Comprehensive investigation of the angular transmission in magnetic spectrometers

Abstract

Magnetic spectrometers are largely used in experimental nuclear physics. These devices allow one to perform precise measurements of the reaction kinematics or to separate and fully identify the reaction residues for nuclear-dynamics and structure studies. They can also be used as separators to produce radioactive nuclear beams. However, all these applications are affected by the limited transmission of the reaction residues through the spectrometer. The final transmission depends on the ion-optical characteristics and the mechanical constraints of the spectrometer but also on the kinematic properties of the considered nuclei. The ion optics are characterised by the bending and focusing powers of the magnetic elements which constitute the spectrometer. The reaction mechanism determines the kinematics of the particles. The transmission losses in any magnetic spectrometer can be attributed to its limited acceptance in longitudinal momentum and angle. The limitation in longitudinal momentum can be overcome by combining different magnetic settings [Nucl. Phys. A 658 (1999) 47; Nucl. Phys. A 683 (2001) 513]. In the case of a zero-degree spectrometer, the angular acceptance can only be estimated with calculations. Standard ion-optical transport codes [Transport, SLAC-R-95-462; Nucl. Instr. and Meth. B 126 (1997) 284] are commonly used to determine the transmission of charged particles through magnetic spectrometers. Although these codes describe precisely the trajectories of the ions according to their kinematics within the spectrometer, they are rather time consuming. Therefore, experiments in which a large amount of different nuclides with specific kinematic properties are investigated [Nucl. Phys. A 658 (1999) 47; Nucl. Phys. A 683 (2001) 513; Phys. Rev. Lett. 84 (2000) 5736] cannot afford for such calculations. In this work we present a new method to estimate the angular transmission in zero-degree magnetic spectrometers [Nucl. Instr. and Meth. A 478 (2003) 493]. This method is based on a parameterisation of the angular aperture of the spectrometer for any possible value of the magnetic rigidity of the transmitted particles. The parameterisation of the angular aperture together with a description of the kinematics of the reaction mechanism allows one to determine the angular transmission analytically, avoiding tedious Monte-Carlo calculations. The analytical solutions are implemented for residual nuclei produced in fission, projectile-fragmentation and fusion-evaporation reactions.