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Development of a multiwire proportional chamber as an area sensitive detector for X-ray protein crystallography
Nuclear Instruments and Methods 201 (i 982) 193-196 North-Holland Publishing Company
193
DEVELOPMENT OF A MULTIWlRE PROPORTIONAL CHAMBER AS AN AREA SENSITIVE DETECTOR FOR X-RAY PROTEIN CRYSTALLOGRAPHY D. BADE, F. PA'RAK, R.L. MC)SSBAUER, W. HOPPE, N. LEVAI and G. C H A R P A K * Max-Planck-lnstitut fiir Biochemie, Abteilung fiir Strukturforschung I, Am Klopferspitz, D-8033 Martinsried, Fed. Rep. Germany and Physikdepartment E15 der Technischen Universittit Mi~nchen, James-Franck-Strasse, D-8046 Garching, Fed. Rep. Germany
The prototype of an area sensitive multiwire proportional chamber operated in series with a spherical drift chamber was investigated for its applicability to protein crystallography, employing 14.4 keV (h =0.86 ,~) "t-radiation of STCo MSssbauer sources. The 10 cm long spherical drift field, in this case, provides high efficiency in order to compensate for the small primary intensity of M6ssbauer sources, The detector system was tested with different gas mixtures. The improvement of the spatial resolution with increasing xenon content is demonstrated using the 603 reflection of a myoglobin crystal. The addition of xenon to the counter gas is shown to be necessary for protein crystallography with short wavelengths. The background is sufficiently low for M6ssbauer experiment~, due to good energy discrimination at the anode (~0.003 counts/(min × mm 2)).
1. Introduction The MiSssbauer scattering on a 57Fe nucleus offers a new possibility for an experimental solution of the phase problem arising in structure determination of protein crystals [1]. The applicability of this method was demonstrated in the case of a myoglobin single crystal [2]. Such experiments have to be performed with STCo gamma sources which yield a much lower intensity of the primary beam than X-ray tubes. In order to get reasonable times for the collection of the intensities of all reflections which are necessary for a structure determination, one has to work with a slightly divergent beam. This way, one excites simultaneously a large number of Bragg reflections. This procedure requires a large area sensitive detector. Because of the low intensity scattered into each Bragg reflection (typically 1 count/min) one has to provide for an optimum efficiency of the counter. For the same reason the background has to be as low as possible in an energy window around 14.4 keV (~ = 0.86 A). The spatial resolution must be sufficient to allow for a separation of closely spaced protein crystal reflections. Optimal efficiency together with low background can be achieved by using small Si(Li) detectors for single reflections [2]. However, the * CERN, Geneva, Switzerland.
0167-5087/82/0000-0000/$02.75
extension of this concept to larger Si(Li) counters for many reflections [3] involves the adaption of special detector assemblies to each reciprocal lattice geometry. Flat multiwire proportional chambers (MWPC) might appear to be adequate for the application in protein crystallography, but for usual anode-cathode distances the achievable efficiency is rather low at 14.4 keV. An increase of the thickness of the counter improves the efficiency, but at the same time the parallax problem reduces the spatial resolution at large scattering angles. The spherical drift chamber in combination with a flat MWPC [4] can provide both optimal efficiency for 14.4 keV radiation and sufficient spatial resolution. First experiments with a special prototype chamber designed for protein crystallography with wavelengths below 1 .~ are described in the following sections.
