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Experience with in-pile fission targets at LOHENGRIN
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 613 (2010) 363–370
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Experience with in-pile fission targets at LOHENGRIN a, ¨ U. Koster , H. Faust a, T. Materna a, L. Mathieu b a b
Institut Laue Langevin, 6 rue Jules Horowitz, F-38042 Grenoble Cedex 9, France CEA Cadarache, DEN/DER/SPRC/LEPh, F-13108 Saint-Paul-lez-Durance, France
a r t i c l e in fo
abstract
Available online 5 November 2009
The LOHENGRIN fission fragment separator uses actinide targets in a neutron flux of about 5 1014 neutrons=cm2 =s in an in-pile position of the high-flux reactor of ILL Grenoble. For fission yield measurements relatively thin targets (tens of mg=cm2 ) are used, while for nuclear spectroscopy applications targets up to 1 mg=cm2 are employed. This leads to fission rates up to 5 1012 =s. The targets are heated by the fission power in vacuum to temperatures of up to 1000 3 C. The radiation damage caused by the fission fragments can reach 50 dpa (displacements per atom) per day, an extremely high value comparable to that caused by irradiation with intense heavy ion beams. Therefore the thick targets that were produced with different methods (painting, spray-painting, electrolysis and molecular plating) all suffer from a burnup that is much quicker than explainable by nuclear transmutation. We discuss physical effects responsible for this additional decrease in fission fragment rate and ways to improve the situation. & 2009 Elsevier B.V. All rights reserved.
Keywords: Actinide targets High intensity beams Radioactive ion beams Radiation damage Nuclear heating Self-sputtering
1. Introduction The fission fragment separator LOHENGRIN [1] at the high-flux reactor of Institut Laue Langevin in Grenoble provides mass- and energy-separated fission fragment beams since 35 years. Part of the experiments are devoted to a precise determination of yields from thermal neutron-induced fission of various actinide targets. Another important application are nuclear decay spectroscopy experiments with short-lived fission fragments. Due to the short transport time through the separator (122 ms) even the decay of isomeric states with half-lives down to 0:5 ms can be studied [2]. For long time LOHENGRIN was mainly used for precise measurements of kinetic energy distributions of fission fragments and their corresponding fission yields. Precise measurements of the kinetic energy distributions require thin targets to minimize the corrections for energy loss in the target. Precise measurements of fission yields are based on many individual measurements with different separator settings where the mass, ionic charge state and kinetic energy are varied. Since there is no way to monitor directly the evolution of the fission rate of the target these individual measurements have to be normalized to the fission rate by repeated measurements of a reference mass. Therefore precise fission yield measurements rely on a smooth evolution of the fission rate with time that is typical for relatively thin targets.
Corresponding author.
¨ E-mail address: [email protected] (U. Koster). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.09.078
In recent years LOHENGRIN became more and more used as radioactive ion beam (RIB) facility where nuclear spectroscopy experiments are performed with mass-separated fission fragments close to the focal plane. The success of such experiments is directly linked to the intensity of these RIBs. This is a strong incitation to boost the intensity of the available RIBs. After a description of typical parameters of LOHENGRIN targets we will discuss in the following which target parameters can be varied to optimize the RIB intensity and which limitations are encountered. Some of these physical limitations are also encountered by actinide targets exposed to intense charged particle beams. Hence, operational experience with LOHENGRIN targets may serve for judging and optimizing the target performance, e.g. for future radioactive ion beam facilities that are planning to irradiate actinide targets with high intensity heavy ion beams.
2. Limitations for fission target performance The LOHENGRIN targets consist of a layer of an actinide compound deposited on a thick backing that is usually made from 0.2 mm thick high purity titanium sheets of 9 2 cm2 . The backing is fixed by spot-welding on a Ti support frame as shown in Fig. 1. Usually a diaphragm with a cover foil is fixed on top of the target, see Section 5. The ensemble is introduced via a glove box, then placed by a gripper on the front part of a trolley that brings the target in an evacuated beam tube to an in-pile position about 0.5 m from the reactor core where the targets are exposed
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and 243Cm the activity limit translates to a limited target mass of 1 and 2 mg, respectively. 2.3. Neutronics limitation A 1 mg=cm2 thick target with 1000 b neutron capture crosssection will result in a local flux depression of less than one percent. Moreover it has no significant effect on the reactor criticality ( o 1 pcm). Therefore neutronics limitations play no role for the fission target design. However, materials with high neutron capture cross-section have to be avoided as thick backing material. 2.4. Target thickness limitation
Fig. 1. Photo of a target mounted on the support frame ready to be placed onto the trolley that brings it into the beam tube. The target is covered with a 0:25 mm thick Ni foil on a 70 5 mm2 diaphragm.
to a thermal neutron flux of about 5 1014 cm2 s1 , see Fig. 5. After irradiation for several days to weeks, the trolley is pulled back, the target is taken by the gripper and dropped into a shielded waste container.
2.1. Geometric limitations The mass dispersion of the LOHENGRIN spectrometer in the focal plane is Dm ¼ 3:24 m and the magnification from the object (target) to the image (focal plane) is 1:1. Hence, a 3 mm wide target with a 4 mm (to account for possible aberrations of the spectrometer) wide mass-defining slit will provide a mass resolution of DA=A 1=950 and a 10 mm wide target with a 11 mm wide slit a still acceptable mass resolution of DA=A 1=350. The energy dispersion of the parabola spectrometer perpendicular to the mass dispersive plane is DE ¼ 7:2 m. Hence, a 7 cm long target will introduce an uncertainty in the kinetic energy measurement of 1% which is usually acceptable. Much longer targets cannot be used due to geometric limitations of the target changing mechanism and the acceptance of the spectrometer (fixed diaphragms). Hence the surface of the actinide layer may vary from few mm2 (for very rare actinide isotopes) to about 75 10 mm2 at maximum.
LOHENGRIN is a recoil separator that separates the fission fragments at the kinetic energy that they receive in the fission process. Light fission fragments (70 oA o 110) have typically kinetic energies of 90–110 MeV and heavy fission fragments (130o Ao 160) have kinetic energies of 50–80 MeV. With increasing target thickness the fission fragments emitted from the back of the target will suffer more energy loss from electronic stopping. Hence, the average kinetic energy of the fission fragments will decrease and the width of the observed kinetic energy distribution will increase compared to the intrinsic one that is determined by the fission process. These effects will directly limit the maximum rate of separated fission fragments since the energy acceptance dE of the LOHENGRIN spectrometer is a fixed percentage of the average kinetic energy: dE=E 5% when using the RED (Reverse Energy Dispersion) magnet [3]. Eventually the addition of more target material will only contribute to the low-energy tail of the kinetic energy distribution but no longer enhance the useful intensity at the peak. Fig. 2 shows simulated kinetic energy distributions of 98Y ions from 235UO2 targets of 5021400 mg=cm2 thickness assuming an intrinsically Gaussian energy distribution with 100 MeV average energy and 10 MeV FWHM. The simulations were performed with the Monte Carlo program SRIM [4,5]. The energy-dependent transmission through the LOHENGRIN spectrometer is already taken into account. Integration of the ion rate in a 5% energy bin at the peak of the distribution shown in Fig. 2 gives the maximum ion rate available
2.2. Legal limitations For nuclear spectroscopy applications usually relatively thick targets of the fissile isotopes 233U, 235U, 239Pu and 241Pu are being used. Due to the very high neutron flux also the fertile isotope 241 Am provides a significant fission rate after breeding to the fissile 242g;m Am. For fission yield studies moreover targets made from the fissile isotopes 229Th, 245Cm, 249Cf, 251Cf and from the fertile isotope 237 Np have been used. New interesting experiments could be performed if targets made from 231Pa, 236Np, 242mAm, 243Cm or 247 Cm would become available. The legal limit for the activity of a single target is 3.7 GBq (0.1 Ci). With sufficient enrichment of the isotopes mentioned above this limitation is usually less tight than the physical limitations discussed below. Only for the relatively short-lived isotopes 241Pu
Fig. 2. SRIM simulation of 98Y kinetic energy distributions from 50 to 1400 mg=cm2 UO2 targets folded with the energy-dependent transmission of LOHENGRIN.
