Electronic and Data Acquisition
3 ELECTRONICS AND DATA ACQUISITION.
An electronics system is needed to process energy and timing information, make trigger decisions and relay data to the acquisition system. The system will be based on VXI and VME. The laboratories involved in EXOGAM have developed this type of electronics for the Eurogam and Euroball projects and will capitilise on the experience from these systems.
The electronics will use the VXI (VME extension for instrumentation) bus standard. This standard allows a large card size to be used and the mixing of both high density analogue signal processing and digital circuits on the same card. VXI cards are built for specific applications, for example, detector signal processing, digitisation and event triggering. In EXOGAM VXI cards are to be developed for the Ge detectors, the BGO suppression elements and the trigger. Full details of the system architecture can be found in the specification document [4.1].
EXOGAM will use the Euroball readout and triggering systems since they can be used without further development. The Euroball style infrastructure (crates, resource managers and crate readout cards) will be used in EXOGAM. This is beneficial since the engineers designing and maintaining the electronics for EXOGAM are familiar with this system. Logic and analogue inspection lines will again be provided from all modules (accessible via the resource manager front panels) for visualisation of internal signals from the modules. Some component designs from Eurogam and Euroball can be re-used with small enhancements or modifications in the new VXI modules.
EXOGAM imposes additional design constraints to those for Euroball: the detectors are different, there will be a high level of background radioactivity to be removed from the spectra, and with an array comprising entirely multi-element detectors, the use of add back becomes more important and therefore good energy resolution and efficiency down to low energies are highly significant.
The VXI modules in the EXOGAM system will be:
- Ge detector card electronics
- BGO card (6 shields per card)
- Master Trigger card (1 card)
- Resource Managers, type STR8032 (1 per crate)
- Crate readout cards, type STR8080 (1 per crate)
The following modules can be added for use with ancillary detectors:
- ADC cards for ancillary detectors (GANIL converters for charge, voltage and time) [4.2]
- FVI Bordeaux's FERA to VXI Interface (1 card per 2 FERA chains)
- Purpose built VXI cards for ancillary detectors
For EXOGAM with 16 segmented CLOVERs the basic system will comprise 8 CLOVER Ge fully instrumented cards, 16 CLOVER Ge position cards, 4 CLOVER shield cards, 1 Master Trigger card and so will fit in 3 VXI crates and still leave space for ancillary detector cards. The 3 crates will require a total of 3 Resource managers, and 3 Crate readout cards.
The VXI cards will include the General Interface Readout (GIR) module. The GIR provides a common VXI interface and readout across all cards in the system, a DSP to manipulate data before readout (for gain matching for example) and a fast serial data link which will be used to transmit the true singles data for the Ge detectors.
Singles histograms which record the 6 and 20 MeV ranges for the detector central contact channels without any trigger dead time losses are required for some EXOGAM experiments. A method of histogramming such "true singles" will be provided. The true singles system will generate 2-d spectra with energy (13 bits) and timing (4 bits) where the timing may be connected either to a TAC measuring CFD-beam pulse (for background rejection) or to an external time frame generator (for beta-decay experiments).
The multiparameter event data are read out and transferred over the VXI backplane into a buffer in the STR8080 crate readout controller. At the end of this cycle the crate is ready for a new event, and when all VXI crates have finished their readout cycles for the current event, the system permits a new event.
The readout of the data from the buffers in the STR8080's takes place over a fast 32 bit data transfer bus known as the DT32 bus. An important VME module to be incorporated into the system is the histogrammer whose function is to spy on every multiparameter in the event by event data on the data transfer bus and increment spectra which can be viewed on a workstation. Full details of VXI backplane readout can be found in EDOC076 [4.3] and full details of DT32 readout are in EDOC075 [4.4] from the Eurogam document server.
The EXOGAM trigger system will use a simple "common dead time" method of operation which means that once EXOGAM starts to process an event the system is dead and no further events will be accepted until the first event is either aborted or stored in the VXI readout buffers. The only effect that another event may have during the dead time is to cause pileup indicators to be set in the detector electronics cards.
