Influence of heavy-ion bombardment on the structure of targets for atomic and nuclear interaction studies

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Nuclear Instruments and Methods in Physics Research A282 (1989) 206-212 North-Holland, Amsterdam

INFLUENCE OF HEAVY-ION BOMBARDMENT ON THE STRUCTURE OF TARGETS FOR ATOMIC AND NUCLEAR INTERACTION STUDIES K.E. STIEBING, D. KRAFT, P. SALABURA and K. BETHGE

Institut für Kernphysik der Universität Frankfurt, August-Euler-Strasse 6, 6000 Frankfurt, FRG

H. BOKEMEYER and H. FOLGER Gesellschaft für Schwerionenforschung, Planckstrasse 1, 6100 Darmstadt, FRG

An accurate knowledge of the absolute magnitude of the energy loss of heavy ions inside targets is necessary in many experiments . The energy loss and target properties are directly correlated . Even if the initial status of a target is known, the heavy-ion bombardment may lead to a drastic change of its properties or even to its total deterioration . The degree and timescale of a possible variation depends on the type of the projectile beam and on the target properties itself, the latter being influenced by the chemical composition, the physical characteristics, and the mechanical arrangement of the target system . Structural changes of a target system limit its useful lifetime ; it can be prolonged by special techniques like the application of target wobblers or target wheels . 1.6 MeV proton backscattering has been employed to investigate series of targets prior to and after their irradiation in heavy-ion beams in order to gain information on structural changes for the interpretation of experimental results obtained in heavy-ion positron spectroscopy. 1. Introduction For studies of atomic and nuclear interactions a thorough knowledge of the target characteristics is essential . Especially for investigations of resonance-like reactions in heavy-ion collisions with a drastic increase of the reaction cross section within a small energy window a careful selection and control of the target are inevitable . This is of particular interest in the heavy-ion positron experiments at the EPOS * apparatus. Besides a well defined total thickness it is important that the energy-loss straggling of the projectiles due to target thickness variations has to be as small as possible . This implies that the microstructure of the target has to be homogeneous . Generally, only thin, solid targets are used for such studies. To be more specific, for reactions in very heavy collision systems the resonance widths may be as small as 10 MeV [1]. Thus, for a 5.9 MeV/u 238U beam and 238U targets thicknesses in the order of Ax = 200 wg/cm2 are needed . In the particular case of heavy-ion positron spectroscopy a demand for heaviest collision systems is imposed by the threshold-like increase of the cross section with the combined charge Z = Zt + Z2 exceeding Z >_ 164 [2-4]. The above mentioned requirements introduce the specific problematics of preparing targets in the actinide

* EPOS stands for electron positron solenoid spectrometer. 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

region . At the same time the need of projectiles of equally heavy elements exists. Collisions of that type will cause severe deteriorations in target systems. A collection of information from proton backscattering measurements on deteriorated targets will be presented in this investigation.

2. Experimental situation 2.1 . In-beam monitoring of targets The most common way of controlling the state of a target during a heavy-ion bombardment is to monitor the elastically scattered heavy projectiles and recoil ions by means of a surface-barrier detector or an ionization chamber. Due to the energy resolution of DE = 30-40 MeV, this in-beam method is clearly limited in its application. Drastic deterioration effects like the partial decrease or increase of the effective target thickness by more than a factor of 2 are easily detected . If an extensive analysis of the spectrum of scattered ions is pursued, even minor changes in target layers with respect to loss of material or growth of contaminations can be detected [1,4]. The monitor detector integrates, however, over the whole size of the heavy-ion beam spot which is typically in the order of 3-10 mm in diameter and hence does not allow to localize inhomogeneities in the sub-mm range .

K.E. Stiebing et al. / Influence of heavy-ion bombardment 2.2 . Backscattering spectroscopy The need for a more accurate characterization of the targets arose with the observation of a very sensitive dependence of monoenergetic positron emission in heavy-ion collisions on the absolute projectile energy [2-4] . This detailed information, including that on the microstructure of a target layer in the sub-mm range, can be obtained by means of backscattering spectroscopy [5] . Using a 1 .6 MeV proton beam of the 2 .5 MV Van de Graaff accelerator at the Institut für Kernphysik, Universitlit Frankfurt, thicknesses and microscopic as well as large-scale homogeneities of multilayer targets can be determined with high accuracy [6] . The same procedure has been applied for the investigation of deteriorated target structures resulting from heavy-ion bombardments . Targets, including radioactive 248 Cm layers, were sampled (and transported) prior to and after heavy-ion experiments in order to specify the relevant parameters.

