Rorward and backward electron emission induced by protons from copper foil
Forward and backward secondary electron emission induced by normal incidence of 1.5 MeV protons from 5 μm copper foil was experimentally studied. Measurements were carried out by means of two low-aperture retarding field energy analyzers, which were installed symmetrically on each side of the target...
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| Zitieren: | Rorward and backward electron emission induced by protons from copper foil / S.I. Kononenko, V.P. Zhurenko, O.V. Kalantaryan, N.O. Zheltopyatova // Вопросы атомной науки и техники. — 2013. — № 4. — С. 320-324. — Бібліогр.: 23 назв. — англ. |
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irk-123456789-1121512017-01-18T03:03:50Z Rorward and backward electron emission induced by protons from copper foil Kononenko, S.I. Zhurenko, V.P. Kalantaryan, O.V. Zheltopyatova, N.O. Приложения и технологии Forward and backward secondary electron emission induced by normal incidence of 1.5 MeV protons from 5 μm copper foil was experimentally studied. Measurements were carried out by means of two low-aperture retarding field energy analyzers, which were installed symmetrically on each side of the target at the angle of 53° with respect to the projectile beam. The relation of electron yields of forward and backward emission was obtained. Electron distribution functions were measured in 0…90 eV energy interval. The comparative analysis of electron distribution for forward and backward cases was performed and possible reasons for the differences observed were discussed. Експериментально досліджена вторинна емісія електронів з мідної фольги 5 мкм на простріл і на відбиття для нормального падіння пучка протонів з енергією 1,5 МеВ. Вимірювання були проведені за допомогою двох малоапертурних енергоаналізаторів з гальмуючим полем, які було розміщено симетрично по обидва боки мішені під кутами 53° до пучка частинок, що бомбардують. Отримано відношення коефіцієнтів вторинної іонно-електронної емісії на простріл і на відбиття. Виміряні функції розподілу електронів за енергіями в інтервалі 0…90 еВ. Проведено порівняльний аналіз розподілів електронів за енергіями на простріл і на відбиття та обговорюються можливі причини різниць, що спостерігаються. Экспериментально исследована вторичная эмиссия электронов из медной фольги 5 мкм на прострел и на отражение при нормальном падении пучка протонов с энергией 1,5 МэВ. Измерения были проведены при помощи двух малоапертурных энергоанализаторов с тормозящим полем, установленных симметрично с обеих сторон мишени под углами 53° по отношению к пучку бомбардирующих частиц. Получено отношение коэффициентов вторичной ионно-электронной эмиссии на прострел и на отражение. Измерены функции распределения электронов по энергиям в интервале 0…90 эВ. Проведен сравнительный анализ распределений электронов по энергиям на прострел и на отражение и обсуждаются возможные причины наблюдаемых различий. 2013 Article Rorward and backward electron emission induced by protons from copper foil / S.I. Kononenko, V.P. Zhurenko, O.V. Kalantaryan, N.O. Zheltopyatova // Вопросы атомной науки и техники. — 2013. — № 4. — С. 320-324. — Бібліогр.: 23 назв. — англ. 1562-6016 PACS: 79.20.Rf http://dspace.nbuv.gov.ua/handle/123456789/112151 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Приложения и технологии Приложения и технологии Kononenko, S.I. Zhurenko, V.P. Kalantaryan, O.V. Zheltopyatova, N.O. Rorward and backward electron emission induced by protons from copper foil Вопросы атомной науки и техники |
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Forward and backward secondary electron emission induced by normal incidence of 1.5 MeV protons from 5 μm copper foil was experimentally studied. Measurements were carried out by means of two low-aperture retarding field energy analyzers, which were installed symmetrically on each side of the target at the angle of 53° with respect to the projectile beam. The relation of electron yields of forward and backward emission was obtained. Electron distribution functions were measured in 0…90 eV energy interval. The comparative analysis of electron distribution for forward and backward cases was performed and possible reasons for the differences observed were discussed. |
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Kononenko, S.I. Zhurenko, V.P. Kalantaryan, O.V. Zheltopyatova, N.O. |
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Kononenko, S.I. Zhurenko, V.P. Kalantaryan, O.V. Zheltopyatova, N.O. |
| author_sort |
Kononenko, S.I. |
| title |
Rorward and backward electron emission induced by protons from copper foil |
| title_short |
Rorward and backward electron emission induced by protons from copper foil |
| title_full |
