ARTICLE IN PRESS
Applied Radiation and Isotopes 61 (2004) 167–172
Underground measurements of radioactivity
M. Laubensteina,*, M. Hultb, J. Gasparrob, D. Arnoldc, S. Neumaierc,
e
.
G. Heusserd, M. Kohler
, P. Povinecf, J.-L. Reyssg,
i
!
M. Schwaigerh, P. Theodorsson
a
Laboratori Nazionali del Gran Sasso, INFN, S.S. 17/bis km 18+910, Assergi(AQ), I-67010 Italy
European Commission, Joint Research Centre, Institute for Reference Materials and Measurements (EC-JRC-IRMM),
Retieseweg, Geel, B-2440 Belgium
c
Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig, D-38116 Germany
d
Max-Planck-Institut fur
. Kernphysik, P.O. Box 103980, Heidelberg, D-69029 Germany
e
Verein fur
. Kernverfahrenstechnik und Analytik Rossendorf e.V., P.O. Box 510119, Dresden, D-01314 Germany
f
IAEA - Marine Environment Laboratory,4 Quaie Antoine 1er, Monte-Carlo, MC-98012, Monaco
g
LSCE/Vall!ee, Centre des Faibles Radioactivit!es, Domaine du CNRS, Avenue de la Terrasse, Gif-sur-Yvette Cedex, F-91198 France
h
ARC Seibersdorf Research GmbH, Health Physics Division/Radiation Protection, Seibersdorf, A-2444 Austria
i
Science Institute, University of Iceland, Dunhaga 3, Reykjav!ık, IS-107 Iceland
b
Abstract
The exceptional sensitivity of gamma-ray spectrometry in underground laboratories has increasing application
because of the important science and technology that it allows to be studied. Early work focussed on rare fundamental
phenomena, e.g. double beta decay, but a growing number of underground measurements is being performed in fields
such as environmental monitoring, surveillance of nuclear activities, benchmarking of other physical techniques and
materials selection for equipment which require materials with extremely low levels of radioactivity. This report
describes the state of the art in underground gamma-ray spectrometry. Backgrounds of HPGe-detectors at various
underground laboratories are presented and compared. Improved techniques and detectors are described and needs of
deep underground facilities for higher sensitivity measurements are discussed.
r 2004 Elsevier Ltd. All rights reserved.
Keywords: Underground laboratory; Low-level gamma-ray spectrometry
1. Introduction
The field of underground gamma-ray spectrometry
with germanium detectors was pioneered by studies of
muon interactions (Heusser and Kirsten, 1972). Soon it
was recognized that as for other ionising detector
devices the limiting factors for achieving low background counting rates are the cosmic-ray muons, the
radioactivity of the detector’s construction materials and
the shield, 222Rn in air and neutrons (Heusser, 1995,
*Corresponding author. Tel.: +39-0862-437278; fax: +390862-437570.
E-mail address: [email protected]
(M. Laubenstein).
1994). Unless specially selected materials, the so-called
radiopure materials (materials with low levels of radioactivity), are used for detector and shield construction,
there is not so much to gain from installing a detector
underground. The progress in radiopure material screening was largely driven by the search for double beta
decay in 76Ge (Fiorini et al., 1967; Alessandrello et al.,
1988; Brodzinski et al., 1988), and soon reached the
point where it was useful to place detectors underground
in order to exploit fully the potential of low background.
Currently, many laboratories are engaged in the field of
ultra low-level gamma-ray spectrometry (ULGS, i.e.
with additional background reduction by using antimuon shields or underground laboratories) (Verplancke,
1992), but for aims other than fundamental physics.
0969-8043/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apradiso.2004.03.039
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M. Laubenstein et al. / Applied Radiation and Isotopes 61 (2004) 167–172
A drawback in applications of ULGS is that the
measurements are very timeconsuming. In order to
measure total activities of less than 1 mBq, it is rarely
enough to measure a sample for a day or two and
consequently measurement throughput is low.
It has been recognised that an international collaboration can help to promote development of underground
laboratories and cooperation between them. The Collaboration of European Low-level Underground Laboratories (CELLAR), which is a network of underground
laboratories, initiated its work in the year 2000.
The scope of the collaboration is wide. Some key
points of it are coordination of measuring programmes,
conducting inter-comparisons between partner institutes, joint analyses of samples, and testing of methods
for reduction and measurement of the radon concentration in the air of the underground laboratories.
