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Testing the absolute beam intensity of the high-energy pulsed electron beam with a double-mode readout ionization chamber
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 592 (2008) 498– 501
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Technical Note
Testing the absolute beam intensity of the high-energy pulsed electron beam with a double-mode readout ionization chamber Q. Gou a,, Z. Feng a, S. Yin a,b, F. Shi a, J. Liu a,c, J. Dong a,c, J. Liao a a b c
IHEP of the Chinese Academy of Sciences, Beijing 100049, People’s Republic China Shandong University, Shandong 250100, People’s Republic China Lanzhou University, Gansu 730000, People’s Republic China
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 February 2008 Received in revised form 8 April 2008 Accepted 28 April 2008 Available online 17 May 2008
We constructed an ionization chamber (IC) to test the absolute intensity of the BEPC-LINAC (Beijing Electron Positron Collider-Linear Accelerator) test beam. The IC was adapted for the 1.89 GeV high-energy electron beam, with pulse time width of 1.2 ns and frequency of 25 Hz, by equipping it with a doublemode readout and choosing the optimum circuit parameters for the readout modes. The measured absolute intensity of the test beam is 7.2 109 electron/s, and is consistent with PSPICE simulations. & 2008 Elsevier B.V. All rights reserved.
Keywords: Ionization chamber Double-mode readout High-energy pulsed electron beam
1. Introduction In order to test the YAC (Yangbajing Air-shower Core detector) with a wide particle-intensity dynamic range (six orders of magnitude), developed by Tibet AS (Air Shower) group, by using the test beam of the BEPC-LINAC [1], both an ionization chamber (IC) (as in Ref. [2]) and a Faraday cup (FC) (as in Ref. [3]) as well as other devices were proposed to monitor the beam intensity in different intensity ranges. The IC has been built fulfilling the following requirements: (1) Equip the IC with double-mode readout: DC (Direct Current) mode and pulse mode. (2) Regard the IC as a parallel connection of a resistor and a capacitor. Optimize the additional RC (Resistor–Capacitor) devices in the readout system. (3) Consider all the equivalent or parasitic resistance and capacitance, to be considered for the FC also. If the time constant of the readout circuits is suitable, the beam intensity could be displayed simultaneously by a pure pulse in the pulse mode and by a DC level with an extra sawtooth waveform in the DC mode. The results obtained via two different readout modes could be a crosscheck and be complementary to each other. Corresponding author. Tel.: +86 10 88236115; fax: +86 10 88233086.
E-mail address: [email protected] (Q. Gou). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.04.081
The main purpose of the beam test described in this paper was to check whether: (1) The chamber dimension (3 3 cm2) is adaptable to the beam profile (2 cm in diameter) and the selected working gas mixture Ar/CO2 meets the requirements of the very short time electron pulse. (2) The parameters of the devices associated with the time constant are optimum. The absolute beam intensity could be obtained with two different readout modes and they are in agreement with each other. In this paper, we describe the design and construction of the detector, its readout system, the performance under the test beam of the BEPC-LINAC and the simulation results obtained with the PSPICE 9.2 software.
