Environmental gamma and radon dosimetry in Venezuela

Radiation Measurements

PERGAMON

Radiation Measurements31 (1999) 283-286

ENVIRONMENTAL

G A M M A A N D R A D O N D O S I M E T R Y 1N VENEZUELA

L. SAJ0-BOHUS, J. P/iuLFALVI 1, F. URBANI 2, D. CASTRO, E.D.GREAVES AND J.A.LIENDO Universidad Sim6n Bolivar, Caracas, Venezuela i Atomic Energy Research Institute - HPD, Budapest, Hungary 2 llniversidad Central, Fac. de Ing., Depto. de Geologia, Caracas, Venezuela ABSTRACT Environmental gamma exposure and radon concentration levels measured in Venezuelan regions are presented. A new generation image analyser was used for a particle track counting in CR-39 detectors. Mineral water wells from where water is supplied for massive consumption have an alpha activity around 0.450 Bq L-T and few of them have concentrations above 50 Bq L~. Coastal potable water activity is on the average around 5.3 + 12% Bq L1. Indoor radon national average is 36 + 5% Bq m3; in two of the 36 monitored sites, the measured average is above 400 + 5% Bq m "3. In air gamma dose values are between 100 and 144 nGy hq. In soil, 137Csconcentration is around 0.5 and 10 Bq kg1 at the depth of dowa] to 20 cm. Building materials were included in this study. 7Be and ~37Cs were measured in low concentration in tropical plants on Tepuy-s (sacred mountains in the Amazonas State). Geological active faults were identified by radon concentration measurements using LR-115. KEYWORDS Gamma radiation; radon; image analyser; dosimetry; environment; geology; CR-39; LR-115,

INTRODUCTION During the past few years an intensive national program was developed to determine the environmental gamma dose and the radon concentrations indoors, in drinking water, in caves and in artificial cavities including the effect of radon transported to the surface with the earth gas, oil and with other natural resources. It is largely accepted that the human activity is the most influencing geomorphic agent that modifies the earth surface hence the radiation dose may be considered as an ever-changing parameter. In Venezuela, the environmental radiation level is modified in regions where the industrial activity is more aggressive i. e. the Zulia State, the Central Region and the Bolivar State. This activity is related to the near-to-the-surface mineral exploitation, deep oil extraction, gold and diamond mining and to the by-product of ore processing (aluminium and iron). The techniques employed include the SSNTDs such as CR-39, LR-115 and alpha counting and gamma spectrometry. An advanced version of the image analyser (Palfalvi et al., 1997), was used to count ~ particle tracks. Presence of man-made and cosmic ray induced radioisotopes were discovered in tropical plants far from industrial areas. It was observed that few places near spring water sources might be considered as areas with higher levels of activity.

MEASUREMENT TECHNIQUES Low level radioactivity was measured with a high-energy resolution, 0.8 KeV for ~7Co and 1.8 for 6°Co, gamma spectrometer utilising a HpGe detector (mod GC-1019 Canberra, USA) with usual electronics and Marinelli pot of one litre. The background radiation was reduced by a set of absorbers 1350-4487/99/$ - see front matter © 1999 ElsevierScience Ltd. All rights reserved. PII: S1350-4487(99)00116-X

284

L. Sajd-Bohus et al. / Radiation Measurements 31 (1999) 283-286

(5 cm lead, 2 mm copper, 1 nun acetate), (Saj6-Bohus et aL, 1994). Data interpretation was made using the system software, Spectra-4 (Canberra Inc, Meriden USA), or the one developed recently under a research contract signed with the International Atomic Energy Agency. The spectrometer was regularly calibrated in energy and counting efficiency; Benavides (1998) and Barros (1998) assessed the dynamic behaviour. On the field, a portable four-channel (238U, 232Th, 4°1( and total count) spectrometer with an internal calibration unit (Scintrex GI5, Canada) was employed. This light-weight system has a built in 2"x 2" NaI(TI) crystal, with a pre-set time for any of the four energy channels or gives the total counts. In air, the gamma dose was measured using CaSO4: Dy - TLD in a depleted potassimn glass bulb developed at the AEKI (KFKI), Hungary. The alpha emitters in water were determined using chemical separation and alpha spectrometry or total counting on paper filters as described by Sajo-Bohus et al., (19973). Radon concentrations were determined in water and in underground at the depth (70cm) using the portable Pylon AB5 (Canada) calibrated at the National Air and Radiation Environmental Laboratory, (NAREL-EPA) Montgomery AI. USA and the accuracy is 3% at 800 kBq m -3 and Markus-10 (Gammadata, Sweden) instrument with accuracy of 20% at 10 kBq m 3. Indoor and outdoor radon concentrations in air were determined by LR-115 from Kodak-Pathd, France and CR-39 (TASL, Bristol, U.K.) utilising a diffusion chamber type holder developed by National Radiation Protection Board (NRPB, Didcot, U.K.). The etching conditions were different for the two type of detectors being respectively 2.5 N (60 °C) and 6 N (70 °C), (Sajo Bohus et. al., 1997b). Tracks were counted and evaluated by an advanced version of the VIRGINIA image analyser system (Palfalvi et al., 1997). Its performance was determined in an International Image Analyser Intercomparison (Pfilfalvi et al., 1998). RESULTS A N D DISCUSSION