Moreover, the CELLAR works towards the implementation of new low level radioactivity reference materials
and the exchange of experience related to improvements in measurement techniques of low levels of
radioactivity. The partner institutes perform also joint
research on systematic background studies, radiopurity studies of materials for all type of underground detectors and low-level dose and dose-rate
measurements.
The characteristics of the underground laboratories
that participate in the CELLAR are listed in Table 1.
The uncertainties assigned to the integral counting rate
are the standard uncertainties determined in accordance
to the Guide to the Expression of Uncertainty in
Measurement (ISO/IEC/OIML/BIPM, 1995); In the
case of the Felsenkeller laboratory the fluctuations in
the integral background counting rate are included, too.
Although all laboratories are involved in many different
fields of measurements, each laboratory has a main
activity, which is also indicated in Table 1.
The number of underground laboratories is still
growing world wide. Shallow laboratories with easy
access have proved to be highly suitable for environmental studies with a high number of sample throughput. Deep laboratories, with the lowest detection limits
are suitable for long term measurements (a few weeks or
longer).
2. Background comparison of the cellar underground
laboratories
The overburden of the partner institutes in the
CELLAR varies widely from zero or a few m w.e.
(m w.e. = meter water equivalent, a shielding height of
the material expressed as the water equivalent) (Austrian
Research Centre (ARC) Seibersdorf, Max-Planck-Institut fur
. Kernphysik in Heidelberg (MPI-K-HD)) up to
several thousands of m w.e. (Laboratori Nazionali del
Gran Sasso (LNGS), Laboratoire des Sciences du
Climat et de l’Environnement (LSCE)), which should
be reflected in the achieved background levels. The
nucleonic component of the cosmic-ray-induced background is reduced to a negligible level already with a few
m w.e., while the much more penetrating cosmic-ray
muon part needs much larger overburdens for substantial reduction.
In Fig. 1 the integral background counting rate from
40 to 2700 keV for High Purity Germanium (HPGe)
detectors used in some of the CELLAR-laboratories is
plotted against the shielding depth. The solid line gives
the muon fluence rate as a function of depth. The
integral background counting rate levels off at a certain
depth while the muon fluence rate still decreases
exponentially. This is a clear indication that other
sources which become more important are environmental radioactivity, neutrons from spontaneous fission
and (a,n)-reactions, and residual cosmogenic activation
from the above-ground production of the detector. In
most underground laboratories, the radon gas is
removed carefully from the environment around the
detector.
Fig. 2 displays background spectra from 4 CELLAR
underground laboratories. The spectra are normalised
to the mass of the Ge-crystal. The figure shows, that
the cosmic-ray-induced continuum is significantly reduced at greater depths. A prerequisite for obtaining
low background is, of course, that the detector is
radiopure.
3. Background reduction and further improvements
The shield of an HPGe-detector in an underground
laboratory is typically composed of a thick (15–25 cm)
lead shield of which the inner 2–5 cm are low in 210Pb
(o5 Bq kg1). Often there is an inner lining of freshly
produced electrolytic copper. If the radioactivity of the
lead is very low, the copper shield need not be very thick
(B1 cm), but it is possible to use 15 cm copper deep
underground as the activation of the copper there is very
low. Above-ground copper linings increase the background for energies above 100 keV.
Detector endcaps are mostly made from ultrapure
aluminium (Kryal) or electrolytic copper (Cu). Other
radiopure materials have also been tried, like magnesium, aluminised Mylar and silicon (Si). With Si for
example, two major problems were encountered which
limits its usefulness. First, because of the brittleness of
pure crystalline silicon and second, because of its
transparency to infrared light. Recently, carbon epoxy
windows have been introduced. They are mechanically
relatively strong, and due to their low density they
offer small attenuation to low-energy gamma-rays
and X-rays. Nevertheless, they have usually a rather
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M. Laubenstein et al. / Applied Radiation and Isotopes 61 (2004) 167–172
169
Table 1
List of the underground facilities of the CELLAR network members
Institute
Underground
laboratory
Depth
(m w.e.)
HPGe-detectors
Integral counting
rate [d1 kg1]
(40–2700 keV)
Detector type
Main activity
ARC Seibersdorf research
(Schwaiger et al., 2002)
Max-Planck-Institut fur
.
