Applied Radiation and Isotopes 81 (2013) 81–86
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
A new versatile underground gamma-ray spectrometry system Guillaume Lutter a,n, Mikael Hult a, Gerd Marissens a, Erica Andreotti a, Ulf Rosengård a, Marcin Misiaszek b, Ayhan Yüksel a,c, Namik Sahin a,d a
European Commission, DG JRC, Institute for Reference Materials and Measurements (IRMM), Retieseweg 111, 2440 Geel, Belgium Smoluchowski Institute of Physics, Jagiellonian University, ul. Reymonta 4, PL-30-059 Krakow, Poland c TAEK-CNAEM, Turkish Atomic Energy Authority, Istanbul, Turkey d TAEK-SANAEM, Turkish Atomic Energy Authority, Ankara,Turkey b
H I G H L I G H T S
A new gamma-ray spectrometer was installed in the underground laboratory HADES. It is a versatile ultra low-background gamma-ray spectrometer system (Pacman). It has a large inner volume and can host large samples as well as several detectors. The background characteristics are presented for different HPGe configurations. Detection limits for various anthropogenic radionuclides are given.
art ic l e i nf o
a b s t r a c t
Available online 6 April 2013
The newest development in IRMM's underground analytical facility is a large lead shield lined with copper that is versatile and can host several detectors of different types. The characteristics and the background performance of the shield are described for four different detector configurations involving HPGe-detectors and NaI-detectors. The shield has been designed to swap detectors, while still maintaining a low background. This enables testing of detectors for other experiments and optimisation of detection limits for specific radionuclides in different projects. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Low-level gamma-ray spectrometry Underground laboratory HPGe-detector Shielding Compton suppression
1. Introduction A main objective of Radionuclide metrology Sector at IRMM is to support a European and world-wide metrology system and therefore radioactivity measurements in a wide range of fields are carried out. Consequently detectors of different types are needed in order to optimise measurement conditions for different radionuclides in different matrices. IRMM is operating ten specially designed ultra low-background HPGe-detectors in the 225 m deep HADES underground laboratory. Four years ago, a sandwich spectrometer system was installed in HADES with the aim to get high detection efficiency by using two HPGe-detectors placed face-to-face inside the same shielding (Wieslander et al., 2009). The “Sandwich spectrometer” has the limitation of being able to measure only small samples that fit in the limited space between the two detector endcaps. Based on this detector a new spectrometer named “Pacman” has been developed to overcome the sample size limitation. Furthermore, the Pacman shield has been designed to be flexible to allow different detector configurations. It is possible to use two HPGe-detectors
face-to-face like for the Sandwich spectrometer but in addition a Compton suppression shield, or other types of detectors, can be installed inside the spectrometer. This study will present the general characteristics of the Pacman shield and performance of different detector configurations with respect to background reduction and Compton suppression factors.
