Highligh in Physics 2005

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• Congresso del Dipartimento di Fisica
• Highlights in Physics 2005
• 1114 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 Complexes
formed 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 samples volume. This rate is not relevant
for examination of radons 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 Angeras 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.
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