Introduction

The historic Jeroným Mine is located near the former Čistá municipality (also known as Lauterbach Stadt) in the Sokolov District of the Czech Republic (Fig. 1). The locality is a part of a protected landscape, the Slavkovsky Les Mountains, which border the Bohemian Massif. The Bohemian Massif comprises part of the Variscan belt of Central Europe and hosts a number of uranium deposits found in both the Czech Republic and eastern Germany. Total historical uranium production reached approximately 350,000 t making the Bohemian Massif the most important uranium ore district in Europe (Kříbek et al. 2009). The Jeroným Mine represents a metalliferous deposit that yielded tin, tungsten, silver, gold, bismuth and uranium (Beran and Sejkora 2006). Mine workings date back to the first half of the sixteenth century and extraction occurred with interruptions until the early twentieth century. The nearby town of Čistá was impacted by several events after World War II and then later completely destroyed during a military training operation (Raška and Kirchner 2011). The long term subsurface mining activities and other human impacts have led to the designation of the Jeroným Mine as a National Heritage Site open to the public as part of the greater Czech Bavarian Jeopard.

Fig. 1
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photos: M. Rösnerová and Z. Kalab

The location of the Jeroným Mine on a map of the Czech Republic with recent photos of its access shafts and surroundings ()

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Because World War II and post-war events destroyed archival documents and written records concerning the mine’s history, large areas of the Jeroným Mine have not been evaluated for safety or access. The cessation of mining activities also led to flooding of some of the workings. The mining museum, opened in the twenty-first century, offers access to mine workings and chambers which feature unique, historical tools and infrastructure (Žůrek and Kořínek 2001; Kaláb et al. 2006). Basic geological and geophysical analysis have been used to evaluate the safety of the site for visitors and museum workers. These studies have documented the structural condition and weathering of the rock massif in order to determine the stability of the mine (Lednická and Kaláb 2012, 2016). The mine itself extends to relatively shallow depths of only about 30–50 m below the surface. In preparation for opening of the site to the public, hydrological monitoring was also conducted to determine outflows, interconnections and the response of mine water levels to influent flows from intensive precipitation (Kaláb et al. 2010a, b). Hydrologic monitoring also addressed waters accumulated in closed, undrained areas of the mine.

The present study reports laboratory gamma-ray spectrometry measurements of natural radioactivity levels from characteristic rocks samples found in the mine. The 40K, 232Th and 238U activity concentrations for representative granites and schists were compared to values obtained for similar rocks types as reported in the literature. Activity concentrations were also used to estimate standard indices for assessing radiological safety of building materials and surroundings.

Geological setting and sample locations

Geologically, the Jeroným Mine accesses metamorphosed rocks belonging to the Slavkov mantle crystalline complex and Variscan granites of the Ore Mountains pluton. The area sampled occurred near the contact of major geological units, the Ore Mountain and Tepla-Barrandium megablock which formed at around 265 Ma. The immediate surroundings as well as the Jeroným Mine itself include altered acidic granites hosting prominent Variscan tin-tungsten mineralization. Intensive weathering and the age of the workings have destabilized some areas of the mine.

The Jeroným Mine is a shaft mine consisting of subsurface galleries, shafts and chambers spread across at least three horizontal levels ranging in depth from 10 to 50 m below the surface. Several recent papers have evaluated of structural stability of the mine (Froňka et al. 2013; Lednická and Kaláb 2013; Kaláb and Lednická 2016; Lyubushin et al. 2014). The lowest level is permanently flooded making its scope is unknown. Some parts of the mine have been re-opened or recently developed. Intensive weathering and the age of the workings have destabilized other areas of the mine. Certain localities consist of fissured and weathered supporting pillars or roof layers in chambers. Figure 2 shows a sketch of the Jeroným Mine with sampling locations while Fig. 3 shows photos of typical sampling sites within the mine.

Fig. 2
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Diagram of the Jeroným Mine with sampling locations marked. Table 1 lists sample descriptions

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Fig. 3
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Photos of typical sampling sites in the Jeroným Mine (photos: M. Rösnerová). A JK3 granite collection site from the pillar in a large chamber; B JK5 granite collection site from the seasonally flooded zone; C JK5K schist collection site from loose rock material; D JCH3 schist collection site from a gallery mined out during World War II.

