Live Time Correction With a Live Source
Lynx DSA User's Manual - 9240227J
191
Live Time Correction With a Live Source
To compensate for events rejected due to pile-up and processing time, a system dead
time is derived by the live-time correction function. The dead time signal controls the
MCA “Live-Time” clock which extends the acquisition time by the appropriate
amount.
The accuracy of the Live Time Correction (LTC) deployed on both traditional analog
electronics and the Lynx is dependent on the operation of the Fast Discriminator (fast
channel) and the pulse evolution time or dead time of the shaped signal (slow
channel). In the case of the Lynx, the slow channel is the digital filtered trapezoid
signal. Accurate Live Time Correction is obtained when the energy threshold and
dynamic range of the fast channel and slow channel are the same. In practice
however, the energy threshold of the fast channel is forced to be much higher
compared to the slow channel. In order to obtain good pulse pair or timing resolution,
the fast channel employs little or no noise filtering. As a result, the signal to noise
ratio is much worse, requiring a higher energy/noise threshold.
To optimize the LTC accuracy on traditional systems, the ADC LLD is adjusted or
optimized to normalize the energy threshold of the slow and fast channels. However,
this has the undesirable effect of affecting the spectral low energy cutoff.
On the Lynx, the “LT Trim” function allows minor adjustment of the pulse evolution
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 500,
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 discriminating user can improve performance further, for the intended
application, by calibrating 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
60
Co 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 interfere with the measurement.
1.
Connect the Lynx and set it up as described in
System Connections for a
Detector
on page 62.
2.
Verify LTC is set ON.
3.
Pole/Zero Compensation
