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Multi-anode resistive wire proportional counter as a focal plane detector
Nuclear Instruments and Methods 171 (1980) 61-66 © North-Holland Publishing Company
61
MULTI-ANODE RESISTIVE WIRE PROPORTIONAL COUNTER AS A FOCAL PLANE DETECTOR
K. IWATANI *, H. YOKOMIZO **, M. TANAKA, S. KATO t , T. HASEGAWA h~stitute for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188, Japan H. HASAI and F. NISHIYAMA Department of Applied Physics and Chemistry, Faculty of lz)lgineering, Hiroshima University, Hiroshima 730, Japan Received 5 October 1979
A resistive-wire position-sensitive proportional counter used in the focal plane of a magnetic spectrograph has been improved by a multi-anode arrangement available for the non-normal incidence of particles. The achieved position resolution is 0.75 mm fwhm for 28 MeV protons and 0.9 mm fwhm for 50 MeV protons with an incidence angle of 35 ° to the wire axis. The non-normal incidence effect is also described with a measurement of the angle dependence of the position resolution in a simple resistivewire proportional counter.
1. Introduction
The position-sensitive proportional counter with single resistive anode, which was first devised by Kuhlmann et al. [1], has been used in many laboratories as a particle detector mounted in the focal plane o f a magnetic spectrograph for nuclear reaction studies. This type o f detector, which we hereafter call SWPC (single wire proportional counter), has advantages in its simplicity o f mechanical construction, adjustment and read-out system. High position resolution better than 0.5 mm fwhm is obtainable when particles are incident normal to the counter. For the non-normal incidence, however, the resolution is often degraded in SWPC. The degradation is quite serious for high energy light particles which have low specific ionization in the counter gas. This phenomenon with non-normal incidence (non-normal effect) has been recognized as being attributable to energy loss fluctuations in thin detector layers such as the Landau effect for electrons. Several workers calculated [2 5] the energy and particle dependence o f this effect but experimental data systematically corn* Present address: Department of Applied Physics and Chemistry, Faculty of Engineering, Hiroshima University, Hiroshima 730, Japan. ** Present address: Japan Atomic Energy Research Institute, Tokai, Ibaragi, Japan. t Present address: Faculty of General Education, Yamagata University, Yamagata, Japan.
pared with the theory have been rare. We are thus in a situation where the non-normal effect can be understood only qualitatively. In most magnetic spectrographs constructed so far, the particles reach the focal plane at angles around 0 = 45 °. Therefore the simple SWPC as an actual focal plane detector, despite its advantages, suffers with respect to the position resolution which can be obtained. Markham and Robertson constructed [4] a proportional counter effectively subdivided into five thinner ones with the multi-anode. They used a lumped constant delay line and inclined cathode stripes, resulting in a remarkable improvement: a position resolution o f 0.25 mm was obtained with 35 MeV protons at 0 = 45 °. The aim o f the present work is to build a focal plane detector o f general use for the INS QDD-type magnetic spectrograph [6] and in high resolution experiments with light particles such as protons, deuterons and so forth. Because o f its simplicity we adopted' the SWPC scheme for this purpose. In the INS spectrograph, when the horizontal size o f the beam on the target is 1 m m and the solid angle is 5 msr, the spread o f image is about 0.7 mm along the focal line. It corresponds to the energy resolving power E / A E = 5000. The mean angle o f particle injection is very acute and 35 ° to the focal plane. In our attempts to reduce the contributions from the nonnormal effect, we found a new design o f the SWPC scheme with the multi-anode.
62
K. Iwatani et al. / A focal plane detector
This report describes not only a 20 cm long counter which was constructed as a test bed for realizing the idea of the multi-anode, but also the non-normal effect in a simple SWPC. In our discussion of counter performances we deal only with position resolution.
2. Non-normal effect in the SWPC When a particle is incident at an angle 0 to the direction of the anode wire axis in a SWPC, the avalanche is developed along the wire in a length ZXL which corresponds to the projection of the particle trajectory within the counter, 2xL = D c o t 0, where D is the depth of the counter (see fig. 1). The induced counter signal includes position information about the center of gravity of the charges liberated with the avalanche length 2xL. The position of the center o gravity, Xc, is always the middle point o f 2xL if the energy loss is uniform along the particle passage. In the usual case of non-uniform energy loss, however, the point X c fluctuates from the geometrical middle point with the fluctuation ZXXc, which results in the degradation of the counter position resolution. One can see this, for instance, by imagining the case when the number o f primary ion-pairs formed at the forward and backward parts of particle passage are different. In the case of normal incidence (0 = 90°), 2xXc can be ignored because 2xL equals zero regardless of the existence of energy loss fluctuations. The extent of ~ g c is affected by the amount of ionization fluctuation in the counter, which depends on
several parameters such as a particle, its energy and incident angle 0, a counter depth D, a counter gas and its pressure. To investigate the incident angle dependence, we measured the position resolution with a SWPC under the following experimental conditions. The test counter has an active size of 1.5 × 1.5 × 20 cm 3 with a resistive anode wire which is a carbon coated quartz fiber 0.025 mm in diameter with a resistivity of 8 K ~2 mm -1. The entrance and exit windows consist of 0.004 m m thick aluminized Mylar. The counter gas is a mixture of 95% Ar and 5% CO2 and is circulated continuously at 1 atm. The anode voltage is 1400 V. Two signals from each side of the anode wire are treated by charge sensitive pre-amplifiers, shaping amplifiers (time constant z = 1/Js) and analog to digital converters. The position spectrum is taken by the charge division method with use of a digital computer. A 51.9 MeV proton beam accelerated using the INS-FM cyclotron is led to a scattering chamber. Elastic scattering protons from polyethylene are defined to the spread of 0.25 mm by means of two tantalum slits in front of and behind the counter and with coincidence method using a NaI (T1) scintillation detector. The scintillation detector is also used for the particle and energy identification. As shown in fig. 2, the measurement of the position resolution at angles of 90 °, 60 ° and 35 ° resulted in 0.28 ram, 1.1 m m and 2.6 mm respectively for 50
1.0
/IL (crn) 2.0
I
I
-/ PARTICLE
E
At.
