Fibre Bragg Gratings to monitor the deformations of a silicon microstrip vertex detector

An important issue concerning tracking detectors is that they undergo deformations, which, if not negligible, may worsen the detector intrinsic resolution. The objective of this paper is to assess the accuracy of Fibre Bragg Grating (FBG) technology to monitor the deformations of a silicon microstrip detector under stationary thermal conditions. For this purpose, a detector module was employed, of the same type as those composing the vertex detector of the FINUDA apparatus at DAFNE. Two bare FBG sensors were glued close to the border of the silicon microstrip detectors and their responses were compared with the responses of two CCD laser displacement sensors over a time period of 24 days. A calibration method to determine the quantitative relationship between the Bragg wavelengths of the gratings and the lateral deflection of the microstrip module was developed. This method does not require either knowledge of the exact position or calibration of the individual FBG sensor. The lateral deflection of the microstrip detector could be reconstructed with an accuracy of approximately 10um. The results obtained suggest that a better accuracy could be achieved by employing a larger number of FBG sensors.