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A position sensitive gas detector system for Coulomb excitation experiments
Nuclear Instruments and Methods in Physics Research A248 (1986) 419-424 North-Holland, Amsterdam
419
A POSITION SENSITIVE GAS DETECTOR SYSTEM FOR COULOMB EXCITATION EXPERIMENTS B j r r n V A R N E S T I G , A n d e r s B,ACKLIN, Claes F A H L A N D E R , A l e x a n d e r E. K A V K A , T o m a s L E N K E a n d Lars Erik S V E N S S O N Tandem Accelerator Laboratory, Uppsalc~ Sweden
Received 19 February 1986
A position sensitive parallel-plate avalanche counter system for detection of heavy ions, primarily from Coulomb excitation experiments, is presented. The angular region is 40° < 0 < 140°, with a division in 0 of 2°.
1. Introduction In Coulomb excitation experiments a beam of accelerated ions interacts electromagnetically with the target nuclei, which become excited. With heavy ions, the Coulomb excitation cross-sections for various excited states depend in a complicated way on the nuclear E2 matrix, the beam energy, the atomic numbers, and the scattering angle 0 [1]. Experimentally, the excitation cross-sections are obtained from the intensities of the de-exciting ,/-rays. The value of every cross section depends on many electromagnetic matrix elements. Some examples of the sensitivity of the excitation probability of a nuclear level to changes in various matrix elements are shown in fig. 1. The figure shows that the sensitivity to a specific matrix dement often d e~ends on the scattering angle in a characteristic way, which may
" - ~ 4
MeV 160 ON 114Cd ~
>- 1.0 >
9+ ~2 p~ . . . . .
~
+ 22
+ 21
22
01
i /
0.2 0.0 -0.2
~,/
" t/
I
--...~ "~ - ~ - ~ '
4b
'
8'o
'
1~o
+ + 22 - 22 +
+
~ . _ . - . ~ 22 " 02 " + 41+ 22 '
i~o
'
THETA (DEGREES) Fig. 1. The calculated sensitivity to various matrix elements, versus scattering angle, of the excitation probability of the second 2 + level in 114Cd. The sensitivity is defined as the ratio between the relative change of the excitation probability and a relative change of a given matrix element. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
be different for different matrix elements. In order to extract information on the many E2 matrix elements involved it is necessary to study the excitation of various levels as a function of the particle scattering angle. This is preferably done by using a position sensitive particle detector covering a large range in 0. Another important use of the information of the scattering anlge is the possibility of correcting the ,/-spectra for the Doppler effect. In order to have a well defined incident ion energy, and "t-peaks that are not distorted by the recoil velocity distribution in the target, thin targets are needed for Coulomb excitation experiments. With a thin target, the excited nuclei will recoil into vacuum before the de-excitation, and the -/-peaks will be Doppler shifted and broadened, to an extent that depends on the magnitude of the recoil velocity, the opening angle of the 3, detector, and the angle between the direction of the recoil and the direction of the detector. With a knowledge of the scattering angle, this can be corrected for in the off-line analysis of the spectra. In this type of experiments, solid-state detectors have often been used. They have several limitations. They are easily destroyed by radiation damage, and the acceptable count rate is typically limited to a few times 10 4 Hz. A gas filled paralld-plate avalanche counter (PPAC) does not have these drawbacks. Furthermore it can be custom made to fit the desired experimental setup [2-6]. Several P P A C systems for the detection of heavy ions in various angles, dedicated for Coulomb-excitation experiments are reported in the literature [5,6]. We have constructed two detector systems covering different regions of the scattering angle. In this report we present a system, covering an angular region centered around 0 = 90 °. The other counter, which is of annular type, is described in ref. [7].
B. Varnestig et al. / A position sensitive gas detector
420 2. The P P A C
110pd +
0.10 -
The PPAC consists of two parallel plane electrodes with a separation of a few mm. The volume between the electrodes is gas filled, e.g. with isobutane. An electric field of the order of 200 V / m m is applied, allowing the counter to work in the proportional region. When a fast ion traverses the gap between the electrodes, an electron avalanche is created. A more detailed description of the avalanche formation is given in refs. [8] and [9]. The width Y of the electron avalanche in a direction perpendicular to the field is given by the following eq. [9]; I
0.08 -
0.06
0.04
o 0.02
I
, , / 8aU~h ~1/2 = a~-3-g--) (= Xk), Lg(1)
where Uth is the thermal energy of the electron, E is the applied electric field strength, X is the distance from the cathode and a is the Townsend coefficient. With isobutane at a pressure of 10 mbar, the value of k is about 0.5. When the electrons are collected on the anode, a mirror charge with the same timing characteristics, but with positive polarity, is induced on the cathode. The surface-density distribution of the induced charge, os is given by [10]: -
os=
Qd
..~[..~
I
[
I
i
I
'
1~0
156Gd
8
2+ 2 +
(x 6)
6
4
!
