CIENCIAS NUCLEARES
Characterization of the InSTEC’s low-background gamma spectrometer for environmental radioactivity studies
Caracterización del espectrómetro gamma de bajo fondo del INSTEC para estudios de radioactividad ambiental
Oscar Díaz
Rizo 1 , Neivy López Pino 1 , Katia D´Alessandro Rodríguez
1 , Henry Reyes 1 , Cármen Ródenas Palomino 2 , Fátima
Padilla Cabal 1 , Juana Orquídea Arado López, Amalia Ofelia Casanova
Díaz 1 , Alina Gelen Rudnikas 1 , José Gómez Arozamena
2 .
1 Instituto Superior
de Tecnologías y Ciencias Aplicadas (InSTEC)
Ave. Salvador Allende y Luaces, POB 6163, Habana 10600, Cuba
2 Universidad de Cantabria (UC), Ave. Los Castros s/n, Santander, España.
odrizo@instec.cu
ABSTRACT
The capabilities
of the LowBackground Gamma Spectrometer (LBGS) at InSTEC were studied for environmental
purposes. Fifty three glines were identified in the LBGS background spectrum.
The Minimum Detectable Activity for ,
,
,
,
and
were calculated using the detector’s volumetric efficiency simulated by
Monte Carlo method. Validation was performed by absolute and relative analysis
of radionuclide activities present in a marine sediment certified material.
RESUMEN
Se determinan las
potencialidades del Espectrómetro Gamma de Bajo Fondo del InSTEC con
fines ambientales. Se identificaron 53 líneas gamma en el espectro de
fondo natural del espectrómetro. Se calculan las actividades mínimas
detectables para los radionucleidos ,
,
,
,
and
empleando la eficiencia volumétrica del detector simulada por Monte Carlo.
Como validación se determinan, por vía absoluta y relativa, las
actividades de los radionucleidos presentes en un estándar de sedimento
marino.
Key words: background radiation, gamma spectroscopy, Monte Carlo method, gamma spectrometers
Introduction
A large number
of users of gamma spectrometry are concerned with measuring low levels of radioactivity
such as those present in environmental samples (soils, sediments, water, food,
etc.). There is a constant pressure to resolve lower and lower concentration
levels. For this reason, the experimental set-up to be used for environmental
radioactivity studies needs as low as possible the Minimum Detectable Activity
(
). The
is defined as the minimum amount of radioactive nuclide which can be determined
[1]. It may vary with the amount, type and geometry of the sample and nuclide
identity, as well regarding the detector (type, crystal dimension, energy resolution,
etc.), detector environment (background) and counting time, i.e. the
have a strong dependence with the detector efficiency and the radioactive background.
Taking these facts into account, to characterize a gamma spectrometer destined
to environmental studies, a detector efficiency well known and a radioactive
background minimization are indispensable.
The experimental
determination of the detector efficiency for volumetric samples (typical in
environmental studies) is not a simple problem. A set of volumetric certified
gamma sources (with density, composition and gamma energy range similar to the
samples of interest) is needed, but they are either too expensive or not for
sale. An alternative way is starting from the detector efficiency using a set
of point standard sources (for example, ,
,
,
and
),
compute the point (
(
))
and volumetric (
(
))
energy-dependent solid angles by Monte Carlo simulations [2] or by a Semiempirical
Method [3], and calculate the volumetric efficiency as follow:
On the other hand,
unwanted background radiation is originated from radionuclides in the detector
assembly and those in surround materials, radionuclides in air and cosmic ray
interactions with both the detector itself and with surrounding materials. Background
reduction methods have been categorized as «active» and «passive».
«Active» procedures block potential background counts in the spectrum
dynamically on a pulse-by-pulse basis [1]; «passive» methods use
absorption in graded shields, by selecting low activity materials with different
atomic number (Z) in order to absorb the high and low energy gamma radiation,
X-rays from the shield materials and cosmic rays reaction products.
The aim of the
present study is to evaluate the capabilities of the recently installed Low-Background
Gamma Spectrometer (LBGS) at InSTEC for environmental radioactivity studies
by the measure of the spectrometer background, the
and the detector efficiency; also the activity of the nuclides present in marine
sediment standard is determined.
Material
and Methods
Low-Background
Gamma Spectrometer: The LBGS of the Nuclear Analytical Laboratory at InSTEC
(figure 1) is composed by a Low-Background Chamber (LBC), using an n-type closed-end
coaxial high-purity germanium detector (DSG, NGC-3018, 130 ,
FHWM = 2.04 keV for 1332 keV
gamma line) equipped with an 8192 channel multichannel analyser (webMASTER TARGET
coupled to PC). The gamma spectra are processed using the Gamma-W version 18.
