DRAFT SANE Gas Cherenkov General Operation and Safety Manual

Transcription

1 DRAFT SANE Gas Cherenkov General Operation and Safety Manual W. Armstrong, E. Kaczanowicz, A. Luhkanin, Z.-E. Meziani, Brad Sawatzky Temple University Abstract This document is written to describe the design and the basic operation of the gas Cherenkov detector built for the SANE experiment in Hall C. We also point out the safety issues and their mitigation in order to prepare any potential user of this device in Hall C. The Cherenkov detector is designed as an electron detector and pion rejector. This version is written in preparation for the SANE testing run in early April

2 Contents 1 Introduction Design Considerations Detector Overview Tank Construction Removable Side Panels Front Snout Photomultiplier Cans Cherenkov Stand Gas Flushing and Filling system Gas System Safety Issues Controller Electronics and Over Pressurization Extreme Over Pressurization Mitigation Lifting and Surveying 9 5 Optics Mirrors Mirror Characterization Mirror Mounts Alignment Target Magnetic Field Compensation Photo-multipliers and Electronics PMT/ mu metal Holder Summary 11 2

3 1 Introduction The threshold gas Cherenkov counter is an integral part of the SANE experiment detector package known as Beta in Hall C. It s primary role is twofold, to identify scattered electrons and to reject pions during the experiment. Nitrogen is the selected radiator gas to fill the detector volume while C02 is an optional radiator that might used if the scintillation becomes a large source of background during the experiment. The detector contains eight mirrors and eight photo-multipliers. These are mounted on the side of the detector tank and the side furthest from the incident beamline. The PMTs are shielded magnetically and a separate shielding from direct view of the target is provided to considerably reduce the background events originating from the target area. Figure 1: SANE Cherenkov. Drawing by E. Kaczanowicz 1.1 Design Considerations The principle requirements for the Čerenkov counter are to provide high efficiency (greater than 90%) for electron detection while maintaining a pion rejection factor of at least 1,000:1. A gas Čerenkov is the logical choice because the low areal density minimizes the probability of δ-rays from π and e scattering. 3

4 Firstly, operation at about atmospheric pressure is assumed in order to simplify the mechanical design and minimize the windows thicknesses. The choice of radiator gas is a trade-off between many parameters. The reference design assumes the use of dry N 2 gas. At 20 C, the index of refraction n of N 2 is approximately , yielding a β threshold for Čerenkov light emission by pions of β threshold = 1 n = , which corresponds to a momentum threshold for pions of 5.9 GeV/c. Pions above this momentum threshold should be extremely rare with 6 GeV beam and would be removed in any case with our software electron definition, >3-5 photoelectrons. This cut will also suppress low energy δ rays and virtually extinguish expected scintillation backgrounds from all non-electron charged particle species. Desiccants will be emplaced in the radiator box, it will be flushed with dry N 2 to remove contaminants, and then hermetically sealed with a slight overpressure. A thin front window of aluminum painted in black provides a light-tight seal. An interior polymer window will provide a gas-tight seal capable of withstanding normally variations in barometric pressure (± 1 of water equivalent). After an extensive detailed optical ray-trace analysis (see Fig. 2) which included among other things the effect of the target magnetic field on the scattered electron trajectories, the shape of the mirrors were determined. The optimized result called for eight, roughly 40 cm by 40 cm glass mirrors arranged in two overlapping columns of four mirrors to cover the rather large acceptance shown in Fig 3. Four spherical mirrors cover one column at large scattering angle and four elliptical mirrors cover another column at small scattering angle. Light from each mirror is focussed onto individual 3 quartz window photomultipliers (Photonis XP4318B) without the use of a Winston cone. The 3 size helps reduce background signals generated on the photomultiplier. It also provides for more space to shield from the magnetic field of the target magnet with thicker µ-metal jackets. All phototubes are located on the large scattering angle side of the BET A detector in order to minimize the background from the target and the beam line (see Fig 3). Efficient shielding of the phototubes in this configuration is possible. This design is different from the preliminary proposal design which considered instead 5 photomutipliers and had the photomultipliers mounted symmetrically on each side of the Čerenkov tank. Glass mirrors are preferred over plastic for their long term stability, they should maintain their curvature without creep and are insusceptible to crazing. The electrons of interest are above 0.7 GeV/c and are deflected by the target field less than a few degrees. Thus, to a good approximation, the mirrors have been designed for point-to-point focusing from the target cell to the photomultiplier photocathodes. This permits the two towers of mirrors to be optimally aligned with a small, bright light bulb located at the same target-mirror distance. This 4