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at the focal plane, see Fig. 3. It is obvious that the rate saturates at about 1 mg=cm2 target thickness. Any addition of non-fissile material in the target layer (i.e. additives or impurities of the target material), or in front of it (e.g. a cover layer of foil, see Section 5) will reduce the kinetic energies and broaden the energy distribution, thus further reducing the transmission. Therefore the observed ion rates are usually lower than these calculated ideal ones. Note that this thickness limitation was derived for typical binary fission fragments. Ternary fission fragments with low Z and high kinetic energy per nucleon have much lower specific energy loss. Thus in principle even thicker targets could be used for measuring yields of very rare ternary fragments like 3He, 11Li, 14 Be, etc. [6].
2.5. Temperature limitation Half of the fission fragments are emitted backwards and deposit their kinetic energy in the thick backing. The forward emitted fragments loose, depending on the target thickness, only a relatively small part of their kinetic energy in the target layer. Thus, e.g. for a 400 mg=cm2 235U target the fission heating amounts to about 4:5 W=cm2 . The nuclear heating by gamma rays from the reactor core amounts to about 1.3 W/g at the target position, i.e. 0:12 W=cm2 for a 0.2 mm thick Ti sheet. The target and the backing are sufficiently thin that thermal conduction keeps the temperature gradient perpendicular to the
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area negligibly small ( o1 3 C), but they are too thin to allow for efficient heat conduction in lateral direction. Instead the heat is mainly dissipated by radiation to the zircalloy beam tube that is kept by external water cooling at about 50 3 C. There is no convective cooling due to the low pressure ( o 104 mbar) in the beam tube. Neglecting heat conduction, the maximum target temperature for large area targets can thus be estimated with the Stefan– Boltzmann law: pheat ¼ sðet þ eb ÞðTt4 323K 4 Þ:
ð1Þ
The target-backing-sandwich radiates to the front side with the emissivity of the target material et and to the back side with the emissivity of the backing material eb . Fig. 4 shows the calculated maximum temperatures as function of target thickness and emissivities. The calculated temperature can be validated by observing the hot target in situ with two instruments [7]: The relative temperature distribution is deduced from the relative brightness of the target surface that is measured with an infrared sensible CCD camera via a telelens, see, e.g. Fig. 8. The absolute temperature is measured with an infrared pyrometer (type KTR2000-L-P with 8.0/500 mm mirror lens from Dr. Georg Maurer GmbH, http://www.maurer-ir.de). To avoid the radiation field of gamma rays and fast neutrons in direction of the beam tube both instruments are observing the target indirectly via a periscope system made from two gold-plated mirrors, see schematic setup in Fig. 5. When a target cover foil (see Section 5) is used it has often a bad thermal contact to the target and backing. Hence, it may act as a heat screen, resulting in a further rise of the target temperature. When the cover foil is viewed with the pyrometer only a lower limit of the real target temperature can be measured. The elevated temperature of the target may have various effects, most of them detrimental to the target survival:
evaporation of volatile target materials (e.g. thick targets made from UF4 cannot be used),
promotion of chemical reactions (e.g. reduction of UO2 target layer in contact with carbon or titanium backing or cover),
thermal diffusion of target atoms into the backing or cover, thermal stress due to different thermal expansion of target and backing material and Fig. 3. Calculated initial ion beam intensity for A ¼ 98 isobars in the focal plane of the RED magnet.
outgassing of volatile fission products stopped in the backing. In principle at an elevated target temperature part of the radiation induced damages (vacancies, etc.) could be annealed, but clearly the negative effects outweigh this possible advantage.
Fig. 4. The maximum target temperature was estimated as function of the target thickness and the emissivities of target and backing: et ¼ 0:4 for the UO2 layer, eb ¼ 0:3 for a Ti backing, eb ¼ 0:8 for a graphite backing and et ¼ 0:1 as effective emissivity when a glossy cover foil is placed in front of the target without direct thermal contact.
Fig. 5. Set-up of the LOHENGRIN fission fragment separator at the high-flux reactor of ILL Grenoble.
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2.6. Outgassing of volatile fission products Only a tiny fraction of the produced fission fragments is emitted in the solid angle covered by the acceptance of the spectrometer (3 105 sterad). Half of the fission fragments are emitted forward. Those not stopped in the diaphragm or cover foil will be implanted into the water cooled beam tube where they will remain until they finally have decayed to a stable isotope. The other half of the fission fragments are stopped in the backing. At sufficiently high temperature a fraction of the most volatile fission products may diffuse to the surface of the backing, desorb and migrate through the beam tube. Thus, unintentionally the backing may act as an ISOL-type (isotope separation on-line) catcher where the reaction products are first stopped, then thermally diffuse and effuse to the ion source [8]. Without ion source the radioisotopes will just effuse as neutral atoms through the vacuum system. For thick targets ( 4 300 mg=cm2 target thickness) of isotopes with high fission cross-section that reach correspondingly high target temperatures, a noticeable increase of the radioactive background in the vacuum system of the LOHENGRIN separator has been observed, creating ‘‘hot spots’’ in turbopumps, charcoal filters and the primary pump. A small fraction of the volatile activity even migrates to the focal plane resulting in gamma-ray background for the Ge detectors. Thus gamma rays of the volatile fission products 85–91Kr, 133–140Xe, 131–135I could be observed. To limit this disturbing emanation of volatile fission products one has to hinder their outdiffusion, i.e. minimize the diffusion coefficient. This can be done by minimizing the target temperature that governs the diffusion coefficient via the Arrhenius equation or by selecting a backing material that is intrinsically hindering diffusion. Existing literature data [8–11] on diffusion of krypton, iodine and xenon in potential backing materials are relatively scarce since such nonmetallic elements have no technological importance as dopants. To complement these data an experiment was performed at LOHENGRIN: mass-separated radioactive tracers of the respective elements were ion-implanted into samples of different materials. After a gamma spectrometry assay of the radiotracer content the samples were annealed under air or vacuum and measured again to deduce the remaining and released fraction, respectively. Qualitatively this corroborates that materials like SiC, Pt, W or Re retain ion-implanted volatile fission products better than Ti, Zr or graphite. For a detailed report see Ref. [12].
3. Target burnup The rate of ions arriving at the focal plane of the spectrometer is usually1 constantly dropping. Part of this drop is due to the nuclear burnup since the amount of fissile nuclides in the target is reduced by neutron capture followed by fission or gamma emission. A purely nuclear burnup would result in an exponential drop of the fission rate r as rðtÞ ¼ r0 expðltÞ
with l ¼ ½sðn;fÞ þ sðn;gÞ F:
ð2Þ
The target burnup of very thin targets is normally well described by a single exponential function close to the expected nuclear burnup. However, for thicker targets a more rapid drop is observed, in particular in the first hours or days after bringing a 1 An exception are fertile targets like 237Np or 241Am where the fissile isotope is being bred in the neutron flux. The fission rate increases for some days until the amount of the fissile isotope reaches equilibrium.
Fig. 6. Observed burnup of various actinide targets. The target thickness, target supplier and burnup time when fitted with a single exponential function are given in the legend.
Fig. 7. Observed burnup of various 235 UO2 targets produced by painting at ILL. The target thickness, backing, cover foil and burnup time when fitted with a single exponential function are given in the legend.
new target into the neutron flux. For convenience the burnup behaviour can be described by a sum of exponentials, but there is no physical model justifying such a description. Figs. 6 and 7 show a collection of experimental burnup curves. Obviously the burnup times t ¼ 1=l do not follow the expected nuclear burnup time but are often much shorter and scatter widely. The burnup becomes significantly faster with increasing target thickness but the deposition method of the actinide layer also plays a role. Electroplated targets (from IRMM and Mainz) usually last much longer than painted targets. Nuclear spectroscopy experiments suffer from a rapid target burnup since it results in a drop of ion rate arriving in the focal plane. In certain experiments each arriving ion has to be identified by an ionization chamber and in searches of microsecond isomers each incoming ion opens a time window in which gamma rays are detected. Such experiments cannot handle count rates above few thousand ions per second. Thus, the total count rate of a fresh thick target would be too high and has to be reduced by cutting deliberately the spectrometer acceptance by closing shutters and diaphragms. When the target burnup reduces the ion rate the drop can be compensated for a while by opening consecutively more shutters but eventually the lower ion rate results in a limitation of the statistics that can be collected in the experiment and the target has to be replaced by a new one. Other experiments that just detect decay radiation (beta or gamma rays) could usually handle much higher ion rates and suffer from the beginning from a rapidly dropping ion rate.
