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Pd targets
Nuclear Instruments and Methods in Physics Research B 318 (2014) 297–305
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Preparation and high intensity heavy ion irradiation tests of intermetallic 243Am/Pd targets I. Usoltsev a,b,⇑, R. Eichler a,b, G.K. Vostokin c, A.V. Sabel’nikov c, N.V. Aksenov c, Y.V. Albin c, G.A. Bozhikov c, V.I. Chepigin c, S.N. Dmitriev c, V.Ya. Lebedev c, O.N. Malyshev c, O.V. Petrushkin c, D. Piguet a, G.Ya. Starodub c, A.I. Svirikhin c, A. Türler a,b, A.V. Yeremin c a b c
Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland University of Bern, CH-3012 Bern, Switzerland Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Russian Federation
a r t i c l e
i n f o
Article history: Received 19 August 2013 Received in revised form 22 October 2013 Available online 8 November 2013 Keywords: Target Electrodeposition Actinides Intermetallic Americium Palladium
a b s t r a c t Previously reported preparation method for Pd-based intermetallic targets (Usoltsev, et al., 2012) [1] has been successfully applied for producing two stationary 243Am/Pd targets. Both targets have been irradiated at the U-400 cyclotron at Flerov Laboratory of Nuclear Reactions Dubna (Russian Federation) using high intensity beams (up to 0.83 lApart) of 48Ca18+. Alpha-particle spectroscopy and light microscopy allowed for a comprehensive characterization of the intermetallic targets before and after irradiation. A natNd/Pd intermetallic target and a solely electroplated 243Am/Ti target were similarly investigated for comparison. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Extremely low production cross-sections of Super Heavy Elements (SHE, Z > 103) [2–4] stimulated severe improvements of heavy ion sources and modernization of accelerators leading thus to a significant increase of heavy ion beam intensities available for irradiation experiments [5]. In order to benefit from this progress and maximize the SHE production rates, development of actinide targets which are able to stand long-term irradiations at high heavy ion beam intensities is of great importance. Normally production of heavy ion targets is based on the electrodeposition [6–11] of thin layers of a target material on thin foils of high-melting elements with low atomic numbers, e.g., C, Be or Ti. Rotating targets [12,13] manufactured this way are perfectly suitable for physics experiments e.g. [14,15], where only thin layer targets are acceptable, and for chemical experiments using more abundant and not too radioactive target materials. However, for chemistry experiments thick targets are advantageous to increase the production rate of SHE and to simultaneously produce several isotopes of the same element in different neutron evaporation channels [16]. Limited target material amounts and high ⇑ Corresponding author. Address: Paul Scherrer Institute, OFLB/105, CH-5232 Villigen PSI, Switzerland. Tel.: +41 056 3105771; fax: +41 056 3104435. E-mail address: [email protected] (I. Usoltsev). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.10.019
radioactivity require the use of stationary targets during the month-long experiments [16,17]. Recently we suggested an efficient approach to produce intermetallic targets for heavy ion irradiations [1], implying that the metallic state of the entire target is superior to electroplated targets in terms of electric conductance, heat conductance and mechanical stability. Based on the molecular plating1 technique followed by the chemical path of coupled reduction2 [18–20], the suggested method allows for producing stable and homogeneous metallic lanthanide and actinide targets. In the first step, the target material is electroplated on a thin Pd foil surface, insuring efficient and homogeneous distribution of the desired element on the target backing. In the second step, the foil with the plated material is heated up in a flow of pure hydrogen. The reduction of the target material with hydrogen at high temperatures becomes thermodynamically feasible due to the formation of an intermetallic compound with the Pd support. In the case of an actinide (An) oxide the coupled reduction reaction occurring on the Pd surface can be written as follows:
xPd þ Anx Oy þ yH2 $ xAnPd þ yH2 O
1 2
In this paper we use this term as a synonym for electrodeposition. Direct translation from German ‘Gekoppelte Reduktion’.
ðiÞ
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It was also observed that the reduced target material diffuses into the depth of the foil immediately after the reaction (i), thus allowing further reduction of the remaining electroplated layer. Once the whole amount of the target material is reduced, it will be broadly distributed within the Pd backing. The final distribution depends on the reduction temperature, reduction time and the thickness of the Pd foil and can be changed by additional heating of the target in either reducing or oxidizing atmosphere. Here, we report on a proof of principle, i.e., the preparation of two 243Am/Pd intermetallic targets with different thicknesses, which were irradiated with high intensity 48Ca beams and characterized before and after irradiation by alpha-particle spectrometry and by optical light microscopy. For comparison with widely used nowadays solely electroplated targets, similar experiments with an electroplated 243Am/Ti target were carried out as well.
and gamma-ray spectrometry in conjunction with acquisition and analysis systems based on Canberra’s Genie2kÒ allowed for a quantitative analysis of the produced targets. 2.1. Nd-target Based on the experience from [1] the 0.7 mg/cm2 Nd-layer (calculated as pure metal, natural isotopic composition) was electroplated in the cell depicted in Fig. 1. Deposited in 10 steps from iPrOH and reduced in hydrogen atmosphere at 900 °C, the target was prepared for test irradiation experiments, the importance of which was conditioned mainly by the possibility of a target meltdown, which is highly undesirable in the case of highly radioactive 243 Am/Pd targets. This target is in the following referred to as Ndtarget. 2.2.
2. Experimental Prior to their use, the light-tested (pinhole-free) 3 lm Pd foil (purchased from GoodFellowÒ) was polished with a diamond polish (1 lm grain size) and subsequently rinsed with distilled water and ethanol. Two 243Am/Pd targets were electroplated by using a 19 ml electrodeposition cell shown in Fig. 1 (deposition area 1.76 cm2). Due to the large effective deposition area and high thickness, the electroplating was performed batch-wise. Multiple-step deposition process was found to be a reliable procedure for obtaining high quality depositions on Pd surface [1]. The target material was dried on a heating plate after each deposition step. A 1 mm thick spiral-shaped Pd wire covering the required circular deposition area served as a counter electrode placed at a distance of about 1 cm from the cathode. The parameters for the electrodeposition of all prepared targets are given in Table 1. Adjustments of the applied voltage were necessary in order to keep the current within the limits given in the table. Deposition homogeneity was examined by optical microscopy using a DinoEyeÒ (magnification: 10–200) and a SomikonÒ (magnification: 20–500) USB digital microscopes. For the reduction in hydrogen atmosphere at high temperatures, the foil with electroplated target material was placed in a tubular quartz apparatus (Fig. 2) on a flat alumina support. The apparatus was heated up by a tubular resistance oven. For safety reasons the quartz tube was flushed with pure He before and after the reduction in H2 atmosphere. Due to the high alpha-activity, all 243Am targets were prepared in a glovebox in the class II radiochemical laboratory. Standard alpha-particle spectroscopy
Am-Target I
The first americium target, in the following referred to as Target I, with the thickness of 0.85 mg/cm2 was electrodeposited from 243 Am nitrate solution in iBuOH in ten consecutive steps with an average yield of 90% on a 3 lm thin Pd foil. Neodymium with natural isotopic composition (natNd) was added to the target (50 lg/ cm2) in order to monitor the target behavior on-line during the irradiation with 48Ca by producing volatile 185Hg in the fusionevaporation reaction natNd(48Ca,xn)185Hg. The deposition yield for 243 Am was quantified after each step by means of gamma spectrometry of an aliquot from the plating solution. 90% deposition yield was assumed for natNd [1]. The target with the plated material was dried up on a heating plate at 100 °C after each deposition step without taking the target out of the cell. Target I was reduced in a tubular quartz reaction chamber (Fig. 2) in a 100 ml/min flow of pure hydrogen at 900 °C. Completeness of the reduction was controlled visually every 10 min by taking the quartz tube out of the resistance oven. Unreduced target material was easily distinguishable from the grayish surface of 243 Am–Pd intermetallic alloy by its black color and by the candoluminescence of the oxide layer on the surface [1]. The reduction process was stopped after 30 min. 2.3.
