Whispering gallery mirrors for the soft x-ray region: properties and applications

Basic properties of whispering gallery mirrors in the soft x-ray region are considered. The following applications of whispering gallery optics are discussed: increasing the utilization efficiency of point source radiation, deflecting a synchrotron radiation beam to the vertical plane and transporting it to another horizontal level, resonators for soft x-ray lasers, and using the whispering gallery effect to investigate the roughness of concave surfaces.     

A Soft X-Ray/EUV Reflectometer Based on a Laser Produced Plasma Source

A soft x-ray reflectometer is described which is based on a laser-produced plasma source and is continuously tunable over the range 40 ? < λ < 400 ?. The source is produced by focusing 0.532-μm light from a Q-switched Nd:YAG laser on a solid target. The x-ray wavelength is defined using a high throughput spherical grating monochromator with moderate resolving power (λ/Δλ ≈ 100 to 500). A time-averaged monochromatized flux of more than 109 photons/s in a 1% bandwidth at 100 eV is obtained. Photon "shot noise" limited measurements are obtained by the use of an I0 detector to normalize out the shot-to-shot variations in source intensity. Measurements with submillimeter spot sizes are readily obtainable. Various detectors have been used and the advantages and disadvantages of each are discussed. The higher order contamination of the monochromator output has been analyzed using a second grating for the purpose of making measurement corrections. The reflectometer thus provides the capability for precision absolute measurements of the reflectance of gratings and multilayer mirrors, the transmittance of thin film filters, or other properties of x-ray optical elements.     

Performance Optimization of a High-Repetition-Rate KrF Laser Plasma X-Ray Source for Microlithography

In order to develop a high-intensity laser plasma x-ray source appropriate for industrial application of x-ray lithography, experiments have been carried out using a high-repetition-rate (up to 40 Hz) excimer laser (249 nm, 300 mJ) with a power density of 2 × 1013 W/ cm2 in the laser focus. In this study emphasis is given to remedying specific problems inherent in operating the laser plasma x-ray source at high repetition rates and in its prolonged operation. Two different methods of minimizing the production of target debris are investigated. First, the use of helium as a quenching gas results in a reduction of the amount of atomic debris particles by more than two orders of magnitude with negligible x-ray absorption. Second, a tape target as opposed to a solid target reduces the production of larger debris particles by a further factor of 100. Remaining debris is stopped by an aluminized plastic or beryllium filter used to avoid exposure of the resist by plasma ultraviolet radiation. The x-ray source has been used to image x-ray transmission mask structures down to 0.3 μm onto general purpose x-ray photo-resist. Results have been analyzed with SEM. The x-ray emission spectrum of the repetitive laser plasmas created from an iron target has been recorded and the conversion efficiency of the laser light into x-rays that contribute to exposure of the resist was measured to be 0.3% over 2π sr.     

Investigation of 125I as an isotopic source for mammography.

An isotopic radiation source 125I was investigated for use in soft tissue radiaography, with particular attention to possible use in mammography for mass screening. Film sensitivities, exposure rates, absorbed doeses, contrast and resolution were determined, and compared to conventional x-ray units. It was found that contrast and resolution were comparable to 30 kVp x-rays (W anode) but that surface absorbed dose was reduced by a factor of two with 125I. These results experimentally verified the advantage to be obtained from monoenergetic radiations, which had been predicted from theoretical considerations by others. Duration of exposure was estimated to be between 4 and 12 sec with a 40 Ci source. The 60-day half life of 125I may necessitate its use in conjunction with an image intensification system or electron radiographic techniques, in order to preclude source replacement at inconveniently short intervals.     

A 10-TW Pulsed Facility "PIRIT" for Investigation of Short-Wave Radiation Generation

The results of experiments with a plasma x-ray source in the PIRIT-2000 facility are presented in this paper. The facility is designed with module capacitive energy storage energizing vacuum inductive storage. The formation of a rapidly growing current pulse as well as its commutation on a load was carried out by a plasma opening switch. A vacuum diode as well as various types of plasma loads can be used for the generation of a high-power x-ray flux. The storage energy of a 54-module capacitive storage is up to 2 MJ, its inductance is 15 nH, and its output voltage is 500 kV. The peak current in the plasma load constituted 4 MA with a 150-ns rise time. The maximum integral energy output of x radiation measured by an open thermocouple calorimeter was as high as 100 kJ, while the primary storage energy was 1 MJ. The plasma load usage at a current of 4 MA ensured a 100-kJ generation in x-ray radiation and the density of the radiation flux at a distance of 1 m from the source was as much as 0.8 J/cm2, while near the source it was 10 J/cm2.     

