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Neutron Choppers 
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Chopper Maintenance Facility located in the Target Building. Allows for maintenance and testing of multiple chopper systems.
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The primary mission of the neutron chopper activity is to provide functional, reliable, safe and operationally proven neutron chopper systems as required by the SNS Instrument Beam Lines.
Chopper Functional Classes
Neutron choppers are rotating mechanical devices designed to block the neutron beam for some fraction of each revolution of the chopper. At least three different functional classes of neutron choppers are envisioned. Most, if not all, of these shall be designed in standard forms that are interchangeable among the instruments. Most instruments will have one or more neutron choppers, and often several will be operational on the same instrument to optimize the operating conditions for particular experiments.
T0 Choppers - These are designed to block the fast neutrons from the prompt pulse. These 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 sub-multiple of the source frequency and be kept in phase with the source.
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POWGEN T0 chopper installed in beamline.
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Bandwidth-limiting Choppers - These 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 also must typically rotate at some multiple or sub-multiple of the source frequency and be kept in phase with the source.
E0 Choppers - These 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 sub-multiple of the source frequency and be kept very precisely in phase with the source.
Sophisticated control systems are required to keep each of these choppers rotating at its particular desired frequency and in phase with the source.
Chopper Activities
The SNS neutron chopper activity includes the following elements:
- Development of performance specifications.
- Construction, either as an in-house activity or by procurement, of chopper systems.
- Testing to assure 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.
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POWGEN T0 chopper under going testing inside secondary confinement chamber with scatter shield installed.
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Accomplishments
The SNS neutron chopper accomplishments to date include the following:
- Chopper Test Facility (CTF)
The SNS Chopper Test Facility (CTF) has been developed in the ANL Bldg. 365 Ring Building. The CTF includes areas for chopper assembly and testing along with the associated control, instrumentation, and predictive maintenance systems.
- T0 Chopper Prototype No. 1 (TOP1)
The initial 60 Hz T0 chopper, TOP1, has been designed, constructed, and tested at the CTF. TOP1 has been installed and is operating in the ANL IPNS GPPD Instrument.
- T0 Chopper Prototype No. 2 (TOP2)
The second 60 Hz T0 chopper, TOP2, has been designed and constructed, and is now under test at the CTF. TOP2 will be employed for the engineering and evaluation of T0 options that include 120 Hz operation and alternate drive systems.
- Fermi Chopper Prototype No. 1 (EOP1)
A prototype Fermi Chopper, EOP1, has been procured and is under test at the CTF. It is planned to install and operate the EOP1 in the ANL IPNS LRMECS or HRMECS instruments.
- 60 Hz Disk Choppers
The procurement of 60 Hz disk choppers is underway. The initial two of the first complement of nine systems will be configured for operation at the ORNL NOTS Facility. Delivery to the CTF is planned for the summer of 2002.
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Fermi choppers undergoing testing in the CMF.
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Chopper Team contacts
Scientific and technical contributors in the chopper activity include the following:
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