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For fission yield measurements usually thinner targets are being used, resulting in a much smoother and slower burnup. However, the burnup behaviour has to be known with good accuracy since it directly affects the corrections of the measurements of the relative abundance of different masses or isotopes. Frequent checks of the target burnup are required to keep the systematic errors due to this correction acceptable. What are the reasons for the observed target burnup that is faster than the nuclear burnup?
Fig. 8. Infrared picture of a 286 mg=cm2 241 PuO2 target (produced in Sarov) taken with a CCD camera showing the evolution of the apparent temperature distribution of the target cover as function of time since the start of the irradiation.
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3.1. Diffusion of actinides into the target backing or target cover Literature data on diffusion coefficients of actinides show that at high temperatures not only fission products may diffuse in or out of the backing but also actinide atoms. The diffusion coefficient for U in aTi is 4 1013 cm2 =s at 1000 K [9]. Thus, after one day at 1000 K the actinide atoms could diffuse about 4 mm into a titanium backing, reducing the fission fragment energies by several 10 MeV. At higher temperatures, after the phase transition to bTi, diffusion gets even faster with a diffusionpcoefficient of 2 109 cm2 =s at 1200 K and a diffusion ffiffiffiffiffiffi length ( Dt ) of several mm per minute. Thus the actinide atoms could in principle diffuse quickly so deep into the backing that the fission fragments are no longer released. Nevertheless, fission fragments will still be produced and they will still deposit their kinetic energy in the backing, actually now even their entire kinetic energy. Thus the target temperature should remain constant or even rise. However, in reality the measured apparent target temperatures drop equally quickly as the fission fragment rates when the targets show a rapid burnup, see, e.g. the dropping target brightness in Fig. 8. To explain such a change the fissile nuclei must leave the visible area of the target laterally, i.e. cover a distance of about 1 cm to quit the area visible with the camera. This is difficult to explain by diffusion or surface diffusion. Still, as long as one cannot exclude losses of target material due to diffusion one should investigate alternative backing materials. Zr is not better in this respect since U diffusion is similarly fast as in Ti. However, diffusion in hosts of group 5, 6 or higher is several orders of magnitude slower, see Table 1. The
Table 1 Characteristics of Possible backing materials and actinide coatings for use at LOHENGRIN. See main text (sections 2.5, 3.1 and 4) for an explanation of the given data. Material
sðnth ; gÞ
l at 1000 K (W/cm/K)
e at 1000 K
Tmelt (1C)
Tðpvap ¼ 1 PaÞ (1C)
0.89 0.6 0.0785 0.312 0.2–0.8 0.21 0.06-0.4 0.386 0.718 3.57 0.237 0.01–0.02 0.644 1.12 E1 1.21 0.755 0.207 0.602 1.21 0.446 1.26 0.786
0.4 0.8 0.45–0.65 0.64 0.5–0.9 0.3 0.7 0.2 0.15–0.5 0.04–0.8 0.2–0.3 0.3–0.5 0.12–0.7 0.15
1287 3727 (subl.) 2046 1410 2700 1668 3140 1900 1453 1083 1852 2700 2468 2610 2250 1966 1552 2233 2996 3410 3180 2446 1769
1140 2300 2050 1630 1860 1786 2200 1850 1530 1240 2370 2400 2700 2490 2400 2035 1530 2560 3040 3200 3030 2460 2100
1750 3050 1135 2500 1300 (dec.) 2790 2350 2805 1036 2390
2600 2300 2200 1940
Density (g=cm3 )
DL=L0
(b)
0.0088 0.0035 0.23 0.166 0.166 6.1 6.1 5 4.5 3.8 0.19 0.19 1.11 2.5 2.6 145 7 106 20 18 90 420 10
1.85 2.2 3.99 2.33 3.16 4.51 4.93 6 8.9 8.96 6.52 5.68 8.57 10.2 12.1 12.4 12 13.3 16.4 19.3 20.8 22.6 21.5
1.19 0.08–0.55 0.565 0.266 0.321 0.736 0.513 0.737 1.126 1.37 0.505 0.489 0.56 0.38 0.528 0.712 0.975 0.475 0.485 0.339 0.46 0.535 0.699
Th ThO2 U UO2 U 3 O8
11.7 10 19.1 10.97 8.38
0.922 0.64 1.737 0.709 0.011
0.51 0.04 0.439 0.01–0.1 0.012
UC UC2 UN UF4 PuO2
13.6 11.3 14.3 6.7 11.5
0.756 1 0.654 0.8 0.7
0.2 0.1 0.2 0.034
Be Graphites Al2 O3 Si SiC Ti TiC V Ni Cu Zr ZrO2 Nb Mo Ru Rh Pd Hf Ta W Re Ir Pt
293–1000 K (%)
0.05 0.08 0.3 0.08–0.4 0.1–0.4 0.2 0.12
0.3 0.35–0.7 0.46
0.45
750 2240
d in 24 h at 1273 K (mm)
200 0.2 40 140 0.2 0.1
1E 3 2E 4
Dose rate at 1 m (mGy/h)
0 0 0 0 0 0 0 0 0 0 1.8 1.1 1.5 3.7 160 1.5 50 6000 3000 10 400 40000 9
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pffiffiffiffiffiffi diffusion length d ¼ Dt at 1273 K was estimated from extrapolating literature data obtained at higher temperatures. Even at such a high temperature the diffusion lengths of U in V, Nb, Ta, Mo or W remain modest. No U diffusion data are available for metal hosts like Ru, Rh, Re, Ir or Pt but from the systematics of diffusion coefficients of other impurities [9,11] one should also expect slow diffusion in these matrices. 3.2. Self-sputtering If the actinide atoms are not lost by diffusion into the bulk of the backing or sidewards diffusion, they might leave the target towards the front side. Due to the permanent bombardment by energetic fission fragments they have a non-negligible probability to be sputtered off from the target surface. The sputter yield k gives the number of target atoms (here only the number of sputtered actinide atoms is of interest) per fission fragment traversing the target layer. The sputter yield can be estimated from SRIM simulations as function of the target thickness. Our result are consistent with previous calculations and measurements [13]. Pouchou et al. [14] observed a UO2 target burnup that was even more pronounced as function of neutron fluence and that increased with the target temperature. Also the systematics by Bouffard et al. [15] indicates a sputter yield k b1 for fission fragments. Self-sputtering modifies the nuclear burnup to
l ¼ ½sðn;fÞ þ sðn;gÞ F þ ksðn;fÞ F:
ð3Þ
Thus the burnup curve becomes steeper but can still be described as a single exponential. Only when a massive change in target thickness occurs, the sputter yield per fission will change. Any observation of a non-exponential burnup behaviour, e.g. a burnup that has to be described by sum of exponentials, therefore indicates an inhomogeneous target change. This could be explained if the target material is lost in form of ‘‘flakes’’ due to insufficient adhesion on the backing. An apparent target temperature distribution evolving towards very irregular shapes points indeed towards such an effect. Fig. 9 shows the observation of a 655 mg=cm2 239PuO2 target that was produced in Mainz by molecular plating. Within few days the fission rate dropped strongly and the remaining temperature distribution became very irregular.
4. Target backing The material of the target backing has to fulfill several conditions:
stability at high temperatures (high melting point, low vapour pressure),
thermal expansion coefficient similar to that of the target material,
Fig. 9. Infrared picture of a 655 mg=cm2 239PuO2 target (produced in Mainz) taken with a CCD camera showing the evolution of the apparent temperature distribution of the target cover.
high emissivity, low activation by thermal neutrons, chemically compatible with target deposition methods and commercially available at affordable cost.
