Aiello, S

Anzalone, A

Arena, N

Cavallaro, S

De Filippo, E

Geraci, E

Geraci, M

Giustolisi, F

Lanzanò, G

Mahboub, D

Pagano, A

Politi, G

Rothard, H

Volant, C

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Fast electrons ejection by swift highly charged ions:absolute cross sections and evidence for \Fermishuttle" accelerationG. Lanzanoa, E. De Filippoa, D. Mahboubb,H. Rothardc, C. Volantd,S. Aielloa, A. Anzaloneb, N. Arenaa, S. Cavallarob,E. Geracib, M. Geracia, F. Giustolisib, A. Paganoa, G. PolitiaaIstituto Naz. Fisica Nucleare and Dipartimento di FisicaCorso Italia 57, 95129 Catania, ItalybINFN-LNS and Dipartimento di Fisica,Via S. Soa 44, 95123 Catania, ItalycCIRIL-Ganil (UMR 6637 CEA/CNRS/ISMRA),BP 5133, 14070 Caen Cedex 05, FrancedDAPNIA/SPhN, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, FranceJuly 27, 2000AbstractWe shall report in this contribution on the results of recent ex-periments performed at the CS (Catania) and Ganil (Caen) cyclotronaccelerators, with 45 A.MeV 19+and 28+ 58Ni and 95 A.MeV 18+ 36Arpulsed beams respectively. Electron velocity spectra were measured ina large angular range with dierent conducting targets (C, Al, Ni, Ag,Au). Besides electrons with the beam velocity (convoy electrons, CE)and two times the beam velocity (binary encounter electrons, BE), inthe case of Au target we observe also a high velocity tail, that can beexplained by a Fermi-shuttle mechanism. We give also BE and CEabsolute production cross-sections. In the Ganil experiment, as verypreliminary result, the lack of the CE component at the most forwardangles for the carbon target, is observed. For the other targets the CEcomponent is increasing as a function of the target atomic number.PACS Ref: 34.50.Fa, 79.20.Rf, 25.70.-ze-mail: lanzano@ct.infn.it11 IntroductionHeavy ion beams at \intermediate energies" (20A.MeV< E <200A.MeV)have extensively been used in nucleus-nucleus interaction studies. On thecontrary, very little attention has been paid to the nucleus-electron inter-action, a eld being at the boundary between nuclear and atomic physics.The knowledge of fast electron ejection properties at these high projectileenergies, like velocity spectra and production cross-section angular distri-butions, are important for testing basic atomic ionisation theories. In theforward beam direction, fast electrons are essentially due to two reactionmechanisms. A binary-encounter (BE) between the incident ion and anatomic electron produces electrons with a centroid velocity of almost twicethe projectile velocity vP. Since electrons are bound to the target nucleus indierent shells, the observed distribution of BE electrons at xed angle is adistribution which reects the initial momentum distribution of the boundelectrons of the target (\Compton prole"). Also, target electrons may becaptured or projectile electrons may be lost into low lying projectile centredcontinuum states. These so-called convoy electrons travel with a velocityclose to that of the projectile and lead to a cusp shaped peak in electronspectra.Studies of BE electron emission at high beam energies above 10 A.MeVare quite scarce. We refer to our recent study [1] for a more completebibliography on the subject.2 Experimental lay-out and detection methodThe experiments were performed at the CS Superconducting Cyclotron ofLNS in Catania and at Ganil in Caen. In the CS experiment pulsed 45A.MeV 19+and 28+ 58Ni beams with a pulse width (burst time resolution)of 1.2 ns were used. We used dierent targets:12C,27Al,58Ni and64Ni,natAg,197Au, of almost the same areal thickness, 300 g/cm2, in order tovary the target nuclear charge ZTwhile keeping secondary eects connectedto electron transport constant as discussed in [2]. Also, the possible depen-dence on the target thickness (electron transport) was investigated by using7 dierent Carbon targets of 10, 20, 90, 300, 1000, 2000 and 8000 g/cm2thickness. The multidetector ARGOS, consisting in about 100 scintillationphoswich detectors, was used inside the big scattering chamber CICLOPEof the LNS for a complete detection and identication of electrons and othernuclear products [1, 3]. Briey, the in-plane electron detection was extendedfrom 4to 170in coincidence with a nearly 0geometry central wall. Someinclusive measurements at few selected angles were also carried out. Elec-trons were identied by shape discrimination of the photomultiplier outputsignals (the \fast" and \slow" components of the detector) and by measuring2their time-of-ight as described in detail in [1]. For their absolute velocitycalibration the prompt -ray peak centroid was measured.In the recent (July 2000) Ganil experiment a pulsed 95 A.MeV 18+ 36Arbeam was used. An overall timing resolution (beam burst and detector)of 500 ps was obtained. The geometry of the ARGOS detectors was verysimilar to the one of the CS experiment. Due to the higher beam energy,threshold eects are here less important, so that convoy electron velocityspectra can be fully measured at the most forward angles.3 ResultsFig. 