- Email: [email protected]
A new type of proportional counter using a capillary plate
Nuclear Instruments and Methods in Physics Research A 374 (1996) 341-344
NUCLEAR INSTRUMENTS 8, METHODS IN PMVSICS RESEARCH Section A
new type of proportional
counter using a capillary plate
H. Sakurai*, T. Tamura, S. Gunji, M. Noma Department of Physics, Yamagata University I-4 12 Kojirakawa,
Yamagata 990. Japan
Received 23 October 1995; revised form received 19 December 1995 Abstract We have developed a new type of proportional counter with the capability of fine position resolution and high gas gain. It consists of a bundle of fine glass capillaries with electrodes on both the inlet and the outlet. The capillaries are 100 pm in diameter and 800 km in length and gas multiplication was found to occur in each capillary with an applied voltage between the inlet and the outlet. The resulting proportional counter was found to operate up to a gain of IO4 without the onset of breakdown and had an energy resolution of 26% for 5.9 keV X-rays at a gas gain of 7000. Its detection efficiency was identical to a conventional single wire device.
Position-sensitive gas proportional counters have been widely used in fields as high energy physics, X-ray astronomy, medical science etc., and new instruments are continually being proposed. The recently developed microstrip gas proportional counter (MSGC). for example, has demonstrated excellent performance including a position resolution of 60 pm [ 1,2]. Another type of instrument images the photoelectron track produced by X-ray absorption in gas, through the combination of a parallel plate proportional counter with an image intensifier, for detection of X-ray polarization [3]. Regardless of the field, the constant challenge is to develop imaging instruments with finer spatial resolution. We have developed a new type of proportional counter with the capability of fine position resolution and high gas gain. We can envision many applications of this new type of instrument for extremely precise imaging in many fields. As shown in Fig. 1, the proportional counter is composed of an absorption and drift region of 7 mm and a capillary plate where gas multiplication occurs. A cathode voltage and an anode voltage are applied to the upper and the lower surfaces of the capillary plate. The capillary plate, which is commercially manufactured by Hamamatsu Photonics, consists of a bundle of fine glass capillaries with uniform length, the ends of which form two flat planes. The diameter and length of a capillary are 100 pm and 800 pm, respectively. The diameter of the plate is effectively 20 mm. On the faces of both ends Inconel metal is evaporated to form electrodes. The penetration of the
* Corresponding author. Tel. + 81 236 28 4553, fax + 81 236 28 4567, e-mail [email protected].
coating into the capillary inlet and outlet is 0.5d and I.Od respectively, where d is the diameter of a single capillary. The capillary plate is similar in structure to a microchannel plate [4], except that the glass is just lead glass and the inner wall surface of each capillary has not received any treatment such as the H,-reduction typical of a microchannel plate. So, the glass has higher resistivity, compared with the typical 200 MR resistance of microchannel plates necessary to prevent surface charge build-up. The characteristics of the capillary plate gas proportional counter (CGPC) were investigated for a gas mixture of argon + 5% methane with 55Fe 5.9 keV X-rays. Fig. 2 shows gas gains as a function of the appIied voltage between the two faces of the capillary plate for gas pressures of 380, 560 and 760 Torr. The gas gains at each pressure increased exponentially with voltage and the CGPC was operated up to a gas gain of IO’ without the onset of breakdown.
Fig. I. A schematic view of a Capillary Plate Gas Proportional Counter (CGPC).