The user may trigger his experiment using a conventional multiplicity sumbus or may provide a single logic pulse to be used as a trigger. It is possible to combine the two methods for more complex triggers, requiring a certain fold and the existence of an external logic input before starting to process an event. Multiple (parallel) trigger conditions may be used.
The trigger system is a 2 level system, the first level being used to provide good timing for stopping TACs and a quick trigger decision. The second level is used to either confirm that the event is good, and so must be read out, or else that it is bad and so must be rejected. The global trigger signals issued by the master trigger card are Fast Trigger (1st level trigger) and then either Validation or Event Reject (2nd level trigger). The global signals are distributed to every module where they are used by Local Triggers to check whether or not its channel is active in coincidence with enough other channels to have caused an event. The Local Trigger controls ADC coding and readout for its channel or module. Full details of the trigger card can be found in the trigger card specification document [4.5]. A schematic layout of the system is shown in figure 12.
Figure 12: A schematic of the electronics and data acquisition system.
3.1.1 Ge detector cards
The segmented CLOVER detectors comprise 4 crystals, each electrically segmented 4 ways. Thus the segmented CLOVERs require electronics for 4 inner contacts and 16 outer contacts.
The EXOGAM Ge electronics will comprise 2 card types: one fully instrumented for 8 centre contacts and one instrumented more simply for 18 outer contacts. This two card approach is flexible since it caters for the different types of segmented CLOVERs (different numbers of position channels) using the same electronics. In addition, different types of detectors can be instrumented, for example, the segmented single-crystal coaxial detectors which have 2 centre contacts and 6 outer position contacts.
Specification for the electronics for the centre contacts.
The centre contact electronics will measure energy, timing and radial hit position. The energy will be measured in 2 ranges from 0-6 MeV and from 0-20 MeV with 14 bits (16 k) ADCs. The timing will measure from the time the CFD fires to either the next beam timing (RF) pulse or to the time of the Fast Trigger (first level trigger, also acting as a global timing reference). The radial hit position determination, combined with the segmentation of the detector, will enable precise hit localisation. The position is determined by analysis of a current pulse obtained by differentiating the charge pulse from the preamplifiers. The pulse analysis method will be either a digital processing using a DSP to analyse samples from a flash ADC, or else an analogue processing using the steepest slope algorithm proposed by the Heidelberg/Cologne groups. The former method is more versatile while the latter is simpler. The choice between the 2 methods will be made after further tests. Full details of the centre contact Ge card can be found in the Ge card specification document [4.6].
A schematic layout of the proposed 8 channel Ge VXI card is shown in figure 13.
Figure 13: A schematic layout of the 8 channel Ge VXI card.
Specification of the outer contact VXI card.
The Ge outer contact electronics will measure only energy, relying on the associated inner contact card for timing, and for triggering (based on inner contact CFD signals).
For simple energy measurement, and ADCs without sliding scale it should be possible to have 18 channels on one card but only the complexity of the electronics will finally determine whether this is possible or not.
The suppression shields for each of the Ge detectors consist of many BGO crystals each with a photomultiplier tube. The outputs of these photomultiplier tubes will be coupled close to the detector to give four signals per CLOVER shield.
The four signals from a shield will be handled by a single channel of electronics which will measure the sum energy and the timing of the OR of the four inputs measured against either the beam pulse (RF) or the 1st level trigger Fast Trigger (FT). The choice of the timing measurement will be controlled by the user. In addition to energy and timing, a 4 bit hit pattern will be generated from each shield. One BGO VXI card will handle six suppression shields.
The signals from the photomultiplier tubes will either be buffered through line drivers or driven by shaping pre-amplifiers before transmission depending on the amount of light generated by the crystals which governs the amplitude of the output signal from the phototubes.
