A photon veto detector for the CKM experiment

Fermilab experiment E921, or Charged Kaons at the Main Injector (CKM), will measure the branching ratio of the ultrarare kaon decay: K⁺→π⁺νν. To do this, CKM will need to reject the far more copious decay K⁺→π⁺π⁰→π⁺γγ by a factor of more than 10 billion. Much of this rejection will come from the detection of either or both of the photons by our photon veto system. The main CKM photon veto system is based on 80-layer sandwiches of 1-mm Pb and 5-mm fast scintillator with wavelength shifting fibers inside a vacuum decay volume, read out with photomultiplier tubes outside the vacuum. We describe the design of the CKM photon veto system and the testing of a prototype in an electron beam at Jefferson National Laboratory, demonstrating an order of magnitude better performance than the required 3×10^-5 inefficiency for a 1 GeV electromagnetic shower.