Recent studies to improve release properties from thick isotope separator on-line fission targets

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1 Nuclear Instruments and Methods in Physics Research B 204 (2003) Recent studies to improve release properties from thick isotope separator on-line fission targets C. Lau *, B. Roussiere, D. Verney, O. Bajeat, F. Ibrahim, F. Clapier, E. Cottereau, C. Donzaud, M. Ducourtieux, S. Essabaa, D. Guillemaud-Mueller, F. Hosni, H. Lefort, F. Le Blanc, A.C. Mueller, J. Obert, N. Pauwels, J.C. Potier, F. Pougheon, J. Proust, J. Sauvage, A. Wojtasiewicz Institut de Physique Nucleaire dõorsay, Groupe Source dõions, B^atiment 106, Division Accelerateur, F Orsay Cedex, France Abstract In the framework of the PARRNe program (Production dõatomes Radioactifs Riches en Neutrons) of IPN Orsay, various techniques are currently used to characterize the release properties of elements of interest from a UC X target. On-line studies have been carried out with two plasma ion sources: a Nier Bernas and a hot plasma ISOLDE-type (the ISOLDE collaboration kindly supplied us a MK5 ion source for these studies). In parallel, the analysis of the chemical and structure properties of some UC X samples as function of heating conditions has been carried out. Such data are essential to determine optimal conditions for the production of isotopes by the isotope separator on-line (ISOL) technique. First results are presented here for Kr and Ag. Investigations for other kinds of fission targets are planned. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: Rm; Ec; Kv Keywords: ISOL fission target; Release process 1. Introduction In order to probe physics with neutron-rich isotopes by isotope separator on-line (ISOL) technique (e.g. [1]), the R&D part of the PARRNe program [2] was initiated in This program of IPN Orsay has supplied substantial experimental data about two production modes: fission induced by fast neutrons and fission induced by bremsstrahlung of electrons [3]. Furthermore, R&D * Corresponding author. Tel.: ; fax: address: lau@ipno.in2p3.fr (C. Lau). studies have been undertaken to characterize the release properties of some fission products from a UC X target. Indeed, such an experimental approach is required to significantly improve the efficiency of beam production by the ISOL technique. On-line measurements have been carried out at the PARRNe isotope separator with two types of plasma source: a Nier Bernas and a hot plasma ISOLDE-type. Production data obtained on neutron-rich isotopes of Kr are used to compare the two units. Data for Ag, obtained with the ISOLDE source, are also presented. In parallel, our first investigations about the properties of the UC X material will be analyzed X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi: /s x(02)

2 C. Lau et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) atom/s/µα Cu Zn Ga Ge As Se Br Kr atom/s/µα Ag Cd In Sn Sb Te I Xe Mass Mass Fig. 1. Most of the yields measured at the end of the PARRNe isotope separator. Data obtained with an incident 26 MeV deuteron beam impinging on a 3 mm graphite converter in contact with the thick UC X target (50 g of 238 U/cm 2 ) at 2000 C. The target is connected to an ISOLDE hot-plasma source at 1800 C. 2. On-line measurements The tandem of IPN Orsay [4] delivers beams of 26 MeV deuterons. A graphite converter placed close to the target fully stops the incident ion beam and generates a flux of fast neutrons towards the fission target [5]. The first target-ion source unit tested was a 6 cm-long UC X target connected to a Nier Bernas ion source by means of a 30 cm-long transfer tube whose overall temperature did not exceed 1400 C. With such a device, beams of Zn, Kr, Rb, Cd, I, Xe and Cs have been produced [6]. To enlarge the variety and the yield of neutronrich isotopes produced, the device has been upgraded with a 20 cm-long UC X target connected to an ISOLDE-type hot plasma source [7] at 1800 C (Fig. 1). Release curves of different elements have been obtained and release times have been determined using two methods [8]. 3. Release studies of Kr The release of an isotope from the target to the ion source can be characterized by the mean time T R which indicates how long it takes to release half of the amount of the isotope considered. The timecontrolling processes of the isotope release are diffusion in the target material and desorption at the various surfaces encountered from the target to the ion source. The release study carried out with the Nier Bernas ion source are presented in [8]. With the ISOLDE-type hot-plasma source, we first analyzed Kr. Indeed, since in the case of noble gases, the time of desorption is completely negligible (e.g. [9,10]), the effusion in a standard ISOL device can be overlooked compared to diffusion. So the data obtained can be compared to a certain extent with those from the first target-ion source unit although the configuration of the two units are quite different (transfer line, arc chamber...). The release curve of 90 Kr obtained with a target at 2100 C is analyzed in Fig. 2. The mathematical expression describing the intensity of the c-rays from the decay of 90 Kr as function of time has been established assuming a pure diffusion process and using the procedure of [11]. This expression being a sum of an infinite number of terms whose last are negligible, we approximated it to a finite number of terms and calculated time spectra for given T R and T 1=2 values. These calculated time spectra were then fitted to the experimental one. For various T 1=2 the v 2 curves as function of T R have been drawn: the lowest v 2 defines the most probable T 1=2 and T R. This way we determined T R ¼ 4:5 2 s. This value fully matches those

