[en] The spectrometer is designed for the JET roof laboratory. The neutrons enter the spectrometer through a collimator. They are scattered by the protons in a thin polyethylene foil. The recoiling protons are detected in the proton detectors, and the scattered neutrons in the neutron detectors. The energy of the protons and the difference between the flight times of the neutrons and the protons are measured. The kinetic energies of the initial neutrons are calculated from these two quantities. Our conclusion from this design work is that a neutron spectrometer with a resolution of 200 keV or better for an efficiency at about 10^-5 cm² can be made. We estimate the time needed for building and testing the spectrometer at three years. The benefits of the new design of TANSY, as compared to the old annular one, is that the efficiency has been increased by a factor of ten and that the background produced by the spectrometer has decreased by a factor of twenty. This has been achieved at the expense of the resolution which has been worsened, but not more than a factor of two. Neutron detectors and proton detectors have been experimentally tested. We have found that, with the technology available, the demands of time and energy resolution can be fulfilled. A test of the complete instrument may be done at our laboratory during the assembly of the spectrometer. However, this test must be done using a special collimator. Therefore, the final judgement on the spectrometer as a plasma diagnostics tool can be done before it has been used at JET. (Author)