Requirements of proposed electronics and data acquisition system for the Indian Neutrino Observatory

B.Satyanarayana

Department of High Energy Physics,

Tata Institute of Fundamental Research, Mumbai, INDIA

A consortium formed by several Indian research organisations has proposed setting up of a major Indian Neutrino Observatory¹(INO). Survey for a suitable location for this laboratory, physics potential from such a facility and other feasibility studies are currently in progress. Aim of this document is to provide an overview of the requirements for the possible electronics, data acquisition, trigger and other ancillary systems for this experiment.

Two types of detectors are being considered at present for this experiment, viz., a magnetised iron tracking calorimeter employing Glass Spark Chambers(GSC) as active elements and a water Èerenkov detector employing large diameter photomultiplier tubes for readout. Former type is the preferred choice at present and so is assumed for this document.

1. Detector and segmentation

GSC is composed of two parallel float glass electrodes, 2mm thick, kept 2mm apart by using appropriate spacers. The gap between electrodes is filled with a suitable flowing, non-flammable gas mixture. The resistive coating (using graphite for example) on the outer surfaces of the glass, connected to the HV power supply creates a strong electric field across the gap. An ionising particle initiates ionisation and a localized spark in the gas volume which induces signals on external pickup strips placed on either side of this assembly. Use of high resistance glass(10¹²Wcm) as electrodes and quenching properties of the gas limit the discharge to a small area around the spark. Output impedance of the strip signal depends on the geometry of the pickup strip.

The proposed INO detector will have a modular structure of lateral size 32m X 15m and height 11.9m with iron plates used as the absorber. It comprises of 140 layers of horizontally arranged iron plates of 6cm thick interleaved with 2.5cm gap between successive layers of iron plates to house the GSC detector elements. There are two sets of orthogonally arranged pickup strips on either side of the spark chamber. Total number of strips for the detector depends on geometry of a single chamber and the pitch of the strips. As mentioned above, the geometry itself will depend on the signal impedance of the pickup strips among other considerations.

Assuming a strip pitch of 20mm(50mm) we get an estimate of 329,000(131,600) total strip readout channels. We typically record latch and timing information of these readout channels on a trigger. While latch information consists of one bit per channel, for timing, groups of channels are logically ORed into one before feeding to time-to-digital converters. Assuming ORing of 16(32) strips for readout, we get an estimate of 20,440(10,220) and 8400(4200) total TDC channels respectively for 20mm and 50mm cases.

1. Pulse characteristics

Detector R&D and fabrication of GSC prototypes are in progress. Before, we arrive at our own bench marked parameters of these chambers, one may quote the following numbers arrived at by other groups working on GSCs of similar design and operation.

1. Front-end electronics

As can be seen from the above table, amplitude of the chamber signal is quite good and hence there may not be any need for further amplification of the same. Assuming that we don't need to record analog information of any kind from the chamber, a simple discriminator may be straight away mounted on this signal. Further, depending on the readout scheme that is going to be employed, one may need to add cable drivers etc. on this logic line. Alternatively, there may be a requirement for faster (for trigger) and slower (for generating latch inputs) copies of the same discriminated signal in addition to even providing a linearly added(all strips from a group) fast analog signal output. Still another approach is to have the strip status latch bits serially output to the readout electronics racks. Typically, all this electronics is integrated on the chamber itself.

1. Readout electronics

We need to essentially record latch and timing information from all the detector channels on trigger. Latch information is formed by strobing the discriminator pulse status using the trigger signal. It indicates whether or not a channel has fired in that event. We need a logic bit per a detector channel to record this information. As mentioned above, however as far as timing information is concerned, one may combine all channels of a side of a layer(or a group in a layer) into one or some manageable number of TDC channels. We need to have one TDC channel per this group of detector channels. The TDC should have multi-hit capability, apart from other on-line programmable features.

As far as readout method is concerned, one can have a 'non-accelerator based' experiment or an 'accelerator based' experiment approach. In a typical non-accelerator based experiments' readout scheme, all the channel signals are brought upto the readout crates without multiplexing. All the readout functions are carried out in these crates from where the data is transferred to the host.

In case of a typical accelerator based experiments' scheme, data is digitised right at the front-end, even on the detector units itself and is transmitted to the readout crates using standard giga-bit serial links. These two schemes have many contrasting advantages and drawbacks each and choosing one or the other scheme may also have significant influence on the trigger and data acquisition sub-systems.