5 top view back view side view Figure 2: Optics simulation to determine the shape and radii of curvatures of the mirrors. geometry also permits good rejection of stray light from scintillation and low energy δ rays (which are preferentially emitted at angles several times larger than the Čerenkov cone). 1.2 Detector Overview The tank s frame is constructed of welded and leak checked two inch square aluminum tubing. Attached to the front of the frame is the snout which narrows down to the front window frame holder. Attached to this is a thin piece of aluminum which is completely opaque, painted black, and framed by an aluminum bracket. The side panels of the frame are detachable, as is the rear window, which is made of 1/16 inch aluminum sheet and framed by a large bracket. Neoprene gaskets and o-rings are used when attaching or sealing any panels of flanges. 5

6 Figure 3: Left: three dimensional open view of the Čerenkov counter with its mirrors and phototubes. Right: open side view of the counter with its wheeled base. By sliding the front part of the tank access to the mirrors is possible even when the BigCal detector and hodoscopes are in place. Figure 4: SANE Cherenkov. Drawing by E. Kaczanowicz 2 Tank Construction 2.1 Removable Side Panels The aluminum side panels are removable and held in place by machine screws. When attaching the panels for pressurization of the tank, these screws must be torqued to about in lbs. It is important that the rubber gasket be properly aligned before mounting the panels as the gasket becomes useless once it is torn. Making use of a few set screws and silicon grease makes this process bit easier. The panels are identical therefore they were marked in the upper left hand corner as viewed from the outside. There is a similar marking on the upper left hand corner of the frame so that the identification can be done consistently. 6

7 2.2 Front Snout The front snout held in place much like the removable panels and marked in the corner for consistency as well. The front window of the snout is removable and uses two gaskets and an aluminum frame to attach the desired window. The front window was chosen to eliminate light leakage and to minimize knock-on electrons. Care must be taken when working near the front window as it is quite thin and can be punctured easily. However, puncturing the front window presents no safety hazard to workers, just a slight puff of air. Before beginning work near the front window, always make sure that the photomultiplier s HV supply is turned off. This is done to protect the photomultipliers photocathode should the front window get punctured. Also after turning off the gas supply replace the window. 2.3 Photomultiplier Cans The photomultiplier Cans each have two BNC and one SHV feed through. Connected to the circular back plate are three ultra torr fittings which feed through the PMT adjustment rods. A few inches of translation are built into the rod, however, care must be taken when initially placing the back plate and PMT such that the preferential angle of rotation is not blocked by the threaded support rods. That is one should be aware of the way one might want to tilt the PMT and thus rotate the back plate to accommodate. 2.4 Cherenkov Stand The tank is accompanied by a stand which allows for translation and rotation. Vertical displacement can be obtained using the screw jacks between the lower and upper stand plates. The tank sits on two four inch square tubing the front of which nests a smaller tube that is held to the bottom by a pin. Care must be taken when lowering the tank onto the stand such that the holes align. A guide pin is used to make the alignment easier. Once on the stand the guide pin can be replaced by a bolt and the four, one inch threaded rods can be attached to the square tubes. These rods can be adjusted to provide translation and rotation. 3 Gas Flushing and Filling system After prforming maintenance work inside the tank and thus exposing it to room environment, the volume must be flushed with dry nitrogen. This can be done two ways, automatically or manually. On the controller panel there is an auto/manual switch. The automatic switch allows the microprocessor based controller to take 7

8 over. Ideally, there are two modes for the automatic operation, flushing and Maritain, which must be written to the driver from the controlling computer. This computer is connected via RS-232. In the flush mode, the vent valve is fixed open and the fill valve opens while the pressure is under a set limit. The electromechanically controlled solenoid vent valve is mounted at the base of the tank and would be used as the primary relief during an over pressure condition in the tank. Furthermore, based on trials at Temple University, the average delivery flow rate of Nitrogen while maintaining a differential pressure of 10 torr is less than 1 Standard Cubic Foot per Hour (SCFH). For an ideal hermetic and perfectly sealed tank this rate would be zero, and therefore this rate is also referred to as a leak rate. This is the rate that gas will enter the Cerenkov Tank when the manometer calls for more gas and automatically opens the fill solenoid valve to maintain set point pressure. 3.1 Gas System Safety Issues Controller Electronics and Over Pressurization When programming the controller, one must take care in choosing an operating pressure setting. Due to atmospheric fluctuations, the differential pressure varies slowly and thus problems can occur long after programming. The suggested setting is 775 torr. In case of depressurization due to overpressure the likely location of failure is the front or back windows because of their small thickness. Due to the low pressures of the tank there should be no issue of safety for those working around the tank, except for the possibility of extreme over pressure, in which case the valves described below will take effect. Furthermore, the solenoid valves are powered by standard 110VAC so care must be taken when connecting, disconnecting, and working around the valves Extreme Over Pressurization Mitigation Two Circle Seal 500 series pop-off valves are installed on the tank, one near the top, one near the bottom. Set at 0.5 to 1 psig cracking pressure according to manufacturers charts each will allow.08 SCFM (x 60 minutes will give a gas relief capability 4.8 SCFH. Total 2 x 4.8 = 9.6 SCFH). One alone should have plenty of capability to keep the tank from over pressurizing. These valves of course do not depend on any external connections and reseal when the overpressure condition is relieved. Because the gas supply is coming from the SOS additional safeguards may be in place - TBD. 8

