SAIC Exploranium – GR-135 System Manual page: 36
This parameter is useful in some applications where low amplitude peaks need to be excluded for
special application. However if this value is set too low then some spurious peaks may occur so the
default setting of 30 usually gives the best results
d) AUTO NI LIM. – selections are 10, 15, 20,
25, 30, 35, 40, 45, 50%
The libraries have been optimized for various applications to suit the majority of normal cases. It is
possible that occasionally the user will come across an unusual isotope that has not been included in
the library. However during routine analysis, spurious peaks are quite often detected (caused by
pile-up events, back scatter or various other scattering phenomena). In the MANUAL mode, peaks
that are not accounted for in the Nuclide Identification process are summed and shown as N.I. (Non
Identified peaks). This is an acceptable procedure for skilled uses as their experience allows them
to asses the relevance of such “extra” peaks. However in the AUTO mode where semi-skilled users
involved these N.I. peaks cause confusion as in most cases they are relatively insignificant.
In software version 1V22 this problem is solved by computing the percentage of NI peaks in the
analysis. After peak analysis when all the low amplitude peaks have been removed via the PEAK
LIMIT parameter, all remaining peak amplitudes (in SD units) are summed. The Nuclide
Identification takes place. Any NI peaks left over from this analysis also have their amplitude
summed. The NI ratio is then computed being (NI amplitude/All peak amplitude). If this amplitude
exceeds the NON-ID THRESHOLD parameter then the display shows UNKNOWN ISOTOPE.
The default setting of 25% is a reasonable compromise for normal operation and prevents semi-
skilled users from having unnecessary concern when operating in the AUTO mode as unimportant
peaks are just suppressed.
10.3.2 NaI
Selections here are :
a) MEAS TIME – sets the sample time in seconds for acquiring a Sodium-Iodide spectrum in the
IDENTIFY mode– selections are 10, 20, 30, 40, 50,
55, 60 .. 100, 200 .. 600, 1200, 1800, 3600 secs
b) MEAS MODE - Selections are: LIVE,
CLOCK
GENERAL COMMENTS regarding LIVE-TIME and DEAD-TIME
. When the spectrometer is acquiring
data, each incoming signal pulse from the detector takes a finite time to process (convert from an
analog amplitude indicative of energy level to the corresponding channel number in the spectrum).
The time taken to process each pulse is referred to as DEAD-TIME and is usually expressed as a %
of the available time. [As an example, see DT in Figure 10]. If the incoming count rate is very high
then the Dead-Time will be very high. As an example, if Dead-Time = 50% then for every second of
sample time, the system is only “available” for 50% of the time = 0.5 seconds - to process new
pulses, so any incoming pulses encountered during signal processing will be lost. Thus when
accumulating data the system is “LIVE” only for a fraction of the time.
At very low count rates, the Dead-Time is relatively insignificant. However at higher rates Dead-
Time correction is important. The standard way of dealing with Dead-Time is to normalize the data
to compensate for this sampling Dead-Time by using the actual system LIVE-TIME, where :
LT (Live-Time) = ST (Sample-Time) - DT (Dead-Time)
The system LIVE-TIME is automatically accumulated by the GR-135 and stored as a data variable
that is output on the data stream to permit data correction on external processing systems. The
displayed DEAD-TIME is computed at a 1/sec rate for display purposes only
by the simple formula -
DT%= [(ST - LT)/ST]*100
To Next Page
Loading...