- Email: [email protected]
Ion beam current dependence of secondary electron emission from thin carbon foils
Nuclear
Instruments
North-Holland,
and Methods
ION BEAM CURRENT FROM THIN CARBON J.C. DEHAES, Vniversitt!
in Physics Research B13 (1986) 627-630
627
Amsterdam
DEPENDENCE FOILS*
J. CARMELIET**
Libre de Bruxelles,
The secondary electron shown that the observed
OF SECONDARY
ELECTRON
EMISSION
and A. DUBUS***
C.P. 165, 50, av. F.D.
Roosevelt,
B-1050,
Brussels,
yield and the mean charge of the beam have been measured current dependence is not a temperature effect.
1. Introduction In 1977, Hight et al. [l] observed that the alignment of the 3p ‘P level of He I, excited by passage of a He’ beam through a thin carbon foil, depends on the beam current. One year later, Gay and Berry [2] proposed an explanation based upon the measurement of the secondary electron yield as a function of beam current and foil temperature. They concluded that the increase of temperature due to ion bombardment decreases the number of secondaries and that the depolarizing effect of the electrons is responsible for the observed variation of the alignment. This explanation rests on two basic assumptions: _ the alignment is perturbed due to the interaction between the emerging excited atom and the outgoing secondaries; _ the electron yield is a function of temperature. The first assumption has not yet been verified experimentally, work is in progress to try to obtain a theoretical estimate of the transient electric field acting on the atom. It is worth noting that the variation of the alignment which is rather small for the 3p ‘P level is very important for the 3d ‘D level [3]. The purpose of .this paper is to give both experimental and theoretical evidence against the second hypothesis i.e. the direct temperature effect is negligible. In the course of these experiments we have also gained some new results on the beam current dependence of the mean charge and light yield of the beam after the target.
Belgium
as a function
We have
of the beam current.
target chamber is shown in fig. 1. It makes possible the measurement of the current in the target holder (I,), the forward (1,) and backward (I,) electron currents and the ion current (I,,) after the target. An ion collector can be put in the beam upstream of the first plate in order to measure the current I, of the wellcollimated incoming He’ beam, which is produced by a 400 kV, 2 mA Sames accelerator. Two deflexion plates located at the exit of the analysing magnet are used either to deviate the beam between two measurements or to pulse the incoming beam. The rise time of the ion pulse is less than 10 ps. The electronic equipment differs according to the working mode: continuous or pulsed beam. In the continuous mode the collected charge is measured by a current-to-frequency converter followed by a scaler. The integral linearity is better than 0.1% and the five converters are identical within 0.2%. In the pulsed mode, two multichannel analysers working in the sampling mode record the facsimile of two selected combinations of currents. The electron collectors were held at a positive bias voltage of 2OOV, although 1OOV was enough to collect all the electrons. In order to avoid spurious effects due to changes of the electrode bias during the measurements of the currents, the resistances at the input of the converters were chosen such FARADAY \
CUP
FOIL HOLDER SHIELDS
\
ELECTRON
MOVABCE FARADAY COLLECTORS /
CUP
BEAM
2. Experimental
setup
The experimental
setup
which
has been
*Work supported by the USN. ** From the Institut Interuniversitaire leaires. *** From the Fonds
National
0168-583X186/$03.50 (North-Holland
Physics
0
des
Sciences
de la Recherche
Elsevier Publishing
put in the
Science Division)
Nuc-
Scientifique.
Publishers
'f Fig. 1. Schematic
B.V. XI. ELECTRON
diagram
‘H
‘b
‘M
of the experimental
EXCHANGE/SECONDARY
setup. EMISSION
J. C. Dehaes et al. I Secondary electron emission
628
that the voltage drop was always less than 0.5 V in the continuous mode and 0.1 V in the pulsed mode. All the experiments have been performed with 6 mm diameter carbon foils (10 pg/cm2 thickness) and with a 4mm He’ beam. Without the foil, the currents If, I, and In are less than 0.1% of the beam current. The vacuum in the target chamber was about 2 x 10d6 Torr.
