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Simultaneous measurement of gamma rays and radon emission (SIMGRAE) for solid samples radioactivity assessment
Radiation Measurements 41 (2006) 695 – 702 www.elsevier.com/locate/radmeas
Simultaneous measurement of gamma rays and radon emission (SIMGRAE) for solid samples radioactivity assessment Igor D’Angelo a , Marco G. Giammarchi b , Lino Miramonti c,∗ , Roberto Scardaoni d a Centre for Blood Research at Life Sciences Centre, University of British Columbia, 2350 Health Sciences Mall, Vancouver, Canada V6T 1Z3 b Istituto Nazionale di Fisica Nucleare-Sezione di Milano, Via Celoria 16, 20133 Milano, Italy c Dipartimento di Fisica dell’Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy d Cosmomatic, Via Ventura 17-20134 Milano, Italy
Received 15 August 2005; received in revised form 30 November 2005; accepted 15 December 2005
Abstract A gamma/radon high-sensitivity radioactivity counter has been built in the frame of a research and development aimed at assessing the effect of construction materials on the annual radioactive dose absorbed by individuals. The counter features simultaneous gamma measurement (by high-resolution germanium spectroscopy) and radon evaluation through electrostatic collection and alpha spectroscopy of radon daughters. © 2006 Elsevier Ltd. All rights reserved.
1. Introduction and motivations The issue of radioactivity in buildings is of particular interest since the use of certain construction materials can result in a significant increase in the absorbed dose for the general population. Two different mechanisms are responsible for the main part of the absorbed dose due to construction materials. First, radioactive content of walls and ceilings generates a variety of gamma rays that can impinge on indoor-living individuals. Secondly, radon noble gas is released from construction materials and can be inhaled by the population generating internally absorbed doses. Gamma rays belong to the most penetrating type of radiation. Long-term exposure to low levels of gamma radiation has been linked to cancer formation (with a multi-fold increased risk of developing leukaemia, thyroid cancer and breast cancer) and decreased effectiveness of antioxidants. Long-term exposure to gamma rays can also induce dominant lethal mutations in a wide variety of cellular systems, recessive autosomal mutations
∗ Corresponding author. Tel.: +39 02 5031 7617.
E-mail address: [email protected] (L. Miramonti). 1350-4487/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.12.006
and sex-linked recessive lethal mutations (IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, 2000). Radon has been recognized by various international health organizations as a major lung carcinogen. A number of biological and epidemiological studies have shown that approximately 12% of annual lung cancers reported worldwide can be linked to radon gas exposure from the environment (EPA Report 402-R-03-003, 2003; Darby et al., 2005). The onset of cellular carcinogenesis involves DNA damage to bronchioepithelial cells by particles emitted by radon progeny. In response, criteria have been defined by the U. N. Scientific Committee on the Effect of Atomic Radiation (UNSCEAR) (http://www.unscear.org) for the acceptability of an effective dose rate due to construction materials. Accordingly, criteria for the radioactivity of a construction material in relation with the acceptable effective dose have been defined by various national agencies (see for instance Steger and Gruen,). For a given sample of a construction material, these criteria are typically expressed in the form of the following inequality: aK40 aRa226 aRa226 G aTh232 + + + 1, 10, 000 1000 1000 600 where the concentration of the various nuclides are expressed in Bq kg−1 . The third factor contains as a product the 226 Ra concentration multiplied by an emission coefficient that takes
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into account radon emanation from the material sample. This is usually the most important term in the equation and is multiplied by a factor (G) that accounts for geometrical conditions and ventilation of living quarters. The numerical coefficients in the equation depend on the energy and characteristics of the emitted radiation and may slightly vary from a given environmental norm to another. This study describes the experimental set-up and performance of a two-detector assembly capable of simultaneously measuring radon and gamma radiation emission from solid construction materials.
2. Detector description 2.1. Detector set-up The structure of our detector set-up consists of a HPGe detector and a “surface barrier” silicon detector assembled in a configuration designed for the simultaneous measurement of gamma-rays and radon emission, respectively. The two detectors are inserted in an “electrostatic collection unit” where the end-cap of the cylindrical HPGe detector is inserted at the bottom of the vessel, while the silicon detector is mounted on top (see Fig. 1). As usual, the endcap of the HPGe detector can be encapsulated in a Marinelli beaker.
Radon emission from the sample generates radioactive daughters nuclei (radon progeny) which are collected in the “electrostatic collection unit”, a cylindrical stainless steel vessel serving as a radon buffer tank and electric field cage. Its dimensions are 230 mm in diameter, 280 mm in height and 5 mm thickness, with a volume of 12 l. An electrostatic field of 2.5 kV between the internal steel walls of the container and the silicon detector drives the (effectively charged) daughter nuclei of 222 Rn near the silicon detector for energy measurement of alpha rays. The silicon detector is electrically isolated from the stainless steel vessel by means of an acrylic plate. In our protocol, it is specifically foreseen that the solid sample be prepared in a Marinelli geometry shape with the actual measurement taking place without the Marinelli beaker itself. The dimensions are: 120 mm height and 130 mm diameter, the bore diameter is 85 mm and the height of annulus is 80 mm. The sample therefore lies “naked” on the top of the HPGe endcap; this allows radon emanation to be effective from most of the sample surface. Samples are prepared starting from raw materials, mixing them in the proper amount according to the specification of the final product and waiting for the complete drying of the mixture before removing them from the Marinelli stamp. Typically, the drying process requires two days. The inside wall of the electrostatic collection vessel is of high-quality electropolished stainless steel, in order to achieve a good-quality surface and a low radioactive background level. Since the “electrostatic collection method” is known to have a significant humidity dependence, the system is provided with a nitrogen supply, so that measurements are made under dry nitrogen at atmospheric pressure. The external detector shielding is made of modular ordinary lead of 100 mm thickness. The upper part of the apparatus is a (compressed air driven) sliding cover for sample insertion. An internal 50 mm thick aluminium completes the detector shielding. The total size of the apparatus (shielding included) is a cube of 800 × 800 × 1200 mm (see Fig. 2). 2.2. Electronics and data acquisition
Fig. 1. Schematic view of the “electrostatic collection unit”. The silicon detector (SSD) and the HPGe detector are also shown.
