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Dr Graham Wilson
Evaluation of a Scintillating Fiber Testing Apparatus,
University of Kansas, Lawrence, KS 66044 USA
The goal of this research is to design and evaluate the performance of a scintillating fiber testing apparatus. The purpose of the apparatus is to measure the properties of scintillating fibers. These properties include the time distribution of the scintillation process, the core and cladding light yields as a function of charged particle path length, and the attenuation length of the fiber.
A scintillator is a material that emits a flash of light as a result of the deposition of energy by ionizing radiation. Many particle detectors use scintillators in the form of long thin fibers because fibers offer several advantages. Scintillating fibers may be arranged into layers to provide information about a particle’s trajectory or time of flight. The fundamental properties of a single fiber measured by this apparatus can be used to simulate the response of a very large detector using thousands of fibers.
A device has been built which uses 16mm outside diameter 1.52m long 1010 steel pipe as a fiber holder. The pipe has a wall thickness of 3mm. The design includes two different sizes of particle windows. One is a 0.79mm diameter particle window drilled through the diameter of the steel pipe. Connected to the design of this small window is a mechanism to move the fiber inside the pipe to a known position relative to the window. The other window is 6.35mm in diameter. This window is large enough to use top and bottom ports to inspect the fiber visually for position. These windows are used to collimate the radiation from a source onto the scintillating fiber.
A Photonis XP2262B SN 37393 photo-multiplier tube is used to detect photons leaving the end of the fiber and a Photonis XP2020 PMT SN 25257 is used to detect photons originating from a scintillating disc placed behind the particle window. Several tests have been performed to find the various rates measured by these PMTs. The variables for these experiments include PMT supply voltage, the presence of a radioactive source, and trigger-source distance. Qualitative examinations of these data indicate that the device is capable of detecting a useful rate of coincidences, 0.34 ± 0.02 Hz, between the fiber and trigger PMTs. Timing measurements have also been performed and qualitative examinations of these data indicate the apparatus is providing reasonable information and is a worthy design to improve upon.
This research was partially inspired by the work of C.M. Hawkes, et al, of the California Institute of Technology1. Hawkes also used a steel pipe to house the fiber; however, his design may have used a separate part that surrounded the radiation source. This additional part would serve to collimate the radiation. Our design relied upon holes drilled through the fiber housing to collimate the radiation. Among other measurements, C.M. Hawkes et al’s research measured the pulse charge distribution detected by the fiber PMT. This is a useful exercise to determine if the setup is capable of resolving single photoelectron events.
One of the problems that one must overcome when making the time decay measurement for a scintillating material is that the decay time one is trying to measure is faster than the recovery time of the PMT that is being used to measure it. One method of overcoming this problem is the “time correlated single photon counting technique”2 as explained by W. Becker. This method relies upon the quantum nature of light. In order to reveal this nature, one must often reduce the light levels to a small number of photons. This is accomplished with the SciFTA design by controlling the amount of material available for scintillation and the use of neutral density filters between the end of the fiber and the fiber PMT.
The basic reference work for this research is Techniques for Nuclear and particle Physics Experiments by W.R. Leo3. This reference contains much of the fundamental theory behind this research and explains some of the practical considerations of the experimental setup. The calibration procedure used for the CAEN TDC was detailed by this text.
Data Reduction and Error Analysis by Philip R. Bevington and D. Keith Robinson4 was used to understand some of the curve fitting done with the TDC calibration and the error analysis in general.
Our method to measure the properties of scintillating fibers begins with the design of the fiber housing. The fiber housing is comprised of a fiber holder and a fiber PMT cup. The fiber holder is a 1.52m long 16mm diameter 3mm thick 1010 steel pipe. Aside from the assorted screws, the rest of the parts are made from black PVC plastic. The physical details of this fiber housing can be seen in Appendix A. The fiber housing was designed to provide a light tight environment for the fiber and fiber PMT assembly, collimate the radiation from a source, position the fiber relative to the collimated radiation’s path, and provide for neutral density filters to be place between one end of the fiber and the fiber PMT face. The fiber housing may also accept a mirror or light absorber at the non-PMT end of the fiber. Figure 1 gives some details of the fiber-PMT-source configuration.
