Borcea, C

Cennini, P

Ferrari, A

Kadi, Y

Lacoste, V

Radermacher, E

Vlachoudis, V

CERN Document Server

CERN n TOF Facility and its Possible Impact onProducing Nuclei with Large Neutron ExcessC. Borceaa;bP. CenniniaA. Ferraria;cY. KadiaV. LacosteaE. RadermacheraV. VlachoudisaaCERN, SL Division, EET Group, CH-1211 Geneve 23, SwitzerlandbIFIN-HH, PO Box MG-6, Bucharest-Magurele, RomaniacINFN Milan, ItalyThe n TOF facility at CERN is a source of neutrons producedby spallation of 20 GeV/c protons onto a lead target. The out-standing characteristics of this source (very high intensity, 200 might path, wide spectral function) make it an extremely useful toolfor studying neutron induced reactions. The neutrons in the energyregion from 1 to 10 MeV are particularly interesting for producingnuclei with large neutron excess by ssion. The onset of an asym-metric ssion mode at these energies seems to considerably enhancethe yield of neutron rich species. Therefore, a method could emergefor producing neutron rich rare earth nuclei in a region up to nowinaccessible.The n TOF is a CERN spallation neutron facility at CERN. Its description,including the main characteristics and also the physics program to be accom-plished can be found [1] in the TOF Proposal and the recent Technical DesignReport. Here, only some relevant parameters for the following discussion willbe given. All the parameters of the installation resulted from a detailed MonteCarlo simulation that made use of the advanced FLUKA-EA-MC[2] programs.This paper is organized as follows: rstly some parameters of n TOF hav-ing a particular relevance for the coming discussion will be presented. Then,some recent data concerning the mass distributions of fragments from ssionof238U and237Np with neutrons of 5 MeV energy will be reviewed with em-phasis on the production of exotic species. Corresponding productivities willbe evaluated for the TOF conditions. They will be also compared to the re-cent results obtained at GSI by ssioning an238U (750 AMeV) incident beamon Pb and Be targets. Conclusions will be drawn concerning the potential ofTOF to produce/study exotic nuclei.The 20 GeV/c beam from the CERN-PS will impinge on a spallation targetmade of high purity lead (a solid block with the dimensions of 608080 cm3).Preprint submitted to Elsevier Preprint 26 April 2001SIS-2001-12917 August 2001The target is immersed in a water pool that serves also as a moderator andwhich attens the spectral shape of the produced neutrons. A ight path ofabout 200 m follows after the target and the moderator water layer (5 cmthick), allowing a precise energy determination of the neutrons by time ofight. The direction of this ight path makes an angle of 10owith respect tothe incoming proton beam direction. A window with a diameter of 80 cm sepa-rates the target-moderator assembly from the ight path tube from which theair is evacuated. The spatial distribution of neutrons at the exit of the windowas expected from the simulations is shown in g.1a [3]. If only the neutronswith energies in the interval 1-10 MeV are selected, then the distribution isnarrower and preserves a memory of the initial proton beam direction (seeg.1b). The simulations indicate that for each incident proton, about 70 neu-trons are produced that come out of the window. Of these, about one neutronhaving the energy in the indicated domain will be placed in a disk with adiameter of 10 cm centered on the peak in g.1b. As the PS can deliver 7 1012protons in a pulse (with a time width  7ns), some 6 1012such neutrons willbe created per proton pulse.In the following one will briey review some recent experimental resultsconcerning the ssion of238U with neutrons of energies between 1 and 5.5MeV [4]. The authors of [4] measured the mass and total kinetic energy dis-tributions for fragments resulting from238U ssion with neutrons of variousenergies and made a multimodal analysis. Their results show that the con-tribution of the so called standard 2 mode is responsible for the productionof masses placed on the wings of the well known two humped distribution.Moreover, the contribution of this mode increases with the neutron incidentenergy and so does its width. This tendency has been followed up to 5.8 MeV;it is likely that it will be preserved for some additional 1-2 MeV. For higherenergies, multiple chance ssion sets in and complicates the analysis. Fromthe les kindly provided by the author one can see mass splits as asymmetricas 63 and 176 at yield levels of about 10 3%. The charge yield curves mea-sured by [5] extend from Z=30 to Z=62 but the yield values at the extremesare dicult to evaluate on the linear scale. The distributions presented in [4]are the initial ones, in the sense that they are corrected for the evaporatedneutrons. This correction can amount to two neutrons for the heavy fragmentand 0.5 neutrons for the light fragment. One should remark at this point thatsuch asymmetric splits lead imperatively to nuclei far from stability. On theisobaric lines 63 and 174 all Z combinations adding up to 92 give rise to veryexotic nuclei. Perhaps the less exotic of all is the pair63Cr-174Er. Trying tocome close to stability