Plate 1. Contents of chamber
Plate 2. not ideal, but shows some of the scattering chamber and support stand
Better picture would have:
1) Wheels with feet
2) Chamber closed up
3) Not a bunch of junk from unpacking
Chamber system needs some kind of name for the documentation.
Overview
The Edinburgh Scattering Chamber is designed for charged-particle induced reaction cross section measurements of astrophysical interest.
One of its main features is radial symmetry, compatible with the TRIUMF UK Detector Array (TUDA) which mainly uses the Micron S2-type silicon strip detectors (SSDs) placed downstream of the target.
The geometric similarity between the two setups allows the employment of similar hardware such as preamplifier PCBs, cooling plates, etc.
The arrangement allows for a large solid-angle coverage for light-ion detection (> 50% of 4-pi in the centre-of-mass frame), light ion particle identification by the deltaE-E method.
At present, our interests are aimed at neutron-deficient, medium-mass beams.
In this case and with the use of a thin target, beam-like recoils can also be detected further downstream by an additional S2 detector in coincidence with light ions with a typical efficiency >25% (depending on the precise geometry).
An adjustable target slider can hold up to ten targets (generally thin-film samples like CH_2, collimators, beam diagnostic devices ...), and we have plans to accommodate a windowed gas cell in the future to serve as a helium target. A CMOS pinhole camera is also provided to allow viewing of targets which will mainly be used in conjunction with a ZnS screen for beam tuning.
The end of the chamber is equipped with a detachable Faraday cup to assist with beam tuning and monitoring the beam intensity during experimental runs (depending on the radioactive beam intensity).
Chamber Design
The chamber is welded aluminium construction in the form of 2 barrels connected coaxially with flanges on each end. The barrels and flanges are secured with M14 bolts and sealed with ISO 400 centring O-rings.
The chamber is designed to be connected to the downstream port of the existing Scattering Chamber Experiments (SEC) installed at HIE-ISOLDE beam line XT03 by a KF50 port on the upstream end of the chamber. This connection has a turbo pump mounted on it to ensure that the vacuum exposed to the downstream systems is as low as possible. Further additions to the connection are a constrained bellow to take up any misalignment, an electrical break to prevent ground loops and a gate vale to allow the chamber to be opened without exposing the beam line to atmosphere.
All of this enables us to shoot through the SEC and have a minimal impact on other existing experimental setups.
The chamber is cylindrical and can be opened on the downstream side by sliding the end flange in the beam direction.
The end flange is mounted perpendicular to a sliding support structure, so that the chamber can be opened and serviced without the use of an overhead crane nor any requirements of heavy lifting.
Internally, the majority of the equipment is mounted on four rods installed on the inside of the end flange, which extend the full length of the chamber when sealed are supported by complimentary receiving sockets on the inner side of the upstream chamber end flange.
The chamber and its associated equipment (chiller, electronics racks) are fitted with wheels for easy positioning, enabling the chamber to be connected and disconnected from the XT03 beamline with relative ease.
The support structure is more than twice as long as the chamber itself, so that the chamber can be fully opened and easily serviced. This results in the chamber being stable in in the fore/aft direction but due to the height of the beam line and a preference for a small footprint the top heavy nature of the chamber and support it is potentially unstable in the side to side direction. To alleviate this outriggers are installed on the bottom of the upstream and downstream structure with retractable wheels. The wheels can be fully lowered when the chamber needs to be relocated, and otherwise the wheels are kept installed with minimal floor clearance to provide mechanical support in the beam left or right directions should the need arise (e.g. to prevent accidental tipping).
Targets:
Targets are mounted on NSF CP Type 3 frames and theses are in turn mounted on a ladder with 10 positions (this ladder will also accommodate NBI Type 2 frames). The ladder also has holes to accept 2 Hamamatsu photo diodes for beam tuning.
