MuID Safety Review Packet

September 11, 1997

Introduction

The purpose of this packet is to provide information to the RHIC Safety Committee for the purpose of reviewing the PHENIX Muon Identifier (MuID) from the standpoint of safety. Included in the packet are MuID background materials and summaries of the mechanical, fire and electrical safety issues (there are no lasers, confined space access, radioactive sources or magnetic fields). Relevant material safety data sheets (MSDS) and product specifications will be made available in the hardcopy version of this document.

Action Items from Previous MuID Safety Review
(February, 1997)

MuID Introduction

The PHENIX Muon Identifier (MuID) serves to distinguish between pions and muons that exit from the back of the Muon Arm magnet. The MuID is a sandwich consisting of several layers of steel absorber and sensitive elements. The absorber preferentially stops non-muons and the sensitive area records the remaining particles. The MuID operates in concert with the Muon Tracker (MuTR) which accurately measures particles' momenta before they enter into the MuID steel absorber which would seriously degrade a momentum measurement. To orient the reader, a picture of the PHENIX hall is shown in figure 1, below. It may not be completely obvious from the picture, but the MuID will be buried behind the PHENIX shield wall. This means that it must be in place before the shield wall goes up (hence the scheduling urgency) and the panel interiors cannot be serviced afterwards.

Figure 1. The PHENIX hall. Note the two muon arms north and south of the central detector. For MuID, both arms are identical.

MuID Panels

The MuID consists of sixty physically separate detectors, called panels. Large panels (40 total) are roughly 5.4 x 5.6 x 0.08 m3 and small panels (20 total) are roughly 2.9 x 4.3 x 0.08 m3. The panels are boxes which have edges formed by specialized aluminum extrusions and covers made of 0.100" thick aluminum covers. There is also a 0.125" thick aluminum mid-plane (bisecting the panel volume in the short dimension). The box structure serves as a fixture on which to mount the sensitive elements (Iarocci tubes, described below) and some readout electronics, and as a secondary gas containment vessel that we can purge if there are leaks from the Iarocci tubes (which operate with an Isobutane/Carbon Dioxide gas mixture). There are isolated signal, power, HV and gas connectors at the edge of the panels to connect to the outside world. Figure 2 shows a full-scale prototype panel in a horizontal (for assembly) position.

Figure 2. Full-scale MuID prototype panel at ORNL, shown in its horizontal (assembly) position. The tubes shown are mounted to the panel mid-plane. The panel gets flipped so that the other side of the mid-plane is exposed (and the tubes shown are on the bottom side) and a series of tubes are mounted in the orthogonal direction.

Iarocci Tubes

The detector technology chosen for the MuID sensitive elements are Iarocci tubes. Iarocci tubes are a relatively standard HEP detector consisting of wires, under high voltage, together with a preformed cathode, all contained inside a hermetic gas volume. Our cathode and gas volume are made of PVC (MSDS included). The PHENIX tubes are 8.5 cm wide with lengths varying from 2.5 - 5.2 meters. A picture of a very short (~4") Iarocci tube without its hermetic gas volume is shown in figure 3, below. The Iarocci tubes and the mating European HV pin connector have already received safety approval.

Figure 3. A short (~4") Iarocci tube. HV, signals and ground are connected through pins in the orange endcaps and the mating European HV pin connector (shown at right). The black "comb" is the cathode. The anode wires (probably not visible) are strung parallel to the cathode comb, centered between the tines.

Summary of MuID Panel Construction Materials

This list only contains combustible materials inside the panels. The Iarocci tubes are made of PVC (MSDS included). The total PVC volume per panel is (large panel: 0.75 m3, small panel: 0.3 m3). There is internal cabling for HV, signal, power and pulser (product specifications included). Cabling details are given below, but note that all chosen cables pass the VW-1 vertical flame test. In addition the signal cables (which are the bulk of the cable volume) pass the N.E.C CL2 test. The total cable volume per panel is (large panel: 0.12 m3, small panel: 0.1 m3). There is polyethylene tubing for gas distribution (MSDS included). The total polyethylene volume per panel is (large panel: 8000 cm3, small panel: 2000 cm3). There is a foam tape (MSDS included). The total foam tape volume per panel is (large panel: 16000 cm3, small panel: 8000 cm3). There are small volumes (<1000 cm3 per panel) of other miscellaneous tapes: kapton, double-sided, vinyl and fiberglass tapes (MSDSs included for foam, double-sided, and fiberglass tapes). There are a few cm3 per panel of epoxy (RHODORSIL Resin #991; MSDS included) for gas sealing at the Iarocci tubes. There are several dozen cable ties per panel. There are a few plastic connectors on the panel edges to connect electrical services to the outside world (for signal, HV and power; large panels: 8, 2 and 8, small panels: 5, 1, and 4). The signal and power connectors have a UL 94V-0 flammability rating. Product specifications are included for all connectors. There are printed circuit cards for HV distribution and signal amplification. We have not included product specifications for the various components. We have included MSDSs for two possible HV potting compounds (G.E. Silicone's RTV 12 and Glyptal 1201B). The RTV is a two-component silicone rubber (RTV12A is the rubber, RTV12C is the curing compound). If we go with RTV12 the total volume per panel is (large panel: 4500 cm3, small panel: 2250 cm3). If we go with Glyptal 1201B the total volume per panel is (large panel: 180 cm3, small panel: 90 cm3).

