  We report experimental measurements of heat transport in rotating
Rayleigh-B{\'e}nard convection in a cylindrical convection cell with aspect
ratio $\Gamma = 1/2$. The fluid was helium gas with Prandtl number Pr = 0.7.
The range of control parameters was Rayleigh number $4 \times 10^9 < {\rm Ra} <
4 \times 10^{11}$ and Ekman number $2 \times 10^{-7} < {\rm Ek} < 3 \times
10^{-5}$(corresponding to Taylor number $4 \times 10^9 < {\rm Ta} < 1 \times
10^{14}$ and convective Rossby number $0.07 < {\rm Ro} < 5$). We determine the
crossover from weakly rotating turbulent convection to rotation dominated
geostrophic convection through experimental measurements of the normalized heat
transport Nu. The heat transport for the rotating state in the geostrophic
regime, normalized by the zero-rotation heat transport, is consistent with
scaling of $({\rm RaEk}^{-7/4})^\beta$ with $\beta \approx 1$. A phase diagram
is presented that encapsulates measurements on the potential geostrophic
turbulence regime of rotating thermal convection.