40
2~r(d 2 + R2) 3 / 2 '
where d is the electrode spacing, Q is the charge collected on the anode (assumed point shaped), and R is the distance from the center of the induced charge. With d = 3 mm, the fwhm of the charge distribution is about 5 mm. One may obtain the position of the impinging ion by subdividing one of the electrodes in a suitable way (either the anode or the cathode can be used for this). Several methods for the readout of the position have been reported, e.g. charge division [11] or the use of delay lines [12].
i
'
do
'
x~o
'
THETA (DEGREES)
Fig. 2. The calculated differential excitation cross section vs scattering angle for ground-band levels in 156Gd, Coulomb excited using a 130 MeV Ar beam, and for various 2 + levels in la°pd, Coulomb excited with a 175 MeV Ni beam. The differential cross section is defined as d o / d 0 = 21r(do/d12)sin& tion on the electrode becomes narrower with a smaller gap, as discussed in the previous section. However, the demand for a well defined electric field distribution over the electrodes area sets a lower limit of the electrode gap, due to the limited accuracy in the definition of the electrode distance.
3. Design 3.2. Construction 3.1. Some design considerations Fig.2 shows the differential Coulomb excitation cross section versus the scattering angle for two different nuclei, in both cases using accelerated ions with low mass compared to the target nuclei. This example illustrates that the angular region around 0 = 90 ° in some cases is the most important, when investigating nuclear levels excited with a " l i g h t " heavy ion beam. The timing characteristics of the PPACs improves with a decreasing distance between the electrodes as shown in ref. [4], where the best time resolution was obtained with the smallest gap tested. Also the charge distribu-
The detector system is shown in fig. 3. It consists of two identical PPAC's that form the side walls of a target chamber. The pressure window consists of a thin plastic foil, glued to an aluminum frame. A typical foil used is 0.2 m g / c m 2 mylar. This is a sufficiently thin foil to allow the entrance of scattered ions into the detector for most experiments [13]. However, if heavy recoil nuclei also are to be detected, thinner foils are normally required for full detection efficiency [5]. One of the electrodes is made of aluminized plastic foil of the same type and thickness as the pressure window, glued to a plastic (Delrin) frame. To obtain
B. Varnestig et al. / A position sensitive gas detector
~
\
C~ --
. . . . . . . . +. . . . . . . . . . . . . . . . . . . . . . . .
\ \
I~ [~ i~ B~
AlumJnium = org / Plastic P = Pressure window / Steel C = Circuit boardelectrode / Heavy metal FE=Foil electrode F= Faraday cup 5= Solid state detector
/
421
~
Ge
Fig. 3. Horizontal cut through the PPAC's and the scattering chamber. electrical insulation, the electrode rests in a plastic holder, which also serves as a guide for the gas circulation. The other electrode, separated 3 mm from the first one, is made from a standard circuit board. A hyperbolic pattern, defining constant scattering angles with a division of 2 degrees, is etched on the board electrode. Fig. 4 shows a side view of the detector system, with one PPAC removed. The pressure window and the foil electrode are not mounted, thus exposing the hyperbolic
electrode structure. Every part of the pattern is connected to taps of a delay line mounted directly on the board electrode. The delay between every two consecutive taps is 2 ns. F r o m the time difference between the prompt foil electrode pulse and the delayed pulse from the board electrode, the scattering angle is deduced. The principle of the design is similar to the counters developed in Rochester [5]. The scattering angular range covered is 40 to 140 °. The azimuthal angular limits are - 60 ° < ~ < 60 °. A crude information of the azimuthal angle is obtained by dividing the foil electrode into three regions, covering the intervals - 6 0 ° < ~ < - 3 0 ° , - 3 0 ° < ~ < 3 0 ° and 3 0 ° < ~ < 60 °. The outer regions are, however, equivalent with respect to the Doppler shift correction (with the present gamma detector geometry), and are therefore interconnected electrically in order to reduce the number of readouts. The accuracy of the information on ~ needed for the Doppler shift correction of the particle-coincident -/-spectra was checked with a Monte Carlo simulation computer code [14,15]. These calculations showed that a division of the @ range into two sections sufficiently improves the line shapes of the deexcitation y-peaks for the experiments intended. A small annular solid-state detector has been added to the P P A C side detector system, thus giving the possibility to extend the detected region further, to include the 169 ° < 0 < 174 ° region.
