- Email: [email protected]
Automated monitoring and calibrating system of gas gain and electron drift velocity
Nuclear Instruments and Methods North-Holland, Amsterdam
in Physics
Research
A245 (1986) 35-44
AUTOMATED MONITORING AND CALIBRATING DRIFT VELOCITY Prototype system and accumulation of reference data
35
SYSTEM OF GAS GAIN AND ELECTRON
K. FUJII, J. FUJIMOTO, H. HAYASHII, R. KAJIKAWA, Y. MASATANI, H. OZAKI, A. SUGIYAMA, S. SUZUKI, T. TAKAHASHI, T.F. TSUKAMOTO, T.Y. TSUKAMOTO, M. UEDA and S. UN0 Depurfment
Received
of Physrcs, Nagoya
University,
Chikusu-ku,
Nqqw
1 July 1985 and in revised form 25 November
464, Japan
1985
A prototype of an automated monitoring and calibrating system of gas gain and electron drift velocity for the end cap detectors in TOPAZ [l] is constructed. Systematic data under various conditions of gas mixture, temperature, pressure and applied high voltage are accumulated using this system. They will be used as reference for a detailed calibration.
1. Introduction Today,
various
kinds
of particle
detectors
based
monitor the variation of the parameters which affect the detector output, and to correct systematic shifts by a premeasured reference at various conditions. At the same time, one can also measure the output of the reference detectors which are set under the same environmental conditions and which are irradiated by standard radioactive sources to cancel out the systematic error and the time dependent shift. In this paper, we describe a prototype of an automated monitoring and calibration system which will be used for the electromagnetic calorimeter [2] and drift chamber [3] in the end cap region of the TOPAZ detector at TRISTAN [l]. We also present the results of systematic measurements of gas gain and electron drift
on
have been developed and utilized in high energy physics experiments. One of the most serious practical problems of this kind of detector is that the fluctuation of environmental conditions (gas temperature, pressure, mixture and purity of each gas component, etc.) affects the detector output and deteriorates the resolution. Considering the long duration of recent experiments, it is not easy to keep the environmental parameters stable enough to get the designed performance. One solution to this problem is to continuously the gas amplification
method
Proportional 55Fe
Tube
Drift
Chamber QoSr
c perature
Pressure Sensor
C,HsOH
out
Mass
Flow
Pressure
Controlled Bubbler
HPDP~
Fig. 1. Schematic
diagram
II23
of the gas gain and drift time monitoring
0168-9002/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
system.
K. Fujii et al. /Automated
36
for various gas mixtures and operational conditions. These will be used as premeasured reference data for calibration and correction.
velocity
monitoring ofgasgain 2.0
1.5
2. Monitoring system 1.0
2.1. Outline of the system The system is composed of a drift chamber specialized for drift time measurements, a reference proportional counter for gain monitoring, temperature and pressure sensors, a mass flow sensor/controller, a programmable high voltage power supply, and several interfacing modules (fig. 1). The drift chamber with uniform electric field in the drift region is irradiated by two collimated P-ray sources at a certain distance, and the drift time from each position is measured by a TDC. The gas gain is monitored by a single cell proportional counter irradiated by a 55Fe source whose pulse height is measured by an ADC. Monitoring and controlling of the high voltage, gas pressure, gas flow and temperature are done by a programmable high voltage power supply, a scanning ADC and a DAC module. All digitization and control is done via CAMAC and a serial interface module linked to the minicomputer PPDP-11/23. The applied high voltage is varied periodically to obtain full scan data on the field strength dependence. This is useful, in particular, for correcting the time dependence of the drift chamber whose inhomogeneous field gradient has a wide range. When this system is applied to the TOPAZ detector, the reference proportional counter and drift chamber will be placed at several places in the gas glow line. Data taken by the FPDP-11 will be transferred to the main data acquisition computer. Also local processing and data logging will be done as a stand-alone mode.
-
0.5
0.0: 0
of a drift chamber
specialized
Anode
Box
(mm)
Fig. 3. Calculated electric field strength distribution in the chamber shown in fig. 2 for various conditions of the supplied high voltage. The field strength near the “anode box” lies between the solid and the dashed curve according to the different values of Vanode. Vanode has no effect on the field gradient in the drift region between 11 and 29 mm from the “anode box”.
2.2. Gain monitoring The reference proportional counter for gas gain monitoring is the same as that used in the TOPAZ end cap calorimeter. This is a single cell counter of 10 mm (height) x 15 mm (width) conductive plastic tube of 1 mm thick, with an anode wire of 50 pm diameter gold plated tungsten. The output pulse from collimated Xrays from a 55Fe source (5.9 keV) is amplified by a preamplifier (Canberra 2006, time constant 1 ps) and shaping amplifier (Oyo-Koken [4] 704-2, shaping time 0.4 ps), and is digitized by a CAMAC ADC (LRS 2249). The ADC is gated by the fast output from a
26
Unit
Fig. 2. Construction
from
30
20
10
Distance
for drift time measurements.
(mm)
See text for details.
K. Fujii et al. / Automated monitoring of gas gain
shaping amplifier. To cover a wide dynamic range, the output of the main amplifier is split into two. One goes directly into the ADC and the other is attenuated before put into the ADC. The overall response of the amplifier and ADC is linear within 1% in the measured range. 2.3.
Drift
velocity
monitoring
A special drift chamber with a uniform field gradient in the drift region is prepared to monitor the electron drift velocity (fig. 2). The field is shaped by two printed circuit boards placed 10 mm apart in parallel, which have shaping strips of 0.3 mm wide and 2.0 mm spacing. At the one end of the chamber, the multiplication and detection region (the “anode box”) consists of an anode wire of 20 pm diameter and a conductive plastic tube (10 mm (h) x 15 mm (w) X 110 mm (1) with a window of 5 mm (h) X 10 mm (1). A similar tube is placed at the other end to keep the electric field uniform at the edge of the chamber. This chamber has a 40 mm long drift region. The printed board has two slits (1 mm wide and 10 mm long) located at 11 mm and 29 mm from the “anode box” to make P-rays from a 90Sr source go through the chamber. Calculation shows the uniformity of the field gradient in the drift region to be within + 0.5% at the measured range of 0.2-1.7 kV/cm (fig. 3). Triggering is done by a coincidence signal from two plastic scintillation counters placed under the chamber. y-rays and very low momentum P-rays are rejected by this method. By requiring a coincidence of two 3.0 mm (0.5 g/cm2) thick counters the momentum of the P-rays must be above 1 MeV/c. Smearing by multiple scattering in the chamber is estimated to be 0.3 mm at the lower slit, which is negligibly small compared with the slit width. A TDC (LRS 4208) is started by the trigger pulse and stopped by the chamber output shaped by a
, 0
40-
2 c :
30-
L : z
20
-
5 8
lo-
ow’*h,B 0
1000
2000 Drift
aw.bluJuw’*l’ 3000 Time
31
discriminator amplifier (LRS DC201). Two peaks in the drift time distribution correspond to two source positions (fig. 4). We calculate the drift velocity by fitting the center of the two peaks assuming a Gaussian distribution. The statistical error is negligibly small in this method. Instead, the field nonuniformity in the drift region dominates the measurement error. The estimated mechanical error of the chamber construction (k50 pm) will cause an error of 1.0% in the field strength. The accuracy of slit positioning is about + 100 pm. In total, the overall error in the drift velocity is estimated to be 3%. 2.4.
