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A new CR-39 nuclear track passive thoron measuring device

A new CR-39 nuclear track passive thoron measuring device. Massimo Calamosca, Silvia Penzo. 24 th International Conference on Nuclear Tracks in Solids. Bologna 1-5 September 2008. Thoron scenario. Insufficient knowledge compared to radon Not explicitly addressed from the national law

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A new CR-39 nuclear track passive thoron measuring device

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  1. A new CR-39 nuclear track passive thoron measuring device Massimo Calamosca, Silvia Penzo 24th International Conference on Nuclear Tracks in Solids Bologna 1-5 September 2008

  2. Thoronscenario • Insufficient knowledge compared to radon • Not explicitly addressed from the national law • No standard measurement method available • Scarcity of available calibration facilities • Absence of intercomparisons up to now • Short T1/2 • Low exhalation probability • In-homogeneity of thoron concentration Usually the thoron risk caused by TnDP is not evaluated 24th International Conference on Nuclear Tracks in Solids

  3. Specific thoron harm >specific radon harm for the same Ceq Thoronscenario Cp thoron (75.66 nJ Bq-1) > Cp radon (5.56 nJ Bq-1) • Some building materials (as for instance in the case of tuff) can cause 220Rn decay products exposure comparable or also greater to 222Rn decay products exposure Interest in risk assessment of thoron exposure is world-wide increasing 24th International Conference on Nuclear Tracks in Solids

  4. The standard procedure proposed by ENEA to assess equivalent dose due to thoron exposure • Sampling selection. • Mean thoron gas concentration measurement by ATD-PD. • Experimental determination of effective room diffusion coefficient. • Thoron exhalation rate from room surfaces. • Thoron decay products (TnDP) concentration computation • Experimental validation of the computed TnPD mean concentrations. • Definition of occupancy factor and exposure assessment. • DE assessment by applying the conversion dose factor 0,50 mSv (mJ h m3)-1. 24th International Conference on Nuclear Tracks in Solids

  5. Start point: ATD-RnPD (patent MI2006A000703) with changes aimed to The new ENEA ATD-TnPD Closed type, under patent, to be used coupled to our ATD-RnPD Improve the thoron sensitivity: Decrease the radon sensitivity: • Increase of the diffusion free surface • Reduction of the diffusion time • Increase of thoron concentration Increase the thickness of the window between the volume of the device and the ATD surface 24th International Conference on Nuclear Tracks in Solids

  6. Diffusion kinetic test facilitiy: schematic diagram Flow rate=1300 l/h The CR-39 detectors in the ATD-RnPD and ATD-TnPD have been substituted by PIPS detectors to record the time development of the a spectra due to the radon exposure of the two ATD-PD 24th International Conference on Nuclear Tracks in Solids

  7. Dwelling time = 600 s Diffusion kinetic test Asymptotic concentration growth in the radon chamber Requirements to perform the test • High radon concentration (~400 kBq/m3) • Concentration in the radon chamber ≥ 90% equilibrium value • The equilibrium in the test chamber is reached very fast: t90%=19 s => the delay time does not affect significantly the measurement uncertainty The alpha energy spectra, sequentially saved with short dwelling times, were analysed and corrected for the 218Po and 214Po spectra spill over the 222Rn ROI 24th International Conference on Nuclear Tracks in Solids

  8. 3.08 min (2.32÷4.58) (k=2) ATD-TnPD 20.8 min (15.3÷32.4) (k=2) ATD-RnPD Mean diffusion time Equilibrium ratio between internal and external 220Rn concentration 6.0 % (4.0÷8.0) (k=2) ATD-RnPD 30.3 % (22.6÷36.6) (k=2) ATD-TnPD Diffusion kinetic test: results Increase of radon concentration in the two devices 24th International Conference on Nuclear Tracks in Solids

  9. Flow rate = 18.0 l/min Thoron exposure: the calibration exposure chamber 24th International Conference on Nuclear Tracks in Solids

  10. Thoron exposure: the 220Rn source Sample of Monazitemineral was crushed, sieved and wet-grinded by a Turbula to reduce the grain size to ~ 1µm, so to increase the thoron emanation. The suspension obtained was filtered and piled up in set of 15 filters. The source is constituted by 3 towers of 15 filters each, 3 mm spaced, connected in series and installed within a stainless-steel vessel. 232Th and 226Ra activities measured by HPGe 232Th=(8.82±0.19) kBq, 226Ra (355±18) Bq, 226Ra/232Th = 4% 24th International Conference on Nuclear Tracks in Solids