DX Magnet Access

The MuID steel overlaps the DX magnet, but the panels do not violate the boundaries of the hole in the MuID steel that was designed to allow for access to the vacuum pumps for maintenance and removal if necessary. This is shown in figure 4 below.

Figure 4. This figure shows three of the muon identifier panels hanging in a gap (all gaps are identical). The six panels in a gap hang in two parallel rails attached to the MuID steel. This figure shows panels A,C and E, all of which hang in a single rail. The green square at the center is the cutout to allow access to the DX magnet. Note that the panels are contained within the MuID steel envelope.

MuID Installation

The MuID panels will be constructed in a factory on the BNL site (building 905). Bill Stokes or Jacques Negrin will summarize a finite element analysis that they performed to validate the strength of the panels themselves and the unistrut rails. The panels will be transported from the factory to the Major Facility Hall (building 1008). At this point they will be installed in the permanent support structure which will be attached to the MuID steel absorber.

There are unistrut rails that are critical to the operation of both a specialized lifting fixture and the permanent support structure. Figure 5 shows a closeup of one of the support rails in the permanent support structure and gives details on the pull-out reinforcements and can be followed along with the text description. These rails are made of standard Unistrut Product P5501 which is two back-to-back pieces of 15/8 inch sections. There are side plates attached to the rails to further increase the safety factor on pull-out. The unistrut rails are attached to a supporting I-beam (in the case of the lifting fixture) or square tubes (in the case of the permanent support structure) with unistrut nuts in the top column of the rail. Panels have cam followers on their top bars that allow them to roll into the bottom column of the rail. (The top of the bottom panels are connected to an aluminum bar with the necessary cam followers with 5 meter steel tubes. This allows the bottom and top panels to be supported in the same rails.) There are stops in the rails to prevent panels from rolling out. The top column of the lifting beam's unistrut rail is filled with a piece that acts like a key and which fits into a lock formed by the top column of the mating rail in the permanent support structure. This ensures alignment and continuity. Analyses of the lifting beam and permanent support structure are included in the hardcopy of this document. These analyses were performed with a factor of three safety margin from the yield point.

The MuID panels are only self-supporting when in a vertical position and supported from their top rail. When they are not in such a position they must be locked into a "strongback" - a self-supporting frame. Together the panels and strongback will weigh less than 5000 pounds. Supported in such a strongback, the panels will be transported from the factory to the major facility hall in a horizontal position on top of a standard ball-hitch flat trailer. Once inside 1008 the panel and strongback will be lifted together by the main crane (15/20/40 ton) using an eye hook welded into the strongback. Together they will then be placed on a vertical dolly which will hold the strongback/panel combination with pins inserted into the strongback. The above-described lifting fixture will then be rolled onto the panel where it is held with the main crane. The panel will then be released from the strongback (which will still be secured to the vertical dolly) and lifted by the main crane via the lifting fixture into position aside the unistrut rail in the permanent support structure. The mating unistrut rails of the lifting fixture and permanent support structure are locked together and their level is verified. At this point the panel will be rolled from the lifting fixture into the permanent support structure, and the lifting fixture will be returned to the floor.

Figure 6 shows a rotated full-length view of the panels supported in the permanent support structure. Figure 7 shows a closeup of the supported panels. These are slightly dated figures, so Gap 6 is shown even though this gap will not be instrumented.

Figure 5. This figure shows the design details of the reinforced unistrut rails that hold the panels in their final positions.

Figure 6. This figure shows a rotated view of the entire height of one arm.

Figure 7. This figure shows details of the attachment of the panels to the MuID steel and the placement of chambers in the gaps. Three different positions for the steel are shown: 1) where it was supposed to go, 2) where it was actually installed in the north arm, and 3) where it was actually installed in the south arm (the width shown indicates the smallest and largest z-excursions). For gaps 2 and 6, the small panels are shown, for all other gaps the large panels are shown. Gap 6 will not actually be instrumented.

