Pergamon Radiation Measurements, Vol. 27, No. 3, pp. 453-456, 1997 ~ 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S1350-4487(97)00013-9 1350-4487/97 $17.00 + 0.00 ON THE CRITICAL ETCHING PARAMETERS OF TRACK DETECTORS K. K. DWIVEDI Department of Chemistry, North-Eastern Hill University, Shillong 793 003, India (Received 11 March 1996; revised 28 November 1996; accepted 24 February 1997) Abstract--The etching characteristics of mica, Lexan and cellulose acetate have been studied for fission fragment tracks. Under suitable etching conditions a few critical etching parameters (viz. critical track diameter, critical cone angle and critical angle of incidence) for these three track detectors have been determined. An empirical relationship between complete etching time and the etching temperature has been established. © 1997 Elsevier Science Ltd 1. INTRODUCTION It is well known that the latent damage trails of heavy ions in solid dielectrics are conveniently revealed by suitable chemical etching processes and that the optimal use of any track detector is largely dependent on standardization of various etch parameters. The formation of etchable tracks in a track detector depends on certain critical etch parameters which must be experimentally determined under suitable etching conditions. The determination of critical etch parameters is essential for employing track detectors in the fields of alpha micro-mapping, micro-analysis and radon dosimetry (Chambaudet et al., 1995); particle identification (Majeed et al., 1993); low energy particle dosimetry (Luszik-Bhadra et al., 1995), and in determination of the true track length of heavy ions (Dwivedi and Mukherji, 1979; Singh et al., 1988; Bhattacharyya et al., 1992; Ghosh et al., 1995). Under given etching conditions, the time required by an etchant to develop damage trails to their maximum length is known as the complete etching time (t~). This parameter has to be known for the determination of the maximum etchable track length (Lc) of a heavy ion in any track detector from the experimentally observed track length (Dwivedi, 1977). Since it requires a laborious experiment at each temperature for obtaining to, an empirical method for calculating tc would be very useful. While studying track-etch kinetics it has been observed that for each solid state track detector there exists a critical value of track diameter D~ at which all the fission fragment tracks were found to be etched to their full lengths. At different temperatures the time to, required to enlarge the track diameter to a value D, have been obtained for mica, Lexan and cellulose acetate. The plots of In(to) vs temperature yield straight lines. An empirical relationship between the complete etching time tc and temperature z has been obtained. From the measured track parameters, the critical angle of incidence for these track detectors have also been determined. 2. FORMULAE AND DEFINITIONS 2.1. True track length (L) A schematic diagram showing track geometry and different track parameters is given in Fig. 1. The true track length (L) after any etching time (t) greater than complete etching time (to) is related to the measured projected length (l) by the following equations (Dwivedi and Mukherji, 1979). (a) For plastic detectors with finite bulk-etch rate (vo): L =//cos • + VG.t/sin • -- VG(t -- tc) (1) where • is the angle of incidence, V~ is the bulk-etch rate, t is the actual etching time larger than the time t¢ required for complete etching of the tracks. (b) For mica detector having a negligible bulk-etch rate perpendicular to the surface plane: L =//cos • (2) where I is the projected length of etched tracks after time t. 2.2. Maximum etchable track length (Lc) The maximum etchable track length (L~) is defined as true track length (L) after an etching time t = t~. Hence equation (1) and equation (2) can be written as Lc =/dcos • + VG'tdsin q) (3) and L~ =/c/cos • (4) 453