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Charge integrating type position-sensitive proportional chamber for time-resolved measurements using intense X-ray sources
Nuclear Instruments and Methods in Physics Research A234 (1985) 593-601 North-Holland, Amsterdam
593
CHARGE INTEGRATING TYPE POSITION-SENSITIVE PROPORTIONAL CHAMBER FOR T I M E - R E S O L V E D M E A S U R E M E N T S U S I N G I N T E N S E X-RAY S O U R C E S
Koh-ichi M O C H I K I a n d Ken-ichi H A S E G A W A Department of Nuclear Engineering, University of Tokyo, 7- 3-1, Hongo, Bunkyo- ku, Tokyo 113, Japan Received 22 May 1984 and in revised form 24 September 1984
A position-sensitive detecting system for time-resolved diffraction measurements with very intensive X-ray sources has been developed. It consists of a charge integrating type gas-filled detector, multichannel analog multiplexers, a signal processor and a memory (120 ch. × 128 phases × 24 bits). The detector is 120 mm long in effective length by 10 mm × 10 mm in cross section with a single anode of 20/~m diameter. One of the cathode planes consists of 120 cathode strips with a pitch of 1 mm. The spatial resolution is equal to the pitch under a certain detector current limit. The gas gain is adjustable to an appropriate value according to the X-ray intensity range of interest. For experiments with 8 keV X-ray sources, maximum absorption rates of 9 × 107 photons/s, mm with low applied voltage and minimum absorption rates of about 3 photons/s- mm with high applied voltage can be achieved. This system was applied to a time-resolvedX-ray diffraction study on frog muscle using a synchrotron radiation source at the Photon Factory and we could collect diffraction patterns with a time resolution of 10 ms and only 10 stimulations.
I. Introduction Synchrotron radiation sources delivering intense Xray beams demand superior quality instrumentation. In time-resolved X-ray diffraction experiments, the main requirement is to reduce the necessary recording times sufficiently to obtain statistically meaningful information in the time scale of interest. To overcome this, three types of one dimensional position-sensitive detector may be considered, that is, (1) the delay line type, (2) the MWPC type equipped with a preamplifier, a discriminator and a scaler for each anode wire and (3) the charge integrating type [1-3] equipped with a multielectrode and external capacitors for charge accumulation. The delay line type [4] can be operated with count rates up to about 5 × 105 counts/s over the whole detection field and the maximum count rate per channel depends on the pattern to be measured. The scaler/wire type [5,6] can cope with count rates up to about 2 × 105 counts/(s wire) and in the case of a 100 channel detector, this implies rates of 2 × 107 counts/s over the whole detection field. In contrast with these detectors, the integrating type has no counting loss, and the applied voltage and the accumulation time are appropriately adjustable according to the intensity of the incident X-ray beams, so the attainable maximum count rate of the integrating type is higher than that of any other type. Several detectors of the charge integrating type have been designed and constructed. They are divided into two groups according to the readout electrode, that is, the cathode readout type with a multistrip cathode and 0168-9002/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
the anode readout type with a multiple anode. The spatial resolution depends on the pitch of these readout electrodes. As far as the gas gain is concerned, the attainable range of the former type is wider than that of the latter, thus the cathode readout type can cope with a very wide range of X-ray intensity. In this paper, we describe a one-dimensional cathode readout type detector and its data acquisition system, and an application to the time-resolved X-ray diffraction study on frog muscle [7] is shown.
2. Charge integrating type position-sensitive detecting system 2.1. Principle The detector is an ordinary proportional counter except for the multistrip cathode equipped with capacitors (Ca'S) for accumulating charges collected by these cathode strips as shown in fig. 1. Each cathode strip is also connected to an input terminal of a CMOS analog multiplexer and the positive charges accumulated in each capacitor are transferred to an IC charge-sensitive amplifier when the associated channel of the multiplexer IC is selected by a binary coded addressing signal. The charge-sensitive amplifier is a capacitive feedback type whose input impedance is very low, so that the charge transfer to the feedback capacitor, Cf, is done almost completely. The output voltage signal of the charge amplier is fed to an analog-to-digital converter (ADC).
