Evaluación a la gestión del aprendizaje del docente por parte del estudiante del

143 Poster Presentation

HIGH PURITY GERMANIUM GAMMA-RAY DETECTOR FOR

144

2. MATERIALS AND METHODS

Three types of calibration were carried out:

(i) The spectrometric system was first calibrated for energy using four different gamma-ray photopeaks from radioactive sources containing the artificial radionuclides ¹³⁷Cs, ⁶⁰Co and ¹³³Ba. A plot of energy (in keV) versus channel number was obtained over a wide spectrum of gamma energies.

(ii) Certified standards provided by the New Brunswick Laboratory of the US Department of Energy, Argonne, IL, consisting of pitchblende and monazite sand with different uranium and thorium concentrations, were utilized for the calibration for uranium and thorium concentration. Pure KCl and different mixtures of this matrix with SiO2 were also used to calibrate for potassium concentration. Eight gamma ray photopeaks were chosen from the ²³⁸U and ²³²Th decay series [4]:

– ²³⁸U decay series: ²¹⁴Bi (1120.19 and 1764.49 keV, beta decay) and

226Ra (186.1 keV, alpha decay);

– ²³²Th decay series: ²⁰⁸Tl (583.19 and 2614.53 keV, beta decay), ²²⁸Ac (911.21 and 968.97 keV, beta decay) and ²²⁴Ra (240.9 keV alpha decay).

The two alpha emitting radionuclides (²²⁴Ra and ²²⁶Ra) were chosen despite overlapping with or being very close to other gamma ray photopeaks, in the absence of more isolated viable peaks. For potassium, the 1461 keV electron capture photopeak for ⁴⁰K was used.

Several plots were generated of concentration (in parts per million) versus effective intensity (the counting rate in counts per second, per unit mass in grams).

(iii) The last calibration was done for activity concentration (in Bq/g), correlating the activity concentrations of ²³⁸U, ²³²Th, and ⁴⁰K with the effective intensity, using the same photopeaks as in (ii) above.

In all readings, the background activity was measured and subtracted from the gamma spectrum of each standard. Subsequently, a percentage efficiency detection curve was generated as a function of the gamma ray energy according to Eq. (1).

A f

BG f cr



 

 (1)

where f is the detection efficiency (%) for the gamma ray energy of a specific radionuclide, crrepresents the measured counting rate (counts per second), BG is the background counting rate (counts per second), f is the peak intensity (%) and Ais the known standard activity. In this case, additional gamma ray photopeaks were adopted, i.e. ²¹⁰Pb (46.54 keV), ²¹⁴Pb (242, 352 and 295.2 keV) and ²¹⁴Bi (609 and 2204.21 keV). The efficiency is a factor directly dependent on the geometry of the detector–sample system [5].

The groundwater samples to be analysed were collected from water wells and pipes located in various municipalities in the Brazilian states of São Paulo, Minas Gerais and Mato Grosso do Sul and stored in sealed polyethylene containers. The analyses were done using an EG&G ORTEC gamma spectrometric system with a coaxial HPGe detector and Gamma Vision Software installed at LARINIonizing Radiation Laboratory at Instituto de Geociências e Ciências Exatas,Universidade Estadual Paulista em Rio Clara (IGCE-UNESP-Rio Claro). The reading for each sample lasted at least 30 000 s (~8 h) using a 1 L capacity Marinelli container.

The calibration curves were used to calculate the alpha and beta activities of the samples, taking into account the separation of the superimposed alpha related gamma-ray peaks, according to Eq. (2).

145 f

f BG A cr





  (2)

The activity value obtained was divided by the sample volume V to yield the activity concentration As of the water sample.

V

As A (3)

The final gross beta activity was calculated by the sum of the averages of partial specific activities of ⁴⁰K and each representative radionuclide from the uranium and thorium decay series, whereas the gross alpha activity was obtained by the ²²⁶Ra and ²²⁴Ra decay peaks belonging to the ²³⁸U and ²³²Th decay series, respectively.

3. RESULTS AND DISCUSSION

The result of the energy calibration is shown in Fig. 1. Examples of the results of the calibrations for concentration (in parts per million) and activity concentration (in becquerels per gram) are shown in Figs 2–4 and 5–7, respectively.

FIG. 1. Calibration for energy

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FIG. 2. Calibration for uranium concentration.

FIG. 3. Calibration for thorium concentration.

FIG. 4. Calibration for potassium concentration.

147 FIG. 5. Calibration for uranium activity concentration.

FIG. 6. Calibration for thorium activity concentration.

FIG. 7. Calibration for potassium activity concentration

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The results for the detection efficiency were represented by two separate polynomial functions, the first covering lower gamma ray energy levels (from 46 to about 1000 keV) and the second covering energies from 1000 to 2600 keV. The general trend, shown in Fig. 8, indicates that higher gamma ray energy values corresponded to lower detection efficiency levels, as expected.