0168-9002/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SO168-9002(96)00226-4
Fig. 3b shows the variation of gas gains for irradiation of 76 counts mm ‘s ’ and 320 counts mm-‘s ‘_ During the first 15 minutes after turning on the high voltages, the gas gains for the high and the low irradiation rates decreased to 90% and 98.5%. respectively. after which time both were stable. These are stable to the charging up effects experienced with the MSGC [5]. The Townsend ionization coefficient cy can be represented by the semi-empirical formula cy = Ap exp[-Bl(El p,]. where E is the electric field, p is the gas pressure and the constants A and B depend on the gas filling. The gas gain of the CGPC. supposing a constant electric field in the multiplication region, is given by G = exp(Ap exp(-B/(V/ fp))/), where V is the applied voltage between the inlet and the outlet and I is the length of the capillary. The constants A and B can be derived from the gas gain vs. voltage data at each pressure and these in turn can be used to generate the corresponding Townsend ionization coefficient as a function of field 161. These data are shown in Fig. 4 along with data both for pure argon from Kruithof [7] and argon + 5.3% methane from Nakamura [S]. The data shown that the Townsend ionization coefficients derived from the gas gain are similar to the published data. This indicates that the electron multiplication of the CGPC is due to gas multiplication and not to secondary emission from the walls. In Fig. 5 a pulse height distribution for 55Fe 5.9 keV X-rays is shown. It clearly shows the separation of the main peak from the escape peak due to argon. The energy resolution was 26% (FWHM) at a gas gain of 7000. This is comparable to that of the standard gas proportional counter. Also, the total counts were 99.7% of those estimated through calibrations with a conventional single wire proportional counter with IO mm X IO mm cross section. This indicates that the CGPC has a similar detection efficiency to the standard gas proportional counrates
800
1000 applied
1200 voltage
1600
1400 [V]
Fig. 3. Gas gain as a function of applied voltage between both electrodes of the CGPC at gas pressures of 380, 560, and 760 Torr for an argon + 5% methane gas mixture. The solid lines are best fit curves obtained with a least squares method.
Fig. 3a shows the gain stability of the CGPC for long term operation. The gas gain and the irradiation rate of 5.9 keV X-rays were IO’ and 76 counts mm-' s-‘, respectively. During the first 15 minutes after applying the high voltage, the gas gain decreased to 98.5% of its initial value, after which time the gain was stable over 10 hours. 1.05
F”“I”“““““4 4
1.00
0.95
0.90 x: 76 counts/mm8s 0.85
0.80
0
200 Time
400 [min]
600 1.00
1.05
0.50
1.00 0.95 X : 780 Torr 0 : 670 Torr 0 : 380 Ton-
0.90 0.85 0.80
10
20
E/p 0.75
20
40 Time [min]
60
Fig. 3. The stability of the gain of CGPC (a) for long term operation and (b) for different irradiation rates of X-rays.
30 40 [V/cm.Torr]
50 60 708090
Fig. 4. Townsend ioniration coefficient as a function of reduced electric field derived for the CGPC in an argon + 5% methane gab mixture. Also. data both for pure argon from Kruithof (71 and argon + 5.3% methane from Nakamura [8] are shown.
H. Sakurai et al. I Nucl. Imtr. and Meth. in Phy. Res. A 374 (1996) 341-344
1260 -
I”“I”“I”“I”“L
600 -
0
100
200
200
100
(100
channel Fig. 5. A pulse height distribution gain of 7000.
for “Fe 5.9 keV X-rays at a gas
Figs. 6a and 6b show the electric fields around the inlet and the outlet, respectively, of a capillary calculated by a numerical method for Laplace equations with boundary conditions. An X-ray absorbed in the absorption and drift region (see Fig. 1) produces a cloud of electrons, which move, under the action of the drift field, to the inlet of the capillary plate. Then, the electrons are shared and enter several capillaries in the plate along the lines of electric force. Once in the capillaries they experience an almost constant electric field of around 2 X 1OhV/m which is
04 Fig. 6. (a) Equipotential contours around the inlet of a capillary. (b) Equipotential counters around the outlet of a capillary.
