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

Instrument responsible: Alex Szakál, Gyula Török

The thermal neutron three-axis spectrometer (TAST) at BNC is installed on the thermal neutron channel No.8 at the Budapest Research Reactor. The instrument provides moderate resolution (~1.0 meV) with sufficient intensity for use in a wide range of problems. It is ideally suited for the study of phonon and magnon dispersion curves in single crystals, phonon density of states for that 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 multipurpose regime, e.g. for high-resolution diffractometry, inelastic scattering as well as for thermal beam irradiation and transmission tests or tests of various kinds of neutron optical components.

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

A highly efficient (90% at 1 Å) 3 He single counter of 1” diameter is applied as detector. A two dimensional position sensitive detector in medium resolution mode is also available. For energy analyzing 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 environment devices up to a weight of 100 kp. Since 2005 TAST can be used also as an instrument for atomic resolution neutron holography measurements both in neutron or gamma ray detection modes. A BGO gamma detector is available with a propoer shielding to filter the rather high gamma background of the environment and detect the small inside detetor holographic signal.

Beam tube:

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


Cu[002], horizontally and vertically focusing


PG[002], horizontally focusing


3He tube

Range of monochromator angle:

14° < 2Θmonochromator < 44°

Range of analyzer angle:

-100° < 2Θmonochromator < 100°

Angular precision


Flux at sample at 1Å wavelength

2x106 n/cm2/sec

Beam size

50x50 / 10x15 mm2 depending on focusing

Momentum transfer

1.5 – 10Å-1

Energy transfer

1 – 60 meV

Table 1: Main parameters of the spectrometer


Instrument fig



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