Rorward and backward electron emission induced by protons from copper foil |
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Rorward and backward electron emission induced by protons from copper foil |
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Rorward and backward electron emission induced by protons from copper foil |
| title_sort |
rorward and backward electron emission induced by protons from copper foil |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Приложения и технологии |
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Rorward and backward electron emission induced by protons from copper foil / S.I. Kononenko, V.P. Zhurenko, O.V. Kalantaryan, N.O. Zheltopyatova // Вопросы атомной науки и техники. — 2013. — № 4. — С. 320-324. — Бібліогр.: 23 назв. — англ. |
| series |
Вопросы атомной науки и техники |
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2025-07-08T03:28:10Z |
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| fulltext |
ISSN 1562-6016. ВАНТ. 2013. №4(86) 320
FORWARD AND BACKWARD ELECTRON EMISSION
INDUCED BY PROTONS FROM COPPER FOIL
S.I. Kononenko, V.P. Zhurenko, O.V. Kalantaryan, N.O. Zheltopyatova
V.N. Karazin Kharkоv National University, Kharkоv, Ukraine
Е-mail: kononenko@htuni.kharkov.ua
Forward and backward secondary electron emission induced by normal incidence of 1.5 MeV protons from 5 μm
copper foil was experimentally studied. Measurements were carried out by means of two low-aperture retarding
field energy analyzers, which were installed symmetrically on each side of the target at the angle of 53° with respect
to the projectile beam. The relation of electron yields of forward and backward emission was obtained. Electron
distribution functions were measured in 0…90 eV energy interval. The comparative analysis of electron distribution
for forward and backward cases was performed and possible reasons for the differences observed were discussed.
PACS: 79.20.Rf
INTRODUCTION
It is well known that fast ions transfer kinetic energy
mainly to electron subsystem of a solid. In this case
energy transfer can occur by means of both close and
distant collisions [1]. A part of projectile energy can
goes into the excitation of plasmons [2].
In close collision primary particle produce fast δ-
electrons. In further collisions these fast electrons pro-
duce slow electrons as a result of cascade process [3].
Moving ion can entrain some electrons of a substance,
so called «convoy» electrons [3].
Part of arisen nonequilibrium electrons can over-
come surface potencial barier and escape into vacuum.
This process named secondary ion induced electron
emission (SIEE) [1].
It is generally accepted that SIEE process is de-
scribed by three-step model:
1) production of nonequilibrium electrons;
2) transport of electrons (diffusion) to a surface of
a solid and collisions;
3) overcoming potential barrier existing on
a surface, and ejection into vacuum [1].
Fast electrons make a substantial energy contribu-
tion to SIEE. We have earlier studied forward-backward
emission and showed that flux of energy (electrons) in
the forward direction exceeds one in the backward di-
rection (anisotropy) [4]. However, we left the question
of energy distribution of SIEE electrons open.
It is proved for fast light ions theoretically and ex-
perimentally that SIEE coefficient (or electron yield) is
directly proportional to the mean specific ionization loss
dE/dx of ion in a matter [5, 6]. A high-energy ion propa-
gating through a matter produces a large amount of
nonequilibrium electrons, whose energy distribution can
be approximated by a power law [7]:
f(E)=A⋅E–s, (1)
where A is constant, s is power law index [7].
Both of the above mentioned mechanisms for energy
transfer from the primary particle to the electrons in the
matter (the collision and plasmon mechanisms) contrib-
ute to the electron energy distribution.
Study of the forward and backward energy spectra of
SIEE will make it possible to obtain new data on the
energy contribution of the fast electrons to the formation
of the nonequilibrium distribution function.
The question is how this energy is distributed be-
tween different electron groups.
The paper deals with experimental study of forward
and backward SIEE from copper foil bombarded by fast
proton beam.
1. EXPERIMENTAL SETUP
Scheme of SIEE study experiment is showed on
Fig. 1. 1.5 MeV proton beam from Van Graaf accelera-
tor impinged on foil target perpendicularly to the sur-
face. Ion beam was limited by diaphragm system, so
diameter of the beam was equal 3 mm. The target was
thin polycrystalline copper foil of high purity with 5 μm
thickness and 15 mm diameter. Target foil was fixed
and mounted on copper holder. Ion beam current regis-
tered by Faraday cup was equal IFC = 0.8 μА. Residual
gas pressure was lower than 10-4 Pa.