03 code (Dr. Westmeier Gesellschaft für Kernspektrometrie mbH). The LBC
is a detector shielded with 55 mm thickness of lead, 35 mm thickness of a steel
frame, and an internal graded shielding consisting of 12 mm thickness of cadmium,
17 mm of copper and 9 mm of aluminium. The inner dimensions of the LBC are 800
mm depth X´718 mm width X´736 mm height.
Sample preparation: In the present study, we have used the certified reference material (CRM) IAEA-375 [4] and the marine sediment standards UC-1 and UC-2, prepared in the University of Cantabria (Spain) [5]. Their radionuclide activities and main characteristics of the corresponding analytical lines are presented in table 1. Sample preparation was carried out by experimentally standardized the samples at 50 grams (dry weight), and stored on a hermetic closed plastic carried out 28 days after sample preparation (to guarantee the secular equilibrium of the U and Th daughters) during 24 hours.
Minimum Detectable
Activity ():
The LBGS background spectrum was measured during 72 hours for
determination. For each radionuclide, the
was calculated as:
where
is the spectrometer detection limit defined by Currie [6] equal to 3
,
where s is the standard deviation (in counts) of the area of the background
windows (peak window at 1.17 times the FWHM), t the counting time,
the absolute emission probability corresponding to the i-th gamma-ray for the
nuclide j and
(
)
is the volumetric detection efficiency of the full-energy absorption peak considered.
is the energy corresponding to the i-th gamma-ray for the nuclide j.
The volumetric
efficiency was determined by semiempirical method using Monte Carlo (MC) simulation
and point standard sources (,
,
,
and
),
produced by AEA Technology QSA GmbH. Simulations were carried out to compute
the point (
(
)) and volumetric (
(
))
energy dependent solid angles of Eq. (1). The MCNPX 2.5 was used as simulation
code [8]. It has been used widely to estimate the efficiency curve of HPGe detectors
for volumetric samples [9-12]. The sample-detector configuration was exactly
reproduced in the MC simulation, taking as data the dimensions, density and
chemical major elemental composition of CRM and standards, as well as the detector
specifications (Ge crystal dimensions, dead layer and Al thickness, distance
from Ge crystal to Al end cap) provided by detector supplier.
Activity Determination: For validation, the radionuclide activities present in UC-1 standard were determined by absolute method as:
where is
the i-th photopeak area for the nuclide j; and by relative method calculated
as:
using the CRM IAEA-375 and UC-2 standard for calibration. The accuracy was evaluated using the SR criterion, proposed by McFarrell et al. [13]:
where –
experimental activity,
–
certified value and
is the standard deviation of
. On the basis of this criterion the similarity between the certified value
and the analytical data obtained by proposed methods falls into three categories:
SR
25% = excellent; 25 < SR
50%= acceptable, SR > 50% = unacceptable.
Results
and Discussion
Table 2 shows 52
gamma lines determined with an area error lesser than 30% in the 72 hours LBGS
background spectrum. All of them were very well identified. The origin of the
major percent of the observed -lines
is the presence of different nuclides from U and Th decay families. A larger
contribution is caused by bremsstrahlung from the b decay of the daughter
(
=
5.013 d,
=1161
keV), which is present in secular equilibrium. Independently of the copper and
aluminium shield present in the LBC, the
and
Pb X-ray lines are observed. The biggest count rate (0,013 cps) was determined
for the
non-analytic 92.38 keV
-line.
During LBC construction,
a difficulty with normal commercial Pb is that it can probably contain some
,
which originates in trace
in lead ore, and it is important to mention that this lead isotope cannot be
removed chemically in lead purification. For this reason, the use of low-level
lead is more recommended. Verplancke [14] takes as «low-level lead»
that lead with less than 10 Bq of
per Pb kg. Considering the count rate observed for 46.54 keV
-line
(0.006 cps) in the spectrometer background, the presence of low-level lead in
the LBGS is confirmed. On the other hand, the presence of the non-natural radionuclide
was unexpected. This isotope usually is associated with the nuclear explosions
occurred after 1945. Then, its presence must be associated to the use of non-sufficient
old steel in the LBC construction.
Figure 2 shows the efficiency curves for point sources located at 30 cm of height (experimental), and for the 50 grams sediment sample at 0 cm. The last curve was obtained computing the energy-dependent solid angles by MC simulations and taking the 30 cm point efficiency as reference (Eq. 1). The low energy gamma rays self-absorption inside the volumetric sample is clearly observed. These curves were well fitted by the following polynomials:
The
calculated by Eq.(2) for each radionuclide of interest after the 72 hours of
background spectrum measurement are shown in table 3. The comparison with other
similar gamma spectrometers (UC–University of Cantabria, Spain; CEAC–Center
for Environmental Studies of Cienfuegos, Cuba) used for environmental studies
shows that the InSTEC LBGS has the lowest detectable activities.
The activities of the UC-1 standard radionuclides determined in the InSTEC LBGS are shown in table 4.
