> **[The FPD Detector]{.underline}**

#### Gilvan A. Alves

#### Lafex/CBPF

In order to well characterize hard diffractive events we need to be able
to measure its kinematic variables to a good precision. External factors
such as beam dispersion and uncertainty in the beam position make a
point resolution of better than 100µm unnecessary. Some desirable
characteristics for this detector are listed bellow:

-   Overall efficiency close to 100%

-   Modest radiation hardness. The detector will be operating at
    approximately 8σ (σ is the beam standard deviation) from the beam
    axis, so the expected radiation dose for one year of running is only
    0.03 Mrad.

-   High Rate capability. The detector needs to be alive at every beam
    crossing.

-   Low sensitivity to accelerator background, in particular particle
    showering along the beam pipe or magnets.

-   Small dead area closer to the beam. This is particularly important
    since the particles are scattered at very low angles, so the
    detector acceptance is heavily dependent on its position relative to
    the beam axis.

**[Detector Design:]{.underline}**

We have considered several scenarios for the position detector,
including Silicon Microstrip Detector, Microstrip Gas Chamber and
Scintillating Fiber Detector. A scintillating fiber based detector with
Multi Anode Photomultiplier (MAPMT) readout was chosen as the best
option for the FPD, considering all the aspects of acceptance, costs,
radiation hardness and trigger capabilities. A detailed description of
these scenarios and the R&D results can be found in ref. \[1\].

The final detector configuration is shown in figure 1. There will be 6
planes in 3 views (UU', VV' and XX'). This configuration not only
minimizes ghost-hit problems but also helps reducing the diameter of the
"Roman Pot", avoiding issues like pressure compensation for these
devices.

The fibers will be mounted on thin plastic frames, which maintains both
the fiber position and a relative separation of about 1/3 of a fiber
width for each channel within a plane. The frames also keep the relative
alignment between the primed and unprimed planes, which consists of a
shift in the direction perpendicular to the fiber length of about 2/3 of
a fiber width.

The XX' planes will have 16 channels each while the UU' and VV' will
have 20 channels, amounting to a total of 112 channels per detector.
Table I summarizes the detector proprieties.

+-----------+-----------+-----------+---------+----------+-------------+
| View      | Channels  | Width     | Gap     | Offset   | Orientation |
|           |           |           |         |          |             |
|           |           | (μm)      | (μm)    | (μm)     | (degree)    |
+-----------+-----------+-----------+---------+----------+-------------+
| U         | 20        | 800       | 270     | \-       | 45          |
+-----------+-----------+-----------+---------+----------+-------------+
| U'        | 20        | 800       | 270     | 540      | 45          |
+-----------+-----------+-----------+---------+----------+-------------+
| V         | 20        | 800       | 270     | \-       | 135         |
+-----------+-----------+-----------+---------+----------+-------------+
| V'        | 20        | 800       | 270     | 540      | 135         |
+-----------+-----------+-----------+---------+----------+-------------+
| X         | 16        | 800       | 270     | \-       | 90          |
+-----------+-----------+-----------+---------+----------+-------------+
| X'        | 16        | 800       | 270     | 540      | 90          |
+-----------+-----------+-----------+---------+----------+-------------+
| Total     | 112       |           |         |          |             |
+-----------+-----------+-----------+---------+----------+-------------+

Table I - Details of the detector geometry, where gap is the separation
between channels, offset the shift between primed and unprimed channels,
and orientation is relative to the bottom of the Roman Pot.

Each channel corresponds to 4 scintillating fibers, which fits nicely in
the 4mm x 4mm-pixel size of the Hamamatsu H6568 MAPMT. A total of 7
MAPMT will be needed for each detector. The analog signals from each
MAPMT channel will be amplified in the tunnel and the resulting signals
routed to the CFT analog boards using a high quality 16-way coaxial
cable saved from the Run I FADC system.

The last component of the detector is a ½ inch thick trigger
scintillator situated between the XX' and VV' planes. The light from
this scintillator will be collected through a standard light guide into
a single channel photomultiplier saved from the Run I Level0. The analog
signal will then be routed to the Luminosity Monitor (LM) boards for
precision timing, using high quality cables. The timing information will
be used mainly for beam halo background rejection and time consistency
with an interaction at D0.

