Flash X-ray Machine Research Articles

Design of bremsstrahlung target of 4 MeV flash X-ray machine

Bremsstrahlung converter target is one of the key factors for flash X-ray machine. The bremsstrahlung radiation process in which the electron beam impinges on the target is simulated utilizing the Monte Carlo method, by which the influence of the effective target thickness on the exposure is analyzed, the optimal target thickness and the exposure are also obtained for the 4 MeV flash X-ray machine. The damages of target impacted by the electron beam are compared and discussed for various electron energy deposit density from different sources, such as 12 MeV LIA, Dragon accelerators and 4 MeV flash X-ray machine. The results show that the tantalum distributed target may be the satisfactory solution for the bremsstrahlung target of 4 MeV flash X-ray machine.

The Chinese nuclear tests, 1964–1996

A combination of intellectual rigor, technical sophistication, hard work, and intelligence gathering brought China into the world’s nuclear club in record-shattering time.

Multimegavolt multiaxis high-resolution flash X-ray source development for a new hydrodynamics research facility at AWE Aldermaston

Summary form only given, as follows. The Atomic Weapons Establishment at Aldermaston, U.K., has a number of pulsed-power-driven flash X-ray machines for diagnosing the hydrodynamics of explosively driven high-Z materials. The most powerful of these machines is Mogul-E which operates at about 10 MV and 30 kA, delivering about 450 R @ 1 in in a 5 mm spot. Longer-term plans envisage the upgrading of our existing facilities through the construction of a multi-axis. Hydrodynamic Research Facility (HRF). It is proposed that the HRF will be furnished initially with three inductive voltage adder (IVA) machines operating at 14 MV, each giving 600 R @ I in in a 5 mm spot. It is envisaged that, following further research, the outputs will be increased towards 1000 R, with, it is hoped, a reduction in X-ray spot size. More speculative proposals involve increasing the number of machines from 3 to 5, and/or the splitting of the output end of one or more machines to drive more than one X-ray source per machine. An overview of the research programme necessary to achieve these aims will be presented. Reference will be made to other, more detailed, presentations at this conference. Topics covered will include: experiments to investigate the performance at /spl sim/5 MV on the paraxial diode (on E-Minor and Superswarf) and the magnetically-immersed diode on superswarf split magnetically-insulated transmission lines (MITLs) where one machine drove two X-ray sources. The design and testing of a prototype IVA module (PIM) operating at /spl sim/1.5 MV. The design of an upgrade from 5 MV to 10 MV of one of our existing single pulse-forming-line machines (E-Minor). Rod pinch diodes at /spl sim/1 MeV on Mevex. Initial, very speculative, experiments (at the Rutherford-Appleton Laboratory) employing picosecond laser pulses rather than conventional pulsed-power to generate MeV X-ray photons.

New developments in paraxial radiographic diode technology for focusing intense relativistic electron beams

The Pulsed Power Group at the Atomic Weapons Establishment (AWE) is interested in the generation of small, intense, multimegavolt X-ray sources for radiographic applications. For this paper, research has been carried out into electron beam transport and focusing studies on one of our single pulse forming line flash X-ray machines. Experiments were undertaken on E Minor to improve the radiographic spot size generated by our "paraxial diode". This is a novel anode/cathode assembly designed to produce a small-diameter electron beam focused onto a high-Z target. Electrons are propagated through a gas-filled cone such that their electrostatic repulsive and magnetic attractive forces are almost cancelled. The cone's length and gas pressure were initially optimized and then specific gas species applied to the drift cone. Helium gas improved spot size reproducibility with respect to variations in drift gas pressure when compared with dry air. The addition of a "secondary cell" to increase the neutralization of the electron beam's self-electric forces yielded a 30% improvement in measured radiographic spot size.

Compact flash X-ray sources and their applications

Compact flash X-ray machines are opening up extended fields of applications. X-ray diodes driven by repetitive small size pulsers have been shown able to deliver high dose rates of X-rays in pulses of nanosecond to microsecond duration. Improvements have been carried out on the lifetime of the diodes and reliability of the systems to allow operation at constant emitted dose over long periods of time. After a brief review of recent developments, described here, in more details, is the progress obtained at the GREMI laboratory in the development of true table-top flash X-ray sources producing strong X-ray doses in nanosecond pulses at a high repetition rate (50 Hz). Doses up to 2 R (5.2 × 10 −4 C/kg), measured at the output window, of X-rays between 5 and 200 keV can be generated from either a linear source (up to 10 cm long) or from a focal spot of less than 300 μm in diameter depending on the electrode configuration. Of the numerous applications of compact flash X-ray sources, an example is given of the use of these devices for the excitation of high pressure gas samples, realized at GREMI. Such an energetic excitation leads to important populations of highly excited ionic species.

Test CMOS/SOS RAM for transient radiation upset comparative research and failure analysis

The test Complementary Metal-Oxide-Semiconductor/Silicon-on-Sapphire Random Access Memory (CMOS/SOS RAM) with eight types of memory cells was designed and tested at high dose rates with a flash X-ray machine and laser simulator. The memory cell (MC) design with additional transistors and RC-chain was found to be upset free up to 2/spl times/10/sup 12/ rad(Si)/s. An "inversion" effect was discovered in which almost 100% logic upset was observed in poorly protected memory cell arrays at very high dose rates.

