The Slanic-Prahova (ROMANIA) underground low-background radiation laboratory

ARTICLE IN PRESS Applied Radiation and Isotopes 66 (2008) 1501– 1506

Contents lists available at ScienceDirect

Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso

The Slanic-Prahova (ROMANIA) underground low-background radiation laboratory R. Margineanu a, C. Simion a, S. Bercea a, O.G. Duliu b,, D. Gheorghiu a, A. Stochioiu a, M. Matei c a b c

Horia Hulubei National Institute of Physics and Nuclear Engineering, P.O. Box MG-06, 077125 Magurele (Ilfov), Romania Department of Atomic and Nuclear Physics, University of Bucharest, P.O. Box MG-11, 077125 Magurele (Ilfov), Romania SALROM RA, 220 Calea Victoriei, 010099 Bucharest, Romania

a r t i c l e in fo

abstract

Article history: Received 24 October 2007 Received in revised form 19 March 2008 Accepted 1 April 2008

A low-background radiation laboratory was constructed and fully commissioned in 2006 in the former Unirea (Slanic-Prahova) salt mine at 208 m below surface (estimated to 560 m water equivalent). Preliminary measurements showed a global reduction of the absorbed dose due to natural factors of about 39 times compared to level on the surface, reaching inside the mine 1.1770.14 nGy/h. The total gamma background spectrum between 40 KeV and 3 MeV was 100 times smaller at laboratory level with respect to the same spectrum recorder at surface, in open field. All these experimental facts recommend the Slanic-Prahova low-background radiation laboratory, at present time fully operational, as very suitable for various measurements needing a low background. & 2008 Elsevier Ltd. All rights reserved.

Keywords: Underground laboratory Low-level dosimetry Low-level gamma spectrometry Low-level radiation metrology

1. Introduction The construction and exploitation of underground low-background laboratories represents a great challenge for nuclear as well as for elementary particle physics because of the increased capacity of thick ground layers to diminish the cosmic radiation, an ubiquitous component of any radiation laboratory. This radiation, and especially secondary muons, in spite of various types of active as well as passive shielding, some of them very elaborated, strongly restricts the minimum measurable activity of a sample. Consequently, one of the best ways to reduce the cosmic background radiation consists of using deep underground mines or tunnels for location of low-background radiation laboratories (LBRL) (Heusser, 1996; Mouchel and Wordel, 1997; Neumaier et al., 2000). On the other hand, the construction of deep underground laboratory for many technical as well as psychical problems due to the extreme conditions of work (small space, higher or lower than normal ambient temperature, presence of different aerosols, complete isolation, etc.). Irrespective of some practical inconveniences, the great advantage offered by deep underground locations has stimulated the construction in different countries, e.g. in Germany, United States, United Kingdom, Japan, Italy, Spain, France, Russia, Ukraine and Canada where various high-class

 Corresponding author. Fax: +021 4574521.

E-mail address: [email protected] (O.G. Duliu). 0969-8043/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2008.04.002

scientific experiments are performed, such as the investigation of solar neutrino emission, neutrino emission from supernovae and black hole formation, dark matter, double beta decay and elusive ˆ ara, 1995; Hult et al., 2000; proton decay (Lauterbach and Peu Neumaier et al., 2000; Avignone et al., 2005; Cebria´n et al., 2006; Daw, 2006). To such performances, it must be added the identification of cosmogenic 26Al radionuclides in meteorites (Arnold et al., 2002) or of the anthropogenic 60Co radioisotope in steel exposed to the Hiroshima atomic bomb explosion (Hult et al., 2004) as well as a better value of the lower bound of natural 80Tam radionuclide whose half-life of 7.1 1015 yr is among the longest ever experimentally determined (Hult et al., 2006). At the same time, underground laboratories offer ideal conditions for investigation of low-level radioactive materials, animal and human contamination with various natural as well as artificial radionuclides or for a more accurate calibration of detectors used in radiation metrology. Beside fundamental physics, other sciences could benefit from the advantages and opportunities of underground environments. Investigations of biological processes in dark and radiation-free conditions, or, as in the case of salt mine, in significantly germ-depleted environment are some other possible applications. To reduce the level of natural rocks background radiation, for low-background underground laboratories are preferred locations in rocks with a low content of natural radionuclides such as limestone, coal or salt. In the last case, the natural process of salt fractional crystallization in a supersaturated marine brine partially remove potassium chloride as well as other heavy metals salts, and

ARTICLE IN PRESS 1502

R. Margineanu et al. / Applied Radiation and Isotopes 66 (2008) 1501–1506

Fig. 1. Romania’s map with the location of possible salt mine underground laboratories.