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The output voltage of the charge-sensitive amplifier can be described by V = k q A I T / C t,
where q = amount of positive charge produced by a primary ionizing event, A = gas gain, I = X-ray intensity per cathode strip, T = accumulating time of the collected charges in the capacitors, Ca's, that is, sampling period, Cf = feedback capacitance of the charge-sensitive amplifier, k = conversion coefficient of the system, including the charge collection efficiency of the readout cathode. This equation shows that the output voltage is proportional to the X-ray intensity, provided the gas gain does not depend on the X-ray intensity and the incident X-ray is monoenergetic or the energy spectrum is independent of the measuring position. The incident position of an X-ray photon can be identified by the strip number.
Fig. 2. Schematic of the detector with a multistrip cathode.
2.3. Signal processing
The block diagram of the signal processing system is shown in fig. 3. It consists of eight kinds of electronic circuit boards, a monitor scope and a microcomputer used as a system controller. The multiplexer board contains capacitors (Ca = 1000 pF) for charge accumulation and 8 chips of 16 ch. analog multiplexers (Harris HI 506). For fast charge transfer, multiplexers without overvoltage protection circuits are used and a simple current limiting circuit with an RC network is installed between a high voltage power supply and the anode wire. Addressing and chip enable signals are sent sequentially from the enable-controller board and the charges are transferred to the charge-sensitive amplifier in the charge amp. & ADC board. The feedback capacitance of the charge-sensitive amplifier is 510 p F which was determined in consideration of the stability of the amplifier, the protection of the over-voltage at the input of the CMOS analog multiplexer and the input voltage range of the ADC (0 to - 1 0 V). In the adder & memory board, each digitized value after AD conversion is additionally integrated to the value stored in the memory
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The detector vessel is schematically shown in fig. 2. The cross section of the detector is 10 mm x 10 mm and a gold-plated tungsten anode wire of 20 # m diameter is mounted at its centre. The readout cathode was made by etching on a printed circuit board and consists of 128 strips with a pitch of 1 mm. The gas flowing in the detector was a mixture of Ar + 8.9% CO 2.
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and restored to the memory at the same address. The data area of the system is divided into 128 segments of 128 channels with 24 bits; therefore, 128 phases in a dynamic phenomenon can be obtained. The timing pulses and the output signal from the charge-sensitive amplifier are shown in fig. 4 in the case of time-resolved experiments. Operating parameters of the accumulation time, T,, corresponding to resolution time and the number of times of the additional integration are set with the microcomputer before the start of measurements. The external trigger pulse starts the sam-
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The values of Tf, Tt, Tc and Tp are 1 ~s, 2/xs, 4 #s and 7 #s, respectively, and T~ is variable in the range of 1-1000 ms.
3. Performance In the following measurements, an X-ray beam of 8 keV from a rotating anode type X-ray source was used except where specially mentioned.
3.1. Gas gain One of the advantageous characteristics of this system is that the applied voltage is adjustable so as to keep an appropriate gas gain over an X-ray intensity to be measured. Gas gain shift therefore restricts the upper limit of the intensity. The gas gain characteristics, at first, were measured with a proportional counter. It has a planer cathode, not a strip cathode, and the same dimension as the integral type detector, and a collimated beam 0.5 mm high x 0.4 mm wide was used. The gas flowing in the detector was a mixture of Ar + 8.9% CO 2. The output current from the cathode was measured with a picoammeter, and the incident X-ray intensity was attenuated by copper foils step by step. The gas gain could be derived from the ratio of the output current to be amount of primary charges produced by X-ray absorption during I s. In estimating the primary charges, a concept of effective X-ray energy was introduced in consideration of the effect of escape X-rays which do not dissipate their energy in the detector gas. For our detector, the effective X-ray energy was estimated to be 7.68 keV from a pulse height spectrum. The results are shown in fig. 5. It is evident that gain shift occurs in the higher region of the intensity for each applied voltage. The dependence of the relative gas gain on the output current is shown in fig. 6. As a cause of the gain shift, Hendricks proposed a space charge model [8] and the relative gain shift can be expressed as follows;
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b2p -I 8V= 4~rc~-~LV " In fig. 6, the solid line means the calculated values of the above formula for the applied voltage of 1600 V and it is found that the measured values fit well with the calculated ones. Output currents for gas gain shifts of 1% and 5% are 0.4 nA and 2 nA, respectively.
