energy density plasmas with ultrafast temporal resolution.

X-ray Observations at Gaisberg Tower

We report the occurrence of X-rays at ground level due to cloud-to-ground flashes of upward-initiated lightning from Gaisberg Tower, in Austria, which is located at an altitude of 1300 m. This is the first observation of X-ray emissions from upward lightning from a tower top located at high altitude. Measurements were carried out using scintillation detectors installed close to the tower top in two phases from 2011 to 2015. X-rays were recorded in three subsequent strokes of three flashes out of the total of 108 flashes recorded in the system during both phases. In contrast to the observations from downward natural or triggered lightning, X-rays were observed only within 10 µs before the subsequent return stroke. This shows that X-rays were emitted when the dart leader was in the vicinity of the tower top, hence during the most intense phase of the dart leader. Both the detected energy and the fluence of X-rays are far lower compared to X-rays from downward natural or rocket-triggered lightning. In addition to the above 108 flashes, an interesting observation of X-rays produced by a nearby downward flash is also presented. The shorter length of dart-leader channels in Gaisberg is suggested as a possible cause of this apparently weaker X-ray production.

Monte Carlo analysis of the oxygen knock-on effects induced by synchrotron x-ray radiation in the Bi2Sr2CaCu2O8+δ superconductor

We investigate the microscopic mechanism responsible for the change of macroscopic electrical properties of the $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{r}}_{2}\mathrm{CaC}{\mathrm{u}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ high-temperature superconductor induced by intense synchrotron hard x-ray beams. The possible effects of secondary electrons on the oxygen content via the knock-on interaction are studied by Monte Carlo simulations. The change in the oxygen content expected from the knock-on model is computed convoluting the fluence of photogenerated electrons in the material with the Seitz-Koehler cross section. This approach has been adopted to analyze several experimental irradiation sessions with increasing x-ray fluences. A close comparison between the expected variations in oxygen content and the experimental results allows determining the irradiation regime in which the knock-on mechanism can satisfactorily explain the observed changes. Finally, we estimate the threshold displacement energy of loosely bound oxygen atoms in this material ${T}_{d}=0.{15}_{\ensuremath{-}0.01}^{+0.025}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$.

Determination of Calibration Cycles Using X-ray Tube Output, Workload and Use Factor for Diagnostic X-rays

The stipulated standard time according to annual inspection done under the Health Facility Act 2008 is once per year however, due to varying workloads, use factor, and tube age the calibration cycles can be adjusted. Significance of this work includes determining the optimal calibration time of diagnostic X-ray tubes, resulting in promotion of new knowledge and influencing policies on the need for timely calibrations, additionally it may be used as a marker for urgent calibration in high workload facilities via diagnostic reference per unit to establish an institutional local standard. Parameters involved in this research are X-ray tube output (mGy/mAs), workload (mAmin/ week), use factor and fluence (mGy/cm2). Comparative studies were done at two local hospitals in Guyana namely Georgetown Public Hospital Corporation (GPHC) and Linden Hospital Corporation (LHC). Barium meals (B.M) and intravenous pyelograms (IVP) were measured at Georgetown Public Hospital Corporation. The RaySafe X2 was placed in the path of the X-ray beam at different settings of peak kilovoltage and millamperage- second and the radiation quantities were displayed on the base unit. This research investigated the effect of variation factor on the calibration of the static radiography on a fluoroscopy unit. Factors measured included kerma, tube output, fluence and age of tube output and workload. The differences obtained were significant in the cases of the elevated R/F sensor at 60 cm and at table top of 115 cm and varying peak kilovoltages and milliamperage. The difference in tube output could be attributed to anode surface damage (pitting), age of the tube, filtration, and collimation. A lower workload requires less calibration. In conclusion facilities with a workload of approximately 50 patients do not require more frequent calibration.

