On the 9660 DSP, the “LT Trim” function allows minor adjustment of the pulse evolu-
tion time or dead time of the digital trapezoid signal to normalize the fast and slow
channel energy thresholds without affecting the spectral low energy cutoff threshold.
The LT Trim has an adjustment value of 0 to 1000 and the default value is 250, which
gives good Live Time correction performance for most applications. In the steps that
follow, Live Time Correction accuracy is measured using the “two source method”
which monitors the area of a reference spectral peak when subjected to varying rates
of background counts. Typical LTC performance (reference peak area variation) using
the default LT Trim setting is typically less than 3% for dead times of 50%. The dis-
criminating user can improve performance further, for the intended application, by cal-
ibrating the system using the “Two Source Method” and optimizing performance
using the LT Trim.
The following steps are designed to demonstrate and verify the effectiveness of the
Live Time Correction function. The verification/optimization process uses the “Two
Source Method” which assumes that source “A” is
60
Co and source “B” is
137
Cs. The
1173.2 keV peak of
60
Co will be used as a reference. The upper peak, at 1332.5 keV, is
not a good choice because the sum peak of
137
Cs at 2 x 661.6 = 1323.2 keV would in-
terfere with the measurement.
1.
Connect the Model 9660 and setup described in “Spectroscopy System
Setup” on page 34.
2.
Verify LTC is set ON.
3.
Pole/Zero Compensation
The Pole/Zero was previously adjusted and it is not necessary to do it again.
If for some reason readjustment is necessary, please follow the directions in
“Detector Matching” on page 38.
4.
Set the MCA’s preset to 500 Live seconds.
5.
Position the
60
Co source near the Ge detector and adjust for an incoming
count rate of 2 to 5 kcps. The 1173.2 keV
60
Co reference peak should be at
approximately 80% of the spectral full scale range. If necessary, adjust the
9660 gain to properly locate the peak.
Note:
Once in place, the source should not be moved or altered in any way for the
remainder of the experiment!
6.
Clear the MCA and acquire a spectrum for 500 live seconds. Record the net
area of the 1173.2 keV
60
Co peak.
7.
To the
60
Co source, add approximately 25 kcps of
137
Cs to make the total
incoming rate 30 kcps.
Chapter 7 - PUR/LTC Operation
50
Model 9660-9660A ICB Digital Signal Processor
