Fabrication of Fiber Apparatus for the BTSM

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					             Fabrication of Fiber Apparatus for the BTSM


This report describes the fabrication procedure for the scintillating fiber apparatus to be
used in the Beam Time Stamp Module (BTSM) of the Charged Kaons at the Main
Injector (CKM) experiment, and the scheme used to map individual scintillating fibers in
the apparatus to Hamamatsu H7546 Photo Multiplier Tubes.


Components of the Fiber Apparatus:

      Bicron multi-clad, 1mm diameter scintillating fibers. A total of 512 of these fibers
       was used.
      Hamamatsu H7546 photo multiplier tubes (PMTs), each with an 8x8 grid of pair
       of pixels. 8 of these tubes were used.


Introduction to the BTSM:
The basic purpose of the fiber apparatus to be used in the BTSM is to tag the arrival time
of Kaon particles in a Kaon-rich beam with a precision of 1ns. The fiber apparatus
consists of two fiber ribbons, each attached at one end to 4 photo multiplier tubes. The
fibers leading up to each PMT are arranged in the form of an 8x8 fiber grid aligned with
the pixels of the PMT. Each individual fiber in the grid is mapped to a unique pixel pair
in the PMT. Each planar fiber ribbon is produced by gluing 256 scintillating fibers next to
each other. One fiber ribbon is placed on top of the other, the centers of the two displaced
by 0.5 mm such that the fibers of one ribbon settle in the depressions formed between
fibers of the other ribbon, and vice versa. The length through which the fibers must be
shaped in the form of a ribbon is 160 mm (6.299 inches). This length marks the effective
area where a Kaon shower will hit the BTSM. Since the fibers are in fact about 26 inches
long, all the rest of the length is available for accommodating the distortions caused in the
fibers by a change in shape from planar to grid-like.
For a detailed introduction, please visit:
 http://galileo.phys.virginia.edu/research/groups/hep/aag/aag_memo_list.html


Fabrication Procedure:

The fabrication procedure described in this report is in the form of a bulleted list, as we
think that this is the clearest way of presenting the conclusions of a detailed effort. The
emphasis of the effort had been to build commendable apparatus using simple ideas and
tools, and likewise to present the information in simple terms.
The fabrication procedure for the fiber apparatus consists of the following steps:
Fabrication of the ‘Fiber Grids’:
    Potting the fibers in the Photo Multiplier Tube Mold (PMTM).
    Sanding and Polishing the fiber ends potted in the PMTM (called fiber grids in
       the report).
Fabrication of Fiber Ribbons:
    Forming a ribbon with the free ends of fiber grids.
    Potting fiber ribbons in the fiber ribbon mold.
    Polishing fiber ribbon end.
Attaching Fiber Grids to PMTs:
    Forming a mapping scheme between individual fibers and PMT pixels.
    Devising a scheme to precisely position a PMT with respect to a fiber grid
       (discussed in a separate report).
    Attaching PMTs to fiber grids.



Fiber Grid Fabrication:

 Potting the Fibers in PMTM:

   Apparatus:

      PMTM: Serves as a “pot” for dipping fibers in a reservoir of potting compound
            and letting the compound cure.




            (Refer to “btsm_pmt_mold_v1_1.dwg” for dimensions and details)

      Potting Compound: UC40 or EP30
    Teflon Plate: Used for making a regular array of scintillation fibers. Also fixes
                into the PMTM.




                (Refer to “btsm_pmt_mold_teflon_v1_1.dwg” for details)

    Hot Knife: Used for cutting scintillation fibers. Brand: Industrial Fiber Optics.
    Polyethylene Tubing: To prevent fibers from dust, oil and other ambient
            detrimental pollutants.

