Characterisation of Scintillating fibres for the LHCb SciFi Tracker. Cesare Alfieri, CERN

1. Characterisation of Scintillating fibres for the LHCbSciFi Tracker. Cesare Alfieri, CERN 15th September 2014

2. Summary • Introduction to SciFi • Fibre’s structure. • Scintillation mechanism. • Attenuation Lengths • Experimental Set-up. • Results for irradiated and non-irradiated fibres. • Absorption and Emission Spectra • Light Yield • Experimental Set-up. • Compared results. • Conclusions and Prospects

3. SCINTILLATING FIBERS • Plastic Fibres. • 0,25mm diameter. • Double cladded (increased trapping efficiency). • Scintillating Polystyrene core (PS). • High NA.

4. SCINTILLATINGMECHANISM A photon signal is produced through a multi-step process. Energy deposited by charged particles excites the polymer core. Relaxation time and light yield of this base material is poor. Energy is transferred (sub-ns) by non-radiative dipole-dipole transmission to an organic fluorescent dye which will relax by emission of a photon. A wavelength shifter absorbs the emitted photon and fluoresces at a longer wavelength. Large Stokes shift allows for lower auto-absorption.

5. Analysed Fibres • Fibres with different wavelength shifter’s concentrations were measured and compared: • Kuraray SCSF-77 (500ppm WLS concentration) • Kuraray SCSF-78 (1000ppm WLS concentration) • Kuraray SCSF-78 (2000ppm WLS concentration) • Kuraray SCSF-78 (5000ppm WLS concentration) (SciFi baseline)

6. SciFi: Attenuation Length Light is attenuated during propagation in the fibre. Light intensity can be described by a double decreasing exponential: Helical Path • : Short component. It represents the rapid attenuation of helical modes and cladding light in the fibre. In the order of 30cm. • : Long component. It represents the attenuation of the core modes. A long component >3m is highly recommended for the SciFi project. Unbound cladding rays Core modes

7. Attenuation Length: Set-Up PIN diode Excitation moves on the rail: current from the PIN read every 5-10 cm. Excitation: Teflon cavity + 4 UV LEDs (WLS is excited).

8. Attenuation Lengths: Results • Single exponential fit from 100cm to 300cm (long component) for comparison with Kuraray. • Attenuation length expected to decrease when WLS↑ (more self-absorption) but other factors are predominant. • Differences between batches of nominally identical fibres. • 2014 production worse than 2010.

9. Absorption and Emission Spectra PS absorption peaks WLS self-absorption Two spectra recorded at different distances from the excitation light source. We obtain the attenuation factor (=1/attenuation length) at each λ.

10. Absorption Spectrum Significant increase of the AttFactor, for the SciFis. ↑WLS concentration AttFactors scaled to the same plateau at longer λ Slight increase of the AttFactor for the clear fibre We observe higher attenuation in the blue-violet region. For the clear fibre (no WLS), this is principally due to Rayleigh Scattering (). ↑WLS concentration ⇨↑Attenuation at short λ (self-absorption). ⇨ High attenuation visible at longer wavelengths.

11. Light Yield • A possible figure of merit to evaluate SciFi’s quality is a combination of • Attenuation (λ). • Light Yield (number of photons produced by a traversing particle). •  A new experimental set-up was built to provide relative light yields measurements and compare performances from different fibres. Electron monochromator: An -Gun is housing a 90Sr source (370MBq). 90Sr irradiates electrons with a wide energy spectrum (0-2.2MeV); we choose with Ekin≈1.1MeV. We select electron’s energy bending the trajectory with a magnetic field produced by an electromagnet. The output beam is sent to the fibres.

12. Light Yield: Set-up Trigger fibre Trigger PMTs e-Beam’s direction wall wall T2 e-gun (e- of ~1,1 MeV) T1 ~ 270 mm On a digital scope, we trigger on T1-T2 coincidence and calculate the integral of the signal coming from the signal PMT: Four fibres were initially read simultaneously to increase the signal’s intensity. Trigger fibre Trigger

13. LY Measurements Example of a charge spectrum obtained measuring LY. Pedestal peak (misalignment, attenuated photons in the signal fibres) Charge spectrum Experimental set-up: LY is measured at different distances between the excitation and detection points.