thus significantly reduce the presence of natural radionuclides (Nenitescu, 1982). Romania is rich in natural deposits of salts, some of them forming massive salt domes, which appear to result from pressure, which pushes the salt up through the rocks from great depths. Such domes, mainly encountered in the sub-Carpathian region have been worked since ancient times and now host a multitude of active as well as decommissioned salt mines, the last one representing ideal location for LBRLs. For this purpose, we have investigated existing facilities as well as the level of background radiation of three former salt mines, all of them located in the Romanian sub-Carpathian region, i.e. Praid, Cacica, and Slanic-Prahova (Fig. 1). A radiological mapping of these mines performed by using a high-resolution gamma spectrometry with a SILENA-WALKLAB portable system equipped a GeHP Canberra detector of 18% relative efficiency and an Eberline FH40G dosimeter provided with a proportional counter indicated the Unirea salt mine in the vicinity of Slanic-Prahova city as the best location (Bordeanu et al., 2008). Our decision was also influenced by the results of some detailed analysis concerning the high purity of salt existing in Unirea mine which showed, for the chosen location of low-background laboratory, an average content of mineral impurities (clay minerals, quartz, feldspars, carbonates, etc.) of about 1.16 (SNS Saltrom, 2006). Further we will present in detail the Slanic-Prahove LBRL.

2. Experimental 2.1. Laboratory Sla˘nic-Prahova is located in the outer Carpathians area (Prahova County), 45 km NE of Ploies- ti. From a geological point of view, this locality belongs to the post-tectogenetic cover of Tarca˘u Nappe, one of the most important ones cropping out in the Eastern Carpathians. It includes Cretaceous, Paleogene and lower Miocene sedimentary successions. Towards Prahova County, this nappe ends on two main anticlines, Homoraˆci and Va˘leni, separated by two synclines, Sla˘nic

and Drajna, both filled with Miocene molasse. The post-tectogenetic sedimentary cover of Tarca˘u Nappe begun to accumulate in early Miocene (23.7 Ma) and ended in Sarmatian (5.3 Ma). The sedimentary environment in the region of actual SlanicPrahova was dominated by the Middle Badenian evaporitic condition that covers the interval from the last stage of carbonates precipitation to halite, however, without reaching the stage of K and Mg salt precipitation (Harr et al., 2006). At the same time, some authigenic evaporitic minerals that precipitated simultaneously with a certain amount of sediments from the adjacent area appear in interbedded layers of salt formation (Fig. 2). The Unirea mine is situated at a depth of 208 m beneath the surface that corresponds to a water-equivalent thickness of about 560 m. Mine itself consists of a hive-like structure composed of more galleries of 34–36-m-wide and 54–58-m-height. The remarkable stability of the microenvironment characterized by a constant temperature all over the years of 12.570.5 1C, a relative humidity of 60–65% and a significantly reduced (with respect to surface) germ load must be pointed out. For this reason, some galleries are used for the treatment of asthma too. Two elevators with a capacity of about 45 tones each connect the mine with surface. The mine is fully electrified and has a small cafeteria (Fig. 3). Due to the presence of salt aerosols, the environment is highly corrosive that required us to provide the laboratory with purifying filters. About 200 m from elevators, on the Southern part of mine, the LBRL was assembled by prefabricated modules transported downstairs with elevators. To reduce further the background radiation, the laboratory was constructed entirely by polyvinyl profiles and Lucite windows. Because the background was too low, the presence of personnel in the vicinity of detectors was able to increase significantly the background. To avoid such unwanted effects, each room was delineated and separated from neighbor ones by 0.5-m-thick walls made also by polyvinyl profiles, but filled with high-purity salt (Fig. 3). Epithermal neutron activation analysis performed at JINR Dubna (Russia) has shown no detectable traces of potassium

ARTICLE IN PRESS R. Margineanu et al. / Applied Radiation and Isotopes 66 (2008) 1501–1506

1503

Fig. 2. A photograph of the salt walls of former Unirea salt mine in the vicinity of the constructed low-background radiation laboratory.

and thorium in salt and a concentration of uranium of about 1 ppm, while the concentrations of these elements in the mineral fraction (Cristache et al., 2007) were comparable to those from the Upper Continental Crust, a good model for sediments of continental origin (Taylor and McLennan, 1985) (Table 1). The 20-m-long and 6-m-wide laboratory consists of four multifunctional rooms, each of them connected to a local electricity network and a room for personnel. It is worth mentioning that there is a full access to Internet by means of a built-in fix phone network (Fig. 3).

a relative efficiency of 22.8% with respect to a standard 300  300 NaI(Tl)and FWHM of 1.76 keV for the 1332.5 keV of 60Co line. To verify at which extent a supplementary lead shield could determine a more reduction of the background, we have performed preliminary gamma-ray measurements in the same conditions with the detector unshielded and shielded by 5-cm-thick lead bricks. In both cases, gamma spectra were recorded for an energy interval between 40 and 3000 keV for 8192 channels and analyzed by means of a DigiDART portable gamma-ray spectrometer provided with Inspector 2000 assisted by Genie 2000 codes.