2. Detector design The detector construction was carried out at CERN with a concept similar to two other counters [5,6]. As schematically shown in fig. 1 it consists of three successive chambers made from epoxy and fiberglass. A cylindrical entrance chamber is filled with helium in order to reduce the absorption of the 7-rays. The ~,-ray is absorbed in a spherical drift chamber between two spherical
© 1982 North-Holland
Iv. GAS C H A M B E R
BASED DETECTORS
D. Bade eta[. / Development of a multiwire proportional chamber
194
A
VD
f'
I
protein reflections for wavelengths below 1 ,~. The size of the entrance window limits the opening angle of the prototype to 18 ° . At both sides of the drift space the radial boundary conditions of the spherical electric field are matched by cylindrical electrodes which are connected to the drift potential by a resistive divider. Electrons generated in the drift space pass through the transfer space and enter perpendicularly into the flat MWPC (50 × 50 cm 2) consisting of three wire planes which are 6 mm apart. Goldplated tungsten wires with 100/Lm diameter and 1 mm separation are used for the rectangular cathode arrays and similar wires with 20 # m diameter and 2 mm separation form the anode plane. Subject to the opening angle of the chamber only the central area of 3 0 × 30 cm 2 of the MWPC is presently being used.
3. Readout and data processing
Fig. 1. Scheme of the M W P C with spherical drift chamber. "/-rays penetrate through a hostaphan window (1) into the entrance chamber containing He and cross the spherical entrance electrode (2) of the drift chamber filled with counter gas. After photoelectric absorption the electrons drift through the spherical electric field maintained by the drift voltage VD ( - 18 kV) and through the exit electrode (3) and the transfer space (4) into the flat M W P C (5). They cause an avalanche at the anode A ( + 3.8 kV). The position is determined at the cathodes (Kll and K ± ) .
electrodes. The crystal under study is placed at the focus of the spheres. All scattered "},-rays will then pass radially through the drift chamber. The electrons produced by absorption of a "/-quantum in the counter gas likewise drift radially. This way, the spatial resolution does no longer depend on the absorption position of a `/-quantum and no parallax problem occurs. The described detector has a spherical entrance electrode of 40 /~m A1. The spherical exit electrode consists of a stainlesssteel grid. The thickness of the drift chamber is 10 cm.
In contrast to the other detectors mentioned above, the spherical drift chamber of our detector was designed for a radius of 90 cm of the entrance electrode in order to resolve the closely spaced
The readout of the MWPC is performed by the analog center-of-gravity readout method [7]. The principle is schematically shown in fig. 2. An avalanche at a single anode wire induces positive pulse distributions on the two rectangular cathode wire planes. The coordinates X and Y of the avalanche can then be determined from the center of gravity which is obtained from a weighted summation of the analog pulses by a resistor network. Diffusion of the primary ionization along the drift path can expand the avalanche over more than one anode wire. Then the analog center-of-gravity method in connection with the spherical drift chamber allows to overcome the spatial quantizing
/
v~ v2 v31 v~.
V5
VI
5 'Y~.
L Fig. 2. Scheme of the analog center-of-gravity readout. An avalanche at the anode (A) gives rise to pulse distributions at the cathodes (Kii and K ± ). The position (X, Y) is obtained by analog summation and division.
D. Bade et al. / Development of a multiwire proportional chamber
Fig. 3. On-line coupling of the detector to a PDP 1 !/40. The 14.4 keV "y-radiation from the 57Co source (1) on the M0ssbauer drive (2) is scattered by the protein crystal (3) into the detector (4). The M0ssbauer drive is synchronized by a general device interface (GDI).
195
which is sufficient for Mrssbauer experiments. It should be mentioned, however, that this limitation comes purely from the readout system and is not imposed by the detector itself. The readout can be speeded up by fast AD-conversion units and transfer to a large (512K × 16 bits) separate semiconductor memory working with microsecond cycle time. In that way the maximum counting rate can be 5 X l05 counts/s. All interrupt service and data processing routines run under a RT-11 operating system and are written in Assembler for high efficiency and speed. For fast information on the crystal orientation the whole detector area is divided into 96 × 96 pixels and the data are stored in the computer core memory. In addition to this possibility the interrupt service routine of the two AR-1 l's is programmed with enough flexibility to restrict the readout to smaller parts of the detector area with a corresponding decrease in pixel size. The smallest pixel size (0.4 X 0.4 mm 2) is then given by the accuracy of the ADC.
