TIME-of-FLIGHT TECHNIQUE
for RILIS SELECTIVITY
IMPROVEMENT
V. I. Mishin
Institute for Spectroscopy (ISAN)
Russian Academy of Sciences
Troitsk, Moscow, 142190 Russia
1st Topical Workshop and Users meeting 2013: Laser Based Particle Sources
CERN, Switzerland
20 – 22 February 2013
Laser Resonant Ionization Spectroscopy of
Radioactive Isotopes in Atomic Beams (1982)
ISAN / LNPI experimental setup
Ion Detector
Laser beams
Target and
Ionizer
А+
Mass separator
А+
Neutralizer
Protons
ISAN & LNPI
Minimal measurable isotope flow
≈
103
–
105
isotopes/s
COMPLIS – ISOLDE
MAINZ UNIVERSITY
Laser Resonant Ionization of Atoms
in a Hot Metal Pipe
(1982)
An excerpt from Mishin’s
scientific log book
ISAN/Troitsk
Photoionization Methods for
LNPI
1. Ionization in the pipe.
laser
ions
Laser Resonant Ionization of Atoms
in a Hot Cavity. Operating Principle (198
(1984)
metal
plates
HOT CAVITY
ηRILIS =
f
laser pulse
repetition rate
of isotopes
l = 1cm
+
Pi
≈ 75%
Pi + Pa
laser beam
l cavity length
υ thermal velocity
atoms
ions
ISAN/Troitsk
insulator
Pi /Pa = 4fl/υKtr
S.V. Andreev,
V.I. Mishin,
S.K. Sekatsky
Laser Resonant Ionization of Atoms in a Hot Cavity
(1984 – 1988)
1. Rise in the efficiency of ionization of atoms by pulse-periodic lasers
S. V. Andreev, V. I. Mishin, S. K. Sekatskiy
Sov. J. Quantum Electron., Vol. 15, Num. 3 (1985) 398-400 (English version)
Kvantovaya Elektronika, Volume 12, № 3 (1985) 611-614 (in Russian)
Abstract. The possibility is investigated of raising the efficiency of particle interception in
the method of resonant photoionization of atoms by laser radiation in a closed hot cavity,
located in vacuum, and subsequently employing an electric field to extract the ions
formed through a small aperture in the wall. It is shown that for realistic laser radiation
parameters (pulse duration ~ 15 nsec, repetition frequency 10 kHz) the cavity geometry
can be chosen in such a way that the interception efficiency exceeds 50%. The possibility
is demonstrated of completely extracting the ions formed by photoionization from the
cavity.
2. High-efficiency laser resonance photoionization of Sr atom in a hot cavity
S. V. Andreev, V. I. Mishin, and V. S. Letokhov
Optics Communications, Volume 57, Issue 5 (1986) 317-320
Abstract. The possibility of high-efficiency photoionization of Sr atoms inside a hot cavity
with lasers of high pulse repetition rate ( 104 pps) has been studied. The produced
photoions were extracted from the cavity through a small hole in its wall for further
analysis and counting. An overall photoion yield of about 0.2 has been achieved.
Laser Resonant Ionization of Atoms in a Hot Cavity.
3.
(1985 – 1988)
Laser resonant photoionization detection of traces of the radioactive isotope
in a sample
S. V. Andreev, V. S. Letokhov, V. I. Mishin
JETP Letters, Volume 43, Issue 12 (1986) 736-739 (English version)
Pis'ma Zh. Eksp. Teor. Fiz., Volume 43, Issue 12 ( 1986) 570-572 (in Russian)
221Fr
Abstract. The hyperfine splitting of the D2 line of the isotope 221Fr (T1/2 = 4.8 min) has
been measured. The ionization potential of the francium atom has been refined:
Ei ≤ 4.154 eV.