Figure 1, the length from the large window to the fiber PMT is 56.6cm.
The original design called for a 0.79mm diameter particle window. A window for this use is a set of holes, inlet and outlet, that are aligned to form a straight path. A mechanism was provided to move the fiber relative to the window. To detect the position of the fiber within the pipe, a laser was directed through the inlet hole and onto a card placed behind the outlet hole. As the fiber positioning screws were advanced a pattern was produced on the card. A representation of the pattern and a picture of the positioning setup can be seen in Appendix B. Due to damage to the fiber positioning system an alternative, larger set of holes were used to collect data. The larger holes are a set of four threaded 6.4mm diameter holes that are arranged circumferentially around the steel pipe. Two of the holes form the particle window, and all four are used to visually inspect the fiber for position. Once the fiber has been positioned to be in the window, the large holes are then sealed with electrical tape to provide a light tight environment.
Part of the fiber housing consists of a black PVC cup that locates the fiber relative to the fiber PMT. Testing indicated that this part did not provide enough protection from ambient light so this part has been surrounded with black felt paper. Additionally, the light levels in SciFTA’s room are kept low during data collection.
The fiber PMT cup provides a 0.5mm thick space to accept two neutral density filters between the fiber and the fiber PMT face. This was not used so a 0.5mm air gap existed between the fiber and the PMT face for all of the data collection.
The schematics for the timing and rate experiments are given in Appendix C. The time data reported by the CAEN TDC was calibrated by using a CAEN N108A delay module. The change in the TDC count was found as a function of CAEN N108A delay and a linear fit was found for this relationship.
Results and Discussion
Since high light levels easily damage photo-multiplier tubes the first evaluation of the SciFTA design was the light tightness of the fiber housing. The results of this measurement can be seen in Table 1.
Table 1 gives details about the fiber PMT rate prior to the addition of light shielding to the fiber PMT cup.
The manufacturer reported the rate for fiber PMT, XP2262 SN37393 at the >0.2 photoelectron threshold was 606 Hz at 1789 V. The rates for the fiber PMT appeared to be too high; also, the rate was affected by shadows moving across the cup area so some black felt paper was cut and fitted around the fiber PMT cup. Table 2 gives some details about the typical fiber PMT rates after the addition of light shielding to the fiber PMT cup.
Table 2 gives information about the fiber PMT rate after the addition of light shielding.
Both Table 1 and 2 give the fiber PMT rates without a source. After the addition of light shielding the fiber PMT rates did not appear to be sensitive to room light levels.
The trigger PMT for SciFTA is a XP2020 Photonis photo-multiplier tube SN 25257 with a scintillating disc mounted to its face. The manufacturer stated the rate for this tube to be 211 Hz at 1932 V for the >0.2 photoelectron threshold level. The trigger PMT-scintillating disc assembly is sealed with black electrical tape. The trigger PMT can be moved freely to any position relative to the rest of the fiber housing. Several calorimetry measurements were made with the trigger PMT and a Bi-207 source. Figure 2 gives the results of these measurements.
Figure 2 was measured with the trigger PMT.
The high-energy particles detected by the trigger PMT from the Bi-207 source are expected to be 1MeV internal conversion electrons. The top left graph of Figure 2 shows a prominent peak for these electrons and the bottom right graph indicates that all of these electrons have lost a significant amount of energy in the fiber housing before reaching the trigger PMT scintillator. The top right and bottom left graphs would seem to indicate that both the small and large particle windows allow some 1MeV electrons to pass through while losing very little energy. This assumption seems valid for the large particle window since a 1mm diameter fiber would obstruct at the most only 20% of the clear space formed by a 6.4mm diameter hole. The small particle window should have no clear space. If the 1mm diameter fiber were perfectly aligned between the 0.79mm inlet and outlet holes, the minimum distance of scintillating material a straight path would encounter would be approximately 0.6mm.