14. Light Yield: Number of Photoelectrons (Npe) • To evaluate the most probable value for the produced photoelectrons we have two possibilities: • Eliminate the pedestal, calculate the spectrum’s centre of gravity and divide it by the “1 photoelectron gain”, previously measured and fixed: • We fit the pedestal with a Gaussian and the spectrum with a sum of Gaussians weighted by a Poisson distribution. • Where are respectively the STD of the pedestal peak • and of the 1 photoelectron signal and ) is the electron’s path • inside the fibre.

15. 4 Fibres or 1 Fibre? LY measurements can be performed by reading 1 single fibre or 4 fibres collectively. Failing fit 1000ppm (2014), d=120cm. 4fibres 1000ppm (2014), d=120cm. 1fibre • 4-fibre measurement • Imperfect model (misalignment between fibres, Xtalk…): hard to fit the spectra. • Prefer to rely only on COG calculations. • Higher signal. • Possible to measure poor LYs at great distance. • Possible to compare LY of different fibres. • 1-fibre measurement • Low signal for fibres with poor LY at long distances. • Set-up not optimised for 1-fibre measurements. • More correct model. • More exact numbers extracted from the fit. • Possible to compare LY of different fibres. Results of 2 approaches are consistent!

16. Light Yield: Results Results obtained with 4fibre measurements, Npe obtained through COG. • No clear correlation between LY and WLS concentration. • SCSF78 1000ppm (2010) show the highest LY. • 2014 Batch below expectations (LY ≈20% lower than 2010).

17. Conclusions… • With our set-ups we can measure: • Attenuation Lengths. • Emission and Absorption Spectra. • Light Yield. • Reliable QA tests can be performed on every new fibre batch. • There is no visible reason nowadays to change the Kuraray standard WLS concentration (1000ppm). • Fibres produced in 2014 don’t reach the quality we had in 2010 in terms of attenuation length, LY and diameter profile’s homogeneity. …and Prospects • Fibres in the LHCbSciFi Tracker will be mostly exposed to low radiation doses (<1kGy). No available experimental data in that range and different models disagree. • We are building set-ups for fibres irradiation at low doses .

18. Backup Slides

19. Attenuation Length vs WLS Concentration Higher WLS concentration->Lower AttLength, but not for the 2014 production. 1000ppm (2010) 1000ppm (2014)

20. Clear Fibre We cannot excite a clear fibre. Two measured spectra at two different distances from the light source. From the ratio we got the Attenuation (λ). Thermal QTH lamp (Oriel)

21. Irradiated fibre Test plate irradiated in November 2012, re-measured in August 2014 UV LED for excitation Background 3 kGy 22kGy Radiation doesn’t harm the scintillation mechanism, but it reduces the attenuation length.

22. E-Gun characterisation We set the current in the monochromator’s coil in order to have the highest electron rate from the 90Sr source and therefore the greatest number of scintillation events.

23. Light Yield Set-up

24. 1pe gain and sigma The 1pe calibration, obtained through a LED enlightenment of the PMT, is fitted with a double Gaussian (one for the pedestal, one for the signal). The found values are used as fixed parameters in the fits and in the COG calculations.

25. MODEL’S IMPERFECTIONS IN THE 4-FIBRE LY MEASUREMENTS • With 4 fibres we underestimate low photoelectrons’ contribution. Due to: Misalignment (Electrons maybe don’t always pass through all the 4 fibres). ≈270μm Xtalk (Fibres are tangled: some helical/cladding photon can pass from one fibre to another)

26. Single fibre and 4 fibre LYs show differences • Single fibre LY (from fit) multiplied by 4 is ≈10% higher than 4fibre LY (from COG). • Ly1fibre*4/Ly4fibre quite stable at all distances. • Still possible to use 4 fibre for relative LY measurements (signals easier to read-out for fibres with low LY at distances >2m).