4. Detector operation and results effect of the anode wires, too. For good linearity of the position determination over the whole detector careful and iterative adjustment of the resistor network was found to be essential. In practice this work was done using trimmable helipot resitors in addition to resistors with fixed values. The negative pulses of the anode wires were used for energy discrimination which is necessary to achieve a low background. The data processing is performed with commercial elements as shown in fig. 3. Whenever an anode puls e appears in the proper energy window and is discriminated by the single channel analyzer, the coordinates X and Y are determined from the cathode signals and become available as analog levels. They can be read into the core memory of the PDP 11//40 computer after analog-to-digital conversion with commercial AR-11 interfaces, which can be started externally. Further data reduction, graphic display or transfer to magnetic tape is performed with special programs residing on a floppy disc. A Mrssbauer drive can be synchronized to the readout by a general device interface. In this configuration the maximum counting rate of the detector is limited by the AD-conversion time of the AR-11 units to 30000 counts/s,
The detector can be operated with a continuous flow of 80% Ar and 20% C 2 H 6. This is however not satisf.ying for measurements with wavelenghts below 1 A because the photoelectric absorption amounts only to 26% for 14.4 keV quanta in the 10 cm path within the drift space. A second problem arises from the practical range of the photoelectrons. At low energy (~<20 keV) photoelectrons are ejected preferentially perpendicular to the direction of the absorbed T-quanta [8]. This effect can spoil the spatial resolution substantially, if the range of the photoelectrons is large. According to ref. 9 the practical range of a 12 keV photoelectron was calculated to be larger than 2 mm in 80% Ar + 20% C2H 6. This effect can be reduced by the addition of Xe to the counter gas. The high cost and large volume of the detector makes it necessary to circulate and purify the counter gas. In the present assembly the detector chamber cannot be evacuated and therefore helium leak testing cannot be done effectively. The purification system must efficiently remove oxygen and water vapour from the counter gas. For this purpose we have applied a gas cleaning and purification syrtem similar to that in ref. 5. Oxygen and water vapour are IV. GAS CHAMBER BASED DETECTORS
196
D. Bade et al. / Development of a multiwire proportional chamber
removed by a copper catalyst (BASF R3-11) and hot calcium, respectively. The circulation of about 100 cm3/min is effected by a small oil-free rotation pump. Continuous operation up to periods of two weeks was obtained with a gas mixture of 24% Xe, 35% Ar, 35% C 2 H 6 and 6% CO 2. This gives 63% efficiency for 14.4 keV ,/-rays in the drift space. Better efficiency was obtained once with 41% Xe, 27% Ar, 27% C 2 H 6 and 5% CO 2. However, the stability of the working point was still not sufficient at these conditions. The spatial resolution of the prototype detector was tested with an oscillating myoglobin crystal and 17 keV MoK~ radiation (X = 0.71 ,~). The profile of the 603 reflection of myoglobin is given in fig. 4 for three different gas mixtures. From comparison with the reflection size on an X-ray film a width of 4 mm is expected because of the crystal dimensions. It can be clearly seen that the counter produces a small additional broadening, particularly when no Xe is added to the counter gas. This effect can be attributed to the practical range of the photoelectrons as discussed above. It should be mentioned, however,
1500 1000
WHM=7Omm
500
6000 p
3000~,~
~FWNM= 7.0m ,i
, 20
-
i 40
[mm] X
Fig. 5. Profiles of the adjacent reflections 820, 720, 620, 520 and 420 of the a* b* plane of myoglobin.
that this broadening can be tolerated in protein crystallography. Moreover, a further increase of the Xe content of the counter gas would make the broadening of the reflections by the counter negligible. Fig. 5 gives the profiles of some adjacent reflections of the a * b * plane of myoglobin. Strong and weak neighbouring reflections are well resolved. The well-known intensities of these reflections are accurately reproduced. The background of the detector was found to be 0.003 counts/(min × mm 2) for the gas mixture with 24% Xe. This value is low enough for M6ssbauer experiments. This work was supported by the Bundesministerium for Forschung und Technologie.