Indoor and outdoor air dose rates were either calculated data from counts per seconds (Scintrex) or measured by TLD-Ca SO3: Dy (Pille) in some of the most densely populated areas of the Northeast region. Results are respectively given in Tables 1 and 2. The data monthly collected since 1993, at 36 meteorological stations of the Venezuelan Air Forces (Saj6-Bohus e t a / . , 1996) indicated values between 100 and 144 nGy h -1.

Table 1. Indoor and outdoor air dose rates (nGy h-l). Indoor

Outdoor

Site

K

U

Th

Total

Site

K

U

Th

Total

Porlamar sea side

13.0

13.0

33.1

59.1

Porlamar

10.1

14.0

36

60.1

Porlamar centre

15.5

16.2

45.7

77.4

Cumana

15.5

11.5

32.4

59.4

Cumana sea side

18.0

15.5

36

69.5

Puerto La Cruz

21.6

16.9

36.0

74.5

Cumana centre

22.0

16.9

41.8

80.7

Puerto Piritu

21.2

19.4

45.4

86.0

Boca de Uchire

22.0

19.4

41.8

83.2

Cupira

22.0

22.7

45.4

90.1

El Guapo

22.0

24.5

36.0

82.5

Guarenas

23.8

19.8

45.7

89.3

We summarise the radon concentrations measured in water of 27 different rivers of the Litoral Central as follows: 19 with a concentration in the range of 0.1 - 1 Bq L "~ and only 8 Central as follows: 19 with a concentration in the range of 0.1 - l Bq L "~ and only 8 with values higher than 1 Bq L -1. The largest concentration of radon in water was found in a spring (576 Bq L ~). The radon level in water requires further studies to establish the seasonal variations. The 137Cs concentration at two different depths, 0-20 cm and 20-40 cm, were also measured obtaining values between 3.5 and 10.1

L. Saj6-Bohus et al. /Radiation Measurements 31 (1999) 283-286

285

Bq kg ~ and 3.2 - 6.9 Bq kg -~, respectively. In Table 3, we present separated sets of data concerning the northern region (Litoral and Central) and the region below Orinoco River (Amazonas State).

