  Counting rate is a key parameter of superconducting nanowire single photon
detectors (SNSPD) and is determined by the current recovery time of an SNSPD
after a detection event. We propose a new method to study the transient
detection efficiency (DE) and pulse amplitude during the current recovery
process by statistically analyzing the single photon response of an SNSPD under
photon illumination with a high repetition rate. The transient DE results match
well with the DEs deduced from the static current dependence of DE combined
with the waveform of a single-photon detection event. This proves that the
static measurement results can be used to analyze the transient current
recovery process after a detection event. The results are relevant for
understanding the current recovery process of SNSPDs after a detection event
and for determining the counting rate of SNSPDs.
