GBS Elektronik MCA-166 Influence of very long detector cables on spectrum measurements - Manual (Page 2)

The point is now that the pile up rejector (PUR) interprets this echo as a close following event and this way
reject everything. The time limit for the PUR to distinguish between two close following signals is about 500 ns.
So for cable length >50 m this effect should be seen.
If the spectrum is measured with PUR switched off, a "normal" can be seen. However, the resolution seems to be
a bit decreased.
Another possibility to get rid of the echo signals is to place a 50 Ohm termination resistor in parallel to the MCA
input. This eliminates the echo signals, but also reduces significantly (66%) the signal amplitude. So with this
method the amplification has to be increased; also here a slight degradation in resolution can be found.
Fig. 2: Co60 spectrum measured with short cable, with long cable, with long cable and PUR off, with long cable
and 50 Ohm termination.
Condition Resolution at 1333 keV Drift
short cable 1.73 keV
250 m cable, PUR off 1.90 keV 0.833
250 m cable, 50 Ohm terminated 2.77 keV 0.34
250 m cable, 50 Ohm terminated, amplification increased 1.87 keV (1)
Conclusions
-Without precautions signal cable length >30m cannot be recommended.
-For cable length >30m the PUR has to be switched off or a 50 Ohm termination resistor has to be placed in
parallel to the MCA signal input.
-For long cable length, a slight resolution degradation has to be taken into account.
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