2.3. Main characteristics of the investigation The backscattering measurements have been carried out mainly for the following purposes : (a) for testing the thickness and homogeneity of nonirradiated targets for their use in heavy-ion experiments ; (b) for providing data on target lifetimes under the bombardment of very heavy ions : and (c) for finding favourable sample compositions and for the development of target preparation techniques [7] . The investigation was applied to targets which have been used in EPOS experiments [2-4] since 1983 . With the exception of a few self-supporting metallic targets (e .g., Ta, Au, Pb, Th) all others were of the "multilayer type", i .e. the target materials had been deposited as spots of diameters ranging from 2 to 20 mm onto supporting foils of carbon (-- 25 Wg/cm2 ) and had been coated with thin protecting carbon layers ( -- 10 ~L g/cm2 ) to reduce radiation-induced diffusion and sputtering of the target material [7] . In order to minimize chemical target deterioration for instance by oxidation, chemically stable compounds like oxides or fluorides were mainly used as target materials . The radiation doses of the sampled targets were determined by the demands of the heavy-ion experiment rather than what would have been desirable for a systematic target-development programme . The following presentation of the results of some of the analyses by backscattering spectroscopy is subdivided into three sections exemplifying steps of development of the target investigations pursued for the EPOS experiments .

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3 . Results 3.1 . Cm 203 targets on C and Ti backings First measurements have been carried out on heavyion-irradiated Ti/ 248 Cm 2 0 3 /C and C/ 248 Cm 2 03/C targets [8]. They had been fabricated by molecular plating [9,10], a method which is very efficient in handling even microgram amounts of target materials . A typical backscattering spectrum of an irradiated target of C/ 248 Cm 20 3/C of 150/430/30 R g/cm2, respectively, is shown in fig. 1 . All targets of this series showed similar extremely asymmetric energy distributions indicating a strongly inhomogeneous microstructure of the Cm 203 layer, where target material disappeared from certain areas and was accumulated in others . From the fact that these asymmetric backscattering profiles do not vary significantly when a target is scanned with the smallest possible proton probe (0 .1 mm x 0 .1 mm) over the whole diameter of the heavy-ion beam spot, one has to conclude that the average size of a region, where target material has disappeared or clustered, is in the order of wm . Therefore, a distinction has been made in this paper between large-scale homogeneity, which can be measured directly by scanning with a thin proton probe, and microscopic homogeneity, which is deduced from energy-loss profiles described in detail in ref. [6] . In the deteriorated areas the energy-loss data of the projectiles were found up to five times larger than the calculated values taking the nominal thickness [9,10] . In nearly all cases the nominal thickness corresponded very well to the total target thickness as derived from the backscattering yields [6] . More systematic investigations, not documented here, have been performed on targets of Gd, chemically the Cm homologous . The corresponding Gd 203 targets on Ti backings showed that even nonirradiated layers had already a coarse microstructure . This can be related to

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Fig . 1 . Proton backscattering spectrum of a C/ 248 Cm 20 3 /C target of 150/430/30 Wg/cm2 (log . scale) . VII . CHARACTERIZING TARGETS

KE. Stiebing et al. / Influence of heavy-ion bombardment

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the fabrication method : there the target material is deposited electrochemically on the Ti backing, and after drying - the target layer shows fine cracks, similar to the ones obtained from comparable 248Cm203 deposits [9,10] .

3.2 . Evaporated' 48CmF3 -targets between C foils A second series of backscattering analyses was carried out with C/ 248 CmF3 /C targets which had been prepared by evaporation at ORNL, Oak Ridge, USA [11] in cooperation with one of us (H .F .). Although the samples showed initially a satisfactory microstructure, the limited number of targets of that type demanded long irradiation times in the heavy-ion beam . As a result, the observed damages in the evaporated C/ 248 CmF3 /C samples were higher than for the electroplated C/ 248 Cm 2 0 3/C targets, which had been exposed to the beam for shorter periods of time . This is demonstrated in fig. 2, where examples are given from a scan over a C/ 248 CmF3 /C target of 60/350/15 wg/cm2 [11], which was irradiated by a 5 .9 MeV/u beam of 238U ions for - 24 h at an average beam flux of -- 1 .5 pnA . In the insert on top of fig . 2 the scan positions are indicated where the spectra have been recorded . From a view over the spectra (left column of fig. 2) two clear transitions can qualitatively be distinguished : the beginning of the spot of target material, going from spectrum pas. 1 to spectrum pas . 2, and the beginning of the

