Tigress/ Bambino Scattering Chamber

Douglas Cline, Robert Flight, Andrew Whitbeck

Department of Physics and Astronomy
University of Rochester
Rochester, NY 14627-0171
Cline@pas.rochester/edu

Introduction

The Tigress Scattering Chamber was designed specifically for the Bambino detector system developed by the Bambino collaboration based at the Lawrence Livermore National Laboratory and the University of Rochester. Bambino comprises a charged-particle position-sensitive silicon detector array for use in conjunction with the Tigress tracking γ-ray detector array. The scientific goals are to exploit the combination of Tigress with Bambino to study nuclear structure of neutron-rich radioactive beams produced by the ISAC-2 radioactive beam facility at Triumf. The Bambino chamber was designed and built at Rochester to mount the Bambino detectors built at LLNL by Dr Ching-Yen Wu.  The scattering chamber design is modeled after the Gammasphere scattering chamber which is modular and versatile allowing it to be adaptable to mount other compact particle detectors inside the Tigress inner cavity. The rational behind the design was presented at the Tigress Meeting held at the Colorado School of Mines, Golden CO, on 24-25 February 2006.

The Bambino Scattering Chamber was designed by Doug Cline and Robert Flight while construction and testing was performed by Andrew Whitbeck and Doug Cline The following is a brief description of the scattering chamber and recommended operating procedures.

 Scattering Chamber

The chamber is based on an 8.00 inch diameter aluminum sphere with 0.064 wall thickness. This sphere has the maximum diameter allowed within the inner cavity of Tigress and minimum mass in order to minimize degradation of the γ-ray detection performance of the Tigress array. The chamber design assumes that the Tigress mount has 180o symmetry. The entrance and exit ports to the chamber have the maximum inner diameter consistent with the geometry of the Tigress modules in order to minimize build up of activity from scattered radioactive ions inside the active volume of Tigress. The design allows for use of up to 14 Tigress detector modules at the closest distance. The beam inlet and outlet ports occupy two locations while another two locations are used for the target ladder and electrical feed through plus mount for the charged particle detectors.

The sphere has a side port that subtends 135o of the sphere for access to charged particle detectors etc. The joint between the side port is at an angle of 22.5o to the vertical axis of the chamber which was chosen since the dividing line between individual Tigress modules is at this angle if the target ladder points along the axis of one detector position on the Tigress corona support. This geometry minimizes the influence of γ-ray absorption in the side-port vacuum flange on the performance of Tigress. The chamber was machined using a wire EDM machine. The side port flanges were glued to the sphere using Hysol EA 9309.3NA two-part epoxy which is optimized for gluing aluminum. This bonding agent was successfully used previously for construction of the CHICO heavy-ion detector. Glue rather than electron beam welding was used because of time constraints and lack of access to an electron-beam welding facility.  The aluminum flanges and sphere were treated with a chromic acid etch prior to bonding to enhance bonding with the glue. The beam ports vacuum seals are modeled after those on CHICO and employ O-ring seals. Compression of the O-ring on the outside of the sphere employs a stainless steel inner ring pressing on the inside of the sphere with pressure on the O-ring applied by 8 socket head screws. The chamber was tested to be vacuum tight using a helium leak detector.

 The chamber is provided with a target ladder that has five positions. The ladder can mount four targets, while the fifth position is used with no target being on the beam axis. The target ladder is mounted in a black Delrin support that is designed to handle secondary electron suppression voltages of up to 5kV. However, care must be taken to ensure no sparking damage occurs to the silicon detectors. Experience using CHICO shows that about +3kV positive bias on the target ladder greatly reduces the influence of secondary electron emission produced using particle-nanoamp heavy ion beams. Electron suppression may not be necessary with weak radioactive beams. For safety always remove the bias voltage and cable from the target ladder when handling the target ladder or when the chamber is not at high vacuum.

 Black Delrin beam line insulators are provided both upstream and down stream of the target chamber. The beam line couplings employ the standard V-clamp used at TRIUMF.

 The lower port was designed for flexibility. Currently the bottom flange uses two special purpose vacuum electrical feedthroughs built at LLNL for Bambino. Different flanges holding other custom built electrical feedthroughs probably will be necessary for use with other charged particle detector systems. The lower port also provides the base flange for supporting charged particle detectors. Again, custom designed support flanges can be screwed to this base flange.

 Operating Procedures

 Mounting: 

Great care must to taken not to distort the thin-walled sphere when mounting the scattering chamber. Flexible bellows must be used both upstream and downstream of the scattering chamber to minimize loads and torques on the Bambino exit ports. Kinematic mounts are required to support and align the chamber both near the entrance and exit port bellows and must not apply torques to the chamber beam line ports..

 The side port to the chamber is not screwed down but is held in place by atmospheric pressure. Experience has shown that it is unnecessary to apply any clamping force when pumping down the chamber, the O-ring seals easily when pumped. Also the tight fit of the side port reduces the probability of the port falling off the chamber. However, care still must be taken that the side port does not fall off when not under vacuum. Note that the side port is thin aluminum that can be damaged easily if dropped on the floor. The Bambino strap, that is provided, wraps around the chamber to hold it closed and ensure that the side port does not fall off the chamber. When the chamber is not evacuated you are responsible to ensure that Bambino wears his belt so that he does not drop his port. Note that the orientation of Tigress is such that the target ladder axis should be mounted at 22.5o relative to the vertical. It is recommended that the orientation of the chamber be such that the normal to the side port points 22.5o above the horizontal plane which will ensure that the side port will not fall off.

 Gripping the side port with the fingers is sufficient to pull the side port off the chamber. DO NOT USE A SCREW DRIVER OR OTHER METAL OBJECTS TO PRY OFF THE SIDE PORT, this will destroy the O-ring seal. If you have trouble removing the lid you can attach a piece of duct tape to the lid to pull it off. Machining the chamber cost US $22,000; so handle the chamber with great care to ensure that you are not saddled with an expensive repair bill.  

 Target ladder

The target frames are mounted by loosening the 0-80 screws and sliding the slotted target frame under the screw heads. Then tighten lightly so as not to strip these fine threads. Do not completely remove the screws from the ladder since it is hard to replace them without breaking the target.

 A pin in the upper section of the black Delrin piece is used to locate the target ladder at each target position relative to the beam axis.

 Always attach a cable to the SHV connector on the target ladder when the target chamber is used to either apply a bias voltage, read the target current, or to electrically ground the target.

 The target frames are cut from 0.016 stainless steel. They have a 0.635 ID hole. For beam alignment purposes, target frames with 3mm, 4mm and 5mm ID holes also are provided.

 Lower detector port

Note that the lower port is made of aluminum. It has 10-32 tapped holes for attaching to the detector cone and also for attaching the electrical feedthroughs. Ensure that the 10-32 screws engage at least 0.3 of thread inside the aluminum and do not overtighten to minimize the chance of stripping the thread.

 The current silicon detector mounts were custom built to mount the Bambino detectors. Similar holders can be made to hold other detector geometries. 

 Chamber activation

 Rolled sheets of Kapton can be inserted into the 0.75 long, 1.63 ID of the exit and exit ports to catch scattered radioactivity. This Kapton plus the residual activity can be removed when necessary simplifying decontamination of the scattering chamber.