Table 1 shows an overview of materials that may potentially serve as backing. Most data are from Refs. [9,16–18,23,24]. The importance of thermal stress between backing and coating can be judged from the difference in thermal expansion DL=L0 . The total emissivity e is given for polycrystalline samples. As an indicator for the importance of longer-lived neutron activation products we give the calculated dose rate at 1 m distance for a standard target backing (2 9 0:025 cm3 ) that has been irradiated for 14 days in the LOHENGRIN neutron flux (including up to 0.1% of fast neutrons) and that decayed for 10 days after end of irradiation. This number can be compared to a dose rate of about 10 mGy/h at 1 m distance due to the fission products of 1 mg 235U that has undergone the same activation and decay cycle. Materials that show excessive activation in the high neutron flux (Ru, Hf, Ta, Re, Ir) or that are not available at affordable cost as self-supporting plates (Ru, Rh, Pd, Re, Ir, Pt) might still serve as thin intermediate layer to increase the adhesion of the actinide material on the backing. Gold has to be avoided completely since it transmutes in the high neutron flux quickly to mercury that evaporates at the high target temperatures. Also platinum transmutes in a thermal neutron flux via gold to mercury, but at such a low rate that Pt is acceptable as a thin intermediate layer. Indeed some of the actinide targets produced for LOHENGRIN at IRMM Geel were deposited on Ti backings after evaporation of an intermediate Pt layer.
5. Target cover To reduce self-sputtering, the targets are usually covered by a protective layer. In most cases 0:25 mm thick Ni foil was used (type NI000080 from Goodfellow, 99.95% purity). For easier handling it is stabilized by an acrylic layer on one side. The foil is clamped between two titanium diaphragms that are then spotwelded. Subsequently the acrylic layer is removed by washing with acetone before placing the foil on the target. The washing has to be repeated at least twice with fresh acetone to avoid a backplating of dissolved acrylic that would cause a low energy tail in the measured energy distributions of the fission fragments [19]. It has been observed that the strong irradiation of certain cover foils may lead to a rapid oxidation of the latter, e.g. for zirconium [20]. Hence the energy loss of the fission fragments can increase with time and the varying energy loss correction results in an increased uncertainty of the real kinetic energies. As alternative to the nickel foil we also tested 0:25 mm thick titanium foils (from ACF-Metals) and 180 mg=cm2 thick carbon foils (ACF-180). With thin targets the carbon foils behave similarly as the nickel foils. When using carbon cover foils or backings with thicker targets the kinetic energy distribution of the detected fission fragments broadens rapidly up to about 30 MeV. The higher target temperature leads probably to rapid diffusion of actinide material into the carbon layer or of carbon atoms into the actinide layer, respectively. Titanium cover foils perform well for thin targets, but for thicker targets a rapid broadening of the kinetic energy distribution is observed. This could be due to interdiffusion between Ti and target material or due to oxidation of the Ti foil. Also the burnup seems to be more rapid with Ti cover foils. Still thinner foils would be desirable to minimize the energy loss of the fission fragments but they are increasingly fragile, in
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Table 2 Comparison of actinide targets exposed to intense heavy ion beams (22Ne, 48Ca and 64Ni) with those used for thermal neutron induced fission at LOHENGRIN. Ion
Ekin (MeV)
22
Ne Ca 64 Ni
120 340 400
0 0 0
96
100 70 170
45 45 45
48
Sr Xe Fission
138
Angle (deg)
DE (MeV)
Damage (dpa/d)
Beam heating (W)
15 20 40
2.4 6.7 12.2
2 5 11
2.4 6.7 12.2
440 1600 2040
21.8 22.4 44.2
53
4.3
Damage (U disp./ion)
A 800 mg=cm2 thick UO2 target is considered in all cases. The fission rate is 6 1011 fissions=cm2 at LOHENGRIN while the beam intensity of the heavy ion beams is 1 particle mA=cm2 . 96Sr and 138Xe were taken as typical fission fragments and an average emission angle of 45 3 C is assumed instead of integrating over 4p. ‘‘Fission’’ represents the sum of the values for the typical fission fragments 96Sr and 138Xe.
particular when keeping in mind the handling with coarse tools in a glove box. Thus, alternatively to a cover foil a layer can be deposited on the target to limit sputter losses. Such a deposited layer has several other advantages. With an optimum thermal conduction between target and cover it prevents the unwanted heat screen effect. If sufficiently thick and dense it may even reduce the outdiffusion of fission products from the front-side of the target backing. Finally it could stabilize the target layer mechanically. IRMM Geel formerly provided targets that were covered with a sputtered layer of, e.g. 125 mg=cm2 tantalum that performed very satisfactorily. Unfortunately at present no in-house facilities exist at ILL that would allow to coat an actinide target with a cover layer.
sputtering directly onto the target material should guarantee better performance compared to separate cover foils. To separate the effects of target heating and radiation damage one could vary the target temperature independently of the fission rate by rendering the surface of target backing and cover ‘‘black’’ or add additional heat screens, respectively. The behaviour of actinide targets in the LOHENGRIN in-pile position can be used as reference for actinide targets that are planned to be exposed to very intense primary beams in future accelerator facilities. Any target that passes successfully the tough ‘‘survival training’’ in the LOHENGRIN beam tube should also qualify for a use with many mA=cm2 of heavy ion beams.
Acknowledgements 6. Comparison to actinide targets exposed to heavy ion beams Actinide targets similar to those used at LOHENGRIN are also being used for fusion–evaporation, fusion–fission, deep inelastic and other reactions induced by heavy ion beams. In these applications the primary beam intensity is limited by similar considerations. Table 2 gives a comparison of target heating and radiation damage at LOHENGRIN and for certain heavy ion beams. The heat deposition is of similar magnitude in both cases. An accelerator beam could be wobbled over a bigger target or the target might rotate while the accelerator beam irradiates a fixed position. Thus, with the exception of thermal cycling most parameters affecting the target lifetime, namely the induced radiation damage and the sputter rate can be directly scaled from the experience at LOHENGRIN. LOHENGRIN can therefore serve to perform already today tests of new actinide targets that are planned to be exposed in future to very intense primary beams, e.g. at the super separator spectrometer S3 of the SPIRAL2 project [21,22].
Conclusion and outlook
We are very grateful to those who prepared and provided the actinide targets for LOHENGRIN experiments, in particular Petra ¨ Thorle (Institute for Nuclear Chemistry, Mainz University), Tatiana Kuzmina (V.G. Khlopin Radium Institute, St. Petersburg), Ilham Almahamid (LBNL Berkeley and Wadsworth Center, Albany, NY), Adeline Bail (CEA Cadarache), Stanislav P. Vesnovskii (AllRussian Scientific Research Institute of Experimental Physics, Sarov, previously Arzamas-16) and the Institute for Reference Materials and Measurements, JRC Geel, Belgium. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Fission targets of various actinide elements have been used since 35 years at the LOHENGRIN recoil separator. The target behaviour shows a strong dependence on thickness, material and preparation method. The rapid burnup of thin actinide targets in a high neutron flux is a serious limitation for experiments but has not yet been fully understood. The performance of alternative target compounds such as metallic U, UCx , USix , UMox , UN, US, etc. should be tested. Special care has to be taken to appropriately choose the material of target backing and cover. Cover layers deposited by physical vapour deposition, i.e. evaporation or
[12] [13] [14] [15] [16] [17] [18]
P. Armbruster, et al., Nucl. Instr. and Meth. 139 (1976) 213. G.S. Simpson, et al., Phys. Rev. C 76 (2007) 041303(R). G. Fioni, et al., Nucl. Instr. and Meth. A 332 (1993) 175. SRIM, The Stopping and Range of Ions in Matter Monte Carlo program by J.F. Ziegler. /http://www.srim.orgS. J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. ¨ U. Koster, et al., Nucl. Phys. A 652 (1999) 371. H. Faust, Report ILL03FA08T, ILL, Grenoble, 2003. R. Kirchner, Nucl. Instr. and Meth. B 70 (1992) 186. H. Mehrer (Ed.), Diffusion in Solid Metals and Alloys, vol. III/26, Landolt¨ Bornstein, Springer, Berlin, 1990. E. Liatard, G. Gimond, G. Perrin, F. Schussler, Nucl. Instr. and Meth. A 385 (1997) 398. G.J. Beyer, E. Hagebø, A.F. Novgorodov, H.L. Ravn, the ISOLDE Collaboration, Nucl. Instr. and Meth. B 204 (2003) 225. J.-C. Thomas, et al., GANIL. J.P. Biersack, D. Fink, P. Mertens, J. Nucl. Mater. 53 (1974) 194. J.L. Pouchou, L. Zuppiroli, N. Housseau, J. Nucl. Mater. 59 (1976) 321. S. Bouffard, J.P. Duraud, M. Mosbah, S. Schlutig, Nucl. Instr. and Meth. B 141 (1998) 372. D.R. Lide, CRC Handbook of Chemistry and Physics, in: D.R. Lide (Ed.), 89th ed., CRC Press, Boca Raton, 2008. Y.S. Touloukian, et al., Thermophysical Properties of Matter, in: Y.S. Touloukian et al.(Ed.), IFI Plenum, New York, 1970. PVD Online Catalog, Technical Data, Umicore Thin Film Products, Liechtenstein, 2009.ohttp://catalog.thinfilmproducts.umicore.com/Portals/0/Content/ PVD_gruen. pdf4.
