Foil Window Research Articles

Dose distributions in tubing irradiated by electron beam: Monte Carlo simulation and measurement

A set of nine tube sleeves was prepared by wrapping polyethylene film around a mandrel. Wall thicknesses ranged from a nominal 0.3 to 0.6 cm and diameters ranged from 2 to 5 cm. The sleeves were irradiated with a scanning 3 MeV electron beam and were dose-mapped by indexing and unwrapping the tube sleeves and measuring the dose by an FTIR spectroscopic technique. A Monte Carlo simulation model is also described which includes all important elements of 3-D geometry necessary to describe this experiment including window foil and beam scan angle. The simulation results were compared with the highly detailed dose distribution measurements in the tubing and the differences were found to be generally within 3 to 6 percent.

Large-area secondary emission electron gun

A high current electron beam processor, using secondary emission electrons from a cold cathode by striking helium ions, has been developed for industrial applications. The helium ions are extracted, through a honeycomb partition, from 10 × 70 cm of a discharge chamber where the plasma is generated by a glow-discharge using the wire-anode of 74 cm length and are accelerated toward a cathode biased at − 250 kV. The secondary electrons emitted from the cathode are accelerated in the inverse direction to ions and pass through the plasma chamber. It was demonstrated that a current of 200 mA was extracted into the air through a thin window foil and irradiated running products. The dose uniformity on the products was ± 5% over the 65 cm width of the beam area. Processing capacities were up to about 500 Mrad · m/min. The characteristics of this gun will be discussed.

High resolution and two-dimensional focal plane detector for intermediate energy heavy ions

A large-area, high-resolution two-dimensional position sensitive gas detector has been developed for intermediate energy heavy ions. The x position information is obtained by a multi-cathode read-out method and the y position by measuring the drift time of electrons. This detector has no left-right ambiguity and very little dead layer in the sensitive area, resulting in a detector system which is thin and homogeneous in the whole sensitive area. The overall position resolution of 0.20 and 0.4 mm (FWHM) were obtained for x and y directions, respectively. The position resolutions at lower gas pressure were also measured to find an optimum operating condition to reduce multiple scattering effect from the window foils and the detector gas. This detector has been shown to be very useful for high-resolution heavy ion experiments at intermediate energies.

Ion beam equipment modification for external beam operation

Equipment for analyses by energetic ions has been modified to allow external beam application. An exit window combined with beam charge monitoring, and a system of accelerator vacuum protection are items of modification. To measure the analysing proton integral charge, backscattering from the exit window foil is used. To protect the accelerator vacuum in the case of exit foil breakthrough, a delay of pressure increase along the beam line and a fast closing of the beam line are employed.

Large-Area Pressure-Enduring Entrance Windows for Soft X-Ray Regime

A technology capable of producing large-area pressure-tolerant x-ray entrance windows of submicrometer thickness is presented. It is based on successive tungsten griddings to support a multilayered membrane consisting of polyimide, aluminum, and aluminum nitride. The aspects of design and fabrication processes are discussed with emphasis on the window foil fabrication. The performance of the windows is presented in terms of x-ray transmission, gas leak, pressure endurance, and radiation hardness properties.

High-energy electron impact apparatus utilizing a simple diode electron gun assembly

An electron impact facility based on a ribbon filament cathode, simple diode gun, and thin aluminium foil window is described. The facility is designed for the study of gas phase electron impact phenomena at pressures up to one atmosphere in a variety of gases. Of particular interest is the simple design of the electron gun for generating electron beams having energies up to 100 keV and currents up to 5 mA. The beam is magnetically collimated and directed through a thin aluminium foil which is used as the vacuum-gas interface to an impact chamber. Maximum power delivered to the impact region depends on the thickness of the foil window. Using an 8.4 mu m thick aluminium foil, approximately 150 W can be delivered to the impact chamber. Thermal heating of the foil by the electron beam is the limiting factor restricting higher effective power transmission.

Vibration of Beryllium Foil Window Caused by Plasma Particle Bombardment in Plasma Focus X-Ray Source

The effect of plasma particle bombardment on beryllium foil windows was investigated by the displacement measurement of the window foil. It was experimentally verified that the beryllium foil began to vibrate after a shot of plasma focus discharge. It was also verified by the displacement measurement that the particle bombardment caused a thermal expansion of the foil. All these dynamic effects of particle bombardment suggested that fatigue fracture of the foil played an important role in the process of the beryllium foil destruction.

