Using Optical Fiber Readout
P. Mantsch, S. Gourlay, J Ozelis
January 15, 1995
INTRODUCTION
Cosmic rays of unknown origin have recently been observed at energies significantly above 10**20 eV.[1, 2] Groups at The University of Chicago and the University of Leeds have proposed that an international collaboration be assembled to design a new detector to attempt to determine source and properties of the highest energy cosmic rays. The initial concept features a 5000 square kilometer surface array with an atmospheric fluorescence detector at the center. The surface array would consist of 3000 detector modules, each with a sensitive area of ten square meters.
At a conference on highest energy cosmic rays held in Adelaide in 1993[3], designs for possible surface detectors were developed. The detector concept was a scintillator/lead/scintillator sandwich. Such a counter could effectively separate the hard (muon) from the soft (electromagnetic) components of the air showers.
This note describes the design, construction and measured performance of a 3m[2 ]scintillator counter proposed as a candidate design for the giant air shower array. The test counter was approximately 2.5 x 1.2 meters[2] and was readout by wave length shifter fibers embedded in grooves machined in the plastic scintillator.
LARGE AREA SCINTILATION COUNTERS
Large area plastic scintillation counters have been used for many years in cosmic ray air shower detector arrays. The 2.2 m**2 detectors, for example, used in the AGASA array[4] in Japan use 5 cm thick scintillator viewed with a photomultiplier tube through the air via mirrors. The CASA array[5] in Utah has 1 mm**2 counters of 0.5 inch scintillator with a 5 inch photomultiplier glued to the center of the counter. Large area counters used in high energy physics experiments have used wave length shifter bars along the counter edges coupled to photomultiplier tubes at opposite corners. Such large area counters are difficult to read out with high efficiency and uniformity while keeping the cost reasonable.
A technique has been developed to read out scintillator tiles in sampling calorimeters in high energy physics experiments using optical fibers.[6] Calorimeters using large scintillator arrays designed for the SDC detector for the SSC[7], the new endplug calorimeters for the CDF detector at Fermilab[8], and for detectors being planned for use at the Large Hadron Collider at CERN use optical fiber readout. Wave length shifting (WLS) optical fibers embedded in grooves in the scintillator collect the light for transmission via clear optical fibers to photomultiplier tubes outside the calorimeter. This method allowed the construction of highly efficient, segmented, and yet hermetic calorimeters.
The largest "tiles" with fiber readout typically used in calorimetry are about 50 x 50 cm**2. Wave length shifter fiber readout can also be used to advantage for scintillation counters of much larger area. As will be shown, counters of 3 m**2 can be effectively readout by a single 3.8 mm diameter photomultiplier with good uniformity and efficiency.
DETECTOR REQUIREMENTS
The proposed scintillator/lead/scintillator sandwich detector is segmented into four parts. Each detector segment is nominally 2.5 m**2. The detector needs to be efficient (>95%) for a single minimum ionizing particle (MIP). The energy of the air shower is inferred from the density of particles measured in the surface detectors. The uniformity of the counter is, therefore, an important contributor to the determination of the shower energy. Also of crucial importance is the unit cost of the detectors in large scale production. Scintillating plastic is the single most costly item in the array detector module, accounting for about $6,000 per unit. It is important, therefore, that the scintillator be as thin as possible consistent with good efficiency and uniformity.
TEST COUNTER DESIGN AND A MODEL COUNTER
An intensive effort went into the development of scintillating tile/fiber calorimetry for the SSC/SDC detector and Fermilab CDF detector end plug upgrade.[9, 10 ] This experience makes the design of a large area counter fairly straightforward. The design starts with the determination of the number of fibers, spacing and groove geometry necessary to ensure adequate light collection. A small test model was used for these studies.
The material to be used for the test counter was a 2.49 x 1.25 m**2 piece of Acrylic scintillator of uncertain properties, discarded from another project. Samples of this material, 30 cm x 30 cm, were used to study the light collection using WLS fiber readout. A series of straight grooves was machined in the scintillator. The small model counters were grooved with depths of 5 mm and 10 mm and spacing of 2.5 mm and 5.0 mm. The grooves were made with a 1.6 mm end mill. The counter edges were sawcut and painted with Bicron BC620 white reflective paint.[11] The light was collected with 1 mm diameter wave length shifter fiber (Kuraray Y11, 200 PPM, double clad.) A bundle of 30, 1 meter fiber loops were glued into a plastic "cookie" and polished so that the test configuration could be easily changed to accommodate different numbers of grooves or fibers per groove without disturbing the joint to the phototube. In each test, the fibers were simply laid in the grooves and the counter was wrapped with Dupont Tyvek. The fibers were constrained in the grooves by a thin sheet of G-10 over the Tyvek. Experience has shown that light yield is insensitive to the depth of the fiber in the groove.[12] The cookie was spring loaded to an EMI 9902KB-38 mm photomultiplier tube with an interface of Dow Corning Q2-3067 silicone optical couplant. This phototube with "green extended" photocathode was chosen to better couple to the spectral character of the wave length shifter fiber. A typical test configuration is shown in Figure 1. The phototube was readout with a LRS model 2249 CAMAC ADC. The CAMAC crate was readout using an AST 386C PC equipped with a DSP 6001 interface card. The ADC was gated with a coincidence between two four-inch square scintillator paddle counters above and below the dark box containing the model counter. Approximately 2.5 cm of steel was placed above the lower paddle to harden the cosmic ray muon spectrum.
