Fast PC-based data acquisition system for gas-filled position sensitive detectors

Nuclear Instruments and Methods in Physics Research A 392 (1997) 384-391

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH

-_ @ ELSEVIER

Sectton A

Fast PC-based data acquisition system for gas-filled position sensitive detectors H. Mio”,*, M. Chemloul”,

P. Laggner”,

H. Amenitscha,

S. Bernstorffb,

“Institute oj”Bioph.vsics and X-ray Structure Research, Austrian Academy of‘ Science, Steyrergasse bSincrotrone Trieste, Padriciano 99, 34012 Trieste. ItaJv

M. Rappoltb

17, A-8010 Graz, Austria

Abstract The high flux of the new generation of synchrotron radiation sources requires fast position sensitive detectors with high count rate data acquisition systems capability. Though the local count rate in a gas-filled position-sensitive detector is limited by the space charge effect, the integral rate will increase with the area of the detector. Thus, more than several lo6 events per second can be achieved. Therefore, we developed a new PC-based histogramming and control interface (HCl) with an intrinsic dead time lower than 200 ns for linear and area X-ray detectors for time-resolved measurement applications. An overview of the complete acquisition system including a fast time-to-digital converter and image processing software will be given. The design principles and operating characteristics including experimental results obtained with a 100 mm linear delay-line detector at the SAXS beamline 5.2 in Elettra (Trieste) will be presented.

1. Introduction Due to developments in synchrotron instrumentation in the last few decades, the potential of synchrotron radiation sources with specialised optics [l] for X-ray diffraction and scattering is increasing. However, the full advantage of high brilliance beamlines for time-resolved small-angle X-ray experiments [2-51 can only be utilised with the corresponding detection systems, combining good linearity, spatial resolution, low noise and high count rate capability. Depending upon the underlying experiment several existing one-dimensional (1D) and two-dimensional (2D) position-sensitive detectors (PSD) [6-lo] can be chosen. Often, the readout and data acquisition systems are not optimised regarding the detector and the application. The primary goal was to design a flexible and reliable data acquisition system for single-event encoding X-ray detectors. Besides, the system must offer a high acquisition rate, on-line data display and easy-to-use image processing software. In the present work a new PC-based data acquisition board with an intrinsic dead time lower than 200 ns operating a 1D delay-line detector at the SAXS-beamline 5.2 at ELETTRA will be described. Tests were performed

* Corresponding author. Tel.: + 43 316 8 12004;fax: + 43 316 812367; e-mail: [email protected].

with a tungsten rotating disc positioned in the primary beam path and before a scattering sample.

2. Principle of operation 2. I. Position

encoding

X-ray photons in the energy range up to 30 keV are mainly absorbed by the photoelectric effect in the detector gas, thereby producing electron emission (photoor/and Auger electrons) or fluorescence [ll]. The primary electrons thermalize by subsequent multiple elastic and inelastic collisions with surrounding gas atoms creating a cloud of secondary electrons, which drifts in the presence of a high electric field to the anode wire. Only the gas-amplified induced signal can be measured electronically with a reasonable signal-to-noise ratio. For position encoding of single-event detectors many different methods have been described in literature [12-191. Generally those methods obtain position information, either by comparing amplitudes (resistive-, charge separation method) or times (delay line method). The readout of the detector used in the following experiments was based on the delay line method. This technique is based on the determination of the time difference between two pulses produced by the same event at the terminal of the built-in delay line [Fig. 11. Therefore, the precision of this method and the attainable rate capability depend directly

016%9002/97/$17.00 Copyright :(‘, 1997 Elsevier Science B.V. All rights reserved PII SO1 68-9002(97)00203-9

H. Mio et al. /Nucl. Ins@. and Meth. in Phvs. Rex A 392 (1997) 3844391 upon the performance of the time-to-digital converter (TDC). The maximum rate (rate,,,) which can be achieved theoretically with a detector having a delay line of zpsD is given by 1 rate,,,

<-

385

(Eq. (1)). Consequently, it is prerequisite to use a TDC with minimum time resolution At. The TDC used for our system was specially developed for applications using gas-filled detectors with delay line readout [20]. A minimum time resolution of 80 ps, a differential nonlinearity less than k 1.5%, and a rate up to 3 MHz are practical.

