Last Revised: November 4, 1998
This note describes the phosphor screen preparation procedures to be used for the AGS-to-RHIC
(ATR) transfer line phosphor screens (Flags).
Phosphor: Gadolinium oxy-sulfide doped with terbium (Gd2O 2S:Tb)
Substrate: Aluminum Foil, 0.001" thick
Epoxy: Torr Seal
Wooden Frame:
Custom Pyrex Trays:
II. PREPARATION OF THE SUBSTRATE
Cut a piece of aluminum foil about 12" on a side, being careful not to wrinkle the foil. A special jig has been
built on which the aluminum foil can be stretched. When the foil is smooth and tight an aluminum disk of slightly smaller diameter than the ring ID, but
the same height, is slid in place and the rings lowered over it. This supports the foil in the next step.
The screen frame, P/N RD58020005A, must be cleaned in acetone, alcohol and finally with distilled water. A thin layer of the Torr Seal epoxy (mix as per
directions using 1/4" long strips of A resin and hardner) is then applied to one side of the frame. The frame is then carefully placed on the foil
so that the epoxy is not rubbed across the foil. Weights are placed over the frame to hold it while the epoxy sets. These consist of small blocks of
aluminum 1" square and 3" long. Let sit for 2 - 3 hours.
The foil must be carefully cleaned again in baths of acetone, alcohol and distilled water then allowed to air dry in an area free of dust and lint. A number 6 camel hair brush should be used to clean the foil before it is put into the Pyrex tray.
The foils should be stored in holders specifically constructed for this purpose, they provide protection and reduce possible contamination.
III. SAFETY CONSIDERATIONS
While the chemicals used are not classified as hazardous according to the MSDS, the manufacturers recommend working with proper ventilation and preventing skin and eye contact. A face shield or, as a minimum, goggles, and plastic disposable gloves should be worn. Inhalation should also be avoided. The deposition does not require a ventilated hood but great care should be taken to avoid dust falling on the screens (keep all containers and the Pyrex settling tray covered with aluminum foil when not actually in use). The MSDS says that the PS-6 can be put into the waste water if it is "sufficiently diluted". However, the AGS safety representative has required that the residual liquid be put in sealed glass jars disposed of through the Safety Office, and any water used to wash the glassware after deposition, be recovered in a closed waste system. A suitable facility is located in the Linac Building (930) on the main floor in the rear of the mechanical shop area. There is a hood and a closed waste water system next to it.
IV. PREPARING THE CHEMICALS
The following recipe is for the custom made phosphor deposition trays and takes account of the real dimensions, including the filleted corners. It allows for the frame being supported on 6-32 stainless nuts and not lying directly on the bottom of the tray to allow drainage. The weight of phosphor shown is calculated for .002-inch thickness of phosphor based on a density of 3 g/cm3 . To eliminate any possible moisture contamination of the phosphor, measure the phosphor needed and store in a sealed (labeled) intermediate container shortly before it will be used. If possible, acquire phosphor material from beneath the top layer from the large phosphor storage bucket.
The following table states the amounts of ingredients needed:
V. APPLYING THE PHOSPHOR
First set up the production apparatus using the following technique.
Put the glass deposition tray in the hood onto the wooden frame which will allow clear access to the bottom drain carefully. Attach the tygon tubing to
the bottom drain and close the stopcock. Place the screen(s) on the bottom of the glass tray with the foil side up, supported on the nuts. The nuts will
keep the screen off the bottom of the tray, allowing the liquid under it to drain. With its large surface area, the light screen will tend to float as the liquid is
added. Weights such as clean aluminum blocks can be used to hold it down but may produce non-uniform deposition over too large an area. It has been
found to be better to use 1/16th inch tinned copper rod to apply pressure to the screens. The rods must be bent at about a 30o angle
and held to the glass side wall by small "binder clips".
Mix the chemicals together.
Measure the water using one of the beakers, and graduated cylinder for accuracy. Put about half of the water into the other beaker. Add the PS-6
to one of the beakers. Several times (until there is no more water) pour some of the remaining water from the other beaker into the graduated cylinder,
and dump into the beaker with the PS-6
water mix, the effect is to rinse the graduated cylinder and have one beaker with all the water and PS-6. Mix the water and PS-6 with the drill/mixing tool
until there is a uniform distribution.
When fully mixed add the phosphor and mix again. Do not mix the phosphor with water first.
There is some evidence that the binder will not hold the phosphor properly in this case. Do not let the PS-6 and water stand for more than a few
minutes before adding the phosphor or the liquid will be too thick to allow the powder to mix fully. Keep stirring or the phosphor will quickly settle
to the bottom.
Transfer liquid to the deposition tray apparatus.
Emptying all the liquid from the beaker in
a single fast pour seems to make the deposition more uniform and does not result in significant bubble formation. (Originally a funnel was used when
pouring in the solution to minimize bubble formation.
It was found that the phosphor would not stay in solution during the time required to drain through the funnel). After pouring all of the liquid into the
tray, cover it with aluminum foil and leave UNDISTURBED for 30 minutes after which the liquid will be clear and the phosphor settled to the
bottom. It is much better to leave the setup undisturbed over night. This results in less perturbation of the phosphor when the hold-down rods are
removed and the solution drained. The binder does not appear to set until the water has been drained, so movement, even after many hours, may
disturb the phosphor.
VI. REMOVING THE LIQUID RESIDUE
The bottom drain makes it very easy to remove the liquid. Place a
quart jar on the floor with the tygon tubing fed into its mouth. Open the stopcock and "slowly" let the liquid out (slowly is of the
order of 5 - 10 drops per second). When the level gets to about 1/4-inch of the screen close the stopcock until there is only about 1 or 2 drops
per second to prevent disturbing the phosphor. When the level is below the frame, the liquid flow can be increased again. After the liquid is
drained from the tray, close the stopcock.
Cover the tray and let air dry for several hours minimum.
VII. DRYING
After the screen has dried for several hours, remove the screen and set it to dry in a safe place supported on 6-32 stainless nuts to allow the
underside of the screen to dry and prevent sticking. Cover the foils with a tent of aluminum foil. A set of tongs specially modified to slide
under the sides of the screen help in lifing the screens out of the tray. Pour some of the liquid in the quart jar back into the tray. Use a "
solder brush" to loosen the phosphor deposited on the tray bottom. While stirring the liquid to pick up as much of the phosphor as
possible open the stop cock again to the liquid back into the jar. Seal the jar and deliver it to the Department Safety Representative.
Form BNL F2974 may have to be filed.
After air drying overnight the screens may be stored in their custom made sleeves. It may be desirable to bake at 125o
C for a few hours before use to drive off any water vapor, but experience at other labs has not confirmed this.
VIII. FIDUCIAL MARKS
A series of small circular pencil marks form the calibration (fiducial) marks made by transferring the pattern to the screen from a numerically controlled laser
drilled metal template. There are 4 patterns which have been used for the AGS to RHIC transfer line. These are given on BNL drawings 508020048 through 508020051.
The transfer procedure is as follows;
Use a Pentel P207 mechanical pencil with a 0.7mm HB lead to transfer the pattern to the screen.
B. Position the screen over the cardboard so that it fits in the frame opening to provide support when the marks are made in the screen.
C. Carefully align the template with the edges of the screen frame.
D. Place the pencil lead in a template hole and twist 360 degrees clockwise, then 360 degrees counter-clockwise, applying no additional downward force (only the weight of the mechanical pencil).
E. When all the marks have been made, remove the template. Inspect carefully.
F. Remove the cardboard support from the bottom of the screen.
G. Return the screen to its custom made sleeve.