243
Am Target II
The second target, in the following referred to as Target II, with the thickness of 1.7 mg/cm2 was electrodeposited from 243Am nitrate solution in iPrOH in five consecutive steps with an average yield of 99%. Similarly, neodymium with natural isotopic composition (natNd) was added to the target (50 lg/cm2). The deposition yield for 243Am was quantified after each step by means of gamma-spectrometry. In contrast to Target I, the second target was not just dried up on a heating plate at 100 °C, but annealed at 350 °C after each deposition step. Attempting to obtain a pinhole-free target the reduction temperature was lowered to 840 °C. This temperature however appeared to be insufficient for the reduction purposes. By gradually increasing the reaction temperature and visually checking the completeness of the reduction process, temperature of 890 °C was finally reached. The overall time of the reduction procedure exceeded 2 h. 2.4.
Fig. 1. Scheme of the cell used for the electrodeposition of 243Am and natNd on 3 lm Pd foils.
243
243
Am/Ti target
The 243Am/Ti target was produced by using an untreated 2 lm thick Ti foil as a backing in a 9 ml cell with water cooled stainless steel electrodes (Fig. 3) based on the suggestions from [21]. A 1.2 mg/cm2 243AmO2 target was prepared from 243Am nitrate solution in iBuOH in five consecutive steps with an average yield of 99%. Annealing at 350 °C between the deposition steps was found
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I. Usoltsev et al. / Nuclear Instruments and Methods in Physics Research B 318 (2014) 297–305 Table 1 Experimental conditions for the molecular plating. Target
243
Target I Target II Am/Ti Nd-target
0.85 1.7 1.2 0.7 (natNd)
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Am amount (mg/cm2)
Plating solution
Voltage (V)
Current (mA)
Number of deposition steps
Duration of one step (min)
Deposition yield (%)
Deposition quality
Nitrate/iBuOH Nitrate/iPrOH Nitrate/iBuOH Nitrate/iPrOH
200–600 250–450 500–1000 200–300
1.2–1.9 0.9–1.3 2.0–3.0 1.0–1.5
10 5 5 10
40 60 120 40
90 99 99 70
Poor Good Good Good
Fig. 2. Scheme of the quartz apparatus used for the reduction in H2 atmosphere. The electroplated target is supported inside the tube by alumina slab at the end of the quartz tube. This part is afterward heated up by means of a resistance oven.
Fig. 4. Scheme of the experimental gas-jet set-up used for investigating adsorption of the transported isotopes during the irradiation [22]. Arrows are indicating the direction of the gas flow. (1) 243Am target. (2) 4 lm Ti vacuum window. (3) Quartz housing of a target chamber. (4) Water cooled beam stop. (5) Target chamber. (6) Transport capillary. (7) IVO oven held at 800 °C. (8) Transport capillary heater. (9) 16 pairs Au(Si) detector array held at ambient temperature. (10) 16 pairs Au(Si) detector array held at 0 °C. (11) Gas cleaning module, which includes drying cartridges and aerosol filters. (12) Buffer volumes. (13) Pump. (14) Quartz wool as an aerosol filter. (15) Thermostate.
Fig. 3. Photograph of the electrodeposition cell used for 243Am/Ti target preparation. The Teflon housing in the middle serves as the containment for the electroplating solution. The large cross-shaped screws in front and at the back of the cell are used for pressing the water-cooled electrodes with mounted Ti foils to the plating windows in the Teflon housing.
to be beneficial for the final target quality. A thick 20 lm Ti foil served as an anode material and was mounted at a distance of 2.5 cm from the counter electrode. The 1.6 cm2 effective deposition area was confined by the Teflon housing of the cell. Neodymium with natural isotopic composition was added on the every deposition step in the form of nitrate to the electroplating solution, so that the overall thickness of natNd layer accounting for 50 lg/cm2 was evenly distributed throughout the entire target layer.
12.5 cm3 by an argon–helium mixture (30:70%-vol.) held at 1.25 bar. Driven by a membrane pump (13), the gas mixture was flushed through the system at a rate of 2 l/min. Transported through an aerosol filter (7) and a 3 m long PFAÒ-Teflon capillary with 2 mm inner diameter (8), produced nuclides reached two consecutively placed silicon detector arrays. Covered with gold, each 32-channel array was forming a 16 cm long rectangular chromatography channel of 2 cm width and 2 mm height (9, 10, see also Fig. 5). The first detector array was kept at room temperature, whereas the second one was cooled by a thermostate (15) to 0 °C. The 64 detectors registered alpha-particles and spontaneous fission fragments in an energy dispersive event-by-event mode. Recirculation of the transport gas in a loop through a drying system (11) assured high purity and dry conditions in the system with a continuously measured dew point of about 60 °C.
2.5. Target setup and gas-jet system
3. Results
A gas-jet set-up depicted in Fig. 4 was used to investigate the transport and deposition of volatile isotopes on the gold-covered surface of two silicon detector arrays. The circular shaped 48Ca18+ beam of 2 cm diameter provided by FLNR’s U-400 cyclotron was collimated through a water-cooled honeycomb-grid shaped copper collimator and passed through a 4 lm Ti vacuum window (2), which separated the recoil chamber from the beam line. Subsequently it passed the target (1), supported by a honeycomb grid mounted in a target holder, the recoil chamber (3), and finally it was stopped in a water-cooled copper beam stop (4). Formed nuclear reaction products were stopped in the recoil chamber of
3.1. Nd-target preparation results The produced Nd-target (Fig. 6) was essentially very similar to the Eu/Pd and the U/Pd targets, preparation of which was in detail described in our previous publication [1]. Due to relatively high Nd layer thickness (0.7 mg/cm2) the problem with the formation of pin-holes upon the reduction procedure was not solved. An upper limit target thickness of 0.3 mg/cm2 was found for obtaining pinhole free Nd intermetallic targets based on 3 lm Pd backings. In the Pd–Nd system Nd is the low-melting component (m.p. 1021 °C) and therefore by increasing its content we are lowering
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Fig. 5. 8-Detector ‘sandwich’ used in the detector array. Left: open ‘sandwich’ with the dark rectangular areas showing the four active Si detectors prepared on one waver chip; right: closed detector ‘sandwich’. Mounted in a row, four such gold covered ‘sandwiches’ form 16 pairs of a Au(Si) detector array.
unacceptable in the case of Pd-based target. A poor quality nonhomogeneous deposition was obtained as illustrated in the micrographic picture shown in Fig. 7. The alpha-particle spectrum of Target I is given in Fig. 8. Micro-optical analysis of Target I revealed pinholes after the reduction step shown in Fig. 9 with an additional light source underneath the target. The pinholes are estimated to cover about 5% of the entire target surface area. Alpha-particle spectra were taken from both sides of the target after the reduction as well (Fig. 10). The high energetic edge of the alpha-particle spectrum is not shifted towards lower energies in both spectra. This additionally proves that the target material was indeed reduced, since only reduced 243Am could diffuse through the whole thickness of the backing foil and reach the opposite side of the target. Fig. 6. Optical microscopy image of the Nd-target. Scale bar is 0.3 mm.
the melting point of the formed alloy, which in turn causes the local melting and formation of pinholes.
