## 3 Sensor Upgrade  
### The KATERINA II Underwater Radioactivity Sensor  
KATERINA II is a compact and autonomous underwater gamma-ray spectrometer capable of monitoring radon \({}^{222}\)Rn and thoron \({}^{220}\)Rn progenies (\({}^{214}\)Bi, \({}^{214}\)Pb, \({}^{208}\)Tl) and radioactive potassium \({}^{40}\)K.  
Figure 2: Scheme of the main components constituting the W1-M3A observatory. For the detection of gamma-rays, a scintillation crystal of NaI(Tl) (7.62 \(\times\) 7.62 cm\({}^{2}\)) is used providing spectra of gamma-rays in a wide range of energy (50-2800 keV) based on a multi-channel analyzer of 1024 channels and other auxiliary electronic and memory modules. The NaI(Tl) crystal is ideal for prolonged application in harsh environmental conditions as it is solid and durable, needs no cooling or extremely high voltage input (5-18 V DC), and it can continuously operate for years without significant deterioration of its lattice. On the other hand, the resolution of the provided spectra is low with a relative full width at half maximum of 6.5% for \({}^{137}\)Cs detection at the energy of 661 keV. The dead time (time needed for data processing) is less than 0.5% when the system operates in aquatic environment and the lower limit of detectability is around 20 Bqm\({}^{-3}\) for an acquisition time of 24 h. All parts of the system are tightly assembled inside a cylindrical housing made of acetal, allowing maximum operation depth of 600 m. Acetal is a light and durable material which weakly interact with gamma-rays ensuring minimum gamma-ray attenuation during operation. The KATERINA II system is capable of easy integration into measuring platforms (both stationary and mobile). For its integration into the W1-M3A observing system the following improvements were implemented prior to the installation: (a) spectrum stability, (b) low power consumption and (c) modern communication protocols. A standard drawback of scintillation-based gamma-ray spectrometers is the spectrum drift during their operation due to temperature and voltage variations or other external conditions. With respect to previous versions, the spectrum variation due to voltage drifts was eliminated in KATERINA II, using digital electronics in the electronic modules to adjust the voltage output of the intense peak of \({}^{40}\)K (at 1461 keV as measured due to the seawater constituents) in the middle of the spectrum (around the 512 channel). Also, due to slow and slight variations of temperature in the marine environment, the phenomenon is further reduced. As result, the stability of the acquired spectra (preset acquisition time of 1 h) allows to set regions of interest (ROI) in specific energy windows in areas of photo-peaks of interest and observe variations of the corresponding counting rates during the day. Also, to improve the precision of the measurements, several 1-h spectra can be summed and a spectrum corresponding to time intervals of 6, 12 or 24 h can be created in the post-process phase of data manipulation. The spectrum stability also ensures the quality of the results obtained by the automated spectrum analysis routines which were embedded in the controller onboard the buoy, providing quantitative results of radon \({}^{222}\)Rn progenies at the gamma-ray energy of 351 keV (\({}^{214}\)Pb), 609 and 1764 keV (\({}^{214}\)Bi). An important upgrade of the KATERINA II system with respect to previous versions concerns the drastic reduction in power consumption and the new communication protocol which allows easy data transfer and two-way communication. The electronic modules responsible for the power supply of the system were re-implemented allowing a power consumption lower than 1 W making possible its integration into the observation system and its continuous operation for periods of several months. Also, the upgraded communication protocol allows the data transfer via USB, RS232 and/or Ethernet protocols. Appropriate software was developed providing two-way communication with any operational data-logging system.