References
5000 ii_ FWHM=5,m5m
t s FwTM.117mm . , 2'0-% Fig. 4. Profile of the 60.] reflection of myoglobin for different gas mixtures; pixel size=0.8 ram. Top: 46% Ar+46% C2H 6 + 8% CO 2. Center:124% Xe+35% Ar+35% C2H6+6% CO 2. Bottom: 41% Xe+27% Ar+27% C2H 6 +5% CO 2.
[1] F. Parak, R.L. M6ssbauer and W. Hoppe, Ber. Bunsenges. Phys. Chemie 74 (1970) 1207. [2] F. Parak, R.L. MOssbauer, W. Hoppe, U.F. Thomanek and D. Bade, J. Phys. (Paris) 37, C6 (1976) 703. [3] U. Biebl and F. Parak, Nucl. Instr. and Meth. 112 (1973) 455. [4] G. Charpak, Z. Hajduk, A. Jeavons, R. Stubbs and R. Kahn, Nucl. Instr. and Meth. 122 (1974) 307. [5] R. Kahn, R. Fourme, B. Caudron, R. Bosshard, R. Benoit, R. Bouclier, G. Charpak, J.C. Santiard and F. Sauli, Nucl. Instr. and Meth. 172 (1980) 337. [6] C. Bolon, M. Deutsch, R. Lanza, G. Quigley and A. Rich, IEEE Trans. Nucl. Sci. NS-26 (1979) 146. [7] G. Charpak, A. Jeavons, F. Sauli and R. Stubbs, CERN Report No. 73-11 (1973). [8] E.J. Williams, J.M. Nuttal and J.M. Barlow, Proc. Roy. Soc. (London) AI21 (1928) 611. [9] R.O. Lane and D.J. Zaffarano, Phys. Rev. 94 (1954) 960.
193
DEVELOPMENT OF A MULTIWlRE PROPORTIONAL CHAMBER AS AN AREA SENSITIVE DETECTOR FOR X-RAY PROTEIN CRYSTALLOGRAPHY D. BADE, F. PA'RAK, R.L. MC)SSBAUER, W. HOPPE, N. LEVAI and G. C H A R P A K * Max-Planck-lnstitut fiir Biochemie, Abteilung fiir Strukturforschung I, Am Klopferspitz, D-8033 Martinsried, Fed. Rep. Germany and Physikdepartment E15 der Technischen Universittit Mi~nchen, James-Franck-Strasse, D-8046 Garching, Fed. Rep. Germany
The prototype of an area sensitive multiwire proportional chamber operated in series with a spherical drift chamber was investigated for its applicability to protein crystallography, employing 14.4 keV (h =0.86 ,~) "t-radiation of STCo MSssbauer sources. The 10 cm long spherical drift field, in this case, provides high efficiency in order to compensate for the small primary intensity of M6ssbauer sources, The detector system was tested with different gas mixtures. The improvement of the spatial resolution with increasing xenon content is demonstrated using the 603 reflection of a myoglobin crystal. The addition of xenon to the counter gas is shown to be necessary for protein crystallography with short wavelengths. The background is sufficiently low for M6ssbauer experiment~, due to good energy discrimination at the anode (~0.003 counts/(min × mm 2)).