Table 2. Outdoor air dose rate (nGy h 1) in the Litoral Central. Site location

Site location

Aricagua

76.2

La Sabana

65.5

Aricagua-Chuspita

65.5

Urama

83.4

Chuspita

74.8

Osma-Los Caracas

48.8

Caruao

58.3

Chichiriviche

138.9

Table 3. Radon concentration in water (Bq L~). Litoral Central

Central Region

Site location

Site location

Amazonas State Site location

Rio Caruao

1.6

Las Tricheras

70

Boca Chico

16.8

Rio Osma

3.8

E1 Castafio

560

Cano Negro

7.7

Aguas Calientes

6.0

Rio Autana

30.3

Chiclfiriviche

18.7

Cuenca Alta

16.1

In Fig. 1 we show the 137Csactivity concentration measured in th~ same Litoral Central region. The lower value is consistently observed for the deepest layer. In the village of Osma (Federal District) a larger survey indicated that for the upper layer of the soil, the values are dispersed and the ~37Cs concentration distribution does not show any particular pattern. Vegetables and other sources of food have shown low levels of radioactivity. However, it is interesting that some plants, nest of leaf cutter ants ,and sediment soil including ashes found in a small Indian hut of the southern region (Guafm, Amazonas State), where industrial activity is almost negligible, the presence of 7Be and 137Cs has a well detectable level ranging respectively from 9.5 + 5.5 -75.4 + 7.3 and 8.3 + 0.8 - 11.3 + 1.1 Bq kg -I . In domestic gas, the radon concentration ranges between 5 - 15 Bq m -3 therefore has no significant contribution to the indoor dose. Venezuela in general term suffers from relatively low level radioactivity. However, the result related to the red brick indicate a specific activity of 1540 + 4 Bq kg -1. For other building materials, as aggregate in concrete, pebble with a commercial size No.2 (around 100 cc in volume) is often used and has a specific activity of 530 Bq m -3. Red slurry intended to be recycled and a by-product from an aluminium factory near the Paraguaza (a tributary stream of the Orinoco river basin), has a specific activity of 2030 + 5% Bq kg "1. Ash collected twice a year from a local oil bunting power station, ranges between 58 + 4% and 293 + 4% Bq kg 1 (Barros 1997). This means that some places due to human activity and planning may have a higher than the expected radiation level. Furthermore, forty-four caves were inspected; some of them were recently discovered. In the caves, the radioactivity of the clay, the organic materials and limestone were 392 + 8%; 178 + 9% and 57 + 9% Bq kg -~, respectively. In conclusion, the results indicate that, in few places, the environmental radioactivity may be considered as unusually high. In most of the inspected places, the soil activity is comparable to the world average.

286

L. Saj6-Bohuset al. /RadiationMeasurements31 (1999)283-286 lO 8 O r-

6

Ut"

4

2 0 1

3 5 Activity, Bq kg 1

7

9

Fig. 1. ~37Csconcentrations in soil at several locations in Venezuela.

A study of radon concentration profiles near Caracas (a high-risk seismic zone), reaching the value of 27.2 +_20% kBq m3, gave tile exact location of the El Avila active fault, hi the north of Caracas, the tectonic feature revealed a relatively higher radon concentration in comparison with sealed faults. Other geological places of interest were the Boyaca tunnel, 13 + 2 kBq m-3 and the Spanish Trail with 9 + 2 kBq m3, suggesting that radon may be a useful tool in active fault studies. A large number of caves (44) where explored using the LR-115, obtaining an average radon concentration of 1.9 + 2 kBq m3. The low indoor radon (36 +_ 5 Bq m3) is close to the world average, so far, only one place has been spotted for its high radiation level: Las Trincheras (51 + 5 kBq m-3). Since there is an interest at national level to set environmental standards, our preliminary results suggest that the intervention level for indoor radon concentration in Venezuela could be around 400 Bq m3; also that the concentration of alpha emitters in water used for massive human consumption could be less than 100 mBq L-~. REFERENCES

Barros H. (1998)A New Calibration Method for Gamma ,Spectrometry. Diploma-Thesis. Universidad Simon Bolivar, Physics Dept., Venezuela. Benavides C. (1998) Dynamic Stability o f a Gamma ,Spectrometer. Diploma-Thesis, Universidad Simon Bolivar, Dept. of Mathematics, Venezuela. Palfalvi J., EOrdOgh I., Sz~isz K. and Saj6-Bohus L. (1997) New Generation Image Analyser for Evaluating SSNTDs. Rad. Meas. 28, 849-852. Pfilfalvi J., Saj6-Bohus L. and EOrd6gh I. (1998) First International Intercomparison of Image Analysers. Rad. Meas., this issue. Saj6-Bohus L., Greaves E.D., Ferrer H., Dehollain P., Guerra M. and Ferrer R.H. (1994) Analisis Quantitativo de Radionucleidos en Algunos de los Productos Alimenticios de la Dieta del Venezolano. Rev. Fac. Agronomia (LUZ) 11,395-409. Saj6-Bohus L., Greaves E.D., Pfilfalvi J., Benitez A., Liendo J., and Faisca C. (1996) Dosim. Ambiental en Venezuela. Protec. Rad. En Am. Latma y Caribe, OIEA, 1, 38-42. Sajo Bohus L. Gomez J. Capote T., Greaves E., Herrera O., Salazar V. (1997a) Gross alpha radioactivity of drinking water in Venezuela. £ Environ. Radioactivity 35, 305-312. Sajo Bohus L., Palfalvi J. and Greaves E.D. (1997b) Response Function Determination for Nuclear Solid State Detector Type CR-39. Acta Cientifica Venezolana 48, 73-76.