(1)

2

7PSD

2.2. Data acquisition

the delay should be kept as small as possible for a high count rate. The relation between time and space is given through the following equation: So,

At _=-

Ap

TPSD

L, ’

(2)

where L, (mm) is the active length of the PSD and Ap the channel-width (bin). In order to achieve a good spatial resolution, the ratio of Ap/L, should be kept as low as possible. This can be done by increasing the delay line tpSD of the detector, thereby decreasing the rate capability

CFD

Fig. I. Operation more information

Constant

Fraction Discriminator

principle of a delay line base readout. see Ref. [23].

For

The data acquisition system was built in a form of a PC-board due to the flexibility and the low cost of this technology. A scheme of the data acquisition system is given in Fig. 2. The PC-board was designed with state-ofart technology circuits as FPGAs (field programmable gate arrays from Altera) and a 8OC167 CPU from Siemens. This combination offers high speed and large flexibility in operation. For data storage, the board uses a built-in SRAM memory modules from Electronic Design Inc. This memory offers faster access time (25 ns) than the dynamic one (60 ns). 2.2.1. Histogramming The PC-board is directly connected via a 2 m shielded 25pin cable to the TDC, which encodes the position of a properly detected event in digital form. The whole pattern is obtained by histogramming into a digital data memory. This process is performed by two FPGAs (histogramming unit). The first (ADR) performs I/O operations to the memory cell using the data encoded by the time-to-digital converter as memory address. The second (INC) increments the content of the corresponding cell. These two operations are pipelined allowing approximately 170 ns for a read-increment-write cycle. For 1D and 2D time-resolved measurements a segmentation of the data memory is necessary. However, the address of the memory cell to be incremented is a combination of the result supplied by the TDC and the time references produced by the time-frame generator. This operation is fully configurable by the software. Table 1 gives some of the possible configurations for 1D and 2D

Fig. 2. Schematic diagram of the PC-board for fast histogramming and time frame organization. Up to 20 input lines can be handled simultaneously for 1D and 2D position sensitive detectors. (Abbreviations: IOC = input&output control circuit; INC = incrementation circuit; ADR = address encoding circuit; RAM = random access memory; CPU = central processing unit; FIFO = first-in first-out buffer.)

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Table 1 Some of the possible configurations of the 4 MB data-memory on the PC board Configuration of data memory

F

X

Y

Sum

2D single: 1 pattern of 1024 x 1024 pixels 2D multi: 16 patterns of 256 x 256 pixels 2D multi: 64 patterns of 128 x 128 pixels 1D single: 1 frame of 1048 576 channels 1D multi: 512 frames of 2048 channels 1D multi: 1024 frames of 1024 channels

