Nuclear Instruments and Methods in Physics Research A265 (1988) 457-460 North-Holland, Amsterdam
A STUDY OF NONFLAMMABLE GAS MIXTURES FOR LIMITED STREAMER TUBES IN THE VENUS DETECTOR AT TRISTAN
T. U E B A Y A S H I , J. H A B A , T. K A M I T A N I , N. K A N E M A T S U , Y. N A G A S H I M A , H. OSABE, S. S A K A M O T O , S. S U G I M O T O , Y. S U Z U K I , A. T S U K A M O T O a n d Y. Y A M A S H I T A
Department of Physics, Osaka University, Toyonaka, Osaka, Japan T. S U M I Y O S H I a n d F. T A K A S A K I
National Laboratory for High Energy Physics, KEK, Ibaraki-ken, Japan Y. H O M M A
School of Allied Medical Sciences, Kobe University, Kobe, Japan Y. H O J Y O a n d H. S A K A E
Graduate School of Science and Technology, Kobe University, Kobe, Japan Received 27 July 1987 and in revised form 16 October 1987
Performances of limited streamer tubes operated with nonflammable gas mixtures composed of argon (Ar), carbon-dioxide (CO2) and isobutane (i-C4H10) with ratios of 1:1.5:0.15, 1:2:0.2, 1:3:0.3, 1:4:0.4 and 1:6:0.6 were studied. It was found that these mixtures were very suitable for the VENUS detector. The charge spectra exhibited very sharp distributions and indicated the possibility for the limited streamer tubes to be applied to calorimetry use. A mixture of 1 : 2 : 0.2 was chosen for the VENUS streamer tubes because of its moderate gain and long efficiency plateau.
2. Selection of gas mixture
Lead glass blocks of 5160 subunits and 1200 streamer tubes are used for the VENUS detector at TRISTAN as a barrel electromagnetic calorimeter. The streamer tubes are placed in front of the lead glass blocks and used for the measurement of the point of incidence of charged particles and converted gammas . Because the VENUS detector is located in an experimental area almost 10 m underground, the danger of explosion and fires must be seriously taken into account. Therefore, the use of nonflammable gas mixtures in gaseous detectors and hermeticity were required. But the gas mixtures previously tested and reported  for the streamer tubes were highly flammable because of their large mixing ratio of explosive quenching gases such as ethane, isobutane, n-pentane etc. We studied nonflammable mixtures composed of argon (Ar), carbon-dioxide (CO2) and isobutane (i-C4H10) in the streamer mode.
In general, most of the "good" quenching gases are hydrocarbons, but they are always very explosive. CO2 can also be used as a nonflammable quencher, but its ability as a quencher is weaker than those of most hydrocarbon gases. When CO2 alone is used as a quencher, some tubes do not work well in the streamer mode depending on their dimensions or the thickness of the wires. Under these circumstances, we have studied several kinds of mixed gases of Ar and CO 2 with small amounts of isobutane whose mixing ratio was determined in order to retain the nonflammability of the mixed gas. The flammability of a certain mixed gas is characterized in terms of its volume concentration in air (known as explosion limit) and is a function of the mixing ratio of the flammable component . Fig. 1 shows the explosion limit curve for a mixture of Ar and CO: (1 : 2) as a function of the additional mixing ratio
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T. Uebayashi et al. / Study of nonflammable gas mixtures
Table ] Summary of gas mixtures tested in the present study
30 ~:i:i:i:i:ii::E:i:i:i!ii ii _iiiiiiiiiiiii E iiiii:i: :i:i:: i : i ~
....... :....... g~::~::~i~::~:
1 2 3 4 5
1 : 1.5 : 0.15 1 : 2 : 0.2 1 : 3 : 0.3 1:4:0.4 1:6:0.6
37.7 31.2 23.2 18.5 13.2
56.6 62.5 69.8 74.1 78.9
5.7 6.3 7 7.4 7.9
lower J limit
A r / C O 2/isobutane
I 10 Isobutone mixing rotio (%)
Fig. 1. A plot of the explosion limit, which is expressed by the volume concentration of the mixed gas in air, for the case of argon+carbon-dioxide(l:2)+ isobutane as a function of the isobutane mixing ratio. The hatched area indicates the explosive condition in air, and the dotted area indicates a nonflammable mixture of argon + carbon-dioxide (1 : 2) + isobutane.