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[19] A. Bail, Mesures de rendements isobariques et isotopiques des produits de fission lourds sur le spectrome tre de masse LOHENGRIN, Ph.D. Thesis, University of Bordeaux I, 2009. [20] N. Be´rerd, et al., Nucl. Instr. and Meth. B 240 (2006) 711. [21] A. Drouart, et al., Nucl. Instr. and Meth. B 266 (2008) 4162.
[22] F. Pellemoine, C. Stodel, Nucl. Instr. and Meth. B (2009), Proceedings of INTDS-2008, doi:10.106/j.nim.2009.10.08. [23] R. De Coninck et al., J. Nucl. Mat. 46 (1973) 213. [24] I.S. Golovnin, Atom. Energy 89 (2000) 627.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Experience with in-pile fission targets at LOHENGRIN a, ¨ U. Koster , H. Faust a, T. Materna a, L. Mathieu b a b
Institut Laue Langevin, 6 rue Jules Horowitz, F-38042 Grenoble Cedex 9, France CEA Cadarache, DEN/DER/SPRC/LEPh, F-13108 Saint-Paul-lez-Durance, France
a r t i c l e in fo
abstract
Available online 5 November 2009
The LOHENGRIN fission fragment separator uses actinide targets in a neutron flux of about 5 1014 neutrons=cm2 =s in an in-pile position of the high-flux reactor of ILL Grenoble. For fission yield measurements relatively thin targets (tens of mg=cm2 ) are used, while for nuclear spectroscopy applications targets up to 1 mg=cm2 are employed. This leads to fission rates up to 5 1012 =s. The targets are heated by the fission power in vacuum to temperatures of up to 1000 3 C. The radiation damage caused by the fission fragments can reach 50 dpa (displacements per atom) per day, an extremely high value comparable to that caused by irradiation with intense heavy ion beams. Therefore the thick targets that were produced with different methods (painting, spray-painting, electrolysis and molecular plating) all suffer from a burnup that is much quicker than explainable by nuclear transmutation. We discuss physical effects responsible for this additional decrease in fission fragment rate and ways to improve the situation. & 2009 Elsevier B.V. All rights reserved.
Keywords: Actinide targets High intensity beams Radioactive ion beams Radiation damage Nuclear heating Self-sputtering
1. Introduction The fission fragment separator LOHENGRIN [1] at the high-flux reactor of Institut Laue Langevin in Grenoble provides mass- and energy-separated fission fragment beams since 35 years. Part of the experiments are devoted to a precise determination of yields from thermal neutron-induced fission of various actinide targets. Another important application are nuclear decay spectroscopy experiments with short-lived fission fragments. Due to the short transport time through the separator (122 ms) even the decay of isomeric states with half-lives down to 0:5 ms can be studied [2]. For long time LOHENGRIN was mainly used for precise measurements of kinetic energy distributions of fission fragments and their corresponding fission yields. Precise measurements of the kinetic energy distributions require thin targets to minimize the corrections for energy loss in the target. Precise measurements of fission yields are based on many individual measurements with different separator settings where the mass, ionic charge state and kinetic energy are varied. Since there is no way to monitor directly the evolution of the fission rate of the target these individual measurements have to be normalized to the fission rate by repeated measurements of a reference mass. Therefore precise fission yield measurements rely on a smooth evolution of the fission rate with time that is typical for relatively thin targets.
Corresponding author.
¨ E-mail address: [email protected] (U. Koster). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.09.078
In recent years LOHENGRIN became more and more used as radioactive ion beam (RIB) facility where nuclear spectroscopy experiments are performed with mass-separated fission fragments close to the focal plane. The success of such experiments is directly linked to the intensity of these RIBs. This is a strong incitation to boost the intensity of the available RIBs. After a description of typical parameters of LOHENGRIN targets we will discuss in the following which target parameters can be varied to optimize the RIB intensity and which limitations are encountered. Some of these physical limitations are also encountered by actinide targets exposed to intense charged particle beams. Hence, operational experience with LOHENGRIN targets may serve for judging and optimizing the target performance, e.g. for future radioactive ion beam facilities that are planning to irradiate actinide targets with high intensity heavy ion beams.
2. Limitations for fission target performance The LOHENGRIN targets consist of a layer of an actinide compound deposited on a thick backing that is usually made from 0.2 mm thick high purity titanium sheets of 9 2 cm2 . The backing is fixed by spot-welding on a Ti support frame as shown in Fig. 1. Usually a diaphragm with a cover foil is fixed on top of the target, see Section 5. The ensemble is introduced via a glove box, then placed by a gripper on the front part of a trolley that brings the target in an evacuated beam tube to an in-pile position about 0.5 m from the reactor core where the targets are exposed
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and 243Cm the activity limit translates to a limited target mass of 1 and 2 mg, respectively. 2.3. Neutronics limitation A 1 mg=cm2 thick target with 1000 b neutron capture crosssection will result in a local flux depression of less than one percent. Moreover it has no significant effect on the reactor criticality ( o 1 pcm). Therefore neutronics limitations play no role for the fission target design. However, materials with high neutron capture cross-section have to be avoided as thick backing material. 2.4. Target thickness limitation
Fig. 1. Photo of a target mounted on the support frame ready to be placed onto the trolley that brings it into the beam tube. The target is covered with a 0:25 mm thick Ni foil on a 70 5 mm2 diaphragm.
to a thermal neutron flux of about 5 1014 cm2 s1 , see Fig. 5. After irradiation for several days to weeks, the trolley is pulled back, the target is taken by the gripper and dropped into a shielded waste container.
2.1. Geometric limitations The mass dispersion of the LOHENGRIN spectrometer in the focal plane is Dm ¼ 3:24 m and the magnification from the object (target) to the image (focal plane) is 1:1. Hence, a 3 mm wide target with a 4 mm (to account for possible aberrations of the spectrometer) wide mass-defining slit will provide a mass resolution of DA=A 1=950 and a 10 mm wide target with a 11 mm wide slit a still acceptable mass resolution of DA=A 1=350. The energy dispersion of the parabola spectrometer perpendicular to the mass dispersive plane is DE ¼ 7:2 m. Hence, a 7 cm long target will introduce an uncertainty in the kinetic energy measurement of 1% which is usually acceptable. Much longer targets cannot be used due to geometric limitations of the target changing mechanism and the acceptance of the spectrometer (fixed diaphragms). Hence the surface of the actinide layer may vary from few mm2 (for very rare actinide isotopes) to about 75 10 mm2 at maximum.
