Electron ejection from beam-tilted-foil experiments

N U C L E A R I N S T R U M E N T S AND METHODS 151 (1978)

563-565 ; ©

N O R T H - H O L L A N D PUBLISHING CO.

ELECTRON EJECTION F R O M BEAM-TILTED-FOIL EXPERIMENTS J. SCHADER, B. KOLB*, K. D. SEVIER t and K. O. GROENEVELD

Institut fiir Kernphysik, Universiti~t Frank]'urt/Main, IV. Germany

Received 24 October 1977 The total yield of secondary electrons (SE) emitted during the passage of heavy ions (H + , He + , N + ) through thin C-foils (thickness <50/zg/cm 2) with incident energies in the MeV range has been measured by a simple method. The number of emitted SE per projectile increases with target tilt angle and with increasing nuclear charge Z of the projectile, while it decreases with increasing incident energy. The results are discussed and compared to theoretical assumptions.

1. Introduction The emission of secondary electrons (SE) induced by fast ion-solid foil interactions has been the subject of several experimental and theoretical investigationsl). Measuring electron yields not only has the purpose of gaining information about the processes leading to SE emission but has also found useful applications. In time-of-flight experiments, for instance, SE emitted from a foil can provide the start signal. Another useful application fbllows from the strong dependence of SE yields on the type of ion, which may be used for the Zidentification of the projectile ion2). Recent experiments on radiation emitted in beam-foil experiments have indicated anisotropies depending on the tilt angle of the target foil to the beam direction3). To elucidate one detail of the question of electrostatic interaction between the beam and the foil surface we have measured the number of electrons emitted in ion-solid foil interaction as a function of tilt angle for different ions, incident energies, and target thicknesses. 2. Experiment Fig. 1 shows the arrangement used in the experiment. The ions with initial charge state q~ and energy Ep traverse a thin carbon foil (thickness < 5 0 / z g / c m 2) and are stopped in a Faraday cup. The target could be tilted by a desired angle 0 to the beam (see fig. 1). At a backward angle of 135°, Rutherford scattered ions were detected to calculate the target thickness. By integrating the current flux it through the insulated target during the emission of SE (integrated to charge Qt =~itdt) and the current flux ifc through the Faraday cup (integrated to charge Qfc = J'ifcdt) the SE yield, i.e. '* Now at M.P.I. ftir Kernphysik, Heidelberg, W. Germany. t Now at GSI, Darmstadt, W. Germany.

the number N of emitted SE per projectile could be calculated by the relation N = i~f Ot/Qfc -I- (f~f- q i ) ,

(1)

which follows from the balance of charges flowing into and out of the target: qiAp+NAp-qfAp-Qt

(2)

= 0.

Here ,Zip is the number of incident projectiles and F]f their averaged final charge state. The SE yields measured this way (typical uncertainty 15%) are total yields, that is, summed over all energies and directions of emission. In fig. 2 SE yields N are shown as a function of the target tilt angle, measured in collisions of H ~ , He + , and N + ions with solid carbon foils (22 ~ g / c m 2 and 43 # g / c m 2) at incident energies of 1 MeV and 2 MeV. As can be seen from fig. 2, the results may be summarized as follows: 1) the yield increases with increasing target tilt angle, 2) it decreases with increasing energy of the incident ions, and 3) it increases with increasing nuclear charge Z of the projectile. At 2 MeV incident energy and target tilt angle 0---0 ° the yield is 1.3 electrons per projectile for incident H ÷, 10 for He + and 150 for N + ions. As far as comparable these values are in target

ion b e a m

•

/ / / ~~

Ep,qi /

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particle

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/

farada,

cup

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detector

Fig. 1. Experimental set-up.

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564

J. SCHADER et al. NUMBER OF ELECTRONS PER PROJECTILE N EJECTED IN H E A V Y - I O N SOLID CARBON COLLISIONS TARGET THICKNESS ( p~lcm 2 )

ENERGY (MeV)

t

2

22

•

Do

43

•

103 ~

PROJECTILE

(dE/dx) of the projectiles and to l/L"0, with E0 [with values in the order of 30 eV 4 ) ] being the mean energy transfer necessary to form one SE. Once formed the electrons lose energy by elastic and inelastic collisions, so that only a small fraction of them can reach the target surface with sufficient energy to overcome the surface potential barrier and to escape from the target. The probability P(x) that a SE formed at a depth x below the surface will be able to escape is then P(x) = A T e x p ( - x/Z),

