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Collaboration

Low Flux Neutron Measurements Using A Large Volume (1m3) Spherical Proportional Counter Ilias Savvidis Aristotle University of Thessaloniki. Collaboration S . Aune , E. Bougamont , M. Chapelier , P. Colas, J. Derré , E. Ferrer , G. Gerbier , I. Giomataris ,

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Collaboration

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  1. Low Flux Neutron Measurements Using A Large Volume (1m3) Spherical Proportional CounterIliasSavvidisAristotle University of Thessaloniki Collaboration S. Aune, E. Bougamont, M. Chapelier, P. Colas, J. Derré, E. Ferrer, G. Gerbier, I. Giomataris, M. Gros, P. Mangier, X.F. Navick,CEA-Saclay, I. Savvidis , University of Thessaloniki P. Piquemal, M. Zampalo, LSM S. Gaffet, P. Salin, LSBB I. Irastorza, University of Saragoza J. D. Vergados, University of Ioannina The outline 1. The spherical proportional counter (SPC) 2. Resolution and He-3 neutron detection 3. Underground neutron flux measurements in LSM (Modane) 4. N-14 neutron detection 5. Conclusions

  2. The three detectors CEA Saclay LSM - Modane Thessaloniki

  3. Radial TPC with spherical proportional counter read-out Saclay-Thessaloniki-Saragoza A Novel large-volume Spherical Detector with Proportional Amplification read-out, I. Giomataris et al., JINST 3:P09007,2008 • 5.9 keV 55Fe signal • Very low electronic noise: low threshold • Good fit to theoretical curve including avalanche induction and electronics E=A/R2 20 s 15 mm • Simple and cheap • single read-out • Robustness • Good energy resolution • Low energy threshold • Efficient fiducial cut C= Rin= 7.5 mm < .1pF

  4. Electrostatic field (simulation results)LEFT: 15 mm sphere, 1mm Cu cable covered with 3mm PE RIGHT: 15 mm sphere, 1mm Cu cable covered with 3mm PE + graphite (ground). Distance sphere to graphite 4mm No field correction With field correction

  5. Some of the sensors we have been used in Thessaloniki

  6. The new glass detector (Φ= 40cm) and the test of the sensors Light production around the ball (P= 100 mbar,air) The beginning of a spark

  7. Experimental results from Rn sourceand the resolutionto the alpha particles (CEA Saclay) • Rn and Rn daughters: • Rn-222: 5.49 MeV alpha • Po-218: 6.00 MeV alpha • Po-214: 7.68 MeV alpha • Resolution: σ=1.5% • Gas: 98% Ar + 2% CH4 • P=200 mbar • HV=2800 Volt

  8. Neutron detection and the wall effect • Neutrons are detecting using the reaction • n +He-3 →p+H-3 +765 keV • The charged particles from the neutron reactions have long range in the gas and interact with the wall of the detector, leaving in the gas a part of their energy. • It is a big problem for the small volume proportional counters. • The large volume spherical detector limit drastically the wall effect for neutrons up to several MeV.

  9. 3-He(n,p)3-H cross section Thermal Neutron flux= 120n/cm2 d 1 MeV< En <10 MeV = 432n/cm2 d 100 MeV<En< 1000 MeV = 43n/cm2 d

  10. Atmospheric neutrons Neutrons of a Cf-252 source Neutron detection in CEA-SaclayAr+1%CH4+34 mgrHe3, P=188mbar, V=2880 Volt

  11. The first installation of the detector in Modane

  12. Neutrons sourcesNeutrons present in the rock are those produced undergroundby cosmic muons (the only particles whichcross the rock, down to a few hundred kilometers),and neutrons induced by spontaneous fission and( α,n) reactions due to uranium and thorium traces in therock . • 1. Neutron production by cosmic muons • in therocks: 2.30 x 10-8neut/year/gofrock. • In the lead : 3.2 x 10-7 neut/year/g of lead of the shield. • We can conclude that neutron flux produced bymuons interaction is negligible. • 2. Neutron production by spontaneous fission • The average number of neutrons emitted per fission event is 2.4 . • The number of spontaneous fission of 238U • is 0.218 /year/g of rock for 1 ppm of uranium in the rock. • The flux is 0.47 neut/year/g of rock, emitted by spontaneous fission in the rock. • 3. Neutron production by (a, n) reactions • Alpha-particles are emitted by uranium, thorium • and their daughter products present in the rock. • The neutron flux is1.93 neut/year/gof rock, inside the rock. • (V. Chazal et all, January 1998)

  13. Results in LSM( 5 weeks): 3 gr of He-3Detector is stable operating in seal modeGas Ar +2% CH4 at p=280 mbarthe 760 keV thermal neutrons and the Rn daughters background 210Po 760 keV 218Po 214Po

  14. Results in LSM(Michel Gros Tango program) Amplitude Rise time (ms)

  15. Ampl. - Date 5 weeks 210Po 760 keV

  16. 2- LSM (run 2) (underground neutrons) Thermal neutron capture rate R=0.0048evts/s = 417 evts/d With3gr He-3inthesphere Thermal neutron flux Φth.neutron = 1.9 10-6 n/cm2/s 1- Saclay (atmospheric neutrons) Thermal neutron capture rate R=0.0375 evts/s = 3240 evts/dwith34 mg He-3inthesphere Thermal neutron flux Φth.neutron = 1.035 10-3 n/cm2/s Thermal neutron fluxDirectlymeasurementwithout any moderator

  17. Events/1.74 days Neutron flux variation87 days run (last results)

  18. Neutron detection with N2 via the reaction 14N(n,p)14C 14 N + n → 14 C + p + 625.87 keV The signals are coming from the protons and the 14C As an example for a gas mixture of 50% Ne and 50% N2 and 500mbar pressure, the proton range for thermal neutrons is 2.2cm and the wall effect 2%. 3He(n,p)3H cross section The cross section disadvantage can be covered sufficiently by the large amount of nitrogen atoms in the sphere. 14N(n,p)14C cross section

  19. Thessaloniki sphere experimental setup The neutron spectrum in the sphere Shielding Pb= 9cm Fe= 5cm PE= 2cm

  20. Neutron irradiation data (Thessaloniki sphere) P=150mbar, Ar 91%, CH4 5%, N2 4% 14N(n,p)14C 625.87 keVpeak 14N(n,p)14C 625.87 keVpeak 210-Po, 5.3 MeV α particles 210-Po, 5.3 MeV α particles

  21. 14N(n,p)14C 625.87 keV peak at higher gain recoils 14N(n,p)14C 625.87 keVpeak 14N(n,p)14C 625.87 keVpeak

  22. Low neutron flux Cf-252 source (CEA Saclay) 400mbar Ne , C2H6 2% , N2 30% 14N(n,p)14C 625.87 keVpeack 210-Po, 5.3 MeV α particles

  23. Conclusions • The spherical proportional counter is a high resolution and low flux He-3 neutron detector • Small decrease in gain≈ 0.2% per day • The detector can measure directly low flux underground thermal neutrons. • The spherical proportional counter can be used successfully as a neutron detector via the 14N(n,p)14C reaction and both the gas mixtures, (Ne, C2H6, N2) and (Ar,CH4,N2) are well working. • The large volume with the possibility to operate at high gas pressure, is an important advantage of our detector, compared to the traditional cylindrical proportional counter. • Prospects for measuring fast neutrons are open.

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