International RICH-Workshop of the CBM Experiment at FAIR Gesellschaft für Schwerionenforschung Darmstadt, GERMANY March Radiator Gases { s m a l l r e f r a c t i v e i n d i c e s } Olav Ullaland (PH, CERN)

The requirements  th  38, good UV transmittance, long radiation length ideal: non inflammable, chemically passive gas potential problem: fluorescence of N 2 ? CH 4 /CO 2 could be used as quenching gas in mixture ?  330  < n-1 < 360  If n-1 « 1 CH 4 [from Air Liquide ] Major hazard : Fire and High Pressure Toxicity: Simple Asphyxiant Flammability limits in air (STP conditions) : vol% [CERN rules:LEL(%): 4.4 UEL(%):16.9] Odour : None Tci values (%) for CH 4 N CO He11.86 Ne 9.2 Ar 6.15 SF CF R134a11.98

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 and the answer is:

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

Anything wrong with dry air? Cheap ! Abundant ! Non flammable ! ~Correct refractive index ! Eigenshaften der Materie in Ihren Aggregatzustanden, 8. Teil Opische Konstanten, ppm Ne, 5.2 He, 1.5 CH 4, 1.14 Kr, 0.5 N 2 O, 0.5 H 2, 0.4 O 3, Xe

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

From: G. R. Cook and B. K. Ching, The Journal of Chemical Physics 43(1965) R. Abjean et al., NIM A292(1990) 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 CO 2 start absorbing around 180 nm. CF 4 around 110 nm. N 2, Ar, Ne.... transparent well below 150 nm.

With a little bit of mixing of CF 4 and Ne : Setting (n-1)  10 6 = 350 at 400 nm gives a mixing ratio of CF 4 :Ne = 67:33 Well described by: at 0 o C and 760 torr ‘The Dutch Chemist’, c 1780s. Copper engraving by J Boydell after a painting by J Stein.

We can do the same with CF 4 and He : Setting (n-1)  10 6 = 350 at 400 nm gives a mixing ratio of CF 4 :He = 695:305 Well described by: at 0 o C and 760 torr n1.html

Do a little comparison: densityX 0 X 0 g/lg/cm 2 cm He  10 5 at 0 o C and 1013 hPa Ne  10 4 CF  10 3 air3.0  10 4 at 20 o C and 1013 hPa Radiation length, X 0, for a 1 m radiator CF 4 /Ne1.05 % CF 4 /He1.14 air0.33 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)

UBE Industries, Specialty Chemicals and Products Division, High Purity Chemicals Business Unit, Ube Europe GmbH, Duseldorf, Germany. Selectivity measurement with different types of membranes. NeoMechs composite hollow fibre GT Generon hollow fibre membrane Model B210 It is therefore (fairly) easy to separate He or Ne from CF 4

Some reasons why NOT having quantum efficiency below ~190 nm. Air contamination ( O 2, H 2 O and CO 2 ) 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 6.5 eV

C2H2C2H2 C6H6C6H6 What some C n H m traces can do to you (and your photons). C n H 2n+2 C n H 2n

The fate of a photon after 8 m with 10 ppm O 2 The (apparent) radiator length will therefore change as function of wavelength.

Two extremes. 1 m N 2 as radiator #photons/m  13 detected  CsI up to ~8 eV RMS MaPMT = 0.43 mrad RMS CsI = 0.45 mrad

What about scintillation and fluorescent? Example: Ar130 nm Kr150 nm Xe175 nm 2 time constants: from a few ns to 1 µs. CF 4 >120 nm20%  [3% + 9% - 6%] of Xe >180 nm 45%  [3% +17% -13%] of Xe NIM 361(1995)543 As it is non-directional, it will (normally) not influence the pattern recognition algorithm. To watch: Cherenkov signal photons to background hits.  ( cm 2 ) = A  ( cm 2 ) = A Perhaps evident, but still: n=  F n: photons emitted/cm 3 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 Relative light yield: Xe:Kr:Ar:Ne:He=1.0:0.52:0.16:0.043:0.33

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, CF 4 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.}