MAPPING THE EXPOSURE OF THE BRAZILIAN POPULATION TO NATURAL BACKGROUND RADIATION COSMIC RADIATION

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1 INSTITUTO MILITAR DE ENGENHARIA Praça General Tiburcio, 80, Praia Vermelha - Rio de Janeiro RJ, Brazil. MAPPING THE EXPOSURE OF THE BRAZILIAN POPULATION TO NATURAL BACKGROUND RADIATION COSMIC RADIATION Elaine R. R. Rochedo Krause C. S. Salles Nadya M. C. Prado

2 INTRODUCTION Exposure to natural sources is due to two main sources, cosmic radiation and terrestrial radiation sources. It is a well known statement that exposure from natural sources is the main contribution to overall exposure to ionizing radiation for most of the world s population.

3 OBJECTIVE The objective of this work was to investigate the doses from exposure to cosmic radiation in Brazil. The exposure to cosmic radiation at ground level is responsible to about 16% of total exposure to natural background radiation.

4 METHODOLOGY The study area was selected to include all municipalities with over 100,000 inhabitants. 252 municipalities were then selected, comprising about 52% of Brazil's total population.

5 METHODOLOGY Doses for each location was estimated at the center of the capital of each municipality, considering the latitude and altitude according to the model provided by UNSCEAR [1]

6 METHODOLOGY The effective dose rate for the directly ionizing component of cosmic radiation at altitude z, e(z), was calculated by: e(z) = e (0) [0,21 exp ( z) + 0,79 exp ( z)] where e(0) - is the effective dose rate due to the dir ectly ionizing component at sea level (z = 0) z - is the altitude, in km.

7 METHODOLOGY The effective dose rate for the neutron component of cosmic radiation at altitude z was calculated by: e n (z) = e n (0) [ exp (a.z)] e n (0) = dose rate component due to neutron s at sea level a = 1 for z < 2 km

8 METHODOLOGY Total dose rate was estimated by the sum of the ionizing and neutron components Dose rates at sea level, e(0) and e n (0), as a function of latitude, was taken from UNSCEAR [1].

9 METHODOLOGY Individual doses were estimated considering an indoor occupancy rate of 80% and a shielding factor of 0.8, as proposed by UNSCEAR for the energy spectrum expected at sea level [1]. Population data was taken from IBGE [2] and altitudes were taken from Google Earth [3].

10 METHODOLOGY The map was confectioned in Arcgis, a software to geographic process. The data was extracted by site of IBGE and the datum to work was South American Datum 1969 (SAD 69). The map utilize the scale 1:

11 RESULTS The number of counties in each state varied from one municipality in the states of Acre, Amapá, Roraima and the Federal District, up to 70 counties in the state of São Paulo. Figure 1 summarizes the results obtained for the average per caput dose for the Brazilian states.

12 RESULTS Figure 1. Average per caput doses from cosmic radiation assessed for all Brazilian states

13 RESULTS The values ranged from 0.24 msv/a at Macapa, AP, up to 0.76 msv/a at Pocos de Caldas, MG. This range is a little smaller than that cited by UNSCEAR, because the largest altitude for the locations analyzed was 1235 m, for Poços de Caldas.

14 RESULTS The estimated average per caput dose for Brazil is 0.37 msv / a. This value is very close to the world average estimated by UNSCEAR (0.38 msv /a).

15 COSMIC RADIATION EXPOSURES

16 CONCLUSIONS Average exposure to cosmic radiation in Brazil was estimated to be 0.37 msv/a, ranging from 0.24 up to 0.76 msv/a. The population considered was 98,636,881 people, roughly 52% of the total population of the country. Considering the average per caput dose, it was estimated a total collective dose around 71,000 mansv per year in Brazil due to exposure to cosmic radiation.

17 CONCLUSIONS The results are be viewed as average values, since the model used has been developed for this purpose, and does not consider the fluctuations associated with usual variability on cosmic rays in the atmosphere and on the effect of solar cycles.

18 REFERENCES 1. United Nations Scientific Committee on the Effects of Atomic Radiation. UNSCEAR Report to the Ge neral Assembly, with scientific annexes. Vol.I, Annex B, (2010). 2. IBGE - Instituto Brasileiro de Geografia e Estatística. Censo /populacao/censo2010/default.shtm (2012) 3. Google Earth. (2012).