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Investigation of breakdown conditions in drift chambers
155
Nuclear Instruments and Methods in Physics Research A245 (1986) 155-158 North-Holland, Amsterdam
INVESTIGATION Paolo
OF BREAKDOWN
GIUBELLINO,
William
CONDITIONS
A. ROWE,
Hartmut
IN DRIFT CHAMBERS
F.-W.
SADROZINSKI
*
and Jeffrey
L. SKALA
Institute for Particle Physics, University of Calijornia, Santa Cruz, California 95064, USA
Received 17 October 1985
The electrostatic conditions for which breakdown and the total gain in the system.
occurs
in multiwire
1. Introduction We have investigated the conditions under which breakdown occurs in multiwire chambers, i.e. when large currents are drawn in the gas volume. One motivation for these investigations is to be able to predict “safe” parameters at which the chamber can be operated. These parameters include diameters of the sense and field wires, their number and arrangement, operating voltages and gas mixtures. The goal of our experiments was to compare the set of measurements to simple models of gas multiplication and derive criteria which indicate that breakdown occurs and that wire chamber performance are degraded.
chambers
Breakdown occurs if the gain on the wires is too large. We recognize that gas amplification can occur at both the sense wire and the field wire and we define an overall breakdown gain g, as the product of the gain on the sense wires g, and field wires g,:
(1)
g, = g&f. Following integrated
Sauli [l], the gain ‘g is determined Townsend coefficient a:
by the
g=exp where a is the wire radius and rc is the radius at which amplification starts. As in ref. [l], we assume that the Townsend coefficient is a function of the electric field E: CX=JkniE,
* Work supported
(3)
in part by the U.S. Department
of Energy.
0168-9002/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
as a function
of wire thickness
where k = 1.8 X 10-l’ (cm* V-‘) for argon, and N is the number of argon atoms per cm3. The amplification takes place in the radial field around the wire which is given by E=_=X’ x 2aror
(4)
r ’
where we introduce the scaled linear charge density h’, whose value is twice the value in electrostatic units. With
we find from eqs. (2), (3) and (4): g=exp(2JkN
2. Theory
are investigated
fl(Jj\‘/E,-
6)).
(6)
Thus the gain on a wire with radius a is a function of the charge per unit length X’ alone if the cutoff field EC is known. The cutoff field EC is the lowest field strength at which amplification occurs.
3. Determination of the cutoff field EC We used a single wire proportional tube of radius r = 3/Y to find the cutoff field as a function of wire diameter in a gas mixture of 89% A, 10% CO, and 1% CH,. The wire of radius a was connected either as an anode (“sense”) wire (wire grounded, negative high voltage on the tube) or as a cathode (“field”) wire (wire at negative high voltage, tube grounded). For different wire radii, the voltage, V,,, was determined at which excessive current was drawn (> 2 PA). For this breakdown point, the charge density Xi was determined according to
P. GiubeSm
156 Table 1 Breakdown
conditions
in a 3/4”
diameter
et al. / Breakdown conditions in drift chambers
tube ‘)
Field wire Diameter
Sense wire Diameter
(elm)
f% (kV)
E, (kV/cm)
(Ek:V/cm)
(elm)
xi? (kV)
E, (kV/cm)
:V/cm)
20 30 50 100 178 304
0.19 0.22 0.30 0.42 0.61 0.82
190 147 118 83 69 54
51 50 55 51 50 43
20 30 50 100 178
0.28 0.33 0.39 0.50 0.62
280 220 154 98 70
89 88 77 62 51
a) Gas: 89% A, 10% CO,,
1% CH,.
