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Spatial resolution of induction chambers
Nuclear Instruments and Methods 207 (1983) 379-388 North-Holland Publishing Company
SPATIAL
RESOLUTION
OF INDUCTION
379
CHAMBERS
A.E. BONDAR, A.P. ONUCHIN, V.S. P A N I N Institute of Nuclear Physics, Novosibirsk 630090, USSR
a n d V.I. T E L N O V
Received 22 March 1982 and in revised form 12 July 1982
The spatial resolution of proportional chambers with the delay line connected to the cathode plane has been studied. A coaxial cable was used as a delay line. For the relativistic electrons a spatial resolution of 25 ~m (rms) for normal incidence and 120 ~m for an angle of inclination of 35 ° has been obtained. For protons with an energy of 65 MeV a spatial resolution of 15 /zm has been achieved for normal incidence. The factors that limit the accuracy of the method are discussed.
1. Introduction The cathode-coupled delay-line readout was suggested in 1970 [1,2]. This a p p r o a c h is attractive due to the fact that the wire spacing does not limit the spatial accuracy. This m e t h o d is now used for p r o p o r t i o n a l and drift chambers. In the work of refs. 3 - 5 a spatial resolution for the m i n i m u m ionizing particles of better t h a n 300 ~ m was obtained. In the work of ref. 3 the spatial resolution was 100 # m for n o r m a l incidence a n d 200 ~ m for an angle of inclination of 30 °. In refs. [6-8] the m e t h o d of coordinate m e a s u r e m e n t using pulses induced in the cathode strips is developed. A spatial resolution o b t a i n e d in such a way was of a b o u t 50 t~m for n o r m a l incidence [8]. Both m e t h o d s are very likely to have the same limiting accuracy, b u t the delay line readout requires the simpler electronics, t h o u g h it is more complex in construction. Let us note for comparison that the best resolution o b t a i n e d for drift c h a m b e r s is 60 /~m at the pressure p = 1 atm [9,10] a n d 30 /~m at p = 4 atm [10]. In the electroluminescent drift c h a m b e r at a xenon pressure of 20 a t m the accuracy o = 16 t~m [11] has been reached. The investigation of the cathode-coupled delay-line r e a d o u t b e g a n in our group in 1972. In the work rep o r t e d in ref. 12 we studied the different types of delay lines. For 5.9 keV X-rays from a 55Fe source a spatial resolution better t h a n 100 t~m (rms) was obtained. Preference was given to the coaxial cable line. Of most interest to us is the spatial resolution for charged particles, which was studied in refs. 13 a n d 14. F o r n o r m a l incidence a spatial resolution of a b o u t 30 t~m (rms) for relativistic electrons and 15 /~m for protons of energy 65 MeV was obtained. In the present p a p e r the results of a new cycle of m e a s u r e m e n t s for normally incident relativistic electrons and data for oblique incidence in the region 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
( 0 - 3 5 ) ° are presented. The c h a m b e r s had a size of the order of 3 cm. W e used gas mixtures based on Ar, isobutane, C O 2 with admixture of methylal, alcohol a n d freon 13-B1. Problems relating to the optimal choice of the c h a m b e r a n d delay line parameters are considered. T h e physical processes in the gas, which determine the resolution, are discussed.
2. The principle of operation of the induction chamber The essence of the m e t h o d is well k n o w n and consists of the following. After passage of the particle t h r o u g h the chamber, the moving electrons and ions of the avalanche induce charges on the cathode planes, with the centre of the charge distribution coinciding with the avalanche coordinate. The induced charge distribution has the form p ( x ) oc 1 / ( x 2 + d 2) with a width at half m a x i m u m of 2 d , where d is the gap between the a n o d e and cathode planes. W h e n the cathode plane with the wires stretched orthogonally to the anode wires is coupled to the delay line the induced signal spreads in b o t h directions. The avalanche coordinate is determined by the time difference of the pulse arrival at the ends of the delay line (DL). Since a signal is induced on several wires the resolution m a y be less than the cathode wire spacing. In accordance with the agreement reached at the I n t e r n a t i o n a l Meeting on wire c h a m b e r technique ( D u b n a , 1975) we refer to chambers with coordinate m e a s u r e m e n t s made by means of the induced charge distribution as induction chambers.
3. Ionization statistics The spatial accuracy of the induction c h a m b e r is limited by the statistical processes during p r o d u c t i o n
A.E. Bondar et aL / Spatial resolution of induction chambers
380
a n d collection of ionization. We shall consider the most i m p o r t a n t of these processes and present an estimation of their c o n t r i b u t i o n for the cases of normal and inclined incidence of the particles. The specific evaluations made for the c h a m b e r with a c a t h o d e - a n o d e gap of 2 m m a n d an anode wire spacing of 2 m m are presented in tables 1 and 2.
3.1. The ranges of primary electrons The primary electrons mostly have ranges of the order of a few microns and their contribution to the spatial accuracy is small. The energetic &electrons produce the main part of ionization, and their ranges may be long. The probability of a large transfer is described by the well k n o w n formula p ( e ) = a / f l Z e 2, a n d the electron range is R cc eL7 [7]. F r o m the known spatial resolution for Fe 55 (3 + 3 keV) o--- 100 ktm in Ar, we obtain the probability for the appearance in the chamber (4 m m thickness) of 6-electrons with a range greater than R : p = 0.2R 06/fl2, where R is expressed in microns. Hence, the probability of an electron with a range greater than 5 0 / ~ m equals 2%. Thus the primary electron ranges result in large coordinate fluctuations with a small probability, but do not practically influence the h a l f - m a x i m u m width of the distribution.
3.2. Diffusion The rms deviation of an electron in the electric field E after passing the length x equals %~f= 2 ~ k x / e E [15], where e k is the characteristic electron energy. Fig. 1 shows the dependence of o'0,dif (O'di f a t x = 1 cm) on concentrations of C O 2 and C4HI0 [15,16]. Note that in the E region of interest in CO 2, ~k CCE a n d in C4H10, e k 0CE 14. Thus, O0.oif depends on E only slightly, and one can neglect the inhomogeneity of E near the anode wires in the evaluation of the c o n t r i b u t i o n of the diffusion. Let us designate by o I and Op the diffusion contribution to the spatial accuracy for the cases when the avalanche is created by the first electron and by all primary electrons, respectively (the " m a g i c " a n d proportional regime). The m e a n distance from the track to the wire equals s/4, where s is the anode wire spacing and hence o I ~ O0,difV~J4. For the uniform distribution of the i o n i z a t i o n a l o n g the gap Op ---0.5% × V:[I + ~(1 - l / n ) ] / n o , where n o is the ionization density, n is the total n u m b e r of ion pairs, ~ is a factor taking into account the fluctuations of the gaseous gain a n d equals about 0.7 [17]. In the gas mixture Ar + 40% C4Hi0 for electrons on ionization plateau n o 130 a n d o 0 - - 2 0 0 /zm (fig. 1), n > > 1, hence o L = 5 0 and Op= 10 /tin. For A r + 2 0 % C O 2, n o = 90, o0 ~ 350 # m and the diffusion contribu-
=
E O ~
(CH ,,o)
~,/40C
~13C
~.
213(3
~5 c-
{00
20
140
60
n %
Fig. 1. R.m.s. deviation of an electron due to diffusion for 1 cm drift in the mixture At+ n%CO2(C4Hio ) at normal proessure: anode wire spacing is 2 ram. tion is somewhat greater: o I ~ 80 # m and op ~ 25 /~m. For the regime of limited proportionality the diffusion contribution to the spatial accuracy has an intermediate value between ol and %.
3.3. Fluctuations of ionization losses For the inclined incidence of charged particles (fig. 2) the spatial resolution gets worse because of fluctuations of the ionization density along the track. The distribution with respect to the distance from the closest electron to the wire has the form p ( ~ ) = ( 2 / X ) e x p ( - 2~/X), where X is the m e a n distance between the primary electrons. F r o m that one obtains o ( = 0.5X2 sin 2 0,. In the mixture Ar + 40%C4Hto on the ionization plateau = 180 ~ m and hence o I = 130 sin0:, /~m; for A r + 20%CO 2 X --- 2 2 0 / x m a n d o I ~ 160 sin 0~ ~m. For the proportional regime one can calculate the fluctuations of the ionization centre-of-gravity using the distribution function of the ionization losses [18]. A
//
oc&bOae,¢~ooym anode,¢ 28tim o
o
o
$
cathode
coQ~id co~&
--de~au
~ine
II
Fig. 2. Scheme of the chamber with the delay line.
A.E. Bondar et aL / Spatial resolution of induction chambers rough estimation shows that at small angles 0x Op ~ 350 sin 0x /~m for A r + 40%C4H10 a n d Op = 600 sin 0~ /~m for Ar + 20%CO 2. It is clear that for inclined incidence in the p r o p o r t i o n a l regime the fluctuations of the avalanche centre-of-gravity are greater t h a n those for the " m a g i c " regime.