Environment
monitoring
As a sensor of gas temperature, an active bridge circuit with heat sensitive resistor (LPTC25, Shinsei Denshi Co. [5]) is provided. Its measuring accuracy is +0.2”C in the range lo-3O“C. As a pressure gauge, we use a piezo semiconductor device (FPSlOA, Fujikura Densen Co. [6]) which can measure the absolute value with an accuracy of kO.5 gf/cm2. The gas flow is monitored and controlled by a mass flow controller (STEC Co. [7]) with an accuracy of +0.2 cm3/min. The flow rate is digitized and read into the computer by a scanning ADC (LRS LG8252), and computer-processed feedback to the mass flow controller is done by a DAC module (Kinetic 3112). The high voltage power supply (LRS HV4032) has a voltage regulation of f 1 V at 1V 1max= 7 kV (+ 0.5 V = 3.3 kV), and provides a field gradient up at IVI,, to 1.7 kV/cm in the drift chamber. It is hooked to the serial line of the PPDP-11 via a home-made interface module. 2.5.
System
controlling
software
The system software is constructed on a minicomputer ( PPDP-11/23) to control all devices mentioned above. Much attention has been paid to making the architecture compatible with the data acquisition system of the TOPAZ detector. The basic function of the gas-monitoring software should be to check the status of each device by collecting calibration data and its environmental condition, and deliver the result to the physics data acquisition. It should work in parallel with the high-speed event-by-event acquisition of physics data. To fulfil these requirements, the software system was constructed on a real time multitask monitor RSXll-M.
** 4000
5000
(nsec)
Fig. 4. Typical drift time distribution obtained by the chamber shown in fig. 2. The two peaks correspond to two source positions.
3. Measured results Using the system described in the previous section, systematic data on gas gain and electron drift velocity were accumulated.
K. Fujii et al. / Automated monitoring of gas gain
38
For the gas gain, various mixtures of Ar/CH,, Ar/C,H, and Ar/i-C,H,, were examined. PR-gas (Ar 90%, CH, 10%) was used to study the temperature and pressure dependence. For the drift velocity, measurements were done using Ar/CH,, Ar/C,H, and Ar/i-C,H,, under various pressures and field strengths. In the case of CzH,, a measurement was also done by mixing with C2H50H vapor, since in many cases C,H,OH is added to minimize ageing effects of chambers which are operated in the high gain mode and under a high flux of incoming particles.
3.1. Gas amplification The gas amplification under various conditions of applied high voltage and gas mixing ratios was measured for gas mixtures of Ar/CH,, Ar/C,H, and Ar/i-C,H,, (fig. 5). No simple correlation with molecular number exists. The quenching effect tends to appear stronger for a gas of larger molecules. Saturation phenomena at higher voltages are very evident by the quenching of larger molecules. A detailed study was carried out for Ar/CH, and Ar/CzHs, because these were the candidates for use in the TOPAZ end cap calorimeter. Figs. 6 and 7 show high voltage response curves for various Ar/C,H, and Ar/CH, mixtures. Fig. 8 shows the fitted results of these high-voltage curves by the empirical formula [LX]:
l
1
1.5
2
H.V.
(kV)
Fig. 6. Detailed high voltage dependence of the gas gain of the reference chamber on the Ar/C,H, mixture. Mixing ratios tested were 90/10, 80/20. 70/30, 60/40, 50/50, 44.4/55.6. 37.5/62.5. 28.6/71.4 and 16.7/83.3. l(
I
I
I
ArICH,
ArIG&-
2.5
H.V.(kV)
Fig. 5. High voltage dependence of the gas gain of the reference chamber irradiated by a 55Fe source. Tested gas mixtures are Ar/CH,, Ar/C,H, and Ar/i-C,H,, with mixing ratios of 90/10, 70/30 and 50/50 each.
L.”
H.V. (kV)
Fig. 7. Detailed high voltage dependence of the gas gain of the reference chamber on the Ar/CH, mixture. Mixing ratios tested were 90/10, 80/20, 70/30, 60/40, 50/50, 37.5/62.5, 28.6/71.4 and 16.7/83.3.
K. Fujri et al. / Au~~muted monirorrng of gas Ruin
25-
b)
x
39
ArICH,
.
8OllO
0
70130
55Fe ; f P
x 50150 d 28.6l71.4
1.0 -
Ar/CH,
x-
Ar/Q-&
l
0.8
‘.6[‘
’
2;
4”
60
80
G=exp[l(V-Vo)l l
-
Ar/CH.,
1
I
0.01
L
0.1
I
l.0
Charge(pC) 2
-
1010
60
40
20
Rate
of
Quencher
mixing
ratios
y-value
for lower
a
I
see saturation
of C,H,
(fig.
mixing
I
phenomena 6)
ratios
and
. 0
for
a rapid
larger
increase
of
( < 5%) (fig. 8b). Ar/CH,
I
I
I
Ar/Cfk
rlaft
SXJL~)
1
Ar/CeHe right scalp
t
dG/G=.*dR/R R:
Rateof Quencher 10
0;o Rate
of Quencher
resolution (fwhm) of ‘5Fe for (a) Ar/C2H6 as a function of the gain at various mixing
(%)
Fig. 8. Fitted results of the high voltage dependence of the gas gain of the reference chamber by the formula G = eY(‘- ‘11’. y and V0 are plotted as a function of quencher mixing ratio.
We can clearly
Fig. 10. Energy and (b) Ar/CH, ratios.
(%)
Fig. 9. a (coefficient of relative change of gain G to relative change of quencher mixing ratio R) as a function of R for Ar/CH, and Ar/C2Hb mixtures.
is quite different, showing no saturation increase of y. This difference becomes much clearer the coefficient (Yof the relative change of relative change of the mixing ratio R (fig. dG/G
and no sharp by examining gain G to the 9).
