Pergamon Radiation Measurements, Vol. 28, Nos I -6 , pp.667-670, 1997

© 1997 Elsevier Science Ltd Printed in Great Britain. All r ights reserved

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R A D O N C O N C E N T R A T I O N S I N T H E P Y R A M I D O F T H E S U N

A T T E O T I H U A C A N "

G. ESPINOSA I, L. MANZANILLA 2, AND R.B. GAMMAGE 3

qnstituto de Fisica, UNAM, Apdo. Postal 20-364, 01000, Mexico, DF, MEXICO

21nst. de lnvestig. Antropol., UNAM, Circuito Exterior, C.U., Mexico, DF, MEXICO

3Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6379. USA

ABSTRACT

Measurements of radon were taken inside the Pyramid of the Sun at the pro-Columbian Teotihuacan site in Mexico. Beneath the Pyramid of the Sun, there is a prehispanic tunnel that was used for ritual ceremonies. We evaluated the radon levels inside this and other tunnels plus tunnels excavated for research purposes by archaeologists. Passive nuclear track detectors were used to measure radon. Measurements were made in winter during the dry season because the tunnels flood in the rainy season. Radon ranged from 70 to 190 Bq m 3. The radon levels decreased with height above ground level which probably reflects relatively easy escape routes for radon through the loosely stacked volcanic stores in the outer wall covering.

KEYWORDS

Radon exhalation; radon concentration; solid state nuclear track methodology; Teotihuacan pyramid.

INTRODUCTION

Teotihuacan, a prehispanic city of 20 km 2 located in Mexico's basin, was the first large-scale urban development in Meso-America. It was well planned as a manufacturing center, trade center, model of the mesoamerican cosmos, and it was developed during the first eight centuries of the Christian era (Millon, 1973). Teotihuacan was the Holy City of population 125,000 and a center of pilgrimage.

Built in the first century AD, the Pyramid of the Sun, the largest monumental structure is basically made with compressed soil, being different from other constructions of Teotihuacan that were made with volcanic remains. The pyramid has a square base of 215 x 215 m 2 and a height of 65 m.

The pyramid is located on a well-leveled terrain and bordered by a platform. On the west side it has a platform built a few centuries afcer the original construction. Its main facade faces west and its sides have a deviation of 15o25 ' east of astronomical north; some people think that this orientation is related to the Pleiades constellation set at the beginning of the summer solstice, when the rainy season begins (Towsend, 1993). The pyramid is situated at latitude 19°41'30 '' north and longitude 98050'30 '' west (Millon, 1973).

This pyramid was a temple probably dedicated to the god Tlaloc (Teotihuacan Fately Deity) in its advocation of Theocatecuhtli, lord of sustenance and fertility, according to XVI century sources telling that the idol on its top was such a god (Manzaniila, 1994).

* Oak Ridge National Laboratory, managed by Lockheed Energy Research Corp. of U.S. Department of Energy under contract number DE-AC05-96OR22464.

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668 PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE

There are three tunnels in the Pyramid of the Sun. The first tunnel (I), winds under the structure below ground level, is excavated into tuff, basaltic blocks, and volcanic pyroplasts and runs from west to east. This tunnel was excavated in prehispanic times at the beginning of the Christian era and has a length of approximately 100 m. It ends in a chamber divided into four sections that were used for ceremonial purposes (Heyden, 1975).

The second and higher tunnel (II) was excavated by the archaeologists Gamio and Noguera in the 1920"s and 1930"s, running from west to east along the first body of the pyramid. It is one meter wide and two meters high with two portions connected by a ladder; the portion from the east to the middle is located six meters above the portion from the west to the middle. The walls are soil mixed with the volcanic tuff fragments from 5 to 10 cm long, and also ceramic and coal fragments. This soil is native material. It has been dated to 80 + 75 years AD using the radiocarbon method (Millon et al, 1965; Rattray, 1974).

The third and higher yet tunnel (III) is located in the fifth body of the pyramid, with an entrance on the east side. It is one meter wide, two meters high and 30 meters long. It was excavated by the archeologist R. E. Smith. It traverses several types of mud and volcanic tuff filling (Smith, 1987).

On the east of the platform of the Pyramid of the Sun there are two large depressions formed by extruction of the volcanic scoria where the pyramid was built. The ensuing extensive excavations produced the "Cave of the Varillas" (tunnel 1, in 1993 and 1994), and "Cave of the Pirul" (tunnel 2, in 1994 and 1995) (Manzanilla et al, 1989; Manzanilla et al, 1994). It was at each of these locations that radon measurements were performed.

METHOD AND MATERIALS

The passive method of integration by Nuclear Tracks in Solids (NTS) (Fleischer et al, 1975) was used in the evaluation of radon levels inside the tunnels of the pyramid (1, I1, III) and the caves CPirui" and "Varillas"). The detector material was CR-39 (allyl diglycol carbonate) chosen because of its high sensitivity.

