Applied Radiation and Isotopes 161 (2020) 109166 Available online 2 April 2020 0969-8043/© 2020 Elsevier Ltd. All rights reserved. Measurements of gamma-ray background radiation in Pyh€ asalmi mine S. Pohuliai a , A. Sokolov a , V. Gostilo a, * , J. Joutsenvaara b , J. Puputti b a Baltic Scientifc Instruments, Ramulu Str. 3, LV-1005, Riga, Latvia b Kerttu Saalasti Institute, University of Oulu, Pajatie 5, 86800, Nivala, Finland A R T I C L E INFO Keywords: HPGe spectrometry Gamma-ray background radiation Underground laboratory ABSTRACT The results of measuring the gamma background radiation in Lab 5 of the underground facility in the Pyh€ asalmi Mine at a depth of 1410 m are presented. The background integral count rate per kg of germanium of the HPGe- detector was 0.028 s -1 � kg -1 . After purging the measuring chamber with nitrogen gas at a rate of 0.15 L/h the count rate was reduced to 0.021 s -1 � kg -1 . 1. Introduction In the previous paper (Gostilo et al., 2020) in the Baltic Sea Under- ground Innovation Network (BSUIN) (Baltic Sea Underground Innova- tion Network (BSUIN), 2020), we presented the results of characterising the gamma-ray background radiation (GBR) level in the Callio Lab, which is intended to organize the low-background laboratory in the Pyh€ asalmi Mine (Finland), which with a depth of 1444 m is the deepest base-metal mine in Europe (Callio Lab, 2020). During subsequent assessment of the GBR level in the halls and corridors of the Pyh€ asalmi Mine, we have found a lower GBR level in one of the halls (Lab 5) than that in the Callio Lab. In present paper we give the results of measuring the GBR characteristics in Lab 5 in comparison with the results previ- ously obtained for Callio Lab. 2. Brief description of Lab 5 The Pyh€ asalmi Mine contains numerous spaces/caverns at various depths (Callio Lab, 2020). Lab 5 is located on the main level of the mine at a depth of 1410 m. It was originally used as a storage facility (named KA1) for the mine. The dimensions of the hall are 53 m � 11–13 m � 5–6 m (length � width � height), making it one of the largest halls in the mine, and its total estimated volume is approximately 3000 m 3 . Lab 5 is part of the cooling and air exchange systems of the main level. The measured rate of air fow at the location of low background HPGe spectrometer is 10–20 m 3 /s. The average temperature is 22 � C and the relative humidity of the air is 42.6%. The measured radon level is 21.9 � 1.1 Bq/m 3 , and this relatively low value for an underground site is because fresh radon-free air is blown from the surface down to the mine at rates of up to 130 m 3 /s. 3. Results and discussion 3.1. Background measurements The background level in Lab 5 was evaluated using the same spec- trometers as in (Gostilo et al., 2020). For example, the preliminary measurements using a MultiSPEC-6000 clustered CdZnTe spectrometer gave an integral count rate in the range of 30–2700 keV is 28.5 � 0.01 s 1 , which is around half the average in the Callio Lab. To characterise the GBR in Lab 5 on a low-background HPGe spec- trometer, the background characteristics were measured with the lead lid of the low-background shield either open (acquisition time is 73 h) or closed (213 h). The acquired spectra are shown in Fig. 1. The compar- ison of obtained results shown that with an open lid of the low- background shield, the count rates for the main peaks of the spectra in Lab 5 were 1.5–3 times lower than those in the Callio Lab. The main peaks are the peaks labeled in Fig.1. A rough estimate of the total count rate for the main peaks with an open lid for a solid angle of 4π gave a value of around 20 s –1 kg –1 for Callio Lab (Gostilo et al., 2020). A similar estimate for the data obtained in Lab 5 gave a value of the order of 10 s –1 kg –1 , which agrees with the approx. 50% lower background observed with portable instruments. With a closed lid, the counting rate for different peaks in Lab 5 was 1.5–7 times lower than that in Callio Lab. The integrated counting rate for the main peaks in the range of 40–2700 keV was 0.028 � 0.0007 s –1 kg –1 , which is much better than our previously obtained result of 0.095 � 0.03 s –1 kg –1 for Callio Lab (Gostilo et al., 2020). * Corresponding author. E-mail address: offce@bsi.lv (V. Gostilo). Contents lists available at ScienceDirect Applied Radiation and Isotopes journal homepage: http://www.elsevier.com/locate/apradiso https://doi.org/10.1016/j.apradiso.2020.109166 Received 4 February 2020; Received in revised form 24 March 2020; Accepted 26 March 2020