where c is the capacity/unit
length of the tube:
1 C=ln(r/a). In table 1 we show the scaled linear charge density XL and the fields E, = Xi/a on the surface of the wires at breakdown for several wire diameters 2a for both the sense wire and field wire configurations. Note that for equal wire diameter, breakdown occurs at lower fields when the wire is connected to the cathode (“field wire”). Electron emission could explain this effect. The cutoff field EC is then calculated in the following way: We have observed in the new MARK II drift chamber [2] a gain of g = lo5 at X’ = 0.28 kV (a = 15 urn). With eq. (6) we get EC = 88 kV/cm. Using this value for EC and Xl, = 0.33 kV from table 1 for breakdown, we get ‘c a dr= 17, / c1 and a gain of about g = 10’ for breakdown. The cutoff fields EC for all other wire diameters are then calculated assuming that breakdown occurs at the same gain, i.e., that /a dr = 17 for all wire diameters and configurations. These cutoff fields, .&, calculated with eqs. (6) and (9), are given in table 1. While EC is constant at 50 kV/cm for the field wires, we see a slight dependence of E, on the sense wire diameter. This could be due to our assumption about the dependence of a on E or effects of saturation of the gain. It is interesting to note that we get very similar values for EC if we extrapolate gain curves as a function of voltage [3] to unit gain and calculate the field E on the wire for that point.
long. The sense wire was grounded and the voltage on the field wires, V,, and the box, V, were varied to produce breakdown with different gains on sense and field wires. As above, a mixture of 89% A, 10% CO,, and 1% CH, was used. From electrostatic simulation, the scaled linear charge density Xi on the field wires and Xi on the sense wires were determined. Using the values for EC from table 1 for field and sense wires, the gains on the wires were calculated with eq. (6). Fig. 2 shows the gain on the field wire, g,, versus the gain on the sense wire, g,, at breakdown for sense wire diameters of 20, 30, 50, 100 and 178 pm. The data shows the correlation between the gain on the sense wires and the field wires, and suggests that breakdown occurs when the overall gain g, is of the order 10’. For large wire diameter, the calculation of the gain using eq. (6) becomes less reliable because it involves the difference of large numbers. Local field emission might also become
j--
f
4cm
0
0
0
0
E (y”
1
4. Breakdown in multiwire ceils To study the interplay of gain on sense and field wires to produce breakdown, the small test chamber sketched in fig. 1 was built: a sense wire of varied diameter was surrounded by four field wires of 178 u-diameter and enclosed by a conducting box, 25 cm
Fig. 1. Endview of the multiwire cell used to test breakdown. The replaceable sense wire of varied diameter (X) is at ground, the field wires (0) of 178 gm diameter are at high voltage Vr, and the outside conducting box is at high voltage Va. The length of the wires is 25 cm.
157
P. Giubellino et al. / Breakdown conditions in drift chambers IO9
I
1
I
I
I
I
I
I
SenseWire
$
_
20um 30um 50um
I OOum 175um
$ 0.4 0.2
0’
x
\e! I
1
I
I
100
I
I
I03 SENSE
WIRE
I
x
I
50
”
100
DURATION OFBREAKDOWN
x
0
Al
”
0
I
IO6
”
” 150
200
AT50pA
250 (hrs)
Fig. 3. Relative gain for two different gases as a function of the duration of breakdown at a current of 50 PA. The 50% points are 40 h (corresponding to 7 C integrated charge) for the 89% A. 10% CO,. 1% CH, mixture and 265 h (corresponding to 48 C integrated charge) for the 93% A, 3% CO,, 4% CH, mixture.
GAIN
Fig. 2. Field wire gain, gr, vs sense wire gain, g,, at breakdown in a mixture of 89% A, 10% CO,, 1% CH, for different sense wire diameters. The straight line, to guide the eyes, corre-
sponds to a total gain of 10’.
more important. We are not surprised from the simple behavior for diameters
to see deviation > 100 pm.
