Proceedings of The National Conference
On Undergraduate Research (NCUR) 2005
Washington and Lee University
Virginia Military Institute
Lexington, Virginia
April 21 – 23, 2005
The MuLan Detector Calibration System
Christopher J. Church
Department of Physics
James Madison University
800 South Main St
MSC 7702
Harrisonburg, VA 22807. USA
Faculty Advisor: Dr. Kevin Giovanetti
Abstract
The MuLan experiment has been designed to make a precision measurement of the muon lifetime to one part per
million. The experiment uses 340 scintillation detectors to detect the positron produced in the muon's decay.
Members of the James Madison University Physics department have been responsible for the design and
implementation of a calibration system for the MuLan detector. The calibration system that was developed utilizes
Light Emitting Diodes and Fiber Optics to simulate the detection of positrons in the scintillators. This system was
designed as a real-time diagnostic to calibrate and test the entire detector array both locally and remotely. The
completed system was integrated into the detector July 2004. Test performed on the installed system have shown
that while some adjustment will be required, the overall design of the system works and the system is capable of
producing results beneficial to the experiment.
1. Introduction
For the past several years members of the MuLan collaboration from James Madison University have been
developing a calibration system for the MuLan project. The goal of this experiment is to make a precision
measurement of the muon lifetime to 1 part per million (ppm). A precision measurement of the muon life time will
lead to a 0.5 ppm value for the Fermi coupling constant1, a value significant to the standard model. This paper,
however, will discuss primarily the development and implementation of a calibration system for the project.
To obtain a one part per million (ppm) measurement, a spherical array of three-hundred and forty scintillation
detectors is being used. These detectors are segregated into pairs to detect the positron produced during the muon
decay. In recent years, members of the James Madison University Physics Department have designed and
constructed a calibration system for the MuLan detector that uses Light Emitting Diodes (LEDs) and fiber optics to
simulate the detection of positrons. The design of the system allows it to be easily integrated into the detector and
allow real-time use.
2. The MuLan Experiment
The MuLan experiment uses the πE3 beam at the Paul Scherrer Institut in Switzerland to deliver muons to a thin
target in the center of the MuLan detector. The beam is directed at the target for a period of approximately 5µs
allowing muons to be collected in the target. After this time, the beam is "kicked" away and the twenty or so muons
collected in the target decay. The beam is then returned and more muons are collected in the target. This process is
repeated until the 1012 decays required to achieve a 1ppm measurement is reached.
The muon decay is identified by detecting the positron produced from the muon decay with scintillation tiles. Each
scintillation tile is attached to a Photo Multiplier Tube (PMT) via a light guide. These components make up the
detector assemblies that are then housed in a series hexagonal and pentagonal shaped "cans". The cans are then
connected to form a large soccer ball shaped detector within which the target is held.
The individual detectors are wrapped to prevent any "light leaks" that would produce erroneous signals on the
PMTs. Each detector assembly is paired with another and the pairs are placed into one of the cans; six pairs in the
hexagonal cans and five pairs in the pentagonal cans. The entire detector thus consists of one-hundred and seventy
of these detector pairs.
The design is to have the PMT signals read out by waveform digitizers (WFDs) that are being developed at Boston
University, however, these will not be installed until the Summer of 2005. This has dictated that another method be
used until the WFDs are completed. Therefore, the PMTs are currently read out using discriminators and time to
digital converters (TDCs).
3. MuLan Calibration System
The calibration system that has been developed by James Madison University was designed to simulate the detection
of the positron by the PMT. When the positron passes through the scintillator it results in a flash of blue light being
produced. This light is then guided to the PMT and an output signal is read. Since the detector assemblies are
wrapped to prevent light leaks it was necessary to develop a system that could deliver a flash of light, similar to the
one detected as a result of the muon decay, to each individual PMT without compromising the integrity of the
detector assemblies. It was determined that the problem could be solved through the use of LEDs and fiber optics.
The LEDs were based off of the KAMLAND 2 LED boards used in the KAMLAND experiment. These boards
require a system to control their operation and consequently an “LED driver” system was developed.
Over the past several years each of these components have been developed and integrated into a complete
functioning system. The system consists of three-hundred and forty individual LED, boards one-hundred and ninety
two driver boards, and twenty four ECL to TTL translator boards. The driver boards and translator boards are house
in rack mountable boxes each of which contains sixteen driver boards and two translators. This allows thirty two
LEDs to be controlled by a single box.
3.1. LEDs and fiber optics
Initially JMU studies3 showed that LEDs produce an acceptable signal and that the KAMLAND pulser, with slight
modification would work for the MuLan experiment. The studies verified that LEDs were capable of producing light
pulses on the order of a few nanoseconds and that the pulses could be repeated at a rate or 50 kHz. These aspects
allow the LEDs pulsers to accurately simulate positron detection.
The LED boards were coupled to the detector via a fiber optic bundle consisting of three individual one millimeter
fibers. This fiber optic cable allowed the LED boards to be attached to the outside of the detector while still
allowing the light to be delivered directly to the PMT. Investigations into fiber polishing methods and light
transmittance allowed the development of an efficient means of producing the 1200 fibers that were required. It was
determined that cutting the fibers to the appropriate length with a hot-knife consistently produced a fiber end with
the desired transmittance. This method significantly cut the production time to a minimum.
