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Laboratory observation of forbidden transitions
following charge exchange collisions between
solar wind ions and neutrals
N. Numadate, K. Shimada, Y. Uchikura, H. Shimaya, T. Ishida,
K. OkadaA, N. NakamuraB, H. Tanuma
Depatrtment of Physics, Tokyo Metropolitan University
ADepartment of Physics, Sophia University
BInstitute for laser science, University of Electro-Communications
Outline
1. Introduction
- SWCX
- CX
2. Experimental setup
3. Results
- Trapping lifetime measurements
- Soft X-ray observations
4. Summary and Future plans
Mysterious soft X-ray emission with a varying intensity,
repeating in cycles of a few days, was observed.
Average all-sky-survey count rate
(15 orbits per day)
All-sky-map observed by ROSAT
S. L. Snowden et al. 1994, 1995
Comparison between the X-ray fluctuation
and the solar wind proton flux
T. E. Cravens et al. 2001
Solar Wind Charge eXchange (SWCX)
Soft X-ray emission from a comet was observed and
the intensity fluctuation corresponded to the solar activity.
Emission from the comet Hyakutake
C. M. Lisse et al. 1996
Charge exchange
Aq+ + B → A(q-1)+* + B+
A(q-1)+ + B+ + hν
Aq+ : Ion
B : Neutral ↓
s p d f
n = 4
3
2
1
Allowed transition
Δl = ±1
Cascades of transitions
Forbidden transition
Δl ≠ ±1
Soft X-ray spectrum observed by
Suzaku satellite R. Fujimoto et al., 2007
Composition and density distribution of
neutrals can be obtained from the spectrum.
Cross section data are needed.
1s2p 1P1
1s2 1S0
1s2s 3S1
1s2p 3P0,1,2 1s2s 1S0
w (E1)
573.9 eV
0.3 ps x (M2)
568.6 eV
3.0 ms
y (E1)
568.6 eV
1.9 ns
z (M1)
561.0 eV
1.0 ms
2 hn
0.4 ms
Level diagram of helium-like Oxygen ion
FWHM ~ 100 eV
O6+(1s2-1s2s/1s2p)
The long-lived, forbidden transitions in the SWCX had not yet
been observed by beam-based experiments in the laboratory.
The ASTRO-H (to be launched on 12 Feb. 2016) can separate
the resonance, inter-combination and forbidden lines.
Reproduction of the observed solar wind charge-
exchange with collision energy of 0.2 - 4.2 keV/u in the
laboratory
Observation of the long-lived, forbidden transitions
following charge exchange collisions with an ion trap
O7+(1s) + He → O6+(1snl) + He+
O6+(1s2) +
hν
↓ O6+(1s2s) ↓
Collision system in our experiment:
Ion trapping
Soft X-ray observation
Ion beam
Target gas inlet
Pcc(He) ~ 5.3×10-3 Pa
Capacitance
manometer
Electrostatic
lenses Collision
cell
• Detection efficiency: ~ 100 %
• FWHM: ~ 80 eV @ ~ 600 eV
• Detection area: 10 mm × 10 mm
• Peltier cooling
Window-less Silicon Drift Detector (SDD)
Ion beam
Trap potential
MCP gate
SDD gate
HV
LV
GND
Storage time
Delay
1 cycle 1 cycle
HV
LV
GND
ON
OFF
0
25
50
75
100
125
150
0 0.2 0.4 0.6 0.8 1
ion
cou
nts
storage time [s]
Lifetime measurement of O6+ ions
τ ~ 270 ms
v = 4.0(0.4)×104 m/s
Ecm = 13.7 eV
σ = 1.2(0.8)×10-14 cm2
Reasons for the ion decay in the trap
• Charge exchange
• Elastic scattering
Trapping lifetime was long enough to
observe the forbidden transition.
t ~ 270 ms >> 1 ms
H2 pressure in the trap : 4.4×10-7 Pa
Fitted by an exponential
I(t) = I0 e -t/t
1/t = snv
s : cross section
n : H2 number density
v : ion velocity
400 450 500 550 600 650 700 750 800
X-ray energy/eV
Inte
nsi
ty /
arb
. u
nit
s
Forbidden line from the
metastable O6+ ions
Forbidden line from O6+ ions produced by
charge exchange of O7+ - He system
1s2 - 1s2s
Level diagram of He-like Oxygen ion
Collision energy : 2.6 keV/u
1s2p 1P1
1s2 1S0
1s2s 3S1
1s2p 3P0,1,2 1s2s 1S0
w (E1)
573.9 eV
0.3 ps x (M2)
568.6 eV
3.0 ms
y (E1)
568.6 eV
1.9 ns
z (M1)
561.0 eV
1.0 ms
2 hn 0.4 ms
400 450 500 550 600 650 700 750 800
Inte
nsi
ty /
arb
. u
nit
s
X-ray energy/eV
Resonance line from O6+ ions
Deconvolution using Gaussian
functions
1s2 - 1s2p
1s2 - 1s3p
1s2 - 1s4p
Resonance line from O6+ ions produced by
charge exchange of O7+ - H2 system
Energy level
s p d f
n = 4
3
2
1
Collision energy : 2.6 keV/u
400 450 500 550 600 650 700 750 800
X-ray energy/eV
Inte
nsi
ty /
arb
. u
nit
s
Forbidden line: 1s2 – 1s2s
Resonance lines: 1s2 – 1snp (n = 1 - 3)
Peak positions
Forbidden line measurement: 560 eV
Resonance line measurement: 570 eV
FWHM: ~ 85 eV
There was a significant difference
between their peak positions.
We succeeded in the observation of the
forbidden lines in the lab.
Comparison of the forbidden and
the resonance lines from O6+ ions
We developed the Kingdon ion trap system and achieved
much longer trapping lifetime than the forbidden transition
lifetime of O6+.
We succeeded in the laboratory observation of the forbidden
transition following the SWCX.
Observation of other forbidden transitions in C4+, N5+ etc.
Perform measurements using a hydrogen atom target
Precise spectroscopy with a soft X-ray spectrometer
Measurements of the absolute values of forbidden emission
cross sections
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