of isobutane. The right hand side of the curve (hatched region) satisfies the explosive condition. For example, the mixed gas of A r / C O 2 (1 : 2) and isobutane (12%) is indicated by a vertical line. It is explosive in air where this line crosses the hatched area. If the mixing ratio of isobutane is chosen to be less than the left edge of the curve, the mixed gas never explodes under any conditions (nonflammable). Fig. 2 shows these flammability limits in a plot of the ratio of C O 2 to Ar and the ratio of isobutane to Ar. The upper region of the curve means
x : mixture studied
a nonflammable gas mixture. We have chosen five mixtures as indicated in the figure and studied them. The compositions of these mixtures are summarized in table 1.
3. Test measurements
3.1. Tube geometry and test setup The tubes used in this study were identical with those in the V E N U S detector except for their longitudinal length. The tube is extruded polystyrene of 1 m m thickness, to which carbon powders were loaded, The cross section of the tube had inner dimensions of 11 m m height and 17 m m width. The inner surface was coated by a silicon grease (KF96, a product of Shinetsu Silicon Co., Japan) in order to suppress spurious discharges. A n o d e wires were gold-plated t u n g s t e n / R e (3%) wires with 6 0 / ~ m diameter . The mixed gas was continuously flowed in the tubes and the mixing ratio of the gas was determined by the volume flow controller which had been previously calibrated by a precise film flow meter (SF101 manufactured by STEC Inc., Kyoto, Japan)
3.2. Anode signals and high voltage plateau
._o 4 '~
Ratio of isobut(]ne to Ar Fig. 2. Flammability limit of a gas mixture of Ar+ C O 2 + isobutane. The mixture is parametrised by two variables, the ratio of CO2 to Ar (Y-axis) and the ratio of isobutane to Ar (X-axis). The mixture in the lower hatched area can explode under some conditions but the mixture in the upper region will never explode in any condition. The mixtures studied here are indicated in this figure by × with the numbers "1" through "5".
Fig. 3 shows a typical example of the pulse shape on a 50 I2 load. The picture was taken with the gas mixture # 3 irradiated by a 55Fe X-rays source. We observed the rise time of the signal to be less than 10 ns, and a fall time of about 30 ns. Though the pulse height depended on both applied high voltage and gas mixture, the pulse shapes themselves were similar for all five gas mixtures. The high voltage and gas mixture dependence of the single counts for 5SFe X-rays were measured. A n o d e signals were fed into a leading edge discriminator of standard N I M module with a threshold of 30 inV. They were counted with a dead time of 1 #s, which eliminated the spurious counting caused by afterpulses. The high voltage was raised till the leakage currents reached 1 /~A. The results are shown in fig. 4. We observed that the lower the concentration of Ar, the higher the voltage
T. Uebayashi et aL / Study of nonflammable gas mixtures
Fig. 3. Pulse shape from a 55Fe X-ray source for the gas mixture # 3 with hv = 3.8 kV (30 mV/div., 20 ns/div.).
to reach the full efficiency, as expected. Though the plateau of stable operation became wider when the mixing ratio of Ar is lowered, all gas mixtures 1 - 5 showed long enough plateaus to be used as practical detector gases. On the other hand, our previous study showed that any nonflammable mixtures composed of Ar + CO2, Ar + CO 2 + methanol or Ar + CO 2 + ethane had no practically stable plateaus with our tube dimensions and wire thickness.
3.3. Charge spectra Charge spectra of anode signals were measured by C A M A C A D C (LRS 2249W) with the 55Fe X-ray source. The gate signal had a width of 1 /xs and was made by the anode signal itself with a threshold of 5 mV. Typical charge spectra thus obtained for the mixture # 2 at several high voltage points are shown in fig. 5. As seen in this figure, the streamer mode appears at
. ~ .
Fig. 5. Charge spectra of signals at different high voltages for mixture #2. The horizontal scale of the three plots on the right hand side is different from that on the left hand side. The transition to the streamer mode and the appearance of the double streamer mode is dearly seen.