Toward The Simultaneous Correction of Spherical and Chromatic Aberration in Electron Optics by Means of an Electrostatic Electron Mirror

Optimum performance in electron and ion imaging instruments, such as electron microscopes and probe-forming instruments, in most cases depends on a compromise either between imaging errors due to spherical and chromatic aberrations and the diffraction error or between the imaging errors and the current in the image. These compromises result in the use of very small angular apertures. Reducing the spherical and chromatic aberration coefficients would permit the use of larger apertures with resulting improved performance, granted that other problems such as incorrect operation of the instrument or spurious disturbances do not interfere. One approach to correcting aberrations which has been investigated extensively is through the use of multipole electric and magnetic fields. Another approach involves the use of foil windows. However, a practical system for correcting spherical and chromatic aberration is not yet available.Our approach to correction of spherical and chromatic aberration makes use of an electrostatic electron mirror. Early studies of the properties of electron mirrors were done by Recknagel. More recently my colleagues and I have studied the properties of the hyperbolic electron mirror as a function of the ratio of accelerating voltage to mirror voltage. The spherical and chromatic aberration coefficients of the mirror are of opposite sign (overcorrected) from those of electron lenses (undercorrected). This important property invites one to find a way to incorporate a correcting mirror in an electron microscope. Unfortunately, the parts of the beam heading toward and away from the mirror must be separated. A transverse magnetic field can separate the beams, but in general the deflection aberrations degrade the image. The key to avoiding the detrimental effects of deflection aberrations is to have deflections take place at image planes. Our separating system is shown in Fig. 1. Deflections take place at the separating magnet and also at two additional magnetic deflectors. The uncorrected magnified image formed by the objective lens is focused in the first deflector, and relay lenses transfer the image to the separating magnet. The interface lens and the hyperbolic mirror acting in zoom fashion return the corrected image to the separating magnet, and the second set of relay lenses transfers the image to the final deflector, where the beam is deflected onto the projection axis.

Helium cooling of a double-foil window for external cyclotron beams

A versatile helium-gas cooling system for beam window foils is described. Measured cooling power densities are reported for a specific double-foil beam window design, which performed well under bombardment with 66 MeV protons at beam currents of up to 100 μA. Calculated temperature profiles for different window materials and beam parameters are also presented.

Characterization of external 3He ion beams used for ex-vacuo nuclear reaction analysis of deuterium

In-air D(3He, p)4He nuclear reaction analysis (NRA) using external 3He ion beams is now routinely used to characterize deuterium in both fusion and fission reactor materials. The depth resolution, lateral resolution, and detection sensitivity attainable in these ex-vacuo analyses is influenced by the energy spread, the spatial divergence, and the nuclear reaction background produced by extraction of the 3He ion beam through a suitable exit window foil and subsequent transmission of the beam through one or more millimeters of air to the target. In the present paper, the energy and spatial distributions of ex-vacuo MeV 3He ion beams are measured for transmission distances in air of 0 to 15 mm. Exit windows made from both commercially produced foils (3.81 μm thick Au/Al) and foils prepared in the laboratory (1.14 μm thick Au/Al) by e-beam evaporation onto NaCl have been examined. The measured distributions are compared to TRIM simulations, and the resolution of in-air D(3He, p)4He NRA is assessed. Finally, interfering background nuclear reaction yields in air are measured as a function of ex-vacuo 3He beam energy, and the resulting deuterium detection sensitivity is discussed.

A polarized 3He target for nuclear physics

A prototype polarized 3He target based on optical pumping of metastables has been constructed and tested. The target employs an infrared laser to polarize 3He atoms in a pyrex cell which is connected by a capillary to a copper cell at 87 K. With 20% polarization and thickness 4 × 10 18cm −2, the target has been routinely operated with 1 μA of 5 MeV protons passing through it. Using this technique it should be possible with increased laser power and lower temperatures to construct targets of 50% polarized 3He with foil windows and thickness 5 × 10 18cm −2, or windowless targets of thickness 10 16cm −2 for use in a storage ring.