1 shows typical inclusive time-of-ight spectra for electrons emittedat 5, following the impact of 95 A.MeV 18+ 36Ar beam on dierent (C,Ni, Ag, Au) targets. The spectra are normalized to the integrated beamcharge and to the number of target electrons per unit area. We observetwo distinct components: fast binary encounter (BE) electrons and elec-trons with velocity close to the beam velocity (convoy electrons). Otherless intense peaks are observed for much longer times of ight, mainly af-fected by electronic threshold (indicated by the shadowed area) eects. Theintegrated absolute values for the BE and CE components are shown inFigs. 2(a) and 2(b) respectively for the 45 A.MeV Ni beam (full circles)and the 95 A.MeV Ar beam (empty squares). As a function of the tar-get atomic number ZTthe BE production cross section per target electronis almost constant within error bars. This means that the BE intensitiesare roughly proportional to the number of electrons \seen" by the projec-tile on its way through the target, i.e. to the number of electrons per unitarea. Note, however, that forward BE production induced by 45 A.MeV28+ 58Ni beam is larger by almost a factor 5 with respect to the case of a95 A.MeV 18+ 36Ar beam. This is probably a pure kinematic eect. Byusing the simple Rutherford scattering formula, we get for the two cases afactor (ZNi=ZAr)2 (AAr=ANi)2 (EAr=ENi)24.15, in good agreementwith the data. Increasing the detection angle , BE production increasesas  1= cos3, while the centroid of the BE velocity spectrum decreasesapproximately as 2vPcos . Therefore, it seems that at these intermediateenergies many of the observed BEE properties are well accounted for bytreating the collision as a binary encounter between the incoming ion and a\free" electron.Concerning the electron convoy component, their systematic study ismade dicult because of important threshold eects. Consider for instancethat in the forward direction convoy electrons of only 25 and 50 keV areproduced with58Ni 45 A.MeV and36Ar 95 A.MeV beams respectively. Forthe58Ni beam experiment, more aected by threshold eects, some generalconsiderations are possible, in connection either with the beam charge state3and the dierent nature of targets. For a bare 28+ 58Ni projectile, convoyelectrons are due essentially to a \pick-up" mechanism of electrons in theatomic medium, with subsequent re-emission. As expected [6], their pro-duction cross-section shows a dependence on the target atomic number (seeFig. 2), but more marked than in the case of BE electrons . For a 19+58Ni beam, a58Ni nucleus with a cloud of 9 electrons, the results are quitedierent [5]. The convoy electrons come essentially from the projectile, witha transfer mechanism to the continuum, being the pick-up component negli-gible. Their production cross-section is independent from the target atomicnumber. In this case, we observe a convoy component almost a factor 80much higher than the BE component.Looking now to Fig. 1 relative to the Ar beam experiment (95 A.MeV),we observe that the trend of the CE production cross section as a function ofthe target atomic number is very similar to the one obtained at lower energy(45 A.MeV). However, absolute values are much lower. Much more striking,in Fig. 1, is the lack of the convoy peak for the carbon target (either 100or 1000 g/cm2thick), at the most forward angles, up to 1.5, speciallyif compared to the case of a gold target equally or less thick. Which is thereaction mechanism responsible for electron capture at these intermediateenergies? Is there an evidence of a \threshold" eect in the case of the carbontarget? Are the convoy electrons in the case of the gold target captured frominner shells, in order to \match" the high projectile velocity?Finally, let us comment on two other results obtained in the experimentwith the 45 A.MeV58Ni beam. The rst one concerns the presence ofhigh energy electrons in the backward direction. For solid thick targets aconsistent part of the initially forward focused electrons interact more or lessstrongly with the atomic medium, so that a fraction of them are deviatedin the backward direction. We expect also that this eect should dependon the size of the scatter atomic center, and hence from the target atomicnumber. Absolute values obtained at -140are shown in Fig. 2(c) as afunction of the target atomic number. At the most backward angles thedetected electrons are aected in a signicant way by threshold eects. Theexperimental points have been obtained by integrating the electron velocityspectra starting from the threshold (7.5 cm/ns) up to the high velocitypart of the spectrum, that extends up to two times the beam velocity. It isalso noteworthy that the spectra are relative to targets of dierent atomicnumbers but with the same areal thickness (300g/cm2), and hence roughlywith the same total number of electrons. In other words, this should beregarded as an evidence of the importance of the target size scattering centerin the production of fast electrons at backward angles.The second result concerns the presence, in the case of a gold target, ofan extended tail in the high energy part