4. Laser resonance photoionization spectroscopy of Rydberg levels in Fr
S. V. Andreev, V. S. Letokhov, and V. I. Mishin
«Physical Review Letters»
Phys. Rev. Lett., Volume 59, Issue 12 (1987) 1274-1276
Abstract. We investigated for the first time the high-lying Rydberg levels in the rare
radioactive element francium (Fr). The investigations were conducted by the highly
sensitive laser resonance atomic photoionization technique with Fr atoms produced at a
rate of about 103 atoms/s in a hot cavity. We measured the wave numbers ofthe
7p2P3/2→nd2D (n=22–33) and 7p2P3/2→ns2S (n=23, 25–27, 29–31) transitions and found
the binding energy of the 7p2P3/2 state to be T=-18 924.8(3) cm-1, which made it possible
to establish accurately the ionization potential of Fr.
Laser Resonant Ionization of Atoms in a Hot Cavity.
(1985 - 1988)
5. Rydberg levels and ionization potential of francium measured by laserresonance ionization in a hot cavity
S. V. Andreev, V. I. Mishin, and V. S. Letokhov
«Journal of the Optical Society of America B: Optical Physics»
J. Opt. Soc. Am. B, Volume 5, Issue 10 (1988) 2190- 2198
Abstract. A highly sensitive method of detecting atoms in samples has been used for
spectral investigations of the rare radioactive element Fr. The method is based on laserresonance photoionization of Fr atoms in a hot quasienclosed cavity. The investigations
have been carried out with samples in which short-lived radioactive 221Fr atoms formed at
a rate of approximately 103 atoms/sec. The data obtained, to our knowledge for the first
time, on the energies of the high-lying Rydberg levels of the 2S½ and 2D series have
made it possible to determine the electron binding energy of the 7p 2P3/2 state and to
establish the ionization potential of Fr accurately.
(V. S. Letokhov and V. I. Mishin)
Laser Photonization
Pulsed Source
of Radioactive Atoms
(1984)
V.S. Letokhov, V.I. Mishin
Laser Ion Sources
(1985)
H.-Jürgen Kluge, and F. Ames,
W. Ruster, K. Wallmeroth
Invited talk, given
at the “Accelerated
Radioactive Beams
Workshop”
Vancouver Island,
Canada
4 – 7 September 1985
Selective Laser Ion Source
Высокоэффективная z-селективная
фотоионизация атомов в горячей
металлической полости с последующим
электростатическим удержанием ионов
Г. Д. Алхазов, В. С. Летохов, В. И. Мишин,
В. Н. Пантелеев, В. И. Романов,
С. К. Секацкий, В. Н. Федосеев
Письма в ЖТФ, том 15, выпуск 10 (1989) 63-66
(1989)
LNPI-ISAN
High efficient z-selective
photoionization of atoms in a hot
metal cavity followed by
electrostatic confinement of the
ions
G.D. Alkhazov, V.S. Letokhov, V.I. Mishin,
V.N. Panteleyev, V.I. Romanov,
S.K. Sekatsky, V.N. Fedoseyev
Fig. 1. Schematic drawing of the selective laser ion
source. The dashed area is the region of ionization.
Pis'ma Zh. Tekhn. Fiz.,
Volume 15, Issue10 (1989) 63-67
A laser ion-source
for on-line isotope separation
V.I. Mishin, V.N. Fedoseev, Yu.A. Kudryavtsev, V.S. Letokhov,
H. Ravn, S. Sundell, H.J. Kluge, F. Scheerer
(1990)
ISAN
ISOLDE-3
Synchrocyclotron
Proceedings of the Fifth International Symposium on “Resonance Ionization
Spectroscopy and its Applications, RIS -90”, Varese, Italy (1990)
Abstract. A laser ion source has been developed for efficient production of
isobarically pure ion beams at the on-line mass separator ISOLDE at CERN. In
first off-line tests with radioactive Yb-169, an efficiency of about 15% was
achieved. An elemental selectivity between 10 and 104 was observed. The
maximum value could be obtained at the off-line separator with TaC as
construction material. A first test at the on-line separator ISOLDE-3 was
performed recently with Yb isotopes. The lasers produced a pulsed ion beam of
about 10 ns pulse length. In order to suppress the continuous background due to
surface ionization a pulsed deflector was used so that the selectivity was
improved by a factor of 10.