Microhardness evaluation of cyclotron foil windows

The production of radiopharmaceuticals for application to positron emission tomography necessitates radionuclides of finite specific activity and chemical purity. Reliable, routine production of the short-lived, biologically useful radionuclides has required careful selection of target system materials. The degenerative effects of high beam currents and use of refrigerated cooling units on metallic cyclotron target windows are of particular interest. Repeated subjection of the metallic windows and target bodies to energetic charged particles induces crystalline lattice imperfections and strain hardening. The reduction in ductility of the foil window appears to correlate with an increase in hardness as evaluated using the Knoop microhardness technique and optical microscopic examination on both aluminum and Havar irradiated cyclotron windows. A summary of our results on the above effects will be presented.

A polarized 3He target for nuclear physics

A prototype polarized 3He target based on optical pumping of metastables has been constructed and tested. The target employs an infrared laser to polarize 3He atoms in a pyrex cell which is connected by a capillary to a copper cell at 87 K. With 20% polarization and a thickness of 4 × 10 18 cm −2, the target has been routinely operated with 1 μA of 5 MeV protons passing through it. Using this technique it should be possible with increased laser power and lower temperatures to construct targets of 50% polarized 3He with foil windows and a thickness of 5 × 10 18 cm −2, or windowless targets of thickness 10 16 cm −2 for use in a storage ring.

Correlation of hardness with radiation induced defects in metal foil windows

Correlation of hardness with radiation induced defects in metal foil windows

The state of the art of target preparation at the Chalk River Nuclear Laboratories

During the past year, the Chalk River target laboratory has developed several new methods for materials preparation related to nuclear research. These include the preparation of large-area tritium sources for paper document imaging and titanium and tungsten foil windows for water targets and high-temperature ion sources, respectively. The fabrication of ultra-thin titanium tritide sources suitable for a neutrino-mass experiment is being investigated. The preparation of radioactive sources by vacuum sublimation has also been carried out to test the focusing properties and to obtain the response function of the Chalk River iron-free β spectrometer, which will be used in the neutrino-mass experiment.

Low-pressure multisteo detector for very low energy heavy ions

We describe a large-area timing and position-sensitive multistep gaseous detection heavy ions. It consists of a preamplification stage operating as a parallel plate avalanche chamber directly coupled to a multiwire proportional chamber. The detector operates at a pressure range of 1–4 Torr, with very thin window foils. The principle of operation and the performance of the detector when used with various ions are presented. It has a high gain and good time resolution (better than 180 ps fwhm) and localization properties (better than 0.2 mm (fwhm)). Its efficiency for low energy high mass ions was tested with 160Gd ions and found to be 93% (equal to the electrode transparency) down to kinetic energies of 1.3 MeV.

Position sensitive multiwire proportional parallel plate detector for heavy ions

We describe in detail a 10 cm × 10 cm gas counter which is able to provide two dimensional localization with an accuracy of 1 mm and 500 ps timing resolution for 5.5 MeV α particles. The technology of this detector is intermediate between parallel plate and multiwire proportional counters. It is operated under a pressure of a few Torr of isobutane. It consists of a common cathode (1.5 μm mylar foil with gold evaporated on both sides) located between two parallel planes or orthogonal wires acting as anodes. The entire detector is enclosed in a container with two 1.5 μm mylar foil windows.

On-line mass determination of α-active products from the reaction natU + 208Pb with a gas jet time-of-flight mass spectrometer

The damped collision natU + 208Pb was investigated with the gas jet transport system at the Darmstadt UNILAC yielding very complex α-spectra. Utilizing a time-of-flight mass spectrometer for α-recoils many reaction products are identified by α-energies, α − γ coincidences and α-recoil masses. Most of the detected nuclides belong to the translead region. Some products with mass numbers around 150 are assigned to rare earth nuclides, which were produced in background reactions of the Pb ions in the Ti window foil or in the Ar thermalization gas. With 30% gas jet yield and 2% detector efficiency the overall efficiency of the system was 0.6%.

Thin metal foil vacuum interface for low energy particle accelerators

A brazing technique has been developed for thin foil windows that enables them to withstand much higher particle beam currents than windows of previous designs.

Generation of XPS Auger lines by bremsstrahlung

Bremsstahlung-induced Auger lines of kinetic energy 1500–2200 eV can be recorded in the normal operation of an X-ray photoelectron spectrometer. The relatively low intensities can be enhanced by substituting a beryllium foil window for the customary aluminum foil. Auger lines of aluminum, silicon, phosphorus, sulfur, and probably chlorine, are recorded most favorably, and should be useful in identification of chemical states.