1. Radiation Damage in Optical Cement
2 August, 2016
Jose Luis Salinas
Joliet Junior College
Fermi National Accelerator Laboratory
Abstract
The booster uses scintillator counters to detect beam loss from RF buckets and to better
understand the beam. After about 9 months, the detectors must be replaced due to optical
component darkening caused by radiation damage; primarily to the optical cement. We outline
a program to measure the transparency of optical cement as a function of radiation dose. We
describe the transparency and dose measurements in the paper.
Introduction
Beam losses from the Fermilab Booster limit the rate at which protons can be delivered to
experiments. In 2014, scintillation counters were installed to measure these losses with
superior time resolution compared with the existing ionization chambers that form the beam
loss monitor (BLM) system [1]. The scintillator detectors are located near aperture restrictions
at the collimators and are capable of seeing losses from single RF buckets that are
approximately 20 ns apart. The booster uses these fast loss monitors as a tool to better
understand the beam [2]. However, after 9 months of operation of the first detectors, their
performance had degraded. When examining the first detectors, we see that the acrylic, the
scintillators, and the adhesive that that holds the optical components together all have a
darkish color to them. Of these components, the adhesive is affected the most. In this paper we
present a program to measure the properties of the optical cement as a function of radiation
dose for two different optical cement manufactures. We describe the setups to measure
transparency and radiation dose to the samples of optical cement. We further describe the
adhesive sample preparation and report results of transparency measurements for multiple
unirradiated samples of cements from two sources.
Method
We use the setup illustrated in Figure 1 to test the transparency of the optical cement. Using
model EIG1000D laser system from Advanced Laser Diode Systems [3], a 405 nm laser will send
light pulses at 10 kHz with a 40 picoseconds wide pulse. We manipulated the tune of the laser
to send out an amplitude that is 10% of the laser’s maximum light intensity. The laser is being
adjusted by a laser driver. The drive comes from Advanced Laser Diode Systems and we are
using model EIG1000D [3]. Light from the laser is sent into a splitter. Half of the light travels to
PMT B. The other half of the light passes through the sample holder, through a filter wheel, and
then into PMT A. The light that hits the PMTs is similar in duration to the light pulses from the
scintillators. PMT B monitors the light intensity pulse by pulse in our setup. We use an
oscilloscope to read the pulse heights from both PMTs.

2. Figure 1: The laser emits a light pulse that is split in half when it passes through a light splitter.
One-half of the light goes to PMT B and the other half passes through the sample, through a
filter wheel, and then into PMT A.
Before any transparency measurements can be made, we must calibrate the PMTs. We
set the high voltage for each PMT to operate in the linear region. We evaluate the linearity by
changing the amount of light a PMT receives. In the setup, PMT B monitors the light pulses
from the laser. Changing the neutral density filter on the filter wheel only alters the amount of
light PMT A detects. Figure 2 shows pulse heights vs transmission for a PMT with the gain set to
high (left) which is showing pulse height saturation and for the same PMT with the gain
adjusted down (right).
Figure 2: Pulse height vs. filter transmission for PMT A. The graph on the left shows it operating
at a gain too high for it be linear for unaltenuated laser light. The graph on the right illustrates
that both PMT A and PMT B are operating in a linear region for the same unaltenuated laser
light.
0
0.2
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1
1.2
0
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0.1
0.15
0.2
0.25
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0.35
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0.0 0.5 1.0
Pulse Height (v)
Filter transmission
PMT A avg
ratio
0
0.2
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0.0 0.5 1.0
Pulse Hight (v)
Filter transmission
PMT 1
ratio
PAE/PBe
PAE/PBe

