The K2K SciBar detector

Abstract

A new near detector, Scintillator Bar (SciBar), for the K2K long-baseline neutrino oscillation experiment was installed to improve the measurement of the neutrino energy spectrum and to study neutrino interactions in the energy region around 1 GeV. SciBar is a `fully active' tracking detector with fine segmentation consisting of plastic scintillator bars. The detector was constructed in the summer of 2003 and is taking data since October 2003. The basic design and initial performance are presented.

Introduction

The KEK-to-Kamioka long-baseline neutrino experiment (K2K) [1] started taking data in 1999. An almost pure muon neutrino beam with average energy of 1.3 GeV is produced with the KEK 12 GeV proton synchrotron and directed toward the Super-Kamiokande detector (SK) located at Kamioka, 250 km away from KEK. The neutrino flux and energy spectrum at SK is estimated from the flux measured by near detectors located 300 m downstream of the production target. The number of events and the spectral shape at SK are compared with the expectations to study neutrino oscillations. The latest K2K results [2] indicate neutrino oscillation, and are consistent with the SK results [3]. In K2K, the neutrino energy at the oscillation maximum is expected to be $\sim$0.6 GeV.

A new near detector, Scintillator Bar (SciBar), was constructed in Summer 2003 to upgrade the near detectors. A main motivation of the new detector is to improve the measurement of the neutrino energy spectrum by using Charged Current Quasi Elastic interaction (CCQE, ${\mathrm{\nu }}_{\mu }+\mathrm{n}\to {\mathrm{\mu }}^{-}+\mathrm{p}$). In order to select CCQE interaction with high purity and high efficiency and to suppress other interactions such as inelastic interactions with pions (${\mathrm{\nu }}_{\mu }+\mathrm{p}\to {\mathrm{\mu }}^{-}+\mathrm{p}+{\mathrm{\pi }}^{+}$), the detector is designed to have high efficiency for all charged particles produced in the interaction. The detector consists of plastic scintillator strips with fine segmentation. The scintillator itself is a neutrino target and has no dead region. Due to the fine segmentation of the detector, short tracks down to 10 cm long can be detected. The detector also has a capability of particle identification (especially for protons and pions) with $\mathrm{d}E/\mathrm{d}x$ information by measuring the energy deposition in each strip.

In addition, we expect to provide many new results for neutrino interactions in the 1 GeV region with SciBar.