2. Description of the chamber and the measurement principle with the readout electronics The chamber is made of two parallel electrode plates, consisting of a 1 mm thick polycarbonate (PCB) layer coated with 35 um copper on the surface. It has a sensitive area of 3 3 cm2 with a gas gap of 7 mm. The gas mixture Ar/CO2 is used for convenience under the pressure of 1 atm. Considering that the proper electric field inside the chamber is 1000 V/cm with the gas mixtures Ar/CO2 ¼ 70/30 as the typical Gas Electron
ARTICLE IN PRESS Q. Gou et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 498–501
Multiplier (GEM) [4,5] used and the gas gap of this chamber is of 7 mm, the saturated operating voltage ought to be 700 V in our case. The energy loss of penetrating 1.89 GeV electrons in the material of the front lucite window and the rear epoxy wall of the chamber and in the material of the anode/cathode PCB electrode underlay is almost negligible compared with the original energy, so that it does not influence the energy effects of the downstream devices. However, the beam intensity can be reduced by the scattering effect from the chamber material which should be considered further. The measurement principle is considered as follows: regarding 1.89 GeV electrons as the minimum-ionizing particles (MIP), their energy loss DE in the chamber gap with the definite gas mixture is expressed by DE ¼ n W e, where n is the number of ion pairs (Ip) produced by one MIP in the chamber, W is the average energy needed for producing one ion pair, and e is the charge quantity of one single electron. Since an MIP produces 94 Ip/cm in Ar and 91 Ip/cm in CO2 under 1 atm [6], n ¼ (0.7 94Ip/cm+0.3 91Ip/cm) 0.7 cm ¼ 65Ip/MIP. The number of particles per bunch (Pb) can be obtained via formula: Pb ¼
Qb ¼ 9:6 1016 Q b ne
(1)
where Qb is the total ionization charge per bunch in the chamber. In the PSPICE simulation, the pulse current source is a trapezoidal function with rise–fall slopes and plateau having several parameters, including I2 (current amplitude nominated by PSPICE), TR (Rise Time) and TF (Fall Time) as well as PW (Pulse Width). Approximately we define: Q b ¼ I2Dt
Ia ¼ fP b ¼ 25P b ðf is the pulse frequencyÞ
(3)
Since the pulse width is 1.2 ns (very short) and the pulse frequency is 25 Hz (with very low beam duty circle, 3 108), the double-mode readout is selected. For convenient and clear description, in several groups of the I2***, Dt***, P***, Q***, I***, parameters, their subscripts (***) with associated conditions are defined as: bds: particle per Bunch in the DC mode associated Simulation; bps: particle per Bunch in the Pulse mode associated Simulation; bde: particle per Bunch in the DC mode associated Experiment; bpe: particle per Bunch in the Pulse mode associated Experiment. Formulae (1)–(3) can be expressed as: P b
Fig. 1 shows the layout of the double-mode readout system including the equivalent devices of the chamber and the additional RC.
3. Results Referring to Fig. 1, in the DC mode, the analog signals are directly picked up by a parallel RC circuit (where R1d ¼ 1 MO, C1d ¼ 0.141 mF), which has a large time constant of 141 ms, and its voltage is detected with a mini-volt meter and an oscilloscope. In the pulse mode, the cathode signals pass through a coupled blocking capacitor, C2p (1000 pF, 6 kV), and a parallel RC circuit (where, R2p ¼ 10 MO, C1p ¼ 390 pF, R1p ¼ 1.02 MO) with small time constant of 398 ms and the output signal is monitored directly with an oscilloscope. All the above-mentioned parameters are finally selected as the optimum ones after trying them group by group. Fig. 2 shows the signals observed with the oscilloscope in both the DC and the pulse modes. In the DC mode: (1) the ratio of the amplitude of the net DC part to the sawtooth part is 3/2. As expected, the output signal is DC-dominated. (2) The interval between any two pulses is 40 ms, consistent with the beam pulse frequency of 25 Hz. In the pulse mode, the rise time (3 ms) is associated with the smaller collecting time (2.1 ms). Fig. 3 shows the measured voltage value vs. the operating voltage in the DC mode and Vd ¼ 68 mV corresponds to the saturated operation voltage of 800 V. Taking into account the load resistance Rd about 909 kO (Rd ¼ R1dJZin, where Zin is the input impendence of the mini-volt meter around 10 MO), the detector current (Id) can be expressed as:
(2)
Here Dt is the estimated FWHM (Full-Width at Half-Maximum). The absolute intensity (Ia) can be given by
499
Id ¼
V d 68 103 ¼ ¼ 7:48 108 A Rd 909 103
Then, the absolute intensity Iade is obtained by using formula (3) (since here Id is just fQbde C/s: Iade ¼
fQ bde 7:48 108 ¼ ¼ 7:2 109 electron=s ne 65 1:602 1019
Concerning the pulse mode, Fig. 4 shows the pulse amplitude (U) vs. the operating voltage and Up ¼ 670 mV corresponds to the saturated operation voltage of 800 V. Taking into account the output capacitance C1p (390 pF), and the additional parasitic capacitance of the 30 m long cable (2.93 nF), the equivalent capacitance C1ps (as named in PSPICE) is 3.32 nF (see Fig. 5). The charge Qbpe on the capacitance C1ps per pulse can be roughly expressed as Up C1ps, ignoring the discharge effect of the RC circuit (Up/Up max). Using formula (1), Pbpe can be expressed as: Pbpe ¼ 9:6 1016 Q bpe
Q ¼ b ; ne
Q b ¼ I2b Dt b
and Ia ¼ fP b
¼ 9:6 1016 670 103 3:32 109 ¼ 2:1 108 electron=pulse
Beam
Cath
C2p C1p
-HV
R1p
R2p
Gas Anod Polycarbonate layer
C1d
Fig. 1. Layout of the double-mode readout system.
R1d
Using formula (3), the absolute intensity (Iape) is given by: Iape ¼ 25P bpe ¼ 25 2:1 108 ¼ 5:3 109 electron=s Comparing the two modes, the Iade in the DC mode is larger than the Iape in the pulse mode (as expected), since for the DC mode, more positive ions are collected due to the larger RC constant. In order to select the optimum circuit parameters and to provide a rough consistency check of the results, we carried out simulations using the PSPICE 9.2. The equivalent circuits are presented in Fig. 5 compared with the devices in Fig. 1: R1ps (or R1ds) is the equivalent resistance of R1p (or R1d) and of the oscilloscope;
ARTICLE IN PRESS 500
Q. Gou et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 498–501
Fig. 2. The DC and pulse mode signals obtained with oscilloscope, the upper plot (CH2) is from the pulse mode, and the lower plot (CH1) is from the DC mode.
C2ps
70
V
60 Amplitude (mV)
1000pF
I1bps=0 I2bps=1.34mA TDbps=0 TRbps=2.1us TFbps=2.1us PWbps=1.2ns PERbps=40ms
50 40 30
C3ps
R3ps
C1ps
R1ps
200pF
8.6meg
3.32nF
505k
20 10 0 0
200
400
600
800
1000
HV (V) Fig. 3. The measured voltage in the DC mode.
I1bps=0 I2bps=1.45mA TDbps=0 TRbps=2.1us TFbps=2.1us PWbps=1.2ns PERbps=40ms
V C3ds
R3ds
C1ds
R1ds
200pF
60meg
143.93nF
500k
Fig. 5. The equivalent circuits used in the simulation (upper circuit for the pulse mode, and lower for the DC mode).
700
Amplitude (mV)
600 500 400 300 200 100 0
200
400
600
800
HV (V) Fig. 4. The measured voltage in the pulse mode.
1000
C1ps (or C1ds) is the equivalent capacitance of C1p (or C1d) and of the long cable; C2ps ¼ C2p; R3ds ( ¼ 60 MO) is the evaluated insulating resistance of the chamber; R3ps ¼ R3dsJR2p; C3ps ( ¼ C3ds ¼ 200 pF) is the equivalent capacitance of the chamber and the parasitic contribution from short connection cables, etc. In Fig. 5, the amplitude of the pulse current source in the pulse mode is I2bps ( ¼ 1.34 mA) assuming an estimated FWHM (Dtbps) of 2.1 ms, determined by the transit time of the ionized electrons between two electrodes (the estimated velocity of the drift electrons is 3.3 106 cm/s; the drift distance is 7 mm). Thus,
ARTICLE IN PRESS Q. Gou et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 498–501
501
800mV
400mV
0V
-20mV
-40mV
-60mV 390ms
400ms
410ms
420ms
430ms
440ms
450ms
460ms
470ms
480ms
490ms
Fig. 6. The DC and pulse mode signals obtained with simulation (upper part for the pulse mode, and lower for the DC mode).