The relative light yield results are shown in the table. For groove depths of 5 mm and 10 mm, the light capture is essentially the same. This is illustrated in Figure 2. This result confirms the findings of deBarbaro, et. al.[13] Figure 2 also shows the light yield for the two different groove spacings as a function of the number of fibers per groove. The yield gains slowly by adding grooves with an increase of only about 35% when doubling the number of fibers and grooves. Given the relative high cost of machining the grooves, the wider spacing is clearly preferable. The light yield with the number of fibers per groove increases approximately linearly. Each additional fiber adds about 50% of the light of the first fiber.
As shown in the table, another counter of the same dimensions as the acrylic counter was made from 6 mm thick Kuraray SCSN38[14]. The counter was made with a 10 mm gap spacing and 3 mm deep grooves. The data shows that the light yield of the polystyrene based SCSN38 scintillator is 80% of that for acrylic even though the SCSN38 counter is one quarter the thickness. The relative brightness is consistent with expectations and points to the choice of material for detectors in production.
THE TEST COUNTER DESIGN AND FABRICATION
The configuration of the 3 m**2 test counter was based on the results from the small model described above. The 25 mm thick plastic was grooved with a 1.2 mm slitting saw on CNC horizontal mill. The material was uneven, varying in thickness by about 10% and in flatness by about 5 mm. The resulting groove depth consequently varied from 5 to 10 mm. As shown above, this variation in groove depth does not affect light gathering performance. The grooves were spaced every 5 cm parallel to the short edge as shown in Figure 3. Two extra grooves were placed at each edge to enhance light levels in order to extend the light yield uniformity to the edge of the counter. The extra grooves were added based on the experience with 1 x 1 m**2 counters used for a test calorimeter built for SDC.[15] The edges of the counters were again painted with Bicron white paint. The readout fibers consisted of 3 m of WLS fiber with 1.5 m of clear fiber (1 mm diameter Kuraray double clad) spliced to each end. The clear fiber was used to decrease attenuation between the counter and the photomultiplier. The splices were made by diamond cutting the ends and fusing the plastic.[16] Although the transmission of these splices was not measured, similar splices exhibit a 90% transmission with a few percent variation.[17] The fibers are looped back into every second groove. The use of the looped fiber results in uniform light yield in the detector parallel to the fibers. Three fibers were placed in each groove to ensure adequate light collection. The fibers were collected, glued into a plastic cookie as in the model, and the assembly was polished using a diamond cutter. The counter was wrapped in Tyvek and enclosed in a shallow light tight plywood box for testing.
TEST COUNTER RESULTS
The test counter was tested in the same way as the small model. The paddle counters used to gate the ADC accepted cosmic rays in a cone of roughly 13 deg. about the vertical. A 21 mm thick steel plate was placed between the test counter and bottom paddle. Figure 4 shows a pulse height spectrum at the center of the counter. The mean pulse height at that position corresponds to about 10 photoelectrons. The uniformity was measured over the surface of the counter. Scans were made perpendicular and parallel to the fiber directions. The results are shown in Figures 5 and 6. In the longitudinal scan, the light yield is minimum at the center of the counter and increases by about 30% at the two ends. The dominant reason is that the plastic is thinnest in the center and is thickest at the edges. Based on measurements of another sheet of scintillator from the same source, the difference is about 25% between center and edge. This thickness variation is typical for large sheets of acrylic scintillator. A second reason for the additional light near the edges is the extra grooves and fibers placed there. The experience of A. Beretras et. al.[18] showed that with uniformly spaced grooves, the light dropped off by 20% at the edges of the counter. By adding two extra grooves at each end, we apparently over compensated somewhat for light loss at the edges. Were the material of consistent thickness, the light yield would be uniform to a few percent.