~
~.
3.C
30
//
CATHODE
D
I
i
'
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I
[ "--4\
ANODE
CATHODE
i
2.C
0~7/ Fig. 1o Schematic illustration of a SWPCwith a position spectrum. D is the counter depth. A particle is incident at an angle 0 and then avalanches occur along the anode wire in a length ~L. The center of gravity of the liberated charge is at Xc, the middle point of AL. By the energy loss fluctuation, the position spectrum has a finite spread AXc.
90
t 60 (~ (degrees)
I 30
Fig. 2. Non-normal effect on the SWPC position resolution
for 50 MeV protons. The raw data (solid circles) are corrected for beam size and then drawn as a function of AL (empty circles).
K. lwatani et al. / A focal plane detector
63
3. Multi-anode resistive wire proportional c o u n t e r
MeV protons. The result is also drawn in the figure as a function of the particle trajectory length AL projecting onto the anode wire. In the latter case, the values of the resolution were corrected for beam size, i.e., the spread along the anode wire, which were 0.25 mm, 0 . 2 9 m m and 0 . 4 4 m m at 90 °, 60 ° and 35 ° respectively. One can see that the resolution degradation is roughly proportional to AL. This means that if the angle of particle injection remains constant, the thinner the counter, the lesser the contribution from the non-normal effect. In table 1, the present results are listed with all the experimental data that have ever been obtained by SWPC scheme. One can see in general that the position resolution AX for normal incidence is better than 1 mm but for non-normal incidence it is worse than 1 m m with two exceptions which are the data with a very thin counter (Borkowski et al. and Ford et al.). The ~60-data, as the results o f Saghai et al show, must be not affected by the non-normal effect because of the small fluctuation in the energy loss of heavy ions.
3.1. Design principles The multi-wire proportional chamber (MWPC) is an example of a thin counter which has a conventional active thickness of about 0.5 mm and relatively restrains the non-normal effect. Furthermore one can confine the sensitive volume to a smaller size by mixing a small amount of Freon gas with the counter gas, and by operating the counter system at high gain and with a short shaping time constant. In the case of SWPC, it is impractical to try to reduce the depth of a counter without changing its height. Because the electric field in such a counter deviates considerably from a uniform cylindrical one, the gas multiplication factor varies with the vertical position of the particle incidence. Another disadvantage of making a counter thinner is the small signal such a counter gives because of small energy loss. This is undesirable especially for light particles in relation to the system noise. Then, we designed a modifica-
Table 1 Position resolution which has been obtained by the SWPC scheme Incident particle
Th C, C'-alpha 21 oPo -alpha Protons from ORIC Alpha Protons Alpha 241 Am-alpha X-ray 160 Alpha from RI source Tritons Protons 160 160 Th C, C'-alpha Alpha
Energy (MeV)
Counter depth D (cm)
6.1, 8.8 5.3
0.35 1.3 (cylinder) 1.0 (cylinder) 10 0.6 ~15 0.35 5.5 / 3.0 (cylinder) 8 keV ~ 60 1.0 / , 1.0 42 1.0 ~56 0.64 (drift type) 52.4 2.2 6.1, 8.8~ 3 8 ~ 6 3 } 1.5
Protons
50
Protons Protons
50 28
1.5 }
1.0 (multi-anode)
Position resolution (fwhm) With normal incidence AX (mm) 1.6 ~ 3.4 0.15
0.27 (X-ray) 0.42 0.6 0.88 0.2
Author and Ref.
With nonnormal incidence AX (ram)
0 (deg.)