2. Materials and methods 2.1. The laboratory The Pacman spectrometer is installed in the underground laboratory HADES (Andreotti et al., 2011). It is located 225 m underground at the premises of the Belgian Nuclear Research Centre SCKCEN and operated by (EURIDICE 2012). The muon flux reduction in HADES is about 4 orders of magnitude compared to ground level. 2.2. The Pacman shield
n
Corresponding author. Tel.: +32 14 571398. E-mail address:
[email protected] (G. Lutter).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.03.079
Fig. 1 shows a drawing of the Pacman shield with two HPGedetectors. The shield is made of 17 cm of lead lined with 3.5 cm
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of electrolytic copper. The lead is divided in three layers depending on the intrinsic 210Pb activity. The outmost 7.5 cm has 50 Bq/kg, the 6 cm middle layer 20 Bq/kg and the inner 3.5 cm part 2.0 Bq/kg. The copper was produced in 2006 and immediately after production transported to HADES and stored there until shortly before machining in order to minimize cosmogenic activation. The shield is a rectangular parallelepiped with an outer sidelength of 72.3 cm and an outer height of 111.8 cm. The corresponding numbers for the inside of the shield are 31.3 cm and 71.0 cm, respectively which gives an inner volume of 70 L. The effective air volume is much smaller since the detectors and sample occupy space. There is also an extra inner copper shield installed when solely Ge10 and Ge11 are in operation. As shown on Fig. 1, the shield splits in two parts when it is opened. One problem with such big shields is that the detector arm needs to be fairly long. In order to operate two HPGe-detectors, they need to be inserted at an angle as seen in Fig. 1. For such a configuration the minimum distance between the preamplifier and the endcap is 365 mm. However, there is an option of inserting one HPGe-detector orthogonally into the shield which enables testing of detectors with arms as short as 265 mm. The main part of the shield is manually translated along a rail. The fixed part corresponds to the bottom of the shield and one of the four walls. The central part of the fixed wall is composed of several removable pieces of lead and copper with different sizes in order to adjust the height of the upper HPGe-detector (the lower HPGe-detector remains in a fixed position). Therefore it is possible to use samples with height up to 8.0 cm when working with two HPGe-detectors. The large inner volume enables the placement of bigger detectors like various types of scintillation detectors as is explained in Section 2.3. Whenever only HPGe-detectors are used an extra inner Cu-shield is installed in order to minimize the background contribution from 222Rn. However, that Cu-shield does not completely fill all the empty space. The boil-off nitrogen from Ge10 and Ge11 is also lead into the shield to more efficiently expel air from the inside.
2.3. Detectors and configurations In this study, to characterise the Pacman spectrometer we used two HPGe-detectors called “Ge10“ and “Ge11“ and two scintillation crystals made of NaI(Tl) called “N6“ and “N7“. All detectors are of low-background type made from selected radiopure materials. However, it is well known that the photomultiplier tubes of the NaI detectors are not very radiopure. Details of the four detectors are given in Table 1. Fig. 2 shows two different possible Pacman configurations with Compton shield used for this study. Note that detector Ge10 is especially suited for a Compton suppression system as it is an n-type crystal with thin deadlayers on all outside surfaces, a thin crystal-holder and a thin aluminium endcap. Detector Ge11 has thick outer deadlayers and a copper endcap and is more suited for gamma-rays of higher energies. 2.4. Electronics The electronics of the Pacman spectrometer is similar to the Sandwich detector (Wieslander et al., 2009) and it consists of standard NIM modules (amplifiers and ADCs) connected to a DAQ2000 multi-parameter system (Gonzalez, 2012). The DAQ2000 is based on LabViews and designed and manufactured by IRMM. It registers up to four input channels and time-stamps all events with 100 ns time step. All the events are stored and the anticoincidence or coincidence is set in the software during offline analysis. 2.5. Samples and measurements The performance of the Pacman spectrometer was studied using three different dried reference soil samples which have different levels of radioactivity. Soil sample #1 (103 g) has a relatively high activity of 137Cs, 368 Bq, soil sample #2 (156 g) has a lower 137Cs activity, 2.2 Bq, and soil sample #3 (77 g) was
Fig. 1. Schematic drawing of the Pacman shield with HPGe-detectors Ge10 and Ge11. Table 1 Details of the two HPGe-detectors and the two scintillation-detectors used in this study.
Detector material Crystal configuration Relative efficiency (%) Crystal mass (kg) Crystal diameter (cm) Crystal height (cm) Manufacturer and year Top deadlayerthickness Endcap Window Crystal holder
Ge10
Ge11
N6
N7
HPGe, n-type Coaxial 62% 1.04 6.7 6.85 Canberra 2011 o 1 μm 1.5 mm Al 1.5 mm Al 0.5 mm Al (base in Cu)
HPGe, p-type Coaxial 85% 1.88 8.0 7.05 Baltic Scientific Instruments 2011 0.7 mm 1.5 mm Cu 0.9 mm Cu 1.2 mm Cu
NaI(Tl) Cylinder – 3.06 10.2 10.2 Scionix 2008 – 0.8 mm Stainless steel
NaI(Tl) Annular – 26.3 22.8 (outer) 10.9 (inner) 22.8 Scionix 2008 – 0.8 mm Stainless steel
n.a.
n.a.
