Documents and Manuals

Quokka manual

This manual describes the characteristics and the use of Quokka, the small-angle neutron scattering instrument at ANSTO. It provides links to various pieces of information. See also the QUOKKA Short Manual in the cabin.

Contents of this manual

1. Instrument features
2. Technical data
3. Instrument control, data acquisition and reduction
3.1. SICS
3.2. GUMTREE
3.3. Data reduction using NIST Igor Macros 
4. Neutron velocity selector
5. Attenuators and source apertures
6. Collimation
7. Multidetector
8. Sample position and sample environments
9. Monitor
10. Printing

1. Instrument features

The main features of Quokka are:

• High brilliance of the cold source, CG1
• High performance selector (peak transmission 94%)
• Large (960 x 960 mm) detector of 5 x 5 mm pixels
• High dynamic q-range (in single configuration typical qmax/qmin of ca. 10 in central and 15 in offset position)

Quokka is the small-angle neutron scattering (SANS) facility at OPAL. It has a peak flux at sample position of 4 x 107 cm2s-1. This is due to a brilliant cold source, a velocity selector of short rotor length (25 cm) and high transmission, the relatively large beam cross-section of 50 x 50 mm of the guide CG1, and with a shortest guide-to-sample distance ("collimation length") of 1 m. The available wavelength range is < 4.5 to > 40 Å.

QUOKKA possesses the largest individual area detector of all small-angle scattering instruments (active area 960 x 960 mm), with a pixel size of 5 x 5 mm, i.e. 38K resolution elements. The detector can be moved laterally by up to 450 mm inside an evacuated tube of 2.5 m diameter and 20 m length. This results in a high dynamic q-range (in single configuration) of typically qmax/qmin of ca. 10 in central and 15 in offset positions. This dynamic range can be extended considerably in the absence of a beamstop for sufficiently weakly scattering samples.

2. Technical data

• Cold guide, CG1.
• Wavelength selection: 2 x Astrium velocity selectors, max. speed 28300 rpm (standard resolution) and 21000 rpm respectively (high resolution) (see below)
• Incident wavelength: < 4.5 Å to > 40 Å (3.0 < λ < 12.0 nm, selector angle = ±8.5 deg.)
• Wavelength resolution: 10% for selector angle = 0 deg (standard); 6.4 % (high resolution)
• Collimation: 9 removable sections of 50 x 50 mm cross section
• Source-to-sample distances: 1, 2, 4, 6, 8, 10, 12, 14, 16, 20 m
• Maximum flux at sample position is 4 x 10⁷ n cm2 s^-1
• Detector: 2-dimensional 3He detector with 192 x 192 pixels of 5 x 5 mm
• Sample-to-detector distances: 1.3 < L < 20.1 m
• Horizontal detector offset: < 450 mm
• q-range (standard): 4x10^-3 Å^-1 < q < 0.7 Å-1  (with largest beamstop), qmin =  6 x 10^-4 Å^-1 with focusing lens optics
• Sample environment: compatible with QUOKKA

o automatic, temperature-controlled sample changer, 10 or 20 positions which may be thermostated
in a range -15 to 300°C.  Sample cells are provided for good thermal contact of samples in a variety
of physical states. “Instructions for use and cleaning of Hellma cells are available here”
o electromagnets up to 1.1 Tesla
o cryostats (1.8 to 300 K)
o Couette shear cell/rheometer.
o Rapid Heat and Quench Cell
o Stopped Flow Mixing Cell
o Eulerian Cradles 

A complete listing of sample environments and their specifications is available here.
 

3. Instrument control, data acquisition and reduction

3.1. SICS

SICS is responsible for the instrument control system coordinates control of devices including:

• Motion control
• Sample environment
• Data acquisition (histogram memory)
• Data archiving in the NeXus format

A manual for SICS may be found here.

3.2. Gumtree

Gumtree contains a client for the instrument control program SICS, a view to the histogram memory server, NeXus file reading, and a data reduction library and user interface. Gumtree allows the user to set multi-sample workflows automating most routine operation of the instrument. Though your instrument scientist may advise you of the functioning see also the QUOKKA Short Manual here.

As part of your experiment, you should liaise with your contact as to the q range you require (if you know) and how featured the scattering is expected to be. The former impacts on the wavelength selected and how many configurations are required while the latter has a bearing on the resolution needed.
 

Figure 4 These screens allow you to define, monitor and/or alter the instrument configuration.  The LHS screen is where preset configurations may be loaded.  Your local contact will provide you with configurations, and this would be preferred mode of interaction with the instrument.  The middle screen allows you to view and alter some configurational items during the experiment.  The RHS screen is an overview (dashboard) of important configuration parameters during the experiment.

