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

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

Sediments 40K (225 Bq/kg)

diffusion

Seawater 40K (12 Bq/kg)

salt particulate

3

40K ≥ 10 Bq/Kg

Dimensions: 190x460 mm Sensor – NaI(Tl) 150x100 mmSupply voltage – 12 VAverage power – not more than 1.2 WInterface – RS232

4

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

6

7

100 200 300 400 500 600 7000

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

ADC Channel

Puls

es

---Background Noise

---- 137Cs Data

Analysis of Calibration Data

•137Cs

•133Ba

•60Co

•22Na

8

Spectrum of 137Cs

Fitmodel: Fermi-Dirac+ Gaussian

100 150 200 250 300 350

0

500

1000

1500

2000

2500

3000

3500

ADC Channel

Pul

ses

Sperimental Data

fit range 50÷350χ²/ndf = 5

9

Spectrum of 60Co

Fitmodel: Fermi Dirac + Gaussian

350 400 450 500 550 600

0

200

400

600

800

1000

1200

1400

ADC Channel

Puls

es

Sperimental Data

fit 350÷700χ²/ndf =2.7

10

Spectrum of 22Na

Fitmodel: Exponential + Gaussian

50 100 150 200 250 300 350 400 450

0

200

400

600

800

1000

1200

ADC Channel

Pul

ses

Sperimental Data

fit range 50÷550

χ²/ndf =1.3

11

Spectrum of 40K Fitmodel: Exponential + Gaussian

400 420 440 460 480 500 520 540 560 580 600

100

150

200

250

300

ADC Channel

Pul

ses

Sperimental Data

fit range 400÷600χ²/ndf =7.4

12

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

100

200

300

400

500

600

700

E [MeV]

E [

AD

C C

hann

el]

Fit: Y=mx+q

m1 = (353.62 ± 0.12) [Channel/MeV ]

q1 = (6.974 ± 0.075)[Channel]

m2=(309.8 ± 9.4) [ Channel/MeV]

q2=(59 ± 14) [Channel]

χ²/ndf = 0.044

χ²/ndf=16

40K

60Co

137Cs

22Na

133Ba

13

experimental datacalibration curve

ADC channel

14

Energy Resolution

15

3200 m depth36°18.915’ N

16°65.531’ E

Deployment in the Capo Passero-site

16

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

0 200 400 600 800 1000 120010

0

101

102

103

104

ADC Channel

Pul

ses

X: 519Y: 459

K-40

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

Bq/l

Sample Number

11.4 Bq/l

12.1 Bq/l

Six Months

20

Monthly Mean Values

Nov 08 Dec 08 Jan 09 Feb 09 March 09 April 09 May 0910

10.5

11

11.5

12

12.5

13

Month

Act

ivity

[B

q/l]

σ = 0.1

11.7 Bq/l

21

0 500 1000 1500 2000 2500 300010

-1

100

101

102

103

104

X: 1462Y: 426

KeV

Puls

es

X: 1758Y: 44.51 X: 2192

Y: 23.75

X: 2668Y: 32.24

X: 550.1Y: 647.3

40K-1460 KeV

214Bi-1760 KeV

214Bi-2614 KeV

208Tl-2190 KeV

208Tl-550 KeV

Preliminary Peaks Identification

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