G. Lutter et al. / Applied Radiation and Isotopes 81 (2013) 81–86
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Fig. 2. Two different configurations of the Pacman spectrometer; (a) Ge10 and Ge11 with N7 and (b) Ge10 with N6 and N7.
Table 2 Characteristics of the three soil samples used in this study. Soil sample
Mass (g)
Density (g/cm3)
Massic activity (Bq/kg) 137
Cs
#1 #2 #3
103 35 77
1.3 0.72 1.0
238
U
228
Th
40
K
35707 130 24.370.4 21.5 70.7 4107 20 13.8 7 1.4 40 712 52.3 72.5 545 7 55 o 0.031 45 75 43.7 71.4 566 7 17
selected for its minute amount of anthropogenic radionuclides (e.g. o2 mBq of 137Cs). The latter is from the new batch of NIST reference material “Peruvian soil“ that is being certified (Inn et al., 2009). The main use of a Compton shield is for detecting weak signals of e.g. 137Cs in samples with environmental levels of 40K and other primordial radionuclides. All soil samples contain the common primordial radionuclides. The densities of the soil samples and massic activities of for example 238U, 228Th and 40K in the three samples are given in Table 2. For the measurements, each soil sample was placed in a small radon-tight Teflon container to assure equilibrium between possible radon daughters and 226Ra in the soil. The radius of the Teflon containers was 4 cm and the height was 4.5 cm for the soil samples #1 and #3 and 3 cm for #2. The thicknesses of the container for the soil samples #1 and #3 were 2 mm on the bottom (in contact with Ge10) and 5 mm on the top (in contact with Ge11) and respectively 0.35 mm and 5 mm for the soil sample #2 container. The thickness of the side part of the containers was 13 mm. The background measurements were performed using a radiopure Teflon cylinder as sample. 2.6. Data analysis Like for the Sandwich spectrometer, the output of the acquisition system consists of files with time-stamped list-mode data in binary format. The list files were converted to ROOT file format (CERN, 2012). The ROOT-based data analysis software made for the Sandwich detector was adapted for the Pacman spectrometer. The software is used for normal gamma-ray spectrometry analysis by summing spectra from several measurements and performing energy calibrations. Since all events are collected and time-stamped,
Fig. 3. Background spectra of Ge10 in different configurations in the Pacman shield; in red, Ge10 alone; in black together with Ge11; in green, in anticoincidence with N7 and Ge11; in blue with Ge11 and N7 in the shield. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the coincidences or anti-coincidences events between the detectors are also performed by the software.
