1.3.12. - Counting
Another important function of the diffractometer program
is to measure the scattered X-ray intensities.
spec
supports several types of timers, scalers
and
multichannel analyzers
(MCAs).
Timers control the count time.
Scalers count detected photons.
MCAs accumulate many channels of counts and
are used with energy-dispersive detectors
and positional-sensitive detectors.
To count the number of X rays
incident on the detector per second,
the counting hardware must be able to accumulate detector counts
accurately within a fixed time period.
The scaler hardware is gated by a
clock
that operates independently of the computer.
Thus, the response time of the computer to interrupts
(real-time events)
does not affect the accuracy of the count.
spec
programs and starts the clock
and senses when the clock period,
and hence the counting, has ended.
spec can then read the contents of the scalers
and save the measurement in a data file.
Clearing the scalers and starting the clock is accomplished
by the function
tcount(seconds). To count for one second, type:
1.FOURC> tcount(1)
2.FOURC>
The contents of the scalers are accessed through the built-in
S[]
array.
The hardware scalers are read and their contents loaded
into the scaler array by the
getcounts command.
A second, associated string array,
S_NA[], is defined in the standard macros and
identifies each scaler:
1.FOURC> getcounts; printf("%s = %g\n%s = %g\n%s = %g\n",\
2.cont> S_NA[0], S[0]/1000, S_NA[1], S[1], S_NA[2], S[2])
seconds = 1
monitor = 347
detector = 35031
3.FOURC>
The first scaler, labeled
seconds, is usually fed a 1 kHz signal, so it actually tracks milliseconds
and is therefore divided by 1000.
The number of scalers available depends on the particular
hardware and the number of detectors and monitors used.
The default scaler channel numbering for the first three scalers
puts a 1 kHz time signal in scaler 0, monitor counts in scaler 1
and detector counts in scaler 2.
You can also count to a fixed number of monitor pulses,
rather than to a fixed time period.
1.FOURC> mcount(1e4)
2.FOURC> getcounts; printf("%s = %.1f\n%s = %g\n%s = %g\n",\
3.cont> S_NA[0], S[0]/1000, S_NA[1], S[1], S_NA[2], S[2])
seconds = 28.8
monitor = 10000
detector = 1.00954e+6
4.FOURC>
Counting is
asynchronous, i.e., the
tcount()
and
mcount()
functions return
immediately
after starting the clock.
They do not wait until the counting period is over.
Use the
wait()
function to determine when counting is finished.
A useful macro has been written to count
and print the scaler contents:
1.FOURC> ct 5
Thu Aug 20 19:11:51 1987
Seconds = 5 Detector = 175103 (35020.6/s) Monitor = 1730 (346/s)
2.FOURC>
If the argument is omitted, a default count time
(stored in the global variable
COUNT) is used.
A
positive
argument to
ct
signifies
seconds;
a
negative
argument signifies
monitor counts.
1.FOURC> ct -10000
Thu Aug 20 19:13:42 1987
Seconds = 28.3 Detector = 1.0434e6 (36869.3/s) Monitor = 10000 (353.36/s)
2.FOURC>
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