Measurement of Light Transmission in Radiation Damaged
Glued Quartz Bars for Qweak
KATIE KINSLEY (Department of Physics, Ohio University, Athens, OH 45701)
DAVID MACK (Jefferson Lab, Newport News, VA 23606), JULIE ROCHE (Ohio University, Athens, OH 45701)
INTRODUCTION
METHODS USED
ANALYSIS
SYSTEMATIC ERROR
The key objective of Qweak is to compare the probability of
scattering between polarized electrons spinning in opposite
directions. In order to do this we take a continuous beam of
electrons and shoot it at a hydrogen target. In doing this,
the electrons will sometimes come close to the electrically
charged parts of the target, causing the beam of polarized
electrons to scatter at different angles. We used Cherenkov
light detectors consisting of large quartz bars and
photomultiplier tubes. These detectors are quite large,
therefore several large quartz bars must be glued together
using Shin-Etsu Silicones 406. However, the continuous
electron beam emits powerful amounts of radiation, and the
transparency of the glue could be compromised due to
radiation damage. The ability of the glue to withstand high
levels of radiation needed to be tested in order to ensure
that the Cherenkov light detectors could measure as much
light as possible, and that the glue would not absorb more
light with radiation damage.
BASIC SETUP
The transmission data collected was analyzed using the
following equation:
The transmission, especially at low frequencies, is well over
100%, which is impossible. Because of this graph:
The setup to measure the radiation damaged quartz
along with the SES406 consists of a monochrometer that
shoots a beam of light with specified frequencies through
the sample bars. After the light travels through the quartz,
it goes though a collimator and then into an integrating
sphere. The integrating sphere has a shutter that can be
opened and closed from outside the dark box. Once the
light passes through the shudder and into the integrating
sphere it bounces the light around inside, and shoots it
into a photomultiplier tube. All of the equipment is
connected to a MS-DOS computer program, which
displays the output current for each run.
( Iin  Idark )
Tuncor 
( Iout  Idark )
Where Tuncor is the transmission percentage without Fresnel
correction factor included, Idark is the output current of the
dark rate, and Iin and Iout are the output currents with the
quartz sample in and out, respectively.
According to
Fresnel’s correction
factors, different
amounts of light are
reflected at different
frequencies of light.
Therefore, we need
to account for this
reflection by
multiplying Tuncor
by its appropriate
correction factor.
QUARTZ
BAR
SAMPLE
Qweak Detector at
Jefferson Lab
MONOCHROMETER
After accounting for Fresnel reflection, the transmission data
could be analyzed.
Transmission in Control Quartz
ABSTRACT
Electron accelerators all over the world are used to study particles
at a subatomic level. In the Qweak experiment, scientists are
attempting to learn more about scattering probability for polarized
electrons with opposite spin. Detectors are being used in Qweak to
measure Cherenkov light. These detectors are extremely large, and
in order to build them quartz bars must be glued together using
SES406. These quartz bars and glue will be exposed to very high
levels of radiation, and it is essential to be sure that the glue will
remain transparent, even after high doses of radiation. Using a
monochrometer and a PMT, we took measurements with control and
experimental quartz bar samples which had been exposed to 1 Mrad
of radiation. The final result showed that the SES406 absorbed <1%
of the light after being damaged from radiation.
With the stated setup, the procedure for measuring the
transmission of light though the quartz bars consisted of
recording the displayed output current at 250nm, 275nm,
300nm, 400nm, and 500nm. First we needed to measure
the dark rate of the box, so we closed the shudder to
prevent the monochrometer from shooting light into the
PMT. Usually the dark rate measured around -14.500 nA.
Additionally, it was necessary to take with the sample in
and out. In order to do this, we would set the desired
frequency and high voltage, and then we would take a
total of 10 runs, alternating the quartz bar sample being
in and out of the beam of light, providing 5 runs with the
sample in and 5 runs with the sample out. We would use
this method for a control sample and an experimental
sample glued together with SES406 that was exposed to
1 Mrad of radiation.
1.004
Transmission
PROCEDURE
1.006
1.002
1
Transmission
0.998
0.996
0.994
250
275
300
400
500
Wavelength
Transmission in the Experimental Quartz
1.005
1
0.995
Transmission
INTEGRATING
SPHERE WITH PMT
ATTACHED
0.99
0.985
Transmission
0.98
0.975
0.97
0.965
0.96
250
275
300
Wavelength
400
500
We realized that this was because of a light leak in our dark
box, which was being analyzed by the PMT. We used a
piece of tape to cover different slit sizes, and also added a
collimator to block room light from reaching the PMT. With
this change to the hardware, we measured the size of the
light leak and found that is was 0.2 nA of room light leaking
in. Our new equation accounts for this error:
( Iin  ( Idark  0.2))
Tuncor 
( Iout  ( Idark  0.2))
CONCLUSION
We resolved all the systematic issues with the addition of the
collimator and the subtraction of stray light. Comparison of
glued and control slide show that glue causes <1% loss of
transmission in the UV.