International RICH-Workshop of the CBM Experiment at FAIR

International RICH-Workshop of the CBM Experiment at FAIR Gesellschaft für SchwerionenforschungDarmstadt, GERMANY March 6 - 7 2006 Radiator Gases{small refractive indices} Olav Ullaland (PH, CERN)

?  330 10-6 < n-1 < 360 10-6 • The requirements • gth 38, • good UV transmittance, • long radiation length • ideal: non inflammable, chemically passive gas • potential problem: fluorescence of N2? • CH4/CO2 could be used as quenching gas in mixture If n-1 « 1 Tci values (%) for CH4 N2 9.9 CO2 22.45 He 11.86 Ne 9.2 Ar 6.15 SF6 50.4 CF4 33.4 R134a 11.98 CH4 [from Air Liquide ] Major hazard : Fire and High Pressure Toxicity: Simple Asphyxiant Flammability limits in air (STP conditions) : 5.0-15.0 vol% [CERN rules: LEL(%): 4.4 UEL(%):16.9] Odour : None

and the answer is: Data from: J.V. Jelly, Čerenkov Radiation and its Application V.P. Zrelov, Čerenkov Radiation in High Energy Physics II DuPont Freon Technical Bulletins B-32, 32A

Journal of the Optical Society of America 59(1969)863 at 0 oC and 760 torr

+ 18 ppm Ne, 5.2 He, 1.5 CH4, 1.14 Kr, 0.5 N2O, 0.5 H2, 0.4 O3, 0.086 Xe Anything wrong with dry air? Cheap! Abundant! Non flammable! ~Correct refractive index! Eigenshaften der Materie in Ihren Aggregatzustanden, 8. Teil Opische Konstanten, 1962

The (possible) drawback: The transparency of a fluid is defined by: where tis the path length in cm,f = f()is the absorption coefficient and pis the pressure in bar. K. Watanabe et al., Absorption Coefficients of Several Atmospheric Gases, AFCRC Technical Report No. 53-23, 1953

CO2 start absorbing around 180 nm. CF4 around 110 nm. N2, Ar, Ne .... transparent well below 150 nm. From: G. R. Cook and B. K. Ching, The Journal of Chemical Physics 43(1965)1794-1797 R. Abjean et al., NIM A292(1990)593-594 H.E. Watson and K.L. Ramaswamy, Proc. R. Soc. London, A156(1936)144 Eigenshaften der Materie in Ihren Aggregatzustanden, 8. Teil Opische Konstanten, 1962

Well described by: at 0 oC and 760 torr With a little bit of mixing of CF4 and Ne: Setting (n-1) 106 = 350 at 400 nm gives a mixing ratio of CF4:Ne = 67:33 ‘The Dutch Chemist’, c 1780s. Copper engraving by J Boydell after a painting by J Stein.

Well described by: at 0 oC and 760 torr We can do the same with CF4 and He: Setting (n-1) 106 = 350 at 400 nm gives a mixing ratio of CF4:He = 695:305 http://www.levity.com/alchemy/cab_min1.html

Do a little comparison: density X0 X0 g/l g/cm2 cm He 0.178 94.32 5.3 105 at 0 oC and 1013 hPa Ne 1.25 37.99 3.0 104 CF4 3.92 33.6 8.6 103 air 3.0 104 at 20 oC and 1013 hPa Radiation length, X0, for a 1 m radiator CF4/Ne 1.05 % CF4/He 1.14 air 0.33 In addition: He and vacuum photo tubes  no good

If using a binary (or more) gas mixture • chose gases which are easy to separate. Or use and discard. • Boiling point • Size The gases considered have all very low boiling point. Rather strong correlation between refractive index and size Kinetic Diameter (A)

Selectivity measurement with different types of membranes. Generon hollow fibre membrane Model B210 UBE Industries, Specialty Chemicals and Products Division, High Purity Chemicals Business Unit, Ube Europe GmbH, Duseldorf, Germany. It is therefore (fairly) easy to separate He or Ne from CF4

6.5 eV • Some reasons why NOT having quantum efficiency below ~190 nm. • Air contamination (O2, H2O and CO2) levels of a few ppm. • Trace contamination of the main radiator gas to levels approaching ppb • Outgassing properties of the main structures to space requirements • Perfect gas flow pattern • Chromatic aberration is important • Rayleigh scattering starts to be important • Expensive optical windows • Photon detector entrance window in contact with the radiator or high quality atmosphere in the photon detector enclosure

What some CnHm traces can do to you (and your photons). CnH2n+2 C2H2 CnH2n C6H6

The fate of a photon after 8 m with 10 ppm O2 The (apparent) radiator length will therefore change as function of wavelength.

Two extremes. 1 m N2 as radiator #photons/m  13 detected  CsI up to ~8 eV RMSMaPMT = 0.43 mrad RMSCsI = 0.45 mrad

(10-19 cm2) = 330 3914 A (10-19 cm2) = 93 4278 A What about scintillation and fluorescent? Example: Ar 130 nm Kr 150 nm Xe 175 nm 2 time constants: from a few ns to 1 µs. CF4 >120 nm 20%  [3% + 9% - 6%] of Xe >180 nm 45%  [3% +17% -13%] of Xe NIM 361(1995)543 Relative light yield: Xe:Kr:Ar:Ne:He=1.0:0.52:0.16:0.043:0.33 Perhaps evident, but still: n=F n: photons emitted/cm3 F: proton flux : cross section for excitation : molecular density In addition n  dE/dx Spectra induced by 200 keV proton impact in nitrogen. Phys.Rev.123(1961)2084 As it is non-directional, it will (normally) not influence the pattern recognition algorithm. To watch: Cherenkov signal photons to background hits.

Conclusion Gases with low refractive index are not (really) different from gases with high refractive index If you want to move down a little, neon is a good gas If you want to move up a little, CF4 is a good gas If you are nearly right with air, use air, but remove the water and the dust. {There will always be somebody who ask if you have included Mie's theory in the simulation.}

International RICH-Workshop of the CBM Experiment at FAIR