102 --

with ~. being the average mean free path of a SE (average, because not a monoenergetic electron but a continuous electron energy distribution contributes), A being an anisotropy parameter with a value of approximately 1.0, and T[with values in the order of 0.9 4)] being a surface transmission coefficient, representing the probability that an electron arriving at the surface from the interior will be able to escape. The differential yield of SE from a layer dx at the depth x can then be expressed by dN(x)

lv

(3)

O0 i i 0o20°

01

, 40 °

02

03 6~0O

04

05

• LOG I/COSGI

70° TARGET T!LT ANGLE O

Fig. 2. Number of electrons per projectile N as a function of target tilt angle 0 for H + (1 MeV) on C-foil (43 ~tg/cm 2 , black triangles), H + (2 MeV) on C-foil (22 ~tg/cm 2, open circles), He + (1 MeV, black squares and 2 MeV, open squares) on Cfoil (22 t t g / c m 2) and N + (2 MeV) on C-foil (43 p g / c m 2 , open triangles).

good agreement with those measured by other methods2). The enhancement of the yield with increasing target tilt angle cannot be explained by a target thickness effect: The effective thickness seen by the ion beam increases by a factor 1/cos 0. Measurements with targets with thicknesses ranging from 20~tg/cm 2 to 300#g/cm 2, however, yield a number of emitted electrons per projectile which is nearly constant over this thickness range. 3. Discussion The experimental results can be compared to theoretical considerations by Sternglass4). In ion-atom collisions the projectiles lose energy by excitation and ionization processes which lead to the formation of secondary electrons. The number Y of SE originating in a layer (Ix at depth x is then proportional to the mean electronic energy loss

=

YP(x) dx.

(4)

Integrating over the whole target thickness d then gives the total yield N = AT/Eo ( d E / d x ) t[1 - ( e x p - d / I ) ] .

(5)

At target thicknesses d<~. the yield increases with increasing d to reach a nearly constant value for d>> ~.. SE therefore are emitted only from a thin surface layer with a thickness of a few ~., i.e. for thick targets the yield does not depend on the target thickness and remains constant as long as the mean energy loss is constant. Now the constant yield at increasing target thickness as has been measured in the present experiment can be explained: some authors 5,6) have found a value of 5 #g/cm 2 for the mean free path ~. of SE in carbon. That means that at the target thicknesses d > 2 0 ~ z g / c m 2 used here the yield has reached its saturation value. The increase of the yield with higher-Z projectile and the decrease with increasing incident energy can be explained by the dependence of the yield on the energy loss of the projectiles. By measuring with different heavy ions using carbon foils Clerc et al. 2) could show that the dependence of the yield on the ion energy is almost identical with the shape of the differential energy loss curve. The theory predicts that turning the target round

ELECTRON EJECTION

by an angle 0 will increase the SE yield by a factor l/cos 0, because the length of the ion's track within the "escape zone" is increased. As can be seen from fig. 2 our results are in good agreement with this prediction. For all projectiles and incident energies used in the present experiment the measured yields can be very well approximated by such 1/cos 0 curves (note the 1/cos 0-scale!).

565

tron detector. Eq. (5) indicates that it is possible to measure average mean free paths ~. of SE with this method by using targets with thicknesses d in the order of magnitude of ~. and measuring at different target tilt angles. The authors wish to thank J. DiSppner for preparing the foils used in this study and K. H. Ziegenhain for aid in carrying out the measurements.

4. Conclusion

By a very simple experimental method, total yields of secondary electrons emitted in collisions of heavy ions with carbon targets have been measured. The dependence of the yield on the type of the ion, the incident energy and the target thickness could be quantitatively explained by a theoretical model. In particular, the dependence of the yield on the target tilt angle, following a 1/cos 0law, is in good agreement with the theory. It should be noted that these results have been obtained without using an energy dispersive elec-

References l) W. Meckbach, Beam-Joil spectroscopy, vol. 2 (eds. I. A. Sellin and D. J. Pegg; Plenum Press, New York, 1976) p. 577, and references therein. 2) H. G. Clerc, H. J. Gehrhardt, L. Richter and K. H. Schmidt, Nucl. Instr. and Meth. 113 (1973) 325. 3) H. G. Berry, L. J. Curtis and R. M. Schectman, Phys. Rev. Lett. 34 (1975) 509. 4) E. J. Sternglass, Phys. Rev. 108 (1957) 1. 5) H. Seiler, Z. Angew. Phys. 22 (1967) 249. 6) U. Werlein, Diplomarbeit IKF - D 167 (Frankfurt/Main, 1973); U. Werlein, R. Bass, E. Dietz and K. O. Groeneveld Verhand. DPG (VI) 8 (1973) 79.