а_ 4JJ H; ел CE IPNO-DRE-e9-15 EQUILIBRIUM CHARGE STATE OF FAST HEAVY IDNS IN SOLIDS. MEASUREMENTS OF POST IONIZATION EFFECTS.

Transcription

1 I.P.N. BP n ORSAY о ел I а_ 4JJ H; ел CE i CO Q_ CNJ IPNO-DRE-e9-15 EQUILIBRIUM CHARGE STATE OF FAST HEAVY IDNS IN SOLIDS. MEASUREMENTS OF POST IONIZATION EFFECTS. A.Brunei le, S.Della-Negra, J.Depauw H.Joret, Y.Le Beyec, K.Wien

2 EQUILIBRIUM CHARGE STATE OF FAST HEAVY IONS IN SOLIDS. MEASUREMENTS OF POST IONIZATION EFFECTS. A. BRUNELLE, S. DELLA-NEBRA, J. DEPAUW, H. JORET, Y. LE BEYEC, K. WIEN* Institut de Physique Nucléaire, F Orsay Cedex Technische Haschschule Darmstadt, D 6100 Darmstadt + Abstract : Measurement a-f the H secondary ion yield -from the surface of solid targets has been used to probe the charge state of fast heavy ion projectiles at the target surface. Results have been obtained for ion beams of S, "Ar, Kr at ~ 1 MeV/u and between 0.5 and 1.5 MeV/u for I. Increases in the charge state after passage through carbon and Au fails Are clearly observed for 84 Kr and I. The variation of charge state as a function of the thickness of matter traversed is compared with calculated values. In recent years, a large number o-f equilibrium mean charge states have been measured -for energetic ions passing through solids. Experimental results have been obtained over a wide a-f beam energies with a variety o-f projectiles and targets range Cl,23. In all cases, the charge state distributions were determined by means o-f a magnetic spectrometer a-fter the beam had passed the solid target. That means, the measured charge states correspond to ions travelling in vacuum behind the target in the order o-f 10 sec. The passible alterations a-f the charge states during the flight behind the target were there-fare not taken into account.

3 In this connection a -fundamental question was raised by Betz : "Is there a difference o-f ionic projectile charge states inside and outside solid stripper targets?" The question deals with stripping processes, the understanding of energy loss in solids and also the differences of equilibrium charge states observed between gas and solids. According to Betz and Grodzins C33, Auger transitions take place once the ion leaves the exit surface of a target. This process of deexcitatian from multi excited states which are populated inside the solid will be responsible for the enhancement of the projectile charge state after passage through a solid target. Experimental results being in qualitative agreement with the Betz Grodzins model have been obtained recently C43 with Ar and Kr beams at 1.16 MeV/u. For the Kr ions an increase of charge states was observed, after the ions had left the solid foils of 32 carbon and gold. New experimental data are now available far S and I ions at various incident energies. The method used in this work allows to determine equilibrium charge states inside a solid. In this paper we present the results which have been obtained in our laboratory. A comparison is also made with theoretical predictions from Maynard and Deutsch C53. EXPERIMENTAL The method is based on the emission of secondary H ions ejected from a surface at the instant a fast ion intersects the surface. It has been shown that the H emission yield is strongly dependent on the incident charge state of a projectile and does not depend on the nature of the projectiles C6D- Fig. 1 shows the

4 variation o-f the H yield as a -function o-f the incident charge state -for the three projectiles Ne, Ar, Kr - all having a velocity o-f 1.16 cm/nsec. It can be seen, -for instance, that Ne and Ar or Ar and Kr give the same value -for the H yield. H ions are ejected -from any surfaces under relatively poor vacuum conditions (~ 10 mbar) when a -fast ion strikes the surface o-f the target. They originate very likely -from layers o-f water adsorbed at the surface and/or organic contaminants. Although the -formation mechanism o-f H is not well understood, its yield dependence on the charge state o-f the incident beam 71 and also recent investigations of its energy distributions CS3 indicate, that H is mainly produced in the close vicinity of the nuclear track surface exit. The same phenomenon is observed -for other light destruction products like С C73- In solids, a time of about 10 elapses between charge state variation for 1 MeV/u ions, thus one could estimate the duration o-f the process which triggers the H emission to be 10 to 10 sec. In recent experiments, ion beams of S and I have been accelerated by the Orsay Tandem machine. The experimental arrangement is similar to the system used with the Linac machine C4J. The ion beam passes through a thin foil of carbon ( ig/cm ) or gold (170 цд/cm ) placed at the center of the "stripping foil chamber". The charge state desired is selected by a magnet. A low intensity beam (~ 50Q ions/sec) is obtained by moving the magnet to a certain angle with respect to the incident beam direction. Aft_r charge state selection the beam enters into the reaction chamber by passing through a 1 mm diameter hole. There, the ions strike a thin target and hit a silicon detector