= (Yd R/R.
In the case of Ar/C,H,, the gain is very sensitive to the rate of quencher for lower mixing ratios (R < 20%). If a large amount of C,H, exists, it dominates the overall collision cross section (o(C,H6) > o(Ar)), and only a change of the molecular density of C,H, contributes to dG/G. In the case of smaller admixtures of C,H,, however, the collision cross section is not necessarily determined by C,H, alone, but is very sensitive to its mixing ratio. This results in a drastic change of the gain. On the other hand, for Ar/CH,, the apparent value of (a[ is not as large as for Ar&H, but rather decreases for mixing ratios below 10%. This is explained by the small collision cross section of CH, (o(CH,) < o(Ar)) with a similar (reversed) discussion as above. The energy resolution as a function of the gain is shown in figs. 10a and b for Ar/C,H, and Ar/CH, respectively. Larger mixing ratios of C,H, give worse resolutions. Since C,H, has a much more complex excitation level than Ar, it gives a larger statistical fluctuation in the gas multiplication process. This is also understood by the fact that Ar/CH, gives a better energy resolution (fig. 10). A bad resolution for Ar/C,H, is caused by the coexistence of a limited
K. Fujii et al. / Automated monitoring
40
r
10.0
ofgasgain I
I
I
I
1.0 ArICH,
0.5 1.0
u I 5:
0.0
;;
dG/G=a+dR/R
,a Q 0 ; 2
R: Rate 01 ouenctler
-0.5 0.1
L
I
I
I
I
1
2
3
4
Rate Fig. 13. Coefficient
I
I
1 1.0
I
I
2.0
1.5 H.V.,
(kV)
Fig. 11. High voltage dependence of the gas gain of the reference chamber for a small amount of CH, mixture with Ar.
streamer
mode
electric
noise
at higher at lower
gains
gains
(higher
(lower
voltages),
and
by
voltages).
The small ) a) value of the Ar/CH, mixture stimulates us to carry out a test for much smaller mixing ratios of CH,. In general, if less quencher is used, many
of
Quencher
-
(%)
a at low mixing ratios of the quencher
gas.
photons from the avalanche hit the cathode and generate more secondary electrons. These electrons cause breakdown of the high voltage in the chamber before obtaining a high gain. Fortunately, the plastic tube we use has a larger work function than metals and does not easily emit secondary electrons. Hence, we can expect stable operation of the high voltage till the high gain region. A small amount of quencher is required, however, not for the photons, but to prevent Arf for making secondary emission on the cathode, since Arf is expected to be neutralized by the reaction Ar++ CH, is not likely to make sec+Ar+CH;, and CH: ondary emission on the plastic cathode. Results obtained for various quencher admixtures are shown in
2.2(
0.8 T z”.7 >” 0.6
1.2 LOI
I o
I
I
I
I
I
2
4
6
8
10
Rate
of
Quencher
i
0.1
1.0 Charge
(%)
Fig. 12. Fitted results of fig. 11 with the same formula s.
55Fe t
as in fig.
5.0
(PC)
Fig. 14. Main/escape peak pulse height ratio of 55Fe as a function of output charge. The higher gain (higher applied voltage) side shows a clear effect of saturation.
K. Fujii et 01. / Automated
figs. 11, 12 and 13. Not much difference is seen down to 4% (figs. 11, 12). Below 18, the same gas gain as at 4% cannot be obtained by the effect of Ar+ as described above. Note that a crosses zero at a mixing ratio around 1% (fig. 13). Using the region of very small 1a 1, we can obtain stable results against fluctuations of the quencher mixing ratio. Using less quencher gas also has the merit of less cost and more safety. Note that no saturation is apparent in the high voltage curves for the Ar/CH, mixtures (figs. 7, 11). However, what we need to obtain a good energy resolution is not saturation with high voltage, but with the number of primary ion pairs. Fig. 14 shows the output gain dependence of the main/escape peak pulse height ratio for various Ar/CH, mixtures. It indicates the effect of saturation by the space charge density of primary ions. The saturation clearly depends on the gain, not on the gas mixture. In the case of Ar/CH, = 98/2 and hv = 1.6 kV, the relative gain difference is expressed as dG/G
= 17 dV/V,
i.e. to keep the gain deviation
to control
1. l-
the hv to better
I
I
to less than l%, we need than 0.06% (+l v). This
I
I
I
PR-Gas
pin
41 1
I
.l-
r
I
I
.
PR-Gas
.
.
\
.
.* .
z
.o-
G==2.
5x104 t,
:..
dGlG
z-4 1.9-
0.96
Fig. 16. Re-plot
1.02
1.00 Relative
P/T
of fig. 15 as a function
of P/T.
accuracy is easily attained by the present high voltage system. The effect of temperature (T) and pressure (P) on the gas gain was measured for a mixing ratio of Ar/CH, = 90/10 (fig. 15). The dots in the figure correspond to the central value of the pulse height distribution measured under various conditions for T and P. Correlations of dG/G, dT/T and d P/P were obtained iteratively: dG/G
1. o-
ofps
moniming
= 4.6 dT/T
and
I
dG/G
$
The absolute values of the two coefficients are almost equal, as expected. A much clearer correlation is ob-
Q, v) 0. 9:
1. 1-
.-z c co
:
7i.i a
= -4.3
dP/P.
0 .-
1. 0’
0. 9’
s;
‘Or-----
..
Relative
Pressure
I
I
0.99
1.00
I
I 1.01
Fig. 15. Central value of the pulse height distribution under various conditions of (a) temperature T and (b) pressure P. The averaged gain is about 2.5 X 104. Coefficients of the relative gain difference to the relative temperature and pressure difference are obtained iteratively. The gas mixture used was Ar/CH, = 90/10.
0.9
Relative
1.0
Pulse
1.1
Height
Fig. 17. The effect of a correction on P and T. The open histogram shows the distribution of the central value of the pulse height distribution for various conditions without any correction, and the shaded one shows the distribution after a correction for P and T by the formula shown in the text.
K. Fujii et al. / Automated monitoring ofgasgain
42
tained by a P/T plot (fig. 16), which relation of gain and number of molecules, dG/G
= -4.1
d( P/T)/(
presents
the
P/T).
Fig. 17 presents the effect of a correction on P and T as obtained above. The spread of the corrected central value is about 1% in 0, which is within the measurement error and negligible compared with the energy resolution of the end cap calorimeter. Also this is a typical demonstration of how effective this kind of monitoring and calibrating system will be. Let us briefly mention the ageing effect. We carried out measurements for 17 days with a rate of 800 Hz. As we did not encounter anything abnormal during this measurement, we estimated the lower limit of the lifetime of the plastic proportional tube to be: No. of pulse x gas multiplication = 1 X lo9 x 2.5 X lo4 ions/cm
1
1.5
Field(kV/cm)
Fig. 19. Electron drift velocity in various as a function of field gradient.
tube.
