Update on Scintillating Fibres

Update on Scintillating Fibres June-July 2014 Cesare Alfieri Christian Joram

Summary • SciFi Emission Spectra • Attenuation Length (λ) & Attenuation Factor (λ) • Irradiated fibres • Clear Fibre • Transmission Spectrum • Attenuation Length (λ) & Attenuation Factor (λ) • Light Yield • Set-up • Fits, produced Photoelectrons, comparison

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) (just received this week)

SciFi Emission Spectra We recorded two spectra, corresponding to different distances from the excitation light source. From the distance and the division of the two intensities we could obtain the attenuation length.

Att Length & Att Factor of non-irradiated fibres

Different behaviour in the blue region • Where does this discrepancy come from? • WLS ? • Polystyrene core? • The green and blue small points correspond to our measurements, the big blue points come from Blake’s measurements. • We remark a lower attenuation in the blue region. Let’s have a look at a clear fibre….

Clear Fibre Two measured spectra at two different distances from the excitation source. From the ratio we got the Attenuation (λ). Thermal QTH lamp (Oriel)

Clear fibre’s Attenuation When compared to SciFis, this clear fibre strangely seems more attenuating. Admitting a 10% error in the measurement (different coupling lamp-fibre, not identical cut for the two points etc.) the difference becomes small.

Comparison with Kuraray catalog plot When compared to the plot in the Kuraray catalog (probably for Ø1 mm), our measurements give a higher attenuation and differences in the spectral shape. Clear fibre appears to be stiffer than a SciFi and easily cracks. We received this fibre 'accidentally', left on a demo spool provided by Kuraray.

Irradiated fibre Test plate irradiated in November 2012, re-measured in June 2014 UV LED for excitation Background 3 kGy 22kGy

Att Factor/Dose We divided the attenuation factor by the dose looking for a relation between the two quantities.

Attenuation (dose) • It seems that a sub-linear dependency exists between the Attenuation Factor and the dose a= 0.85 ?

Light Yield • The figure of merit that best evaluates a SciFi is a combination of • Attenuation (λ). • Light Yield. •  A new experimental set-up to provide relative light yields and compare performances from different fibres. We’re using an electron gun consisting of a Sr-90 electron source with a magnetic filter. From the wide spectrum of electron’s energies given by the Sr-90, we choose electrons with Ekin≈1.1MeV. Their trajectory is bent through a magnetic field produced by an electromagnet.

Light Yield: Set-up e-Beam’s direction On a digital scope, we trigger on PMT1 and calculate the integral of the signal coming from PMT2: Three fibres are read to increase the signal’s intensity. • Our set-up is not yet finalised: • Fibres aren't perfectly aligned in the slit (slightly larger than 0.25mm). • The plastic collimator must be better fixed to the electron gun. • UV light coming from the scintillating trigger fibre could maybe cause extra scintillation in the read-out fibres.

Light Yield: Number of Photoelectrons (Npe) • To evaluate the most probable value for the produced photoelectrons we have two possibilities: • Eliminate the pedestal (made of zeros), calculate the spectrum’s centre of gravity and divide it by the “1 photoelectron gain”, previously fixed: • Develop a theoretical model of the statistic of the process that fits the spectrum. 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. • First of all it is so necessary to fix this 1pe gain and sigma.

1pe gain and sigma The 1pe calibration, obtained through an LED enlightenment of the PMT, was fitted with a double Gaussian (one for the pedestal, one for the signal). The found values will be used as constants in the fits and in the COG calculations.

Spectra: Examples

Light Yield Spectra Extra 1pe Examples of Light Yield spectra. The spectrum at 90 cm is more narrow due to attenuation. We observed an excess of single-pe's, probably due to UV light escaped from the trigger fibre (see below). The fits are not perfect: we are actually considering the path differences of the particles inside the fibre but, for the time being, we don’t consider different angles in their trajectory nor differences in the energy deposition (Landau tail).

Excess of 1pe due to inter-fibre Xtalk? A trigger fibre painted in black reduces the extra 1pe effect.

Light Yield and Attenuation Length SCSF-78 SCSF-77 To explore the reliability of light yield measurement, we checked that Npe scales consistently with the attenuation length. The results are good for both the fibres if we assume a 5% error on Npe and a 5cm error on the distance excitation-PMT. Only the 30cm-point for SCSF-78 is not compatible (measured twice with the same result).

Light Yield: Results We prefer the results given by the Centre Of Gravity (COG) method, since the fitting model is still imperfect. We see that a comparison between two fibres is possible and reproducible. SCSF-78 has in general a higher light yield compared to SCSF-77.

Light Yield: comparison Work in progress: measurements of 2000 ppm fibre were done today. All measurements are to be repeated with a finalised set-up…

Attenuation Length vs WLS Concentration Higher WLS concentration->Lower AttLength, but still in a range of good values (≈3m)

Summary and prel. conclusions • CERN/HD puzzle of high attenuation in region below 450 nm persists. • Clear fibres don’t show the increased attenuation below 450 nm.  It must be related to the WLS. • LY set-up starts to give reasonable results, but needs further improvement (mechanical alignment, collimation). • Consistency of LY and attenuation data generally good (+/- 5%). • First LY results show a positive correlation between WLS concentration and LY. • High WLS concentration fibres have still reasonable att. Length (~3 m).

Backup Slides

E-Gun characterisation The Sr-90 source being ≈ 25% less active compared to another previously used, we set the current in the coil in order to have the highest electron rate and the greatest number of scintillation events

Update on Scintillating Fibres