3.2. Background In any experiment, background data have to be measured and deducted from diffraction data to obtain precise information on X-ray intensity. The characteristics of the background data were measured and it was found that several background components are involved in the input voltage of the charge-sensitive amplifier. They originate from switching noise of the analog switches, leakage currents from the multiplexer input and offset voltage of multiplexer channels. These contributions depend on the characteristics of the multiplexer channels and the charge accumulating time. The sum of these components results in a positive voltage at the input of the charge-sensitive amplifier. To prevent the input signal from being out of range of the ADC, the analog input range of the ADC was shifted by applying negative currents to the comparator of the ADC. The leakage currents from the input pin of the 8th channel of the multiplexer are dominant because the input pin for the negative power supply is adjacent to it. Therefore, we do not use the 8th channel of each chip and this system is operated for 120 cathode strips. In case the accumulation time is less than 1 s, the fluctuation of these components can be suppressed to be lower than 1 LSB of the ADC. In addition to these background components, there were external noises. To suppress this effect under 1 LSB, additional integration of at least 4 times is needed.
3.3. Collection efficiency As the multistrip cathode occupies only one side of the square cross section, ions generated near the anode
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K. Mochiki, K. Hasegawa / Charge integrating type proportional chamber
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characteristics of the avalanche distribution around the anode wire. From the results it may be recommended that in the case of diffraction experiments using narrow beams the detector should be adjusted so the diffracted beams enter the region near the readout cathode.
3.4. Digital output vs. X-ray intensity The dependency of the sum of ADC out on the intensity of the monochromized 8 keV X-ray beam from the storage ring in the Photon Factory was measured and the result is shown in fig. 8. Beam profiles obtained in the measurement are also shown in figs. (9a, b and c where the applied voltage was 500 V and the beam intensities were 4.8 x 10 5, 2.4 x 107 and 8.4 x 10 9 photons/s, respectively. In fig. 8, the solid and broken lines mean the sum of the profile and the peak channel, respectively. As concerns the sum of the profile, good linearity is preserved over 108 photon/s.
3.5. Spatial resolution Responses of the adjacent channels to a narrow X-ray beam of 30 #m diameter was measured by scanning the detector step by step. The results are shown in figs. 10a and b where the applied voltages were 500 and 1000 V, respectively. In the case of low gas gain, the spatial resolution is determined by the pitch of the cathode strip. However, degradation is observed when the output currents are increased.
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The dependence of line broadening of an observed peak on the applied voltage was measured with a collimated beam of 500 #m diameter. The beam intensity was 107 photons/s and the applied voltage was changed in 5 steps and the operating parameters were 10 ms for Ta and 103 times for additional integration. Output distributions normalized to peak values are shown in figs. 11 and 12. From these figures, an increase in the peak width and growth of the feet can be seen. When the applied voltage was 1300 V, the output current of the peak channel was 2.5 nA and the upheaval of the feet was less than 1%. Another measurement was carried out with a broad beam and the same characteristics of the upheaval were observed for the same average output current. From these results it may be derived that the average output current of the peak region should be lower than about 2.5 nA to suppress the upheaval of the feet to less than 1%. These phenomena seem to be caused by diffusion of ions and emission of photons by excited atoms and molecules; however, a more detailed investigation is the subject of a future study.
3. 7. Uniformity The response of the output charge from each cell was measured by scanning the detector at a fixed speed across a broad X-ray beam. A relative standard deviation of 1.4% was obtained over three fourths of the
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Fig. 13. 100 ms resting pattern from frog muscle.
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effective region of the detector with an applied voltage of 1000 V. However, concerning the whole region, the value was 3.7%. The main factor of the inhomogeneity appears to be caused by errors in fabricating the detector.