3. Results 3.1. Backgrounds Fig. 3 compares the background spectra of Ge10 for different configurations of the Pacman spectrometer. The associated numbers of counts per day for the main gamma-rays are given in Table 3. The background of detector Ge11 (520 counts per day, 40–2400 keV, together with Ge10 in the shield) is lower than for Ge10 (559 counts per day, 40–2400 keV), when operated without the NaI detector (Lutter et al., in press). One reason for this is its thicker deadlayers, crystal-holder and endcap. With only Ge10 and Ge11 inside the Pacman spectrometer and a piece of ultra pure Cu between them, the background count rates in the energy interval 40–2400 keV was 719 counts per day for Ge10 and 474 counts per day for Ge11. Although it was checked that the contribution from radon-daughters had decayed to steady-state, the general radon-
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Table 3 Count–rates of main background gamma lines for Ge10 in different configurations in the Pacman shield. Eγ
Radionuclide Alone (counts/ day)
With Ge11 With Ge11+ In anti-coincidence N7 (counts/ with Ge11+N7 (counts/ (counts/day) day) day)
46 210Pb o1.8 1.9 7 1.2 63 234Th o1.6 o 2.2 234 93 Th o1.4 o 2.0 186 226Ra+235U 1.0 7 0.5 o 2.1 238 212Pb o0.9 o 1.6 242 214Pb 0.9 70.5 o 1.6 295 214Pb 1.5 7 0.4 o 1.3 338 228Ac o0.6 o 1.4 214 351 Pb 2.9 70.4 o 1.3 511 Annihilation 1.5 7 0.3 2.4 7 0.9 o0.3 1.3 7 0.6 583 208Tl 609 214Bi 1.9 7 0.3 2.7 7 0.8 661 137Cs o0.3 o 1.5 911 228Ac o0.3 o 0.8 228 969 Ac o0.3 o 0.6 214 1120 Bi 0.84 70.24 o 0.8 60 1173 Co o0.2 o 0.6 o0.2 o 0.5 1332 60Co 1460 40K o0.6 1.4 7 0.6 1764 214Bi 0.517 0.16 o 0.6 2614 208Tl o0.1 0.7 7 0.4 40–2400 keV 719 559 40–400 keV 559 351 40–1400 keV 697 515 1400– 22 43 2400 keV Measurement time 30.4 6.1 (d)
1.5 71.3 1.5 71.4 4.5 7 1.5 6.5 7 1.6 16.5 7 1.7 13.5 7 1.6 24.57 1.7 2.8 7 1.1 49.7 72.0 13.7 7 1.2 7.0 71.0 42.17 1.7 o1.3 8.0 7 0.9 3.6 7 0.8 9.17 1.0 10.8 7 1.0 10.2 7 0.9 49.4 71.7 11.17 0.8 9.7 7 0.8 3395 2048 3180 216
2.3 71.0 o 1.9 3.3 71.0 7.0 7 1.1 15.2 7 1.3 13.6 7 1.3 24.9 7 1.4 2.6 70.7 49.37 1.8 4.6 70.7 3.9 70.7 25.17 1.3 o 0.8 7.17 0.8 3.5 70.6 5.17 0.7 7.17 0.7 6.7 70.7 49.67 1.7 10.17 0.8 6.2 70.6 1540 971 1409 133
17.4
Fig. 4. Spectra from detector Ge10 (a) up to 3000 keV and (b) up to 1500 keV. The red and black lines are background measurements while the other 3 lines are from the measurement of soil sample #1. The numbers indicate the suppression factor obtained with N6+N7 in anti-coincidence.
concentration in air may still have affected these measurements and contributed to the higher values obtained for Ge10 when shielded from Ge11 with copper. Compared to the Sandwich spectrometer, the Pacman spectrometer has a much bigger volume of air inside and data in Table 3 show the presence of the 222Rn daughters 214Bi and 214Pb at the level of 1–4 counts per day for Ge10. The radon monitoring in the laboratory showed during the background measurement period a mean level of 5 Bq/m3 in the air. For the measurements presented here it was first verified that the integral count rate was down to the count rate obtained during stable conditions. Immediately after closing the shield the background is somewhat higher. This decays rather quickly due to the combination of flushing with boil-off nitrogen and natural decay of 222Rn (Hult et al., 2013). The introduction of N7 in the Pacman spectrometer gave an important increase of the background counting rates of Ge10 and Ge11 to 3395 and 21104 counts per day, respectively, in the energy window 40–2400 keV. The higher value in background countingrates between Ge10 and Ge11 can be explained by the position of Ge11 in the setup. As Ge11 is positioned closer to the four PMTs of N7, its background is consequently higher. The background counting-rates with the active Compton suppression shield in the energy window 40–2400 keV were reduced to 1540 counts per day for Ge10 and 20668 counts per day for Ge11. The active shield reduces thus the total number of background counts a factor of 2 for Ge10. For Ge11 the effect is much less due to the combination of having a thicker deadlayer, holder and endcap and not being positioned as well as Ge10. 3.2. Soil measurements Fig. 4 shows the effect of the active Compton shield on a Ge10 measurement for the #1 soil sample with high 137Cs content. It is
Fig. 5. Spectra from detector Ge10 from the measurement of soil sample #3 (NIST-Peruvian soil). The anticoincidence with Ge10 is set by (a) N6+N7 in the upper spectrum and (b) Ge11+N7 in the lower spectrum. The numbers near the arrows give the suppression factor.
