TAST

TAST/HOLO - Thermal Neutron Three-Axis Spectrometer and Neutron Holographic Instrument

Instrument responsible: Alex Szakál, Gyula Török

The thermal neutron three-axis spectrometer (TAST) of BNC is installed on the thermal neutron channel No.8 at the Budapest Research Reactor. The instrument provides (~1.0 meV) moderate resolution with sufficient intensity for use in a wide range of problems. It is ideally suited for the study of phonon and magnon dispersion in single crystals, phonon density of states for a large class of materials, which contain hydrogen. Independent control of the momentum (Q) and energy transfer (E) is routine if required as opposed to the time of flight spectrometer in which Q and E are related by the instrumental configuration. This triple axis spectrometer can also be used in a multi-purpose regime, e.g. for high-resolution diffractometry, inelastic scattering as well as for thermal beam irradiation and transmission tests as well as tests of neutron optical components of various kinds.

The monochromatic beam is provided by a horizontally and vertically  focusing Cu[002] monochromator of 90mm height and 150mm width. In order to suppress the intensity of fast neutrons, a 15 cm long sapphire single crystal is inserted in the primary shutter. A Ge analyzer can be optionally used for higher order filtering in the incident monochromatic beam. The beam divergence is determined by thin film Soller-type Mylar collimators coated with Gd2O3 .

A highly efficient (90% at 1 Å) 3He single counter of 1” diameter is applied as detector. A two-dimensional position-sensitive detector is also available to use in medium resolution mode. For energy analysis a horizontally focusing pyrolytic graphite crystal assembly is used.

The spectrometer can be equipped by an Eulerian Cradle, or a tilt table that can hold various sample environments up to a mass of 100 kg. In 2005 the TAST instrument was upgraded to be optionally used for atomic resolution neutron holography in neutron- or gamma-ray detection modes. A BGO gamma detector is available with a proper shielding to filter the rather high gamma background of the environment and detect the low intensity holographic signal.

   Beam tube:

   Channel No. 8. (radial channel with sapphire filter)

   Monochromator

   Cu[002], horizontally and vertically focusing

   Analyzer

   PG[002], horizontally focusing

   Detector

   3He tube

   Range of monochromator angle:

   14° < 2Θmonochromator < 44°

   Range of analyzer angle:

   -100° < 2Θmonochromator < 100°

   Angular precision

   0.01°

   Flux at sample at 1Å wavelength

   2x106 n/cm2/sec

   Beam size

   50x50 / 10x15 mm2 depending on focusing

   Momentum transfer range

   1.5 – 10Å-1

   Energy transfer range

   1 – 60 meV

Table 1: Basic parameters of the TAST/HOLO spectrometer at BNC.

 

Instrument fig

 

 

Figure 1. Layout of the thermal neutron three-axis spectrometer