THERMAL NEUTRON MODERNIZATION PROGRAM
We are presently in a multi-year safety modernization program for the thermal neutron instruments. The goal is to design the necessary safety features into the new instruments in order to accommodate new radiation safety guidelines, to substantially reduce planned exposures of personnel during normal operation, and dramatically reduce the possibility of an accidental radiation exposure. These features need to be incorporated while developing a suite of modernized instruments that would have the highest scientific and technological impact in the long term. Considerable progress has been made with these plans, and we describe below the overview and current status of the project.
New Instrument Design PhilosophyFor the design of the new instruments, a number of them should be able to utilize similar experimental beam shutters and monochromator drums. Therefore initial design efforts were focused on developing a new standard beam-shutter system that will allow utilization of the full beam tube sizes available at NCNR. This required abandoning the concept of having both shutter and collimation selections that are contained together inside the biological shield, as was the case with the original beam shutters.
External to the main beam shutters, all the beam tubes in the biological shield are identical in design. We have now designed a standard beam tube liner and shutter that will operate inside the biological shield, and will allow a beam as large as 16cm tall and 7.5 cm wide; the detailed beam optics will depend on the particular instrument under consideration. New shutters have been installed at BT-5 and BT-7.
A second common safety problem concerns radiation streaming along the wall at the instrument positions. A permanent face plate has been designed so that the shielding along the wall will be "stepped" in order to suppress this streaming, and it is planned to install such a face plate on all the new instruments.
The final element that is common to a number of instruments is the monochromator drum. With the 16 cm high beams available at NCNR, a monochromator height of 20 cm was chosen as a design parameter. An engineering design for a monochromator drum that will accommodate a beam size of 20 cm in height and 16 cm in width has been completed. Two units have been delivered, for the BT-7 and BT-4 instruments.
Instruments and TimetableTable I gives the description of the neutron scattering instruments in the confinement building. The High Resolution Powder Diffractometer (BT-1) has already been constructed, installed, and is operational. The Residual Stress/Single Crystal Diffractometer (BT-8) has also recently been completed, and both of these instruments are considered finished as far as the project is concerned.
The triple-axis spectrometer is the work-horse for inelastic scattering studies at steady-state neutron sources. Originally we had four triple axis spectrometers in the hall, with various capabilities. The current plan is to replace these with two double-focusing thermal neutron triple-axis spectrometers, one dedicated Be-filter spectrometer (FANS), and one horizontally focused sub-thermal (2-20 meV) triple axis spectrometer. The FANS spectrometer will be positioned at BT-4, while the first of the new thermal triple axis instruments will be placed at the BT-7 beam port. In addition to these inelastic instruments, a Perfect Crystal SANS instrument is located at the BT-5 position. These instruments will be followed by the second of the thermal triple axis instruments, to be located at BT-9, and a Doubly-Focused Cold Neutron Triple Axis Spectrometer at NG-0. During the construction phase of these new instruments it is planned to keep the triple axis spectrometer at BT-2 operational, and a decision about the type of new instrument to be placed at this beam port will be taken at a future date.
Table I. Proposed Neutron Spectrometers
Beam Tube Diameter at Source (cm) Spectrometer BT-1 16 High Resolution Powder Diffractometer BT-2 13 Conventional Polarized Triple Axis BT-3 13 truncated Depth Profiling BT-4 16 Filter Analyzer Neutron Spectrometer BT-5 13 Perfect Crystal SANS (USANS) BT-6 13 BT-7 16 Double-Focusing Thermal Triple Axis BT-8 13 truncated Residual Stress Spectrometer (DARTS) BT-9 16 Double-Focusing Thermal Triple Axis NG-0 17 Double-Focusing Cold Triple Axis
Contact: Jeff Lynn, (301)975-6246, Jeff.Lynn@nist.gov
Last modified 18-August-2003 by website owner: NCNR (attn: Bill Kamitakahara)