Journal of Nuclear Materials 431 (2012) 60–65
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Journal of Nuclear Materials
journal homepage: www.elsevier.com/locate/jnucmat
Post-irradiation analysis of the tantalum container of an ISOLDE LBE target q
E. Noah a,⇑, V. Boutellier b, R. Brütsch b, R. Catherall a, D. Gavillet b, J. Krbanjevic c, H.P. Linder b, M. Martin b,
J. Neuhausen b, D. Schumann b, T. Stora a, L. Zanini b
a
CERN – European Organization for Nuclear Research, CH-1211 Geneva, Switzerland
PSI – Paul Scherrer Institute, CH-5232 Villigen, Switzerland
c
EPFL – Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
b
a r t i c l e
i n f o
Article history:
Available online 16 November 2011
a b s t r a c t
CERN–ISOLDE operates a range of oxides, carbides, refractory metal foils and liquid metal targets for the
production of radioactive ion beams. Following irradiation with a pulsed beam of 1 GeV and 1.4 GeV protons at temperatures reaching 600 °C, the tantalum container of a liquid lead bismuth eutectic (LBE) target was examined. A thin layer of Pb/Bi was observed on the inner surface of the container. A sample of
the surface prepared using the focused ion beam technique was investigated using SEM and EDX. Results
show a higher concentration of bismuth at the interface with tantalum and micron-sized cracks in the
tantalum filled with LBE. Implications of these results for the lifetime of the target container which
has been known to fail under pulsed beam operation are discussed.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
Since the early 1990s, the lifetime of CERN–ISOLDE [1] liquid
metal targets has been of concern to the operation of the facility
and in particular the reliable delivery of radioactive ion beams.
ISOLDE operates many different types of targets each of which is
matched to one of a number of different ion sources to optimize
the production, release and ionization of specific radioactive species whilst simultaneously suppressing unwanted contaminants.
Most of these targets are in the solid state: oxides, carbides and
refractory metal foils. Each year however, a few liquid metal
targets are operated, usually lead for the production of mercury
isotopes or tin for the production of cadmium isotopes for solid
state physics. The transition of incident proton beams from the
quasi-continuous 600 MeV proton beam of the Synchrocyclotron
(SC) to the pulsed 1 or 1.4 GeV beam of the Proton-Synchrotron
Booster (PSB) led to a dramatic increase in the failure rate of liquid
metal targets. Proton-induced pressure waves led to the breaking
of welds on stainless steel and later tantalum containers with
post-irradiation analysis showing the first signs of pitting damage
due to cavitation and cracks (0.05 by 1 mm) caused by thermomechanical cycling [1].
Changing the geometry of the target container and the characteristics of the pulsed proton beam led to a significant reduction
in the failure rate of liquid metal targets. Post-irradiation analysis
q
IWSMT – 10th International Workshop on Spallation Materials Technology,
October 18–22, 2010, Beijing, China.
⇑ Corresponding author. Present address: European Spallation Source ESS AB, SE
22100 Lund, Sweden. Tel.: +41 227678781.
E-mail address: [email protected] (E. Noah).
0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnucmat.2011.11.034
of the improved target container had yet to be carried out until
dedicated tests of the release of volatile products from a lead–
bismuth eutectic target were conducted online at ISOLDE within
the framework of the MEGAPIE target.
Although the release of volatiles was investigated online at ISOLDE [2,3], further measurements were deemed necessary to determine whether the release of noble gases had been complete.
Transport of the target to PSI was organized and the following programme of post-irradiation analysis was established:
– Investigation of target structure and LBE.
– Search for damage due to pulsed-beam operation (cavitation,
thermo-mechanical stress-induced cracks).
– Search for other signs of pulsed-beam operation (splashing of
liquid metal).
First results of this post-irradiation analysis of the structural
components have been presented elsewhere [4], further results
and their relevance for ISOLDE liquid metal targets are presented
in this document.
2. Irradiation conditions
The ISOLDE LBE target #264 was irradiated with 1.4 GeV protons
at ISOLDE in 2004 (28th April – 5th May & 26th August – 29th August) and again in 2005 (9th August – 23rd August), with
1.3 1017 and 1 1018 protons in 2004 and 2005 respectively.
A cross-section through the target and its enclosure is shown in
Fig. 1. It consists of a horizontal cylindrical structure made of
tantalum 200 mm long with an outer diameter of 22 mm,
E. Noah et al. / Journal of Nuclear Materials 431 (2012) 60–65
61
bunch spacings which can be varied. The target discussed here
operated with the 2004/2005 settings, the ‘‘STAGISO’’ settings,
which are applied for liquid metal targets and differ from the settings applied to solid state targets in that the pulse duration is increased by a factor >10 by varying the bunch-to-bunch spacing.
