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3 1293 01026 9730

This is to certify that the

thesis entitled

"New Methods in Optical Fiber
Preparation For Scintillating
Tile Calorimetry"

presented by

Benn Harold Tannenbaum

has been accepted towards fulfillment
of the requirements for

M. S . degree in Physics

 

 

Major professor

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Date November 16, 1993

 

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

 

 

 

LIBRARY
Michigan State
University

 

 

 

PLACE IN RETURN BOX to remove We checkout from your record.
TO AVOiD FINES return on or before dete due.

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NEW METHODS IN OPTICAL FIBER
PREPARATION FOR SCINTILLATING
TILE CALORIMETRY

By

Ben Harold Tannenbaum

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the Degree of

MASTER OF SCIENCE

Department of Physics and Astronomy

1993

ABSTRACT

NEW METHODS IN OPTICAL FIBER PREPARATION FOR SCINTILLATIN G
TILE CALORIMETRY

By

Benn Harold Tannenbaum

Michigan State University has deve10ped a semi-automatic splicing machine to weld
clear, wavelength shifting and scintillating polystyrene fibers. The splicing technique
has been refined to produce splices with an average transmission of 95.7% and a a of
3.7%. The splicer is easily modified to work with a variety of fiber types and sizes and
will be of great value as the use of fibers in calorimetry and tracking increases. Also, a
multiple fiber linear scanner has been developed to allow quick and easy feedback on
the quality of the spliced fibers. Finally, an x-y scanner has been developed to examine
photocathode response uniformity of photomultiplier tubes and the effectiveness of

light mixers.

ACKNOWLEDGEMENTS

I thank Carl Bromberg and Joey Huston, my advisors, and John Mansour, for all
their help, Ron Richards and Sharon Joy for designing and building the splicer, Dean
Shooltz for building the 93-3; scanner, Ewa Skup and Pam Schmidt for helping splice

fibers and collect data and Aleks Kuzminski for helping with the figures.

iii

Contents

LIST OF TABLES

LIST OF FIGURES

1

6

7

Introduction

History

The Splicer: A Description

The Splicer: Operation and Maintenance
Linear Scanners: Methods of Data Collection
The Splicer: Research and Results

XY Scanners: Photomultiplier Response Uniformity

LIST OF REFERENCES

iv

vi

13

18

32

38

45

List of Tables

6.1

6.2

6.3

6.4

Tensile Strength: Kuraray versus Bicron. A long heating time and
high pressure make stronger splices. The apparent greater strength of
the Bicron fiber is due to the mismatch in sizes for the Kuraray fiber
(no clear Kuraray of the proper size was available during this test). .

Comparison of Kuraray Y11(250) Non-S and S type fibers. The only
property in which the S type fiber excels is in bending radius. For our
application, this was secondary to light transmission and Non-S type
was chosen. ................................

Fiber End Preparation: Razor Blade versus Diamond Polisher. The
Diamond Polisher significantly improves the splice quality and adds
only 2 minutes of preparation time per splice. .............

Study of a large batch of spliced fibers. After a 40 fiber learning curve,
the quality of the splice improves. While the bad splices seem worse
than the good, some of the worst splices are in the good group.

36

36

37

37

List of Figures

1.1

2.1

3.1

3.2

4.1

5.1

5.2

5.3

5.4

Overview of the planned CDF Endplug Upgrade. The light from the
tile is routed though Y11(250) 0.83 mm¢ fiber spliced to 0.83 mmq’)
clear fiber connected to 0.9 mmqb clear fiber. The 0.9 mm¢ fiber leads
to the decoder box where the light is summed by tower .........

The original coil splicer, used to construct a spaghetti calorimeter. The
user placed the ends of the fiber to be spliced in the glass tube, applied

pressure with his fingers and depressed the foot pedal to start the cycle.

Overview of the semi-automatic splicer. During the splicing cycle the
glare shield is lowered to protect the eyes of the user. .........

Closeup of the splicer: view of the halftubes, clamps and lamp. . . . .
Timing diagram for the semi-automatic splicer. ............

Data from manual scans of spliced fibers. By plotting on a semi-log
scale, the data is linear and the splice easier to see. The bump is due
to light being trapped in the sleeve ....................

Data from manual scans of a spliced fiber. The upper plot is the
raw data: the solid line is the unspliced fiber and the dashed line the
fiber after splicing. The lower plot is unspliced/ spliced and is easier to
interpret. It also makes visible any change in attenuation length due
to a poor splice ...............................

Scan of fiber using linear scanner. The top figure is a plot of the
normalized data: the data for the fiber with splice is divided by the
data for the fiber without the splice. Two lines are fit to the data— one
between the splice and the PMT, and one beyond the splice to the end
of the fiber. By dividing the fit to the data beyond the splice by the
fit to the data before the splice, the transmission is obtained. This is
shown in the bottom plot. A poor splice was intentionally selected to
emphasize the location of the splice ....................

Photodiode box for use with the multiple fiber linear scanner. The
fiber is inserted in the nylon bolt until it reaches the photodiode. The '
bolt is then firmly screwed on. This applies pressure to the rubber
washer and holds the fiber well but does not damage the cladding. . .

vi

6

9
10

20

21

22

24

5.5

5.6

5.7

5.8

6.1

7.1

7.2

7.3

7.4

Fit of the function par(1)exp(-:r/par(2))+par(3)exp(-z/pa1(4)) to the
data for the unmirrored fiber. par(2) is the short attenuation length of
the fiber; par(4) is the long attenuation length ..............

Fit of the function parfl)(75-:r) +par(2) to calculated values for Rm.
This functional form is then used in the program to determine Rm at
the desired position. ...........................

Comparison of expected light output and measured light output. The
agreement is quite good for the entire length of the fiber. .......

Plot of pigtail data taken with multiple fiber linear scanner. The first
dip is due to a piece of tape holding the calibration fibers; the second
is due to the connector ...........................

Comparison of good, reasonable and bad splices. The bad splices are
indeed lower than average, but some of the good splices are equally
poor. Many of the fibers were not measured before splicing and used
a standard fiber for normalization. This results in a few splices with
apparent transmission over 100%. ....................