- -
i
381 i
dUs
/
S C
4. Chamber, delay line, electronic noise 0
The resolution due to the electronics noise can be evaluated using the following formula ool = U.tr/Us'fd
where U~ is a pulse height, Un is a r.m.s, amplitude of noise, t r is the rise time of the pulse, ~a is the propagation delay of the delay lines. A s s u m i n g that Un is determined by the thermal noise of the i n p u t resistor, one has Un ac v/Z00Af, where Z 0 is the i n p u t resistance of the preamplifier (the characteristic i m p e d a n c e of DL), A f is the transmission b a n d of the preamplifier which equals - l / t r at o p t i m u m integration. The rise time of the pulse can be taken to be equal to the pulse d u r a t i o n at half m a x i m u m : t r = t s ~ ( t 2 + 4d2"r2) 1/2, where t i is the d u r a t i o n of the current pulse. Taking into account that ~ cc Z o Q J t ~ one obtains oel0c (t 2 + 4d2,rz)3/4/,ra~oQs, where Q~ is the charge induced on the cathode. The resolution is minimal at ~a = ti/v/2d, then o~l ~ d t~i/Zo/Q~. I neretore ~t ~s preleral31e to have a delay line with a high characteristic i m p e d a n c e to use a gas mixture providing a small time d u r a t i o n and a large height of the current pulse to have a c h a m b e r with a small gap. Decreasing the gap increases the charge fraction induced on the cathode. There exist several types of delay lines (see the review in ref. 19). Flat spiral delay lines with capacitive coupling to the c h a m b e r are widely used. They have a high value of Z o (up to 1500 1"2). ~-~= 10-100 n s / c m and small attenuation. However, such delay lines have large dispersion for short signals, which leads to nonlinearity a n d considerable worsening of the resolution caused by the electronics noise. There is also one undesirable feature of this type of delay line consisting of a considerable change of Z 0 a n d ~'d when coupled to the chamber [20]. N o n - u n i f o r m pressing of the delay line to the lamellas results in the nonlinearities of the delay line. The simplest delay line that transmits short signals well, is one made from coaxial cable. Despite a small line impedance ( 5 0 - 1 0 0 $2) such a delay line has advantages. Because of its large capacity per unit length the properties of the delay line do not change with coupling to the chamber. Fig. 3 shows the propeorties of a cable RC-100-1-41 with Z 0 = 93 $2. The pulse-height slew rate and fU~ dt are shown as a function of the time of passage of the
t
~o0
/~o0
,
o3o
8oo ~7 flS
Fig. 3. Attenuation in the coaxial cable RC-100-1-41: u~ is the
pulse height, S-fu s dt.
i n p u t signal with a d u r a t i o n at a half m a x i m u m of 25 ns. The slew rate influencing the resolution decrease by the law e x p ( - t/'r) with ~"= 280 ns. With this law of the signal a t t e n u a t i o n for long delay lines the optimal resolution is obtained at ~'d = "r/x, where x is the length of the delay line. U n d e r this condition oel cc t~/2x/ZoQs, that is the resolution gets worse proportionally to the length of the delay line. Thus, at the optimal ~'d for t i = 15 ns a n d d = 2 m m the resolution oeL is worse than for a short delay line by a factor of k = 0.25x (cm) (at k >> 1). We have chosen the following parameters for the chambers: the gap between anode a n d cathode planes d = 2 mm, the anode wire spacing 2 m m a n d the diameter of the anode wires 20/~m, the cathode wire spacing 1 m m and the diameter 100 ~m. The wires of opposite cathode planes were displaced by half a step, thus the effective cathode spacing was 0.5 mm. The fiducial size of the c h a m b e r was 3 × 3 cm 2, the delay line length was 2.2 cm. The cable RC-100-1-41 was used as the delay line, the p r o p a g a t i o n delay of the line was 84 n s / c m . The cable was soldered alternatively to the lamellas of b o t h cathodes. F o r reducing the electronics noise the choice of the preamplifier is of great significance. As shown by R a d e k a [21], with the help of "electron cooling" one can decrease the noise by a factor of ~ 5 (Oel
A.E. Bondar et a L / Spatial resolution of induction chambers
382
5. The differential nonlinearity (modulations)
6. Experimental set-up
Although the spatial resolution of an induction c h a m b e r is less than the cathode wire spacing, nevertheless small periodic deviations from a linear dependence of the measured coordinate from the avalanche position are retained. Thus in ref. 22 the measured value of such m o d u l a t i o n s for Fe 55 was _+ 70/~m. For the method u n d e r consideration the main reasons for m o d u l a t i o n s are the following: (a) A form of the signal from the delay line has modulations with period equal to a delay line step, which results in the periodic deviation from the linear t i m e - p o s i t i o n relationship. The value of these deviations is AX
The spatial resolution of the c h a m b e r s has been measured at the e -+ b e a m with an energy of 200 + 500 MeV, obtained by the conversion of b r e m s s t r a h l u n g T-quanta extracted from the VEPP-2 storage ring (fig. 5). Three induction c h a m b e r s were placed in a box with a gas mixture, the gap between the c h a m b e r s was 3 cm. The middle c h a m b e r could be rotated up to an angle of 35 ° a r o u n d the axis parallel to the cathode wires. Mechanical inaccuracies of the c h a m b e r s themselves and their positioning were at the level of 10 #m. The b r e m s s t r a h l u n g v - q u a n t a were converted in counter 1 into the e + e - - p a i r . The lead collimator 2 served for cutting the y-beam sizes a n d selecting single h a r d particles. The counter 4 detected the electrons passed through the chambers. The particles with an energy higher than 200 MeV were selected by the NaI(T1) counter. The size of the electron b e a m along the anode wires was a b o u t 4 m m and its intensity was about 1 kHz. To select the particles with 0 v --- 0, firing of the central wires of the edge c h a m b e r s was required. The electronics consisted of the preamplifiers with the "electron cooling", amplifier-shapers, discriminators a n d TDC. The shapers with an integration constant of 10 ns together with the zero crossing discriminators were employed as a constant fraction discriminator with a fraction of 0.6 and an internal split channel delay of 40 ns. This provided a close to o p t i m i u m slew-rate-tonoise ratio. A timing walk of the discriminators of less t h a n 300 ns has been achieved for an amplitude dyn a m i c range of 40 dB; this corresponded to a position shift of 20/~m. For the real pulse-height spectrum this value is considerably smaller. The T D C had a channel width of 50 ps, that corresponded to 3 ~ m in a position. The threshold of the electronics, reduced to the preamplifier input, was 0.03 mV. The electronics was calibrated by means of generator pulses put in the delay line and 55Fe source. The uncollimated source irradia-
x~0~
5
3 ..
*o
~
5
.,
/ /
/'
CATHODE WIlkES
2
/
/'
5
X (ram]
Fig. 4. Modulation at the irradiation of the chamber by a wide electron beam; the gas mixture: Ar+40%C4H10 +3%C3H802 + 0.25:CI~Br.
Fig. 5. Scheme of the measurement: 1, 4 - scintillation counters, 2 - lead collimator, 3 - induction chambers, 5 the total-absorption NaI(TI) counter.
383
A.E. Bondar et al. / Spatial resolution of induction chambers
7. The experimental results, normal incidence 7.1. Magic gas mixture
0 0 i
i
I
I
I
I
~CATHODE channaP.,
I
l
Wl'R E 5
nurn~:~ r
Fig. 6. The distribution of events over the measured coordinate at irradiation of the chamber by the uncollimated 55Fe source.
ted all the chambers. The distribution of the events in the coordinate measured has well defined peaks, corresponding to the wire position and due to the electrons coming from the region close to the cathode plane and outside the chambers (fig. 6). The linearity of the delay lines, measured in this way, was better than 10 ~ m (excluding the delay line edges). The spatial resolution was measured in the following way. For each event we calculated the quantity 4, the deviation of the coordinate in the middle chamber (no. 2), from the straight line coming through the coordinates in the edge chambers. The spatial resolution in the middle chamber was determined by the formula o# = o,~ - °2 + 032 + 4"S2CAT , 4 COS20x where ol. 3 is the resolution of the edge chambers, determined when high voltages on all the chambers were equal. The value of % was obtained from the width of the distributions: % = fwhm/2.36; OSCAT is the contribution due to the multiple scattering in the gas between the chambers and equals 10/xm. The spatial resolution was measured with a proton beam of energy E = 65 MeV, extracted from the N A P - M [23] intended for studying the electron cooling of protons. The intensity of the beam was about 1 kHz, the transverse size 1 mm, the angular divergency 0.3 × 10 - 4 rad. The profilometer [14], manufactured for the beam diagnostics, consisted of two chambers identical to that described above. Owing to a small angular spread, two chambers allowed one to measure the spatial accuracy. The gap between the anode planes was 10 m m and that between the first chamber and the entrance window (150 # m Be) was also 10 ram. Under these conditions the contribution of the multiple scattering at the entrance window and gas was about 15/~m. The measurements were performed only for normal incidence of protons.
The magic gas mixture [24,4,26] is well known by its large pulse height and the highly specific character of the gas amplification. The pulse height of the signal in this regime does not depend on the primary ionization, the time duration of the current pulse is less than 10 ns. The gas components of the " m a g i c " gas are argon, isobutane, freon 13B1 and methylal; the latter serves to prevent polymerization [26]. The nature of the magic gas was thought to be dependent on the electronegativity of freon. However, we observed a similar regime in the mixture without freon but with methylal. Apparently, the necessary condition is not the electronegativity but good photon absorption. The magic gas has been studied in the works of refs. 4 and 26 where a conclusion about the photon nature of the avalanche growth was made, moreover, most probably, the avalanche grows from the anode wire and reaches distances of about 1 mm. The large value of the fast component of the signal is due to the large distances of the electrons from the wire. For chambers with a delay line readout, the magic gas presents special interest, since the large pulse heights, small time duration and uniformity of the pulse shape lead to a decrease of the electronics noise contribution. The coaxial cable delay line allows one to use these advantages in the most complete way. We have carried out the measurements with the following mixtures: (i) Ar + 40%C4H10 + 3%C3H802 + 0.25%CF3Br (ii) Ar + 40%C4H10 + 3%C3H802 In figs. 7 and 8 we show the result of the measurements of the dependence of the anode puls height
r
.J c~o
t~8 /
cO 0- 2
/o/
/
~/°
A r +407oC.H,o+5~'oC~HsO~ ° - Fe- ~5 - e, (E =200+500 ]'4eV)
C
i
2.2
o
i
i
2.~
2.6
~ V
Fig. 7. Pulse height on the anode plane vs high voltage on the chamber; the integration constant of the electronics is 1 t~s.
