A small multiparameter data acquisition system for nuclear spectroscopy

Nuclear Instruments and Methods in Physics Research A267 (1988) 161-164 North-Holland, Amsterdam

A SMALL MULTIPARAMETER DATA ACQUISITION SYSTEM FOR NUCLEAR SPECTROSCOPY Wania WOLFF and Hans E. WOLF Instituto de Fisica, Unioersidade Federal do Rio de Janeiro, C.P. 68528, 21945 Rio de Janeiro - RJ, Brazil

Received 14 August 1987 A small computer-based system for multiparametric data acquisition in nuclear spectroscopic work is described. The system supports up to eight analog-to-digital converters (ADCs) and permits the detection of time correlations between up to eight channels . Single- and two-parameter spectra are generated and displayed on-line, while three-parameter events are recorded on magnetic tape for off-line analysis. Several independent spectra may be generated concurrently . The system can be used in a "stand-alone" configuration, the only external modules needed in this case being the ADCs. All internal modules have been constructed using easily obtainable components only. The system software has been optimized towards small size and fast data processing by coding all routines in Assembly language . The system has been developed on a PDP 11 computer, but system hardware is largely independent of the specific computer bus architecture . 1. Introduction Construction of the data acquisition system has been motivated by the need to use the Nuclear Physics Departments' PDP 11/40 computer for real-time data acquisition in nuclear and atomic physics experiments, with emphasis on the generation of two- or multiparameter spectra requiring large amounts of storage space, a requirement which multichannel analyzer-based systems cannot meet. The experiments are often characterized by rather small counting rates so that spectrum genera-

tion may be carried out concurrently with the data acquisition process. The PDP 11/40 was chosen because of its availability, but with only minor alterations the system hardware may also be connected to a microcomputer bus. The main features of the PDP 11/40 include a 248 Kbytes main memory, a system disc drive RL02 (10.5 Mbytes storage capacity), a magnetic tape drive and a graphic display terminal Tektronix 4010 . The operational system employed is DEC's RSX-11M. A typical application consists of two detectors work-

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Fig. 1 . A 2D experimental configuration . 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

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ing in coincidence to produce a two-dimensional spectrum, as would be the case for angular correlation work or for a telescope detector for particle identification. The way in which such a configuration may be connected to the system is sketched in fig. 1. Each detector's preamplifier signal is fed both into a spectroscopy amplifier and into a fast timing filter amplifier. The spectroscopy amplifier's output may be connected directly to the ADC, while the timing filter amplifier's output goes into a time pick-off unit, such as a constant fraction timing discriminator. If very high coincidence time resolution is required, the timing signals thus derived are fed into a fast coincidence unit, whose output is used to open the ADC gates. It is also fed into the system's "ADC-TIMING" inputs. If coincidence resolution requirements are less severe, the timing signals from the time pick-off units may be used directly as "ADC-TIMING" inputs . In this case rejection of noncoincident events is optional, so that noncoincident spectra may also be taken. The coincidence resolution time is given by the internal resolution of the precoincidence module (100 ns) or, if this module is not active, by the resolution of the 8-ADC multiparameter and multiplexer unit (2 jLs, see below) . It is also possible to discard the time pick-off units altogether and use as "ADC-TIMING" signals the ADCs' "ADC-BUSY" signals . In this simple environment the software may be instructed to generate simultaneously three independent spectra : the spectrum of all events of ADCI, the spectrum of all events of ADC2 and the 2D spectrum of coincident events of ADC1 and ADC2 . 2. The hardware configuration The data acquisition system consists of a doublebuffered DMA interface housed within the PDP's cabinet and the "experimental electronics", a collection of modules installed in a separate cabinet close to the experimental setup (see fig. 2). The latter comprises at present the 8-ADC multiparameter and multiplexing unit, the precoincidence module and eight ADC live time clocks . An experimental configuration, such as selection of ADCs, specification of precoincidence and multiparameter groups and presetting of live time clocks, is defined by the setting of internal flip-flops. There are no mechanical switches . 2.1 . The 8-ADC multiparameter and multiplexing unit