Qbps can be obtained from formula (2), Pbps is given by formula (1), and Iaps is calculated using formula (3) and is roughly in agreement with Iape ( ¼ 5.3 109 electron/s): Q bps ¼ I2bps Dt bps ¼ 1:34 103 2:1 106 ¼ 2:81 109 C=pulse P bps ¼ 9:6 1016 Q bps ¼ 9:6 1016 2:81 109 ¼ 2:7 108 electron=pulse Iaps ¼ 25P bps ¼ 25 2:7 108 ¼ 6:8 109 electron=s For the DC mode, the total charge should include the contribution of both positive ions and fast electrons. Thus Qbds in the DC mode should be larger than Qbps in the pulse mode. Again, we calculate Qbds from formula (2), Pbds is given by formula (1), and Iads is calculated using formula (3) and is approximately equal to Iade ( ¼ 7.2 109 electron/s):
experiment, and serve as a crosscheck. Using the optimum parameters of the readout system, the results of the PSPICE simulation are consistent with the experimental results. As a result of this experiment the double-mode readout system will be used for the FC, which is currently under development, but with the ionized charge quantity divided by a factor of about 65 (the number of ion pairs produced by an MIP interaction in the IC) because there is no gas ionization inside the FC. Different types of IC (such as two-dimensional, multi-strip chambers, designed for measuring lower beam intensities and the beam profile), together with the FC and other beam monitors (for example, micro-pattern gas chambers, such as Micro-megas and GEM) are currently under development and, when complete, will be suitable for a large range of beam intensities and for crosschecks.
Acknowledgments
Q bds ¼ I2bds Dt bds ¼ 1:45 103 2:1 106 ¼ 3:05 109 C=pulse P bds ¼ 9:6 1016 Q bds ¼ 2:93 108 electron=pulse Iads ¼ 25P bds ¼ 7:3 109 electron=s The simulated plots presented in Fig. 6 are consistent with the experimental results (see Fig. 2).
4. Conclusions The absolute intensity of the BEPC-LINAC test beam, measured with a specifically developed IC with double-mode readout, is Iade ¼ 7.2 109 electron/s. As expected, the absolute intensity measured in the pulse mode, Iape (or Iaps), is slightly smaller than that measured in the DC mode, Iade (or Iads) due to the contribution of positive ions. The measured absolute intensities from the two modes are consistent to within the limits of the
The authors are in debt to Prof. Yigang Xie, Prof. Hongbo Hu, Prof. Qun Ouyang, Prof. Shuchen Zheng, Prof. Jiacai Li, Dr. Peng Chen, Dr. Wan Xie, Dr. Paolo Camarri and Dr. Nick Schurch for their kind help and discussions. The authors would like to thank the ATLAS group of IHEP, China, for their great support in the construction of the detector, to thank the test beam group of IHEP, China, for their skilled and helpful cooperation. This work is partly supported by the Chinese Academy of Sciences and the National Science Foundation of China (Funding No. 10120130794). References [1] Z. Feng, et al., Nucl. Phys. B (Proc. Suppl.). 175–176 (2008) 251. [2] A. La Rosa, et al., Nucl. Instr. and Meth. A 565 (2006) 833. [3] D. Yount, Nucl. Instr. and Meth 52 (1) (1967) 1 D. Yount, SLAC_PUB_0264, January 1967. [4] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [5] M. Inuzuka, et al., Nucl. Instr. and Meth. A 525 (2004) 529. [6] Y. Xie, et al., Particle Detector and Data Acquisition, Science Press, China, 2003, p. 33–34 (in Chinese).