384
A.E. Bondar et aL / Spatial resolution of induction chambers
2~
2
c
O
i
40 ~ o
~2
/ / /
Ck
o./
o../"
2
I
A r +40%C. H4o+3%C3HsOa+ O,25V~£F3 , - FeSS
/ 4"
/
•
- 2.bKV-Ar" ~Q~C.~he+St/oC~HeOe
o - e(.E = 200-500 MeV)
/
I
o
2LIKV .'~ , }Ar+40%C4F!m+57oC3HsOa+O.25CF~B
•
O.UKV J
I
2.2
i
i
i
2.4
2.6
2.8
t
1 #s) on the high voltage on the chamber for p h o t o n s from the 55Fe source and for electrons. The transition to the magic regime is clearly seen where the pulse heights become equal. In the intermediate region the pulse height distribution has two maxima; this is also shown in the figure. The pulse height spectra from an electron at angles of incidence of 0 and 35 ° obtained with the mixture I are displayed in fig. 9. F r o m here one can estimate the (Tin t =
~v
2."i'KV Z
0°
V, ~V
Fig. 8. Pulse height on the anode plane vs high voltage, Tint = 1 ffs.
~2h
,
o
~
O* 2 6 KV
i
L
20 °
(0 °
50 °
O~
Fig. 10. The ratio of the mean pulse height at inclined and normal incidence vs the angle of incidence.
avalanche size along the wire at high voltage. Since the pulse heights at 0~ = 0 and 0x = 35 ° became equal, the avalanche size at normal incidence reaches 2.5 m m the projection of the track onto the wire at 0X = 35". At lower voltages the nonproportionality of the gas gain is clearly seen - the ratio of the pulse height at inclined and normal incidence is larger than that following from the geometry. In fig. 10 the dependence of the anode pulse height on the incident angle is shown for the mixtures I and II. The spatial resolution as a function of high voltage is shown in fig. 11. In the low voltage region the degradation of the resolution is explained by the electronics noise; this was checked experimentally. The spatial resolution with the electronics noise contribution subtracted is also shown in the figure. With increasing high voltage,
2B ~V 13"
o
N 60
~ o x o
o
ELECTRONS E = 2C~+ ~ M e v
2
W
50
-
/ o
aA
o
E
oI
"
"
z,o
• - A t *40YoC~H,o+5YoC3HeOa+ 025% CF3Br" o
d?
Ar+hO~C.l-ho + 3%C3HBOe
~
CHANNA L
NUMBEe, A R B SCALE
Fig. 9. The pulse height spectra at 0~ = 0 and 35 ° at the various high voltages obtained on the electron beam; the gas mixture: Ar +40%C4Hi0 + 3%CBHsO2 +0.25%CF3Br.
.2
i
2.q
J
J
2.6
i
i
2.8
i
i
~,
KY
Fig. 11. The depencence of the spatial resolution on the high
voltage on the chamber; • - the resolution same as • but with the electronics noise subtracted.
385
A.E. Bondar et a L / Spatial resolution of induction chambers
N
ELECTR~ONS E=200-5001YIeV
aoo
DP0TONS
E = 6 5MeV
~, ~ 2 5 ~ r n
~,
= ,I 5 r m
z uJ > b3
i
200
SJ
200 X,~m
O
Fig. 12. The distribution of events over the difference between the measured coordinate in the middle chamber and that calculated with respect to the coordinates in the edge chambers; the gas mixture: Ar + 40%C4H1o + 3%C3H802 + 0.25%CF~ Br.
the deterioration of the resolution is connected with the fluctuation of the avalanche coordinate itself. The beginning of the growth coincides approximately with the transition to the magic regime. This can be explained by an increase of the diffusion c o n t r i b u t i o n and by the avalanche growth. The best resolution for mixture I is o = 25 # m and for mixture II o ~ 40/xm. Fig. 12 shows the distribution over A o b t a i n e d with the mixture I at the optimal voltage V = 2600 V. The n o n - G a u s s i a n tails are caused by &electrons. In fig. 13 the d e p e n d e n c e of the resolution on the high voltage measured with the p r o t o n b e a m is presented. The minimal resolution is 15 #m. The distribution over the difference of the coordinates measured in
i
,
,
CHANNAL
NUMBER'
Fig. 14. The distribution of events over the difference of the coordinates in two chambers, obtained on the proton beam; the gas mixture: Ar + 4 0 % C 4 H Jo + 7 % C 3 H 802 + 0.35 % C F 3 Br.
two c h a m b e r s is shown in fig. 14. Again, the long tails are due to the 8-electrons, which a p p e a r here more often t h a n in the m e a s u r e m e n t s with relativistic electrons. The resolution in o p t i m u m is better than with electrons owing to the higher density of the ionization a n d hence larger pulse height and smaller c o n t r i b u t i o n from diffusion. A t high voltage the resolutions became almost equal. Here the resolution is defined by the diffusion, a n d its c o n t r i b u t i o n in the magic regime is density-independent. So for n o r m a l incidence one should work at moderate voltages without the transition to the magic regime. 7.2. A r - C O 2
In fig. 15 the results of the m e a s u r e m e n t s on the mixture A r + 20%CO 2 for the n o r m a l incidence are T
(50
q
o - ~qr + 207. C0z
t00 E
E qo L %0"
•
o
,qr+2(Y'/*C02 +3%C~Ns0H
o
7 "/
5( 20
0 J
i
22
i
,
2q
i
i
26
i
i
28
i
~/
KV
Fig. 13. Dependence of the spatial resolution on the high voltage on the chamber.
,17
i
t8
t
~9
2 V, kV
Fig. 15. The spatial resolution vs high voltage; O - the resolution same as © but with the electronic noise subtracted.
386
A.E. Bondar et al. / Spatial resolution of induction chambers
p r e s e n t e d . It s h o u l d be n o t e d t h a t with t h e d i f f e r e n t g a s filling we h a d shifts in v o l t a g e o f u p to 70 V, a n d t h e r e s o l u t i o n , in o p t i m u m , c h a n g e d f r o m 57 to 70 /~m. A p p a r e n t l y , this is c o n n e c t e d with a fairly s h a r p d e p e n d e n c e o f t h e p u l s e h e i g h t o n the h i g h v o l t a g e a n d w i t h s t r o n g s e n s i t i v i t y to the u n c o n t r o l l e d a d m i x t u r e s . T h e a d d i t i o n of a l c o h o l i m p r o v e s t h e r e s o l u t i o n w h i c h testifies to the p h o t o n c o n t r i b u t i o n in t h e a v a l a n c h e dev e l o p m e n t a n d f l u c t u a t i o n s of its c o o r d i n a t e . A p p a r e n t l y , this effect is entirely r e s p o n s i b l e for the resolution d e g r a d a t i o n w i t h the h i g h v o l t a g e increase. In fig. 15 t h e s p a t i a l r e s o l u t i o n w i t h the e l e c t r o n i c s n o i s e s u b t r a c t e d is also s h o w n . It is seen t h a t a n i n t r i n s i c c h a m b e r r e s o l u t i o n at low v o l t a g e is o f a b o u t 25 /xm a n d b e c o m e s w o r s e with the h i g h v o l t a g e g r o w t h . T h e s p a t i a l r e s o l u t i o n o b t a i n e d at o p t i m a l h i g h voltage with t h e n o r m a l i n c i d e n c e a n d t h e c o n t r i b u t i o n o f
v a r i o u s f a c t o r s are collected in table 1. A l s o p r e s e n t e d h e r e is t h e best r e s o l u t i o n o b t a i n e d with t h e m i x t u r e A r + 2 0 % C O 2 for t h e p r o t o n b e a m . T h e a l c o h o l a n d f r e o n a d m i x t u r e s i m p r o v e s o m e w h a t t h e s p a t i a l resolution. I n c o m p a r i s o n with t h e e l e c t r o n s the r e s o l u t i o n is b e t t e r by a p p r o x i m a t e l y 30%. T h e result for C O 2 w i t h a l c o h o l a n d f r e o n a d m i x t u r e s is also p r e s e n t e d . T h i s m i x t u r e is i n t e r e s t i n g d u e to the fact t h a t t h e d i f f u s i o n in it is twice as s m a l l as t h a t in A r + 2 0 % C O 2. H o w e v e r , t h e r e s u l t s in o p t i m u m are a p p r o x i m a t e l y equal, t h a t c a n be a g a i n e x p l a i n e d by t h e p h o t o n m e c h a n i s m . It is n o t e x c l u d e d that, d u e to t h e large drift velocity in A r + 2 0 % C O 2 , s o m e d e t e r i o r a t i o n of the r e s o l u t i o n at a n y v o l t a g e s is c o n n e c t e d w i t h the e l e c t r o n s w h i c h c a m e f r o m t h e c a t h o d e wires in t h e vicinity with t h e n o n - u n i f o r m electric field.
