1999 - Determination of 210Pb concentration in the air at ground

Applied
Radiation and
Isotopes
PERGAMON
Applied Radiation and Isotopes 51 (1999) 239±245
Determination of 210 Pb concentration in the air at groundlevel by gamma-ray spectrometry
F. Cannizzaro, G. Greco *, M. Raneli, M.C. Spitale, E. Tomarchio
Dipartimento di Ingegneria Nucleare, UniversitaÂ di Palermo, Viale delle Scienze, 90128 Palermo, Italy
Received 5 August 1998
Abstract
210
Pb activity concentrations in the air of Palermo were determined by g-ray spectrometric analysis of 323
particulate samples collected in the period September 1995±December 1996. For each sample, the air ®ltered
through a cellulose ®lter paper was 8600 m3 on average. The values of the daily activity concentration of 210 Pb were
ranging from 136 to 3390 mBq/m3. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
Lead-210 is formed in the atmosphere by the normal
process of decay of 222 Rn, which diuses as a gas from
the soil. Moreover, all the 222 Rn descendants are solids
and are rather rapidly adsorbed on atmospheric particulates; their removal process from the atmosphere
occurs by precipitation scavenging and dry deposition.
Due to its half-life of 22.3 yr, measurements of lowlevel 210 Pb have been utilized in many ®elds as dating
Ð in the time scale of the past 100±150 yr Ð of sediment in seas or lakes and ice as well as study of the
behaviour of aerosols in the air. In most cases, quantitative determinations of 210 Pb are performed by
measuring b emission of the daughter 210 Bi (T1/2=5.01
d) or by measuring a emission of its grand-daughter
210
Po (T1/2=138.4 d). These methods are rather sensitive, but they require laborious radiochemical separation procedures and/or long waiting times for
radioactive equilibrium reasons (Nevissi, 1991; Sheets
and Thomson, 1992; Bou-Rabee et al., 1995; Fisenne
et al., 1996). A rapid and nondestructive analysis of
210
Pb is also possible by direct measurement of 46.5
keV g-rays. It does not require preliminary chemical
separations in preparing the samples but is limited by
* Corresponding author. Fax: +39-091-232-215; e-mail:
[email protected]
the low emission probability of the g-line and the often
dicult corrections for self-absorption in the sample
matrix (Cutshall et al., 1983; Ishikawa et al., 1994).
In this paper, we report the results of a study aimed
at determining air-borne concentrations of 210 Pb at
ground-level in the period September 1995±December
1996 by measuring its 46.5 keV g-line. At this end, we
submitted again to g-ray spectrometry 323 atmospheric
particulate samples which had been collected in such a
period in the context of one of our research projects
addressed usually to determine the 137 Cs and 7 Be concentrations in the air of Palermo (Agelao et al., 1984;
Cannizzaro et al., 1989, 1994, 1995). Two g-ray spectrometers with HPGe detectors were nearly always utilized contemporaneously in order to complete the
analysis in a shorter time. Lead-210 was identi®ed and
quanti®ed in all the particulate samples examined.
2. Sampling and spectrometric measurements
Particulate collection was performed by aspiration
of atmospheric air through 45 45 cm So®ltra±
Poelman HYN-75 (blu type) cellulose ®lter paper using
a high-volume air sampler (015,000 m3 dÿ1) located on
the roof of our department 20 m above ground-level
(Agelao et al., 1979). The sampling time for all the
particulate samples considered for this work was 14 h
from 6 p.m. to 8 a.m. the next day (local time); the ®l-
0969-8043/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 9 - 8 0 4 3 ( 9 8 ) 0 0 1 7 7 - 8
240
F. Cannizzaro et al. / Applied Radiation and Isotopes 51 (1999) 239±245
tered air was 8600 m3 on average. After particulate
sampling, the ®lters were sprayed with a suitable ®xer,
cut into strips, folded and pressed into 6 6 0.7 cm
packets by a 15-ton press. These samples are referred
to as packet-samples.
In order to detect the 46.5 keV g-line of 210 Pb, the
packet-samples were analyzed by a planar HPGe detector and a coaxial HPGe detector. The planar HPGe is
a LEPS (low energy photon spectrometer) ORTEC
with an 16 mm active diameter and 7 mm depth,
energy resolution (FWHM) of 240 eV at 5.9 keV and
530 eV at 122 keV. The detector is located in a shielding consisting of lead and iron lined with 2 cm of
OFHC (oxygen-free, high-conductivity) copper plus 1
cm polyethylene; the measurement cavity is 1.6 L. The
coaxial HPGe detector is a GEM-50195 ORTEC
selected in the LLB series (very-low background detector). The crystal, 60% relative eciency and 1.75 keV
FWHM at 1.33 MeV, has 700 mm inactive germanium
and at a distance of 4 mm from the magnesium end
cup. The detector is shielded by 12 cm thick lead lined
with 3 cm OFHC copper and surrounded by polyethylene bricks 9 cm in thickness and plexiglas panels containing boric acid. The measurement cavity (018 L) is
¯ushed continuously with about 1.5 L/min gaseous
nitrogen from the dewar to prevent radon infusion.
