Preparation of the Plastic Veto Detector for the Pion Beta Decay Experiment

Here is some information and suggestions for:

The plastic veto scintillator staves and light guides(LG). The light guides are Lucite while the staves were made by Bicron out of BC-400 scintillator.


The LG were made at PSI in the light guide shop. Each LG is comprised of two parts:

Each stave has two LGs, one for each end.

As the LGs have broken off from time to time, it was neccesary to remove any residual glue from the broken joint so the joint would be clean when it was re-glued. This was initially done using a razor blade and that is still the recommended method. Unfortunately, razors have an tendency to scratch the surface quite a bit. At one point in time, fine grit sandpaper (600) was used to remove any glue which was not easily remove by a razor so the deep scratches in the surface could be avoided. THE RECOMMENDATION IS NOT TO USE SANDPAPER BUT TO USE AS MANY NEW RAZOR BLADES AS NECESSARY TO REMOVE ANY RESIDUAL GLUE FROM A BROKEN JOINT.

If it turns out to be necessary to polish a stave or a LG, here is the procedure:

  1. Sandpaper: Use wet (plain water is fine) 600 grit or higher sandpaper until the surface is more or less even. It should have the appearance of frosted glass. If possible, place the sandpaper on a table or attach it to flat object as this will help prevent wearing the surface unevenly. Some of the LGs in use have somewhat uneven surfaces already so it might be a good idea to use a belt sander or the like to even up the surface before using the 600 grit sandpaper. Just remember that the length may be an issue so if you're at that point, you probably want to consider having a new piece made.

  2. Aluminum Oxide sanding Gel: This is sold as a car polish but claims to be equivalent to 10000 grit sandpaper. It does a very nice job. I have always used this with a buffing wheel attached to a drill which I had mounted on a table. You will probably want to wet the buffing wheel first with water and then apply the Alu. Oxide gel to it while it's spinning. Get the buffing wheel nice and coated before bringing in your piece. The most important thing to remember here is that you can overheat the piece by pressing too hard, having the wheel spin too fast, or letting the wheel get too dry. Try and keep the wheel moist with water and keep the speed at around 120 rpm. It's perhaps worth noting that when I was originally consulting with people in various fields about polishing plastics, I was told by one person that the best way to get a smooth shiny surface was to use a blow torch. Though this may not be feasible, if you did get the surface of an LG close to the melting point (e.g. while using the buffing wheel, the surface would likely come out very polished. You just have to worry about distortions in the surface. It's also good to keep in mind that while it won't hurt the LGs too much to be overheated, it is much easier and potentially damaging to overheat the scintillator.

Final Remarks: I would again like to stress that in my opinion it is not worth polishing anything. In the case of the staves, you tend to do more damage from handling them while polishing (i.e the very noticable crazing). For the LGs, you will almost certainly end up with a much better piece if you have a new one made. The 1996/1997 beam data showed the half-staves all preformed more or less the same while some had dramatically more aesthetic glue joints and/or phototube coupling than others. About the only noticable difference was in the overall gains which were easily adjusted via H.V.


The staves and LGs are glued together using Bicron BC-600 Optical Cement This is a two part epoxy that should be mixed at a ratio of 100/28 parts base to hardener by weight. From the information supplied to me by Bicron, this comes out to 3/1 by volume (with insignificant volume dependance on temperature). I have used syringes to measure each part and mixed them in one of the small aluminum cups which can be found in the Plastic veto polishing materials box.

The pieces are best held together using one of the small teflon squares contained in the Plastic veto polishing materials box. Rubber bands (2) should be placed on the teflon square so that one rubberband (wrapped around 3 or 4 times) will hold one of the pieces to be glued securely in place. It's important not to make the rubberband too tight since that may prove an obstacle when you're ready to remove the pieces (unless you're willing to sacrifice the rubber band and just cut it off). After both rubberbands are secured, slip in each of the pieces and make sure they line up. You will probably need to support the phototube end of the LG (i.e. LG2) so the LG is flush with the teflon. Also, make sure the stave is well supported so it doesn't get stressed any more than necessary adding to the crazing problem.

Once the pieces are prepared and mounted on a jig (i.e. piece of teflon) mix the glue and let it sit for a few minutes (~10). This lets it thicken up a little so it will stay where you put it as well as giving some of the air bubbles a chance to rise to the surface. Due to the nature of the pieces involved and the way in which they are glued, it has been my experience that using a vaccuum chamber to remove air bubbles from the glue is not necessary. When all is ready, apply the glue to both surfaces and press the. pieces together. Use the rubberbands to keep the pieces pressing against each other while the glue is drying. Make sure you leave a nice bead of glue across the top and sides of the joint so if the glue inside the joint starts to drain out, it will not leave an air bubble in the joint.

Allow the glue to dry for approx. 24 hrs. Remove the rubberbands and prepare yourself for the most stressfull part of the procedure ... removing the pieces from the jig. At my best, I still experience about a 5% breakage rate at this point. You will probably see glue that has worked its way under the pieces to either side of the joint. When viewed from the correct angle, this will change color as it is pulled away from the teflon. It is best to first lift gently on one side watching the discoloration until it gets to the joint. Repeat for the other piece. Keep working on each piece by gently pulling it up until the pieces break free (hopefully, still together). Sometimes, it has helped to wedge the end of the small metal spatula I use to mix the glue under one of the pieces and let it sit for a minute.

Once the pieces are glued, use a razor blade to remove any glue that could get in the way while mounting the piece in the detector. It's very important to remove any glue that will be on the carbon fiber cylinder side of the stave since it is a very tight fit there. Any glue that won't physically conflict with other parts of the detector could be left in place to provide strength for the joint.


The staves are wrapped using the same aluminized mylar used in wrapping the crystals (in B11). A couple of extra strips are in the Plastic veto polishing materials box. They are about 104mm wide. The wrapping is taped (Scotch tape) along the top (outside) of the stave. Don't let any tape wrap over the sides since space there is very tight. The LG2 pieces are shaped oddley enough that is difficulty to wrap these in a clean matter. To be consistent with the others, use black electrical tape around this part and just crush the aluminized mylar to make it conform to the shape of the LG. You may find it easier to insert the stave if the black tape doesn't extend more than 100mm from the phototube end of the LG.


Mounting the staves is a difficult task to say the least. I WOULD HIGHLY RECOMMEND NOT USING THE PINS WHICH DEFINE THE POSITION OF LG1. I believe these pins are no longer necessary considering the carbon fiber cylinder, elastic band(s), springs,... which should hold the array firmly in place. I believe these lead to most of the LG breaks experienced in the test mountings. Additionally, without these pins, it would appear much easier to use the stave support mechanisim to support the stave until it is completely inserted into the detector. One important thing to note while inserting the staves is to make sure you have them all oriented the same way (w.r.t. the angled side.)

This page written by
David Lawrence
Arizona State University
March 29,1998