Table 1 -0:, = 0 Gas mixture ~)
Particles b)
1
e
2 4 5 3 4 6 7 8
e e e p p p p p
O.min c) [p.m]
%1 d) [/~m]
25 40 60 + 70 50 15 45 40 35 45
15 20 30 25 13 35 30 23 15
Omin c) [~m]
Ofluct [~m]
O'diff"[/~m] ~J Prop. regime f~
1st el. g)
10 l0 25 25 5 12 12 12 3
50 50 80 80 50 80 80 80 25
Table 2 - 0x = 30 ° Gas mixture
1 2 4 ~) 1. 2. 3. 4. b) c) d) c) f) g)
Particles b) ~>
e e e
120 130 180
Prop. regime fl
1st el. gl
200 200 300
70 70 80
Gas mixtures: Ar+40%C4HI0 + 3%C3H802 +0.25%CF3Br 5. Ar + 20%CO 2 + 3 % C 2 H s O H Ar + 40%C 4 H 10 + 3%C3 HsO2 6. Ar + 20%CO 2 + 0.3%CF 3 Br Ar + 40%C4 Hlo + 7%C3HSO2 + 0.35%CF3Br 7. Ar + 20%CO2 + 3%C2 H s O H + 0.35%CF3 Br Ar + 20%CO 2 8. CO 2 + 3%C 2 H 5OH + 0.35 %CF~ Br Ee = 200 MeV, Ep = 65 MeV. °rain is the best resolution obtained at the optimal voltage. %1 is the electronics noise contribution at the optimal high voltage. %iff. is the diffusion contribution to the spatial resolution. Proportional regime. From the first electron.
387
A.E. Bondar et al. / Spatial resolution of induction chambers
8. Experimental results: inclined incidence ~,
The dependence of the resolution on the incident angle of a particle for mixtures I and II is shown in figs. 16 and 17. The curves are presented for the high voltage close to the optimal one at 0x = 0 and for the maximal voltage. It is seen that at large angles of incidence the resolution is better in the magic regime. The dependence of the resolution on the high voltage for mixture I at an incident angle of 35 ° is shown in fig. 18. Similar data for the mixture Ar + 20%CO 2 are presented in fig. 19. Here, again, at large angles the resolution is better at higher voltages. Moreover, the timing to the leading edge of the signal improves resolution as well (curve 3 - the internal delay of the constant fraction discriminator (see sect. 6) is 10 ns instead of 40 ns). In this case it is useful to work with the faster delayline. As it has been pointed out in sect. 3, at the inclined incidence the main contribution to the resolution is given by the fluctuation of the ionization losses, therefore it is preferable here to work at the highest voltage in the region of restricted proportionality or in the magic regime.
=35 °
(so
E
'LO. t00
~r+qOXCuH~o ~ 3VoC~HaOe+0 25FoCr3
22
26
28
V, K~/
.5
Fig. 18. The dependence of the spatial resolution on the high voltage at the incident angle of electrons, 8~ = 35 °.
o
Ar* 2o%coe 300
o-
t 6 KV
n
• - t98
,
t00
;;~'P~
.
o
~
+--
,,
O"
/
o
.
ELECTRO NS E:
i ~10~
t
J
i 20 °
200+
500MeV L
30 °
ex
Fig. 19. The spatial resolution vs the incident angle of electrons. '150
At,_~26KV40% c.N~o+3%CsHe.02 0 2 5 % C ~ +
o- 30KV
L
,~
/ "~o_ The main results for a 30 ° incident angle are presented in table 2. Just as in table 1 for diffusion, the estimations of the contribution of the fluctuation of ionization losses to the resolution are given for twolimiting cases: proportional and magic regimes.
tOO
50
ELECTI~'ONS
E = 200 + 50OMeV
i
0°
i 2~
~0 ~
i ,30"
Ox
9. Conclusion Fig. 16. The spatial resolution vs the incident angle of electrons. 1
r
i
A r- + ho%C. H4~+3%C..~4e,Oz
./" I
t 50
•
2q
o
26KV
/
KV
q
~///Io/-~
/
E7100
w50
ELECTRONG E
0°
L O
"1 °
i 20 °
= 200 ~ 500Me~
I -30°
0.
Fig. 17. The spatial resolution vs the incident angle of electrons.
On the chambers with the readout by the coaxial cable delay line in the measurement on the beam of relativistic electrons a spatial resolution of 25 tim (rms) at normal incidence and 120/~m at the angle 35 ° has been obtained. This resolution has been achieved with the magic gas. On the mixtures without electronegative admixtures a 40/~m resolution for normal incidence and 140/~m for inclined incidence (35 °) has been obtained. For the normal incidence of particles at the optimal high voltage on the chamber the resolution achieved is determined by the electronics noise and diffusion. At low voltages the main contribution is due to the electronics noise. With the growth of the voltage, the electronics noise decreases, but when passing to the magic regime the role of diffusion increases. Besides this, the avalanche in the magic gas reaches a size of 2 mm along the anode wire; during the growth of the avalanche due
388
A.E. Bondar et al. / Spatial resolution of induction chambers
to photons,noticeable fluctuations in the avalanche position arise. In the mixture Ar + 20%CO 2 at optimum voltage the resolution is connected, to a considerable extent, with fluctuations of the avalanche position caused by photons; the quenching admixtures improve the resolution. For inclined incidence the resolution is determined by the fluctuations in the distribution of ionization along the track, The best resolution is achieved with high voltages on the chamber, when the avalanche is created mainly by the electrons closest to the wire. There are some reserves of improving the resolution here. In particular, it is helpful to use fast delay lines and gases with a high density of primary ionization, The spatial accuracy obtained with the small chamber can be achieved for large chambers by dividing the cable into groups. Working with the magic gas, one can have a resolution of 50 > m for the group of 20 cm width, the delay per unit length should be smaller. For many applications induction chambers have notable advantages over drift chambers, They are simpler in operation and calibration, have lower requirements for gas stability, are less sensitive to the magnetic field and there is no left-right problem. The disadvantages are the complicated construction and the impossibility of manufacturing the cylindrical chambers which are used in the detectors with colliding beams. On the basis of the experience of this work, the scattered electron tagging system in the detector MD-1 has been created. The system contains 6 induction chambers of size 60 × 20 cm 2. The delay lines are 20 cm long. In the preliminary testing with electrons at an energy of 1.8 GeV the 100 ~ m resolution has been obtained. We wish to thank V.M. Aulchenko and V.I. Fominukch for design of electronics construction, A.A. Zhivalev, C.P. Pachin, R.G. Snopkov, V,S. Filippov and V.A. Shochin for their help in construction of the chambers and with measurements, V.A. Sidorov for support of this work and VEPP-2 and N A P - M staff for the possibility of performing the beam testings.
References [1] A. Rindi, V. Perez-Mendez and R. Wallace, Nucl. Instr. and Meth. 77 (1970) 325. [2] R. Grove, K. Lee, V. Perez-Mendez and J. Sperinde, Nucl. Instr. and Meth. 89 (1970) 257. [3] D. Lee and S. Sobotka, Nucl. Instr. and Meth. 104 (1972) 179. [4] J.L. Lacy and R.S. Lindsey, Nuct. Instr. and Meth. 119 (1974) 483. [5] H. Anderhub, J. B,Scklin, H. Hofer, D. Makowiecki, 13. Sapp, P. Seller and G. Wemmer, Nucl. Instr. and Meth. 142 (1977) 595. [6] G. Charpak and F. Sauli, Nucl. Instr. and Meth. 113 (1973) 381. [7] F. Sauli, Nucl. Instr. and Meth. 156 (1978) 147. [8] G. Charpak, G. Melchart, G. Petersen and Sauli, Nucl. Instr. and Meth. 167 (1979) 455. [9] N.A. Filatova et al., Nucl. Instr. and Meth. 143 (1977) 17. [10] W. Farr, J. Heintze, K.H. Hellenbrand and A.H. Walenta, Nucl. Instr. and Meth. 154 (1978) 175. [11] V.I. Baskakov et al., Nucl. Instr. and Meth. 158 (1979) 129. [12] A.E. Bondar, E.L. Panina, A.P. Onuchin, V.I. Telnov, Proc. Int. Mtg on Proportional and Drift Chambers, Dubna (1975) p. 219. [13] A.E. Bondar, A.P. Onuchin and V.I. Telnov, Proc. Int. Meeting on Proportional and Drift Chambers, Dubna (1978) p. 184. [14] V.M. Aulchenko, A.P. Onuchin. V.I. Telnov and V.I. Fominukch, ibid, p. 258. [15] V. Palladino, B. Sadoulet, Nucl. Instr. and Meth. 128 (1975) 323. [16] A.G. Engherardt and A.V. Phelps, Phys. Rev. 133A (1964) 375; R.D. Hake and AN.Phelps, Phys. Rev. 158 (1967) 70. [17] G.D. Alkhazov, Nucl. Instr. and Meth. 89, 155 (1970). [18] V.C. Ermilova, L.P. Kotenko and G.I. Merson, Nucl. Instr. and Meth. 145 (1977) 555. [19] Lecome, V. Perez-Mendez and G. Stoker, Nucl. Instr. and Meth. r53 (1978) 543. [20] R. Crove, V. Perez-Mendez and J. Sperinde, Nucl. Instr. and Meth. 106 (1973) 407. [21] V. Radeka, IEEE Trans. Nucl. Sci. 21 (1974) 51. [22] F. Swanson, F. Kuchne and A. Favale, IEEE Trans. Nucl. Sci. 20 (1974) 160. [23] V. Anashin et al. Proc. IV Nat. Conf. on Accelerators, vol. II (Nauka, Moscow, 1975) p. 304. [24] R. Bouclier et al., Nucl. Instr. and Meth. 88 (1970) 149. [25] G. Charpak et al., Nucl. Instr. and Meth. 97 (1971) 377. [26] J. Fischer, H. Okono and A. Walenta, Nucl. Instr. and Meth. 151 (1978)451.
