Waisman Center Xray Microbeam

PSL first became involved in the project in the late 1970’s, during the proposal phase. At that time a precursor machine developed by JEOL in Tokyo was in operation. JEOL declined to develop a second machine, and PSL was awarded the development project (incorporating significant improvements over the JEOL design thanks to the advice of its users).

The instrument weighs about 20 tons, most of which resides on the second floor of the Waisman Center in Madison WI. Most of the weight is due to thick lead xray shielding and the support structures required. The shielding is up to 4.5″ (114mm) of lead plus 0.25″ (6.4mm) of steel cladding. The high voltage power supply is on the first floor, and stands 12 feet (3.7m) tall. At the other extreme of size is a gold/nickel coating about 0.8 millionths of an inch (200 Angstroms) thick, which is partially transparent yet conductive.

Challenging aspects of the microbeam’s development included ensuring subject safety, machine safety, and excellent long term performance. Specialized skills required included thin film deposition, UHV system design, precision mechanism design, very high voltage handling (up to 600,000 volts), radiation safety design and monitoring, electron optics in vacuum and simulation of scattering in solids, high speed analog and digital electronics design and fabrication, optical alignment, as well as many others.

Up to 3 kilowatts of electron beam power are concentrated into a 0.020″ (0.5mm) diameter area, striking a thin tungsten sheet. Electronics monitor the motion of the electron beam, since if it becomes stationary for more than a few tens of microseconds, it will melt through the tungsten sheet. The heating of the target is so rapid that the rate of thermal expansion is equivalent to running a tensile test to fracture 60 times per second. The power density in the beam is higher than that of a normal electron beam welder, and the maximum available power is comparable. On one side of the 0.020″ (0.5 mm) thick tungsten is 10^-7 Torr vacuum; on the other is a thin layer of atmospheric pressure flowing cooling water. A closed loop cooling system with pressure regulation, ballast tank, and water purification is included to ensure that cooling continues for a short time after power failure, and to minimize the contamination of the vacuum system if a melt-through of the target does occur. Uninterruptible power was included at the control console to allow orderly shutdown.

When striking the target, the electrons produce xrays in all directions; a pinhole in the shielding allows a thin pencil of xrays to pass through the experimental subject to a detector. By successively directing the electron beam to rectangular arrays of spots on the target, areas in the subject where pellets are predicted to have moved are illuminated by the xrays, somewhat like the raster scanning of a tv tube. By the combination of limited area scans near where the pellets were last seen, pattern recognition, and adaptive scheduling of the beam spot, xray dosage to the patient is minimized, allowing a wider variety of subjects and longer experiments than would otherwise be possible.

The instrument was intended to have a 10 year life. We are pleased that the reliability of this complex machine has exceeded expectations, such that the design life has been considerably exceeded, the instrument is still in operation, and a number of spare components built at the same time as the original parts have yet to be used. This instrument has over time become a standard by which others are evaluated, thanks to its longevity, experimental database, resolution, tracking rate, and repeatability: Magnetometer and X-ray Microbeam Comparison

The xray generator’s electrical and mechanical components were designed and constructed in the early 1980’s. On April 13, 1984, it was assembled with a small radiation cooled target. By August 20, 1984, tests were under way with radiation cooled targets. In October 1985 the system was disassembled, shipped from PSL to the Waisman Center, and reassembled. After verification of safety and controls, and operation on static and moving mechanical phantoms, the first tests with live subjects were performed in early 1987.

The project’s principal PSL electronics engineer, mechanical engineer, and physicist all remain at PSL, providing technical support to the project when required, without the much higher cost of maintaining such personnel on staff at Waisman Center.

Above is the operator’s console for the Xray generator. Power supplies for the vacuum system’s pumps and gauges occupy a separate third rack placed next to the xray generator.

The xray generator is housed inside the blue painted steel clad lead shielding to provide the many millions of attenuation required to provide radiation safety. The shields’ steel cladding protects the soft lead and also partially shields the beam from the effects of the earth’s magnetic field as well as an MRI machine located in the same building. The electron source is at left in the photo above. It contains a hairpin filament in a triode gun and electrostatic accelerator. At its outlet there are the first in a series of sets of static steering coils to compensate for the beam curvature caused by the earth’s magnetic field. Following the steering coils are a focus coil, ferrite magnetic field shields, and beam dump. In the taller central section there is an axially positionable 4-jaw independently adjustable collimator with a second set of steering coils. This is followed by a limiting aperture, second focus coil, and main deflection coils. The enlarged near section contains an aluminum vacuum tank with viewports and water cooled transmission xray target.

