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This content was downloaded from IP address 194.27.125.37 on 20/11/2023 at 05:25
Published by IOP Publishing for Sissa Medialab
Received: February 23, 2018
Accepted: March 20, 2018
Published: April 4, 2018
Experimental ion mobility measurements in Xe-CF4
mixtures
A.F.V. Cortez,a,b,1 M.A. Kaja,c J. Escada,a,b M.A.G. Santos,a,b R. Veenhof,d,e, f
P.N.B. Neves,g F.P. Santos,a,b F.I.G.M. Borgesa,b and C.A.N. Condea,b
aLaboratory of Instrumentation and Experimental Particle Physics — LIP,
Rua Larga, 3004-516 Coimbra, Portugal
bDepartment of Physics, Faculty of Science and Technology, University of Coimbra,
Rua Larga, 3004-516 Coimbra, Portugal
cFaculty of Physics, Warsaw University of Technology,
Koszykowa 75, PL 00-662 Warsaw, Poland
dNational Research Nuclear University MEPhI,
Kashirskoe Highway 31, Moscow, Russia
eCERN PH Department,
Geneve 23, CH-1211 Switzerland
fDepartment of Physics, Uludağ University,
16059 Bursa, Turkey
gCloser Consultoria, LDA,
Av. Engenheiro Duarte Pacheco, Torre 2, 14o-C, Lisboa, 1070-102 Portugal
E-mail: andre.f.cortez@gmail.com
Abstract: In this paper we present the results of the ion mobility measurements made in gaseous
mixtures of xenon with carbon tetrafluoride (Xe-CF4) for pressures ranging from 6 to 10 Torr
(8-10.6 mbar) and for low reduced electric fields in the 10 to 25 Td range (2.4-6.1 kV·cm−1·bar−1),
at room temperature. The time-of-arrival spectra revealed one or two peaks depending on the gas
relative abundances, which were attributed to CF+ +3 and to Xe2 ions. However, for Xe concentrations
above 60%, only one peak remains (Xe+2 ). The reduced mobilities obtained from the peak centroid
of the time-of-arrival spectra are presented for Xe concentrations in the 5%-95% range.
Keywords: Charge transport and multiplication in gas; Gaseous detectors; Ion sources (positive
ions, negative ions, electron cyclotron resonance (ECR), electron beam (EBIS)); Ionization and
excitation processes
1Corresponding author.
©c 2018 CERN. Published by IOP Publishing Ltd on behalf of Sissa
Medialab. Original content from this work may be used under the terms
of the Creative Commons Attribution 3.0 licence. Any further distribution of this work https://doi.org/10.1088/1748-0221/13/04/P04006
must maintain attribution to the author(s) and the title of the work, journal citation
and DOI.
2018 JINST 13 P04006
Contents
1 Introduction 1
1.1 Ion mobility 2
1.2 Langevin’s theory 2
1.3 Blanc’s law 2
2 Method and experimental setup 3
3 Results and discussion 3
3.1 Xenon (Xe) 4
3.2 Carbon tetrafluoride (CF4) 4
3.3 Xenon-carbon tetrafluoride (Xe-CF4) mixture 5
4 Conclusion 8
1 Introduction
Measuring the mobility of ions in gases is relevant in several areas, from physics to chemistry, e.g. in
gaseous radiation detectors modelling and in the understanding of the pulse shape formation [1–3].
One of these examples is the upcoming LCTPC (Linear Collider TPC) for the International Linear
Collider (ILC), where the estimate of the ion disk composition and evolution during the drift is
essential to see where to stop the ions during the 1st millisecond of their drift (i.e. during a bunch
train crossing) for the design of the gating device and dimensioning of the gap between the gating
device and the readout the information on the ion mobility is essential.
Xenon (Xe) is commonly adopted as the main filling gas for several detectors, while the choice
of the quencher is determined by different parameters [3]. Nowadays carbon tetrafluoride, CF4, has
been thought for the LCTPC mixture.
So, in the present work we studied the mobility of ions in xenon-carbon tetrafluoride (Xe-CF4)
gas mixtures in the 6 to 8 Torr (8 to 10.6 mbar) range and for reduced electric fields commonly used
in gaseous detectors, extending previous studies developed in our group for other gases [4–18].
The experimental setup used in the present work (described in detail in [4]) allows the measure-
ment of ion mobility in gas mixtures. Initially thought for high pressure, it was converted into a low
pressure gas system, since lowering the operation pressure provided a wider scope of application
and more detailed information on the fundamental processes involved in the ion transport and also
allowed to reduce the inherent operation cost. Still, the results have been consistently in accordance
with data obtained at high pressure [12].