0 bit 4 bit 6 bit 0 bit 9 bit 10 bit

10 bit 8 bit 7 bit 20 bit 11 bit 10 bit

10 bit 8 bit 7 bit 0 bit 0 bit 0 bit

20 bit 20 bit 20 bit 20 bit 20 bit 20 bit

applications. For any segmentation the sum of F + X + Y must be less than or equal to 20 bits, the maximum number of digital input lines (Fig. 2). The corresponding configuration file is transferred to the PCboard once during the initialisation routine. 2.2.2. Time-jkame generator The input-output control and switching from one frame to another is done by a third FPGA (IOC) in co-operation with the CPU, working as a time-frame generator (TFG) [21]. To ensure accurate timing the clock of a 20 MHz quartz-oscillator is pre-scaled in the FPGA by a factor of 20. This 1 ps clock is the input of a pulse width modulator PWM in the CPU, which defines the active and wait periods of a single time-frame. The output of the PWM is compared with a status register and synchronised in the FPGA with respect to possible delays resulting from interrupt response times from the CPU. So, the precision of is defined by the crystal oscillator only, which is typically in the ppm domain. Therefore, the minimum time interval between successive frames is 1 pts and the minimum active time 10 vs. The experiment settings, like number of frames, active and wait times for a single acquisition and the output trigger, etc. are controlled from the CPU in the status register. However, all control signals are delivered by the FPGA itself. Consequently, this co-operation between the CPU and the FPGA allows also operation of the data acquisition system in remote mode: the PC-board is fully controlled and triggered from external devices with response times lower than 200 ns. A more detailed description and circuit layout is given in Ref. [22]. 2.2.3. On-line data disphy The use of dual-port memory was not necessary. However, no losses are visible during the experiment because a measurable decrease in intensity is within the statistical fluctuations. The on-line data transfer is controlled by the CPU. A request from the PC, is not immediately executed, but

as soon as the histogramming unit has finished its operation the CPU transfer the data from the on-board RAMmodules into a 2 kB FIFO to avoid delays due to the slow bus of the PC. The access time of the CPU to the data memory for one channel (32 bit) is 300 ns. The minimal cycle time of sequential accesses from the CPU is 2 ps. Data transfer rates between 1 and 2 MB per second from the PC-board into the memory of the computer have been measured, depending on the AT-bus clock and speed of the PC’s CPU.

2.2.4. Image processing and control sofbvare A powerful data acquisition software combines all functions in just one program as defining experiment times (active and wait times of frames and cycles), programming the time-frame generator (output signals), starting the acquisition, storing, printing and displaying data. Beside the control of the hardware, it is also required to evaluate the data qualitatively during the experiment without interrupting the histogramming process. Therefore, on-line display of acquired data and image processing tools are a indispensable part of the whole data acquisition system. Simple operations as background rejection, radial integration, line profile cut and scaling options as zooming, linear and logarithmic plots are essential prerequisites. The introduction of colours reveals more details, otherwise not seen on a monitor. Any coloration of intensities corresponds to a mathematical transformation of the raw data array to the visualised pattern and so becomes an important interface. In order to create individual colour tables a very powerful colour palette editor has been implemented in the program. Adjustment of the amplitude, repetition rate and phase shift of each main RGB-colour determines the final colour table. The effect of the lookup table in use can be seen simultaneously in the image. During the experiment simple 3D-plots, meshes, (hidden) dots, (hidden) lines and contour plots with adjustable view by rotating and tilting the image for visualisation of patterns with a large amount of data of typically 1 Mb are possible.

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H. Mio et al. INucl. Instr. and Meth. in Phys. Res. A 392 (1997) 3844391

situation by inserting a tungsten disc in the synchrotron beam before passing the sample stage. The direct beam was chopped in defined exposure (Topen) and recovery (Tc,ased) times and the X-ray scattering of the sample measured at various velocities. The detector is not capable of detecting each single bunch in the synchrotron beam. Moreover it would behave as low-pass filter with an upper frequency cut in the MHz-region. This means that only the mean value of the Fourier transform of the synchrotron beam wave form is detected (Fig. 4). Consequently, variable exposure and relaxation times should be applied to the detector to find the behaviour of the detector and associated data acquisition system.

3. Test and results

The aim of tests performed with the rotator was to investigate the space charge effect on the scattering curves obtained and to optimise for a certain diffraction pattern. While the maximum rate capability of this detector, which can be in the order of lo6 counts/s, is directly related to the dead time of the system, the influence of the space charge effect depends on geometrical parameters of the detector, especially the anode-cathode gap, gas mixture, high voltage, and the shape of the diffraction pattern itself, e.g. diffuse small-angle scattering versus sharp Bragg-reflexes. However, this leads to count rate capabilities which are quite below the theoretical limit, calculated by Eq. (l), and are never reached in experiments. Even with this limitation, it will be shown that it is practicable to achieve a time resolution in the microsecond domain, if care is taken that the detector is not saturated by high local intensities.

