Congresso del Dipartimento di Fisica Highlights in Physics 2005

1. Congresso del Dipartimento di Fisica • Highlights in Physics 2005 • 11–14 October 2005, Dipartimento di Fisica, Università di Milano • 222Rn in the house in soil and in atmosphere • M. Carnevale*, G. Polla*, L. Sesana*, U. Facchini* and L. De Capitani† • * Istituto di Fisica Generale Applicata, dipartimento di Fisica, Università di Milano • † Dipartimento di Scienze della Terra “A. Desio”, Università di Milano Radon measurements in Milan and in Ispra Natural radionuclides in the rocks of Valle del Cervo Pluton Atmospheric level of 222Rn, a natural radioactive tracer, can be used to characterize the conditions of turbulence and stability of lower layers of the atmosphere to investigate the structure of PBL and, in some cases, to observe the movements of air masses. In particular, measurements have been carried out in Milan and Ispra on the shores of Lago Maggiore over an entire year. Radon concentrations were measured by means of continously Scintillator detectors which detect alpha particles emitted by radon and its products decay. The radionuclides from 238U and 232Th series represent, with 40K, the main source of gamma radiation in the continental crust. The specific levels are related to the rock types. Highest radiation levels are mainly associated with felsic rocks, due to the geochemical behavior of U, Th and K. These elements are more abundant in the continental crust than in the earth mantle and prefer the liquid phase during a process of crustal anatexis (partial melting) and subsequent fractional crystallization. Valle del Cervo Pluton is a composite body consisting of Monzonitic, Syenitic and Granitic Complexesformed by strongly K-enriched rocks. The areas, where these rocks crop out, are characterized by high natural radiation background. Gamma spectroscopy at high resolution, with HPGe, is the most used technique for quantitative analysis of gamma emitters radionuclides.The following hypothesis has been made: 1- Isotopic ratio between 235U and 238U is 7,2 ‰. 2- 238U and 226Ra are in secular equilibrium.3- The exhalation rate of radon gas has been determined to be less than 1% of the total radon contained inside sample’s volume. This rate is not relevant for examination of radon’s daughters. UP: investigated rock samples RIGHT: geological sketch map of V.d.C. Down: Map in which location of V.d.C. is shown For all the analysed rocks the U concentrations are lower than the Th ones. The rocks of the Granitic Complex show the lowest contents in elemental Th, whereas the elemental U values are similar to those of the Syenitic and Monzonitic Complexes. A frequently encountered daily pattern is characterized by accumulation of radon in night until the early hours of the morning, followed by a decrease. Nocturnal accumulation of radon occurs under stable atmospheric conditions as the result of the formation of the ground-based temperature inversion; radon mixing takes place in increasingly higher layers in the daily hours. Figures on the right side show the trends in radon concentration measured at EMEP station in Ispra for the same period shown for Milan. It can be seen that nocturnal radon accumulation is far more frequent in Ispra, indicating greater atmospheric stability. Nocturnal accumulation of radon indicates the formation of the Nocturnal Stable Layer, where mixing is limitated but not completely inhibited. The Equivalent Mixing Height (EMH) can be obtained by analysing radon concentration. The specific activity related to syenite PM 26 is particularly interesting: 764 Bq/kg for 238U and 478 Bq/kg for 232Th. This rock is the most active. Moreover, the syenite LB 99, collected at the La Balma quarry, shows specific activities of 468 Bq/kg for 238U and 381 Bq/kg for 232Th, respectively. These values are relevant compared with those of other granitoids used as building stones. Radon-Prone areas in the Lombard plain Study of 222Rn levels in the Angera area (Lake Maggiore) Radon measurements were performed in a sample of 411 one-family houses in seven villages located in the southern area of Bergamo and Brescia. This area lies in the Po plain, and is delimitated by the Adda river to the West and the Chiese river to the East and extends southwards from the Bergamo and Brescia areas to the Po river. This area is flat. The area near to the town of Angera, on the eastern coast of the Lake Maggiore (Lombardy), is characterized by hills ranging from 200 to 350 m.s.l.. The main geological feature of the area is the presence of glacial deposits (moraine) referred to the Wurm glacial episode and of post-glacial flood deposits; these form a thick cover of sands and pebbles that make difficult the geological interpretation. The outcrops of pre-quaternary rocks are scarce; some isolated hills of Triassic dolomitic limestone come out from the moraines and a late Permian volcanic felsic rock (granofiro) rises out by fault near the town of Angera. The location of the selected dwellings can be divided in two areas: - In south area, situated in the alluvial plain degrading to Po river, silt and clay are predominant. - In north area sands and gravels are predominant near the hilly terrain of Bergamo and Brescia provinces. Geological maps of the Lombardy administrative region show that this area is characterized by quaternary deposits mainly made up of loose material with granulometry varying from gravel to sand and silt. Measurements of the average radon concentration has been performed in Angera’s area, by means of a suitable track detectors. With a ZnS scintillator detectors, variation has been monitorated continuosly over a long period. Variation in relation to meteorological parameters has been analysed. In particular an one-family house has been investigated because of the high-level of radon concentration (103 Bq/m3 ), connected to the presence of an hole sited in the cellar. The following figures show the statistical distribution of the radon concentration in the investigated areas. Results are relative to summer and winter semesters for houses without cellars. The results of measurements performed on the ground floor confirmed the correspondence between degree of permeability of the ground and indoor radon concentration. The following patterns show the variation of radon concentration over a period of 8 days. Of particular interest is that the maximum radon levels have been observed during the nocturnal hours, on the other hand minimum values have been observed during morning hours. This alternation is due to convective movements of air connected to the difference of indoor-outdoor temperatures.    