1. WLS FIBER-SCINTILLATOR GLUING UPDATE
For the Pcal collaboration,
Jose Riso, Aji Daniel, Wei Tang, Stepan Stepanyan

2. Summary
Extensive R&D has been done in order to maximize the homogeneity of the epoxy injected to serve as interface between the WLS fiber and scintillator’s hole surface. The tests had been conducted in strips ranging from 3 to 3.8 meters long (supplied by FNAL). Air bubbles and chambers had been targeted.

3. Optical Epoxy used for the tests & main technical specifications:
BICRON BC-600
Refractive index: 1.56
Bond strength: 18000psi > 125Kg/cm2
Coefficient of linear expansion: 7.2x10- 5cm/deg. C*
Light transmission above 400 nm >98%, between 340 and 400 nm >95% and between 308 and 340 nm >90%
*Typical coefficient of linear expansion of plastic scintillator is ~7.8x10-5cm/deg. C

4. Scintillator used for the tests:
Extruded scintillator strips 50 mm x 10mm, ten pieces ranging from 3 to 4 metes in length, with TiO2 co-extruded surface layer and hole in the center (the hole is not round, it’s shape is an elongated slot with rounded ends 2x4mm)

5. Fiber used for the glue tests:
Kuraray WLS fiber
Type: Y-11 (200)MSJ
Size: 1mm Dia.
Cladding: Multiclad
Cross section: Rd.

6. Weighting scale used to prepare the epoxy and vacuum system used for the removal of air trapped in the mixed epoxy

7. Epoxy dispenser

8. Preliminary epoxy injection tests using standard procedures with syringe-needle sealed with RTV, silicon rubber and combinations. The tests results indicated introduction of air bubbles and leak of epoxy at the injection point.

9. Design of a fixture to inject epoxy in the medium to long scintillator strips with adjustment of up to 25 degrees inclination. The injection is at the lower side in order to keep air flowing above the epoxy level. The horizontal position is used for mounting of strips and other operations.

10. Prototype used for the injection of epoxy.

11. Design of fixture for the epoxy injection point

12. Prototype used for injection of epoxy, needle and RTV has been replaced by o-rings and fix adapter, this design proved to be epoxy leak-free and air bubbles-injection-free

13. Epoxy diverted used at the top end of the strip, this serves to compensate shrink of the epoxy during polymerization, it’s about 2%, this means a drop in the epoxy column of 8cm for a 4 mt strip during the first hours of the cure. The diverter serves the purpose of compensate for the drop.

14. Epoxy diverter in use.

15. A HAMAMATSU R6095 photomultiplier tube has been used for the epoxy homogeneity tests as well as light yield and attenuation length tests. A dark box had been implemented for the long strips.

16. Epoxy or not Epoxy…
In view of the very labor intensive and time consuming operations required to inject epoxy in the scintillator strips assuring that no air bubbles are trapped in the volume and with results of recent multiple measurements on hand I will ask the question: why we need to use epoxy to fill-up the space scintillator-fiber in the strips?
At first we’ll respond about the index of refraction of air with the consequent reduction of light collection, etc.
In the next slide I’ll like to explain the tests conducted to advocate for not to use epoxy as interface media.

17. Materials used for the tests
For the following tests the same 3.2mts long scintillator strip has been used, it has been provided by FNAL for our epoxy tests, it have a slot-like hole 2mm wide x 4mm long rounded at the ends in the center of the cross-section running along it’s length, the dimensions of the strip are 5x1x320cm.
Also the same WLS fiber KURARAY Type: Y-11 (200)MSJ, 1mm Dia.

18. The tests: Test 1: 2 fibers inserted in the scintillator’s
hole, no epoxy used.
Longitudinal scan with Sr-90 source measuring anode current at 10cm intervals.
Test 2: 1 fiber is removed from the strip.
Test 3: 1 fiber with epoxy injected.
Longitudinal scan with Sr-90 source measuring anode current at 10cm intervals.

19. Results will be presented by Aji
As Aji will show in his presentation the results for 2 fibers with no epoxy shows similar light yield than the case of 1 fiber with epoxy, even showing some improvement in attenuation length and uniformity.

20. Benefits: Reduction of intensive labor Cleaner operations
Time reduction (multiple curing times)
No dedicated fixtures required
No risk of non-uniformity produced by air bubbles introduced if non-skilled people is involved in the injection.
No increase of risk on bending, the fibers are still 1mm. The pmt’s and adapters have room to accommodate the increase.

21. Some remarks
The routing of fibers through scintillator not using epoxy is a well proven technology. It’s used in every shashlik calorimeter as for KOPIO, LHB-c, ALICE (Jose), EMC for STAR (Jose) and many other. The electromagnetic calorimeter for STAR was commissioned 11 years ago and is still running.
The factor for the choice is usually light yield.
The budget factor has to be considered

22. Thanks,
Jose