Measuring the ions momentum vector with a large acceptance magnetic spectrometer

A technique to measure the momentum vector of the ions produced in a nuclear reaction and analyzed by a magnetic spectrometer is presented. It consists of a powerful procedure for the reconstruction of the ions trajectory based on the use of a focal plane detector, accurate three-dimensional models of the magnetic fields and efficient algorithms to solve the highly non-linear transport equations. The technique is quite general and reliable even with large acceptance optical devices. Experimental results obtained with the MAGNEX spectrometer show a remarkable resolution of about 0.2° in the horizontal direction, 0.7° in the vertical one and 1/1800 in the momentum modulus. A high degree of accuracy is also found in the reconstruction of both the direction and the modulus of the momentum vector, with the latter strongly depending on the order of reconstruction.