SPATIAL DISTRIBUTION OF NEUTRON FLUX IN GEOLOGICAL LARGER SAMPLE ANALYSIS AT CDTN/CNEN, BRAZIL 1 Maria Ângela de B. C. Menezes 2 Radojko Jaćimović 3 Cláubia Pereira 1 Nuclear Technology Development Center/Brazilian Commission for Nuclear Energy (CDTN/CNEN), Belo Horizonte, Brazil 2 Jožef Stefan Institute (JSI), Ljubljana, Slovenia 3 Federal University of Minas Gerais, Department of Nuclear Engineering, Belo Horizonte, Brazil
Geometrical point-source sample: Larger sample: more several simplifications disregard the representative, analysis takes neutron self-shielding, neutron-flux less time, can reach lower gradients and self-attenuation of gamma detection limit, analysis rays considered negligible sensitivity and quality can be increased and cost decreased, way to compensate for low flux of neutrons and for non-homogeneity ~ 5g 0.2 g 25 times higher than the routine size!
Larger sample Three parameters were considered: i) detector efficiency over the volume source, ii) gamma ray attenuation within the sample during counting and iii) neutron self-shielding and neutron flux gradients during irradiation Objective: to show how was the evaluation of the contribution of the spatial neutron flux gradients to the final elemental concentration results during the establishment of the larger sample methodology
Importance to experimentally evaluate these factors: - is to make a real evaluation of the neutron interaction through a sample during irradiation - to know the neutron flux distribution - it should be specific for each kind of matrix, once the composition of the sample is not completely known This establishment: geological matrix à the most used matrix in routine elemental analysis biological matrix à comparison
Experimental Two experiments iron wires (99.9% Fe from Mallinckrodt, USA) 0.4 mm in diameter and 5 cm in length
Experimental The characterization of neutron flux gradients (axial and radial) based on the specific activities of 59 Fe and 54 Mn 59 Fe (thermal neutrons) 58 Fe(n t, ) 59 Fe, 44.50 d half-live, 1099.3 kev and 1291.6 kev 54 Mn (fast neutrons) 54 Fe(n f,p) 54 Mn, 312.2 d half-live, 834.8 kev
NEUTRON ACTIVATION ANALYSIS TRIGA MARK I IPR-R1 reactor at CDTN/CNEN carrousel facility: IC-7 and IC-6, f and (22.32 and -0.0022, respectively) Thermal neutron flux 6.35 10 11 n cm -2 s -1
Each wire à cut into five 1 cm sections and submitted to gamma spectrometry on an HPGe detector, 50% relative efficiency Peak area evaluation: HyperLab 2009 program Activity calculation: KayWin v.2.42, 2011
Experiment 1: in a polyethylene vial filled with clay and air Five iron wires were placed in the vial: - one in the centre and - four near the wall Clay Air 2 cm high 3 cm high Experiment 2: filled with plant wire was inserted in the middle Plant 5 cm high
1,06 TRIGA I, IC- 7 (Clay) and IC- 6 (Biomat) Normal. 59 Fe 1,04 2%/cm 1,02 1,00 0,98 ~2.5%/cm 0,96 0,94 A B C D E Posi0on: Bo4om - Top Clay (3 cm) - Air (2 cm) Biomat (5 cm) Axial gradient for thermal neutrons in clay-air and plant samples
1,06 TRIGA I, IC- 7 1,04 Normal. 59 Fe 1,02 1,00 0,98 ~2%/cm 0,96 0,94-60 0 60 120 180 240 300 360 420 Angle, Clay (3 cm) - Air (2 cm) Radial gradient for thermal neutrons in clay-air sample
1,10 TRIGA I, IC- 7 (Clay) and IC- 6 (Biomat) 1,05 Normal. 54 Mn 1,00 2.5%/cm 5%/cm 0,95 0,90 A B C D E Posi0on: Bo4om - Top Clay (3 cm) - Air (2 cm) Biomat (5 cm) Axial gradient for fast neutrons in clay-air and plant samples
1,10 TRIGA I, IC- 7 1,05 1,00 Normal. 54 Mn ~5%/cm 0,95 0,90-60 0 60 120 180 240 300 360 420 Angle, Clay (3 cm) - Air (2 cm) Radial gradient for fast neutrons in clay-air sample
Spa0al distribu0ons in a 5g- geological sample were about: 2%/cm for radial and axial gradients for thermal neutrons and 5%/cm for radial and 2.5%/cm for axial gradients for fast neutrons In biological sample, the thermal neutrons axial gradient was about 2.5%/cm and about 5%/cm for fast neutrons Is the total contribution of neutron flux gradients, significant?!
Verify whether gradients were significant in the final concentration values IAEA-SOIL-7 analysed as small sample (0.2 g) and larger sample (5 g) applying the k 0 -method Statistical evaluation : Relative Bias and E n -number Relative Bias informs the distance of the experimental results from the assigned values for IAEA-SOIL-7 RB Value Lab Value Value Assigned Assigned 100% Small sample presented 72.2% of results with deviation from the recommended values, lower than 10% Larger sample, 88.9%
E Experimental n Statistical Evaluations E n -number: - evaluates the performance of the procedure applied - takes into account the expanded uncertainty of experimental and certified values with a 95 % confidence interval Value U Experimental 2 Experimental Lab_Comb Value U 2 Assigned U 2 U k=2 Assigned Evaluation of the performance: satisfactory: En 1 unsatisfactory En > 1 Sta0s0cal methods for use in proficiency tes0ng by interlaboratory comparisons, ISO 13528:2005
1,5 IAEA Soil-7 by k 0 -INAA small sample satisfactory: En 1 En-number 1,0 0,5 0,0 As Ce Co Cr Cs Eu Hf La Nd Rb Sb Sc Sm Ta Tb Th U Yb Elements En-number (IAEA Soil-7)
1,5 IAEA Soil-7 by k 0 -INAA larger sample satisfactory: En 1 En-number 1,0 0,5 0,0 As Ce Co Cr Cs Eu Hf La Nd Rb Sb Sc Sm Ta Tb Th U Yb Elements En-number (IAEA Soil-7)
Conclusions IAEA-SOIL-7: analysed by k 0 -method as small and larger samples Statistical evaluation pointed out all results were in good agreement with the assigned values Although there are neutron flux gradients in 5g-sample, the contribution may be negligible for larger sample or, at least, within the uncertainty range It is not necessary to apply correction factors Acknowledgments IAEA, FAPEMIG, Slovenian Research Agency
Thank you! Muito obrigada!