2.1.1. Instruments To obtain as many as possible confident data concerning the radiation background within the laboratory with respect to the surface, we have used three different kinds of detectors: GR 200A (LiF: Mg, Cu, P) thermoluminescence (TL) dosimeters, an Eberline FH–40G dose ratemeter, a Cannbera HPGe stationary detector with

3. Results and discussion 3.1. TL dosimetry TL dosimeter, due to their high versatility and mobility were used to determine the background radiation in different parts of

ARTICLE IN PRESS 1504

R. Margineanu et al. / Applied Radiation and Isotopes 66 (2008) 1501–1506

Fig. 3. Schematic diagram of Unirea salt mine with the location of the low-background radiation laboratory (inset).

Table 1 Experimental concentrations (in ppm) as determined by Epithermal Neutrons Activation Analysis of main natural radionuclides in salt as well as in sediments collected from the walls of Slanic-Prahova former salt mine Element

Salt

Sediment

UCC

Potassium Thorium Uranium

Non-detectable Non-detectable 0.9270.3

15407700 5.5670.07 9.3271.02

2820 10.7 2.8

the Unirea salt mine, in order to establish the best location for the laboratory. For each measuring point we have used a set of three detectors, including those located outside the mine for comparison. The exposure times were 888, 1587 and 2232 h. Thus measured, the average absorbed dose debit was 1.1770.14 nGy/h inside the mine with respect to an average value of 87.8727.4 nGy/h outside it, i.e. 80–90 times smaller. For each case, the measurements were concordant within the experimental errors (Fig. 5). It is worth mentioning that the average value inside mine is comparable with similar data reported for Asse salt mine near Braunschweig (Neumaier et al., 2000).

Irrespective of these precautions, we only were able to estimate the background radiation to be between 1.6 and 9.8 nGy/h, lower than 12.7 nGy/h, the equivalent of the electronic noise of the instrument. In these conditions, we come to the conclusion that Eberline FH radiation dose debit ratemeters, in spite of their excellent performance on ground laboratories, are not suitable for measuring low radiation field and only specially designed detectors for this purpose will allow to perform confident measurements. 3.3. Gamma-ray spectrometry Three representative gamma-ray spectra recorder in the abovementioned conditions in the laboratory within the mine and outside it are reproduced in Fig. 4. Both LBRL spectra (unshielded and shielded by 5 cm lead) have been recorded for 267,500 s, while for the outside spectrum a total acquisition time of 10,000 s was sufficient. From these spectra, the most significant data are reproduced in Table 2. By analyzing both these data and spectra some remarks can be made:

 The total count rates from 40 to 3 MeV of collected spectra in 3.2. Low-background ionization chamber The significant low background of the Unirea mine enabled us to use for the ionization chamber measurements an extrapolation technique to estimate the equivalent dose debit. For this purpose, we carefully calibrated an Eberline FH–40G dose ratemeter by using 60Co and 137Cs sources in both collimated and panoramic exposures. In this way, we have been able to determine the calibration factor equal to 0.17 nGy/imp. By extrapolating the impulses number versus time calibration curve for measurements performed in the laboratory, we estimated the electronic noise of the instrument at 1.43 imp/h.





the laboratory is of 2.19 cps, about 80 times lower compared with a spectrum collected in the same conditions in an open field of 175.6 cps, but still high with respect to other similar laboratories (Table 3). Moreover, in the energy interval between 2615 (the maximum energy of 208Tl ) and 3000 keV, the laboratory background is about 40 times smaller than the corresponding background measured outside mine, this fact representing the best illustration of the shielding of cosmic radiation by the earth stratum that separates the laboratory from exterior. In these conditions, the 46.52 keV line of 210Pb can be observed without any supplementary shielding, thing impossible to

ARTICLE IN PRESS R. Margineanu et al. / Applied Radiation and Isotopes 66 (2008) 1501–1506

1505

Fig. 4. Box and whiskers diagram (average, one and two standard deviations) of the distribution of absorbed dose debit within (left) and outside (right) the Unirea salt mine. A, B and C measuring points as well as the location of the low-background radiation laboratory are reproduced in Fig. 3.