This corresponds to 200 years for the estimated tion of the TOPAZ end cap calorimeter.
mixtures
of Ar/C,H,
condi-
3.2. Electron drifr velocity A measurement of electron drift velocity was done with various mixing ratios of the following gas mixtures: (1) Ar/CH,, (2) Ar&H,, (3) Ar/i-C,H,,, (4) Ar/C, H,/C, H, OH. They were selected considering the TOPAZ end cap drift chamber. Gas mixture (4) was specifically meant to prevent possible ageing effects caused by the limited streamer mode operation. Data were taken at field gradients of 0.2-l .7 kV/cm, and at 1.0, 1.07 and 1.12 atm pressure. The temperature dependence was not examined this time. During the C&/Ar
““‘*.#.‘,“I’*0.5
0
measurements, the deviation of the temperature was small and negligible. Figs. 18, 19 and 20 show the drift velocity in gas mixtures (l), (2) and (3) respectively, as a function of field gradient E, leaving the mixing ratio R as a parameter. Figs. 21, 22 and 23 are re-plots of the same data as a function of R at fixed field gradient. In general, a gas mixture which gives slow drift velocity Vd with a small dependence on E and R is suitable for a drift chamber. Ar/C,H, is the best in every respect. Ar/i-C,H,, and Ar/CH, follow in this order. To get a position resolurequired an accuracy of tion a, - 200 pm in Ar/C,H, the high voltage power supply of typically l%, and this is achieved easily. Vd is fairly stable against dR/R in
Mixture
i-C&i,,,/Ar
Mixture
62
‘*s
5
4
3
2
1
.
1
L..I”.“..“‘-~
OO
in various
1
z 1.5
Field(kVlcm)
Field(kV/cm)
Fig. 18. Electron drift velocity as a function of field gradient.
0.5
100
mixtures
of Ar/CH,
Fig. 20. Electron drift velocity in various C,H,” as a function of field gradient.
mixtures
of Ar/i-
K. Fujii et al. / Automated CK/Ar WC
I
monitoring
ofgas
43
pm
Mixture
I
I
I
4
10 -
: 6_ v) _ z _ 2 ‘, o 6 >I .Z ;4:
-
2-
0.3677
Y"/wn
0
0.6129
0.2451 Y”,wn w/en
A
0.9806
w/cm
0
1.3403
IN/em
.
1.7180
k"/Cfn
I 40 CH, Mixing
I 20
OO
+ x
I 60 Ratio(%)
I 60
Fig. 21. Electron drift velocity in Ar/CH, at various strengths as a function of Ar/CH, mixing ratio. C&/AT 61
3JT
field
0
Mixture
5-
:‘E 3+ *
0.2451 0.3677
Y"/wn
>
0
0.6129
k"/Cnl
A
0.9906
k"/wIl
o
1.3483
Y"/cm
1.7160
w/cm
-
l-
.
I
t
I
I
20
OO
60 Ratio(%)
40 C,H, Mixing
k”/cm
I
1
60
100
Fig. 22. Electron drift velocity in Ar/C,H, at various strengths as a function of Ar/C,H, mixing ratio. i-C.,H,,/Ar 6
I
I
I
I
I1
1 I
0.5 Field/Pressure(kV/cm.atm)
I, 1
1
I
a
,I, 1.5
Ar/C2H, over the entire region. Also Ar/CH, and are usable without difficulty in the apAr/i-C,H,, propriate mixing ratio. Figs. 24a and b show the drift velocity in Ar/C,H, (50/50 mixture) as a function of E and E/P leaving the pressure P as a parameter. Here we can see a clear scaling by E/P. Finally we took a brief look at how C,H,OH affects
-z 4: z .
21 .Z 0 50 2- -
I I
Fig. 24. Electron drift velocity in an Ar/C,H, SO/50 mixture as a function of (a) field gradient and (b) field gradient/pressure.
I
I
I
I
L
100
6
,,,,,,,,,,,,,,
I
field
Mixture
I
I
I l
Ar/C&
=50/50
A bubblmg5"C
Q-,&,,,
-z 4: El _ ; 3” > .Z :: 2-
08 0
-
+
0.2451
k"/C
>
-
x 0
0.3w7 0.8129
k"/cm w/cm
, _
h
0.9906
k"/CIII
-
0
1.3483
Y"/Crn
1.7180
WlErn
-.
0
0
1
1.5
Field(kVlcm)
ar
-
0.5
I 20
I 40 i-C,H,o Mixing
Fig. 25. Field gradient dependence of electron drift velocity in an Ar/C,H, SO/50 mixture for various conditions of the C, H,OH vapor pressure.
I 60 Ratio(%)
I 60
100
Fig. 23. Electron drift velocity in Ar/i-C,H,o at various strengths as a function of Ar/i-C,H,, mixing ratio.
field
44
K. Fujii et al. / Automated monitoring
the drift velocity in Ar/C,H, (fig. 25). The drift velocity is so sensitive to the vapor pressure of C,H,OH that we are now trying to find a method to avoid ageing effects without C,H,OH admixture.
stability ratio.
ofgas gain against fluctuations
References
111TOPAZ Collaboration, 4. Summary and conclusion A prototype of a monitoring and calibrating system practically applicable to the TOPAZ end cap calorimeter and drift chamber was constructed. This is proved to be effective in obtaining a high resolution. Based on the accumulation of data obtained by a practical test of this system, we can conclude (1) Ar with a small admixture of CH, is most suitable for the TOPAZ proportional tube calorimeter, and (2) Ar/C,H, is the best for the drift chamber in terms of
of electric field and mixing
TRISTAN-EXP-002, KEK proposal (Jan. 1983). PI K. Fujii et al., Nucl. Instr. and Meth. 236 (1985) 55. [31 K. Fujii et al., Nucl. Instr. and Meth. 225 (1984) 23. [41 Oyo Koken Co., 3-25-2 Hyakunin-cho, shinjuku-ku, Tokyo 160, Japan. I51 Shinsei Denshi Co., 12229 Minowa-machi, kami-ina-gun, Nagano-ken 399-46, Japan. 161Fujikura Densen Co., l-5-1 Kiba, Koutou-ku, Tokyo 135, Japan. Minami-ku, 171STEC Co., 2 Kisshouin-Miyanohigashi-machi, Kyoto 601, Japan. PI For the theoretical formula, see, for example, F. Sauli, CERN 77-09 (May, 1977).