Bremsstrahlung risetime shortening by diode geometry reconfiguration

The Aurora flash X-ray machine was recently modified to offer an unfocused mode so that ionizing radiation is spread over a larger volume. An attempt was made to ascertain whether a faster bremmstrahlung risetime can be achieved. Experimental studies on one arm of the Aurora flash X-ray machine indicated that it is possible to lower the risetime of the bremsstrahlung produced at a single Aurora diode by increasing the anode-cathode (AK) gap of the diode and by increasing the length of the magnetically insulated transmission line (MITL) leading up to the diode. In an experimental setup using a cylindrically symmetrical toroidal cathode, the local spatial distribution of bremsstrahlung pulse shapes at the thick tantalum converter was measured at 10 radial positions by 10 Compton diodes placed at the converter. Additionally, bremsstrahlung produced at a distance of 2 m from the converter was measured by plastic scintillation counters. As the AK gap was widened, the early part of the bremsstrahlung pulse was diverted in the radial direction of the MITL feeding the diode. The bremsstrahlung risetime at 3 m from the bremsstrahlung target was reduced from about 60 to 18 ns by increasing the AK gap from 11 to 19 in. or more.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

New capabilities for the Aurora flash X-ray machine

A series of recent upgrades enable the Aurora flash X-ray machine to provide a wide variety of different radiation pulses. The original design gives a single γ-ray pulse with 50 ns risetime and 135 ns width that irradiates a volume of around 1 m 3 with up to 50 krad (Si). The upgrades allow for two pulses with arbitrary separation in time, each with half the output. The pulse widths can be shortened to around 30 ns (with reduced energy), and the risetime can be reduced to 10 ns. These upgrades are discussed in some detail. Other upgrades are described elsewhere: the moderate energy bremsstrahlung option is described by J.A. Anderson et al. [Nucl. Instr. and Meth. B40/41 (1989) 1189]; the high power microwave option is described by G.A. Huttlin et al. [IEEE Trans. Plasma Sci. PS-18 (1990) 618 and other papers in this series].

The energy dependence of CaF/sub 2/:Mn/Teflon thermoluminescent dosimeters

Thermoluminescent dosimeters (TLDs) which consist of Teflon disks, 0.635 cm in diameter by 0.038 cm thick, containing 5% CaF/sub 2/:Mn in the form of 20- mu m grains distributed throughout the volume, are convenient, disposable, single-reading dosimeters which are frequently used in radiation hardness testing. Their photon energy response differs markedly from that of solid CaF/sub 2/:Mn TLDs for photon energies below approximately 500 keV. An accurate model of the energy response is required for the application of such dosimeters to testing at moderate energy flash X-ray machines (E/sub e/ approximately=1.0-1.5 MeV). The energy response of these dosimeters was measured using heavily filtered X-rays from a 300-keV constant potential X-ray generator and calculated for photon energies from 10 to 500 keV using the transport code ACCEPT. The results indicate good agreement between measurements and calculations. >

Excitation of a Canonical RF Shelter with Relativistic Electrons and with Enhanced Risetime Bremsstrahlung

In two tests at the Aurora flash x-ray machine, a cavity (canonical rf shelter) was excited in two ways: (1) with relativistic electrons, and (2) with enhanced-risetime bremsstrahlung. Differences in cavity excitation resulting from the two types of sources are explored. The relativistic electrons excite the cavity producing large electric fields (>80 kV/m), but are not expected to produce much of a SGEMP response in individual cables and electronic boxes inside the shelter, because the electrons are easily shielded by such enclosures. The relativistic electron current density in the case of direct e-beam injection is about two orders of magnitude greater than the Compton electron current density produced by enhanced risetime bremsstrahlung. The direct e-beam currents, therefore, excite significantly larger electromagnetic fields than the bremsstrahlung produced Compton currents. The greater penetrating power of the bremsstrahlung radiation, on the other hand, excites both the larger cavity and smaller interior sub-cavities and cables. The two methods thus provide a possible means of distinguishing between different types of SGEMP excitation. Conclusions are drawn from the results concerning the feasibility of using relativistic electrons in SREMP simulation schemes.

Enhanced-Rise-Time Bremsstrahlung Aurora Environment

The technique of beam nose erosion has been employed at the Aurora flash x-ray machine to obtain a modified bremsstrahlung pulse with a shorter rise time. When the Aurora is operated in its conventional mode the individual thick target bremsstrahlung sources each have a rise time of about 40 to 50 nanoseconds. The modifications described here involve the insertion of a beam erosion chamber between one of the four anodes and its bremsstrahlung target. Bremsstrahlung rise times varied, depending on the anode-cathode gap and the chamber pressures, between 10 nanoseconds and 100 nanoseconds.

