SEPD - Special Environment Powder Diffractometer
Instrument Scientist - Bob Von Dreele
Scientific Associate - Simine Short
SEPD Web Page
| Beam Line |
F5 |
| Moderator |
100K liquid methane |
| Wavelength Range (frame overlap limit) |
0.5 - 7.55 Å |
| SEPD Characteristics for typical diffraction
data |
| 2-theta |
delta-d/d (FWHM) |
Number of Detectors |
Detector Area (srd) |
dmin(Å) |
dmax(Å) |
| ±145° |
0.0034 |
80 Detectors |
0.172 |
0.33 |
4.02 |
| ±90° |
0.0054 |
40 Detectors |
0.086 |
0.45 |
5.41 |
| ±44° |
0.0098 |
24 Detectors |
0.052 |
0.85 |
10.21 |
| ±22° |
0.047 |
14 Detectors |
0.030 |
1.7 |
20.25 |
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| Beam Size |
1.27 cm wide, 5 cm high |
| Thermal Flux on Sample |
1 x 107 neutrons cm-2 s-1 |
| Detector Angles |
±145, ±90, ±44, ±22 |
| Moderator-to-Sample Distance |
14 meter |
| Scattered Flight Path |
1.5 meter |
| Detectors |
10 atm 3He proportional counters; 1.27 cm dia x 38 cm long |
| Sample Size |
0.05 - 20 grams |
| Typical Data Collection Times |
5 h for 5 g, 24 h for 0.1 g for precision Rietveld refinements |
| Time-Dependent Studies |
useful data in ~5 minutes for large samples in favorable cases |
Sample Environments:
| Displex |
10 K -350 K |
| Three Furnaces |
300 K - 1700 K, one with controlled atmosphere |
| Helium Gas Pressure Cell |
90,000 psi, 40 - 300 K |
| Ten-position Sample Changer |
for room temperature runs |
The SEPD is a time-of-flight powder diffractometer that is optimized for diffraction in special environments, where a 90° scattering angle is often used to allow optimum collimation in order to eliminate scattering from the sample environment. The SEPD and GPPD scattered flight paths are the same, but the incident flight paths are different. Thus, the SEPD has nominally twice the flux at the sample position, but achieves lower resolution in back scattering. However, at the 90° scattering angle, the two instruments have similar resolution. Thus, the SEPD is the instrument of choice for special-environment experiments done at 90° because of its higher count rate. The SEPD has also proven to be an effective instrument for small samples (<100 mg in favorable cases) because of its very low backgrounds. The back-scattering detector banks offer sufficient resolution for general structural problems of modest complexity, where Rietveld refinement is done. For such problems, the fast count rate of the SEPD is often desirable when a number of closely related samples are to be run. When large samples (10-20 g) are used, data collection is fast enough for time-dependent studies on the time scale of minutes. Meaningful Rietveld refinements have been done for 5-minute data sets. The SEPD is also often used for studies of non-crystalline materials and pair-distribution-function studies of the local structure crystalline materials, when higher resolution than that available on GLAD is desired.
Neutron powder diffraction data and Rietveld refinement profile for Tl2Ba2CuO6+x at 0.609 GPa and 60 K (H. Takahashi et al., Physica C 191, 248-254 (1992).
References
(Instrument) J. D. Jorgensen et al., J. Appl. Cryst. 22, 321-333 (1989)
(Pressure Cell)
J. D. Jorgensen et al., Physica C 171, 93-102 (1990)
(Time-dependant)
H. Shaked et al., Phys. Rev. B 51 547-552 (1995)
(Small samples)
H. Shaked et al., Phys. Rev. B 51 11784-11790 (1995)
Y. Shimakawa et al., Phys. Rev. B 50 16008-16014 (1994)
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