4. The gas system Fig. 4. Side view of the detector system, with one PPAC removed,
The purpose of the gas system is to keep a well defined pressure of the gas in the detector. This is
422
B. Varnestig et al. / A position sensitive gas detector
important, since both the break-down voltage and the amplitude distribution are dependent on the pressure. It is also important to have a continous flow of gas through the counter for removal of traces of oxygen and molecular fragments resulting from the dissociation of the gas. Pure hydrocarbons have shown to be well suited as filling gases for avalanche counters [4]. We have operated the counters with isobutane at a pressure of typically 8 mbar. The gas pressure is stabilized by means of a micro-processor unit, which together with a step-motor operated valve, and a capacitive pressure gauge form a regulating feedback system. This system has a higher reliability and accuracy than simple manually operated valves, and it is less expensive than comparable commercially available systems.
5. Performance 5.1. Test results
The detector system was tested by detecting various Rutherford scattered heavy ions. For this purpose thin targets were used (e.g. 0.5 m g / c m 2 2°spb). The foil electrode was usually connected as anode. Due to statistics, the fwhm of the anode pulse distribution is expected to decrease with a higher gas pressure (and thus an increased number of primary electrons). The signal to noise ratio is expected to improve with a higher gas gain (if the main noise contribution is due to the electronics). This behaviour is experimentally observed, and also the position resolution is observed to improve with the pressure (and voltage), up to about 8 mbar. A suitable operating pressure, also with respect to the strain on the pressure windows, was found to be 8 mbar. Fig. 5 shows the anode amplitude distribution for a setting of the gas pressure of 8 mbar, with the bias voltage just below the break-down value. This distribu-
TIME RIGHT - LEFT
12000
1 Ch = 0.24 nS
g 8000 0
4000
CHANNEL
Fig. 6. Time difference between a scattered Se projectile and the corresponding Pb recoil. tion was obtained using a 30 MeV oxygen beam. The time resolution of the counter can be estimated from a measurement of the time difference between a scattered ion and the corresponding recoil, as detected in the opposite counter. Fig. 6 shows the time spectrum obtained when bombarding 2°spb with 312 MeV S2Se ions. Here, a multi-stop TAC was used to measure the 8000 POSITIVE BIAS 6000 o
4000
2000
100
2000
8000
200 300 CHANNEL
400
NEGATIVE BIAS
8 mbar 590 V
6000 o
4000
3 tooo
2000
I00 40
80 120 CHANNEL
160
Fig. 5. The anode pulse amplitude distribution.
200 300 CHANNEL
400
Fig. 7. Particle position spectrum, recorded using positive and negative bias voltage respectively.
1o3[ 102~
lO
1o°~
I,
1obo 2000 3oo~" CHANNEL UNCORRECTEDFOR DOPPLERSHIFT +
(~= 400 _ 730
I
e = 730 - 105° 103I
÷
21 - 01 103
~
423
B. Varnestig et aL / A position sensitive gas detector
+
z~ 102
+
22 - 21
,i-2;
101
i02 I
+
_ 0+
10O
'
101
t
103[ 100 I000 2000 3000 CHANNEL CORRECTEDFOR DOPPLERSHIFT
Fig. 8. Experimental ",/ spectrum from the projectile excitation of 312 MeV S2Se on a 2°Spb target, with and without Doppler correction.
102
+
(~= +
61
1050 .