Kuhlmann et al. [ 1l 0.3 2.5 1.6 ~ 1.0 0.7
45 40 45 45 45
0.28
Miller et al. [2] Ford et al. [81 Hough et al. [91 Harvey et al. [10]
1.1 1.1
45 45
0.6 ~
Borkowski et al. [7]
1.0 ~ 1.5 ~<1.0 1.1 4~5
45 45 30 30
~ 1.1 / 2.6 0.9 0.75
60 35 }
35
Fulbright et al. [ 11 l Homeyer et al. [12] Erskine et al. [13] Saghai et al. [141 M611er et al. [5]
Present
64
K. lwatani et al. / A focal plane detector
tion o f the SWPC which should not so change the counter depth and height, and which should match the SWPC advantages o f construction, adjustment and read-out method. The design principles o f the counter are described as follows: (1) the counter layer is effectively subdivided into thinner layers b y replacing the single resistive anode wire with several resistive wires in the plane o f particle trajectory. (2) Each wire is electrically connected together at b o t h ends and the connection lines are inclined to coincide with the angle o f particle incidence to the focal plane, so that the relative positions o f induced charge are the same on all wires. (3) Adding guard wires in front o f and behind the active region, the electric field in the active layer is arranged so as to be macroscopically similar to that made b y a parallel plate. (4) Signal handling out o f the counter is just the same as the method o f SWPC. Thus the counter is virtually made up o f several thin counters in tandem. For the number o f wires, N, the non-normal effect on each wire is expected to be 1/N o f that found with the SWPC, being proportional to the gas thickness undertaken b y one wire. Connecting N wires, the overall reduction becomes to 1 / N v ~ . For example, its amount is 0.09 for N = 5. Consequently, with this idea a multi-anode system o f the counter constructed by Markham and Robertson can be realized in a quite simple way. 3.2. Construction and operation Based on above principles, we constructed the counter which had five anode and four guard wires with 2 mm spacing as shown in fig. 3. The active size o f the counter was 20 cm in length, 1.5 cm in height and 1.0 cm in depth. This depth corresponds to the particle pass length o f 1.74 cm when the particle is incident at 0 = 35 °. The distance between entrance and exit windows was 3.0 cm. These windows were o f a 0.012 mm thick aluminized Mylar foil to endure a 1 atm pressure difference in a vacuum chamber. The anode wires consisted o f the same material as those o f the SWPC. The b e r y l l i u m - c o p p e r guard wires were 0.1 m m in diameter and were biased at 80% o f the anode voltage to prevent the multiplications from occurring in them. The wires were fastened at each end with epoxy resin in metal pipes. The pipes were 0.3 m m in inside diameter and were held b y a Teflon
particle
active region /
c°nnecti~ineS ~
cathode
~%'~\ \ ~ , x ~ - - - ~ ~.
~
k guard cathode
Fig. 3. Schematic of the multi-anode resistive wire proportional counter.
stand. A tiny spot o f conducting paint at the nib o f the pipe ensured the electrical continuity and determined the end o f wire resistivity. These end points were aligned at 0 = 35 ° with an accuracy o f 1.5 °, i.e., the connection lines on b o t h sides were inclined according to the angle o f particle incidence. Figure 4 shows an illustration o f the electrostatic field in the counter cross section, which was calculated by a fourth-order image method. Because the electric field in the active region is similar to that o f the parallel plate, the working anode voltage was 2900 V and about two times as high compared with that o f the SWPC. The signals on the guard wires short-circuited through the high voltage capacitors and resistors.
~THODE WINDOW o I
E.~UAR D WIRE ANODEWIRE
1 I I
Fig. 4. Calculated electric field in the counter cross section. Curved solid lines indicate the actual electric lines of force at the boundaries of the wires and these appear only in a quarter part of the counter cross section due to geometrical asymmetry. Dotted lines indicate electric lines of force assuming a perfect parallel plate field.
65
K. Iwatani et al. / A focal plane detector
4. Result and discussion
particle
Two experiments were carried out for the multianode resistive wire proportional counter. In the first one, the experimental set-up was the same as described in section 2. The particles were incident to the counter at 0 = 35 °. The measured position spectra for 50 MeV protons are shown in fig. 5 and the position resolution o f 0.9 mm was obtained with the beam collimation o f 0.5 mm. The second experiment was performed under actual conditions where the counter was used in the focal plane. Protons accelerated by the INS-SF cyclotron were scattered from 100/ag cm -2 thick gold foil and analyzed by the INS QDD-type magnetic spectrograph• The position resolution o f 1.0 ram, which is shown in fig. 6, was obtained at 0 = 35 ° for 28 MeV protons and somewhat limited b y the spread o f beam image o f about 0.6 ram. Both results showed a remarkable improvement over the simple SWPC in which the best resolution o f 1.6 mm can be estimated from fig. 2 for a 1.0 cm depth counter• The results also showed a difference from the expected resolution o f 0.69 mm, which consisted o f contributions from the spread o f the image o f 0.6 ram, the residual non-normal effect o f 0.15 mm and circuit noise o f 0.2 ram. In order to check the angle dependence o f resolution in this counter, the second experiment was carried out also varying the detector setting angle from the designed one o f 35 °. As shown in fig. 6, the o p t i m u m setting angle shifted 5 ° to the large angle, where the resolution came up to 0.75
Ep = 50 MeV
1oo 5mm C
_~ 50
g
"
•
-•
0 1090
0.9ram fwhm
.'.
" 1140 Channel
~
I
1190 Number
" 1240
Fig. 5. T w o p o s i t i o n s p e c t r a w i t h t h e n e w c o u n t e r /'or 5 0 MeV p r o t o n s . P o s i t i o n shifts o f 5 m m a l o n g t h e f o c a l p l a n e c o r r e s p o n d s to 5 0 c h a n n e l s .
