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Scintillator-Based Muon System R&D


The identification and precise measurement of muons is critical to the physics program of the ILC. The muons produced from decays of W and Z bosons provide key signatures for the Higgs and possible new particles. Muons may also be produced directly from decays of new particles. Our R&D project addresses three critical areas that have emerged from discussions inside the ILC detector community.

1. What is the additional capability for muon identification that an instrumented iron magnetic flux return can provide beyond that from a finely segmented particle flow capable hadron calorimeter?

2. What performance for muon identification (efficiency and purity) can be provided by a strip scintillator detector with barrel and endcap pieces combined with the hadron calorimeter?

3. What is the best candidate for photon detection for scintillator readout among the established and newly developed devices: multi-anode photomultiplier, Geiger-mode avalanche photo-diode, silicon photomultiplier and silicon avalanche photodiodes?

Question 1 above is relevant to all of the detector concepts which include an instrumented flux return muon detector. Scintillator technology is considered as a candidate technology in some of the concepts, and in all cases provides a benchmark for comparison.

Institutional Responsibilities

The Fermilab group has developed software for muon tracking, integrating the calorimeter and muon systems. The tracking employs a Kalman filter which takes into account multiple scattering, energy loss and magnetic field. Results have been presented on the efficiency and purity of muon identification for the barrel detector. See the talk by Caroline Milstene at Snowmass, 22 August, 2005. Since then, preliminary results show that the instrumented flux return muon detector significantly enhances the efficiency and purity.

The Fermilab group coordinates the fabrication and operation of the prototype detector. Fermilab purchased the scintillator and optical fiber and provides laboratory space, mechanical infrastructure and electronics instrumentation. The splicing of WLS and clear fiber is performed at Fermilab. See the talk by Eugene Fisk at Fermilab, June 3, 2005.

The Fermilab group coordinates the muon detector design with the Si-D detector collaboration.

The Indiana University group tests the prototype detector modules at Fermilab using radioactive sources and cosmic rays. See the talk by Robert Abrams at Snowmass, August 23, 2005.

The Northern Illinois University group is developing a tail-catcher, muon tracker (TCMT) detector using silicon photomultiplier readout. This effort is now integrated into the scintillator muon project. The NIU group fabricates scintillator bars using an extrusion facility operated jointly by Fermilab and the Northern Illinois Center for Accelerator and Detector Development. The bars are loaded with WLS fibers and assembled into complete detector planes with Si-PM readout and electronics. See talks by Gerry Blazey at SLAC, 18 March, 2005 and Snowmass, 14 August, 2005.

The Notre Dame University group fabricates the prototype scintillator planes, including cutting of the scintillator and assembly into modules, which are shipped to Fermilab. The spliced fibers are tested at Notre Dame. The optical interface between the clear fibers and the multi-anode PMT is fabricated at Notre Dame. See the talk by Mitchell Wayne at Snowmass, 22 August, 2005.

The University of California at Davis group built the readout interface between the Lecroy TDC system and the data acquisition PC and established its operation at Fermilab. See the talk by Mani Tripathi at SLAC, March 21, 2005.

The Wayne State University group develops test and calibration methods for the multi-anode photo-tubes and helps coordinate the work of the collaboration. See the talk by Paul Karchin at Victoria, July 29, 2004 and the article by Paul Karchin in proceedings of the DPF meeting at UC Riverside, August 28, 2004.

Associate collaborators from Colorado State University develop a geiger mode avalanche photodiode detector in a package that will be compatible with the optical interface of our prototype system.

Associate collaborators from Rice University have expressed interest in working with us in the future.

Associate collaborators from the University of Texas have loaned us spare PMT assemblies from MINOS which were used in earlier tests. They also provided advice on the initial specification of the prototype detectors.

Recent Results

A recent (Fall 2005) result of the collaboration is the operation of two 1/4 size prototype planes and the response to a radioactive source (Cs-137) and to cosmic rays.

Figure 1 of the publicity graphics shows two prototype muon scintillator detector planes under test at Fermilab. Each plane has 64 strips of cross section 1 by 5 cm with each strip readout by a wavelength shifting fiber fused to a clear fiber. All fibers in the top plane are routed to a single anode photomultiplier tube. Each fiber in the bottom plane is routed to one element of a 64-channel Hamamatsu multi-anode photomultiplier tube. A cosmic ray trigger is defined by scintillator paddles and absorbers above and below the planes.

Figure 2 shows typical phototube anode signals from the prototype planes of Figure 1. CH1 corresponds to the plane where all fibers are routed to a single anode phototube. CH2 corresponds to one fiber readout by one channel of the multi-anode phototube. The multiple peak structure is due, in part, to the approximately 7 ns decay time of the wavelength shifting fluor. Reflections inside the scintillator may also contribute to the multiple peak structure.

A plane of the tail-catcher muon tracker was recently operated (in October 2005) in an electron beam at DESY. A silicon photomultiplier assembly is shown in Figure 3. Scintillator bars extruded at the NICADD facility and assembled arrays are shown in Figure 4. Data from electron beam scans and LED pulsing is under analysis.

One Year Plans

In the next year, we plan to continue fabrication of prototype planes and tests with radioactive sources and cosmic rays. We want to obtain a detailed understanding of the relative contribution to the multiple peak signal structure from the fluorescence decay time in the WLS fiber and reflections inside the scintillator. We want to collect charge integral data from all the strips with sufficient statistics to measure the distribution of the mean number of photoelectrons. Furthermore, we want to study the dependence of the mean number of p.e.'s on strip position and strip length for single and doubled-ended readout. We plan to test whether it is possible to route two 1.2 mm diameter fibers to a single 2 mm X 2 mm photocathode cell. If detection efficiency is not degraded by this scheme, we could halve the number of photo-detector channels needed for strips with double-ended readout.

We hope to operate the prototypes in a test beam at Fermilab, before the March 1, 2006 accelerator shutdown. We want to measure position and timing resolution using upstream tracking as a position reference and upstream beam counters as a time reference.

Simulation studies will continue towards establishing the efficiency and purity for a barrel detector with and without an instrumented flux return detector. We would like to begin simulation of the endcap detectors.

2-3 Year Plans

On the time scale of 2-3 years, we hope to have well-established performance data from beam tests as well as realistic estimates from simulation studies of efficiency and purity for both barrel and endcap detectors.

Also, on the time scale of 2-3 years, we want to compare the performance of multi-anode photo-tube readout with the emerging solid state technologies employing avalanche photo-diodes and silicon photomultipliers. Of particular interest are the photo-electron yield, noise rate, and time accuracy. Unique requirements on the WLS fiber may be required for each type of optical detector.

We want to develop (or adapt) a dedicated readout chip (application specific integrated circuit) that measures both time of arrival and integrated charge. The Fermilab schedule calls for test beam operation in 2007 with a full EM and hadronic calorimeter with tail-catcher. We will explore the possibility for a common readout architecture between the Si-PMT's used for the tail-catcher and the muon system.

We expect to establish techniques for mechanical support systems, optical fiber splicing, routing of fibers and the interface between the scintillator and the various types of photodetectors.

We plan to establish realistic cost estimates for construction, testing and installation of an ILC muon detector system.

Funding Limitations

Currently our progress is limited by lack of personnel. The university groups have no external funding for students, support staff (engineers and technicians) or physicists (postdocs and research scientists). The universities have provided personnel through their own, limited institutional funding. We cannot answer questions 1-3 without enough personnel to operate the equipment, analyze the data and perform computer simulations.