1. Introduction The MiSssbauer scattering on a 57Fe nucleus offers a new possibility for an experimental solution of the phase problem arising in structure determination of protein crystals [1]. The applicability of this method was demonstrated in the case of a myoglobin single crystal [2]. Such experiments have to be performed with STCo gamma sources which yield a much lower intensity of the primary beam than X-ray tubes. In order to get reasonable times for the collection of the intensities of all reflections which are necessary for a structure determination, one has to work with a slightly divergent beam. This way, one excites simultaneously a large number of Bragg reflections. This procedure requires a large area sensitive detector. Because of the low intensity scattered into each Bragg reflection (typically 1 count/min) one has to provide for an optimum efficiency of the counter. For the same reason the background has to be as low as possible in an energy window around 14.4 keV (~ = 0.86 A). The spatial resolution must be sufficient to allow for a separation of closely spaced protein crystal reflections. Optimal efficiency together with low background can be achieved by using small Si(Li) detectors for single reflections [2]. However, the * CERN, Geneva, Switzerland.
0167-5087/82/0000-0000/$02.75
extension of this concept to larger Si(Li) counters for many reflections [3] involves the adaption of special detector assemblies to each reciprocal lattice geometry. Flat multiwire proportional chambers (MWPC) might appear to be adequate for the application in protein crystallography, but for usual anode-cathode distances the achievable efficiency is rather low at 14.4 keV. An increase of the thickness of the counter improves the efficiency, but at the same time the parallax problem reduces the spatial resolution at large scattering angles. The spherical drift chamber in combination with a flat MWPC [4] can provide both optimal efficiency for 14.4 keV radiation and sufficient spatial resolution. First experiments with a special prototype chamber designed for protein crystallography with wavelengths below 1 .~ are described in the following sections.
2. Detector design The detector construction was carried out at CERN with a concept similar to two other counters [5,6]. As schematically shown in fig. 1 it consists of three successive chambers made from epoxy and fiberglass. A cylindrical entrance chamber is filled with helium in order to reduce the absorption of the 7-rays. The ~,-ray is absorbed in a spherical drift chamber between two spherical
© 1982 North-Holland
Iv. GAS C H A M B E R
BASED DETECTORS
D. Bade eta[. / Development of a multiwire proportional chamber
194
A
VD
f'
I
protein reflections for wavelengths below 1 ,~. The size of the entrance window limits the opening angle of the prototype to 18 ° . At both sides of the drift space the radial boundary conditions of the spherical electric field are matched by cylindrical electrodes which are connected to the drift potential by a resistive divider. Electrons generated in the drift space pass through the transfer space and enter perpendicularly into the flat MWPC (50 × 50 cm 2) consisting of three wire planes which are 6 mm apart. Goldplated tungsten wires with 100/Lm diameter and 1 mm separation are used for the rectangular cathode arrays and similar wires with 20 # m diameter and 2 mm separation form the anode plane. Subject to the opening angle of the chamber only the central area of 3 0 × 30 cm 2 of the MWPC is presently being used.
3. Readout and data processing
Fig. 1. Scheme of the M W P C with spherical drift chamber. "/-rays penetrate through a hostaphan window (1) into the entrance chamber containing He and cross the spherical entrance electrode (2) of the drift chamber filled with counter gas. After photoelectric absorption the electrons drift through the spherical electric field maintained by the drift voltage VD ( - 18 kV) and through the exit electrode (3) and the transfer space (4) into the flat M W P C (5). They cause an avalanche at the anode A ( + 3.8 kV). The position is determined at the cathodes (Kll and K ± ) .
electrodes. The crystal under study is placed at the focus of the spheres. All scattered "},-rays will then pass radially through the drift chamber. The electrons produced by absorption of a "/-quantum in the counter gas likewise drift radially. This way, the spatial resolution does no longer depend on the absorption position of a `/-quantum and no parallax problem occurs. The described detector has a spherical entrance electrode of 40 /~m A1. The spherical exit electrode consists of a stainlesssteel grid. The thickness of the drift chamber is 10 cm.
