Alamanos, N

Mittig, W

Ridikas, D

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RNB production using a powerful protonacceleratorD. Ridikas1, W. Mittig2, N. Alamanos11) CEA Saclay, F-91191 Gif-sur-Yvette, France2) GANIL, BP 5027, F-14076 Caen, FranceAbstractThe aim of the considerations below is to estimate the orders of magni-tude for the production of intensive RNBs by the use of the powerful pro-ton accelerator if available. We try to take into account present technicallimitations of ISOL-techniques by limiting our discussion to the produc-tion of ssion products. These possibilities are compared to existing orprojected RNB facilities.1 IntroductionA signicant increase of the intensities of RNBs will certainly open access to a newdomain of physics. Future extension of the present facilities or the constructionof new facilities would allow to reach RNBs with the intensities of 1012pps, andthus to study the cross sections down to the b or even nb level.Here we assume that a powerful proton accelerator (1GeV; a few MW) isavailable, what is about 3 orders of magnitudes more than currently used in anyISOL and fragmentation of the beam facilities, all below 10kW power limit. Onthe other hand, proton beams of 160kW are already available at RAL (UK) andmore than 1MW at PSI (Switzerland). The research to develop high-intensitybeams of several MW is in progress, mainly stimulated by the projects such asAccelerator Transmutation of Waste (ATW), Spallation Neutron Sources (SNS),Accelerator Production of Tritium (APT), etc.Unfortunately, an increase of primary beam intensity does not necessarily in-crease the intensity of the secondary RNBs as was shown in [1]. The authorsargue that the maximum RNBs will be obtained actually for a limited incidentbeam power. This is an important warning that there may be quite severe limi-tations with respect to the admissible power on the target. For the RNB facilitiesbased on the charged particle induced reactions, this limitation is of the orderof 20-30kW in the RNB production target [2]. An alternative way of pro-ducing the RNBs in the mass region of 75<A<160 can be achieved utilizing atarget-converter (neutron source) [3]. The emitted neutrons then interact with assionable target. Contrary to the charged particles, the neutrons will heat thetarget indirectly and mainly by the "useful" ssion reactions.ridikas@cea.fr1In this context we will discuss mainly two possibilities: a) the neutron inducedssion, where the neutrons are produced in a converter from a primary protonbeam; b) the proton induced ssion/spallation, where the primary beam directlyhits a heavy target. The major goal is to reach at least 1014ssions/s in orderto be compatible with reactor-driven or other future RNB facilities. Anotherequally important possibility { a spallation of intermediatemass targets that mayprovide very broad range of isotopic production { will be considered elsewhere.We note in this context that the isotopic distributions of low energy ssion andhigh energy spallation are very dierent; a high energy spallation provides a verybroad production range, whereas a low energy ssion gives the highest yields inthe region covered by this process.We employ a coupled LAHET+ MCNP+ CINDER code system [4] for allnumerical calculations presented in this work.2 The present and projected RNB facilitiesThe production targets for the future ISOL facility should be able to work athigh beam power, and at the same time, be coupled eciently to ion sources forthe production of the secondary beams of short-lived isotopes. Beam power of100kW or more involves extrapolation beyond current experience at any ISOL-type facility. Indeed, the highest charged particle beam power dissipated so farin a RNB target is 9kW (at Louvain-la-Neuve), and scheduled values at otherfacilities are summarized in Table 1 [5]y. We note that the power given in thisProject, place Particle Energy Intensity Power Target(s) Fiss. rates(MeV) (pA) (kW) Z (#/s)KEK-JAERI, Japan p 3000 333 1000 4-92 ?Rex-ISOLDE, CERN p 1000 3 3 4-92 31012IRIS, Russia p 1000 100 100 4-92 1014SIRIUS/RAL, UK p 800 100 80 4-92 1014ISAC/TRIUMF, Canada p 500 100 50 4-92 1014GANIL(*), France p or d 200 70 14 4-92 1014Louvain-la-Neuve, Belgium p 30 300 9 3-17 ?Table 1: Characteristics of