1400
+
-
41
6;
1o;-8;
101 100
time difference between the foil-electrode pulses from the two detector halves. The pulses originate from the induced mirror charges. This results in two time peaks, corresponding to the recoil detected either in the left detector or in the right one. The average fwhm of the time peaks is 1.2 ns. The contribution to the width from the uncertainty in the time difference due to the finite scattering angular region is 0.5 ns. Correcting for this, using standard formulas for propagation of errors, the resulting fwhm per counter (including amplifiers) is 0.8
~
41 - 21
260
600
1000
CHANNEL
Fig. 9. y yield following the Coulomb excitation of 172yb using 115 MeV 32S ions, for different particle scattering regions.
indicating that the induced pulse usually covers more than one antenna when the positive voltage is applied. 5.2. Experimental performance
ns.
The position resolution was observed to improve when the sign of the bias voltage was reversed from positive to negative. This is shown in fig. 7, where two position spectra are recorded using positive and negative bias respectively. This is in accordance with the calculations in section two, which show that the extension of the charge distribution in the electrode plane should be larger for the induced cathode pulse than for the direct anode pulse (about 5 mm and 1.5 mm respectively). Here, the widths of the hyperbolic copper antennas are about 2 mm (varying with 0 and ~k), thus
Fig. 8 shows an experimental y-spectrum obtained when bombarding 2°spb with 312 MeV S2Se ions. The off-line Doppler-shift correction is observed to improve the resolution considerably. The dependence of the differential excitation cross-section on the scattering angle for excited levels of different spins is illustrated in fig. 9. The y-emitting nucleus is here z72yb, after Coulomb excitation using 115 MeV 32S ions. This example shows that the excitation probabilities of the levels change considerably with the scattering angle, within the angular interval covered by the detector.
424
B. Varnestig et a L / A position sensitive gas detector
References [1] K. Alder and A. Winther, Theory of Coulomb excitation with heavy ions, (North-Holland, Amsterdam, 1975). [2] G. Hempel, F. Hopkins and G. Schatz, Nucl. Instr. and Meth. 131 (1975) 445. [3] A. Breskin and N. Zwang, Nucl. Instr. and Meth. 144 (1977) 609. [4] H. Steltzer, Nucl. Instr. and Meth. 133 (1976) 409. [5] D. Cline and B. Kotlinski, Annual Report NSRL Rochester (1982-83) 363. [6] S. Brbsserman, K.P. Lieb and P. Sona, Annual Report GSI Darmstad (1982) p. 46. [7] B. Varnestig et al., TLU Report 119/85.
[8] G.F. Knoll, Radiation detection and measurement (Wiley, New York, 1979). [9] H. Raether, Electron avalanches and breakdown in gases (Butterworth & Co, London, 1964). [10] W. Panofsky and M. Philips, Classical electricity and magnetism (Addison-Wesley, Reading MA, 1978). [11] Y. Eyal and H. Stelzer, Nucl. Instr. and meth. 155 (1978) 157. [12] D. v. Harrach and H. Specht, Nucl. Instr. and Meth. 164 (1979) 477. [13] B. Varnestig et al., TLU Report, To be published. [14] B. Varnestig et ai., TLU Report 130/85. [15] B. Kotlinski, Ph.D. Thesis, NSRL Rochester (1984).
419
A POSITION SENSITIVE GAS DETECTOR SYSTEM FOR COULOMB EXCITATION EXPERIMENTS B j r r n V A R N E S T I G , A n d e r s B,ACKLIN, Claes F A H L A N D E R , A l e x a n d e r E. K A V K A , T o m a s L E N K E a n d Lars Erik S V E N S S O N Tandem Accelerator Laboratory, Uppsalc~ Sweden
Received 19 February 1986
A position sensitive parallel-plate avalanche counter system for detection of heavy ions, primarily from Coulomb excitation experiments, is presented. The angular region is 40° < 0 < 140°, with a division in 0 of 2°.
1. Introduction In Coulomb excitation experiments a beam of accelerated ions interacts electromagnetically with the target nuclei, which become excited. With heavy ions, the Coulomb excitation cross-sections for various excited states depend in a complicated way on the nuclear E2 matrix, the beam energy, the atomic numbers, and the scattering angle 0 [1]. Experimentally, the excitation cross-sections are obtained from the intensities of the de-exciting ,/-rays. The value of every cross section depends on many electromagnetic matrix elements. Some examples of the sensitivity of the excitation probability of a nuclear level to changes in various matrix elements are shown in fig. 1. The figure shows that the sensitivity to a specific matrix dement often d e~ends on the scattering angle in a characteristic way, which may
" - ~ 4
MeV 160 ON 114Cd ~
>- 1.0 >
9+ ~2 p~ . . . . .
