SiO x coated fiber optic sensor for gas monitoring in RPC S. Grassini 1, M. Parvis 2, M. Ishaiwi 2 1 Dip. di Scienza Applicata e Tecnologia 2 Dip. di Elettronica e Telecomunicazioni, Politecnico di Torino, Italy L. Benussi, S. Bianco, S. Colafranceschi, D. Piccolo INFN- Istituto Nazionale di Fisica Nucleare, Frascati, Italy

Project supported by Italian Research Ministery PRIN2008 Innovative sensors for gas monitoring  Politecnico di Torino  Università di Roma la Sapienza  INFN- Istituto Nazionale di Fisica Nucleare, Frascati

3 The Higgs Boson Hunt

RPC: Resistive Plate Chambers Fast gaseous detectors that provide a muon trigger system Bakelite plates (  = 2-5x10 10 Ω∙cm)  V = 9.0 kV GAS GAP 2 mm Detecting strips HV Al foil Gas mixture: 96% of C 2 H 2 F 4 / 3.5% of iC 4 H 10 / 0.5% SF 6 ; 45% RH µ+µ+µ+µ+

 It is known that an increase of the leakage current with time is observed, which may impair the muon detection  The chemical analysis showed a correlation between leakage current increase and F - concentration in the gas  The F - concentration can be reduced by renewing the gas, but the gas mixture is expensive and the volumes are huge so that a closed loop recirculation system with gas filters was designed  New gas fraction variable from 30% (currently) to 2% RPC issue

The RPC Closed-loop system New gas fraction (30% -> 2%) Gas Mixer RPC RPC Chemical analyses (GC, ICP-MS) Ni-Al2O 3 CuO/ZnOZeolite

New proposed approach New gas fraction (30% -> 2%) Gas Mixer RPC RPC Ni-Al2O 3 CuO/ZnOZeolite Plastic Optic Fiber Sensor

POF sensor  immune to electrostatic discharges and magnetic fields  fire safety compliance  robust and flexible  cheap  easy to obtain high detection capabilities via cumulative output responses  immune to electrostatic discharges and magnetic fields  fire safety compliance  robust and flexible  cheap  easy to obtain high detection capabilities via cumulative output responses

LED PD Plastic Optic Fiber Sensor Sensor principle  Cladding removal (in ethylacetate)  Deposition on the core of a sensitive layer capable of reacting with the gas  Selective and non-reversible The chemical reaction must alter the fiber light transmittance capability

Radiation the Gamma Irradiation Facility (GIF) - CERN  A very intense (740 GBq) 137 Cs source  A continuous photon load  Six months test exposing the fiber to 2 Gy (corresponding to several months of CMS operation) No light transmittance changes detected both on bare and coated fibers

Sensitive Layer deposition via plasma Glow discharge

Sensitive Layer deposition via plasma Glow discharge  Ag layer by plasma sputtering  SiO x by PECVD of organosilicon compounds

OSi(CH X ) Y Si CH X CH + H CO CO 2 H(OH)C H 2 O, OH O 2, O H CH 4 C2H2C2H2C2H2C2H2 Si(CH x ) y (CH 3 ) 2 C SiO OHHO Si O O Si CH 2 O O Si O O Si O Si O Si OH Si Si O Si O OH O SiOHO Si CH 3 Ion bombardment SiO x layer (a glass-like layer) SiC 8 H 20 O 4 Electron bombardment TEOS + Ar + O 2 TEOS + Ar + O 2

SiO x layer (a glass-like layer) SiO HF  SiF H 2 O The SiO x has an high reflective index which makes the fiber dispersive The F - reacts with SiO x corroding the layer: 200 nm TEOS:O 2 :Ar 5 Pa - 50  100 W

Measuring set-up POF SENSORS REACTION CHAMBER HF solution PT100

HF vapor exposition The current increases as the time passes until a plateau P HF  200 Pa

SiO x coated SiO x coated + HF Uncoated fiber HF effect on the fiber dispersion

HF effect on the fiber surface 50 µm 1 µm before

HF effect on the fiber surface 50 µm 1 µm 50 µm 1 µm before after

Plasma chemistry and fiber response to HF TEOS:O 2 : Ar=1:20:10 100W TEOS:O 2 : Ar=1:0:30 50W

Plasma chemistry and fiber response to HF TEOS:O 2 : Ar=1:20:10 100W TEOS:O 2 : Ar=1:0:30 50W 1 µm

Plasma chemistry and fiber response to HF TEOS:O 2 : Ar=1:20:10 100W TEOS:O 2 : Ar=1:0:30 50W 1 µm

Plasma chemistry and fiber response The FTIR shows the presence of organic groups which make the coating not completely glass-like

Plasma chemistry and fiber response Increasing O 2 a fraction and discharge power leads to SiO 2 -like films The sensitivity significantly increases

Conclusions  Plasma modified POFs can be successfully used for measurement of F - concentration in gas mixtures  The sensors are cumulative so they directly estimate the filter exposition  First prototypes showed a good sensitivity (the light changes 3 times after an exposition of ppm per hour)  The overall sensor cost is less than 1€  Experiments are being performed to assess the sensor repeatability

CMS building – CERN - February 2011