3.2 Detector Requirements

The various detector partitions exploit different techniques to achieve their aims of identifying or measuring the position, momentum, or energy of the products of electron-positron collisions. Consequently they have different requirements for their Slow Controls. Here only those aspects relevant to the Slow Controls are detailed. Full details of the detector itself may be found in chapter 2 and the references given therein.

3.2.1 General Principles

Most modern particle detectors rely on the detection of charged particles by their ionizing effect on the material they pass through. The charged particles are either those from the physics interactions or part of a shower of particles formed when either charged or neutral particles pass through a dense medium. The electrons (or ions) liberated by this ionization are drawn to an electrode by an electric field. These signals can then be amplified, digitized, and recorded for subsequent analysis.

To control and monitor the provision of these conditions is one of the major tasks of a slow controls system. Different types of detector use different ionizing materials, usually gases. Careful monitoring is required of the gas supply, mixing, and distribution, particularly as a number of the gases are flammable. To provide sufficiently strong electric fields, high voltages of thousands of volts are often required. The electronics used to process the detected signals requires carefully controlled (low) voltages. All these systems must be capable of being switched off quickly in the event of a dangerous condition. Finally, the environment has to be monitored carefully for conditions, such as a high temperature, which could damage equipment or indicate burning electronics.

There are thousands of these quantities which require monitoring or control -- far far too many to oversee manually. Hence the need for computer control.

3.2.2 General Features

Apart from the RICH fluids, all gases used in DELPHI, despite the different compositions required by differing detection techniques, are provided by a unified gas system, summarized in section 3.9.1. The gases used in the various detector partitions are summarized in table 3.1.

Table 3.1: The gases used by each DELPHI detector partition. In addition, argon and a hydrogen-argon mixture (7%/93%) are used for regeneration of the active copper purification columns; and nitrogen and carbon dioxide are used for cooling and purging. The RICH Fluids (C$ _{6}$F$ _{14}$ liquid, C$ _{5}$F$ _{12}$/C$ _{4}$F$ _{10}$ gas, and TMAE vapour) are supplied by a separate system, described briefly in section 3.2.5.
\begin{abox*}{\textwidth}
\begin{tabular}[t]{\vert l\vert\vert*{4}{lr@{}l\vert}}...
...iC$_{4}$H$_{10}$&15& & C$_{3}$H$_{7}$OH& 1&.7\\
\hline
\end{tabular}\end{abox*}


Most high voltages are supplied by the CAEN high voltage unit, described in section 3.3.1. When large numbers of stray particles are produced by LEP (i.e. while filling the machine), high voltages of a number of detector partitions have to be ramped to a lower value to prevent excessive currents due to large amounts of ionization. This is necessary for the Inner Detector, Time Projection Chamber (TPC), Outer Detector, forward tracking chambers, Barrel and Forward RICHes, barrel electromagnetic calorimeter (HPC), Forward Muon Chambers, and STIC. Consequently, speed and reliability of ramping for these detector partitions is particularly important.

Except for the HPC, the voltages and currents of the Fastbus Data Acquisition crate power supplies are all monitored, and can be switched on or off under computer control. Most detector partitions provide similar monitoring and control for their front-end electronics.

Temperature monitoring inside the detector is performed by the DELPHI Slow Controls. In the electronics counting rooms, the environment (including rack temperatures) is monitored by the GSS system, summarized in section 3.9.2.

A summary of the general requirements for each detector partition is shown in the Elementary Process function columns of table 3.2 and, in more detail, in table C.1. Specific details of each detector partition, as they relate to the Slow Controls, are given below.

3.2.3 Barrel Tracking Detectors


3.2.3.1 Vertex Detector (VD)

The VD requires a bias voltage of about 60 V, and detector cooling. The Slow Controls hardware is described in [88], although the dedicated VAX software described therein is now supplemented by standard Elementary Processes.

Monitoring of temperature is vital, both in order to prevent damage to the detectors due to overheating, and to keep track of temperature variations which can lead to movement and consequent degradation of the precise alignment. These movements relative to the Inner Detector are monitored both by capacitive probes [89]3.1and by lasers.


