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High Sensitivity Radon Detector

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					2nd Topical Workshop in Low Radioactivity Techniques
           October 1 - 4, 2006 - Aussois




Radon reduction facility and Radon
  measurements at the Modane
     Underground Laboratory
                Abdellatif NACHAB
                        and
              the NEMO Collaboration
                NEMO3 Detector
The NEMO3 (Neutrino Ettore Majorana Observatory)             experiment
investigates neutrinoless double beta decay (2β0ν).



                                 7 kg Mo100
                                 1 kg Se82
                                 0,4 kg Cd116
                                 1 kg Te130
                                 50 g Nd150
                                 20 g Zr96
                                 7 g Ca48
                                 1 kg Tenat
                                 0,6 kg Cu


NEMO3 is now running in the (LSM). It is a very low radioactive background
detector.
Origin of the background in NEMO3


                                                                   Energy (MeV)

      0         ~ 1-2                 3              10
 Natural radioactivity
                        208Tl    et   214Bi

                                              neutrons
                                                          cosmic rays

Cosmic rays   Underground lab (1700 m of rocks)
10.000.000 muons/m2/day (>200 MeV)
LSM : 4 muons/m2/day !!

Neutrons coming from rock and materials surrounding the detector :
Shielding: water, wood, paraffin …

Natural radioactivity   208Tl   , 214Bi
All construction materials are selected according their radio-purity   !   Rn
           Radon level in NEMO 3
In the LSM the level of radon activity is about 16 Bq/m3
First data have shown inside NEMO3 a level of Rn around 15 mBq/m3
This level gives 8 events/year in the 100Mo (7 kg) ββ0ν energy bin
Need to be reduced by at least a factor of 10
The origin of this Rn was suspected to be air leaks between the sectors


 In 2004 :the detector has been surrounded by an airtight plastic tent.




 The radon value inside the tent went
 down from 16 Bq/m3 to 5 Bq/m3



              Not enough
                Free-Radon air factory
                                   Principe:
Air circulation (150 m3/h) trough a column of charcoal cooled down at -50°C




The specifications of the radon trapping facility are :
      Compressor (7 bars)
      filtration with oil separator (0.03 µm) and dust separator (0.1 µm)
      Air Dryer with a dew point -70°C for 8.5 bars -30°C at maximum value
      Cooling unit
      Two adsorption columns, with internal diameter of 600 mm and 3 m high
      Charcoal: activated carbon 2x500 kg
  The Rn level at exit of the column: 18 mBq/m3           air sent into the tent
               Rn control at the exit of the anti-radon factory
                                                             (from 28/07/05 to 13/09/06)

Evolution of the radon activity at the exit of the free-radon air factory between the
28 July 2005 to 13 September 2006. One point corresponds to 24h measurement.
The average activity in this period (~ 1 year) is around 18 mBq/m3.
              100

                                                                                                               Anti-radon
                                                          Alpha 7.7 M eV                                      factory "off"

                                                                                                               17/07/06
               80
                                       N o acq              1 counts/day ≈ 1 mBq/m3                                    Anti-radon
                                                                                                                      factory "off"
                                               18/10/05
                                    05/10/05                                                                           11/08/06
               60
 Counts/day




                                                                        Anti-radon
               40                                                      factory "off"
                    Pow er cut
                                                                        08/03/06               Anti-radon
                         10/08/05
                                                                                              factory "off"
                                                                                                16/06/06

                        28/07/05
               20




                                     Background level = 5.8 counts/day
                0
                    0               50                100        150        200         250       300           350               400          450
                                                                                                                                        days
Comments:
                                                                          1 Point / day

- Natural variations of the radon level in the LSM                                                                1 measurement / day
- Temperature variation of the charcoal column
                    Rn control in the NEMO tent                                                          (11/01/06 to 13/09/06)
Evolution of the radon activity inside the airtight tent with the radon detector between the
11 January 2005 to 13 September 2006. One point corresponds to 6h measurement.
         8000

                                                                                          Anti-radon factory "off"
                           Anti-radon factory "off"                                           16/06/06
         7000                08/03/06



         6000                                Anti-radon factory "off"
                                                                                                                                  Anti-radon
                                                                                                                                 factory "off"

         5000
Counts




         4000


         3000
                                                      Calibration
                                                      15/03/0
                                                        22/03/06
         2000
                    11/01/06                                  Power cut                                  Calibration
                                                               31/03/06                                    11/07/0
                         Power cut
                                                                      Power cut    calibration                         Calibration
         1000                                                                           31/05/06                       27/07/0



            0
                0              100         200            300           400         500            600         700               800             900   1000
                                                                                  Point / 6h
           The average activity in this period is around 170 mBq/m3.
           Several accidents due to different events (energy calibration, power cuts, …)
           Note that after each accident we come back to the low Rn level within ~1 day .
   Current situation for NEMO3

                                                 The difference between
                 ≃ 18     mBq/m3    Anti-radon   the activity injected
                                     factory     and measured inside the
Tent                                             tent due to:
                                                    air leaks in the tent
                NEMO3
                                                    degassing     of   the
                                                 NEMO electronics

       ~ 2 mBq/m3


                                                    We suspect some
                                                    220Rn (Thoron) in
       ~ 170 mBq/m3                                     NEMO3 ?!
                                   ≃ 16 Bq/m3     We planned to study
       NEMO electronics                            this component with
                                                  a new radon detector
                                       LSM
     High Sensitivity Radon Detectors
             Built by H.Ohsumi, Saga University, Japan


• Electrostatic collection of the 222Rn daughter (-1500 V)
• Energy measurement of the alpha decay on the PIN photodiode
• Large (70 l) and clean vessel to get high sensitivity



       Typical sensitivity
           ~1mBq/m3
                                                          -1500
     1 α detected in 24h                                    V
                                                 218Po+



                                         222Rn
                             Typical α spectra
         The decays of 218Po,214Po, 210Po (daughter of 222Rn in the U-series), and
         212Po (daughter of 220Rn in the Th-series) are distinguished by
         monoenergetic peaks in the energy spectrum. (7.7 MeV and 8.8 MeV)
counts




                                                              counts
                                214                                                                           212
                                   Po                                                                            Po
                               7.7 MeV                                                                       8.8 MeV
                                                                                               212
                                      B                                                          Bi                     Thoron
                                                                                              6 MeV
                       218
                          Po                  Radon
                       6 MeV                                                                  C      Small
                                                                                                     peak           E
                       A
                                                                                                      D




                                                    channel                                                                channel



                                 210Po     218Po 212Bi             216Po 214Po       212Po
                               (5.30MeV) (6.03MeV) (6.05MeV) (6.77MeV) (7.68MeV) (8.78MeV)

                                                  C      D                       E



                                                A                      B
                                      5         6             7            8            9    MeV
                 Example of Rn monitoring in the NEMO tent

                        After accident (truck in fire in the tunnel)
       180


       160


       140                    6 MeV
                              7.7 MeV

       120


       100
3
Bq/m




       80
                                                                   07/07/05 at 20h
       60                                                          New ventilation



       40

                            04/06/05 18h
       20                     accident
                 17/05/05

        0
             0         50   100      150   200   250   300   350      400            450   500


         After the accident in the tunnel, ventilation and compressor went off

				
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