Forecasting Solar Energetic Particle (SEP) events with Flare X-ray peak ratios

Solar flare X-ray peak fluxes and fluences in the 0.1–0.8 nm band are often used in models to forecast solar energetic particle (SEP) events. Garcia (2004) [Forecasting methods for occurrence and magnitude of proton storms with solar soft X rays, Space Weather, 2, S02002, 2004] used ratios of the 0.05–0.4 and 0.1–0.8 nm bands of the X-ray instrument on the GOES spacecraft to plot inferred peak flare temperatures versus peak 0.1–0.8 nm fluxes for flares from 1988 to 2002. Flares associated with E > 10 MeV SEP events of >10 proton flux units (pfu) had statistically lower peak temperatures than those without SEP events and therefore offered a possible empirical forecasting tool for SEP events. We review the soft and hard X-ray flare spectral variations as SEP event forecast tools and repeat Garcia’s work for the period 1998–2016, comparing both the peak ratios and the ratios of the preceding 0.05–0.4 nm peak fluxes to the later 0.1–0.8 nm peak fluxes of flares >M3 to the occurrence of associated SEP events. We divide the events into eastern and western hemisphere sources and compare both small (1.2–10 pfu) and large (≥300 pfu) SEP events with those of >10 pfu. In the western hemisphere X-ray peak ratios are statistically lower for >10 pfu SEP events than for non-SEP events and are even lower for the large (>300 pfu) events. The small SEP events, however, are not distinguished from the non-SEP events. We discuss the possible connections between the flare X-ray peak ratios and associated coronal mass ejections that are presumed to be the sources of the SEPs.

X-ray and optical pulse interactions in GaAs.

Absorption of hard x-rays in GaAs creates excitations that can dramatically alter the propagation of optical laser pulses with photon energies near the bandgap. Measurements of optical transmission through a thin crystalline wafer of GaAs after absorption of an intense x-ray synchrotron pulse demonstrate how x-ray induced optical transparency depends on the recombination of excited conduction band electrons and valence band holes via Auger, spontaneous emission, and especially stimulated emission processes. The x-ray induced band fluorescence spectrum also reveals amplified spontaneous emission at the high x-ray fluences used, confirming the importance of stimulated emission. For laser pulses with sufficiently high fluence, the interaction of optically excited electrons with x-ray excited electrons can quench the enhanced laser transmission.

A Monster CME Obscuring a Demon Star Flare

Abstract We explore the scenario of a coronal mass ejection (CME) being the cause of the observed continuous X-ray absorption of the 1997 August 30 superflare on the eclipsing binary Algol (the Demon Star). The temporal decay of the absorption is consistent with absorption by a CME undergoing self-similar evolution with uniform expansion velocity. We investigate the kinematic and energetic properties of the CME using the ice cream cone model for its three-dimensional structure in combination with the observed profile of the hydrogen column density decline with time. Different physically justified length scales were used that allowed us to estimate lower and upper limits of the possible CME characteristics. Further consideration of the maximum available magnetic energy in starspots leads us to quantify its mass as likely lying in the range g and kinetic energy in the range erg. The results are in reasonable agreement with extrapolated relations between flare X-ray fluence and CME mass and kinetic energy derived for solar CMEs.

Evaluation of Cable SGEMP Response Using Monte Carlo and Finite-Difference Time-Domain Methods

In this paper, the system-generated electromagnetic pulse (SGEMP) response generated on the shielded cable after X-ray irradiation in laser inertial confinement fusion facility is studied by the Monte Carlo and finite-difference time-domain (FDTD) method. A layered cable model is established according to the radiation environment in the target chamber of the SG-III laser facility, the Monte Carlo software MCNP5 is used to calculate the number of deposited charges, and a program is developed based on the FDTD method to calculate the electromagnetic field in the dielectric layer. Finally, the current response on the cable conductor is obtained using Ampere’s circuit law. The results showed that the peak current response on the cable with a length of 1 m is as large as 1.26 A with a frequency range of 0–5 GHz. Furthermore, some important parameters, such as the X-ray fluence, pulsewidth, X-ray energy, cable material, thickness of the shielding layer, and length of the cable, all have effects on the cable SGEMP.