3.3. Target II preparation results The use of iPrOH solution and annealing at higher temperatures was found to be beneficial to the deposition quality. As a result
3.2. Target I preparation results The procedure described in the experimental part and successfully applied for preparation of 243Am/Ti target was found to be
Fig. 7. Optical microscopy image of the Pd foil after electrodeposition of 0.85 mg/ cm2 243Am layer from the nitrate solution in iBuOH. Scale bar is 1 mm.
Fig. 8. Alpha-particle spectrum of Target I taken after electrodeposition. A peak at 5.275 MeV corresponds to the main component 243Am; low intensity alpha-lines at 5.485 and 5.813 MeV correspond to 241Am and contamination with 249Cf, respectively. Origin of the 241Am impurities was traced back to the 243Am stock solution. The 249Cf contamination is artificial.
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Fig. 9. Optical microscopy image of Target I after reduction in H2; an additional light source was placed underneath the target. Scale bar is 1 mm.
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Fig. 12. Optical microscopy image of Target II after reduction in H2; an additional light source underneath the target. The light scattering virtually increases the size of the pin holes, which were estimated to cover about 5% of the target area. Scale bar is 1 mm.
Fig. 10. Alpha-particle spectra of Target I taken from both sides before irradiation. Fig. 13. Alpha-particle spectra of Target II taken from both sides before irradiation.
provided unambiguous evidence of the reduction process as discussed before. 3.4.
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Am/Ti target preparation results
Fig. 14 shows a well adhesive layer and highly homogeneous distribution of the thick 1.2 mg/cm2 target material on the Ti surface backing which was obtained upon electrodeposition.
Fig. 11. Optical microscopy image of Target II after electrodeposition of 1.7 mg/cm2 Am layer from the nitrate solution in iPrOH. Scale bar is 1 mm.
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homogenous layer of the target material was deposited on a wrinkled surface of the 3 lm Pd foil (Fig. 11). Despite of the fairly good deposition layer and the lowered reduction temperature, the problem with pinholes formation persisted. A micrograph of the final product is shown in Fig. 12 with an additional light source underneath the target. An alphaparticle spectrum (Fig. 13) of the target after the reduction
Fig. 14. Optical microscopy image of the Ti foil after electrodeposition of 1.2 mg/ cm2 243Am layer from the nitrate solution in iBuOH. Scale bar is 1 mm.
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3.5. Nd/Pd target irradiation results The irradiation conditions of the Nd-target are collected in Table 2. The stability of the target was monitored by the yield of 185 Hg, produced in hot-fusion reactions with Nd isotopes, e.g., nat Nd(48Ca,xn)185Hg and deposited in the first detector array. A constant production yield of 185Hg was a strong indication for the integrity of the target (Fig. 15). At the beginning of the experiment, a silica frit held at 800 °C was used as an aerosol particle filter in the gas-jet transport system. It appeared to be ineffective letting pass aerosol particles formed in the target chamber. As a result alpha-particles of nonvolatile 150,151Dy isotopes formed in hot-fusion reactions, e.g., 108Pd(48Ca,5n)151Dy were detected along both detector channels in the 4.0–4.2 MeV energy region (Fig. 16). Replacement of the frit by a quartz-wool based filter (Fig. 414) heated to 800 °C removed efficiently this unwanted transport product. Thus, no significant amount of Dy was observed in all subsequent experiments (Fig. 16B).
Fig. 15. On-line monitoring of the Nd-target. 185Hg yield in the first detector array at the given beam intensities during the irradiation.
3.6. Target I irradiation results The irradiation conditions of Target I are given in Table 2. Although the natNd/Pd target withstood the high intensity beam, there was no guarantee that the slightly thicker 243Am intermetallic Target I would stand similar irradiations conditions. Therefore, the beam current was increased gradually from 1 lAelectr slowly up to 15 lAelectr (0.83 lApart). After 2 days of irradiation at low beam intensity the first optical examination revealed an unchanged target. Subsequently, the Target I was irradiated for another 2 days at the highest beam intensities up to an integral dose of 1.1 1018 48Ca particles at a primary beam energy of 269 ± 1 MeV. Sharp drop of the production yield at a day 4.5 was due to temporal technical issues, while subtle variations of the beam intensity acconted for the erratic pattern of the yield on a short time-scale. The integrity of the target was monitored by means of alpha spectroscopy yields of volatile products as well as nuclides of their decay chains (185Hg, 211At, 219Rn, 220Rn and 211–212 Po) (Fig. 17). The natNd nitrate admixture to the 243Am stock solution accounted for production of 185Hg, while multinucleon transfer reactions between 243Am (with Pb contaminations) with 48 Ca lead to the production of isotopes of Rn and At transported to the detector, thus allowing for an direct on-line measurement during the irradiation. In Fig. 18 deposition yields of 212Po and 185 Hg are plotted against the irradiation time. The amount of the target material left after irradiation was quantified by alpha-particle spectroscopy, revealing that less than 5% of the initial activity was lost during the irradiation. An accidental 30 min interlock in the gas-jet system and irradiation without gas cooling may account for such a loss. The alpha-particle activity distribution within the target did not change dramatically during the irradiation (similar to Target II, see Fig. 21). This points towards target temperatures at irradiation conditions, at which the Am diffusion in Pd is still slow, i.e., well below 900 °C.
Fig. 16. Sum alpha-particle spectra measured during natNd/Pd target irradiation. (A) Alpha spectrum of the first part revealing Dy transport by aerosol particles using an inefficient silica frit filter. Applied 48Ca beam dose reached 2 1015 particles. (B) Alpha spectrum obtained using the efficient aerosol particle filter at an integral 48Ca beam dose of 2.5 1017 particles (for details see text).
3.7. Target II irradiation results The irradiation conditions of Target II are compiled in Table 2. The stability of the second 243Am target was also assessed by continuously monitoring 185Hg and 220Rn?212Po production (Fig. 19). Light microscopy pictures were taken after the irradiation for examination of the integrity of the target. Fortunately, no significant enlargement or increase of the number of pinholes was observed (Fig. 20). Some visible upper surface melting indicates local high temperatures, which nevertheless did not lead to a target meltdown. The alpha-particle spectra did not show a significant 243Am displacement in the target (Fig. 21), proving that the
Table 2 Irradiation parameters and calculated stability for investigated targets. Target (mg/cm2) 243
Am/Pd 0.85 Am/Pd 1.7 243 Am/Ti 1.2 nat Nd/Pd 0.7 243
a b
48
Ca18+ beam dose 18
1.1 10 6 1017 3.2 1018 2.5 1017
Beam energy (MeV)
Mean/max. beam current value (lApart)
Stabilitya
269 270 273 270
0.50/0.83 0.56/0.78 0.50/0.56 0.23/0.67
95% 97% 95% N/Ab
Stability of the target is expressed in percentage value and refers to overall The target did not contain any radioactive isotopes before irradiation.
243
Am amount preserved in a target after irradiation.
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Fig. 17. Sum spectrum of alpha-particles recorded during the irradiation of Target I.
Fig. 19. On-line monitoring of Target II. (A) 185Hg production yields during the irradiation. The structure of the 185Hg yield represents the beam intensity variations. (B) 212Po yields during the irradiation. The in-grow of the long-lived 212 Po precursor 212Pb (T1/2 = 10 h) can be seen.
