Time of Flight Instruments
IN5
The instrument was used for experiments in polymer physics looking at the dynamics of long chains in the rubbery state and in solution.
In liquid crystal studies the smectic phase dynamics were elucidated explicitly, beating considerable competition from the less direct magnetic resonance experiments. The elastic incoherent structure factor measurements (EISF) became a standard tool for investigating the geometry of reorientation in molecular solids.
The dynamics of 4He, the roton minumum were investigated at high resolution, and in 1976 the first measurements on the dynamics of the highly absorbing 3He were made. At the Gatlinberg meeting these were compared directly with measurements made on a statistical chopper instrument at the Argonne National Laboratory (Skold, et al). For later measurements the Argonne group provided their much superior crysostat for use on IN5.
Initially the IN5 time of flight spectrometer (Scherm),a four choppper monochromator
with a 4m sample-detector flight path) ran at 20000 rpm, but the specially
selected ball bearings were repeatedly subject to failure and for most of the
early years the maximum speed was limited to 10000 rpm; this could resolve
energy changes of less than 10 microelectron volts.
To achieve the planned resolution with the choppers at their maximum speed
high pressure 1 cm diameter 3He detectors were used. These were mounted in boxes
of four, side by side, and attached to a spherical framework at a distance
of 4 m from the sample. Four boxes could be placed in a vertical column, and
there were sufficient boxes to offer twelve complete columns, effectively
twelve scattering angles. Each year it was planned to increase the
array by a further square metre of detectors; this was one of the major
continuing investment items for the Vercors Group.
The detector framework was improved at an early stage. Problems had arisen in
1975 during the 4He experiments because a support for the
gas-filled bag in the chamber between sample and detectors had a support
at about 12 degrees, blocking the path to the detectors over
several degrees. To complete measurements required using two
wavelengths, 10 and 8 Å. The differences in the resolution
were a major problem in treating the data to obtain the roton position,
and look for deviations in the linear dispersion with Q.
This was resolved by cutting out the support, replacing it with a beam
suspending the box roof. At the same time a thin-walled gas-tight aluminium box was
permanently added between sample and detectors, to be filled with argon,
to reduce intermediate scattering in the 4 m flight path.
The chopper problems were partially solved by the introduction of
magnetic bearings designed at Juelich, though these only proved
really reliable once a controlled temperature environment had been
added around them.
For small angles rings of shortrer tubes were installed about the straight
through beam on the 4 m support structure. These were later replaced
with a BF3 64x64x1cm multidetector (from D11). Three
low efficiency detectors, one before and two after the sample in the line
of the beam could be used (when working) to assess sample transmissions.
In later years the instrument was completely rebuilt with four pairs
of contra-rotating disk choppers and a 3m high cylindrical array
of position sensitive detector tubes.
IN6
A new TOF instrument, IN6, (Scherm, Dianoux, Ghosh)
was operational in 1982 with a larger detector area but lower resolution.
This employed three graphite crystal monochromators consecutively using the full
height of the H15 guide. (Lights on D11 indicated the wavelength, 4.1 4.6,
5.1 or 5.9 Å
being used by IN6!). Time focussing was employed (Scherm) with a
Fermi chopper turning in the sense to pick the highest angle monochromator
(longest wavelength, slowest neutrons first). The rotational speed
ensured that sample elastic scattering from the three sets of crystals arrived
simultaneously at the detectors. A second choppper could be used in phase
before the sample. The sample was at the centre of a box with detectors
2.3 m from the sample. All space was filled with some 350 detectors.
These were 6 atmosphere 3He detectors with a nominal 25mm
diameter, but squashed shortening the distance in the neutron path.
The electronics allowed some 256 input channels. The three rows of
detectors were divided into 80 inputs and control software used to
reduce the recorded inputs, typically to 32 angles, though individual
inputs could be stored, or as top+bottom, and central blocks.
With the fixed wiring and limited configurations, which required
moving the whole instrument to a different take-off angle, the machine
was very simple to use and was very reliable, delivering high quality
data usually without changes for whole sequences of experiments.
The original 3He experiments had counted continuously for some 10 days to measure appreciable scattering on IN5. On IN6, again using the Argonne sample configuration, the scattering could be seen after half an hour of counting, though some background neutron leakage through the chopper system was noted and subsequently corrected by covering the inside of the chopper housing with absorbing paint.
Backscattering IN10
The backscattering spectrometer IN10 (Heidemann) was initially used for measuring hyperfine coupling in Cobalt, in the micro-eV energy range, but later was used extensively for measuring polymer dynamics, extending the measurements made on IN5. Another new field was measurement of proton tunnelling which is very sensitive to the protons local environment. Initially limited to a few compounds which had measurable frequncies in the micro-electron volt level of IN10, chemists developed new classes of compounds and systems lead to measurements even on IN5, for methane etc. adsorbed on surfaces. This enabled a range of measurements varyng pressure, temperature and compositions for which the tunnelling is a sensitive probe for eliciting the local environment. The energy range was extended with IN13 on a thermal guide but with incipient problems of high backgrounds.
This instrument used the top part of the H15 guide. A Si monochromator crystal was mounted normal to reflect the incident beam (more or less) back along its axis, where it was deflected into the secondary spectrometer to the sample. This was in the centre of arrays of thin Si crystal wafers mounted on spherical plates about 1.6 m from the sample. Scattered neutrons from the sample thus travelled to these reflectors, and then back through the sample to detectors placed behind the sample. To scan the incident energy the monochromator crystal was moved back and forth along the incident beam using the doppler effect to change the wavelength sent into the secondary spectrometer. A velocity sensor was mounted on the monchromator mount. This was used by the acquisition computer to build a delay table of velocities; it was hence possible to lookup the original incident velocity of the detected neutron simply by taking into account the flight time through the instrument.
Spin Echo IN11
High resolution measurements benefited from a second improvement step with the introduction of the Neutron Spin-echo spectrometer (Mezei), IN11. Measuring in the frequency rather than energy domain slow relaxation phenomena were massively extended, almost closing the gap with photon correlation spectroscopy, but with much larger range range in momentum space. For the polymer world it was the opportunity to verify explicitly De Gennes theories of polymer reptation, which were shown to be just outside the measurement ranges of IN5 and IN10. Again the width of the 4He roton could be examined in even finer detail. These experiments were long in preparation and execution, with the analysis then performed by the few experts in the field.
Neutrons passed through the guide to a 15% helical slot velocity selector. These were then polarised using a supermirror, then flipped through 90 degrees and precess, passing through a long solenoid to the sample. After scattreing they passed through a flipper, changing the direction of polariastion by 180 degrees, then passing through a second solenoid, before being analysed by a second supermirror to the detector. The function of the spin echo technique was that the neutrons performed a number of precessions in the constant solenoid field in the first half of the instrument, then, after the sample, after reversing the spin direction by 180 degrees, the precessions unwound in the second solenoid field. If the sample produced no change in neutron velocity the intensity remained unchanged. The solenoids were programmed to change field together. As the field increased the total number of precessions increased. The apparatus thus became more and more sensitive to any phase shift (difference in number of precessions) induced by the sample, since there would then be a reduction of the final intensity. Using air-cored solenoids the fields were proportional to the current which could reach 200 a, with a sensitivity to effective relaxation times from 0.01 to 100 nsecs. A breakthrough in measurements came with the introduction of fresnel coils (Mezei) which allowed more than tenfold increase in beam area to be used through the instrument. The dynamic range arises from the broad wavelength range used: the net phase change is only sensitive to the change in velocity of the neutron due to the sample.