I!
’
’
’
’
’
0
I
2
3
L
5
INCOMING
’
’
’
6
7
8
CURRENT
’
’
9
10
IM(~JA
’
‘I
I1
12
3. Experimental
results
In the continuous mode, the four currents Z,, Z,, Z, and ZFc were measured simultaneously and the following quantities calculated: -the sum of all the currents: Z, = I, + I,, + I, + ZFc: -the mean charge of the outgoing beam: q = Z,,iZ,; -the total electron yield (e-/ion): y = IZ,,+ Z,llZ,,; -the forward to backward yield ratio: R = Z,/Z,,. The incoming current was measured before and after each experiment to verify the beam stability, a maximum of 5% variation is allowed. The difference between I,, and Z, is then always less than 2%. During these experiments we have observed a foil aging effect which manifests itself through a slow evolution of all the currents. This effect may be due to the poor vacuum in the target chamber: ~2 x 10m6Torr obtained by a turbomolecular pump. During a one hour period, the variation of q and y is less than 5% but the variation of R is 3 times larger. To account for this effect, the measurements as a function of beam current have been done both for increasing and decreasing current. A typical result is shown in fig. 2. It is worth noting than in most experiments there was a kind of hysteresis in the current dependence of y : the curve for increasing current is above the one for decreasing current.
1
Fig. 2. Beam current dependence of the secondary electron yield (a), forward to backward ratio (b) and mean beam charge (c). Four consecutive measurements on the same target are shown: -increasing I,, --- decreasing I,, I . . increasing I, and - . - decreasing IM. The indicated error bars have been calculated assuming a systematic error of 0.2% for each measured current.
0
.l
I
I
1
I
,
I
I
I
.2
.3
.L
.5
.6
.7
.8
.9
INCOMING
CURRENT
I
1.8
I&A)
Fig. 3. Beam current dependence of the light yield for the n = 4 to n = 3 transition in He II (-) and the 2p ‘P-4d ‘D transition in He I (---). The error bars represent ?2 standard deviations.
J. C. Dehaes et al.
0
100
50
i Secondary
150
TIME fms)
Fig. 4. Relative electron yield (y(l) - y(m))ly(m) as a function of time. y(m) has been estimated from y(150ms). The experimental data have been smoothed. The statistical error is less than 0.005. Nothing can be said about the current dependence of the forward to backward ratio since our measurements were not reproducible, although this ratio was always between 1.1 and 1.2. We also measured the light intensity for the 2p ‘P4d ‘D transition in He I and for the n = 4 to n = 3 transition in He II, the resuit is shown in fig. 3. In the pulsed mode, there is a small transient effect on the electron yield, as is seen in fig. 4. The time constant of the observed decay varies from 20ms for 0.5 PA to 5 ms for 8 PA. The amplitude is maximum for 8 PA and amounts to about 2.5% of the equilibrium value of y. No such transient effect has been observed either on the mean charge or on the light intensity. In order to have an estimate of the temperature transient, we measured the facsimile of the light emitted by the foil in the 7000-8500 A range. For a beam current of 2 PA (5 mm beam diameter, 45” tilted foil), this light intensity reaches almost its equilibrium value after 30 ms.
