Neutron guide task force
1.1 At the meeting
Phil Bentley, Ken Andersen, Werner Schweika, Feri Mezei, ESS; Jochen Stahn, Uwe Filges, PSI; Kim Lefmann, Kaspar Klenø, Henrik Jacobsen, Uni Cph; Jan Saroun, NPI; Nicolo Violini, FZJ; Klaus Lieutenant, Carolin Zendler, Katarina Rolfs, Leo Cussen, HZB; Peter Willendrup, DTU;
Mads Bertelsen, Uni Cph;
For the instruments at the long-pulsed ESS, long neutron guides are essential. Although it has been shown that different types of ballistic guides (parabolic, elliptic) can transport neutrons very efficiently over long distances, it is still not clear which geometries are in practice preferable. We therefore set out a task force to study the implications of real-life constraints on different guide systems for a number of particular instrument requirements (wavelength band, divergence).
The protocol below is the agreed outcome of the task force workshop in Berlin, 17th-18th April, 2012.
This wiki serves to collect and present definitions and results for the guide designs from the task force.
Mailinglist and calendar are accessible through the Infrastructure page.
4 Agreed Scheme for Presentation of the Results
- Graph Type 0) Instrument Layout
- Graph Type 1) Four 1-D plots, divergence and position distributions, integrated along the other dimensions, will be presented as brilliance transfer.
- Graph Type 2) Four acceptance diagrams. In the 2-D plots (pos-pos, pos-div 1, pos-div 2, div-div).
- Graph Type 3) A 1-D plot: Integrated brilliance transfer as a function of wavelength.
The integration boundaries of the parameter space will be defined in each instrument case.
For graph types 1 and 2, wavelength range will be covered in pseudo-monochromatic snapshots of 0.1 Å rather than white beams over the full bandwidth. The particular wavelength chosen will be defined in each instrument case.
5 Instrument Definitions & Design Criteria
5.1 Powder Diffraction
Beam size at sample: height=20mm x width=6mm. Divergence: 0.3 deg horizontal and ? degrees vertical FWHM, approximately gaussian distribution independent of wavelength. length: 6m to PS chopper, then 150m from PS chopper to sample
5.1.2 Other Criteria
Wavelength spectrum: 0.3-6Å. If using numerical optimiser, weight wavelengths as 1/lambda^0.5 in order to properly emphasise the short wavelengths. Uniform divergence profile when averaging over the full beam size or the central part (10x3mm2) of the beam.
5.1.3 Results Required
Wavelength snapshots are centred on 0.3, 0.5, 2, 6, each snapshot is 0.1 Å wide. Integration box size for position plus minus 10 mm vertical, plus minus 3 mm horizontal. Integration box plus minus 0.15 degrees.
Solution by Kaspar Hewitt Klenø: Powder_diffractometer_Kaspar
Solution by Kaspar Hewitt Klenø, where guide has been curved to avoid LoS: Powder_diffractometer_curved_Kaspar
5.2 Protein crystallography (Solved, but results need uploading)
Beam size at sample: 1mm x 1mm. Divergence: +/-0.1deg square distribution independent of wavelength length: 6m to PS chopper, then 150m from PS chopper to sample
5.2.2 Other criteria
wavelength spectrum: 1.5-3.3Å, with uniform weighting No direct line of sight, good rejection of shorter wavelengths, small beam at sample (for background) specific question: can we keep guide costs low by having m=1 for most of the length?
5.2.3 Results Required
Wavelength snapshots are centred on 1.5, 2.4, 3.3, each snapshot is 0.1 Å wide. Integration box size for position plus minus 1 mm vertical and horizontal. Integration box plus minus 0.1 degrees.
Solution by Kaspar Hewitt Klenø: Protein_crystallography_Kaspar
A simple straight, curved guide is sufficient for this instrument concept. A single ellipse was not competitive with the straight concepts.
5.3 Cold chopper spectrometer
beam size at sample: height=50mm x width=30mm divergence: +/-0.4deg square distribution independent of wavelength length: 6m to PS chopper, then 150m from PS chopper to sample
5.3.2 Other criteria
wavelength spectrum: 2-10Å, with uniform weighting no direct line of sight, good rejection of shorter wavelengths, less than 10% intensity variation over beam profile
5.3.3 Results Required
Wavelength snapshots are centred on 2, 4, 6, 10 Å, each snapshot is 0.1 Å wide. Integration box size for position plus minus 5 mm vertical and horizontal. Integration box plus minus 0.4 degrees.
Possible solutions presented at the meeting in june: Cold Chopper suggestion by Mads Bertelsen
5.4 Further Work
5.4.1 Action Groups
- Ken Andersen prepares another instrument to work on to replace solved protein crystallography. Around 2-3 options will be presented to the group at the next meeting in June, and one will be selected as a problem to work on.
- Phil Bentley & Ken Andersen feed back information and refine parameters on the other two instruments. New instrument problem circulated in next week or so.
- Phil Bentley, Ken Andersen, Klaus Lieutenant work on resolution functions and optimisation figures of merit.
- Berlin group investigate where the cutoff is located in source size (between 1 and 12 cm) where gravity provides an observable effect on the vertical neutron guide performance for elliptic guides.
- Copenhagen group to investigate when a feeder is needed for an eye of the needle solution.
- Uwe Filges and the PSI group begins a study of the background attenuation offered by eye of the needle solutions for the chopper spectrometer concept studied so far. “High energy neutrons” we define as > 0.5 eV for this purpose.