Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Study of light yield for different configurations of plastic scintillators and wavelength shifting fibers
Introduction
Since the start of data collection in 2004, the Pierre Auger Observatory [1] improved the understanding of ultra-high energy cosmic rays significantly. Fundamental results and unexpected discoveries were achieved, e.g. the verification of the strong flux suppression for cosmic rays above eV, the indication of a mixed mass composition at the highest energies, or the observed anisotropy of cosmic rays above eV [2]. Still, the origin of these discoveries and their consequences on the understanding of the acceleration and propagation processes in the universe are unknown. Furthermore, the ultimate goal of the Observatory, the identification of sources of ultra-high energy cosmic rays, is still not reached.
For this reason, the Observatory will be upgraded in the following two years. The upgrade, called AugerPrime, is realized with several changes of detector electronics, as well as an installation of new detectors and detection devices increasing the exposure time and providing additional measurements to the already existing detectors. In addition, the enhanced mass sensitivity of the upgraded detectors will improve the determination of the characteristics of cosmic particles significantly [3].
One major step in the upgrade is the installation of a Scintillator Surface Detector (SSD) on top of each of the Water-Cherenkov Detectors (WCD) which originally form the surface detector (SD) [3]. With the new SSDs, complementary measurements of the extensive air shower particles arriving at the ground level are possible. Due to the different sensitivity to different shower components, the combination of scintillator detectors and water-Cherenkov detectors provides a significant improvement in the discrimination of shower particles, and therefore, facilitates the analysis of cosmic rays. Each SSD consists of an aluminum box filled with plastic scintillator bars, which are separated into two active modules with an area of approximately each. The scintillator bars are coupled with wavelength-shifting (WLS) plastic fibers which guide the scintillation photons by utilizing total internal reflection effects towards a single photomultiplier tube (PMT) placed in-between the two modules. The use of a scintillator-fiber combination and the WLS characteristics of the optical components are necessary to reduce the attenuation effects inside large sized detectors, such as the SSDs.
In general, the light yield of a scintillator-fiber combination, i.e., the detection efficiency, is the major criterion to describe the quality of the detection set-up. In this study, the performance of multiple configurations of scintillators and fibers is tested under equal conditions in a dark box experiment. Each configuration contains short scintillator bars coupled with plastic fibers in different combinations which were candidates for the installation inside the SSDs. The components differ in their profiles and dimensions, as well as in their material purity depending on the production process. In addition, the performance of scintillators and fibers experiencing aging effects is studied. The sample tests were performed independently from each other in two different laboratories, at the Institut de Physique Nucléaire d’Orsay (IPNO), France, and Institut für Kernphysik (IKP), Germany.
The final selection of the best fitting configuration to optimize the detector design is mainly based on the absolute light yield but also takes the total costs and the long-term stability of the material into account. In order to analyze the effect of altering processes of the material, several aging studies were run within our scintillator performance tests. While the presented work has been carried out in the framework of the AugerPrime R&D activity, the outcome of this study is of general interest for large-scale cosmic-ray experiments which aim to measure the electromagnetic or muonic components of EAS. The choice to collect the scintillation light by means of optical fibers on a single photo-detector represents a cost-effective solution to reduce the number of read-out channels, especially for a large detection area.
Section snippets
Plastic scintillators
For the construction of the samples, we used a large number of plastic scintillator bars from two different producers varying in their profiles, dimensions, and fabrication processes. In detail, we measured the light yield of plastic scintillators produced by the Fermi National Accelerator Laboratory (FNAL) in the USA and by the Beijing Gaonengkedi Science and Technology Co., Ltd. (GNKD) in China.
Plastic scintillators produced by FNAL are used for particle detection in several large-scale
Wavelength-shifting fibers
The second major component for this light yield study, besides the scintillator bars, is the optical plastic fibers used for the readout of the scintillation photons. Depending on the scintillator profile, the fibers are either inserted in the holes through the bars or kept into the groove on top of the bars with Aluminum tape and, possibly, optical glue. For the light yield tests, we used wavelength-shifting fibers based on polystyrene as core material from two producers, Saint-Gobain in
Sample preparation
To achieve comparable results between individual samples and between the studies of the two laboratories, the preparation of the scintillator-fiber configurations follows a uniform design and procedure.
Before installing the scintillators in the dark box, all bars were cut to a fixed length of mm. The scintillators from GNKD which are not co-extruded with a reflective surface layer were wrapped in different materials, in Tyvek foil, in aluminum foil or in Teflon tape.
After preparing the
General set-up and measurement procedure
The light yield studies of both laboratories follow a general set-up and procedure for a dark box experiment. Each sample configuration consists of two scintillator bars connected with one or more optical fibers depending on the scintillator profile. For each run, a single sample is placed in between or on top of two plastic scintillator paddles which are used as coincidence trigger for the data acquisition. The readout of the sample is realized by coupling the fiber ends to the entrance window
Evaluation of sample components
The different ways to couple the fiber ends to the PMT entrance window and the different PMT models used in the IPNO and IKP laboratories result, respectively, in a different light collection efficiency (LCE) and a different quantum efficiency (QE) for the light yield measurements performed in the two laboratories. For this reason, a direct comparison of the light yield measured in the two laboratories for the different tested scintillator/fiber configurations is somewhat difficult. Anyway, a
Light collection optimization
It was of interest in this study to propose an effective way to couple the fiber to the PMT entrance window in order to optimize the light collection efficiency. To this aim we used one pair of FNAL 45 10 mm 2-hole scintillators, two pieces of Kuraray Y11(200)-MSJ WLS fibers bent in U-route mode at the ends of the scintillators and two different PMTs, the R9420-10 from Hamamatsu and XP5300 from Photonis. The scintillator detector was then coupled to the PMTs’ entrance window using five
Light-collection efficiency estimate for differentscintillator/WLS-fiber configurations
In order to explain the outcomes of this study, we performed an estimate of the number of photoelectrons yielded by different scintillator/WLS-fiber configurations. We followed the procedure proposed by M. Crow et al. [20] and by B. Loehr et al. [21].
When a cosmic muon passes through the scintillator bar it deposits a well-defined amount of energy and, as a consequence, the scintillator produces scintillation light. This light is then collected, by means of wavelength-shifting optical fibers,
Aging studies of scintillators and fibers
The estimate and analysis of the signal development over several years of data talking plays an important role for the long-term stability and performance of the scintillator detectors. Especially the aging of the used material can influence the light output of the scintillator-fiber combinations significantly. The observed aging effects are mainly caused by high temperatures and large temperature changes that affect the chemical and mechanical properties of the plastic material resulting in a
Conclusions
We have performed several detailed studies on each component in a variety of scintillator/WLS-fiber configurations and discussed the effects of scintillator cross-section properties, WLS fiber type, and optical coupling method on the final signal amplitude. A numerical estimate of LCE is also presented to explain the experimental results of the scintillator tests. We evaluated also the temperature variation and aging effect on the detector response. These results offer a quantitative comparison
Acknowledgments
The authors are grateful to many Pierre Auger collaborators for useful discussions. The authors would like to acknowledge the Fermilab staff producing custom-made scintillators and the Beijing Gaonengkedi Science and Technology Co. for providing scintillator samples which have been used in this study. One of the authors, Zizhao Zong, gratefully acknowledges the financial support from the China Scholarship Council PhD Program, China (No. 201406170006). The authors thank Anna Pla-Dalmau for
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