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Materials and methods

Granite and schist samples from the Jeroným Mine were dried, crushed and placed in Marinelli-450 beakers. Samples were analyzed a few months after collection using a GX3020 HPGe detector in a lead and copper shield (60 mm) with a multichannel InSpector 2000 DSP buffer. The GX3020 HPGe system uses a coaxial HPGe Extended Range detector with 32% relative efficiency, a detector bias voltage of 3000 V and energy resolutions of 0.86 keV at 122 keV and 1.76 keV at 1332 keV. The LabSOCS (Laboratory Sourceless Calibration Software) and Genie 2000 v.4 software packages performed efficiency calibration and estimated radionuclides and their activities. The spectrometer energy was calibrated using homogeneously dispersed 241Am, 109Cd, 139Ce, 57Co, 60Co, 137Cs, 113Sn, 85Sr, 88Y and 203Hg radioisotopes in a silicone resin [certificate source type Marinelli Beaker Standard Source (MBSS) supplied by the Czech Metrological Institute]. Activities for the target radionuclides were calculated from the following gamma transitions (energy in keV): 40K (1460.8), 208Tl (277.4, 583.1, 860.6 and 2614.5), 212Pb (238.6 and 300.0), 214Pb (241.9, 295.2 and 351.9), 214Bi (609.3, 768.3, 1120.3 and 1764.5) and 228Ac (338.3, 911.6, 964.6 and 969.1). A single measurement lasted 24 h. Measurements were performed at the Laboratory of Natural Radioactivity (Institute of Earth Sciences, University of Silesia). Figure 4 shows typical gamma-ray spectra for granite and schist samples JK3 and JCH3.

Fig. 4
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Gamma-ray spectra from granite sample (JK3) and schist sample (JCH3). Characteristic gamma-ray emitters are marked above the corresponding peaks

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Results and discussion

Table 1 lists measured 40K, 208Tl, 212Pb, 228Ac, 214Pb, 214Bi, and 226Ra activity concentrations for the nine Jeroným Mine rock samples.

Table 1 Activity concentrations of primordial radionuclides as measured in rock samples under laboratory conditions
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40K

As seen in Fig. 5 and Table 1, sample K9 (weathered granite) gave the lowest observed 40K activity concentration of 595 Bq kg−1. The granite K2 gave the maximum 40K activity observed of 1244 Bq kg−1. Weathered granite samples JK3 and K42 containing large, visible amounts of potassium feldspar gave the next highest 40K values of 1141 Bq kg−1 and 1136 Bq kg−1 (respectively). Two schist samples JK5K and JCH3 gave similar 40K activity concentrations of 1101 and 912 Bq kg−1, respectively. Three other samples of weathered granite gave relatively low 40K activity values ranging from 668 to 692 Bq kg−1. These rock samples showed very little in the way of darker mineral content. As shown in Fig. 5, the average 40K activity value of 907 Bq kg−1 slightly exceeds the average 40K activity concentration of 850 Bq kg−1 estimated for the continental crust (Eisenbud and Gessel 1997). This indicates that the study area is characterized by a normal 40K radiation levels.

Fig. 5
figure 5

Measured 40K activity values. Thick solid line: average 40K value measured from all samples. Thin solid line: average 40K value reported for the continental crust

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Granites

Figure 6 shows the 40K activity concentrations for granite samples (excluding samples JK5K and JCH3), which gave an arithmetic mean value of 878 Bq kg−1. As seen in Fig. 6, this value falls below the average value of 1200 Bq kg−1 reported for typical granites (Eisenbud and Gessel 1997; Van Schmus 1995) and below values of 1100 Bq kg−1 measured for Čistá type granites found in the study area (Krešl and Vaňková, 1978).

Fig. 6
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Measured 40K activities from granite samples (excluding schist samples JK5K and JCH3). Thick solid line: average 40K activity measured from granite samples. Thin solid line: average 40K activity reported for typical granites

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Albitized and greisenized granites K9, JK5 and K1 gave the lowest 40K values observed from sampling sites located in the eastern part of the Jeroným Mine. The low values may reflect the albitization process in which the granites experienced hydration. A major increase in Na and loss of K could have resulted in considerably lower 40K activity values for these granite samples. The highest 40K activity of 1244 Bq kg−1 (K2) slightly exceeded the average value for typical granites. This sample along with granites JK3 and K42 occurred in the western part of the mine.