3.2. Rotating disc In order to find the optimum times for Top_, and T c,osedwe installed a mechanical X-ray chopper directly in front of the sample and measured the diffraction pattern of rat-tail tendon at room temperature at different velocities of the rotating disc. To change the relation of TO,,, and Tclased (duty cycle) three tungsten discs with 4, 45 and 90 holes, each 1 mm in diameter, were mounted optionally on the rotator. The maximum rotating speed achieved was 36 000 rpm, hence allowing a minimum Topen of approximately 9 ps, as

3.1. Experimental set-up The experimental set-up, including the detection system, for a standard time-resolved small-angle X-ray scattering experiment is shown in Fig. 3. We modified this

T-,p-jump one bunch

observed scattering range

/

..... .............................. synchrotron beam

sample stage beam stop

Trigger Signals

t position sensitive detector

readout electronics

J

data acquisition and control unit

Display

system transit time

Fig. 3. Typical set-up in time-resolved small-angle X-ray scattering experiments. To obtain the behaviour of a sample far off the thermodynamic equilibrium. an energy jump (temperature, pressure) is induced and the diffraction pattern recorded with a position sensitive detector in short time-frames. To attain a satisfactorily counting statistic the measurement must be repeated several times in so called ‘cycles’. Therefore the data acquisition system should include the possibility of producing trigger signals synchronous with the data acquisition.

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tri.qqer signal for syiihronkation with data acquisition

rotating tungsten disc

synchrotron beam

beam stop guard pinhole

Detector:

Fig. 4. Schematic of the X-ray chopper. The direct synchrotron beam is chopped through a rotating tungsten disc far off from the bunch frequency. The detector only records the average intensity during one passage of a single hole over the beam. To have a defined illumination of the sample a second guard pin-hole can be inserted.

calculated from proportional to related to both, of the disc (Eq.

Topen =

Tclosed

Eq. 3(a). It should be noted that Topen is the rotation speed only, while Tclosed is the number of holes (N) and the velocity 3(b)).

arcsin(R/r) (3a)

rcu 1

=

-

UN

-

Topen,

where N is the number of holes in the disc with r = 28.65 mm distance from the centre of the disc, R = 0.5 mm the radius of a single aperture, and u the speed of revolution in rps. So in one turn the sample is exposed for N. Topen s. The results with the three discs are summarized in Fig. 5, showing the changing of intensity of the 2ndorder peak as a function of the rotation speed with different duty cycle. To adapt to possible fluctuations of the synchrotron beam the maximum was normalised to the integral area between detector channel 500 and 800.

The increase of the counting efficiency with velocity at a fixed duty cycle (Figs. 5 and 6), first seems to be contradictory to the expected behaviour. If the rotation speed is increased both Top_, and Tclased will be lowered. However, at smaller recovery periods not all of the positive ions will find enough time to drift to the cathode, and the remaining ions will cause spare charge effects. It seems that the influence of recovery time in that case is considerably small compared to the effect of decreasing the exposure time. At low velocities, the detector is heavily saturated with just one shot so that the following recovery time is not capable of restoring the detector totally. The subsequent recovery time at a duty cycle ) of 1:44 is long enough, even with the (Topen : Tclosed fastest rotation speed, to restore the detector again. The detector behaves like a low-pass filter with a cut-off frequency of a few micro seconds. With a duty cycle of 1: 1 and rotation speeds over 100 rps the detector is not capable to recover within one Tclosed period and remains in a saturated state. The influence of different recovery times becomes significant especially at short exposure times, higher

389

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0.07

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400

300

200

100

600

rotation speed [rps] -a-duty

cycle I:44 -A-duty

Fig. 5. Maximum of 2nd order peak of rat-tail tendon, normalised rotation speed and the recovery time as parameters corresponding

cycle I:3 -.-duty cycle

,:,

on the region between detector channel to three different duty cycles.

