Karol Jędrzejczak, PhD, is a physicist at the National Centre for Nuclear Research (NCBJ) in Poland, and at the Cosmic Ray Laboratory in Łódź. He has been associated with NCBJ (former name: IPJ) since 2000, with a 3-year break when he worked at the Faculty of Physics of the Jagiellonian University in Krakow (Poland). He obtained his doctoral degree in physical sciences at IPJ, in 2011, for his work on the measurement of the natural neutron background in the Gran Sasso underground laboratory (Italy). The main focus of his scientific interests is on the measurements of low neutron fluxes in underground laboratories and the physics of cosmic rays. He deals with measurements, preparation, and data analysis. He is Chairman of the Branch of the Polish Physical Society in Łódź (Poland).