target frame bean spot CmF3 spot carbon foils 1234 scan position pos . 2

pos. 3

pos .4

10ô0 lzoo 1600 backscattering energy IkeVI

1wo

o

2o 4o w e0 100 energy loss of II-projectile IMeV1

Fig . 2. Proton backscattering spectra (left column) from a scan over a C/ 248 CmF3 /C sandwich target of 60/350/15 p g/cm2 . The relative scan positions are indicated in the insert (top right). Peaks (1) and (2) in the spectrum pas. 2 indicate impurities, in the right column the analysis of the energy-loss distribution of Cm is given for scan pas . 2-4.

heavy-ion bombarding zone, going from spectrum pas . 2 to spectra pas . 3 and 4 in fig. 2 . Spectrum pas . 1 represents the backscattering spectrum of a carbon foil of the summed thickness of supporting and covering foils . It also shows some negligible amount of 248 Cm which was probably deposited outside the target spot during the evaporation process . Other backscattering signals originate from wall material of the chamber ; they are produced by secondary backscattering [6] at the brass of the LN2 trap, which surrounded the highly radioactive target to protect other components of the beam line . Spectrum pas. 2 is a typical example for a microscopically homogeneous target of C/ 248 CmF3/C of 60/350/15 Wg/cm2 [11] . The peaks of the two carbon foils are now separated by the energy loss of the protons in the 248 CmF3 layer . The relative amounts of curium and fluorine atoms as calculated from the total backscattering yields are close to the expected stoichiometric ratio of 1 : 3 . Besides a very small content of oxygen (A x < 20 h g/cm2 ) some other contaminations at higher masses can also be seen in spectrum pas . 2. Due to the limited mass resolution in this range and due to the lack of additional information about the depth of location of that layer inside the target it is ambiguous to relate the impurity to an element only by means of the information provided by the backscattering spectrum . In this case a unique identification would have been possible by means of an additional PIXE analysis [6], which had not been performed for this particular target . In the spectra pas . 3 and pas . 4, both the covering layer as well as the impurity peaks seem to have disappeared . This fact is due to the massive deterioration of the 248 CmF3 layer which manifests itself by a drastic broadening of the 248 Cm and F backscattering peaks . Only the carbon backing foil, being localized upstream in the proton beam relative to the 248 CmF3 , appears as a well defined distribution . Contributions from other target layers are hardly visible because in addition to the energy loss of protons in that respective target component the energy loss from trespassing the deteriorated CmF3 layer has to be added . The quantitative analysis shows that no damage has happened to the carbon backing foil and that loss of material from the thin protecting layer just starts. No noticeable loss of the absolute amount of curium has been detected in this target, so that the enormous change in the energy-loss profiles of the Cm and the F backscattering peaks have to be assigned to the already mentioned change of the target layer towards a "granular" structure . The obvious structure influence of the target deterioration on the definition of the heavy-ion impact energy is demonstrated in the right-hand column of fig . 2, where energy distributions are displayed for three scan positions of 248 Cm as taken from the measured backscattering profiles .

K.E. Stiebing et at. / Influence of heavy-ion bombardment 3.3. ThF, and UF4 targets sandwiched between C foils

been discussed in an earlier paper [12] . Here, two representative examples of radiation-induced changes of structures in fluoride targets will be discussed with the help of the backscattering spectra displayed in figs . 3 and 4 (left columns) for a C/ThF4/C target of 35/280/5 Wg/cmz and for a C/UFq/C target of 35/270/5 wg/cmz , respectively . Both targets have been exposed to the heavy-ion beam for a period which was an order of magnitude shorter than that of the C/ 24s CmF3/C targets mentioned above. Even at these low total doses radiation-induced changes can clearly be seen and the heavy-ion-bombarded area can be well localized on both lateral target scans given in figs . 3 and 4 (right columns). The radiation damages are such that, due to the much thinner protecting carbon foils, a loss of material started already and partly compensated the pileup of target material . Therefore the energy-loss distributions of deteriorated regions show still a confined shape of an average width (at FWHM) comparable to those of nonirradiated regions of the target . Their shapes, however, deviate clearly from the more rectangular distributions of the nonirradiated areas of spectrum pos . 23 .0 in fig. 3 and spectrum pos . 1 .5 in fig. 4 . The spectra taken from the most destroyed target spots