3. After setting the high voltage for both PMTs, we can begin making transparency
measurements. Transparency is the amount of light passing through the sample (RS) divided by
the amount of light passing through an empty holder (RE).
. (1)
We measure the light passing through the empty holder as the ratio of PMTA/PMTB pulse
heights.
. (2)
We then measure the light passing through a sample as the ratio of PMTA/PMTB pulse
heights.
. (3)
Taking the ratio of the sample divided by the ratio of the empty holder results in the
transparency of the optical cement. Care must be taken not to alter the alignment of the setup
during a measurement to assure consistent results.
We evaluate optical cement from two manufactures. The BC-600 from St.-Gobain
Crystals [4] and the EJ-500 from Eljen Technologies [5]. Both are two component adhesives.
When preparing the adhesives, one must keep in mind not to stir too quickly to prevent air
bubbles. Once the optical cement is mixed well, we wait approximately 20 minuets to let air
bubbles rise to the surface. After the bubbles have risen, the cement was poured into a ½ inch
long, ¾ inch section of clear PVC pipe and set aside to cure. After a few days, we removed the
samples from the molds and had their optical surface diamond polished. A few imperfections
such as air bubbles and swirls caused by the difference in index of refraction from incomplete
mixing of cements remained.
Optical Transparency Measurements
Figure 3 shows the graphs of the light transmission of the samples as a function of the
orientation of the sample. The left graph is for the BC-600. The right graph is for the EJ-500. We
evaluated the variation in transmission due to these defects by rotating the sample in the
holder. An initial angle was set by a mark on the sample. The figure shows that most of the
optical cements have an approximately 80% transmission. About 10% of the light was lost due
to imperfections in the optical cement. The remaining 10% was calculated. We calculated the
amount of light that should be reflected due to the index of reflection. BC-600 should reflect
around 10.37% of the light passing through and the EJ-500 should reflect about 9.83%.
Neglecting the reflection at the optical surfaces, ~90% of the light is transmitted though each
sample.

4. Figure 3. Light transmission vs sample angle. The graph on the left evaluates the transparency
as a function of orientation of the BC-600. The graph on the right evaluates the EJ-500. Both
graphs have a suppressed zero on vertical axis.
Dosimetry
In addition to transparency, we intend to use a 1 cm x 1 cm x 300 μm thick silicon photodiode
to measure radiation dose. Equation 4 [6] shows that the diode leakage current is linear in the
particles radiation dose, or fluence, Ileak(Φ). V in the equation is the volume of the diode, and
αdamage has a value of 3.0x10-17
A/cm.
(4)
Temperature must also be considered because it also correlates to the leakage current.
Equation 5 [5] measures the leakage current as a funtion of temperature, where T is the
temperature in Kelvin and kb is the Boltzmann constant.
(5)
We use a thermocouple read out using an Arduino microprocessor [7] to record the
temperature as a function of time. The temperature will be recorded on an SD card that is
attached to the Arduino.
Future measurements.
Once the initial transparency of the samples has been measured, the samples will be exposed
to radiation in the booster. A silicon photodiode will measure the radiation dose and an
Arduino microprocessor will record the temperature. After some period of time, some of the
samples will be harvested and measure their transparency as a function of dose
I(T ) ⇠ T 2
e
1.23(eV)
2kB T
Ileak( ) = Ileak(0) + ↵damage · V ·
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
-100 100 300 500
Transmission
degrees
EJ-500 #1
EJ-500 #2
EJ-500 #3
EJ-500 #4
EJ-500 #5
EJ-500 #11
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
-100 100 300 500
Transmission
degrees
BC-600 #7
BC-600 #8
BC-600 #9
BC-600 #10
BC-600 #6
BC- #0
.
.

5. Conclusion
We have developed a test setup that allows us to measure the transparency of optical cement.
We fabricated samples of two different adhesives and measured its initial transparency before
exposing them to radiation. Once the measurements have been taken, we will put them into
the booster and later on measure the transparency as a function of radiation.
Acknowledgments
This research was made possible with the help of Fermilab employees. Thank you to Eileen
Hahn for helping prepare the optical cement samples, as well as cutting them and polishing
them. Also, thank you to Anatoly Ronzhin for allowing us to use his laser and work area to
measure the transparency of our samples. This work was supported in part by the U. S.
Department of Energy, Office of Science, Office of Workforce Development for Teachers and
Scientists (WDTS) under the Community College Internship Program (CCI).
References
[1] M. Karagoz-Unel, R. J. Tesarek. Nucl.Instrum.Meth. A506:7-19,2003. (arXiv:hep-
ex/0211055v1)-what is used for beam diagnostics
[2] Woodworth, Brian, “Booster Beam Profile Measurements via Stripping Foil Losses.” Fermilab
Undergraduate Poster Session. Aug 11, 2016
[3] Advanced Laser Diode Systems, A.L.S. GmbH, Schwarzschildstr. 6, D-12489 Berlin,
Germany
[4] Saint-Gobain Crystals, 17900 Great Lakes Pkwy, Hiram, OH 44234-9661
[5] Eljen Technology, 1300 W Broadway, Sweetwater, TX 79556
[6] Barberis, E., et al. Nucl. Instrum. and Meth. A326 (1993) 373-380.
[7] www.arduino.cc