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PE vs. Water and requirements on wall materials

PE vs. Water and requirements on wall materials. Béla Majorovits for the Max-Planck-Institut für Physik, München. OUTLINE:. Alternative to a third wall: PE instead of water Radiopurity of PE Estimated background due to PE and how could we avoid it?

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PE vs. Water and requirements on wall materials

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  1. PE vs. Water and requirements on wall materials Béla Majorovits for the Max-Planck-Institut für Physik, München

  2. OUTLINE: • Alternative to a third wall: PE instead of water • Radiopurity of PE • Estimated background due to PE and how could we avoid it? • What can we learn? MaGe simulations for: copper, water, superisolation

  3. Third wall required by LNGS for safety reasons Several disadvantages: • Less water to shield against external gammas and neutrons • More (potentially dirty) material in the vicinity of the detectors • More complicated structure

  4. Alternative design: Use PE and LAr instead of water and LN2: Advantagesof PE: • PE can not mix with LN2  No catastrophic evaporation possible • Self supporting: can be stacked around cryo tank  easy handling But what is the influence of PE material to expected background rate?

  5. Radiopurity of PE: Values taken from: Recent GERDA measurement and http://radiopurity.in2p3.fr  Assume 10 mBq/kg 208Tl to estimate overall contribution of PE

  6. Analytical estimate of background contribution I: • Calculate the number of emitted 2.6 MeV gammas from unit volume per unit time that are emitted towards the detector volume • Take into account self absorption • Integrate over thickness and sphere

  7. Analytical estimate of background contribution II: • Scale this number with reduction factor due to nitrogen and copper in the way • Scale this number with the peak to background ratio (from simulation) • Take into account anticoincidence and detection efficiency

  8. GERDA sensitivity(see K. Kroeninger)  We need to obey severe constraint: Achievable sensitivity of the experiment degrades rapidly with Btot≥10-3 Counts/kg/keV/y Bmax,contr≤ 10-4Counts/kg/keV/y

  9. Expected background contribution of PE with 2 m LAr tank 2.6MeVAPE = 10 mBq/kg  2.6MeVBPE= 1.9 * 10-2 Counts/kg/keV/y Reduction of factor 190 required in order to meet the requirement of 10-4Counts/kg/keV/y r = e -μCu L Cu= 1/190  we need to have additional copper shield of dCu,ana=133 mm Independent cross check with MC simulation: dCu,sim=138 mm

  10. PE contribution is less than 10-4Counts/kg/keV/y for liquid Argon as shield with vessel of more than 3000 mm radius  PE seems feasible, but makes sense only with tank radius >> 2000 mm

  11. We have to be aware:  Results calculated for PE with liquid Argon shield will be even stricter for any surrounding materials with liquid nitrogen shield!  Check for radiopurity requirements of shielding materials: • Water • Copper • Superisolation (~30 layers of MYLAR)

  12. Inner Copper wall • Vacuum (30 layers super-isolation) • Outer Copper wall • Water around the outer Copper vessel Simulations made with MaGe 2.6 MeV gammas randomly distributed in each volume:

  13.  !  We need to be extremely carefull with (surface) contamination of superisolation! 0.36 km2 of very-clean surface Constraints for different materials:   Water needs to be of very high purity! doable: achieved for BOREXINO and SNO  Same requirements hold for PE: 10mBq/kg could be compensated by 200 mm of Cu shield  Copper has to be pure, but OF01 and NOSV copper meet requirements

  14. CONCLUSIONS • PE design with liquid Argon seems reasonable, but only with increased vessel radius r >> 2000mm • Restrictions for all materials are severe for LN2 • Beware of the superisolation: 0.36km2 of (electrostatically easily chargable) very-clean surface: 260 μBq/m2 • Internal note with details will be published soon

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