– 1 –
2018 JINST 13 P04006
1.1 Ion mobility
As a result of an interaction process in a gas mixture, a group of ions is originated and during their
movement under the influence of low uniform electric field, these ions will collide with neutral
gas molecules, losing energy in collisions while gaining energy from the electric field, eventually
reaching a steady state. The resulting average velocity of the group of ions, vd, also known as drift
velocity can be given by:
vd = KE (1.1)
where K is the mobility of the ions and E is the intensity of the drift electric field. For low E/N , vd
is proportional to the electric field, i.e., when the energy gained from the field between collisions
is below the thermal energy [19, 20]. K is usually expressed in terms of reduced mobility K0,
suppressing the dependence of the mobility values on the gas pressure. Thus
K0 = KN/N0 (1.2)
where N is the gas number density and N0 is the Loschmidt number (N0 = 2.68678 × 1019 cm−3
for 273.15 K and 101.325 kPa according to NIST [21]). The mobility measurements are usually
presented as a function of the reduced electric field E/N in units of Td (1 Td = 10−21 V·m2) and
the reduced mobility obtained from equation (1.2) is expressed in terms of cm2·V−1·s−1.
1.2 Langevin’s theory
According to the Langevin’s theory [22], one limiting value of the ions’ mobility is reached when
the electrostatic hard-core repulsion becomes negligible compared to the neutral polarization effect.
This limit is given by the following equation, ( ) 1
1 2
Kpol = 13.88 (1.3)
αµ
where α is the neutral polarisability in cubic angstroms (α = 4.044±0.01 Å3 for Xe [23] and
α = 3.86±0.01 Å3 for CF4 [24]) and µ is the ion-neutral reduced mass in unified atomic mass units.
Although the Langevin limit is meant to be applied for real ion-neutral systems in the double
limit of low E/N and low temperature, it predicts the low-field mobility at room temperature with
relatively good accuracy [20]. Although generally accepted, Langevin theory has some known
limitations in its application, namely with ions that may interact by resonant charge transfer, in
which case it fails to provide correct values for the ion’s mobility [12].
1.3 Blanc’s law
Blanc’s empirical law, which resulted from Blanc’s work in the mobility of ions in binary gaseous
mixtures, has proven to be most useful when determining ions’ mobility using mixtures of gases.
Blanc found that the mobility of ions in gaseous mixtures, obeyed a simple relationship which can
be expressed as follows:
1 f1 f2
= + (1.4)
Kmix Kg1 Kg2
where Kmix is the reduced mobility of the ion in the binary mixture; Kg1 and Kg2 the reduced
mobility of that same ion in an atmosphere of 100% of gas #1 and #2 respectively; f1 and f2 are the
molar fraction of each gas in the binary mixture [25].
– 2 –
2018 JINST 13 P04006
2 Method and experimental setup
The mobility measurements presented in this study were obtained using the experimental system
described in [4]. A UV flash lamp with a frequency of 10 Hz emits photons that impinge on
a 250 nm thick CsI film deposited on the top of a Gas Electron Multiplier (GEM) placed inside
a gas vessel. The photoelectrons released from the CsI film are guided through the GEM holes
by the electric field created by applying an adequate voltage across its electrodes. After gaining
enough energy, the electrons will ionize the gas molecules encountered along their paths. While
the electrons are collected at the bottom of the GEM electrode, the cations formed will drift across
a uniform electric field region towards a double grid; the first one acts as Frisch grid while the
second one, at ground voltage, collects the ions’ charge. This charge collected at the collecting
grid is converted from current to voltage by a pre-amplifier originating a time-of-arrival spectrum
that is recorded in a digital oscilloscope (Tektronix TDS 2022B), set to continuously average 128
pulses, and fed to a PC for further processing. After subtracting the background spectra, obtained
without the voltage applied to the GEM (i.e. without drifting ions), to these time-of-arrival spectra,
Gaussian curves are fitted to the peaks in the spectra using Matlab. The trigger in the system is set
by the UV flash lamp, providing the initial time information.
Since the peaks’ centroid corresponds to the average drift time of the ions along a known
distance (4.273 cm), the drift velocity is determined, and the mobility can then be calculated using
expression (1.1). The system relies on the voltage across the GEM (VGEM) to control the maximum
energy of the electrons, which helps in the primary ion identification. Identifying the primary ions
will allow to pinpoint secondary reaction paths that lead to the identification of the detected ions.