RNB (ISOL) facilities in operation or projected, inwhich the direct production method is based on either p or d induced reactions.Table is an incident beam power but not the beam power actually dissipated inthe RNB target. Fission rates are estimated for 62.5g/cm2of UCx (=2.5g/cm3yGANIL(*) estimates are from the case study reported in [7] as requested by the ConseilScientique du GANIL.2and 25cm long) except for the GANIL(*) project where 54g/cm2of molten U(=18g/cm3and 3cm long) is taken.The actual RNB target design problem can be viewed as one of minimizingtarget size and geometry while still handling the high beam power. Minimizingtarget size and geometry is essential to optimize diusion and eusion ecienciesfor the very short-lived radionuclides. In conventional ISOL production schemes(direct method), the target and ion source are integrally coupled so that theprimary target can not be massively cooled without interfering with the ion sourceperformance. With two-step schemes (converter method) these functions arephysically separated. Table 2 lists a few examples based on this method [5, 6, 7].Project, place Particle Energy Intensity Power Target(s) Fiss. rates(MeV) (pA) (kW) (#/s)ANL, USA d 200 500 100 Be,W+U 5.51013LNL, Italy p or d 40 2500 100 ?+U ?GANIL(*), France d 35 5700 200 Li,C+U 1.01014ORELA/ORNL, USA e 150 333 50 92 1.21013Photossion, Russia e 25 20 0.5 W+U 1.51011MAFF/FRM-II, Germany n thermal 1014ns cm23235U 1.01014Table 2: Characteristics of RNB (ISOL) projected facilities which are based onconverter production method.Finally, Table 3 presents a few examples of other projects based on a directhigh energy projectile/target ssion/fragmentation [5]. We note that both RIAProject, place Projectile Energy Intensity Power Target(s) Fiss. rates(MeV/u) (pA) (kW) (#/s)RIA, USA238U 400 1/1 100 Li, C 1013GSI, Germany238U 1000 1/6 40 Be, C 1012GANIL(#), France C 100 5 6238U 1012GANIL(#), France  100 15 6238U 1013Table 3: Characteristics of 3 typical RNB projected facilities which are based ondirect projectile ssion/fragmentation method. Note: GANIL actually operateswith heavy ion beams up to 2kW. The beams of 6kW power and acceleration of particles might be considered in the (near!?) future.and GSI plan the slowing down of energetic ssion fragments. If successful, theseRNB factories will be able to provide intensive secondary beams both at highand low energies.33 Comparison of the C- and D-methodsWe assume that the main goal for the future RNB factory is to reach ssionrates 1014ssions/s. If one comes back to the numbers presented in Tables 1- 3,1014ssions/s might be considered "only" as a minimum requirement. Indeed,1012{1013ssions/s could be obtained even with present GANIL facility if 6kWbeam power including acceleration of -particles were allowed. We also take forgranted that a powerful proton accelerator is available (say, protons of 1GeVand up to a few MW power). Tables 4 and 5 summarize the main beam andtarget characteristics within these two constraints for both converter (C) anddirect (D) methods. In the case S1 (see Table 4), protons interact directly withScenario Particle Energy Min. primary beam In target Target typetype power (current) power (material)D S1 p 500MeV 42kW (84A) 16kW solid UCxC S2 p 500MeV 141kW (282A) 7kW solid W+UCxTable 4: Projectile/target combinations to produce high ssion yields denedby 1014ssions/s in the RNB production target. Also see Table 5. Note: C -Converter, D - Direct.Target238U density Volume238U mass Geometrytype (g/cm3) (cm3) (g) (cylinder)D 2.5 177 443 "full": r=1.5cm, l=25cmC 2.5 942 2355 "empty": r1(r2)=2cm(4cm), l=25cmTable 5: Target parameters for the production of ssion yields by the s-sion/spallation reactions. See Table 4 for details.a solid UCx target (see line D of Table 5). In the case S2 (see Table 4), primarybeam of protons impings upon a well-cooled W target to produce an intense uxof secondary neutrons. The secondary neutrons are of relatively low energy andnearly isotropic. To optimize the solid angle of the secondary UCx target, acylindrical blanket is chosen (see line C of Table 5). The length and diameter ofthe secondary target are kept to a minimum for optimal extraction of short-livedproducts. Using thicker and longer secondary target can increase the