343
sufficient to produce charge multiplication. The multiplied electrons are then either collected at the outlet electrode, or directed to readout electrodes. A standard microchannel plate (MCP) is operated in a vacuum with the electron multiplication taking piace by collisions with the wall of the capillaries [9]. If the electron multiplications of CGPC were similarly generated by collisions with the wall then the CGPC could not operate in the manner described here. First, as mentioned in the foregoing section, the gains of the electron multiplications depend on the gas pressure, and the gain curves as a function of applied voltage between the inlet and the outlet fit well to the gas gain formula using the semi-empirical formula of the Townsend ionization coefficient. This means that the electron multiplication in the CGPC is through collisions with the gas. Second, the observed rise times of the output pulse were around 800 ns for the times between 10% and 90% of the pulse heights. As the size of a charge cloud at the inlet would be typically around 0.6 mm, taking account of the diffusion for the 7 mm drift at 5.9 keV, the rise time due to the electron cloud only, i.e. that observed if multiplication were from secondary wall emission, would be around 26 ns for a drift velocity of 2.3 X lO’m/s of electrons in argon + 5% methane [IO]. The calculation of the electric fields for a capillary indicates that most of the gas multiplications occur in the middle of the capillary between the inlet and the outlet. The drift path length of the ions is approximately 350 pm from the center of the capillary to its inlet. As the ion drift velocity is around 3.7 X IO’ m/s. using an ion mobility of 1.7 cm’ V-l s-’ [I I]. the drift time is about 1 p,s which is in agreement with the observed rise time of 800 ns. Hence the longer rise time presumably shows the motion of ions generated by gas multiplication travelling to the inlet. From these arguments, we can rule out electron multiplication through collisions with the walls in the CGPC. We conclude that electron multiplication in the CGPC is generated by collisions with gas molecules in each capillary. An important point of the CGPC is that the gas multiplication occurs in each capillary and hence each part of the electron cloud is identified in a very small domain. The position sensitivity of gas X-ray detectors is mainly limited by two factors, namely electronic noise and electron diffusion. The operation of the CGPC would provide advantages over both these limitations. The fluctuation of the centroid in the charge cloud depends on the drift length. particularly for low energy X-rays. For instance, for a 5.9 keV X-ray in argon + 10% methane at a pressure of 760Torr the range of the photoelectron is around 75 km (rms) and the diffusion is about 17.5 pm (rms) after only 1 mm drift [12]. However, by arranging the configuration of the CGPC such that the initial interaction of an incoming X-ray occurs in the capillary tube, we can eliminate pre-diffusion in a drift region. As for the electronic noise limitation, a large gas gain, say of around IO”, would give rise to -pm position
344
H. Sukumi
et 01. I Nucl. Instr. and Meth. ill Phg.
over a IOmm distance. By stacking the CGPC we anticipate achieving these gains. Thus the CGPC in these modes of operation would provide precise imaging detectors. In the near future we will investigate the characteristics of photon emission, the operation of the driftless CGPC, and also CGPCs with finer capillary plates such as 20 km. resolution
References [l] A. Oed, Nucl. Instr. and Meth. A 263, (1988) 351. [2] F. Angelini et al., Nucl. Instr. and Meth. A 323 (1992) 229.
Res. A 374 (1996) 341--744
I31 R.A. Austin and B.D. Ramsey. Optical Engineering 31 (1993) 1990. 131 J.L. Wiza. Nucl. Instr. and Meth. 162. ( 1979) 587. I51 R. Bouclier et al., Nucl. Instr. and Meth. A 315 (1992) 521. 161 H. Sakurai, B.D. Ramsey and M.C. Weisskopf, Nucl. Instr. and Meth. A 307 (1991) 504.
171 A.A. Kruithof, Physica 7 (1940) 519. 181 Y. Nakamura, private communication. I91 G.W. Fraser et al., Nucl. Instr. and Meth. 224 (1984) 272. t101 1. Lehraus et al., IEEE Trans. Nucl. Sci. NS-30 (1983) 50. [III F. Sauli, CERN, 77-09 (1977). 1121 P.J. Gilvin et al., IEEE Trans. Nucl. Sci. NS-28 (1) (1981) 835.