LOHENGRIN is a recoil separator that separates the fission fragments at the kinetic energy that they receive in the fission process. Light fission fragments (70 oA o 110) have typically kinetic energies of 90–110 MeV and heavy fission fragments (130o Ao 160) have kinetic energies of 50–80 MeV. With increasing target thickness the fission fragments emitted from the back of the target will suffer more energy loss from electronic stopping. Hence, the average kinetic energy of the fission fragments will decrease and the width of the observed kinetic energy distribution will increase compared to the intrinsic one that is determined by the fission process. These effects will directly limit the maximum rate of separated fission fragments since the energy acceptance dE of the LOHENGRIN spectrometer is a fixed percentage of the average kinetic energy: dE=E 5% when using the RED (Reverse Energy Dispersion) magnet [3]. Eventually the addition of more target material will only contribute to the low-energy tail of the kinetic energy distribution but no longer enhance the useful intensity at the peak. Fig. 2 shows simulated kinetic energy distributions of 98Y ions from 235UO2 targets of 5021400 mg=cm2 thickness assuming an intrinsically Gaussian energy distribution with 100 MeV average energy and 10 MeV FWHM. The simulations were performed with the Monte Carlo program SRIM [4,5]. The energy-dependent transmission through the LOHENGRIN spectrometer is already taken into account. Integration of the ion rate in a 5% energy bin at the peak of the distribution shown in Fig. 2 gives the maximum ion rate available
2.2. Legal limitations For nuclear spectroscopy applications usually relatively thick targets of the fissile isotopes 233U, 235U, 239Pu and 241Pu are being used. Due to the very high neutron flux also the fertile isotope 241 Am provides a significant fission rate after breeding to the fissile 242g;m Am. For fission yield studies moreover targets made from the fissile isotopes 229Th, 245Cm, 249Cf, 251Cf and from the fertile isotope 237 Np have been used. New interesting experiments could be performed if targets made from 231Pa, 236Np, 242mAm, 243Cm or 247 Cm would become available. The legal limit for the activity of a single target is 3.7 GBq (0.1 Ci). With sufficient enrichment of the isotopes mentioned above this limitation is usually less tight than the physical limitations discussed below. Only for the relatively short-lived isotopes 241Pu
Fig. 2. SRIM simulation of 98Y kinetic energy distributions from 50 to 1400 mg=cm2 UO2 targets folded with the energy-dependent transmission of LOHENGRIN.
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at the focal plane, see Fig. 3. It is obvious that the rate saturates at about 1 mg=cm2 target thickness. Any addition of non-fissile material in the target layer (i.e. additives or impurities of the target material), or in front of it (e.g. a cover layer of foil, see Section 5) will reduce the kinetic energies and broaden the energy distribution, thus further reducing the transmission. Therefore the observed ion rates are usually lower than these calculated ideal ones. Note that this thickness limitation was derived for typical binary fission fragments. Ternary fission fragments with low Z and high kinetic energy per nucleon have much lower specific energy loss. Thus in principle even thicker targets could be used for measuring yields of very rare ternary fragments like 3He, 11Li, 14 Be, etc. [6].
2.5. Temperature limitation Half of the fission fragments are emitted backwards and deposit their kinetic energy in the thick backing. The forward emitted fragments loose, depending on the target thickness, only a relatively small part of their kinetic energy in the target layer. Thus, e.g. for a 400 mg=cm2 235U target the fission heating amounts to about 4:5 W=cm2 . The nuclear heating by gamma rays from the reactor core amounts to about 1.3 W/g at the target position, i.e. 0:12 W=cm2 for a 0.2 mm thick Ti sheet. The target and the backing are sufficiently thin that thermal conduction keeps the temperature gradient perpendicular to the
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area negligibly small ( o1 3 C), but they are too thin to allow for efficient heat conduction in lateral direction. Instead the heat is mainly dissipated by radiation to the zircalloy beam tube that is kept by external water cooling at about 50 3 C. There is no convective cooling due to the low pressure ( o 104 mbar) in the beam tube. Neglecting heat conduction, the maximum target temperature for large area targets can thus be estimated with the Stefan– Boltzmann law: pheat ¼ sðet þ eb ÞðTt4 323K 4 Þ:
ð1Þ
The target-backing-sandwich radiates to the front side with the emissivity of the target material et and to the back side with the emissivity of the backing material eb . Fig. 4 shows the calculated maximum temperatures as function of target thickness and emissivities. The calculated temperature can be validated by observing the hot target in situ with two instruments [7]: The relative temperature distribution is deduced from the relative brightness of the target surface that is measured with an infrared sensible CCD camera via a telelens, see, e.g. Fig. 8. The absolute temperature is measured with an infrared pyrometer (type KTR2000-L-P with 8.0/500 mm mirror lens from Dr. Georg Maurer GmbH, http://www.maurer-ir.de). To avoid the radiation field of gamma rays and fast neutrons in direction of the beam tube both instruments are observing the target indirectly via a periscope system made from two gold-plated mirrors, see schematic setup in Fig. 5. When a target cover foil (see Section 5) is used it has often a bad thermal contact to the target and backing. Hence, it may act as a heat screen, resulting in a further rise of the target temperature. When the cover foil is viewed with the pyrometer only a lower limit of the real target temperature can be measured. The elevated temperature of the target may have various effects, most of them detrimental to the target survival:
evaporation of volatile target materials (e.g. thick targets made from UF4 cannot be used),
promotion of chemical reactions (e.g. reduction of UO2 target layer in contact with carbon or titanium backing or cover),
thermal diffusion of target atoms into the backing or cover, thermal stress due to different thermal expansion of target and backing material and Fig. 3. Calculated initial ion beam intensity for A ¼ 98 isobars in the focal plane of the RED magnet.
outgassing of volatile fission products stopped in the backing. In principle at an elevated target temperature part of the radiation induced damages (vacancies, etc.) could be annealed, but clearly the negative effects outweigh this possible advantage.
Fig. 4. The maximum target temperature was estimated as function of the target thickness and the emissivities of target and backing: et ¼ 0:4 for the UO2 layer, eb ¼ 0:3 for a Ti backing, eb ¼ 0:8 for a graphite backing and et ¼ 0:1 as effective emissivity when a glossy cover foil is placed in front of the target without direct thermal contact.
Fig. 5. Set-up of the LOHENGRIN fission fragment separator at the high-flux reactor of ILL Grenoble.
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2.6. Outgassing of volatile fission products Only a tiny fraction of the produced fission fragments is emitted in the solid angle covered by the acceptance of the spectrometer (3 105 sterad). Half of the fission fragments are emitted forward. Those not stopped in the diaphragm or cover foil will be implanted into the water cooled beam tube where they will remain until they finally have decayed to a stable isotope. The other half of the fission fragments are stopped in the backing. At sufficiently high temperature a fraction of the most volatile fission products may diffuse to the surface of the backing, desorb and migrate through the beam tube. Thus, unintentionally the backing may act as an ISOL-type (isotope separation on-line) catcher where the reaction products are first stopped, then thermally diffuse and effuse to the ion source [8]. Without ion source the radioisotopes will just effuse as neutral atoms through the vacuum system. For thick targets ( 4 300 mg=cm2 target thickness) of isotopes with high fission cross-section that reach correspondingly high target temperatures, a noticeable increase of the radioactive background in the vacuum system of the LOHENGRIN separator has been observed, creating ‘‘hot spots’’ in turbopumps, charcoal filters and the primary pump. A small fraction of the volatile activity even migrates to the focal plane resulting in gamma-ray background for the Ge detectors. Thus gamma rays of the volatile fission products 85–91Kr, 133–140Xe, 131–135I could be observed. To limit this disturbing emanation of volatile fission products one has to hinder their outdiffusion, i.e. minimize the diffusion coefficient. This can be done by minimizing the target temperature that governs the diffusion coefficient via the Arrhenius equation or by selecting a backing material that is intrinsically hindering diffusion. Existing literature data [8–11] on diffusion of krypton, iodine and xenon in potential backing materials are relatively scarce since such nonmetallic elements have no technological importance as dopants. To complement these data an experiment was performed at LOHENGRIN: mass-separated radioactive tracers of the respective elements were ion-implanted into samples of different materials. After a gamma spectrometry assay of the radiotracer content the samples were annealed under air or vacuum and measured again to deduce the remaining and released fraction, respectively. Qualitatively this corroborates that materials like SiC, Pt, W or Re retain ion-implanted volatile fission products better than Ti, Zr or graphite. For a detailed report see Ref. [12].
3. Target burnup The rate of ions arriving at the focal plane of the spectrometer is usually1 constantly dropping. Part of this drop is due to the nuclear burnup since the amount of fissile nuclides in the target is reduced by neutron capture followed by fission or gamma emission. A purely nuclear burnup would result in an exponential drop of the fission rate r as rðtÞ ¼ r0 expðltÞ
with l ¼ ½sðn;fÞ þ sðn;gÞ F:
ð2Þ
The target burnup of very thin targets is normally well described by a single exponential function close to the expected nuclear burnup. However, for thicker targets a more rapid drop is observed, in particular in the first hours or days after bringing a 1 An exception are fertile targets like 237Np or 241Am where the fissile isotope is being bred in the neutron flux. The fission rate increases for some days until the amount of the fissile isotope reaches equilibrium.
