Sy
clone Portable Gamma-Ray Spectrometer, Revision 10 5.0 MANUAL MODE
Ra
dComm – Proprietary Page 41 Revision Date: March 3, 2016
very significantly improved with very little down side.
5.5.3.2 NaI
The selections are:
a) MEASURING 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, 70, 80, 90,
100, 200, 300, 400, 500, 600, 1200, 1800,
3600, 5400secs
b) MEASURING MODE - Selections are: LIVE,
CLOCK, REP
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. 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 Syclone 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
For example, if a 100 second sample had a Live-Time of 91.373 secs then:
ST - SAMPLE -TIME = 100.000 sec
LT - LIVE-TIME = 91.373 sec
Therefore DT = 8.6% from the above formula.
The DEAD-TIME is only computed for display purposes to advise the user in the
ANALYSIS mode of the approximate intensity. As mentioned previously for best data
analysis Dead-Time below 20% are advised as above this level the local radiation field is
so intense that scattering may cause data analysis problems. Since the radiation falls off
as the inverse square - moving a few feet away from a “hot” source will reduce the DT
thus giving better data analysis AND reducing the users’ exposure.
To give an idea of Dead-Time:
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