Since impurities play an important role in the ions’ mobility, before each experiment the vessel
was vacuum pumped down to pressures of 10−6 to 10−7 Torr and a strict gas filling procedure was
carried out. No measurement was considered until the signal stabilised, and all measurements were
done in a 2-3 minutes time interval to ensure minimal contamination of the gas mixture, mainly due
to outgassing processes.
Despite the fact that no direct identification of the ions is possible with the present system,
using an elaborate procedure based on the method described together with the knowledge of
the dissociation channels, product distribution and rate constants it is possible to solve the ion
identification problem.
3 Results and discussion
The mobility of the ions originated in Xe-CF4 mixtures has been measured for different reduced
electric fields E/N (from 10 to 25 Td), for a total pressure of 8 Torr and at room temperature (293K).
The range of the reduced electric field values used to determine the ions’ mobility is limited
by two factors: the occurrence of electric discharges at high E/N and the deterioration of the
time-of-arrival spectra for very low E/N values (below 5 Td or 1.2 kV·cm−1·bar−1), which has been
attributed to collisions between the ions and impurity molecules.
The range of E/N values considered in this work is within the conditions of low reduced field,
i.e. E/N < 30 Td for the working pressures used.
– 3 –
2018 JINST 13 P04006
3.1 Xenon (Xe)
For pure Xe, with the experimental setup and conditions used of reduced electric fields E/N (10–25
Td) and for a pressure of 8 Torr, at room temperature (293 K) and for a constant VGEM of 25 V, two
types of ions are produced: Xe+ and Xe+2 . The atomic ion Xe
+ is produced directly by electron
impact. The homonuclear diatomic ion Xe+2 is produced either by Hornbeck-Molnar process (a
two-body association involving an excited atom and another in the ground-state), the dominant
formation process below about 1 Torr,
Xe + Xe**→− Xe+2 + e− (3.1)
or by conversion of the primary ions Xe+ through a three body process, which is favoured at high
pressure,
Xe+ + 2Xe→− Xe+2 + Xe. (3.2)
The corresponding K0 values, compatible with those obtained in [4].
3.2 Carbon tetrafluoride (CF4)
In pure carbon tetrafluoride (CF4), only one peak was observed for different reduced electric fields
E/N (10–25 Td) and for a pressure of 8 Torr, at room temperature (293 K) and for a constant VGEM
of 25 V.
In table 1 the possible reactions resulting from the electron impact in CF4 for electron energies
up to 25 eV, together with their (respective) cross-sections, appearance energies and the product
distribution are summarized. The probabilities indicated for the product distributionwere calculated
using the partial and total electron impact cross sections for CF4 provided in [26], which allowed to
infer the product distribution of the primary ionization. To the best of our knowledge these primary
ions (CF+3 , CF
+, CF+2 , F
+ and C+) are long-lived at our working pressures, which means that once
formed they will remain unaltered during their drift time.
Table 1. Ionization products, ionization cross sections for electron impact 25 eV on CF4 [26], appearance
energies (A.E.) [27], and respective product distribution.
Reaction Cross Sec. (10−16cm2) A. E. (eV) Prod. Dist.
e− + CF4→− CF+ −3 + F + 2e 1.011 15.69 96.1 %
e− + CF4→− CF+2 + 2F + e− 0.038 21.47 3.6 %
e− + CF4→− CF+ + F + e− 0.0017 29.14 0.16 %
e− + CF4→− C+ + 2F2 + e− 0.0007 34.77 0.007 %
e− + CF →− F+ + CF + e−4 3 0.0007 35 0.007 %
As seen in table 1, the most probable ion is CF+. In addition CF+ + + +3 2 , CF , C and F are also
produced but with a much lower probability, up to 3.9 % of the total number of primary ions.
Considering the relative abundance expected and our experimental system’s limitations (e.g. signal-
to-noise ratio), it is highly probable that it will only be possible to observe the peak corresponding
– 4 –
2018 JINST 13 P04006
to CF+3 ion, eventually with the residual (up to 3.6%) contribution of CF
+
2 , which would be expected
to have a slightly higher mobility due to its smaller mass.
Comparing the experimental mobility values with the polarization limit from Langevin’s for-
mula (equation (1.3)) it was possible to verify that the measured mobility tends to 1.12 cm2 V−1
s−1, a value in good agreement with the calculated value from equation (1.3) for CF+3 , 1.13 cm
2
V−1 s−1, although it deviates from those obtained by other authors [28, 29]. The difference can
be explained by the possibility of the cluster formation [30], CF+3 (CF4), that would be favoured
at higher pressures (close to atmospheric) and whose expected value is close to those observed
experimentally by these authors.
