Measuring absolute cross sections with HELIOS*

The HELIOS device at Argonne National Laboratory provides a means of analyzing outgoing ions produced in direct reactions in inverse kinematics, with good energy resolution. To best exploit this device it is imperative that there is a means to measure absolute cross sections: ingredients essential, for instance, to the extraction of spectroscopic factors. A method has been developed to determine absolute cross sections and to monitor the luminosity in the course of a measurement, which relies on the magnetic properties of HELIOS and the elastic scattering cross section at small center-of-mass angles. This method was tested recently using a 680-MeV $^{136}$Xe beam impinging a CD$_2$ target of nominal thickness 110~$\mu$g/cm$^2$---here ($d,d$) elastic scattering was measured in the Rutherford regime. Using the same system the deuterium content of the target was monitored at higher (10~MeV/u) beam energies to obtain information on the extent to which the target degrades with beam dose.