4. Application This system was applied to a time-resolved X-ray diffraction study on frog muscle using the synchrotron radiation source of the Photon Factory in the National Laboratory for High Energy Physics in Tsukuba [9]. The study was performed with collaborators Prof. H. Sugl, Drs. H. Tanaka, Y. Amemiya and T. Kobayashi and from among the preliminary results we only show two diffraction data in this paper. Figs. 13 and 14 show the diffraction patterns recorded from frog muscle in resting state and during contraction, respectively. Fig. 13 shows a diffraction pattern recorded in only 100 ms from muscle in the resting state with an X-ray intensity about 105 photons/s over the whole detector and about 6 x 103 photons/s at the peak channel. Fig. 14 shows the time course of the diffraction peak intensity during contraction where the data were summed from 10 twitches, each time slot being of 10 ms duration, that is, corresponding to 100 ms total exposure. In this case the memory area is able to cover 1280 ms and only onefourth of the total diagram is shown. The X-ray intensity was about 6 x 104 photons/s over the whole detector and about 5 X 103 photons/s at the peak channel.
5. Discussion In this readout method, the background data have to be deduced from the diffraction data obtained with the same operating parameters. Previously, it was described
that the fluctuation in the background data can be suppressed to below 1 LSB of the ADC if samplings are carried out more than 4 times and the charge accumulation time is set between 1 ms and 1 s. In this time range, the ADC output has a good linear relation to the accumulation time over the full range of the ADC. Therefore, the resolution of the signal processing system can be defined to be 1/4095. Concerning the statistical error, there is no difference between the pulse type and the charge integrating type and it means that the charge integrating type can be expected to have better accuracy than the pulse type for experiments using very intense X-ray beams where the counting loss is dominant in the pulse type system. For low intensity beams, the detector is operated with a high gas gain and the minimum detectable absorption rate may be defined to be 1 LSB of the ADC for these operating conditions and it is estimated to be 2.8 p h o t o n s / s - c h for a gas gain of 104 and an X-ray energy of 8 keV which corresponds to an incident intensity of 18 photons/s- ch. The maximum measurable intensity depends on the experimental requirements for accuracy and is restricted by the gas gain shift and the degradation of the peak profile. Output currents causing gas gain shifts of 1% and 5% are 0.4 nA and 2 nA for a narrow beam and in the case of unity gas gain these values correspond to absorption rates of 8.7 x 106 and 4.4 x 107 photons/s • ch, respectively and incident intensities of 5.7 × 107 and 2.9 x 108 p h o t o n s / s . c h , respectively. Degradation of the peak profile can be suppressed within 1% by limiting the output currents to be lower than 2.5 nA. These effects may be decreases by changing the single anode to a multiple anode. The collection efficiency can also be improved to have a flat response by using a couple of multistrip cathodes. Better spatial resolution would be achieved with a multistrip cathode of shorter spacing. Further improvements will be carried out on these points and the application to the study on frog muscle will be continued. This work was supported by a scientific research fund from the Ministry of Education, Science and Culture. The authors are deeply indebted to Prof. H. Sugi, Drs. H. Tanaka, and T. Kobayashi of Teikyo University and Dr. Amemiya of K E K for their collaboration in the time-resolved experiments and useful discussions. The authors would like to thank Mr. K. Ono for assistance with the X-ray sources, and to thank Prof. A. Sekiguchi of the University of Tokyo and Prof. H. Hashizume of the Tokyo Institute of Technology for several useful discussions.
K. Mochiki, K. Hasegawa / Charge integrating type proportional chamber References [1] K. Hasegawa, K. Mochiki and A. Sekiguchi, IEEE Trans. Nucl. Sci. NS-28 (1981) 3660. [2] K. Mochiki, K. Hasegawa, A. Sekiguchi and Y. Yoshioka, Adv. X-ray Anal. 24 (1981) 155. [3] K. Hasegawa and K. Mochiki, in: X-ray Instrumentation for Photon Factory (KTK-Reidel, Tokyo, in press). [4] J. Bordas, M.H.J. Koch, P.N. Clout, E. Dorrington, C. Boulin and A. Gabriel, J. Phys. E: Sci. Instr. 13 (1980) 938.
