1. GEMS: UNDERWATER SPECTROMETER FOR LONG-TERM RADIOACTIVITY MEASUREMENTS
Sartini L., Simeone F., Pani P., Lo Bue N.,Marinaro G., Grubich A., Lobko A., Etiope G., Capone A., Favali P.,Gasparoni F., Bruni F.
for the KM3NeT Consortium

2. Need of radioactivity analysis and monitoring
GEMS (Gamma Energy Marine Spectrometer) is a prototype of an autonomous radioactivity sensor for underwater measurement, developed in the framework of the KM3NeT Dessign Study Project.
The spectrometer is sensitive to gamma rays produced by 40K decay and it’s suitable to detect total gamma spectrum in ocean seawater
Need of radioactivity analysis and monitoring
40K (source of background for PMT's) is not
only contained in sea salt but also
in suspended matter and sediments
Benthic sediment mobilisation, bottom currents,
far-field landslides,
volcanic fall-out and other processes
may induce 40K variations
Seawater 40K (12 Bq/kg)
particulate
salt
diffusion
Sediments 40K (225 Bq/kg)

3. Average power – not more than 1.2 W Interface – RS232
40K ≥ 10 Bq/Kg
Dimensions: 190x460 mm
Sensor – NaI(Tl) 150x100 mm
Supply voltage – 12 V
Average power – not more than 1.2 W
Interface – RS232

5. “Marinisation”
The process of adaptation of the Sensor into a version capable to operate in deep-sea conditions, into a Seafloor Observatory or integrated into a multisensor probe

7. Analysis of Calibration Data
---Background Noise
Cs Data
137Cs
133Ba
60Co
22Na

8. Spectrum of 137Cs
Fitmodel: Fermi-Dirac+ Gaussian
χ²/ndf = 5

9. Spectrum of 60Co
Fitmodel: Fermi Dirac + Gaussian
χ²/ndf =2.7

10. Fitmodel: Exponential + Gaussian
26/04/2017
Spectrum of 22Na
Fitmodel: Exponential + Gaussian
χ²/ndf =1.3

11. Spectrum of 40K
Fitmodel: Exponential + Gaussian
χ²/ndf =7.4

12. 26/04/2017
Fit: Y=mx+q
40K
m1 = ( ± 0.12) [Channel/MeV ]
q1 = (6.974 ± 0.075)[Channel]
60Co
χ²/ndf = 0.044
22Na
m2=(309.8 ± 9.4) [ Channel/MeV]
q2=(59 ± 14) [Channel]
137Cs
χ²/ndf=16
133Ba

13. experimental data
calibration curve
ADC channel

14. Energy Resolution

15. Deployment in the Capo Passero-site
3200 m depth
36°18.915’ N
16°65.531’ E

17. Quality check on data collected
GEMS, previously calibrated and set for an autonomous acquisition of six months-period, was deployed in mooring on the 4th November 2008 and recovered on 11th May 2009.
During this period GEMS acquired 751 spectra (one spectra every 6h)
A preliminary quality-check on the data shows that there aren’t any gaps or lacks in the acquisition series

18. The peak of 40K is visible in every spectrum for the entire period of acquisition

19. Time series analysis of 40K activity shows a very stable behaviour during the six months of deployment
12.1 Bq/l
Bq/l
11.4 Bq/l
Six Months
Sample Number

20. Monthly Mean Values
σ = 0.1
11.7 Bq/l

21. Preliminary Peaks Identification
40K-1460 KeV
214Bi-2614 KeV
214Bi-1760 KeV
208Tl-550 KeV
208Tl-2190 KeV

22. Conclusions:
the instrument was built, calibrated and employed in a long-term monitoring with good results.
- Strong electronics
- No acquisition series incompleted
- Clean spectra
More refined analisys are in progress
Next Developments:
perform a Monte Carlo Simulation
extend the analysis to the other radionuclides
perform the study of the correlations with the oceanographic variables