3.2.3.2 Inner Detector (ID)

The ID uses LeCroy power supplies to provide high voltages for the drift field (not sensitive to LEP conditions), the anodes, and the MWPC. They are controlled using RS232-C connected to the ID VAXstation (via a terminal server), where a special process emulates a G64 system controlling a CAEN. This allows standard VAX software to be used with only minor changes. For more precise measurement of the detector voltages than can be provided by the LeCroys, a digital voltmeter is used.


3.2.3.3 Time Projection Chamber (TPC)

Both drift field and sense wire high voltages (25.3 and 1.435 kV respectively) are provided by CAEN units. Only the sense wire voltages need to be lowered during LEP filling. Special modules are used to measure the current in each sector. High voltage channels are `daisy-chained' together in the CAEN in such a way that if one channel trips, then all channels of the same polarity trip. Trips are minimized by automatically lowering the volts if the current becomes too high.

Due to the proximity of the heated Barrel RICH, the temperatures are monitored and, if they are too high, the preamplifiers are switched off.


3.2.3.4 Outer Detector (OD)

High voltages are required for the OD anode wires. Since the OD is attached to the outside of the heated Barrel RICH, the temperatures and positions of the planks are monitored to check that the alignment does not change.


3.2.4 Forward Tracking Chambers

The high voltage systems of both FCA and FCB provide automatic trip-recovery. When a channel trips (due to a large current being drawn by excessive ionization in the chamber), this system automatically ramps the channel up again (after a short delay to allow the chambers to recover). If this occurs repeatedly, then the system gives up, leaving further action to the operator (who is kept informed via SMI and EMU).

In addition, the software ensures that ramping is always done in groups of channels so that there are no delays between the start of ramping for different channels within an endcap (FCA) or module (FCB).

These functions were implemented by changes to the standard Elementary Process, which treats all channels independently.

Special precautions are taken to prevent the possibility of significant voltage differences between the FCB wire planes, which are only 1 cm apart. The CAEN high voltage channel for each plane is daisy-chained with the others in the same module in such a way that if one channel trips, then they all trip.

Monitoring of the FCB preamplifier low voltages is required to maintain a balance between sufficient amplification of the signals and noise reduction.


3.2.5 Ring Imaging Cherenkov Counters (RICH)

The RICH fluids [90] (C$ _{6}$F$ _{14}$ liquid, C$ _{5}$F$ _{12}$/C$ _{4}$F$ _{10}$ gas, methane and ethane used as drift gases, and TMAE vapour) are supplied by a special system controlled by five Siemens process controllers, which perform the particularly careful control and monitoring required by these sensitive detectors. The radiator ultraviolet transparency is checked with a monochromator controlled by G64.


3.2.5.1 Barrel RICH (RIB)

The Barrel RICH gases are heated to 40 $ \ensuremathbox{^\circ}$C. This allows the normally liquid C$ _{5}$F$ _{12}$ to be used as a gas radiator, and a greater quantity of TMAE vapour to be present.3.2The temperature has to be controlled and monitored very carefully to prevent condensation of the TMAE by cooling, damage to the detector by overheating, or expansion or contraction which would destroy the detector alignment.

An 80 kV Heinzinger very high voltage unit (controlled, via an IEEE bus, by G64) provides the electric field to drift photoelectrons to multiwire proportional chambers (MWPC), which are supplied by CAEN units.


3.2.5.2 Forward RICH (RIF)

The Forward RICH uses C$ _{4}$F$ _{10}$, which has a lower boiling point than C$ _{5}$F$ _{12}$, as its gas radiator, and thus does not require the elevated temperature used in the barrel, considerably simplifying the Slow Controls. A 35 kV FUG very high voltage unit (controlled via a CAEN unit) provides the drift fields, and CAENs are used for the MWPCs. The temperatures of the gas radiator, drift gas, front-end electronics, and fastbus crates are monitored by G64s, which can cut the TMAE flow or crate power in the event of problems.

3.2.6 Calorimetry


3.2.6.1 High-density Projection Chamber (HPC)

Due to the fairly large number of HPC CAEN channels (144) and to particular features of the switching on/off procedure, special software has been developed for the high voltage control. This optimizes the time needed to ramp up the chambers' high voltages and performs extensive checks on the power supply hardware to ensure safe operation of the chambers.

Since energy and position measurements depend critically on the gas mixture, continuous monitoring of the drift velocity and chamber gain is performed on external drift tubes connected to the gas system [91]. These measurements are performed using CAMAC devices, which are then read out by a G64 acting as a crate controller.