Several rate measurements were taken to determine if SciFTA was able to detect a useful rate of coincidences between the fiber and trigger PMTs. Figures 3 and 4 give some detail about these measurements.
Figure 3 gives the rates of the fiber and trigger PMTs as a function of trigger PMT-source distance.
A diagram of the setup used for the rate experiments can be seen in Appendix C Figure 1.
Figure 4 gives the rates of rate of coincidences between the fiber and trigger PMTs
A calibration was found for the CAEN Time to Digital Converter or TDC. A pulse signal generated by the VM-USB module was sent through a delay and then supplied to the “common” or comm. channel of the TDC. A diagram of the setup used for this calibration can be seen in Appendix C Figure 3. The change in the TDC count was found as a function of the change in the delay setting of the CAEN N108A Delay module. The results from this experiment can be seen in Appendix D. Looking at Appendix D one can also see relatively large root mean square values for all the channels in runs 6 and 15. In each case this is the result of a single invalid count well outside of the range of the remaining counts. The majority of the error in this measurement is in the abscissa or the delay value. To find the error in the TDC count the error in delay value was propagated into the “y” direction. The results from this calibration were applied to the timing measurements via the data analysis program ROOT5.
Several timing measurements were taken. The measurements were taken in common stop mode using a delayed CFD trigger PMT signal as the stop. A diagram of setup used in this experiment can be seen in Appendix C Figure 2. The results of these measurements can be seen in Appendix E. The lower left graph is the most important to the scintillating decay time measurement. This is a histogram showing the difference between the delayed CFD trigger signal and the CFD fiber signal using Bi-207 as a source. In Appendix E, Comparing the dt_CFD graph to the dt_STD graph one can see the affect of using a constant fraction discriminator to that of a leading edge discriminator, the root mean square of the data has been reduced from 2.359 ± 0.005 to 1.647 ± 0.005.
The elements of the scintillating fiber testing apparatus SciFTA have been built and testing indicates that the present design is capable of producing useful data; however, several opportunities exist for improvement.
Photomultiplier tubes generate electrons from photons by the photoelectric effect. Photons with the right energy strike the PMT’s photocathode and liberate an electron called a photo-electron. This photo-electron continues through the tube and causes a cascade of secondary electrons. Since it is desirable to operate the fiber PMT with a very low light input, the single photon response of this PMT should be determined.
Presently, the large particle window geometry is poorly controlled. The alignment of the source-pipe-trigger assembly is done by eye. This procedure produces an inconsistent alignment from one experiment to the next. To maintain a light tight environment, tape was added to the inlet and outlet holes of the large particle window. This tape has the undesirable effect of smearing the energies of the particles delivered by a calibration source, such as Bi-207. Both of these problems could be made less severe if a light tight cup, similar to the fiber PMT cup, were designed for the window area. A preliminary design for a trigger-source cup is given in Appendix F.
1 C.M Hawkes, M. Kuhlen, B. Milliken, R. Stroynowski and E. Wicklund :Decay Time and Light Yeild Measurements for Plastic Scintillating Fibers (Nuclear Instruments and Methods in Physics Research A292 1990 329-336 North-Holland).
2 W. Becker, Advanced Time-Correlated Single Photon Counting Techniques, (Springer 2005 Berlin Germany).
3 W.R. Leo, Techniques for Nuclear and particle Physics Experiments, (Springer-Verlag 1994 Berlin Germany).
4 Philip R. Bevington, D. Keith Robinson, Data Reduction and Error Analysis for the Physical Sciences, (McGraw-Hill 2003 New York NY USA).
5 Rene Brun, Fons Rademakers, Nenad Buncic, Valery Fine, Philippe Canal, Suzanne Panacek, :http://root.cern.ch/
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