5 placed behind the target. The secondary ions including H - ions are ejected -From the target sur-face, accelerated to В kv and detected by a microchannel plate detector (MCP). Their masses are measured by time of -flight, with a start signal delivered by the silicon detector. An energy window is de-fined electronically an the silicon energy spectrum in order to accept events only i-f the primary ion has the energy expected. For a given target under fixed bombarding conditions, the rate o-f H emission is constant For several hours. Each time o-f -Flight (TOF) spectrum measurement at a selected charge state is made in a short time C-from a -few 10 seconds to a -few minutes -For the small charge state projectiles). Fig. 2 illustrates the experimental setup. Several targets can be successively put in the irradiation position without opening the vacuum chamber. The target and the MCP detectors are mounted on a rotating plat-form. Therefore the beam can bombard the same target sur-face either directly or -from the back a-fter passage through the target. In both cases the secondary ions originate from the same area. About 70 cm upstream -from the target, a foil (equilibrium charge state -foil) of the same nature and thickness as the target can be inserted into the beam. At the highest beam velocity used (corresponding td 1.5 MeV/u) the time of flight of an ion projectile is about 40 nsec. In the "Secondary Ion TOF chamber" carbon foils of 20 4O pg/cm and gold foils of 170 цд/crn are also used. For charge state measurements inside a solid, very thin foils of nitrocellulose have been prepared as targets (100 A to 5OO A).

6 The experimental procedure is as -follows : i) Measurement of the calibration yield curve Y(H ) = f(q.). For each incident charge state the H yield per incident ion is measured. This is -for example curve 1 in Fig. 3. ii) In a second series of measurements, the "equilibrium charge state foil" is inserted into the beam and the H yield (from the same point of the target) is again measured as a function of the incident charge state hitting the equilibrium charge state fail. If the equilibrium charge state is reached in this foil the H yield curve exhibits a flat dependence versus the charge state. This is shown with curve 3 in Fig. 3. The intersection between curve 1 and 3 gives the equilibrium charge state of an ion more than 40 nsec after passage through a solid fail ("<q >" in the figure). iii) The third step is to turn the target by ISO", remove the "equilibrium charge state fail", and measure once more the H yield curve as a function of incident charge states. Results are shown in curve 2 in Fig. 3. The crossing between curve 1 and 2 gives the equilibrium charge state measured at the exit surface of the target when the projectile leaves the surface ("<q > exit" in the figures). The results from Fig. 3 indicate that the charge state at the exit surface is smaller than the charge state measured more than 40 nsec later. When the target is thick enough to achieve charge state equilibrium, the H yield measured at the exit surface does not depend on the projectile charge state at the entrance surface and Y(H ).. = f(q.) is flat as shown by curve 2 in Fig. 3. This ч exit ^i ' means that the memory of the entrance charge state is lost. On the

7 contrary, i-f the target thickness is smaller than the equilibrium distance the equilibrium charge state is not reached, and the H yield increases with q.. This case will be shown in the -fallowing section where very thin targets are used. The experimental yields were reproducible, their values checked several times. During the experiments the target chamber was not opened to air, all the targets being kept under vacuum. RESULTS AND DISCUSSION The abjective of this work was ta determine experimentally the equilibrium charge states a-f an ion inside " 1 04 outside a -foil. Results have been obtained with S, "Ar, Кг and Г projectiles at around 1 MeV/u. One set of measurements was also per-formed with and I at 0.5 and 1.5 MeV/u. As demonstrated in Fig. 3, outside a carbon foil the equilibrium charge state <q > ohf I ions was -found ta be +27.0, whereas inside the -foil eq <q > exit is only These values are average values, which eq have not been corrected -for the charge state distribution. However, based on the calibration curve Y(H ), which varies as q, and the shape of the charge state distributions measured by us, we have calculated that the mean equilibrium charge state values (inside or outside) extracted from the present results are ovei estimated by only a -few percent. Similar results are presented in Fig. 4 for I projectiles at MeV and a gold target. The equilibrium charge state in this case is around outside and inside the foil.