The acceptable proton pulse intensity is a factor 3 lower for liquid
metal targets than it is for all other targets. For a pulse intensity of
1 1013 protons per pulse (ppp), the peak energy density in LBE is
15.7 J/g. To illustrate the ongoing developments with liquid metal
targets at ISOLDE, note that the bunch-to-bunch spacing was increased in 2009 from 10 ls to 16 ls when it was demonstrated
that constructive interference of successive bunches led to increased pressure pulse peaks with the 10 ls setting [5].
3. Post-irradiation analysis
Fig. 1. LBE target container in its enclosure unit, the cross at the level of the target
container indicates the proton beam axis. Isotopes generated in the target container
volume travel through the transfer line to the ion source.
Fig. 2. The beam entrance window is up to 6 mm thick, reinforced
to mitigate the effects of the pressure wave generated by the proton beam, the thickness of the barrel section is 1.0 mm. A ring is
added to the outer rim of the barrel section to strengthen the point
where it is connected to the beam entrance window.
Approximately 550 g of LBE was loaded into the target container by inserting solid LBE pieces roughly 0.1 cm3 into the target
container through the vertical chimney, in several separate stages,
heating the target to beyond the melting point of LBE after each
stage. The LBE did not fill the container completely so that a free
surface exists, a crucial feature to allow radioisotopes to diffuse
out of the target melt and effuse through to the ion source. The
path from target to ion source is assured by the so-called ‘‘transfer
line’’, a temperature-controlled tube located halfway down the target container axis and perpendicular to it (and hence to the beam
axis). A special helix is inserted into the vertical transfer line with
the primary function of stopping any metal (liquid or vapour) from
reaching the ion source. A copper block installed around the transfer line is cooled with a constant flow of water to ensure that all
LBE vapours condense on the helix. A tantalum filament is also
integrated into the copper block to regulate the transfer line temperature, ensuring a large range of operating temperatures either
side of the melting point of lead or LBE. On some lead or LBE targets, the transfer line is occasionally blocked by condensed lead
or LBE and must be heated to melt residues and allow them to flow
back into the target container. The target container is wrapped in
several layers of refractory metal foils that provide some thermal
insulation when the target is heated to operational temperatures.
The proton beam conditions are shown in Table 1. Each proton
pulse consists of a microstructure of many bunches separated by
The target was allowed to cool down for a couple of years from
2005. It was then removed from its enclosure and shipped to PSI in
2007 with cooling block and insulation foils as shown in Fig. 2. The
measured activity for 20 isotopes and dose rate at 10 cm when
transport occurred were 3.8 108 Bq and 3 mSv/hr compared with
estimated values from FLUKA [6,7] calculations of 1.9 108 Bq and
1.7 mSv/hr.
Work on the target started in May 2008 at the PSI hot cell 5. First
the insulation foils were removed, the copper cooling/heater block
was then disconnected from the transfer line. The target was cut
as described in Fig. 2 enabling further investigations of the beam entrance window (cuts 4, 6, 7 and 8), the rear of the target (cuts 1 and
2), the transfer line (cuts 3, 5 and 10) and the free surface (cut 9).
3.1. Visual inspection
Large droplets of LBE up to 3 mm in diameter are clearly visible
on the tip of the helix within the transfer line, Fig. 3. This tip lies
approximately 10 mm above the free surface of the LBE. The presence of these droplets could be due to condensation of lead/bismuth vapours. If this were the case, an enrichment of bismuth
would be expected due to its higher concentration in the vapour
phase (Bi/Pb ratio of 3.0 in the vapour phase compared with 1.2
in LBE [8]), detectable by measuring the specific activity of bismuth
radioisotopes. Another plausible explanation for the presence of
these droplets is that the LBE splashes around in the target container upon impact of the pulsed proton beam. This effect has been
investigated on lead and tin targets and reported in [5] with both
simulations of the dynamic response of the target and measurements of the release of radioactive isotopes as a function of proton
beam time structure and intensity.
The free surface of the LBE exposed following cut 9 in Fig. 2 is
shown in Fig. 4. The LBE was operated well above its melting point
and then cooled down progressively to room temperature over
several hours. Needle-like features can be observed on the surface
likely due to recrystallisation of lead/bismuth.
