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Three Axis instrument for Low Energy Spectrometry

ThALES is the replacement for IN14. It will implement the results of technical progress of the last two decades and will be fully optimized to address present scientific problems especially in the field of quantum magnetism and the physics of highly correlated electron systems.

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Characteristics

Thales in short

reactor hall, cold guide H53

18.84 m from the cold source

monochromator


Crystal

dhkl(Å)

Focusing

polarised

Heusler (111)

3.437

horizontally

unpolarised

PG (002)

3.355

horizontally


Si (111)

3.135


analyser

polarised

Heusler (111)

3.437

horizontally

unpolarised

PG (002)

3.355

horizontally and vertically

detectors

standard three-axis setup

He tube

multiplexed secondary spectrometer

to be defined

The ThALES instrument will be situated at the end of the cold neutron guide H53 at a distance of 19.0 m from the horizontal cold source of ILL high-flux reactor. It comprises the primary spectrometer, with the incident beam casemates and the monochromator shielding, and the secondary spectrometer, with the sample table and the analyzer-detector assemblies. The sample table, analyzer and detector move around the monochromator shielding drum within the experimental zone.

Primary spectrometer

To achieve an unprecedented sensitivity in studies of weak signals, the primary spectrometer of ThALES will offer high incident monochromatic flux at sample position, free of contamination by high-order reflections and parasitic scattering from the monochromator and its environment. To this end a velocity selector (2), acting as a broadband filter suppressing neutrons with wavelength of λ/2, λ/3 etc. will be placed at the exit of the neutron guide in the casemate. The incident beam casemate as well as the monochromator will be enclosed in a massive shielding consisting of a sandwich of lead (300 mm) and high-density polyethylene (HDPE, 600 mm), optimized to provide not only an efficient biological protection, but also to minimize background counts in the experimental data. The choice of shielding material is dictated by the requirement of a nonmagnetic environment in experiment area.
For experiments requiring a high momentum resolution the monochromator curvature can be reduced both in the horizontal and in the vertical plane (down to flat positions) and the elliptically tapered guide end-section can be replaced by a straight m = 1 guide section to provide an incoming beam-divergence smaller 60’. To this end a three-stage lifting system is foreseen in the casemate, with the third position being reserved for an eventual future pre-polarizing guide section.

ThALES is expected to be used both for traditional TAS investigations of the inelastic response and its evolution with external parameters at a selected point of reciprocal space and for mapping of extended areas of the reciprocal space in search of inelastic signal. To provide optimum conditions for both types of experiments ThALES will provide two interchangeable analyzer-detector systems:

  • conventional single analyzer/detector unit for classical three-axis experiments
  • multiplexed analyzer/detector system for mapping inelastic response over larger phase space areas


IN14 - Classical secondary spectrometer with the sample table (1), the analyzer unit (2) and the housing of the single tube detector (3).

Thales – Multiplexed secondary spectrometer with the sample table (1), the housing of the multiplexed analyzer-detector unit (2).

Polarized neutron investigations

The demand in beam time for cold neutron polarized spectroscopy on IN14 increases steadily - from 10% in 2006 to 30% in the first half of 2009 - but is still limited by neutron count-rate losses of about an order of magnitude related to the lower efficiency of devices used to produce polarized neutron beams and to analyze the polarization of the scattered beam. ThALES will provide an important step forward by employing a state of the art doubly focusing polarizing monochromator and analyzer (Heusler alloy, analogous to those successfully used on IN20). Moreover there will be a possibility to employ the newly developed wide-angle polarized 3He-filter in connection with the FlatCone multianalyzer option. In both cases the luminosity of the polarization analysis set-up of ThALES should be comparable to that of the present unpolarized IN14 and, hence, make the polarization analysis technique accessible to a wide class of experiments.