~
+ 22
+ 21
22
01
i /
0.2 0.0 -0.2
~,/
" t/
I
--...~ "~ - ~ - ~ '
4b
'
8'o
'
1~o
+ + 22 - 22 +
+
~ . _ . - . ~ 22 " 02 " + 41+ 22 '
i~o
'
THETA (DEGREES) Fig. 1. The calculated sensitivity to various matrix elements, versus scattering angle, of the excitation probability of the second 2 + level in 114Cd. The sensitivity is defined as the ratio between the relative change of the excitation probability and a relative change of a given matrix element. 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
be different for different matrix elements. In order to extract information on the many E2 matrix elements involved it is necessary to study the excitation of various levels as a function of the particle scattering angle. This is preferably done by using a position sensitive particle detector covering a large range in 0. Another important use of the information of the scattering anlge is the possibility of correcting the ,/-spectra for the Doppler effect. In order to have a well defined incident ion energy, and "t-peaks that are not distorted by the recoil velocity distribution in the target, thin targets are needed for Coulomb excitation experiments. With a thin target, the excited nuclei will recoil into vacuum before the de-excitation, and the -/-peaks will be Doppler shifted and broadened, to an extent that depends on the magnitude of the recoil velocity, the opening angle of the 3, detector, and the angle between the direction of the recoil and the direction of the detector. With a knowledge of the scattering angle, this can be corrected for in the off-line analysis of the spectra. In this type of experiments, solid-state detectors have often been used. They have several limitations. They are easily destroyed by radiation damage, and the acceptable count rate is typically limited to a few times 10 4 Hz. A gas filled paralld-plate avalanche counter (PPAC) does not have these drawbacks. Furthermore it can be custom made to fit the desired experimental setup [2-6]. Several P P A C systems for the detection of heavy ions in various angles, dedicated for Coulomb-excitation experiments are reported in the literature [5,6]. We have constructed two detector systems covering different regions of the scattering angle. In this report we present a system, covering an angular region centered around 0 = 90 °. The other counter, which is of annular type, is described in ref. [7].
B. Varnestig et al. / A position sensitive gas detector
420 2. The P P A C
110pd +
0.10 -
The PPAC consists of two parallel plane electrodes with a separation of a few mm. The volume between the electrodes is gas filled, e.g. with isobutane. An electric field of the order of 200 V / m m is applied, allowing the counter to work in the proportional region. When a fast ion traverses the gap between the electrodes, an electron avalanche is created. A more detailed description of the avalanche formation is given in refs. [8] and [9]. The width Y of the electron avalanche in a direction perpendicular to the field is given by the following eq. [9]; I
0.08 -
0.06
0.04
o 0.02
I
, , / 8aU~h ~1/2 = a~-3-g--) (= Xk), Lg(1)
where Uth is the thermal energy of the electron, E is the applied electric field strength, X is the distance from the cathode and a is the Townsend coefficient. With isobutane at a pressure of 10 mbar, the value of k is about 0.5. When the electrons are collected on the anode, a mirror charge with the same timing characteristics, but with positive polarity, is induced on the cathode. The surface-density distribution of the induced charge, os is given by [10]: -
os=
Qd
..~[..~
I
[
I
i
I
'
1~0
156Gd
8
2+ 2 +
(x 6)
6
4
!
40
2~r(d 2 + R2) 3 / 2 '
where d is the electrode spacing, Q is the charge collected on the anode (assumed point shaped), and R is the distance from the center of the induced charge. With d = 3 mm, the fwhm of the charge distribution is about 5 mm. One may obtain the position of the impinging ion by subdividing one of the electrodes in a suitable way (either the anode or the cathode can be used for this). Several methods for the readout of the position have been reported, e.g. charge division [11] or the use of delay lines [12].
i
'
do
'
x~o
'
THETA (DEGREES)
Fig. 2. The calculated differential excitation cross section vs scattering angle for ground-band levels in 156Gd, Coulomb excited using a 130 MeV Ar beam, and for various 2 + levels in la°pd, Coulomb excited with a 175 MeV Ni beam. The differential cross section is defined as d o / d 0 = 21r(do/d12)sin& tion on the electrode becomes narrower with a smaller gap, as discussed in the previous section. However, the demand for a well defined electric field distribution over the electrodes area sets a lower limit of the electrode gap, due to the limited accuracy in the definition of the electrode distance.