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Technical Note
Testing the absolute beam intensity of the high-energy pulsed electron beam with a double-mode readout ionization chamber Q. Gou a,, Z. Feng a, S. Yin a,b, F. Shi a, J. Liu a,c, J. Dong a,c, J. Liao a a b c
IHEP of the Chinese Academy of Sciences, Beijing 100049, People’s Republic China Shandong University, Shandong 250100, People’s Republic China Lanzhou University, Gansu 730000, People’s Republic China
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 February 2008 Received in revised form 8 April 2008 Accepted 28 April 2008 Available online 17 May 2008
We constructed an ionization chamber (IC) to test the absolute intensity of the BEPC-LINAC (Beijing Electron Positron Collider-Linear Accelerator) test beam. The IC was adapted for the 1.89 GeV high-energy electron beam, with pulse time width of 1.2 ns and frequency of 25 Hz, by equipping it with a doublemode readout and choosing the optimum circuit parameters for the readout modes. The measured absolute intensity of the test beam is 7.2 109 electron/s, and is consistent with PSPICE simulations. & 2008 Elsevier B.V. All rights reserved.
Keywords: Ionization chamber Double-mode readout High-energy pulsed electron beam
1. Introduction In order to test the YAC (Yangbajing Air-shower Core detector) with a wide particle-intensity dynamic range (six orders of magnitude), developed by Tibet AS (Air Shower) group, by using the test beam of the BEPC-LINAC [1], both an ionization chamber (IC) (as in Ref. [2]) and a Faraday cup (FC) (as in Ref. [3]) as well as other devices were proposed to monitor the beam intensity in different intensity ranges. The IC has been built fulfilling the following requirements: (1) Equip the IC with double-mode readout: DC (Direct Current) mode and pulse mode. (2) Regard the IC as a parallel connection of a resistor and a capacitor. Optimize the additional RC (Resistor–Capacitor) devices in the readout system. (3) Consider all the equivalent or parasitic resistance and capacitance, to be considered for the FC also. If the time constant of the readout circuits is suitable, the beam intensity could be displayed simultaneously by a pure pulse in the pulse mode and by a DC level with an extra sawtooth waveform in the DC mode. The results obtained via two different readout modes could be a crosscheck and be complementary to each other. Corresponding author. Tel.: +86 10 88236115; fax: +86 10 88233086.
E-mail address: [email protected] (Q. Gou). 0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.04.081
The main purpose of the beam test described in this paper was to check whether: (1) The chamber dimension (3 3 cm2) is adaptable to the beam profile (2 cm in diameter) and the selected working gas mixture Ar/CO2 meets the requirements of the very short time electron pulse. (2) The parameters of the devices associated with the time constant are optimum. The absolute beam intensity could be obtained with two different readout modes and they are in agreement with each other. In this paper, we describe the design and construction of the detector, its readout system, the performance under the test beam of the BEPC-LINAC and the simulation results obtained with the PSPICE 9.2 software.
