Subsections

• HMS Gas Cerenkov
• SOS Gas Cerenkov
• Møller Vacuum System
• HMS Vacuum System
  • Hazard Mitigation
  
  
• SOS Vacuum System
  • Hazard Mitigation
  
  
• Evacuated Beam Dump Line
  • Hazard Mitigation
  
  
• Working Near Vacuum Windows

Vacuum and Pressure Hazards

The greatest safety concern for the vacuum and/or pressure vessels possibly in use in Hall C are the thin Aluminum or kevlar/mylar windows that close the entrance and/or exit of these vessels. Examples are the HMS and SOS Gas Cerenkov tanks, the HMS and SOS spectrometer vacuum cans, and the Hall C Scattering Chamber. The Hazards associated with the Hall C Scattering Chamber will be dealt with in the Section on the Hall C cryotargets. Note that the HMS Gas Cerenkov can operate both above and below atmospheric pressure. All work on vacuum windows in Hall C must occur under the supervision of appropriately trained Jlab personnel.

HMS Gas Cerenkov

The HMS Cerenkov Detector is a 1.5 m long by 1.5 m diameter cylinder made of 0.5 inch thick aluminum walls with two photomultiplier tube (PMT) ports and a pump port. When in operation, the vessel is evacuated and then filled with a gas, typically CO$_2$, N$_2$, or C$_4$F$_{10}$ at underpressure, or Freon-12 (CCl$_2$F$_2$) at overpressure.

The greatest safety concerns for the Cerenkov Detector are the thin aluminum windows that close off the ends of the tank. These windows must be thin to minimize multiple scattering of the particles we are trying to observe, but thick enough to be structurally safe. The yield strength for the type 2024-T3 aluminum we are using is 50000 psi, and the ultimate strength is 70000 psi. If using the HMS Gas Cerenkov at overpressure (underpressure) 60 (40) mil thick window will be used. According to stress calculations at the maximum overpressure of 35 psig, the load on a 60 mil thick window will be 36770 psi and will cause a center deflection of approximately 3.5 inches. For the 14.7 psi underpressure case, the load on a 40 mil thick window will be 20356 psi with a deflection of 4 inches. Comparing these loads with the yield and ultimate strengths of the material shows that there is a considerable safety margin. In addition, the windows have been hydrostatically preformed to the shape they will assume under stress [1]; this procedure forces the load to be purely tensile, and is known to enhance the strength of the material. Extensive pressure tests of both the tank and the windows were performed prior to installation of the tank in the HMS Detector Hut [5].

SOS Gas Cerenkov

The SOS Gas Cerenkov Counter is a box, approximately 1 cubic meter in volume, made of 0.5 inch thick aluminum walls. There are two end windows each composed of one layer of 0.010 inch lexan (for gas tightness) and a thin layer of tedlar (for light tightness). There are four ports for photomultiplier tubes (PMTs), six holes for gas I/O, and two 0.5 inch thick aluminum access windows. When in operation, the vessel is flushed with nitrogen and filled with Freon-12 at atmospheric pressure.

Because the detector is designed to run at zero overpressure, the bursting of the lexan windows is of minimal concern. The gas system incoporates a large bladder that allows for over and under-pressure relief. The gas pressure of the cerenkov is monitored closely by shift personnel and experts are on call in the event that the pressure deviates by more than 0.025 PSI relative to atmospheric pressure. Calculations show that the stress in the lexan is approximately 800 PSI at a typical overpressure of 0.125 PSI. Given DuPont's failure rating of 8000 PSI for lexan, this seems to be quite adequate.

Møller Vacuum System

The space between the Møller target and the two-arm detectors is evacuated to minimize multiple scattering. At the target the vacuum furthermore isolates the cold coil vessel from room temperature. These volumes represent an implosion hazard. This is especially true at the exit of the two arms in front of the Møller detectors where thin vacuum windows are mounted. The vacuum volume is 1.5 m$^3$ representing a stored energy of 1.5$\times$10$^5$ Joule. The diameter of the exit windows is 20 cm. Experience from HMS window testing is used and applied to the Aluminum windows. Despite this, it is recommended to wear ear protection plugs, which are available at the entrances to the alcove. If working within 3ft. of the vacuum windows the wearing of ear protection plugs is mandatory.

HMS Vacuum System

The space between the magnet poles of each spectrometer is evacuated in order to diminish multiple scattering. The HMS also has vacuum volumes which are used to thermally isolate the cold coils from room temperature. All of these volumes represent an implosion hazard. The hazard is particularly serious at the entrances and exits of the main spectrometer volumes as these are covered by relatively thin vacuum windows. Catastrophic window failure would generate a significant shock wave as air rushed to fill the evacuated volume. It would also cause a loud noise which could cause hearing damage to anyone in the immediate vicinity.

The HMS spectrometer vacuum can has a volume of approximately $6$ m$^3$, representing a stored energy of $6 \times 10^5$ Joules. The exit window is a circle with a center-to-center bolt hole diameter of $40$ inches and a $38$ inch diameter vacuum opening. It is located in the HMS detector hut and is the largest vacuum window required for Hall C. Under vacuum, this window must support 16,785 lbs (74,425 N).

The vacuum window material is composed of 0.020 inch titanium. Bolt holes are cut into the material and it is then mounted on a ring flange using the standard O-ring vacuum seal technique. The ring flange is mounted on the 8" long HMS hut vacuum extension piece. This window has been used successfully in the HMS, holding a vacuum of $\approx 10^{-5}$ Torr.