3. Design 3.2. Construction 3.1. Some design considerations Fig.2 shows the differential Coulomb excitation cross section versus the scattering angle for two different nuclei, in both cases using accelerated ions with low mass compared to the target nuclei. This example illustrates that the angular region around 0 = 90 ° in some cases is the most important, when investigating nuclear levels excited with a " l i g h t " heavy ion beam. The timing characteristics of the PPACs improves with a decreasing distance between the electrodes as shown in ref. [4], where the best time resolution was obtained with the smallest gap tested. Also the charge distribu-
The detector system is shown in fig. 3. It consists of two identical PPAC's that form the side walls of a target chamber. The pressure window consists of a thin plastic foil, glued to an aluminum frame. A typical foil used is 0.2 m g / c m 2 mylar. This is a sufficiently thin foil to allow the entrance of scattered ions into the detector for most experiments [13]. However, if heavy recoil nuclei also are to be detected, thinner foils are normally required for full detection efficiency [5]. One of the electrodes is made of aluminized plastic foil of the same type and thickness as the pressure window, glued to a plastic (Delrin) frame. To obtain
B. Varnestig et al. / A position sensitive gas detector
~
\
C~ --
. . . . . . . . +. . . . . . . . . . . . . . . . . . . . . . . .
\ \
I~ [~ i~ B~
AlumJnium = org / Plastic P = Pressure window / Steel C = Circuit boardelectrode / Heavy metal FE=Foil electrode F= Faraday cup 5= Solid state detector
/
421
~
Ge
Fig. 3. Horizontal cut through the PPAC's and the scattering chamber. electrical insulation, the electrode rests in a plastic holder, which also serves as a guide for the gas circulation. The other electrode, separated 3 mm from the first one, is made from a standard circuit board. A hyperbolic pattern, defining constant scattering angles with a division of 2 degrees, is etched on the board electrode. Fig. 4 shows a side view of the detector system, with one PPAC removed. The pressure window and the foil electrode are not mounted, thus exposing the hyperbolic
electrode structure. Every part of the pattern is connected to taps of a delay line mounted directly on the board electrode. The delay between every two consecutive taps is 2 ns. F r o m the time difference between the prompt foil electrode pulse and the delayed pulse from the board electrode, the scattering angle is deduced. The principle of the design is similar to the counters developed in Rochester [5]. The scattering angular range covered is 40 to 140 °. The azimuthal angular limits are - 60 ° < ~ < 60 °. A crude information of the azimuthal angle is obtained by dividing the foil electrode into three regions, covering the intervals - 6 0 ° < ~ < - 3 0 ° , - 3 0 ° < ~ < 3 0 ° and 3 0 ° < ~ < 60 °. The outer regions are, however, equivalent with respect to the Doppler shift correction (with the present gamma detector geometry), and are therefore interconnected electrically in order to reduce the number of readouts. The accuracy of the information on ~ needed for the Doppler shift correction of the particle-coincident -/-spectra was checked with a Monte Carlo simulation computer code [14,15]. These calculations showed that a division of the @ range into two sections sufficiently improves the line shapes of the deexcitation y-peaks for the experiments intended. A small annular solid-state detector has been added to the P P A C side detector system, thus giving the possibility to extend the detected region further, to include the 169 ° < 0 < 174 ° region.
4. The gas system Fig. 4. Side view of the detector system, with one PPAC removed,
The purpose of the gas system is to keep a well defined pressure of the gas in the detector. This is
422
B. Varnestig et al. / A position sensitive gas detector
important, since both the break-down voltage and the amplitude distribution are dependent on the pressure. It is also important to have a continous flow of gas through the counter for removal of traces of oxygen and molecular fragments resulting from the dissociation of the gas. Pure hydrocarbons have shown to be well suited as filling gases for avalanche counters [4]. We have operated the counters with isobutane at a pressure of typically 8 mbar. The gas pressure is stabilized by means of a micro-processor unit, which together with a step-motor operated valve, and a capacitive pressure gauge form a regulating feedback system. This system has a higher reliability and accuracy than simple manually operated valves, and it is less expensive than comparable commercially available systems.
