We report an experiment measuring simultaneously the temperatureand the flux
of ions produced by a cloud of triplet metastablehelium atoms at the
Bose-Einstein critical temperature. The onsetof condensation is revealed by a
sharp increase of the ion fluxduring evaporative cooling. Combining our
measurements withprevious measurements of ionization in a pure BEC,we extract
an improved value of the scattering lengtha=11.3−1+2​ nm. The analysis
includes corrections takinginto accountthe effect of atomic interactions on the
criticaltemperature, and thus an independent measurement of the
scatteringlength would allow a new test of these calculations