2.9 kV and the transition is completed at about 3.3 kV. Above 3.7 kV the double streamer mode appears and becomes d o m i n a n t when the high voltage exceeds 4.0 kV. Fig. 6 shows a plot of the charge at the peaks corresponding to the proportional, single streamer and the double streamer mode for two gas mixtures. Though the charges corresponding to the single streamer depend on both the gas mixtures and applied voltage as shown in fig. 6, the evolution of the charge spectrum with increasing high voltage was similar for all mixtures studied. To make this similarity clearer, we defined the
500 1000 400
~" 300 8
~2 0 appearance of o 0 o double streamer--,- o o
o /2 U ,
/ transition to the • • J• streamermode
3.5 4.5 High voltage (kV} Fig. 4. Single counts vs high voltage for five mixtures. The threshold was set to 30 mV on a 50 $2 load. Counting rates at the plateau depend on the mixing ratios of Ar which determine the effective absorption length of X-rays.
High voltage (kV) Fig. 6. High voltage dependence of peak charges corresponding to proportional, single streamer and double streamer mode for the mixtures # 2 and # 5.
7~ Uebayashiet al. / Study of nonflammable gas mixtures
~ ~ _ . ~ 1 , ~ 0 2.75
flammability which showed enough uniformity of pulse height , but they could not find a suitable gas mixture with low flammability. Our present results of the pulse height distribution was better by a factor of 3 than even their results with flammable gas mixtures containing heavy hydrocarbon gases. In this respect, the mixtures studied here are very suitable for calorimetry use and are nonflammable. Of course, the resolution of the sampling calorimeter with streamer mode is also determined by many other factors such as multitrack separation. Further studies are needed for the application to calorimetry.
I i I i 3.00 .5.25
High voltoge (kV)
Fig. 7. High voltage dependence of AQ/(Q> for the mixtures #2, #3 and # 5. The curves are very similar to each other except for their horizontal positions. This indicates that, for all mixtures studied, there are no significant differences in the evolutions of charge spectra. value AQ/(Q), where AQ means the minimum width which contains 68% of total entries of the spectrum shown in fig. 5 and ( Q ) the mean charge of the entries inside AQ. Fig. 7 shows how AQ/(Q) depends on the high voltage. AQ/(Q) decreases drastically when the transition to the streamer mode begins and reaches a minimum when the transition is completed. By raising the high voltage further, the double streamer mode appears and AQ/(Q) increases again till the double streamer mode exceeds the single one. For still higher voltages, AQ/(Q) decreases again because the double streamer mode becomes dominant. This behavior is similar in all gas mixtures except for the starting voltage where the transition begins. At the minimum of AQ/(Q) the standard deviation o of the charge distribution ( o - AQ/2
10%). Recently, an application of the streamer tube to sampling calorimetry was discussed . One of the key points to improve the energy resolution of such a calorimeter is the uniformity of the pulse height for a single track in sampling devices. In this respect, the streamer mode is superior to the proportional one. Some groups were seeking for a gas mixture with low
4. Conclusion We studied streamer tube operation with nonflammable mixed gases and found that a mixture composed of Ar, CO 2 and a small amount of isobutane was very suitable for the streamer tubes in the VENUS detector. We determined the mixing ratio of Ar, CO 2 and isobutane to be 1 : 2 : 0.2, though all mixtures studied here were applicable. The charge spectra obtained with these mixtures shows a deviation of the distribution (of
Acknowledgements We would like to thank The Safety Group headed by Prof. K. Morimoto at K E K for their sincere advice. We also thank Mr. H. Kaneko at Osaka University and Mr. K. Hayashi at KEK for their excellent technical support.
References  Y. Arai et al. TRISTAN-EXPOO1 (1983).  G. Battistoni et al., Nucl. Instr. and Meth. 164 (1979) 57; G.D. Alekseev et al., Nucl. Instr. and Meth. 177 (1980) 385; M.A. Atac, A.V. Tollestrup and D. Potter, Nucl. Instr. and Meth. 200 (1982) 345; E. Iarocci, Nucl. Instr. and Meth. 217 (1983) 30.  B. Lewis and G. von Elbe, Combustion, Flame and Explosions of Gases (Academic Press, New York, 1961).  The design and construction of the barrel streamer tube in the VENUS detector will be described elsewhere.  P. Campana, Nucl. Instr. and Meth. 225 (1984) 505.  P. Rapp, Nucl. Instr. and Meth. A244 (1985) 430.