Rotating crystal cube as a variable shutter for use with synchrotron radiation. Page: 8 of 9
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Each of the pulses in Figure 3(a) is really an envelope of several pulses as shown in Figure 3(b) and Figure 3(c),
where the time-scale resolution is increased. (The noise beginning about 500 ns after the trigger pulse in (b) is
due to reflection problems resulting from impedance mismatch in the detector circuit. This problem was later
corrected as illustrated by the clean pulses in (c).) The trigger pulse in each example is usually the sextet
because it serves as an easily identified reference pulse. In Figure 3(b) the sextet is followed by 25 triplets
covering a total time of about 2.65 ps. Note the beginning of a second set of pulses 3.68 ps after the trigger pulse
sextet, which represents transmission by the tail of the rocking curve width of the rotating crystal. Figure 3(c)
illustrates a loading pattern of 1 sextet followed by 20 singlets. Note that the trigger pulse is the first singlet
and that a second set of pulses begins about 3.68 ps after the sextet. Increasing the time scale further, as in
Figure 3(d), illustrates the characteristics of the sextet signal used to identify it, namely its increased width of
about 17 ns, and the known spacing to the next pulse within the envelope, about 165 ns.
As the rotation frequency of the Si crystal is increased, the beam chopper transmission window time decreases.
In Figure 4, the rotation frequency of the cube is about 62 - 63 Hz and the storage ring loading pattern is the same
as illustrated in Figure 3(c) and 3(d). Each pulse in the top trace of Figure 4(a) now represents either the sextet
or a singlet. The amplitude of the transmitted pulse varies because of asynchronous timing between the orbital
frequency of the storage ring and the rocking curve width of the Si crystal. Note that some pulses were missed
because the timing of the rotation of the Si crystal was completely out of phase with respect to the arrival of
an x-ray pulse. Figure 4(b) illustrates that each pulse in Figure 4(a) represents either the sextet or a singlet
because of the short open time of the transmission window, approximately 90 ns. The first singlet following the
sextet, as illustrated in Figure 3(d), is missing here because the transmission window of the beam chopper closed
before arrival of the first singlet.* I z,....,:.
. ~ ~ ~ ~ -~- I .:o m1 .. . . . r5f:rt.. . . . .a b
Figure 4. As the rotation frequency of the Si cube is increased to about 63 Hz, the transmission
window time decreases to about 90 ns. Asynchronous timing results in the variable x-ray pulse
amplitude observed in (a). Each x-ray pulse in (a) is either the sextet or a singlet because the
transmission window time is too short to transmit two adjacent pulses, as illustrated by the lone -
sextet in (b).
Figure 4(a) illustrates both the strength and the weakness of operating the beam chopper motor asynchronously
with respect to the orbital frequency of the storage ring. On the plus side, an x-ray pulse can be transmitted
with a high duty cycle (number of pluses transmitted vs. the number of possible pulses). On the negative side,
the amplitude of the transmitted pulses will vary greatly from pulse to pulse depending upon the correlation
between the arrival time of the x-ray pulse from the storage ring and the rocking curve width of the rotating
crystal. For experiments that need short x-ray pulses, but not necessarily a constant pulse amplitude,ff stable
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McPherson, A. Rotating crystal cube as a variable shutter for use with synchrotron radiation., article, July 8, 1998; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc628279/m1/8/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.