Fig. 6. Observed burnup of various actinide targets. The target thickness, target supplier and burnup time when fitted with a single exponential function are given in the legend.
Fig. 7. Observed burnup of various 235 UO2 targets produced by painting at ILL. The target thickness, backing, cover foil and burnup time when fitted with a single exponential function are given in the legend.
new target into the neutron flux. For convenience the burnup behaviour can be described by a sum of exponentials, but there is no physical model justifying such a description. Figs. 6 and 7 show a collection of experimental burnup curves. Obviously the burnup times t ¼ 1=l do not follow the expected nuclear burnup time but are often much shorter and scatter widely. The burnup becomes significantly faster with increasing target thickness but the deposition method of the actinide layer also plays a role. Electroplated targets (from IRMM and Mainz) usually last much longer than painted targets. Nuclear spectroscopy experiments suffer from a rapid target burnup since it results in a drop of ion rate arriving in the focal plane. In certain experiments each arriving ion has to be identified by an ionization chamber and in searches of microsecond isomers each incoming ion opens a time window in which gamma rays are detected. Such experiments cannot handle count rates above few thousand ions per second. Thus, the total count rate of a fresh thick target would be too high and has to be reduced by cutting deliberately the spectrometer acceptance by closing shutters and diaphragms. When the target burnup reduces the ion rate the drop can be compensated for a while by opening consecutively more shutters but eventually the lower ion rate results in a limitation of the statistics that can be collected in the experiment and the target has to be replaced by a new one. Other experiments that just detect decay radiation (beta or gamma rays) could usually handle much higher ion rates and suffer from the beginning from a rapidly dropping ion rate.
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For fission yield measurements usually thinner targets are being used, resulting in a much smoother and slower burnup. However, the burnup behaviour has to be known with good accuracy since it directly affects the corrections of the measurements of the relative abundance of different masses or isotopes. Frequent checks of the target burnup are required to keep the systematic errors due to this correction acceptable. What are the reasons for the observed target burnup that is faster than the nuclear burnup?
Fig. 8. Infrared picture of a 286 mg=cm2 241 PuO2 target (produced in Sarov) taken with a CCD camera showing the evolution of the apparent temperature distribution of the target cover as function of time since the start of the irradiation.
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3.1. Diffusion of actinides into the target backing or target cover Literature data on diffusion coefficients of actinides show that at high temperatures not only fission products may diffuse in or out of the backing but also actinide atoms. The diffusion coefficient for U in aTi is 4 1013 cm2 =s at 1000 K [9]. Thus, after one day at 1000 K the actinide atoms could diffuse about 4 mm into a titanium backing, reducing the fission fragment energies by several 10 MeV. At higher temperatures, after the phase transition to bTi, diffusion gets even faster with a diffusionpcoefficient of 2 109 cm2 =s at 1200 K and a diffusion ffiffiffiffiffiffi length ( Dt ) of several mm per minute. Thus the actinide atoms could in principle diffuse quickly so deep into the backing that the fission fragments are no longer released. Nevertheless, fission fragments will still be produced and they will still deposit their kinetic energy in the backing, actually now even their entire kinetic energy. Thus the target temperature should remain constant or even rise. However, in reality the measured apparent target temperatures drop equally quickly as the fission fragment rates when the targets show a rapid burnup, see, e.g. the dropping target brightness in Fig. 8. To explain such a change the fissile nuclei must leave the visible area of the target laterally, i.e. cover a distance of about 1 cm to quit the area visible with the camera. This is difficult to explain by diffusion or surface diffusion. Still, as long as one cannot exclude losses of target material due to diffusion one should investigate alternative backing materials. Zr is not better in this respect since U diffusion is similarly fast as in Ti. However, diffusion in hosts of group 5, 6 or higher is several orders of magnitude slower, see Table 1. The
Table 1 Characteristics of Possible backing materials and actinide coatings for use at LOHENGRIN. See main text (sections 2.5, 3.1 and 4) for an explanation of the given data. Material
sðnth ; gÞ
l at 1000 K (W/cm/K)
e at 1000 K
Tmelt (1C)
Tðpvap ¼ 1 PaÞ (1C)
0.89 0.6 0.0785 0.312 0.2–0.8 0.21 0.06-0.4 0.386 0.718 3.57 0.237 0.01–0.02 0.644 1.12 E1 1.21 0.755 0.207 0.602 1.21 0.446 1.26 0.786
0.4 0.8 0.45–0.65 0.64 0.5–0.9 0.3 0.7 0.2 0.15–0.5 0.04–0.8 0.2–0.3 0.3–0.5 0.12–0.7 0.15
1287 3727 (subl.) 2046 1410 2700 1668 3140 1900 1453 1083 1852 2700 2468 2610 2250 1966 1552 2233 2996 3410 3180 2446 1769
1140 2300 2050 1630 1860 1786 2200 1850 1530 1240 2370 2400 2700 2490 2400 2035 1530 2560 3040 3200 3030 2460 2100
1750 3050 1135 2500 1300 (dec.) 2790 2350 2805 1036 2390
2600 2300 2200 1940
Density (g=cm3 )
DL=L0
(b)
0.0088 0.0035 0.23 0.166 0.166 6.1 6.1 5 4.5 3.8 0.19 0.19 1.11 2.5 2.6 145 7 106 20 18 90 420 10
1.85 2.2 3.99 2.33 3.16 4.51 4.93 6 8.9 8.96 6.52 5.68 8.57 10.2 12.1 12.4 12 13.3 16.4 19.3 20.8 22.6 21.5
1.19 0.08–0.55 0.565 0.266 0.321 0.736 0.513 0.737 1.126 1.37 0.505 0.489 0.56 0.38 0.528 0.712 0.975 0.475 0.485 0.339 0.46 0.535 0.699
Th ThO2 U UO2 U 3 O8
11.7 10 19.1 10.97 8.38
0.922 0.64 1.737 0.709 0.011
0.51 0.04 0.439 0.01–0.1 0.012
UC UC2 UN UF4 PuO2
13.6 11.3 14.3 6.7 11.5
0.756 1 0.654 0.8 0.7
0.2 0.1 0.2 0.034
Be Graphites Al2 O3 Si SiC Ti TiC V Ni Cu Zr ZrO2 Nb Mo Ru Rh Pd Hf Ta W Re Ir Pt
293–1000 K (%)
0.05 0.08 0.3 0.08–0.4 0.1–0.4 0.2 0.12
0.3 0.35–0.7 0.46
0.45
750 2240
d in 24 h at 1273 K (mm)
200 0.2 40 140 0.2 0.1
1E 3 2E 4
Dose rate at 1 m (mGy/h)
0 0 0 0 0 0 0 0 0 0 1.8 1.1 1.5 3.7 160 1.5 50 6000 3000 10 400 40000 9
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pffiffiffiffiffiffi diffusion length d ¼ Dt at 1273 K was estimated from extrapolating literature data obtained at higher temperatures. Even at such a high temperature the diffusion lengths of U in V, Nb, Ta, Mo or W remain modest. No U diffusion data are available for metal hosts like Ru, Rh, Re, Ir or Pt but from the systematics of diffusion coefficients of other impurities [9,11] one should also expect slow diffusion in these matrices. 3.2. Self-sputtering If the actinide atoms are not lost by diffusion into the bulk of the backing or sidewards diffusion, they might leave the target towards the front side. Due to the permanent bombardment by energetic fission fragments they have a non-negligible probability to be sputtered off from the target surface. The sputter yield k gives the number of target atoms (here only the number of sputtered actinide atoms is of interest) per fission fragment traversing the target layer. The sputter yield can be estimated from SRIM simulations as function of the target thickness. Our result are consistent with previous calculations and measurements [13]. Pouchou et al. [14] observed a UO2 target burnup that was even more pronounced as function of neutron fluence and that increased with the target temperature. Also the systematics by Bouffard et al. [15] indicates a sputter yield k b1 for fission fragments. Self-sputtering modifies the nuclear burnup to
l ¼ ½sðn;fÞ þ sðn;gÞ F þ ksðn;fÞ F:
ð3Þ
Thus the burnup curve becomes steeper but can still be described as a single exponential. Only when a massive change in target thickness occurs, the sputter yield per fission will change. Any observation of a non-exponential burnup behaviour, e.g. a burnup that has to be described by sum of exponentials, therefore indicates an inhomogeneous target change. This could be explained if the target material is lost in form of ‘‘flakes’’ due to insufficient adhesion on the backing. An apparent target temperature distribution evolving towards very irregular shapes points indeed towards such an effect. Fig. 9 shows the observation of a 655 mg=cm2 239PuO2 target that was produced in Mainz by molecular plating. Within few days the fission rate dropped strongly and the remaining temperature distribution became very irregular.
