Track length measurements

The application of fission track length studies to the interpretation of fission track ages depends on three properties of spontaneous fission tracks. 

All tracks in apatite have a very similar initial length (Gleadow et al. 1986), which is controlled by the energetics of the fission decay and the nature of the track recording material (e g., apatite). 

Tracks become progressively shorter during exposure to elevated temperatures so that the final length is controlled principally by the maximum temperature that each track has experienced. 

New tracks are continually added to the sample through time so that each one has experienced a different fraction of the total thermal history. 

When combined with the apparent fission track age, length distributions can be used to reconstruct the variation of temperature through time. 

---

The positioning scan is designed to accurately position every spot on a measuring track. The parameters for measuring, like track length, track distance, number of tracks and spots per track, mode of evaluation as well as step sizes for the search of spots and positioning, spot width and data per spot for the integration are entered into the computer. If a method is elaborated, these parameters can be stored and need not be repeatedly entered for each scan. 

Figure 10. Representative track length distributions for spontaneous tracks in the various apatite length groups recognized by Gleadow et al. (1986). The top row represents measurements on horizontal confined tracks for which the differences between the different types are more distinctive than for the corresponding projected length distributions (bottom row). 100 confined tracks and 500 projected track lengths were measured in each case. After Gleadow et al. (1986).

Laslett et al. (1987) based their model on a fanning Arrhenius relationship (Fig. 14) between reduced track length r (= l/lo where / is the measured track length and lo is the initial length), log time i), and inverse absolute temperature (7). They derived the following equation for constant temperature annealing that accounted for 98% of the variation in the observed data ... 

Bertel E, Mark TD, Pahl M (1977) A new method for the measurement of the mean etchable track length and of extremely liigh fission track densities in minerals. Nucl Track Detection 1 123-126... 

Laslett GM, Kendall WS, Gleadow AJW, Duddy IR (1982) Bias in measurement of fission-track length distributioa Nucl Tracks 6 79-85... 

The motion of the needle tip was measured experimentally and compared with predictions of the analytical model. The tip motion is measured with a magnetic position sensor (miniBIRD model 80 from Ascension Technology). This electromagnetic tracking system measures the three positions and three orientations of a small sensor (1.3 mm in diameter, 6.5 mm in length) with RMS accuracies of 1.4 mm and 0.5°. The sensor is small compared to the manipulator and does not affect its performance. 

The cost of a cylindrical drift chamber is much less than that of a calorimeter, and not a strong function of radius. The charged particle transverse momentum resolution depends on where L is the measured transverse track length for a given axial... 

Mesospheric sodium atoms excited at the 3Ps/2 level scatter light in every direction. The backscattered beam observed at an auxiliary telescope B meters away from the main one looks like a plume strip with an angular length (p B 8h / where 8h stands for the thickness of the sodium layer. The tilt of the wavefront at the auxiliary telescope and vibrations equally affects the plume and the NGS. Thus departures of the plume from the average NGS location is due to the only tilt on the upward laser beam. Therefore measuring this departure allows us to know the actual location of the LGS, and to derive the tdt. Because of Earth rotation and of perspective effects, the auxiliary telescope has to track the diurnal rotation, and simultaneously to move on the ground to keep aligned the NGS and the LGS plume. Two mobile auxiliary telescopes are necessary for the two components of the tilt. 

---