Preparation of the Plastic Veto Detector for the Pion Beta Decay Experiment
Here is some information and suggestions for:
- and Installing
The plastic veto scintillator staves and light guides(LG). The
light guides are Lucite while the staves were made by
Bicron out of
The LG were made at PSI in the light
guide shop. Each LG is comprised of two parts:
LG1 : the piece that is coupled directly with the scintillator
Each stave has two LGs, one for each end.
LG2 : the piece that is glued to LG1 and couples it to the
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
- 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.
- 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
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
Arizona State University