Fig. 18. On-line monitoring of Target I. (A) 185Hg production yields during the irradiation of Target I. After the intermediate target analysis (day #3) beam intensity reached its maximum value of 0.83 lApart. (B) 212Po represents the production and transport yields of 220Rn during the irradiation of Target I. The slopes after the beginning of the irradiation and after the intermediate target analysis on day #3 are due to the scheduled slow beam intensity increase and the in-grow of the long-lived 212Po precursor 212Pb (T1/2 = 10 h) which is a descendant of in-flight decaying 220Rn.
target temperature during the irradiation was not high enough for reaching significant diffusion within the Pd foil. This observation leads to an assumption that the target temperature at the applied beam intensities (up to 0.78 lApart) did not exceed 900 °C.
Fig. 20. Optical microscopy image of Target II after irradiation; an additional light source is placed underneath the target. Scale bar is 1 mm.
3.8.
243
Am/Ti target irradiation results
The 1.2 mg/cm2 solely electroplated 243Am/Ti target was irradiated for 14 days and experienced a three and six times higher integral beam dose in comparison to Target I and Target II, respectively (see Table 2). However, only maximum intensities of 0.5–0.56 lApart have been applied. On-line monitoring of 185Hg
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Fig. 21. Comparison of the alpha-particle spectra of top sides of Target I and Target II taken before and after irradiation.
Fig. 23. Optical microscopy image of the 243Am/Ti target after irradiation; (A) without an additional light source placed underneath the target; and (B) with light source. Scale is given by honeycomb grid pattern – the hexagon side length is 2 mm. It can be clearly seen, that the Ti backing foil is destroyed by a chemical reaction with the target material under the harsh irradiation conditions. A translucent unidentified crystalline product is obtained.
3.9. Post-irradiation analysis The performance evaluation of the investigated targets included not only the on-line 185Hg and 212Po monitoring and light microscopy but also post-irradiation alpha-particle measurements (see Fig. 21). In Table 2 we summarized irradiation parameters for all examined targets. The stability of the irradiated targets was assessed by means of alpha-particle spectroscopy except for the non-radioactive natNd/Pd target. The conducted measurements allowed for determination of the target material loss after irradiations. The percentage of overall 243Am alpha activity remaining on the target after irradiation is shown in the last column of Table 2.
4. Discussion
Fig. 22. (A) 185Hg production yields during the irradiation of the 243Am/Ti target. After 6 days of irradiation the Ti backing is considerably damaged and the recoiling products are not pre-stopped anymore by Ti, thus implanting into the walls of the recoil chamber. (B) 212Po yields during the irradiation of the 243Am/Ti target show a slight decrease after day #10. This was attributed to the target damage as well, which is however delayed due to the decay of the long-lived precursor of 220 Rn–224Ra, accumulated in the recoil chamber and in the quartz wool filter.
and 212Po (Fig. 22), alpha-particle spectroscopy and light microscopy pictures (Fig. 23) allowed for quantitative assessment of the target integrity.
From the target stability point of view intermetallic targets are expected to be superior to electroplated targets for a number of reasons; among them higher chemical and mechanical stability and better electrical conductivity. In this study we first of all aimed at testing intermetallic targets proposed earlier [1] at real experimental irradiation conditions. Due to time constraints the integral 48Ca beam dose was much less for both intermetallic targets in comparison to the electroplated 243Am/Ti target. However, the higher integral beam dose on the 243Am/Ti target is counterbalanced by higher maximum beam intensities for the intermetallic targets (Table 2). Therefore, the obtained results appear to be sufficient for making definitive conclusions on the performance of Pd-based intermetallic targets. All three 243Am targets seem to be stable in terms of target material loss after irradiation. Additional information on target stability
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These constraints lead to a significant part of the target material that is not effectively used during irradiation. Only a layer of approximately 1.25 lm of the target is operational in terms of production and pre-stopping of the nuclear reaction products. However, it can be also shown that in this layer the energy loss of Ca equals the energy range of a typical excitation function of the fusion reaction: 243Am(48Ca,3n)115288 [25]. 5. Conclusions
Fig. 24. Simulation of the 48Ca beam energy loss in a 4 lm thick model 2 mg/cm2 U/ Pd intermetallic target (U is assumed to be evenly distributed within the whole depth of the target). Only a 1.25 lm layer indicated by arrows between 2.25 and 3.5 lm (600 lg/cm2) is in superposition to the productive part of the corresponding excitation function (243Am(48Ca,3n)115288 [25]) neglecting here the slightly lower Z of U compared to Am.
came from 185Hg and 220Rn?212Po on-line production and deposition yields. During the irradiation intermetallic targets showed constant (for the given beam intensity) production rates of 185Hg and 220Rn. In contrast, the 243Am/Ti electroplated target shows a considerable decline of 185Hg production rate beginning after 6 days of irradiation (Fig. 22). As seen from the alpha spectroscopic measurement of the target after irradiation, this observation cannot be attributed to the target material loss. 185Hg as produced in a nuclear fusion reaction between 48Ca and natNd, has an average recoil energy of 70 MeV. As it is obvious from the micrographs of the 243Am/Ti target after irradiation, significant part of the target backing (Ti) is missing after the irradiation. Without target backing as energy degrading material it is very probable that a larger part of 185Hg will be implanted into the walls of the target chamber and in the beam stop [23]. Therefore, the continuous decrease of the 185Hg yield after 6 days of irradiation could be ascribed to the effective loss of backing material. This problem was not observed with the 243Am/Pd targets. Despite better performance in terms of 185Hg production and higher beam intensities, we must point out some drawbacks associated with Pd-based intermetallic targets. Since the target area covered with pinholes does not exceed 5% of the total area it is not strongly affecting its operation. However, with increasing amount of a target material or/and changing the backing foil thickness the formation of pinholes might have a much larger impact and has therefore to be taken into account. In experiments with 25 lm Pd backings it was shown that the target material has a broad distribution within the foil after the complete reduction in hydrogen atmosphere [1]. From the alpha spectra taken from both sides of the target after the reduction (Fig. 21) we observed exactly the same behavior of 243Am. This wide spread of 243Am within the target depth and the considerable energy loss of heavy ions in the target material represent another disadvantages of intermetallic targets, which could be especially serious when even more expensive actinide targets are prepared using e.g., 244Pu, 249Bk or 248Cm. A SRIM-2012 [24] particle tracking simulation of 48Ca ions passing through a 4 lm foil consisting of a model target (238U/Pd 1:1 mixture with an assumed density of 12.5 g/cm3) is shown in Fig. 24. The incoming beam energy was adjusted to provide an efficient production of the SHE in a layer where the recoiling products are efficiently pre-stopped, but are still able to recoil out of the target.