Because of target thickness was lower than path
length of 1.5 MeV protons in a copper, there were pos-
sibility to measure both backward and forward emis-
sion. Emission electrons from both surfaces of the target
(beam entrance surface − «backward», beam exit sur-
face − «forward») were registered by means of two
identical low aperture cylindrical energy analyzers
based on retarding field principal.
Fig. 1. Scheme of experiment
ISSN 1562-6016. ВАНТ. 2013. №4(86) 321
The analyzers were mounted symmetrically with re-
spect to target surface (53° angle with respect to the
beam direction) at the distance of 48 mm from emitting
surfaces. Dimensions of the analyzer were the follow-
ing: length − 65 mm, diameter − 19 mm, entrance aper-
ture − 3 mm. It was consisted of metallic case with three
high transparence (>90 %) grids and Faraday cup col-
lector. The first and last grids were grounded, while
middle grid was at retarding potential U (linear varying
voltage of 0…90 V range with 1 V step). Emission cur-
rents of analyzer collectors were amplified by electro-
metric amplifiers. In the sequel, amplified signals were
registered by controlling computer by means of analog-
digital converter. Measuring complex enabled to per-
form averaging of 100 findings (7 second exposure) for
each value of retarding voltage. Construction of energy
analyzers and measuring channels satisfied electromet-
ric circuit [8].
Relation of analyzer aperture to the distance from
target was much lower than 1. Therefore, emission elec-
trons were registered in narrow solid angle. Hence, such
analyzer gave information about velocity vector magni-
tude. Consequently, it is possible to obtain energy dis-
tribution function of emission electrons by means of
differentiation of retarding characteristics (dependence
of emission current on retarding voltage) IF/B(U). Proce-
dure of retarding characteristic treatment and obtaining
of energy distribution functions one can find in our ear-
lier papers (see, for example [9]).
It should be noted, in the case of power-law depend-
ence of energy distribution functions it is convenient to
use logarithmic scale. Then power-law dependence
should be represented by straight line with slope, de-
fined by power-law index s (see (1)).
Observation angle differential SIEE coefficient can
be found by mean of energy analyzer at 0 retarding
voltage mode:
γF/B = IF/B / IFC , (2)
2. RESULTS
2.1. MODELLING OF ION BEAM PASSAGE
THROUGH THE TARGET
Passing through a target, ion losses part of its en-
ergy. Proton energy losses are described with high accu-
racy by Yu. Gott’s empiric formula for projectile energy
range of 10 keV…50 MeV [10]. Moreover, formula has
simple compact form.
In our case ion energy at the exit of the foil target
was calculated as a solution of integral equation with
use of Yu. Gott’s empiric formula for ion energy loss in
a substance [10]. Numerical solution was obtained by
means of our program in MathCad software. Calculated
value of ion energy at the exit of the target was equal
approximately 1.04 MeV.
We performed modeling for protons moving in cop-
per target using SRIM2006 software [11]. There was
significant scattering of ions at the end of tracks
(Fig. 2).
It is accepted that dependence of SIEE coefficient on
incident angle ϕ of ion (ϕ is measured from normal to
the target plane) is defined as [12, 13]:
γ(ϕ) = γ(0) / cos(ϕ), (3)
where γ(0) is SIEE coefficient for normal incidence of
ion on target.
Secondary electrons are emitted into vacuum from
thin layer with thickness on the order of electron path
length. Considerable ion scattering results in increase of
average ion path in the emitting layer of the target.
Therefore, the number of emission electrons is enlarged
in forward direction. This ion scattering causes addi-
tional enlargement of ion energy losses in the emitting
layer.
Fig. 2. 1.5 MeV proton trajectories in copper target (SRIM2006 software)
ISSN 1562-6016. ВАНТ. 2013. №4(86) 322
2.2. EXPERIMENTAL RESULTS
The experiments showed that intensity of forward
emission induced by proton from copper foil was more
than one of backward emission at the same observation
angle.