3.3 Xenon-carbon tetrafluoride (Xe-CF4) mixture
In xenon-carbon tetrafluoride (Xe-CF4) mixtures, one or two peaks can be observed depending on
the mixture composition, from pure Xe to pure CF4, as can be seen in figure 1, where the drift
spectra for several Xe-CF4 mixtures (20%, 50%, 80% and 90% of Xe) are displayed, at 8 Torr, 293
K and 15 Td with a VGEM of 25 V.
10 10
20% Xe 50% Xe
8 8
Xe + +
2 Xe2
6 6
CF +
3
4 4
CF +
3
2 2
0 0
0.25 0.50 0.75 1.00 1.25 1.50 1.75 0.25 0.50 0.75 1.00 1.25 1.50 1.75
10 10
80% Xe 90% Xe
8 8
Xe +
2 Xe +
6 6 2
4 4
2 2
0 0
0.75 1.00 1.25 1.50 1.75 2.00 2.25 0.75 1.00 1.25 1.50 1.75 2.00 2.25
Drift time (ms) Drift time (ms)
Figure 1. Time-of-arrival spectra of an average of 128 pulses recorded for several Xe-CF4 mixtures (20%,
50%, 80% and 90% of Xe) at a total pressure of 8 Torr, a reduced electric field of 15 Td, a VGEM of 25 V and
at room temperature (293 K).
In figure 1 there are two stricking features both of which are a result of the increasing Xe
concentration in the mixture: the decrease in mobility of the different ions observed — more
pronounced in the lower mobility peak — and the change in the relative abundace of ion species
present (even changing the dominant ion species), which can be perceived by a decrease in the
area of the peak corresponding to the higher mobility, and an increase in the area of the other
– 5 –
Signal Amplitude (mV) Signal Amplitude (mV)
2018 JINST 13 P04006
peak. Considering the shift of the peaks towards higher drift times (decreasing ion mobility) with
increasing Xe concentration, it can be explained by the higher Xe mass when compared to that of the
CF4 molecule, which implies a higher reduced mass in ion-neutral collisions (µ in equation (1.3)),
thus a lowermobility. Regarding the changes observed in the different peaks’ area, it suggests that the
faster group of ions comes from CF4, while the second, and slower group, is originated by Xe atoms.
The evolution of the proportion of the peaks observed (from CF4 and Xe ions) with the mixture
composition was compared with the relative abundance of primary ions after traversing the GEM
holes, using aMonte Carlo simulation code described in detail in [31]. The simulation code uses the
cross-sections for the scattering of electrons by Xe atoms and CF4 molecules described in [32]. As
in [31], a uniform field between anode and cathode inside the GEM holes was considered. Although
this is not the real field geometry, it can provide sufficient information for the present purpose. The
simulation reproduces the drift of 108 photoelectrons released from a CsI photocathode into Xe-CF4
mixtures (at a typical pressure of 8 Torr and temperature of 293 K) until they reach the anode at a
distance of 50 mm from the photocathode and calculates the proportion of primary ions (CF+3 or
Xe+) after this drift for the different gas mixtures. The results are displayed in figure 2, where it can
be seen that, even at low Xe concentrations (down to about 15% of Xe), Xe ions are still the ones
preferentially produced.
1
CF + Xe
+
3
0.8
0.6
0.4
0.2
0
0 20 40 60 80 100
Xe %
Figure 2. Monte Carlo calculated relative abundance of the fraction of ions produced at the GEM holes as a
function of the Xe percentage in the mixture.
Comparing the results from figure 2 for the relative abundance of the ions in Xe-CF4 calculated
through the Monte Carlo simulation with the peak area, obtained by adjusting a Gaussian fit to each
peak in the experimental time spectra (figure 1), it is possible to see a good agreement between the
two, indicating that once the primary ions are formed the relative abundance of CF4 and Xe ions
will remain approximately constant.
– 6 –
Relative Abundance
2018 JINST 13 P04006
Blanc’s law was also used as a cross checking method to verify the ion identification.
Figure 3 shows the reduced mobility of the ions produced in different Xe-CF4 mixtures at
8 Torr and for E/N of 15 Td at room temperature, together with Blanc’s law prediction for
the main candidate ions — CF+3 (blue), Xe
+ (green) and Xe+2 (red). Kg1 and Kg2 in Blanc’s
law (equation (1.4)), were obtained either using experimental values from literature or, when not
existing, by using the Langevin limit formula (equation (1.3)).