Detail of the :r-y scanner. Light from the LED passes through the
1 mm¢ clear fiber to the calibration tube and to scanning point. The
scanner has 1 mm resolution. ......................

The collimator is used when determining the uniformity. However,
when studying light mixers, it must be removed. Light from a real
fiber will not be collimated and collimated light cannot be mixed. . .

The upper plot is a scan of a bare photomultiplier using the collimator.
The lower plot is a scan of the same photomultiplier with a 40 mm light
mixer, the collimator, and a 5 mm¢ black dot placed in one corner.
Each line on the contour plot is 5% ....................

Both plots are with a 40 mm light mixer but without the collimator.
The upper plot is with the dot; the lower is without it. The light mixer
effectively mixes the light and removes the effect of the dot, with a 4%
reduction in output. Each line on the contour plot is 5% ........

vii

26

28

29

30

35

39

41

43

44

Chapter 1

Introduction

The ability to investigate certain physics topics using the Colliding Detector Facility
(CDF) detector at Fermi National Accelerator (FN AL) is limited by the speed and
resolution of the current endplugs. When the Main Injector Upgrade is completed,
providing more proton and anti-proton bunches and a smaller crossing time, the per-
formance of the gas calorimetry used in the current endplugs will degrade significantly.
To provide faster, better endplugs, an upgrade to a (lead + iron) /scintillating sam-
pling calorimeter has been designed and is being constructed. The performance crite-
ria require a calorimeter whose resolution should be approximately 80-90%/ VIE-695%.
This requirement has implication on the light yield since photostatistics must be
folded into the intrinsic resolution due to sampling. Also, the 5% constant term re-
quires that variation of the tile-to-tile light yield be better than about 10% and the
transverse uniformity be better than 5%. Second, the calorimeter should be able to
clearly identify muons. Finally, the segmentation should allow physics topics, such
as b-physics, W, Z, and '7 physics and jet physics, to be done in an efficient and
productive fashion. The criteria must be weighed against cost, radiation hardness

properties and ease of construction.1

In the new plugs, the scintillating tiles are made of Kuraray SCSN38. Optical

 

18cc Reference [1]

fibers are used to transmit the light from the tiles to the photomultiplier tubes. The
light in the tile is collected by 0.83 mqu Kuraray Y11 (250) green wavelength shifting
(WLS) fibers which lie in grooves machined in the scintillator, and passes through
0.83 mm¢ clear fibers to the edge of the megatile. From there, 0.9 mmqb clear fibers
transmit the light to the decoder box, where 1.0 mm¢ fibers are used to sum the
signals by tower (Figure 1.1). Michigan State University is responsible for producing
all of the necessary fiber optics for the hadron calorimeter, from the fibers in the tiles
to the photomultiplier tube decoder boxes. The 0.83 mm¢-to—0.9 mm¢ and 0.9 mm¢-
to-1.0 mm¢ joints are accomplished using injection molded flat connectors. The fibers
are glued into these connectors in groups of 8 or 9 (known as a pigtail) and the face
is polished using a diamond polisher. The conversion from WLS fiber to clear fiber

is more complex.2

Since a given fiber is extruded from a single boule, it is impossible to manufacture
a single fiber that simply stops wavelength shifting at some point. As the fibers are
made of a plastic with a reasonable melting point, it is easy to imagine heating the
ends of two fibers while pushing them together, and allowing them to cool. This
would produce a single fiber that is wavelength shifting for part of its length and still
has an overall attenuation length that is long enough to allow the fiber to deliver

sufficient light to a photomultiplier tube.

The construction of the detector will include two plugs and a test beam module.
Each plug has 22 hadron layers, each layer 12 megatiles, each megatile four pigtails
and each pigtail 9 fibers. This totals approximately 20,000 splices for the hadron
calorimeter. For this operation to be successful, an dependably repeatable, easy to

use device had to be constructed. The Michigan State University semi-automatic

 

2The transition from WLS fiber to clear fiber is necessary as the doping agents in WLS fiber
result in a short attenuation length-2.2 meters. The attenuation length of clear fiber is 8.6 meters.

 

 

 

 

Figure 1.1: Overview of the planned CDF Endplug Upgrade. The light from the tile
is routed though Y11(250) 0.83 mm¢ fiber spliced to 0.83 mm¢ clear fiber connected
to 0.9 mqu clear fiber. The 0.9 mm¢ fiber leads to the decoder box where the light
is summed by tower.

splicer is such a device and it is presently being used in the endplug assembly.

Chapter 2

History

The first splicer built at Michigan State University bears little semblance to the final
product. Based on a machine designed at Fermilab‘, it consisted of a simple coil of
nichrome connected to a transformer (Figure 2.1) and was used to build a spaghetti
calorimeter for T-840. The user placed the ends of the fibers to be welded in a
glass tube and pressed them together while the coil heated and melted them. The
first refinement replaced the fingers with a spring loaded clamp to provide somewhat
more consistent pressure on the fibers. This device did produce spliced fibers: the
quality of the splice was acceptable but the physical strength of the weld was very

poor.2

In an SSC / GEM related effort, a split welding tube machine was designed to con-
sistently hold and push the fiber while providing uniform heating. A clamp firmly
holds the fiber and gently applies pressure to maintain contact between the fiber ends
being spliced. A treated slide projector bulb is used to provide uniform, controllable
heat at the splice joint (see below for a complete description of the splicer). Refine-
ments to this machine include a cooling cycle, adjustable heating time and horizontal

pressure, and myriad safety features.

 

18cc Reference [2]
2See Reference [3]

glass tube

 

 

 

transformer

to foot
pedal

Figure 2.1: The original coil splicer, used to construct a spaghetti calorimeter. The
user placed the ends of the fiber to be spliced in the glass tube, applied pressure with
his fingers and depressed the foot pedal to start the cycle.

At present, seven final splicing machines exist— four at Fermilab, two at Michigan
State and one in Japan at the University of Tsukuba. Michigan State is inithe process

of building more to be sent to various laboratories around the country.