**[Fiber Splicing:]{.underline}**

The scintillating fibers are spliced to clear fibers just on the border
of the detector active area. The fiber splicing will be done using a
version of the MSU splicing machine modified to handle square fibers.
The setup using scintillating fibers spliced to clear fibers was chosen
bearing in mind the effects of the halo background if only scintillating
fibers were used. A comparison of scintillating fibers only (fig.2a)
with scintillating plus clear fibers (fig.2b) shows that the
scintillating plus clear fiber combination actually improves the light
output due to the lower attenuation length of the smaller scintillating
fiber. In addition the cross talk was measured to be lower for the
scintillating plus clear fiber option.

The splicing procedure is a delicate task since the usual procedure of a
"shrinking tube" to strengthen the joint can not be used in our case, as
it would make the joint too bulky. Even so any person with adequate
training could perform this task. However, this condition requires that
the assembling of the fibers in the frames should be as close as
possible to the splicing area in order to avoid unnecessary handling
which could damage the fibers.

Light transmission measurements will be done at random during the
production phase in order to assure the quality of the splicing. The
same tests were done during the prototype construction and yielded light
transmissions in excess of 92%.

**[Detector Prototype:]{.underline}**

A preliminary version of the detector consisting of ½ UU' plane view was
built in the spring of 1998 in order to verify some aspects of the
design. Efficiency measurements done using a cosmic ray setup resulted
in a value of 98%, where the deviation from 100% was consistent with the
geometrical acceptance of the setup.

A full-scale detector prototype has been built during the winter of 1999
and is in the test phase. The construction of the prototype involved the
following steps:

1.  Cutting and polishing of the square scintillating and clear fibers
    provided by LAFEX. Polishing square fibers is only possible using
    the ice-polishing method, available at Fermilab Lab 7 facility.

2.  Splicing of the scintillating to clear fibers at Lab 5 using the MSU
    splicing machine modified for square fibers.

3.  Assembling and fixing the spliced fibers in the plastic frames and
    MAPMT masks (provided at no cost by the plastic shop at Lab 5).

4.  Polishing of frame ends and MAPMT masks and Aluminum coating of the
    frame ends, done at Fermilab Lab 7 facility.

5.  Final assembly and alignment of the detector frames and trigger
    scintillator.

6.  Connection of the readout cables and MAPMT in a light tight box.

The MAPMT for the detector readout are being provided by LAFEX (3) and
Northwestern (4). This prototype is being used for efficiency and
resolution studies using a cosmic ray setup at Fermilab. The development
of the prototype is also important as it gives us the opportunity for
testing the methods and tools for the production phase, as well as a
more accurate time estimate for each of the steps involved. Figure 3
shows some pictures of the prototype construction.

**[Prototype Studies:]{.underline}**

In order to perform the resolution and efficiency studies, a cosmic ray
setup is being used, which consists of:

1.  A coincidence logic using the trigger scintillator plus 2 external
    trigger elements.

2.  A Camac Data Acquisition System, with a PC interface provided by
    Northwestern.

3.  MAPMT readout using 11 bit FADC modules (LeCroy 4300) connected
    through the aforementioned high quality 16-way coaxial cable.

The basic idea for these studies is to do the track finding using only 2
views and then search for hits in the third view close to the predicted
track position. The RMS of the track residual distribution will give us
the detector resolution, which can be compared to the optimal
theoretical value of 78 μm.

As we have to use the U and V views to extract the x coordinate, the
combined errors on these coordinates (assuming uncorrelated errors) will
give:

$\delta x\  \simeq \ \sqrt{2}\ \delta u$

This means that the resolution measured by this method will be worsened
by the fact that we do not have 2 direct measurements of the x
coordinate. Figure 4 shows a preliminary plot of the residual
distribution for the x coordinate. The RMS of 201 ± 11 μm in this
distribution implies a resolution of about 140 μm for the x coordinate.
This value is a bit worse than expected and can be attributed to errors
on the relative alignment of the planes. To minimize this problem we
have redesigned the frame assembly in order to provide more constraints
for the relative alignment.

Using the same approach as described above, we can also determine the
plane efficiency, by counting how many tracks did not leave a signal in
the XX' plane having a match in the UU', VV' planes. The efficiency
determined by this method is (99.7 ± 0.3)%.