This CR-39 material is 500 p.m thick, cut in 2.5 x 1.0 cm 2 chips, and labeled with a laser beam. The device used was the cup system (Espinosa and Gammage, 1993) with a 330 cm 3 volume, and a drying material to keep the relative humidity constant. The cups were placed along tunnels I, II and III and the two caves at approximately every 10 steps. The temperature during the three months of the experiment ranged between 7 and 20°C and the relative humidity between 60 and 80%. The humidity was highest in Tunnel I. All the detectors were subject to the same treatment and handling. The etching conditions were KOH solution 6.25M, 60°C and 16 hours of chemical etching. The readings of etch pits were done atterwards with an automatic system (Espinosa and Gammage, 1996)

The measurements were done during the winter avoiding the rainy season, carrying out three sets of measurements, one per month for each position and with two detectors each time. The calibration of the detectors was performed at the Oak Ridge National Laboratory.

RESULTS AND DISCUSSION

The radon concentration is due fundamentally to the contribution of 222Rn arising from the subsoil and the large mass of the compressed soil forming the pyramid. Unlike other types of pyramid, this one is covered with fragments of volcanic stone and not with massive blocks. There are, therefore, more or less easy routes for the radon gas to escape into the atmosphere. The overall radon levels are not excessively high. Only Tunnel I has a tightly closing door and thus the ventilation is not good. In Tunnels II and III there are fence gates, so that there is a free access of air from the outside, but without production of a real current of fresh air because both tunnels are closed at their ends. The net result is that the air in Tunnel I has the highest radon concentration. Table I shows the radon level measurements in Tunnels I, II and Ill in the pyramid and in the caves "La Varilla" and "El Pirul".

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Table 1.- Radon Levels Inside the Pyramid of the Sun.

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Cup Position Cup Location Average Radon Level

( Bq m "3 )

1 - 5 Tunnel I 177 + 13

6-10 Tunnel II 144 + 16

1 !-15 Tunnel III 93 + 17

16-20 Varilla Cave 89 + 17

21-25 Pirul Cave 149 + 17

Unlike the levels reported for the interior of the Cheops Pyramid in Egypt (Kenawy and Morsy, 1991), the radon levels in the Pyramid of the Sun in Teotihuacan diminish with the height in the Pyramid according to the position of the tunnels. This is an indication that the Pyramid of the Sun is, in all probability, a great mass of compressed soil, and that in the vicinity of the Tunnel I there are unlikely to be additional undiscovered chambers. The exterior cover of thin volcanic stones presents large cracks allowing the escape of radon gas.

The concentration of radon along the tunnels is more or less homogeneous. The results of the evaluation of radon in "Las Varillas" and "El Pirul" caves yield between 70 and 190 Bq m "3. The radon-related health consequences to persons working in these tunnels and caves are expected to be quite small, provided that the personnel for limited perods of time.

CONCLUSION

In contrast to the Great Pyramid of "Cheops" at Giza, Egypt, the radon concentration in the Pyramid of the Sun, Teotihuacan, diminishes as a function of height in the pyramid. This difference is probably due to the covering of the walls being permeable to the radon gas. This covering presents a high porosity, being thin and with large spaces between the volcanic stones of the outer covering.

These measurements give some indications that inside the Pyramid of the Sun there do not exist undiscovered, interior chambers and that most of the compositional material is characteristic soil of the region.

Acknowledgements-The authors wish thank to JT Golzarri for his technical help. We extend our appreciation to Dr. E. Ley-Koo for his valuable discussions.

REFERENCES

Espinosa, G. and Gammage, R.B.. (1993). Measurements methodology for indoor radon using passive track detectors. Appl. Radiat. lsot. 4, 719-723.

Espinosa, G. and Gammage, R.B. (1996). Digital image system for track measurements. Radiation Measurements. (in press).

Fieischer, R.L., Price, P.B. and Walker, R.M. (1975). Nuclear tracks in solids, principles and applications. Berkeley: University of California Press.

Heyden, D. (1975), An interpretation of the cave underneath the pyramid of the sun in Teotihuacan, Mexico, American Antiquity 40, 131-147.

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Kenawy, M.A. and Morsy, A. (1991). Radon measurements in the interior of the great pyramid. Nuclear Track Radiat. Meas. 19, 347-349.

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Manzaniila, L., Barba, L., Chfivez, R., Arzate J. and Flores, L. (1989), El inframundo de Teotihuacan, geofisica y arqueologia, Cienciay Desarrollo, XV, 85, 21-35.

Manzanilla, L., Barba, L., Chfivez, R., Tejero, A., Cifuentes G. and Peraita, N. (1994), Caves and geophysics; an approximation to the underworld of Teotihuacan, Mexico, Archaeometry 36, 141- 157.

Millon, R., (1973), The Teotihuacan map, part one: Text, University of Texas Press., Austin.

Millon, R., Drewitt, B. y Bennyhoff, J.A. (1965), The pyramid of the sun at Teotihuacan: 1959 Investigations. The American Philosophical Society, Philadelphia. Transactions n.s, 55, 6, September.

Rattray, E.C., (1974), Some clarifications on the early Teotihuacan ceramics sequence, XL1 Congreso Internacional de Americanistas, M6xico, D.F. 364-368.

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Towsend, R.F. (1993), La antigua Am6rica, Grupo Azabache, M6xico.