High Count Rates

Something in the neighborhood of 10,000 counts per second (cps) is the maximum that can be transmitted up a typical moderate length logging line / cable.  Extremely long lines, lines designed for H2S service, and very small diameter logging lines may be limited to as few as 4,000 cps.  Pulse width is a factor in both the maximum number of counts per second possible, and in the longest possible line that can be used.  If a pulse is 50 microseconds wide (a common pulse width for very long logging lines), then the absolute theoretical maximum would be limited to 20,000 cps, but real world physics prevents anything even close to the theoretical maximum.  The old GO differential temperature tools produce 20 cps per degree, and logging engineers sometimes have trouble with even the count rates produced by these tools in their upper temperature ranges.

Because of high count rates, most neutron tools employ a circuit to divide the raw He-3 neutron detector counts by two or four before sending them up the logging cable.  This same technology can be used to advantage with gamma ray tools, especially those equipped with scintillation detectors.  Comprobe ships their natural gamma ray tools stock configured to divide by two (¸2), neutron tools ¸4, high resolution density ¸16, 4p omni directional density ¸16, etc. (Comprobe uses a universal radiation tool building block module that can be easily factory or service shop configured to do many jobs).

Radioactive tracer survey (RATS) logging can generate very high count rates.  Gamma ray tools are often set for ¸8 or ¸16 operation when configured for RATS work.  The beauty of the "divide by" circuit is that it theoretically has no effect on tool statistics (a scintillation detector tool with a "divide by" circuit is still statistically superior to a G-M detector tool).  AnaLog Services, Inc. can install a "divide by" circuit in most gamma ray tools not already equipped with same.  Divide by 1, 2. 4, 8, 16, 32, 64, and 128 are easily jumper or switch selectable.  Here are a couple of Divide-By and Doubler Schematics; we offer these as part of the 106 Radiation Tool™ Printed Circuit Boards (PCBs) line.

Also see Anti-Coincidence Considerations for a discussion of this related topic.

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Last 10-20-10