GEMS: UNDERWATER SPECTROMETER FOR LONG-TERM RADIOACTIVITY MEASUREMENTS
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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.

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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


GEMS MEASUREMENTS(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)


40 MEASUREMENTSK ≥ 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


MEASUREMENTSMarinisation”

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


Analysis of Calibration Data MEASUREMENTS

---Background Noise

---- 137Cs Data

  • 137Cs

  • 133Ba

  • 60Co

  • 22Na


Spectrum of MEASUREMENTS137Cs

Fitmodel: Fermi-Dirac+ Gaussian

χ²/ndf = 5


Spectrum of MEASUREMENTS60Co

Fitmodel: Fermi Dirac + Gaussian

χ²/ndf =2.7


Spectrum of MEASUREMENTS22Na

Fitmodel: Exponential + Gaussian

χ²/ndf =1.3


Spectrum of MEASUREMENTS40K

Fitmodel: Exponential + Gaussian

χ²/ndf =7.4


Fit: Y=mx+q MEASUREMENTS

40K

m1 = (353.62 ± 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


experimental data MEASUREMENTS

calibration curve

ADC channel


Energy Resolution MEASUREMENTS


Deployment in the Capo Passero-site MEASUREMENTS

3200 m depth

36°18.915’ N

16°65.531’ E


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


The peak of MEASUREMENTS40K is visible in every spectrum for the entire period of acquisition


Time series analysis of MEASUREMENTS40K 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


Monthly Mean Values MEASUREMENTS

σ = 0.1

11.7 Bq/l


Preliminary Peaks Identification MEASUREMENTS

40K-1460 KeV

214Bi-2614 KeV

214Bi-1760 KeV

208Tl-550 KeV

208Tl-2190 KeV


  • Conclusions: MEASUREMENTS

  • 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


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