1 Introduction

Naturally occurring radioactive materials (NORM) are ubiquitous in the environment. The NORM contents in most natural substances are low^[1], but higher contents may arise as the result of human activities, such as mineral extraction and processing, phosphate fertilizer application, forest products, thermal electric production etc.^[2-4] High activity levels of NORM may increase risks to human health and the environment, causing high exposure level to workers or the public, such as typical occupational exposure is 16 mSv/a in part of non-ferrous metal underground mines, but it exceeds the occupational dose limit of 20 mSv/a in a large percentage of the mines. As the result of human activities, rising level of public exposure to NORM radiation is 0.07 mSv/a per person, and collective dose is 7.98×10⁴ Sv/a per person, which is 2.82% of the total radiation dose^[5].

The long-lived radioactive elements, e.g. uranium, thorium and their radioactive decay products, such as radium and radon in minerals, receive more concern for the human health ^[6,7,8,9], particularly in some regions of China^[10,11]. A common terrestrial radiation source is radon gas, which comes from uranium in the soil and accumulates in buildings. Radon inhaled into the pulmonary system may cause cancer. The α-ray it emits is a particle specie of big linear energy transfer, generating high concentration of biologically harmful OH-radicals, which damage cell membranes by oxidation and any other cell constituents that are vital for complex and correct cell functions.

However, there are limited data which can be used to evaluate the U and Th contents in minerals and to establish a regulatory framework to reduce potential dangerous exposure to workers. In this paper, inductively coupled plasma mass spectrometry (HR-ICP-MS), with its advantages of multielement characteristics, speed of analysis and low detection limits^[12], is used to measure U and Th contents in a 60 material samples from 16 mines in China.

2 Materials and Methods

3 Results and Discussion

The 60 samples collected from 16 active mines include coal and mineral ores of iron, copper, tin, zinc, manganese, nickel, gold, mercury, antimony and aluminum. The analytical results are given in Table 2. The content ranges are as follows: U, 0.17 ±0.04–15.3 ±2.39 μg·g⁻¹, averaged at 3.17±3.95 μg·g⁻¹; and Th, 0.19±0.04 – 19.6±7.56 μg·g⁻¹, averaged at 4.03±4.67 μg·g⁻¹. The radioactivity of U and Th was calculated by their natural abundance (U²³⁸, 99.2745%; U²³⁵, 0.7200%; U²³⁴, 0.0055%; and Th²³², 100%): U, 0.43±0.01–38.8±6.08 Bq·g⁻¹, averaged at 8.03±10.05 Bq·g⁻¹; and Th²³², 0.0007±0.0001–0.0790±0.0306 Bq·g⁻¹, averaged at 0.0163±0.0189 Bq·g⁻¹.

The results showed that the U content was low in nickel and antimony ores, but it was high in aluminum and tin ores. The Th content was low in copper ores and high in aluminum ores. The results are in agreement with reported data in the literatures. For example, high radioactivities of U and Th were observed in tin mine area, ranging from 8.7 Bq·g⁻¹ to 51 Bq·g⁻¹ for ²³⁸U and from 16.8 Bq·g⁻¹ to 98 Bq·g⁻¹ for ²³²Th^[13]. Chang et al. reported the natural radioactivity in industrial raw mineral commodities (domestic, 17 kinds; and imported, 18 kinds), radioactivity for ²³²Th (0.357±0.059 Bq·g⁻¹) in Bauxite was higher than other minerals ^[14].

Specific activity is calculated as follows：

N = N₀ e^−λ.t

−dN/dt = N₀ λ t = N₀ (ln2/t_1/2) t = N_A/m (ln2/t_1/2) t =A t

So, specific activity A= N_A/m (ln2/t_1/2), where m is content of the isotope in question in this application.

A weak correlation was observed between Th and U in all samples (R² = 0.6006), indicating that U activities may be related to the presence of Th. Other authors found similar weak correlations ^[15]. The specific activities of Th and U in raw minerals mainly depend on their geological sites of origin and their geochemical properties ^[16].

The guidelines related to the NORM have been introduced by in many countries. Th activities in the 60 samples were much lower than the limits of EU and Canada (Table 3), while the U activities exceeded 10 Bq·g⁻¹ in four minerals and 5 Bq·g⁻¹ in 8 samples. The 12 samples include 3 samples each of the tin and aluminum ores, and one sample each of the zinc, iron, tungsten, copper, coal, and mercury ores.

²³²Th is an alpha-ray emitter with a long half-life and therefore low specific activity. Because of its radiation properties and the biokinetics of ²³²Th following incorporation, it is one of the radioisotopes with the highest radiotoxicity. The previous studies have shown that the activity of ²³²Th in monazite and zircon is 182.425±9.870 Bq·g⁻¹ and 1.195±0.048 Bq·g⁻¹, respectively (Table 4), whereas the other raw minerals show the low activities of Th with the similar ranges of determined activities in our minerals samples.

Since the half-life of uranium is shorter than thorium, the natural radioactivity of uranium is much larger than the contribution of thorium if their contents were little difference. It is difficult to analyze the radioactivity of ²³⁸U by direct γ-ray measurements, instead, radioactivity of Ra is used ^[16]. The ²³⁸U radioactivities in different mineral ores reported in the literatures are given in Table 5. The high radioactivity for ²³⁸U was observed in monazite. In other minerals the radioactivity for ²³⁸U was lower than 0.05 Bq·g⁻¹. With the exception of monazite, our results indicate that the radioactivity content of ²³⁸U is higher than the reported values..

4 Conclusions

Since the high activity levels of NORM increase risks to human health, particularly the long-lived radioactive elements, such as uranium, thorium and the radioactive decay products in minerals receive more concern for the potential dangerous exposure to employees overall the country^[17,18]. The radioactivity content of U and Th in a total of 60 mineral samples collected from 7 provinces was analyzed. The results demonstrate that the radioactivity content of U in at least 12 minerals samples is beyond the limited values of European commission and suggest that the further evaluation of dangerous exposure of U to employee should be taken into account.