CONCLUSIONS
Optical wave length shifting fibers have been demonstrated to effectively readout the large area (2.5 m**2) scintillation counters proposed for the Giant Airshower surface detector array.
On the basis of these tests, a counter configuration for the Giant Airshower Detector surface can be designed. The details of the design are driven by cost. Given the large quantities of scintillator, about 60,000 m[2], the mass of the constituent material plastic will determine the cost. This fact, together with the insensitivity to light attenuation in the scintillator when using WLS fiber readout, leads to the use of the thinnest possible scintillator. Polystyrene based scintillator with a brightness over three times that of the acrylic is the material of choice as shown in the above tests. Polystyrene has the further advantage that it can be cut and machined at tool speeds twice as fast as can be used with acrylic. On the basis of experience with styrene scintillator used in high energy physics experiments, light yield degradation with time is at least as good as with acrylic. Quantitative information needs to be obtained on longevity for candidate scintillators, particularly in the environment of large temperature swings to which the counters will be subjected.
The styrene scintillator tested (Kuraray SCSN38) or one of equivalent brightness with about 8 mm thickness and 3 fibers per groove spaced 50 mm apart would give a counter of excellent performance. To be sure, further studies involving the economies of groove machining, groove spacing, and the number of fibers are necessary to achieve an optimum design.
Table
Small Model Counter Results
Groove Groove Number of Light
Scintillator Thickness Depth Spacing Tiles Yield (pe)
(mm) (mm)
Acrylic 2.5 cm 5 5.0 1 15.6
Acrylic 2.5 cm 5 5.0 2 23.9
Acrylic 2.5 cm 5 5.0 3 32.4
Acrylic 2.5 cm 10 5.0 1 15.5
Acrylic 2.5 cm 10 5.0 2 24.8
Acrylic 2.5 cm 10 5.0 3 31.5
Acrylic 2.5 cm 10 5.0 4 37.7
Acrylic 2.5 cm 5 2.5 1 22.0
Acrylic 2.5 cm 5 2.5 2 32.2
Acrylic 2.5 cm 5 2.5 3 39.9
SCSN38 6 mm 3 5.0 1 13.3
SCSN38 6 mm 3 5.0 2 19.0
[2]N. Hayashida, et. al., "Observation of a Very Energetic Cosmic Ray well beyond the Predicted 2.7 K Cutoff in the Primary Energy Spectrum," (Preprint August 1994).
[3]Adelaide workshop, Jan. 1993, unpublished.
[4] N. Chiba, et. al., "Akeno Giant Air Shower Array (AGASA) Covering 100 km**2 Area," NIM A311 (1992) pp. 338-349.
[5] A. Borione, et. al., "A Large Air Shower Array to Search for Astrophysical Sources Emitting X-rays With Energies >= 10**14 eV", NIM A346 (1994) pp. 329-352.
[6] M. G. Albrow, et.al. , Nuclear Instrum. Methods A256 (1987) p. 23.
[7]Solonoidal Detector Collaboration Technical Design Report, SSCL-SR-1215, (April 1, 1992).
[8]G. Apollinari, et. al., "CDF End Plug Calorimeter Upgrade Project," Proceedings of the 4th International Conference in Calorimetry in High Energy Physics, (LaBiodalas, Italy, Sept 1993).
[9]G. W. Foster, et. al. , "Scintillating Tile/Fiber Calorimetry Development at FNAL," Nuclear Physics B 23 A (1991) pp. 92-99.
[10]P. de Barbaro, et. al.. "RFD Results on Scintillating Tile/Fiber Calorimetry for CDF and SDG Detectors," Nuclear Instrum. Methods A315 (1992) pp. 317-321.
[11] Bicron, Inc., 12345 Kinsman Rd., Newbury, Ohio 44065-9677.
[12] P. de Barbaro, et. al., "Recent R&D Results in Tile/Fiber Calorimetry." SDC-93-407 (Jan 1993).
[13] Ibid.
[14]Kuraray International Corp., 200 Park Ave., New York, NY 10166.
[15]A. Beretras, et. al., "Beam Tests of Composite Calorimeter Configurations from Reconfigurable-Stack Calorimeter," Nuclear Instrum. Methods A329 (1993), pp. 50-56.
[16]G. Apollinari, et. al., "Plastic Optical Fiber Splicing by Thermal Fusion," Nuclear Instrum. Methods A311 (1992), pp. 520-528.
[17]P. de Barbaro, "CDF Plug Upgrade Hadron Calorimeter Design, " CDF/DOC/PLUG-UPGR/CDFR12545, 25 July 1994.
[18]A. Beretras, et. al., ibid.