Natural radioactivity of rocks from the historic Jeroným Mine in the Czech Republic

This study reports the natural radioactivity of characteristic rocks found in the historic Jeroným Mine of the Czech Republic as measured under the laboratory conditions. The rocks analyzed included granites and schists weathered to varying degrees and collected from different levels of the underground workings of the Jeroným Mine. The mine itself has been subject to metal extraction (mainly tin and tungsten) since the sixteenth century and has recently been developed as a cultural and scientific attraction open to the public. Activity concentrations of 40K, 232Th and 238U were measured from nine rock samples using gamma-ray spectrometry. The activity concentrations of 40K varied from 595 Bq kg−1 to 1244 Bq kg−1, while 232Th varied from 25 Bq kg−1 to 55 Bq kg−1. The activities associated with 238U ranged from 46 Bq kg−1 to 386 Bq kg−1. The measured activities were used to estimate two radiation hazard indices typically applied to building materials, the activity concentration index I and the external hazard index Hex. Mean respective values of 1.02 and 0.77 for I and Hex indicate that the rocks found in the Jeroným Mine meet radiological safety standards for building materials and do not pose a risk to potential tourists and staff.


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.
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 1 3 650 Page 2 of 11 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 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 40 K, 232 Th and 238 U 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.  Table 1 lists sample descriptions

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 241 Figure 4 shows typical gammaray spectra for granite and schist samples JK3 and JCH3. Table 1 lists measured 40 K, 208 Tl, 212 Pb, 228 Ac, 214 Pb, 214 Bi, and 226 Ra activity concentrations for the nine Jeroným Mine rock samples.

K
As seen in Fig. 5 and Table 1, sample K9 (weathered granite) gave the lowest observed 40 K activity concentration of 595 Bq kg −1 . The granite K2 gave the maximum 40 K activity observed of 1244 Bq kg −1 . Weathered granite samples JK3 and K42 containing large, visible amounts of potassium feldspar gave the next highest 40 K values of 1141 Bq kg −1 and 1136 Bq kg −1 (respectively). Two schist samples JK5K and JCH3 gave similar 40 K activity concentrations of 1101 and 912 Bq kg −1 , respectively. Three other samples of weathered granite gave relatively low 40 K 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 40 K activity value of 907 Bq kg −1 slightly exceeds the average 40 K 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 40 K radiation levels. Figure 6 shows the 40 K 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).

Granites
Albitized and greisenized granites K9, JK5 and K1 gave the lowest 40 K 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 40 K activity Gamma-ray spectra from granite sample (JK3) and schist sample (JCH3). Characteristic gamma-ray emitters are marked above the corresponding peaks values for these granite samples. The highest 40 K 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 40 K 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(Malczewski et al. , 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 40 K activity values of 1560 Bq kg −1 (Dżaluk et al. 2018).

Schists
The two schist samples, JK5K and JCH3, gave an average 40 K 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. It also exceeds values of 822 Bq kg −1 given for the USGS mica schist standard SDC-1.

232
Th series ( 228 Ac, 212 Pb, and 208 Tl) Table 1 shows that rock samples have achieved radioactive equilibrium among 232 Th series daughter products. Since 228 Ac represents the second radionuclide in the thorium decay series, 232 Th activity is assumed to equal 228 Ac activity. Figure 7 shows that sample K9 (weathered granite) gave the lowest 232 Th activity of 25 Bq kg −1 , whereas sample JCH3 (schist) gave the highest 232 Th activity value observed of 55 Bq kg −1 . All samples gave an average 232 Th 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.

Granites
The seven granite samples from the Jeroným Mine gave 232 Th 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 232 Th activity values or the 70 Bq kg −1 value reported for typical granites (Eisenbud and Gessel 1997). None of the 232 Th activities measured in this study exceeded this value. However, the average 232 Th activity exceeded mean values of 18 Bq kg −1 reported for Ĉistá type granites (Krešl and Vaňková, 1978 (Anjos et al. 2011;Chen and Lin 1996). Typical commercial granites from Greece and Sardinia gave respective 232 Th activity values of 77 and 66 Bq kg −1 (Papadopoulos et al. 2012;Dentoni et al. 2020). Commercial granites from Japan gave lower 232 Th activity values of 40 Bq kg −1 (Hassan et al. 2010).

Schists
Two schists (samples JK5K and JCH3) gave respective 232 Th 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 232 Th activity values resembles the 232 Th activity reported for the USGS mica schist standard SDC-1 (46 Bq kg −1 ).

U series ( 214 Pb, 214 Bi and 226 Ra)
Activity concentrations for 238 U were estimated assuming radioactive equilibrium within the 238 U → 226 Ra → 222 R n → 214 Pb → 214 Bi decay chain. We estimated 238 U activities from 226 Ra activity determined from 214 Pb and 214 Bi activities. Table 1 and Fig. 9 show that two weathered granites (JK3 and K42) gave the minimum 238 U activity value of 46 Bq kg −1 . The sample JK5 gave the maximum observed 238 U activity of 386 Bq kg −1 . All samples gave an average 238 U activity value of 166 Bq kg −1 which exceeds the continental crust of 36 Bq kg −1 (Eisenbud and Gessel 1997). The 238 U radiation background thus appears elevated relative to typical background. Figure 10 shows granite sample 238 U 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 238 U activity value of 120 Bq kg −1 (measured in situ) (Malczewski et al. 2005).

Granite
Samples JK3 and K42 gave the lowest observed 238 U activity value of 46 Bq kg −1 , which resembled that measured from typical granites. The sample JK5 gave the highest observed 238 U 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 238 U 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   Figures 2 and 10 show that the lowest 238 U activities observed come from samples collected from the northern part of the Jeroným Mine. Table 2 lists 238 U activities for granites used as building materials from different global localities. Samples from Sardinia (Italy) gave the lowest mean value of 32.
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 238 U 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 226 Ra ( 238 U) 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 238 U activity concentration of 386 Bq kg −1 . Granites from the Um Taghir region (eastern desert, Egypt) gave the highest 238 U activity concentration value reported in the literature of 9087 Bq kg −1 (El-Arabi 2007).

Schists
Schist samples from the Jeroným Mine gave 238 U 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(Malczewski et al. , 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: where A Ra , A Th and A K represent 226 Ra, 232 Th and 40 K (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 226 Ra, 232 Th and 40 K contributions for rock samples analyzed in this study.
The external hazard index H ex also represents a commonly used index for evaluating radiological risk of building materials. It is calculated as follows: where A Ra , A Th and A K represent 226 Ra, 232 Th and 40 K (Bq kg −1 ) activities in the building material or surroundings   Table 3 and Fig. 12 list H ex estimates for the rock samples analyzed here. As seen in Table 3 and Fig. 11 (Hassan et al. 2010). Granites from Egypt gave H ex 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 40 K, 232 Th and 238 U of 907, 33 and 166 Bq kg −1 , respectively. Average 226 Ra ( 238 U) activity concentrations exceeded average values measured for typical granites and schists. The estimates of I and H ex 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.
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