The multiparameter and multiplexing unit serves eight data channels . Each channel is connected to an ADC, which provides a 13-bit data word and the control signals "ADC-BUSY" and "ADC-READY", and

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Fig. 2. The system's hardware configuration . to a time pick-off or coincidence unit providing the "ADC-TIMING" signal (see fig . 3). The "ADC-TIMING" signals, after having passed through the precoincidence module, are used internally to establish timing relationships between the various channels . At present correlation time resolution has been fixed to 2 gs, but any value down to approximately 100 ns may also be chosen . The "ADC-BUSY" signals are also used internally to make sure that an event which has given rise to an "ADC-TIMING" signal has also been accepted for analysis by the ADC . The ADC data words and "ADC-READY" signals are multiplexed and passed on to the interface. The multiplexer adds 3 identification bits to the 13-bit ADC data word. The channels which are active and participate in the correlation detection mechanism are selected under program control . Multiparameter operation means the direction of coincident (or time-correlated) events and the grouping of these events in a form suitable for processing by computer software. The data words of correlated events are combined into clocks headed by a unique marker word as indicated in fig. 4 and transmitted in this sequence to the computer. The data words of those ADCs which have not been declared to participate in a multiparameter analysis scheme may appear in any of these blocks of correlated data without having any connection with the latter. It is the responsibility of the data-processing software to extract these noncorrelated events from the blocks. In fig . 4, ADC1 and ADC2 have been declared to

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correlated by the data processing software. The second block contains a data word of ADCI only. This may happen because the unit does not reject single events. The appearance of a single word means that either it is a noncoincident event or that ADC2 has not carried out its conversion because of threshold or overflow problems. The third block is again a "normal" block in that it contains a pair of data words from ADCI and ADC2. In addition it also contains two data words from ADC3, which have no correlation whatsoever with the pair above . Block four lacks a data word from ADC1, for the same reasons explained above for block two. Finally the last block shown in the figure contains only a data word from ADC3. In this case the unit has received at least one "ADC-TIMING" and ADC-BUSY" signal from either ADC1 or ADC2, but analysis of the analog signal has not been successfully completed so that no "ADC-READY" signal was received. 2.2. The precoincidence module

Fig . 3. The structure of a data channel . participate in the multiparameter detection mechanism, while ADC3 does not . Various possible combinations are shown . The first block to start with a marker word is a "normal" block, i.e., it contains a data word from both ADC1 and ADC2. Both words are considered

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Fig. 4. A sequence of correlated data blocks.

The precoincidence module serves the purpose of optionally rejecting noncoincident events by keeping the associated "ADC-TIMING" signal from reaching the multiparameter unit and by resetting the associated ADC . As it is built up from standard TTL circuits its coincidence resolution is moderate (> 100 ns) . It may be employed in experimental situations where the noncoincident background is not too high. If this is not the case an external fast coincidence module must be used. The module receives 8 "ADC-TIMING" signals from external time pick-off or coincidence units. The "ADCBUSY" signal may also be used. From these signals it derived 8 new "ADC-TIMING" output signals, some of which, depending on the setting of internal flip-flops, may just be derived without further processing from the original inputs, while others may be coincidence signals . These output signals are connected to the corresponding "ADC-TIMING" inputs of the 8-ADC multiparameter and multiplexing unit. The unit is completely programmable . The possible combinations for noncoincident rejection are the ADC groups (1,2), (3,4), (5,6), (7,8), (1,2,3) and (4,5,6) . 2.3. The ADC live time clocks All live time clocks feature two 24-bit accumulators, one for accumulating the ADC's live time and the other to establish a preset time interval, at the end of which an interrupt is generated . Both accumulators are clocked by a 1 Hz signal derived from scaling down the original clock frequency of 8.3886 MHz . 2.4. The double-buffered DMA interface The system's interface features DMA data transfer in three different modes: buffered automatic, buffered