Chapter 3

The Splicer: A Description

The splicer is a deceptively simple looking device (Figure 3.1). The heart of the
machine is two half—round glass tubes (Figure 3.2). These tubes, plus the use of an
FEP sleeve around the splice joint, are what make the Michigan State splicer unique.
The bottom tube is anchored and the top tube has one inch of vertical motion.
In the open position, the fiber can easily be inserted and removed. In the closed
position, the half tubes meet to form a whole tube whose inner diameter is exactly
the outer diameter of the fiber plus FEP sleeve. This holds the fiber during the
splicing operation without allowing the joint to buckle or swell. Since the half tube
is quite fragile and susceptible to breaking, a whole tube is positioned underneath to
add support. The ends of the whole and half tubes are in a U-shaped piece of steel,
collectively referred to as a cassette. Two cassettes are in a set (top and bottom) and
are perfectly matched— if one is broken, the entire set must be replaced. On either
side of the cassettes are a set of rubber clamps. The upper and lower clamps have
a 1 mm deep groove milled along the center. In addition, the lower grooves have
holes drilled in them. The insides of the lower clamps are hollow and connected to
a small pump, which is used to draw a light vacuum to hold the fiber in place prior
to closing the clamp. The right clamps are firmly fixed, while the left clamps have

0.25” movement to push the ends of the fiber together.
8

 

 

 

steel slit
to vecmm pttmp

 

x-y-z mount

 

I '-
-: :-.- -.-.-.
:3-c43:-:-'::«:~. ~:-:-:-

 

 

 

 

 

......

........
..........

,
:
cassette

 

 

protective glare shield

cooling
fl!

' *\ // i3

/
it

Figure 3.1: Overview of the semi-automatic splicer. During the splicing cycle the
glare shield is lowered to protect the eyes of the user. _

tofootpedal

10

front view of a cassette

W
.5 whole glass tube

tube steel

side view of a clamp

SCTCW

fiber groove

    
  
   
  

steel slit _ ,
aluminium

CHSSCIICS

1" vertical
clamps

travel

 
 

0.25" lateral
movement

 
  
 

x-y-z mount

lamp

Figure 3.2: Closeup of the splicer: view of the halftubes, clamps and lamp.

11

Directly in front of the center of the cassette is the slide projector lamp. It is a
standard bulb, with the inner side of the reflector coated with 500A of chrome under
1300A gold. Gold is used because it is very reflective in the near-infrared region; the
chrome provides a better surface to which the gold adheres. The reflector is coated
since the original coating is a special dichromatic film that reflects visible light and
passes infrared light (enabling one to view one’s slides without melting them). The
lamp is positioned roughly 2” from the cassette placing the focal point directly on the
middle of the cassette. The lamp is attached to a :r-y-z mount that allows the lamp
to be easily adjusted in all three directions. Between the lamp and cassette, attached
to the upper clamp, is an 0.008” thick steel shim, bent and slotted to reduce the
amount of fiber exposed to the light. The slit allows only a small fraction of the fiber
in the half tube to be heated and melted, resulting in a better splice. In front and
to the left of the lamp is a fan, used to cool the spliced fiber and the lamp assembly

after heating.

The remainder of the splicer is timers, switches and valves. In the front corners
are the start and stop switches. As a safety feature, both start switches must be
pressed at the same time; the stop buttons function independently. In the left rear
are the pressure regulators, connected to house air. The right one controls the vertical
pressure; it is generally set to 100 psi. The front left regulator is used to reduce the
pressure reaching the rear regulator, which controls the horizontal pressure. A 10:1
converter is used to reduce the pressure to the needed level and the horizontal pressure
can be adjusted in 0.05 psi increments from 0 psi to 3.0 psi. In the right rear corner
of the splicer are the 5 timers. They have adjustable ranges, from 0-6 seconds, 0-60
seconds and 0-6 minutes. They allow complete control of the various cycles of the
splicer, easily changed to accommodate the times needed for various types of fibers.

The timers used in the US. version are 60 Hz; the Japanese splicer uses 50 Hz in

12

accordance with local electrical supply.

There are a few additional parts needed for the splicer. A single box contains
the switches for main power and a separate control switch for the heater lamp, as
well as the 110V to 22V transformer to supply the lamp. The cable from this box to
the splicer provides the 22V for the slide projector bulb and the 110V to power the
timers, pressure valves and an overhead lamp. In the floor is a small vacuum pump

controlled by a foot switch. This pump provides the vacuum for the fiber chuck.

These three components— the pump, the transformer box and the splicer itself—
form the package as produced by Michigan State University. The only other necessary
supplies are 110 VAC and house air (100 psi minimum). The splicer can be uncrated,

set up and used easily and quickly.

Chapter 4

The Splicer: Operation and
Maintenance

For a general user, the splicer is quite simple to operate. The prepared ends1 of the
fibers to be fused are placed in an F EP sleeve and centered in the bottom half tube
and the foot pedal is depressed. The fiber is held in place by the light vacuum from
the floor pump. Next, both start buttons are simultaneously pressed and the glare
shield is lowered (to reduce the risk to the user and other people in the room). The
upper clamp lowers to secure the fibers and timer 1 starts. After timer 1 stops, the
horizontal pressure is applied and timer 2 starts. These steps are done separately to
ensure grasp of the fiber is complete before attempting to apply horizontal pressure.
After timer 2 stops, the heater bulb comes on for the time set by timer 3. When
the heating is done, the fan cools the light and the fiber/cassette assembly for the
time set by timer 4. When cooling is complete, the horizontal pressure is removed.
Finally, after time set by timer 5, the vertical pressure is removed and the clamp
opens. Again, these operations are separate tasks to prevent any possible damage to
the spliced fiber. As soon as the clamp Opens, the fiber can be removed. The timers
automatically reset themselves and the splicer is ready to begin again. Figure 4.1 is

the timing diagram for all five timers.

 

1The ends are prepared using a diamond polisher; see below for details.

13

14

I
T
I
l
l

Judas

____C____
I
I

TIMEZ

_—-_-—T——_—i—__——r____
|

 

VERTICAL
PRESSURE

i.