The signal processing electronics for nuclear applications is nowadays for the greater part tailored to the specific characteristics of the detectors. For our instrument, we have used general-purpose electronics available from specialized companies like Ortec, Silena, Thesys, which regularly take care of developments dedicated to a given family of detectors. The topology of the system is mostly “traditional” with the preamplifiers for the Ge and Si mounted on the detectors while the signal shaping and digitizing processes are performed a few meters from the detectors (see Fig. 3). The HPGe detector must be cooled, because, due to the small band-gap (0.7 eV) at room temperature, it will produce a high thermal leakage current that would strongly degrade the energy resolution. In order to minimize the leakage current and the electronic noise, the detector, the “front-end” of the preamplifier (which include the input field-effect transistor, FET) and the
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697
Fig. 2. Schematic view of the shielding. The global size is 800 × 800 × 1200 mm.
feedback elements of the “charge-sensitive preamplifier” (passive resistor and capacitor) are located in a cryogenic vacuum enclosure (see Fig. 4). Usually, HPGe detectors are cooled with liquid nitrogen (LN2 ) to 85–105 K inside a specifically designed cryostat. For our specific application, in order to avoid inconveniences related to Dewar filling, manual operations, LN2 cost and availability issues, we relied on an alternative X-cooler system (Ortec Industries), which is electrically powered. This system allows the use of standard mixedgas refrigerants and compressors in the cooling process by removing residual oil and other contaminants from the refrigerant. 2.2.1. Basic pulse shaping and processing consideration Usually when the detector and preamplifier noise predominates, the “main amplifier” noise is a relatively unimportant factor, since it represents an inconsequential quantity in the total system performance. All the useful information in the preamplifier output signal is included in the step function rise time and amplitude
of each pulse. The pulse shaping circuit in the subsequent “main amplifier” operating with time constants much shorter than the decay of the preamplifier signal and much longer than its rise time, effectively removes the slow component of the preamplifier signal and produce individual pulses whose amplitudes convey the quantity of interest, usually the energy. The main characteristics of the preamplifier that we are using with the HPGe detector are: pulse “rise time” typically of 25 ns, nominal conversion gain of 175 mV MeV−1 with a maximum pulse output of 10 V. Incorporated into the “preamplifier electronics” is a hybrid monitoring circuit connected to a temperature sensing element; this circuit provides a signal (bias shutdown) for the high voltage bias supply of the detector. This supply is designed to prevent the application of detector bias voltage if the “bias shutdown” signal is not present, and moreover, it also reduces the detector bias voltage to zero if the “bias shutdown” signal indicates a warm condition. Finally, the circuit will also prevent the accidental application of bias to a detector that has not yet reached operating temperature.
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Fig. 3. Electronic interconnections for HPGe and silicon detector.
Fig. 4. Front-end electronics for HPGe detector.
I. D’Angelo et al. / Radiation Measurements 41 (2006) 695 – 702
A hybrid circuit within the preamplifier checks if the event rate exceeds a given level. In fact, when a condition of excessively high rate exists, the detector system may suffer resolution degradation and peak shift, requiring corrective measures. The preamplifiers used for both (HPGe and silicon detectors) can operate also in differential mode, to suppress induced noise when long connecting cables between the “detector/preamplifier” and “main shaping amplifier” are used. 2.2.2. Data acquisition system Since in our system a HPGe detector is used together with a “surface barrier silicon detector”, a practical solution for data acquisition is to employ an Ortec multichannel buffer (mod. 919E) which presents great flexibility. This module can be used with up to four separate detection systems, each with up to 16K channel resolution. A personal computer is interfaced to a task-specific hardware system, and an interface-card gives access to the data memory. 3. Gamma rays detection 3.1. HPGe detector characteristics The gamma rays detector is an intrinsic high-purity 0.150 l germanium crystal manufactured by Ortec. It is a coaxial detector of cylindrical shape with an external diameter of 64.7 mm and a length of 45.0 mm. The crystal is enclosed in a low radioactive background aluminium layer with a thickness of 1 mm; the distance between the end-cap and the crystal is 4 mm. The inactive germanium layer is 700 m. The measured relative efficiency at 1333 keV is 37%, while the peak to Compton ratio, measured at the same energy, is 69:1. The measured resolutions (FWHM) at 122 keV (57 Co) and 1333 keV (60 Co) are 0.78 and 1.71 keV, respectively. Concerning the peak shape, we obtained values of 1.9 for the FWTM/FWHM ratio, and 2.5 for FWFM/FWHM ratio, both at the 1333 keV (60 Co) ray energy. The performance of the detector (warranted and measured) are presented in Table 1. 3.2. Calibration measurements 3.2.1. Calibration counting efficiency source The counting efficiency of the HPGe has been estimated with a commercial calibration source. This source has a Marinelli
