Spallation experimentsA. Boudard, J.E. Ducret, B. Fernandez, S. Leray, C. Villagrasa, C. Volant

Collaboration: IPN Orsay, CENBG Bordeaux (France), GSI Darmstadt, T.U. Munich (Germany), Univ. of Jyvaskyla (Finland),Univ. of Santiago de Compostela (Spain), ANL Argonne, Univ. of Chicago, CIT Pasadena, JPLCIT Pasadena,Univ. of Washington (USA).

IntroductionThe spallation experimental program initiated by thegroup at LNS SATURNE (measurement of neutrondouble differential cross sections) is now continuingat GSI where a wide collaboration takes advantage ofthe energetic ion beams delivered by the synchrotronSIS to measure the spallation residues isotopic crosssections. This is possible thanks to the reverse kine-matics (residues are focussed in the forward directionwith velocities close to the beam one) and to the qua-lity of the FRS magnetic spectrometer that allows toidentify each isotope produced less than 300 ns afterits production, well before most of the β decays. Theliquid hydrogen target constructed and maintainedby the DAPNIA complements nicely this set-up andmakes possible clean studies of the p-Nucleus inter-action. Except just at the Bρ (or momentum overcharge) of the beam, all isotopes produced are mea-sured down to ~µb cross sections, giving a full mapof residues produced by surface interactions, byevaporation of nucleons after a stage of internalexcitation, or by a fission process if the nucleus isheavy enough.

Three experiments have been recently performed.The Pb (500 MeV/A) on proton complements by anenergy variation the Pb experiment at 1 GeV/A [1].Whereas lead is most of the time chosen to producespallation neutrons in Accelerator Driven Systems,iron is a crucial material used in the composition ofthe delicate window between the proton acceleratorand the reactor. So an experiment, iron on proton hasbeen measured at 5 energies between 300 MeV/Aand 1.5 GeV/A. More recently, 136Xe on proton at200 MeV/A, 500 MeV/A and 1 GeV/A was measuredin fall 2002. It will provide an intermediate step inthe target mass variation.

Experiment Pb(500 MeV/A) + pThe lead experiment at 500 MeV/A is now fully analysedand has lead to two PhD thesis, one at IPNO [2] on

the evaporation residues, and one in SPhN byB. Fernandez [3] on the fission residues. Experimentallythis last part had to face two main difficulties. For theabsolute attribution of Z, starting from the welldefined peak corresponding to the beam in theenergy loss measurement, there is a gap of a fewunproduced elements between evaporation resi-dues and fission ones. Actually, three different

methods have been found to solve coherently thisproblem. The other difficulty comes from the lowtransmission of the spectrometer for fission residues.In that case, the fissioning energy leads to the emissionof the fission products in a substantial cone out ofwhich the limited acceptance of the spectrometer

Recent measurements of residues performed at GSI for the Fe+p and the Pb+p system arepresented and discussed.

Fig. 1: Mean value of the N/Z ratio for isotopes of a given mass fordifferent systems measured at GSI, Pb+p at 500 MeV/A and 1000 MeV/A,Pb+d at 1000 MeV/A and Pb+p at 600 MeV/A measured at ISOLDE(solid curve between dashed-dotted curves showing the uncertaintyregion). The dashed curve represents the valley of stability.

64

Physics f

or

the n

uclear energ

y

selects only ~10%. This acceptance is a steep functionof the residue mass but can be accurately derivedfrom the measured velocity distribution of the residues,the ion optics and assuming isotropy of the fissionprocess. It remains however the dominant contributionto the total error (15% to 20%).

From experimental data (more than 400 crosssections), characteristics of the average fissioningsystem can be reconstructed in charge (Z=80), in mass(A=194) and in excitation energy (E*=107 ± 20 MeV).In addition, the number of post fission neutrons emittedby the fission fragments has been determined (ν=8 ± 2)and is compatible with the value of the excitationenergy.