Fig. 2. Left: Photo of the target vessel shipped to PSI showing the horizontal cylinder of target container wrapped in insulating refractory metal foils and showing the copper
cooling block around the vertical transfer line. Right: Main cuts applied to the PbBi target, labelled according to cutting sequence.
62
E. Noah et al. / Journal of Nuclear Materials 431 (2012) 60–65
Table 1
Evolution of ISOLDE proton beam parameters on liquid metal targets. Corresponding values for solid state targets are shown in brackets. The repetition frequency and pulse
intensities are maximum values. The beam profile is indicated for different beam intensities: i6.5 1012 ppp, ii2 1012 ppp, iii7 1012 ppp, iv3 1013 ppp.
Parameter
1990s
2004/2005
2009
Energy (GeV)
Number of bunches
Bunch width (ns)
Bunch spacing (ls)
Pulse duration (ls)
Repetition frequency (Hz)
Pulse max. intensity (1012 protons)
FWHM, v h (mm)
1, 1.4
5 3 (20)
48
4 0.072, 2 10 (0.072)
20 (2.4)
0.833
10 (33)
11 7i
1, 1.4
3 (4)
200
10 (0.6)
20 (2)
0.833
10 (33)
7.8 7.4ii
1, 1.4
3 (4)
200
16 (0.6)
32 (2)
0.833
10 (33)
6.7 6.4ii
8.3 10.7iii
17.7 19.5iv
Fig. 3. Left: LBE droplets on the tip of the tantalum helix and the inner surface of the transfer line. Right: Autodyn simulations showing splashing due to the pulsed proton
beam in an ISOLDE lead target [5].
to optimize production of radioisotopes as close as possible to the
free surface where the LBE diffusion path is shortest.
3.2. LBE removal
In order to further characterize the beam entrance window, the
LBE had to be removed from it. It was heated and the LBE ingot detached easily from its tantalum containment, as shown in Fig. 5.
This is in sharp contrast with the rear of the target where much
of the LBE remained on the tantalum after heating, Fig. 6.A dark deposit can be seen in Fig. 6 (right). An X-ray powder diffractogram
showed this material to be amorphous. Thus, no information about
the present phases could be obtained. The dark deposit was also
studied by c-spectrometry. The results indicate a strong enrichment of spallation products in the sample, compared to the bulk
of LBE. A complete chemical analysis was not performed so far.
The target container had a slight inclination with respect to the
horizontal plane with the front of the target lower than the rear as
is apparent from the empty volume above the free surface of LBE,
Figs. 5 and 6 (left). During operation, maintaining the free surface
of the LBE parallel to the axis of the proton beam is a requirement
Fig. 4. Free surface of the LBE.
3.3. Beam entrance window analysis
The beam entrance window was cut into two parts, both of
which were investigated with a scanning electron microscope
(SEM) and neither of which showed any obvious signs of degradation. Fig. 7 shows a thin layer of PbBi covering the tantalum. This
layer does not cover evenly the tantalum surface and a sub-mm
structure of roughly parallel channels 5–10 lm wide extending
over lengths >1 mm can be seen. The PbBi layer is thought to be
too thin to cover up any significant cracks such as the one (0.05
by 1 mm) reported in [1]. Its thickness was confirmed by SEM/
EDX studies on samples prepared by the focused ion beam technique described in [4]. The samples were mounted into the FIB
apparatus (Zeiss, NVision 40) such that the incident ion beam
was perpendicular to the LBE layer. With 3–13 nA 30 kV FIB, a
30 lm deep window was cut into the tantalum, through the LBE.
The PbBi layer on the surface of the tantalum is 5–15 lm thick,
with a migration of Bi towards the tantalum container forming a
pure Bi layer 1–2 lm thick, Fig. 8 [4]. Data in the literature concerning the interaction of tantalum with either lead or bismuth
are sparse, in particular no phase diagrams could be found. The
available literature sources state that no tantalum was detectable
in bismuth that was in contact with tantalum cladding at temperatures up to 950 °C for up to 5000 h (detection limit 6 ppm) [9].
According to theoretical predictions the solubility of tantalum in
lead or bismuth should be <8 atom-ppm (in Pb) and <170 atomppm (in Bi) at 500 °C [10]. For the homologue niobium, the
solubility in bismuth is reported to be 50 ppm at 500 °C. In consequence, only very small amounts of tantalum are expected to be
dissolved from the target hull. Furthermore, tantalum is also
E. Noah et al. / Journal of Nuclear Materials 431 (2012) 60–65
63
Fig. 5. Left: Beam entrance window seen from the cut plane before LBE removal. Middle: Beam entrance window seen after LBE removal by heating. Right: LBE ingot that fell
out of the beam entrance window.