3.2.6.2 Forward Electromagnetic Calorimeter (EMF)

All phototriode high voltages on each side of the EMF are supplied by a single Kepco high voltage unit. A splitter allows the voltage and current for each quadrant to be individually controlled and monitored directly by G64, and the 2560 currents drawn by individual groups of phototriodes are also monitored. A water cooling system is employed and temperatures are monitored, allowing the detector to be automatically switched off if the temperature rises too high.


3.2.6.3 Hadron Calorimeter (HAC)

The high voltage [92] for each HAC tower is provided by a single CAEN channel, for which automatic trip-recovery is provided. Each of the 1872 layers can be disconnected separately by relay. This prevents a single short putting an entire tower out of action. To achieve this, the current drawn by each layer is monitored [93]; if it is too high, the relay is switched off directly by the G64 (for speed). The front-end electronics supplies are also controlled [94]. Test streamer tubes are used to monitor the gas mixture quality and drift velocity.

3.2.7 Muon Chambers

Both Barrel and Forward Muon Chamber high voltage control includes automatic trip-recovery, similar to that described for the Forward Tracking Chambers in section 3.2.4 (though without the form of channel grouping used there).


3.2.7.1 Barrel Muon Chambers (MUB)

The Barrel Muon Chambers' high voltages are applied to both the anode wires (6150 V) and the cathode (grading) strips (graded with voltage between 4000 V and ground). Hardware interlocks ensure that both anode and grading will trip if the current drawn by either is too large. The voltage difference between anode and grading is further protected by automatically ramping the voltages in 500 V steps. Special conditioning logic automatically comes into operation for sectors tripping repeatedly. This reduces, for a time, the target voltages to find a level where the chambers can operate without tripping. The voltages are ramped down if the gas supply is stopped or the mixture is bad (in addition to the general switch-off in the event of a gas loss).


3.2.7.2 Forward Muon Chambers (MUF)

The Forward Muon Chambers' anode wire voltages are provided by CAEN and the cathode strips by FUG power supplies, which are controlled directly by the G64s. The anode voltages are varied (by the Elementary Process) as a function of atmospheric pressure in order to maintain a constant efficiency. The drift velocity is monitored [95] with a special chamber supplied with the same gas mixture as the detector.


3.2.7.3 Surround Muon Chambers (MUS)

Since the MUS streamer tubes are of the same design as those of the Hadron Calorimeter, the same gas supply can be used for the two detectors. However, the smaller number of planes allows each one to be provided with high voltage by a single CAEN channel.


3.2.8 Scintillators


3.2.8.1 Scintillator Trigger Counters (SCI)

High voltages for the SCI photomultiplier tubes are provided by non-standard CAEN voltage dividers, controlled using the HPC G64 system and special VAX software.


3.2.8.2 Time of Flight Counters (TOF)

High voltages are used for the photomultiplier tubes.


3.2.8.3 Forward Scintillator Hodoscope (HOF)

The HOF Slow Controls are considered a subsystem of the Forward Muon Chambers (section 3.2.7), which provides high voltages for the photomultiplier tubes.


3.2.9 Luminosity Monitors

The Slow Controls of the SAT and VSAT are described in [97]. Bias voltages for both detector partitions are provided by special low voltage crates, which are connected via RS232-C to a shared G64. The G64 Skeleton program and the Elementary Process have been adapted to control these bias channels. This system is also used for the bias voltages of the STIC, which replaced the SAT at the start of 1994.

Monitoring is performed on the currents drawn by each of the 320 STIC phototetrodes, which are supplied by a system based on that of the EMF (see section 3.2.6). Control is performed on each endcap as a whole: this is emulated in the G64 as a 2-channel CAEN crate. The veto hodoscope photomultiplier high voltages are provided by a CAEN SY403 high voltage unit, at present controlled by hardwired signals. The STIC fastbus monitoring and control are provided by the old SAT system.


3.2.10 Other Systems

Monitoring is also performed for the central Data Acquisition and Trigger system fastbus crate power supplies, temperatures at various places round the DELPHI barrel, cavern temperature and humidity, and the detector cooling water temperatures, flows, and vessel condition.

Tim Adye 2002-11-06