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Exploring the Use of Polyethylene as an Alternative to Water for Wall Materials in Physics Experiments

This paper by Béla Majorovits, presented at the Max Planck Institute for Physics, discusses the potential of using polyethylene (PE) instead of water for the construction of walls in physics experiments. It reviews the radiopurity of PE and its estimated background contributions, addressing safety concerns associated with the traditional third wall requirement. The advantages and implications of using PE and liquid argon (LAr) over water are examined, emphasizing design considerations and necessary shielding to meet stringent radiopurity standards.

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Exploring the Use of Polyethylene as an Alternative to Water for Wall Materials in Physics Experiments

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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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