601
[5] J. Hendrix, H. Fuerst, B. Hartfield and D. Dainton, Nud. Instr. and Meth. 201 (1982) 139. [6] A.R. Faruqui, IEEE Trans. Nucl. Sci. NS-30 (1983) 358. [7] H.E. Huxley, A.R. Farugi, J. Bodas, M.H.J. Koch and J.R. Milch, Nature (London) 284 (1980) 140. [8] R.W. Hendricks, Rev. So. Instr. 40 (1969) 1216. [9] K. Hasegawa and K. Mochiki, Photon Factory Activity Report 1982/1983 (1984) VI-93.
593
CHARGE INTEGRATING TYPE POSITION-SENSITIVE PROPORTIONAL CHAMBER FOR T I M E - R E S O L V E D M E A S U R E M E N T S U S I N G I N T E N S E X-RAY S O U R C E S
Koh-ichi M O C H I K I a n d Ken-ichi H A S E G A W A Department of Nuclear Engineering, University of Tokyo, 7- 3-1, Hongo, Bunkyo- ku, Tokyo 113, Japan Received 22 May 1984 and in revised form 24 September 1984
A position-sensitive detecting system for time-resolved diffraction measurements with very intensive X-ray sources has been developed. It consists of a charge integrating type gas-filled detector, multichannel analog multiplexers, a signal processor and a memory (120 ch. × 128 phases × 24 bits). The detector is 120 mm long in effective length by 10 mm × 10 mm in cross section with a single anode of 20/~m diameter. One of the cathode planes consists of 120 cathode strips with a pitch of 1 mm. The spatial resolution is equal to the pitch under a certain detector current limit. The gas gain is adjustable to an appropriate value according to the X-ray intensity range of interest. For experiments with 8 keV X-ray sources, maximum absorption rates of 9 × 107 photons/s, mm with low applied voltage and minimum absorption rates of about 3 photons/s- mm with high applied voltage can be achieved. This system was applied to a time-resolvedX-ray diffraction study on frog muscle using a synchrotron radiation source at the Photon Factory and we could collect diffraction patterns with a time resolution of 10 ms and only 10 stimulations.
I. Introduction Synchrotron radiation sources delivering intense Xray beams demand superior quality instrumentation. In time-resolved X-ray diffraction experiments, the main requirement is to reduce the necessary recording times sufficiently to obtain statistically meaningful information in the time scale of interest. To overcome this, three types of one dimensional position-sensitive detector may be considered, that is, (1) the delay line type, (2) the MWPC type equipped with a preamplifier, a discriminator and a scaler for each anode wire and (3) the charge integrating type [1-3] equipped with a multielectrode and external capacitors for charge accumulation. The delay line type [4] can be operated with count rates up to about 5 × 105 counts/s over the whole detection field and the maximum count rate per channel depends on the pattern to be measured. The scaler/wire type [5,6] can cope with count rates up to about 2 × 105 counts/(s wire) and in the case of a 100 channel detector, this implies rates of 2 × 107 counts/s over the whole detection field. In contrast with these detectors, the integrating type has no counting loss, and the applied voltage and the accumulation time are appropriately adjustable according to the intensity of the incident X-ray beams, so the attainable maximum count rate of the integrating type is higher than that of any other type. Several detectors of the charge integrating type have been designed and constructed. They are divided into two groups according to the readout electrode, that is, the cathode readout type with a multistrip cathode and 0168-9002/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
the anode readout type with a multiple anode. The spatial resolution depends on the pitch of these readout electrodes. As far as the gas gain is concerned, the attainable range of the former type is wider than that of the latter, thus the cathode readout type can cope with a very wide range of X-ray intensity. In this paper, we describe a one-dimensional cathode readout type detector and its data acquisition system, and an application to the time-resolved X-ray diffraction study on frog muscle [7] is shown.
2. Charge integrating type position-sensitive detecting system 2.1. Principle The detector is an ordinary proportional counter except for the multistrip cathode equipped with capacitors (Ca'S) for accumulating charges collected by these cathode strips as shown in fig. 1. Each cathode strip is also connected to an input terminal of a CMOS analog multiplexer and the positive charges accumulated in each capacitor are transferred to an IC charge-sensitive amplifier when the associated channel of the multiplexer IC is selected by a binary coded addressing signal. The charge-sensitive amplifier is a capacitive feedback type whose input impedance is very low, so that the charge transfer to the feedback capacitor, Cf, is done almost completely. The output voltage signal of the charge amplier is fed to an analog-to-digital converter (ADC).