SPATIAL
RESOLUTION
OF INDUCTION
379
CHAMBERS
A.E. BONDAR, A.P. ONUCHIN, V.S. P A N I N Institute of Nuclear Physics, Novosibirsk 630090, USSR
a n d V.I. T E L N O V
Received 22 March 1982 and in revised form 12 July 1982
The spatial resolution of proportional chambers with the delay line connected to the cathode plane has been studied. A coaxial cable was used as a delay line. For the relativistic electrons a spatial resolution of 25 ~m (rms) for normal incidence and 120 ~m for an angle of inclination of 35 ° has been obtained. For protons with an energy of 65 MeV a spatial resolution of 15 /zm has been achieved for normal incidence. The factors that limit the accuracy of the method are discussed.
1. Introduction The cathode-coupled delay-line readout was suggested in 1970 [1,2]. This a p p r o a c h is attractive due to the fact that the wire spacing does not limit the spatial accuracy. This m e t h o d is now used for p r o p o r t i o n a l and drift chambers. In the work of refs. 3 - 5 a spatial resolution for the m i n i m u m ionizing particles of better t h a n 300 ~ m was obtained. In the work of ref. 3 the spatial resolution was 100 # m for n o r m a l incidence a n d 200 ~ m for an angle of inclination of 30 °. In refs. [6-8] the m e t h o d of coordinate m e a s u r e m e n t using pulses induced in the cathode strips is developed. A spatial resolution o b t a i n e d in such a way was of a b o u t 50 t~m for n o r m a l incidence [8]. Both m e t h o d s are very likely to have the same limiting accuracy, b u t the delay line readout requires the simpler electronics, t h o u g h it is more complex in construction. Let us note for comparison that the best resolution o b t a i n e d for drift c h a m b e r s is 60 /~m at the pressure p = 1 atm [9,10] a n d 30 /~m at p = 4 atm [10]. In the electroluminescent drift c h a m b e r at a xenon pressure of 20 a t m the accuracy o = 16 t~m [11] has been reached. The investigation of the cathode-coupled delay-line r e a d o u t b e g a n in our group in 1972. In the work rep o r t e d in ref. 12 we studied the different types of delay lines. For 5.9 keV X-rays from a 55Fe source a spatial resolution better t h a n 100 t~m (rms) was obtained. Preference was given to the coaxial cable line. Of most interest to us is the spatial resolution for charged particles, which was studied in refs. 13 a n d 14. F o r n o r m a l incidence a spatial resolution of a b o u t 30 t~m (rms) for relativistic electrons and 15 /~m for protons of energy 65 MeV was obtained. In the present p a p e r the results of a new cycle of m e a s u r e m e n t s for normally incident relativistic electrons and data for oblique incidence in the region 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
( 0 - 3 5 ) ° are presented. The c h a m b e r s had a size of the order of 3 cm. W e used gas mixtures based on Ar, isobutane, C O 2 with admixture of methylal, alcohol a n d freon 13-B1. Problems relating to the optimal choice of the c h a m b e r a n d delay line parameters are considered. T h e physical processes in the gas, which determine the resolution, are discussed.
2. The principle of operation of the induction chamber The essence of the m e t h o d is well k n o w n and consists of the following. After passage of the particle t h r o u g h the chamber, the moving electrons and ions of the avalanche induce charges on the cathode planes, with the centre of the charge distribution coinciding with the avalanche coordinate. The induced charge distribution has the form p ( x ) oc 1 / ( x 2 + d 2) with a width at half m a x i m u m of 2 d , where d is the gap between the a n o d e and cathode planes. W h e n the cathode plane with the wires stretched orthogonally to the anode wires is coupled to the delay line the induced signal spreads in b o t h directions. The avalanche coordinate is determined by the time difference of the pulse arrival at the ends of the delay line (DL). Since a signal is induced on several wires the resolution m a y be less than the cathode wire spacing. In accordance with the agreement reached at the I n t e r n a t i o n a l Meeting on wire c h a m b e r technique ( D u b n a , 1975) we refer to chambers with coordinate m e a s u r e m e n t s made by means of the induced charge distribution as induction chambers.
3. Ionization statistics The spatial accuracy of the induction c h a m b e r is limited by the statistical processes during p r o d u c t i o n
A.E. Bondar et aL / Spatial resolution of induction chambers
380
a n d collection of ionization. We shall consider the most i m p o r t a n t of these processes and present an estimation of their c o n t r i b u t i o n for the cases of normal and inclined incidence of the particles. The specific evaluations made for the c h a m b e r with a c a t h o d e - a n o d e gap of 2 m m a n d an anode wire spacing of 2 m m are presented in tables 1 and 2.
3.1. The ranges of primary electrons The primary electrons mostly have ranges of the order of a few microns and their contribution to the spatial accuracy is small. The energetic &electrons produce the main part of ionization, and their ranges may be long. The probability of a large transfer is described by the well k n o w n formula p ( e ) = a / f l Z e 2, a n d the electron range is R cc eL7 [7]. F r o m the known spatial resolution for Fe 55 (3 + 3 keV) o--- 100 ktm in Ar, we obtain the probability for the appearance in the chamber (4 m m thickness) of 6-electrons with a range greater than R : p = 0.2R 06/fl2, where R is expressed in microns. Hence, the probability of an electron with a range greater than 5 0 / ~ m equals 2%. Thus the primary electron ranges result in large coordinate fluctuations with a small probability, but do not practically influence the h a l f - m a x i m u m width of the distribution.
3.2. Diffusion The rms deviation of an electron in the electric field E after passing the length x equals %~f= 2 ~ k x / e E [15], where e k is the characteristic electron energy. Fig. 1 shows the dependence of o'0,dif (O'di f a t x = 1 cm) on concentrations of C O 2 and C4HI0 [15,16]. Note that in the E region of interest in CO 2, ~k CCE a n d in C4H10, e k 0CE 14. Thus, O0.oif depends on E only slightly, and one can neglect the inhomogeneity of E near the anode wires in the evaluation of the c o n t r i b u t i o n of the diffusion. Let us designate by o I and Op the diffusion contribution to the spatial accuracy for the cases when the avalanche is created by the first electron and by all primary electrons, respectively (the " m a g i c " a n d proportional regime). The m e a n distance from the track to the wire equals s/4, where s is the anode wire spacing and hence o I ~ O0,difV~J4. For the uniform distribution of the i o n i z a t i o n a l o n g the gap Op ---0.5% × V:[I + ~(1 - l / n ) ] / n o , where n o is the ionization density, n is the total n u m b e r of ion pairs, ~ is a factor taking into account the fluctuations of the gaseous gain a n d equals about 0.7 [17]. In the gas mixture Ar + 40% C4Hi0 for electrons on ionization plateau n o 130 a n d o 0 - - 2 0 0 /zm (fig. 1), n > > 1, hence o L = 5 0 and Op= 10 /tin. For A r + 2 0 % C O 2, n o = 90, o0 ~ 350 # m and the diffusion contribu-
=
E O ~
(CH ,,o)
~,/40C
~13C
~.
213(3
~5 c-
{00
20
140
60
n %
Fig. 1. R.m.s. deviation of an electron due to diffusion for 1 cm drift in the mixture At+ n%CO2(C4Hio ) at normal proessure: anode wire spacing is 2 ram. tion is somewhat greater: o I ~ 80 # m and op ~ 25 /~m. For the regime of limited proportionality the diffusion contribution to the spatial accuracy has an intermediate value between ol and %.
3.3. Fluctuations of ionization losses For the inclined incidence of charged particles (fig. 2) the spatial resolution gets worse because of fluctuations of the ionization density along the track. The distribution with respect to the distance from the closest electron to the wire has the form p ( ~ ) = ( 2 / X ) e x p ( - 2~/X), where X is the m e a n distance between the primary electrons. F r o m that one obtains o ( = 0.5X2 sin 2 0,. In the mixture Ar + 40%C4Hto on the ionization plateau = 180 ~ m and hence o I = 130 sin0:, /~m; for A r + 20%CO 2 X --- 2 2 0 / x m a n d o I ~ 160 sin 0~ ~m. For the proportional regime one can calculate the fluctuations of the ionization centre-of-gravity using the distribution function of the ionization losses [18]. A
//
oc&bOae,¢~ooym anode,¢ 28tim o
o
o
$
cathode
coQ~id co~&
--de~au
~ine
II
Fig. 2. Scheme of the chamber with the delay line.
A.E. Bondar et aL / Spatial resolution of induction chambers rough estimation shows that at small angles 0x Op ~ 350 sin 0x /~m for A r + 40%C4H10 a n d Op = 600 sin 0~ /~m for Ar + 20%CO 2. It is clear that for inclined incidence in the p r o p o r t i o n a l regime the fluctuations of the avalanche centre-of-gravity are greater t h a n those for the " m a g i c " regime.