As part of the experiment, your local contact will select configurations containing the following parameters: (i) collimation distance (ii) lateral detector position; (iii) transverse detector position; (iv) source aperture; (v) sample aperture; (vi) beamstop size; (vii) attenuator for transmission (and possibly scattering) measurements.

3.3. Data reduction using NIST Igor Macros

Users may visualize their data, reduce to an absolute scale of intensity and perform data analysis and modeling using Igor Pro macros based on those of NIST Center for Neutron Research (Gaithersburg, USA) and modified to accept Quokka data. There is a functioning copy of Igor Pro for data reduction in the instrument cabin. Information and the appropriate files will be provided to reduce the data . Users are requested to reference the paper by Steve Kline describing the software (Kline J. Appl. Cryst. 2003).

Igor Pro is propriety software published by Wavemetrics Inc. It is possible, but not desirable to download a 30 days demonstration copy of the program for data reduction. More details on using demo version of Igor are provided in the Reduction and Analysis help files.

4. Neutron velocity selector

One of two Astrium velocity selectors is used to determine the distribution of neutron wavelengths reaching the sample. The incident neutron beam is defined by an average wavelength, λ, and a distribution, Δ λ / λ.  The value of the wavelength depends on the rotation speed and on the tilt angle of the selector around its vertical axis.

- 4.5 to 12.0 Å (10600 to 28300 RPM)
- 16.5 to 25.5 Å (5000 to 7700 RPM)
- 36.4 to 41.1 Å (4625 to 5300 RPM)

Allowed values of the wavelength for the high resolution velocity selector (NVS043) at 0° tilt are:

- 4.6 to 10.6 Å (9700 to 21400 RPM)
- 12.9 to 19.9 Å (5000 to 7700 RPM)
- 28.3 to 41.1 Å (3100 to 3500 RPM)

Values which fall outside these ranges are blocked and forbidden by the velocity selector software.

*Note that the velocity selector are exchanged every 6 months for maintenance. Users do not have the option of selecting the velocity selector for thier experiment.

For users wishing to conduct experiments at non-standard wavelengths or at non-zero tilt angle, we advise you to make contact with the instrument scientist well ahead of your experiment. The scattering from a sample of silver behenate (AgBe) is used to calibrate the wavelength. A measurement time of the order of 200 s is normally sufficient.

Velocity selector program
The neutron velocity selector (NVS) is controlled by a remote PC (opposite side of the guide hall from instrument cabin).  A window with control screen of the NVS is on the instrument control PC.  A screen shot of the control is shown below.  It is possible to monitor the correct functioning of the velocity selectors, identify potential faults, identify forbidden values and control the velocity selector from this window.  The standard wavelength used on Quokka is 5 Åwithout lenses and 8.9 Å with lenses.

5. Attenuators, source and sample apertures

There are 12 attenuators available on QUOKKA, these are mounted on a wheel (fabricated from Plexiglas with graduated thickness, 0 mm to 25 mm thickness).

During normal operation the attenuator wheel is controlled automatically, to provide maximum flux for measurements, while protecting the area detector from excess count rates (both local and global) and to conduct transmission measurements.  It may be possible to shorten the automated attenuation selection in the interests of optimizing the experimental collection time in consultation with the instrument scientist.  Values of the attenuation are shown for each thickness of Plexiglas and 5 Å neutrons

- 1; 0°, 0cm, attenuation factor 1
- 2; 30°, 0.13 cm, attenuation factor 0.494
- 3; 60°, 0.33 cm, ,attenuation factor 0.173
- 4; 90°, 0.49 cm, ,attenuation factor 0.0730
- 5; 120°, 0.64 cm,  attenuation factor 0.0339
- 6; 150°,0.82 cm, attenuation factor 0.0124
- 7; 180°, 0.97 cm, attenuation factor 0.00548
- 8; 210°, 1.12 cm, attenuation factor 0.00243
- 9; 240°, 1.30 cm, attenuation factor 8.34 x 10^-4
- 10; 270°, 1.50 cm, attenuation factor 3.14 x 10^-4
- 11; 300°, 1.80 cm, attenuation factor 6.18 x 10^-5
- 12; 330°, 2.50 cm, attenuation factor 1.12 x 10^-6

Remotely controlled entrance slits

In front of the neutron guide and after the beam monitor and attenuator, there is a remotely controlled beam-defining aperture selected as part of the experimental configuration:

- 1; 0°, circular 5 mm
- 2; 30°, circular 10 mm
- 3; 60°, circular 20 mm
- 4; 90°, circular 30 mm
- 5; 120°, circular 40 mm
- 6; 150°, circular 50 mm
- 7; 180°, square 50 x 50 mm (effectively open)
- 8; 210°, open
- 9; 240°, blocked beam position
- 10; 270°, 2.5 mm slit
- 11; 300°, 5 mm slit
- 12; 330°, 10 mm slit