In contrast to the other detectors mentioned above, the spherical drift chamber of our detector was designed for a radius of 90 cm of the entrance electrode in order to resolve the closely spaced
The readout of the MWPC is performed by the analog center-of-gravity readout method [7]. The principle is schematically shown in fig. 2. An avalanche at a single anode wire induces positive pulse distributions on the two rectangular cathode wire planes. The coordinates X and Y of the avalanche can then be determined from the center of gravity which is obtained from a weighted summation of the analog pulses by a resistor network. Diffusion of the primary ionization along the drift path can expand the avalanche over more than one anode wire. Then the analog center-of-gravity method in connection with the spherical drift chamber allows to overcome the spatial quantizing
/
v~ v2 v31 v~.
V5
VI
5 'Y~.
L Fig. 2. Scheme of the analog center-of-gravity readout. An avalanche at the anode (A) gives rise to pulse distributions at the cathodes (Kii and K ± ). The position (X, Y) is obtained by analog summation and division.
D. Bade et al. / Development of a multiwire proportional chamber
Fig. 3. On-line coupling of the detector to a PDP 1 !/40. The 14.4 keV "y-radiation from the 57Co source (1) on the M0ssbauer drive (2) is scattered by the protein crystal (3) into the detector (4). The M0ssbauer drive is synchronized by a general device interface (GDI).
195
which is sufficient for Mrssbauer experiments. It should be mentioned, however, that this limitation comes purely from the readout system and is not imposed by the detector itself. The readout can be speeded up by fast AD-conversion units and transfer to a large (512K × 16 bits) separate semiconductor memory working with microsecond cycle time. In that way the maximum counting rate can be 5 X l05 counts/s. All interrupt service and data processing routines run under a RT-11 operating system and are written in Assembler for high efficiency and speed. For fast information on the crystal orientation the whole detector area is divided into 96 × 96 pixels and the data are stored in the computer core memory. In addition to this possibility the interrupt service routine of the two AR-1 l's is programmed with enough flexibility to restrict the readout to smaller parts of the detector area with a corresponding decrease in pixel size. The smallest pixel size (0.4 X 0.4 mm 2) is then given by the accuracy of the ADC.
4. Detector operation and results effect of the anode wires, too. For good linearity of the position determination over the whole detector careful and iterative adjustment of the resistor network was found to be essential. In practice this work was done using trimmable helipot resitors in addition to resistors with fixed values. The negative pulses of the anode wires were used for energy discrimination which is necessary to achieve a low background. The data processing is performed with commercial elements as shown in fig. 3. Whenever an anode puls e appears in the proper energy window and is discriminated by the single channel analyzer, the coordinates X and Y are determined from the cathode signals and become available as analog levels. They can be read into the core memory of the PDP 11//40 computer after analog-to-digital conversion with commercial AR-11 interfaces, which can be started externally. Further data reduction, graphic display or transfer to magnetic tape is performed with special programs residing on a floppy disc. A Mrssbauer drive can be synchronized to the readout by a general device interface. In this configuration the maximum counting rate of the detector is limited by the AD-conversion time of the AR-11 units to 30000 counts/s,