- -
i
381 i
dUs
/
S C
4. Chamber, delay line, electronic noise 0
The resolution due to the electronics noise can be evaluated using the following formula ool = U.tr/Us'fd
where U~ is a pulse height, Un is a r.m.s, amplitude of noise, t r is the rise time of the pulse, ~a is the propagation delay of the delay lines. A s s u m i n g that Un is determined by the thermal noise of the i n p u t resistor, one has Un ac v/Z00Af, where Z 0 is the i n p u t resistance of the preamplifier (the characteristic i m p e d a n c e of DL), A f is the transmission b a n d of the preamplifier which equals - l / t r at o p t i m u m integration. The rise time of the pulse can be taken to be equal to the pulse d u r a t i o n at half m a x i m u m : t r = t s ~ ( t 2 + 4d2"r2) 1/2, where t i is the d u r a t i o n of the current pulse. Taking into account that ~ cc Z o Q J t ~ one obtains oel0c (t 2 + 4d2,rz)3/4/,ra~oQs, where Q~ is the charge induced on the cathode. The resolution is minimal at ~a = ti/v/2d, then o~l ~ d t~i/Zo/Q~. I neretore ~t ~s preleral31e to have a delay line with a high characteristic i m p e d a n c e to use a gas mixture providing a small time d u r a t i o n and a large height of the current pulse to have a c h a m b e r with a small gap. Decreasing the gap increases the charge fraction induced on the cathode. There exist several types of delay lines (see the review in ref. 19). Flat spiral delay lines with capacitive coupling to the c h a m b e r are widely used. They have a high value of Z o (up to 1500 1"2). ~-~= 10-100 n s / c m and small attenuation. However, such delay lines have large dispersion for short signals, which leads to nonlinearity a n d considerable worsening of the resolution caused by the electronics noise. There is also one undesirable feature of this type of delay line consisting of a considerable change of Z 0 a n d ~'d when coupled to the chamber [20]. N o n - u n i f o r m pressing of the delay line to the lamellas results in the nonlinearities of the delay line. The simplest delay line that transmits short signals well, is one made from coaxial cable. Despite a small line impedance ( 5 0 - 1 0 0 $2) such a delay line has advantages. Because of its large capacity per unit length the properties of the delay line do not change with coupling to the chamber. Fig. 3 shows the propeorties of a cable RC-100-1-41 with Z 0 = 93 $2. The pulse-height slew rate and fU~ dt are shown as a function of the time of passage of the
t
~o0
/~o0
,
o3o
8oo ~7 flS
Fig. 3. Attenuation in the coaxial cable RC-100-1-41: u~ is the
pulse height, S-fu s dt.
i n p u t signal with a d u r a t i o n at a half m a x i m u m of 25 ns. The slew rate influencing the resolution decrease by the law e x p ( - t/'r) with ~"= 280 ns. With this law of the signal a t t e n u a t i o n for long delay lines the optimal resolution is obtained at ~'d = "r/x, where x is the length of the delay line. U n d e r this condition oel cc t~/2x/ZoQs, that is the resolution gets worse proportionally to the length of the delay line. Thus, at the optimal ~'d for t i = 15 ns a n d d = 2 m m the resolution oeL is worse than for a short delay line by a factor of k = 0.25x (cm) (at k >> 1). We have chosen the following parameters for the chambers: the gap between anode a n d cathode planes d = 2 mm, the anode wire spacing 2 m m a n d the diameter of the anode wires 20/~m, the cathode wire spacing 1 m m and the diameter 100 ~m. The wires of opposite cathode planes were displaced by half a step, thus the effective cathode spacing was 0.5 mm. The fiducial size of the c h a m b e r was 3 × 3 cm 2, the delay line length was 2.2 cm. The cable RC-100-1-41 was used as the delay line, the p r o p a g a t i o n delay of the line was 84 n s / c m . The cable was soldered alternatively to the lamellas of b o t h cathodes. F o r reducing the electronics noise the choice of the preamplifier is of great significance. As shown by R a d e k a [21], with the help of "electron cooling" one can decrease the noise by a factor of ~ 5 (Oel
A.E. Bondar et a L / Spatial resolution of induction chambers
382
5. The differential nonlinearity (modulations)
6. Experimental set-up
Although the spatial resolution of an induction c h a m b e r is less than the cathode wire spacing, nevertheless small periodic deviations from a linear dependence of the measured coordinate from the avalanche position are retained. Thus in ref. 22 the measured value of such m o d u l a t i o n s for Fe 55 was _+ 70/~m. For the method u n d e r consideration the main reasons for m o d u l a t i o n s are the following: (a) A form of the signal from the delay line has modulations with period equal to a delay line step, which results in the periodic deviation from the linear t i m e - p o s i t i o n relationship. The value of these deviations is AX
The spatial resolution of the c h a m b e r s has been measured at the e -+ b e a m with an energy of 200 + 500 MeV, obtained by the conversion of b r e m s s t r a h l u n g T-quanta extracted from the VEPP-2 storage ring (fig. 5). Three induction c h a m b e r s were placed in a box with a gas mixture, the gap between the c h a m b e r s was 3 cm. The middle c h a m b e r could be rotated up to an angle of 35 ° a r o u n d the axis parallel to the cathode wires. Mechanical inaccuracies of the c h a m b e r s themselves and their positioning were at the level of 10 #m. The b r e m s s t r a h l u n g v - q u a n t a were converted in counter 1 into the e + e - - p a i r . The lead collimator 2 served for cutting the y-beam sizes a n d selecting single h a r d particles. The counter 4 detected the electrons passed through the chambers. The particles with an energy higher than 200 MeV were selected by the NaI(T1) counter. The size of the electron b e a m along the anode wires was a b o u t 4 m m and its intensity was about 1 kHz. To select the particles with 0 v --- 0, firing of the central wires of the edge c h a m b e r s was required. The electronics consisted of the preamplifiers with the "electron cooling", amplifier-shapers, discriminators a n d TDC. The shapers with an integration constant of 10 ns together with the zero crossing discriminators were employed as a constant fraction discriminator with a fraction of 0.6 and an internal split channel delay of 40 ns. This provided a close to o p t i m i u m slew-rate-tonoise ratio. A timing walk of the discriminators of less t h a n 300 ns has been achieved for an amplitude dyn a m i c range of 40 dB; this corresponded to a position shift of 20/~m. For the real pulse-height spectrum this value is considerably smaller. The T D C had a channel width of 50 ps, that corresponded to 3 ~ m in a position. The threshold of the electronics, reduced to the preamplifier input, was 0.03 mV. The electronics was calibrated by means of generator pulses put in the delay line and 55Fe source. The uncollimated source irradia-
x~0~
5
3 ..
*o
~
5
.,
/ /
/'
CATHODE WIlkES
2
/
/'
5
X (ram]
Fig. 4. Modulation at the irradiation of the chamber by a wide electron beam; the gas mixture: Ar+40%C4H10 +3%C3H802 + 0.25:CI~Br.
Fig. 5. Scheme of the measurement: 1, 4 - scintillation counters, 2 - lead collimator, 3 - induction chambers, 5 the total-absorption NaI(TI) counter.
383
A.E. Bondar et al. / Spatial resolution of induction chambers
7. The experimental results, normal incidence 7.1. Magic gas mixture
0 0 i
i
I
I
I
I
~CATHODE channaP.,
I
l
Wl'R E 5
nurn~:~ r
Fig. 6. The distribution of events over the measured coordinate at irradiation of the chamber by the uncollimated 55Fe source.
ted all the chambers. The distribution of the events in the coordinate measured has well defined peaks, corresponding to the wire position and due to the electrons coming from the region close to the cathode plane and outside the chambers (fig. 6). The linearity of the delay lines, measured in this way, was better than 10 ~ m (excluding the delay line edges). The spatial resolution was measured in the following way. For each event we calculated the quantity 4, the deviation of the coordinate in the middle chamber (no. 2), from the straight line coming through the coordinates in the edge chambers. The spatial resolution in the middle chamber was determined by the formula o# = o,~ - °2 + 032 + 4"S2CAT , 4 COS20x where ol. 3 is the resolution of the edge chambers, determined when high voltages on all the chambers were equal. The value of % was obtained from the width of the distributions: % = fwhm/2.36; OSCAT is the contribution due to the multiple scattering in the gas between the chambers and equals 10/xm. The spatial resolution was measured with a proton beam of energy E = 65 MeV, extracted from the N A P - M [23] intended for studying the electron cooling of protons. The intensity of the beam was about 1 kHz, the transverse size 1 mm, the angular divergency 0.3 × 10 - 4 rad. The profilometer [14], manufactured for the beam diagnostics, consisted of two chambers identical to that described above. Owing to a small angular spread, two chambers allowed one to measure the spatial accuracy. The gap between the anode planes was 10 m m and that between the first chamber and the entrance window (150 # m Be) was also 10 ram. Under these conditions the contribution of the multiple scattering at the entrance window and gas was about 15/~m. The measurements were performed only for normal incidence of protons.
The magic gas mixture [24,4,26] is well known by its large pulse height and the highly specific character of the gas amplification. The pulse height of the signal in this regime does not depend on the primary ionization, the time duration of the current pulse is less than 10 ns. The gas components of the " m a g i c " gas are argon, isobutane, freon 13B1 and methylal; the latter serves to prevent polymerization [26]. The nature of the magic gas was thought to be dependent on the electronegativity of freon. However, we observed a similar regime in the mixture without freon but with methylal. Apparently, the necessary condition is not the electronegativity but good photon absorption. The magic gas has been studied in the works of refs. 4 and 26 where a conclusion about the photon nature of the avalanche growth was made, moreover, most probably, the avalanche grows from the anode wire and reaches distances of about 1 mm. The large value of the fast component of the signal is due to the large distances of the electrons from the wire. For chambers with a delay line readout, the magic gas presents special interest, since the large pulse heights, small time duration and uniformity of the pulse shape lead to a decrease of the electronics noise contribution. The coaxial cable delay line allows one to use these advantages in the most complete way. We have carried out the measurements with the following mixtures: (i) Ar + 40%C4H10 + 3%C3H802 + 0.25%CF3Br (ii) Ar + 40%C4H10 + 3%C3H802 In figs. 7 and 8 we show the result of the measurements of the dependence of the anode puls height
r
.J c~o
t~8 /
cO 0- 2
/o/
/
~/°
A r +407oC.H,o+5~'oC~HsO~ ° - Fe- ~5 - e, (E =200+500 ]'4eV)
C
i
2.2
o
i
i
2.~
2.6
~ V
Fig. 7. Pulse height on the anode plane vs high voltage on the chamber; the integration constant of the electronics is 1 t~s.