Within measurement uncertainty, the average 40K activity of 878 Bq kg−1 for granite samples strongly resembled the 887 Bq kg−1 average value determined from in situ measurements of Izera Block granites (Malczewski et al. 2004, 2005). Located 250 km away in SW Poland, the Izera Block exhibits similar geological structure to that of the Slavkovsky Les Mountains. The block also hosts metalliferous deposits of tin, cobalt, copper and bismuth. Laboratory measurements of a similar weathered granite from the Sławniowice quarry in the Opava Mountains (Poland) gave significantly higher 40K activity values of 1560 Bq kg−1 (Dżaluk et al. 2018). Papadopoulos et al. (2016) reported 40K activity values ranging from 148 Bq kg−1 to 2518 Bq kg−1 with a mean value of 1097 Bq kg−1 for 70 samples of granites collected from plutons in western Anatolia (Turkey). Commercial granite rock used as building materials investigated by Pavlidou et al. (2006; 16 samples from Greece) and Tzortzis et al. (2003; 28 samples from Cyprus) gave mean 40K activity values of 1104 Bq kg−1 and 1215 Bq kg−1, respectively. Commercial granite samples from Brazil gave mean 40K activity values ranging from 190 to 2029 Bq kg−1 and an arithmetic mean of 1320 Bq kg−1 (Anjos et al. 2011). One of the highest mean values reported for granite in the literature came from a rock in the Wadi Karim area of Egypt, which gave a 40K activity value of 4849 Bq kg−1. Samples collected in the Um Taghir region of Egypt (El-Arabi 2007) and from India, gave the maximum observed values of 10,230 and 10,990 Bq kg−1, respectively.

Schists

The two schist samples, JK5K and JCH3, gave an average 40K activity value of 1007 Bq kg−1. This value slightly exceeds that measured from Izera Block schists (960 Bq kg−1) using in situ methods (Malczewski et al. 2004 and 2005). It also exceeds values of 822 Bq kg−1 given for the USGS mica schist standard SDC-1.

232Th series (228Ac, 212Pb, and 208Tl)

Table 1 shows that rock samples have achieved radioactive equilibrium among 232Th series daughter products. Since 228Ac represents the second radionuclide in the thorium decay series, 232Th activity is assumed to equal 228Ac activity. Figure 7 shows that sample K9 (weathered granite) gave the lowest 232Th activity of 25 Bq kg−1, whereas sample JCH3 (schist) gave the highest 232Th activity value observed of 55 Bq kg−1. All samples gave an average 232Th activity value of 33 Bq kg−1, which fell below the continental crust of 44 Bq kg−1. This indicates relatively low and safe levels of background radiation within the Jeroným Mine.

Fig. 7
figure 7

Measured 232Th activity values. Thick solid line: average 232Th value from all samples. Thin solid line: average 232Th value reported for the continental crust

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Granites

The seven granite samples from the Jeroným Mine gave 232Th activity values that fell within a narrow 25 Bq kg−1 (K9) to 33 Bq kg−1 (JK5) range. Figure 8 shows that the 28 Bq kg−1 arithmetic mean for these granites falls significantly below average 232Th activity values or the 70 Bq kg−1 value reported for typical granites (Eisenbud and Gessel 1997). None of the 232Th activities measured in this study exceeded this value. However, the average 232Th activity exceeded mean values of 18 Bq kg−1 reported for Ĉistá type granites (Krešl and Vaňková, 1978). Similar to 40K values, the average 232Th activity for the Jeroným Mine granites resembled average values within uncertainties for Izera Block granites (29 Bq kg−1) as measured by in situ methods (Malczewski et al. 2005). Granites from the easternmost part of the Sudetes (Opava Mountains) gave 232Th values ranging from 7 (weathered granite) to 54 Bq kg−1 (granite) (Dżaluk et al. 2018). Papadopoulos et al. (2016) reported 232Th activities ranging from 0.14 to 241 Bq kg−1 with a mean value of 90 Bq kg−1. These values significantly exceeded average 232Th values reported here for mine samples. Seven granite samples found in southeastern Eskisehir (Kaymaz, Turkey) gave higher values ranging from 165 to 352 Bq kg−1 (average of 248 Bq kg−1; Örgün et al. 2005). The highest 232Th activity value reported (3834 Bq kg−1) derived from anomalous granite samples from the Um Taghir region of Egypt (El-Arabi 2007).

Fig. 8
figure 8

Measured 232Th activities from granite samples (excluding schist samples JK5K and JCH3). Thick solid line: average 232Th activity from granite samples. Thin solid line: average 232Th activity reported for typical granites

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The 232Th activities measured from Jeroným Mine granites fell below activities measured from granites used as building materials and quarried from various global localities. Brazilian, Indian and Swedish commercial granites gave respective average 232Th activity values of 106, 172 and 110 Bq kg−1 (Anjos et al. 2011; Chen and Lin 1996). Typical commercial granites from Greece and Sardinia gave respective 232Th activity values of 77 and 66 Bq kg−1 (Papadopoulos et al. 2012; Dentoni et al. 2020). Commercial granites from Japan gave lower 232Th activity values of 40 Bq kg−1 (Hassan et al. 2010).