-—--—l_—__"l"_—--l'"

 

I

l

HORIZONTAL I
PRESSURE

 

I
|

 

I
—————r—————:.—_———lu--

 

 

LAMP

 

 

 

 

I
4____4__-__L____L____L.

 

______L____J____

Figure 4.1: Timing diagram for the semi-automatic splicer.

15

Daily maintenance is necessary to keep the splicer operating at peak efficiency.
Every time a fiber is handled, some skin oil is left on the fiber. When the fiber is
placed in the half tubes, some of this oil is transferred to the glass and is baked
on. Burnt skin oil is dark and opaque and will cause uneven heating of the fiber.
This results in uneven melting and poor splice quality. To remove these deposits,
the cassettes must be removed. First, the steel slit is removed and the retaining
screws in the cassettes are unscrewed. Next, the cassettes are gently pulled from the
alignment posts. If this is difficult, it is possible to gently pry them loose with a razor
blade. The deposits are now gently scraped off, using a 0.046” gauge pin— this pin
should have a slightly smaller diameter than the inner diameter of the half tubes. At
this time all four glass tubes should be visibly inspected for wear and damage. If
no damage is found, the cassettes are replaced. Since the cleaning and replacement
process requires handling of the cassettes, they must be cleaned again. Now a cotton
applicator with flux remover is used — this removes un-heated skin oil quite nicely
and does not damage the cassette in any way. Finally, the steel slit is replaced. The

splicer is now ready for use.

After many splices, the steel slit will become carbonized and the slit itself will
widen. This, too, causes a degradation in splice quality. Another slit must be fash-
ioned from 0.008” steel. When returning the slit to the splicer, care must be taken
to ensure proper placement. The slit must be centered on the cassette, and it must

be placed with the bend towards the cassette.

In general, the maintenance of the splicer is as easy as the operation. However,
from time to time serious problems arise, although none will result in permanent
damage. The most common serious problem is a broken cassette. This is usually
caused by user error— if the fiber is not exactly centered in the half tubes, it will be

pinched and crack or chip the half tube. It can also simply wear out- although it

16

rarely lasts that long. Cassettes are easily replaced, but as this takes a minimum
of four hours, it is recommended to have at least two cassettes for each splicer. To
replace the cassette, they are first removed from the splicer. The broken tubes are
removed. With care, the whole tubes, if not damaged, can be salvaged. However, the
half tubes must be removed. If they are unbroken, they can be remeasured and reused
later. It is not necessary to remove the whole tubes, but as much RTV is scraped
out as possible.2 If building a new cassette, or if the whole tubes are replaced, the
retaining screws are inserted first. Next, some RTV is applied on the cassette and
the ends of the whole tubes. A small jewellers screwdrivers works well for applying
RT V. Some more RTV is put on the top of the whole tubes and the half tubes are
placed the proper position. [See below for instructions of how to choose half and
whole tubes and cassettes.] A cotton applicator is used to remove any excess RTV
and the cassettes are returned to the machine. This is easier if the bottom cassette is
inserted first. A 0.047” gauge pin (or the appropriate size for the half tubes used) is
placed in the lower half tube and a wire jumper in the appropriate holes for timer 1.
The vertical and horizontal pressures are turned off. The splicer, but not the lamp, is
turned on and the upper jaw is manually lowered. The horizontal pressure is slowly
increased to 80 psi. The cassettes are inspected for cracks in the glass and left to cure
for at least four hours. When curing is complete, the horizontal pressure is slowly
reduced, the upper jaw lifted, and the cassette removed. Using a razor blade, any
excess RTV is scraped off. The cassette can now be returned to the splicer, timer 1
replaced, and the horizontal and vertical pressures returned to normal. The splicer

is ready for use.

When building a cassette, careful selection of all components is important. Since

 

2RTV is used as it has a 20-minute working time, cures reasonably rapidly, has reasonable flex-
ibility during curing, can be heated and cooled repeatedly with no loss of strength, and can easily
be removed with a razor blade.

17

each cassette can be used only in the machine in which it was built, each cassette
must be labeled. Information on the cassette should include both the machine in
which it belongs and whether it is for the top or bottom jaw. When the cassette is in
use, 100 psi are exerted on the bottom jaw. The glass half tubes will quickly break
if they protrude above the edges of the metal cassette. The level of the half tubes is
controlled by the outer diameter of the whole tube— 0.088”. The inner diameter of the
half tubes is constant; the outer diameter can vary by as much as 0.002”. This slop
can be removed by pairing an oversized half tube with an undersized one. Properly

sized parts make a cassette that will generally last until user error damages it.

The only other easily replaced item is the slide projector bulb. Our bulbs have
500A chrome under 1300A gold. Careful degreasing and an overnight session in an
evaporator will properly prepare the bulbs. After a bulb is in place, it must be
positioned so the hottest spot is centered on the middle of the cassette. Silphos3 will
ignite quickly at the temperature provided by the bulb and thus provides an easy
locator for the focal point. Stopping the light cycle as soon as the metal burns and
moving the bulb in all three dimensions using the :r-y-z mount will allow the bulb to
be properly positioned. The :r- and y-axes are easily done; the z-axis is harder to do
but equally important. It can take some time to align the bulb but practice makes

this easier.

 

3Silver phosphorous copper brazing alloy, used in refrigeration.

Chapter 5

Linear Scanners: Methods of Data
Collection

When building a device as complex as the plug hadron calorimeter upgrade, it is
necessary to have methods of testing and quality control at ever step of the assembly.
The splices and pigtails must be tested to ensure consistent quality. There are two
possible methods to stimulate the spliced fiber and thus measure the light output. One
is to place the end of the fiber in a scintillating tile and place a radioactive source
near the tile. The other is to shine an ultraviolet light directly on the fiber. The
second method is preferable, as it is safer, and can be used to provide measurements

of position versus light output.

Our early measurements were accomplished with a simple device. A UV light
bulb was wrapped in vinyl electrical tape, leaving only a small hole exposed. A 1
mm inner diameter glass tube was positioned over the hole, tangent to the bulb, and
the fiber to be tested was drawn through the tube. The end of the fiber was placed
in contact with a photomultiplier tube. By moving the UV bulb along the length
of the fiber, position versus light output measurements were made. The data were
entered into a computer and a graph plotted using a semi-log scale. By comparing

the value directly before the splice to the value directly after, a rough calculation of
18

19

the transmission was calculated (Figure 5.1).