  11. How to determine the thoron exposure: the thoron monitor (TM) A new monitor based on the two-filter method has been devised to absolutely measure the 220Rn concentration directly at the exit of the chamber. It consists of a small aluminium cylindrical tube with two filters in series separated by a delay volume to permit 220Rn decay, 216Po growth and a negligible presence of 218Po. 24th International Conference on Nuclear Tracks in Solids

  12. 1° sector (sampling head & 1° filter) 5° sector (Volume & 2° filter) 2° sector (Body) 3&4° sectors (PIPS) The thoron monitor: sectors 24th International Conference on Nuclear Tracks in Solids

  13. Thoron exposure: the thoron monitor The thoron concentration is determinated by 220Rn and 216Po counts counted during the sampling (1000 s). 24th International Conference on Nuclear Tracks in Solids

  14. Thoron monitor: uncertainty budget 24th International Conference on Nuclear Tracks in Solids

  15. Thoron exposure: calibration results 24th International Conference on Nuclear Tracks in Solids

  16. 7 different etching times Removed Layer range : (7.84 ± 0.04) ÷ (12.54 ± 0.05 ) µm 114 ATD-PDs Thoron exposure: calibration test design 3 thoron exposures (360 ± 44) kBq h m-3 (109 ± 11)  10 kBq h m-3 (301 ± 22)  10 kBq h m-3 12 ATD-TnPDs 6 ATD-RnPDs 6 ATD-RnPDs (off position) 3 radon exposures (273 ± 11) kBq h m-3 (1365 ± 53) kBq h m-3 (395 ± 15)  10 kBq h m-3 6 ATD-TnPDs 6 ATD-RnPDs Reference radon exposure (260 ± 10)  10 kBq h m-3 6 ATD-RnPDs 24th International Conference on Nuclear Tracks in Solids

  17. The thoron chamber exposure: procedure requirements A high (18 lpm) circulating flow must be hold for all the course of exposure To minimize 220Rn decay the volume of the tubing has to be kept as the minimum To minimize in-homogeneity exposure within the chamber, the use of a fan to mix air has been adopted To prevent the growing of radon in the circuit Nitrogen has to be fluxed through the chamber at least every 24 h The ATD-PD were stored for at least 70 h in a radon proof container to get the irradiation equilibrium for 212Bi (212Pb) 24th International Conference on Nuclear Tracks in Solids

  18. Thoron exposure: net track density evaluation Where D(XPD,Tn+Rn) is the track density measured in the X passive device exposed to mixed irradiation (Tn+Rn, or Tn or Rn) Where SENS(X,Y) is the response or sensitivity of the X passive device to the Y (Tn or Rn) gas 24th International Conference on Nuclear Tracks in Solids

  19.  = 0. 056+0.00300D95% R2 = 0.67, P<0.05, N=7 Thoron exposure results: w dependence on etching time 24th International Conference on Nuclear Tracks in Solids

  20.  = 0.940 - 0.0218D95% R2 = 0.912, P<0.01, N=7 Thoron exposure results:  dependence on etching time 24th International Conference on Nuclear Tracks in Solids

  21. Thoron exposure results: response of both devices to radon vs. etching time 24th International Conference on Nuclear Tracks in Solids

  22. Thoron exposure results: response of radon passive device to toron vs. etching time 24th International Conference on Nuclear Tracks in Solids

  23. SENS = 1.840(1-Exp(-0.1826D95%)) R2 = 0.941, P<0.01, N=7 Thoron exposure results: response of toron passive device to toron vs. etching time 24th International Conference on Nuclear Tracks in Solids

  24. Conclusion The new ATD-TnPD has been completely characterised. The responses (SENS) to toron and radon result a little better than our previsions based on the preliminary tests We are always looking for an external calibration The new thoron source and monitor have resulted very effective, so to be considered fit to be used in the calibration step. And for the future… A finer determination of the diffusion losses, that result the primary sources of uncertainty, is to be worked out, … Thank You for your attention… and patience 24th International Conference on Nuclear Tracks in Solids

  25. Here is what did occur at PTB ! 24th International Conference on Nuclear Tracks in Solids

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