We prepared and successfully irradiated two 243Am/Pd intermetallic targets (0.85 and 1.7 mg/cm2) with intense beams of 48 Ca at the U-400 cyclotron at FLNR and conclude that in contrast to 243Am/Ti electroplated target, Pd based intermetallic targets can withstand higher 48Ca beam intensities of up to 0.83 lApart. Intermetallic targets proved to be more stable under the given irradiation conditions. At the same time we point out disadvantages related to such Pd-based intermetallic targets which could limit their use in the SHE chemistry research field. Broad distribution of the target material, pinhole formation, strong attenuation and scattering of the products and the heavy ion beam in the target may at certain circumstances, e.g., experiments at physical separators, not be acceptable. However, for pure production experiments connected to chemical investigations, such as e.g. [16,17], these targets might be useful in the future, especially if the available amount of rare target materials or its radioactive decay properties limits the experiment to the use of stationary targets instead of rotating targets. Further research will have to show whether it is possible to achieve an enriched intermetallic phase on the surface of a thin noble metal foil. However, for pure production schemes in chemical experiments with SHE the stability of targets during month-long irradiations is of outmost importance overruling some of the given drawbacks. Acknowledgment This research project was supported by Swiss National Science Foundation Grant 200020_126639. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
[23] [24] [25]
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Preparation and high intensity heavy ion irradiation tests of intermetallic 243Am/Pd targets I. Usoltsev a,b,⇑, R. Eichler a,b, G.K. Vostokin c, A.V. Sabel’nikov c, N.V. Aksenov c, Y.V. Albin c, G.A. Bozhikov c, V.I. Chepigin c, S.N. Dmitriev c, V.Ya. Lebedev c, O.N. Malyshev c, O.V. Petrushkin c, D. Piguet a, G.Ya. Starodub c, A.I. Svirikhin c, A. Türler a,b, A.V. Yeremin c a b c
Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland University of Bern, CH-3012 Bern, Switzerland Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Russian Federation
a r t i c l e
i n f o
Article history: Received 19 August 2013 Received in revised form 22 October 2013 Available online 8 November 2013 Keywords: Target Electrodeposition Actinides Intermetallic Americium Palladium
a b s t r a c t Previously reported preparation method for Pd-based intermetallic targets (Usoltsev, et al., 2012) [1] has been successfully applied for producing two stationary 243Am/Pd targets. Both targets have been irradiated at the U-400 cyclotron at Flerov Laboratory of Nuclear Reactions Dubna (Russian Federation) using high intensity beams (up to 0.83 lApart) of 48Ca18+. Alpha-particle spectroscopy and light microscopy allowed for a comprehensive characterization of the intermetallic targets before and after irradiation. A natNd/Pd intermetallic target and a solely electroplated 243Am/Ti target were similarly investigated for comparison. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Extremely low production cross-sections of Super Heavy Elements (SHE, Z > 103) [2–4] stimulated severe improvements of heavy ion sources and modernization of accelerators leading thus to a significant increase of heavy ion beam intensities available for irradiation experiments [5]. In order to benefit from this progress and maximize the SHE production rates, development of actinide targets which are able to stand long-term irradiations at high heavy ion beam intensities is of great importance. Normally production of heavy ion targets is based on the electrodeposition [6–11] of thin layers of a target material on thin foils of high-melting elements with low atomic numbers, e.g., C, Be or Ti. Rotating targets [12,13] manufactured this way are perfectly suitable for physics experiments e.g. [14,15], where only thin layer targets are acceptable, and for chemical experiments using more abundant and not too radioactive target materials. However, for chemistry experiments thick targets are advantageous to increase the production rate of SHE and to simultaneously produce several isotopes of the same element in different neutron evaporation channels [16]. Limited target material amounts and high ⇑ Corresponding author. Address: Paul Scherrer Institute, OFLB/105, CH-5232 Villigen PSI, Switzerland. Tel.: +41 056 3105771; fax: +41 056 3104435. E-mail address: [email protected] (I. Usoltsev). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.10.019
radioactivity require the use of stationary targets during the month-long experiments [16,17]. Recently we suggested an efficient approach to produce intermetallic targets for heavy ion irradiations [1], implying that the metallic state of the entire target is superior to electroplated targets in terms of electric conductance, heat conductance and mechanical stability. Based on the molecular plating1 technique followed by the chemical path of coupled reduction2 [18–20], the suggested method allows for producing stable and homogeneous metallic lanthanide and actinide targets. In the first step, the target material is electroplated on a thin Pd foil surface, insuring efficient and homogeneous distribution of the desired element on the target backing. In the second step, the foil with the plated material is heated up in a flow of pure hydrogen. The reduction of the target material with hydrogen at high temperatures becomes thermodynamically feasible due to the formation of an intermetallic compound with the Pd support. In the case of an actinide (An) oxide the coupled reduction reaction occurring on the Pd surface can be written as follows:
xPd þ Anx Oy þ yH2 $ xAnPd þ yH2 O
1 2
In this paper we use this term as a synonym for electrodeposition. Direct translation from German ‘Gekoppelte Reduktion’.
ðiÞ
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It was also observed that the reduced target material diffuses into the depth of the foil immediately after the reaction (i), thus allowing further reduction of the remaining electroplated layer. Once the whole amount of the target material is reduced, it will be broadly distributed within the Pd backing. The final distribution depends on the reduction temperature, reduction time and the thickness of the Pd foil and can be changed by additional heating of the target in either reducing or oxidizing atmosphere. Here, we report on a proof of principle, i.e., the preparation of two 243Am/Pd intermetallic targets with different thicknesses, which were irradiated with high intensity 48Ca beams and characterized before and after irradiation by alpha-particle spectrometry and by optical light microscopy. For comparison with widely used nowadays solely electroplated targets, similar experiments with an electroplated 243Am/Ti target were carried out as well.
and gamma-ray spectrometry in conjunction with acquisition and analysis systems based on Canberra’s Genie2kÒ allowed for a quantitative analysis of the produced targets. 2.1. Nd-target Based on the experience from [1] the 0.7 mg/cm2 Nd-layer (calculated as pure metal, natural isotopic composition) was electroplated in the cell depicted in Fig. 1. Deposited in 10 steps from iPrOH and reduced in hydrogen atmosphere at 900 °C, the target was prepared for test irradiation experiments, the importance of which was conditioned mainly by the possibility of a target meltdown, which is highly undesirable in the case of highly radioactive 243 Am/Pd targets. This target is in the following referred to as Ndtarget. 2.2.
2. Experimental Prior to their use, the light-tested (pinhole-free) 3 lm Pd foil (purchased from GoodFellowÒ) was polished with a diamond polish (1 lm grain size) and subsequently rinsed with distilled water and ethanol. Two 243Am/Pd targets were electroplated by using a 19 ml electrodeposition cell shown in Fig. 1 (deposition area 1.76 cm2). Due to the large effective deposition area and high thickness, the electroplating was performed batch-wise. Multiple-step deposition process was found to be a reliable procedure for obtaining high quality depositions on Pd surface [1]. The target material was dried on a heating plate after each deposition step. A 1 mm thick spiral-shaped Pd wire covering the required circular deposition area served as a counter electrode placed at a distance of about 1 cm from the cathode. The parameters for the electrodeposition of all prepared targets are given in Table 1. Adjustments of the applied voltage were necessary in order to keep the current within the limits given in the table. Deposition homogeneity was examined by optical microscopy using a DinoEyeÒ (magnification: 10–200) and a SomikonÒ (magnification: 20–500) USB digital microscopes. For the reduction in hydrogen atmosphere at high temperatures, the foil with electroplated target material was placed in a tubular quartz apparatus (Fig. 2) on a flat alumina support. The apparatus was heated up by a tubular resistance oven. For safety reasons the quartz tube was flushed with pure He before and after the reduction in H2 atmosphere. Due to the high alpha-activity, all 243Am targets were prepared in a glovebox in the class II radiochemical laboratory. Standard alpha-particle spectroscopy
Am-Target I
The first americium target, in the following referred to as Target I, with the thickness of 0.85 mg/cm2 was electrodeposited from 243 Am nitrate solution in iBuOH in ten consecutive steps with an average yield of 90% on a 3 lm thin Pd foil. Neodymium with natural isotopic composition (natNd) was added to the target (50 lg/ cm2) in order to monitor the target behavior on-line during the irradiation with 48Ca by producing volatile 185Hg in the fusionevaporation reaction natNd(48Ca,xn)185Hg. The deposition yield for 243 Am was quantified after each step by means of gamma spectrometry of an aliquot from the plating solution. 90% deposition yield was assumed for natNd [1]. The target with the plated material was dried up on a heating plate at 100 °C after each deposition step without taking the target out of the cell. Target I was reduced in a tubular quartz reaction chamber (Fig. 2) in a 100 ml/min flow of pure hydrogen at 900 °C. Completeness of the reduction was controlled visually every 10 min by taking the quartz tube out of the resistance oven. Unreduced target material was easily distinguishable from the grayish surface of 243 Am–Pd intermetallic alloy by its black color and by the candoluminescence of the oxide layer on the surface [1]. The reduction process was stopped after 30 min. 2.3.