Study of Short-Lived
101-108Sn
50
Isotopes with RILIS
at Heavy Ion Accelerator UNILAC/GSI
(1992)
laser beams
on-line
mass separator
Ø 1 mm
ion beam
106-xSn
+ 2p + xn
+
+
E = 1.0 V
E = 0.01 V
+
40 particle•nA of
+
58Ni14+(5MeV/u)
FEBIAD
50Cr
T ≈ 2400 K
extraction
electrode
The acronym RILIS was enacted for the first time
in
1993
at the ISOLDE/BOOSTER
by Slava Mishin, Valentine Fedosseev and Ulrich Köster
Operation of a RILIS
atoms
surface ions
+ ++
source
container
cavity
+
+
photo ions
+
+
laser beam
RESONANT LASER
IONIZATION
of an ATOM
high-temperature
pipe
--- + -- + + - + + ----
A+
n2
hot
metal
n1
A
Selectivity of the RILIS
Hot Metal Cavity
Two basic factors define RILIS selectivity:
*** LASER IONIZATION of studied atoms
*** SURFACE IONIZATION of interfering atoms
S(Ag/In) =
ηLASER (Ag)
βSURFACE(In)
ISOL
DE
Overall RILIS efficiencies
for elements available
at ISOLDE
RILIS overall efficiency, %
30
25
20
15
10
5
0
B
e
M
g Al Ca Sc n o Ni u Z n a g d Sn Sb Dy m Yb u
C
G A C
A
M C
T
T l Pb Bi
Selectivity of the RILIS
Hot Metal Cavity
ηLASER (Ag)
S(Ag/In)
=
βSURFACE (In)
Wall sticking times
Frenkel
equation
Ed
 sorp   0 exp(  )
kT
1/τ0 – frequency factor
Ed – interaction energy
of the atom with the surface
τ, c
TEMPERATURE, oC
The number of collisions
of atoms with a wall of
the RILIS ionizer prior to
atoms fly out is
Swall = πDL
N=
= 4L/D
Shole =
πD2/4
L = 3 cm, D =3 mm
(length and diameter of the
ionizer)
Lifetimes of the Sc, Y, Zr, Hf and some lanthanide
atoms on the polycrystalline Ta surface
N = 40
J. Beyer, A. F. Novgorodov and V. A. Halkin.
JINR preprint Р6 – 9917, 1976
Z
Proton Number
Neutron Number
N
Selectivity of a RILIS can be increased considerably
providing
laser produced ions are separated from thermal ions
τions creation time = τlaser pulse duration
T laser pulse-repetition interval
time
Maximum RILIS selectivity, which can be reached by laser ions separation
from thermal ions, is equal to
flaser = 104 pps
T = 1/flaser = 100 μs
τions = τlaser ≈ 10 ns
S = T / τions
S ≈ 10000
It makes sense to hunt for this number
ISOL
ISOLDE
DE
RILIS
≈ 140 mm
30 mm
IONIZER
Ions to mass separator magnets
Laser beams in ionizer
TARGET
≈+2V
Repelling electrode
- 60 kV
Ground plate
Acceleration electrode
COMPARISON of an ISOL Time-of-Flight RILIS and
the Time-of-Flight Mass Spectrometer
There is a significant
discrepancy
between the TOF
mass spectrometer
and ISOL TOF RILIS
The Wiley-McLaren TOF mass spectrometer
Source
Extraction
region
Drift region
Acceleration
region
+
+
+
+
+
+
+
+
0.2 cm
1.2 cm
40cm
0V
- 1600 V
- 64 V
Ground
plate
+
+
+
+
Extraction
grid
- 1600 V
Acceleration
grid
Detector
2. Voltage applied to
the TOF electrodes
TOF MS ≈ 5000 V
TOF RILIS ≥ 50 V
An ISOL TOF RILIS
Source – Hot cavity
Drift region
+
+
+
+
+
3 cm
Acceleration region
+
+
+
+
+
+
+
3 cm
Repelling electrode
+
+
+
+
80 cm
30 V
- 60 000 V
Ground grid
Ground grid
1. Initial ion spatial
distributions
TOF MS ≈ 0.3 mm
TOF RILIS 30 mm
Acceleration electrode
3. linear dimension
TOF MS 40 - 200 cm
TOF RILIS ≈ 12 cm
Ion packets width
τ ion peaks ≈ τ spatial distributions + τ thermal energy distributions
Broadening of ion packets
by initial spatial distributions
acceleration region
+V
E
V
L
field free drift-region
E=0
τ spatial distributions
Broadening of ion packets
by initial thermal energy distributions
acceleration region
E
+V
V
L
field free drift-region
E=0
τ turn-around time
t1
t0
t0
t1
L
τturn-around time
2v0 m
= t1 – t0 
eE
υ0 - initial thermal velocity
m - mass of ions
e - charge of electron
Duration of ion packets at the output
of the mass separator in relation
to the voltage drop across the RILIS ionizer
M = 100 a. e.