8 The same experiment has been repeated with I ions at 63 MeV on carbon and on gold -foils. Fig. 5 and 6 present the results at this energy : <q > outside is +23 and <q > exit inside is far the carbon -Fail and and +18.4, respectively, -for the gold -Foil. A di-f-ference Aq between (q) outside and (q) inside is thus observed again at this energy. In most cases the equilibrium charge state values outside the target have also been measured by analyzing th charge state distribution with the magnet. Fig. 7 shows a measurement -for I at 63 MeV on gold and carbon -foils. The mean charge state is <q > = T. q F(q), FCq) being the relative intensities of the various charge states. We have obtained in these cases in carbon and in gold. These values are in agreement (within less than ±5 '/.) with the values determined by the H secondary ion method (see Tab le 1). The results on <q > inside and outside targets obtained eq with I at 190 MeV are presented in Table I. This table summarizes the experimental results obtained with Kr ions at 98 MeV, Ar ions at 46 MeV and S ions at 32 MeV. For low mass projectiles ( S and Ar) the charge state values <q > exit eq inside and <q > outside are the same. A systematic di-f-ference 04 exists -for Kr and I primary ions. So -far, we have used the desorption yield o-f H ta determine the charge state di-fferences. But the С ion - a destruction product o-f organic contaminants - could have also been used as a probe o-f the projectile charge states. As shown in re-ference C7] + 4 the pronounced С yield dependence charge scales like q. and is between q. = +9 and +45 nearly independent o-f the nature o-f the

9 projectile. The yield is, however rather small - generally below 1 '/.. As an example, in Fig. 8 results deduced from С data are presented. A gold -foil was irradiated by 63 MeV I ions. In good agreement with the H results, the measured equilibrium charge states are inside and outside. The increase o-f charge state o-f a -fast ion leaving a -foil is clearly observed in our experiments. In the Betz and Grodzins model, when an ion travels inside a solid, a large number of electrons is excited and this contributes to the ion excitation. As a consequence o-f this excitation, the ejection o-f Auger electrons is very likely responsible -for the increase of charge states in vacuum a-f*er the ion has le-ft the exit sur-face. In table 1 we have compared our experimental results with the theoretical predictions -from Maynard who has calculated the variation o-f a projectile charge state as a -function o-f the thickness traversed. An overall good agreement is -found. The calculations are described elsewhere С5Л. The present experimental method allows also to measure the charge state a-f a projectile at the exit sur-face as a -function o-f the thickness o-f matter traversed and as a -function o-f the incident charge state q.. Measurements have been per-formed with Kr ions at 98 MeV and various thin -foils o-f nitrocellulose. The thicknesses were determined by in-frared light absorption measurements. Fig. 9 shows the experimental results together with the calculated variation o-f charge states derived -from Maynard ' s calculations. The in-frared absorption measurements have been per-formed by F. Rocard -from the R. Bernas Laboratory at Drsay

10 CONCLUSION Equilibrium charge states in gases are smaller than in solids С13, Far I at 110 MeV С 1,93 the difference between gas and carbon target is approximately 7 charge units- The observation of the increase o-f charge states after the ions have emerged from the solid is consistent with a post ionisation process via Auger electron cascades, and with the differences observed between gases and solids. The post foil increase Aq is however only around of the equilibrium charge at the exit surface of a solid. Therefore only a part of the difference between gases and solids can be due to post deexcitation. The estimation of the post deexcitation effect in 15О MeV Cu carbon collisions by Shima et al С103 is of the same order of magnitude ( + 19 to +20 inside compared with outside) as observed by us. The post foil effect iiq for S and Ar Cat about 1 MeV/u) passing through carbon or gold foils is small (< 0.3 charge unit). This is not surprising since the difference Aq between gas and solid targets 32 is smaller than one in S on carbon and is estimated to be as small in 40 Ar on carbon С13. Systematic measurements with various projectiles at different energies need to be done. Measurements of the variation of the equilibrium charge state as a function of thicknesses of the matter traversed, with different combinations of projectiles would also be very informative. They would targets and facilitate the comparison with theoretical models and refine our understanding of complex atomic collisions and excitations in sal ids.

11 One o-f us, А.В., wishes to acknowledge the -financial support ai Instruments SA (Division Riber). H.J. wishes to acknowledge the financial support a-f Nermag. We thank the sta-f-f o-f the Tandem machine and E.Davanture -for the preparation o-f the manuscript. Dr G.Maynard has performed charge state calculations -far us. The authors are grate-ful to him for his e-f-ficient contribution and -for many interesting discussions. 10

12 Ian Energy (MeV) Target <q > eq a С? yi Л X + L (inside) <q > eq (outside) <q > on eq with magnet (outside) q Maynard (inside) Re-F. C5D Sh i ma j! (outside) Re-F. C23 S 32 С Ar С Au Кг 9B 9B С Ni trace Coranen Au IB O.B I О 19О С Au С Au С Au 21.4 IB ir.-' В ! 27.4 ;. 25 г! 30.8 ' 28.2! Table Г, ficperimental and theoretical mean equilibrium charge states. Values o-f mean charge states determined outside a target a-fter magnetic de-flexion are also presented. It is observed that <q> outside and <q> with magnet sre in ver> goad agreement. In this table the mean equilibrium cha.-ge states calculated by C,.Maynard and C.Deutsch C53 and those derived -from a semi-empirical formula 121 are also shown. The agreement with recent calculations -From MaynarJ and Deutsch is very satis-factory.