Fig. 6. Left: Rear of target seen from cut plane with LBE. Right: Rear of target after partial removal of LBE by heating.
Fig. 7. Scanning electron micrographs of the inner surface of the tantalum beam window at two different scales.
Fig. 8. Left: SEM picture of sample cut by FIB. Right: EDX showing layers of Bi then Pb + Bi on tantalum bulk, the tantalum is blue, the intermediate bismuth layer (thickness
2 mm) is green and the orange is PbBi. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
produced by nuclear reactions in the liquid target material itself.
Moreover, the most likely detectable tantalum isotope, 182Ta with
a half-life of 114 d, had largely decayed by the time the c-spectroscopy was carried out. Therefore, this isotope was not detected in
any of the c-spectra taken from LBE samples. It is uncertain that
a high precision analysis technique could determine whether a
small concentration of tantalum originates from dissolution or
from a nuclear reaction process.
64
E. Noah et al. / Journal of Nuclear Materials 431 (2012) 60–65
Fig. 9. Left: SEM picture of sample cut by FIB. Right: EDX showing a small Pb/Bi-filled crack, the tantalum is shown in blue, the green is bismuth and the orange is PbBi. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
A PbBi-filled crack can be seen in Fig. 9. The crack is approximately 20 lm deep, with a crack opening of 1 lm. Although
the observed crack would likely not affect the structural integrity
of the target, it is apparent that the PbBi wets down to those lm
scales, raising the prospect of crack propagation by liquid metal
embrittlement once a crack develops.
3.4. LBE sample analysis
Analysis of samples extracted from the irradiated LBE was
planned for the following items:
– Noble gas isotopes: precision mass spectrometry on 1 mg
samples.
– Various nuclides: gamma spectroscopy.
– Po: alpha spectroscopy.
A strong enrichment of electropositive elements, e.g. alkaline
earth metals, lanthanides and early transition metals, represented
by radioactive nuclides such as 133Ba, 172,173Lu and 172Hf was
observed at both the former liquid metal-vacuum interface
(Fig. 6) and the interface between liquid metal and the tantalum
hull, while noble metals such as silver (110mAg) seem to be homogeneously distributed in LBE [4,11,12].
Earlier investigations on polonium-containing LBE samples
showed enrichment of Po in a thin layer of a few mm thickness
near the surface of LBE droplets [4,11–13]. For Po-analysis in
the ISOLDE target, samples of approximately 100 lm thickness
were taken at four different depths in the LBE: free surface, just
below free surface, bulk, interface with inner bottom surface of
tantalum container. These samples have been analyzed for
208,209,210
Po by alpha-spectrometry [11,12]. The results show
slightly increased concentrations of all three polonium isotopes
in the near surface samples taken from the free surface and the
bottom surface of the container, but the limited precision of the
alpha-spectrometry results as well as the low spatial resolution
of the sample taking procedure preclude a definite conclusion
on whether the above mentioned surface enrichment phenomena
occurred also in the present target. The absolute values of the
measured polonium activity concentrations agree reasonably well
with predictions using nuclear codes [4].
4. Discussion
Post-irradiation analysis of a LBE target irradiated at ISOLDE
indicated that noble gases had been almost completely released
from the melt during operation when the target reached 600 °C.
Enrichment of specific nuclides close to the free surface was observed and specific activities of 208,209,210Po were measured at different depths within the melt.
There is a marked reduction in the failure rate of liquid metal
target systems online at ISOLDE since the early 1990s. An interlock
system has been implemented and is activated when liquid metal
targets are operational at ISOLDE setting a proton beam intensity
threshold to 1 1013 ppp and locking the proton beam time structure to the ‘‘STAGISO’’ mode with extended bunch-to-bunch spacings. The bunch-to-bunch spacing was recently extended from
10 lm to 16 lm to avoid potential constructive interference of
pressure peaks due to the pulsed beam.
Analysis of the tantalum target container showed no damage to
the tantalum beam entrance window, a regular failure site for past
liquid metal targets. A 5–15 lm layer of PbBi was observed on the
tantalum, with a transition region between tantalum and PbBi consisting of a 1–2 lm layer of Bi.
