The dependences of neutron yield on incident energy and atomic mass of the target have been studied by bombardment of 44GeV or 18GeV 12C ions with Cu or Phtargets. The CR-39 stacks are used to detect fast neutrons. The neutron yield ratios obtained for 44GeV and 18GeV are 2.12± 0.19(Cu target) and 2.04±0.15(Pb target).The neutron yield ratios obtained for Ph and Cu target are 2.54±0.20(44GeV) and 2.56 ± 0.20(18GeV)
showing that Pb target produces more neutrons than Cu target in the same conditions.