This is a report concerning the fission-track dating of archaeological remains excavated at the Douara Cave in Syria. One of the most important reasons for the study of the remains was to clarify the age of the cave. Some C14 dating was carried out by K. Kigoshi (1973) and H. Kobayashi (1973), but the samples exceeded the limit of 43,900 years. However a baked pebble of barite was discovered in a hearth deposit, and fission-track dating was carried out: the age was determined to be 75,000 y.B.P.
INTRODUCTION
Dating by fission-track counting (Price et al., 1963) has been used by several workers to date rock minerals and baked relics (Brill et al., 1964; Watanabe and Suzuki, 1969; Nishi-mura and Tosi, 1976). This report is the result of an experiment carried out in order to explore the possibility of application of the technique to archaeological remains. Whereas fission-track dating is well-suited to measuring the ages of materials which are in the order of millions of years old, it can be applied to much younger man-made or man-used materials only if they have large crystal surfaces or contain appreciable amounts of uranium.
The Douara Cave site, spanning the Middle and Epi-Paleolithic, is located about 200 km north-east of Damascus, Syria. This cave was seen from the beginning as the most promising of the newly discovered cave site because many flint implements of types from the Middle to the Post-Paleolithic were scattered on its terrace surface. Therefore, the Fourth Tokyo University Expedition to Western Asia carried out a systematic excavation of the Douara Cave for two months, August and September in 1970. Preliminary results were presented in 1973.
PRINCIPLE OF THE TECHNIQUE
The fission-track dating method depends on the spontaneous fission of U238
atoms in minerals or glass taking place at a constant rate and leaving fission-tracks.
Once formed, the fission-tracks disappear if the material is heated above
a critical temperature. The fission-track age, T y, can be represented by
the following equation,
(1)
where ρs is the fossil track density (cm-2), ρ1 is the track density induced by bombardment with thermal neutrons (cm-2), λf is the fission decay constant for U238 (we use 6.85× 10-17 y-1, Nishimura, 1975), λ is the decay constant for uranium (y-1), φ is the thermal neutron dose (cm-2), σ is the thermal neutron cross section for fission of U235 (cm2), η is the isotope ratio U235/U238, and Rs and Ri are the mean values of the etchable length of the fragments produced by the spontaneous fission and the induced fission (cm).
If T is smaller than 109 y and Rs=Ri, equation (1) can be written
(2)
For the reactor run, a small amount of barite was packed in a plastic container and irradiated in a reactor. Neutron dose φ was also obtained by the fission track method (Hashimoto et al., 1969).
APPLICATION TO THE BAKED PEBBLE
About 1.4 m deep in the cave a large fireplace (7 × 4 m) was excavated. Surrounding the fireplace many round pebbles were found. One of these pebbles has a star-like cavity filled with large barite crystals. Barite crystals were collected by hand-picking. They are colorless and transparent and have about 5×5 mm crystalline surfaces.
Part of these were irradiated by neutron flux and etched by 70% HNO3 at 100°C for the determination of a suitable etching time.
Testing for thermal fading of fission-tracks in these minerals was carried
out. A sample having a large uniform induced track density was selected and
irradiated with a 1 × 1015 thermal neutron dose in the research reactor
of Kyoto University. After etching, the density of the tracks was determined
in the annealed and unannealded samples and variation of the density studied
as a function of time and temperature. Figure 1 shows that the track-length
decreases to a greater degree than track density is reduced at all temperatures.
A similar non-linear relation (Fig. 1) has also been observed between track
length reduction and track density reduction in apatite.
Annealing experiments were performed to determine the stability of fission-tracks
in this mineral. From these results (Fig. 2) it was determined that tracks
in this mineral could be easily annealed around a fire place, so the fission-track
age of this mineral may be esti-mated as the last age of heating.
RESULTS
The results of the experiment (Table 1) give 75,000 y.B.P. as the fission-track
age of the barite in the baked pebble. Among the pebbles, only one pebble
has a star-like cavity filled with large crystals; the others do not have
these large crystals.
Compared with the C14 method, the fission-track technique may offer better dates for archeological discoveries, provided that finds include high concentrations of uranium and/or large crystal surfaces which show spontaneous fission-track densities enough to be measured.
LITERATURE CITED
- Brill, R.B., Fleischer, L., Price, P.B. and Walker, R.M. (1964)
- The fission track dating of man-made glasses.: Preliminary results. Jour. Glass Stud., 6: 151-155.
- Hashimoto, T., Iwata, S., Nishimura, S., Nakanishi, T. and Sakanoue, M. (1969)
- Measurements of reactor neutron flux by fission track method. Proc. 9th Jap. Conf. Radioisotopes, 133-134.
- Kigoshi, K. (1973)
- Gakushuin University Radiocarbon Datings. In: The Palaeolithic Site at Douara Cave in Syria, Part I (Suzuki, H. and Takai, F., eds.), Bull. Univ. Museum, Univ. Tokyo, 5: 143-144.
- Kobayashi, H. (1973)
- The radiocarbon datings of TK-111 (a), TK-111 (b), and TK-112, by the University of Tokyo. In: The Palaeolithic Site at Douara Cave in Syria, Part I. (Suzuki, H. and Takai, P., eds.), Bull. Univ. Museum, Univ. Tokyo, 5: 145-146.
- Nishimura, S. (1975)
- On the value of the decay constant for spontaneous fission of Uranium-238. Mem. Eac. Sci., Kyoto Univ., Ser, Geol. Min., 16: 15-19.
- Nishimura, S. and Tosi, M. (1976)
- Fission-track ages of the remains excavated at Shar-i Sokhta and Kangavar, Iran. Mem. 6th Intern. Congr. Iranian Art Archaeol, 281-285.
- Price, P.B. and Walker, R.M. (1963)
- Fossil tracks of charged particles in mica and age of minerals. Jour. Geophys. Res., 68: 4847-4852.
- Watanabe, N. and Suzuki, M. (1969)
- Fission track dating of archaeological glass materials from Japan. Nature, 222: 1057-1058.