Long-term measurements of equilibrium factor with electrochemically etched CR-39 SSNTD

Abstract

Recently, our group proposed a method (proxy equilibrium factor method) using a bare LR 115 detector for long-term monitoring of the equilibrium factor. Due to the presence of an upper alpha-particle energy threshold for track formation in the LR 115 detector, the partial sensitivities to 222Rn, 218Po and 214Po were the same, which made possible measurements of a proxy equilibrium factor Fp that was well correlated with the equilibrium factor. In the present work, the method is extended to CR-39 detectors which have better-controlled etching properties but do not have an upper energy threshold. An exposed bare CR-39 detector is first pre-etched in 6.25N NaOH solution at 70°C for 6h, and then etched electrochemically in a 6.25N NaOH solution with ac voltage of 400V (peak to peak) and 5kHz applied across the detectors for 1h at room temperature. Under these conditions, for tracks corresponding to incident angles larger than or equal to 50°, the treeing efficiency is 0% and 100% for incident energies smaller than and larger than 4MeV, respectively. A simple method is then proposed to obtain the total number of tracks formed below the upper energy threshold of 4MeV, from which the proxy equilibrium factor method can apply.