Schists

Two schists (samples JK5K and JCH3) gave respective 232Th activity values of 51 and 55 Bq kg−1 and an average value of 53 Bq kg−1. These values exceed values measured from granites by as much as a factor of two. The values resemble those measured in situ from Izera Block schists (43 and 48 Bq kg−1) (Malczewski et al. 2005). The 53 Bq kg−1 average for 232Th activity values resembles the 232Th activity reported for the USGS mica schist standard SDC-1 (46 Bq kg−1).

238U series (214Pb, 214Bi and 226Ra)

Activity concentrations for 238U were estimated assuming radioactive equilibrium within the 238U → 226Ra → 222Rn → 214Pb → 214Bi decay chain. We estimated 238U activities from 226Ra activity determined from 214Pb and 214Bi activities.

Table 1 and Fig. 9 show that two weathered granites (JK3 and K42) gave the minimum 238U activity value of 46 Bq kg−1. The sample JK5 gave the maximum observed 238U activity of 386 Bq kg−1. All samples gave an average 238U activity value of 166 Bq kg−1 which exceeds the continental crust of 36 Bq kg−1 (Eisenbud and Gessel 1997). The 238U radiation background thus appears elevated relative to typical background.

Fig. 9
figure 9

Measured 238U activity values. Thick solid line: average 238U value from all samples. Thin solid line: average 238U value reported for the continental crust

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Granite

Figure 10 shows granite sample 238U activity concentrations that give an average value of 161 Bq kg−1. This average exceeds average values for typical granites (40 Bq kg−1) (Eisenbud and Gessel 1997) by a factor of four but falls slightly below the average value for Ĉistá type granites of 214 Bq kg−1 (Krešl and Vaňková, 1978). The nearby and geologically similar Izera Block hosts a leucogranite that gave the highest observed 238U activity value of 120 Bq kg−1 (measured in situ) (Malczewski et al. 2005).

Fig. 10
figure 10

Measured 238U activities from granite samples (excluding schist samples JK5K and JCH3). Thick solid line: average 238U activity from granite samples. Thin solid line: average 238U activity reported for typical granites

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Samples JK3 and K42 gave the lowest observed 238U activity value of 46 Bq kg−1, which resembled that measured from typical granites. The sample JK5 gave the highest observed 238U activity value of 386 Bq kg−1. The difference between the highest and lowest values was 340 Bq kg−1. Sample JK5 was a weathered granite collected from a chamber in the central part of the mine subject to seasonal flooding (Figs. 2 and 3). Four other granite samples exhibited relatively high 238U activities of 215 Bq kg−1 for CH41, 153 Bq kg−1 for K9, 150 Bq kg−1 for K2 and 140 Bq kg−1 for K1. Figures 2 and 10 show that the lowest 238U activities observed come from samples collected from the northern part of the Jeroným Mine.

Table 2 lists 238U activities for granites used as building materials from different global localities. Samples from Sardinia (Italy) gave the lowest mean value of 32.

Table 2 Examples of 226Ra (238U) activities measured from commercial granites used in construction from different global localities
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Bq kg−1 (Dentoni et al. 2020). Granites from Egypt and India gave the highest mean values of 118 and 119 Bq kg−1 (Harb et al. 2012; Chen and Lin 1996). Tzortzis et al. (2003) reported an average value of 77 Bq kg−1 and 238U concentrations of up to 588 Bq kg−1 for rocks from Cyprus. Sakoda et al. (2008) analyzed granite samples from Misasa (Japan) and Badgastein (Austria), which both host well-known radon therapy spas. Those samples gave extremely high 226Ra (238U) activity concentrations of 895 Bq kg−1 for the Misasa granite and 7064 Bq kg−1 for the Badgastein granite. For comparison, the JK5 granite from the Jeroným Mine gave a 238U activity concentration of 386 Bq kg−1. Granites from the Um Taghir region (eastern desert, Egypt) gave the highest 238U activity concentration value reported in the literature of 9087 Bq kg−1 (El-Arabi 2007).

Schists

Schist samples from the Jeroným Mine gave 238U activities of 167 Bq and 196 Bq kg−1 with an average value of 181 Bq kg−1, which slightly exceeds that measured from the granite samples. This value also greatly exceeded the 36 Bq kg−1 value measured from the Clarke (Eisenbud and Gessel 1997) (Fig. 9), the 38 Bq kg−1 value measured from USGS standard SDC-1 and the 43 Bq kg−1 value measured in situ for Izera Block schists (Malczewski et al. 2004, 2005).

