The measurements were performed on the M15 surface
beamline
at TRIUMF. The beam consisted of low momentum (28.6MeV/c),
backward spin polarized positive muons. The short range of these muons
makes this beamline ideal for the study of thin samples. A crossed-field
separator was used to rotate the muon spins perpendicular to their
momentum direction. After passing through a 1cm diameter collimator and a thin
muon defining counter, a small fraction of the incoming beam came to
rest in the crystals (
stops out of
incoming).
Great effort was devoted to reducing the background signal in this
experiment, which superimposes itself
on the measured field distribution originating
from the sample. The background signal originates from muons stopping in the
sample holder or in the cryostat walls and windows, which then precess
in the externally applied field. Previous SR experiments
on single crystals have been
seriously plagued by the inseparable nature of the often large background signal
[77]. A novel experimental setup was used to eliminate
most of the background signal produced by those muons which missed the
sample [86]. The three crystals were
mounted on a thin layer of aluminized
mylar using a minute portion of Apiezon N grease, with their
c-axes parallel to the applied field.
Recall with this orientation of the field
it is
which is measured.
The thin aluminized
mylar provides no appreciable signal. The three
crystals together provided a total area of
for the incoming
muon beam. Data taken with only one of the three single crystals of
,
showed no further appreciable reduction in the background signal
and gave the same foreground signal as the 3-crystal sample.
Consequently, a mosaic consisting of all three single crystals was used
to maximize the counting rates for muons striking the sample.
The mylar with the mosaic of crystals was stretched over the end of a
hollow 4.45cm-diameter, cylindrical, aluminum sample holder, as indicated
in Fig. 4.1.
A horizontal 4He gas-flow cryostat with an internal
diameter of 4.92cm, allowed cooling of the crystalline
down
to
K. A 7T warm-bore superconducting
magnet called Helios was
used to produce magnetic fields transverse to the initial muon spin
direction.
The positrons emitted from the muon decay were readily detected by a
cylindrical arrangement of positron counters, coaxial with the magnet
axis. Overlapping forward (F) and
backward (B) counters were used to define
a solid angle for muon decay events originating from the sample.
A veto (V) counter in the
form of a cylindrical scintillator cup, was employed to discriminate
against those
muons which missed the sample.
A ``good'' muon stop was defined as
(
muon
counter). A ``good'' positron stop
(i.e. a positron originating from a muon that stopped in the sample)
was defined as
.