In the next iteration of tests, the light output for an uncut fiber was measured
as a function of position. The fiber was then cut, spliced and re—measured. As can
be seen in Figure 5.2, this made the data much easier to interpret and removed such
systematic errors as fiber placement near the photomultiplier and small variations in
the position of the UV bulb. However, this method of data collection was very time

consuming and subject to positional errors.

To improve the quality of the data collected and reduce the time necessary to scan
a fiber, an automated linear scanner was built. A six-foot long section of 1” PVC
pipe with a groove milled along the length serves as a vacuum chuck for the fiber to
be tested. A UV light source attached to a cart moves along the length of the fiber.
The motion of the cart is accomplished by a threaded rod which runs beneath the
pipe and passes though a stepper motor which is attached to the cart. By spinning
a nut inside the motor, the cart can be made to move along the rod. The entire
device is controlled and read out using CAMAC. The position of the light was easily
controlled to less than 0.25 mm. A program was written so the data is plotted on a

terminal as it is collected.

For this device, a clear fiber is spliced to a green fiber. The free end of the clear
fiber is placed near the face of a photomultiplier and the program is run. The fiber
was removed, the green fiber cut and spliced and the fiber then rescanned.l The two
scans are compared and the transmission across the splice calculated. A typical scan

takes 20 minutes. A sample fiber scan is shown in Figure 5.3.

For the construction of the plug upgrade, it is impractical to scan each individual

fiber, especially since this would take over 1000 days. Instead, a pigtail scanner was

 

1Merely scanning the green-clear splice will not provide any useful information. The clear fiber
does not respond to the UV lamp and 0 is read until the light is over the green fiber.

20

K1 green-green weld 1

 

 

 

 

 

 

 

 

 

 

' I l I
\d
f \
/ \
§ / ‘
0.844 V
17% changeojoo V

 

 

 

0.1 I Al

O 20 4O 6O 80 100
Disrance to PMT

Figure 5.1: Data from manual scans of spliced fibers. By plotting on a semi-log scale,
the data is linear and the splice easier to see. The bump is due to light being trapped

in the sleeve.

21

-——K3 Unwelded ----- K3 Weld 1

 

10
l
0.1
0 ' 20 40 60 80 100
DistancetoPM'Hcm)
—K3 Weld 1 corrected
.,
1.6
1.2
0.8
0.4
0

 

O 20 4O 60 80 100
- . Distance to PM!" (cm)

Figure 5.2: Data from manual scans of a spliced fiber. The upper plot is the raw
data: the solid line is the unspliced fiber and the dashed line the fiber after splicing.
The lower plot is unspliced/spliced and is easier to interpret. It also makes visible
any change in attenuation length due to a poor splice.

Normalized Light Output

O
N.O\l
mm .1

L”

O
O

0.75
0.5
0.25

22

 

 

 

 

 

 

 

 

 

P I

i.» e

.- .................. [... .............................
3 ‘__1.
L. ............................................................ f .............................
,.. . :

I: 3 g

__ ................................................................................................................... g ..............................
l— : 2

b . .

'- : :
r..........................,.-.--.---o--cocoo-u..o-...-.€....-.....---.-c..--.--..o-.-'..............-.------..---o-.E ............................. 1
l: : :

"' 1 1 1 1 l 1 1 n 1 l 1 1 1 1 l 1 1 1 1 l 1 1 1 4

0 200 40 800 1000

O
Distonce from Splice (mm)

 

IIIIIIIIIIIIIIIIITII

llllllllllllmLLLlllllllllllllLLllIll lllllIJLLllll

I

 

 

 

 

 

0
0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1

Percent Tronsmissuon

Figure 5.3: Scan of fiber using linear scanner. The top figure is a plot of the normalized
data: the data for the fiber with splice is divided by the data for the fiber without
the splice. Two lines are fit to the data— one between the splice and the PMT, and
one beyond the splice to the end of the fiber. By dividing the fit to the data beyond
the splice by the fit to the data before the splice, the transmission is obtained. This
is shown in the bottom plot. A poor splice was intentionally selected to emphasize
the location of the splice.

23

built. This device is capable of scanning an entire pigtail (8-9 fibers) in 20 minutes,
keeping pace with the assembly of the pigtails. It it necessary to have the ability
to determine the percent transmission of each green to clear splice. Using a similar
design to the single fiber scanner, a pigtail scanner was constructed to make these
measurements possible. A 2” by 8’ piece of PVC with 10 grooves milled along the
length and holes drilled at regular intervals is glued to a chamber that is attached
to a large vacuum pump. This serves as a fiber chuck to hold the pigtail stationary
during the scan. A 3” UV pen lamp, inside a box with only a 1 mm by 3” slit
opening, is attached to a belt driven cart. The cart moves in 1 cm steps along the
fibers. For general use, the pigtail is placed in the grooves and the connector snapped
into a connector box. The other side of the connector box is occupied by a connector
containing 9 green fibers, which lead to a photodiode box (Figure 5.4). Each fiber is
connected to a photodiode, which in turn is connected to a switchbox.2 The switchbox
is controlled through CAMAC and feeds a single, switchable output to a picoammeter.

The picoammeter is read out through GPIB at 2Hz, the maximum usable rate.3

After the data is taken, it must be examined to determine the quality of the
splice. Each fiber is normalized by dividing all the data for that fiber by the value at
a point close to the connector on the photodiode side. This removes the dependance
on the photodiode and any small variation in the light source. Next, the transmission
through the splice and connector is calculated, using the formula

normalized light output(a:)
melt/A...» eXP(-12//\clear)(F($) + RmFeIs — x»

where normalized light output(:c) is the measured value at point 2:, 11 is the length

 

Transmission =

of the green fiber between the calibration point and the connector (constant), Agree"

 

2Designed and constructed by Bill Kinney, FNAL.