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The output voltage of the charge-sensitive amplifier can be described by V = k q A I T / C t,
where q = amount of positive charge produced by a primary ionizing event, A = gas gain, I = X-ray intensity per cathode strip, T = accumulating time of the collected charges in the capacitors, Ca's, that is, sampling period, Cf = feedback capacitance of the charge-sensitive amplifier, k = conversion coefficient of the system, including the charge collection efficiency of the readout cathode. This equation shows that the output voltage is proportional to the X-ray intensity, provided the gas gain does not depend on the X-ray intensity and the incident X-ray is monoenergetic or the energy spectrum is independent of the measuring position. The incident position of an X-ray photon can be identified by the strip number.
Fig. 2. Schematic of the detector with a multistrip cathode.
2.3. Signal processing
The block diagram of the signal processing system is shown in fig. 3. It consists of eight kinds of electronic circuit boards, a monitor scope and a microcomputer used as a system controller. The multiplexer board contains capacitors (Ca = 1000 pF) for charge accumulation and 8 chips of 16 ch. analog multiplexers (Harris HI 506). For fast charge transfer, multiplexers without overvoltage protection circuits are used and a simple current limiting circuit with an RC network is installed between a high voltage power supply and the anode wire. Addressing and chip enable signals are sent sequentially from the enable-controller board and the charges are transferred to the charge-sensitive amplifier in the charge amp. & ADC board. The feedback capacitance of the charge-sensitive amplifier is 510 p F which was determined in consideration of the stability of the amplifier, the protection of the over-voltage at the input of the CMOS analog multiplexer and the input voltage range of the ADC (0 to - 1 0 V). In the adder & memory board, each digitized value after AD conversion is additionally integrated to the value stored in the memory
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The detector vessel is schematically shown in fig. 2. The cross section of the detector is 10 mm x 10 mm and a gold-plated tungsten anode wire of 20 # m diameter is mounted at its centre. The readout cathode was made by etching on a printed circuit board and consists of 128 strips with a pitch of 1 mm. The gas flowing in the detector was a mixture of Ar + 8.9% CO 2.
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and restored to the memory at the same address. The data area of the system is divided into 128 segments of 128 channels with 24 bits; therefore, 128 phases in a dynamic phenomenon can be obtained. The timing pulses and the output signal from the charge-sensitive amplifier are shown in fig. 4 in the case of time-resolved experiments. Operating parameters of the accumulation time, T,, corresponding to resolution time and the number of times of the additional integration are set with the microcomputer before the start of measurements. The external trigger pulse starts the sam-
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The values of Tf, Tt, Tc and Tp are 1 ~s, 2/xs, 4 #s and 7 #s, respectively, and T~ is variable in the range of 1-1000 ms.
3. Performance In the following measurements, an X-ray beam of 8 keV from a rotating anode type X-ray source was used except where specially mentioned.
3.1. Gas gain One of the advantageous characteristics of this system is that the applied voltage is adjustable so as to keep an appropriate gas gain over an X-ray intensity to be measured. Gas gain shift therefore restricts the upper limit of the intensity. The gas gain characteristics, at first, were measured with a proportional counter. It has a planer cathode, not a strip cathode, and the same dimension as the integral type detector, and a collimated beam 0.5 mm high x 0.4 mm wide was used. The gas flowing in the detector was a mixture of Ar + 8.9% CO 2. The output current from the cathode was measured with a picoammeter, and the incident X-ray intensity was attenuated by copper foils step by step. The gas gain could be derived from the ratio of the output current to be amount of primary charges produced by X-ray absorption during I s. In estimating the primary charges, a concept of effective X-ray energy was introduced in consideration of the effect of escape X-rays which do not dissipate their energy in the detector gas. For our detector, the effective X-ray energy was estimated to be 7.68 keV from a pulse height spectrum. The results are shown in fig. 5. It is evident that gain shift occurs in the higher region of the intensity for each applied voltage. The dependence of the relative gas gain on the output current is shown in fig. 6. As a cause of the gain shift, Hendricks proposed a space charge model [8] and the relative gain shift can be expressed as follows;
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3.2. Background In any experiment, background data have to be measured and deducted from diffraction data to obtain precise information on X-ray intensity. The characteristics of the background data were measured and it was found that several background components are involved in the input voltage of the charge-sensitive amplifier. They originate from switching noise of the analog switches, leakage currents from the multiplexer input and offset voltage of multiplexer channels. These contributions depend on the characteristics of the multiplexer channels and the charge accumulating time. The sum of these components results in a positive voltage at the input of the charge-sensitive amplifier. To prevent the input signal from being out of range of the ADC, the analog input range of the ADC was shifted by applying negative currents to the comparator of the ADC. The leakage currents from the input pin of the 8th channel of the multiplexer are dominant because the input pin for the negative power supply is adjacent to it. Therefore, we do not use the 8th channel of each chip and this system is operated for 120 cathode strips. In case the accumulation time is less than 1 s, the fluctuation of these components can be suppressed to be lower than 1 LSB of the ADC. In addition to these background components, there were external noises. To suppress this effect under 1 LSB, additional integration of at least 4 times is needed.