of the BE peak. It extends up tovelocities as high as 24-25 cm/ns with an intensity of almost 1/1000 of theBE peak maximum intensity. This behaviour, peculiar of the gold target, is4observed for all the forward angles up to 60. This can only in part be dueto the very complex Compton prole for the gold target. An additional pos-sible mechanism involves multiple collision sequences of electrons betweentarget and projectile nuclei. This is often referred to as \Fermi-shuttle"mechanism, already invoked to explain high energy cosmic rays [4]. A partof the BE electrons produced in the collision interacts with the target atomsalong the ion trajectory. Possibly, they are scattered back with a certainvelocity distribution, and a certain probability of colliding again with thesame incident nucleus. We emphasize that the probability of such higherorder processes may be sharply enhanced in ion- solid compared to ion-atom collisions, because of the high target nucleus density. We made simpleFermi-shuttle calculations, described in [3, 5] that accounts in a satisfactoryway for the tails of the BE velocity spectra.4 ConclusionIn conclusion, we have shown the complexity of the origin of fast electronsat intermediate energies. At the most forward angles they are due in greatamount to two distinct direct mechanisms, wich give origin to electrons withbeam velocity (convoy electrons) or two times the beam velocity (BE, binaryencounter electrons). Their relative production depends on the beam chargestate as well on the target atomic number. The convoy electron captureproduction cross section at most forward angles strongly depends on thetarget atomic number, as already observed [5, 6], and on the beam energy.In particular we observe a lack of convoy electron in the case of a Carbontarget for a 95 A.MeV36Ar beam. Also (see Fig. 1), an increasing amountof electrons, with velocity intermediate between the beam velocity and twotimes the beam velocity, is apparent with the increasing atomic number ofthe target.For heavy target, like Au, an enhancement of the high velocity tail in theBE peak is present, that can be explained with a simple \multiscattering"or \Fermi Shuttle" mechanism.Finally we have shown that fast electrons are present also at most back-ward angles. Very probably they are due to transport eects in the atomicmedium of the target. Their production is very sensitive to the size of thetarget atom.AcknowledgementsWe would like to thank the CS and Ganil sta for providing the 45 A.MeV58Ni and 95 A.MeV36Ar beams respectively, N. Giudice, N. Guardone,V. Sparti and S. Urso from INFN Catania and J. Cacitti from Ganil for5helping during the mounting of the experiment, C. Marchetta for targetspreparation.References[1] G. Lanzano, E. De Filippo, S. Aiello, M. Geraci, A.Pagano, Sl. Caval-laro, F. Lo Piano, E. C. Pollacco, C. Volant, S. Vuillier, C. Beck,D. Mah-boub, R. Nouicer, G. Politi, H. Rothard, D. H. Jakubassa-Amundsen,Phys. Rev. A58, 3634 (1998).[2] H. Rothard, D. H. Jakubassa-Amundsen, A. Billebaud, J. Phys. B31,1563 (1998).[3] G. Lanzano, E. De Filippo, D. Mahboub, H. Rothard, S. Aiello, A.Anzalone, S. Cavallaro, E. Geraci, M. Geraci, F. Giustolisi, A.Pagano,and G. Politi, Phys. Rev. Lett. 83 4518 (1999).[4] E. Fermi, Phys. Rev. 75 1169 (1949).[5] G. Lanzano et al., submitted to Phys. Rev. A[6] M. Breinig et al., Phys. Rev. A25 3015 (1982).Figure captionFig 1. For the reaction 18+ 36Ar (95 A.MeV) + C, Ni, Ag, Au the electroninclusive time-of-ight spectra are shown. The binary encounter (BE)and convoy components of the spectra are indicated. The detector isplaced at 282.1 cm from the target, at a laboratory angle of 5. Theshadowed area indicates the electronic threshold. Note the lack of theconvoy component in the case of the carbon target and the presenceof several structures in the region of longer times of ight.Fig 2. For the reaction induced by 28+ 58Ni (45 A.MeV) (lled circles) and18+ 36Ar (95 A.MeV) (empty squares) on dierent targets, as belowindicated, we show the absolute electron production cross section at6(circles) and 5(squares) in (a) for the BE component, in (b) forthe convoy component, in (c) for backward electrons at -140(ve 7:5cm/ns) respectively, as a function of the target atomic number (respec-tively C, Al, Ni, Ag, Au). Note that the absolute production cross-section scale is divided by the target atomic number Z. The emptycircles in (b) are values taken from [6] and normalized to our data.600.20.40.60.811.21.41.61.82350 400 450 500 550 600 650 700TIME OF FLIGHT (arb. units)YIELD (arb. units)ConvoyBEAuAgNiCΘ = 5o95 A.MeV  36Ar 18+710 310 4a)10 210 310 4b)10 310 40 10 20 30 40 50 60 70 80 90 100c)Target Atomic Number Zdσ/(Z dΩ) (b/(e sr))Binary Encounter electronsConvoy electronsBackward electrons at -140oC Al Ni Ag Au8

Fast electrons ejection by swift highly charged ions: absolute cross sections and evidence for Fermi shuttle acceleration

Fast electrons ejection by swift highly charged ions: absolute cross sections and evidence for Fermi shuttle acceleration

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