Full details of the BGO card can be found in the BGO card specification document [4.7].
EXOGAM has been designed to operate with different ancillary detectors. There are three possible methods for connecting ancillary detectors into the acquisition system.
The first is that users bring their own custom built EXOGAM-compatible VXI cards and plug them into EXOGAM VXI crates. This is not likely for the majority of users, but will provide the most ßeamless" route in to the EXOGAM system for those who have the resources.
The second method is for the users who already have a system with FERA ADCs. A module called an FVI (FERA-VXI Interface) designed by CENBG, Bordeaux can interface FERA chains into the VXI readout. Each FVI handles two FERA chains. This would require users supplying the FVI module, a CAMAC crate and the necessary expertise to configure and support the FERA system and interface.
The third method is for users who do not have their own ADCs. 64 channel QDC/ADC/TDC VXI cards can be supported by GANIL [4.2].
The data acquisition system for EXOGAM will be based on the systems developed for Eurogam and Euroball and will incorporate the existing systems at GANIL. The experience from acquisition systems on the spectrometers LISE and SPEG will be utilised. The systems will be designed to handle a data rate of 2 Mbytes/sec. A schematic diagram of the data acquisition system is shown in figure 14.
The data acquisition system will be based on the MIDAS package (Multiple Instance Data Acquisition System) which is a development of the software system initially developed for Eurogam Phase I. MIDAS is a very flexible, highly modular experiment control, monitoring and acquisition system which is in use in a number of laboratories.
A Graphical User interface (GUI) running within a Unix environment is used for all interaction between the user and the data acquisition electronics. All configurations of the system can be performed dynamically using the graphical tools. Setup, control and monitoring of electronics in VXI, CAMAC and VME is possible remotely using the GUI.
Each EXOGAM VXI crate will contain a Resource Manager which has a built in processor board containing an on-board ethernet interface. Each VME crate will contain a similar single board processor. All processors will run essentially the same software with class specific modules loaded appropriate to the type of hardware. The software has been designed with the aim of allowing new hardware to be added quickly and easily.
The VXI crates will be read out via a 32 bit data bus (DT32) into a VME crate (event builder) where the data is checked and formatted for output to tape. The event builder passes the formatted data via a broadcast data network (FDDI) to a VME based dedicated Tape Server which manages a farm of tape drives and outputs the data to one or more of the drives.
The existing GANIL data acquisition system and software can be used by transferring the data, via the FDDI network, directly from the Data Processor VME crate to the GANIL VMS based data acquisition system. The two systems can run in parallel if required and data can be written to tape in either or both of the tape servers and the VMS system.
Online data analysis (Sorting) is possible by workstations which spy on the data network. Any number of workstations can concurrently work in this way running codes supplied either as a part of the system or by the user.
All data processing and standard data analysis programs can be controlled by the same GUI as used for hardware control.
Particular attention will be given at all stages to the provision of diagnostic tools (both for hardware and software) using as far as possible graphical presentation of the information in a way meaningful to non specialists. The aim will be to both detect as quickly as possible errors in the system and to help mimimise the time to fix them.
The actual electronique and data acquisition of the EXOGAM array aim to be renewed in the coming years. This project called EXOGAM2 will equip the array with a state-of-the art digital electronic for germanium array. This project is managed by the GANIL laboratory in an international collaboration in the contexte of SPIRAL2. The project status and documentation can be found at the EXOGAM2 web page.
[4.1] The EXOGAM Electronics System Architecture, I.Lazarus.
[4.2] The GANIL ADC/QDC/TDC VXI module.GIP/ELEC02/97, QDC6414V-53, M.Tripon.
[4.3] VXI backplane readout, EDOC076
[4.4] DT32 readout, EDOC075
[4.5] The EXOGAM Trigger System, I.Lazarus.
[4.6] The EXOGAM Ge detector card, I.Lazarus.
[4.7] The EXOGAM Escape Suppression Shield (BGO) card, I.Lazarus.