Furthermore, the detector with its shielding is located
in a room with walls and ceiling made of 75 cm thick
concrete (Cannizzaro et al., 1997). Both detectors are
coupled to SILENA multichannel analyzers, each
equipped with spectrum stabilizer mod. 8915/P. The
two spectrometric systems will be referred to as LEPS
and LBS (low background system).
The eciencies of the two systems for 210 Pb were
determined as described below.
An appropriate analysis of spectra detected by a
coaxial HPGe on a packet-sample pertinent a particulate collection following the Chernobyl accident had
enabled us to identify and quantify with good precision several radionuclides, including 103 Ru, 134 Cs,
136
Cs, 137 Cs emitting also low-energy g-rays as well as
X-rays (Cannizzaro et al., 1990). Since several spectra
had been detected on this packet-sample also by the
LEPS in contact geometry, we had recourse to such a
spectrum in order to deduce calibration points for
evaluating the full-energy peak eciency at 46.5 keV.
From the selected spectrum (Fig. 1a) we determined
calibration points at the 36.4 and 37.3 keV energies of
Ba X-rays, originated essentially from the 134 Cs (T1/2
=2.06 yr) and 137 Cs (T1/2=30 yr) decay, as well as at
the 53.3 keV energy of the 103 Ru (T1/2=39.25 d) grays. Summing-coincidence corrections for photopeak
areas were considered to be negligible. Emission probabilities adopted for X-rays were 0.139% and 0.031%
Fig. 1. 33±60 keV region of g-ray spectra detected on Chernobyl packet-sample by the LEPS. (a) Spectrum taken in July 1986 (two
months after particulate sampling). (b) Spectrum taken in December 1996.
F. Cannizzaro et al. / Applied Radiation and Isotopes 51 (1999) 239±245
for 36.4 and 37.3 keV, respectively, from 134 Cs (Chand
et al., 1988) and 1.116% and 0.273% for the same
energies from 137 Cs (Metha et al., 1987); emission
intensity of 0.384% was adopted for the 53.3 keV grays from 103 Ru (SchoÈtzig, 1994). Being the intrinsic
eciency of HPGe-LEPS nearly constant in the 35±55
keV energy range, a linear ®t was adopted to obtain
the detector eciency at 46.5 keV so the value of
0.88% was deduced. In this way self-absorption corrections were not necessary. By assuming the value of
4.26% for the 46.5 keV g-line emission probability of
210
Pb (Hino and Kawada, 1990), the absolute eciency
of the LEPS for 210 Pb resulted in 3.74 10ÿ4 cps/Bq.
Such a value was adopted in the context of this work
as the same measurement geometry was used.
Fig. 1 shows the 33±60 keV region of the spectra
taken on the Chernobyl packet-sample by the LEPS in
the same measurement conditions in July 1986 and
December 1996. Taking into account the decays, the
full agreement of the photopeak areas at 36.4 and 37.3
keV in both spectra gave further evidence of unaltered
system characteristics and measurement geometry as
previous periodic checks had proved too.
The absolute eciency of the LBS for 210 Pb was
determined using as standard a packet-sample in which
the 210 Pb activity had been measured by the LEPS
241
with one-sigma counting uncertainty of 1.7%. The eciency for adopted measurement geometry resulted in
2.59 10ÿ4 cps/Bq.
Fig. 2 shows the 0±200 keV region of the spectra
detected on the standard packet-sample by the two systems as well as the background spectra detected on a
blank packet-sample. The spectrum detected by the
LEPS presents also 3 photopeaks related to internal
conversion of the 210 Pb g-line; the background spectra
do not highlight any photopeak.
For both systems, the detection level LD expressed
in terms of counts was computed by the expression
proposed by Strom and Stansbury (1992) to extend
that of Currie (1968) also to cases in which the background counting is carried out for a time dierent
from that used for the sample counting. Using values
taken from the continuum of the background spectra
of Fig. 2, the LD(LEPS) values resulted equal to 26
and 79 counts for packet-sample counting times of
8 104 and 5 105 s, respectively; likewise, the
LD(LBS) values resulted equal to 66 and 210 counts.