The HMS entrance window is located near the pivot and has a center-to-center bolt hole diameter of $10.5$ inches. It must support a load of 6200 N (1400 lbs). As the load for these entrance windows is far less than that on the exit windows, the vacuum window material is in this case composed of 0.0045 inch ballistic Kevlar 29 style 713 (31x 31 count, plain weave) with 0.002 inch Mylar on both sides.

Hazard Mitigation

An interlock prevents access to the HMS detector hut unless one inch thick aluminum safety shutter has been moved to its `down' position, such that it covers the vacuum window. This is to prevent people from accidentally touching the large vacuum window. Installation of vacuum windows can only be done by the responsible personnel following detailed instructions in the Hall C Operating Manual [1]. The vacuum windows are replaced periodically to prevent accidents due to structural damage of the window material over time.

SOS Vacuum System

The SOS spectrometer vacuum can has a volume of approximately $2$ m$^3$ representing a stored energy of $2 \times 10^5$ Joules. The entrance window (near the pivot) is round and has a diameter of $9$ inches. It must support a load of 929 lbs. In some cases a small vacuum extension snout is used to minimize the air distance between the scattering chamber vacuum and the SOS spectrometer vacuum. In that case a rectangular entrance window is used with even smaller load. The vacuum window material is composed of 0.0045 inch ballistic Kevlar 29 style 713 (31x 31 count, plain weave) with 0.002 inch Mylar on both sides.

The SOS exit window is the second largest window in Hall C. This window is located in the SOS detector hut. It is rectangular. The opening has a length of $30$ inches and a width of $10$ inches. The SOS exit window must support a load of $4,400$ lbs ($19,687$ N) under vacuum. The vacuum window material is composed of 0.015 inch ballistic Kevlar 29 style 713 (31x 31 count, plain weave) with 0.004 inch Mylar on one side and 0.001 inch thick Mylar on the other. Bolt holes are cut into the material and it is then mounted on a ring flange using the standard O-ring vacuum seal technique.

Catastrophic window failure would generate a significant shock wave as air rushed to fill the evacuated volume. It would also cause a loud noise which could cause hearing damage to anyone in the immediate vicinity. Extensive testing has been performed to minimize the likelihood of such an occurrence [1].

Hazard Mitigation

Installment of vacuum windows can only be done by the responsible personnel (Thia Keppel or Paul Hood) following detailed instructions in the Operation Safety Procedures[1]. The vacuum windows are replaced approximately every 6 months to prevent accidents due to structural damage of the window material over time. The wearing of earplugs is required when accessing the SOS shield house to work closer than 3ft. from the SOS vacuum exit window.

Evacuated Beam Dump Line

The Hall C beam dump line has been changed to an evacuated beam line, although specialized dump lines with Helium may still be used for low-current polarized target experiments. In the evacuated beam dump line configuration there will be no Be window separating the scattering chamber vacuum from this beam dump line. Rather we will use a thin foil with hole to allow for differential vacuum. The end window of the beam dump line will be an Al window, with inner 3.75 inches consisting of 30 mil Be. The beam dump line has been strengthened with braces to prevent collapse during evacuation (the safety factor in the present configuration is more than five). Calculations have shown the Be window to reach a peak temperature of 270 F, even when the unlikely scenario occurs of an unrastered, 180 $\mu$A beam current hitting the window with no target in place. Such a scenario is unlikely, as we will have less than 100 $\mu$A beam currents, and the fast raster is in the accelerator fast shutdown system. The safety factor in the mentioned unlikely situation is still a factor of three. A Be beam diffuser resides at the entrance of the beam dump alcove, to enlarge the beam spot a the beam dump.

Hazard Mitigation

A beam interlock prevents beam delivery with the fast raster off. This interlock is properly checked at the end of a long (installation) down time. In addition, an administrative cureent limit is imposed on beam delivery conditions, as given in the accelerator operational restrictions

(see http://opweb.acc.jlab.org/internal/ops/ops_webpage/restrictions/ops_restrictions.php).

Working Near Vacuum Windows

Before entering the detector huts or pivot area, all personnel should check the spectrometer and/or scattering chamber vacuum gauges. The HMS gauge is located under and near the Q3 quadrupole. The SOS gauge hangs from the carriage beneath the quadrupole. The scattering chamber vacuum gauge is under the pivot point. Please note that if Hall C uses a polarized target special safety rules restrict access to the pivot area (see Section on Polarized Target).

If the spectrometers and/or scattering chamber are under vacuum:

1. Before entering the detector hut or pivot area, put on hearing protection. It is recommended that nobody should be closer than 3 feet from the windows without ear protection and that only those personnel who need to approach the windows be in their immediate vicinity.
2. If entering the pivot area, check both spectrometer windows visually (from a distance greater than 3 feet if possible). If entering the SOS detector hut, check the SOS exit window visually. If entering the HMS detector hut, verify the HMS shutter is really down in front of the vacuum window. If you observe any of the following problems, vacate the area and contact one of the responsible personnel:
   • Visual defects, particularly wrinkles, discoloration, or uneven fiber stress.
   • Date of removal on tag near spectrometer window indicates a date near or after current date.
   • HMS shutter malfunction (troubles opening or closing).
   • The pre-traced ring drawn onto the window perimeter has moved away from the perimeter.
3. Never touch the vacuum windows, neither with your hands nor with tools.
4. Never attempt to bypass the HMS shutter interlock system.
5. Use careful judgement if it is necessary to work near the vacuum windows. Do not place objects so that they may fall on the windows, etc.
6. Do not work near the windows any longer than is absolutely necessary.