FIG. 8. Detection efficiency represented by two polynomial functions.

The final gross beta activity results obtained through ²⁰⁸Tl were significantly higher than those obtained through ²²⁸Ac, contrary to expectations. The World Health Organization guideline criteria [1] were exceeded by a significant number of groundwater samples, but especially so when using the ²⁰⁸Tl calibration procedure — about 77% of the samples showed activities above 1 Bq/L whereas, using the ²⁰⁸Ac calibration procedure, 80% of them were below the criteria. In general, water from fractured aquifers from Caxambu, Cambuquira, São Lourenço, Lambari, Poços de Caldas and Serra Negra municipalities exhibited higher gross beta activity values. Practically all the samples resulted in gross alpha activities above 0.5 Bq/L, with large contributions from ²²⁴Ra and, again, the most significant values were associated with those same municipalities and fractured aquifers. In some samples, the activity of ²²⁶Ra was so low that the ²²⁶Ra–²³⁵U photopeak was barely identifiable in the gamma spectrum, especially for the samples collected in the Paraná Basin aquifer systems, where comparatively low radioactivity levels could be expected.

The difference between the results calculated through ²⁰⁸Tl and those calculated through

228Ac could be explained by the type of efficiency curve used and/or by a deviation from equilibrium conditions as a result of the escape of radon gas from the radioactive sources and samples. Results calculated through ²⁰⁸Tl would be affected by the escape of radon, whereas results based on the use of a ‘pre-radon’ radionuclide belonging to the Th decay series (²²⁸Ac) would not be affected. The geological and geochemical characteristics of the aquifers are certainly the main factors affecting the radioactivity levels in the groundwater. The cities with relatively high gross alpha and beta activities have their groundwater associated with orthogneiss, migmatites, garnet schists, pegmatites, and intrusive alkaline rocks [6]. In contrast,

149 water samples from the Paraná Basin showed lower gross alpha and beta activity results, as would be expected from the geology-related factors. The behaviour of radionuclides of natural origin under different geological and geochemical conditions may explain the gross alpha and beta values. However, they should be better investigated considering the mobility and/or solubility of uranium, thorium and their progeny and taking into account anthropogenic inputs such as agrochemical products and/or industrial contamination.

4. CONCLUSIONS

The results verified the viability of the proposed technique as an alternative to the previous method developed for gross alpha and gross beta radioactivity determination. It allows water samples to be analysed without the use of chemicals, sample destruction, problems caused by salinity or even the adoption of more than one spectrometric system for the data acquisition.

Shortcomings of the technique can be attributed to a lack of isolated and intense photopeaks for alpha calibration and also to potential problems caused by non-equilibrium conditions resulting from to the escape of radon gas. In this case, the use of a ‘pre-radon’ radionuclide for calibration, such as ²²⁸Ac in the Th series, is highly recommended. Some of the groundwater samples indicating high gross alpha and beta activity values could be re-evaluated considering seasonal changes or even possible anthropogenic inputs perhaps associated with agriculture.

REFERENCES

[1] WORLD HEALTH ORGANIZATION, Guidelines for Drinking Water Quality:

Radiological Aspects, 4th Edition, Vol. 1, WHO, Geneva (2011).

[2] BONOTTO, D.M., BUENO, T.O., TESSARI, B.W., SILVA, A., The natural radioactivity in water by gross alpha and beta measurements, Radiat. Meas. 44 (2009) 92–101.

[3] KHANDAKER, M.U., LATIF, S.A., KARIM, A.N.M., UDDIN, N., MURAD, H., JOJO, P.J., Characterization of HPGe-Detector for gamma-ray spectrometry and its application for analyzing natural samples, Asian J. Phys. Sci. 1 (2012) 47–61.

[4] CHU, S.Y.F., EKSTRÖM, L.P., FIRESTONE, R.B., The Lund/LBNL Nuclear Data Search (1999), http://nucleardata.nuclear.lu.se/nucleardata/toi/index.asp.

[5] RODRIGUES, J.L., KASTNER, G.F., FERREIRA, A.V., “Determinação de curvas de eficiência para detector HPGe em diferentes geometrias de contagem”, International Nuclear Atlantic Conference (INAC 2011) (Proc. Conf. Belo Horizonte, 2011), Associação Brasileira de Energia Nuclear (ABEN), Rio de Janeiro (2011).

[6] BEATO, D.A., OLIVEIRA, F.A., VIANA, H.S., Projeto Circuito das Águas do Estado de Minas Gerais, Geological Survey of Brazil (CPRM), Belo Horizonte (1999).

150 Poster Presentation

NaI(Tl) DETECTORS FOR ENVIRONMENTAL GAMMA SURVEY

AND BACKGROUND RADIATION MAPPING OF VENUES AND

OTHER STRATEGIC LOCATIONS FOR MAJOR PUBLIC