Plasma x-ray radiation source

This paper gives the results of studies on a plasma x-ray source, which enables one to obtain a 2.5-krad radiation dose per pulse over an area of 100 cm2 in the quantum energy range from 20 to 500 keV. Pulse duration is 100 ns. Spectral radiation distributions from a diode under various operation conditions of a plasma are obtained. A Marx generator served as an initial energy source of 120 kJ with a discharge time of T/4 = 10-6 s. A short electromagnetic pulse (10-7 s) was shaped using plasma erosion opening switches.     

Adult Fingers Visualized on Neonatal Intensive Care Unit Chest Radiographs: What You Don’t See

Purpose

In a previous publication, it was revealed that a disturbingly high incidence of adult fingers were seen on pediatric intensive care unit radiographs, an example of inappropriate occupational exposure to diagnostic radiation. The present study examined adult fingers seen on neonatal intensive care unit (NICU) radiographs to assess the frequency of this occupational radiation exposure. During this study, we encountered an unexpected issue. The inappropriately exposed fingers appeared on the raw images but were sometimes cropped during technologist image processing before being sent to the picture archiving and communication system (PACS) for interpretation. Our audit describes the frequency of cropping adult fingers from images before display on PACS, with the intent of unmasking this source of occupational radiation exposure, of which quality assurance personnel may not be aware.

Methods

At the x-ray workstation, the raw NICU source x-ray images were analysed for the visibility of adult fingers and then were compared with the final processed images sent to PACS by the x-ray technologist.

Results

Of 230 radiographs audited, 30 (13%) contained fingers directly in the x-ray beam that remained visible on PACS, 22 (10%) contained fingers in the direct beam that were cropped before being sent to PACS for analysis, and 44 (19%) contained fingers in the coned area.

Conclusions

A significant number of adult fingers are being exposed to radiation during the acquisition of NICU radiographs. Cropping NICU radiographs before sending them to PACS can conceal a significant source of occupational radiation exposure.     

Power loading limitations in soft x-ray projection lithography

Soft x-ray projection lithography (SXPL) is an attractive technique for the fabrication of high-speed, high-density integrated circuits. In an SXPL stepper, the x-ray imaging mirrors consist of multilayer coatings deposited onto high precision substrates. The stepper is intended to fabricate ultra-high spatial-resolution structures with a minimum feature size of <0.1 μm. To achieve this resolution, the imaging mirrors must maintain a very precise surface figure while being exposed to x radiation. Failure to achieve and maintain the mirror surface figure will distort the wavefront propagating through the imaging system and will degrade system resolution. The required surface figure accuracy for each imaging mirror depends upon the required resolution, the wavelength, and the optical design. For conventional SXPL stepper designs, the total (peak-to-valley) surface figure error budget per mirror is approximately ±1 nm. Due to material properties at soft x-ray wavelengths and practical fabrication considerations, x-ray multilayer mirrors have limited reflectivities. A fraction of the incident x radiation is absorbed in the multilayer coating. This absorbed radiation constitutes a thermal load on the mirror, thereby distorting its shape and compromising the accuracy of its surface figure. In this paper, we analyze the thermally induced distortion on the imaging optics and conclude that the maximum allowable thermal distortion limits the maximum allowable x-ray power transported to the wafer and limits the minimum acceptable multilayer mirror reflectivity. The penalty for either insensitive x-ray resists or inefficient mirror reflectivity is a decrease in system throughput which cannot be compensated with increased source power either collected by condenser optics or generated by the source.     

Design of a normal incidence multilayer imaging x-ray microscope

Normal incidence multilayer Cassegrain x-ray telescopes were flown on the Stanford/MSFC Rocket X-Ray Spectroheliograph. These instruments produced high spatial resolution images of the Sun and conclusively demonstrated that doubly reflecting multilayer x-ray optical systems are feasible. The images indicated that aplanatic imaging soft x-ray /EUV microscopes should be achievable using multilayer optics technology. We have designed a doubly reflecting normal incidence multilayer imaging x-ray microscope based on the Schwarzschild configuration. The Schwarzschild microscope utilizes two spherical mirrors with concentric radii of curvature which are chosen such that the third-order spherical aberration and coma are minimized. We discuss the design of the microscope and the results of the optical system ray trace analysis which indicates that diffraction-limited performance with 600 ? spatial resolution should be obtainable over a 1 mm field of view at a wavelength of 100 ?. Fabrication of several imaging soft x-ray microscopes based upon these designs, for use in conjunction with x-ray telescopes and laser fusion research, is now in progress. High resolution aplanatic imaging x-ray microscopes using normal incidence multilayer x-ray mirrors should have many important applications in advanced x-ray astronomical instrumentation, x-ray lithography, biological, biomedical, metallurgical, and laser fusion research.     

