The Main Injector Neutrino Oscillation Search (MINOS) experiment uses an acceleratorproduced neutrino beam to perform precision measurements of the neutrino oscillation parameters in the "atmospheric neutrino" sector associated with muon neutrino disappearance. This long-baseline experiment measures neutrino interactions in Fermilab's NuMI neutrino beam with a near detector at Fermilab and again 735 km downstream with a far detector in the Soudan Underground Laboratory in northern Minnesota. The two detectors are magnetized steel-scintillator tracking calorimeters. They are designed to be as similar as possible in order to ensure that differences in detector response have minimal impact on the comparisons of event rates, energy spectra and topologies that are essential to MINOS measurements of oscillation parameters. The design, construction, calibration and performance of the far and near detectors are described in this paper.

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Nucl. Phys. B22, 131 (1970). 1 9 0 . Czyiewski and M. Szeptycka, Phys. Lett. 25B, 482 (1967). 2 0 B r u x e l l e s -~~~~-~r a c o w Collaboration, A. Eskreys et al. , Nucl. Phys. (to be published).Measurements of differential cross sections for quasielastic electron-deuteron scattering e + de + p + n have been made in which recoil protons were detected in coincidence with the scattered electrons. The ratios of the elastic electron-neutron to the electron-proton scattering cross sections are derived from the proton coincidence data. For comparison, these ratios are also determined from the scattered-electron momentum spectra using the peak and area methods. The theory developed by Renard, Tran Thanh Van, and LeBellac, which includes corrections for the final-state interactions, is found to explain the proton coincidence results. The present measurements, made at an electron scattering angle of 90" for four-momentum transfers of q = 7, 10, 15, and 29 F-', and at 80' for q = 45 F-', are combined with previous results to obtain the electromagnetic form factors of the neutron.

Elastic electron-proton scattering cross sections were measured at backward angles (80"-90") in the laboratory for four-momentum transfers between 7 F -~ and 45 F-'. Experimental errors range from 3.1% to 5.3%, including a systematic error estimated to be 1.9% added in quadrature. Electric and magnetic form factors are computed from all the recent data in this q2 range, with allowance made for possible normalization differences. The results show a deviation from the scaling law.

We report a measurement of the atmospheric neutrino flavor ratio, R, using a sample of quasi-elastic neutrino interactions occurring in an iron medium. The flavor ratio (tracks/showers) of atmospheric neutrinos in a 3.9 fiducial kiloton-year exposure of Soudan 2 is 0.64 ± 0.11(stat.) ± 0.06(syst.) of that expected. Important aspects of our main analysis have been checked by carrying out two independent, alternative analyses; one is based upon automated scanning, the other uses a multivariate approach for background subtraction. Similar results are found by all three approaches.