Improvements of a low-level gamma-ray spectrometry system at the underground laboratory “UDO”

ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 726–730

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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso

Improvements of a low-level gamma-ray spectrometry system at the underground laboratory ‘‘UDO’’ S. Neumaier a,, M. Wojcik b, H. Dombrowski a, D. Arnold a a b

Physikalisch-Technische Bundesanstalt, Braunschweig, Germany Institute of Physics, Jagellonian University, Cracow, Poland

a r t i c l e in f o

Keywords: Underground laboratory Low-level g-ray spectrometry Radon suppression

a b s t r a c t The Physikalisch-Technische Bundesanstalt (PTB) operates various low-background g-ray spectrometry systems at the underground laboratory for dosimetry and spectrometry ‘‘UDO’’ in the Asse salt mine. Experiences gained with these detector systems within 10 years of operation have led to technical changes and improvements of the most sensitive detector system at UDO. Key aspects are the precautions to suppress radon (and its progenies) and the performance of the detector system after exchanging the end cap. In addition, a brief summary of recent applications of this g-ray detector system will be presented. & 2009 Elsevier Ltd. All rights reserved.

1. Introduction The Physikalisch-Technische Bundesanstalt (PTB), the German National Metrology Institute, operates an underground laboratory for dosimetry and spectrometry (UDO) in the Asse II research salt mine, near Braunschweig. This laboratory is used for dosimetry applications at very low dose rates as well as for the measurement of very low activities (Neumaier et al., 2000). For the calibration of dosimetry detector systems at very low dose rates, UDO provides an irradiation facility to generate collimated photon radiation fields which are traceable to PTB’s primary standards. This worldwide unique facility has especially been used to investigate environmental radiation monitoring systems based on various kinds of dose and dose rate meters. In the last decade, several European intercomparison exercises took place at UDO and at further PTB reference sites for environmental dosimetry (Saez-Vergara et al., 2006). UDO is very suitable for low-level measurements because of its high shielding against cosmic radiation and the extremely low ambient dose equivalent rate of only 1 nSv/h inside the laboratory, which is due to the very low activity concentration of 40K of the rock salt. In addition to the dosimetry installations, UDO is presently equipped with three low-level detector systems for the measurement of very low activities. In the following chapter, the performance of the lowlevel gamma-ray spectrometry systems at UDO will be described. Due to the backfilling of the Asse mine, in 2004 the whole UDO facility had to be moved within the mine from its former location at a depth of 925 m to the present depth of 490 m. Therefore, the  Corresponding author. Fax: +49 531 592 6015.

E-mail address: [email protected] (S. Neumaier). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.01.025

muon component of the secondary cosmic radiation increased by about a factor of 10. Nevertheless, the remaining reduction of the muon fluence by four orders of magnitude relative to the ground level is still sufficient for many low-level measurements. The contribution of muons to the background of the g-ray spectrometry systems is still negligible. In addition, the ambient dose equivalent rate at UDO increased to 2 nSv/h, due to the higher 40K activity concentration of the salt. Unfortunately, also the radon activity concentration in air increased considerably (by a factor of 4!). At the 490 m level it amounts to about 60 Bq/m3 on average and may even exceed the level of 100 Bq/m3 for shorter periods of time. Therefore, it became necessary to significantly improve the shielding against radon, at least for the most sensitive detector system. In addition, this detector system was equipped with a new end cap to minimise the inherent background of the detector. The influence of the improved radon shielding and of the replacement of the end cap on the detector performance is described in the subsequent sections.

2. Low-level c-ray spectrometry at UDO At UDO three low-background g-ray spectrometry systems are presently in operation: A Si(Li) detector and two coaxial ‘‘extended range’’ HPGe detectors (p-type crystal, 2 kg each). All systems are equipped with a passive shielding. The Si(Li) detector is mainly used for the measurement of L X-rays from various Pu isotopes in order to determine their isotopic ratios in combination with results from alpha-particle spectrometry (Arnold, 2006). The continuous background of the Si(Li) detector at UDO is lower by a factor of 10 than the background observed at ground level.

