Determination of fundamental mode decay constant out of a decay curve with higher mode contamination

A practical new method is presented for the determination of a fundamental mode decay constant out of a decay curve with higher mode contamination. Suppose one attempts to determine the decay constant of a certain physical quantity whose fundamental component decreases exponentially with time. If higher modes are included in the measured values the logarithmic curve of the quantity deviates from a straight line but approaches asymptotically to the latter, which represents the decay of the fundamental mode. The authors introduced an appropriate function which approximated the deviation, and devised a practical method to get the fundamental decay constant. The new method was applied successfully to the neutron population decay experiments, which was carried out with a neutron generator with pulsing function and a graphite stack. The calculatiorial method may apply to any decaying phenomena.