500 to 800, as a function

of

duty cycle 1.44

detector

channel

Fig. 6. 3D-plot of rat-tail tendon at various rotation speeds with a constant duty cycle of 1:44. The counting efficiency is increasing at higher velocities, corresponding to shorter exposure times during the passage of a single hole. Data have been normalised on the region between detector channel 500 to 800.

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-

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duty

cycle

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-. . . . . .

380

420

460

500

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cycle

1: 1

Fig. 7. Comparison of the diffraction pattern of rat-tail tendon recorded with three different recovery times (duty cycles) at 200 rps: 1.22ms (1: 44), 83.3 ps (1: 3). and 27.78 ps (1: 1). The exposure time during the passage of a single hole was 27.78 ps. The plot shows a zoom in the whole spectra of raw data (channel 180 to 500 @ 1024). Data have been normalised on the region between detector channel 500 to 800. The integral exposure time was 6.67 s @ 1:44, 15 s @ 1: 3 and 30 s @ 1: 1 for the three different discs.

Fig. 8. Practical simulation of a time-resolved experiment. During the passage of single aperture over the synchrotron beam the sample was exposed for about 360 ps and several frames, each of 10 ps recording time. have been taken. The plot shows the sum of 1000 cycles, repetitions, respectively. The time difference between two adjacent frames is 11 ps (1 ps frame switching time).

respectively. Fig. 7 shows the detector 180 to 500 of rat-tail tendon measured at duty cycles of 1:44, 1: 3 and 1: 1 at a rotation speed of 200 rps. With a duty cycle of 1: 1 (90 holes) the detector is totally saturated and an effective rate (= integral intenrotation

speeds,

sity/effective

channels

from

was calculated. The situation is becoming just a little bit better with a duty cycle of 1: 3 (45 holes) with an effective rate of 147 k counts/s but changes dramatically with the 4-hole disc and a computed rate of 179 kcounts/s.

integral

exposure

time)

of

111 k counts/s

H. Mio et al. iNucl. Instr.. and Meth. in Phw. Res. A 392 (1997) 384-391

However, the influence of the space charge effect is not only manifested in a drop down of intensity at the corresponding location, but also in a shift of peak positions towards higher detector channels. Additionally the intensity of Bragg reflexes above the third order of the saturated detector are higher than in the unsaturated state, which is an effect produced through the finite conversion time of the time-to-digital converter and well known with any digitizing unit. In a second experiment we tried to simulate a timeresolved experiment practically. Therefore we attached a tungsten disc with 4 apertures, each 1 mm in diameter, on the rotator and additionally a fixed guard pinhole near to the rotating disc (Fig. 4). A photoelectric beam on the opposite aperture of the synchrotron beam produced a trigger signal at each time the hole was passing the synchrotron beam. For synchronisation with the data acquisition we adjusted a variable delay on the PC-board in that way so we were able to start a cycle just before the next aperture was intersecting the beam. In one cycle about 40 frames, each with 10 us active time, have been accumulated and summed in 1000 cycles. The rotation speed of disc was 50 rps. Fig. 8 gives the diffraction pattern of rat-tail tendon during the passage of one aperture over the synchrotron beam. The exposure time of the sample during a single passing was calculated to be approximately 360 ps.

4. Conclusions and future developments It has been shown that experiments in the us-domain can be performed with a fast PC-based data acquisition system including external trigger facilities. Through insertion of the X-ray chopper it is possible to adapt to different samples thereby optimising the counting efficiency of the detector. The influence of different exposure and recovery times on the diffraction pattern of rat-tail tendon has been shown with three tungsten discs with 90.45 and 4 apertures, each 1 mm in diameter. Saturation of the detection system is mainly due to space charge effects which leads to a drop in counting efficiency, depending on both exposure and recovery time. Due to the construction of the chopper, it was not possible to investigate the dependence of exposure and recovery time separately,

rising the demand for a more variable chopper independently adjustable Topen and Tclosed times.

391

with

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