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 any of the standard analyzer crystals.

            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 will be under computer control and can be selected and interchanged by the experimenter without requiring any mechanical changes or user intervention.  A separate diffraction detector is provided in front of the analyzer for Bragg peak measurements, and a series of 11 detectors 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

• PG filter in reactor beam, remotely insertable and tunable.
• Double focusing PG(002) (d=3.35416 Å) or Cu(220) (d=1.273 Å) monochromator crystals
• Flat PG or horizontally focused PG (13 blades, 2 cm wide each)
• Built-in guide fields, and insertable spin rotators, He³ polarizers
• Monochromator take-off angles 2q_M from 16 to 75 degrees
• Variable incident energy from 5.0 to 500 meV
• Scattering angles from 0 to 120 degrees
• Söller slit collimation of 10', 25', 50', and open at each position.
• Radial collimators are available before and after the analyzer.
• Computer controlled vertical or horizontal guide field at sample position.

 

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.

 

Fluxes

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, Double Focus, 1.8 ´10⁸ n/cm²/s
14.7 meV, Open'-50' (PG filter) 2.4 ´10⁷n/cm²/s

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

 

Cu(220)

100 meV, Open'-50',  2.0 ´10⁷n/cm²/s 

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

---

Main Page  •   Design Specifications   •   Polarized Beam Operation

Last modified 21-February-2008