Table 3 Integral gamma-ray counting rate within the interval 40– 2700 keV as determined in some low background laboratories Laboratory

Slanic-Prahova (Romania)a ARS Seibersdorf (Austria) Heidelberg (Germany) CAVE (Monaco)b Felsenkeller (Germany) HADES (Belgium) Salt mine Asse (Germany) Gran Sasso (Italy) Modane (France) Fig. 5. Three experimental gamma spectra (Cannbera 22.8% efficiency HPGe stationary detector) of the background radiation measured on the ground outside mine (upper spectrum) and in the low-background radiation laboratory without (middle spectrum) and with 5 cm lead shielding (lower spectrum).

a b

Depth (w.e.)

Integral counting rate (keV1 kg1 day1)

Reference

–

Present work

82007200 2012723

Schwaigeret al. (2002) Heusser (1986)

225– 340 3870730

Povinec (2004) Niese et al. (1998)

26074 27774

Hult et al. (2003) Neumaier et al. (2000) Arpessella (1996) Reyes et al. (1995)

560 1 15 35 110 500 2100 3800 4800

8771 3071

Shielded by 5 cm lead. Few HPGe detector without anticosmic shielding.

40 K radionuclide in mine is mainly due to traces of potassium from the mineral impurities of the salt as determined by Neutron Activation Analysis (Cristache et al., 2007) as well as to the human presence in the vicinity of detectors.

 The presence of Table 2 The most significant data concerning the gamma spectra measured in our underground laboratory (box #4, shielded by 5 cm lead) as well as outside the salt mine (count/1000 s) Energy (keV)

Radionuclide

Laboratory Net peak area

Outside Net peak area

46.57 185.98 209.36 238.71 351.99 661.8 1460.75 2613.86

210

25.473.4 72.473.9 3.773.2 272.575.3 14.272.1 193.672.9 158.472.5 30.871.2

– 2002.57397.8 998.67793.0 18254.87630.1 9550.67288.4 9590.67227.0 59394.27429.9 5468.27130.8



Pb Ra Ac 212 Pb 214 Pb 214 Bi 40 K 208 Tl 226 228

be realized without appropriate shielding on the surface laboratories. The discrepancy between 226Ra and 222Rn descendents activity can be better explained by the radon introduced in mine by elevators that act as a sui generic air pump, by taking into account that there is no artificial ventilation in the mine.

Compared with other low-background laboratories (Table 3) in our laboratory, in spite of a significant reduction of radiation background with respect to surface, the use of a commercially HPGe detector, designed for normal laboratories, still determine a considerable higher-level background. To reduce it at levels comparable with other similar laboratories, it will need special passive as well as active measures such as carefully selected ultralow radioactivity materials for detectors, triple wall shield made by actual lead, old lead and electrolytic copper as well as an active anti-Compton counting system.

4. Concluding remarks By its natural conditions, the LBRL is comparable with the other deep-ground laboratories. Preliminary measurements showed a global reduction of the absorbed dose due to natural factors of

ARTICLE IN PRESS 1506

R. Margineanu et al. / Applied Radiation and Isotopes 66 (2008) 1501–1506

about 39 times which reached a value of 1.1770.14 nGy/h, perfectly comparable with similar laboratories. The integral gamma spectrum between 40 and 3 MeV was at the laboratory level 100 times lower compared with the spectrum collected in the open field. Moreover, the laboratory gamma-ray background energy between 2.615 and 3.0 MeV was reduced by a factor of about 40 times with respect to the same spectrum measured outside the mine. At the same time, the gamma spectrometry unit to be full comparable with other similar laboratories will need a careful selection of low-background HPGe detectors as well as supplementary active and passive shieldings. But, irrespective of the last remarks, all experimental facts recommend the Slanic-Prahova LBRL, at present time fully operational, as very suitable for various measurements needing low-radiation background.

