ION CHAMBERS FOR MONITORING THE NUMI BEAM AT FNAL

Figure 1: Layout of the secondary and tertiary beam monitors. The Neutrinos at the Main Injector (NuMI) beamline will deliver an intense muon neutrino beam by focusing a beam of mesons into a long evacuated decay volume. The beam must be steered with 1 mrad angular accuracy toward the Soudan Underground Laboratory in northern Minnesota. We have built 4 arrays of ionization chambers to monitor the neutrino beam direction and quality. The arrays are located at 4 stations downstream of the decay volume, and measure the remnant hadron beam and tertiary muons produced along with neutrinos in meson decays. The hadron monitor sits downstream of the NuMI decay volume, immediately in front of the beam absorber. It measures approximately 1m×1m, and is segmented to measure the spatial profile of particles arriving at the detector. Monte Carlo calculations indicate that the charged particle rate at the center of the decay pipe is approximately 109particles/cm2/spill, and is dominated by protons which did not interact in the target. The total radiation expected at the hadron monitor is approximately 2 × 109 Rad/year, dominated by protons and neutrons backscattered from the beam absorber