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 technician Bill Jordan recording chopper balance data.
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.
Cold Neutron Chopper Spectrometer high-speed, double-disk 300 Hz chopper system (housing cover removed).
MaNDi beam line bandwidth chopper no 2 of 3.
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.
Fermi Choppers
These choppers, also known as E0 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. Fermi choppers must typically rotate at some multiple or submultiple of the source frequency and be kept very precisely phased 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 Maintenance Facility, located in the SNS Experiment Hall.
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, maintenance, and repair of chopper ancillary systems
- Quality assurance
- Safety considerations, analyses, and protective measures implementation
- Documentation of all testing, maintenance, and repair
- 24/7 operation and support of chopper systems during SNS experiment run cycles
Facilities
The Chopper Maintenance Facility (CMF) is located in the SNS Experiment Hall. The majority of all chopper maintenance and testing is performed in the CMF. By design, choppers are exposed to all energies of neutrons. As a result, special care is taken to ensure that radiation (activation) levels are monitored before and during disassembly and repair of all choppers.
The Chopper Lab is located in the SNS Central Laboratory and Office Building (CLO). This laboratory serves as office space for chopper team personnel as well as a secondary assembly and test area for low speed, non-activated chopper systems.
Equipment
Currently installed and operational on SNS experimental beam lines:
- Vertical axis t-zero choppers–magnetic bearings (high-energy chopper)
- Two 180 Hz systems
- One 60 Hz system
- Horizontal axis t-zero choppers–mechanical bearings (high-energy chopper)
- Four 60 Hz systems
- Fermi choppers–magnetic bearings (high-energy chopper)
- Four 600 Hz systems
- One 480 Hz system
- One 300 Hz system
- Bandwidth-Limiting choppers–mechanical bearings (low-energy chopper)
- Six 60 Hz systems
- Bandwidth-Limiting choppers–magnetic bearings (low-energy chopper)
- Twenty-nine 60 Hz systems
- One double-disk 60 Hz system
- One double-disk 300 Hz system