243
Am Target II
The second target, in the following referred to as Target II, with the thickness of 1.7 mg/cm2 was electrodeposited from 243Am nitrate solution in iPrOH in five consecutive steps with an average yield of 99%. Similarly, neodymium with natural isotopic composition (natNd) was added to the target (50 lg/cm2). The deposition yield for 243Am was quantified after each step by means of gamma-spectrometry. In contrast to Target I, the second target was not just dried up on a heating plate at 100 °C, but annealed at 350 °C after each deposition step. Attempting to obtain a pinhole-free target the reduction temperature was lowered to 840 °C. This temperature however appeared to be insufficient for the reduction purposes. By gradually increasing the reaction temperature and visually checking the completeness of the reduction process, temperature of 890 °C was finally reached. The overall time of the reduction procedure exceeded 2 h. 2.4.
Fig. 1. Scheme of the cell used for the electrodeposition of 243Am and natNd on 3 lm Pd foils.
243
243
Am/Ti target
The 243Am/Ti target was produced by using an untreated 2 lm thick Ti foil as a backing in a 9 ml cell with water cooled stainless steel electrodes (Fig. 3) based on the suggestions from [21]. A 1.2 mg/cm2 243AmO2 target was prepared from 243Am nitrate solution in iBuOH in five consecutive steps with an average yield of 99%. Annealing at 350 °C between the deposition steps was found
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I. Usoltsev et al. / Nuclear Instruments and Methods in Physics Research B 318 (2014) 297–305 Table 1 Experimental conditions for the molecular plating. Target
243
Target I Target II Am/Ti Nd-target
0.85 1.7 1.2 0.7 (natNd)
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Am amount (mg/cm2)
Plating solution
Voltage (V)
Current (mA)
Number of deposition steps
Duration of one step (min)
Deposition yield (%)
Deposition quality
Nitrate/iBuOH Nitrate/iPrOH Nitrate/iBuOH Nitrate/iPrOH
200–600 250–450 500–1000 200–300
1.2–1.9 0.9–1.3 2.0–3.0 1.0–1.5
10 5 5 10
40 60 120 40
90 99 99 70
Poor Good Good Good
Fig. 2. Scheme of the quartz apparatus used for the reduction in H2 atmosphere. The electroplated target is supported inside the tube by alumina slab at the end of the quartz tube. This part is afterward heated up by means of a resistance oven.
Fig. 4. Scheme of the experimental gas-jet set-up used for investigating adsorption of the transported isotopes during the irradiation [22]. Arrows are indicating the direction of the gas flow. (1) 243Am target. (2) 4 lm Ti vacuum window. (3) Quartz housing of a target chamber. (4) Water cooled beam stop. (5) Target chamber. (6) Transport capillary. (7) IVO oven held at 800 °C. (8) Transport capillary heater. (9) 16 pairs Au(Si) detector array held at ambient temperature. (10) 16 pairs Au(Si) detector array held at 0 °C. (11) Gas cleaning module, which includes drying cartridges and aerosol filters. (12) Buffer volumes. (13) Pump. (14) Quartz wool as an aerosol filter. (15) Thermostate.
Fig. 3. Photograph of the electrodeposition cell used for 243Am/Ti target preparation. The Teflon housing in the middle serves as the containment for the electroplating solution. The large cross-shaped screws in front and at the back of the cell are used for pressing the water-cooled electrodes with mounted Ti foils to the plating windows in the Teflon housing.
to be beneficial for the final target quality. A thick 20 lm Ti foil served as an anode material and was mounted at a distance of 2.5 cm from the counter electrode. The 1.6 cm2 effective deposition area was confined by the Teflon housing of the cell. Neodymium with natural isotopic composition was added on the every deposition step in the form of nitrate to the electroplating solution, so that the overall thickness of natNd layer accounting for 50 lg/cm2 was evenly distributed throughout the entire target layer.
12.5 cm3 by an argon–helium mixture (30:70%-vol.) held at 1.25 bar. Driven by a membrane pump (13), the gas mixture was flushed through the system at a rate of 2 l/min. Transported through an aerosol filter (7) and a 3 m long PFAÒ-Teflon capillary with 2 mm inner diameter (8), produced nuclides reached two consecutively placed silicon detector arrays. Covered with gold, each 32-channel array was forming a 16 cm long rectangular chromatography channel of 2 cm width and 2 mm height (9, 10, see also Fig. 5). The first detector array was kept at room temperature, whereas the second one was cooled by a thermostate (15) to 0 °C. The 64 detectors registered alpha-particles and spontaneous fission fragments in an energy dispersive event-by-event mode. Recirculation of the transport gas in a loop through a drying system (11) assured high purity and dry conditions in the system with a continuously measured dew point of about 60 °C.
2.5. Target setup and gas-jet system
3. Results
A gas-jet set-up depicted in Fig. 4 was used to investigate the transport and deposition of volatile isotopes on the gold-covered surface of two silicon detector arrays. The circular shaped 48Ca18+ beam of 2 cm diameter provided by FLNR’s U-400 cyclotron was collimated through a water-cooled honeycomb-grid shaped copper collimator and passed through a 4 lm Ti vacuum window (2), which separated the recoil chamber from the beam line. Subsequently it passed the target (1), supported by a honeycomb grid mounted in a target holder, the recoil chamber (3), and finally it was stopped in a water-cooled copper beam stop (4). Formed nuclear reaction products were stopped in the recoil chamber of
3.1. Nd-target preparation results The produced Nd-target (Fig. 6) was essentially very similar to the Eu/Pd and the U/Pd targets, preparation of which was in detail described in our previous publication [1]. Due to relatively high Nd layer thickness (0.7 mg/cm2) the problem with the formation of pin-holes upon the reduction procedure was not solved. An upper limit target thickness of 0.3 mg/cm2 was found for obtaining pinhole free Nd intermetallic targets based on 3 lm Pd backings. In the Pd–Nd system Nd is the low-melting component (m.p. 1021 °C) and therefore by increasing its content we are lowering
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Fig. 5. 8-Detector ‘sandwich’ used in the detector array. Left: open ‘sandwich’ with the dark rectangular areas showing the four active Si detectors prepared on one waver chip; right: closed detector ‘sandwich’. Mounted in a row, four such gold covered ‘sandwiches’ form 16 pairs of a Au(Si) detector array.
unacceptable in the case of Pd-based target. A poor quality nonhomogeneous deposition was obtained as illustrated in the micrographic picture shown in Fig. 7. The alpha-particle spectrum of Target I is given in Fig. 8. Micro-optical analysis of Target I revealed pinholes after the reduction step shown in Fig. 9 with an additional light source underneath the target. The pinholes are estimated to cover about 5% of the entire target surface area. Alpha-particle spectra were taken from both sides of the target after the reduction as well (Fig. 10). The high energetic edge of the alpha-particle spectrum is not shifted towards lower energies in both spectra. This additionally proves that the target material was indeed reduced, since only reduced 243Am could diffuse through the whole thickness of the backing foil and reach the opposite side of the target. Fig. 6. Optical microscopy image of the Nd-target. Scale bar is 0.3 mm.
the melting point of the formed alloy, which in turn causes the local melting and formation of pinholes.