5. Performance 5.1. Test results
The detector system was tested by detecting various Rutherford scattered heavy ions. For this purpose thin targets were used (e.g. 0.5 m g / c m 2 2°spb). The foil electrode was usually connected as anode. Due to statistics, the fwhm of the anode pulse distribution is expected to decrease with a higher gas pressure (and thus an increased number of primary electrons). The signal to noise ratio is expected to improve with a higher gas gain (if the main noise contribution is due to the electronics). This behaviour is experimentally observed, and also the position resolution is observed to improve with the pressure (and voltage), up to about 8 mbar. A suitable operating pressure, also with respect to the strain on the pressure windows, was found to be 8 mbar. Fig. 5 shows the anode amplitude distribution for a setting of the gas pressure of 8 mbar, with the bias voltage just below the break-down value. This distribu-
TIME RIGHT - LEFT
12000
1 Ch = 0.24 nS
g 8000 0
4000
CHANNEL
Fig. 6. Time difference between a scattered Se projectile and the corresponding Pb recoil. tion was obtained using a 30 MeV oxygen beam. The time resolution of the counter can be estimated from a measurement of the time difference between a scattered ion and the corresponding recoil, as detected in the opposite counter. Fig. 6 shows the time spectrum obtained when bombarding 2°spb with 312 MeV S2Se ions. Here, a multi-stop TAC was used to measure the 8000 POSITIVE BIAS 6000 o
4000
2000
100
2000
8000
200 300 CHANNEL
400
NEGATIVE BIAS
8 mbar 590 V
6000 o
4000
3 tooo
2000
I00 40
80 120 CHANNEL
160
Fig. 5. The anode pulse amplitude distribution.
200 300 CHANNEL
400
Fig. 7. Particle position spectrum, recorded using positive and negative bias voltage respectively.
1o3[ 102~
lO
1o°~
I,
1obo 2000 3oo~" CHANNEL UNCORRECTEDFOR DOPPLERSHIFT +
(~= 400 _ 730
I
e = 730 - 105° 103I
÷
21 - 01 103
~
423
B. Varnestig et aL / A position sensitive gas detector
+
z~ 102
+
22 - 21
,i-2;
101
i02 I
+
_ 0+
10O
'
101
t
103[ 100 I000 2000 3000 CHANNEL CORRECTEDFOR DOPPLERSHIFT
Fig. 8. Experimental ",/ spectrum from the projectile excitation of 312 MeV S2Se on a 2°Spb target, with and without Doppler correction.
102
+
(~= +
61
1050 .
1400
+
-
41
6;
1o;-8;
101 100
time difference between the foil-electrode pulses from the two detector halves. The pulses originate from the induced mirror charges. This results in two time peaks, corresponding to the recoil detected either in the left detector or in the right one. The average fwhm of the time peaks is 1.2 ns. The contribution to the width from the uncertainty in the time difference due to the finite scattering angular region is 0.5 ns. Correcting for this, using standard formulas for propagation of errors, the resulting fwhm per counter (including amplifiers) is 0.8
~
41 - 21
260
600
1000
CHANNEL
Fig. 9. y yield following the Coulomb excitation of 172yb using 115 MeV 32S ions, for different particle scattering regions.
indicating that the induced pulse usually covers more than one antenna when the positive voltage is applied. 5.2. Experimental performance
ns.
The position resolution was observed to improve when the sign of the bias voltage was reversed from positive to negative. This is shown in fig. 7, where two position spectra are recorded using positive and negative bias respectively. This is in accordance with the calculations in section two, which show that the extension of the charge distribution in the electrode plane should be larger for the induced cathode pulse than for the direct anode pulse (about 5 mm and 1.5 mm respectively). Here, the widths of the hyperbolic copper antennas are about 2 mm (varying with 0 and ~k), thus
Fig. 8 shows an experimental y-spectrum obtained when bombarding 2°spb with 312 MeV S2Se ions. The off-line Doppler-shift correction is observed to improve the resolution considerably. The dependence of the differential excitation cross-section on the scattering angle for excited levels of different spins is illustrated in fig. 9. The y-emitting nucleus is here z72yb, after Coulomb excitation using 115 MeV 32S ions. This example shows that the excitation probabilities of the levels change considerably with the scattering angle, within the angular interval covered by the detector.
424
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