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The background reduction of the two coaxial detectors is even more pronounced: The suppression factor lies in the order of 50, averaged over the relevant energy range (a factor of 30 at low energies and a factor of 100 at energies above 1 MeV). The two coaxial detector sys-tems have about the same germaniumcrystal geometry and size, but differ in detector materials and in their shielding. One of the germanium detectors is a standard lowbackground detector (in the following referred to as ‘‘LB’’), the other one a so-called ‘‘customer-designed’’ or ultra-low-background detector (in the following referred to as ‘‘ULB’’). More details on the LB detector systems can be found in (Neumaier et al., 2000). Descriptions of the ULB detector system and of recent improvements follow in the next chapter. The main applications of these two detector systems are related to the following interdisciplinary investigations:

 Dosimetry system DS86







 

The activity concentrations of 60Co in steel samples from Hiroshima, activated by the neutrons of the atomic bomb explosion in 1945, were measured and compared with predictions of the dosimetry system from 1986 (DS86) (Hult et al., 2004). Tukaimura accident The 60Co activities of steel samples from Tukaimura were measured in order to determine the neutron flux (and thereby the neutron dose to the public) at various places around the nuclear fuel element factory in Tukaimura, where a severe criticality accident killed two workers and exposed members of the public to neutron irradiation in 1999 (Gasparro et al., 2004). Radionuclides in meteorite samples PTB participated in an international intercomparison between laboratories using accelerator mass spectrometry and gammaray spectrometry to improve the quality of 26Al measurements in meteorite samples (Arnold et al., 2002). In 2004, a meteorite had entered the earth’s atmosphere only a few weeks before it was found in Spain. At UDO, this meteorite was investigated with emphasis on the detection of short-lived isotopes (Neumaier and Arnold, 2004). Detector materials Hyper-pure aluminium and other materials used for the production of low-level detectors as well as various thermoluminescence detector materials were investigated with respect to their use. Steel contaminations The activity concentrations of 60Co, 137Cs and other nuclides in various steel samples were determined for the steel industry. Fusion-plasma diagnostics Various targets irradiated by the plasma of the JET fusion experiment were analysed with regard to fusion-plasma diagnostics (Wieslander et al., 2008).

3. An ultra-low-background detector system The most sensitive detector system at UDO (called ‘‘ULB’’ detector) consists of a 95% coaxial p-type ‘‘extended range’’ HPGe detector (FWHM: 2.15 keV at 1.33 MeV) built in 1999 by Canberra Industries. Until the end of 2004, this detector was equipped with an electrolytic copper end cap having a carbon–epoxy entrance window (+ ¼ 86 mm, 0.5 mm thickness). In 2005, this end cap was replaced by a radiopure aluminium (‘‘KryAl’’) end cap. The germanium detector itself was described more detailed by Neumaier et al. (2000). The ULB detector is surrounded by a passive shielding composed of three layers: two outer layers of lead with a 210Pb

Fig. 1. Sketch of the set-up of the ULB detector system.

content of 6 and 2 Bq/kg, respectively, and an inner layer of 10 cm of electrolytic copper. The latter was exposed to the secondary cosmic radiation at ground level only for a few days before it was brought to UDO. In order to reduce the diffusion of radon and its progenies into the system, the complete detector system is housed in an almost radon-tight glove box. A sketch of the ULB detector system is shown in Fig. 1. The overall count rate of the ULB system of about 22 counts/h is lower by a factor of 4 than that of the LB system. In addition, most of the background peaks present in the spectra of the LB detector are significantly lower in the spectra of the ULB system (see Fig. 2). A summary of background peak count rates of the ULB detector system is listed (for the two different end caps) in Table 1.

3.1. Radon suppression Radon and its progenies are a source of a strong time-dependent background component in low-level radioactivity measurements. After the detector is shielded against external gamma-rays and detector as well as shield materials of low intrinsic radioactivity have been selected, the radon component is often the limiting factor when striving for a further background reduction. In highly sensitive experiments performed underground, radon suppression is therefore a prerequisite. The radon shield of the ULB system consists of an almost airtight box which surrounds the detector and the whole passive shielding. In addition, an improved nitrogen flushing of the inner volume of the detector chamber was realised by various changes of the detector system design. Especially the radon tightness of the inner part of the passive shielding at the feedthrough of the detector’s cooling finger is crucial. It was achieved by mounting a

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Table 1 Background count rates of the ULB detector system equipped with a compact KryAl end cap (‘‘KryAl’’) and—in comparison—with an electrolytic copper end cap having a carbon—epoxy entrance window (‘‘E-copper’’). Nuclide

Peak energy (keV)

Aluminium end cap count rates (d1)

Copper end cap count rates (d1)

210

46.5 186 239 338 352 511 583 609 662 911 965/969 1332 1461 2615 40–2750

0.2 o0.2 18.6 4.4 4.1 2.2 6.1 1.8 o0.2 5.7 4.8 o0.2 3.6 2.5 19 h1

1.5 2.7 1.7 0.9 6.9 o0.2 o0.2 4.6 o0.2 o0.2 o0.2 0.7 11.1 0.2 22 h1

Pb Ra/235U Pb 228 Ac 214 Pb Annihil./208Tl 208 Tl 214 Bi 137 Cs 228 Ac 228 Ac 60 Co 40 K 208 Tl Overall count rate 226 212

Count rate / (d · keV · kg)-1

10000



 

Counting time: 60 d

1000 100

Peak count rates per day 6.9 4.6

10

214

0.8

11

Pb

40 214

1

0.2

K

Bi

1

60

Co

214

Bi

208

0.1

Tl

0.01 0

500

1000

1500

2000

 The glove box is equipped with an airlock chamber. In the

2500

3000

Energy / keV Fig. 2. Background spectrum of the ULB detector system with copper end cap.

