Gadolinium-148 production cross section measurements for 600- and 800-MEV protons

In a series of experiments at LANSCE's WNR facility, 148Gd production was measured for 600- and 800-MeV protons on tungsten, tantalum, and gold. These experiments used 3 μm thin W, Ta, and Au foils and 10 μm thin Al activation foils. Gadolinium spallation yields were determined from these foils using alpha spectroscopy and compared with the LANL codes CEM2k+GEM2 and MCNPX. When heavy metal targets, such as tungsten, are bombarded with protons greater than a few hundred MeV many different nuclides are produced. These nuclides are both stable and radioactive and are created by spallation, proton activation, or secondary reactions with neutrons and other nuclear particles made in the target. These products are distributed somewhat heterogeneously throughout a thick target because of the energy dependence of the cross sections and energy loss of the proton beam within the target. From this standpoint, it is difficult to measure nuclide production cross sections for a given energy proton in a thick target. At the Los Alamos Neutron Science Center (LANSCE) accelerator complex, protons are accelerated to 800 MeV and directed to two tungsten targets, Target 4 at the Weapons Neutron Research (WNR) facility and 1L target at the Manuel Lujan Jr. Neutron Scattering Center. DOE requires hazard classification analyses to be performed on these targets and places limits on radionuclide inventories in the target as a means of determining the 'nuclear facility' category level. Presently, WNR's Target 4 is a non-nuclear facility while the Lujan 1L target is classified as a Category 3 nuclear facility. Gadolinium-148 is a radionuclide created from the spallation of tungsten and other heavy elements. Allowable isotopic inventories are particularly low for this isotope because it is an alpha-particle emitter with a 75-year half-life. The activity level of 148Gd is generally low, but it encompasses almost two-thirds of the total inhalation dose burden in an accident scenario for the two tungsten targets at LANSCE based on present yield estimates. From a hazard classification standpoint, this severely limits the irradiation lifetime of these tungsten targets. As 800-MeV protons pass through the tungsten targets at WNR and the Lujan Center, the proton energy is degraded to 600 MeV upon exiting the target. Since the facility classification is partly driven by the inventory of 148Gd, a better estimate of the true production rate in tungsten targets is needed.