  Quasi-simultaneous arrival (QSA) effects in secondary ion mass spectrometry
can create mass-indepedent inaccuracies in isotope measurements when using
electron multiplier detectors (EMs). The simple Poisson statistical model of
QSA does not explain most experimental data. We present pulse-height
distributions (PHDs) and time-series measurements to better study QSA. Our data
show that PHDs and the distribution of multiple arrivals on the EM are not
consistent with the Poisson model. Multiple arrivals are over-dispersed
compared to Poisson and are closer to a negative binomial distribution. Through
an emission-transmission-detection model we show that the QSA correction
depends on the non-Poisson emission of multiple secondary ions, the secondary
ion energy distribution, and other factors, making an analytical correction
impractical. A standards-based correction for QSA is the best approach, and we
show the proper way to calculate standards-normalized $\delta$ values to
minimize the effect of QSA.