4. Discussion Our measurements of the beam current dependence of the secondary electron yield (fig. 2.) are in agreement with those of Gay and Berry [2] and Garnir et al, [4]: there is a 30% decrease between 0.1 PA and 10 PA. If this effect was only due to a change of temperature, the amplitude of the transient effect should be about ten times larger than the one observed (fig. 4). So we can conclude that the direct temperature effect is small. The time constants measured in fig. 4 are
electron
629
emission
in good agreement with those calculated from the heat equ$tion. It is worth noting however that this calculation is based upon the temperature measurements of Gay and Berry [2] from which a totaf emissivity of 0.04 has been deduced. From a theoretical point of view, the temperature dependence is expected to be small. Indeed the electron-phonon interaction is an elastic process which contributes very little to the total electron yield [5]. This is due to the fact that the isotropic part of the elastic cross section does not appear in the slowingdown equation for an infinite medium. Some preliminary calculations made in the frame of the models described in the previous paper [6] show that the decrease of the electron yield is less than 5% when the elastic mean free path is decreased from 6 A to 3 A, a result which is in contradiction with the one derived from the simple electron transport model of Sternglass 171. It is interesting to note that the electron yield is much more sensitive to the inelastic cross section: a 6% increase of the free electron density increases the total electron yield by about 10%. It is well known that the dielectric function of carbon depends on its structure [8] and that the stopping cross section for a particles is different for graphite and vapor-deposited carbon 191. Hence if the structure of the carbon target changes during ion bombardment, changes of the ion-electron and electron-electron cross sections are also expected. Such structural changes have been observed [lo, 111 but no evidence exists that they are reversible as needed to explain that new or irradiated foils have almost the same beam current dependence of the electron yield. Surface contamination or other surface changes may also occur during ion bombardment. In our experiments, contamination of the surfaces and thickening of the target may play an important role, although these effects are also irreversible. The beam current effect on the mean charge is also a puzzling feature. It would be interesting to test if small structural changes in the target or surface contamination can be responsible for a 10% variation of the mean charge. The beam current dependence of the light yield shows that the excited atoms are formed at the surface of the target. Indeed the light yield is proportional to the mean charge, i.e. the fraction of He’, even for the 2p ‘P-4d ‘D transition in neutral helium. The same trend has been observed by Dynefors et al. [12] below 100 keV: the relative level populationsof He I states increase with ion energy while the neutral fraction decreases.
5. Conclusion Both
experimental
XI. ELECTRON
and theoretical
arguments
EXCHANGE/SECONDARY
have
EMISSION
630
J.C.
Dehaes et al. I Secondary electron emission
led to the conclusion that the temperature effect explains only 10% of the beam current dependence of the secondary electron yield. Further experiments are needed to explain the beam current effect: for instance, measurement of the ion stopping power which is mainly sensitive to bulk effects. Anyhow, our experiments must be repeated in UHV conditions in order to avoid the contamination of the target surfaces. Work is in progress in these directions.
References [l] R.D. Hight, R.M. Schectman, and T. Gay, Phys. Rev. Al6
H.G. Berry, G. Gabrielse (1977) 1805.
[2] T.J. Gay and H.G.
Berry, Phys. Rev. Al9 (1979) 952. [3] H. Winter, Nucl. Instr. and Meth. 194 (1982) 357. [4] H.P. Garnir, P.D. Dumont and Y. Baudinet-Robinet, Nucl. Instr. and Meth. 220 (1982) 187. [S] P. Sigmund and S. Tougaard, in: Inelastic ParticlesSurface Collisions, eds., E. Taglauer and W. Heiland (Springer Verlag, Berlin, 1981) p. 2. [6] A. Dubus, J. Devooght and J.C. Dehaes, these Proceedings (ICACS ‘85) Nucl. Instr. and Meth. B13 (1986) 623. [7] E.J. Sternglass, Phys. Rev. 108 (1957) 1. [8] C.J. Tung and C. Lin, Radiat. Eff. 80 (1984) 261. [Y] S. Matteson, E.K.L. Chau and D. Powers, Phys. Rev. Al4 (1976) 169. [lo] U. Sander and H.H. Bukow, Radiat. Eff. 40 (1979) 143. [ll] H.H. Bukow, B. Gross-Kreul and U. Sander, Nucl. Instr. and Meth. 182/183 (1981) 383. [12] B. Dynefors, I. Martinson and E. Veje, Phys. Scripta 13 (1976) 308.