The detector can be operated with a continuous flow of 80% Ar and 20% C 2 H 6. This is however not satisf.ying for measurements with wavelenghts below 1 A because the photoelectric absorption amounts only to 26% for 14.4 keV quanta in the 10 cm path within the drift space. A second problem arises from the practical range of the photoelectrons. At low energy (~<20 keV) photoelectrons are ejected preferentially perpendicular to the direction of the absorbed T-quanta [8]. This effect can spoil the spatial resolution substantially, if the range of the photoelectrons is large. According to ref. 9 the practical range of a 12 keV photoelectron was calculated to be larger than 2 mm in 80% Ar + 20% C2H 6. This effect can be reduced by the addition of Xe to the counter gas. The high cost and large volume of the detector makes it necessary to circulate and purify the counter gas. In the present assembly the detector chamber cannot be evacuated and therefore helium leak testing cannot be done effectively. The purification system must efficiently remove oxygen and water vapour from the counter gas. For this purpose we have applied a gas cleaning and purification syrtem similar to that in ref. 5. Oxygen and water vapour are IV. GAS CHAMBER BASED DETECTORS
196
D. Bade et al. / Development of a multiwire proportional chamber
removed by a copper catalyst (BASF R3-11) and hot calcium, respectively. The circulation of about 100 cm3/min is effected by a small oil-free rotation pump. Continuous operation up to periods of two weeks was obtained with a gas mixture of 24% Xe, 35% Ar, 35% C 2 H 6 and 6% CO 2. This gives 63% efficiency for 14.4 keV ,/-rays in the drift space. Better efficiency was obtained once with 41% Xe, 27% Ar, 27% C 2 H 6 and 5% CO 2. However, the stability of the working point was still not sufficient at these conditions. The spatial resolution of the prototype detector was tested with an oscillating myoglobin crystal and 17 keV MoK~ radiation (X = 0.71 ,~). The profile of the 603 reflection of myoglobin is given in fig. 4 for three different gas mixtures. From comparison with the reflection size on an X-ray film a width of 4 mm is expected because of the crystal dimensions. It can be clearly seen that the counter produces a small additional broadening, particularly when no Xe is added to the counter gas. This effect can be attributed to the practical range of the photoelectrons as discussed above. It should be mentioned, however,
1500 1000
WHM=7Omm
500
6000 p
3000~,~
~FWNM= 7.0m ,i
, 20
-
i 40
[mm] X
Fig. 5. Profiles of the adjacent reflections 820, 720, 620, 520 and 420 of the a* b* plane of myoglobin.
that this broadening can be tolerated in protein crystallography. Moreover, a further increase of the Xe content of the counter gas would make the broadening of the reflections by the counter negligible. Fig. 5 gives the profiles of some adjacent reflections of the a * b * plane of myoglobin. Strong and weak neighbouring reflections are well resolved. The well-known intensities of these reflections are accurately reproduced. The background of the detector was found to be 0.003 counts/(min × mm 2) for the gas mixture with 24% Xe. This value is low enough for M6ssbauer experiments. This work was supported by the Bundesministerium for Forschung und Technologie.
References
5000 ii_ FWHM=5,m5m
t s FwTM.117mm . , 2'0-% Fig. 4. Profile of the 60.] reflection of myoglobin for different gas mixtures; pixel size=0.8 ram. Top: 46% Ar+46% C2H 6 + 8% CO 2. Center:124% Xe+35% Ar+35% C2H6+6% CO 2. Bottom: 41% Xe+27% Ar+27% C2H 6 +5% CO 2.
[1] F. Parak, R.L. M6ssbauer and W. Hoppe, Ber. Bunsenges. Phys. Chemie 74 (1970) 1207. [2] F. Parak, R.L. MOssbauer, W. Hoppe, U.F. Thomanek and D. Bade, J. Phys. (Paris) 37, C6 (1976) 703. [3] U. Biebl and F. Parak, Nucl. Instr. and Meth. 112 (1973) 455. [4] G. Charpak, Z. Hajduk, A. Jeavons, R. Stubbs and R. Kahn, Nucl. Instr. and Meth. 122 (1974) 307. [5] R. Kahn, R. Fourme, B. Caudron, R. Bosshard, R. Benoit, R. Bouclier, G. Charpak, J.C. Santiard and F. Sauli, Nucl. Instr. and Meth. 172 (1980) 337. [6] C. Bolon, M. Deutsch, R. Lanza, G. Quigley and A. Rich, IEEE Trans. Nucl. Sci. NS-26 (1979) 146. [7] G. Charpak, A. Jeavons, F. Sauli and R. Stubbs, CERN Report No. 73-11 (1973). [8] E.J. Williams, J.M. Nuttal and J.M. Barlow, Proc. Roy. Soc. (London) AI21 (1928) 611. [9] R.O. Lane and D.J. Zaffarano, Phys. Rev. 94 (1954) 960.