10 cm silicon tube between the inner electrolytic copper shield and the feedthrough as a seal. In addition, the performance of the N2 flushing itself was improved. It is important that the N2 gas inlet is very close to the detector’s end cap (see Fig. 3) and that the gas outlet does not allow the re-diffusion of radon into the inner detection volume. The latter means that only tiny gaps allow the gas to escape from the inner volume. For the ULB system, this is achieved by a few scores of 0.1 mm diameter each, at the plug, which otherwise completely closes and tightens the inner volume due to its slightly conical shape. As a result of the slight gas overpressure inside the glove box, nitrogen escapes to the outside through the scores in the plug and through numerous slots between lead bricks constituting the passive shielding. Radon generated inside the detector chamber and in the slots of the shielding is transported outside the glove box by the nitrogen stream. Once the nitrogen inside the glove box has been purified it cannot be interfused by radon-loaded air from outside. Note: The boil-off nitrogen from the detector cryostat used for flushing the detector chamber (‘‘inner detection volume’’), has a typical radon activity concentration of only 0.5 mBq/m3. For the ULB detector system, the following additional measures have been undertaken to minimise the influence of the radon background component on the spectra:

present stage of operation, this airlock chamber is not yet flushed with nitrogen before it is opened to the inner side of the glove box in order to transfer samples. Nevertheless, the limited amount of air inside the airlock chamber considerably reduces the amount of air (and radon) that can enter the inner volume of the system during the sample handling. In a future approach, this pre-flushing of the airlock chamber shall be installed to further reduce radon penetration into the system. The gloves are manufactured from butyl rubber (0.4 mm in thickness) having a radon permeability approximately 50 times lower than standard gloves made of soft rubber. The gloves are nevertheless not completely radon-tight. Therefore, immediately after they have been used to transfer samples from outside via the airlock into the system, the openings of the gloves are closed with acrylic plates. All components inside the box were carefully cleaned in many steps and were selected for low radon emanation. The evaporated nitrogen is guided to the inner detection volume via a nylon (supronyl) pipe to avoid radon diffusion from the ambient air. The radon diffusion coefficient in nylon is very low (in the order of 1012 cm2 s1).

Two experiments were made to check the radon tightness of the glove box. In both experiments, the radon activity concentration of the gas filling the inner detector volume (see Fig. 3) was observed as a function of time, by plotting the count rate of the 609 keV peak of the radon progeny 214Bi as a measure of the radon concentration (Fig. 4). In both experiments, the N2 flushing of the inner detection volume was ‘‘switched off’’ and the plug as well as the glove box which protects the inner detection volume from the diffusion of radon were opened for 100 h (from 100 to 0 h). In the next step (from 0 to 500 h), both the plug and the glove box were closed. The decrease of 214Bi is therefore dominated by the decay of radon and its progenies. In Fig. 4a no N2 flushing was applied. An acrylic glass dummy with the size of a typical sample was placed inside the inner detection volume (Fig. 3) in front of the detector (until 500 h). In Fig. 4b, the dummy was removed and N2 flushing was applied (starting at ‘‘0 h’’). The fast decrease of 214Bi observed is due to the replacement of the primary gas of the inner detection volume by nitrogen. These results reveal that the acrylic glass dummy caused an unexpected additional contribution of radon progenies to the spectra (resulting from a small Ra

Fig. 3. The inner detection volume, a cavity amidst the electrolytic copper shield, can house the sample holder (fixed at the bottom of the plug).

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10.00

10.00 no flushing / dummy flushing / dummy flushing / no dummy

Count rate / h-1

Count rate / h-1

729

1.00

0.10 -200

0

200

400

600

no flushing / dummy flushing / dummy flushing / no dummy

1.00

0.10 -200

800

0

200

400

600

800

Time / h

Time / h

Fig. 4. (a and b) Tests of the radon suppression performed by evaluating the count rate at the 609 keV peak of 214Bi detected by the ULB system (for further explanations see text).

