We report on reproducible shock acceleration from irradiation of a $lambda = 10$ $mu$m CO$_2$ laser on optically shaped H$_2$ and He gas targets. A low energy laser prepulse ($Ilesssim10^{14}, {rm Wcm^{-2}}$) was used to drive a blast wave inside the gas target, creating a steepened, variable density gradient. This was followed, after 25 ns, by a high intensity laser pulse ($I>10^{16}, {rm Wcm^{-2}}$) that produces an electrostatic collisionless shock. Upstream ions were accelerated for a narrow range of prepulse energies ($> 110$ mJ & $< 220$mJ). For long density gradients ($gtrsim 40 mu$m), broadband beams of He$^+$ and H$^+$ were routinely produced, whilst for shorter gradients ($lesssim 20 mu$m), quasimonoenergetic acceleration of proton was observed. These measurements indicate that the properties of the accelerating shock and the resultant ion energy distribution, in particular the production of narrow energy spread beams, is highly dependent on the plasma density profile. These findings are corroborated by 2D PIC simulations.
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