MuID Gas System

We will use a mixture of Isobutane/Carbon Dioxide with a maximum isobutane concentration of 50%. Gas will vented into the atmosphere - there will not be a recirculating system. The total gas volume of the detector is 50 m3, although the gas volume of the largest individual panel is <1 m3. Normal operation will require a maximum flow rate of 50 m3 / day (35 l/min), assuring one volume gas exchange per day. The panels will operate at pressures very slightly above atmospheric pressure (~10 mbar). They can withstand a 60 mbar overpressure without inflating/deforming too much (a safety factor of 6). The gas system will be designed (with bellows, etc.) to track ambient pressure. These may appear to be small pressure differences but the volume (25 cubic meters / arm) is very large

While the gas mixing system has yet to be designed, it is understood that this will reside outside the collision hall. The gas distribution system requires that gas is presented in a manifold at the edge of a panel and then distributed in parallel using polyflow tubing to groups of 24 tubes each. (This gives 12 gas circuits for a large panel and 6 for a small panel.)

Leak tests were performed on tubes from the selected tube vendor. After proper tightening of the gas connectors at the edge of the tubes a chain of 22 tubes was pressurized to 30 cm H2O and attached to a manometer. A pressure drop of 1.7 cm was observed over a period of 29 hours. This represents a leak rate of approximately 1.6 cm3/hr/tube.

The tubes will be pneumatically leaktested at the manufacturer by placing them under a foot of water and demanding that there be no more than two bubbles of gas per minute leaking out of the tubes. We estimate this to be a leak rate of 0.6 cm3/hr/tube. After the tubes arrive at BNL, but before they are installed in the panels, the tubes will be pneumatically leaktested with a manometer system similar to that used for the leak test described above. Each gas chain (including all internal distribution) will be similarly leaktested before the panels are installed in the major facility hall. We can make this pneumatic leaktest arbitrarily sensitive by observing the pressure loss over an arbitrarily long time. But by demanding less than a 1 cm pressure drop over 16 hours we limit any leaks to be less than 1.6 cm3/hr/tube, or 400 cm3/hr/panel (0.016 ft3/hr/panel).

As recommended by the tube manufacturer, gas connectors on the tubes will be sealed with RHODORSIL Resin #991 in order to prevent leaks from developing over time (in a manner similar to teflon tape).

The panel frames serve as secondary gas containment vessels. The panel edges are designed to allow for a constant purge of the secondary containment vessel with room air at a rate sufficient to remove accumulated residual gas and render it non-flammable. Although the purging system has not been designed, the concept is to feed the purged volume into gas sensors that will detect the presence of chamber gas in the purged volume.

MuID Electronics

Summary of Electrical Hazards

HV: 5000 V with currents software limited to 100 uA/line.

Stored Energy (on HV capacitors): 1.5 J/ HV line.

Power: +/- 12V with 0.2 A/line.

High Voltage

The MuID high voltage system feeds an identical partitioning as the gas system to both muon arms (i.e., ~24 tubes per control). The operating voltage is 5000 Volts and the expected current is <1 micro amp per tube. A LeCroy HV supply will be used that provides a maximum of 6000 Volts per channel at a maximum current of 100 micro amp. The latter will be limited by software control at the supply. 400 MOhm resistors on each tube will passively limit the current to 12.5 micro amp per tube. External HV cables have not been chosen yet. We know they will be multiconductor cables made from appropriate HV corona resistant wires. The HV cables enter the panel through isolated feedthroughs (2 per large panel, 1 per small panel; AMP 15 kV Seven-Pin Metal Shell Circular Connector (862004-1)). Internal HV distribution will be through corona-resistant AWG 20 HV wire (Rowe Industries R1800-1520). This wire passes the VW-1 vertical flame test. The HV lines will be secured to the mid-plane with occasional cable ties. They will be physically isolated from signal and power lines both inside and outside the panel. Product specifications for wire and connectors are included.

Power and Fusing

There are in-panel amplifiers (described below) that will require low-voltage (+/- 12 V) power supplies. The power system feeds an identical partitioning as the gas and HV systems. Large panels will have 6 lines (small panels 3 lines) at each voltage, each of which will carry 200 mA. Power on each line will be fused at the supply. The power supplies have not been chosen, nor has the distribution to the panel. Power cables enter the panel through isolated feedthroughs (AMP 206061 and associated pins/sockets and cable clamps). Internal power distribution will be via four-conductor AWG 22 cable (Dearborn 882204) secured to the mid-plane with occasional cable ties. The four conductors carry +/-12V and grounds from each supply. The wires pass the VW-1 vertical flame test and the connectors have a UL 94V-0 flammability rating. Product specifications for wire and connectors are included.