699
shape with a total volume of approximately 1000 ml and a density of approximately 1.0 g cm−3 . The beaker is constructed from polypropylene and the radioactive material is homogeneously diluted in a special water-equivalent plastic matrix. The source dimensions are: 120 mm height and 130 mm diameter, the bore diameter is 82 mm and the height of annulus is 76 mm. Table 2 shows the nuclei of the source and the activity at the reference date (i.e. 1 July 2004 at 12 GMT). The relative uncertainty on the activity is 3.0%. Fig. 5 shows the spectrum obtained with the Marinelli source for a 5000 s measurement time. This spectrum source clearly shows a drop in the efficiency at energies below 100 keV. Although the spectrum shows the 210 Pb gamma peak at 46.5 keV and its four X-rays excitation maxima, the efficiency below 100 keV suffers from considerable uncertainty. In addition, this part of the spectrum does not convey important information for our applications. For these reasons, we will always consider gamma energies above an operational threshold of 100 keV. In order to check the reliability of the calibration source we made a Monte Carlo simulation (based on Geant code). The difference between the efficiency values obtained with the simulation and those measured with the calibration source is within 10%. 3.2.2. Correction to the calibration efficiency for different samples In order to determine the calibration efficiency for material having a density and/or element composition different with respect to the one of the calibration source (i.e. water-equivalent plastic matrix) we employ the commercially available software GamatoolTM . This is a software package for calculating correction factors for gamma-rays self-absorption (Debertin and Helmer, 1988; Debertin and Jianping, 1989). Fig. 6 shows the counting efficiency plot obtained with the Marinelli-shaped source and the corrected plot with the GamatoolTM software for a concrete sample of well known composition (i.e. 50% of “cemento pozzolanico” and 50% of SiO2 ; this mixture has a volume of 1.7 kg and a density of about 1.39 g cm−3 . The percentage composition is: 52.7% of CaO, 24.42% of SiO, 7.32% of Al2 O3 , 3.52% of Fe2 O3 , 1.7% of MgO, 3.48% of SO3 , 0.35% of Na2 O, 2.04% of K2 O, 0.35% of TiO2 , 0.01% of Cl2 and 1.2% not determined).
Table 1 Performance specifications (warranted and measured) of the HPGe detector Warranted
Measured
Amplifier time constant (s)
Relative efficiency at 1333 keV Peak to Compton ratio at 1333 keV
30% 60:1
37% 69:1
6 6
Resolution (FWHM) at 1333 keV Resolution (FWHM) at 122 keV
1.85 keV 0.93 keV
1.71 keV 0.78 keV
6 6
Peak shape (FWTM/FWHM) at 1333 keV Peak shape (FWFM/FWHM) at 1333 keV
1.9 —
1.9 2.5
6 6
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Table 2 Characteristics of the source as given by the manufacturer Nuclide
-ray energy (MeV)
Activity (Bq)
Emission rate (s−1 )
Americium-241 Cadmium-109 Cobalt-57 Cerium-139 Mercury-203 Tin-113 Strontium-85 Caesium-137 Yttrium-88 Cobalt-60 Cobalt-60 Yttrium-88
0.060 0.088 0.122 0.166 0.279 0.392 0.514 0.662 0.898 1.173 1.333 1.836
3.48 × 103 1.85 × 104 7.09 × 102 8.14 × 102 1.70 × 103 3.03 × 103 3.16 × 103 3.26 × 103 6.05 × 103 3.65 × 103 3.65 × 103 6.05 × 103
1.25 × 103 6.70 × 102 6.07 × 102 6.50 × 102 1.38 × 103 1.96 × 103 3.11 × 103 2.77 × 103 5.69 × 103 3.64 × 103 3.64 × 103 6.01 × 103
The following upper limits were also quoted: 243Am < 1 Bq; 153 Gd < 1 Bq; 113m Cd < 30 Bq.
1.0E05
10000
Counts
1000
100
10
0.00
898.00
1797.00
2698.00
3600.00
Energy (keV)
Fig. 5. Spectrum obtained with the Marinelli source for a 5000 s measurement time. In abscise the energy in keV and in ordinate the counts in arbitrary scale are given.
3.2.3. Background and sensitivity The total background counting rate, obtained with measurement time span of 4.08 days is 363,372 count per day per kg of germanium in the 40–2700 keV range. Table 3 summarizes the total background counting rate and the background counting rate for the 214 Pb peak (352 keV), the 208 Tl peak (583 keV) and the 40 K peak (1461 keV).
4. Radon detection Radon is at the same time an element of the natural radioactive chains and a noble gas. As such, it is highly soluble in liquids and diffusive in solids. In a solid material, the radon diffusion length has a typical value of D/ where D is the gas diffusivity and is the radon decay constant. However, in situations of environmental interest, radon convection due to
air currents (or water currents in water) plays a dominant role. Convection effect will in general spread radon at a much faster rate than suggested by the D/ ratio alone. In general, 222 Rn of the 238 U family is the most important isotope, having a half-life of 3.82 days. By comparison, 220 Rn of the 232 Th family has a half-life of 56 s only. In a construction material, radon is important because it is continuously emitted and significantly affects the radiation exposure of individuals. For highly sensitive measurement of 222 Rn diffusion and emanation see for instance (Zuzel, 2005). Measurement of radon content in air is usually performed by Lucas cells or by means of SSNTD integrating devices. In our case the use of a SSNTD device would imply a problem of collection of radon daughters (low efficiency) and the lack of spectroscopy information. On the other hand, the real-time Lucas cell can in principle be used also for the study of radon emission of materials.