SIEE coefficient for protons with energy under study
is well known to be proportional to the specific ioniza-
tion energy losses of ion in a substance [5]:
γ = Λ dE/dx , (4)
In our case, in order to appreciate the effect of emis-
sion direction (forward-backward) we defined k coeffi-
cient as a relation between specific ionization energy
losses of protons dE/dx at the beam entrance (dE/dx)B
and exit (dE/dx)F in the target. Value for k coefficient
calculated by SRIM2006 was approximately equal 0.8
[11].
Thus,
R = k·γF/γB ≈ 1,8. (5)
This R relation is usually called as Meckbach factor
[14].
10 100
0.1
1
lg
(f(
E)
)
lgE
forward
backward
Fig. 3. Double logarithmic scale
energy distribution function of electrons
It should be noted that obtained value of R relation
for copper is close to one observed for other species of
light ions with energy from some hundred keV to some
MeV and others targets, particularly, carbon (see, for
example [15]). Thus, for normal incidence of 250 keV
proton beam on thin carbon foil [12] it was obtained the
relation of forward and backward SIEE coefficients
(output angle integral coefficient) was close to 1.55. For
4 MeV Li+ ions and carbon target R was approximately
1.72 [14].
We presented earlier the experimental results on ani-
sotropy of energy losses of fast α-particles in some met-
als (forward and backward), which were obtained by
means of measurements of output angle integral SIEE
coefficient γF и γB [4]. It is was shown, that R ≈ 1.7,
therefore ionization losses associated with energy trans-
fer to the electrons that move in the same direction as
the primary ion approximately 1.7 times greater than
one in the opposite direction [4].
R value for copper under study showed that in the
case of light ions with energy of some MeV the relation
between γF and γB is slightly changed both for integral
and differential on output angles values.
Typical energy distribution function of SIEE is
curve with maximum at low energy of some eV and
subsequent power-law decrease [5, 16, 17]. Energy dis-
tribution functions had the same structure in our case.
Fig. 3 shows double logarithmic scale typical energy
distribution functions of electrons emitted from copper
for both forward and backward emission (start plot of
the distribution up to 10 eV (maximum) is not presented
on the graph).
One can see that the distributions are well approxi-
mated by linear functions with different slope angles
(solid and dotted line). It means that the distributions
have power-law dependences with s power index vary-
ing from –2 to –3. Junction point of the distributions
(intersections of approximating solid and dotted lines),
where power index changes its value, is located at
25…30 eV energy interval both for forward and back-
ward case. Peacewise power-law dependence of SIEE
energy distribution functions was also observed for
some other metals [5 - 7, 16, 17].
20 40 60 80 100
0
1
2
3
4
r,
a.
u.
E, eV
Fig. 4. Relation of forward-backward energy distribu-
tion functions of electrons
In order to reveal differences in energy distribution
functions for forward and backward emissions we found
relation r = fF(E)/fB(E) (Fig. 4). R value is more than 1
for energy intervals under study. This fact is well con-
formed by other forward-backward experiments. For
example, experiments with normal incidence of atomic
and molecular hydrogen beam with 0.8 MeV/a.m.u.
energy on thin Au foil showed that intensity of forward
spectrum (0° observation angle) was approximately
double greater than one for backward spectrum (180°
observation angle) [18]. The intensity of the 65° forward
spectra was much lower than the intensity of the 0°
spectra [18].
Let’s consider features of r(E) dependence. For elec-
trons with energies more than 10 eV we found slow
growth of r curve. To guide the eye we presented by
dotted curve the results of linear approximation of r
dependence in 10…90 eV energy interval on Fig. 4
(equation of straight line: y = 1.455+0.006⋅x). As it can
be seen, the part of fast electrons is increased with
growth of energy. Authors of paper [12] observed simi-
lar behavior r dependence, though they found stronger
increase of fast electron part in forward spectra in com-
parison with backward one.
ISSN 1562-6016. ВАНТ. 2013. №4(86) 323
Greater emission observed in forward direction
(beam exit side of target foil) in comparison with back-
ward one can be explained by asymmetric angular dis-
tribution of electrons produced by ion in a solid. In our
case the major source of electron production in a solid is
ionization energy losses of moving fast ion, therefore
asymmetry of electron distribution points out on anisot-
ropy of energy losses of moving ion. It is well known
(see, for example, [19]) that for forward emission case
there is group of fast convoy and δ-electrons, which
possess of significant part of energy transferred from
moving ion to slow electrons of a solid. Having quite
large energy these electrons can result in production of
new nonequilibrium electrons and cause appearance of
collision cascades. Directed motion of convoy and
δ-electrons apparently causes anisotropy of ion energy
losses in a matter. This anisotropy causes difference in
electron emission from thin foil in forward and back-
ward directions.