By looking at the integrated charge for each channel (70 ns gate) we can
determine the expected noise level for the detector, which can come from
optical and electronic cross talk, pickup in cables, etc. Previous
measurements \[2\] indicate an optical/electronic cross talk on the
order of 2%. For the prototype tests we use black or white dyes, to
color the polyurethane frames. In all tests the frames with the white
dye show a higher level of cross talk than the ones with the black dye.
Swapping of MAPMTs shows a large cross talk variation between tubes,
reaching a factor of 2 in the worst cases, so for the production phase
we need to set strict limits for the manufacturer and verify that in our
test bench.

Figure 5 shows the number of hits for a particular detector plane, where
a hit is defined as a signal above a threshold of 10 pC in a channel
(the average single photoelectron peak sits at 3 pC). As it can be seen,
the V plane, with the black dye, has the lowest cross talk (≈2%). The
difference between the X and U planes can be accounted for by the larger
electronic cross talk of some MAPMTs.

**[Detector Construction:]{.underline}**

The steps involved in the detector construction are basically the same
as described for the prototype. The main difference will be at the last
phase, as the light tight box, which will hold the detectors and
photomultipliers, needs to have an interface to the Roman Pots in order
to facilitate the insertion and assure the precise location of the
detector in the Pot. It might also be necessary to improve the test
bench in order to allow for testing a large number of photomultipliers
involved in this phase.

**[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 6 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.

**[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.

**[Detector Calibration and Monitoring:]{.underline}**

Calibration and monitoring of the FPD detector will be done mainly in
two different modes of operation:

During normal operation, data from all streams will be run through
online programs, like Examine, to check for dead or noisy channels,
verify the alignment constants by looking at track residuals, monitor
backgrounds due to halo, multiple interactions, etc.

In the interval between runs, a calibration program will fire an LED
based system, which will illuminate all photomultipliers to check for
dead channels. The same system will also be used to check for aging
effects and gain variation on the MAPMT. The idea is to run the LED
pulser verifying for each channel the MAPMT characteristics, like charge
distribution, single PE peak, etc.

The LED system envisaged for this latter task is basically the same as
for the muon system, but using smaller fibers in diameter and blue
instead of green LEDs due to different spectral response of the MAPMTs.

###  References

\[1\] -- *The Forward Proton Detector at D0*, G. A. Alves (for the D0
collaboration), Lishep98 -- Workshop on Diffractive Physics, Rio de
Janeiro, RJ, FERMILAB-CONF-98-107-E.

\[2\] -- *The Forward Proton Detector at D0*, G. A. Alves (for the D0
collaboration), Lishep98 Talk Slides:
<http://www-d0.fnal.gov/~gilvan/talks/Lishep98.ppt>

![](media/image1.png)

**Fig. 1 -- Plane design for each FPD detector. Also shown are the
frames for fiber alignment and the fiber to photomultiplier interface.**

![](media/image3.png){width="2.7729166666666667in"
height="3.5881944444444445in"}![](media/image4.png){width="2.9451388888888888in"
height="3.811111111111111in"}

**Fig.2 -- Integrated charge distribution from MAPMT readout of a
detector element consisting of (a) 4 scintillating fibers 0.8mm square x
50cm long, and (b) 4 scintillating fibers 0.8mm square x 5cm long
spliced to 50cm long clear fibers as a waveguide.**

![](media/image5.jpeg){width="5.75in" height="3.8375in"}

![](media/image6.jpeg){width="5.75in" height="3.845833333333333in"}

![](media/image7.jpeg){width="5.9944444444444445in"
height="4.495833333333334in"}

![](media/image8.jpeg){width="5.9944444444444445in"
height="4.495833333333334in"}

**Fig.3 -- Pictures from the prototype construction. Assembled UU' and
VV' planes showing the Aluminum coated ends (a). Same for the XX'
planes, also showing the slot for the trigger scintillator (b). Full
detector mounted including trigger scintillator and alignment pins (c).
Detector and PMTs mounted on the light-tight box (d).**

![](media/image9.wmf){width="5.992361111111111in"
height="5.992361111111111in"}

**Fig.4 -- Fit to the residual distribution on the x coordinate as
measured from the UV planes and compared to the X plane measurement.**

![](media/image10.wmf){width="5.992361111111111in"
height="5.992361111111111in"}

**Fig.5 -- Cross talk measurement for the V plane (solid), X plane
(dashed) and U plane (dotted).**

![](media/image11.png){width="5.881944444444445in"
height="8.989583333333334in"}

**Fig.6 -- Light tight box drawing showing its main elements.**