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701
Fig. 6. Example of the counting efficiency plot obtained with the Marinelli source and the corrected plot with the GammatoolTM . Table 3 Total and peak background rate (d−1 kg−1 Ge) 40–2700 keV
352 keV
583 keV
1461 keV
363,372
918 ± 54
785 ± 33
980 ± 20
For this particular application we have chosen to use a solidstate (silicon) detector because of the following reasons: • The solid-state detector can be used in a collection mode as a pole of an electrostatic Rn-daughter collector. This technique has been used, for instance, in particle physics research to measure low-level radio-impurities (Mitsuda et al., 2003; Simgen et al., 2003). • The energy resolution of the silicon detector allows the spectroscopic study of the alpha particle energies. Because of this reason, one does not need to make assumptions on the status of local equilibrium between 222 Rn and its immediate daughters. • The spectroscopy technique can easily convey additional information. For instance, the presence of 220 Rn of the 232 Th chain. In general, because of the characteristics of the radon electrostatic collection, the efficiency for radon detection can be significantly higher. While a value of about 100 counts per day per Bq m−3 of radon activity is quoted for typical commercially available Lucas-cell systems, the electric collection method allows to reach higher sensitivity levels. For instance, a sensitivity of 14,000 counts per day per Bq m−3 is quoted in the detector system developed to measure 222 Rn in the water of the Superkamiokande detector (Mitsuda et al., 2003).
As pointed out before, in our case the efficiency was increased by measuring radon emission from the solid sample positioned on the top of the HPGe end-cap and in absence of the Marinelli beaker. Additional critical factors in radon daughters collection are the controlled atmosphere conditions (nitrogen) and the 2.5 kV collection potential. The efficiency of this configuration was measured by means of a calibrated radonemission source (Collé et al., 2003) and was found to be of 17% for a radon decay taking place in the unit volume. This value has been quoted by studying the 214 Po alpha peak and is equivalent to a sensitivity of about 10,000 counts per day per Bq m−3 of radon in the volume of the chamber. 5. Results and discussion Figs. 7 and 8 show an example of spectrum taken with the HPGe detector and the silicon detector, respectively, for a typical concrete sample. The uranium and thorium content is of the order of some tens of Bq kg−1 ; the gamma peak at 1461 keV in the gamma spectrum reveals also the presence of 40 K. In the alpha spectrum, taken with the silicon detector, five peaks are present: 222 Rn at 5490 keV, 216 Po at 6778 keV, 214 Po at 7687 keV and 212 Po at 8785 keV. A peak at about 6065 keV is composed by three nonresolved peaks; one from 218 Po at 6002 keV and two from 212 Bi at 6051 and 6090 keV. This particular run has allowed the detection of activity from the Th chain, due to the isotope 220 Rn, an information that can be obtained only by the spectroscopy study. In conclusion, the combined technique of gamma and radon emission measurement allows to completely characterize a solid construction sample from the radioprotection viewpoint. High sensitivity is reached for gamma’s thanks to the germanium spectroscopy technique while for radon emission, electrostatic collection is used.
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10000
Counts
1000
100
10
0.00
900.00
1800.00
2700.00
3600.00
Energy (keV)
Fig. 7. Spectrum obtained with the HPGe detector (after a time of measurement of 42.1 h). In ordinate the counts are in arbitrary scale. The gamma peaks are those from 238 U, 235 U and 232 Th chains and from 40 K at 1461 keV.
180
Counts
135
90
45
0.00
2750.00
5500.00
8250.00
11000.00
Energy (keV)
Fig. 8. Spectrum obtained with the silicon detector (after a time of measurement of 26.7 h). The five alpha peaks visible are, from left to right: 222 Rn, 218 Po+212 Bi, 216 Po at 6778 keV, 214 Po at 7687 keV and 212 Po at 8785 keV. In ordinate the counts are in arbitrary scale.
Acknowledgements We acknowledge the help of Mr. Daniele Cipriani in the setup of the apparatus and Matthias Laubenstein for the Monte Carlo simulations. We thank Kimia for the financial support and we thank the Physics Department of Milano University for its hospitality. References Collé, R., et al., 2003. J. Res. Nat. Inst. Standards Technol. 100, 629. Darby, S., et al., 2005. Biomed. J. 330 (7485), 223.
Debertin, K., Helmer, R.G., 1988. Gamma- and X-ray Spectrometry with Semiconductor Detectors. North-Holland, Amsterdam. Debertin, K., Jianping, R., 1989. Measurement of the activity of radioactive samples in Marinelli beakers. Nucl. Instrum. Methods A 278, 541. EPA Report 402-R-03-003, June 2003. http://www.unscear.org. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, 2000, vol. 75. http://monographs.iarc.fr. Mitsuda, C., et al., 2003. Development of super-high sensitivity radon detector for the super-Kamiokande detector. Nucl. Instrum. Methods A 497, 414. Simgen, H., et al., 2003. Nucl. Instrum. Methods A 497, 407. Steger, F., Gruen, K., Radioactivity in building materials. Oesterreich Norm S 5200. Zuzel, G., 2005. AIP Conference Proceeding, vol. 785, p. 142.
