The 92-inch Chamber

Air stops beams of nuclei, so experiments have to be done in vacuum. The 92-inch chamber is a steel vessel with strong walls and powerful pumps to create that vacuum, so the particles may travel—and be studied—unencumbered. The chamber is a tank big enough to hold very large arrays of particle detectors.

Expanded Description

The 92-inch scattering chamber is designed to house complex detector arrays in vacuum. It is a large cylinder —92 inches in diameter and 120 inches in length —mounted on its side. The front end of the vessel is stationary. The rear part can be rolled back on the support frame to open up the chamber for the installation of equipment. There are ports for windows and feedthroughs for electronic connections. The chamber holds moveable mounts for detectors, and it contains a target mechanism for changing targets without opening the chamber. The target mechanism includes a small “ladder” that will hold seven targets—metal foils about one inch on a side—in a column, one above another. A remote-controlled motor can shift the height of the ladder up or down to move any one of the seven foils into the path of the beam from the cyclotrons.

To enable an experimenter to use the floor space occupied by the chamber for an independent apparatus, the base plate and internal mechanisms are removable. With the moveable part of the chamber rolled back, a large area becomes available for setup; the only parts which must remain are the front quarter of the chamber and the support rails.

Technical Information

The 92” Scattering chamber is designed to house complex detector arrays in vacuum, with two independent rotation mounts and a target ladder. The back of the chamber and mounts can be removed to use the location for a general experimental endstation when self-contained chambers are used.

Status: Operational

Location: N2 Vault

Contact person: Dave Sanderson

Reference: The N3 Vault—A General Purpose User Station; D. P. Sanderson, NSCL Annual Report 1989, p 198. (PDF 222 kByte).

Technical Detail

The vacuum vessel is a large, 234 cm diameter by 304.8 cm long stainless steel right circular cylinder mounted on its side with its main flange in the vertical plane. When installing equipment, the rear 3/4 of the chamber is rolled back on the support frame. There are 17 small ports and 8 large ports for windows and feedthroughs. The chamber has vacuum feedthroughs for high-voltage target bias, 9 different gas or liquid lines, 40 SHV cables, and about 200 BNC signal lines. The number of signal cables between the chamber and the Data- U is 200. A 6-way cross on the front of the chamber is available for beam diagnostic devices or alternative target mechanisms.

The rough pumping of the chamber is accomplished by a belt-driven 90 m³/hr two-stage mechanical vacuum pump coupled to a 500 m³/hr roots blower. They are used to lower the chamber vacuum to 150 mTorr. At this level, two turbo pumps rated at 2000 liters/sec each for air and a liquid nitrogen cold trap take over. With an empty chamber, this system can pump the chamber to 150 mTorr in 40 minutes, to 0.01 mTorr in 60 additional minutes.

The detector mounting platform is a square plate, 2.3 m² in area, resting on two rails cantilevered through bellows from the outside so that detector positioning is independent of vacuum loading. The mounting plate is approximately 71.4 cm below the beamline and contains a rectangular grid of 1/4-20 threaded inserts spaced 6 inches apart. The target mechanism is mounted in the center and has capacity for 7 of the laboratory's standard targets. A 152 cm diameter turntable rated for 200 kg is mounted to the baseplate 58.2 cm below beam height with a radial pattern of 1/4-20 threaded inserts spaced every 2 inches and in 10 degree steps. The angular resolution of the arm and the turntable encoders is 0.01 degree, corresponding to a position resolution of 0.03 cm at 76 cm radius. A radial arm rated for 20 kg (without edge supports) sits above the turntable at approximately 46.8 cm below the beam. The arm consists of two parallel optical bench rails extending to the edge of the turntable. These mounts provide a very flexible system for installing movable detector systems. The motion control system operates through a computer in the vault which can be remotely operated from the data-U.

The exit beamline contains a diagnostic chamber for mounting a scintillator and a large Faraday cup. To protect the detectors in the chamber from neutrons scattered at backward angles toward the chamber, the Faraday cup is surrounded by 46 cm of steel and further upstream, a stack of concrete blocks surrounds the beampipe to shield against neutrons near the beam axis.

For the alignment of detectors and collimators in the chamber, a high precision telescope and mirror system is used. The alignment telescope is set with its optic axis along the beam axis on a removable mount in front of the Faraday cup. A mirror mount on a precision rotary-motion stage is placed at the target position. By setting the mirror at two angles, the telescope can check the position of any forward-angle detector.

To enable the experimenter to use the floor space occupied by the chamber for an independent apparatus, the base plate and internal mechanisms are easily removed and a beamline attached to the entrance cross. With the vacuum chamber rolled back, a large area becomes available for setup. The only parts that must remain are the front 1/4 of the chamber and the support rails.

Figure: Schematic drawing of the base plate, target mechanism, turntable and arm for the 92-inch scattering chamber.