Acknowledgments This work was supported by grant CEx no. 65/2005 of the Romanian Ministry for Education and Research. We also want to thank Adrian Socolov for the photograph of Unirea Mine. References Arnold, D., Neumaier, S., Sima, O., 2002. Deep underground gamma spectrometric measurement of 26Al in meteorite samples. Appl. Radiat. Isot. 56, 405–408. Arpesella, C., 1996. A low background counting facility at laboratory Nazionali del Gran Sasso. Appl. Radiat. Isot. 47, 991–996. Avignone, F.T., King, G.S., Zdesenko, G.Yu., 2005. Next generation double-beta decay experiments: metrics for their evaluation. New J. Phys. 7, 6–46 /www.njp.org/S. Bordeanu, C., Rolfs, C., Margineanu, R., Negoita, F., Simion, C., 2008. Saltmine underground accelerator lab for nuclear astrophysics. J. Phys. G: Nucl. Part. Phys. 35, 1–6 /stacks.iop.org/JPhysG/35/1S. Cebria´n, S., Garcı´a, E., Gonzalez, D., Irastorza, I.G., Morales, A., Morales, J., Ortiz, A., Peruzzi, A., Puimedon, J., Sarsa, M.L., Scopel, S., Villar, J.A., 2000. Cold dark matter searches at the Canfranc underground laboratory. New J. Phys. 2, 13.1–13.20 /www.njp.org/S. Cristache, C., Simion, C.A., Margineanu, R.M., Matei, M., Duliu, O.G., 2008. Epithermal neutrons activation analysis, radiochemical and radiometric

investigations of evaporitic deposits of Slanic-Prahova (Romania) salt mine. Radiochim. Acta, submitted. Daw, E., 2006. Status of the Boulby dark matter programme,. J. Phys. Conf. Ser. 39, 89–95. Harr, N., Barbu, O., Codrea, V., Petrescu, I., 2006. New data on the mineralogy of the salt deposit from Sla˘nic-Prahova (Romania). Stud. Univ. Babes- -Bolyai (Geol) 51, 29–33. Heusser, G., 1986. The background components of germanium low-level spectrometers. Nucl. Instrum. Methods B 17, 418–422. Heusser, G., 1996. Low radioactivity background techniques. Annu. Rev. Nucl. Part. Sci. 45, 543–590. Hult, M., Canet, M.J.M., Ko¨hler, M., das Neves, J., Johnston, P.N., 2000. Recent developments in ultra low-level g-ray spectrometry at IRMM. Appl. Radiat. Isot. 53, 225–229. Hult, M., Gasparro, J., Johansson, L., Johnston, P.N., Vasselli, R., 2003. In: Warwick, P. (Ed.), Ultra Sensitive Measurements of Gamma-Ray Emitting Radionuclides using HPGe Detectors in the Underground Laboratory HADES. Envir. Radiochem. Anal II. Royal Society of Chemistry, p. 375. Hult, M., Gasparro, J., Vasselli, R., Shizuma, K., Hoshi, M., Arnold, D., Neumaier, S., 2004. Deep underground measurements of 60Co in steel exposed to the Hiroshima atomic bomb explosion. Appl. Radiat. Isot. 61, 173–177. Hult, M., Gasparro, J., Marissens, G., Lindahl, P., Wa¨tjen, U., Johnston, P.N., Wagemans, C., Ko¨hler, M., 2006. Underground search for the decay of 180Tam. Phys. Rev. C74, 054311.1–054311.5. ˆ ara, W., 1995. Investigation of environmental dosimeters Lauterbach, U., Peu in the ultra low background facility UDO. Radiat. Prot. Dosimetry 61, 71–72. Mouchel, D., Wordel, R., 1997. The IRMM low-level underground laboratory in HADES. In: Fietz, J. (Ed.), Proceedings of the LOWRAD 1996 Workshop at Rossendorf (Dresden), 7–8 November 1996, Forschungszentrum, Rossendorf. FZR-Report 170, pp. 25-29. Nenitescu, C., 1982. Inorganic Chemistry. Didactic and Pedagogic Press, Bucharest (in Romanian). Neumaier, S., Arnold, D., Bo¨hm, J., Funck, E., 2000. The PTB underground laboratory for dosimetry and spectrometry. Appl. Radiat. Isot. 53, 173–178. Niese, S., Ko¨hler, M., Gleisberg, B., 1998. Low-level counting techniques in the underground laboratory Felsenkeller, Dresden. J. Radioanal. Nucl. Chem. 233, 167–172. Povinec, PP., Comanducci, J.-F., Levy-Palomo, I., 2004. IAEA-MEL’s underground counting laboratory in Monaco—background characteristics of HPGe detectors with anti-cosmic shielding. Appl. Radiat. Isot. 61, 85–93. Reyes, J.-L., Schmidt, S., Legeleux, F., Bonte´, P., 1995. Large low background welltype detectors for measurements of environmental radioactivity. Nucl. Instrum. Methods A 357, 391–397. Schwaiger, M., Steger, F., Schro¨ttner, T., Schmitzer, C., 2002. An ultra low level laboratory for nuclear test ban measurements. Appl. Radiat. Isot. 56, 375–378. SNS Saltrom, 2006. Internal analysis bulletin no. 145B, 2006. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Geoscience Texts, Blackwell, Oxford.