Wear Measurements By Means Of Radioactive Ion
Implantation.
L. Gialanella, G. Imbriani, V. Roca, M. Romano
Dipartimento di Fisica, Università di Napoli “Federico II” and INFN Naples, Naples,Italy
N. De Cesare, A. D’Onofrio, F. Terrasi
Facoltà di Scienze Ambientali, Seconda Università di Napoli and INFN Naples, Naples,Italy
H. W. Becker, D. Rogalla, A. Stephan, F. Strieder
Institut fuer Experimentalphysik III, Ruhr Universitaet Bochum, Bochum, Germany
Z. Fulop, G. Gyurky, E. Somorjai
Institute of Nuclear Research (ATOMKI), Debrecen, Hungary
M. Russo
Dipartimento di Ingegneria dei Materiali per l’Energetica, Università di Napoli “Federico II”, Naples,Italy
D. Daliento, N. Sanseverino
Dipartimento di Ingegneria Elettronica e delle Telecomunicazioni, Università di Napoli “Federico II”, Naples,Italy
Abstract. Radioactive ion implantation allows non-contacting, online wear measurements with sub micrometric
sensitivity to be performed, by monitoring the removed and/or residual activity on parts subject to wear. Comparative
studies of different materials, including those who exhibit a low resistance to radiation damage, can be easily performed
by means of this technique. A project has started at the TTT3 tandem of the University “Federico II” in Naples, Italy, in
order to exploit such technique using a 7Be ion beam, that was developed to study the astrophysical important reaction
7
Be(p,γ)8B. The 7Be ion beam is produced using a ion sputter source and accelerated to an energy up to 8.0 MeV. A
novel setup allows control of the implanted ion depth distribution, and is a crucial point for the application of this
technique. The 7Be ion beam production and implantation procedures are discussed. Finally, the results of some test
measurements are presented.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
469
the radioactive ion beam production easier in respect
to the case of short lived nuclides, that necessarily
need an on line production and acceleration. A good
case is 7Be (T1/2=53 d), that decays by electron capture
to 7Li, with a probability of about 10% by emitting a γray (Eγ=478 keV), corresponding to the population of
the first excited state in 7Li.
At the 3MV tandem accelerator of the University of
Naples Federico II a 7Be beam was developed to
investigate the astrophysical important reaction
7
Be(p,γ)8B. Here we report about the results of a
preliminary study about its possible use for wear
measurements and, in particular, about a novel ion
beam energy degrader, which allows a continuous
variation of the ion beam energy during the
implantation.
INTRODUCTION
Radioactive tracer techniques are nowadays routinely
used for non contacting, on line wear and corrosion
measurements and represent an extremely powerful
tool in material science and engineering. Indeed, they
provide accurate and real-time data, monitoring wear
or corrosion without disassembly and physical
inspection. The basic principle is to incorporate radioisotopes (preferably γ-ray emitters) in the sample to be
tested and to monitor the loss of material due to wear
or corrosion by measuring the resulting decrease of the
residual activity or, in a complementary way, the
increase of the radioactivity accumulated in the
lubricant fluid. A basic requirement is an accurate
knowledge of the density distribution of the radioisotopes in the direction in which the wear proceeds, in
order to infer the wear parameters from the observed
variations of radioactivity. Presently, two radioactive
tracer techniques are typically used: bulk activation
(BA) and surface-layer activation (SLA).
In both cases, radio-isotopes are produced directly in
the sample by irradiating it with neutrons, for BA, or
light charged particles (protons, deuterons and alphas),
for SLA. A common problem of these techniques is
that their application depends on the material to be
tested and it is limited to those materials that exhibit a
high activation cross section and, in particular in the
case of SLA, strong resistance to radiation damage.
Moreover, the radio-isotope depth distribution is
governed by the energy dependence of the activation
cross section, that strongly limits the possibility of
optimising it in view of specific requirements of the
test.
Radioactive ion implantation (RII) was suggested as a
possible solution to such problems [1,2, and references
therein]. In this approach a radioactive ion beam (RIB)
is used to implant radio-isotopes in the surface layer of
the sample. The implantation depth is determined by
the beam energy and the stopping power of the
radioactive ion in the sample: therefore a proper
modulation of the ion beam energy during the
implantation in a given material allows a quite wide
range of possible radio-isotope depth distributions.