3.3. Collection efficiency As the multistrip cathode occupies only one side of the square cross section, ions generated near the anode
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characteristics of the avalanche distribution around the anode wire. From the results it may be recommended that in the case of diffraction experiments using narrow beams the detector should be adjusted so the diffracted beams enter the region near the readout cathode.
3.4. Digital output vs. X-ray intensity The dependency of the sum of ADC out on the intensity of the monochromized 8 keV X-ray beam from the storage ring in the Photon Factory was measured and the result is shown in fig. 8. Beam profiles obtained in the measurement are also shown in figs. (9a, b and c where the applied voltage was 500 V and the beam intensities were 4.8 x 10 5, 2.4 x 107 and 8.4 x 10 9 photons/s, respectively. In fig. 8, the solid and broken lines mean the sum of the profile and the peak channel, respectively. As concerns the sum of the profile, good linearity is preserved over 108 photon/s.
3.5. Spatial resolution Responses of the adjacent channels to a narrow X-ray beam of 30 #m diameter was measured by scanning the detector step by step. The results are shown in figs. 10a and b where the applied voltages were 500 and 1000 V, respectively. In the case of low gas gain, the spatial resolution is determined by the pitch of the cathode strip. However, degradation is observed when the output currents are increased.
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The dependence of line broadening of an observed peak on the applied voltage was measured with a collimated beam of 500 #m diameter. The beam intensity was 107 photons/s and the applied voltage was changed in 5 steps and the operating parameters were 10 ms for Ta and 103 times for additional integration. Output distributions normalized to peak values are shown in figs. 11 and 12. From these figures, an increase in the peak width and growth of the feet can be seen. When the applied voltage was 1300 V, the output current of the peak channel was 2.5 nA and the upheaval of the feet was less than 1%. Another measurement was carried out with a broad beam and the same characteristics of the upheaval were observed for the same average output current. From these results it may be derived that the average output current of the peak region should be lower than about 2.5 nA to suppress the upheaval of the feet to less than 1%. These phenomena seem to be caused by diffusion of ions and emission of photons by excited atoms and molecules; however, a more detailed investigation is the subject of a future study.
3. 7. Uniformity The response of the output charge from each cell was measured by scanning the detector at a fixed speed across a broad X-ray beam. A relative standard deviation of 1.4% was obtained over three fourths of the
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effective region of the detector with an applied voltage of 1000 V. However, concerning the whole region, the value was 3.7%. The main factor of the inhomogeneity appears to be caused by errors in fabricating the detector.
4. Application This system was applied to a time-resolved X-ray diffraction study on frog muscle using the synchrotron radiation source of the Photon Factory in the National Laboratory for High Energy Physics in Tsukuba [9]. The study was performed with collaborators Prof. H. Sugl, Drs. H. Tanaka, Y. Amemiya and T. Kobayashi and from among the preliminary results we only show two diffraction data in this paper. Figs. 13 and 14 show the diffraction patterns recorded from frog muscle in resting state and during contraction, respectively. Fig. 13 shows a diffraction pattern recorded in only 100 ms from muscle in the resting state with an X-ray intensity about 105 photons/s over the whole detector and about 6 x 103 photons/s at the peak channel. Fig. 14 shows the time course of the diffraction peak intensity during contraction where the data were summed from 10 twitches, each time slot being of 10 ms duration, that is, corresponding to 100 ms total exposure. In this case the memory area is able to cover 1280 ms and only onefourth of the total diagram is shown. The X-ray intensity was about 6 x 104 photons/s over the whole detector and about 5 X 103 photons/s at the peak channel.