A 100 kW liquid metal target [14] was proposed for the EURISOL facility, a future Isotope Separation Online facility for Europe
[15]. The layout for such a target is no longer a static container as is
currently operated at ISOLDE but a liquid metal loop with a diffusion chamber for extraction of radioisotopes, pump and heat exchanger to manage the heat deposited by the beam. The
prototype for such a loop is planned for tests at ISOLDE. Careful
investigations are required before lifting the 1 1013 ppp threshold on such a target, especially in the context of potentially higher
pulse intensities available through the HIE-ISOLDE project than the
3.3 1013 ppp currently available at ISOLDE.
5. Summary
Cavitation-induced pitting and thermal stress-induced cracks
were confirmed by previous workers to be the origin of liquid metal target containment failures at ISOLDE in the 1990s. Mitigation
of these effects was addressed by lengthening the proton beam
pulse from sub-ls to 16 ls, by defocusing the beam hence reducing the beam energy density and by modifying the geometry of
the target container. The post-irradiation examination of the latest
geometry target reported here did not reveal any signs of cavitation-induced pitting or thermal stress-induced cracks. Although
the exact threshold at which cavitation-induced pitting occurs is
not determined, it is demonstrated that for equivalent average
beam powers, for a pulsed proton beam, by carefully choosing proton beam parameters and optimizing the target geometry
cavitation-erosion can be mitigated.
E. Noah et al. / Journal of Nuclear Materials 431 (2012) 60–65
Acknowledgements
We acknowledge the financial support of the European Community under the FP6 ‘‘Research Infrastructure Action – Structuring the European Research Area’’ EURISOL DS Project Contract
No. 515768 RIDS. The EC is not liable for any use that can be made
on the information contained herein.
References
[1] J. Lettry et al., Nucl. Instrum. Methods Phys. Res. Sect. B 204 (2003) 251–256.
[2] L. Zanini et al., Volatile elements production rates in a 1.4-GeV protonirradiated molten lead-bismuth target, in: R.C. Haight et al. (Eds.),
International Conference on Nuclear Data for Science and Technology, AIP
Conference Proceedings, vol. 769, Melville, New York, 2005, pp. 1525.
[3] Y. Tall et al. (Eds.), Proceedings of the International Conference on Nuclear Data
for Science and Technology, Nice, 2007, pp. 1069.
[4] L. Zanini et al., Post-irradiation analysis of a LBE-filled ta target irradiated at
ISOLDE, in: Proceedings of TCADS Conference ‘‘Technology and Components of
Accelerator-Driven Systems’’, Karlsruhe, 2010 <www.oecd-nea.org/science/
wpfc/tcads/1st/index.html>.
[5] E. Noah et al., Nucl Instrum Methods Phys Res Sect B 266 (2008) 4304–4307.
65
[6] G. Battistoni et al., The FLUKA code: description and benchmarking, in: M.
Albrow, R. Raja (Eds.), Proceedings of the Hadronic Shower Simulation
Workshop 2006, Fermilab 6–8 September 2006, AIP Conference Proceeding,
vol. 896, 2007, pp. 31–49.
[7] A. Fassò et al., Fluka: a multi-particle transport code, CERN-2005-10, INFN/
TC_05/11, SLAC-R-773, 2005.
[8] S. Ohno et al., J. Nucl. Sci. Tech. 42 (7) (2005) 593–599.
[9] Landolt-Börnstein, Numerical data and functional relationships in science and
technology, new series, in: O. Madelung (Ed.), Group IV: Macroscopic and
Technical Properties of Matter, vol. 5, Phase Equilibria, Crystallographic and
Thermodynamic Data of Binary Alloys, Springer; Berlin, 1991.
[10] J. Neuhausen, Report on Semi-Empirical Modeling of Thermodynamical Data
for the Assessment of the Solubility and Chemical Behavior of Nuclear Reaction
and Corrosion Products in a liquid Lead-Bismuth Spallation Target, Deliverable
D4.32, DM4 DEMETRA, EUROTRANS.
[11] J. Neuhausen et al., Ann. Rep. Lab. of Radio- and Environ. Chemistry, Uni. Bern
& PSI, 2010, pp. 34–37.
[12] J. Neuhausen et al., Proceedings of DAE-BRNS Symposium on Nuclear and
Radiochemistry NUCAR-2011, Visakhapatnam, India, 2011.
[13] T. Miura et al., Appl. Radiat. Isot. 61 (2004) 1307–1311.
[14] E. Noah et al., Driver beam-led EURISOL target design constraints, EPAC08.
CERN-AB-2008-052, in: Eleventh European Particle Accelerator Conference,
Genoa, Italy, 2008 (MOPC090).
[15] J.C. Cornell (Ed.), Final Report of the EURISOL Design Study, Ganil, BP 55027,
14076 Caen cedex 5, France, 2009.