NUCLEAR INSTRUMENTS 8, METHODS IN PMVSICS RESEARCH Section A
new type of proportional
counter using a capillary plate
H. Sakurai*, T. Tamura, S. Gunji, M. Noma Department of Physics, Yamagata University I-4 12 Kojirakawa,
Yamagata 990. Japan
Received 23 October 1995; revised form received 19 December 1995 Abstract We have developed a new type of proportional counter with the capability of fine position resolution and high gas gain. It consists of a bundle of fine glass capillaries with electrodes on both the inlet and the outlet. The capillaries are 100 pm in diameter and 800 km in length and gas multiplication was found to occur in each capillary with an applied voltage between the inlet and the outlet. The resulting proportional counter was found to operate up to a gain of IO4 without the onset of breakdown and had an energy resolution of 26% for 5.9 keV X-rays at a gas gain of 7000. Its detection efficiency was identical to a conventional single wire device.
Position-sensitive gas proportional counters have been widely used in fields as high energy physics, X-ray astronomy, medical science etc., and new instruments are continually being proposed. The recently developed microstrip gas proportional counter (MSGC). for example, has demonstrated excellent performance including a position resolution of 60 pm [ 1,2]. Another type of instrument images the photoelectron track produced by X-ray absorption in gas, through the combination of a parallel plate proportional counter with an image intensifier, for detection of X-ray polarization [3]. Regardless of the field, the constant challenge is to develop imaging instruments with finer spatial resolution. We have developed a new type of proportional counter with the capability of fine position resolution and high gas gain. We can envision many applications of this new type of instrument for extremely precise imaging in many fields. As shown in Fig. 1, the proportional counter is composed of an absorption and drift region of 7 mm and a capillary plate where gas multiplication occurs. A cathode voltage and an anode voltage are applied to the upper and the lower surfaces of the capillary plate. The capillary plate, which is commercially manufactured by Hamamatsu Photonics, consists of a bundle of fine glass capillaries with uniform length, the ends of which form two flat planes. The diameter and length of a capillary are 100 pm and 800 pm, respectively. The diameter of the plate is effectively 20 mm. On the faces of both ends Inconel metal is evaporated to form electrodes. The penetration of the
* Corresponding author. Tel. + 81 236 28 4553, fax + 81 236 28 4567, e-mail [email protected].
coating into the capillary inlet and outlet is 0.5d and I.Od respectively, where d is the diameter of a single capillary. The capillary plate is similar in structure to a microchannel plate [4], except that the glass is just lead glass and the inner wall surface of each capillary has not received any treatment such as the H,-reduction typical of a microchannel plate. So, the glass has higher resistivity, compared with the typical 200 MR resistance of microchannel plates necessary to prevent surface charge build-up. The characteristics of the capillary plate gas proportional counter (CGPC) were investigated for a gas mixture of argon + 5% methane with 55Fe 5.9 keV X-rays. Fig. 2 shows gas gains as a function of the appIied voltage between the two faces of the capillary plate for gas pressures of 380, 560 and 760 Torr. The gas gains at each pressure increased exponentially with voltage and the CGPC was operated up to a gas gain of IO’ without the onset of breakdown.
Fig. I. A schematic view of a Capillary Plate Gas Proportional Counter (CGPC).