in Physics
Research
A245 (1986) 35-44
AUTOMATED MONITORING AND CALIBRATING DRIFT VELOCITY Prototype system and accumulation of reference data
35
SYSTEM OF GAS GAIN AND ELECTRON
K. FUJII, J. FUJIMOTO, H. HAYASHII, R. KAJIKAWA, Y. MASATANI, H. OZAKI, A. SUGIYAMA, S. SUZUKI, T. TAKAHASHI, T.F. TSUKAMOTO, T.Y. TSUKAMOTO, M. UEDA and S. UN0 Depurfment
Received
of Physrcs, Nagoya
University,
Chikusu-ku,
Nqqw
1 July 1985 and in revised form 25 November
464, Japan
1985
A prototype of an automated monitoring and calibrating system of gas gain and electron drift velocity for the end cap detectors in TOPAZ [l] is constructed. Systematic data under various conditions of gas mixture, temperature, pressure and applied high voltage are accumulated using this system. They will be used as reference for a detailed calibration.
1. Introduction Today,
various
kinds
of particle
detectors
based
monitor the variation of the parameters which affect the detector output, and to correct systematic shifts by a premeasured reference at various conditions. At the same time, one can also measure the output of the reference detectors which are set under the same environmental conditions and which are irradiated by standard radioactive sources to cancel out the systematic error and the time dependent shift. In this paper, we describe a prototype of an automated monitoring and calibration system which will be used for the electromagnetic calorimeter [2] and drift chamber [3] in the end cap region of the TOPAZ detector at TRISTAN [l]. We also present the results of systematic measurements of gas gain and electron drift
on
have been developed and utilized in high energy physics experiments. One of the most serious practical problems of this kind of detector is that the fluctuation of environmental conditions (gas temperature, pressure, mixture and purity of each gas component, etc.) affects the detector output and deteriorates the resolution. Considering the long duration of recent experiments, it is not easy to keep the environmental parameters stable enough to get the designed performance. One solution to this problem is to continuously the gas amplification
method
Proportional 55Fe
Tube
Drift
Chamber QoSr
c perature
Pressure Sensor
C,HsOH
out
Mass
Flow
Pressure
Controlled Bubbler
HPDP~
Fig. 1. Schematic
diagram
II23
of the gas gain and drift time monitoring
0168-9002/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
system.
K. Fujii et al. /Automated
36
for various gas mixtures and operational conditions. These will be used as premeasured reference data for calibration and correction.
velocity
monitoring ofgasgain 2.0
1.5
2. Monitoring system 1.0
2.1. Outline of the system The system is composed of a drift chamber specialized for drift time measurements, a reference proportional counter for gain monitoring, temperature and pressure sensors, a mass flow sensor/controller, a programmable high voltage power supply, and several interfacing modules (fig. 1). The drift chamber with uniform electric field in the drift region is irradiated by two collimated P-ray sources at a certain distance, and the drift time from each position is measured by a TDC. The gas gain is monitored by a single cell proportional counter irradiated by a 55Fe source whose pulse height is measured by an ADC. Monitoring and controlling of the high voltage, gas pressure, gas flow and temperature are done by a programmable high voltage power supply, a scanning ADC and a DAC module. All digitization and control is done via CAMAC and a serial interface module linked to the minicomputer PPDP-11/23. The applied high voltage is varied periodically to obtain full scan data on the field strength dependence. This is useful, in particular, for correcting the time dependence of the drift chamber whose inhomogeneous field gradient has a wide range. When this system is applied to the TOPAZ detector, the reference proportional counter and drift chamber will be placed at several places in the gas glow line. Data taken by the FPDP-11 will be transferred to the main data acquisition computer. Also local processing and data logging will be done as a stand-alone mode.
-
0.5
0.0: 0
of a drift chamber
specialized
Anode
Box
(mm)
Fig. 3. Calculated electric field strength distribution in the chamber shown in fig. 2 for various conditions of the supplied high voltage. The field strength near the “anode box” lies between the solid and the dashed curve according to the different values of Vanode. Vanode has no effect on the field gradient in the drift region between 11 and 29 mm from the “anode box”.
2.2. Gain monitoring The reference proportional counter for gas gain monitoring is the same as that used in the TOPAZ end cap calorimeter. This is a single cell counter of 10 mm (height) x 15 mm (width) conductive plastic tube of 1 mm thick, with an anode wire of 50 pm diameter gold plated tungsten. The output pulse from collimated Xrays from a 55Fe source (5.9 keV) is amplified by a preamplifier (Canberra 2006, time constant 1 ps) and shaping amplifier (Oyo-Koken [4] 704-2, shaping time 0.4 ps), and is digitized by a CAMAC ADC (LRS 2249). The ADC is gated by the fast output from a
26
Unit
Fig. 2. Construction
from
30
20
10
Distance
for drift time measurements.
(mm)
See text for details.
K. Fujii et al. / Automated monitoring of gas gain
shaping amplifier. To cover a wide dynamic range, the output of the main amplifier is split into two. One goes directly into the ADC and the other is attenuated before put into the ADC. The overall response of the amplifier and ADC is linear within 1% in the measured range. 2.3.
Drift
velocity
monitoring
A special drift chamber with a uniform field gradient in the drift region is prepared to monitor the electron drift velocity (fig. 2). The field is shaped by two printed circuit boards placed 10 mm apart in parallel, which have shaping strips of 0.3 mm wide and 2.0 mm spacing. At the one end of the chamber, the multiplication and detection region (the “anode box”) consists of an anode wire of 20 pm diameter and a conductive plastic tube (10 mm (h) x 15 mm (w) X 110 mm (1) with a window of 5 mm (h) X 10 mm (1). A similar tube is placed at the other end to keep the electric field uniform at the edge of the chamber. This chamber has a 40 mm long drift region. The printed board has two slits (1 mm wide and 10 mm long) located at 11 mm and 29 mm from the “anode box” to make P-rays from a 90Sr source go through the chamber. Calculation shows the uniformity of the field gradient in the drift region to be within + 0.5% at the measured range of 0.2-1.7 kV/cm (fig. 3). Triggering is done by a coincidence signal from two plastic scintillation counters placed under the chamber. y-rays and very low momentum P-rays are rejected by this method. By requiring a coincidence of two 3.0 mm (0.5 g/cm2) thick counters the momentum of the P-rays must be above 1 MeV/c. Smearing by multiple scattering in the chamber is estimated to be 0.3 mm at the lower slit, which is negligibly small compared with the slit width. A TDC (LRS 4208) is started by the trigger pulse and stopped by the chamber output shaped by a
, 0
40-
2 c :
30-
L : z
20
-
5 8
lo-
ow’*h,B 0
1000
2000 Drift
aw.bluJuw’*l’ 3000 Time
31
discriminator amplifier (LRS DC201). Two peaks in the drift time distribution correspond to two source positions (fig. 4). We calculate the drift velocity by fitting the center of the two peaks assuming a Gaussian distribution. The statistical error is negligibly small in this method. Instead, the field nonuniformity in the drift region dominates the measurement error. The estimated mechanical error of the chamber construction (k50 pm) will cause an error of 1.0% in the field strength. The accuracy of slit positioning is about + 100 pm. In total, the overall error in the drift velocity is estimated to be 3%. 2.4.