5. Deposits of polymers at breakdown In order to study the growth of polymers (“whiskers”) on the wires, we studied breakdown with different gases, mainly with A/CO&H, mixtures with more than 85% A, and also 50% A/50% C,H,. The assumption is that we can simulate prolonged irradiation with ionizing particles by forcing the chamber to discharge for a long time, and thus examine the gas stability. A chamber similar to the one shown in fig. 1, but with Lucite walls to facilitate visual observation of gas discharges and growth of polymers, was left at the breakdown voltage with a current of 50 PA for extended periods of time. Within less than 5 min, very long Z’whiskers” were observed in 50% A/50% C,H,, and 89% A, 10% CH,, 1% CO,. Both gas mixtures are known to grow polymers under experimental conditions in drift chambers. All other gases fall into two categories: if the CH, admixture was more than 5% polymerization was observed within 15 min, independent of the CO, content (between 1 and 10%). On the other hand, if the CH, content was less than 58, no polymerization was detected, and no changes in gain were observed even after the chamber was held at breakdown for several hours. To be more quantitative, we subjected two gas mix-
tures which did not grow whiskers and which are candidate gases for the new MARK II drift chamber [2], to continuous large current (50 PIA) in our test chamber shown in fig. 1. Every few hours the gain was monitored by measuring the pulse height produced by the photon from a 55Fe source. Fig. 3 shows the relative gain in 89% A, 10% CO,, 1% CH,, and 93% A, 3% CO, and 4% CH, as a function of the duration of breakdown at 50 PA. As can be seen, the two gases are degraded at different rates: the mixture with 93% A, 3% CO, and 4% CH, seems to survive about 7 times longer, possibly due to better quenching. The time at which the gain is reduced to 50% of its original value corresponds to an integrated charge of 7 C (40 h) for the 89% A, 10% CO,, 1% CH, mixture and to 48 C (265 h) for the 93% A, 3% CO,, 4% CH, mixture. We suggest that prolonged breakdown can be used as a necessary test to eliminate gases which are inclined to “whisker” growth.
6. Conclusion Our investigation of breakdown in multiwire chambers indicates that breakdown occurs when the overall gain on sense nnd field wires combined is about 10’. We used prolonged breakdown to test the inclination of gases to degrade the performance of the wire chamber.
Acknowledgment We thank Abraham Seiden, Patricia Ron Tyroler for their help and interest.
Burchat
and
158
P. Giubellino er al. / Breakdown conditions in drift chambers
References [l] F. Sauli, CERN 77-09 (1977). [2] Patricia R. Burchat, Gail G. Hanson and Hartmut F.-W. Sadrozinski, SLAC PUB 3475, Contribution to the IEEE Nuclear Science Symposium, Orlando, Florida (1984).
[3] William A. Rowe, Ionization Chamber for Use in High Energy Physics Experiments and Current Division for the MARK III End Caps, UCSC Senior Thesis (June 1980).
Nuclear Instruments and Methods in Physics Research A245 (1986) 155-158 North-Holland, Amsterdam
INVESTIGATION Paolo
OF BREAKDOWN
GIUBELLINO,
William
CONDITIONS
A. ROWE,
Hartmut
IN DRIFT CHAMBERS
F.-W.
SADROZINSKI
*
and Jeffrey
L. SKALA
Institute for Particle Physics, University of Calijornia, Santa Cruz, California 95064, USA
Received 17 October 1985
The electrostatic conditions for which breakdown and the total gain in the system.
occurs
in multiwire
1. Introduction We have investigated the conditions under which breakdown occurs in multiwire chambers, i.e. when large currents are drawn in the gas volume. One motivation for these investigations is to be able to predict “safe” parameters at which the chamber can be operated. These parameters include diameters of the sense and field wires, their number and arrangement, operating voltages and gas mixtures. The goal of our experiments was to compare the set of measurements to simple models of gas multiplication and derive criteria which indicate that breakdown occurs and that wire chamber performance are degraded.
chambers
Breakdown occurs if the gain on the wires is too large. We recognize that gas amplification can occur at both the sense wire and the field wire and we define an overall breakdown gain g, as the product of the gain on the sense wires g, and field wires g,:
(1)
g, = g&f. Following integrated
Sauli [l], the gain ‘g is determined Townsend coefficient a:
by the
g=exp where a is the wire radius and rc is the radius at which amplification starts. As in ref. [l], we assume that the Townsend coefficient is a function of the electric field E: CX=JkniE,
* Work supported
(3)
in part by the U.S. Department
of Energy.
0168-9002/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
as a function
of wire thickness
where k = 1.8 X 10-l’ (cm* V-‘) for argon, and N is the number of argon atoms per cm3. The amplification takes place in the radial field around the wire which is given by E=_=X’ x 2aror
(4)
r ’
where we introduce the scaled linear charge density h’, whose value is twice the value in electrostatic units. With
we find from eqs. (2), (3) and (4): g=exp(2JkN
2. Theory
are investigated
fl(Jj\‘/E,-
6)).
