Why is PMT specialty testing so important?
The specialty tests study several variables involved in the interaction between events in the MiniBooNE tank and the
equipment that allows us to detect those events. By better understanding these variables, we can gain a better
picture of the true nature of the events.
Low Light Level Test
Conducted by Christi Bohmbach
The objective of this PMT Specialty Test was to determine how the PMTs behave when under low light levels.
Much of the data that we currently have for the PMTs does not apply specifically to MiniBooNE.
So, testing the PMTs under low light (producing less than one photoelectron) allows MiniBooNE to more
accurately understand the data that comes out of the detector.
Question:
What charge (in picoCoulombs) does one photoelectron yield in the PMT output?
Purpose:
To determine the mean charge that one photo-electron produces in a PMT’s electronics. Perform the test on both the
LSND tubes and the new Hamamatsu PMTs.
Location:
New Muon Lab darkroom "wine racks".
Pre-testing:
In order to begin, the test needed a filter which has a high probability to allow only enough light to produce
one photoelectron. This was done by finding a filter that has roughly a .001 probability for producing 2 photoelectron
wave forms using Poisson statistics 
. This effectively makes all
waveforms of 2 photoelectrons and above negligible.
Using a PMT (LSND id# 21n10), Christi tested each of five different filters individually and in
combination to find the best candidate to produce only one photoelectron. Once this was completed, that filter
was used in the test.
Setup & Procedure:
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Before running the equipment, the tubes were dark adapted for 12 hours.
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10 sets of data were taken for each of eight different PMTs. Each data set contains 1000 wave forms.
Each wave form contains 500 separate data points.
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Tubes used:
LSND: 14n18, dc40, 9s4, and hh3001
New: sa2510, sa2614, sa1027
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Using a computer program, the data is condensed into a more workable size statistically and graphically.
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When all the data was collected, Christi analyzed the results for the mean charge (in pico-Coulombs)
and their standard deviation (charge resolution).
Significance:
This test provides a model for the uncertainty/error of the PMT electronics for use in conjunction with the detector
Monte Carlo. This test also helps the Monte Carlo translate the final
charge signature into an energy signature for the event in question.
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Angular Dependence Tests
Conducted by Kelli Crews
This test is important for its ability to ensure the accuracy of the detector simulation. The angle at which
light travels through the detector tells us about the particle's speed and direction. It is therefore important
to test the PMTs for angular dependence. An event occurring toward the sides of the detector would yield scintillation
light that would interact with various PMTs at different angles (see below). If there is a dependence to the
angle of incidence, this must be accounted for in order to obtain relevant data.
| Diagram of the angular conditions inside the MiniBooNE tank |
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Note the difference in the angle theta at different positions around the tank
Theta being the angle between the photon incidence vector (yellow) and the tank radii (black).
All the PMTs are oriented so their central axes align to the tank's center (along the radii). |
Question:
How does the quantum efficiency of the PMTs vary with respect to the angle of incoming photons?
Purpose:
Define the effects of PMT response when the angle of incident light with respect to the PMT-axis varies. This
includes testing inside oil (Duo Prime 70, non-synthetic) to further simulate conditions inside the detector.
Define a function to describe the
average quantum efficiency among the PMTs as a function of the angle of incidence.
Location:
New Muon Lab darkroom
Setup & Procedure:
Note, color is for reference only, the actual setup is painted completely black.
- Pretesting
- First, a test was performed to ensure that the length of a given testing run did not affect test results.
Tube 15s9 was used to test overnight for twelve hours, then for three hours, and finally two. No
significant differences were observed and three hours was chosen as the length of a single testing run.
- The light source was place at a distance 33in or about .8 meters away from the PMT. The light source was
then rotated (not the PMT). This was to insure that the intensity of the light source did not change by more than
10%. Results were within this range and 33in was used.
- Test One
- The test setup is as shown in the diagram above. The tube was rotated around a pivot located
on a harness that fits snugly over the photo-cathode. The pivot axis is along the very front of the photo-cathode.
