- If a sample is surrounded with neutron detectors and neutrons are
detected in one detector at the same time, then each such coincident
event will represent the detection of one fission.
- The rate of such coincidences appearing is proportional to the
concentration at the spontaneous fissioning material.
- A protocol such as the following is used.
- a neutron is detected;
- there is a short delay to allow the detector to stabilise (0.25
- 64 ms);
- count for a short period to collect real and accidental
coincidences (4 - 1024 ms). These
counts are collected in an up down counter;
- wait for a 'long' period (4 ms);
- collect another count for the same time interval as the first,
but in this case collect counts negatively in the up down counter,
thus subtracting the accidental rate;
- repeat until the net number of counts, the real coincidences, is
significant to the required statistical degree.