(6)
Thus the gain on a wire with radius a is a function of the charge per unit length X’ alone if the cutoff field EC is known. The cutoff field EC is the lowest field strength at which amplification occurs.
3. Determination of the cutoff field EC We used a single wire proportional tube of radius r = 3/Y to find the cutoff field as a function of wire diameter in a gas mixture of 89% A, 10% CO, and 1% CH,. The wire of radius a was connected either as an anode (“sense”) wire (wire grounded, negative high voltage on the tube) or as a cathode (“field”) wire (wire at negative high voltage, tube grounded). For different wire radii, the voltage, V,,, was determined at which excessive current was drawn (> 2 PA). For this breakdown point, the charge density Xi was determined according to
P. GiubeSm
156 Table 1 Breakdown
conditions
in a 3/4”
diameter
et al. / Breakdown conditions in drift chambers
tube ‘)
Field wire Diameter
Sense wire Diameter
(elm)
f% (kV)
E, (kV/cm)
(Ek:V/cm)
(elm)
xi? (kV)
E, (kV/cm)
:V/cm)
20 30 50 100 178 304
0.19 0.22 0.30 0.42 0.61 0.82
190 147 118 83 69 54
51 50 55 51 50 43
20 30 50 100 178
0.28 0.33 0.39 0.50 0.62
280 220 154 98 70
89 88 77 62 51
a) Gas: 89% A, 10% CO,,
1% CH,.
where c is the capacity/unit
length of the tube:
1 C=ln(r/a). In table 1 we show the scaled linear charge density XL and the fields E, = Xi/a on the surface of the wires at breakdown for several wire diameters 2a for both the sense wire and field wire configurations. Note that for equal wire diameter, breakdown occurs at lower fields when the wire is connected to the cathode (“field wire”). Electron emission could explain this effect. The cutoff field EC is then calculated in the following way: We have observed in the new MARK II drift chamber [2] a gain of g = lo5 at X’ = 0.28 kV (a = 15 urn). With eq. (6) we get EC = 88 kV/cm. Using this value for EC and Xl, = 0.33 kV from table 1 for breakdown, we get ‘c a dr= 17, / c1 and a gain of about g = 10’ for breakdown. The cutoff fields EC for all other wire diameters are then calculated assuming that breakdown occurs at the same gain, i.e., that /a dr = 17 for all wire diameters and configurations. These cutoff fields, .&, calculated with eqs. (6) and (9), are given in table 1. While EC is constant at 50 kV/cm for the field wires, we see a slight dependence of E, on the sense wire diameter. This could be due to our assumption about the dependence of a on E or effects of saturation of the gain. It is interesting to note that we get very similar values for EC if we extrapolate gain curves as a function of voltage [3] to unit gain and calculate the field E on the wire for that point.
long. The sense wire was grounded and the voltage on the field wires, V,, and the box, V, were varied to produce breakdown with different gains on sense and field wires. As above, a mixture of 89% A, 10% CO,, and 1% CH, was used. From electrostatic simulation, the scaled linear charge density Xi on the field wires and Xi on the sense wires were determined. Using the values for EC from table 1 for field and sense wires, the gains on the wires were calculated with eq. (6). Fig. 2 shows the gain on the field wire, g,, versus the gain on the sense wire, g,, at breakdown for sense wire diameters of 20, 30, 50, 100 and 178 pm. The data shows the correlation between the gain on the sense wires and the field wires, and suggests that breakdown occurs when the overall gain g, is of the order 10’. For large wire diameter, the calculation of the gain using eq. (6) becomes less reliable because it involves the difference of large numbers. Local field emission might also become
j--
f
4cm
0
0
0
0
E (y”
1
4. Breakdown in multiwire ceils To study the interplay of gain on sense and field wires to produce breakdown, the small test chamber sketched in fig. 1 was built: a sense wire of varied diameter was surrounded by four field wires of 178 u-diameter and enclosed by a conducting box, 25 cm
Fig. 1. Endview of the multiwire cell used to test breakdown. The replaceable sense wire of varied diameter (X) is at ground, the field wires (0) of 178 gm diameter are at high voltage Vr, and the outside conducting box is at high voltage Va. The length of the wires is 25 cm.