384
A.E. Bondar et aL / Spatial resolution of induction chambers
2~
2
c
O
i
40 ~ o
~2
/ / /
Ck
o./
o../"
2
I
A r +40%C. H4o+3%C3HsOa+ O,25V~£F3 , - FeSS
/ 4"
/
•
- 2.bKV-Ar" ~Q~C.~he+St/oC~HeOe
o - e(.E = 200-500 MeV)
/
I
o
2LIKV .'~ , }Ar+40%C4F!m+57oC3HsOa+O.25CF~B
•
O.UKV J
I
2.2
i
i
i
2.4
2.6
2.8
t
1 #s) on the high voltage on the chamber for p h o t o n s from the 55Fe source and for electrons. The transition to the magic regime is clearly seen where the pulse heights become equal. In the intermediate region the pulse height distribution has two maxima; this is also shown in the figure. The pulse height spectra from an electron at angles of incidence of 0 and 35 ° obtained with the mixture I are displayed in fig. 9. F r o m here one can estimate the (Tin t =
~v
2."i'KV Z
0°
V, ~V
Fig. 8. Pulse height on the anode plane vs high voltage, Tint = 1 ffs.
~2h
,
o
~
O* 2 6 KV
i
L
20 °
(0 °
50 °
O~
Fig. 10. The ratio of the mean pulse height at inclined and normal incidence vs the angle of incidence.
avalanche size along the wire at high voltage. Since the pulse heights at 0~ = 0 and 0x = 35 ° became equal, the avalanche size at normal incidence reaches 2.5 m m the projection of the track onto the wire at 0X = 35". At lower voltages the nonproportionality of the gas gain is clearly seen - the ratio of the pulse height at inclined and normal incidence is larger than that following from the geometry. In fig. 10 the dependence of the anode pulse height on the incident angle is shown for the mixtures I and II. The spatial resolution as a function of high voltage is shown in fig. 11. In the low voltage region the degradation of the resolution is explained by the electronics noise; this was checked experimentally. The spatial resolution with the electronics noise contribution subtracted is also shown in the figure. With increasing high voltage,
2B ~V 13"
o
N 60
~ o x o
o
ELECTRONS E = 2C~+ ~ M e v
2
W
50
-
/ o
aA
o
E
oI
"
"
z,o
• - A t *40YoC~H,o+5YoC3HeOa+ 025% CF3Br" o
d?
Ar+hO~C.l-ho + 3%C3HBOe
~
CHANNA L
NUMBEe, A R B SCALE
Fig. 9. The pulse height spectra at 0~ = 0 and 35 ° at the various high voltages obtained on the electron beam; the gas mixture: Ar +40%C4Hi0 + 3%CBHsO2 +0.25%CF3Br.
.2
i
2.q
J
J
2.6
i
i
2.8
i
i
~,
KY
Fig. 11. The depencence of the spatial resolution on the high
voltage on the chamber; • - the resolution same as • but with the electronics noise subtracted.
385
A.E. Bondar et a L / Spatial resolution of induction chambers
N
ELECTR~ONS E=200-5001YIeV
aoo
DP0TONS
E = 6 5MeV
~, ~ 2 5 ~ r n
~,
= ,I 5 r m
z uJ > b3
i
200
SJ
200 X,~m
O
Fig. 12. The distribution of events over the difference between the measured coordinate in the middle chamber and that calculated with respect to the coordinates in the edge chambers; the gas mixture: Ar + 40%C4H1o + 3%C3H802 + 0.25%CF~ Br.
the deterioration of the resolution is connected with the fluctuation of the avalanche coordinate itself. The beginning of the growth coincides approximately with the transition to the magic regime. This can be explained by an increase of the diffusion c o n t r i b u t i o n and by the avalanche growth. The best resolution for mixture I is o = 25 # m and for mixture II o ~ 40/xm. Fig. 12 shows the distribution over A o b t a i n e d with the mixture I at the optimal voltage V = 2600 V. The n o n - G a u s s i a n tails are caused by &electrons. In fig. 13 the d e p e n d e n c e of the resolution on the high voltage measured with the p r o t o n b e a m is presented. The minimal resolution is 15 #m. The distribution over the difference of the coordinates measured in
i
,
,
CHANNAL
NUMBER'
Fig. 14. The distribution of events over the difference of the coordinates in two chambers, obtained on the proton beam; the gas mixture: Ar + 4 0 % C 4 H Jo + 7 % C 3 H 802 + 0.35 % C F 3 Br.
two c h a m b e r s is shown in fig. 14. Again, the long tails are due to the 8-electrons, which a p p e a r here more often t h a n in the m e a s u r e m e n t s with relativistic electrons. The resolution in o p t i m u m is better than with electrons owing to the higher density of the ionization a n d hence larger pulse height and smaller c o n t r i b u t i o n from diffusion. A t high voltage the resolutions became almost equal. Here the resolution is defined by the diffusion, a n d its c o n t r i b u t i o n in the magic regime is density-independent. So for n o r m a l incidence one should work at moderate voltages without the transition to the magic regime. 7.2. A r - C O 2
In fig. 15 the results of the m e a s u r e m e n t s on the mixture A r + 20%CO 2 for the n o r m a l incidence are T
(50
q
o - ~qr + 207. C0z
t00 E
E qo L %0"
•
o
,qr+2(Y'/*C02 +3%C~Ns0H
o
7 "/
5( 20
0 J
i
22
i
,
2q
i
i
26
i
i
28
i
~/
KV
Fig. 13. Dependence of the spatial resolution on the high voltage on the chamber.
,17
i
t8
t
~9
2 V, kV
Fig. 15. The spatial resolution vs high voltage; O - the resolution same as © but with the electronic noise subtracted.
386
A.E. Bondar et al. / Spatial resolution of induction chambers
p r e s e n t e d . It s h o u l d be n o t e d t h a t with t h e d i f f e r e n t g a s filling we h a d shifts in v o l t a g e o f u p to 70 V, a n d t h e r e s o l u t i o n , in o p t i m u m , c h a n g e d f r o m 57 to 70 /~m. A p p a r e n t l y , this is c o n n e c t e d with a fairly s h a r p d e p e n d e n c e o f t h e p u l s e h e i g h t o n the h i g h v o l t a g e a n d w i t h s t r o n g s e n s i t i v i t y to the u n c o n t r o l l e d a d m i x t u r e s . T h e a d d i t i o n of a l c o h o l i m p r o v e s t h e r e s o l u t i o n w h i c h testifies to the p h o t o n c o n t r i b u t i o n in t h e a v a l a n c h e dev e l o p m e n t a n d f l u c t u a t i o n s of its c o o r d i n a t e . A p p a r e n t l y , this effect is entirely r e s p o n s i b l e for the resolution d e g r a d a t i o n w i t h the h i g h v o l t a g e increase. In fig. 15 t h e s p a t i a l r e s o l u t i o n w i t h the e l e c t r o n i c s n o i s e s u b t r a c t e d is also s h o w n . It is seen t h a t a n i n t r i n s i c c h a m b e r r e s o l u t i o n at low v o l t a g e is o f a b o u t 25 /xm a n d b e c o m e s w o r s e with the h i g h v o l t a g e g r o w t h . T h e s p a t i a l r e s o l u t i o n o b t a i n e d at o p t i m a l h i g h voltage with t h e n o r m a l i n c i d e n c e a n d t h e c o n t r i b u t i o n o f
v a r i o u s f a c t o r s are collected in table 1. A l s o p r e s e n t e d h e r e is t h e best r e s o l u t i o n o b t a i n e d with t h e m i x t u r e A r + 2 0 % C O 2 for t h e p r o t o n b e a m . T h e a l c o h o l a n d f r e o n a d m i x t u r e s i m p r o v e s o m e w h a t t h e s p a t i a l resolution. I n c o m p a r i s o n with t h e e l e c t r o n s the r e s o l u t i o n is b e t t e r by a p p r o x i m a t e l y 30%. T h e result for C O 2 w i t h a l c o h o l a n d f r e o n a d m i x t u r e s is also p r e s e n t e d . T h i s m i x t u r e is i n t e r e s t i n g d u e to the fact t h a t t h e d i f f u s i o n in it is twice as s m a l l as t h a t in A r + 2 0 % C O 2. H o w e v e r , t h e r e s u l t s in o p t i m u m are a p p r o x i m a t e l y equal, t h a t c a n be a g a i n e x p l a i n e d by t h e p h o t o n m e c h a n i s m . It is n o t e x c l u d e d that, d u e to t h e large drift velocity in A r + 2 0 % C O 2 , s o m e d e t e r i o r a t i o n of the r e s o l u t i o n at a n y v o l t a g e s is c o n n e c t e d w i t h the e l e c t r o n s w h i c h c a m e f r o m t h e c a t h o d e wires in t h e vicinity with t h e n o n - u n i f o r m electric field.