It should be noted that besides anisotropy effect the
differences in energy spectra can be explained by en-
ergy losses of ion in a target matter. As it was men-
tioned above, non-equilibrium power-law distribution
function (NPDF) can be formed in a fast ion track [20].
Moreover, so-called «universal» distribution function of
electrons can be realized under certain conditions. Thus
papers [21, 22] presented conditions under which distri-
bution function became universal with power-law index
s = –5/4. Formation of such NPDF depends first of all
on density of non-equilibrium electrons. Density of non-
equilibrium electrons is in direct proportion to dE/dx.
Density of non-equilibrium electrons in forward emis-
sion case is larger than in backward one. Therefore,
power index of NPDF should be approached to –5/4,
which results in increase of relative part of fast elec-
trons.
Similar results (increase of fast electron relative part
in energy spectra) were observed with atomic and mo-
lecular hydrogen ions [23]. It is well-known, that energy
losses of molecular hydrogen ions is greater than energy
losses of two protons, so-called «molecular effect» [23].
CONCLUSIONS
Experimental study of secondary electron emission
induced by fast protons from thin copper foil at small
solid angle showed differences in emission characteris-
tics in forward and backward directions. The differences
can be explained by anisotropy of ion energy losses in a
matter (directed motion of convoy and δ-electrons,
which have quite large energy). The results obtained are
in good agreement with earlier emission experiments
performed for fast light ions with integral measurements
on output angles of electrons.
An enlargement of part of fast electrons in forward
distribution function can be explained for the following
reasons:
• anisotropy of ion energy loss in a matter;
• greater specific energy loss of ion at the exit of a
target;
• considerable ion scattering at the exit of the foil tar-
get.
The authors want to express great thanks to
V. I. Karas’ for devotion much attention to the paper
and a number of valuable remarks.
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253.
Article received 14.05.2013
ЭЛЕКТРОННАЯ ЭМИССИЯ, ИНДУЦИРОВАННАЯ ПРОТОНАМИ
ИЗ МЕДНОЙ ФОЛЬГИ НА ПРОСТРЕЛ И НА ОТРАЖЕНИЕ
С. И. Кононенко, В.П. Журенко, О.В. Калантарьян, Н.А. Желтопятова
Экспериментально исследована вторичная эмиссия электронов из медной фольги 5 мкм на прострел и на
отражение при нормальном падении пучка протонов с энергией 1,5 МэВ. Измерения были проведены при
помощи двух малоапертурных энергоанализаторов с тормозящим полем, установленных симметрично с
обеих сторон мишени под углами 53° по отношению к пучку бомбардирующих частиц. Получено отноше-
ние коэффициентов вторичной ионно-электронной эмиссии на прострел и на отражение. Измерены функции
распределения электронов по энергиям в интервале 0…90 эВ. Проведен сравнительный анализ распределе-
ний электронов по энергиям на прострел и на отражение и обсуждаются возможные причины наблюдаемых
различий.
ЕЛЕКТРОННА ЕМІСІЯ, СПРИЧИНЕНА ПРОТОНАМИ
З МІДНОЇ ФОЛЬГИ НА ПРОСТРІЛ І НА ВІДБИТТЯ
С.І. Кононенко, В.П. Журенко, О.В. Калантар’ян, Н.О. Желтопятова
Експериментально досліджена вторинна емісія електронів з мідної фольги 5 мкм на простріл і на відбит-
тя для нормального падіння пучка протонів з енергією 1,5 МеВ. Вимірювання були проведені за допомогою
двох малоапертурних енергоаналізаторів з гальмуючим полем, які було розміщено симетрично по обидва
боки мішені під кутами 53° до пучка частинок, що бомбардують. Отримано відношення коефіцієнтів вто-
ринної іонно-електронної емісії на простріл і на відбиття. Виміряні функції розподілу електронів за енергія-
ми в інтервалі 0…90 еВ. Проведено порівняльний аналіз розподілів електронів за енергіями на простріл і на
відбиття та обговорюються можливі причини різниць, що спостерігаються.
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