Optical and scintillation properties of Lead Tungstate crystals: a statistical approach

Transcription

1 Optical and scintillation properties of Lead Tungstate crystals: a statistical approach Università "La Sapienza" Dipartimento di Fisica, INFN Sezione di Roma (on behalf of CMS ECAL Collaboration)

2 Index 1. Introduction - PWO crystals for ECAL-CMS - problems related to large scale production 2. Optical parameters statistics - parameters stability - correlations 3. Correlation Longitudinal Transmission - Light Yield 4. Conclusions - LT LY correlation characteristics - semi-empirical model - results

3 Largest number of crystals ever used for a single electromagnetic calorimeter: Introduction Barrel: Xtals Endcaps: Xtals Uniformity of Xtals properties needed for intercalibration monitoring PWO is a tunable scintillator scintillation properties can be modified through crystal doping and/or modifications of the crystal growth technology difficult control of scintillation and optical transmission parameters along as grown crystal ingots and from crystal to crystal, inside batches of large scale production

4 Introduction More: production extended over several years possible technology modification and/or involvement of different producers Production database were specially designed and built for the crystals measured in ECAL Regional Centers. initial purpose: further use: ensure the quality of ECAL detector and tune the quality of crystals produced at industrial scale find possible correlations between optical and scintillation parameters to be used for the crosscheck between LY and transmission measurements and the improvement of the ECAL crystals inter-calibration precision give hints on scintillation characteristics of large PWO crystals, otherwise impossible to be put put in evidence (direct measurements are out of discussion given the dimensions and number of crystals) aim of this talk

5 REDACLE database, June 2005 only delivery batches statistically significant (N > 20 crystals) (98.87%) Optical parameters statistics Longitudinal transmission (LTO) statistics Made in Russia LY statistics #Xtals correlation LY-LTO C LTO LY ( λ) = N i LTOi ( λ ) LTO ( λ ) LYi LY N σ σ LTO LY

6 Optical parameters statistics Xtals belonging to the same batch have generally uniform behaviour Batch S4 Batch S8

7 LTO-LY correlation characteristics example: Batch P8 (363 Xtals) LY=light production+light transport

8 LTO-LY correlation characteristics highest C value doesn t correspond to emission peak (420nm) as one may expect: ( λ1 ) Cmax C = r C = C 420nm max batch λ1 (nm) Cλ1 C420nnm r P P P P10all P P P P15all P P P S S S S S5all S6all S8all S9all S10all allxtals % 31% Xtal transparency: Batch transparency: τ τ 1 λ λ ( ) = 2 % LTO( λ) dλ 2 =<τ 1 λ λ Batch > NXtals for the 23 batches under study, λ 1 =350nm ; λ 2 =375nm λ 1 =400nm ; λ 2 =440nm anomalous LY Batch τ Batch correlation LTO LY correlation is not driven by light transport 1

9 LTO-LY correlation characteristics? LTO - LY correlation origin reminder: c a Pb 2+ blue emission: around 420 nm (WO 4 ) 2- (WO 4 ) 3- green emission: around nm Real, possibly doped crystals may have different relative concentration and effectiveness of these two emission centers. The overall emission spectrum of large PWO crystals may vary in: green and blue peaks positions green/blue intensity ratio overall spectrum position overall spectrum intensity PWO crystals under discussion are specially produced for ECAL use, result of a dedicated R&D activity. It is therefore expected that only the fourth possibility is effective.

10 Semi-empirical model Light Yield of a PWO crystal is intended as LY@8X0 (ECAL-CMS Collaboration). It may be expressed as: (1) n LY = c S( λ) T ( λ) ε ( λ) dλ with: S(λ) emission spectrum, T(λ) longitudinally measured transmission, n effective light path inside the crystal expressed in crystal length (L) units, ε PM (λ) PM quantum efficiency (Hamamatsu R1847) and c calibration constant. The intensity of the emission spectrum is: (2) S( λ) = ( n 0 q nt ) f ( λ) with: n 0 and n t concentrations of luminescent centres and respectively centres acting with efficiency q as traps for secondary carriers generated by gamma excitation. Trapping centres may put their fingerprint in the region of the absorption band edge of PWO crystals: (3) α( λ 0 ) = α 0 = const ( n t + n b ) with: α absorption coefficient, n t concentration of absorption centres supposed to work as traps for free carriers, n b concentration of other absorption centres active at the same wavelength λ 0. PM The longitudinally measured transmission T is: 2 αλ L tλ e (4) Tλ = 2 2α λ L 1 rλ e with: L crystal length, r reflectance, t transmittance of PWO Considering the values of r=0.004 and t=0.996 and typical values for T measured in the region close to the absorption edge on PWO crystals (0.25<T 0 <0.55) one has: 2 α0 L (5) T0 t0 e which allows for the calculation of the absorption coefficient and therefore the concentration of colour centres: (6) n t = a lnt0 + b + n b with: a and b parameters with the same value for all PWO crystals under discussion. Equation (1) becomes: n (7) LY = A + B lnt ) f ( λ) T ( λ) ε ( λ) dλ ( 0 with: n, A, B parameters to be fixed for a given batch of PWO crystals of the same nature (same raw material, growth technology and samples processing) by fitting (7) computed values with LY experimental data. PM

11 Fit program LTOspectra f ( ) λ PWO Hamamatsu R1847 ε PM fixed ( ) λ ε exp = ε LY = 0.49 batch dependent λ 0 CLTO LY P8 ( ) λ LY exp values 2 exp n LY= A+ B lnt ) f ( λ) T ( λ) ε ( λ) dλ ( 0 ( ) χ = LY j LY j ε j exp PM A, B, n

12 Results-1 A= ± B= ± n= ± λ = nm A + B lnt k = < A + B lnt 0 0 > k max -k min =0.250 selected delivery batches Xtals

13 A= ± B= ± n= ± Results-2 λ = nm k max -k min =0.180 Batch S6all 1399 Xtals

14 A= ± B= ± n= ± Results-3 λ 0 = 364nm k max -k min =0.056 Batch Xtals

15 Conclusions Analysis of data stored in ECAL-CMS construction databases may give hints on scintillation characteristics of large PWO crystals, otherwise impossible to be put in evidence. Correlation found between and LTO near the fundamental absorption edge is not a light transport effect i.e. cannot be attributed (only) to variations of the optical transmission. A semi-empirical model was developed in order to explain the characteristics of the correlation LTO-LY. The model has an overall 6% guess precision ( crystals processed in Rome RC), going to 4% for batches with a high uniformity of crystals characteristics. The model predicts an overall variation of light production efficiency in a range of 25% of the mean value for the crystals analysed. The variation range may drop to 5% for uniform batches. The model offers an analysis tool for large number of scintillators, particularly interesting in cases where the LY measurement cannot be made with sufficiently high precision and transmission measurement brings the necessary improvement.