# [FPD Detector Production]{.underline}

Gilvan A. Alves

Lafex/CBPF

This document intends to give an overview of the necessary steps for the
production of the FPD fiber tracker detector. An overview of the
detector design and components can be found at the FPD Technical Design
Report.

The FPD fiber tracker uses square double-clad plastic scintillating
fibers to detect the passage of a charged particle. Outside of the
detector active area, matching square clear fibers transport the
collected photons up to the photon detector, in our case a 16 channel
Multi Anode Photomultiplier (MAPMT) developed by Hamamatsu (H6568-MOD).
From there on the electronic signal is amplified, shaped and sent to the
D0 trigger electronics.

Before any detector assembly, the square scintillating and clear fibers
need to be cut to size and polished at one end, where the splicing of
the two will take place. Polishing square fibers is only possible using
the ice-polishing method, available at Fermilab Lab 7 facility, Eileen
Hahn is the person responsible for the job requisitions on this
facility, which includes also the vacuum deposition system used latter
on.

The scintillating fibers are cut to 10cm length in order to allow for
the handling in the polishing and splicing machines. The clear fibers
are cut to 45cm, which should leave a 3cm margin for assembly in the
cartridge.

Bicron has been committed to deliver the fibers within the tolerances of
2% and fiber width no greater than 0.8mm. We need to verify that this
condition is met in every batch we receive. Usually the variations of
width within a batch are well bellow the 2% quoted, but it is good to
verify that too.

Before splicing production starts, it is a good idea to verify the
transmission efficiency of the process. The method for that is as
follows:

1.  Chose the appropriate glass cassette for the fiber width. There are
    two assembled cassettes at Lab 5, one with 0.78mm width and other
    with 0.82mm width. There is also a supply of glass pieces in case a
    new cassette needs to be made. The fiber should not be too tight or
    too loose inside the glass piece or the splicing will fail.

2.  Using a sample of about 8 scintillating fibers cut at about 50cm and
    polished at one end, fill and glue 2 channels of a MAPMT mask,
    polishing any excess at the mask end. Insert the individual fibers
    in the source collimator and measure the charge spectrum using
    either a multichannel analyzer or an ADC module. Save the contents
    in a histogram.

3.  Mark the fibers at both ends and have them cut at the middle and ice
    polished (the ends that were cut) at Lab 7.

4.  Splice individually each fiber to its counterpart, changing the heat
    time settings within ± 3 seconds, and redo the charge spectrum with
    the source collimator. Compare the histograms and mean charge
    distributions to get the transmission through the splice.

Once the fibers become available after polishing, and the transmission
verified, splicing could commence using the best transmission settings
from the earlier test. It should be noted that the splicing point is
very weak to bending stress, so transportation of the fibers should be
avoided. If possible the fibers should be readily assembled in the
frames to avoid any damage.

In order to avoid any problems with dirt deposits, the splicing machine
should be cleaned after about 100 splices or so with soft cotton swabs
and alcohol (glass pieces) and dry clean air jet.

While the splicing is taking place, fiber assembly in the frames can be
done using a regular microscope (100X) with light spot. The task can be
eased using the fiber holding devices developed for this purpose.

Before loading the fibers in the frames, the primed planes need to be
isolated with kapton tape to avoid the epoxy from running into the
unloaded plane, which would fill in the gaps for the fibers.

The fibers need to be grouped in four, verifying the alignment of the
splicing points, labeled according to channel number and fixed in place
using a small drop of fast curing glue (plasti-zap or similar). The
detector active area (17.39mmx17.39mm) can be marked in the frame using
a precise cut mask and thin graphite in a granite table. This allows the
border to be seen in the microscope for correct placement of the
splicing point in the frame.

After the fiber loading in one plane, this can be filled with low
viscosity epoxy, which fills the gaps and strengthen the splice joint
making less risky any further handling of the frame. In order to assure
a flush upper surface after the epoxy filling, the plane has to be
covered with a piece of mylar (or other anti-adhesive material) and
gently pressed with a flat piece of glass and a weight of around 200
grams. The epoxy takes about 8 hours to cure at room temperature and any
handling should be avoided at this time.

After the epoxy curing, the same procedure should be repeated for the
primed plane completing a whole view (XX', UU' or VV'). For the X' plane
the procedure differs in the last stage, because the trigger
scintillator and light guide should be inserted in place during the
epoxying of this plane in order to avoid any residual epoxy to cause
problems in the insertion of this scintillator.

The trigger scintillator is a ½" thick solid piece of BC-408 plastic
scintillator cut at the dimensions of the detector active area
(17.39mmx17.39mm). In order to avoid the light from the scintillator to
cause false signals in the fibers, this scintillator must have all its
sides aluminized, with exception of the top, where the light guide is
connected, and the bottom which is to be polished later in the assembly
process.

After the assembly of the frames, the individual channels, which were
labeled in that process, have to be inserted in the MAPMT cookies in the
following order:

The 16 channels from XX', UU' and VV' are inserted in their individual
cookies. For the U and V views, the channels 17-20 are mixed into a
single cookie with the following map:

The X view has its fibers running straight into the MAPMT, but for the U
and V views the fibers have to be bent to reach the MAPMT. In order to
avoid excessive stress on the fibers, they have to be bent prior to the
epoxying on the cookie. In this way the fibers will keep the bending
shape after the curing of the epoxy. At this stage the 200 μm fibers for
the LED calibration, one for each channel, have to be inserted and fixed
in place with epoxy. These fibers have to be bundled into a 1 mm fiber
connector, which serves as the interface to the 1 mm fibers coming from
the LED Mixing Block (LMB).

The next stage in the assembly is the polishing of the cookies, followed
by polishing and aluminizing of the sides of the U and V views. The
bottom of these views will be polished latter on together with the X
view after the whole detector is assembled. This is to ensure the
flatness of the whole detector bottom.

At this point the whole detector can be assembled using the mounting
screws, and the whole bottom can be polished and aluminized.

The whole detector can now be mounted in the Detector Cartridge/Light
Tight Box, following specific procedures for that.

**[Light Tigh Box:]{.underline}**

The light tight box for the production detector is divided in 2 parts:
The detector cartridge and the photomultiplier box. Figure 1 shows a
drawing of these elements. The detector cartridge holds the detector
frames and fibers in place, providing a mechanism to fix the detector at
the bottom of the Pot, by the way of 2 hollow rods, which will slide
into the threaded guiding rods, fixed at the alignment balls at bottom
of the Pot. The photomultiplier box houses the trigger and multi anode
photomultipliers and the cabling to the MAPMT amplifier/shaper. In order
to allow for small deviations on the position of the fiber mask holders,
the MAPMT will be encased in silicon rubber, which will allow only for
lateral movements, since it is necessary to preserve the contact between
the MAPMT window and the masks. At the same time this scheme also
prevents excessive pressure on the photomultipliers window.

![](media/image1.png){width="5.881944444444445in"
height="8.989583333333334in"}

Fig.1 -- Detector Cartridge for the FPD

**[Cable Routing:]{.underline}**

The routing of each amplified signal from the MAPMT to the trigger
boards, situated on the west platform, will proceed through long (\~50m)
16-way flat coaxial cables, which will need to be joined for the
farthest dipole detectors. An interface for these cables to the CFT
analog board is under development. Unlike the VLPC case however this
interface can be very simple since there is no special temperature needs
involved. Cable trays will need to be added in the tunnel for these and
the trigger scintillator signal cables. The latter ones will have to be
routed to the Moving Counting House, where the Luminosity Monitor boards
are located.