3The scanner was originally designed to take measurements using a LeCroy 2249A ADC through
CAMAC. This would have resulted in a device capable of scanning 10 channels of 2+ meters of
fiber in less than two minutes. Unfortunately, the photodiodes need do not produce a strong enough
signal and must be read out using the picoammeter.

24

 

 

 

 

 

   

 

' to
switchbox
—>
i

i i

nut nylon rubber white photodiode
washer nylon

Figure 5.4: Photodiode box for use with the multiple fiber linear scanner. The fiber
is inserted in the nylon bolt until it reaches the photodiode. The bolt is then firmly
screwed on. This applies pressure to the rubber washer and holds the fiber well but
does not damage the cladding.

25

is the long attenuation length of the green fiber (constant), 12 is the length of the
clear fiber between the connector and the splice (determined by the program), Aclear
the attenuation length of the clear fiber (constant), and I3 is the length of the spliced
green fiber (determined by the program). All of these parameters are determined by

the program during execution and are generally different for different pigtails.

F(a:) is

F(:r) = 0.27 exp(—:r//\1) + 0.73 exp(—:r/).2)

and is the expected light output when the fiber is excited at point 1:. A1 is 12.3 cm, the
short attenuation length of the green fiber, and A2 is 222.2 cm, the long attenuation
length. These constants were determined by scanning ten 70 cm long fibers each
spliced to a 1 m long clear fiber. The average values for all ten fibers were read into

a histogram and the formula

par(1) exp(-x/par(2)) + par(3) eXP(-x/par(4))

was fit to the data. The resulting plot is Figure 5.5.

Naively, one would expect R,,,, the effective reflectance of the mirror, to be a
constant. However, because of the two different attenuation lengths and imperfections
in the mirrored surface, the effective R... is a function of x. Rm was determined by
scanning ten 70 cm mirrored fibers. The mirrored ends of the fibers were then cut off
at 45°, the ends were blackened, and the fibers rescanned. The expected light output

for an unmirrored fiber is

F1($) = FCC)

and the expected light output for a mirrored fiber is

172(3) = £92) +EmF(2L - as)

transmitted ref l:cted

 

26

 

in’ 030195—04
gm 0.2;697

 

 

.0
oo

 

Normalized Light Output
0
on

I
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I
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I I ‘ 0 0 V U
I I I I I I '
O I ' I ' I I
O 2 l_""""""'"°;‘ ................... ;...................;...................;...................',...................;...................,' ........ .
I . . . . I a I

 

 

 

O 11 lml lit 1 l llLLllJIl 1 ll llgllllmlmlllll Ill

O 10 20 3O 4O 5O 60 70
Distance From Splice (cm)

 

Figure 5.5: Fit of the function par(1)exp(-z/par(2))+par(3)exp(-:c/par(4)) to the data
for the unmirrored fiber. par(2) is the short attenuation length of the fiber; pa1(4) is
the long attenuation length.

27

By solving for Rm, one gets

 

_ F205) FCC)
Rm”) - (am - 1) —F<2L — .)

F2(x) is the measured light for the mirrored fiber and 171(3) is the measured light
output for the unmirrored fibers. The values for Rm(:c) were placed in a histogram

and the function

par(1)(L - x) + par(3)

was fit to the data. The results from this fit are in Figure 5.6 and produce the

functional form

mm = 0.0016303 — 2:) + 0.82

that is used in the program. This function is valid for 30 cm< 13 < 100 cm; for 13 < 30
cm R,,.(:c) simply becomes 0.80, a constant. A comparison between the calculated

and measured light output for a mirrored fiber is shown in Figure 5.7.

The transmission through the splice and connector is calculated at three points
and averaged. This information, as well as the raw light output 5 cm beyond the
splice, is output to a file. The file also contains the normalized data. Plots of the
normalized data are shown in Figure 5.8. The information in the plot is saved for

future reference.

This linear scanner will perform additional functions needed in the plug upgrade
construction. Each batch of Y11(250) fiber must be checked to ensure consistent
quality. The attenuation lengths of the Y11(250) fiber can be easily measured using
the scanner and then compared with previously measured values. To test the clear
fiber, 1 m green fiber is spliced to 2 m of clear fiber. The fiber is scanned and 1 m clear
fiber removed. The fiber is re—scanned and the results are compared to determine the
attenuation length of the clear fibers. In this way, the mean attenuation length for a

sample of 10 fibers can be compared with the standard value. The other main use of

28

 

~ g X2 go.1233E—;o3
1 __ ------------------ m-------—----~--§-e.-+6-26-E-{02—-
e 3 P2 0.8204

 

 

 
   

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O lllll1llllllLllLlllllJlJJlllllllllll

O 10 20 3O 4O 5O 60 70
Distance From Splice (cm)

 

Figure 5.6: Fit of the function par(1)(75-z) +par(2) to calculated values for Rm. This
functional form is then used in the program to determine Rm at the desired position.

 

29

 

Calculated Ligtit Output (Dashed lipe)

.0
ao

 

Normalized Light Output
0
07

.0
41>

I D I I I I I

I I I n I h I

n a n I I I I

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l I 4 I I I I

n I I I I I l

I I I t I I I

 

 

 

O lllllllllllllLilll11111414414LL4111L

O 10 20 3O 4O 50 60 70
Distance From Splice (cm)

Figure 5.7: Comparison of expected light output and measured light output. The
agreement is quite good for the entire length of the fiber.

 

30

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52

L 51
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mem M

fit
:2 ~ 2 2 a 2 °
(umm)mam

Figure 5.8: Plot of pigtail data taken with multiple fiber linear scanner. The first
dip is due to a piece of tape holding the calibration fibers; the second is due to the

connector.

31

the scanner is to measure daily splice quality. Each morning, two fibers are spliced
and scanned. The transmission of these fibers is compared with standards to ensure

the equipment is producing consistently good splices.

Chapter 6

The Splicer: Research and Results

The first tests done on spliced fibers were measurements of their tensile strength,
as a relationship between splice quality and the strength of the fiber had previously
been noted. A variety of heating times and horizontal pressures were examined. The
results for these tests are in Table 6.1. These results were used to narrow the search

for the best heating times and pressures.