The drastic reduction of the radiation damage (up to a
factor 106) in comparison with SLA allows application
of RII in principle to any material, therefore providing
a powerful tool for comparative studies. The main
problem of this technique is the availability of a
suitable RIB. As in the case of activation techniques,
half lives of the same order of magnitude as the
duration of the tests, typically some weeks, are
preferable, in order to minimize the amount of activity
needed to achieve the required sensitivity. This makes
7
Be ION BEAM PRODUCTION
Details about the 7Be ion beam production have been
reported elsewhere [3]. Shortly, 7Be is produced via
the 7Li(p,n)7Be reaction using a 11 MeV proton beam
on a 7Li metallic target. Then 7Be is chemically
extracted from the 7Li matrix and put in form of 7BeO
in a cathode of a sputter ion source, whose average
activity is 10 GBq. The ion source delivers a 23 keV
7
BeO- beam, which is injected into the tandem
accelerator, together with an accompanying 7LiObeam, originating from the residual Li in the cathode.
The interaction with the carbon stripper foil in the
terminal, breaks the 7Be and 7LiO molecules and the
resulting 7Be and 7Li ions emerge from the accelerator
in all their possible charge states, with the
corresponding energies. The beam of interest, 7Be3+
E=8.0 MeV, is selected by means of a 90° analysing
magnet. Actually, in order to filter out the 8.0 MeV
7 3+
Li contamination, a post stripper carbon foil is used
to form the 7Be4+ at 8 MeV before the analysing
magnet. Using cathodes of about 10 GBq, typical
beam intensities are of several ppA over some tens of
hours. This corresponds to a ratio between the total
number of 7Be ions obtained from the integrated
current after the analysing magnet and the number of
ions available in the cathode of the order of 6 · 10-5,
i.e. about 0.6 MBq of 7Be are available for
implantation. It should be pointed out that for those
applications in which a Li contamination of about 50%
and somewhat lower beam energy is acceptable, e.g. 5
MeV, the amount of 7Be could be increased by a
factor of 5.
470
7
equation 1 it is assumed that the wear proceeds along
the z direction and dz/dt represents the wear speed,
while dN/dz is the radio-nuclide depth distribution,
that, in the case of ion implantation, is given by the
following formula:
BE ION IMPLANTATION
As already mentioned, a crucial point in radioactive
tracer techniques is an accurate knowledge of the
distribution of the radio-isotopes. Indeed, if A is the
observed activity on the sample at the time t, corrected
for radioactive decay, then its variation due to loss of
material is given by:
dA/dt=ε⋅dN/dz⋅dz/dt
dN/dz = ∫dN/dE⋅f(z ;E) dE
where E is the ion beam energy, f(z;E) is the
implantation depth distribution for a monoenergetic
ion beam with energy E, and dN/dE is the radioactive
ion beam energy distribution.
(1)
where ε includes the detection efficiency and the
eventual branching to non detectable transitions: in
50 mm
40 mm
gas
outlet
mylar foil
=10 mm
φ
7
gold coated
mylar foil φ =8 mm
Si 2
Be ion
beam
target
Si 1
pressure
transducer
pressure read pressure
control
unit
set valve
(2)
gas
inlet
solenoid
valve
MCA1
MCA2
SCA1
SCA2
LOG FILE
(P,t,Nel1,Nel2) set
(P,t,Nel1,Nel2) read
Ar gas
bottle
set pressure
computer
read pressure
implantation
profile
parameters
a0,..,a8
energy loss
data
b0,..,b4
FIGURE 1. Schematic representation of the implantation setup. See text for details.
471
Therefore a proper modulation of the beam energy
allows to tailor the implantation profile to match
specific requirements of the measurement, within the
limits set by the range fluctuations for a given incident
beam energy, i.e. f(z;E).
This can be accomplished setting the highest necessary
beam energy and degrading it by means of a gas
absorber. The main advantage of this technique is a
drastic reduction of the time required for tuning the
beam and the possibility of a continuous variation of
the beam energy. On the other hand, the ion beam
quality is worsened by the energy and angular
straggling in the absorber. In this approach, one gets
the following expression for dN/dE:
dN/dE=∫dN/dP⋅g(P ;E)dP
system by means of an interface (National PC1024).