Numerical Simulation of the SGEMP Inside a Target Chamber of a Laser Inertial Confinement Facility

After X-ray irradiate a metal cavity, a large number of photoelectrons are emitted into the internal space, thereby producing a very strong system-generated electromagnetic pulse (SGEMP). The X-ray environment inside a laser inertial confinement device is very complex. Even if the diagnostic equipment working inside the target chamber has a good electromagnetic shielding capability, it would still face severe SGEMP interference. In this paper, the X-ray environments inside target chambers of National Ignition Facility, optimized method for estimated guidance accuracy (OMEGA), and Shenguang-III (SG-III) facility were obtained through a survey of the literature. In light of the survey results, the time-biased finite-difference time-domain method and the particle-in-cell method were used to numerically simulate SGEMP inside a 2-D cylindrical cavity model. Besides, the relationships between the SGEMP in the cavity and parameters such as X-ray fluence, energy spectrum, pulsewidth, and spatial size of the model were studied. The fluence and pulsewidth of the X-ray were found to have a more significant effect on the time-domain waveform of the electromagnetic field. Finally, based on the calculation results, the SGEMP of the SG-III facility in the target chamber was calculated to approximately be 0.75 MV/m.

О корреляции околоземных протонных возрастаний >100 МэВ с параметрами солнечных микроволновых всплесков

We analyze the relations between various combinations of peak fluxes and fluences of solar microwave bursts at 35 GHz recorded with the Nobeyama Radio Polarimeters during 1990–2015, and corresponding parameters of proton enhancements with E>100 MeV exceeding 0.1 pfu registered by GOES monitors in near-Earth environment. The highest correlation has been found between the microwave and proton fluences. This fact reflects a dependence of the total number of protons on the total duration of the acceleration process. In the events with strong flares, the correlation coefficients of proton fluences with microwave and soft X-ray fluences are higher than those with speeds of coronal mass ejections. The results indicate a statistically larger contribution of flare processes to acceleration of high-energy protons. Acceleration by shock waves seems to be less important at high energies in events associated with strong flares, although its contribution probably prevails in weaker events. The probability of a detectable proton enhancement was found to directly depend on the peak flux and duration of the microwave burst, that can be used for diagnostics of proton enhancements based on microwave observations.

Locking and Unlocking the Molecular Spin Crossover Transition.

The Fe(II) spin crossover complex [Fe{H2 B(pz)2 }2 (bipy)] (pz = pyrazol-1-yl, bipy = 2,2'-bipyridine) can be locked in a largely low-spin-state configuration over a temperature range that includes temperatures well above the thermal spin crossover temperature of 160 K. This locking of the spin state is achieved for nanometer thin films of this complex in two distinct ways: through substrate interactions with dielectric substrates such as SiO2 and Al2 O3 , or in powder samples by mixing with the strongly dipolar zwitterionic p-benzoquinonemonoimine C6 H2 (-⋯ NH2 )2 (-⋯ O)2 . Remarkably, it is found in both cases that incident X-ray fluences then restore the [Fe{H2 B(pz)2 }2 (bipy)] moiety to an electronic state characteristic of the high spin state at temperatures of 200 K to above room temperature; that is, well above the spin crossover transition temperature for the pristine powder, and well above the temperatures characteristic of light- or X-ray-induced excited-spin-state trapping. Heating slightly above room temperature allows the initial locked state to be restored. These findings, supported by theory, show how the spin crossover transition can be manipulated reversibly around room temperature by appropriate design of the electrostatic and chemical environment.

