Electron beams in a good vacuum are invisible, but a screen coated with a material that fluoresces when hit by electrons can be used to make the beam visible. These fluorescent materials are called phosphors, and are used most commonly in cathode-ray oscilloscope and TV screens to produce an image by steering an electron beam inside the CRT. Phosphors don’t contain the element phosphorous. They are usually fine powders of zinc sulfide (ZnS) that have been “activated” with a tiny bit (a few parts per million) of either copper or silver. The ZnS(Ag) or ZnS(Cu) powders are then mixed with a binder (an adhesive to help them stick to a surface) into a suspension and deposited onto the screen. Another popular phosphor for electron-tube screens is zinc silicate activated with manganese.
We made a screen for our Ace Glass set of components from an Ace 1,000 mL Gledhill flask (catalog number 14205-09), the base of which we coated with a phosphor (Figure 40). The bottom of the flask is not perfectly even, so we first poured 20 mL (around 3/4 oz.) of clear, slow-cure epoxy on the flask and let it sit for 3 days to form a flat, transparent surface.‡
After that, we thoroughly mixed 3 g of phosphor powder sold for recoating electron-microscope screens (made by Structure Probe, as their catalog number 04129-AB) with 0.75 g of Beacon 527 multi-use glue (from a craft store) and 10 g of acetone (from the paint thinner aisle in a hardware store). We then poured the suspension onto the bottom of the flask (on top of the cured epoxy), swirled it for even distribution, and allowed it to sit undisturbed until the acetone completely evaporated. We also painted a 1-cm wide fluorescent stripe on the side wall of the flask between the screen and the Ace-Thred #7 side port of the Gledhill flask.
Commercial phosphors made for electron microscopes are somewhat expensive, so you could try other materials that fluoresce under electron bombardment. These include many highlighter pen inks, UV pen and stamp-pad inks, and glow-in-the-dark paints.
An alternative to making your own screen is to use one from an oscilloscope CRT that has been cut away and matched to an Ace-Thred connector. As shown in Figure 40, we cut a 3HP7 oscilloscope CRT in half and mated the screen to an Ace Glass #25 threaded connector (Ace catalog number 7644-20), using a piece of 1-in. ID rubber hose (Kurt J. Lesker catalog number T100 coated on the inside with Dow Corning high-vacuum grease). We tried to cut the neck of the CRT with many of the common bottle-cutting methods (hot wire, score with diamond cutting wheel, etc.), but had the best results cutting it with a Dremel rotary tool using a diamond grind disk from Micro-Mark (catalog number 82259 or 84611).
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