Initial splicing was done with Bicron BCF 91A 0.75 mm¢ fiber. Bicron has a
problem maintaining a consistent fiber diameter and only splices moderately well.
After a few comparison tests, Kuraray Y11(250) fiber was selected. Initially, a single
clad fiber was used, but more experiments showed double clad fiber spliced better and
captured more light. A fiber spliced without a sleeve has a minimum bending radius
of 15 cm before damage occurs. This is unaccaptable in any practical application.
Adding an F EP shrink tube over the joint before splicing— the tube is left in place
after splicing— decreases the minimum bending radius to 2 cm. The shrink tube is
purchased large and shrunk to the proper inner diameter using a heat gun and an
appropriately sized gauge pin. Experimentation revealed that a tube larger than the
fiber results in large bulges at the splice and poor transmission; a. small tube makes

assembly difficult and can result in splice failures.

32

33

A variety of heating times and horizontal pressures were tried. After finding the
best combination for 0.75 mmd) fiber, the hadron group for the plug upgrade decided
to switch to 0.83 mm¢ fibers. After more experimentation, it was determined that 15
seconds of heat and 1.5 psi consistently gave splices with excellent transmission. In
late December 1992, as a comprehensive study of the effects of pressure and heating
time on splice quality began, a dramatic drop in percent transmission was observed.
After much frustration and contact with Kuraray, it was determined that this drop
was due to a change in the extrusion process used to make the fiber. The new type
of fiber, called S-Type, is more flexible (it has a much smaller bending radius) but
conducts 25% less light than does the older type of fiber. The old type, referred to as
Non S-Type, splices better and was selected as the fiber of choice for the plug hadron
calorimeter upgrade.l Table 6.2 is a comparison of various attributes for fibers spliced

with the semi-automatic splicer.

The preparation of the ends to be spliced was also examined. Careful end prepa-
ration results in both a better average transmission and a smaller sigma. Table 6.3 is
a comparison of razor blade polishing versus diamond polishing. The former is done
using a brass block with a hole of the proper diameter and a razor blade; the latter is
done with a machine designed by engineers at Fermilab and built by AVTech.2 Any

debris left on the ends or in the shrink tube is blown off using compressed nitrogen.

To study the transmission behavior for a large batch of fibers, 100 splices were
made. Since these were the first splices made by a new technician, this was also used

to determine the learning curve for splicing. The results for this study are in Table 6.4.

The technician also attempted to identify which fibers had bad splices and which had

 

1The plug electromagnetic calorimeter upgrade group, in Japan, are able to successfully splice
S—Type fiber using a very different splicing technique. See Reference [3] for details.

2Carl W. Lindenmeyer, John B. Korienek and Masanori Mishina. A paper is to be published in
NIM.

34

good splices. Unfortunately, there was little correlation between appearance of the
splice (how much light leaks out at the splice) and the percent transmission across
it. Figure 6.1 is a comparison of the good, reasonable, and bad splices. While the
bad fibers have some of the lower percent transmissions, many would pass as good

splices.

Timing studies were done using this large batch of splices. It was determined that
to cut to length, polish and splice a fiber takes 6 minutes. Two minutes of this time is
the time for one splicing cycle. This resulted in the need for 4 technicians to produce

the fibers so the endplugs will be completed in time for installation prior to Run 2.

These results lead 'us to believe it is possible to successfully build a calorimeter
that will meet the criteria presented in the Introduction. The transmission across
the splice is 95.7%, with a variation of 3.7%. This small variation allows for further
variation in the tiles, while still meeting the criteria to have tile-to-tile variation of
less than 10%. Also, we believe it is possible to construct a quality device in the time

allotted.

 

35

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

12
'- 1'—
10
i— : Good Splices
_ m m Reasonable Splices
63 Bad Splices
8 r- —-J
U)
Q)
.9 b r— F—
'3.
(n
35 ._
8
.0
g L — _
z
4 L /
2 — ¢ _
L; I 1% / // , j WI 1 fl 1 L l l l l l l l l l l l
0.95

1 1.05 1.1 1.15 1.2
Percent Transmission

0 ,
0.8 0.85 0.9

Figure 6.1: Comparison of good, reasonable and bad splices. The bad splices are
indeed lower than average, but some of the good splices are equally poor. Many of the
fibers were not measured before splicing and used a standard fiber for normalization.
This results in a few splices with apparent transmission over 100%.

36

Table 6.1: Tensile Strength: Kuraray versus Bicron. A long heating time and high
pressure make stronger splices. The apparent greater strength of the Bicron fiber is
due to the mismatch in sizes for the Kuraray fiber (no clear Kuraray of the proper
size was available during this test).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pressure Heating Time Breaking Pressure
(psi) (seconds) (pounds)
Kuraray Y11(250) 0.75 mmd) to Kurrary clear 0.83 mm¢

1.5 20 3.30

1.5 10 5.65

1.2 10 4.30

1.2 7 3.30

1.0 10 3.75

1.0 7 3.00

0.8 10 3.60

0.6 15 3.95

0.6 10 1.50

Bicron 0.75 mm¢ to Bicron clear 0.75 mm¢

1.5 15 6.45

1.5 10 6.45

1.2 10 6.30

1.2 7 2.70

1.0 15 6.25

0.8 15 2.65

0.6 20 2.75

0.6 10 2.70

 

 

 

 

 

 

Table 6.2: Comparison of Kuraray Y11(250) Non-S and S type fibers. The only
property in which the S type fiber excels is in bending radius. For our application,
this was secondary to light transmission and Non-S type was chosen.

 

 

 

 

 

 

 

 

 

Fiber Sample Transmission 0 Bending Normalized
Size % m Radius (cm) Light Transmission
S Type 15 82.6 6.0 .5 0.75
Non-S Type 13 94.2 1.7 2 1.0

 

 

37

Table 6.3: Fiber End Preparation: Razor Blade versus Diamond Polisher. The Di-
amond Polisher significantly improves the splice quality and adds only 2 minutes of
preparation time per splice.

