Prototyping Megaton-Scale n Detectors

1. Prototyping Megaton-ScalenDetectors Developing a New Lower-Cost Scintillator Design • Jason Trevor • DOE Review • July 25, 2007

2. The Problem • Neutrino and cosmic ray physics continue to require ever larger detectors for measurements of interest. • All existing technologies have limitations • Water Cerenkov – Cheap, but limited by low energy cut-off • Liquid Scintillator – High light yield, but difficult to work with and environmentally hazardous • Solid Scintillator – Many desirable characteristics, but too expensive for use in very large detectors • Water Soluble Scintillator – Highly desirable, but no practical scintillators of this type are currently known • Conclusion: Construction of future large-scale n detectors will require the development of new technologies which will lower unit detector cost.

3. An idea inspired by MACRO and MINOS Scintillator • In a MINOS scintillator strip, only 5-10% of the light produced actually makes it into the WLS fiber. • There are three main cause of light absorption before the WLS fiber: • Self absorption by the fluors and polystyrene in the scintillator. • Imperfect surface reflectivity. • Absorption through either of the preceding processes after the light has reflected off the fiber/glue/polystyrene interface. WLS Fiber Polystyrene TiO2 Cladding MINOS Scintillator Strip

4. Plastic Scintillator Granules in Water? • What would happen if granules of plastic scintillator are mixed into water at a 5% concentration with a grid of WLS fibers? • The effective attenuation length of the bulk material is increased by about a factor of 20 • Absorption in the reflector is reduced • A better optical coupling is achieved for light at the WLS fiber boundary • Water is free - Cost per unit mass is reduced by 80-90% (est.) WLS Fibers PMT ~Water Scintillator Granules

5. Proof of Principle • 19cm x 19cm x 13cm • Constructed using left-over MINOS scintillator and WLS Fiber • Water and scintillator granules were circulated by small pumps • Light output in this prototype was lower than the nominal goal for a practical large detector, but • The scintillator was of poor quality • The prototype was too small… losses were still dominated by absorption in the walls • Solution: Scale up volume by a factor of 200

6. One Cubic Meter Tank Detector WLS Fibers • Our ADR grant proposal was funded (1yr $48k) • Scintillator strands replaced granules • Easier to extrude high quality scintillator • No circulation system required • More realistic configuration for larger tank detectors • But, more difficult to construct • Construction is complete(with the help of Caltech Undergrads) PMT (A 1 m cube) Scintillator Strands

7. Detector Construction

8. Detector Construction

9. Completed Detector

10. Readout Topology • Tank is divided into eight regions • All WLS fibers from a given region are routed to one of eight phototube boxes • Muon triggers are centered over the inner four regions • Inner regions are 30cm x 30cm • Muon Triggers are 18cm x 18cm 0 7 1 6 5 2 4 3 1 Meter

11. A Few Events • Two typical events • Event 897 – Single trigger event – These are mostly muon events • Event 4 – Multiple trigger event – Other stuff (Electrons, hadrons, etc) • Numbers shown are estimated light output in P.E.s – exact calibration still needs to be completed • Preliminary numbers suggest the light output is ~2 – 5 times that of MINOS – A more detailed analysis is necessary to confirm this

12. Preliminary Analysis 0 1 2 3 4 6 7 5

13. Summary • Construction of the one cubic meter prototype is complete – undergrad student labor was an essential part of the construction effort • Initial results suggest the light output is 2 – 5 times that of MINOS, but more detailed analysis is necessary • Recent addition of Leon Mualem and Alex Himmel has accelerated progress. • This is a promising new technology • More R&D is necessary – The design is far from optimal • We plan to apply for further ADR funding • A paper is in the works