ED : 2 8 M e V
E
~
2.0
erector
tO 0 tO ¢U
~ 1.0 ._o
0 30 0
I 40 (degrees)
50
Fig. 6. Position resolution as a function of the detector setting angel.
mm. The shift suggests that the inclination angle o f the wire connection lines is not appropriate and there is a discrepancy between the trajectory o f the incident particle and that "felt" b y the counter. It can be seen from the following considerations that this phenomenon is caused by the deviation o f the electric field from the parallel plate one, i.e., by fringe field effect, and that it is determined b y the geometrical arrangement o f wires and cathodes• Comparing the two kinds o f lines illustrated in fig. 4, it can be seen that the space for each wire, except the central wire, shifts to the outside (near window) in the actual counter• The mean amount o f space shift for the outermost anode wire was calculated to be 1.0 mm at the middle height o f a n o d e - c a t h o d e gap. Suppose that primary ion-pairs are produced at this height• Electrons then drift toward the anode along the curved electric line o f force and cause an avalanche in the vicinity o f the anode. The positive ions formed by the avalanche drift back to the cathode along the same line and generate a counter signal• The velocity o f the positive ions is so small that, for the signal shaping time o f a few/~s, they are only separated from the wire by a distance o f about 0.5 ram, i.e., the center o f gravity o f the charge induced on the wire exists almost at the middle point o f avalanche length. Accordingly, a difference occurs in the sites o f the primary ion-pairs and the resulting induced charge• Fig. 7 illustrates the t o p view o f the counter and two inclined lines o f 0 = 35 ° and 29•3 ° . The former represents a particle trajectory and the latter a line connecting each center o f gravity o f charge on the wires under the condition o f the above-calculated
66
K. Iwatani et al. / A focal plane detector p/ particle lralectory
able for the focal plane detector for the INS QDDtype magnetic spectrograph.
anode
wires
Fig. 7. Schematic of the sites of the primary ion-pairs (A) and induced charges (B) for each anode wire. The particle is supposed to enter at the middle height of anode-cathode gap and the primary ion-pairs are affected in the field as shown in fig. 4.
shifts. The angle difference o f 5.7 ° is nearly consistent with the experimental value shown in fig. 6. One solution in order to reduce the fringe field effect is to make the a n o d e - c a t h o d e gap narrower or to make the distance between the entrance and exit windows large enough to add more guard wires. A simple method is to coincide the inclination of connection line with that of induced charge. It can also be achieved by connecting the resistors to the ends of anode wires and thus compensating the connection lines. We adopted the last mentioned modification not changing the counter geometry and verified the above hypothesis by another test, achieving the optim u m position resolution in actual use. We tested a modification of the SWPC scheme, the multi-anode resistive wire proportional counter, by using high energy protons under rather severe conditions where the non-normal effect is serious, and confirmed the design principles for the counter. As a result, the 40 cm long counters have been constructed and are currently being used with a performance suit-
We are indebted to Mr. Y. Fujita of INS and Professor J. Sanada of the University of Tsukuba for helpful comments and suggestions. We gratefully acknowledge Mr. K. Omata and Dr. M. Yasue for their preparation of the computer digital division program.
References [1] W.R. Kuhlmann, K.H. Lauterjung, B. Schimmer and K. Sistemich, Nucl. Instr. and Meth. 40 (1966) 118. [2] G.L. Miller, N. Williams, A. Senator, R. Stensgaard and J. Fischer, Nucl. Instr. and Meth. 91 (1971) 389. [31 T.H. Braid and S.A. Zawadzki, Bull. Am. Phys. Soc. 17 (1972) 120. [41 R.G. Markham and R.G.H. Robertson, Nucl. Instr. and Meth. 129 (1975) 131. [5] P. M611er and H.V. Klapdor, Nucl. Instr. and Meth. 142 (1977) 447. [6] S. Kate, T, Hasegawa and M. Tanaka, Nucl. Instr. and Meth. 154 (1978) 19. [7] C.J. Borkowski and M.K. Kopp, IEEE Trans. Nucl. Sci. NS-17 (1970) 340. [8] J.L.C. Ford, Jr., P.H. Stelson and R.L. Robinson, Nucl. Instr. and Meth. 98 (1972) 199. [9] J. Hough and R.W.P. Drever, Nucl. Instr. and Meth. 103 (1972) 365. [10] B.G. Harvey , J. Mahoney, F.G. Piihlhoffer, F.S. Goulding, D.A. Landis, J.C. Faivre, D.G. Kovar, M.S. Zisman, J.R. Meriwether, S.W. Cosper and D.L. Hendrie, Nucl. Instr. and Meth. 104 (1972) 21. [1 l] H.W. Fulbright, R.G. Markham and W.A. Lanford, Nucl. Instr. and Meth. 108 (1973) 125. [12] H. Homeyer, J. Mahoney and B.G. Harvey, Nucl. Instr. and Meth. 118 (1974) 311. [13] J.R. Erskine, T.H. Braid and J.C. Stoltzfus, Nucl. Instr. and Meth. 135 (1976) 67. [14] B. Saghai and P. Roussel, Nucl. Instr. and Meth. 141 (1977) 93.