Simultaneous measurement of gamma rays and radon emission (SIMGRAE) for solid samples radioactivity assessment Igor D’Angelo a , Marco G. Giammarchi b , Lino Miramonti c,∗ , Roberto Scardaoni d a Centre for Blood Research at Life Sciences Centre, University of British Columbia, 2350 Health Sciences Mall, Vancouver, Canada V6T 1Z3 b Istituto Nazionale di Fisica Nucleare-Sezione di Milano, Via Celoria 16, 20133 Milano, Italy c Dipartimento di Fisica dell’Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy d Cosmomatic, Via Ventura 17-20134 Milano, Italy
Received 15 August 2005; received in revised form 30 November 2005; accepted 15 December 2005
Abstract A gamma/radon high-sensitivity radioactivity counter has been built in the frame of a research and development aimed at assessing the effect of construction materials on the annual radioactive dose absorbed by individuals. The counter features simultaneous gamma measurement (by high-resolution germanium spectroscopy) and radon evaluation through electrostatic collection and alpha spectroscopy of radon daughters. © 2006 Elsevier Ltd. All rights reserved.
1. Introduction and motivations The issue of radioactivity in buildings is of particular interest since the use of certain construction materials can result in a significant increase in the absorbed dose for the general population. Two different mechanisms are responsible for the main part of the absorbed dose due to construction materials. First, radioactive content of walls and ceilings generates a variety of gamma rays that can impinge on indoor-living individuals. Secondly, radon noble gas is released from construction materials and can be inhaled by the population generating internally absorbed doses. Gamma rays belong to the most penetrating type of radiation. Long-term exposure to low levels of gamma radiation has been linked to cancer formation (with a multi-fold increased risk of developing leukaemia, thyroid cancer and breast cancer) and decreased effectiveness of antioxidants. Long-term exposure to gamma rays can also induce dominant lethal mutations in a wide variety of cellular systems, recessive autosomal mutations
∗ Corresponding author. Tel.: +39 02 5031 7617.
E-mail address: [email protected] (L. Miramonti). 1350-4487/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.12.006
and sex-linked recessive lethal mutations (IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, 2000). Radon has been recognized by various international health organizations as a major lung carcinogen. A number of biological and epidemiological studies have shown that approximately 12% of annual lung cancers reported worldwide can be linked to radon gas exposure from the environment (EPA Report 402-R-03-003, 2003; Darby et al., 2005). The onset of cellular carcinogenesis involves DNA damage to bronchioepithelial cells by particles emitted by radon progeny. In response, criteria have been defined by the U. N. Scientific Committee on the Effect of Atomic Radiation (UNSCEAR) (http://www.unscear.org) for the acceptability of an effective dose rate due to construction materials. Accordingly, criteria for the radioactivity of a construction material in relation with the acceptable effective dose have been defined by various national agencies (see for instance Steger and Gruen,). For a given sample of a construction material, these criteria are typically expressed in the form of the following inequality: aK40 aRa226 aRa226 G aTh232 + + + 1, 10, 000 1000 1000 600 where the concentration of the various nuclides are expressed in Bq kg−1 . The third factor contains as a product the 226 Ra concentration multiplied by an emission coefficient that takes
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I. D’Angelo et al. / Radiation Measurements 41 (2006) 695 – 702
into account radon emanation from the material sample. This is usually the most important term in the equation and is multiplied by a factor (G) that accounts for geometrical conditions and ventilation of living quarters. The numerical coefficients in the equation depend on the energy and characteristics of the emitted radiation and may slightly vary from a given environmental norm to another. This study describes the experimental set-up and performance of a two-detector assembly capable of simultaneously measuring radon and gamma radiation emission from solid construction materials.
2. Detector description 2.1. Detector set-up The structure of our detector set-up consists of a HPGe detector and a “surface barrier” silicon detector assembled in a configuration designed for the simultaneous measurement of gamma-rays and radon emission, respectively. The two detectors are inserted in an “electrostatic collection unit” where the end-cap of the cylindrical HPGe detector is inserted at the bottom of the vessel, while the silicon detector is mounted on top (see Fig. 1). As usual, the endcap of the HPGe detector can be encapsulated in a Marinelli beaker.
Radon emission from the sample generates radioactive daughters nuclei (radon progeny) which are collected in the “electrostatic collection unit”, a cylindrical stainless steel vessel serving as a radon buffer tank and electric field cage. Its dimensions are 230 mm in diameter, 280 mm in height and 5 mm thickness, with a volume of 12 l. An electrostatic field of 2.5 kV between the internal steel walls of the container and the silicon detector drives the (effectively charged) daughter nuclei of 222 Rn near the silicon detector for energy measurement of alpha rays. The silicon detector is electrically isolated from the stainless steel vessel by means of an acrylic plate. In our protocol, it is specifically foreseen that the solid sample be prepared in a Marinelli geometry shape with the actual measurement taking place without the Marinelli beaker itself. The dimensions are: 120 mm height and 130 mm diameter, the bore diameter is 85 mm and the height of annulus is 80 mm. The sample therefore lies “naked” on the top of the HPGe endcap; this allows radon emanation to be effective from most of the sample surface. Samples are prepared starting from raw materials, mixing them in the proper amount according to the specification of the final product and waiting for the complete drying of the mixture before removing them from the Marinelli stamp. Typically, the drying process requires two days. The inside wall of the electrostatic collection vessel is of high-quality electropolished stainless steel, in order to achieve a good-quality surface and a low radioactive background level. Since the “electrostatic collection method” is known to have a significant humidity dependence, the system is provided with a nitrogen supply, so that measurements are made under dry nitrogen at atmospheric pressure. The external detector shielding is made of modular ordinary lead of 100 mm thickness. The upper part of the apparatus is a (compressed air driven) sliding cover for sample insertion. An internal 50 mm thick aluminium completes the detector shielding. The total size of the apparatus (shielding included) is a cube of 800 × 800 × 1200 mm (see Fig. 2). 2.2. Electronics and data acquisition
Fig. 1. Schematic view of the “electrostatic collection unit”. The silicon detector (SSD) and the HPGe detector are also shown.