   Spray the recommended amount of mold release (Mann Ease Release 300) on the
    inner side of the PMTM after cleaning it thoroughly.
            Caution:
                    If UC40 is to be used as the potting compound, a more-than-
                    recommended amount of mold release might be needed as UC40
                    expands considerably when curing, and hence has a tendency to
                    get stuck in the mold.
   Pass fibers through two rows of holes in the PMT Teflon plate with an 8x8 array
    of holes.
            Recommendation:
                    The diameter of the holes in the Teflon plate is quoted to be 0.041
                    inches. The second batch of these plates had a larger diameter of
                    approximately 0.042 inches, which worked better in that it the
                    fibers could be easily passed through the holes.
   Cut the fiber ends passed through the plate with a hot knife.
   Repeat the procedure so as to fill all the holes in the plate.
   Fix the Teflon plate in the PMTM.
            Note:
                    Make sure all the fibers are touching the bottom plate of the
                    PMTM. This can be done by gently pushing the fibers downwards
                    before fixing the plate in the PMTM so that all the fiber ends come
                    out from the plate to about 0.5 inches. When the plate is fixed in
                    the PMTM, the bottom plate will push the fibers back as the depth
                    of the mold is less than 0.5 inches.
    Mix parts of the potting compound (either UC40 or EP30) in the recommended
     proportions.
             Recommendation:
                    EP30 is the recommended compound. The mixing ratio of Part A
                    and B for EP30 is 100:50. In the actual experiment, 0.040 lb of
                    Part A was mixed with 0.020lb of Part B with an uncertainty of
                    0.005 lb.
    Fill the PMTM with the potting compound through the two holes in the Teflon
     plate.
             Caution:
                    If UC40 is used as the potting compound, care must be taken to
                    minimize air bubbles in the compound. The issue of air bubbles is
                    not that severe with EP30.
    Let the potting compound dry.
             Recommendation:
                    UC40 should be left for at least 8 hours, while it is recommended
                    to leave EP30 for about 40 hours.
    Take the final product (PMT fiber grid) out of the mold.
    Enclose the fibers in polyethylene tubing.
             Note:
                    The polyethylene tubing can be glued to the Teflon plate with the
                    Five Minute epoxy. The epoxy is not only a good sealant, but it
                    sticks to the Teflon plate very mildly, so it can be very easily
                    scratched off if needed.


Sanding and Polishing PMT fiber grids:

The objective of the exercise is to create a polished, level fiber grid surface, to which
PMTs can be attached.

Apparatus:
    Sand Papers: Silicon Carbide, waterproof #240, #400a & #400b, #600, #1500, and
                0.6 micron
    Sanding Cookie: A metal plate that can be fixed to fiber grids, meant to make sure
                     that the sanding produces a flat fiber grid surface.


 Sanding:

    Fix the sanding cookie on the PMT fiber grid so that it represents a level
     horizontal surface, which can be used as a reference during polishing of fiber
     grids.
    Sand the PMT fiber grid with a sander until all the fiber ends are completely
     visible.
             Recommendations:
                 Use a hand sander with a rigid base.
                 Must not use a sand paper under 220 Grit.
             Note:
                   If looked under a microscope, the fiber ends at this stage might
                   exhibit the following damages:
                        Detachment of the cladding from the inner core.
                        Cracks in the cladding.
                        Scratches on the sanded ends of the fibers.
                   The next phase of the process is designed to eliminate these
                   problems.

Polishing:

   Put 400-A Grit sand paper flat on a table.
   Start sanding the grid surface in a figure „8‟ motion with the fiber grid inverted on
    the sand paper (i.e. facing the table).
            Recommendation:
                          Water was used when polishing the fiber ends as is
                            recommended by Bicron.
   Sand off 1 mm to 2 mm of PMT fiber grid surface.
   Change to 400-B/600-A sand paper and keep polishing until the sandpaper stops
    reducing the scratches on the fiber end.
   Continue sanding with 600-B sandpaper until all the damages, other than
    scratching have been eliminated.
   As a last step, the scratches on fiber cores are to be removed. Start polishing with
    1500 Grit sand paper. Keep sanding for as long as the scratches keep diminishing.
    Stop when any further sanding does not seem to have any bearing on both the
    number and the depth of scratches.
            Note:
                    To be sure that the grid surface is flat, the grid was polished down
                    all the way to the level of the sanding cookie. The finer the sanding
                    paper used, the less efficiently is the surface sanded. Hence, if
                    polishing down to the sanding cookie is desired, it is a bit
                    subjective to decide when to switch to a finer sand paper.
   Switch to a sheet of soft plastic (e.g. 0.6 micron sand paper), and polish the fiber
    grid with it in conjunction with Novus Fine Scratch Remover polishing
    compound. Keep polishing the fiber grid until the polishing compound has dried.
   Clean the surface with a clean, soft, cotton cloth by rubbing it in circular motion.
   Look under a microscope with at least 50 times magnification to see if all the
    scratches have been removed. If there still are scratches, continue polishing with a
    plastic sheet using polishing compound until all scratches have been removed.
            Note:
                    At such high magnification, marks left behind by the cloth might
                    appear as scratches. Discount these marks.
Fabrication of a Fiber Ribbon:

   Forming a Fiber Ribbon:

   Apparatus:
   Grooved Metal Plate: To position fibers in the form of a ribbon.




          (Refer to “groove_plate_blow_v1.dwg” and “groove_plate_v1.dwg”)

   Steel Clamp:




                    (Refer to “btsm_bot_mold_v3_4.dwg” for details)

   Single Row Teflon Strip:




                  (Refer to “btsm_bot_mold_v3_4.dwg” for details)
Double Row Teflon Strip:




                  (Refer to “btsm_bot_mold_v3_4.dwg” for details)

   Fiber Ribbon Mold: to pot the free ends of the fiber ribbons.




            (Refer to “btsm_bot_mold_v3_4.dwg” for dimensions and details)

   Fiber Ribbon Jig:

   Adhesive: Bicron BC-626 Fiber Ribbon Adhesive.
   Polyurethane Enamel: To coat the metal grooved plate. Brand: Zynolite Premium
             Hi-Gloss.

   Two techniques are currently under consideration for forming the required fiber
   ribbons:

      1- Single Ribbon Fabrication (SRF) Technique:
            The technique depends on making two fiber ribbons individually, and then
            putting them together.
            SRF technique can be branched out into two sub techniques. The
            techniques are called 1A and 1B for the time being. The main difference
            between the two is that while in 1A the two ribbons are clamped to each
            other such that there is no distance between the two, in 1B the two ribbons
         are held in place at some distance from each other. The difference in
         procedure for the two is mentioned at every step where it occurs.
   2- Double Ribbon Fabrication (DRF) Technique:
         This technique relies on making two identical fiber ribbons
         simultaneously, while keeping them in place relative to each other by
         mechanical means.

A Comparison Between the Two Techniques:

The advantages of DRF technique are:
   1- Less time required to make the fiber apparatus than that required in the SRF
       technique.
   2- Greater accuracy in the alignment of the two fiber ribbons than SRF
       technique.

The advantage of SRF technique is:
   1- Lesser care and skill needed than DRF technique.
   2- Availability of the important option of using low viscosity adhesives for
       making a fiber ribbon.


Single Ribbon Fabrication Technique:

      Fix four fiber grids in a jig, so that they are held in place rigidly. Fix the jig in
       a stand such that the fibers should be coming out of the Teflon plate
       horizontally.
      Pass the fibers of the grids through the holes of the single row Teflon strip one
       by one according to some regular scheme such that each fiber is mapped to a
       uniquely determined hole.
           Note:
                   This mapping scheme helps in identifying the fiber hit by a
                   charged particle when an electrical signal is received from a photo
                   multiplier tube attached to the fiber grid. The mapping scheme for
                   both SRF and DRF techniques is discussed later in the report.
           Caution:
                   Do not bend the fibers more than the safe limit guidelines set by
                   Bicron. Please see the manual for specific details.
      Slide the Teflon strip up the fibers gently until it is no less than 14 inches from
       the end of the shortest fiber (6.5 inches for ribbon + 0.5 inch for mold + 1 inch
       for cutting fiber ends + 1 inches for clamp + 1.5 inches for accommodating
       the stress of placing two fiber ribbons on top of each other in the case of 1A).
           Recommendation:
                   Sliding the strip up only some fibers at a time might be easier than
                   sliding up all the fibers at once. Pushing only some fibers at a time
                   down the strip might be even easier.
    Once all the fibers have been passed through the Teflon strip, put a steel
     clamp right above the Teflon strip in case of 1A. This clamp will serve as a
     temporary clamp.
 Move the Teflon strip down the fibers to make room for placing the metal
     grooved plate underneath the ribbon in between the clamp and the strip in case
     of 1A.
 Coat the metal plate grooves with Polyurethane Enamel so that they don‟t
     scratch the fibers.
 Lay the fibers coming out of the clamp (in 1A) or Teflon strip (in 1B) in the
     grooves of the metal plate designed to hold the fibers in the form of a ribbon.
     The clamp/strip should be touching against the side of the metal plate.
 Place a sheet of soft rubber over the fibers laid in the plate. Screw the sheet to
     the metal plate.
     OR: place another grooved plate on top of the ribbon, such that the ribbon is
     held in-between the groves of each plate. This is the method adopted for
     making a prototype fiber ribbon.
 Make a channel for the metal plate in which it can be moved in a straight line
     perpendicular to the metal clamp.
         Note:
                  For building a prototype, an optical table was used for fixing the
                  fiber grids and for making a temporary channel for the movement
                  of the metal plate. The grid holding jig was fixed to the optical
                  table, and so was the steel clamp, so that the fibers were precisely
                  positioned and the clamp was positioned precisely at right angle to
                  the length of the ribbon. Positioning the Teflon strip can be more
                  challenging owing to relative immobility of Teflon strip with
                  respect to the fibers. Once the clamp was positioned, holes in the
                  optical table were used to make a channel perpendicular to the
                  clamp for moving the metal plate.
 Slide the grooved plate about 1 inch away from the metal clamp/Teflon strip.
 Put a streak of BC-626 on the fibers using a paintbrush numbered -----. Let it
     dry.
         Note:
                  BC-626 takes about 40 minutes to dry. See its manual for details.
         Caution:
                  Be sure to give the adhesive ample time to dry, as some adhesives,
                  including BC-626, contract considerably during curing.
 Again slide the grooved plate an inch away from the metal clamp.
 Put on another streak of glue on the fibers an inch away from the first streak
     and let it dry.
 Keep gluing the fibers together until the clamp is about 12 inches (and no less
     than 11.5 inches) away from the metal plate.
         Note:
              For 1B, the ribbon will be about 12 inches long.
----------------------See if making ribbon shorter is a higher priority--------------
        Place a weight of about ½ kg on top of the rubber sheet/grooved plate
         covering the metal plate underneath.
             Note:
                 The weight is to serve as a protective measure against the fibers
                 moving around under the mild stress of a hot knife cutting the fiber
                 ends.
       Mount a hot knife on a stand, which will allow the knife to move smoothly in
         a straight line.
       Put a glass plate under the fiber ends.
       Cut the ends of the fibers in a straight line using the hot knife mounted on the
         stand.
             Caution:
                      Don‟t cut the fibers so deep that the distance between the last
                      streak of glue and fiber ends becomes less than 1.5 inches.
       Make another fiber ribbon identical to the first one.
       Place the two fiber ribbons in a stand that holds the fiber apparatus vertically.
             Note:
                      In 1B: The stand should use the Teflon strips in the two fiber
                      ribbons to hold the fiber apparatus, such that the strips are held
                      next to each other lengthwise at the same height. A jig will have to
                      be designed that can place the two ribbons offset from each other
                      by 0.5 mm.
       In case of 1A:
              Move one of the clamp parallel to the length of the other clamp such
                 that the fiber ribbons get offset from each other by a distance equal to
                 the radius of a typical fiber (i.e. 0.5 mm).
              Put a clamp just above the first glue line that will hold the two fiber
                 ribbons pressed against each other.
              Take the temporary clamps off the two ribbons.
              Attach another clamp to the ribbons about 1 inch from the ribbon end,
                 which will, again, serve to keep the fibers in place.
              Put the ribbon mold underneath the fiber ends on a lab jack.
              Raise the jack until the fiber ends touch the bottom of the fiber ribbon
                 mold.
              Pot the fiber ends in the fiber ribbon mold in either UC40 or EP30.
              Take out the fiber ribbon from the mold once the potting compound
                 has dried.
**********************need to correct alignment bugs in 1B*******************
       In case of 1B:
              Fix a slotted Teflon strip (yet to be designed) in the fiber ribbon mold.
                 This strip can then be used to align the two ribbon relative to each
                 other.
              Put the ribbon mold on a lab jack underneath the fiber ends of one of
                 the ribbons.
              Raise the jack until the fiber ends from one of the ribbons touch the
                 bottom of the fiber ribbon mold.
             Pot the fiber ends in the fiber ribbon mold in either UC40 or EP30.
             Take out the fiber ribbon from the mold once the potting compound
              has dried.
             Pot fibers from the other ribbon the same way.
             Attach a jig to the holed Teflon strips fixed in the two ribbons, which
              keeps one strip off set with respect to the other by a distance equal to
              the radius of a typical fiber.
             Attach a similar jig to the slotted Teflon strips fixed near the end of the
              fibers.