3.3. Target II preparation results The use of iPrOH solution and annealing at higher temperatures was found to be beneficial to the deposition quality. As a result
3.2. Target I preparation results The procedure described in the experimental part and successfully applied for preparation of 243Am/Ti target was found to be
Fig. 7. Optical microscopy image of the Pd foil after electrodeposition of 0.85 mg/ cm2 243Am layer from the nitrate solution in iBuOH. Scale bar is 1 mm.
Fig. 8. Alpha-particle spectrum of Target I taken after electrodeposition. A peak at 5.275 MeV corresponds to the main component 243Am; low intensity alpha-lines at 5.485 and 5.813 MeV correspond to 241Am and contamination with 249Cf, respectively. Origin of the 241Am impurities was traced back to the 243Am stock solution. The 249Cf contamination is artificial.
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Fig. 9. Optical microscopy image of Target I after reduction in H2; an additional light source was placed underneath the target. Scale bar is 1 mm.
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Fig. 12. Optical microscopy image of Target II after reduction in H2; an additional light source underneath the target. The light scattering virtually increases the size of the pin holes, which were estimated to cover about 5% of the target area. Scale bar is 1 mm.
Fig. 10. Alpha-particle spectra of Target I taken from both sides before irradiation. Fig. 13. Alpha-particle spectra of Target II taken from both sides before irradiation.
provided unambiguous evidence of the reduction process as discussed before. 3.4.
243
Am/Ti target preparation results
Fig. 14 shows a well adhesive layer and highly homogeneous distribution of the thick 1.2 mg/cm2 target material on the Ti surface backing which was obtained upon electrodeposition.
Fig. 11. Optical microscopy image of Target II after electrodeposition of 1.7 mg/cm2 Am layer from the nitrate solution in iPrOH. Scale bar is 1 mm.
243
homogenous layer of the target material was deposited on a wrinkled surface of the 3 lm Pd foil (Fig. 11). Despite of the fairly good deposition layer and the lowered reduction temperature, the problem with pinholes formation persisted. A micrograph of the final product is shown in Fig. 12 with an additional light source underneath the target. An alphaparticle spectrum (Fig. 13) of the target after the reduction
Fig. 14. Optical microscopy image of the Ti foil after electrodeposition of 1.2 mg/ cm2 243Am layer from the nitrate solution in iBuOH. Scale bar is 1 mm.
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3.5. Nd/Pd target irradiation results The irradiation conditions of the Nd-target are collected in Table 2. The stability of the target was monitored by the yield of 185 Hg, produced in hot-fusion reactions with Nd isotopes, e.g., nat Nd(48Ca,xn)185Hg and deposited in the first detector array. A constant production yield of 185Hg was a strong indication for the integrity of the target (Fig. 15). At the beginning of the experiment, a silica frit held at 800 °C was used as an aerosol particle filter in the gas-jet transport system. It appeared to be ineffective letting pass aerosol particles formed in the target chamber. As a result alpha-particles of nonvolatile 150,151Dy isotopes formed in hot-fusion reactions, e.g., 108Pd(48Ca,5n)151Dy were detected along both detector channels in the 4.0–4.2 MeV energy region (Fig. 16). Replacement of the frit by a quartz-wool based filter (Fig. 414) heated to 800 °C removed efficiently this unwanted transport product. Thus, no significant amount of Dy was observed in all subsequent experiments (Fig. 16B).
Fig. 15. On-line monitoring of the Nd-target. 185Hg yield in the first detector array at the given beam intensities during the irradiation.
3.6. Target I irradiation results The irradiation conditions of Target I are given in Table 2. Although the natNd/Pd target withstood the high intensity beam, there was no guarantee that the slightly thicker 243Am intermetallic Target I would stand similar irradiations conditions. Therefore, the beam current was increased gradually from 1 lAelectr slowly up to 15 lAelectr (0.83 lApart). After 2 days of irradiation at low beam intensity the first optical examination revealed an unchanged target. Subsequently, the Target I was irradiated for another 2 days at the highest beam intensities up to an integral dose of 1.1 1018 48Ca particles at a primary beam energy of 269 ± 1 MeV. Sharp drop of the production yield at a day 4.5 was due to temporal technical issues, while subtle variations of the beam intensity acconted for the erratic pattern of the yield on a short time-scale. The integrity of the target was monitored by means of alpha spectroscopy yields of volatile products as well as nuclides of their decay chains (185Hg, 211At, 219Rn, 220Rn and 211–212 Po) (Fig. 17). The natNd nitrate admixture to the 243Am stock solution accounted for production of 185Hg, while multinucleon transfer reactions between 243Am (with Pb contaminations) with 48 Ca lead to the production of isotopes of Rn and At transported to the detector, thus allowing for an direct on-line measurement during the irradiation. In Fig. 18 deposition yields of 212Po and 185 Hg are plotted against the irradiation time. The amount of the target material left after irradiation was quantified by alpha-particle spectroscopy, revealing that less than 5% of the initial activity was lost during the irradiation. An accidental 30 min interlock in the gas-jet system and irradiation without gas cooling may account for such a loss. The alpha-particle activity distribution within the target did not change dramatically during the irradiation (similar to Target II, see Fig. 21). This points towards target temperatures at irradiation conditions, at which the Am diffusion in Pd is still slow, i.e., well below 900 °C.
Fig. 16. Sum alpha-particle spectra measured during natNd/Pd target irradiation. (A) Alpha spectrum of the first part revealing Dy transport by aerosol particles using an inefficient silica frit filter. Applied 48Ca beam dose reached 2 1015 particles. (B) Alpha spectrum obtained using the efficient aerosol particle filter at an integral 48Ca beam dose of 2.5 1017 particles (for details see text).
3.7. Target II irradiation results The irradiation conditions of Target II are compiled in Table 2. The stability of the second 243Am target was also assessed by continuously monitoring 185Hg and 220Rn?212Po production (Fig. 19). Light microscopy pictures were taken after the irradiation for examination of the integrity of the target. Fortunately, no significant enlargement or increase of the number of pinholes was observed (Fig. 20). Some visible upper surface melting indicates local high temperatures, which nevertheless did not lead to a target meltdown. The alpha-particle spectra did not show a significant 243Am displacement in the target (Fig. 21), proving that the
Table 2 Irradiation parameters and calculated stability for investigated targets. Target (mg/cm2) 243
Am/Pd 0.85 Am/Pd 1.7 243 Am/Ti 1.2 nat Nd/Pd 0.7 243
a b
48
Ca18+ beam dose 18
1.1 10 6 1017 3.2 1018 2.5 1017
Beam energy (MeV)
Mean/max. beam current value (lApart)
Stabilitya
269 270 273 270
0.50/0.83 0.56/0.78 0.50/0.56 0.23/0.67
95% 97% 95% N/Ab
Stability of the target is expressed in percentage value and refers to overall The target did not contain any radioactive isotopes before irradiation.
243
Am amount preserved in a target after irradiation.
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Fig. 17. Sum spectrum of alpha-particles recorded during the irradiation of Target I.
Fig. 19. On-line monitoring of Target II. (A) 185Hg production yields during the irradiation. The structure of the 185Hg yield represents the beam intensity variations. (B) 212Po yields during the irradiation. The in-grow of the long-lived 212 Po precursor 212Pb (T1/2 = 10 h) can be seen.
