1. RICH MEETING - CERN - April 21 st 2004 Università degli Studi di Milano Bicocca Variation of Refractive Index inside an Aerogel Block Davide Perego

2. CERN – April 21 st 2004 The Refractive Index of aerogel and its density are related by the following relation: k=0.21 @ =632 nm Local inhomogeneities (occurring during production) lead to variations of the Refractive Index within monoliths These variations can give non-negligible contribution to the  C measurement accuracy Introduction n max allowed for LHCb RICH1 aerogel tiles is ~ 3∙10 -4, corresponding to  C ≈1.17 mrad How such variations can be evaluated? Two methods available: (I) Laser Beam Deflection and (II) APACHE

3. Davide PeregoCERN – April 21 st 2004 Local variations of the Refractive Index in a uniformly transparent medium are gradient indexes which affect the propagation of laser rays Laser Beam Method Assuming parallel faces for the block, the deviation angle  is proportional to the gradient of the Refractive Index variation

4. Aerogel CCD camera laser

5. Davide PeregoCERN – April 21 st 2004 Measurements performed with a 100×100×42 mm 3 block & with L=75.8 cm Laser Beam Method n≈1.031  n ≈4.6∙10 -4 which means   ≈1.8 mrad   ~0.35 mrad  n ~0.8∙10 -4 =543.5 nm

6. Davide PeregoCERN – April 21 st 2004 Why not using a particle beam to characterize Aerogel tiles? APACHE 500 MeV electron beam at the BTF Beam Test Facility (DANE LNF-Frascati) photographic film in a “proximity focusing” configuration…

7. Davide PeregoCERN – April 21 st 2004 Why not using a particle beam to characterize Aerogel tiles? APACHE 500 MeV electron beam at the BTF Beam Test Facility (DANE LNF-Frascati) photographic film in a “proximity focusing” configuration… APACHE Aerogel Photographic Analysis by CHerenkov Emission

8. The radius of the ring, R, is a function of: Refractive Index n=n() aerogel thickness t aerogel-film distance L APACHE Davide PeregoCERN – April 21 st 2004 Very simple geometry The dimensions of the photographic film define the maximum R 8”×10” Kodak TRI-X B/W 8”×10” Kodak EKTACHROME (wavelength range: 300÷600 nm) L≈30.0 cm z (cm) e-e- R Aerogel Film N2N2 L n~1.031 with =543.5 nm

9. APACHE Davide PeregoCERN – April 21 st 2004 Assuming a simplified scenario without scattering, chromatic dispersion, absorption effects, etc Parameters n=1.031 =1 t=4.2 cm d=31.4 cm R min =8.14 cm R max =9.19 cm

10. Aerogel Ring N 2 Ring + beam APACHE Davide PeregoCERN – April 21 st 2004 Aerogel n=1.031 t=4.2 cm A=95.58% C=0.0060 m 4 /cm Beam & Geometry 520 MeV e - Beam L=31.4 cm Beam spot size  x = y =4.0 mm

11. Procedure for the analysis of the films: 1.digitization with a (8bit) scanner 2.subtraction of background 3.search of the center of gravity, then integration 4.comparison of the positions of peaks for different input n APACHE Davide PeregoCERN – April 21 st 2004 Resolution for 3∙10 -4 ~0.25 mm

12. Experimental Setup: portable dark room, ~60×60×60 cm 3 variable height aerogel holder N 2 flux inside the dark room 0.1 mm D263 filter downstream the aerogel (UV photons) APACHE Davide PeregoCERN – April 21 st 2004 d~30 cm e-e- Aerogel holder Film holder N2N2 Film

13. Magnet Transfer Line APACHE e - beam

14. APACHE

15. Aerogel Film e - beam Aerogel Holder

18. APACHE Davide PeregoCERN – April 21 st 2004 data taking: March 2004 @ BTF, LNF-Frascati ~10 10 electrons/run scan on different entrance points 0.1 mm D263 filter used for run D (compare to run C) films processed by a professional PhotoLab in Frascati digitization for the analysis see the processed films… e-e- V. Kandinsky, Einige Kreise (1926)

19. Run CRun D Davide PeregoCERN – April 21 st 2004

20. APACHE Davide PeregoCERN – April 21 st 2004 1.Gaussian fit restricted to the rising and falling edges of the distributions around the peak 2.Center of gravity of the distribution’s peaks 3.Radius corresponding to the maximum of the differential distributions There is not yet an analytical description of the shape of the distributions (i.e. how to parametrize the film efficiency, etc) Preliminary results available with the following methods: e-e-

21. APACHE Davide PeregoCERN – April 21 st 2004 No filter

22. APACHE Davide PeregoCERN – April 21 st 2004 With filter

23. APACHE Davide PeregoCERN – April 21 st 2004 Preliminary Results e-e- correpsonding to The difference between the position of the peaks is assumed to be independent of the detailed shape of distributions

24. Possible improvements for APACHE: 1.use of a spherical mirror tilted wrt the beam axis  photographic film in a RICH configuration  distorsions of the ring due to the spherical aberration  no aerogel thickness effects 2.separation of the different chromatic components by the use of filters Davide PeregoCERN – April 21 st 2004 APACHE e-e- Film (see TB 2003 Vessel)

25. Summary Davide PeregoCERN – April 21 st 2004 1.local inhomogeneities of the density lead to variations of n 2.the maximum n allowed for LHCb aerogel tiles is ~3∙10 -4 3.two methods are presented to evaluate those variations: Laser Beam Deflection APACHE 4.the results from the two methods agree with each other within the experimental resolution 5.the APACHE method is very promising and artistic too! Special thanks to Giovanni Mazzitelli & the Beam Test Facility (LNF Frascati)