8 Mössbauer Spectroscopy in External Magnetic Fields
393
to fully polarize the spectra, which makes interpretation of the measurements easier.
In Figs. 8.12 and 8.13 a sketch of the 15 T high field Mössbauer equipment at the
Institute of Solid State Physics, TU Wien is shown, which was installed in 1984 by
Oxford Instruments. The field is generated by a system of several concentric superconducting coils. The inner ones made of Nb 3 Sn and the outer ones by NbTi. The
coils are in the liquid helium reservoir of a bath cryostat, which is isolated from
the outside by vacuum and a liquid nitrogen shield. The field value is measured by
the voltage drop over a shunt resistance placed in series with the coil set and monitored by a Hall sensor. By reducing the temperature of the liquid helium to 2.2 K
by the installed lamda fridge, fields up to 15 T can be produced. The maximum field
is reduced to 13.6 T if the bath temperature is 4.2 K. To hold fields constant over
long time, the magnet can be switched to persistent mode, where a superconducting shortcut over the coil decouples the coil from the power supply and the current
is confined in the coil. In this persistent mode no field reduction over a period of
several days is obtained. Accuracy of field is ±0.01 T. Field homogeneity of 1%
is reached in a cylinder volume 2 mm height 15 mm diameter, where the sample is
positioned. To avoid splitting of the source spectrum the
57 CoRh source is positioned in a field compensated area, which is produced by a small compensation coil.
The driving unit, based on a loudspeaker system, is situated on top of the cryostat.
Because of stiffness of the rather long (∼ 150 cm) rod which connects the source
with the driving unit, only sinusoidal movement is possible. Both sample and source
are mounted in the variable temperature insert (VTI), which is inserted in the bore of
the magnet (Fig. 8.14). The VTI is separated from the He bath by vacuum. Via two
valves liquid He can be inserted into a pot which is in thermal contact with the inner
tube where the sample is located, allowing to produce temperatures at the sample
between 1.5 K (by pumping above the liquid He) up to room temperature (by heating
the He gas). Source and sample are in two rooms, which are separated by a window.
With its own heater, the source temperature can be hold constant, independent of
the temperature of the sample. Temperature of the absorber is measured by a carbon glass and a SrTiO 3 sensor. The first one allows to determine very precisely the
temperature, whereas the second one is necessary to correct for the field dependence
of the carbon glass. SrTiO 3 is a capacitive sensor which is not as sensitive as the
carbon glass, but is practically field independent (±1 mK at 15 T). The combination
of both sensors allows to stabilize temperature during a field sweep. As detector a
proportional counter is placed on bottom outside of the cryostat. Due to the large
distance between source and detector of 50 cm and the seven windows, which the
γ -ray has to pass on its way out of the cryostat to the detector, sources with higher
activity are necessary. On the other hand the active area of the source is rather small
due to the given geometry. Therefore sources with activities of approx. 35 mCi are
used. Depending on the type of sample, measuring times of up to two weeks for one
spectrum are not seldom, if samples are not enriched with
57 Fe. To change samples
the whole VTI has to be removed. To avoid air entering the system, a bellow is
installed at the top, which can be flushed with helium gas during the removal of the
insert. For calibration of the velocity a second source is mounted on the upper side
of the driving head with an α-Fe foil, a second proportional counter, and a second
393
to fully polarize the spectra, which makes interpretation of the measurements easier.
In Figs. 8.12 and 8.13 a sketch of the 15 T high field Mössbauer equipment at the
Institute of Solid State Physics, TU Wien is shown, which was installed in 1984 by
Oxford Instruments. The field is generated by a system of several concentric superconducting coils. The inner ones made of Nb 3 Sn and the outer ones by NbTi. The
coils are in the liquid helium reservoir of a bath cryostat, which is isolated from
the outside by vacuum and a liquid nitrogen shield. The field value is measured by
the voltage drop over a shunt resistance placed in series with the coil set and monitored by a Hall sensor. By reducing the temperature of the liquid helium to 2.2 K
by the installed lamda fridge, fields up to 15 T can be produced. The maximum field
is reduced to 13.6 T if the bath temperature is 4.2 K. To hold fields constant over
long time, the magnet can be switched to persistent mode, where a superconducting shortcut over the coil decouples the coil from the power supply and the current
is confined in the coil. In this persistent mode no field reduction over a period of
several days is obtained. Accuracy of field is ±0.01 T. Field homogeneity of 1%
is reached in a cylinder volume 2 mm height 15 mm diameter, where the sample is
positioned. To avoid splitting of the source spectrum the
57 CoRh source is positioned in a field compensated area, which is produced by a small compensation coil.
The driving unit, based on a loudspeaker system, is situated on top of the cryostat.
Because of stiffness of the rather long (∼ 150 cm) rod which connects the source
with the driving unit, only sinusoidal movement is possible. Both sample and source
are mounted in the variable temperature insert (VTI), which is inserted in the bore of
the magnet (Fig. 8.14). The VTI is separated from the He bath by vacuum. Via two
valves liquid He can be inserted into a pot which is in thermal contact with the inner
tube where the sample is located, allowing to produce temperatures at the sample
between 1.5 K (by pumping above the liquid He) up to room temperature (by heating
the He gas). Source and sample are in two rooms, which are separated by a window.
With its own heater, the source temperature can be hold constant, independent of
the temperature of the sample. Temperature of the absorber is measured by a carbon glass and a SrTiO 3 sensor. The first one allows to determine very precisely the
temperature, whereas the second one is necessary to correct for the field dependence
of the carbon glass. SrTiO 3 is a capacitive sensor which is not as sensitive as the
carbon glass, but is practically field independent (±1 mK at 15 T). The combination
of both sensors allows to stabilize temperature during a field sweep. As detector a
proportional counter is placed on bottom outside of the cryostat. Due to the large
distance between source and detector of 50 cm and the seven windows, which the
γ -ray has to pass on its way out of the cryostat to the detector, sources with higher
activity are necessary. On the other hand the active area of the source is rather small
due to the given geometry. Therefore sources with activities of approx. 35 mCi are
used. Depending on the type of sample, measuring times of up to two weeks for one
spectrum are not seldom, if samples are not enriched with
57 Fe. To change samples
the whole VTI has to be removed. To avoid air entering the system, a bellow is
installed at the top, which can be flushed with helium gas during the removal of the
insert. For calibration of the velocity a second source is mounted on the upper side
of the driving head with an α-Fe foil, a second proportional counter, and a second