5. Discussion In this readout method, the background data have to be deduced from the diffraction data obtained with the same operating parameters. Previously, it was described
that the fluctuation in the background data can be suppressed to below 1 LSB of the ADC if samplings are carried out more than 4 times and the charge accumulation time is set between 1 ms and 1 s. In this time range, the ADC output has a good linear relation to the accumulation time over the full range of the ADC. Therefore, the resolution of the signal processing system can be defined to be 1/4095. Concerning the statistical error, there is no difference between the pulse type and the charge integrating type and it means that the charge integrating type can be expected to have better accuracy than the pulse type for experiments using very intense X-ray beams where the counting loss is dominant in the pulse type system. For low intensity beams, the detector is operated with a high gas gain and the minimum detectable absorption rate may be defined to be 1 LSB of the ADC for these operating conditions and it is estimated to be 2.8 p h o t o n s / s - c h for a gas gain of 104 and an X-ray energy of 8 keV which corresponds to an incident intensity of 18 photons/s- ch. The maximum measurable intensity depends on the experimental requirements for accuracy and is restricted by the gas gain shift and the degradation of the peak profile. Output currents causing gas gain shifts of 1% and 5% are 0.4 nA and 2 nA for a narrow beam and in the case of unity gas gain these values correspond to absorption rates of 8.7 x 106 and 4.4 x 107 photons/s • ch, respectively and incident intensities of 5.7 × 107 and 2.9 x 108 p h o t o n s / s . c h , respectively. Degradation of the peak profile can be suppressed within 1% by limiting the output currents to be lower than 2.5 nA. These effects may be decreases by changing the single anode to a multiple anode. The collection efficiency can also be improved to have a flat response by using a couple of multistrip cathodes. Better spatial resolution would be achieved with a multistrip cathode of shorter spacing. Further improvements will be carried out on these points and the application to the study on frog muscle will be continued. This work was supported by a scientific research fund from the Ministry of Education, Science and Culture. The authors are deeply indebted to Prof. H. Sugi, Drs. H. Tanaka, and T. Kobayashi of Teikyo University and Dr. Amemiya of K E K for their collaboration in the time-resolved experiments and useful discussions. The authors would like to thank Mr. K. Ono for assistance with the X-ray sources, and to thank Prof. A. Sekiguchi of the University of Tokyo and Prof. H. Hashizume of the Tokyo Institute of Technology for several useful discussions.
K. Mochiki, K. Hasegawa / Charge integrating type proportional chamber References [1] K. Hasegawa, K. Mochiki and A. Sekiguchi, IEEE Trans. Nucl. Sci. NS-28 (1981) 3660. [2] K. Mochiki, K. Hasegawa, A. Sekiguchi and Y. Yoshioka, Adv. X-ray Anal. 24 (1981) 155. [3] K. Hasegawa and K. Mochiki, in: X-ray Instrumentation for Photon Factory (KTK-Reidel, Tokyo, in press). [4] J. Bordas, M.H.J. Koch, P.N. Clout, E. Dorrington, C. Boulin and A. Gabriel, J. Phys. E: Sci. Instr. 13 (1980) 938.
601
[5] J. Hendrix, H. Fuerst, B. Hartfield and D. Dainton, Nud. Instr. and Meth. 201 (1982) 139. [6] A.R. Faruqui, IEEE Trans. Nucl. Sci. NS-30 (1983) 358. [7] H.E. Huxley, A.R. Farugi, J. Bodas, M.H.J. Koch and J.R. Milch, Nature (London) 284 (1980) 140. [8] R.W. Hendricks, Rev. So. Instr. 40 (1969) 1216. [9] K. Hasegawa and K. Mochiki, Photon Factory Activity Report 1982/1983 (1984) VI-93.