The signal processing electronics for nuclear applications is nowadays for the greater part tailored to the specific characteristics of the detectors. For our instrument, we have used general-purpose electronics available from specialized companies like Ortec, Silena, Thesys, which regularly take care of developments dedicated to a given family of detectors. The topology of the system is mostly “traditional” with the preamplifiers for the Ge and Si mounted on the detectors while the signal shaping and digitizing processes are performed a few meters from the detectors (see Fig. 3). The HPGe detector must be cooled, because, due to the small band-gap (0.7 eV) at room temperature, it will produce a high thermal leakage current that would strongly degrade the energy resolution. In order to minimize the leakage current and the electronic noise, the detector, the “front-end” of the preamplifier (which include the input field-effect transistor, FET) and the
I. D’Angelo et al. / Radiation Measurements 41 (2006) 695 – 702
697
Fig. 2. Schematic view of the shielding. The global size is 800 × 800 × 1200 mm.
feedback elements of the “charge-sensitive preamplifier” (passive resistor and capacitor) are located in a cryogenic vacuum enclosure (see Fig. 4). Usually, HPGe detectors are cooled with liquid nitrogen (LN2 ) to 85–105 K inside a specifically designed cryostat. For our specific application, in order to avoid inconveniences related to Dewar filling, manual operations, LN2 cost and availability issues, we relied on an alternative X-cooler system (Ortec Industries), which is electrically powered. This system allows the use of standard mixedgas refrigerants and compressors in the cooling process by removing residual oil and other contaminants from the refrigerant. 2.2.1. Basic pulse shaping and processing consideration Usually when the detector and preamplifier noise predominates, the “main amplifier” noise is a relatively unimportant factor, since it represents an inconsequential quantity in the total system performance. All the useful information in the preamplifier output signal is included in the step function rise time and amplitude
of each pulse. The pulse shaping circuit in the subsequent “main amplifier” operating with time constants much shorter than the decay of the preamplifier signal and much longer than its rise time, effectively removes the slow component of the preamplifier signal and produce individual pulses whose amplitudes convey the quantity of interest, usually the energy. The main characteristics of the preamplifier that we are using with the HPGe detector are: pulse “rise time” typically of 25 ns, nominal conversion gain of 175 mV MeV−1 with a maximum pulse output of 10 V. Incorporated into the “preamplifier electronics” is a hybrid monitoring circuit connected to a temperature sensing element; this circuit provides a signal (bias shutdown) for the high voltage bias supply of the detector. This supply is designed to prevent the application of detector bias voltage if the “bias shutdown” signal is not present, and moreover, it also reduces the detector bias voltage to zero if the “bias shutdown” signal indicates a warm condition. Finally, the circuit will also prevent the accidental application of bias to a detector that has not yet reached operating temperature.
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Fig. 3. Electronic interconnections for HPGe and silicon detector.
Fig. 4. Front-end electronics for HPGe detector.
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A hybrid circuit within the preamplifier checks if the event rate exceeds a given level. In fact, when a condition of excessively high rate exists, the detector system may suffer resolution degradation and peak shift, requiring corrective measures. The preamplifiers used for both (HPGe and silicon detectors) can operate also in differential mode, to suppress induced noise when long connecting cables between the “detector/preamplifier” and “main shaping amplifier” are used. 2.2.2. Data acquisition system Since in our system a HPGe detector is used together with a “surface barrier silicon detector”, a practical solution for data acquisition is to employ an Ortec multichannel buffer (mod. 919E) which presents great flexibility. This module can be used with up to four separate detection systems, each with up to 16K channel resolution. A personal computer is interfaced to a task-specific hardware system, and an interface-card gives access to the data memory. 3. Gamma rays detection 3.1. HPGe detector characteristics The gamma rays detector is an intrinsic high-purity 0.150 l germanium crystal manufactured by Ortec. It is a coaxial detector of cylindrical shape with an external diameter of 64.7 mm and a length of 45.0 mm. The crystal is enclosed in a low radioactive background aluminium layer with a thickness of 1 mm; the distance between the end-cap and the crystal is 4 mm. The inactive germanium layer is 700 m. The measured relative efficiency at 1333 keV is 37%, while the peak to Compton ratio, measured at the same energy, is 69:1. The measured resolutions (FWHM) at 122 keV (57 Co) and 1333 keV (60 Co) are 0.78 and 1.71 keV, respectively. Concerning the peak shape, we obtained values of 1.9 for the FWTM/FWHM ratio, and 2.5 for FWFM/FWHM ratio, both at the 1333 keV (60 Co) ray energy. The performance of the detector (warranted and measured) are presented in Table 1. 3.2. Calibration measurements 3.2.1. Calibration counting efficiency source The counting efficiency of the HPGe has been estimated with a commercial calibration source. This source has a Marinelli
