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On a new method for testing and calibrating wire chambers
Nuclear Instruments and Methods 176 (1980) 323 324 © North-Holland Publishing Company
O N A NEW M E T H O D F O R T E S T I N G A N D C A L I B R A T I N G WIRE C H A M B E R S
H. ANDERHUB, M. DEVEREUX and P.G. SELLER
Laboratory/'or ttigh Energy Physics of the ETtl Zurich, c/o SIN, Ctl-5234 Villigen, Switzerland
The latent tracks caused by ionizing particles in wire chambers can be simulated by a pulsed laser beam, if molecules having appropriate ionization potential, absorption band structure and vapour pressure are admixed to the chamber gas. This can be used for chamber calibration. First tests [1] with N2-1aser light (337 nm) and Ni(Cs Hs)2, were extended to include several organic compounds using a pumped tunable dye laser (428 450 nm).
SCINTILLATOR
1. I n t r o d u c t i o n
PM TUBE
The best statistical space resolution for minimum ionizing particles in wire chambers is now Ox = 50/~m [2], To make full use of this resolution, a method is needed, which allows the calibration of chambers with an absolute accuracy of, say, Ax < 10/ira. We developed a method, which may fulfill this requirement in the future : a pulsed laser beam in a multistep process ionizes molecules admixed to the chamber gas. This offers the following advantages: - a narrow beam can be produced, whose position and timing are very well defined; since the simulated "particle" beam will not be affected by a magnetic field the influence o f such a field on a chamber can be studied. (Often a crucial point in spectrometer calibration.) These favourable features are, however, accompanied by some drawbacks: - the diffraction limitation of the laser beam, the photoeffect on the wires. First measurements with a N~-laser (hu = 3.7 eV) and Ni(CsHs)2 (I = 6.5 eV) have been published. [1].
0,ELASE i LENS P.HBLE L;NS t , Anode
SHUTTER
~ ADJEURSTT#R BLE window
DRIFT CHAMBER
Fig. 1. Setup for ionization investigation. The chamber gas: 60% At, 20% Ctt4, 20% CO2 + admixtures. Wires: anode~ 20 ~m, cathode: 100 ~m, spacing: 8 ram. Voltage: anode: +1800 V, cathode : -600 V, vessel: on ground. Gain and drift time have been monitored by a S4Mn source.
which may result in irreproducible effects * For these re'asons we extended our studies to the visible light region. A dye laser [4] tunable in the range 4 2 8 - 4 5 0 nm (huma x = 2.90 eV, humi n = 2.76 eV) was used. Different molecules, each with an ionization potential [ < 3huma x (4-amino-azobenzene, azobenzene, 2-6-diethylaniline, ferrocene) were successively admixed to the chamber gas in the way described in ref. 1. The chamber and the gas system were cleaned after each run by evacuation. Fig. 1 shows the setup. The laser beam had a peak power of about 50 kW and a diameter of about 0.5 mm inside the chamber. Only one of the chemicals showed a pronounced effect: 4-amino-azobenzene. Two ionization maxima with a width of about 0.08 nm have been found. At
2. Studies with a tunable pumped dye laser The use of ultraviolet light, however, causes some problems: - m y l a r windows absorb part of the light and are destroyed; - many electrons are liberated when the laser beam hits a wire (photoeffect on a metallic surface); -unknown impurities in the gas may be ionized,
* We have seen such ionization exposing a "normal" MWPC to the N2 laser beam. Similar observations have recently been reported [ 3 ]. 323
VII. SPECIAL DEVICES
324
H. Anderhub et al. / Testing wife chambers
428.7 nm and 447.2 nm ionization in the gas mixture occured but the pulse height achieved was still 20 times smaller than in the case where the whole beam hit a wire.
We thank Mr. R. Stehle and his brothers (SOPRA) for their excellent support during the experiment with the pumped dye laser. Dr. S. Leutwyler, University of Bern (now at Tel-Aviv University) is acknowledged for information about the molecules.
3. Conclusions
References Wire chamber gas containing appropriate molecules can be ionized by a pulsed laser beam. This effect may be used to calibrate such chambers precisely. Care has to be taken because of the photoeffect on metallic surfaces.
[1] H. Anderhub, M. Devereux and P.G. Seiler, Nucl. Instr. and Meth. 166 (1979) 581. [2] For example: F. Sauli, Nucl. Instr. and Meth. 156 (1978) 147. [3] J. Bourotte and B. Sadoulet, Preprint CERN-EP/80-14. [4] Manufactured by SOPRA, Bois Colombes, France.