6

5 Count rate / (d · keV · kg)-1

ULB at 490 m level (copper end cap) ULB at 925 m level (aluminium end cap)

4

3

2

1

0 0

500

1000

1500

2000

2500

3000

Energy / keV Fig. 5. Background spectra of the ULB detector system with different end caps and radon shieldings.

contamination). A quantitative analysis of the data plotted in Fig. 4a shows that the decrease of 214Bi is much faster than expected, if 214Bi was in equilibrium with 222Rn (T1/2 ¼ 91 h). This finding may be explained by the deposition of radon progenies (not being in equilibrium with radon) on various surfaces inside the glove box, which results in an additional contribution to the 214 Bi activity, with a much shorter half-life.

unexpected (although very small) 232Th contamination of the KryAl material, some additional background lines, originating from the thorium decay chain, appear as shown in Fig. 5. In Tab. 1, the peak count rates for both end caps are given. Although this result is unsatisfactory, the overall count rate (from 40 to 2750 keV) is nevertheless slightly lower (by about 15%) when the KryAl end cap is compared with the electrolytic copper end cap having a carbon–epoxy entrance window (see Table 1).

3.2. Different end caps 4. Summary and conclusions Due to the removal of UDO in 2004, all detector systems had to be disassembled. This opportunity was used to exchange the end cap of the ULB detector with the aim to further reduce the inherent background of this system (especially the contribution from 40K to the count rate). In this section, results achieved with two different end caps will be compared. In Fig. 5, a background spectrum measured when the detector was equipped with an electrolytic copper end cap (furnished with a thin carbon–epoxy entrance window) is compared with a spectrum obtained after the change to a new low-activity aluminium end cap. As aimed at, the 40K peak is significantly lower (2 d1) when the end cap is made of KryAl, but due to an

The removal of the underground facility UDO within the Asse salt mine in 2004 required special efforts to improve the radon suppression of the ULB detector system at UDO. Although the radon concentration in air increased by a factor of 4, the count rates of the radon progenies found in the background spectra of the improved ULB detector system decreased by a factor of 2. The exchange of the detector’s end cap reduced the 40K count rate from 11 to 3.6 d1 but enhanced other peaks originating from thorium contaminations, most probably of the KryAl end cap. Therefore, the overall count rate of the system has been improved only slightly.

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Acknowledgements We gratefully acknowledge the valuable discussions with Dr. G. Heusser (Heidelberg) concerning the design of the ULB detector system. We should like to thank the following PTB staff members for their kind support in the construction, improvement and operation of g-ray detector systems at UDO: A. Ho¨hne, B. Ho¨hne, J. Kretzer, W. Schrader and R. Zwiener. Personal note: The authors deeply regret that Dr. W. Kolb, former head of the section Radiation Protection Metrology at the PTB and enthusiastic promoter of low-level measurement techniques as well as one of the founders of the UDO underground laboratory, unexpectedly passed away on September 6, 2008. References Arnold, D., Neumaier, S., Sima, O., 2002. Deep underground gamma spectrometric measurements of 26Al in meteorite samples. Appl. Radiat. Isot. 56, 405–408.

Arnold, D., 2006. Improved determination of plutonium content and isotopic ratios in low activity samples by a-particle and underground L X-ray measurement. Appl. Radiat. Isot. 64, 1137–1140. Gasparro, J., et al., 2004. Measurements of 60Co in spoons activated by neutrons during the JCO criticality accident at Tokai-mura in 1999. J. Environ. Radioact. 73 (3), 307–321. Hult, M., et al., 2004. Deep underground measurement of 60Co in steel exposed to the Hiroshima atomic bomb explosion. Appl. Radiat. Isot. 61, 173–177. Neumaier, S., Arnold, D., Bo¨hm, J., Funck, E., 2000. The PTB underground laboratory for dosimetry and spectrometry. Appl. Radiat. Isot. 53, 173–178. Neumaier, S., Arnold, D., 2004. Low level gamma-ray spectrometry at the PTB underground laboratory UDO. In: International Conference on Isotopes in Environmental Studies—Aquatic Forum 2004, IAEA-CN-118/157, Monte Carlo, Monaco, 25–29 October 2004, pp. 533-537. Saez-Vergara, J.C., Thompson, I.M.G., Gurriaran, R., Dombrowski, H., Funck, E., Neumaier, S., 2006. The second EURADOS intercomparison of national network systems used to provide early warning of a nuclear accident. Radiat. Prot. Dosim. 123, 190–208. Wieslander, J.S.E., et al., 2008. Low-level gamma-ray spectrometry for analysing fusion plasma conditions. Nucl. Instrum. Methods A 591, 383–393.