A pulsed nanosecond x-ray generator based on an actively pumped field emission x-ray tube is described. The x-ray source is based on a high voltage Marx generator that drives a field emission tube without the need for an intermediate energy store. The Marx generator stores 12 Joules in ceramic capacitors and produces a voltage pulse > 380 kilovolts with a rise time of < 4 nanoseconds from an equivalent generator-impedance of 52 W. A numerical model is used in which the x-ray tube's cathode width and anode-cathode gap (AK) are permitted to change with time while electron current between the cathode and anode is treated as non relativistic and space-charge-limited (SCL). By coupling this model to an equivalent circuit representation of the Marx generator a calculation of the cathode current, anode-cathode potential and the output x-ray spectrum can be made. The radiation dose is 55 millirems at 30.4 cm from the anode of the x-ray tube and is Gaussian in shape with a 35 nanosecond (full width at half maximum) FWHM. The measured x-ray dose, pulse shape and width are consistent with model predictions. The source was successfully used to study high-velocity projectile induced cavatation in human tissue.
Spectrometers have been developed to record x-ray spectra in the energy range 50 eV to 60 keV. The dispersion elements are transmission crystals for energies higher than approximately 10 keV, reflection crystals for 1 keV to 20 keV, and transmission gratings for energies less than 1 keV. The two-dimensional spectral images are recorded on a CCD or CMOS sensor with a phosphor conversion screen. Silicon photodiodes are positioned in front of the 2D sensor to provide absolute x-ray flux calibrations. The diodes have 1 mm2 area and sub-nanosecond time response. The diodes, transmission gratings, and attenuation filters were absolutely calibrated using synchrotron radiation. In addition, the diodes were calibrated in pulsed mode using the soft x-ray (70 eV to 250 eV) pulses from individual electron bunches circulating in the synchrotron storage ring, and a self-calibration model extends the calibration to higher energy. X-ray and extreme ultraviolet spectra were recorded at the OMEGA and NIKE laser facilities. A hard x-ray spectrometer is being built for the National Ignition Facility (NIF) that covers the 1 keV to 20 keV range with one transmission crystal channel and four reflection crystal channels.
The fabrication and the fundamental study of a repetitive nanosecond x-ray generator having a sealed field emission x-ray tube is described. A compact Marx generator storing 12 Joules directly drives a field emission tube with voltage pulses > 380 kV and with < 4 nanosecond risetime from an equivalent generator-impedance of 52 Ω. A numerical model is used in which the x-ray tube's cathode width and anode-cathode gap (AK) spacing are permitted to change with time while electron flow between the cathode and anode is space-charge-limited (SCL) and nonrelativistic. Coupling this model to an equivalent circuit representation of the Marx generator, which includes the voltage variation of the BaTiO3 Marx capacitors, an estimation of the cathode current, anode-cathode potential and the x-ray spectrum was obtained and compared with measured values.
The ability to nondestructively image living organisms, examine biological materials, and inspect packages without the necessity of removing the contents are applications well suited to x-ray radiography. We have developed a portable flash x-ray source having adjustable peak x-ray photon energy, which delivers a 34 milliroentgen x-ray dose at 30 cm in a 60-ns full-width at half-maximum pulse using less than 5 joules of stored energy. This technology has generated considerable interest in the biomedical imaging community and promises to replace older x-ray system that were not readily portable.
A facility for calibrating far ultraviolet and extreme ultraviolet instruments has recently been completed at the Naval Research Laboratory. Our vacuum calibration vessel is 2-m in length, 1.67-m in diameter, and can accommodate optical test benches up to 1.2-m wide by 1.5-m in length. A kinematically positioned frame with four axis precision pointing capability of 10 microns for linear translation and .01 degrees for rotation is presently used during vacuum optical calibration of SSULI. The chamber was fabricated from 304 stainless steel and polished internally to reduce surface outgassing. A dust-free environment is maintained at the rear of the vacuum chamber by enclosing the 2-m hinged vacuum access door in an 8 ft. by 8 ft. class 100 clean room. Every effort was made to obtain an oil-free environment within the vacuum vessel. Outgassing products are continually monitored with a 1 - 200 amu residual gas analyzer. An oil-free claw and vane pump evacuates the chamber to 10-2 torr through 4 in. diameter stainless steel roughing lines. High vacuum is achieved and maintained with a magnetically levitated 480 l/s turbo pump and a 3000 l/s He4 cryopump. Either of two vacuum monochrometers, a 1-m f/10.4 or a 0.2-m f/4.5 are coaxially aligned with the optical axis of the chamber and are used to select single UV atomic resonance lines from a windowless capillary or penning discharge UV light source. A calibrated channeltron detector is coaxially mounted with the SSULI detector during calibration. All vacuum valves, the cooling system for the cryopump compressor, and the roughing pump are controlled through optical fibers which are interfaced to a computer through a VME board. Optical fibers were chosen to ensure that complete electrical isolation is maintained between the computer and the vacuum system valves-solenoids and relays.
A program is underway at the Naval Research Laboratory (NRL) to develop a high-resolution spectrometer for the study of astrophysical sources at EUV/soft X-ray wavelengths. The spectrometer design is simple in that the sole optic is a multilayer-coated spherical grating or mosaic of co-aligned gratings used at near-normal incidence, allowing large effective collecting area without the strict tolerance requirements of grazing incidence optics. Therefore, both high resolution and high throughput can be obtained over several selected narrow bandpasses. We present efficiency and resolving power measurements of spherical gratings which have parameters similar to that intended for our flight instrument. Two gratings were replicated from the same ruled master and then coated with a multilayer of molybdenum and silicon. A third sister grating was used as a control and overcoated with gold.
Gregory Saulnier, Robert Zacher, Deborah Van Vechten, Craig Boyer, Michael Lovellette, Gilbert Fritz, Robert Soulen, Joonhee Kang, Mark Blamire, Eugenie Kirk, Robert Somekh
We report on our ongoing work using Nb/Al/AIOx!Nb junctions for the detection ofx-rays. Detectors based on superconducting tunneling junctions offer the prospect of resolution over an order of magnitude higher than is obtainable with the current generation of semiconductor-based detectors. Unlike Sn junctions, these Nb-based devices are not degraded by repeated thermal cycling and are known to be exceptionally "radiation hard". We present results on measurements taken at 1.85 K, a temperature achievable with current space flight technology. These measurements include the current-voltage (1-V) curve, subgap current vs. temperature, the dependence of the superconducting current on the applied magnetic field (Fraunhofer pattern), x-ray pulses, and the spectra from a 6 keV x-ray source which gave an intrinsic device resolution of approximately 700 eV. A comparison of the x-ray spectrum peak with a known injected pulse of 105 electrons indicates the collection of more than 105electrons per 6 keV photon.
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