BAS-ION Istituto di Radioprotezione 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

2 24 th International Conference on Nuclear Tracks in Solids Thoron scenario  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 Usually the thoron risk caused by TnDP is not evaluated  Short T 1/2  Low exhalation probability  In-homogeneity of thoron concentration

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

4 24 th International Conference on Nuclear Tracks in Solids 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 m 3 ) -1.

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

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

7 24 th International Conference on Nuclear Tracks in Solids Diffusion kinetic test Requirements to perform the test High radon concentration ( ~ 400 kBq/m 3 ) High radon concentration ( ~ 400 kBq/m 3 ) Concentration in the radon chamber ≥ 90% equilibrium value Concentration in the radon chamber ≥ 90% equilibrium value The equilibrium in the test chamber is reached very fast: t 90% =19 s => the delay time does not affect significantly the measurement uncertainty The equilibrium in the test chamber is reached very fast: t 90% =19 s => the delay time does not affect significantly the measurement uncertainty Dwelling time = 600 s The alpha energy spectra, sequentially saved with short dwelling times, were analysed and corrected for the 218 Po and 214 Po spectra spill over the 222 Rn ROI Asymptotic concentration growth in the radon chamber

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

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

10 24 th International Conference on Nuclear Tracks in Solids Thoron exposure: the 220 Rn source Sample of Monazite mineral 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. 232 Th and 226 Ra activities measured by HPGe 232 Th=(8.82±0.19) kBq, 226 Ra (355±18) Bq, 226 Ra/ 232 Th = 4%

11 24 th International Conference on Nuclear Tracks in Solids 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 220 Rn 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 220 Rn decay, 216 Po growth and a negligible presence of 218 Po.

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

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

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

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

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

17 24 th International Conference on Nuclear Tracks in Solids The thoron chamber exposure: procedure requirements A high (18 lpm) circulating flow must be hold for all the course of exposure To minimize 220 Rn 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 212 Bi ( 212 Pb)

18 24 th International Conference on Nuclear Tracks in Solids 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

19 24 th International Conference on Nuclear Tracks in Solids Thoron exposure results:  dependence on etching time  =  D 95% R 2 = 0.67, P<0.05, N=7

20 24 th International Conference on Nuclear Tracks in Solids  =  D 95% R 2 = 0.912, P<0.01, N=7 Thoron exposure results:  dependence on etching time

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

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

23 24 th International Conference on Nuclear Tracks in Solids SENS =  (1-Exp(  D 95% )) R 2 = 0.941, P<0.01, N=7 Thoron exposure results: response of toron passive device to toron vs. etching time

24 24 th International Conference on Nuclear Tracks in Solids 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 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, … 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

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