- The tube was rotated every three hours going from 0 to 90 degrees at 10 degree increments.
- Data was taken with respect to the average quantum efficiency over the surface for each orientation
- Test Two
- For this test, the setup is the same with the addition of a mask over the photo-cathode with a small hole
at the cathode's center. This hole stays at the center of the photocathode as the tube rotates.
- First, the tube was rotated every three hours going from 0 to 30 degrees at 10 degree increments.
- The test was also done again every three hours going from 0 to 90 degrees at 15 degree increments.
- The same form of data was taken.
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Significance:
The results of this test will allow us to ensure that the detector Monte Carlo simulation of PMT
behavior has the correct response to angular information. This is done by determining the formulae for
modeling the quantum efficiency of the PMTs from this test.
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Structural Dependence Tests
Conducted by Justin May & Matt Wysocki
The only way to fully understand the data that we receive is to understand the tools that we are using to
obtain that data. The statistics available for the LSND and Hamamatsu PMTs apply to testing conditions that are not
specific to MiniBooNE. We need to understand how the structure and makeup of the PMTs affect their accuracy.
If we know the exact behavior and limitations of the PMTs, then we can understand
the data that comes out of the electronics much better.
Question:
What is the geometric acceptance (angular dependence for small areas on the photocathode), the timing resolution
and quantum efficiency of the PMTs with respect to their orientation, their shape, and the angle of incidence of
the photons?
Purpose:
Experiment with the PMTs to determine a model specific to MiniBooNE that augments the information from Hamamatsu.
Determine a model for the MiniBooNE specific effects for PMT response conditions of the following:
The orientation of the "venetian blinds". The venetian blinds are the small metal objects
between the PMT focusing region and the first dynode. These blinds are oriented in one specific direction and may
have a bias against photoelectrons entering from certain sides over others. Analyzing the possible
bias resulting from this is one of the primary concerns of this specialty test.
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The angle (theta) between the central axis of the PMT and the angle of incidence for
incoming photons. We want to make sure that the uniformity of the photocathode is precise enough to not
produce a bias for this effect. Here, the incident vector is always normal to the photocathode.
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The shape of the PMT which can be most nearly approximated by an ellipsoid.
In reality, its geometry is a combination of a sphere on top and an ellipsoid aroung the sides. |
Location:
New Muon Lab
Setup & Procedure:
- The following are some key specifications for this test setup
- The distance from protractor to the front of the PMT is .131 meters . This is the radius of the
spherical portion of the PMT's surface.
- The collimator is constructed of an approximately .09 meter long cylinder of aluminum.
- The PMT harness is a left-over harness from the tank installation. [same used in tank]
- The harness is fitted in the apparatus so that the tube rotates on an axis between the PMT 'stem'
and the elliptical (side) portion of the PMT.
- The PMT is made to rest within a few millimeters of the "light stop" or "mask".
- This test is performed in Duo Prime 70 (non-synthetic) oil
- When testing the PMT is moved in 15 degree intervals.
- When adjusting the PMT, the PMT moves while the mask and collimator remain the same.
- The PMT photocathode is kept as close to being normal to the collimator as possible.
- Measurements are taken from -45 degrees to +45 degrees along a swing perpendicular to the orientation
of the venetian blinds. This accounts for seven measurements.
- Parallel to the venetian blinds, measurements are taken only for 0 to +45 degrees. This adds 3
more measurements to make 10.
Significance:
The primary goal of this series of tests is to map out the geometric acceptance over various small areas and
varying orientations. This data is valuable because it applies specifically to the conditions and problems we
face in our experiment. This series of tests will provide a proper model for the behavior of the PMTs to
further understand the data from the detector.
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This website was created by Richard (Trip) Page with assistance from the following people:
Bonnie Fleming, Darrel Smith, Christi Bohmbach, Kelli Crews, Justin May, and Matt Wysocki.
Last updated: August 2001