2. Description of the chamber and the measurement principle with the readout electronics The chamber is made of two parallel electrode plates, consisting of a 1 mm thick polycarbonate (PCB) layer coated with 35 um copper on the surface. It has a sensitive area of 3 3 cm2 with a gas gap of 7 mm. The gas mixture Ar/CO2 is used for convenience under the pressure of 1 atm. Considering that the proper electric field inside the chamber is 1000 V/cm with the gas mixtures Ar/CO2 ¼ 70/30 as the typical Gas Electron
ARTICLE IN PRESS Q. Gou et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 498–501
Multiplier (GEM) [4,5] used and the gas gap of this chamber is of 7 mm, the saturated operating voltage ought to be 700 V in our case. The energy loss of penetrating 1.89 GeV electrons in the material of the front lucite window and the rear epoxy wall of the chamber and in the material of the anode/cathode PCB electrode underlay is almost negligible compared with the original energy, so that it does not influence the energy effects of the downstream devices. However, the beam intensity can be reduced by the scattering effect from the chamber material which should be considered further. The measurement principle is considered as follows: regarding 1.89 GeV electrons as the minimum-ionizing particles (MIP), their energy loss DE in the chamber gap with the definite gas mixture is expressed by DE ¼ n W e, where n is the number of ion pairs (Ip) produced by one MIP in the chamber, W is the average energy needed for producing one ion pair, and e is the charge quantity of one single electron. Since an MIP produces 94 Ip/cm in Ar and 91 Ip/cm in CO2 under 1 atm [6], n ¼ (0.7 94Ip/cm+0.3 91Ip/cm) 0.7 cm ¼ 65Ip/MIP. The number of particles per bunch (Pb) can be obtained via formula: Pb ¼
Qb ¼ 9:6 1016 Q b ne
(1)
where Qb is the total ionization charge per bunch in the chamber. In the PSPICE simulation, the pulse current source is a trapezoidal function with rise–fall slopes and plateau having several parameters, including I2 (current amplitude nominated by PSPICE), TR (Rise Time) and TF (Fall Time) as well as PW (Pulse Width). Approximately we define: Q b ¼ I2Dt
Ia ¼ fP b ¼ 25P b ðf is the pulse frequencyÞ
(3)
Since the pulse width is 1.2 ns (very short) and the pulse frequency is 25 Hz (with very low beam duty circle, 3 108), the double-mode readout is selected. For convenient and clear description, in several groups of the I2***, Dt***, P***, Q***, I***, parameters, their subscripts (***) with associated conditions are defined as: bds: particle per Bunch in the DC mode associated Simulation; bps: particle per Bunch in the Pulse mode associated Simulation; bde: particle per Bunch in the DC mode associated Experiment; bpe: particle per Bunch in the Pulse mode associated Experiment. Formulae (1)–(3) can be expressed as: P b
Fig. 1 shows the layout of the double-mode readout system including the equivalent devices of the chamber and the additional RC.
3. Results Referring to Fig. 1, in the DC mode, the analog signals are directly picked up by a parallel RC circuit (where R1d ¼ 1 MO, C1d ¼ 0.141 mF), which has a large time constant of 141 ms, and its voltage is detected with a mini-volt meter and an oscilloscope. In the pulse mode, the cathode signals pass through a coupled blocking capacitor, C2p (1000 pF, 6 kV), and a parallel RC circuit (where, R2p ¼ 10 MO, C1p ¼ 390 pF, R1p ¼ 1.02 MO) with small time constant of 398 ms and the output signal is monitored directly with an oscilloscope. All the above-mentioned parameters are finally selected as the optimum ones after trying them group by group. Fig. 2 shows the signals observed with the oscilloscope in both the DC and the pulse modes. In the DC mode: (1) the ratio of the amplitude of the net DC part to the sawtooth part is 3/2. As expected, the output signal is DC-dominated. (2) The interval between any two pulses is 40 ms, consistent with the beam pulse frequency of 25 Hz. In the pulse mode, the rise time (3 ms) is associated with the smaller collecting time (2.1 ms). Fig. 3 shows the measured voltage value vs. the operating voltage in the DC mode and Vd ¼ 68 mV corresponds to the saturated operation voltage of 800 V. Taking into account the load resistance Rd about 909 kO (Rd ¼ R1dJZin, where Zin is the input impendence of the mini-volt meter around 10 MO), the detector current (Id) can be expressed as:
(2)
Here Dt is the estimated FWHM (Full-Width at Half-Maximum). The absolute intensity (Ia) can be given by
499
Id ¼
V d 68 103 ¼ ¼ 7:48 108 A Rd 909 103
Then, the absolute intensity Iade is obtained by using formula (3) (since here Id is just fQbde C/s: Iade ¼
fQ bde 7:48 108 ¼ ¼ 7:2 109 electron=s ne 65 1:602 1019
Concerning the pulse mode, Fig. 4 shows the pulse amplitude (U) vs. the operating voltage and Up ¼ 670 mV corresponds to the saturated operation voltage of 800 V. Taking into account the output capacitance C1p (390 pF), and the additional parasitic capacitance of the 30 m long cable (2.93 nF), the equivalent capacitance C1ps (as named in PSPICE) is 3.32 nF (see Fig. 5). The charge Qbpe on the capacitance C1ps per pulse can be roughly expressed as Up C1ps, ignoring the discharge effect of the RC circuit (Up/Up max). Using formula (1), Pbpe can be expressed as: Pbpe ¼ 9:6 1016 Q bpe
Q ¼ b ; ne
Q b ¼ I2b Dt b
and Ia ¼ fP b
¼ 9:6 1016 670 103 3:32 109 ¼ 2:1 108 electron=pulse
Beam
Cath
C2p C1p
-HV
R1p
R2p
Gas Anod Polycarbonate layer
C1d
Fig. 1. Layout of the double-mode readout system.