157
P. Giubellino et al. / Breakdown conditions in drift chambers IO9
I
1
I
I
I
I
I
I
SenseWire
$
_
20um 30um 50um
I OOum 175um
$ 0.4 0.2
0’
x
\e! I
1
I
I
100
I
I
I03 SENSE
WIRE
I
x
I
50
”
100
DURATION OFBREAKDOWN
x
0
Al
”
0
I
IO6
”
” 150
200
AT50pA
250 (hrs)
Fig. 3. Relative gain for two different gases as a function of the duration of breakdown at a current of 50 PA. The 50% points are 40 h (corresponding to 7 C integrated charge) for the 89% A. 10% CO,. 1% CH, mixture and 265 h (corresponding to 48 C integrated charge) for the 93% A, 3% CO,, 4% CH, mixture.
GAIN
Fig. 2. Field wire gain, gr, vs sense wire gain, g,, at breakdown in a mixture of 89% A, 10% CO,, 1% CH, for different sense wire diameters. The straight line, to guide the eyes, corre-
sponds to a total gain of 10’.
more important. We are not surprised from the simple behavior for diameters
to see deviation > 100 pm.
5. Deposits of polymers at breakdown In order to study the growth of polymers (“whiskers”) on the wires, we studied breakdown with different gases, mainly with A/CO&H, mixtures with more than 85% A, and also 50% A/50% C,H,. The assumption is that we can simulate prolonged irradiation with ionizing particles by forcing the chamber to discharge for a long time, and thus examine the gas stability. A chamber similar to the one shown in fig. 1, but with Lucite walls to facilitate visual observation of gas discharges and growth of polymers, was left at the breakdown voltage with a current of 50 PA for extended periods of time. Within less than 5 min, very long Z’whiskers” were observed in 50% A/50% C,H,, and 89% A, 10% CH,, 1% CO,. Both gas mixtures are known to grow polymers under experimental conditions in drift chambers. All other gases fall into two categories: if the CH, admixture was more than 5% polymerization was observed within 15 min, independent of the CO, content (between 1 and 10%). On the other hand, if the CH, content was less than 58, no polymerization was detected, and no changes in gain were observed even after the chamber was held at breakdown for several hours. To be more quantitative, we subjected two gas mix-
tures which did not grow whiskers and which are candidate gases for the new MARK II drift chamber [2], to continuous large current (50 PIA) in our test chamber shown in fig. 1. Every few hours the gain was monitored by measuring the pulse height produced by the photon from a 55Fe source. Fig. 3 shows the relative gain in 89% A, 10% CO,, 1% CH,, and 93% A, 3% CO, and 4% CH, as a function of the duration of breakdown at 50 PA. As can be seen, the two gases are degraded at different rates: the mixture with 93% A, 3% CO, and 4% CH, seems to survive about 7 times longer, possibly due to better quenching. The time at which the gain is reduced to 50% of its original value corresponds to an integrated charge of 7 C (40 h) for the 89% A, 10% CO,, 1% CH, mixture and to 48 C (265 h) for the 93% A, 3% CO,, 4% CH, mixture. We suggest that prolonged breakdown can be used as a necessary test to eliminate gases which are inclined to “whisker” growth.
6. Conclusion Our investigation of breakdown in multiwire chambers indicates that breakdown occurs when the overall gain on sense nnd field wires combined is about 10’. We used prolonged breakdown to test the inclination of gases to degrade the performance of the wire chamber.
Acknowledgment We thank Abraham Seiden, Patricia Ron Tyroler for their help and interest.
Burchat
and
158
P. Giubellino er al. / Breakdown conditions in drift chambers
References [l] F. Sauli, CERN 77-09 (1977). [2] Patricia R. Burchat, Gail G. Hanson and Hartmut F.-W. Sadrozinski, SLAC PUB 3475, Contribution to the IEEE Nuclear Science Symposium, Orlando, Florida (1984).
[3] William A. Rowe, Ionization Chamber for Use in High Energy Physics Experiments and Current Division for the MARK III End Caps, UCSC Senior Thesis (June 1980).