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program requested and single word transfer . Eight status-data registers are used for controlling interface operation and programming experiment-related modules . Data transfer rate is one 16-bit word every 8 ls, leaving sufficient time for other DMA devices to gain access to the UNIBUS. The size of the double buffer is 2048 data words of 16 bits . In buffer mode incoming ADC data words are first written into the interface's buffer and as soon as one of the two 1024-word segments has been fully written, the contents of this segment is transferred by direct memory access to PDP memory . Concurrently with this process new incoming data words are written into the complementary 1024-word segment, so that reading from and writing into the buffer may take place in a "handshaking" manner . After the whole segment has been transferred an interrupt is generated and further transfer is disabled . Data transfer from the buffer may also be started at any time by a "programmed transfer request" . In this case all the available data in the current segment are transmitted and afterwards an interrupt is generated . After completion of a data transfer the buffer control logic switches automatically to the starting address of the complementary segment, so that the doublebuffered feature of the hardware permitting concurrent reading and writing may be emulated by the software . While the program is working on one segment transmitted previously, the hardware may be enabled to (concurrently) write new data into the complementary PDP memory segment . In simple software environments without much system overhead, data processing speed may almost equal interface transmission speed, so that as soon as the software has finished its work on he current segment, a fresh segment of data is immediately available. In the case that counting rate is low the use of the internal buffer may be disabled . In this "single-word" transfer mode each incoming data word is transferred immediately to PDP memory and an interrupt is generated. The computer's processor derives its internal "realtime" from a "line frequency clock" which does not possess an accumulator of its own, but simply produces interrupts at regular intervals ("ticks"), counted and accumulated by the executive . Since critical parts of the data acquisition and spectrum generation process are carried out in processor kernel mode at the non-interruptable priority 7, the Executive starts to lose "ticks" when the flux of incoming data is high . This effect causes the Executive's "real-time" to lag behind . For this reason an independent . Real-time clock for exclusive use of the data acquisition system has been added to the interface . It features a 24-bit accumulator, with a total count capacity of 194 d .

3 . The system software The data acquisition and display software has been completely written in DEC's Assembly language MACRO-11 version 4 .0 (M1200) for higher speed and efficiency and runs under RSX-11M . It may be divided into three functional parts : (1) The data acquisition package . Through it the user defines an experimental configuration, like precoincidence groups, selection of ADCs and sizes of spectra. In addition the package is responsible for data reception and on-line generation of 1D and 2D spectra. It occupies only 32 Kbytes of main memory thanks to extensive use of overlay techniques . (2) The display package . This allows the display and manipulation of 1D and 2D spectra both during the acquisition process and after termination. In the case of 2D spectra it displays several types of cuts through the spectrum's surface for survey and analysis purposes . In addition the package allows simple analysis of spectra, like peak energy determination, calculation of peak areas with and without background subtraction, and the generation of several 1D spectra from a single 2D spectrum . This last feature is particularly useful in extracting single particle spectra from a 2D spectrum produced by a particle identification system. The display package needs 56 Kbytes of computer memory for running . (3) The auxiliary package . This package provides services like summing or subtracting spectra, conversion of spectra from 32-bit binary to FORTRAN accessible floating point format or to decimal ASCII for printing . 4. Performance Data processing time depends strongly on where a spectrum is resident : either in computer main memory or on disc . In the first case the software is able to process up to 6000 events per second, independent of the number of active spectra . In the case of a 256 x 256channel 2D spectrum completely resident on disc this number may go down to 200 events per seconds . For this reason the system offers the user the option of maintaining that part of the spectrum which receives the highest counting rate in memory, while the rest of it is kept and updated on disc . Even the number quoted above for a memory resident spectrum may seem rather low compared with the acquisition speed of a multichannel analyzer. The reason is that the system has been designed to accommodate several 1D and 2D spectra simultaneously, so that system overhead has become quite high. For every data word received the processor executes approximately 40 machine instructions . If the need arises to accommodate 1D spectra with higher counting rates, external memory modules and control logic to generate 1D spectra by hardware can be easily added to the existing design .