61
MULTI-ANODE RESISTIVE WIRE PROPORTIONAL COUNTER AS A FOCAL PLANE DETECTOR
K. IWATANI *, H. YOKOMIZO **, M. TANAKA, S. KATO t , T. HASEGAWA h~stitute for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188, Japan H. HASAI and F. NISHIYAMA Department of Applied Physics and Chemistry, Faculty of lz)lgineering, Hiroshima University, Hiroshima 730, Japan Received 5 October 1979
A resistive-wire position-sensitive proportional counter used in the focal plane of a magnetic spectrograph has been improved by a multi-anode arrangement available for the non-normal incidence of particles. The achieved position resolution is 0.75 mm fwhm for 28 MeV protons and 0.9 mm fwhm for 50 MeV protons with an incidence angle of 35 ° to the wire axis. The non-normal incidence effect is also described with a measurement of the angle dependence of the position resolution in a simple resistivewire proportional counter.
1. Introduction
The position-sensitive proportional counter with single resistive anode, which was first devised by Kuhlmann et al. [1], has been used in many laboratories as a particle detector mounted in the focal plane o f a magnetic spectrograph for nuclear reaction studies. This type o f detector, which we hereafter call SWPC (single wire proportional counter), has advantages in its simplicity o f mechanical construction, adjustment and read-out system. High position resolution better than 0.5 mm fwhm is obtainable when particles are incident normal to the counter. For the non-normal incidence, however, the resolution is often degraded in SWPC. The degradation is quite serious for high energy light particles which have low specific ionization in the counter gas. This phenomenon with non-normal incidence (non-normal effect) has been recognized as being attributable to energy loss fluctuations in thin detector layers such as the Landau effect for electrons. Several workers calculated [2 5] the energy and particle dependence o f this effect but experimental data systematically corn* Present address: Department of Applied Physics and Chemistry, Faculty of Engineering, Hiroshima University, Hiroshima 730, Japan. ** Present address: Japan Atomic Energy Research Institute, Tokai, Ibaragi, Japan. t Present address: Faculty of General Education, Yamagata University, Yamagata, Japan.
pared with the theory have been rare. We are thus in a situation where the non-normal effect can be understood only qualitatively. In most magnetic spectrographs constructed so far, the particles reach the focal plane at angles around 0 = 45 °. Therefore the simple SWPC as an actual focal plane detector, despite its advantages, suffers with respect to the position resolution which can be obtained. Markham and Robertson constructed [4] a proportional counter effectively subdivided into five thinner ones with the multi-anode. They used a lumped constant delay line and inclined cathode stripes, resulting in a remarkable improvement: a position resolution o f 0.25 mm was obtained with 35 MeV protons at 0 = 45 °. The aim o f the present work is to build a focal plane detector o f general use for the INS QDD-type magnetic spectrograph [6] and in high resolution experiments with light particles such as protons, deuterons and so forth. Because o f its simplicity we adopted' the SWPC scheme for this purpose. In the INS spectrograph, when the horizontal size o f the beam on the target is 1 m m and the solid angle is 5 msr, the spread o f image is about 0.7 mm along the focal line. It corresponds to the energy resolving power E / A E = 5000. The mean angle o f particle injection is very acute and 35 ° to the focal plane. In our attempts to reduce the contributions from the nonnormal effect, we found a new design o f the SWPC scheme with the multi-anode.
62
K. Iwatani et al. / A focal plane detector
This report describes not only a 20 cm long counter which was constructed as a test bed for realizing the idea of the multi-anode, but also the non-normal effect in a simple SWPC. In our discussion of counter performances we deal only with position resolution.
2. Non-normal effect in the SWPC When a particle is incident at an angle 0 to the direction of the anode wire axis in a SWPC, the avalanche is developed along the wire in a length ZXL which corresponds to the projection of the particle trajectory within the counter, 2xL = D c o t 0, where D is the depth of the counter (see fig. 1). The induced counter signal includes position information about the center of gravity of the charges liberated with the avalanche length 2xL. The position of the center o gravity, Xc, is always the middle point o f 2xL if the energy loss is uniform along the particle passage. In the usual case of non-uniform energy loss, however, the point X c fluctuates from the geometrical middle point with the fluctuation ZXXc, which results in the degradation of the counter position resolution. One can see this, for instance, by imagining the case when the number o f primary ion-pairs formed at the forward and backward parts of particle passage are different. In the case of normal incidence (0 = 90°), 2xXc can be ignored because 2xL equals zero regardless of the existence of energy loss fluctuations. The extent of ~ g c is affected by the amount of ionization fluctuation in the counter, which depends on
several parameters such as a particle, its energy and incident angle 0, a counter depth D, a counter gas and its pressure. To investigate the incident angle dependence, we measured the position resolution with a SWPC under the following experimental conditions. The test counter has an active size of 1.5 × 1.5 × 20 cm 3 with a resistive anode wire which is a carbon coated quartz fiber 0.025 mm in diameter with a resistivity of 8 K ~2 mm -1. The entrance and exit windows consist of 0.004 m m thick aluminized Mylar. The counter gas is a mixture of 95% Ar and 5% CO2 and is circulated continuously at 1 atm. The anode voltage is 1400 V. Two signals from each side of the anode wire are treated by charge sensitive pre-amplifiers, shaping amplifiers (time constant z = 1/Js) and analog to digital converters. The position spectrum is taken by the charge division method with use of a digital computer. A 51.9 MeV proton beam accelerated using the INS-FM cyclotron is led to a scattering chamber. Elastic scattering protons from polyethylene are defined to the spread of 0.25 mm by means of two tantalum slits in front of and behind the counter and with coincidence method using a NaI (T1) scintillation detector. The scintillation detector is also used for the particle and energy identification. As shown in fig. 2, the measurement of the position resolution at angles of 90 °, 60 ° and 35 ° resulted in 0.28 ram, 1.1 m m and 2.6 mm respectively for 50
1.0
/IL (crn) 2.0
I
I
-/ PARTICLE
E
At.