699
shape with a total volume of approximately 1000 ml and a density of approximately 1.0 g cm−3 . The beaker is constructed from polypropylene and the radioactive material is homogeneously diluted in a special water-equivalent plastic matrix. The source dimensions are: 120 mm height and 130 mm diameter, the bore diameter is 82 mm and the height of annulus is 76 mm. Table 2 shows the nuclei of the source and the activity at the reference date (i.e. 1 July 2004 at 12 GMT). The relative uncertainty on the activity is 3.0%. Fig. 5 shows the spectrum obtained with the Marinelli source for a 5000 s measurement time. This spectrum source clearly shows a drop in the efficiency at energies below 100 keV. Although the spectrum shows the 210 Pb gamma peak at 46.5 keV and its four X-rays excitation maxima, the efficiency below 100 keV suffers from considerable uncertainty. In addition, this part of the spectrum does not convey important information for our applications. For these reasons, we will always consider gamma energies above an operational threshold of 100 keV. In order to check the reliability of the calibration source we made a Monte Carlo simulation (based on Geant code). The difference between the efficiency values obtained with the simulation and those measured with the calibration source is within 10%. 3.2.2. Correction to the calibration efficiency for different samples In order to determine the calibration efficiency for material having a density and/or element composition different with respect to the one of the calibration source (i.e. water-equivalent plastic matrix) we employ the commercially available software GamatoolTM . This is a software package for calculating correction factors for gamma-rays self-absorption (Debertin and Helmer, 1988; Debertin and Jianping, 1989). Fig. 6 shows the counting efficiency plot obtained with the Marinelli-shaped source and the corrected plot with the GamatoolTM software for a concrete sample of well known composition (i.e. 50% of “cemento pozzolanico” and 50% of SiO2 ; this mixture has a volume of 1.7 kg and a density of about 1.39 g cm−3 . The percentage composition is: 52.7% of CaO, 24.42% of SiO, 7.32% of Al2 O3 , 3.52% of Fe2 O3 , 1.7% of MgO, 3.48% of SO3 , 0.35% of Na2 O, 2.04% of K2 O, 0.35% of TiO2 , 0.01% of Cl2 and 1.2% not determined).
Table 1 Performance specifications (warranted and measured) of the HPGe detector Warranted
Measured
Amplifier time constant (s)
Relative efficiency at 1333 keV Peak to Compton ratio at 1333 keV
30% 60:1
37% 69:1
6 6
Resolution (FWHM) at 1333 keV Resolution (FWHM) at 122 keV
1.85 keV 0.93 keV
1.71 keV 0.78 keV
6 6
Peak shape (FWTM/FWHM) at 1333 keV Peak shape (FWFM/FWHM) at 1333 keV
1.9 —
1.9 2.5
6 6
700
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Table 2 Characteristics of the source as given by the manufacturer Nuclide
-ray energy (MeV)
Activity (Bq)
Emission rate (s−1 )
Americium-241 Cadmium-109 Cobalt-57 Cerium-139 Mercury-203 Tin-113 Strontium-85 Caesium-137 Yttrium-88 Cobalt-60 Cobalt-60 Yttrium-88
0.060 0.088 0.122 0.166 0.279 0.392 0.514 0.662 0.898 1.173 1.333 1.836
3.48 × 103 1.85 × 104 7.09 × 102 8.14 × 102 1.70 × 103 3.03 × 103 3.16 × 103 3.26 × 103 6.05 × 103 3.65 × 103 3.65 × 103 6.05 × 103
1.25 × 103 6.70 × 102 6.07 × 102 6.50 × 102 1.38 × 103 1.96 × 103 3.11 × 103 2.77 × 103 5.69 × 103 3.64 × 103 3.64 × 103 6.01 × 103
The following upper limits were also quoted: 243Am < 1 Bq; 153 Gd < 1 Bq; 113m Cd < 30 Bq.
1.0E05
10000
Counts
1000
100
10
0.00
898.00
1797.00
2698.00
3600.00
Energy (keV)
Fig. 5. Spectrum obtained with the Marinelli source for a 5000 s measurement time. In abscise the energy in keV and in ordinate the counts in arbitrary scale are given.
3.2.3. Background and sensitivity The total background counting rate, obtained with measurement time span of 4.08 days is 363,372 count per day per kg of germanium in the 40–2700 keV range. Table 3 summarizes the total background counting rate and the background counting rate for the 214 Pb peak (352 keV), the 208 Tl peak (583 keV) and the 40 K peak (1461 keV).
4. Radon detection Radon is at the same time an element of the natural radioactive chains and a noble gas. As such, it is highly soluble in liquids and diffusive in solids. In a solid material, the radon diffusion length has a typical value of D/ where D is the gas diffusivity and is the radon decay constant. However, in situations of environmental interest, radon convection due to
air currents (or water currents in water) plays a dominant role. Convection effect will in general spread radon at a much faster rate than suggested by the D/ ratio alone. In general, 222 Rn of the 238 U family is the most important isotope, having a half-life of 3.82 days. By comparison, 220 Rn of the 232 Th family has a half-life of 56 s only. In a construction material, radon is important because it is continuously emitted and significantly affects the radiation exposure of individuals. For highly sensitive measurement of 222 Rn diffusion and emanation see for instance (Zuzel, 2005). Measurement of radon content in air is usually performed by Lucas cells or by means of SSNTD integrating devices. In our case the use of a SSNTD device would imply a problem of collection of radon daughters (low efficiency) and the lack of spectroscopy information. On the other hand, the real-time Lucas cell can in principle be used also for the study of radon emission of materials.
