History

Since its first operation in 1985, almost 500 experiments were carried out on TFXA at ISIS. Its wide energy range (16 to 4000 cm^-1), good resolution and ease of operation attracted users from many disciplines. TFXA was designed to study hydrogenous samples, however, there have also been notable successes with non-hydrogenous systems such as fullerenes and minerals. 

• A good quality spectrum could be obtained from 2 - 3 gm of a hydrogenous material in 8 - 12 hours, thus applications where a few samples were sufficient were well-suited to TFXA. In contrast, where there are many variables, a large number of samples is vital. Examples of such systems are the preparation of amorphous hydrogenated carbon and ceramics based on the sol-gel process.

• Another area of great technological interest is applied catalysis. This requires the study of supported metal catalysts with low metal loadings, 1 - 10 wt%, thus per gram of sample there are only a small number of active sites.

• A third example is new developments; novel materials are only available in small quantities, thus to be at the leading edge necessitates the ability to study small samples. All these applications demanded an instrument of greatly increased sensitivity.

To address the problems of sensitivity encountered with TFXA it has been replaced with the new instrument TOSCA. The instrument is a collaboration between the CNR of Italy, the University of Kent at Canterbury and ISIS.

TFXA

The Time Focused Crystal Analyser (TFXA) was an indirect geometry instrument at 12m from an ambient water moderator. At every ISIS pulse a white spectrum of incident neutrons illuminated the sample. Some of those neutrons with sufficient energy to promote an internal transition in the sample were scattered towards the secondary spectrometer that viewed the sample at a scattering angle of 135° and used a pyrolytic graphite (002) crystal to select the final neutron energy of ca. 3 meV. (24 cm-1) These neutrons were diffracted onto the detector assembly, passing through a beryllium filter on the way to eliminate the higher order Bragg reflections. 

Each detector assembly consisted of 14 ‘squashed’ 3He detector tubes. These were specially selected to be reasonably inefficient and so discriminate disproportionately against the high energy background neutrons. The time versus counts spectrum of each detector tube was corrected for the incident flux distribution (as measured by an incident beam monitor) and transformed into the more conventional energy versus S(Q,w) spectrum by standard programmes. The individual spectra were then co-added. 

Typically spectra were accumulated over several hours or overnight for weak samples. Because of the low final energy and the large energy transfers TFXA had a specific trajectory in (Q2[Å-2], w [meV] ) space and Q2»w/2. The low final energy also meant that Q is almost parallel to the incident beam. This meant that with oriented samples experiments analogous to optical polarisation measurements are possible.

TFXA was superceded by TOSCA in June 1998. The new instrument offers significantly increased sensitivity and a modest improvement in resolution.