Design of microcomputer-based data acquisition system for the time-of-flight ion scattering spectrometer

Nuclear Instruments and Methods 186 (1981) 637 640 North-ftolland Publishing Company

637

DESIGN OF MICROCOMPUTER-BASED DATA ACQUISITION SYSTEM FOR THE TIME-OF-FLIGHT ION SCATTERING SPECTROMETER * Hao-yung LO and Ching-shen SU Institute of Nuclear Engineering, National Tsing Hua University, Itsinchu, Taiwan

Received 10 November 1980

A microcomputer-based data aquisition system used on a time-of-flight ion scattering spectrometer is described. The flight time of 90°-scattered ions from target atom determined directly with a 30 MHz crystal-controlled oscillator and its associated circuit. The ion intensity is detected by a channel multiplier, and its output signal pulse is converted from the analog form into digital torm by an ADC. Both flight time and ion intensity are stored in the microcomputer.

1. Introduction The time-of-flight technique used for the analysis of scattered energy of ions has been investigated by Buck et al. [1]. A time-to-amplitude converter was used by them to supply voltage pulses to a multichannel pulse height analyzer with the start and stop inputs from the ion beam pulsing system and the electron multiplier, respectively. The application o f a microcomputer in recent years on instruments appears very promising both for the simplicity and for the relatively low cost. We report here a new design of the data acquisition system using a microcomputer for a time-of-flight ion scattering spectrometer designed and constructed by this Laboratory [2]. 2. Design considerations Tile primary ion pulse from the ion source of the time-of-flight ion scattering spectrometer (TOFISS) is scattered by the target atoms and the 90°-scattered ions with reduced energies will enter the flight tube and be detected at the end of the flight tube by a channel multiplier. The time of flight represents the energy o f tile scattered ion which in turn represents the mass of the scattering atoms in the sample. The number of scattered ions with tiffs flight time determines the relative concentration of the scattering atoms in the sample. For this spectrometer, the width of the scattered ion peak of 4°Ar is estimated to be * This research work was supported by the National Science Council under grant NSC 70-0404-E007-I 1. 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland

1.8/as and 0.3 /as for the scattering atoms o f a a T i and 9°Zr respectively; the separation of mass peaks between 47Ti and 48Ti is 3.5/as and between 9°Zr and 92Zr is 0.32/as [3]. Considering this fact, a timer with a time base o f variable span, say from 0 to 100/as, was designed to measure the time duration from the start signal, the driving pulse for the primary ions from the ion source system, to the stop signal, the voltage pulse from tile ion detector at the end of the flight tube. A window of adjustable width, say from 0.1 /as to 2/as, was designed which advances one after another beginning at a manually preset time to ending at full span of the time base. The preset time is determined by the mass range to be covered. If there is no mass peak in a window position, the window keeps advancing until a stop signal generated from the ion detector of the TOFISS by the scattered ions is received. Then the flight time of the scattered ions which is related to a certain mass of atom in the target is recorded, and the height of this signal pulse is converted into digital form by an amplitude-to-digital converter (ADC) and stored in a microcomputer. 3. Description of the circuitry The block diagram shown in fig. 1 describes the principle of this system. There are two input signals, one the driving pulse for the primary ions generated from a pulse generator of the ion source system, and the other the signal pulse from the ion detector, a channel multiplier. The driving pulse after a certain delay is also used as a trigger pulse to initiate the

638

H. Y. Lo, C.S. Su ~Microcomputer-based data acquisition system

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Fig. 1. Block diagram of the data acquisition system. starting action of the time sequence of the data acquisition system. The signal pulse from the ion detector is shaped by a preamplifier and a linear amplifier, then held by a pulse height holder for the pulseheight-to-digital conversion by an ADC of 12-bit digital number, HI2B. It is also used to start the conversion and to end the time sequence through a pulse formed circuit, as shown in fig. 1. The conversion sig-

nal is synchronized to the window pulse from the monostable SN74121. After the conversion, an end of conversion (EOC) signal is issued by the ADC to interrupt the microcomputer in order that the time of flight and the digitized pulse height of the input signal are stored in an Intel SDK-85 microcomputer. Since each I/O port of the SDK-85 has only 8 bits, it needs two memory cycles to access memories for

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Fig. 2. Schematic diagram of the pulse height measurement.

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each pulse amplitude conversion. A specified program is then started by the interruption to process the data, and a TTY is used as console input/output control for this system. The schematic diagram of the timer for the timeof-flight measurement is shown in fig. 3. The timer works as a time base which measures the time duration from the start signal to the stop signal as described above. The most important requirement for this system is that the stability of the frequency in temperature variation should be within the limit of 50 ppm. With this stability, the time interval can be controlled to better precision. A 30 MHz crystal-controlled oscillator (block 1 in fig. 3), which is based on the interval-crystal-controlled circuit is chosen for this purpose. The leading edge of the driving pulse, the start signal from the ion source system, is used to load the manual preset data to into counters A, B, and C, and its trailing edge is used to initiate the system operation. It sets the J-K flip-flop 1 and enables the gate G1. This causes the clock pulses passing through G1 to clock the high speed synchronous up/ down binary (or decade) counters SN74S193 (or SN74S192). The operation of counters A and C is connected in

count-down mode, while the counter B is in countup mode. Since these counters are manually preset to to and loaded by the leading edge of the driving pulse, thus, when the counter A decreases to zero, a signal is produced from Br of IC2 to increase a value equal to the window width to the counter B and to reload the preset data to into counter A itself. Simultaneously, the counter C is also decreased to zero and an output pulse is produced from Br of IC4 to latch the data from the output of the counter C to the output of IC5 (SN74S100 latches), which resets IC2 and IC1. A

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Fig. 4. The window position in time scanning.

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tt. Y. Lo, C.S. Su /Microcomputer-based data acquisition system

window is subsequently generated by the monostable vibrator of IC6 for detecting the stop signal. After the opening of tile window, the cycle is started again by the triggering of the driving pulse, the same process is repeated as above except the input data to loaded by 1C4 are increased by a value equal to the width of the window. The process repeats with the window advancing one after another until the end of the time span, say 100/as, as shown in fig. 4. A clear pulse is produced at the end of the scan by the last stage counter to clear counter B and' this completes one operational cycle of the system. In case of a stop signal pulse being received, a starting conversion signal is coincident with the window at G3 and the SDK85 microcomputer is subsequently interrupted by the EOC signal. The time at this window position will then be stored by the interrupted program through the 1/O port.

4. Conclusion The use of a microcomputer for the data acquisition of the time-of-flight ion scattering spectrometer has the advantage that it can record both the ion

flight time and the ion intensity at relatively fast scanning rate. It takes only 0.1 s to scan tile whole time span of 100 /as which covers the mass range interesting to most surface researches. The time span can be extended by the additional cascaded up/down counter if necessary. Tile width of the window can be adjusted to the desired value either by changing the frequency of the crystal, or by using a frequency divider. Furthermore, if only masses of low mass number are of interest, the time span can be set shorter, and the scanning rate can be even faster. The circuitry of this system is relatively simple, and no expensive equipment such as a nmltichannel pulse height analyzer is needed. The authors would like to thank Mr. L. Wang for his help in the construction of the hardware.

References [1] T.M. Buck, Y.S. Chcn and GAl. Wheatley, Surf. Sci. 47 t1975) 244. [2] C.S. Su and H.Y. Lo, to be published. [3] C.S. Su, to be published.