Dierent counting times were used in order to
obtain net countings largely higher than detection
levels. Counting times of 8 104 s were usually
adopted with the LEPS; generally longer times were
used with the LBS. The photopeak area of 210 Pb was
Fig. 2. 0±200 keV region of g-ray spectra detected on the standard packet-sample and on the blank by both the LEPS and the
LBS. Counting time: 5 105 s.
Fig. 3. Daily air concentration of
210
Pb in the period September 1995±December 1996.
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F. Cannizzaro et al. / Applied Radiation and Isotopes 51 (1999) 239±245
F. Cannizzaro et al. / Applied Radiation and Isotopes 51 (1999) 239±245
243
determined by summing-up all counts in the
21.1 FWHM (46.5 keV) interval and subtracting the
background contribution estimated on the basis of
countings in the contiguous intervals.
3. Results and discussion
Gamma-ray spectrometric analysis performed on the
323 atmospheric particulate samples collected in the
period September 1995±December 1996 highlighted the
presence of 210 Pb in all samples. The activity values,
corrected for decay between sampling and analysis,
were determined with one-sigma uncertainties ranging
from 1.7 to 15% for 244 samples and to 20% at most
for the rest. Daily air activity concentration of 210 Pb
versus sampling-end day is shown in Fig. 3. Data are
missing on days the particulate sampling was not carried out (holidays, the month of August, technical
reasons). The daily concentrations present a behaviour
which appears to have a oscillatory characteristic with
a marked enhancement all days of the month of
October 1995. The values vary from 136 to 3390 mBq/
m3, with an arithmetic and geometric mean of 980 and
791 mBq/m3, respectively.
Fig. 4 shows the distribution histogram of daily concentrations together with the best ®t curve obtained by
the log±normal function
Fig. 5. Monthly daily averages of the air concentration of
each month.
210
Fig. 4. Distribution of daily air concentrations of 210 Pb. Fit
curve obtained by a log±normal function (correlation coecient = 0.993).
ln x ÿ b2
y a exp ÿ
:
c
Fig. 5 reports the monthly daily averages of 210 Pb
concentrations as well as the monthly total rain; also
the number of rainfall days for each month is given.
Pb, and total monthly rain. The numbers give the rainfall days for
244
F. Cannizzaro et al. / Applied Radiation and Isotopes 51 (1999) 239±245
The singular 210 Pb concentration value of 2134 mBq/m3
obtained in October 1995 is about 4 higher than the
value of February 1996 (minimum value). An inverse
correlation seems to be between monthly averages of
210
Pb concentration and rain if the number of rainfall
days are considered besides the total rain.
We examined also the possible in¯uence of other
meteorological conditions on the concentration behaviour of 210 Pb. Only a relationship with wind directions appeared of interest. With a windrose including
eight directions, the analysis of the continuous recording of the wind direction enabled us to determine the
direction predominant during the 14 h of particulate
sampling. The average concentrations of 210 Pb corresponding to the eight wind directions are given in
Fig. 6. Two thirds of the samplings were taken with
winds west-southwest. Because of the geographical location of Palermo city, the lowest average values of
concentrations correspond to the four wind directions
bringing maritime air masses.
4. Conclusions
Daily activity concentration of 210 Pb in the air of
Palermo from September 1995 to December 1996 was
lower than 1500 mBq/m3 for 83% of the determinations with an average value of 737 mBq/m3 and
more than 2500 mBq/m3 for 4% of the determinations.
Gamma-ray spectrometric analysis of the packetsamples for detecting 210 Pb was carried out when 7 Be
Fig. 6. Average air concentration of 210 Pb versus wind direction. The numbers give the frequency of the samples.
(T1/2=53.3 d, emitting 477.6 keV g-rays in 10.39% of
its disintegrations) associated with atmospheric particulate was almost totally decayed. However, measurements of packet-samples performed about 10 days
after the end of the particulate sampling have highlighted a background increase in the 30±100 keV
energy range lower than 2 and 60% for the LEPS and
the LBS, respectively, in the presence of 7 Be activity
even of several tens Bq. The negligible or modest consequent increase of LD values has practically no in¯uence on quantifying 210 Pb activity also in packetsamples analyzed a few days after particulate collection.
The results obtained lead us to extend the investigation into the air concentration behaviour of 210 Pb in
the months following the period considered in this
work as well as in previous years as particulate
samples are collected since 1982. At this end, the setting up of another spectrometric system with a planar
HPGe is in progress.
Acknowledgements
This work was supported by Ministero UniversitaÁ
Ricerca Scienti®ca e Tecnologica. Meteorological data
were supplied by the Osservatorio Astronomico
dell'UniversitaÁ di Palermo, situated near our department.
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