Fig. 18. On-line monitoring of Target I. (A) 185Hg production yields during the irradiation of Target I. After the intermediate target analysis (day #3) beam intensity reached its maximum value of 0.83 lApart. (B) 212Po represents the production and transport yields of 220Rn during the irradiation of Target I. The slopes after the beginning of the irradiation and after the intermediate target analysis on day #3 are due to the scheduled slow beam intensity increase and the in-grow of the long-lived 212Po precursor 212Pb (T1/2 = 10 h) which is a descendant of in-flight decaying 220Rn.
target temperature during the irradiation was not high enough for reaching significant diffusion within the Pd foil. This observation leads to an assumption that the target temperature at the applied beam intensities (up to 0.78 lApart) did not exceed 900 °C.
Fig. 20. Optical microscopy image of Target II after irradiation; an additional light source is placed underneath the target. Scale bar is 1 mm.
3.8.
243
Am/Ti target irradiation results
The 1.2 mg/cm2 solely electroplated 243Am/Ti target was irradiated for 14 days and experienced a three and six times higher integral beam dose in comparison to Target I and Target II, respectively (see Table 2). However, only maximum intensities of 0.5–0.56 lApart have been applied. On-line monitoring of 185Hg
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Fig. 21. Comparison of the alpha-particle spectra of top sides of Target I and Target II taken before and after irradiation.
Fig. 23. Optical microscopy image of the 243Am/Ti target after irradiation; (A) without an additional light source placed underneath the target; and (B) with light source. Scale is given by honeycomb grid pattern – the hexagon side length is 2 mm. It can be clearly seen, that the Ti backing foil is destroyed by a chemical reaction with the target material under the harsh irradiation conditions. A translucent unidentified crystalline product is obtained.
3.9. Post-irradiation analysis The performance evaluation of the investigated targets included not only the on-line 185Hg and 212Po monitoring and light microscopy but also post-irradiation alpha-particle measurements (see Fig. 21). In Table 2 we summarized irradiation parameters for all examined targets. The stability of the irradiated targets was assessed by means of alpha-particle spectroscopy except for the non-radioactive natNd/Pd target. The conducted measurements allowed for determination of the target material loss after irradiations. The percentage of overall 243Am alpha activity remaining on the target after irradiation is shown in the last column of Table 2.
4. Discussion
Fig. 22. (A) 185Hg production yields during the irradiation of the 243Am/Ti target. After 6 days of irradiation the Ti backing is considerably damaged and the recoiling products are not pre-stopped anymore by Ti, thus implanting into the walls of the recoil chamber. (B) 212Po yields during the irradiation of the 243Am/Ti target show a slight decrease after day #10. This was attributed to the target damage as well, which is however delayed due to the decay of the long-lived precursor of 220 Rn–224Ra, accumulated in the recoil chamber and in the quartz wool filter.
and 212Po (Fig. 22), alpha-particle spectroscopy and light microscopy pictures (Fig. 23) allowed for quantitative assessment of the target integrity.
From the target stability point of view intermetallic targets are expected to be superior to electroplated targets for a number of reasons; among them higher chemical and mechanical stability and better electrical conductivity. In this study we first of all aimed at testing intermetallic targets proposed earlier [1] at real experimental irradiation conditions. Due to time constraints the integral 48Ca beam dose was much less for both intermetallic targets in comparison to the electroplated 243Am/Ti target. However, the higher integral beam dose on the 243Am/Ti target is counterbalanced by higher maximum beam intensities for the intermetallic targets (Table 2). Therefore, the obtained results appear to be sufficient for making definitive conclusions on the performance of Pd-based intermetallic targets. All three 243Am targets seem to be stable in terms of target material loss after irradiation. Additional information on target stability
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These constraints lead to a significant part of the target material that is not effectively used during irradiation. Only a layer of approximately 1.25 lm of the target is operational in terms of production and pre-stopping of the nuclear reaction products. However, it can be also shown that in this layer the energy loss of Ca equals the energy range of a typical excitation function of the fusion reaction: 243Am(48Ca,3n)115288 [25]. 5. Conclusions
Fig. 24. Simulation of the 48Ca beam energy loss in a 4 lm thick model 2 mg/cm2 U/ Pd intermetallic target (U is assumed to be evenly distributed within the whole depth of the target). Only a 1.25 lm layer indicated by arrows between 2.25 and 3.5 lm (600 lg/cm2) is in superposition to the productive part of the corresponding excitation function (243Am(48Ca,3n)115288 [25]) neglecting here the slightly lower Z of U compared to Am.
came from 185Hg and 220Rn?212Po on-line production and deposition yields. During the irradiation intermetallic targets showed constant (for the given beam intensity) production rates of 185Hg and 220Rn. In contrast, the 243Am/Ti electroplated target shows a considerable decline of 185Hg production rate beginning after 6 days of irradiation (Fig. 22). As seen from the alpha spectroscopic measurement of the target after irradiation, this observation cannot be attributed to the target material loss. 185Hg as produced in a nuclear fusion reaction between 48Ca and natNd, has an average recoil energy of 70 MeV. As it is obvious from the micrographs of the 243Am/Ti target after irradiation, significant part of the target backing (Ti) is missing after the irradiation. Without target backing as energy degrading material it is very probable that a larger part of 185Hg will be implanted into the walls of the target chamber and in the beam stop [23]. Therefore, the continuous decrease of the 185Hg yield after 6 days of irradiation could be ascribed to the effective loss of backing material. This problem was not observed with the 243Am/Pd targets. Despite better performance in terms of 185Hg production and higher beam intensities, we must point out some drawbacks associated with Pd-based intermetallic targets. Since the target area covered with pinholes does not exceed 5% of the total area it is not strongly affecting its operation. However, with increasing amount of a target material or/and changing the backing foil thickness the formation of pinholes might have a much larger impact and has therefore to be taken into account. In experiments with 25 lm Pd backings it was shown that the target material has a broad distribution within the foil after the complete reduction in hydrogen atmosphere [1]. From the alpha spectra taken from both sides of the target after the reduction (Fig. 21) we observed exactly the same behavior of 243Am. This wide spread of 243Am within the target depth and the considerable energy loss of heavy ions in the target material represent another disadvantages of intermetallic targets, which could be especially serious when even more expensive actinide targets are prepared using e.g., 244Pu, 249Bk or 248Cm. A SRIM-2012 [24] particle tracking simulation of 48Ca ions passing through a 4 lm foil consisting of a model target (238U/Pd 1:1 mixture with an assumed density of 12.5 g/cm3) is shown in Fig. 24. The incoming beam energy was adjusted to provide an efficient production of the SHE in a layer where the recoiling products are efficiently pre-stopped, but are still able to recoil out of the target.
We prepared and successfully irradiated two 243Am/Pd intermetallic targets (0.85 and 1.7 mg/cm2) with intense beams of 48 Ca at the U-400 cyclotron at FLNR and conclude that in contrast to 243Am/Ti electroplated target, Pd based intermetallic targets can withstand higher 48Ca beam intensities of up to 0.83 lApart. Intermetallic targets proved to be more stable under the given irradiation conditions. At the same time we point out disadvantages related to such Pd-based intermetallic targets which could limit their use in the SHE chemistry research field. Broad distribution of the target material, pinhole formation, strong attenuation and scattering of the products and the heavy ion beam in the target may at certain circumstances, e.g., experiments at physical separators, not be acceptable. However, for pure production experiments connected to chemical investigations, such as e.g. [16,17], these targets might be useful in the future, especially if the available amount of rare target materials or its radioactive decay properties limits the experiment to the use of stationary targets instead of rotating targets. Further research will have to show whether it is possible to achieve an enriched intermetallic phase on the surface of a thin noble metal foil. However, for pure production schemes in chemical experiments with SHE the stability of targets during month-long irradiations is of outmost importance overruling some of the given drawbacks. Acknowledgment This research project was supported by Swiss National Science Foundation Grant 200020_126639. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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