Kernphysik, Heidelberg
(Heusser, 1986)
IAEA-MEL (Povinec,
2002)
VKTA (Niese et al., 1998)
(Austria)
ca. 1
8200 7 200
Low-level
laboratory
(Germany)
CAVE (Monaco)
15
2012 7 23
p-type extended
range
p-type coaxial
Environmental
radioactivity, CTBT
Rare events research and
detector development
35
840 7 50
p-type well
110
3870 7 30
p-type well
350
—
—
260 7 4
277 7 4
p-type coaxial
p-type extended
range
Environmental
radioactivity
Environmental
radioactivity
Studies of background
components in radiation
detectors
Reference measurements
Reference measurements
87 7 1
p-type coaxial
University of Iceland
!
(Theodorsson,
2003)
Felsenkeller
(Germany)
(Iceland)
IRMM (Hult et al., 2003)
PTB (Neumaier et al.,
2000)
HADES (Belgium)
UDO in the salt
mine Asse
(Germany)
500
2100
LNGS (Arpesella, 1996)
Gran Sasso (Italy)
3800
LSCE (Reyss et al., 1995)
Modane (France)
4800
Radiopurity of
construction materials to
support to rare event
experiments
30 7 1
186 7 2
p-type coaxial
p-type well
Environmental
radioactivity
105
above ground
104
VKTA Rossendorf
103
PTB
LSCE
JRC-IRMM
102
LNGS
101
100
Muon fluence rate [a.u.]
Normalised counting rate [d-1 kg-1 ]
106
10-1
10-2
0
1000
2000
3000
4000
5000
Depth [m w.e.]
Fig. 1. The integral background counting rate from 40 to
2700 keV divided by the mass of the Ge-crystal for the best
HPGe-detectors in some CELLAR laboratories. The solid line
shows the muon fluence rate in arbitrary units normalised to the
background counting rate above ground. All detectors have
only passive shielding.
high intrinsic contamination in 40K and sometimes
in 226Ra, which can limit their use in deep underground
set-ups.
Fig. 2. The background counting rate divided by mass of the
Ge-crystal for different detectors in four CELLAR laboratories
compared to a detector above ground.
Background reduction for low background systems is
achieved first and foremost by careful selection of the
materials for the detector and the passive shield. Many
studies have been made on selecting radiopure materials
for underground detector systems, not only HPGedetectors (Arpesella et al., 2002). Table 2 gives examples
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Co: o18 106
Co: (26 7 17) 106
60
Co: (1.8 7 0.2) 105
207
Bi: (9.6 7 2.6) 105
210
Pb: (2.8 7 0.8) 101
—
210
Pb: (1.1 7 0.3) 101
—
of measurements of radiopure material. The uncertainties stated are the expanded uncertainties obtained by
multiplying the standard uncertainty by the coverage
factor k ¼ 2 (ISO/IEC/OIML/BIPM, 1995), decision
thresholds were calculated according to (ISO, 2000)
using a ¼ 0:5 and b ¼ 2:5 FWHM. The main contribution to the uncertainty is from counting statistics,
followed by the uncertainty of the Monte-Carlo simulations for the calculation of the efficiencies (B5%). The
uncertainties of the emission probability vary from one
radionuclide to another, but are usually small compared
to the first two contributions. Recently, at the LNGS in
collaboration with the MPI-K-HD, some materials used
for shielding have been measured. The copper and the
lead were freshly made and very briefly exposed to
cosmic radiation in order to avoid activation. The Cu is
electrolytic copper (quality NOSV from Norddeutsche
Affinerie, Germany) and the Pb is of Dow Run
quality, produced by JL Goslar, Germany. The measurements reach sensitivities of a few tenths of mBq kg1
for natural uranium and thorium, which is close to
classical mass spectrometry analysis. The ULGS in deep
laboratories (Neder et al., 2000) used for material
screening, has reached the radiopurity level like systems
specially designed for nuclear physics experiments (e.g.
.
double beta decay (Bohm
et al., 1990; Gunther
.
et al.,
1997)).
In order to further lower the detection limits it can be
worthwhile adding active shields. The background of
detector systems located above ground or at depth down
to about 100 m w.e. might be reduced by adding an
active shield, which could reach a reduction factor in the
range from 4 to 10 as for an above-ground system (e.g.