Impact of radiation induced crosslinking on structural, morphological, mechanical and optical properties of Polymethylmethacrylate thin films

Abstract This project is dedicated to investigate the optimal fluence of laser produced X-rays for crosslinking in thin Polymethylmethacrylate (PMMA) films. These films were deposited at room temperature using Pulsed Laser Deposition (PLD) technique. These deposited films were then irradiated with various X-ray fluence (produced from Laser induced Plasma) ranging from 2.5 to 5.76 mJ-cm−2. X-ray Diffractometer, Optical microscope, hardness tester and UV-VIS spectroscopy techniques have been employed to characterize the films. The X-ray fluence 4.48 mJ-cm−2 comes out to be the optimal fluence for crosslinking that not only improved the crystallization of the films but also caused the generation of new phases. The surface exhibited larger fragments at this fluence. A sharp enhancement in hardness as well as in absorption in the UV–vis range have also been observed at this fluence. These cross linked PMMA films have their potential applications in biomedical, optoelectronic, lithography, coating, packing and medical field.

X-Ray-induced Deuterium Enrichment of N-rich Organics in Protoplanetary Disks: An Experimental Investigation Using Synchrotron Light

Abstract The deuterium enrichment of organics in the interstellar medium, protoplanetary disks, and meteorites has been proposed to be the result of ionizing radiation. The goal of this study is to simulate and quantify the effects of soft X-rays (0.1–2 keV), an important component of stellar radiation fields illuminating protoplanetary disks, on the refractory organics present in the disks. We prepared tholins, nitrogen-rich organic analogs to solids found in several astrophysical environments, e.g., Titan’s atmosphere, cometary surfaces, and protoplanetary disks, via plasma deposition. Controlled irradiation experiments with soft X-rays at 0.5 and 1.3 keV were performed at the SEXTANTS beamline of the SOLEIL synchrotron, and were immediately followed by ex-situ infrared, Raman, and isotopic diagnostics. Infrared spectroscopy revealed the preferential loss of singly bonded groups (N–H, C–H, and R–N≡C) and the formation of sp3 carbon defects with signatures at ∼1250–1300 cm−1. Raman analysis revealed that, while the length of polyaromatic units is only slightly modified, the introduction of defects leads to structural amorphization. Finally, tholins were measured via secondary ion mass spectrometry to quantify the D, H, and C elemental abundances in the irradiated versus non-irradiated areas. Isotopic analysis revealed that significant D-enrichment is induced by X-ray irradiation. Our results are compared to previous experimental studies involving the thermal degradation and electron irradiation of organics. The penetration depth of soft X-rays in μm-sized tholins leads to volume rather than surface modifications: lower-energy X-rays (0.5 keV) induce a larger D-enrichment than 1.3 keV X-rays, reaching a plateau for doses larger than 5 × 1027 eV cm−3. Synchrotron fluences fall within the expected soft X-ray fluences in protoplanetary disks, and thus provide evidence of a new non-thermal pathway to deuterium fractionation of organic matter.

Characterization of the ESR response of alanine dosimeters to low-energy Cu-target X-tube photons

This article describes Electron Spin Resonance (ESR) response measurements of Kodak BioMax alanine films exposed to low-energy X-rays from a Cu-target tube operating at 20 kV. Commercial alanine detectors were used to ensure maximum reproducibility of the results, while the choice of a film was due the low penetration of the soft X-rays. X-ray energy spectra and fluences were determined with an innovative digital semiconductor detector system. These data were used to quantify the irradiation of the alanine films in terms of absorbed dose to water. The alanine films were found to present a stable response, highly linear with dose. To our knowledge, these data have not been previously reported and they may be extremely relevant, e.g., when mapping the high-gradient treatment fields used in microbeam radiation therapy (MRT) with synchrotron radiation.

Solar Activity from 2006 to 2014 and Short-term Forecasts of Solar Proton Events Using the ESPERTA Model