~
~.
3.C
30
//
CATHODE
D
I
i
'
"t...
I
[ "--4\
ANODE
CATHODE
i
2.C
0~7/ Fig. 1o Schematic illustration of a SWPCwith a position spectrum. D is the counter depth. A particle is incident at an angle 0 and then avalanches occur along the anode wire in a length ~L. The center of gravity of the liberated charge is at Xc, the middle point of AL. By the energy loss fluctuation, the position spectrum has a finite spread AXc.
90
t 60 (~ (degrees)
I 30
Fig. 2. Non-normal effect on the SWPC position resolution
for 50 MeV protons. The raw data (solid circles) are corrected for beam size and then drawn as a function of AL (empty circles).
K. lwatani et al. / A focal plane detector
63
3. Multi-anode resistive wire proportional c o u n t e r
MeV protons. The result is also drawn in the figure as a function of the particle trajectory length AL projecting onto the anode wire. In the latter case, the values of the resolution were corrected for beam size, i.e., the spread along the anode wire, which were 0.25 mm, 0 . 2 9 m m and 0 . 4 4 m m at 90 °, 60 ° and 35 ° respectively. One can see that the resolution degradation is roughly proportional to AL. This means that if the angle of particle injection remains constant, the thinner the counter, the lesser the contribution from the non-normal effect. In table 1, the present results are listed with all the experimental data that have ever been obtained by SWPC scheme. One can see in general that the position resolution AX for normal incidence is better than 1 mm but for non-normal incidence it is worse than 1 m m with two exceptions which are the data with a very thin counter (Borkowski et al. and Ford et al.). The ~60-data, as the results o f Saghai et al show, must be not affected by the non-normal effect because of the small fluctuation in the energy loss of heavy ions.
3.1. Design principles The multi-wire proportional chamber (MWPC) is an example of a thin counter which has a conventional active thickness of about 0.5 mm and relatively restrains the non-normal effect. Furthermore one can confine the sensitive volume to a smaller size by mixing a small amount of Freon gas with the counter gas, and by operating the counter system at high gain and with a short shaping time constant. In the case of SWPC, it is impractical to try to reduce the depth of a counter without changing its height. Because the electric field in such a counter deviates considerably from a uniform cylindrical one, the gas multiplication factor varies with the vertical position of the particle incidence. Another disadvantage of making a counter thinner is the small signal such a counter gives because of small energy loss. This is undesirable especially for light particles in relation to the system noise. Then, we designed a modifica-
Table 1 Position resolution which has been obtained by the SWPC scheme Incident particle
Th C, C'-alpha 21 oPo -alpha Protons from ORIC Alpha Protons Alpha 241 Am-alpha X-ray 160 Alpha from RI source Tritons Protons 160 160 Th C, C'-alpha Alpha
Energy (MeV)
Counter depth D (cm)
6.1, 8.8 5.3
0.35 1.3 (cylinder) 1.0 (cylinder) 10 0.6 ~15 0.35 5.5 / 3.0 (cylinder) 8 keV ~ 60 1.0 / , 1.0 42 1.0 ~56 0.64 (drift type) 52.4 2.2 6.1, 8.8~ 3 8 ~ 6 3 } 1.5
Protons
50
Protons Protons
50 28
1.5 }
1.0 (multi-anode)
Position resolution (fwhm) With normal incidence AX (mm) 1.6 ~ 3.4 0.15
0.27 (X-ray) 0.42 0.6 0.88 0.2
Author and Ref.
With nonnormal incidence AX (ram)
0 (deg.)