9 4 Lifting and Surveying Ideally the final alignment of the tank optics will take place on the Hall floor. Before closing of the tank it will be situated on our adjustable tank stand (not the JLab platform). The optical alignment source assembly (with bulb mimicking slightly greater target location for the test) will be temporarily mounted on the front of the tank. In that configuration the surveyors map the combined tank, pseudo-target (the bulb) and tank stand off tooling balls located on each part. This should include the vertical adjustments of our tank stand to the required height of the JLab platform to match the beam line height. This could be done days (or weeks in advance). The tank is then lifted off the stand and set aside. The empty stand would be lifted onto the JLab platform. At that time the surveyors would locate the stand alone on the platform. The platform would be drilled and stand would be bolted to the platform. The tank is then be lifted and reattached to the stand at its previously determined position. Surveyors would verify location and any necessary adjustments are be made. 5 Optics 5.1 Mirrors The mirrors a specially coated with aluminum and MgF2 for maximum reflectivity down into the far UV to compliment the of the photomultiplier tube at these wavelengths. The mirrors, while not mounted inside the tank should be stored in two plexiglas crates which is then flushed with dry nitrogen. The desicant should monitored inside the volume to ensure a dry environment. When handling the mirrors, always use appropriate gloves. Furthermore, while mounting the mirrors inside the cherenkov tank, work in pairs. One person is responsible for holding the mirror in place while the other attaches the two thumb screws that holds the mirror channel to the mirror mount Mirror Characterization The mirrors are marked in the upper left hand corner while facing the reflective side. The mirrors were characterized and documented. More information can be found on the SANE wiki. 9

10 5.1.2 Mirror Mounts The mirror mounts are used to hold the mirror in the appropriate location and to make small adjustments during the alignment procedure. The mount attaches to a curved aluminum U-channel. The U-channel is attached to the mirror prior to placing the mirror inside the tank. When attaching the U-channel to the mirror, first remove the neoprene rubber channel insert from the aluminum channel. Carefully attach the rubber to the side mirror that will be used for mounting. Next carefully slide it into the aluminum channel. Make sure that the brass strips are properly positioned between the walls of the aluminum channel and the rubber such that the thumb screws do not make contact with the rubber. Tighten the thumb screws along the front and back of the channel. Do not over tighten the thumb screws. The U-channel attaches to the mount via two thumbscrews. As previously mentioned, be sure to work in pairs when placing the mirror inside the cherenkov tank. It is best to familiarize yourself with the mirror mounting apparatus prior to handling the mirrors. When adjusting the mirrors do not use the collars to fix the final position of the mirror. Also do not overtighten the small allen screws on the collars. Use the larger 3/16 allen wrench screws to do the real work of holding the mount in place. The collars with the finger attachment should be used for alignement only. Once the alignment is complete, securly tighten the 3/16 screws. 5.2 Alignment The alignment of the mirrors can be done using the light bulb at the pseudo-target position. An apparatus is provided which attaches to the front of the tank a light source. Depending on which mirrors are being aligned, it should be mounted to the top of the front window frame or the bottom. Using the mirror mount adjustments described in the previous section, imaging the target on the face of the tube becomes trivial once the mirrors are mounted. Make sure, when looking at the mirrors from the front, that the entire solid angle is covered by mirrors. That is ensure that there are no gaps between mirrors Target Magnetic Field Compensation With a high target magnetic field (5 Tesla at the center of the target), the scattered electrons tend to bend from straight tragectories. This causes the method of alignment described in the previous section to become just a reference for particles at very high momenta (5 GeV and greater) less effective. However, using the simulation software GEANT4 one can maximize the detection efficiency for particle 10

11 with momentum above 0.7 GeV/c. Depending on the desired minimum momentum acceptance, the mirrors can be slightly adjusted to compensate for the effect of the magnetic field on the trajectories. This can be done by determining through simulation where the image should fall on the tube once the magnetic field is turned off. The adjustment should only be significant for low momentum electrons. For simulation information please contact Whitney Armstrong or consult the SANE wiki. 6 Photo-multipliers and Electronics 6.1 PMT/ mu metal Holder The photomultiplier tubes (Photonis XP4318B) are held to the mu-metal shield by a PMT holder that grabs the tube and the mumetal. The holder delicately attaches to the PMT and uses compressed O-rings to attach the mumetal. These attachements should be done ahead of time to ensure the proper mounting of the PMT sheilding. Also it is important not to over tighten any screws as this could damage the PMT. 7 Summary 11