A reasonable means to connect the LEDs to the fiber optics was required. This problem was solved by using plastic
rods as the LED couplers. The rods were cut to approximately 2 cm in length and a hole large enough for the LED
to fit into was drilled halfway into each piece. A smaller hole was drilled from the base of the LED hole through the
opposite end just large enough to fit the three fibers. The fiber bundles were then inserted halfway into the coupler
and glued into place with UV glue. The couplers and fiber bundles were then inserted into a heat shrink tube which
was sealed only around the coupler end to prevent the fibers from melting. The entire assembly was then attached to
the base of the light guide at the PMT as shown in Figure 1.
2
Figure 1 Drawing of the unwrapped detector pair with scintillation tiles, light guide, PMT, fiber optic LED coupling
assembly.
3.2. driver boards
The LED boards used require a trigger in order to flash. This trigger is supplied by the LED Driver boards. These
boards produce a TTL pulse that is sent to the LED board causing it to fire. Each driver board contains two channels
allowing it to control two separate LEDs. Along with the trigger pulse, the driver boards also supply power to the
LED boards. Each channel on the driver is equipped with an adjustable pot that controls the intensity of the LED
flash allowing the individual LEDs to be adjusted individually.
3.3. translator boards / flight simulator
To generate a trigger pulse for a specific LED, Flight Simulators developed by Boston University are used. A flight
simulator is a VME module that can be controlled by the data acquisition system to produce patterns of trigger
pulses. These pulses are cabled to driver boxes that translate the ECL to TLL and route the triggers to the
appropriate LED driver board. The system is used to determine the channels and time to pulse each LED. An
online interface was developed to display and control each flight simulator channel. The interface allows the
individual channel status to be both controlled and displayed according to detector identification number. The
control can be accessed remotely allowing the LED system to be used from a remote location.
As stated above, in order for the flight simulators to be used with the LED system the ECL pulse generated has to be
converted into a TTL pulse. To accommodate this, a circuit was developed that converts each of the 16 triggers
produced by the flight simulators are translated into TTL signals. These TTL signals are then sent by the driver
boards which route them to the LEDs. The translator boards were designed so that several driver boxes could be
connected together and use a single flight simulator if necessary by passing the ECL signal to an output prior to
converting it. This signal is then fed into another driver box.
4. Implementation and Analysis
During July of 2004 the calibration system was completed and installed on the detector. While the entire system
was completely installed, only fifty percent was brought online and tested. Some initial problems were found with
several channels but were generally found to be caused by bad solder connections on the driver boards or translator
boards or a bad cable connection and were easily corrected. One box had several issues that were not readily solved
and it was ultimately determined that a faulty power supply was responsible. The system in its present condition
has, with the exception of some minor problems, demonstrated reasonable stability.
During an experimental run in December 2004 several experimental runs were made using the LED system. These
runs provided data with which a quantitative description of the system can be made. A sample plot of this data is
shown in Figure 2.
3
Figure 2 Sample data taken form the LED in detector 167 on run 23754 in December 2004.
Currently an effort is being made to analyze the data acquired during December and determine the consistency of
the LED system and determine if there are any unexpected effects that might compromise its usefulness. Several
obstacles must, however, be overcome before any definite conclusions can be made. One of these is that the binning
factor for the data has produced a regular pattern of bins containing zero counts. Every fourth bin contains no data
as shown in the sub-set of data shown in Table 1.
Table 1 Sample set of data from detector 167 showing the regular pattern of zero counts in every fourth bin.
Counts
0
17449
85257
124265
0
150493
149529
83088
0
136994
341488
388362
0
264965
133111
107406
0
31407
66330
87937
Time (ns)
25765.5
25766.5
25767.5
25768.5
25769.5
25770.5
25771.5
25772.5
25773.5
25774.5
25775.5
25776.5
25777.5
25778.5
25779.5
25780.5
25781.5
25782.5
25783.5
25784.5
This effect is most likely due to the binning factor used during data acquisition but must be corrected to portray the
behavior of the system.
Initial analysis will consist of fitting the pulses and examining width for a large sample of detectors and evaluating
consistency in the behavior of each. Figure 2 reveals a small feature between 25790 ns and 25800 ns whose origin
is not clear. Along with quantitatively describing the pulses generated a description of this small after pulse is of
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immediate interest.
5. Conclusions
Further analysis on the data that has already been acquired will be done with the goal of quantifying the LED
system. During the summer of 2005 the remaining portion of the calibration system will be brought online and
tested. Once the entire system is functioning data can be acquired with the fully functioning system and a more
accurate statement can be made about the system as a whole. However, based on the initial analysis of the system
the indication is that it will prove to be capable of producing a consistent simulation of the muon decay detection
with the potential of being used to examine more of the detector systems.
6. References
1. D.W. Hertzog et al., “MuLan Progress Report 2004” (2005)
2. “KamLAND”, http://www.awa.tohoku.ac.jp/html/KamLAND/
3. Chris Church Summer 2003, \\Csm1\PhysWeb\Giovanetti\GiovanettiResearchGroupWeb\Chris\Chris2003.htm
7. Acknowledgements
A host of students have directly contributed to the design and final production of the calibrations system. The author
would like particularly acknowledge two students with whom he has worked closely, Eric Bartel and Matt Miller.
This system is part of a much larger detector system and the author would also like to acknowledge professors and
students at collaborating universities for help in developing an overview of muon lifetime experiment as well as
considerable aid in implementing and performing measurements with the calibration system. A full list of
collaborators can be found at:
http://ten.npl.uiuc.edu/exp/mulan/muLanMain.html
PSI Experiment R99.07.01
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