Characterization of pinhole transmission gratings

Gold pinhole transmission gratings fabricated by Heidenhain GmbH primarily for the purpose of studying the radiation of intense soft x-ray sources have been tested with the synchrotron radiation of BESSY. Typical results for the spectral dependence of the grating efficiency into the various diffraction orders are presented in a wavelength region ranging from 4 to 20 nm. Also the influence of grating irregularities has been studied. With appropriate grating parameters quite good agreement between the experimental results and theoretical Calculations is Obtained.     

Radiation dose to the operator's hands during non-vascular radiology involving surgical technique-comparison between under-table method and over-table method

In non-vascular interventional radiology (IVR) such as percutaneous transhepatic cholangio-drainage (PTCD) and nerve block, the operator's hands are irradiated in primary x-ray field. In Over-table tube fluoroscopy system, operator's hands inevitably are exposed excessively to intensive primary radiation, whereas in the Under-table tube fluoroscopy system, they are irradiated weakly by attenuated radiation through the patient's body. For this reason, the dose to the operator's hands in Under-table tube fluoroscopy is less than in Over-table tube fluoroscopy. This paper proposes general formulas for estimating the absorbed dose on the operator's hands in two types of fluoroscopy. The formulas include factors affecting the absorbed dose on the operator's hand; distance from source to the operator's hands, x-ray transmittance of the patient and patient's bed, and back-scatter factor of the patient. Absorbed dose to imitated operator's hand was measured and estimated by formulas in two types of fluoroscopy for various phantom thicknesses and two field sizes using an ionization chamber. The difference between estimated absorbed dose and measured absorbed dose was less than 10%.     

Soft x-ray emission from millimeter-wave electron cyclotron resonance discharge

This work is devoted to the experimental investigation of incoherent soft x-ray radiation from an electron cyclotron resonance discharge with pumping by a millimeter-wave beam from a gyrotron. The basic contribution to the x-ray spectrum was shown to be produced by a plasma emission in the wavelength region 4.5-12 nm. The power of the x-ray emission from the ECR discharge was about 35 kW, and the efficiency of conversion of the gyrotron radiation into an x-ray emission exceeded 25%.     

Generation of Soft X-Ray/VUV Photons with a Hybrid/Bias Micropole Undulator on the LLNL Linac

The recently introduced class of micropole undulators, i.e., undulators with submilli-meter periods, promises a wide variety of useful applications in x-ray science and technology. One of the most important of these could turn out to be the efficient generation of highly coherent soft x rays on economical low-energy storage rings or linacs. Over the last year and a half, an intensive experimental effort has been in progress at the Lawrence Livermore National Laboratory (LLNL) to characterize the radiation emitted on a linac by a novel hybrid/bias micropole undulator fabricated at the Stanford Synchrotron Radiation Laboratory. In this initial report, the properties of this new 706-μm-period insertion device and its generation of 66-eV soft x rays on the LLNL linac are described. Although in the present experiment power levels of a fraction of a picowatt at linac currents of several tens of picoamperes were generated, this was in agreement with theoretical predictions. Operation of our prototype device on a higher energy machine at currents of several milliamperes could consequently be expected to produce milliwatt levels of soft x-ray output, making it an x-ray source potentially competitive with bending magnets on high-energy storage rings.     

Soft x-ray instrumentation and its applications at the advanced photon source

Knowledge of the intermediate energy range from 0.5-4 keV, bridging the "soft" and "hard" x-ray regions, is relatively underdeveloped. However, recent developments in the techniques of microscopy and magnetic circular dichroism have emphasized the need to operate in this energy range for microelectronic, biological, and materials science related experiments. The strong dipole-allowed 3d to 4f transitions in rare-earth magnetic materials fall in this region, as do the K-shells of many of the second and third row elements of the periodic table. Two beamlines to be constructed at the Advanced Photon Source (APS) have been designed to cover this energy region. The proposed undulator source, the beamline layout, and the experimental programs for these beamlines are described.     

Soft X-Ray Emission of Laser-Produced Plasmas: Comparison for 30-ps and 20-ns Laser Pulses

Soft x-ray emission spectra (250-875 eV) are presented for plasmas, produced by picosecond and nanosecond frequency-doubled Nd:YAG-glass laser pulses incident on 14 different target materials. The emitted spectra have been corrected for various apparatus functions which enables a direct comparison between plasmas produced by pico- and nanosecond laser pulses. The relative integrated emission intensity as a function of Z number, obtained from the corrected spectra, shows an oscillatory behavior, with distinct maxima for those elements exhibiting a dominant line emission in our photon energy window. We found for our two pulse lengths an approximately equal conversion efficiency from laser light into x-ray photons. General suggestions are given as to what target material should be used for different applications using the laser plasma as x-ray source in the energy range Studied.     

