The ratio of the tritium and deuterium concentrations (nt/nd) in the core plasma is a key parameter in future burning plasma experiments, such as ITER. However, performing measurements of this parameter in the plasma core is difficult. In this paper we present a method to derive nt/nd using the ratio of the thermonuclear neutron emission to the beam-target neutron emission. The method we use is thus not limited to low tritium concentrations, and we apply it to neutron spectroscopy data from the Magnetic Proton Recoil spectrometer taken during the deuterium tritium experiment at JET. nt/nd-values obtained using neutron spectroscopy are in qualitative agreement with those from other diagnostics measuring the isotopic composition of the exhaust in the divertor.