Foil degradation in a salt solution target

Abstract

The TR13 cyclotron at TRIUMF has been used to irradiate liquid solutions of dissolved nitrate salts in 0.8M nitric acid. When rebuilding the target, discoloration patterns were found on many of the Havar® foils used as beam windows. Additionally, two Havar® foils ruptured during routine irradiations, both at pressures below the safety cutoff for the target used. Irradiated foils were imaged and discoloration was quantified based on pixel saturation. Discoloration was compared to irradiation parameters for each foil. It was found that the maximum operating current applied to a foil correlated with foil discoloration. Bench tests were performed which heated Havar® foils to high temperatures while in contact with nitrate salt solutions. These tests failed to replicate the discolorations noted on irradiated foils. An electrochemical test cell was created to produce some of the radical species found in the target solution during irradiation. Havar® foils were submerged in nitrate salt solutions and used as both positive and negative electrodes with a potential of 20 volts between them. Multiple experiments showed discoloration on the cathode foil resembling the discoloration on irradiated foils. Observations suggest the production of NO2 in solution which may drive discoloration and foil degradation. Profilometry on the cathode foil showed significant material loss in areas of discoloration. Further tests must be performed to confirm that the mechanism of foil degradation in irradiated foils is the same as that demonstrated in the electrochemical test cell.