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

We report loading of $1.5\times {10}^9$ metastable triplet helium atoms in a large magneto-optical trap, using far-red-detuned laser beams. We fully characterized this trap by measuring trap losses and absorption of a probe beam. From the highly nonexponential trap decay we derive Penning ionization loss rate coefficients for two detunings: $5.3\left(9\right)\mathrm{}\times {10}^{-9}{\mathrm{cm}}^3/\mathrm{s}$ at $-35\mathrm{MHz}$ and $3.7\left(6\right)\mathrm{}\times {10}^{-9}{\mathrm{cm}}^3/\mathrm{s}$ at $-44\mathrm{MHz}.$ Also, we find that the loss rate is maximum at $-5\mathrm{MHz}$ detuning, where the rate is $1.3\left(3\right)\mathrm{}\times {10}^{-8}{\mathrm{cm}}^3/\mathrm{s},$ much larger than recent theoretical and experimental values. In the absence of light the $S-S$ ionization rate constant is measured to be $1.3\left(2\right)\mathrm{}\times {10}^{-10}{\mathrm{cm}}^3/\mathrm{s}.$