yields oflonger-lived isotopes.The calculated in-target ssion yields, normalized to a primary beam intensityresulting in 1014ssions/s, are presented in Table 6 for both S1 and S2 scenarios.It is important to note that scenario S2 gives higher in-target ssion yields on theneutron rich-side by a factor of 5 when normalized per successful ssion. Theyield of neutron-rich isotopes depends strongly on the excitation energy which istaken by neutron emission before and after ssion. It is known that post-scission4Element T1=2UCx (S1) W+UCx (S2)(s) (at./s 84Ap) (at./s 282Ap)9136Kr 8.57 6.510112.310129436Kr 0.21 1.410114.910118037Rb 33.4 9.4109<19737Rb 0.17 7.410101.5101113250Sn 40.0 1.910111.1101214254Xe 1.22 1.510119.0101114454Xe 1.10 1.110107.0101014455Cs 1.02 2.210111.1101221387Fr 34.6 6.710116.3109Table 6: Estimate of projected in-target ssion yields, normalized to a primarybeam intensity resulting in 1014ssions/s in the RNB production target. Also seeTable 4.neutrons are emitted by ssion fragments with large neutron excess. That iswhy the ssion of weakly excited nuclei (S2 scenario) is favourable for producingneutron-rich ssion fragments. On the other hand, the S1 method will providewith higher intensities of certain spallation products (e.g. see21387Fr ) and a broadrange of neutron decient nuclei (e.g. see8037Rb).In both S1 and S2 scenarios examined we expect overal RNB source e-ciency (releasedelayion-source eciency) similar to typical numbers given byISOLDE target [8] due to similar target size and material compositions. Thisassumption gives 10% for some isotopes of the same element with T1=2>1s andaround 1%-0.1% for T1=2<1s. In other words, the numbers in Table 6 have tobe corrected accordingly for the expected nal RNB intensities.Due to 1014ssions/s the total activity of about 10-20kCi is expected. Var-ious parts of the installation will become highly radioactive. Hence, the man-agement of this radioactivity is an important element in the design of the in-strument. High radioactivity due to the noble gases and halogens will require aspecial treatment. Table 7 gives the maximum activity of the source after 90daysof the irradiation. It is clear that this source will still be highly radioactive evenCooling period 0s 1s 1min 1hour 1day 14days 30days 90daysActivity S1 (kCi) 11.6 10.7 8.3 4.8 2.6 1.0 0.7 0.3Activity S2 (kCi) 17.7 16.6 11.5 6.2 3.3 1.0 0.6 0.2Table 7: In-target activity of the UCx ion source after 90days of irradiation inthe case of direct method (S1) and converter method (S2). See Tables 4-5 forirradiation conditions and target specications for both scenarios.after 90days of cooling. Consequently a remote control system will be needed todismount a used source, and to mount its replacement.5We note separately that in the case S1 a number of long-lived -radioactivespallation products will be produced on the proton rich side, which will denetarget activity in a long run (t=90days in Table 7). On the other hand, neutroninduced ssions (case S2) will create more short-lived isotopes (see Table 6) onthe neutron rich side resulting in higher target activity during operation (t=0sin Table 7).4 DiscussionThere are a few additional points to be discussed in the context as above. First ofall, we think it is instructive to give the following example. The proton acceleratorat PSI delivers 600MeV protons at 1MW power, i.e. 1.67pmA1016p/s, whichinteracting with a massive Pb-Bi target surrounded by heavy water will resultin 1017n/s over 4. At the radial distance of 50cm from the spallation targetthe thermal neutron ux will be of the order of 31012n/s/cm2to be comparedwith 1.51014n/s/cm2as projected for MAFF/FRM-II (Germany) [5]. In otherwords, in the case of the PSI neutron source the235U (100%) mass has to beincreased from 1g up to 50g to get 1014ssions/s, or the primary beam power hasto be increased by a factor of 50 (!?). In brief, the moderation of fast neutronsmakes the powerful neutron source inecient, simply because the RNB targetrequires a compact geometry and as little of ssionable material as possible. Areactor driven facility is strongly favoured in this particular case.Secondly, if one thinks of the future RNB facility which is able do delivernot only intensive ssion yields but also much broader range of isotopes, in