LiF:Mg,Cu,P phosphors (commercially designated as MCP-N) are very sensitive thermoluminescent detectors and are considered by many laboratories to hold great promise for low level radiation monitoring. It is their high efficiency relative to TLDs based on LiF:Mg,Ti which make MCP-N particularly attractive for low dose measurements. The currently investigated detector material made of LiF:Mg,Cu,P bears the code name MCP-7s and has a very thin active layer and consequently an improved response to low energy beta radiation. Detailed studies have been carried out to characterise MCP-7s detectors in terms of glow curve, beta and gamma response, reusability, batch homogeneity and fading. Based on the positive outcome of these investigations, the dosimetry service at PSI plans to replace the TL material LiF:Mg,Ti presently used for extremity dosimetry with MCP-7s.