Target dependence of charge distributions in spallation reactions of medium-mass nuclei with 12 GeV protons

Spallation yields of Ti, Fe, Co, Ni, Cu, and Zn by 12 GeV protons were measured with a Ge(Li) $\gamma$-ray spectrometer followed by computer analysis of the spectra. The measured cross sections of about 30 product nuclides for each target sample are in general agreement with existing data in the same energy range and cover a wider mass range of the product nuclides for most of the targets. The present data were used to obtain the charge-dispersion and mass-yield curves by the leastsquares analysis. The target dependence of charge distributions in spallation reactions for product nuclides with a mass range $22\lesssim A\lesssim 58$ was determined as a function of the $\frac{N}{Z}$ ratio of the target, ${\left(\frac{N}{Z}\right)}_T$. The $\frac{N}{Z}$ ratio at the charge-dispersion peak, ${\left(\frac{N}{Z}\right)}_p$, can be expressed as ${\left(\frac{N}{Z}\right)}_p=(0.783\mathrm{}\pm 0.038)+(0.304\pm 0.033){\left(\frac{N}{Z}\right)}_T$. The present results of the target dependence of charge distributions do not seem to agree with the earlier data for production of argon isotopes by 24 GeV protons in the same target mass range.