Finally the heavy metal pinhole collimates the xrays produced from the spot where the deflected electron beam strikes the vacuum side of the tungsten target. The green chair was a dentist’s chair and is where the speech research subject sits, conveniently providing positioning of the subject’s vocal tract onto the machine’s optical axis. There are vacuum ion pumps near the source outlet, above the electron collimator, and on the target tank. Visible beneath the blue frame is a modified arc welder adapted to inexpensively power a Varian Ti-Ball sublimator pump. Many sections of the beamline provide current sensing and require water cooling. Visible in the lower left is a section of removed shielding. All shields weigh less than the room crane’s 1 ton limit. Each major shield is labeled with lift weight and instructions. Redundant microswitches sense the absence or improper seating of any major shield and prevent electron beam power via interlock circuitry. Part of the machine maintenance protocol is to survey the machine exterior at low beam current for xray levels after any cycle of shield removal & reinstallation. The machine frame provides mounting for much of the shield tonnage even while removed from the beamline, since the load rating of the floor is low.

After passing through the subject, with variable attenuation & scattering depending on the amount and atomic number of material encountered, the xrays reach the detector cabinet shown at left in the photo above, where a large area scintillator backed by an array of photomultiplier tubes detect them. The detector cabinet is movable on rails to vary the spacing from the subject while maintaining alignment to the source’s optical axis. Computers gather the data and correlate it via timing with where the electron beam was steered and the pinhole camera properties on the xray side to map density, and via template recognition, estimate the new position of gold tracking pellets on the subject. Pellet velocities and accelerations are calculated to forecast future positions. The next set of raster positions for electron spot exposures are calculated from the pellet position forecasts and rescan schedules. Dwell times are selected to control subject dosage, provide adequate signal to noise ratio, and allow reasonable xray target life. Rescan schedules are set on a per-pellet basis; slowly moving locations such as skull and lower-jaw pellets are scanned less frequently than fast moving locations such as the tongue tip.

Above is a photo of the wetted side of the water cooled tungsten target after it has been in use for some time & installed for years. It is about 305mm wide × 310mm high, and about 0.5mm thick. The electron-beam-addressable area is 256 × 256mm out of a slightly larger area exposed to vacuum. The sheet is clamped between two stainless steel flanges into a cylindrical shape. The wet side is the concave side of the bent target. Water flows horizontally across this face of the entire beam-addressable area of the target sheet in a layer about 1.6mm (1/16″) thick. A thin layer of glass reinforced plastic provides the other side of the water flow channel, selected for adequate strength & stiffness & acceptable xray transparency. (Water & plastic must be minimized to control blurring of the very small xray source.) The colored deposits are believed to be oxides of tungsten, although some migration of copper from other parts of the cooling system may be occurring (despite the water purification system) and account for the greenish tinge. The horizontal transition line near center is the water level when not running. Note that the humid air environment in the top half produces a different oxide appearance. The irregular shaped area in the center is the area most used for speech tracking. Thin layers and frequent oxide shedding occur in areas that run hot during tracking. Boiling may sometimes be occurring at the more active areas. It has been cost effective to use bare tungsten targets, rather than plate them on the wet side to limit corrosion. Most platings would not be suitable for xray production on the vacuum side.

In the photomicrograph above, scale is 1mm between large horizontal bars. Cracks & crevices in the (probably tungsten trioxide) bands can be seen at 1 to 2 mm separation, along with spalling of about 50 to 100 microns wide very thin gray oxide. In other areas these spalls are about 250 microns wide.

The thermal strain caused by electron beam heating of the vacuum side is considerable. Beam impact spots sustain damage over time as the same x,y addresses are digitally addressed repeatedly. Spots recrystallize, darken and develop central cracks which grow and cause delamination. The image above is about 4mm square, showing numerous cracks.

With further use the surface layer will eventually peel. The peeling can be seen in xray production differences. Because the system performs background subtraction (a necessity to deal with the presence of teeth, bone and fillings in the tracking area), slow changes in the target that are essentially static during the course of a speech data run do not affect tracking as long as there are enough xrays passing through the subject to the detector. It is also practical to seat the subjects to use different areas of the target. A spent target may have several areas where peeling of about 1cm has occurred. In the example above, numerous arrays of spots can be seen many cm away from the 3 large peeled areas. Given time these too would crack and peel. The photo shows an area of about 11cm square, or 1/8 of the total area.

In an extreme case of peeling the peel will further distort and curl up because once delaminated its cooling is very poor. These can be seen by the local change in xray production. In effect the damaged target provides a radiograph of its own failed areas. This chip (shown in the 3 photos above) was observed as an area of reduced xray production, leading to scheduled target replacement. Note that it exhibits elongated spots, wrinkling, scale of over 1cm, and about 270 degrees curl. Previously buried layers will also peel. To prevent further weakening of the target and consequent introduction of water into the vacuum system, damaged targets are replaced.

Simulations indicated most of the electron beam’s power deposited in the target is absorbed in the first 150 microns. About half the current (containing one third the power) is eventually scattered back out the vacuum side of the target and strikes the aluminum vacuum chamber, creating a diffuse secondary source of xrays. Aluminum was chosen as an economical material to limit production of xrays at this secondary collision. Some of the back-scattered electrons are sufficiently energetic to create Cerenkov radiation visible through the target tank’s viewports. Several inches of high-lead-content shielding glass is used at the viewports to make viewing the vacuum side of the target safe.

Visit the UW Medical School Microbeam website.