On Fig. 1, the mean value of the N/Z ratio foreach isobar is plotted and compared to results thatwe have obtained for similar systems at differentenergies. This shows that fission fragments are moreneutron-rich as the excitation energy after the cascadestage gets lower due to smaller neutron evaporationprior to the fission. Note that the new experimentPb (500 MeV/A)+p is quite compatible with theISOLDE experiment measured at 600 MeV andshown as lines, but clearly more precise.

Experiment Fe + pThis experiment was the PhD thesis of C. Villagrasa [4].The main difficulty in the experimental analysis has beenthe late discovery of a non conventional beam opticused during data taking at 300 MeV/A and 500 MeV/A.In this experiment, there is no fission, and all residualnuclei are strongly forward peaked, so that all of them

are detected when they are not much lighter than theprojectile. Actually this acceptance falls down to ~50%for the lightest nuclei and for low beam energies. So itwas important to correct for the optic actually used atlow energy.

In Fig. 2 top view, cross sections are shown summedby elements for all energies. On the bottom part, someisotopic distributions are shown at 1 GeV/A. Data arecompared with predictions of the INCL4 intra-nuclearcascade model [5] (developed in the group in collabo-ration with J. Cugnon from Liege University) coupledwith the evaporation-fission code GEM [6]. The generaltrend is quite similar with all other systems studied upto now. Cross sections are accurately predicted on 4orders of magnitude for residues not far from theprojectile, but systematically under predicted for lightresidues. Iron is a rather light nucleus, but this was alsotrue for Lead and Uranium. What we learn here in additionis that the situation becomes worst when the energy islowered. So a mechanism is probably missing in themodel, but it could hardly be a multifragmentationprocess. It should be noted that we obtain quite similarresults with the ABLA code [7] developed at GSI andbased on a different approach of the evaporation–fissionstage.

For applications to the window of a hybrid reactor,these data gives a direct access to the quantity ofchemical impurities accumulated after irradiation ofiron by the proton beam. We are confident enough inthe model and the precise nuclear structure is so washedout in the spallation that we could compute theamount of impurities in a real alloy (made in additionwith Cr, Mn, Mo…) with possible empirical correctionsbased on this experiment for impurities far from thetarget nucleus charge. We can expect a precision of15%-20% on each chemical impurity to be used asinput for further investigation of chemical effects in theaccelerator window.

Another important practical output of this experimentis the precise measurement of the residues velocitiesrelative to the beam. After a trivial kinematicaltransformation, this gives an experimental knowledgeof the recoiling energy for each residual nucleus formedin the window. This is the basic quantity needed toevaluate the number of DPA (displacements peratoms of the target), a crucial parameter to predictpractical embrittelment of the window. It should besaid that, contrary to heavy target nuclei, the modelover predicts largely the recoil energy.

[1] T. Enqvist et al., Nucl. Phys. A686 (2001) 481.[2] L. Audouin, PhD thesis, Univ. Paris-Sud 2003.[3] B. Fernandez-Dominguez, PhD thesis, Univ. Caen 2003. [4] C. Villagrasa-Canton, PhD thesis, Univ. Paris-Sud 2003.[5] A. Boudard et al., Phys. Rev. C66 (2002) 044615.[6] S. Furihata, Nucl. Inst. & Meth. B171 (2000) 251.[7] J.J. Gaimard and K.H. Schmidt, Nucl. Phys. A531 (1991) 709.

A.R. Junghans et al., Nucl. Phys. A629 (1998) 635.

Fig. 2: Cross sections of residues produced in Fe+p andmeasured at GSI are presented as points and INCL4-GEMpredictions as lines. On the top part, cross sectionssummed for constant atomic number (Z) are shown foreach beam energy and multiplied by arbitrary powers of 10as indicated. On the bottom part, a selection of isotopiccross sections is shown at 1 GeV/A.

Contact: Alain Boudard aboudard@cea.fr 65