R1d
Using formula (3), the absolute intensity (Iape) is given by: Iape ¼ 25P bpe ¼ 25 2:1 108 ¼ 5:3 109 electron=s Comparing the two modes, the Iade in the DC mode is larger than the Iape in the pulse mode (as expected), since for the DC mode, more positive ions are collected due to the larger RC constant. In order to select the optimum circuit parameters and to provide a rough consistency check of the results, we carried out simulations using the PSPICE 9.2. The equivalent circuits are presented in Fig. 5 compared with the devices in Fig. 1: R1ps (or R1ds) is the equivalent resistance of R1p (or R1d) and of the oscilloscope;
ARTICLE IN PRESS 500
Q. Gou et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 498–501
Fig. 2. The DC and pulse mode signals obtained with oscilloscope, the upper plot (CH2) is from the pulse mode, and the lower plot (CH1) is from the DC mode.
C2ps
70
V
60 Amplitude (mV)
1000pF
I1bps=0 I2bps=1.34mA TDbps=0 TRbps=2.1us TFbps=2.1us PWbps=1.2ns PERbps=40ms
50 40 30
C3ps
R3ps
C1ps
R1ps
200pF
8.6meg
3.32nF
505k
20 10 0 0
200
400
600
800
1000
HV (V) Fig. 3. The measured voltage in the DC mode.
I1bps=0 I2bps=1.45mA TDbps=0 TRbps=2.1us TFbps=2.1us PWbps=1.2ns PERbps=40ms
V C3ds
R3ds
C1ds
R1ds
200pF
60meg
143.93nF
500k
Fig. 5. The equivalent circuits used in the simulation (upper circuit for the pulse mode, and lower for the DC mode).
700
Amplitude (mV)
600 500 400 300 200 100 0
200
400
600
800
HV (V) Fig. 4. The measured voltage in the pulse mode.
1000
C1ps (or C1ds) is the equivalent capacitance of C1p (or C1d) and of the long cable; C2ps ¼ C2p; R3ds ( ¼ 60 MO) is the evaluated insulating resistance of the chamber; R3ps ¼ R3dsJR2p; C3ps ( ¼ C3ds ¼ 200 pF) is the equivalent capacitance of the chamber and the parasitic contribution from short connection cables, etc. In Fig. 5, the amplitude of the pulse current source in the pulse mode is I2bps ( ¼ 1.34 mA) assuming an estimated FWHM (Dtbps) of 2.1 ms, determined by the transit time of the ionized electrons between two electrodes (the estimated velocity of the drift electrons is 3.3 106 cm/s; the drift distance is 7 mm). Thus,
ARTICLE IN PRESS Q. Gou et al. / Nuclear Instruments and Methods in Physics Research A 592 (2008) 498–501
501
800mV
400mV
0V
-20mV
-40mV
-60mV 390ms
400ms
410ms
420ms
430ms
440ms
450ms
460ms
470ms
480ms
490ms
Fig. 6. The DC and pulse mode signals obtained with simulation (upper part for the pulse mode, and lower for the DC mode).