Environment
monitoring
As a sensor of gas temperature, an active bridge circuit with heat sensitive resistor (LPTC25, Shinsei Denshi Co. [5]) is provided. Its measuring accuracy is +0.2”C in the range lo-3O“C. As a pressure gauge, we use a piezo semiconductor device (FPSlOA, Fujikura Densen Co. [6]) which can measure the absolute value with an accuracy of kO.5 gf/cm2. The gas flow is monitored and controlled by a mass flow controller (STEC Co. [7]) with an accuracy of +0.2 cm3/min. The flow rate is digitized and read into the computer by a scanning ADC (LRS LG8252), and computer-processed feedback to the mass flow controller is done by a DAC module (Kinetic 3112). The high voltage power supply (LRS HV4032) has a voltage regulation of f 1 V at 1V 1max= 7 kV (+ 0.5 V = 3.3 kV), and provides a field gradient up at IVI,, to 1.7 kV/cm in the drift chamber. It is hooked to the serial line of the PPDP-11 via a home-made interface module. 2.5.
System
controlling
software
The system software is constructed on a minicomputer ( PPDP-11/23) to control all devices mentioned above. Much attention has been paid to making the architecture compatible with the data acquisition system of the TOPAZ detector. The basic function of the gas-monitoring software should be to check the status of each device by collecting calibration data and its environmental condition, and deliver the result to the physics data acquisition. It should work in parallel with the high-speed event-by-event acquisition of physics data. To fulfil these requirements, the software system was constructed on a real time multitask monitor RSXll-M.
** 4000
5000
(nsec)
Fig. 4. Typical drift time distribution obtained by the chamber shown in fig. 2. The two peaks correspond to two source positions.
3. Measured results Using the system described in the previous section, systematic data on gas gain and electron drift velocity were accumulated.
K. Fujii et al. / Automated monitoring of gas gain
38
For the gas gain, various mixtures of Ar/CH,, Ar/C,H, and Ar/i-C,H,, were examined. PR-gas (Ar 90%, CH, 10%) was used to study the temperature and pressure dependence. For the drift velocity, measurements were done using Ar/CH,, Ar/C,H, and Ar/i-C,H,, under various pressures and field strengths. In the case of CzH,, a measurement was also done by mixing with C2H50H vapor, since in many cases C,H,OH is added to minimize ageing effects of chambers which are operated in the high gain mode and under a high flux of incoming particles.
3.1. Gas amplification The gas amplification under various conditions of applied high voltage and gas mixing ratios was measured for gas mixtures of Ar/CH,, Ar/C,H, and Ar/i-C,H,, (fig. 5). No simple correlation with molecular number exists. The quenching effect tends to appear stronger for a gas of larger molecules. Saturation phenomena at higher voltages are very evident by the quenching of larger molecules. A detailed study was carried out for Ar/CH, and Ar/CzHs, because these were the candidates for use in the TOPAZ end cap calorimeter. Figs. 6 and 7 show high voltage response curves for various Ar/C,H, and Ar/CH, mixtures. Fig. 8 shows the fitted results of these high-voltage curves by the empirical formula [LX]:
l
1
1.5
2
H.V.
(kV)
Fig. 6. Detailed high voltage dependence of the gas gain of the reference chamber on the Ar/C,H, mixture. Mixing ratios tested were 90/10, 80/20. 70/30, 60/40, 50/50, 44.4/55.6. 37.5/62.5. 28.6/71.4 and 16.7/83.3. l(
I
I
I
ArICH,
ArIG&-
2.5
H.V.(kV)
Fig. 5. High voltage dependence of the gas gain of the reference chamber irradiated by a 55Fe source. Tested gas mixtures are Ar/CH,, Ar/C,H, and Ar/i-C,H,, with mixing ratios of 90/10, 70/30 and 50/50 each.
L.”
H.V. (kV)
Fig. 7. Detailed high voltage dependence of the gas gain of the reference chamber on the Ar/CH, mixture. Mixing ratios tested were 90/10, 80/20, 70/30, 60/40, 50/50, 37.5/62.5, 28.6/71.4 and 16.7/83.3.
K. Fujri et al. / Au~~muted monirorrng of gas Ruin
25-
b)
x
39
ArICH,
.
8OllO
0
70130
55Fe ; f P
x 50150 d 28.6l71.4
1.0 -
Ar/CH,
x-
Ar/Q-&
l
0.8
‘.6[‘
’
2;
4”
60
80
G=exp[l(V-Vo)l l
-
Ar/CH.,
1
I
0.01
L
0.1
I
l.0
Charge(pC) 2
-
1010
60
40
20
Rate
of
Quencher
mixing
ratios
y-value
for lower
a
I
see saturation
of C,H,
(fig.
mixing
I
phenomena 6)
ratios
and
. 0
for
a rapid
larger
increase
of
( < 5%) (fig. 8b). Ar/CH,
I
I
I
Ar/Cfk
rlaft
SXJL~)
1
Ar/CeHe right scalp
t
dG/G=.*dR/R R:
Rateof Quencher 10
0;o Rate
of Quencher
resolution (fwhm) of ‘5Fe for (a) Ar/C2H6 as a function of the gain at various mixing
(%)
Fig. 8. Fitted results of the high voltage dependence of the gas gain of the reference chamber by the formula G = eY(‘- ‘11’. y and V0 are plotted as a function of quencher mixing ratio.
We can clearly
Fig. 10. Energy and (b) Ar/CH, ratios.
(%)
Fig. 9. a (coefficient of relative change of gain G to relative change of quencher mixing ratio R) as a function of R for Ar/CH, and Ar/C2Hb mixtures.
is quite different, showing no saturation increase of y. This difference becomes much clearer the coefficient (Yof the relative change of relative change of the mixing ratio R (fig. dG/G
and no sharp by examining gain G to the 9).
= (Yd R/R.