Schwaiger et al., 2002). For 500 m w.e. one can obtain a
40% reduction in the background counting rate from
40–2700 keV (Hult, 2003). At a depth of 3800 m w.e. one
can expect still a reduction of a few percent (Heusser,
2003). This certainly depends on the achieved overall
radiopurity level.
(4.3 7 4.0) 105 (7.5 7 6.4) 105 o2.8 105
(3.4 7 0.9) 103 o1.2 103
(1.5 7 0.6) 103 o9.6 103
1
(1.170.2) 10
—
(0.4 7 0.2)
(2.1 7 0.4) 101
(4.0 7 1.0) 102 (6.3 7 1.4) 102 o0.04
(3.2 7 0.4)
o1.1 102
o3.6 102
—
o1.2
(4.5 7 1.8) 104
58
60
124.7
144.6
Coppera
Leada
o1.8 105
o1.2 105
o1.1 104
o1.6 105
o7 103
Measured at LNGS.
Measured at PTB Braunschweig.
c
Measured at JRC-IRMM.
b
a
21.42
13.57
6.80
1.80
0.001
0.8066
89.89
100.48
4. Discussion
ULB Aluminiumb
Glue & Silver epoxyb
Carbon-epoxyc
Other radionuclides (Bq kg1)
K (Bq kg1)
40
Th (Bq kg1)
228
Ra (Bq kg1)
228
Ra (Bq kg1)
226
U (Bq kg1)
238
Mass (kg) Measuring
live time (d)
Material
Table 2
Radionuclides determined using ULGS in bulk copper and bulk lead for shielding, and some other detector components (coverage factor k = 2 (ISO/IEC/OIML/BIPM, 1995), upper
limits are decision thresholds (ISO, 2000)
170
Currently, deep underground laboratories (> 1000 m
w.e.) are driven by the needs in fundamental physics
with large-scale experiments which require very low
background conditions. The development of ULGS
resulted, indeed, from the need of these experiments to
perform effective materials selection.
In order to fully exploit the gain in lowering the
detection limit obtained by going deep underground, it is
necessary to do long lasting measurements. This causes a
problem with the capacity because of the low throughput. There are, however, detailed plans for some of the
large and deep laboratories (e.g. Gran Sasso and
Modane) for extensions of the ULGS capabilities. This
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is not only because they provide essential support to ‘big
science’ experiments, but also because of new projects
aiming to combine other measurement techniques with
ULGS trying to measure radionuclides at the level of a
few atoms. One could argue that going deep underground is very costly in comparison with the gain of
lowering the detection limits. The pure cost of building
and installing a good system for ULGS is roughly
100,000h. Even though access to the detector may be
difficult, one need only to change samples a few times
each year. The running costs can thus be kept low
because of the limited resources required to operate a
detector. The space required for a detector is small and
recent developments in cooling technique can ease the
frequent filling of liquid nitrogen (LN2). A nice feature
with these HPGe-systems is that they get better with
time as the activation products decay (e.g. 58Co, 60Co,
65
Zn etc.). By pumping the cryostat regularly and by
following strictly the procedures for changing samples,
such that the risk of contamination is kept as low as
possible, it is possible to obtain a lifetime of a HPGedetector of more than 20 years. Performing 10 analyses
per year for 25 years, a lifetime which is has already been
achieved, results in a cost per sample of 400h (not taking
into account interest, deprecation, etc.). If the running
costs can be kept low it can be shown that also deep
underground laboratories can provide ULGS at a
reasonable cost. This will enable the use of these
facilities also in fields outside fundamental physics. A
deep laboratory is, e.g. the obvious place to test the
background performance of new HPGe-detectors to be
installed in more shallow laboratories.
Further reduction of the background of HPGedetectors in the very deep laboratories may be achieved
either by germanium enriched in the stable isotopes
72,73,74
Ge, at least for very special applications, or
growing the germanium crystal directly underground.
Already today, construction and shielding materials are
stored as long as possible underground in order to
reduce the activation of cosmogenic radionuclides.
Commercial detector manufacturers can today also
provide special HPGe-systems like e.g. multi-segmented
crystals or detectors housing multiple crystals. These
new systems can apply Compton suppression or coincidence operations for background reduction in order to
optimise the sensitivity for certain radionuclides.
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