0168-9002/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SO168-9002(96)00226-4
Fig. 3b shows the variation of gas gains for irradiation of 76 counts mm ‘s ’ and 320 counts mm-‘s ‘_ During the first 15 minutes after turning on the high voltages, the gas gains for the high and the low irradiation rates decreased to 90% and 98.5%. respectively. after which time both were stable. These are stable to the charging up effects experienced with the MSGC [5]. The Townsend ionization coefficient cy can be represented by the semi-empirical formula cy = Ap exp[-Bl(El p,]. where E is the electric field, p is the gas pressure and the constants A and B depend on the gas filling. The gas gain of the CGPC. supposing a constant electric field in the multiplication region, is given by G = exp(Ap exp(-B/(V/ fp))/), where V is the applied voltage between the inlet and the outlet and I is the length of the capillary. The constants A and B can be derived from the gas gain vs. voltage data at each pressure and these in turn can be used to generate the corresponding Townsend ionization coefficient as a function of field 161. These data are shown in Fig. 4 along with data both for pure argon from Kruithof [7] and argon + 5.3% methane from Nakamura [S]. The data shown that the Townsend ionization coefficients derived from the gas gain are similar to the published data. This indicates that the electron multiplication of the CGPC is due to gas multiplication and not to secondary emission from the walls. In Fig. 5 a pulse height distribution for 55Fe 5.9 keV X-rays is shown. It clearly shows the separation of the main peak from the escape peak due to argon. The energy resolution was 26% (FWHM) at a gas gain of 7000. This is comparable to that of the standard gas proportional counter. Also, the total counts were 99.7% of those estimated through calibrations with a conventional single wire proportional counter with IO mm X IO mm cross section. This indicates that the CGPC has a similar detection efficiency to the standard gas proportional counrates
800
1000 applied
1200 voltage
1600
1400 [V]
Fig. 3. Gas gain as a function of applied voltage between both electrodes of the CGPC at gas pressures of 380, 560, and 760 Torr for an argon + 5% methane gas mixture. The solid lines are best fit curves obtained with a least squares method.
Fig. 3a shows the gain stability of the CGPC for long term operation. The gas gain and the irradiation rate of 5.9 keV X-rays were IO’ and 76 counts mm-' s-‘, respectively. During the first 15 minutes after applying the high voltage, the gas gain decreased to 98.5% of its initial value, after which time the gain was stable over 10 hours. 1.05
F”“I”“““““4 4
1.00
0.95
0.90 x: 76 counts/mm8s 0.85
0.80
0
200 Time
400 [min]
600 1.00
1.05
0.50
1.00 0.95 X : 780 Torr 0 : 670 Torr 0 : 380 Ton-
0.90 0.85 0.80
10
20
E/p 0.75
20
40 Time [min]
60
Fig. 3. The stability of the gain of CGPC (a) for long term operation and (b) for different irradiation rates of X-rays.
30 40 [V/cm.Torr]
50 60 708090
Fig. 4. Townsend ioniration coefficient as a function of reduced electric field derived for the CGPC in an argon + 5% methane gab mixture. Also. data both for pure argon from Kruithof (71 and argon + 5.3% methane from Nakamura [8] are shown.
H. Sakurai et al. I Nucl. Imtr. and Meth. in Phy. Res. A 374 (1996) 341-344
1260 -
I”“I”“I”“I”“L
600 -
0
100
200
200
100
(100
channel Fig. 5. A pulse height distribution gain of 7000.
for “Fe 5.9 keV X-rays at a gas
Figs. 6a and 6b show the electric fields around the inlet and the outlet, respectively, of a capillary calculated by a numerical method for Laplace equations with boundary conditions. An X-ray absorbed in the absorption and drift region (see Fig. 1) produces a cloud of electrons, which move, under the action of the drift field, to the inlet of the capillary plate. Then, the electrons are shared and enter several capillaries in the plate along the lines of electric force. Once in the capillaries they experience an almost constant electric field of around 2 X 1OhV/m which is
04 Fig. 6. (a) Equipotential contours around the inlet of a capillary. (b) Equipotential counters around the outlet of a capillary.