Kuhlmann et al. [ 1l 0.3 2.5 1.6 ~ 1.0 0.7
45 40 45 45 45
0.28
Miller et al. [2] Ford et al. [81 Hough et al. [91 Harvey et al. [10]
1.1 1.1
45 45
0.6 ~
Borkowski et al. [7]
1.0 ~ 1.5 ~<1.0 1.1 4~5
45 45 30 30
~ 1.1 / 2.6 0.9 0.75
60 35 }
35
Fulbright et al. [ 11 l Homeyer et al. [12] Erskine et al. [13] Saghai et al. [141 M611er et al. [5]
Present
64
K. lwatani et al. / A focal plane detector
tion o f the SWPC which should not so change the counter depth and height, and which should match the SWPC advantages o f construction, adjustment and read-out method. The design principles o f the counter are described as follows: (1) the counter layer is effectively subdivided into thinner layers b y replacing the single resistive anode wire with several resistive wires in the plane o f particle trajectory. (2) Each wire is electrically connected together at b o t h ends and the connection lines are inclined to coincide with the angle o f particle incidence to the focal plane, so that the relative positions o f induced charge are the same on all wires. (3) Adding guard wires in front o f and behind the active region, the electric field in the active layer is arranged so as to be macroscopically similar to that made b y a parallel plate. (4) Signal handling out o f the counter is just the same as the method o f SWPC. Thus the counter is virtually made up o f several thin counters in tandem. For the number o f wires, N, the non-normal effect on each wire is expected to be 1/N o f that found with the SWPC, being proportional to the gas thickness undertaken b y one wire. Connecting N wires, the overall reduction becomes to 1 / N v ~ . For example, its amount is 0.09 for N = 5. Consequently, with this idea a multi-anode system o f the counter constructed by Markham and Robertson can be realized in a quite simple way. 3.2. Construction and operation Based on above principles, we constructed the counter which had five anode and four guard wires with 2 mm spacing as shown in fig. 3. The active size o f the counter was 20 cm in length, 1.5 cm in height and 1.0 cm in depth. This depth corresponds to the particle pass length o f 1.74 cm when the particle is incident at 0 = 35 °. The distance between entrance and exit windows was 3.0 cm. These windows were o f a 0.012 mm thick aluminized Mylar foil to endure a 1 atm pressure difference in a vacuum chamber. The anode wires consisted o f the same material as those o f the SWPC. The b e r y l l i u m - c o p p e r guard wires were 0.1 m m in diameter and were biased at 80% o f the anode voltage to prevent the multiplications from occurring in them. The wires were fastened at each end with epoxy resin in metal pipes. The pipes were 0.3 m m in inside diameter and were held b y a Teflon
particle
active region /
c°nnecti~ineS ~
cathode
~%'~\ \ ~ , x ~ - - - ~ ~.
~
k guard cathode
Fig. 3. Schematic of the multi-anode resistive wire proportional counter.
stand. A tiny spot o f conducting paint at the nib o f the pipe ensured the electrical continuity and determined the end o f wire resistivity. These end points were aligned at 0 = 35 ° with an accuracy o f 1.5 °, i.e., the connection lines on b o t h sides were inclined according to the angle o f particle incidence. Figure 4 shows an illustration o f the electrostatic field in the counter cross section, which was calculated by a fourth-order image method. Because the electric field in the active region is similar to that o f the parallel plate, the working anode voltage was 2900 V and about two times as high compared with that o f the SWPC. The signals on the guard wires short-circuited through the high voltage capacitors and resistors.
~THODE WINDOW o I
E.~UAR D WIRE ANODEWIRE
1 I I
Fig. 4. Calculated electric field in the counter cross section. Curved solid lines indicate the actual electric lines of force at the boundaries of the wires and these appear only in a quarter part of the counter cross section due to geometrical asymmetry. Dotted lines indicate electric lines of force assuming a perfect parallel plate field.
65
K. Iwatani et al. / A focal plane detector
4. Result and discussion
particle
Two experiments were carried out for the multianode resistive wire proportional counter. In the first one, the experimental set-up was the same as described in section 2. The particles were incident to the counter at 0 = 35 °. The measured position spectra for 50 MeV protons are shown in fig. 5 and the position resolution o f 0.9 mm was obtained with the beam collimation o f 0.5 mm. The second experiment was performed under actual conditions where the counter was used in the focal plane. Protons accelerated by the INS-SF cyclotron were scattered from 100/ag cm -2 thick gold foil and analyzed by the INS QDD-type magnetic spectrograph• The position resolution o f 1.0 ram, which is shown in fig. 6, was obtained at 0 = 35 ° for 28 MeV protons and somewhat limited b y the spread o f beam image o f about 0.6 ram. Both results showed a remarkable improvement over the simple SWPC in which the best resolution o f 1.6 mm can be estimated from fig. 2 for a 1.0 cm depth counter• The results also showed a difference from the expected resolution o f 0.69 mm, which consisted o f contributions from the spread o f the image o f 0.6 ram, the residual non-normal effect o f 0.15 mm and circuit noise o f 0.2 ram. In order to check the angle dependence o f resolution in this counter, the second experiment was carried out also varying the detector setting angle from the designed one o f 35 °. As shown in fig. 6, the o p t i m u m setting angle shifted 5 ° to the large angle, where the resolution came up to 0.75
Ep = 50 MeV
1oo 5mm C
_~ 50
g
"
•
-•
0 1090
0.9ram fwhm
.'.
" 1140 Channel
~
I
1190 Number
" 1240
Fig. 5. T w o p o s i t i o n s p e c t r a w i t h t h e n e w c o u n t e r /'or 5 0 MeV p r o t o n s . P o s i t i o n shifts o f 5 m m a l o n g t h e f o c a l p l a n e c o r r e s p o n d s to 5 0 c h a n n e l s .