3 248 C. Lau et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) Kr T 1/2 =36s T R =4.5s Φ / Φ T 1.E+00 1.E-01 1.E-02 Count s Beam 200 s Time [s] Fig. 2. Time spectrum of the 122 kev c-ray of the 90 Kr decay for a UC X target at 2100 C connected to the ISOLDE-type hot plasma source. The full line curve is the best fit obtained with the consideration that the release process of Kr is fully controlled by diffusion. Let us notice that the T 1=2 required for the best fit is longer than the value given in the literature. Since such a deviation has been systematically observed in all our release measurements of 90 Kr, a dedicated measurement of the T 1=2 of 90 Kr is planned. measured with target temperatures of 2000 and 2200 C with the Nier bernas source (T R ¼ 3:6 2 s for a target at 2200 C) [8]. These results confirm that for a noble gas such as Kr, only diffusion characterizes the release delay. A release curve has also been measured for 92 Kr. A similar analysis yields T R ¼ 3 2 s. Furthermore, comparing the yields measured at the end of the ISOL line for the radio-isotopes 85m 93 Kr to the in-target yields expected, we can deduce both T R and the releasecontrolling process. The asymptotic slope of the release efficiency as a function of the isotope halflife reveals the release-controlling process since the efficiency depends on the square root of the halflife in the diffusion limited process and the efficiency dependence is linear for the effusion limited process. Then fitting the data with the suitable mathematical expression gives T R which is constant for an isotopic series of a single element [8]. The mean release time deduced for Kr is T R ¼ 6:2 3:7 s (Fig. 3). Although less precise, this re- 1.E-03 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 sult supplies a fair estimation and agrees with the T R obtained in the direct measurement. The evolution of the T R obtained for Kr between 2000 and 2200 C goes along with the Arrhenius law of the diffusion coefficient (according to our observations the UC X grain size hardly changes in this temperature range) but the temperature range must be enlarged to get quantitative details. 4. Target properties 1/T 1/2 [s] Fig. 3. U T =U values as function of the half-life for the 85m 92 Kr isotopes. U T is the yield measured on the tape, U is the in-target production calculated, and e S is the efficiency of the ISOL line (including the ion source). U was estimated by taking into account the cumulative yields of isotope data given for fission of 238 U induced by 14 MeV neutrons [12] and the number of fissions induced in the target [13]. The fit of the experimental data gives T R ¼ 6:2 3:7 s and e S ¼ 5:2%. For elements whose release is mainly controlled by diffusion, the target structure plays a key role (e.g. [14 16]). To get fastest release, one should keep the structure as fine as possible at high temperature. Our UC X targets have been manufactured according to the technique developed at ISOLDE [17]. With this manufacturing process we got porous lm-large UC 2 clusters in graphite at 2200 C (Fig. 4). The low amount of pure graphite left at 2200 C confirms that the ratio of the initial mixture is optimum. However, it still looks like it may be possible to reduce slightly the amount of graphite without degrading the porous structure. Around 2000 C, the target structure is no longer really granular (Fig. 4) and 10 4 magnified-sem pictures show that the lm UC 2 bulks are also porous with sintered grains of a few microns. However if we

4 C. Lau et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) a combustion must be carried out at about 800 C to make sure that only U 3 O 8 is left after combustion. 5. Release studies of Ag Fig. 4. Structure of a UC X pellet heated at 2200 C for about 5 h. The main picture is 300 magnified and the small window is 10 4 magnified. By back-scattered X-ray spectroscopy the white spots have been identified as UC 2 clusters and the black spots have been identified as being graphite. The size of the UC 2 bulks is about lm. The structure is nearly identical to samples heated at 2000 C for about 10 h. The temperature has been determined by using both an optical pyrometer and a calibration with a thermocouple. Such results have been obtained from a powdery mixture made of graphite and UO 2 with the atomic ratio C=U ¼ 6. The average size of the powder grains is initially 22 lm. Let us notice that from other X-ray analyses, UC molecules (whose melting point is higher than UC 2 Õs [18]), produced since 1300 C fully turn into UC 2 in this graphite-excess medium above 1900 C. consider the structure to be granular with an average grain size of about 20 lm, using T R values determined from experimental data we deduce the diffusion coefficient of Kr in the UC 2 component of the target to be on the order of 10 8 cm 2 /s. This estimate goes along with the diffusion coefficient of Kr in UC from the literature [19]. Because of the disparity in the size of the UC 2 bulks, the effective diffusion time is an average over a range of diffusion times and using diffusion data available for a given element only gives an estimation of its release time. The pictures of our UC X samples and the X-ray spectroscopy of the carbides show that the C/U ratio is roughly about 4 and close to the optimum value. If needed, a more precise measurement of the C/U ratio is in principle possible by combustion in air. This technique has been used for various uranium carbides [19]. However, because of the excess of graphite in our samples, such In addition to its physics interest (e.g. to probe the N ¼ 82 nuclei for the r-process), the development of neutron-rich Ag beams is also useful for better understanding the release from thick ISOL targets. In tracer amount, Ag does not combine neither with U [19] nor with C and it is rapidly released from uranium-carbide-type targets (e.g. [20]). The release efficiency determined as function of the half-life of the Ag isotopes unambiguously shows an asymptotic slope characteristic of an effusion-dominated release (Fig. 5) for a target at 2100 C. The release time deduced from the analysis of Fig. 5 is T R ¼ 5:6 1:2 s. Fitting the release spectrum of 117m Ag with the mathematical expression of a desorption-controlled release [8] gives T R ¼ 3:2 1 s (Fig. 6). This result completes observations from ISOLDE and OSIRIS [20,21]. The fact that Ag beams were not well produced while using the 30 cm long transfer line at lower temperature (with the Nier Bernas source) indicates that Ag release is mainly delayed by effusion in the transfer line. However, whereas for Kr the effusion Φ / Φ T 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1/T 1/2 [s] Fig. 5. e R :e S ¼ U T =U as function of the half-life for the Ag isotopes measured (e R being the release efficiency, cf. legend of Fig. 3 for the definition of U T, U and e S ). The fit of the experimental data (taking into account the error bars) gives T R ¼ 5:6 1:2 s and e S ¼ 5:6%.

