Exploranium GR-135, Системное руководство, Страница: 37 / 80

SAIC Exploranium – GR-135 System Manual page: 37
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:
10% Dead-Time
= a DOSE Rate for Cs-137 of approx. 0.5mR/h
= a count rate of approx. 5000cps in the SURVEY mode
The LIVE-TIME data is what is important and it is used to correct the data for the “lost” sampling
time as shown in the following example:
ST = 100.000 secs
LT = 91.373 secs
Channel 128 = 1850 counts after the 100 secs sample = 18.5 counts/sec
The REAL value for Channel 128 = 1850/91.373 = 20.25 counts/sec
As can be clearly seen – LIVE-TIME correction is essential for proper data analysis.
To make data analysis simpler, the user has 2 options on how to correct this effect by as follows.
LIVE - This setting enables an automatic correction method and this correction essentially
increases the sample-time by the Dead-Time to account for all the pulses that would have
been lost because the electronics were “too busy” to process the incoming pulse.
If LIVE mode is selected, the preset sample period is AUTOMATICALLY extended by the
Dead-Time so the final data is automatically Dead-Time corrected. Thus if you set a 100
second sample period and there is a 10% Dead-Time, then the actual sample period will be
approximately 110 seconds thus all the “lost” time has been compensated for and the data
represents a true 100 second sample. The selection of LIVE mode is an advantage to users
who want “automatic” operation to minimize data processing. In low count areas extending
the sample period also gives slightly more accurate data. The main drawback to this mode of
operation is that the actual duration of the sample is variable.
CLOCK
- In this mode, the preset sample time is the actual sample time and any Dead-Time
occurring must be corrected manually. The data output includes the system Live-Time so
that correction is quite simple especially if using a spreadsheet program.
(As an example the Exploranium IdentiView program described in Appendix B provides the
option to output the spectra to a spreadsheet. In the process the data is automatically Live-
Time corrected and normalized to counts/minute.) The CLOCK mode is used primarily when
several analyses are being compared without time normalization and it is necessary to have
the same sample time.
NOTE
USERS ARE REMINDED THAT IN THE LIVE MODE – ACTUAL SAMPLE
TIMES WILL BE MUCH SLOWER IN HIGH COUNT RATE AREAS. IF
ONLY NUCLIDE-ID IS REQUIRED THEN USE OF CLOCK WILL
ENSURE THAT THE PRESET SAMPLE TIME IS THE ACTUAL SAMPLE
TIME.