 

 

 

 

 

 

 

 

End Sample Transmission 0
Preparation Size % Mean
Razor Blade 20 91.9 5.2

Diamond Polisher 19 95.1 3.0

 

 

Table 6.4: Study of a large batch of spliced fibers. After a 40 fiber learning curve,
the quality of the splice improves. While the bad splices seem worse than the good,
some of the worst splices are in the good group.

p . . a
% Mean
100 bers 95.7 3.7

First 40 94.0 3.9

60 96.8 3.
79 96.0 3.8
17 94.9 3.7
4 91.7 2.0

 

Chapter 7

XY Scanners: Photomultiplier
Response Uniformity

The completed endplug calorimeter will use over 2000 photomultiplier tubes. Also,
the surface of the photomultiplier must have a uniform response over the entire sur-
face. To ensure this uniformity, a light mixer is fixed to the face of the photomul-
tiplier. Thus, instead of light from the fibers entering the photomultiplier directly,
it must pass through the light mixer. This removes any position dependance for the

photomultiplier response. [5,6]

To investigate the surface response uniformity, an z-y scanner was constructed. It
consists of a green LED attached to two 1 mmd> clear fibers. One of these fibers leads
to a fixed phototube; it is used to calibrate of the light output of the LED. The other
fiber is placed in a mount aimed at the phototube to be scanned (Figure 7.1). The
LED can moved over an area of 28 by 28 cm”. The entire LED assembly is attached to
rollers controlled using an IBM PC. The phototubes are read out through CAMAC.
The current PC-based software allows the user to scan the entire surface of a 1 inch
diameter phototube, in 1mm steps, in 15 minutes. Larger tubes can be scanned in

correspondingly longer times.

Two different modes exist for the z-y scanner. In the first mode, an 8 mm long

38

39

1mm clear fiber

   
 

calibration phototube

LED
scanning point

Figure 7.1: Detail of the z-y scanner. Light from the LED passes through the 1 mmd)
clear fiber to the calibration tube and to scanning point. The scanner has 1 mm
resolution.

40

by 1 mmq’) tube is placed over the end of the clear fiber. The inside of this tube
is painted black and the tube acts as a collimator. Light from the end of the fiber
diverges in a cone with half-angle 22°. By placing the collimator over the end of the
fiber, the angle of divergence is reduced to 11°. If the face of the photomultiplier is
placed within 1-2 mm of the end of the tube, the light reaching the photocathode is a
circle of diameter 2 mm (Figure 7.2). Scans made with the collimator in place will be
accurate for scans of position versus output and are useful in examining photocathode
response uniformity. However, light leaving a fiber does diverge in a 22° cone. Thus,
for light mixer studies, the collimator must be removed. If it is not removed, the light

does not sufficiently diverge and renders the light mixer useless.

To determine the effectiveness of a light mixer, four scans are made. First, a scan
of the phototube, using the collimator is made (Figure 7.3). This allows the user to
see if the phototube is acceptably uniform for this type of study— there are limits on
how well even the best light mixer will perform. Next, a small, black, opaque dot is
placed on the surface of the photomultiplier and the tube is again scanned with the
collimator in place. The next scan is done with both the light mixer and dot in place.
The collimator is removed so the light mixer can function. In the final scan, the dot
is removed and the tube is scanned with just the light mixer. For the plots shown,
the tube used was a Hamamatsu R4125, which has a 1.5 cm diameter photocathode.
The dot used with this tube had a diameter of 5 mm. By placing the dot on the
photocathode, a defect is created. If the light mixer can remove this defect, than it is
an effective light mixer. As can be seen in Figure 7.4, the defect is indeed removed.
The only effect of the dot is to lower the overall response of the tube by 5%. Since
the dot effectively decreases the active area of the photocathode by 0.52/152 = 0.11,
this loss is expected (the loss is less than expected as the dot only reduces the light

by z 50%). For this test, the light mixer used was 40 mm long, and the side placed

41

 

1mm clear fiber

3.5 deg

 

collimator

 

 

1mm clear fiber
scanning surface

Figure 7.2: The collimator is used when determining the uniformity. However, when
studying light mixers, it must be removed. Light from a real fiber will not be colli-
mated and collimated light cannot be mixed.

42

on the photomultiplier was a square 10 mm on a side. As can be seen in Figure 7.4,

an area of roughly 100 mm2 has the same response (to within 5%).

The x-y scanner is an effective device for testing both light mixers and photomul-
tipliers. Further tests are underway to examine a variety of differently shaped and
sized light mixers. Also, the plug upgrade PMT group plans to scan a fraction of the

incoming photomultiplier tubes as a quality control.

 

43

 

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Figure 7.3: The upper plot is a scan of a bare photomultiplier using the collimator.
The lower plot is a scan of the same photomultiplier with a 40 mm light mixer, the
collimator, and a 5 mm¢ black dot placed in one corner. Each line on the contour
plot is 5%.

44

 

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Figure 7.4: Both plots are with a 40 mm light mixer but without the collimator. The
upper plot is with the dot; the lower is without it. The light mixer effectively mixes
the light and removes the effect of the dot, with a 4% reduction in output. Each line
on the contour plot is 5%.

Bibliography

Bibliography

[1] Deninno, M. et. al. CDF Plug Upgrade Draft 1.

[2] M. Atac, W. Foster, M. Lundin, “A Simple Method for Fusing Plastic
Fibers”, Fermilab technical note FN-537, March 1990.

[3] C. Bromberg, J. Huston, R. Miller, S. Pollack, D. Shooltz, K. Tollefson,
B. Williams, “A Dichromatic Scintillating Fiber Calorimeter”, Proceed-
ings of the Calorimetry Conference, 1990.

[4] Hara, K., Horiuchi, K., Kim, S., Takikawa, K., Yasouka, K., Koyano, M.,
Matsui, S., Okumura, H, “Heat Splicing of Plastic Fibers Using PEEK
Tube”. CDF /DOC/PLUG-UPGR/CDFR/ 1950, February 5, 1993.

[5] Koksa, Wayne, “Light Mixer Study”, Fermilab technical note TS-DET
93-038, June 1993.

[6] Koksa, Wayne, “Photocathode Scans of Hamamatsu R4125 PMTs”, Fer-
milab technical note TS-DET 93-045, June 1993.

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