Transcript Slide 1
International RICH-Workshop of the CBM Experiment at FAIR
Gesellschaft für Schwerionenforschung
Darmstadt, GERMANY
March 6 - 7
2006
Radiator Gases
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Olav Ullaland (PH, CERN)
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The requirements
gth 38,
good UV transmittance,
?
If
long radiation length
ideal: non inflammable, chemically passive gas
potential problem: fluorescence of N2?
CH4/CO2 could be used as quenching gas in mixture
n-1 « 1
1
n 1 2
2g th
330 10-6 < n-1 < 360 10-6
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
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
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
Anything wrong with dry air?
!
Abundant!
Non flammable!
~Correct refractive index!
Cheap
dry air 760 torr 0 o C
6
(n 1) 10
N
0.053191
0.7809
74.36 2 2
2
O2
0.2095
5.496 10
5
4
1
20.275
2
0.003755
0.050854
73.82 2 2
CO
0.068681
0.0003
80.10 2 2
Ar
0.0093
2
Eigenshaften der Materie in Ihren Aggregatzustanden,
8. Teil Opische Konstanten, 1962
+ 18 ppm Ne, 5.2 He, 1.5 CH4, 1.14 Kr, 0.5 N2O, 0.5 H2, 0.4 O3, 0.086 Xe
The (possible) drawback:
The transparency of a fluid is
t f p
defined by:
where t is the path length in cm, f = f() is the absorption coefficient and p is the
pressure in bar.
T e
1000
Water
Oxygen
CO2
Absorption (/cm/bar) .
100
10
1
0.1
100
125
150
175
Wavelength (nm)
K. Watanabe et al., Absorption Coefficients of Several Atmospheric Gases, AFCRC Technical Report No. 53-23, 1953
200
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
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:
(n 1)106
at 0 oC and 760 torr
0.091553
61.102 2
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_mi
n1.html
Well described by:
(n 1)106
at 0 oC and 760 torr
0.09050
61.452 2
Do a little comparison:
He
Ne
CF4
air
density
g/l
0.178
1.25
3.92
X0
g/cm2
94.32
37.99
33.6
X0
cm
5.3 105 at 0 oC and 1013 hPa
3.0 104
8.6 103
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.
(C4F9)3N
)3N
(C4H9)3N
(C2H5)3N
)3N
(C4F5)2NC3F7
C6F14
F14
C5H12
H12
c-C6H12
CCl4
Rather strong correlation between
refractive index and size
C6H6
6H6
n-C4F10
SF6
SF6
i-C4H10
CF4
CF4
C4H8
C3H6
3H6
CF2Cl2
H10
n-C4H10
C3H8
c-C3H6
3H6
Xe
2H4
C2H4
CH4
CO
CO
N2
SO2
Kr
SO2
H2S
H2S
CS2
HBr
Br2
O2
Ar
HBr
O2
N2O
N2O
CO2
2H2
HCl
C2H2
Cl2
Cl2
H2
H2
NO
Ne
H2O
H2O
NH3
He
He
0
1
2
3
4
5
Kinetic diameter (A)
Kinetic Diameter
(A)
6
7
8
9
10
NeoMechs
composite hollow fibre
Selectivity measurement with
different types of membranes.
GT-0212-0025-50308
Selectivity relative to N2 .
100
100
10
O2
Ar
He
Ar
O2
Ne
1
N2
CF4
0.1
C4F10
N2
1
0.01
2
2.5
3
3.5
4
4.5
5
5.5
6
5.5
6
Kinetic diameter (A)
0.1
CF4
C4F10
Generon
hollow fibre membrane
Model B210
0.01
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Kinetic diameter (A)
100
He
It is therefore (fairly) easy to
separate He or Ne from CF4
CO2
.
UBE Industries, Specialty Chemicals and Products
Division, High Purity Chemicals Business Unit, Ube
Europe GmbH, Duseldorf, Germany.
Selectivity relative to N2
Selectivity relative to N2 .
CO2
CO2
10
Ne
10
O2
Ar
1
N2
0.1
C4F10
CF4
0.01
2
2.5
3
3.5
4
4.5
Kinetic diameter (A)
5
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
6.5 eV
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
RMSCsI
= 0.43 mrad
= 0.45 mrad
What about scintillation and fluorescent?
Example:
Relative light yield:
Ar
130 nm
Xe:Kr:Ar:Ne:He=1.0:0.52:0.16:0.043:0.33
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
Spectra induced by 200 keV
proton impact in nitrogen.
Phys.Rev.123(1961)2084
(10-19 cm2) = 93
4278 A
(10-19 cm2) = 330
3914 A
Perhaps evident, but still:
n=F
n: photons emitted/cm3
F: proton flux
: cross section for excitation
: molecular density
In addition
n dE/dx
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.}