LAD Specifications

LAD Specifications

The LAD detector banks are positioned in the horizontal plane at seven different scattering angles which range from 5° to 150°. The structure factor S(Q) can thus be measured over a wide range of momentum transfer Q, which enables structural information of high real-space resolution to be extracted. The wide range of angles allows measurements to be made successfully for samples containing elements with nuclear resonances.

The detector arrangement on the two sides of the instrument is the same and thus the two 90° banks enable S(Q) for a planar sample mounted at 45° to the incident beam to be determined simultaneously for scattering vector Q parallel and perpendicular to the plane of the sample.

The backward angle detector banks have a high Q-resolution which can be of particular advantage when studying amorphous samples of uncertain quality.

A transmission monitor enables the wavelength-dependent total cross-section of the sample to be studied simultaneously with the differential cross-section.

The instrument includes a large amount of shielding between detectors, thus minimising backgrounds from sample environment equipment such as furnaces.

Typical results are given for vitreous germania, showing the distinct scattering i(Q) and its corresponding correlation function T(r).

Specification

Incident Wavelength	0.25-6.5Å
Minimum momentum transfer Q	Minimum achievable is 0.25Å^-1, but in practice the minimum useful value is usually in the range 0.3-0.8Å^-1.
Maximum momentum transfer Q	Maximum useful value depends on sample, but values in the range 30-50Å^-1 are typical.

Detector Bank	Scattering Angle	Total Flight Path	Resolution	Maximum d-spacing	Detector Type
1	4.937°	11.047m	14%	~80Å	10 atm ³He
2	9.704°	11.033m	7%	~40Å	10 atm ³He
3	20.562°	11.040m	3.5%	~17Å	Li glass scintillator
4	35.183°	11.043m	1.8%	~11Å	Li glass scintillator
5	58.103°	11.046m	1.2%	~7Å	Li glass scintillator
6	90.131°	11.039m	0.8%	4.8Å	Li glass scintillator
7	148.164°	11.128m	0.6%	3.7Å	10 atm ³He
Table 1 LAD detector bank parameters (incident flight path L₁=10.0m)

Instrument

Beam	S7
Moderator	100K CH₄
Sample Position	10m from the moderator (L₁)
Beam Size	40mm high, 20mm wide, reducible by apertures
Count rate	5×10³ (5°) - 7×10⁴ (150°)
Sample environment	The sample tank is smaller than the ISIS standard
Data acquisition	Vaxstation 3200

A comprehensive data analysis suite (the 'ATLAS' suite) is available to reduce the data and apply full corrections so as to yield a final single S(Q) structure factor curve.

References

W.S.Howells, 'A Diffractometer for Liquid and Amorphous Materials at the SNS', RAL Report RAL-80-017 (1980).
A.K.Soper, W.S.Howells and A.C.Hannon, 'ATLAS - Analysis of Time-of-Flight Diffraction Data from Liquid and Amorphous Samples', RAL Report RAL-89-046. (1989).
A.C.Hannon, W.S.Howells and A.K.Soper, 'ATLAS: A Suite of Programs for the Analysis of Time-of-flight Neutron Diffraction Data from Liquid and Amorphous Samples', in: Neutron Scattering Data Analysis 1990, ed. M.W.Johnson, IOP Conf. Series 107(1990)193.

Instrument Scientists

Alex Hannon, Spencer Howells

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LAD Page

Last Updated 15 Aug 2007