Qbps can be obtained from formula (2), Pbps is given by formula (1), and Iaps is calculated using formula (3) and is roughly in agreement with Iape ( ¼ 5.3 109 electron/s): Q bps ¼ I2bps Dt bps ¼ 1:34 103 2:1 106 ¼ 2:81 109 C=pulse P bps ¼ 9:6 1016 Q bps ¼ 9:6 1016 2:81 109 ¼ 2:7 108 electron=pulse Iaps ¼ 25P bps ¼ 25 2:7 108 ¼ 6:8 109 electron=s For the DC mode, the total charge should include the contribution of both positive ions and fast electrons. Thus Qbds in the DC mode should be larger than Qbps in the pulse mode. Again, we calculate Qbds from formula (2), Pbds is given by formula (1), and Iads is calculated using formula (3) and is approximately equal to Iade ( ¼ 7.2 109 electron/s):
experiment, and serve as a crosscheck. Using the optimum parameters of the readout system, the results of the PSPICE simulation are consistent with the experimental results. As a result of this experiment the double-mode readout system will be used for the FC, which is currently under development, but with the ionized charge quantity divided by a factor of about 65 (the number of ion pairs produced by an MIP interaction in the IC) because there is no gas ionization inside the FC. Different types of IC (such as two-dimensional, multi-strip chambers, designed for measuring lower beam intensities and the beam profile), together with the FC and other beam monitors (for example, micro-pattern gas chambers, such as Micro-megas and GEM) are currently under development and, when complete, will be suitable for a large range of beam intensities and for crosschecks.
Acknowledgments
Q bds ¼ I2bds Dt bds ¼ 1:45 103 2:1 106 ¼ 3:05 109 C=pulse P bds ¼ 9:6 1016 Q bds ¼ 2:93 108 electron=pulse Iads ¼ 25P bds ¼ 7:3 109 electron=s The simulated plots presented in Fig. 6 are consistent with the experimental results (see Fig. 2).
4. Conclusions The absolute intensity of the BEPC-LINAC test beam, measured with a specifically developed IC with double-mode readout, is Iade ¼ 7.2 109 electron/s. As expected, the absolute intensity measured in the pulse mode, Iape (or Iaps), is slightly smaller than that measured in the DC mode, Iade (or Iads) due to the contribution of positive ions. The measured absolute intensities from the two modes are consistent to within the limits of the
The authors are in debt to Prof. Yigang Xie, Prof. Hongbo Hu, Prof. Qun Ouyang, Prof. Shuchen Zheng, Prof. Jiacai Li, Dr. Peng Chen, Dr. Wan Xie, Dr. Paolo Camarri and Dr. Nick Schurch for their kind help and discussions. The authors would like to thank the ATLAS group of IHEP, China, for their great support in the construction of the detector, to thank the test beam group of IHEP, China, for their skilled and helpful cooperation. This work is partly supported by the Chinese Academy of Sciences and the National Science Foundation of China (Funding No. 10120130794). References [1] Z. Feng, et al., Nucl. Phys. B (Proc. Suppl.). 175–176 (2008) 251. [2] A. La Rosa, et al., Nucl. Instr. and Meth. A 565 (2006) 833. [3] D. Yount, Nucl. Instr. and Meth 52 (1) (1967) 1 D. Yount, SLAC_PUB_0264, January 1967. [4] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [5] M. Inuzuka, et al., Nucl. Instr. and Meth. A 525 (2004) 529. [6] Y. Xie, et al., Particle Detector and Data Acquisition, Science Press, China, 2003, p. 33–34 (in Chinese).