13 REFERENCES Cl] H.D.Betz, Review o-f Mod. Physics, vol. 44, 3 (1972) 465. C23 K.Shima, T.Ishihara, T.Miyashi and T.Mikumo, Phys. Rev. A, vol. 28, 4 (1983) C33 H.D.Betz and L.Grodzins, Phys. Rev. Lett. 25 (1970) 211. H.D.Betz, Nucl. Instr. and Meth. 132 (1976) 19. C43 S.Della-Negra, Y.Le Beysc, B.Manart, K.G.Standing and K.Wien, Phys. Rev. Lett. 58., 1 (1987) 17. C53 B.Maynard and C.Deutsch, "4th Int. Workshop on Atomic Physics for Ion Driven Fusion", J. Phys. Ç7. (1988) 89. G.Maynard, Thesis 3436, Université Paris Sud (1987). C63 S.Della-Negra, Q.Becker, R.Cotter, Y.Le Beyec, B.Manart, K.G.Standing and K.Wien, J. Phys. 49 (1987) 151. C73 K.Wien, O.Becker, W.Guthier, S.Della-Negra, Y.Le Beyec, M.Manart, K.Standing, G.Maynard and C.Deutsch, Int. J. o-f Mass Spectrom. and Ian Processes 73, (1987) 273. C83 R.Mosshammer, R.Matthaeus and K. Wien, Abstracts o-f the 197th ACS meeting in Dallas, 9-14 April C9J S.Datz, C.D.Maak, H.O. Lutz, L. C. Narthcl i -t-fe and L. B. Br idwel 1, Atomic data 2 (1971) 273. C1Q3 K.Shima, K.Umetani, T.Mikumo, H.Kano, Y.Tagishi, M.Yamanouchi, Y.Iguchi and H.Yamaguchi, in "Inner shell and X ray physics Atoms and solids", edited by D.J.Fabian, H.Kleinpoppen and L.M.Watson Plenum, NY 1981, p. 1B9. 11

14 FIGURE CAPTIONS Figure 1_ : Variation o-f H ion yield as a -function o-f the incident charge state -for the three projectiles Ne, Ar and Кг at 1.16 MeV/u (-from re-f. C6D). Fiqurg 2. : Experimental set up used at the Orsay tandem machine. Figure 3. : I at MeV. Equilibrium charge state in a carbon 2 fail (40 цд/cm ). + Curve i'. Variation o-f H yield as a -function o-f the I projectile charge state. The sur-'uce is hitted directly by the beam. Curve. H yield -from the same target srea. when the primary ions leave this sur-face. The beam has passed through the target thickness -first. Cuz-u<s; 3. H yield -from the same target ares a-fter charge state equilibration o-f the primary ion in a -foil (same as the target) placed 70 cm upstream. Figure 4_ : I at MeV. Equilibrium charge state in a gold foil. Curves 1,2 and 3 were taken as -far Fig. 3. Figure 5. : I at 63 MeV. Equilibrium charge state in a carbon foil. Curves 1, 2 and 3 were taken as -for Fig. 3. Figure 6_ : I at 63 MeV. Equilibrium charge state in a gold -fail. 12

15 Figure 7_ : I at 190 MeV. Equilibrium charge state in a carbon fail Figure 8. : I at 63 MeV. The С ions emitted -from the surface are taken as a probe o-f the incident charge states. The values o-f equilibrium charge states inside (<q > exit) and outside (<q > are equivalent to those obtained with H ions. Figure 9. : Comparison o-f the experimental charge states measured at the exit surface o-f thin -foils o-f nitrocellulose with calculated charge state variations as a -function o-f the thickness traversed. 13

16 Charge state q Figure 1

17 ф

18 i i г i г 80 27MeV Р(Н + М(р ) carbon foil (40 ns after equi I ibration.) i <q. q )exit= i i I i i Incident charge stale Figure 3

19 "Г I I I! I I I I I I i Г 120 l 2 7 I q +!27MeV Gold foil <q eq >exit= i i i Incident charge state Figure 4

20 100-63MeV Carbon foi I "О ф 50 -Wn <<q eq >=23.0 Q <q eq >exit=2l4 (2) I Incident charge state Figure 5

21 100 l 2 7 I q + i i I I i i i i i г Gold 63MeV foil 50 -*- Q <q eq > =20.9 <q eq >exit=l8.4 0 I I j I I I I i i I Incident charge state 30 Figure 6

22 i i i i i i27jq * 63MeV Gold foil <D О A u / Z 1 / / \ Carbon C_ foil 0.5 " / Г \ 0 1 i t i i 1 1 1! Incident charge state Figure 7

23 ю со

24 S. 1 1 II ш о "55 о ^ i * _ X \ 1 1 Kr ^ ^ ^ ^ ^! Distance (A) Figure 9