Table 1 -0:, = 0 Gas mixture ~)
Particles b)
1
e
2 4 5 3 4 6 7 8
e e e p p p p p
O.min c) [p.m]
%1 d) [/~m]
25 40 60 + 70 50 15 45 40 35 45
15 20 30 25 13 35 30 23 15
Omin c) [~m]
Ofluct [~m]
O'diff"[/~m] ~J Prop. regime f~
1st el. g)
10 l0 25 25 5 12 12 12 3
50 50 80 80 50 80 80 80 25
Table 2 - 0x = 30 ° Gas mixture
1 2 4 ~) 1. 2. 3. 4. b) c) d) c) f) g)
Particles b) ~>
e e e
120 130 180
Prop. regime fl
1st el. gl
200 200 300
70 70 80
Gas mixtures: Ar+40%C4HI0 + 3%C3H802 +0.25%CF3Br 5. Ar + 20%CO 2 + 3 % C 2 H s O H Ar + 40%C 4 H 10 + 3%C3 HsO2 6. Ar + 20%CO 2 + 0.3%CF 3 Br Ar + 40%C4 Hlo + 7%C3HSO2 + 0.35%CF3Br 7. Ar + 20%CO2 + 3%C2 H s O H + 0.35%CF3 Br Ar + 20%CO 2 8. CO 2 + 3%C 2 H 5OH + 0.35 %CF~ Br Ee = 200 MeV, Ep = 65 MeV. °rain is the best resolution obtained at the optimal voltage. %1 is the electronics noise contribution at the optimal high voltage. %iff. is the diffusion contribution to the spatial resolution. Proportional regime. From the first electron.
387
A.E. Bondar et al. / Spatial resolution of induction chambers
8. Experimental results: inclined incidence ~,
The dependence of the resolution on the incident angle of a particle for mixtures I and II is shown in figs. 16 and 17. The curves are presented for the high voltage close to the optimal one at 0x = 0 and for the maximal voltage. It is seen that at large angles of incidence the resolution is better in the magic regime. The dependence of the resolution on the high voltage for mixture I at an incident angle of 35 ° is shown in fig. 18. Similar data for the mixture Ar + 20%CO 2 are presented in fig. 19. Here, again, at large angles the resolution is better at higher voltages. Moreover, the timing to the leading edge of the signal improves resolution as well (curve 3 - the internal delay of the constant fraction discriminator (see sect. 6) is 10 ns instead of 40 ns). In this case it is useful to work with the faster delayline. As it has been pointed out in sect. 3, at the inclined incidence the main contribution to the resolution is given by the fluctuation of the ionization losses, therefore it is preferable here to work at the highest voltage in the region of restricted proportionality or in the magic regime.
=35 °
(so
E
'LO. t00
~r+qOXCuH~o ~ 3VoC~HaOe+0 25FoCr3
22
26
28
V, K~/
.5
Fig. 18. The dependence of the spatial resolution on the high voltage at the incident angle of electrons, 8~ = 35 °.
o
Ar* 2o%coe 300
o-
t 6 KV
n
• - t98
,
t00
;;~'P~
.
o
~
+--
,,
O"
/
o
.
ELECTRO NS E:
i ~10~
t
J
i 20 °
200+
500MeV L
30 °
ex
Fig. 19. The spatial resolution vs the incident angle of electrons. '150
At,_~26KV40% c.N~o+3%CsHe.02 0 2 5 % C ~ +
o- 30KV
L
,~
/ "~o_ The main results for a 30 ° incident angle are presented in table 2. Just as in table 1 for diffusion, the estimations of the contribution of the fluctuation of ionization losses to the resolution are given for twolimiting cases: proportional and magic regimes.
tOO
50
ELECTI~'ONS
E = 200 + 50OMeV
i
0°
i 2~
~0 ~
i ,30"
Ox
9. Conclusion Fig. 16. The spatial resolution vs the incident angle of electrons. 1
r
i
A r- + ho%C. H4~+3%C..~4e,Oz
./" I
t 50
•
2q
o
26KV
/
KV
q
~///Io/-~
/
E7100
w50
ELECTRONG E
0°
L O
"1 °
i 20 °
= 200 ~ 500Me~
I -30°
0.
Fig. 17. The spatial resolution vs the incident angle of electrons.
On the chambers with the readout by the coaxial cable delay line in the measurement on the beam of relativistic electrons a spatial resolution of 25 tim (rms) at normal incidence and 120/~m at the angle 35 ° has been obtained. This resolution has been achieved with the magic gas. On the mixtures without electronegative admixtures a 40/~m resolution for normal incidence and 140/~m for inclined incidence (35 °) has been obtained. For the normal incidence of particles at the optimal high voltage on the chamber the resolution achieved is determined by the electronics noise and diffusion. At low voltages the main contribution is due to the electronics noise. With the growth of the voltage, the electronics noise decreases, but when passing to the magic regime the role of diffusion increases. Besides this, the avalanche in the magic gas reaches a size of 2 mm along the anode wire; during the growth of the avalanche due
388
A.E. Bondar et al. / Spatial resolution of induction chambers
to photons,noticeable fluctuations in the avalanche position arise. In the mixture Ar + 20%CO 2 at optimum voltage the resolution is connected, to a considerable extent, with fluctuations of the avalanche position caused by photons; the quenching admixtures improve the resolution. For inclined incidence the resolution is determined by the fluctuations in the distribution of ionization along the track, The best resolution is achieved with high voltages on the chamber, when the avalanche is created mainly by the electrons closest to the wire. There are some reserves of improving the resolution here. In particular, it is helpful to use fast delay lines and gases with a high density of primary ionization, The spatial accuracy obtained with the small chamber can be achieved for large chambers by dividing the cable into groups. Working with the magic gas, one can have a resolution of 50 > m for the group of 20 cm width, the delay per unit length should be smaller. For many applications induction chambers have notable advantages over drift chambers, They are simpler in operation and calibration, have lower requirements for gas stability, are less sensitive to the magnetic field and there is no left-right problem. The disadvantages are the complicated construction and the impossibility of manufacturing the cylindrical chambers which are used in the detectors with colliding beams. On the basis of the experience of this work, the scattered electron tagging system in the detector MD-1 has been created. The system contains 6 induction chambers of size 60 × 20 cm 2. The delay lines are 20 cm long. In the preliminary testing with electrons at an energy of 1.8 GeV the 100 ~ m resolution has been obtained. We wish to thank V.M. Aulchenko and V.I. Fominukch for design of electronics construction, A.A. Zhivalev, C.P. Pachin, R.G. Snopkov, V,S. Filippov and V.A. Shochin for their help in construction of the chambers and with measurements, V.A. Sidorov for support of this work and VEPP-2 and N A P - M staff for the possibility of performing the beam testings.
References [1] A. Rindi, V. Perez-Mendez and R. Wallace, Nucl. Instr. and Meth. 77 (1970) 325. [2] R. Grove, K. Lee, V. Perez-Mendez and J. Sperinde, Nucl. Instr. and Meth. 89 (1970) 257. [3] D. Lee and S. Sobotka, Nucl. Instr. and Meth. 104 (1972) 179. [4] J.L. Lacy and R.S. Lindsey, Nuct. Instr. and Meth. 119 (1974) 483. [5] H. Anderhub, J. B,Scklin, H. Hofer, D. Makowiecki, 13. Sapp, P. Seller and G. Wemmer, Nucl. Instr. and Meth. 142 (1977) 595. [6] G. Charpak and F. Sauli, Nucl. Instr. and Meth. 113 (1973) 381. [7] F. Sauli, Nucl. Instr. and Meth. 156 (1978) 147. [8] G. Charpak, G. Melchart, G. Petersen and Sauli, Nucl. Instr. and Meth. 167 (1979) 455. [9] N.A. Filatova et al., Nucl. Instr. and Meth. 143 (1977) 17. [10] W. Farr, J. Heintze, K.H. Hellenbrand and A.H. Walenta, Nucl. Instr. and Meth. 154 (1978) 175. [11] V.I. Baskakov et al., Nucl. Instr. and Meth. 158 (1979) 129. [12] A.E. Bondar, E.L. Panina, A.P. Onuchin, V.I. Telnov, Proc. Int. Mtg on Proportional and Drift Chambers, Dubna (1975) p. 219. [13] A.E. Bondar, A.P. Onuchin and V.I. Telnov, Proc. Int. Meeting on Proportional and Drift Chambers, Dubna (1978) p. 184. [14] V.M. Aulchenko, A.P. Onuchin. V.I. Telnov and V.I. Fominukch, ibid, p. 258. [15] V. Palladino, B. Sadoulet, Nucl. Instr. and Meth. 128 (1975) 323. [16] A.G. Engherardt and A.V. Phelps, Phys. Rev. 133A (1964) 375; R.D. Hake and AN.Phelps, Phys. Rev. 158 (1967) 70. [17] G.D. Alkhazov, Nucl. Instr. and Meth. 89, 155 (1970). [18] V.C. Ermilova, L.P. Kotenko and G.I. Merson, Nucl. Instr. and Meth. 145 (1977) 555. [19] Lecome, V. Perez-Mendez and G. Stoker, Nucl. Instr. and Meth. r53 (1978) 543. [20] R. Crove, V. Perez-Mendez and J. Sperinde, Nucl. Instr. and Meth. 106 (1973) 407. [21] V. Radeka, IEEE Trans. Nucl. Sci. 21 (1974) 51. [22] F. Swanson, F. Kuchne and A. Favale, IEEE Trans. Nucl. Sci. 20 (1974) 160. [23] V. Anashin et al. Proc. IV Nat. Conf. on Accelerators, vol. II (Nauka, Moscow, 1975) p. 304. [24] R. Bouclier et al., Nucl. Instr. and Meth. 88 (1970) 149. [25] G. Charpak et al., Nucl. Instr. and Meth. 97 (1971) 377. [26] J. Fischer, H. Okono and A. Walenta, Nucl. Instr. and Meth. 151 (1978)451.