Targets used are polyethylene foils as a proton target (CH2) and carbon foils for background subtraction. The polyethylene foils are two sandwiched foils of ~150 ug/cm^2 (300 ug/cm^2 total) are custom-manufactured by Paul Morrall of the STFC Daresbury Laboratory, while the carbon foils were bought commercially from Arizona Carbon Foil Co.
A silver/copper doped ZnS target is also provided for beam-spot viewing.
Collimators with various opening apertures (phi 1 mm to 10 mm) are also placed on the target slider to assist during beam tuning.
Detector Mounts
The SSDs and preamplifiers are mounted on opposite sides of one (or more) metallic disk(s), which can then in turn be mounted on the four support rods provided inside the chamber.
Only the detector is located on the front side of the disk, so that it is impossible for the beam to directly strike any of the electronics.
Thus, the printed circuit board, preamplifiers, and cooling plates are located on the beam downstream side of the support disk.
The preamplifiers are housed inside the vacuum chamber during normal operation, as this enables us to reduce the cable path length between the detectors and preamplifiers while allowing the chamber itself to serve as a Faraday cage, which both reduce the electronic noise.
However, electronics in a vacuum have only limited means to cool, thus we use a forced-flow external refrigeration system to cool the preamplifiers. This takes the form of a chiller using a 50/50 mixture of ethanol and water as the heat transfer medium, operated between -5 and -10 degrees when the preamplifiers are under vacuum.
To protect the detectors during beam tuning remote controlled irises are fitted to each detector.
Detectors and photodiode also have provision for degrader foils to be mounted in front of them.
We prepared several Al degrader foils purchased commercially from Goodfellow Inc. All foils were tested to be light-tight prior to and after mounting. All foils are mounted on 1 mm thick Al stock with vacuum-rated epoxy, left to dry for 24 hours for proper curing. Foils prepared for the photodiode have an opening of 1 cm diameter and include 10, 15 and 20 um nominal thicknesses. Foils prepared for the heavy ion detection with Micron S2 DSSDs have an opening of ~72 mm in diameter are 6, 10, 15, and 20 um nominal thicknesses; these foils have a 20 mm diameter gap punched in the center, to avoid being struck directly by (and thus scattering) the beam.
Associated hardware
The chamber is complimented with a vacuum system comprising mainly of a 500l/s turbo pump and dry scroll pump in addition as previously mentioned a small turbo is provided on the beam line connection and shares the scroll pump. Various gauges provide monitoring of the pressure in the chamber from atmosphere down to 10^-6mbar.
2 off 42U 19” racks provide space for amplifiers and diagnostic equipment. The amplification chain consists of:
1) RAL 109 8 channel amplifier and discriminators (24 units) with custom power supplies;
2) Analog signals are sent to Silena ADCs in a VME
3) Discriminator signals are split, one line sent to one of five Logic Shaped Outputs, which feed to logical signal processing in a NIM crate generating the trigger signal for the DAQ
4) The second of the split discriminator signals are sent to a CAEN V767 TDC in the VME crate.
The data are then collected and processed on a Sun Microsystems Tower via an ethernet hub which relays the data via a Silena 9419 ACQ Controller in the VME crate.
Remote control of the irises is performed with a custom control unit powered by a 12V PSU
To provide power to all the systems 3 power strips, 10 way each, are used. Nominally one provides power to pumps and vacuum systems, one preamps and 19" rack equipment, and the last one computers and test equipment. The total load of the system is approximate 15A @ 230V. All power is distributed through BS1363 plugs and distribution boards that are fitted with SEV 1011 plugs for local power compatibility. All equipment categorised as IEC 61140 Class 1 is therefore earthed through their power connections. The chamber is electrically isolated from the beam line by an electrical break and from its frame by nonconductive spacers. The chamber forms part of the electromagnetic noise protection for the detectors so is electrically connected to the electronics.
All equipment has been tested for electrical safety at Edinburgh before sending to CERN.
|