In the case of Ar/C,H,, the gain is very sensitive to the rate of quencher for lower mixing ratios (R < 20%). If a large amount of C,H, exists, it dominates the overall collision cross section (o(C,H6) > o(Ar)), and only a change of the molecular density of C,H, contributes to dG/G. In the case of smaller admixtures of C,H,, however, the collision cross section is not necessarily determined by C,H, alone, but is very sensitive to its mixing ratio. This results in a drastic change of the gain. On the other hand, for Ar/CH,, the apparent value of (a[ is not as large as for Ar&H, but rather decreases for mixing ratios below 10%. This is explained by the small collision cross section of CH, (o(CH,) < o(Ar)) with a similar (reversed) discussion as above. The energy resolution as a function of the gain is shown in figs. 10a and b for Ar/C,H, and Ar/CH, respectively. Larger mixing ratios of C,H, give worse resolutions. Since C,H, has a much more complex excitation level than Ar, it gives a larger statistical fluctuation in the gas multiplication process. This is also understood by the fact that Ar/CH, gives a better energy resolution (fig. 10). A bad resolution for Ar/C,H, is caused by the coexistence of a limited
K. Fujii et al. / Automated monitoring
40
r
10.0
ofgasgain I
I
I
I
1.0 ArICH,
0.5 1.0
u I 5:
0.0
;;
dG/G=a+dR/R
,a Q 0 ; 2
R: Rate 01 ouenctler
-0.5 0.1
L
I
I
I
I
1
2
3
4
Rate Fig. 13. Coefficient
I
I
1 1.0
I
I
2.0
1.5 H.V.,
(kV)
Fig. 11. High voltage dependence of the gas gain of the reference chamber for a small amount of CH, mixture with Ar.
streamer
mode
electric
noise
at higher at lower
gains
gains
(higher
(lower
voltages),
and
by
voltages).
The small ) a) value of the Ar/CH, mixture stimulates us to carry out a test for much smaller mixing ratios of CH,. In general, if less quencher is used, many
of
Quencher
-
(%)
a at low mixing ratios of the quencher
gas.
photons from the avalanche hit the cathode and generate more secondary electrons. These electrons cause breakdown of the high voltage in the chamber before obtaining a high gain. Fortunately, the plastic tube we use has a larger work function than metals and does not easily emit secondary electrons. Hence, we can expect stable operation of the high voltage till the high gain region. A small amount of quencher is required, however, not for the photons, but to prevent Arf for making secondary emission on the cathode, since Arf is expected to be neutralized by the reaction Ar++ CH, is not likely to make sec+Ar+CH;, and CH: ondary emission on the plastic cathode. Results obtained for various quencher admixtures are shown in
2.2(
0.8 T z”.7 >” 0.6
1.2 LOI
I o
I
I
I
I
I
2
4
6
8
10
Rate
of
Quencher
i
0.1
1.0 Charge
(%)
Fig. 12. Fitted results of fig. 11 with the same formula s.
55Fe t
as in fig.
5.0
(PC)
Fig. 14. Main/escape peak pulse height ratio of 55Fe as a function of output charge. The higher gain (higher applied voltage) side shows a clear effect of saturation.
K. Fujii et 01. / Automated
figs. 11, 12 and 13. Not much difference is seen down to 4% (figs. 11, 12). Below 18, the same gas gain as at 4% cannot be obtained by the effect of Ar+ as described above. Note that a crosses zero at a mixing ratio around 1% (fig. 13). Using the region of very small 1a 1, we can obtain stable results against fluctuations of the quencher mixing ratio. Using less quencher gas also has the merit of less cost and more safety. Note that no saturation is apparent in the high voltage curves for the Ar/CH, mixtures (figs. 7, 11). However, what we need to obtain a good energy resolution is not saturation with high voltage, but with the number of primary ion pairs. Fig. 14 shows the output gain dependence of the main/escape peak pulse height ratio for various Ar/CH, mixtures. It indicates the effect of saturation by the space charge density of primary ions. The saturation clearly depends on the gain, not on the gas mixture. In the case of Ar/CH, = 98/2 and hv = 1.6 kV, the relative gain difference is expressed as dG/G
= 17 dV/V,
i.e. to keep the gain deviation
to control
1. l-
the hv to better
I
I
to less than l%, we need than 0.06% (+l v). This
I
I
I
PR-Gas
pin
41 1
I
.l-
r
I
I
.
PR-Gas
.
.
\
.
.* .
z
.o-
G==2.
5x104 t,
:..
dGlG
z-4 1.9-
0.96
Fig. 16. Re-plot
1.02
1.00 Relative
P/T
of fig. 15 as a function
of P/T.
accuracy is easily attained by the present high voltage system. The effect of temperature (T) and pressure (P) on the gas gain was measured for a mixing ratio of Ar/CH, = 90/10 (fig. 15). The dots in the figure correspond to the central value of the pulse height distribution measured under various conditions for T and P. Correlations of dG/G, dT/T and d P/P were obtained iteratively: dG/G
1. o-
ofps
moniming
= 4.6 dT/T
and
I
dG/G
$
The absolute values of the two coefficients are almost equal, as expected. A much clearer correlation is ob-
Q, v) 0. 9:
1. 1-
.-z c co
:
7i.i a
= -4.3
dP/P.
0 .-
1. 0’
0. 9’
s;
‘Or-----
..
Relative
Pressure
I
I
0.99
1.00
I
I 1.01
Fig. 15. Central value of the pulse height distribution under various conditions of (a) temperature T and (b) pressure P. The averaged gain is about 2.5 X 104. Coefficients of the relative gain difference to the relative temperature and pressure difference are obtained iteratively. The gas mixture used was Ar/CH, = 90/10.
0.9
Relative
1.0
Pulse
1.1
Height
Fig. 17. The effect of a correction on P and T. The open histogram shows the distribution of the central value of the pulse height distribution for various conditions without any correction, and the shaded one shows the distribution after a correction for P and T by the formula shown in the text.
K. Fujii et al. / Automated monitoring ofgasgain
42
tained by a P/T plot (fig. 16), which relation of gain and number of molecules, dG/G
= -4.1
d( P/T)/(
presents
the
P/T).
Fig. 17 presents the effect of a correction on P and T as obtained above. The spread of the corrected central value is about 1% in 0, which is within the measurement error and negligible compared with the energy resolution of the end cap calorimeter. Also this is a typical demonstration of how effective this kind of monitoring and calibrating system will be. Let us briefly mention the ageing effect. We carried out measurements for 17 days with a rate of 800 Hz. As we did not encounter anything abnormal during this measurement, we estimated the lower limit of the lifetime of the plastic proportional tube to be: No. of pulse x gas multiplication = 1 X lo9 x 2.5 X lo4 ions/cm
1
1.5
Field(kV/cm)
Fig. 19. Electron drift velocity in various as a function of field gradient.
tube.