Abstract To evaluate the solar energetic proton (SEP) forecast model of Laurenza et al., here termed ESPERTA, we computed the input parameters (soft X-ray (SXR) fluence and ∼1 MHz radio fluence) for all ≥M2 SXR flares from 2006 to 2014. This database is outside the 1995–2005 interval on which ESPERTA was developed. To assess the difference in the general level of activity between these two intervals, we compared the occurrence frequencies of SXR flares and SEP events for the first six years of cycles 23 (1996 September–2002 September) and 24 (2008 December–2014 December). We found a reduction of SXR flares and SEP events of 40% and 46%, respectively, in the latter period. Moreover, the numbers of ≥M2 flares with high values of SXR and ∼1 MHz fluences (>0.1 J m−2 and >6 × 105 sfu × minute, respectively) are both reduced by ∼30%. A somewhat larger percentage decrease of these two parameters (∼40% versus ∼30%) is obtained for the 2006–2014 interval in comparison with 1995–2005. Despite these differences, ESPERTA performance was comparable for the two intervals. For the 2006–2014 interval, ESPERTA had a probability of detection (POD) of 59% (19/32) and a false alarm rate (FAR) of 30% (8/27), versus a POD = 63% (47/75) and an FAR = 42% (34/81) for the original 1995–2005 data set. In addition, for the 2006–2014 interval the median (average) warning time was estimated to be ∼2 hr (∼7 hr), versus ∼6 hr (∼9 hr), for the 1995–2005 data set.

Focal spot size reduction using asymmetric collimation to enable reduced anode angles with a conventional angiographic x-ray tube for use with high resolution detectors.

The high-resolution requirements for neuro-endovascular image-guided interventions (EIGIs) necessitate the use of a small focal-spot size; however, the maximum tube output limits for such small focal-spot sizes may not enable sufficient x-ray fluence after attenuation through the human head to support the desired image quality. This may necessitate the use of a larger focal spot, thus contributing to the overall reduction in resolution. A method for creating a higher-output small effective focal spot based on the line-focus principle has been demonstrated and characterized. By tilting the C-arm gantry, the anode-side of the x-ray field-of-view is accessible using a detector placed off-axis. This tilted central axis diminishes the resultant focal spot size in the anode-cathode direction by the tangent of the effective anode angle, allowing a medium focal spot to be used in place of a small focal spot with minimal losses in resolution but with increased tube output. Images were acquired of two different objects at the central axis, and with the C-arm tilted away from the central axis at 1° increments from 0°-7°. With standard collimation settings, only 6° was accessible, but using asymmetric extended collimation a maximum of 7° was accessed for enhanced comparisons. All objects were positioned perpendicular to the anode-cathode direction and images were compared qualitatively. The increasing advantage of the off-axis focal spots was quantitatively evidenced at each subsequent angle using the Generalized Measured-Relative Object Detectability metric (GM-ROD). This anode-tilt method is a simple and robust way of increasing tube output for a small field-of-view detector without diminishing the overall apparent resolution for neuro-EIGIs.

Initial experimental demonstration of the principles of a xenon gas shield designed to protect optical components from soft x-ray induced opacity (blanking) in high energy density experiments

The design principles of a xenon gas shield device that is intended to protect optical components from x-ray induced opacity (“x-ray blanking”) have been experimentally demonstrated at the OMEGA-60 Laser Facility at the Laboratory for Laser Energetics, University of Rochester. A volume of xenon gas placed in front of an optical component absorbs the incoming soft x-ray radiation but transmits optical and ultra-violet radiation. The time-resolved optical (532 nm) transmission of samples was recorded as they were exposed to soft x-rays produced by a gold sphere source (1.5 kJ sr−1, 250–300 eV). Blanking of fused silica (SiO2) was measured to occur over a range of time-integrated soft x-ray (<3 keV) fluence from ∼0.2–2.5 J cm−2. A shield test device consisting of a 30 nm silicon nitride (Si3N4) and a 10 cm long volume of 0.04 bar xenon gas succeeded in delaying loss of transmission through a magnesium fluoride sample; optical transmission was observed over a longer period than for the unprotected sample. It is hoped that the design of this x-ray shield can be scaled in order to produce a shield device for the National Ignition Facility optical Thomson scattering collection telescope, in order to allow measurements of hohlraum plasma conditions produced in inertial confinement fusion experiments. If successful, it will also have applications in many other high energy density experiments where optical and ultra-violet measurements are desirable.