Design and Performance of the 1.5 MV Injector for FXR

The new flash x-ray machine (FXR) at LLNL is scheduled for completion in late 1981. This is a 54 module, linear induction accelerator, designed to deliver 500 Roentgen at 1 m as bremsstrahlung from a 20 MeV, 4 kA, 60 ns pulsed electron beam. The 1.5 MV, cold cathode injector makes use of six accelerator modules as voltage sources. High voltage testing of the accelerator modules and their associated pulse forming lines and Marx banks has been completed, and beam tests of the complete injector assembly are in progress. Design information and preliminary test results are presented.

Determination of nanosecond high-voltage pulse shapes at the surface of needle emitters

A method is described for determining, in situ, the shapes of nanosecond, positive high-voltage desorption pulses at the surface of a field-ion specimen, without the use of waveform-distorting probes or voltage dividers. In practice, a negative pulse known to be identical in shape to the positive desorption pulse is used to initiate field electron emission from the specimen surface. The resulting time-varying field emission current is recorded by a detector, and the Fowler–Nordheim equation is iterated to obtain the actual time-varying specimen potential. The pulse shape obtained can be used to predict accurately the arrival time of unknown surface species in time-of-flight atom-probe instruments, or to monitor the effect of changes in the pulse transmission line to the specimen. When applied to pulsed e-beam devices and flash x-ray machines, time-varying potentials at field-emission cathodes can be determined with nanosecond resolution.

Simulation for radiation effects in electronics

Adequate simulation testing for radiation effects requires knowledge of radiation effects mechanisms to relate test environments to operational environments. In order to scale the effect with radiation type, spectrum, or time dependence, the response must be separated into different radiation effects, The basic separation is into displacement, charge-transfer, and ionization effects. Within each category short-lived and long-term responses must be considered. Specific simulation facilities can be used to produce particular combinations of effects. Pulsed and steady-state neutron sources (reactor or accelerators), flash X-ray machines, and electron linear accelerators are particularly useful for producing the effects of the radiation from a nuclear explosion. Steady-state electron and proton accelerators are especially appropriate for simulating space radiation. In the first case the problem is to produce the high radiation rates; in the second to qualify accelerated testing. Many other simulation problems fall between these two limits.

Design of test fixtures for electron‐beam tree experiments

Abstract In this paper, we discuss the experimental problems and radiation noise sources associated with flash X-ray machine (FXR) and linear accelerator (LINAC) electron-beam TREE experiments. Special testing techniques and test fixture design guidelines are presented. Wiring diagrams for a properly designed general-purpose electron-beam test fixture and control unit are shown, in addition to photographs which illustrate the geometry-dependent shielding and noise reduction techniques. Techniques for improving both electron radiation shielding and RF shielding efficiencies are represented.

Magnetic Electron Deflection Tube

The deflection tube described here is used to harden the electron output spectrum of the Febetron 705 Flash X-ray (FXR) machine. The deflection tube is intended for use when performing electron-beam transient radiation effects on electronics (TREE) experiments. High-energy-electron dose rates in silicon are variable from 109 to 3 × 1010 rads(Si)/sec. A magnet is used to deflect all low-energy (E < 0.5 Mev) electrons out of the horizontal electron beam. The remaining high-energy electron spectrum can be used to irradiate test samples with more uniform dose-depth profiles in the test sample. More than 99.9 percent of the electrons are returned to the FXR machine via the deflection tube walls, thereby satisfying an operational requirement of the Febetron.

A Magnetic Compton Spectrometer for High Intensity Pulsed Gamma Ray Environments

A magnetic Compton spectrometer is described which was designed to measure the gamma ray spectra from distributed, intense pulsed sources in the presence of gama ray, electron, and/or neutron backgrounds. It has been used to measure spectra of a nuclear device, the Sandia Pulsed Reactor, and the Hermes II flash x-ray machine. A discussion will be given of the design features, the data reduction, and the results of the Hermes II measurement.

Drift Tube Techniques for Active Semiconductor Testing in the Electron Beam of the Field Emission 705 Flash X-Ray Machine

An evacuated drift tube has been designed and fabricated which enables the experimentor to use the electron beam of a 2 MeV flash X-ray machine for active, low noise irradiations of semiconductor devices. This paper gives design details of the drift tube and indicates the dose levels available as a function of tube parameters.

The determination of X-ray spectra using attenuation measurements and a computer program

A computer program has been written and tested which will rapidly produce reliable X-ray spectral distributions in intensity or photon flux density from measurements of attenuation data through metal filters under narrow beam conditions. Spectra of two 2 Mev sources have been produced which may be compared with a spectrum derived by the Laplace Transform method of analysing attenuation data, and with a spectrum produced by crystal scintillation spectrometry. Convenient and relatively cheap determinations of X-ray spectra using LiF dosemeters are discussed, and the spectrum from a 500 kev single-shot flash X-ray machine is determined as a demonstration of the method.

Flash X-ray machines.

The flash x-ray machine produces a pulsed x-ray beam of short duration (0.1 microsecond) and very high intensity (107 R/sec). Pulses may be repeated at one minute intervals. Voltages as high as 750,000 may be applied to the tube. The machine provides a very penetrating source for high-speed radiography. Problems of adequate shielding and procedures to provide an acceptable installation and of measurement of dose delivered are discussed.