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Detector Design Questions For CD-1

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Some basic questions that drive fundamental detector design need answering for CD-1. This topic poses these questions and describes the studies needed. The questions are organized by their effect on the detector. For each question we need a list of consequences on the physics that can be done. Some of the questions are interrelated. For example, energy threshold and PMT coverage.

The Geant4 application WCSim will be useful for answering some of these questions.

Is a 4 pi veto needed?

SK uses 2 meters of water around the inner detector to tag incoming events. Along with the 55cm dead space it also shields against radiation from the rock. If a veto is not used it allows additional fiducial volume for the same cavern size.

Cosmic muon veto by reconstruction

With no veto, incoming cosmic muons must be removed by reconstructing their entry point at the wall.

  • Will we have sufficient CPU to run this reconstruction?
  • What vertex resolution can be achieved?

In SK the veto is inefficient where it is pierced by the PMT cable bundles. We should be able to answer these questions from that experience.

Some Results

Low energy background

  • What are the consequences of not having ~2.5m of water shielding the main fiducial volume?

What PMT coverage is needed?

PMTs are a cost driver so we must know how many are needed as a function of physics capabilities. Recent developments in PMT design has seen the quantum efficiency increase from 25% to 35% and 45% with accompanying increase in dark rate.

  • Collect PMT operational parameters
  • Evaluate physics capability as a function of:
    • Total photo-cathode coverage
    • Effective PC coverage (using higher eff PC)
    • Granularity for fixed coverage (more smaller vs. fewer larger)

What is the effect of detector size and shape?

What are the trade offs between a 100kton, 150kton and 300kton (fiducial mass) sized detectors and how do their shapes matter. Compare:

  • Cylindrical vs. rectangular
  • Elongated vs. having all linear dimensions comparable

What fiducial cut can be made?

SK used a fiducial cut of 2 meters from the PMT plane. The PMT plane is defined by the optical barrier (black sheet). The PMT face penetrates this plane into the water region. Can this be fiducial cut be decreased? What will suffer? Some work on this is done in Ryan Terri's Ph.D. thesis available here.

What energy threshold can be achieved

Given all the above options, what is the lowest energy threshold that can be achieved. This needs to be evaluated in terms of a simple N-hit hardware trigger and a SK super-low-E type software (fitter based) trigger.

Are adding Winston cones a good idea

Winston cones have the possibility to increase light collection but may degrade reconstruction for events with high-angle light.