Experimental study of the electric charge
of a radioactive nanometric aerosol
R. Abou-Khalil(1,2) - S. Della-Negra(3) - N. Michielsen(1)
A. Nourreddine(2) - E. Baussan(2)
(1)- IRSN/DSU/SERAC/LPMA
(2)- IPHC/DRS/RaMsEs – Université Louis Pasteur Strasbourg I
(3)- Institut de Physique Nucléaire d’Orsay (IPNO)
24th International Conference on Nuclear Tracks in Solids
Bologna – September 1-5, 2008
Outline

Research goals

Introduction

Experimental setup

Time-of-Flight measurements

Results and discussion

Conclusions and perspectives
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Research goals
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Research goals
Context
The behavior of radioactive aerosols (radon’s progeny) in a
medium depends on:
• Their size
• Their electric charge
Cohen et al. (1998) show the effect of the particles’ charge
on their deposition in the breathing apparatus. Their results
show a deposition from 5 to 6 times greater, for 20 and 125 nm
charged particles, than for neutral particles of the same size.
Industry
Deposition on surfaces and
sampling systems
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Health consequences
• Deposition in dwellings
• Deposition in the breathing apparatus
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Introduction
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Decay chain of Radon and its progeny
4.47.109 y
2.46.105 y
Uranium-238
Decay chain
1.17 m
7.54.104 y
24.1 d
1600 y
3.82 d
1.65.10-4 s
3.05 m
138 d
5.01 d
19.9 m
26.8 m
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2.22 y
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Deposition on surfaces
Behavior of the airborne
218Po and 214Pb in the air
aerosol
α
aerosol
β
Pbm
Pon
+
+
Pon
+
Cluster
formation
Pon
+
Pbm
α
+
neutralisation
neutralisation
Rn
Po0
Cluster
formation
Po0
β
β
α
Pb0
Po0
Pb0
β
α
aerosol
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Deposition on surfaces
aerosol
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Literature review
The electric charge of radon progeny has two forms: total charge fraction and charge distribution
Pierre & Marie Curie 1904, were
the first to measure the electric
charge of radon’s progeny
Wellisch 1913, was the first to
measure the total charge fraction
of 218Po using a Zeleny Tube:
Electrode
226
Ra
Tension
f chrg
Cc
Cn
Cc
88% positive
12% neutral
All results in the literature concerning radon’s progeny treated the total
charge fraction, but none of them studied its charge distribution state.
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Experimental setup
Radioactivité : origine des émissions
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ACCELERATION CHAMBER
Time-of-Flight
Target
Si Detector
MCP detector
Secondary ions
α
α recoil
P
A
C
F
D
High Voltages
Start
64 Stops
TDC
64 Channels
• PA: Preamplifier
• CFD: Constant Fraction Discriminator
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64
Anodes
• MCP: Micro Channel Plates
• TDC: Time Digital Converter
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Time-of-Flight measurement
 Time-of-Flight of 218Po before its detection by the Micro Channel Plates is:
T = Ta + TL
Ta: Time-of-Flight in the accelerator chamber 10 mm (10-8 s)
TL: Time-of-Flight between the accelerator chamber and the MCP 46 cm (>10-7 s)
 Velocity and kinetic energy of the recoiling ions 218Poq while applying a differential
potential U in the acceleration chamber:
Vq = γTa + Vr
E = qΔU + Er
: Acceleration of 218Po due to the differential potential
Vr and Er: Velocity and energy recoil of 218Po
 Velocity Vq is constant for all the L distance, this implicates that:
TL =
M: atomic mass unit of
L
Vq
M
T =
L
2
1/2
.(E)-1/2.L
218Po
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Results and discussion
Radioactivité : origine des émissions
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Time-of-Flight measurement obtained without differential
potential to define the recoiling 218Po ions spectrum
100
Radioactive recoils
Counts/channel
The centroid peak represents:
• the mean velocity of the recoiling ion 218Po
resulting from the energy loss in the matter
• an average energy loss of 40 keV, equivalent
to a layer of 10 - 20 nm of the organic deposit
10
H,C recoils
1
0
2000
4000
6000
Time (0.8 ns/channel)
8000
The longest Time-of-Flight is
attributed to ions that lost
about 90 keV in a material about
40 to 50 nm thick
We can also note the presence of light elements such as
hydrogen and carbon atoms; these are ejected
by the radioactive recoil colliding the organic molecules
of the source deposit
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Time-of-Flight spectra for secondary and recoiling ions
Ba
+
+
C2Hn , Al, Si
H
+
Positive secondary Ion
Na
+
CpHn
+
Time-of-Flight
Counts/channel
100
CHn
+
Alpha recoil
Time-of-Flight
+
C
10
1
0
2000
4000
6000
8000
Time (0.8 ns/channel)
 Time-of-Flight spectra show small differences, which reveal the neutral and singly
positive charged states of recoiling ions
 No multi-charged ions greater than +10 appear, otherwise they would have been
located between H and C peaks
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Time-of-Flight spectra for secondary ions for two
different 226Ra sources
Ba
+
1000
Counts/channel
+
H
100
10
1
0
2000
4000
6000
8000
Time (0.8 ns/channel)
The secondary ion emission for both sets of measurements differ by the
presence or not of barium in the source solution; the second set lacks the barium
peak because the decrease of the concentration in the solution
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Time-of-Flight spectra for secondary and recoiling ions
Positive secondary ions
+
H
1000
Time-of-Flight
Counts/channel
Na
100
H2+
CHn
+
+
To adjust the Time-of-Flight deficit, a
contribution of 10 keV was
attributed for 20% of the recoiling
218Po atoms.
10
20% singly charged
218Po
Positive and neutral alpha recoils
1
0
2000
4000
Time (0.8 ns/channel)
+
CHn
To fill the gap near the peak
, the
same rough estimation was made:
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6% multicharged 218Po
Maximum +10 charges
Average +5 charges
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Conclusions and perspectives
Rn
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Conclusion and Perspectives
 These results show, for the first time, the charge distribution state of a
radon progeny (218Po): 74% neutral, 20% singly charged, 6% multicharged (with
a maximum +10 and an average +5), they confirm the decay of 218Po to the
ground state
 These results show how the energy loss (10 - 100 keV) in the matter emits a
high secondary ions yield and how it neutralizes recoiling ions
 Otherwise, we can notice the large influence of the matter (barium) and the
medium surrounding the emitting source
 We must mention as well that our results can not be compared to those of
the literature (88% of 218Po positively charged, Wellisch) as we measured the
charge distribution in the vacuum and not in the ambient air (Wellisch)
 These data are interesting because they show several ways of matter’s
ionization, and will help investigations on the electric charge of radioactive
element attached to nanometric aerosols
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Experimental study of the electric charge of a
radioactive nanometric aerosol
R. Abou-Khalil - S. Della-Negra - N. Michielsen
A. Nourreddine - E. Baussan
24th International Conference on Nuclear Tracks in Solids
Bologna – September 1-5, 2008
Thank You!
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