Abstract
New methods for efficient and unambiguous interconnection between electronic position sensitive detectors and target units based on nuclear photographic emulsion films have been developed. The application to the OPERA experiment,that aims at detecting νμ⇌ντ oscillations in the CNGS neutrino beam, is reported in this paper. In order to reduce background due to latent tracks collected before installation in the detector, on-site large-scale treatments of the emulsions (''refreshing'') have been applied. Changeable Sheet (CSd) packages, each made of a doublet of emulsion films, have been designed, assembled and coupled to the OPERA target units (''ECC bricks''). A device has been built to print X-ray spots for accurate interconnection both within the CSd and between the CSd and the related ECC brick. Sample emulsion films have been extensively scanned with state-of-the-art automated optical microscopes. Efficient track-matching and powerful background rejection have been achieved in tests with electronically tagged penetrating muons. Further improvement of in-doublet film alignment was obtained by matching the pattern of low-energy electron tracks. The commissioning of the overall OPERA alignment procedure is in progress.
Highlights
In 5 years of CNGS run about 25,000 neutrino interactions will be collected by OPERA, out of which about 12 identified ντ charge current interactions are expected with a background of less than 1
The local offset resulting from the global X-ray alignment which has been exemplified in the previous Section can be measured by using a local alignment method based on low-energy electron tracks crossing both films
Number of CSd tracks has been realized for the purposes of the OPERA experiment. It is made of a doublet of emulsion films called CSd
Summary
The OPERA experiment [3] has been designed to directly validate the flavor-mixing neutrino oscillation model that explains the disappearance of νμ in atmospheric neutrino experiments [4][5][6] and in accelerator experiments [7][8]. OPERA is a large detector (10 m 10 m 20 m) placed in the underground LNGS Hall C [10] It consists of two identical super-modules aligned along the CNGS beam direction, each made of a target section and a muon spectrometer. Available at several OPERA institutions in Japan and Europe [12][13], allow performing the second emulsion-based data taking This is a complex, multi-step task consisting of the location of a neutrino interaction vertex by scanning backwards (scan-back) along the path of the selected tracks, the vertex reconstruction by scanning a volume of emulsion around the presumed end points of the tracks, the identification and validation of any τ decay topology and further optional measurements for efficiency study and kinematical analysis
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