A proposal for position monitoring and alignment of pixel detector at LHC using FBG sensors

1. A proposal for position monitoring andalignment of pixel detector at LHC using FBG sensors L. Benussi a, M. Bertani a, S. Bianco a, M.A. Caponero c,D. Colonnab, D. Donisi b, F.L. Fabbri a, F. Felli b, M. Giardoni a,A. La Monaca a, B. Ortenzi a, A. Paolozzib,L. Passamonti a, B. Ponzio a, D. Pierluigi a, C. Pucci b, A. Russo a, G. Saviano b antonio.paolozzi@uniroma1.it a Laboratori Nazionali di Frascati of INFN bLaboratori Nazionali di Frascati of INFN and Sapienza Universita’ di Roma cLaboratori Nazionali di Frascati of INFN and ENEA Frascati

2. Objective • Show the possibility of obtaining an accurate position monitoring of particle detector supporting structures by strain measurements • Strain can be measured by using fiber optic sensors such as Fiber Bragg Gratings (FBG) • FBGs are immune from electromagnetic interferences, can therefore work inside strong magnetic fields and can be multiplexed (10 or more) along one single fiber • Show the possibility of monitoring alignment during the re-positioning of a detector supporting structure, such as a pixel detector.

3. Deformation of a structure can be monitored by stuck/embedded FBGs. l Stressed structure FBG signal l l1=l(s1) l0 Back diffracted signal l Structure ‘at rest’ FBG signal Stuck FBG Input optical signal (before diffraction) Forward signal after diffraction l l0= l(s0) Monitored structure l l Typical values: D = 1 meDl = 1 pm DT = 1 K Dl = 10 pm D = 1 - 0

4. In-plane displacement monitoring of silicon strip detector: BTeV case study n.1 Monitoring the displacements of a specimen relative to a stable frame • Specimen is positioned in the frame by three axially strained braces • Braces are provided with cardan joints to avoid bending and torsion. Y X Loads are applied pulling/pushing the specimen along perpendicular directions.

5. Displacement due to a step increasing force in the X direction X displacement Y X

6. Carbon fiber fabrics Optical fiber Out-of-Plane Position Monitoring Two sensors embedded in a Carbon Fiber Reinforced Plastic (CFRP) slab made by 8 pre-peg fabrics. Axial deformation and Bending of the slab can be monitored. • Experimental measurements: • ‘Cantilever like’ disposition. • Micrometric screw for imposing tip displacement. • Optical comparator for displacement monitoring.

7. Out-of-Plane Position Monitoring Experimental data obtained for a ‘step-by-step’ tip displacement. Linear fit of data show that tip displacement can be monitored with 10mm resolution (versus of bending is discriminated). • Bending test periodically repeated to verify ‘long-term’ stability and temperature dependence. • No temperature dependence has been found • ‘long-term’ stability applies

8. Displacement monitoring of pixel support cylinder: BTeV case study n.2 Pixel Support Cylinder (PCS): FBG sensors will monitor deformations and repositioning of PSC PSC deformations affect Pixel detector position PSC removing and repositioning is required at each proton-antiproton beam storing

9. An omega shaped elastic structure can be used as a device to improve the sensitivity in determination of displacement. Omega shaped displacement device FBG sensor • Finite Element Analysis has been performed to: • determine where to glue the FBG sensors to maximize • sensitivity of omega device • evaluate the minimum displacement detectable. • Minimum FBG wavelength shift detectable = 1 pm • Minimum detectable tip displacement of omega shaped device = 6.5 µm Geometry of Omega device FE analysis results

10. Experimental set-up To validate the procedure a Coordinate Measuring Machine (CMM) has been used. Activity has been performed at FermiLab for BTeV program Omega with FBG under thermo-mechanical deformations Movable point with CMM sensor Omega with FBG under thermal deformation only. The reading of such sensor will be used to subtract the thermal deformation. Fixed point

11. Data Analysis displacement [mm] Thermal strain has been removed micron Maximum deviation from 4th order fit is 4µm

12. Conclusions • POSITION MONITORING • Axial deformation and Bending can be effectively discriminated,allowing for monitoring both in-plane and out-of-plane displacements • FBG sensors can be efficiently embedded in CFRP components, thus providing detector supporting structure with ‘built-in’ structural monitoring system. • Results of experimental tests show that specimen position can be monitored with resolution • better than 0.5µm for in-plane displacement (X,Y) • better than 10µm for out-of-plane displacement (Z) • MONITORING OF ALIGNMENT DURING RE-POSITIONING • Omega-like device allows macroscopic displacement (up to 10 mm). • Position can be determined with 4 micron resolution. • Such a device is well suited for pixel detector repositioning.