5 250 C. Lau et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) Count s m Ag T 1/2 =5.34s T R =3.2s of interest, the extension of our database with other units is also in progress with a W-surface ionization source. Further off-line heating tests are planned to clearly determine the maximum operational temperature of the UC X used and observe long duration heating effects. Our studies of fission targets are not limited to the UC X type. Our team has also worked out a thick molten U target [6,8,24] and plans to investigate other compounds of fission material (e.g. ThC X which is more refractory) process is completely negligible comparing to the diffusion process, for Ag we have not yet determined whether the release can be considered as purely effusive. By treating the release efficiencies (Fig. 5) as being limited by the effusion, desorption times (on the Ta of the line and the cathode) of the order of ms are deduced, which is about two orders of magnitude higher than expected from the Eichler Miedema model [22,23]. 6. Perspectives Beam 30 s Time [s] Fig. 6. Time spectrum for the sum of the 205, 220 and 322 kev c-rays of the 117m Ag decay for a UC X target at 2100 C connected to the ISOLDE-type hot plasma source. The full line curve is the best fit obtained with the mathematical expression of an effusion-dominated release. The mean release time deduced is T R ¼ 3:2 0:7 s. The release program initiated is in progress with the analysis of data for Sn, Cd and I. These results will allow us to complete the comparison between the two target-ion source units. In order to characterize as much as possible the release of elements References [1] RIA project, [2] F. Clapier et al., Phys. Rev. ST AB (1998) [3] F. Ibrahim et al., Eur. Phys. J. A 15 (2002) 357. [4] B. Waast et al., Nucl. Instr. and Meth. A 287 (1990) 26. [5] S. Menard et al., Phys. Rev. ST AB 2 (1999) [6] C. Lau, These de lõuniversite Paris 7 (2000) IPNOT [7] S. Sundell, H.L. Ravn, and the ISOLDE collaboration, Nucl. Instr. and Meth. B 70 (1992) 160. [8] B. Roussiere et al., Nucl. Instr. and Meth. B 194 (2002) 151. [9] R. Kirchner, Nucl. Instr. and Meth. B 126 (1996) 135. [10] R. Kirchner, Nucl. Instr. and Meth. B 26 (1987) 204. [11] G. Rudstam, Nucl. Instr. and Meth. A 256 (1987) 465. [12] R.T. England, B.F. Rider, Los Alamos Laboratory report LA UR , [13] M. Mirea et al., Eur. Phys. J. A 11 (2001) 59. [14] L.C. Carraz et al., Nucl. Instr. and Meth. 148 (1978) 217. [15] M. Fujioka, Nucl. Instr. and Meth. 186 (1981) 409. [16] F. Landre-Pellemoine et al., Nucl. Phys. A 701 (2002) 491. [17] H.L. Ravn et al., Nucl. Instr. and Meth. B 26 (1987) 183. [18] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 81st Ed., CRC Press, [19] P. Pascal, Nouveau Traite de Chimie Generale, Vol. 15, fasc.1 et 4, [20] G. Rudstam et al., Radiochim. Acta 49 (1990) 155. [21] E. Hagebo et al., Nucl. Instr. and Meth. B 70 (1992) 165. [22] H. Roßbach, B. Eichler, Tech. Report ZfK 527, Zentralinstitut f ur Kernforschung, Rossendorf, [23] B. Eichler, S. H ubener, H. Roßbach, Tech. Report ZfK 560 and ZfK 561, Zentralinstitut f ur Kernforschung, Rossendorf, [24] S. Kandri-Rody et al., Nucl. Instr. and Meth. B 160 (2000) 1.