Optimization of Cd1-xZnxTe Detectors for Digital Radiography

In this study, measurements of the electrical and detection parameters of the Cd1-xZnxTe detectors, within the x-ray diagnostic energy range, have been performed with the aim of optimizing the image quality parameters of these solid-state-ionization detectors. Namely, the leakage current and system capacitance of the x-ray imaging system have been measured as they relate to signal parameters. Similarly, the detected signal and noise contributions were measured and related to the radiation exposure and tube current setting. Furthermore, the detector contrast has been experimentally determined. The experimental results indicate that Cd1-xZnxTe detectors have low leakage current, high resistivity, and high detector contrast resolution. Therefore, they appear to be very attractive for imaging applications with applications in x-ray digital radiography.     

Comparison of ultrathin x-ray window designs

In order to fabricate entrance windows for soft x-ray detectors, various technologies have been developed. Depending on the x-ray-detector type and the environment in which the windows are used, entrance windows must meet several, often contradictory, requirements: while good pressure tolerance and durability as well as gas tightness require thicker structures, good x-ray transmission can only be achieved with thin membranes. In this paper, the suitability of different window types for various applications is discussed. The applicability discussion is based on the results of tests performed on prototype windows, as well as on calculated and measured x-ray transmission properties. A comparative study of endurance vs transmission properties of some candidate membrane materials is also presented. Test results include pressure tolerance and leakage rates as well as some measurements of radiation damage to the window materials. The window technologies presented include coated polyimide membranes with two different supporting schemes as well as submicrometer beryllium membranes.     

Thermal x-ray emission from ion-beam-heated matter

The conversion of ion beam energy into thermal x-ray radiation by means of stretched cylindrical plasma volumes is studied by analytic modeling and numerical simulation. The analysis is restricted to one-dimensional radiation hydrodynamics. Scaling relations for different materials are derived. Due to equation-of-state properties, high-Z material turns out superior for achieving high conversion efficiency. The paper provides practical answers for thermal x-ray generation with intense heavy ion beams. Deposition powers in the order of 10l6 W/g are required for applications to ion beam inertial confinement fusion.     

Pitfalls of radiation survey after brachytherapy implant removal preceding Tl-201 study

An unexpected elevated postimplant radiation survey is described in an elderly patient with an interstitial low-dose-rate iridium-192 (Ir-192) needle implant for endometrial cancer. The elevated activity was related to prolonged clearance of Tl-201 from a cardiac study that had been performed 7 days earlier. The Tl-201 accumulated in the soft tissue, particularly the colon, resulting in increased survey readings over the abdomen and raising concern that an Ir-192 source remained within the patient. This case shows that delayed excretion of a diagnostic radionuclide agent can cause elevated activity high enough to confound postradiotherapy implant survey readings. The estimated surface exposure from a single iridium source left in the pelvis was determined using a phantom study. Possible factors causing decreased excretion of Tl-201 in a patient with heart disease, arteriosclerotic vascular disease, previous pelvic radiation therapy, and a brachytherapy procedure are discussed. A preloading radiation survey is recommended in patients who have had previous nuclear medicine studies involving radionuclides with long half-lives.     

Evaluation of the microangioarchitecture of tumors by use of monochromatic x-rays.

RATIONALE AND OBJECTIVES: The purpose was to compare differences in the depiction of small vessels in tumors seen on microangiograms from a conventional soft x-ray system with those from a synchrotron radiation system and to evaluate the microangioarchitecture of these tumors and the growth of neovascularization. METHODS: VX2 carcinomas transplanted to the auricles of 15 rabbits randomized into three groups were investigated after 1, 3, and 7 days. Five normal rabbits were the controls. Barium sulfate, to which sufficient gelatin had been added, was injected into the auricular artery. Microangiograms of auricle specimens were obtained with both a conventional soft x-ray system and a synchrotron radiation system. RESULTS: The conventional x-ray system could detect vessels with diameters of approximately 100 microm, whereas the monochromatic synchrotron radiation system could detect small vessels with diameters of less than 25 microm. On day 1, there was moderate vascularization and flexure vessels were present in the transplantation area. On day 3, dilated vessels were present in the peripheral areas of the tumors and tortuous vessels in the central areas. On day 7, hypovascular areas had increased in the central area. CONCLUSIONS: The synchrotron radiation system confirmed the growth of neovascularization in the tumors. This system should provide a useful tool for evaluating the microangioarchitecture of tumors.