50
J. Lettry et al. (2002) Δτ(Ag) ≈ 50 – 60 μs
Uacceleration
Uionizer
- 60 kV
30 mm
ionizer
target
Duration of ion bunches, s
45
τ spatial distributions + τ turn-around time
40
35
30
The voltage
25
range
20
affect
15
the mass
10
separator
5
resolution
0
M. Koizumi et al. (2002)
Δτ(Al) ≈ 10 μs
0
10
20
30
40
50
60
Voltage applied to the graphite ionizer, Volts
70
Melting Points and Resistivity
of the Refractory Metals and Carbon
Niobium
Molybdenum
Tantalum
2750 K
2896 K
3290 K
Melting point
Tungsten
Rhenium
3695 K
3459 K
Resistivity
Tungsten
5.6×10−8 Ω•m at 20 °C
Carbon (crystalline)
2.5×10−6 to 5.0×10−6 Ω•m
// basal plane
3.0×10−3 Ω•m
⊥ basal plane
Carbon remains solid at higher temperatures
than the highest melting point metals such as tungsten or rhenium.
Carbon sublimation point about 3900 K.
The temperature of the crystalline
graphite pipe in relation to the voltage drop
2000
o
Temperature, C
1800
1600
1400
1200
1000
amorphous graphite
20
22
24
26
28
30
32
Voltage, V
The electrical resistance of the pipe about 2.7 Ω
35,5
Ø5
Ø3
crystalline graphite
Primary Space Focus for Single-Stage
Source Region Configuration
Source – Hot cavity
Drift region
E = E0
E=0
+
+
+
+
s
E = E 1 ≈ 60 000 kV/L
+
+
+
+
Acceleration
plate
Acceleration region
+
D = 2s
Ground
grid
Ground
grid
τion peak ≈ τturn-around time
Experimental Setup
Carbon (amorphous)
L = 37 mm
Ø3 mm
atomic vapour
source
s
3,7 cm
D = 2s
3,7
cm
Time-of-flight mass spectrum
of Li+, Na+, K+ and Tm+
current generator triggering pulse
Uionizer = 15.3 V
photodiode response on the laser
pulse
Li
Na
K
Tm (thermal)
laser ablation of the grid
5 s
Tm (laser)
Tm (thermal)
and
Tm (laser)
peaks
are created
through Tm
ionization on
the hot cavity
surface or by
the laser
Primary Space Focus for Single-Stage
Source Region Configuration
E=0
+
5
+
+
+
+
+
+
+
+
D=
2s
s
Acceleration
Ground
plate
Ground
grid
grid
Broadening of ion peaks
by initial spatial distributions
Broadening of ion peaks
by initial thermal energy distributions
Duration of ion peaks, μs
E = E0
M = 100 a. e.
4
40
35
3
30
2
25
1
20
0
0
10
20
30
40
50
60
15
10
5
0
=
+
0
10
20
30
40
50
Voltage applied to the ionizer, Volts
60
Summary
• • • The hot cavity made of crystalline graphite can operate
stable at high temperatures (≥2000oC)
• • • The voltage applied to the cavity may be as much as 30 V
• • • Short ion pulses approaching 3 μs can be prepared
by the use of the crystalline graphite hot cavity
• • • The RILIS selectivity can be increased by a factor of
30 – 50 for isotopes of mass 100 with the crystalline
graphite hot cavity and single-stage TOF configuration