Neutron Choppers
The primary mission of the Neutron Chopper Team is to provide functional, reliable, safe, and operationally proven neutron chopper systems as required by the SNS instrument beam lines.
Chopper Maintenance Facility located in the SNS Experiment Hall. This facility allows for maintenance and testing of multiple chopper systems.
Types of Choppers
Neutron choppers are rotating mechanical devices designed to block the neutron beam for some fraction of each revolution of the chopper. Our goal is to have at least three different functional classes of neutron choppers available for user experiments. Most, if not all, of these will be designed in standard forms that are interchangeable among the instruments. Most instruments will have one or more neutron choppers; often several will be operational on the same instrument to optimize operating conditions for a particular experiment.
T0 Choppers
These are designed to block fast neutrons from the prompt pulse. Fast neutrons arrive at the chopper position essentially immediately after the proton pulse strikes the target. To block the fast neutrons, the T0 chopper must be able to place a large mass of high-cross-section material in the neutron beam during the brief time the fast neutrons are present. Such choppers must also rotate sufficiently rapidly to leave the beam completely unblocked when neutrons of the desired wavelengths are traveling through the chopper position. T0 choppers must typically rotate at some multiple or submultiple of the source frequency and be kept in phase with the source.
POWGEN T0 chopper undergoing testing inside a secondary confinement chamber with a scatter shield installed.
POWGEN T0 chopper installed on beam line 11A.
Bandwidth-Limiting Choppers
These choppers are designed to block all neutrons except those in the wavelength bandwidth of interest. For this purpose, these choppers typically require an open angle that can be varied to meet the wavelength requirements of a particular experiment. However, bandwidth-limiting choppers do not need to be designed to block fast neutrons since this is done by the T0 choppers or by the optical design of the neutron beam line. Bandwidth-limiting choppers must also typically rotate at some multiple or submultiple of the source frequency and be kept in phase with the source.
E0 Choppers
These choppers are designed to transmit only a very narrow bandwidth of neutrons. They are used on certain inelastic scattering spectrometers to select the incident neutron energy and/or to define very narrow pulses of neutrons so that the scattered neutron wavelengths or energies can also be determined by time of flight. E0 choppers must typically rotate at some multiple or submultiple of the source frequency and be kept very precisely in phase with the source.
Sophisticated control systems are required to keep each of these types of choppers rotating at its particular desired frequency and in phase with the source.
Activities
Chopper Team activities include the following:
- Development of performance specifications
- Construction, either as an in-house activity or by procurement, of chopper systems
- Testing to ensure that performance specifications are met, along with evaluation of operating modes, controls, reliability, and maintenance
- Development of ancillary systems that include activated cooling water systems, chopper support fixtures, rigging fixtures, and predictive maintenance systems
- Quality assurance
- Safety considerations, analyses, and protective measures implementation
- Documentation
Facilities
Fermi choppers undergoing testing in the Chopper Maintenance Facility
The Chopper Lab is located in Building 8600, the SNS Central Laboratory and Office Building. After delivery, all 60-Hz disk choppers are assembled and tested in the lab. The choppers are then prepped for installation and aligned by the Survey and Alignment Group
The Chopper Maintenance Facility is located in Building 8700, the SNS Experiment Hall, and is where all chopper maintenance is performed. This facility is also used for testing of all high-energy-type choppers to ensure that we can provide adequate personnel protection in the unlikely event of a failure.
Equipment
- Vertical axis-t0 choppers (VA-t0)-magnetic bearings (high-energy chopper)
- Two 180-Hz systems currently installed and in use by two instruments
- One 60-Hz system delivered for installation during the January shutdown of 2010
- One 180-Hz system delivered as a spare
- Horizontal axis-t0 choppers (HA-t0)-mechanical bearings (high-energy chopper)
- 60 Hz
- t0P3 being assembled, will be used as a test bed for HA-t0 chopper bearings, resolvers, speed sensors, etc.
- Two systems currently installed and in use by two instruments
- One system delivered as a spare for the currently installed choppers
- One system delivered for installation during the January shutdown of 2010
- Two systems awarded for manufacture
- Fermi choppers-magnetic bearings (high-energy chopper)
- SNS rotor design
- 600 Hz
- Five currently installed and in use by three instruments
- Mirrotron rotor design
- 300 Hz
- One system delivered for installation during the July shutdown of 2009
- 480 Hz
- One system to be delivered in spring 2010
- Disk choppers (low-energy choppers, with one exception)
- Mechanical bearing
- 60 Hz
- Six systems installed and in use by two instruments
- One system delivered awaiting assembly and testing as a spare system
- Magnetic bearings
- 300-Hz double disk (high-energy chopper)
- One system installed and in use
- 120-Hz single disk
- Two systems installed and in use by one instrument
- 60-Hz single disk
- Eleven systems installed and in use by five instruments
- Five systems installed and ready for use by two instruments
- Four systems delivered and awaiting assembly, testing, and installation on two instruments
- One system delivered, awaiting assembly, and testing as a spare system
- 60-Hz double disk
- Two systems installed and in use by two instruments
