Rutherford Backscattering Spectrometry (RBS) is a powerful ion beam analysis technique in frequent use within materials science. The recent emergence of targets incorporating complex features such as ultra-scaled diffusion barriers and isotopically-enriched films has started to push the limits of the technique, necessitating the expanded use of systems with a higher energy resolution. The increased cost and complexity of these systems based on already well-established schemes may present a barrier to their more widespread adoption. Here, we present an RBS detection system specifically designed for simple integration into existing nuclear probe infrastructure, but with a considerably higher energy resolution than a standard passivated planar detector. We employ a modified silicon p-i-n photodiode integrated within an optimised in-vacuum preamplifier to achieve a relative energy resolution of 3 x 10(-3) with 2.2 MeV He+ ions. In a glancing angle geometry, this corresponds to a depth resolution of similar to 1.5 nm and a mass resolution of <1 u from light elements up to Cu. We demonstrate the use of this detector to measure the thermal diffusion of near-surface As implants in a Si substrate, important for the formation of ultra-shallow junctions; and the residual Si-29 concentration in an isotopically enriched Si-28 specimen, a promising platform for quantum computation.