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Operational Notes

The new instrument features the choice of either a Cu(220) or PG(002) doubly-focusing monochromator, providing a continuous incident neutron energy range from 5 to 500 meV.  The 400 cm2 reflecting area for each monochromator yields as much as an order-of-magnitude gain of neutrons onto the sample compared with the old thermal triple-axis spectrometers.  The reactor beam and post monochromatic beam elements offer a wide range of choices to optimize the resolution and intensity of the instrument, with available fluxes well into the 108 n/cm2/s range.

            The sample stage of the instrument includes two coaxial rotary tables, one for sample rotation and one for the independent rotation of magnetic field coils, and a computer controlled sample goniometer and elevator.  He3 cells are available to provide full polarization capability with both monochromators and PG analyzer.

            The BT-7 spectrometer is designed to be used with interchangeable customized analyzer/detection systems supported on air pads.  The first new analyzer system has a multi-strip PG(002) analyzer array that can be used in a horizontally focused mode, or in a flat configuration either with a linear position-sensitive detector or with conventional Söller collimators.  All options are be under computer control and can be selected and interchanged by the experimenter without requiring any mechanical changes or user intervention inside the analyzer system.  PLEASE DO NOT TOUCH ANYTHING INSIDE THE ANALYZER--SOME PARTS ARE EXTREMELY DELICATE AND YOU CAN CAUSE CATASTROPHIC DAMAGE!  A separate diffraction detector is provided in front of the analyzer for Bragg peak measurements, and a series of 11 detectors are imbedded in the shielding behind the analyzer to continuously monitor the neutron flux entering the analyzer system.  These detectors can also be used for measurements of the instantaneous correlation function, for example, or with a radial collimator to determine a diffraction pattern over a limited angular range.  The PSD can also be used with a radial collimator to determine the diffraction pattern or instantaneous correlation function, with higher resolution. 

General Specifications for BT-7

The overall dimensions of the instrument are:

Source to Monochromator:                488 cm

Monochromator to Sample:               206 cm

Sample to Analyzer:                            Variable, from 165 cm to 229 cm

Analyzer (center) to detector:            35 cm



Wavelength contaminations in the incident beam using the PG monochromator with no filter in the reactor beam (variable incident energy).

Monitor Correction factor for the PG monochromator.  This correction factor is available in DAVE as a data reduction choice.This correction factor is to be used only if the velocity slector is NOT employed.  When the velocity selector is used, typically there is no need for a correction.  However, at the lowest energies a small correction can be applied if needed, and those details are in the velocity selector webpage.



Some neutron fluxes measured at the sample position are given below:

PG(002) monochromator

40 meV, Open'-50' (no filter), 1.0 ×108 n/cm2/s

40 meV, Open'-80' (no filter), 1.4 ×108 n/cm2/s

40 meV, Double Focus, 1.8 ×108 n/cm2/s

14.7 meV, Open'-50' (PG filter) 2.4 ×107n/cm2/s
14.7 meV, Open'-80' (PG filter) 3.4 ×107n/cm2/s

13.7 meV, Double Focus (PG filter), 4.6 × 107n/cm2/s



100 meV, Open'-50',  2.0 ×107n/cm2/s 

50 meV, double focus, 6.1 ×107n/cm2/s 

BT-7 Main Page  •   Design Specifications      Polarized Beam Operation

Last modified 28-July-2018 by website owner: NCNR (attn: Jeff Lynn)