Radiological hazard assessment

Basic indices used to evaluate building materials provided estimates and dose criteria for the radiological hazards related to mine tours and working conditions. The European Union standard index I, as defined in Council Directive 59 (2013), represents the sum of three isotopic fractions expressed as:

$$I= \frac{{A}_{Ra}}{{300 Bq kg}^{-1}}+\frac{{A}_{Th}}{{200 Bq kg}^{-1}}+\frac{{A}_{K}}{{3000 Bq kg}^{-1}}$$

where ARa, ATh and AK represent 226Ra, 232Th and 40K (Bq kg−1) activities in surroundings or building material (EC RP 112 1999; Nuccetelli et al. 2012). Bulk material amounts give indoor dose rate which should not exceed a value of 1 mSv y−1 (unity). Table 3 and Fig. 11 show calculated I values along with individual 226Ra, 232Th and 40 K contributions for rock samples analyzed in this study.

Table 3 Calculated I and Hex index values with contributions from 40K, 232Th and 226Ra components as measured from Jeroným Mine samples
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Fig. 11
figure 11

Calculated I index values showing contributions from 40K (IK), 232Th (ITh) and 226Ra (IRa) components. The solid line shows the average value for all samples

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The external hazard index Hex also represents a commonly used index for evaluating radiological risk of building materials. It is calculated as follows:

$${H}_{ex}= \frac{{A}_{Ra}}{{370 Bq kg}^{-1}}+\frac{{A}_{Th}}{{259 Bq kg}^{-1}}+\frac{{A}_{K}}{{4810 Bq kg}^{-1}}$$

where ARa, ATh and AK represent 226Ra, 232Th and 40K (Bq kg−1) activities in the building material or surroundings as before. An Hex index equal to unity corresponds to an external gamma dose of 1.5 mSv y−1 from a material. The Hex utilizes principles of radium equivalent activity (Raeq). The estimate assumes equivalent gamma-ray dose rate produced by 370 Bq kg−1 for 226Ra, 259 Bq kg−1 for 232Th and 4810 Bq kg−1 for 40K (Beretka and Mathew 1985; Monged et al. 2020). Table 3 and Fig. 12 list Hex estimates for the rock samples analyzed here.

Fig. 12
figure 12

Calculated Hex index values showing contributions from 40K (HK), 232Th (HTh) and 226Ra (HRa) components. The solid line shows the average value for all samples

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As seen in Table 3 and Fig. 11, only the JK5 granite sample exceeded critical values of 1 for index I (I = 1.68). Samples JK5K, JCH3, CH41 and K2 slightly exceeded the critical value and all samples gave an average I of 1.02. This average I value fell to 0.94 when JK5 was excluded from calculation. The critical I = 1 threshold corresponding to the dose of 1 mSv derives from an annual exposure time of 7000 h. For the seven samples analyzed, 226Ra made the largest contribution to the I estimate. For the two remaining samples of K42 and JK3, 40K made the largest contribution to I. For all samples, 232Th made the lowest contribution to I.

Similar to the I index, the Hex index exceeded unity only for the JK5 granite sample, which gave a Hex = 1.3 (Tab. 3, Fig. 12). The remaining samples gave Hex values of less than one. All samples gave an average Hex value of 0.77. The largest contribution to Hex values again came from 226Ra except for the two samples K42 and JK3, for which the largest contribution came from 40K. The calculated Hex values for the Jeroným Mine rocks resemble typical values calculated for granites used in construction. For example, commercial granites from Italy, Greece and Sweden give average Hex values of 0.56, 0.73 and 0.97, respectively. Granites from Japan give Hex values ranging from 0.33 to 1.88 with an average value of 0.66 (Hassan et al. 2010). Granites from Egypt gave Hex values ranging from 0.7 to 1.77 with an average value of 1.12 (Harb et al. 2012).

Conclusions

Granites and schists from the Jeroným Mine gave mean activity values for 40K, 232Th and 238U of 907, 33 and 166 Bq kg−1, respectively. Average 226Ra (238U) activity concentrations exceeded average values measured for typical granites and schists. The estimates of I and Hex indices used to assess radiological hazard indicate that the rocks from Jeroným Mine represent safe environmental materials. Gamma ray radiation from the rock surroundings in the Jeroným Mine does not pose a risk to potential tourists and staff. Future analyses should include in situ radon measurements to confirm the low level of the radiological risk.