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Explicitly Radiation Hard Fast Luminosity Monitor
E.Torrence1, O.Atramentov2, J.Hauptman
1

2

University of Oregon, 2Iowa State University


NLC requirements on performance NLC
The NLC design luminosity places rather tight constraints on the performance of NLC detectors: ...bunch-to-bunch time interval of 1.4ns suggests almost speedof-light response... ...large background of low energy e±,g suggests a detector with a 10-20 MeV energy threshold ...large IR radiation dose will radioactivate the detector mass, suggesting an energy threshold above 8 MeV ...large radiation dose will damage detector components, requiring radiation-hard detector
Arlington Linear Collider Workshop, January 9-11, 2003


Gas Cherenkov calorimeter satisfies these four requirements:
· The Cherenkov photon signal exits the calorimeter volume at the velocity of light · Gas has index of refraction n = 1+d, (d~10-3), therefore Cherenkov angle is small
( sin Х C Є 2d Є .05

and energy threshold for electrons is high

· Decay products from radioactivation of the calorimeter mass are below Eth and therefore invisible · A calorimeter made wholly of gas and metal cannot be damaged by any dose of radiation.
Arlington Linear Collider Workshop, January 9-11, 2003

me Eth Є Є 11.2 MeV 2d


"Lasagna" ("accordion ") geometry

Arlington Linear Collider Workshop, January 9-11, 2003


Calorimeter design
· The Cherenkov light is generated by shower particles that cross gas gaps between absorber elements.

e-

· Shower particles co-move with the Cherenkov light as two overlapped pancakes. The width of these pancakes is about 50 ps. · Inside surfaces must be highly reflective at grazing incidence.

Arlington Linear Collider Workshop, January 9-11, 2003


Low angle LC geometry

Y

Z X

Arlington Linear Collider Workshop, January 9-11, 2003


A one-foot deep calorimeter is emptied of Cherenkov light in one ns, however care should be taken to avoid any residual long-lived scintillation light generated in the gas. b-butylene is a good choice for a Cherenkov gas: · Index of refraction is large: d = 1.31 10-3 · Scintillation light yield is a factor of 10-5 of Cherenkov light · Scintillation light is isotropic and only a fraction exits the calorimeter Therefore the calorimeter volume is emptied of light and becomes quiescent between the 1.4 ns beam crossings.

Arlington Linear Collider Workshop, January 9-11, 2003


Design Challenges
· Cherenkov light will typically make N~10 small-angle reflections each with reflectivity R, therefore the surviving light fraction is RN: R should be as close to unity as possible · Cherenkov photons must be transported from the rear of the module to fast PMTs · Fast digitization and storage every 1.4 ns

Arlington Linear Collider Workshop, January 9-11, 2003


Current Status
· Techniques for obtaining highly reflective metallic surfaces are being developed; · Quality control techniques for R are ready to go; · Geant4 simulations are being implemented (nontrivial geometry); · Geometry: new cylindrical design ­ similar complexity as planar, might allow an extension down to 50 mrad; · 16-anode fast(200ps) PMTs and sub-nanosecond FPGA-base ADCs are commercially available; · We will work with other LC collaborators on DAQ chain.

Arlington Linear Collider Workshop, January 9-11, 2003