4. Target backing The material of the target backing has to fulfill several conditions:
stability at high temperatures (high melting point, low vapour pressure),
thermal expansion coefficient similar to that of the target material,
Fig. 9. Infrared picture of a 655 mg=cm2 239PuO2 target (produced in Mainz) taken with a CCD camera showing the evolution of the apparent temperature distribution of the target cover.
high emissivity, low activation by thermal neutrons, chemically compatible with target deposition methods and commercially available at affordable cost.
Table 1 shows an overview of materials that may potentially serve as backing. Most data are from Refs. [9,16–18,23,24]. The importance of thermal stress between backing and coating can be judged from the difference in thermal expansion DL=L0 . The total emissivity e is given for polycrystalline samples. As an indicator for the importance of longer-lived neutron activation products we give the calculated dose rate at 1 m distance for a standard target backing (2 9 0:025 cm3 ) that has been irradiated for 14 days in the LOHENGRIN neutron flux (including up to 0.1% of fast neutrons) and that decayed for 10 days after end of irradiation. This number can be compared to a dose rate of about 10 mGy/h at 1 m distance due to the fission products of 1 mg 235U that has undergone the same activation and decay cycle. Materials that show excessive activation in the high neutron flux (Ru, Hf, Ta, Re, Ir) or that are not available at affordable cost as self-supporting plates (Ru, Rh, Pd, Re, Ir, Pt) might still serve as thin intermediate layer to increase the adhesion of the actinide material on the backing. Gold has to be avoided completely since it transmutes in the high neutron flux quickly to mercury that evaporates at the high target temperatures. Also platinum transmutes in a thermal neutron flux via gold to mercury, but at such a low rate that Pt is acceptable as a thin intermediate layer. Indeed some of the actinide targets produced for LOHENGRIN at IRMM Geel were deposited on Ti backings after evaporation of an intermediate Pt layer.
5. Target cover To reduce self-sputtering, the targets are usually covered by a protective layer. In most cases 0:25 mm thick Ni foil was used (type NI000080 from Goodfellow, 99.95% purity). For easier handling it is stabilized by an acrylic layer on one side. The foil is clamped between two titanium diaphragms that are then spotwelded. Subsequently the acrylic layer is removed by washing with acetone before placing the foil on the target. The washing has to be repeated at least twice with fresh acetone to avoid a backplating of dissolved acrylic that would cause a low energy tail in the measured energy distributions of the fission fragments [19]. It has been observed that the strong irradiation of certain cover foils may lead to a rapid oxidation of the latter, e.g. for zirconium [20]. Hence the energy loss of the fission fragments can increase with time and the varying energy loss correction results in an increased uncertainty of the real kinetic energies. As alternative to the nickel foil we also tested 0:25 mm thick titanium foils (from ACF-Metals) and 180 mg=cm2 thick carbon foils (ACF-180). With thin targets the carbon foils behave similarly as the nickel foils. When using carbon cover foils or backings with thicker targets the kinetic energy distribution of the detected fission fragments broadens rapidly up to about 30 MeV. The higher target temperature leads probably to rapid diffusion of actinide material into the carbon layer or of carbon atoms into the actinide layer, respectively. Titanium cover foils perform well for thin targets, but for thicker targets a rapid broadening of the kinetic energy distribution is observed. This could be due to interdiffusion between Ti and target material or due to oxidation of the Ti foil. Also the burnup seems to be more rapid with Ti cover foils. Still thinner foils would be desirable to minimize the energy loss of the fission fragments but they are increasingly fragile, in
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Table 2 Comparison of actinide targets exposed to intense heavy ion beams (22Ne, 48Ca and 64Ni) with those used for thermal neutron induced fission at LOHENGRIN. Ion
Ekin (MeV)
22
Ne Ca 64 Ni
120 340 400
0 0 0
96
100 70 170
45 45 45
48
Sr Xe Fission
138
Angle (deg)
DE (MeV)
Damage (dpa/d)
Beam heating (W)
15 20 40
2.4 6.7 12.2
2 5 11
2.4 6.7 12.2
440 1600 2040
21.8 22.4 44.2
53
4.3
Damage (U disp./ion)
A 800 mg=cm2 thick UO2 target is considered in all cases. The fission rate is 6 1011 fissions=cm2 at LOHENGRIN while the beam intensity of the heavy ion beams is 1 particle mA=cm2 . 96Sr and 138Xe were taken as typical fission fragments and an average emission angle of 45 3 C is assumed instead of integrating over 4p. ‘‘Fission’’ represents the sum of the values for the typical fission fragments 96Sr and 138Xe.
particular when keeping in mind the handling with coarse tools in a glove box. Thus, alternatively to a cover foil a layer can be deposited on the target to limit sputter losses. Such a deposited layer has several other advantages. With an optimum thermal conduction between target and cover it prevents the unwanted heat screen effect. If sufficiently thick and dense it may even reduce the outdiffusion of fission products from the front-side of the target backing. Finally it could stabilize the target layer mechanically. IRMM Geel formerly provided targets that were covered with a sputtered layer of, e.g. 125 mg=cm2 tantalum that performed very satisfactorily. Unfortunately at present no in-house facilities exist at ILL that would allow to coat an actinide target with a cover layer.
sputtering directly onto the target material should guarantee better performance compared to separate cover foils. To separate the effects of target heating and radiation damage one could vary the target temperature independently of the fission rate by rendering the surface of target backing and cover ‘‘black’’ or add additional heat screens, respectively. The behaviour of actinide targets in the LOHENGRIN in-pile position can be used as reference for actinide targets that are planned to be exposed to very intense primary beams in future accelerator facilities. Any target that passes successfully the tough ‘‘survival training’’ in the LOHENGRIN beam tube should also qualify for a use with many mA=cm2 of heavy ion beams.
Acknowledgements 6. Comparison to actinide targets exposed to heavy ion beams Actinide targets similar to those used at LOHENGRIN are also being used for fusion–evaporation, fusion–fission, deep inelastic and other reactions induced by heavy ion beams. In these applications the primary beam intensity is limited by similar considerations. Table 2 gives a comparison of target heating and radiation damage at LOHENGRIN and for certain heavy ion beams. The heat deposition is of similar magnitude in both cases. An accelerator beam could be wobbled over a bigger target or the target might rotate while the accelerator beam irradiates a fixed position. Thus, with the exception of thermal cycling most parameters affecting the target lifetime, namely the induced radiation damage and the sputter rate can be directly scaled from the experience at LOHENGRIN. LOHENGRIN can therefore serve to perform already today tests of new actinide targets that are planned to be exposed in future to very intense primary beams, e.g. at the super separator spectrometer S3 of the SPIRAL2 project [21,22].
Conclusion and outlook
We are very grateful to those who prepared and provided the actinide targets for LOHENGRIN experiments, in particular Petra ¨ Thorle (Institute for Nuclear Chemistry, Mainz University), Tatiana Kuzmina (V.G. Khlopin Radium Institute, St. Petersburg), Ilham Almahamid (LBNL Berkeley and Wadsworth Center, Albany, NY), Adeline Bail (CEA Cadarache), Stanislav P. Vesnovskii (AllRussian Scientific Research Institute of Experimental Physics, Sarov, previously Arzamas-16) and the Institute for Reference Materials and Measurements, JRC Geel, Belgium. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Fission targets of various actinide elements have been used since 35 years at the LOHENGRIN recoil separator. The target behaviour shows a strong dependence on thickness, material and preparation method. The rapid burnup of thin actinide targets in a high neutron flux is a serious limitation for experiments but has not yet been fully understood. The performance of alternative target compounds such as metallic U, UCx , USix , UMox , UN, US, etc. should be tested. Special care has to be taken to appropriately choose the material of target backing and cover. Cover layers deposited by physical vapour deposition, i.e. evaporation or
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