Instruments
North-Holland,
and Methods
ION BEAM CURRENT FROM THIN CARBON J.C. DEHAES, Vniversitt!
in Physics Research B13 (1986) 627-630
627
Amsterdam
DEPENDENCE FOILS*
J. CARMELIET**
Libre de Bruxelles,
The secondary electron shown that the observed
OF SECONDARY
ELECTRON
EMISSION
and A. DUBUS***
C.P. 165, 50, av. F.D.
Roosevelt,
B-1050,
Brussels,
yield and the mean charge of the beam have been measured current dependence is not a temperature effect.
1. Introduction In 1977, Hight et al. [l] observed that the alignment of the 3p ‘P level of He I, excited by passage of a He’ beam through a thin carbon foil, depends on the beam current. One year later, Gay and Berry [2] proposed an explanation based upon the measurement of the secondary electron yield as a function of beam current and foil temperature. They concluded that the increase of temperature due to ion bombardment decreases the number of secondaries and that the depolarizing effect of the electrons is responsible for the observed variation of the alignment. This explanation rests on two basic assumptions: _ the alignment is perturbed due to the interaction between the emerging excited atom and the outgoing secondaries; _ the electron yield is a function of temperature. The first assumption has not yet been verified experimentally, work is in progress to try to obtain a theoretical estimate of the transient electric field acting on the atom. It is worth noting that the variation of the alignment which is rather small for the 3p ‘P level is very important for the 3d ‘D level [3]. The purpose of .this paper is to give both experimental and theoretical evidence against the second hypothesis i.e. the direct temperature effect is negligible. In the course of these experiments we have also gained some new results on the beam current dependence of the mean charge and light yield of the beam after the target.
Belgium
as a function
We have
of the beam current.
target chamber is shown in fig. 1. It makes possible the measurement of the current in the target holder (I,), the forward (1,) and backward (I,) electron currents and the ion current (I,,) after the target. An ion collector can be put in the beam upstream of the first plate in order to measure the current I, of the wellcollimated incoming He’ beam, which is produced by a 400 kV, 2 mA Sames accelerator. Two deflexion plates located at the exit of the analysing magnet are used either to deviate the beam between two measurements or to pulse the incoming beam. The rise time of the ion pulse is less than 10 ps. The electronic equipment differs according to the working mode: continuous or pulsed beam. In the continuous mode the collected charge is measured by a current-to-frequency converter followed by a scaler. The integral linearity is better than 0.1% and the five converters are identical within 0.2%. In the pulsed mode, two multichannel analysers working in the sampling mode record the facsimile of two selected combinations of currents. The electron collectors were held at a positive bias voltage of 2OOV, although 1OOV was enough to collect all the electrons. In order to avoid spurious effects due to changes of the electrode bias during the measurements of the currents, the resistances at the input of the converters were chosen such FARADAY \
CUP
FOIL HOLDER SHIELDS
\
ELECTRON
MOVABCE FARADAY COLLECTORS /
CUP
BEAM
2. Experimental
setup
The experimental
setup
which
has been
*Work supported by the USN. ** From the Institut Interuniversitaire leaires. *** From the Fonds
National
0168-583X186/$03.50 (North-Holland
Physics
0
des
Sciences
de la Recherche
Elsevier Publishing
put in the
Science Division)
Nuc-
Scientifique.
Publishers
'f Fig. 1. Schematic
B.V. XI. ELECTRON
diagram
‘H
‘b
‘M
of the experimental
EXCHANGE/SECONDARY
setup. EMISSION
J. C. Dehaes et al. I Secondary electron emission
628
that the voltage drop was always less than 0.5 V in the continuous mode and 0.1 V in the pulsed mode. All the experiments have been performed with 6 mm diameter carbon foils (10 pg/cm2 thickness) and with a 4mm He’ beam. Without the foil, the currents If, I, and In are less than 0.1% of the beam current. The vacuum in the target chamber was about 2 x 10d6 Torr.