343
sufficient to produce charge multiplication. The multiplied electrons are then either collected at the outlet electrode, or directed to readout electrodes. A standard microchannel plate (MCP) is operated in a vacuum with the electron multiplication taking piace by collisions with the wall of the capillaries [9]. If the electron multiplications of CGPC were similarly generated by collisions with the wall then the CGPC could not operate in the manner described here. First, as mentioned in the foregoing section, the gains of the electron multiplications depend on the gas pressure, and the gain curves as a function of applied voltage between the inlet and the outlet fit well to the gas gain formula using the semi-empirical formula of the Townsend ionization coefficient. This means that the electron multiplication in the CGPC is through collisions with the gas. Second, the observed rise times of the output pulse were around 800 ns for the times between 10% and 90% of the pulse heights. As the size of a charge cloud at the inlet would be typically around 0.6 mm, taking account of the diffusion for the 7 mm drift at 5.9 keV, the rise time due to the electron cloud only, i.e. that observed if multiplication were from secondary wall emission, would be around 26 ns for a drift velocity of 2.3 X lO’m/s of electrons in argon + 5% methane [IO]. The calculation of the electric fields for a capillary indicates that most of the gas multiplications occur in the middle of the capillary between the inlet and the outlet. The drift path length of the ions is approximately 350 pm from the center of the capillary to its inlet. As the ion drift velocity is around 3.7 X IO’ m/s. using an ion mobility of 1.7 cm’ V-l s-’ [I I]. the drift time is about 1 p,s which is in agreement with the observed rise time of 800 ns. Hence the longer rise time presumably shows the motion of ions generated by gas multiplication travelling to the inlet. From these arguments, we can rule out electron multiplication through collisions with the walls in the CGPC. We conclude that electron multiplication in the CGPC is generated by collisions with gas molecules in each capillary. An important point of the CGPC is that the gas multiplication occurs in each capillary and hence each part of the electron cloud is identified in a very small domain. The position sensitivity of gas X-ray detectors is mainly limited by two factors, namely electronic noise and electron diffusion. The operation of the CGPC would provide advantages over both these limitations. The fluctuation of the centroid in the charge cloud depends on the drift length. particularly for low energy X-rays. For instance, for a 5.9 keV X-ray in argon + 10% methane at a pressure of 760Torr the range of the photoelectron is around 75 km (rms) and the diffusion is about 17.5 pm (rms) after only 1 mm drift [12]. However, by arranging the configuration of the CGPC such that the initial interaction of an incoming X-ray occurs in the capillary tube, we can eliminate pre-diffusion in a drift region. As for the electronic noise limitation, a large gas gain, say of around IO”, would give rise to -pm position
344
H. Sukumi
et 01. I Nucl. Instr. and Meth. ill Phg.
over a IOmm distance. By stacking the CGPC we anticipate achieving these gains. Thus the CGPC in these modes of operation would provide precise imaging detectors. In the near future we will investigate the characteristics of photon emission, the operation of the driftless CGPC, and also CGPCs with finer capillary plates such as 20 km. resolution
References [l] A. Oed, Nucl. Instr. and Meth. A 263, (1988) 351. [2] F. Angelini et al., Nucl. Instr. and Meth. A 323 (1992) 229.
Res. A 374 (1996) 341--744
I31 R.A. Austin and B.D. Ramsey. Optical Engineering 31 (1993) 1990. 131 J.L. Wiza. Nucl. Instr. and Meth. 162. ( 1979) 587. I51 R. Bouclier et al., Nucl. Instr. and Meth. A 315 (1992) 521. 161 H. Sakurai, B.D. Ramsey and M.C. Weisskopf, Nucl. Instr. and Meth. A 307 (1991) 504.
171 A.A. Kruithof, Physica 7 (1940) 519. 181 Y. Nakamura, private communication. I91 G.W. Fraser et al., Nucl. Instr. and Meth. 224 (1984) 272. t101 1. Lehraus et al., IEEE Trans. Nucl. Sci. NS-30 (1983) 50. [III F. Sauli, CERN, 77-09 (1977). 1121 P.J. Gilvin et al., IEEE Trans. Nucl. Sci. NS-28 (1) (1981) 835.