Double Ribbon Fabrication Technique:

   Adhesive: Needs to be highly viscous. No research has been done in selecting
             such an adhesive.

      Fix the potted fiber grid in a jig on a table such that fiber lengths are held
       horizontal.
      Clamp a Teflon strip with two rows of 256 holes to the table such that the
       rows of holes are horizontal and face the ends of fibers in the fiber grid.
      Pass the fibers from a grid through the holes in the Teflon strip one by one
       according to some regular scheme such that each fiber is mapped to a
       uniquely determined hole.
           Note:
                   This mapping scheme helps in identifying the fiber hit by a
                   charged particle when an electrical signal is received from a photo
                   multiplier tube connected to the potted end of the fiber grid. Upon
                   knowing the anode from which the signal is received, the fiber hit
                   by a charged particle can be uniquely determined if the fibers have
                   been shaped into a fiber ribbon according to a regular scheme. The
                   mapping scheme for both SRF and DRF techniques is discussed
                   later.
           Caution:
                   Do not bend the fibers more than the safe limit guidelines set by
                   Bicron. Please see the Bicron manual for specific details.
      Repeat the process to fill all the holes in Teflon strip.
      Remove the Teflon strip from its clamp and slide it up the fibers gently until it
       is about 26 inches from the end of the shortest fiber.
           Recommendation:
                   Sliding the strip up only some fibers at a time might be easier than
                   sliding it up all the fibers at once.
      Pass the fiber ends through another Teflon strip with two rows of 256 holes
       the same way as the ends were passed through the first Teflon strip.
      Slide the second Teflon strip up the fibers until the distance between the two
       Teflon strips is about 11 inches.
           Note:
                The second Teflon strip works as a clamp to keep fiber ends in
                shape when they are being potted in a fiber ribbon mold.
                The distance of 11 inches between the two Teflon strips takes into
                account the fact that actual gluing of fibers, and hence the portion
                of fibers with a strict ribbon-like shape, will start about 1 inch
                away from the first Teflon strip.
*****************Correct all distances….they‟re not updated**************
 Put a glass plate underneath the fiber ends.
 Mount a hot knife on a stand that allows the hot knife to slide in a straight
   line.
 Cut the fiber ends in a straight line using the hot knife mounted on the stand.
        Caution:
                Cut the fiber ends so that the distance between the end and the
                second Teflon strip is no less than ¾ inches after cutting, so that
                the strip does not hinder the potting of fibers in the fiber ribbon
                mold.
 Clamp the fiber apparatus to a stand that holds the fiber ribbons vertically.
        Recommendation:
                The stand should use the first Teflon strip to hold on to the
                apparatus so that the ribbons are held somewhat rigidly.
 Place a plate with grooves on both sides in between the two fiber ribbons and
   below the first Teflon strip at a distance of about 1 inch from the strip.
 Arrange the fibers of one of the fiber ribbons in the grooves of the plate.
 Cover the fibers arranged in the grooved plate with a sheet of rubber and
   screw the sheet of rubber to the grooved plate.
 Repeat the last two steps with the other fiber ribbon.
 Put a streak of epoxy ------ on both the fiber ribbons in the area between the
   first Teflon strip and the grooved plate.
 Let the epoxy dry.
 Slide the grooved plate down the fibers by about 1 inch.
 Put a streak of glue on both the fibers at a distance of about 1 inch from the
   first streak and let the adhesive dry.
 Repeat the process until the entire fiber length between the two Teflon strips
   has been glued.
 Assemble the fiber ribbon mold and put it on a jack underneath the fiber
   ribbons.
 Raise the jack until the fiber ends touch the bottom of the mold.
 Prepare the potting compound mixture by mixing the right amounts of Part A
   and B of either UC40 or EP30.
        Recommendation:
                EP30 is the recommended potting compound.
 Pour the potting compound in the fiber ribbon mold.
 Let the potting compound in the mold dry.
 Take out the fiber ribbon from the mold once the potting compound has dried.
   Sanding and Polishing the Fiber Ends Potted In the Fiber Ribbon Mold:

    Sanding:

         Flip the fiber apparatus and fix it to the stand used for both SRF and DRF
          techniques.
         Cover the apparatus with possibly a sheet of plastic, so that the fiber ribbons
          are isolated from any dust particles emanating during sanding.
         Sand the potted fiber ends with a sander until all the fiber ends are completely
          visible.
              Recommendations:
                           Sand along the length of the ribbon.
                             It must be noted that in new trials of the sanding technique,
                             an electric sander was successfully used without causing
                             damage to the fibers. We now believe that cladding
                             detachment was because of very tight holes of the Teflon
                             plates.

    Polishing:
      Follow the steps of the polishing procedure for fiber grids.



Attaching Fiber Grids to PMT‟s:

   Mapping Schemes:

      The diagrams below show two possible mapping schemes between the PMT
      pixels and the fibers in the ribbon. The numbers shown for the PMT and the fiber
      ribbon correspond to each other i.e. „1‟ corresponds to „1‟, „2‟ to „2‟ and so on.
      Note that the PMTs are attached to the fiber ribbon through fiber grids. Hence, the
      order shown for the PMTs is actually the order of the fibers in the fiber grid. In
      other words, when the fibers are passed through the Teflon strips to be shaped
      into a ribbon, the correspondence shown below should be maintained between the
      holes of the Teflon plate and the holes of the Teflon strip. Of course, the sides and
      holes of the Teflon plate are not numbered a priori, but once the positions of the
      grids with respect to each other are fixed (i.e. the grids are fixed in the grid
      holding jig), the left side of each grid is distinct from its right side. Hence, once
      the fiber grids have been fixed in the jig, the numbering of fibers/holes in the grid
      should always start from the left hand side as shown in the figure.
   Mapping Scheme For SRF Technique:




      Top View of Fiber Ribbon Made Using SRF Technique
            Superimposed On Top View of PMT Array


Mapping Scheme For DRF Technique:




      Top View of Fiber Ribbon Made Using DRF Technique
            Superimposed On Top View of PMT Array




General Directions:
   Touch the fibers as sparingly as possible.
   Keep fibers away from oil, grease, alcohol and dust.
   Be cautious of not scratching the fibers with any of the tools used.
   Use cotton gloves when handling scintillating fibers.

				
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