with one fragment will automatically make its partnermore exotic. The above mentioned yield will be distributed among dierentpairs of fragments corresponding to dierent Z splits and the more exotic willbe one of the partners, the lower will be the yield. As charge measurementsare not easy to perform at this sensitivity level one can only suppose that theproducts with very asymmetric mass splits will be distributed over a limitednumber of charge splits and that moving away from stability with one partner2will imply a sharp decrease in the yield, e.g. one order of magnitude per step.In terms of cross sections, taking for238U a ssion cross section of 0.66 b, themass split 63-174 will appear with a cross section of 0:66 10 5= 6:6 b. Con-sidering another 3 orders of magnitude decrease for more exotic Z splits onestill have a cross section of the order of 1nb. This value is comparable to thecross sections for production of exotic species recently measured at GSI [6] inthe ssion of238U projectiles of 750 AMeV on a Be target. In particular, themass split that lead to the observation of78Ni in that experiment,78Ni-159Gdis not very exotic; instead, the charge split is a very exotic one and it has paidthe price of a very low cross section.237Np is also a good candidate for producing exotic nuclei by ssion. Siegleret al. [7] reported data that present a broad mass distribution and a similarmultimodal analysis. The advantage of237Np is a higher ssion cross section( 1b) but the disadvantages are its lower N=Z ratio and its radioactivity andcost.Some estimations can be made for the production of exotic species using theinduced ssion of238U by the spallation neutrons of the TOF installation. Fora disk-shaped UC2target with a diameter of 10 cm and a thickness of 10 mmplaced on the position of maximum ux for neutrons of 1-10 MeV describedabove, about 6 1012neutrons will traverse the target at each PS pulse. Takinga mean ssion cross section value of 0.66 b, one expects about 1011ssionevents per burst. If the yield of very asymmetric mass splits is taken (following[4]) as 10 5, then 106such cases will occur at each burst. Taking out another3 orders of magnitude for also producing exotic Z splits one arrives at 103perburst. The experience accumulated at ISOLDE [8] shows that the extractioneciency for rare earths vary between 10 and 30% while the post-accelerationhas a 10% eciency. This means that in an ISOLDE type installation, about10 very exotic species per burst can be expected (whose production cross sec-tion is about 1 nb). The lifetimes in the region of interest are expected to berather long and therefore no additional decay losses are considered. In this es-timative calculation, the thermal ssion yield has not been considered, thoughits contribution is very important in view of the high ux for thermal neutrons.For less exotic mass and charge splits this contribution will add to the abovedescribed one, such that, for \moderately" exotic nuclei (yields  10 3), theproductivity may reach as many as 3 106per pulse. The energy release of theproduced ssion events in the target (including thermal ssion) will amount toabout 7 J/pulse. In one day's irradiation, as many as 5000 pulses can be usedwhich casts rather optimistic perspectives for attaining even such exotic splitsas60Ca-177Hf. A highly enhanced neutron ux (about one order of magnitude)can be obtained by inserting the UC2target assembly into the lead spallationblock. This solution is currently under study.In conclusion, some perspectives have been presented for using the newTOF facility at CERN for producing nuclei far from stability. In particular,this method could provide a bridge to ll up the "gulf" existing in the chartof nuclides on the neutron rich side of the rare earth region. At the other3extreme, the attempt to reach60Ca, if successful, could cast a new light onthe persistence of shall closure at very high neutron excess.-0.4-0.3-0.2-0.100.10.20.30.4-0.4 -0.2 0 0.2 0.4x (m)y (m)-0.4 -0.20 0.20.4-0.4-0.200.20.4-0.4 -0.20 0.20.4-0.4-0.200.20.40100200300400500600700800900x (m)y (m)(a) (b)Fig. 1. Neutron spatial distribution at the exit of the window: a) all the neutrons;b) neutrons with energies in the range from 1 to 10 MeV.References[1] TOF Proposal, CERN/ISPSC/99-8 SPC/P 310,TOF Technical Design Report, CERN/INTC/2000-04.[2] A. Fasso et al. in \Intermediate Energy Nuclear Data: Models and Codes",Proceedings of a Specialists Meeting, Issy les Moulineaux (France) 30 May -1-June 1994, p.271, published by OECD, 1994 and references thereinC. Rubbia et al. Conceptual Design of a Fast Neutron Operated High PowerEnergy Amplier CERN/AT/95-44(ET)[3] V. Vlachoudis et al., CERN/ET/Internal Note/2000-06[4] F. Vives et al., Nucl. Phys. A 662 (2000) 63F. Vives, These, Univ. Bordeaux, 1998[5] G. Rudstam, Proc. Int. Conf. On Nuclear Data for Science and Technology,Gatlinburg, 1994 (ed J.K. Dickens)[6]M. Bernas et al., Phys. Lett. B 450 (1997) 111[7] P. Siegler et al., Nucl. Phys. A 594 (1995) 45[8] H. Ravn, private communication4

CERN n-TOF facility and its possible impact on producing nuclei with large neutron excess

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