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clear that turning on the active shielding reduces the overall count-rate and the continuum level. The effect of detector N6 is rather small and only discernible at the Compton edges (from backscatter in Ge10) at 476 keV (from the 662 keV line) and 1243 keV (from the 1460 keV line). One can note that the count rate from the primordial radionuclides in the sample was much higher than the background count rate in Ge10 so the extra background from the PMTs had only marginal effect in this case. Fig. 5 shows the spectra obtained in Detector Ge10 for various configurations when measuring soil sample #3 (Peruvian soil). Not only the continuum level is affected by the Compton suppression, also full energy peaks of cascading gamma-rays are reduced, which may cause problems when quantitative analysis is performed. In Table 4 the suppression factors of peaks and certain energy intervals are listed. Note that none of the single gamma-rays, like 662 keV and 1460 keV, show any sign of reduction. As expected, the suppression factors obtained when using Ge11 as main detector are limited for the reasons previously explained. In addition, N7 covers a smaller solid angle outside Ge11 compared to what it covers outside Ge10. Also as expected, the count rate increases when N7 is installed compared to measuring the soil sample without N7. However, when anti-coincidence is used the count rate is lower compared to the one using Ge10 alone and without N7 inside the shield. It is of great practical importance to compare the detection limits for various anthropogenic radionuclides that can be obtained in the different configurations. This is presented in Table 5. The calculations of detection limits (with α¼0.05) were performed following the ISO (2000). The efficiencies used in the detection limit
Table 4 Reduction factors when using the Compton suppression (CS), i.e. anti-coincidence, for the measurement of the Peruvian soil sample (#3). Eγ
Radionuclide
Detector:Ge10
Detector: Ge11
85
Table 5 The detection limits (α¼ 0.05) of activity (mBq) and massic activity (mBq/g dry weight) of an 80 g soil sample (#3) measured for 5 days for different detector configurations in the Pacman shield. Activity (mBq)
228 keV (132Te)
605 keV (134Cs)
662 keV (137Cs)
796 keV (134Cs)
1596 keV (140Ba–140La)
2.06 y 15 0.19 11 0.13 58 0.72 64 0.81 29 0.37
30.05 y 14 0.18 9 0.12 8 0.10 8 0.10 6 0.07
2.06 y 27 0.33 18 0.22 107 1.33 131 1.64 49 0.61
12.7 d 16 0.20 11 0.13 70 0.87 71 0.89 29 0.36
Massic activity (mBq/g) Half-life: Ge10 (alone)
2.3 d 32 0.40 Ge10+Ge11 23 0.28 Ge10 in anti-coinc. 156 w. (N7+Ge11) 1.94 Ge10 in anti-coinc. 147 w. (N7+N6) 1.84 (Ge10+Ge11) in 69 anti-coinc. w. N7 0.87
calculations included correction for coincidence summing and Compton suppression and were determined using the Monte Carlo code EGS4. For 137Cs, the most favourable configuration is when the signals of Ge10 and Ge11 are added (not in coincidence though) and N7 is in anti-coincidence with each of them. Also the other two anti-coincidence configurations are a factor of 2 lower compared to measuring the sample only on Ge10. For 134Cs and 140Ba/140La the situation is tricky due to cascading gamma-rays and one looses some peak counts when operating the Compton shield. Therefore it is advantageous to use the list mode analysis so one can optimise the peak to background ratio off-line. One can argue that the relative small reduction in background of the Compton shield does not motivate its use for radionuclides with cascading gamma-rays as the detection limit is proportional to the square root of the background but inversely proportional to the efficiency.
