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A Silicon TARget (STAR) for NOMAD

STAR was a silicon tracking detector that was installed in front of NOMAD at the beginning of 1997. Its aim was to serve as a prototype for a new generation neutrino experiment called TOSCA in which the combination of emulsion and silicon detectors can provide improved efficiency in the identification of tau-neutrino interactions. More information is provided in the STAR page at CERN.

STAR consisted of five silicon layers interspersed with four plates of boron carbide with a total mass of 45 Kg. It was installed inside the NOMAD magnet, replacing the first three drift chambers of STAR. A diagram of the set-up is shown below.


Each layer consisted of 10 ladders of silicon detectors, with 12 detectors per ladder. The 640 readout channels of each detector were bonded together to form the ladder. The total length of the ladders was 72 cm. They represented the longest working ladders of silicon microstrip detectors ever built with a signal to noise ratio of the order of 15:1.


The following image shows the construction and components of each of the ladders.


A full survey of each of the ladders was performed to enable less that 10 micrometre precision in the position of each of the detectors. The survey equipment used was a moveable table with stepping motors that move the coordinates in the X and Y direction. An image of this equipment can be seen below.


STAR was designed to observe the vertex position of neutrino interactions with a resolution of 10 micrometres (10 millionths of a metre!) or less. With this resolution, it will be possible to distinguish a tau lepton from a tau-neutrino interaction from a normal muon-neutrino interaction by the small kink caused by the decay of the tau. This decay happens within a few hundred micrometres after the tau is formed, so the resolution of the silicon vertex detector will be able to isolate the tau-neutrino interactions with great accuracy.

A fully reconstructed STAR event is visible in the following image.


The use of a silicon detector in conjunction with emulsion is an even better proposition because the silicon detector is able to accurately pin-point the position of the tau-vertex and a 3D image is able to be formed with the emulsion (which has an even better resolution of the order of 1 micrometre). This is the most significant feature of the proposed TOSCA experiment, a new generation short-baseline muon to tau-neutrino oscillation experiment at CERN. We were able to study the accuracy of the vertex determination under realistic conditions within STAR and test the feasibility of the silicon-tracking-emulsion technique to be used in TOSCA for searching for neutrino oscillations.