Large numbers of cold metastable helium atoms in a magneto-optical trap

We report loading of 1.5x10(9) metastable triplet helium atoms in a large m agneto-optical trap, using far-red-detuned laser beams. We fully characteri zed this trap by measuring trap losses and absorption of a probe beam. From the highly nonexponential trap decay we derive Penning ionization loss rat e coefficients for two detunings: 5.3(9)X10(-9) cm(3)/s at -35 MHz and 3.7( 6)X10(-9) cm(3)/s at -44 MHz. Also, we find that the loss rate is maximum a t -5 MHz detuning, where the rate is 1.3(3)X10(-8) cm(3)/s, much larger tha n recent theoretical and experimental values. In the absence of light the S -S ionization rate constant is measured to be 1.3(2)X10(-10) cm(3)/s. [S105 0-2947(99)51408-X].