The bigger dispersion
(between 10-15%) and deviation from the reference value (between 4.4 –
10.7%) are observed for absolute analysis. This is in correspondence with the
difficulties in detector volumetric efficiency calculation, especially for low
energies (the bigger dispersions are obtained for analytical lines with energy
less than 100 keV). To solve this problem, a finest data (for example, about
sample composition) and/or large calculations during Monte Carlo simulation
are necessary. Independently of the above criteria, the SR accuracy test shows
values between 25 and 50% for all determined activities, i.e., using the absolute
method we obtained «acceptable» results.
On the other hand,
the application of the relative method shows «excellent» results
(SR < 25% for all determined activities). For the studied radionuclides,
the obtained deviation from the reference values is always less than 5%. It
is an excellent precision for natural radioactivity measurements.
According to the
obtained results, for environmental radioactive studies using InSTEC Low Background
Gamma Spectrometer, relative measurements are more recommended. Unfortunately,
this method requires certified standards with a matrix similar to the studied
sample matrix. For environmental studies many different certified standards
(soils, sediments, water, food, etc.) are needed. Therefore, the alternative
way is the use of absolute measurements expecting, until now,
acceptable activity accuracy.
Conclusions
The Low-Background
Gamma Spectrometer at InSTEC is an accurate facility for environmental radioactivity
studies. The combination of low-level lead and Cd-Cu-Al internal graded shields
allow obtaining a very low minimum detectable activity for those radionuclides
of ecological interest. Relative and absolute activity measurements can be carried
out with excellent and acceptable accuracies, respectively.
References
[1] GILMORE G,
HEMINGWAY J. Practical Gamma-ray Spectrometry. John Wiley & Sons, 1995.
[2] VIDMAR T, AUBINEAU-LANIECE I, ANAGNOSTAKIS, et. al. An intercomparison of
Monte Carlo codes used in gamma-ray spectrometry. Applied Radiat. Isot. 2008;
66(6-7): 764-768.
[3] LEPY MC, ALTZITZOGLOU T, ARNOLD D, et. al. Intercomparison of efficiency
transfer software for gamma-ray spectrometry. Applied Radiat. Isot. 2001; 55(4):
493–503.
[4] IAEA. Intercomparison run IAEA-375: Determination of radionuclides in soil.
Report IAEA/AL/075. Vienna: IAEA, 1994.
[5] GÓMEZ J, SOTO J. Ejercicio de intercomparación de resultados
de medida de radiactividad en la Red de Vigilancia Radiológica Ambiental.
Madrid: Consejo de Seguridad Nuclear, 1998. (in Spanish).
[6] CURRIE LA. Limits for quantitative detection and quantitative determination.
Anal. Chem. 1968; 40(3): 586.
[7] FIRESTONE RB. Table of Isotopes. Wiley Interscience, 1996. Eighth edition.
[8] PELOWITZ DD. MCNPX TM User’s manual. LACP-05-0369. Version 2.5.0. Los
Alamos National Laboratory Report, 2005.
[9] RÓDENAS J, MARTINAVARRO A, RIUS V. Validation of the MCNP code for
the simulation of Ge-detector calibration. Nucl. Instr. Methods Phys. Res. A.
2000; 450(1): 88-97.
[10]KARAMANIS D. Efficiency simulation of HPGe and Si(Li) detectors in g- and
X-ray spectroscopy. Nucl. Inst. and Meth. in Phys. Res. A. 2003; 505(1-2): 282-285.
[11]MARQUES SALGADO C, CONTI CC, BECKER PHB. Determination of HPGe detector
response using MCNP5 for 20–150 keV X-rays. Applied Radiat. Isot. 2006;
64(6): 700–705.
[12]RÓDENAS J, GALLARDO S, BALLESTER S, et. al. Application of the Monte
Carlo method to the analysis of measurement geometries for the calibration of
a HP Ge detector in an environmental radioactivity laboratory. Nucl. Instr.
Methods Phys. Res. B. 2007; 263(1): 144-148.
[13]QUEVAUVILLER PH, MARRIER E. Quality Assurance and Quality Control for Environmental
Monitoring. Weinheim: VCH, 1995.
[14] VERPLANCKE V. Low level gamma spectroscopy: low, lower, lowest. Nucl. Instr.
Methods Phys. Res. A. 1992; 312(1-2): 174-182.
[15]GELEN A, SOTO J, DÍAZ RIZO O, SIMON MJ, BELTRÁN J, RAMÍREZ
M, HERRERA E, GÓMEZ J, RÓDENAS C. Radiological evaluation of sediments
from the bay of Havana. Nucleus. 2002; (32): 17-22.
[16]ALONSO C. Environmental Radioactivity Sources, Distribution and Risk in
Cuban Marine Ecosystem, Ph. D. Thesis. La Habana: InSTEC, 2010. (in Spanish)
Recibido: 3 de
agosto de 2009
Aceptado: 12 de noviembre de 2009