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Prototyping Megaton-Scale

Detectors

Jason TrevorDOE ReviewJuly 25, 2007

Developing a New Lower-Cost Scintillator Design

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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 detectors will require the development of new technologies which will lower unit detector cost.

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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.

MINOS Scintillator Strip

WLS Fiber

TiO2 Cladding Polystyrene

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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.)

PMT

~Water

Scintillator Granules

WLS Fibers

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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, butThe scintillator was of poor qualityThe prototype was too small…

losses were still dominated by absorption in the walls

Solution: Scale up volume by a factor of 200

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One Cubic Meter Tank Detector

Our ADR grant proposal was funded (1yr $48k)

Scintillator strands replaced granulesEasier to extrude high quality

scintillatorNo circulation system requiredMore realistic configuration for

larger tank detectorsBut, more difficult to construct

Construction is complete(with the help of Caltech Undergrads)

WLS Fibers

(A 1 m cube) Scintillator Strands

PMT

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Detector Construction

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Detector Construction

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Completed Detector

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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

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52

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43

1 Meter

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A Few EventsTwo 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

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Preliminary Analysis

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2 3 4

5 6 7

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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