CS: N7+N6 CS: N7+Ge11 CS: N7+Ge10
4. Discussion and conclusion PEAKS 210 46 Pb 234 63 Th 234 93 Th 226 186 Ra+235U 212 238 Pb 242 214Pb 295 214Pb 338 228Ac 351 214Pb 511 Annihilation 583 208Tl 609 214Bi 228 911 Ac 969 228Ac 1120 214Bi 1173 60Co 1332 60Co 1460 40K 1764 214Bi 2614 208Tl Continuum below peaks 228 232Te 605 134Cs 662 137Cs 796 134Cs 1243 Compton edge of 1460 keV 1596 140Ba 2381 Compton edge of 2614 keV Integrated over wide intervals 40–2400 keV 40–400 keV 40–1400 keV 1400–2400 keV Measurement time
0.98 7 0.05 0.977 0.05 1.667 0.08 0.977 0.03 1.007 0.01 1.017 0.02 1.007 0.02 1.247 0.02 1.017 0.01 13.7 7 1.9 3.65 7 0.08 3.55 7 0.06 1.177 0.02 1.137 0.03 4.187 0.19 3.7 7 1.7 41.2 1.007 0.01 1.03 70.03 5.59 7 0.22
0.86 70.05 1.03 7 0.04 1.667 0.04 1.03 7 0.02 1.017 0.01 1.03 7 0.02 0.99 70.01 1.28 7 0.02 1.017 0.01 8.17 0.6 3.497 0.06 3.69 70.06 1.247 0.02 1.247 0.02 5.03 70.21 2.8 71.7 1.8 7 0.6 1.00 70.01 1.03 7 0.02 5.95 70.21
41.1 41.1 1.0 7 0.3 1.077 0.14 1.00 70.20 0.98 70.13 1.00 70.04 1.017 0.05 1.00 70.02 1.117 0.11 1.09 7 0.04 1.09 7 0.03 1.02 7 0.03 1.03 7 0.04 1.157 0.07 0.9 70.4 1.0 7 0.3 1.00 70.01 1.00 70.04 1.147 0.08
1.89 70.04 3.477 0.08 3.36 7 0.18 2.49 70.15 3.69 7 0.17 4.17 0.5 147 6
1.96 7 0.03 3.647 0.07 3.80 70.19 2.99 70.17 4.677 0.21 4.9 70.6 13 74
1.09 7 0.03 1.117 0.07 1.117 0.07 1.077 0.07 1.107 0.06 1.067 0.14 1.17 0.3
1.7 1.5 1.7 1.5 5.3 d
1.8 1.6 1.8 1.5 6.8 d
1.1 1.1 1.1 1.0
The Pacman shield has proven to be working well and enables different detector configurations to be studied. The big inner volume opens up for measuring big samples or for opportunities of installing other types of detectors than those presented here, like small plastic scintillators or liquid scintillation cells for beta/ alpha-gamma coincidence counting. A general problem with scintillation detectors is that the PM tubes contain relatively high amounts of radioactivity. Possibilities of using longer radiopure light-guides will be investigated and studies into finding more radiopure materials for the PM-tubes will be performed. Other possible future developments could involve the use of pulse-shape to identify single site events (Gonzalez de Orduna et al., 2010). Furthermore, we will try to improve the design of the inner copper shield to minimise further empty space and thereby reduce radoninduced background when Ge10 and Ge11 are used without any other detectors.
Acknowledgements The work done by EURIDICE and the HADES team of SCKCEN in Mol, Belgium, is gratefully acknowledged. Many thanks to Ken Inn (NIST) for granting use of the Peruvian soil data. References Andreotti, E., Hult, M., Gonzalez de Orduña, R., Marissens, G., Mihailescu, M., Wätjen, U. and Van Marcke, P., 2010. Status of underground radioactivity measurements in HADES. In: Proceedings from 3rd International Conference
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