The investigations mentioned above clearly demand substantially reduced irradiation doses per unit area in a heavy-ion experiment . This means that the whole target or the target spot has to be changed more frequently either with help of mechanical devices like target wobblers or target wheels [4], or simply by exchanging the whole target at shorter time intervals . Thorium targets have been chosen for many consecutive EPOS studies, partly because there was only a small and limited amount of curium available, and partly because collision systems including thorium offered to gain new aspects of the positron production process . These targets mostly have been fabricated as C/ThF4 /C multilayer targets [13], which were proven to be of excellent initial microscopic homogeneity . A scan over a nonirradiated C/ThF4 /C target is given in ref. [6] together with a high accuracy backscattering spectrum. The sharp profile of the target spot and the very good large-scale as well as microscopic homogeneities are evident and typical of these targets . Studies of radiation-induced changes of target properties in a C/ThF4/C sample of this series have already

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Fig. 3 . Proton backscattering spectra from a scan over a C/ThF4 /C sandwich target of 35/280/5 Wg/cmz (left column) . Analysis of the scan data (right column) : (a) Measured energy loss for protons (left scale) converted into a Th thickness (right scale) . (b), (c) Target thicknesses derived by the yield analysis of (b) Th, (c) C backing (squares), and C protection foil (triangles), respectively . VII .

CHARACTERIZING TARGETS

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K.E. Stiebing et al / Influence of heavy-ion bombardment

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Fig. 5 . Proton backscattering spectrum and analysis from a scan over a C/U/C target of 40/380/5 pg/cmz . Left column : (top) backscattering spectrum, (middle) comparison of the energy loss distribution for O and U, and (bottom) relative depth distribution of 0 in U . Right column : Analysis of scan data for U and C (as in fig. 3).

K.E. Stiebing et al. / Influence of heavy-ion bombardment exhibit profiles of even triangular shapes . The extraordinarily large destruction displayed in spectrum pos. 2 .5 of fig. 1 may possibly be caused by an inhomogeneous beam profile, leading to a distinctly higher radiation dose at that point of the target than calculated on the average from cup measurements. A particularly interesting consequence can be drawn from the spectra given in fig . 4. An increase of the thickness of the initially very thin covering foil (nominal thickness 5 ~Lg/cm2 ) develops simultaneously with the degree of destruction seen in the uranium- and fluorine profiles . For the most destroyed layer in spectrum pos . 2 .5 the supporting foil seems even to have vanished, whereas the total number of carbon scattering centres (both foils added) remains more or less constant as can be seen from the scan in fig . 3c . This fact again points to a very specific property of the microstructure of the C/UF4/C target : it indicates that major parts of the carbon foils of the target spot are no longer separated by UF4 material, and therefore appear at the "nonshifted" position of the covering foil, which was upstream in the proton beam with respect to the UF4 . The "missing" UF4 target material on the other hand has piled up on the remaining parts of that target area . The average ratio R n of "covered" to "uncovered" target areas can be estimated for every backscattering spectrum of a lateral scan (index n), if the original scattering yield from the carbon foil that faces the proton beam is known, e.g ., by means of a reference spectrum (index 0) taken from a nonirradiated region of the same target :

Rn =

A Fco, _ Nn ( C2 ) . N nA Funco, .(Cl )No(C l )F

Here Nn(C2) denotes the integrated yield of the "downstream layer" in the proton beam (i .e., the backing in the example given) and Nn ,o (C l ) the integrated yields of the "upstream layer" for the considered (n) and referenced spectrum (0), respectively . F is a normalization constant that takes possible differences in proton doses of spectra (n) and (0) into account . For the example given above (pos . 2 .5 in fig . 4) a value of R 2 .5 = 0 .25 was calculated being subjected to large statistical errors as N2_5 (C2) is a small number and therefore only represents a rough estimate of the reduced density averaged over the layer depth . More quantitative information of the actual distribution of the target layer can be obtained by analyzing the differential scattering yield dNP/dE BS , which is the yield of backscattered protons per unit energy loss for the target backscattering profile . In pos . 2 .5 in fig. 4 a variation of this yield is seen, ranging from 30% to almost 0% of the yield ina nonirradiated spot (pos . 1 .5 in fig . 4) .