I. D’Angelo et al. / Radiation Measurements 41 (2006) 695 – 702
701
Fig. 6. Example of the counting efficiency plot obtained with the Marinelli source and the corrected plot with the GammatoolTM . Table 3 Total and peak background rate (d−1 kg−1 Ge) 40–2700 keV
352 keV
583 keV
1461 keV
363,372
918 ± 54
785 ± 33
980 ± 20
For this particular application we have chosen to use a solidstate (silicon) detector because of the following reasons: • The solid-state detector can be used in a collection mode as a pole of an electrostatic Rn-daughter collector. This technique has been used, for instance, in particle physics research to measure low-level radio-impurities (Mitsuda et al., 2003; Simgen et al., 2003). • The energy resolution of the silicon detector allows the spectroscopic study of the alpha particle energies. Because of this reason, one does not need to make assumptions on the status of local equilibrium between 222 Rn and its immediate daughters. • The spectroscopy technique can easily convey additional information. For instance, the presence of 220 Rn of the 232 Th chain. In general, because of the characteristics of the radon electrostatic collection, the efficiency for radon detection can be significantly higher. While a value of about 100 counts per day per Bq m−3 of radon activity is quoted for typical commercially available Lucas-cell systems, the electric collection method allows to reach higher sensitivity levels. For instance, a sensitivity of 14,000 counts per day per Bq m−3 is quoted in the detector system developed to measure 222 Rn in the water of the Superkamiokande detector (Mitsuda et al., 2003).
As pointed out before, in our case the efficiency was increased by measuring radon emission from the solid sample positioned on the top of the HPGe end-cap and in absence of the Marinelli beaker. Additional critical factors in radon daughters collection are the controlled atmosphere conditions (nitrogen) and the 2.5 kV collection potential. The efficiency of this configuration was measured by means of a calibrated radonemission source (Collé et al., 2003) and was found to be of 17% for a radon decay taking place in the unit volume. This value has been quoted by studying the 214 Po alpha peak and is equivalent to a sensitivity of about 10,000 counts per day per Bq m−3 of radon in the volume of the chamber. 5. Results and discussion Figs. 7 and 8 show an example of spectrum taken with the HPGe detector and the silicon detector, respectively, for a typical concrete sample. The uranium and thorium content is of the order of some tens of Bq kg−1 ; the gamma peak at 1461 keV in the gamma spectrum reveals also the presence of 40 K. In the alpha spectrum, taken with the silicon detector, five peaks are present: 222 Rn at 5490 keV, 216 Po at 6778 keV, 214 Po at 7687 keV and 212 Po at 8785 keV. A peak at about 6065 keV is composed by three nonresolved peaks; one from 218 Po at 6002 keV and two from 212 Bi at 6051 and 6090 keV. This particular run has allowed the detection of activity from the Th chain, due to the isotope 220 Rn, an information that can be obtained only by the spectroscopy study. In conclusion, the combined technique of gamma and radon emission measurement allows to completely characterize a solid construction sample from the radioprotection viewpoint. High sensitivity is reached for gamma’s thanks to the germanium spectroscopy technique while for radon emission, electrostatic collection is used.
702
I. D’Angelo et al. / Radiation Measurements 41 (2006) 695 – 702 1.0E05
10000
Counts
1000
100
10
0.00
900.00
1800.00
2700.00
3600.00
Energy (keV)
Fig. 7. Spectrum obtained with the HPGe detector (after a time of measurement of 42.1 h). In ordinate the counts are in arbitrary scale. The gamma peaks are those from 238 U, 235 U and 232 Th chains and from 40 K at 1461 keV.
180
Counts
135
90
45
0.00
2750.00
5500.00
8250.00
11000.00
Energy (keV)
Fig. 8. Spectrum obtained with the silicon detector (after a time of measurement of 26.7 h). The five alpha peaks visible are, from left to right: 222 Rn, 218 Po+212 Bi, 216 Po at 6778 keV, 214 Po at 7687 keV and 212 Po at 8785 keV. In ordinate the counts are in arbitrary scale.
Acknowledgements We acknowledge the help of Mr. Daniele Cipriani in the setup of the apparatus and Matthias Laubenstein for the Monte Carlo simulations. We thank Kimia for the financial support and we thank the Physics Department of Milano University for its hospitality. References Collé, R., et al., 2003. J. Res. Nat. Inst. Standards Technol. 100, 629. Darby, S., et al., 2005. Biomed. J. 330 (7485), 223.
Debertin, K., Helmer, R.G., 1988. Gamma- and X-ray Spectrometry with Semiconductor Detectors. North-Holland, Amsterdam. Debertin, K., Jianping, R., 1989. Measurement of the activity of radioactive samples in Marinelli beakers. Nucl. Instrum. Methods A 278, 541. EPA Report 402-R-03-003, June 2003. http://www.unscear.org. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, 2000, vol. 75. http://monographs.iarc.fr. Mitsuda, C., et al., 2003. Development of super-high sensitivity radon detector for the super-Kamiokande detector. Nucl. Instrum. Methods A 497, 414. Simgen, H., et al., 2003. Nucl. Instrum. Methods A 497, 407. Steger, F., Gruen, K., Radioactivity in building materials. Oesterreich Norm S 5200. Zuzel, G., 2005. AIP Conference Proceeding, vol. 785, p. 142.