Signal Cables

Signal cables will go out to front-end electronics via multiconductor shielded twist-and-flat cable (Belden 9M28334). The design of the front-end electronics is only just beginning, so they will not be addressed here. The cables exit the panel through an isolated feedthrough (3M 4634-7300). Internal distribution of signals will be through the same Belden cable secured to the mid-plane with occasional cable ties. The signal cable passes both the VW-1 Vertical Flame Test and the N.E.C. CL2 test. The connector has a UL 94V-0 flammability rating. Product specifications for cable and connectors are included.

Test Pulser

There is a test pulser capability built into the in-panel amplifiers. This requires 2 lemo connectors per large panel and 1 Lemo connector (HGW.00.250.CTLP) per small panel; and less than 10 meters of RG174 cable (Alpha 9174; passes VW-1 vertical flame test). In addition, large panels require 2 (small panels 1) additional power connectors, as described above.

In-panel Amplifiers/HV distribution

There are printed circuit cards for high-voltage distribution and signal amplification mounted on the panel's mid-plane. Figure 8 shows a schematic of the board layout. Figure 9 shows the mounting scheme. Click here to see a closeup of one corner of the panel showing how the preamps attach to the Iarocci tubes, where the cables are, etc. Click here to see the in-panel HV/amplifier circuit diagram.

Each card services six pairs of Iarocci tubes - several cards will be jumpered together to form HV and power busses to service the dozens of pairs of tubes in a panel. The jumpers are short (few inch) pieces of insulated wires that connect solder pads on the distribution busses of two different cards. The HV jumpers are made from the same HV wire as the rest of the internal HV distribution. Other jumpers are made from AWG 22 hookup wire (Alpha 1551; passes VW-1 vertical flame test).

The HV distribution portion of the board will be laid out according to the Institute for Interconnecting and Packaging Electronic Circuits specification #IPC-D-275 which defines the required spacing for DC voltage differences between conductors on pc boards to be 0.00012 inch/Volt. HV is delivered from the cards to the Iarocci tubes by way of a flying lead (a 6" jumper made from the same HV wire as the rest of the internal HV distribution) connected to the mating European HV pin connector. The signals are read from the anode wire, capacitively decoupled. The solder joints connecting the jumpers and the flying leads to the card (all pads, no plated-through holes), along with all other HV components, will be potted in an isolating compound. Although we have not chosen an isolating compound at this moment, two look promising: GE Silicone's RTV-12 and Glyptal 1201B. This potting is more for protection of the electronics than for protection of people because the HV side of the board will be mounted facing the aluminum mid-plane, so that all HV components are thoroughly inaccessible. Furthermore, we do not anticipate needing to operate a chamber without its coverplates which further isolate the amplifiers behind an aluminum wall.

There are a maximum of 26 channels on an HV chain, each of which has two 2.2 nF HV capacitors plus the tube capacitance of roughly 0.5 nF. At 5000 V, this adds up to a maximum stored energy in each line of 1.7 J. This is significantly below the threshold of 10 J, but to prevent unnecessary shocks the HV inputs will have a 1 GOhm bleeder resistor to ground in parallel with the Iarocci tubes. This effectively short-circuits the caps, discharging them with a time constant of 7.5 sec after the HV supply is shut off.

Figure 8. Layout of the MuID amplifier/HV distribution card showing board-to-board jumpering and the flying leads. HV and power/amplification components are shown to be on opposite sides of the board.

Figure 9. Mounting scheme for the MuID amplifier/HV distribution card. The solder joints (all pads, no plated-through holes) connecting the jumpers and "flying leads" along with all other HV components will be potted in an isolating compound. This side of the board is then mounted facing the aluminum mid-plane, so that all components are thoroughly inaccessible.

External Cable Plant

Since the external cables have not been finalized, the external cable plant cannot be finalized. But, we can get a good idea of the sizes involved. We assume for this purpose that the internal and external cables will be identical. Each arm has a total of 210 signal cables (0.41" diameter each), 50 HV cables (~0.4" diameter each) and 150 power cables (0.18" diameter each). Front-end electronics and power/HV supplies will likely be located along the west wall of the Major Facility Hall and to get there will require cable lengths of 5-20 meters depending on the panel.

Grounding

All panel grounds will be isolated from the aluminum box which is electrically attached to the MuID steel through the rails from which the panels are hanging. Each multiconductor HV cable has one AWG 20 ground wire. Each multiconductor power cable has 2 AWG 22 ground wires. Each signal cable has 2 AWG 28 ground wires. HV, signal and power ground wires will all be connected at the panel. The HV ground wire will be connected to earth ground at the HV supply. Power grounds will not be connected to earth ground at the supply and signal grounds will not be attached to the FEM grounds.

Cooling

Cooling is not an issue for the panels - the amplifiers on a panel side will dissipate 11 Watts; their area is 4 ft2. External cooling requirements are not addressed.

Enclosures