ouropinion, the p-driver could also provide it. For example, p(1GeV)+U would resultin rather similar isotopic distributions as12C(100MeV/u)+U both on neutron-rich and decient sides [8]. Thicker targets in the case of proton induced reactions(due to longer proton range) would simply compensate lower isotope productioncross sections. In order to produce nuclei further on the proton-rich side of themass valley, other reactions than nuclear ssion should be explored. For thispurpose lower mass targets than uranium or even very light mass targets shouldbe used with 1GeV or higher energy protons [8].This second point is extremely important in dening the production method(converter, direct, or both), which will directly depend on the main goals of thefuture RNB facility: delivering either the most intensive ssion yields or being abroad range isotope production factory or both.It has also been shown that thin Th targets (1mg/cm2) may stand veryhigh beam intensities (>1mA) of high energy protons. A combination of a largenumber of such thin targets would allow eventually a chemically-independentproduction of RNBs (see [9] and Refs. therein for further details). Certainly,more R&D still has to be done in the case of this unconventional ISOL method.65 ConclusionsThe results we present show that very high ssion yields may be obtained bythe use of the powerful proton driver. For massive UCx targets (2.3 kg of238U) the neutron induced ssion from a converter can provide 1014ssions/swith a primary beam intensity of 141kW for protons of 500MeV. The total powerdissipated in the RNB target is 7kW. For less massive targets (0.3 kg of238U),a direct p is preferred. 1014ssions/s are reached with a proton beam of 42kWat 500MeV. The total power dissipated in the target is 16kW what is alreadyvery close to the present target technologies with allowable heat deposition due tothe primary beam and ssions all together. It seems that the converter-methodmay reach the highest ssion yields: 1015ssions/s can be obtained employing theproton beam of 1.4MW at 500MeV, still compatible with the characteristics of ahigh intensity proton accelerator. On the other hand, in the case of the direct-method other targets than U could be used (e.g. Nb, La, Ta, Th or even muchlighter mass targets) in order to explore dierent spallation/ssion/fragmentationregions and proton-rich isotopes in particular. So the direct-method provides ahigher versatility than the converter-method.In brief, the converter method could provide the highest ssion yieldsdened by 1015ssions/s in the RNB target. A combination of the directmethod in addition to the converter method at the same time would producea broad range of other isotopes of interest at the intensities compatiblewith other projected facilities. If all the suggested cases are technologicallyfeasible is another important question that remains to be answered. Therefore,more detailed calculations and some exploratory experiments are necessary inthis domain. Finally we add that the radioactivity problems will be crucialin the construction of the future RNB facility.References[1] W. Mittig, J. of Phys. G: Nucl. Part. Phys. 24 (1998) 1331.[2] J.R.J. Bennett et al., Nucl. Instr. & Meth. B 126 (1997) 105; Nucl. Instr.& Meth. B 126 (1997) 126.[3] J.A. Nolen, Proc. of the 3rd Int. Conference on RNB, Gif-sur-Yvette,France, 24-27 May 1993, Ed. D.J. Morrissey, Editions Frontiers (1993) 111.[4] D. Ridikas, "Optimisation of Beam and Target Combinations for HybridReactor Systems and Radioactive Ion Beam Production by Fission", PhDthesis, report GANIL-T 99 04 (October 1999) and Refs. therein.[5] Book of Abstracts of the 5th Int. Conference on RNB-2000, Divonne,France, 3-8 April (2000); proceedings to be published in Nucl. Phys. A.7[6] J.A. Nolen et al., Proc. of the 8th Int. Conference on Heavy Ion AcceleratorTechnology, ANL, USA, 5-9 October 1998, in press.[7] D. Ridikas, W. Mittig, A.C.C. Villari, "Comparative study of the direct andconverter methods for high ssion yields", CEA report DAPNIA/SPhN-00-51 (August 2000).[8] H.L. Ravn et al., Nucl. Instr. & Meth. B 88 (1994) 441.[9] W. Mittig et al., report of the TWIST workshop, GANIL (2000).8

RNB production using a powerful proton accelerator

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