Three Axis instrument for Low Energy Spectrometry

Reactor hall, cold guide H53

18.84 m from the cold source

Guide changer for 2 different guide end-sections:


straight section

m=33 m

Exit window: width x height = 6,0 x 12,0 cm2

elliptically tapered section

m=33 m

Exit window: width x height = 2,5 x 12,0 cm2

Velocity selector for higher order filtering


carbon fiber with B4C coating

windows entrance and exit

width x height = ~ 7,0 x 12,0 cm2


up to 26 000 rpm, synchronized with incident energy Ei

Virtual source


horizontal variable 0 to 4 cm, vertical fix 12 cm

Angular range

Monochromator scattering angle 34-133 degrees

Sample scatting angle up to 120 degrees (depending on config.)





Heusler (111)3.437

Variable horizontal, fixed vertical


PG (002)3.355

double focusing

 Si (111)3.135

double focusing



Heusler (111)3.437

Variable horizontal, fixed vertical


PG (002)3.355

double focusing

 Si (111)

double focusing


standard three-axis setup

He tube

FlatCone multiplexed option

Horizontally individual He-tubes for all 31 channels

Options (others than sample environment)


Pg002 filter


Main components

[existing table in yellow-book style]

Flux and resolution

The lay-out of the primary spectrometer respects Rowland geometry, giving best possible energy resolution for focusing Bragg neutron optics. The energy resolution δE is typically about 3% of the incoming neutron energy Ei for all monochromators. Typical values are:

δE = 0.04 (Ei= 2 meV)
δE = 0.16 (Ei= 4 meV)
δE = 0.30 (Ei= 8 meV)

With standard focusing conditions the momentum resolution δ|Q| is about δ|Q| ≈ 0.02 Å-1. If needed, δ|Q| can be considerably enhanced by beam collimation with compromises in neutron flux.

Sample environment

ThALES is compatible with most of the ILL sample environment :

  • Low temperatures Tmin:
    Tmin  ≈ 30 mK
  • Vertical magnetic fields HV:
    HV ≤ 15 T
  • Horizontal magnetic fields HH:
    HV ≤   4 T
  • Hydrostatic pressure (clamp cells) pmax:
    pmax ≤ 20 kbar
  • Hydrostatic pressure (PE-cells) pmax:
  • pmax ≤ 100 kbar


Neutrons are polarized and analyzed with large focusing Heusler monochromators and analyzers.
Polarization analysis can be done either with Helmholtz coils or with the CryoPad (for spherical polarization analysis).

Choice of monochromator and analyzer

PG-002 provides the highest neutron flux.

With the Si-111 monochromator higher incident energies are accessible than with PG-002 (due to smaller d-spacing and because it can be used without velocity selector), but at very small incident or final energies its flux is strongly reduced. It generally gives better signal to noise ratio, in particular at energy transfers lower than 0.5 meV (due to the very clean tails of the elastic line).

Changing the monochromator is automatic, while the analyzer has to be changed manually.


The FlatCone analyzer-detector unit covers a scattering angle of about 75 degrees, separated into 31 individual channels. Every channel is equipped with a bent Si-111 analyzer with fixed final energy of 4 meV. By rotating the crystal the complete scattering plane can be mapped out in a very efficient way for energy transfers up to about 10 meV. Also, vertical momentum transfers up to about 0.3·ki are possible. The Flatcone setup is particularly advantageous for measuring excitations which are broad in momentum space.