1.2
CF +
3
1.1
1.0
0.9
0.8
0.7 Xe +
2
0.6
Xe+
0.5
0.4
0 20 40 60 80 100
Xe %
Figure 3. Reduced mobility of the ions produced in the Xe-CF4 mixture for a pressure of 8 Torr and for a
E/N of 15 Td at room temperature. The dotted lines represent the mobility values expected from Blanc’s
law for CF+3 (blue), Xe
+ (green) and Xe+2 (red).
Figure 3 shows that while the peaks corresponding to higher drift times follow closely Blanc’s
law curve for Xe+2 , those corresponding to lower drift times are also well decribed (3% maximum
deviation) by the same law for CF+3 . The slight deviation observed may be caused by the collision-
induced dissociation reaction between CF+3 and Xe [33],
CF+3 + Xe→− CF+2 + Xe + F (3.3)
which is energetically favourable and would lead to an increasing formation of CF+2 with increasing
concentration of Xe in the mixture and for which there is no reaction rate available.
In table 2 the results obtained for the ion mobilities of CF+3 and Xe
+
2 in Xe-CF4 mixtures are
summarized.
No significant variation of the mobility was observed in the range of E/N values (10-25 Td)
studied.
– 7 –
K (cm2.V-1.s-10 ) 
2018 JINST 13 P04006
Table 2. Mobility of the peaks observed for the Xe-CF4 mixture ratios studied, obtained for E/N of 15 Td,
a pressure of 8 Torr at room temperature (293 K).
Mixture Mobility (cm2·V−1·s−1) Ion
5% Xe 1.106 +− 0.009 CF+3
0.843 + +− 0.008 Xe2
10% Xe 1.113 + 0.012 CF+− 3
0.827 +− 0.011 Xe+2
15% Xe 1.122 +− 0.009 CF+3
0.819 +− 0.008 Xe+2
20% Xe 1.125 +− 0.008 CF+3
0.809 +− 0.006 Xe+2
25% Xe 1.114 +− 0.009 CF+3
0.792 +− 0.007 Xe+2
30% Xe 1.100 +− 0.013 CF+3
0.780 +− 0.009 Xe+2
40% Xe 1.108 +− 0.001 CF+3
0.761 +− 0.008 Xe+2
50% Xe 1.107 + +− 0.009 CF3
0.742 +− 0.005 Xe+2
60% Xe 1.095 +− 0.021 CF+3
0.721 +− 0.006 Xe+2
70% Xe 0.700 +− 0.006 Xe+2
75% Xe 0.693 +− 0.005 Xe+2
80% Xe 0.685 +− 0.004 Xe+2
85% Xe 0.675 +− 0.006 Xe+2
90% Xe 0.668 +− 0.005 Xe+2
95% Xe 0.661 +− 0.004 Xe+2
4 Conclusion
In the present work we measured the reduced mobility of ions originated by electron impact in
Xe-CF4 mixtures under different pressures (from 6 to 10 Torr), low reduced electric fields (from 10
to 25 Td) and different mixture ratios, for electron impact ionization energies up to 25 eV.
Only one peak was consistently observed for Xe concentrations above approximately 60%,
thought to be originated by Xe+2 , while a second peak starts to appear for Xe concentrations below
this value and which was attributed to CF+3 , with the expected abundance of both ions being in
accordance with a Monte Carlo simulation carried out for the whole range studied.
– 8 –
2018 JINST 13 P04006
Also, our experimental results seem to be relatively consistent with the ones predicted by the
Blanc law throughout the mixture range studied (from 0 to 100% Xe) for the peak with lower
mobility, Xe+2 , while displaying a slight deviation for CF
+
3 .
Increasing CF4 concentration in the mixture resulted in a higher mobility of both ions observed.
The experimental mobility values did not display a significant dependence over the studied range
of pressure and E/N (6-10 Torr and 10-25 Td, respectively).
Future work is expected with other gaseous mixtures. It is our intention to proceed this line
of investigation using mixtures such as Ne-CF4, Ar-IsoButane, Ar-CF4-IsoButane (T2K mixture)
— the last one being considered for the LCTPC collaboration, of the International Linear Collider
(ILC) experiment.
Acknowledgments
Thisworkwas supported by theRD51Collaboration/CERN, through the common project “Measure-
ment and calculation of ion mobility of some gas mixtures of interest”. André F.V. Cortez received
a PhD scholarship from FCT-Fundação para a Ciência e Tecnologia (SFRH/BD/52333/2013)..
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