  Based on an improved multiphase transport (AMPT) model, which gives a good
description of proton production with a smooth quark matter to hadronic matter
transition in relativistic heavy ion collisions, we study deuteron and triton
production from the coalescence of nucleons at the kinetic freezeout of these
collisions. For Au+Au collisions at center-of-mass energies
$\sqrt{s_\text{NN}}$ from 7.7 GeV to 200 GeV available at the Relativistic
Heavy Ion Collider (RHIC), we find that the yield ratio
$N_\text{t}N_\text{p}/N_\text{d}^2$ of proton, deuteron\com{,} and triton is
essentially a constant as a function of collision energy. Our result confirms
the expectation that without a first-order quark to hadronic matter phase
transition in the produced matter or its approach to the critical point of the
QCD matter, this yield ratio does not show any non-monotonic behavior in its
collision energy dependence.
