Gamma radiation measurements and dose rates in commercially-used natural tiling rocks (granites)

Abstract

The gamma radiation in samples of a variety of natural tiling rocks (granites) imported in Cyprus for use in the building industry was measured, employing high-resolution γ-ray spectroscopy. The rock samples were pulverised, sealed in 1-l plastic Marinelli beakers, and measured in the laboratory with an accumulating time between 10 and 14 h each. From the measured γ-ray spectra, activity concentrations were determined for ²³²Th (range from 1 to 906 Bq kg⁻¹), ²³⁸U (from 1 to 588 Bq kg⁻¹) and ⁴⁰K (from 50 to 1606 Bq kg⁻¹). The total absorbed dose rates in air calculated from the concentrations of the three radionuclides ranged from 7 to 1209 nGy h⁻¹ for full utilization of the materials, from 4 to 605 nGy h⁻¹ for half utilization and from 2 to 302 nGy h⁻¹ for one quarter utilization. The total effective dose rates per person indoors were determined to be between 0.02 and 2.97 mSv y⁻¹ for half utilization of the materials. Applying dose criteria recently recommended by the EU for superficial materials, 25 of the samples meet the exemption dose limit of 0.3 mSv y⁻¹, two of them meet the upper dose limit of 1 mSv y⁻¹ and only one clearly exceeds this limit.

Introduction

Gamma radiation from radionuclides which are characterised by half-lives comparable to the age of the earth, such as ⁴⁰K and the radionuclides from the ²³⁸U and ²³²Th series, and their decay products, represents the main external source of irradiation to the human body. The absorbed dose rate in air from cosmic radiation outdoors at sea level is about 30 nGy h⁻¹ (UNSCEAR, 2000 Report). External exposures to gamma radiation outdoors arise from terrestrial radionuclides occurring at trace levels in all ground formations. Therefore, the natural environmental radiation mainly depends on geological and geographical conditions (Florou and Kritidis, 1992). Higher radiation levels are associated with igneous rocks, such as granite, and lower levels with sedimentary rocks. There are exceptions, however, as some shales and phosphate rocks have relatively high content of radionuclides (UNSCEAR, 2000 Report).

Granites are the most abundant plutonic rocks of mountain belts and continental shield areas. They occur in great batholiths that may occupy thousands of square kilometres and are usually closely associated with quartz monzonite, granodiorite, diorite, and gabbro. They are extremely durable and scratch resistant; their hardness lends themselves for the stone to be mechanically polished to a high gloss finish. Their variety of colour and unique heat and scratch resistant properties makes them ideal for use as work-surfaces, flooring or external and internal cladding. They mainly consist of coarse grains of quartz, potassium feldspar and sodium feldspar. Other common minerals that granites consist of include mica and hornblende. Typical granites are chemically composed of 75% silica, 12% aluminium, less than 5% potassium oxide, less than 5% soda, as well as by lime, iron, magnesia, and titania in smaller quantities. Originally, it was widely believed that granites were formed mainly from magmatic differentiation of basaltic magma, evidence that was considered to indicate a metamorphic origin. However, because of the large quantities of granites that occur in nature, geologists believe now that most of the granites have been formed either by melting, partial melting, or metamorphism of deeply buried shale and sandstone. Granites, therefore, are the result of rapidly injected coalescing sheets of magma, each of which cooled independently of the other sheets. Evidence of intrusion or great mobility indicates an igneous origin that stems from melting of sediments, and consequently granite dykes are clearly igneous (Snelling and Woodmorappe, 1998).

In terms of natural radioactivity, granites exhibit an enhanced elemental concentration of uranium (U) and thorium (Th) compared to the very low abundance of these elements observed in the mantle and the crust of the Earth. Geologists provide an explanation of this behaviour in the course of partial melting and fractional crystallisation of magma, which enables U and Th to be concentrated in the liquid phase and become incorporated into the more silica-rich products. For that reason, igneous rocks of granitic composition are strongly enriched in U and Th (on an average 5 ppm¹ of U and 15 ppm of Th), compared to rocks of basaltic or ultramafic composition (<1 ppm of U) (Faure, 1986, Mènager et al., 1993).

In this paper, the results from gamma radiation measurements in samples of a variety of natural tiling rocks imported in Cyprus and sold under the commercial name of “granites” are presented. These results are of general interest since such rocks are globally used as building and ornamental materials. The measurements have been carried out in the Nuclear Physics Laboratory of the Department of Physics, University of Cyprus, using a high-resolution γ-ray spectroscopic system.