I!
’
’
’
’
’
0
I
2
3
L
5
INCOMING
’
’
’
6
7
8
CURRENT
’
’
9
10
IM(~JA
’
‘I
I1
12
3. Experimental
results
In the continuous mode, the four currents Z,, Z,, Z, and ZFc were measured simultaneously and the following quantities calculated: -the sum of all the currents: Z, = I, + I,, + I, + ZFc: -the mean charge of the outgoing beam: q = Z,,iZ,; -the total electron yield (e-/ion): y = IZ,,+ Z,llZ,,; -the forward to backward yield ratio: R = Z,/Z,,. The incoming current was measured before and after each experiment to verify the beam stability, a maximum of 5% variation is allowed. The difference between I,, and Z, is then always less than 2%. During these experiments we have observed a foil aging effect which manifests itself through a slow evolution of all the currents. This effect may be due to the poor vacuum in the target chamber: ~2 x 10m6Torr obtained by a turbomolecular pump. During a one hour period, the variation of q and y is less than 5% but the variation of R is 3 times larger. To account for this effect, the measurements as a function of beam current have been done both for increasing and decreasing current. A typical result is shown in fig. 2. It is worth noting than in most experiments there was a kind of hysteresis in the current dependence of y : the curve for increasing current is above the one for decreasing current.
1
Fig. 2. Beam current dependence of the secondary electron yield (a), forward to backward ratio (b) and mean beam charge (c). Four consecutive measurements on the same target are shown: -increasing I,, --- decreasing I,, I . . increasing I, and - . - decreasing IM. The indicated error bars have been calculated assuming a systematic error of 0.2% for each measured current.
0
.l
I
I
1
I
,
I
I
I
.2
.3
.L
.5
.6
.7
.8
.9
INCOMING
CURRENT
I
1.8
I&A)
Fig. 3. Beam current dependence of the light yield for the n = 4 to n = 3 transition in He II (-) and the 2p ‘P-4d ‘D transition in He I (---). The error bars represent ?2 standard deviations.
J. C. Dehaes et al.
0
100
50
i Secondary
150
TIME fms)
Fig. 4. Relative electron yield (y(l) - y(m))ly(m) as a function of time. y(m) has been estimated from y(150ms). The experimental data have been smoothed. The statistical error is less than 0.005. Nothing can be said about the current dependence of the forward to backward ratio since our measurements were not reproducible, although this ratio was always between 1.1 and 1.2. We also measured the light intensity for the 2p ‘P4d ‘D transition in He I and for the n = 4 to n = 3 transition in He II, the resuit is shown in fig. 3. In the pulsed mode, there is a small transient effect on the electron yield, as is seen in fig. 4. The time constant of the observed decay varies from 20ms for 0.5 PA to 5 ms for 8 PA. The amplitude is maximum for 8 PA and amounts to about 2.5% of the equilibrium value of y. No such transient effect has been observed either on the mean charge or on the light intensity. In order to have an estimate of the temperature transient, we measured the facsimile of the light emitted by the foil in the 7000-8500 A range. For a beam current of 2 PA (5 mm beam diameter, 45” tilted foil), this light intensity reaches almost its equilibrium value after 30 ms.