This corresponds to 200 years for the estimated tion of the TOPAZ end cap calorimeter.
mixtures
of Ar/C,H,
condi-
3.2. Electron drifr velocity A measurement of electron drift velocity was done with various mixing ratios of the following gas mixtures: (1) Ar/CH,, (2) Ar&H,, (3) Ar/i-C,H,,, (4) Ar/C, H,/C, H, OH. They were selected considering the TOPAZ end cap drift chamber. Gas mixture (4) was specifically meant to prevent possible ageing effects caused by the limited streamer mode operation. Data were taken at field gradients of 0.2-l .7 kV/cm, and at 1.0, 1.07 and 1.12 atm pressure. The temperature dependence was not examined this time. During the C&/Ar
““‘*.#.‘,“I’*0.5
0
measurements, the deviation of the temperature was small and negligible. Figs. 18, 19 and 20 show the drift velocity in gas mixtures (l), (2) and (3) respectively, as a function of field gradient E, leaving the mixing ratio R as a parameter. Figs. 21, 22 and 23 are re-plots of the same data as a function of R at fixed field gradient. In general, a gas mixture which gives slow drift velocity Vd with a small dependence on E and R is suitable for a drift chamber. Ar/C,H, is the best in every respect. Ar/i-C,H,, and Ar/CH, follow in this order. To get a position resolurequired an accuracy of tion a, - 200 pm in Ar/C,H, the high voltage power supply of typically l%, and this is achieved easily. Vd is fairly stable against dR/R in
Mixture
i-C&i,,,/Ar
Mixture
62
‘*s
5
4
3
2
1
.
1
L..I”.“..“‘-~
OO
in various
1
z 1.5
Field(kVlcm)
Field(kV/cm)
Fig. 18. Electron drift velocity as a function of field gradient.
0.5
100
mixtures
of Ar/CH,
Fig. 20. Electron drift velocity in various C,H,” as a function of field gradient.
mixtures
of Ar/i-
K. Fujii et al. / Automated CK/Ar WC
I
monitoring
ofgas
43
pm
Mixture
I
I
I
4
10 -
: 6_ v) _ z _ 2 ‘, o 6 >I .Z ;4:
-
2-
0.3677
Y"/wn
0
0.6129
0.2451 Y”,wn w/en
A
0.9806
w/cm
0
1.3403
IN/em
.
1.7180
k"/Cfn
I 40 CH, Mixing
I 20
OO
+ x
I 60 Ratio(%)
I 60
Fig. 21. Electron drift velocity in Ar/CH, at various strengths as a function of Ar/CH, mixing ratio. C&/AT 61
3JT
field
0
Mixture
5-
:‘E 3+ *
0.2451 0.3677
Y"/wn
>
0
0.6129
k"/Cnl
A
0.9906
k"/wIl
o
1.3483
Y"/cm
1.7160
w/cm
-
l-
.
I
t
I
I
20
OO
60 Ratio(%)
40 C,H, Mixing
k”/cm
I
1
60
100
Fig. 22. Electron drift velocity in Ar/C,H, at various strengths as a function of Ar/C,H, mixing ratio. i-C.,H,,/Ar 6
I
I
I
I
I1
1 I
0.5 Field/Pressure(kV/cm.atm)
I, 1
1
I
a
,I, 1.5
Ar/C2H, over the entire region. Also Ar/CH, and are usable without difficulty in the apAr/i-C,H,, propriate mixing ratio. Figs. 24a and b show the drift velocity in Ar/C,H, (50/50 mixture) as a function of E and E/P leaving the pressure P as a parameter. Here we can see a clear scaling by E/P. Finally we took a brief look at how C,H,OH affects
-z 4: z .
21 .Z 0 50 2- -
I I
Fig. 24. Electron drift velocity in an Ar/C,H, SO/50 mixture as a function of (a) field gradient and (b) field gradient/pressure.
I
I
I
I
L
100
6
,,,,,,,,,,,,,,
I
field
Mixture
I
I
I l
Ar/C&
=50/50
A bubblmg5"C
Q-,&,,,
-z 4: El _ ; 3” > .Z :: 2-
08 0
-
+
0.2451
k"/C
>
-
x 0
0.3w7 0.8129
k"/cm w/cm
, _
h
0.9906
k"/CIII
-
0
1.3483
Y"/Crn
1.7180
WlErn
-.
0
0
1
1.5
Field(kVlcm)
ar
-
0.5
I 20
I 40 i-C,H,o Mixing
Fig. 25. Field gradient dependence of electron drift velocity in an Ar/C,H, SO/50 mixture for various conditions of the C, H,OH vapor pressure.
I 60 Ratio(%)
I 60
100
Fig. 23. Electron drift velocity in Ar/i-C,H,o at various strengths as a function of Ar/i-C,H,, mixing ratio.
field
44
K. Fujii et al. / Automated monitoring
the drift velocity in Ar/C,H, (fig. 25). The drift velocity is so sensitive to the vapor pressure of C,H,OH that we are now trying to find a method to avoid ageing effects without C,H,OH admixture.
stability ratio.
ofgas gain against fluctuations
References
111TOPAZ Collaboration, 4. Summary and conclusion A prototype of a monitoring and calibrating system practically applicable to the TOPAZ end cap calorimeter and drift chamber was constructed. This is proved to be effective in obtaining a high resolution. Based on the accumulation of data obtained by a practical test of this system, we can conclude (1) Ar with a small admixture of CH, is most suitable for the TOPAZ proportional tube calorimeter, and (2) Ar/C,H, is the best for the drift chamber in terms of
of electric field and mixing
TRISTAN-EXP-002, KEK proposal (Jan. 1983). PI K. Fujii et al., Nucl. Instr. and Meth. 236 (1985) 55. [31 K. Fujii et al., Nucl. Instr. and Meth. 225 (1984) 23. [41 Oyo Koken Co., 3-25-2 Hyakunin-cho, shinjuku-ku, Tokyo 160, Japan. I51 Shinsei Denshi Co., 12229 Minowa-machi, kami-ina-gun, Nagano-ken 399-46, Japan. 161Fujikura Densen Co., l-5-1 Kiba, Koutou-ku, Tokyo 135, Japan. Minami-ku, 171STEC Co., 2 Kisshouin-Miyanohigashi-machi, Kyoto 601, Japan. PI For the theoretical formula, see, for example, F. Sauli, CERN 77-09 (May, 1977).