Two procedures, based on experience with the SLIM experiment , have been defined for the chemical etching of CR 39 detectors: the strong and soft etching conditions:
The threshold for CR39 is Z/β ~ 14 in strong etching corresponding to a minimum RELmin ∼ 200 MeV g-1 cm2. In soft etching conditions the threshold is at Z/β∼ 7 which corresponds to RELmin 50 MeV g−1 cm2. In the case where CR 39 is polymerized with 0.1% DOP additive the thresholds are raised to Z/β ∼ 21 (RELmin ∼ 460 MeV g−1 cm2) for strong etching and to Z/β ∼ 13 (RELmin ∼ 170 MeV g−1 cm2) for soft etching. The etching conditions for Makrofol are 6N KOH + 20% ethyl alcohol at 50C for 10 h, and the corresponding threshold is Z/β∼50, RELmin ∼ 2.5 GeV g−1 cm2. The chemical etching is performed in the Nuclear Track Etch Detector Laboratory of INFN, in Bologna. Stainless steel tanks are used: two large tanks of 40 x 55 x 40 cm3 dimensions, a medium tank 40 x 4 x 26 cm3 and two small tanks 22 x 34 x 16.5 cm3 having different types of stirring and heating systems and of temperature control. The stability of the etching temperature throughout the solution is maintained to within 0.01%. The detector foils are placed in the stainless steel detector holders in the etching solution inside the tanks. There are three other tanks which are used for the presoaking and washing of the detectors.