ED : 2 8 M e V
E
~
2.0
erector
tO 0 tO ¢U
~ 1.0 ._o
0 30 0
I 40 (degrees)
50
Fig. 6. Position resolution as a function of the detector setting angel.
mm. The shift suggests that the inclination angle o f the wire connection lines is not appropriate and there is a discrepancy between the trajectory o f the incident particle and that "felt" b y the counter. It can be seen from the following considerations that this phenomenon is caused by the deviation o f the electric field from the parallel plate one, i.e., by fringe field effect, and that it is determined b y the geometrical arrangement o f wires and cathodes• Comparing the two kinds o f lines illustrated in fig. 4, it can be seen that the space for each wire, except the central wire, shifts to the outside (near window) in the actual counter• The mean amount o f space shift for the outermost anode wire was calculated to be 1.0 mm at the middle height o f a n o d e - c a t h o d e gap. Suppose that primary ion-pairs are produced at this height• Electrons then drift toward the anode along the curved electric line o f force and cause an avalanche in the vicinity o f the anode. The positive ions formed by the avalanche drift back to the cathode along the same line and generate a counter signal• The velocity o f the positive ions is so small that, for the signal shaping time o f a few/~s, they are only separated from the wire by a distance o f about 0.5 ram, i.e., the center o f gravity o f the charge induced on the wire exists almost at the middle point o f avalanche length. Accordingly, a difference occurs in the sites o f the primary ion-pairs and the resulting induced charge• Fig. 7 illustrates the t o p view o f the counter and two inclined lines o f 0 = 35 ° and 29•3 ° . The former represents a particle trajectory and the latter a line connecting each center o f gravity o f charge on the wires under the condition o f the above-calculated
66
K. Iwatani et al. / A focal plane detector p/ particle lralectory
able for the focal plane detector for the INS QDDtype magnetic spectrograph.
anode
wires
Fig. 7. Schematic of the sites of the primary ion-pairs (A) and induced charges (B) for each anode wire. The particle is supposed to enter at the middle height of anode-cathode gap and the primary ion-pairs are affected in the field as shown in fig. 4.
shifts. The angle difference o f 5.7 ° is nearly consistent with the experimental value shown in fig. 6. One solution in order to reduce the fringe field effect is to make the a n o d e - c a t h o d e gap narrower or to make the distance between the entrance and exit windows large enough to add more guard wires. A simple method is to coincide the inclination of connection line with that of induced charge. It can also be achieved by connecting the resistors to the ends of anode wires and thus compensating the connection lines. We adopted the last mentioned modification not changing the counter geometry and verified the above hypothesis by another test, achieving the optim u m position resolution in actual use. We tested a modification of the SWPC scheme, the multi-anode resistive wire proportional counter, by using high energy protons under rather severe conditions where the non-normal effect is serious, and confirmed the design principles for the counter. As a result, the 40 cm long counters have been constructed and are currently being used with a performance suit-
We are indebted to Mr. Y. Fujita of INS and Professor J. Sanada of the University of Tsukuba for helpful comments and suggestions. We gratefully acknowledge Mr. K. Omata and Dr. M. Yasue for their preparation of the computer digital division program.
References [1] W.R. Kuhlmann, K.H. Lauterjung, B. Schimmer and K. Sistemich, Nucl. Instr. and Meth. 40 (1966) 118. [2] G.L. Miller, N. Williams, A. Senator, R. Stensgaard and J. Fischer, Nucl. Instr. and Meth. 91 (1971) 389. [31 T.H. Braid and S.A. Zawadzki, Bull. Am. Phys. Soc. 17 (1972) 120. [41 R.G. Markham and R.G.H. Robertson, Nucl. Instr. and Meth. 129 (1975) 131. [5] P. M611er and H.V. Klapdor, Nucl. Instr. and Meth. 142 (1977) 447. [6] S. Kate, T, Hasegawa and M. Tanaka, Nucl. Instr. and Meth. 154 (1978) 19. [7] C.J. Borkowski and M.K. Kopp, IEEE Trans. Nucl. Sci. NS-17 (1970) 340. [8] J.L.C. Ford, Jr., P.H. Stelson and R.L. Robinson, Nucl. Instr. and Meth. 98 (1972) 199. [9] J. Hough and R.W.P. Drever, Nucl. Instr. and Meth. 103 (1972) 365. [10] B.G. Harvey , J. Mahoney, F.G. Piihlhoffer, F.S. Goulding, D.A. Landis, J.C. Faivre, D.G. Kovar, M.S. Zisman, J.R. Meriwether, S.W. Cosper and D.L. Hendrie, Nucl. Instr. and Meth. 104 (1972) 21. [1 l] H.W. Fulbright, R.G. Markham and W.A. Lanford, Nucl. Instr. and Meth. 108 (1973) 125. [12] H. Homeyer, J. Mahoney and B.G. Harvey, Nucl. Instr. and Meth. 118 (1974) 311. [13] J.R. Erskine, T.H. Braid and J.C. Stoltzfus, Nucl. Instr. and Meth. 135 (1976) 67. [14] B. Saghai and P. Roussel, Nucl. Instr. and Meth. 141 (1977) 93.