There are 13 sample apertures (1 mm Cd) - motor apx – under computer control:

- 1; 0 mm ,  circular 2.5 mm
- 2; -23 mm, circular 5 mm

- 3; -47 mm, circular 7.5 mm
- 4; -72 mm, circular 10 mm
- 5; -98 mm, circular 12.5 mm
- 6; -125 mm, circular 15 mm
- 7; -153 mm, circular 17.5 mm
- 8; -183 mm, circular 20 mm
- 9; -215 mm, circular 25 mm
- 10; -250 mm, circular 30 mm

There is an opportunity to use further source-sample aperture combinations as required for specific sample geometries e.g. tangential scattering in Couette shear conditions.

6. Collimation

The incoming beam on QUOKKA is defined by a collimation system consisting of 10 sections on translation tables. The first two sections form a coupled pair (numbered 1 below).  Each section has 5 positions, the last being empty. With the insertion of each neutron guide into position, the flux on sample increases but with corresponding broadening in angular resolution. With no guides in position (i.e. maximum angular resolution setting), the approximate distance to the sample position is 20 m. The precise distance will be determined by the position of the sample stage (on an position encoded rail) and the sample relative to the sample stage centre.

Collimation Section (TABLE)
 

* Approximate distance to standard sample position if guide is inserted.
The lens system is used at 8.9 Å and is intended to enable more efficient access to lower qmin with pinhole geometry. Discuss this option with your instrument contact if you believe that this would be relevant. For further information ref. Choi et al, J. Appl. Cryst. 2000.
 

7. Multidetector

QUOKKA is equipped with Ordela 21000N High Count-Rate Two-Dimensional Neutron Counter a 192 x 192 pixel 3He multidetector (pixel size 5 mm x 5 mm), running at a high voltage of + 2250 V.

Dead time

To first-order, dead-time effects can be corrected assuming that the whole detector behaves like a single detector with a dead time of 2.14 µs. Corrected counts per cell are obtained by
I(x,y,true) = I(x,y,measured) / (1 - n'*tau)
where n' is the observed overall detector count rate and tau the detector dead time.

Beamstops

Six different circular beamstops are available on QUOKKA:

  1. 100 mm
  2. 80 mm
  3. 60 mm
  4. 40 mm
  5. 20 mm
  6. 11 mm

The beam stop number can be chosen by the selbs command with co-ordinates defined by bsx and bsz . The beam-stop movement is independent of the lateral detector position. These values will have already been selected by your local contact. Alignment is normally achieved by placing a strong isotropic scatterer in the sample position (e.g. Teflon) under attenuated beam conditions. Under no circumstances should you change these values without first contacting the instrument scientist.

8. Sample position

Figure 11 A schematic representation of the QUOKKA sample position which may accommodate and position a variety of sample environments while minimizing air scatter and optimizing the scattering angle. The sample area is equipped with a sample table that has an electrical lifting device (z-direction), translation in and out of the page direction (x-direction) and  translation along the direction of neutron beam (y-direction).

9. Monitor

A beam monitor is situated immediately after the velocity selector. You can collect data either per unit time or, monitor counts. Collecting using monitor counts will enable data to be normalized with respect to any changes in reactor power or cold source temperature during your measurements.

10. Logbook and Printing

We request that you record experimental details in the quokka log book. This assists us and will also assist you if you need us to check any details post-experiment. At the start of each experiment the local contact will fill in a sheet containing the specifics of your instrument configuration. There is a printer in the quokka cabin as well as scissors and sticky tape. It would be appreciated if you could paste/tape in print outs of instrument configurations as well as notes about experimental conditions and observations. You should copy this information before you leave. There is a photocopier and scanner near the entrance to the guide hall where you can either make hard copies or send files electronically. If in doubt, ask your local contact.

References:

Choi, S.-M. et al, "Focusing cold neutrons with multiple biconcave lenses for small-angle neutron scattering¨ J. Appl. Cryst.
(2000) 33, 793.

Crystallogr. (1993) 26, 180.

Gilbert, E.P. et al., "Quokka" - the Small-Angle Neutron Scattering Instrument at OPAL.", Physica B (2006) 385-386, 1180.

Huang, T.C. et al., "X-ray Powder Diffraction Analysis of Silver Behenate, a Possible Low-Angle Diffraction Standard" J. Appl.
 

Kline, S. R. "Reduction and Analysis of SANS and USANS Data using Igor Pro", J. Appl. Cryst. (2006) 39, 895.