Deeper understanding at Lab 2 : the new experimental hall at Callio Lab underground centre for science and R & D in the Pyhäsalmi Mine, Finland

Abstract

In this work I introduce Callio Lab, an underground centre for science and R & D in the Pyhäsalmi Mine, Finland, and the new underground measurement hall Lab 2. Furthermore I present the world’s deep underground laboratories (DULs). In addition I cover the main sources of the background radiation for underground laboratories including their effects to specific low background research topics. As a case study I describe the required steps for the concretisation of a deep underground measuring hall and the methods to reduce the radiative background in Lab 2, especially related to radon.

Callio Lab is one of the few deep underground laboratories in the world offering facilities with over-burden of more than 2 000 m.w.e (metres water equivalent), maximum being at 4 000 m.w.e. The deepest currently operating facilities are in Canada (SNOLab, 6 000 m.w.e.) and China (JingPing underground laboratory, 6 800 m.w.e.).

The new experimental hall Lab 2 is located at the depth of 1 430 m (approx. 4 000 m.w.e.) in the Pyhäsalmi Mine. The overburden makes the Lab 2 an optimal site for low (muon) background experiments. The value is based on the measurements presented in the Measurements of muon flux in the Pyhäsalmi underground laboratory (T. Enqvist et al., NIM A 554, 2005).

Lab 2 was finished during the spring 2016. The Lab 2 consists of two halls: the entrance hall (120 square metres) for handling cargo and the experimental hall (120 square metres). My involvement in the realization of the Lab 2 started in spring 2015 with the preliminary design and ended with the final design. During the design phases I contacted several Finnish suppliers to find documented, low background construction materials to be used in the construction. At the end of the construction I was also involved in the instrumentation of the experimental hall.

In the preliminary design the idea was to build a low background experimental hall using low background materials. As these materials were rather expensive the requirements had to be lowered. The main background source in the Lab 2 is the shotcrete walls and the ceiling due to relatively high concentration of uranium and thorium in the additive used in the concrete. Radon, Rn-222, emanating from the surrounding rock and concrete is the biggest challenge for the low background experiments. The radon is radioactive, noble gas and it can diffuse into every setup. The problem comes with the radioactivity of radon, as also the progenies of radon are radioactive all the way to the stable Pb-206.

Other DULs have also been challenged by the radon contamination, and several methods have been developed to mitigate the radon levels. Based on the example radon traps presented in the Low background techniques and experimental challenges for Borexino and its nylon vessels (A. Pocar, Ph. D. Thesis, Princeton University, 2003), a decision was made on the type of a radon trap most suitable for Lab 2. I made a schematic design for an active radon trap, a pressure swing adsorption filter.

Although first experiment, C14, is already using the Lab 2, background screening of the site has to be performed to fully understand the different background sources. This would help to define what kinds of experiments are feasible to be hosted in the Lab 2, and to define the types and thicknesses of radiation shielding needed for these experiments.