4. Discussion Our measurements of the beam current dependence of the secondary electron yield (fig. 2.) are in agreement with those of Gay and Berry [2] and Garnir et al, [4]: there is a 30% decrease between 0.1 PA and 10 PA. If this effect was only due to a change of temperature, the amplitude of the transient effect should be about ten times larger than the one observed (fig. 4). So we can conclude that the direct temperature effect is small. The time constants measured in fig. 4 are
electron
629
emission
in good agreement with those calculated from the heat equ$tion. It is worth noting however that this calculation is based upon the temperature measurements of Gay and Berry [2] from which a totaf emissivity of 0.04 has been deduced. From a theoretical point of view, the temperature dependence is expected to be small. Indeed the electron-phonon interaction is an elastic process which contributes very little to the total electron yield [5]. This is due to the fact that the isotropic part of the elastic cross section does not appear in the slowingdown equation for an infinite medium. Some preliminary calculations made in the frame of the models described in the previous paper [6] show that the decrease of the electron yield is less than 5% when the elastic mean free path is decreased from 6 A to 3 A, a result which is in contradiction with the one derived from the simple electron transport model of Sternglass 171. It is interesting to note that the electron yield is much more sensitive to the inelastic cross section: a 6% increase of the free electron density increases the total electron yield by about 10%. It is well known that the dielectric function of carbon depends on its structure [8] and that the stopping cross section for a particles is different for graphite and vapor-deposited carbon 191. Hence if the structure of the carbon target changes during ion bombardment, changes of the ion-electron and electron-electron cross sections are also expected. Such structural changes have been observed [lo, 111 but no evidence exists that they are reversible as needed to explain that new or irradiated foils have almost the same beam current dependence of the electron yield. Surface contamination or other surface changes may also occur during ion bombardment. In our experiments, contamination of the surfaces and thickening of the target may play an important role, although these effects are also irreversible. The beam current effect on the mean charge is also a puzzling feature. It would be interesting to test if small structural changes in the target or surface contamination can be responsible for a 10% variation of the mean charge. The beam current dependence of the light yield shows that the excited atoms are formed at the surface of the target. Indeed the light yield is proportional to the mean charge, i.e. the fraction of He’, even for the 2p ‘P-4d ‘D transition in neutral helium. The same trend has been observed by Dynefors et al. [12] below 100 keV: the relative level populationsof He I states increase with ion energy while the neutral fraction decreases.
5. Conclusion Both
experimental
XI. ELECTRON
and theoretical
arguments
EXCHANGE/SECONDARY
have
EMISSION
630
J.C.
Dehaes et al. I Secondary electron emission
led to the conclusion that the temperature effect explains only 10% of the beam current dependence of the secondary electron yield. Further experiments are needed to explain the beam current effect: for instance, measurement of the ion stopping power which is mainly sensitive to bulk effects. Anyhow, our experiments must be repeated in UHV conditions in order to avoid the contamination of the target surfaces. Work is in progress in these directions.
References [l] R.D. Hight, R.M. Schectman, and T. Gay, Phys. Rev. Al6
H.G. Berry, G. Gabrielse (1977) 1805.
[2] T.J. Gay and H.G.
Berry, Phys. Rev. Al9 (1979) 952. [3] H. Winter, Nucl. Instr. and Meth. 194 (1982) 357. [4] H.P. Garnir, P.D. Dumont and Y. Baudinet-Robinet, Nucl. Instr. and Meth. 220 (1982) 187. [S] P. Sigmund and S. Tougaard, in: Inelastic ParticlesSurface Collisions, eds., E. Taglauer and W. Heiland (Springer Verlag, Berlin, 1981) p. 2. [6] A. Dubus, J. Devooght and J.C. Dehaes, these Proceedings (ICACS ‘85) Nucl. Instr. and Meth. B13 (1986) 623. [7] E.J. Sternglass, Phys. Rev. 108 (1957) 1. [8] C.J. Tung and C. Lin, Radiat. Eff. 80 (1984) 261. [Y] S. Matteson, E.K.L. Chau and D. Powers, Phys. Rev. Al4 (1976) 169. [lo] U. Sander and H.H. Bukow, Radiat. Eff. 40 (1979) 143. [ll] H.H. Bukow, B. Gross-Kreul and U. Sander, Nucl. Instr. and Meth. 182/183 (1981) 383. [12] B. Dynefors, I. Martinson and E. Veje, Phys. Scripta 13 (1976) 308.