[en] Probability distributions of the size of ion clusters created in 'nanometric' cylindrical volumes of nitrogen by single 4.6 MeV α particles were measured and compared with those calculated by Monte Carlo simulation. The diameter of the sensitive volume had a mass per area of between 0.015 and 1.3 μg.cm^-2 which, for a material at unit density, corresponds to a diameter of between 0.15 nm and 13 nm. These nanometre sizes were simulated experimentally in a device called Jet Counter. The measured or calculated cluster size probabilities confirmed that the formation of ionisation clusters along a 'nanometre' track can be characterised by Poisson's distribution only for very small targets. The present ionisation cluster probabilities produced in 'nanometric' volumes, 2 to 10 nm in diameter, are the first ever determined experimentally and confirmed by Monte Carlo simulation. (author)