3.4. Metallic layers sandwiched between carbon foils It was found in these investigations that metallic layers represent the most stable targets, if no chemical or thermal deterioration like oxidation or melting occurs during heavy-ion bombardments. As an example of the influence of oxidation on the target structure the results from a backscattering analysis of a uranium multilayer target of C/U/C of 40/380/15 Wg/cm2 , respectively, are shown in fig . 5 . The evaporation of uranium onto the carbon backing and the sequential deposition of a protecting carbon layer are always both performed in the same vacuum cyclus [14] . Two weeks after this preparation such a target was measured in the proton beam. An oxygen content is clearly visible in the backscattering spectrum (fig. 5) and amounts to (4 ± 1) wg/cm2 . Of particular interest are the profiles of both the oxygen and the uranium backscattering distributions. From the identical widths of the distributions, as demonstrated in the middle of the left column of fig . 5, follows that oxydation proceeds equally strong through both carbon layers . The ratio of oxygen to uranium as a function of the layer depth is displayed in the lower graph of fig. 5 (left column) indicating a minimum towards the middle of the layer . Together with the oxidation a deterioration of the uranium layer itself takes place as is evident from the shape of the slope of the uranium backscatteringenergy distribution (Fig . 5, left column) . The distinctly smoother high-energy edge as well as the tailing of the profile towards low energies both indicate a coarse structure of the oxidized layers.

4. Concluding remarks It has been shown that radiation damages from heavy ions of - 6 MeV/u cause massive changes of the microstructure in targets . The extent and speed of this destruction are obviously dependent on the target material itself and on its structure . It is drastic for thin unprotected targets (Ax <_ 300 Wg/cm2 ), where electrically nonconducting materials (e .g . oxides) are brought onto carbon backings . Also coating with a thin protection layer does not slow down the deterioration significantly and is important only to conserve the total amount of target material for a prolonged amount of time . The destruction is proceeding in such a way that a "granularisation" of the microstructure leads to a distribution where material disappears from certain areas to be piled up to a multiple of the original thickness in others . No drastic overall changes in the total scattering yield have been observed though. This confirms that the major damage to the targets is indeed effected by radiation-induced processes and that, e .g ., no "flow" of target material along the surface of the backing foil VII . CHARACTERIZING TARGETS

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takes place, which would have been expected if macroscopic effects like target temperature were the driving mechanisms . This conclusion is also supported by the fact that no target temperatures above - 620 K have been observed with an infrared-sensitive camera installed at the EPOS experiment . The consequences of the structural changes are essential for a heavy-ion experiment . Compared to a homogeneous target one has to consider : (a) much lower effective reaction rates and (b) a much lower differential yield dNp/dE BS , i .e., effective target range, but : (c) the same absolute background from the reactions of uranium projectiles and carbon target atoms, and (d) higher background from elastic scattering and reactions originating from the largely extended regions of the target, where the projectile energy is off the resonance. From the large number of targets investigated during this study (- 200) it can be estimated that an irradiation dose of D = 1013 particles/mm2 will not strongly deteriorate the signal-to-background ratio. This value, therefore, was set as an upper limit for heavy-ion applications . It implies that for 6 MeV/u of Pb, Th, or U at ion currents of typically 0 .5 pnA/mmz fixed targets have to be changed about every 2 hours and wobbler targets every 15 hours, unless the monitor detector indicates a massive loss of material prior to this time . In critical experiments, as in the case of using 24sCm targets, a warm-up procedure has been performed, i .e ., the beam intensity has been increased in small steps from zero to full within a few minutes . The information achieved on target structures and structural changes using the backscattering spectroscopy influenced conclusions drawn from the results of the heavy-ion positron experiments and initiated new developments of heavy-ion target preparation techniques. Further, systematic studies are necessary for a deeper understanding of the mechanisms leading to the observed deterioration of thin heavy-ion target layers .

targets. With the permission of the US DOE under the contracts WC-EU-136 and WC-EU-241 248Cm targets could be investigated, kindly prepared in part at the Institut für Kernchemie, Universität Mainz, and at the IRML, Oak Ridge . The work has been supported by the Bundesministerium für Forschung und Technologie (FRG) .

Acknowledgements

[10] [11]

The support of this work within the EPOS collaboration is acknowledged . Of big help has been the assistance of the radiation safety groups of GSI and IKF . We also thank M . Kl0ver and M . Begemann-Blaich for their participation in the investigations, and the staff of the GSI Targetlabor for the preparation of numerous

[12] [13]

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