O N A NEW M E T H O D F O R T E S T I N G A N D C A L I B R A T I N G WIRE C H A M B E R S
H. ANDERHUB, M. DEVEREUX and P.G. SELLER
Laboratory/'or ttigh Energy Physics of the ETtl Zurich, c/o SIN, Ctl-5234 Villigen, Switzerland
The latent tracks caused by ionizing particles in wire chambers can be simulated by a pulsed laser beam, if molecules having appropriate ionization potential, absorption band structure and vapour pressure are admixed to the chamber gas. This can be used for chamber calibration. First tests [1] with N2-1aser light (337 nm) and Ni(Cs Hs)2, were extended to include several organic compounds using a pumped tunable dye laser (428 450 nm).
SCINTILLATOR
1. I n t r o d u c t i o n
PM TUBE
The best statistical space resolution for minimum ionizing particles in wire chambers is now Ox = 50/~m [2], To make full use of this resolution, a method is needed, which allows the calibration of chambers with an absolute accuracy of, say, Ax < 10/ira. We developed a method, which may fulfill this requirement in the future : a pulsed laser beam in a multistep process ionizes molecules admixed to the chamber gas. This offers the following advantages: - a narrow beam can be produced, whose position and timing are very well defined; since the simulated "particle" beam will not be affected by a magnetic field the influence o f such a field on a chamber can be studied. (Often a crucial point in spectrometer calibration.) These favourable features are, however, accompanied by some drawbacks: - the diffraction limitation of the laser beam, the photoeffect on the wires. First measurements with a N~-laser (hu = 3.7 eV) and Ni(CsHs)2 (I = 6.5 eV) have been published. [1].
0,ELASE i LENS P.HBLE L;NS t , Anode
SHUTTER
~ ADJEURSTT#R BLE window
DRIFT CHAMBER
Fig. 1. Setup for ionization investigation. The chamber gas: 60% At, 20% Ctt4, 20% CO2 + admixtures. Wires: anode~ 20 ~m, cathode: 100 ~m, spacing: 8 ram. Voltage: anode: +1800 V, cathode : -600 V, vessel: on ground. Gain and drift time have been monitored by a S4Mn source.
which may result in irreproducible effects * For these re'asons we extended our studies to the visible light region. A dye laser [4] tunable in the range 4 2 8 - 4 5 0 nm (huma x = 2.90 eV, humi n = 2.76 eV) was used. Different molecules, each with an ionization potential [ < 3huma x (4-amino-azobenzene, azobenzene, 2-6-diethylaniline, ferrocene) were successively admixed to the chamber gas in the way described in ref. 1. The chamber and the gas system were cleaned after each run by evacuation. Fig. 1 shows the setup. The laser beam had a peak power of about 50 kW and a diameter of about 0.5 mm inside the chamber. Only one of the chemicals showed a pronounced effect: 4-amino-azobenzene. Two ionization maxima with a width of about 0.08 nm have been found. At
2. Studies with a tunable pumped dye laser The use of ultraviolet light, however, causes some problems: - m y l a r windows absorb part of the light and are destroyed; - many electrons are liberated when the laser beam hits a wire (photoeffect on a metallic surface); -unknown impurities in the gas may be ionized,
* We have seen such ionization exposing a "normal" MWPC to the N2 laser beam. Similar observations have recently been reported [ 3 ]. 323
VII. SPECIAL DEVICES
324
H. Anderhub et al. / Testing wife chambers
428.7 nm and 447.2 nm ionization in the gas mixture occured but the pulse height achieved was still 20 times smaller than in the case where the whole beam hit a wire.
We thank Mr. R. Stehle and his brothers (SOPRA) for their excellent support during the experiment with the pumped dye laser. Dr. S. Leutwyler, University of Bern (now at Tel-Aviv University) is acknowledged for information about the molecules.
3. Conclusions
References Wire chamber gas containing appropriate molecules can be ionized by a pulsed laser beam. This effect may be used to calibrate such chambers precisely. Care has to be taken because of the photoeffect on metallic surfaces.
[1] H. Anderhub, M. Devereux and P.G. Seiler, Nucl. Instr. and Meth. 166 (1979) 581. [2] For example: F. Sauli, Nucl. Instr. and Meth. 156 (1978) 147. [3] J. Bourotte and B. Sadoulet, Preprint CERN-EP/80-14. [4] Manufactured by SOPRA, Bois Colombes, France.