The pressure ramp is determined in order to obtained
the desired implantation depth profile dN/dz, which
has to be provided in form of polynomial, taking into
account the energy loss data and range data, which are
again provided in form of polynomials. The beam
current, and therefore the amount of ions implanted,
and energy are monitored on-line as a function of the
gas pressure by means of two silicon detectors, that
detect the 7Be ions elastically scattered at 45° by a thin
Au layer deposited on the outer surface of the exit
window of the gas cell. The signals from the detectors
are split and sent to two multichannel analysers, to
observe the energy spectra of the detected particles,
and two single channel analysers, which generate a
logic signal and send it to the control computer. The
number of detected ions at a given pressure, corrected
for the energy dependence of the elastic scattering
cross section, is proportional to the number of ions
impinging on the target at the corresponding energy,
i.e. to the number of ions implanted at the depth
corresponding to their range. Figure 2 shows the
results of a test implantation performed on a stack of
Ni foils (thickness 0.25 µm), where we intended to
obtain an implantation profile linearly decreasing with
the depth. The number of ion implanted on each foil
was determined detecting the γ-rays originating from
their decay using a high purity Germanium detector.
The overall agreement is good, except for the first
foils. This is due to the fact that the maximum pressure
in the gas absorber reached during the test was limited
to 250 mbar, in order to keep the energy of the
scattered 7Be ions at 45° above the threshold of the
silicon detectors used for monitoring the beam current.
(3)
where g(P;E) represents the energy distribution of the
ion beam energy with incident energy E0 after passing
through the gas absorber at pressure P. The weight
function dN/dP is determined by the modulation of the
gas pressure in the absorber during implantation. It is
clear that a proper variation of the pressure in the gas
absorber produces the desired implantation profile. On
the basis of these considerations, a novel implantation
set-up has been realized, which is shown schematically
in figure 1. Shortly, the beam passes through a gas
absorber and hits the target: the absorber consists of a
8
N (4x109 ion/µm)
7
experimental
6
5
expected
4
3
CONCLUSIONS
2
1
Direct implantation of 7Be ions provides a tool for sub
micrometric,
non-contacting,
on-line
wear
measurements that can be used also for those
materials, e.g. polymers, for which the presently
available radioactive tracer techniques cannot be
applied. The control of the implantation depth profile
is a crucial point for this technique. An implantation
set-up has been realized and tested, that uses a gas
absorber in order to obtain a wide variety of
implantation depth profiles by a proper variation of the
gas pressure in the absorber. The first results of a test
measurement show a fair agreement between the
measured implantation depth profile and the expected
one. Further work is in progress to improve this
technique and extend its use to other fields of ion
implantation, where also implantation profile tailoring
0
0
1
2
3
4
depth (µm)
FIGURE 2. Result of a test implantation of 7Be ions
on a stack of Ni foils. See text for details.
gas cell: the total energy loss in the absorber is given
by the energy loss in the entrance and exit windows of
the cell (mylar, thickness 2 µm) plus the energy loss in
the gas (Ar, l=50 mm, p=0-500 mbar). The pressure in
the gas cell is regulated by a system composed of a
capacitance manometer (MKS, mod 626A), a control
unit (MKS250E) and an electromagnetic valve (MKS,
mod 248). The pressure set point is provided by a
computer, which is connected with the pressure control
472
is needed. In particular, a program has started to
exploit the advantages of implantation profile tailoring
in lifetime engineering processes, needed in power
electronics devices, able to modify in a selective way
the value of the recombination lifetime in
semiconductor materials.
ACKNOWLEDGEMENTS
The authors thank L. Campajola of the Università di
Napoli Federico II, Naples, for his help during the
beam time at the tandem accelerator in Naples and
M.Roth, and the staff at the Isotop-Labor of the RuhrUniversitaet Bochum for their help during the
preparation of the 7Be cathodes. We also thank L.Ando
and the staff at the cyclotron of Atomki, Debrecen, for
the production of 7Be.
REFERENCES
1. McHarris,Wm.C. et al., Nucl. Instr. and Meth.A 353,
583-587 (1994).
2. Fehsenfeld, P. et al., Nucl. Phys.A 701, 235c-239c
(2002).
3. Gialanella, L. et al., NIMB 65, in press
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