1. Trigger system

This is heart of the data acquisition system for any experiment. Since, typically one cannot completely freeze on the physics possibilities from an experiment at the time of design of the data acquisition system, enough flexibility and programmability has to be built into the trigger system, so that it can be reconfigured with minimum efforts later if needed.

Typically sharing the same data path as that of main data acquisition system, trigger system independently runs various algorithms on the data to determine if a particular pattern of the data represents an interesting physics phenomenon and should be recorded permanently. Often, due to complexity of these algorithms that the system needs to run, the trigger system is segmented into various levels. As the trigger level increases, the sophistication in the algorithms and hence the data filtering capability increases, but each higher level takes more time for computing. Often, starting from a completely hardware based simple trigger level, a highly computing intensive software trigger schemes are implemented.

Typical trigger schemes relevant to INO are vertical triggers that indicate passage of a particle through all or several layers of the detectors. The trigger system should be able to determine whether the particle is traversing from top to bottom of the detector or vice-versa. In addition, we need also to recognise events forming short tracks, typically hitting large number of chambers and strips in a few layers. These are called large-angle triggers. These are only a few suggestive types of triggers. As mentioned before, the trigger system has to be versatile enough to implement any trigger scheme in principle during the course of experiment.

1. Monitor and slow control

Monitor and slow control system is one of the most important aspects of this type of data acquisition system. It monitors the complete health of the entire detector elements so that one can maintain highest level of reliability on the data collected from the experiment. Typically, detector channels are monitored by relaying on their individual counting rates which are due to the background radioactivity at the experiment site. Various pre-trigger signals are also monitored. All the electronics and data acquisition elements such as power supplies, gas systems etc. need to be monitored on continuous basis. Since the expected trigger rate from these type of experiments is typically low, usually these monitoring jobs are taken up at the background. Recording of event data however takes highest priority.

One also needs to control various parameters of the experiment from a central place. Typical parameters that need to be on-line monitored and controlled are thresholds for the front-end discriminators, high and low voltage supplies, gas regulators, gas flow valves etc.

A common low-level interface standard should be designed and built so that applications for monitor and control sub-systems for various parameters based on this standard could be developed effortlessly later by even non-specialists.

1. Bus standards

Choosing a readout crate bus standard is one of the critical decisions into which ease of availability of hardware and other interfaces, module size and densities, compatibilities with other sub-systems and even other experiments are taken as inputs. While VME which satisfies all these aspects, is a reasonable choice to begin with, one needs to make a comprehensive study of all the available standards before arriving at the final choice.

1. Networking issues

This is again an important issue which affects the way the data is managed or system is accessed. With emerging new network standards and protocols, it is necessary to arrive at suitable topology of the network, network protocols to be employed and other related issues. Thus, interconnection of either multiple hosts from the same sub-system or hosts controlling different sub-system forms a major topic of study and evaluation.

1. On-line software

Starting from deciding on the operating system for the on-line hosts, programming language and standards for all the software development, various alternate schemes available at all these stages again needs considerable study, investigation and bench marking before freezing one or the other standard. Processing speed, support of other interfaces and database standards etc. may also influence this choice.

1. Data storage and archival

Commercial databases are gaining immense popularity and lots of applications and products are increasingly available in the market for managing these databases. So, it is probably a better idea to use one of the commercial database standards for this experiment as far as the data storage and its archival are concerned. But, at the same time, one can't ignore the potential of non-commercial scientific analysis tools that are currently available in the community. Taking advantage of these tools, may impose certain constraints on the database choice and hence one may need to adapt a judicial blend of these two choices on data at different levels of abstraction for example.

1. Gas system

The design of the gas system for the chambers must address a number of important considerations. Number of individual channels, maintaining uniform gas levels in the chambers, quality of gas, safety issues, built-in automatic monitor and control capabilities and finally cost of operation are some of these issues. Designing an appropriate scheme which satisfies these challenges deserves also a lot of work.

1. Power supplies

Though often, not given adequate importance in a system, design of appropriate high and low voltage power supplies is very important not only for meeting the specifications, but also in maintaining the experiment in the long run. Quality of power supplies in some sense decides quality of the data recorded. Maintaining a potential of more than 10KV across a 2mm gap in the GSCs could itself a big challenge considering the large number of channels. Again, appropriate control and monitoring of the voltage and current etc. are some of the key design issues.

References

1. N.K.Mondal, Neutrino Physics: Current Status and Future Prospects, Internal note