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A new highly efficient selective laser ion source
Nuclear Instruments and Methods in Physics Research A280 (1989) 141-143 North-Holland, Amsterdam
141
Letter to the Editor A NEW HIGHLY EFFICIENT SELECTIVE LASER ION SOURCE G . D . A L K H A Z O V , E.Ye. B E R L O V I C H a n d V.N. P A N T E L E Y E V
Leningrad Nuclear Physics Institute, 188350, Gatchina, Leningrad district, USSR Received 17 March 1989
A new high-temperature resonant laser ion source is described to be used at on-line mass separators.
For studies of nuclei far from beta-stability and for some other applications it is necessary to produce intense mass-separated ion beams uncontaminated by isobars from neighbouring Z elements. This Z-selectivity may be easily achieved for some elements making use of the appropriate combination of the physical and chemical properties of the given element and the target material. For example, very pure atomic beams may be produced for alkali metals and halogens due to chemical selectivity of positive and negative ion formation by surface ionization. A more universal method of selective ion production may be based on the resonant laser ionization. Here a high element selectivity is achieved by tuning the laser frequencies in resonance with the frequencies of the atomic transitions. Such an ion source was proposed by Andreev et al. and is described in refs. [1,2]. This source provides a rather good efficiency of ionization; however, its application is restricted only to elements with sufficiently high volatility at temperatures lower than 1800-2000 ° C since the enclosure of the ion source contains insulators. In ref. [3] we have proposed a new version of a highly efficient selective laser ion source which is free of the above limitation. Here we describe this ion source in more detail. The ion source (fig. 1) consists of a vessel fabricated from refractory metal. The atoms to be ionized are fed from the target into the ion source through transfer tube 3. To reduce the time of sticking of the atoms to the surface, the vessel is kept at a high enough temperature (2200-2500 ° C) by heating it with a d c current flowing in the axial direction. This way of heating produces an axial electric field in the ion source which pushes the ionized atoms to exit opening 2. Two or three laser rays tuned to appropriate transition frequencies merged together in one ray are fed into the ion source through inlet opening 1. The laser system consists of pulsed pumping laser L1-L 3 with a rather high repetition rate (in our experiments we use copper-vapour lasers with a repetition rate f = 10 kHz and an average power - 8 0168-9002/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
W) and tunable dye lasers L'I-L 3. The atoms are excited to intermediate states and finally to some autoionization state which decays producing a free electron and a positive ion. The probability Pion of ionization of the atom, provided it is exposed to the resonant laser radiation, is very high. It depends on the laser power and the excitation scheme of the given element and may be from 0.1 to 1. To estimate the total efficiency of the ion source one has to take into account that atoms may in principle escape laser irradiation in case the repetition rate of laser pulses is too low and that the ionized atoms may PUMPING LASERS
E H TUNABLE LASE~tS
HOT CAVITY
",,i
~
l/1//H/I/1////_ 3
bI
=
B~'"
EXTRACTION ,ELECTRODE
Fig. 1. Schematic drawing of the high-temperature resonant laser ion source. The dashed area is the region of ionization.
142
G.D. Alkhazov et a L / A new highly efficient selective laser ion source
be lost due to their neutralization on the surface. The rather high efficiency of the described ion source is due to the fact that the atoms before they leave the ion source through the holes are exposed to the laser radiation several times and that the probability of neutralization of the ionized atoms, as will be discussed later, is very low. The total efficiency of the ion source may be estimated by the formula P =Pexit [1
--
(1
-- XpionPgeom)
,,,/xl ] .
(1)
Here x is the ratio of the atoms to be ionized in the volume of the source except those which are sitting on the surface of the cavity to the total number of these atoms, Pgeom is the geometrical factor which is equal to the ratio of the volume irradiated by the laser light in the ion source to the total volume of the cavity Pgeorn = ( d l / / D ) 2,
(2)
where d 1 is the diameter of the inlet opening 1 and D is the diameter of the ion source (we suppose the cross section of the laser beam in the source to be circular with the diameter of the inlet opening); Pexit is the probability of exit of the ionized atoms out of the ion source and n is the number of laser pulses at the mean time r which the atoms wander in the ion source before they leave it (this time does not include the time of sitting on the surface). The value of n may be found as follows: n =fr,
(3)
where r =
NI
--~-.
(4)
Here N is the mean number of flights of atoms from wall to wall in the cavity of the ion source, i is the corresponding mean length of flight and g is the effective mean velocity of the atoms. These values may be estimated as [4] N
4DL
(5)
d? +
(6)
i=D, and -1 6~-(v
-t)
1/'rrkT =V2M '
(7)
where d 2 is the diameter of the outlet hole 2, L is the length of the ion source cavity, M is the mass of the atom, k is Boltzmann's constant and T is the temperature of the ion source. Finally we obtain n
4 D 2L d? + d~
2~T
f"
(8)
An essential feature of this ion source is the extensive thermionic electron emission from the hot walls which causes space charges inside the source so that a sharp potential drop AU at the walls appears while the main part of the volume is free of electric field. This potential drop AU depends on the temperature and the wall material properties and may amount to 2.5 V [5] which is quite enough to imprison the positive ions inside the volume of the source. Due to this potential drop the ionized atoms cannot get on the walls of the source and thus they may not be neutralized on the surface. The axial electric field (due to heating of the source by the dc current) pushes the ions to the outlet hole from which they are extracted by the extraction field. Thus the efficiency of extraction of ionized atoms is high and we suppose the value of Pexit to be close to unity: Pexit = 1.
Usually the product X p i o n P g e o m is small and the value of n i x is large. Hence the formula for the total ion source efficiency may be written in the form
P=l-exp
4dr 2/--~ ] --Pio~d~lT'-d~fLV~ ].
(9)
N o t e that the value of P according to eq. (9) does not depend on the diameter D of the ion source and, for a given ratio of d~ and d 2 (for example for d I = d2) , depends only on the single geometrical parameter of the source its length L. However, since the laser power is limited, it is evident that to ensure a considerable probability of ionization Pion one has to use narrow laser beams and therefore an ion source with a relatively small diameter opening 1. Let us estimate the value of P for the case of d I = d2, L = 50 mm, f = 104 Hz, M = 150 au, T = 2500 K. For the value of Pion we take here Pion = 0.2 w h i c h is easily accessible in resonant laser ionization spectroscopy. Using eq. (9) we obtain P = 35%. Thus the efficiency of the ion source may be high enough. The concepts of the given work have been checked in a joint experiment performed with the laser group from the Moscow Institute of Spectroscopy of Academy of Sciences of the USSR. The ion source as described here has shown high efficiency in good agreement with the presented formulas [6]. As for the chemical selectivity, the proper resonant laser ionization selectivity is extremely high. However, the practical selectivity is limited by a contribution of nonresonant surface ionization of elements with relatively low ionization potential. To suppress this surface ionization one may use sources with the walls made of materials with low work functions (Nb, Hf, LaB 6, BaO the work function for the last case is ~ = 1.7 V). To reduce the work function, the surface of an ion source -
-
G.D. Alkhazov et aL / A new highly efficient selectioe laser ion source
fabricated from a refractory metal (for example, W) may be carbidized [7] or coated with a metal with low work function (for example, yttrium [8]). The ion source described may be employed for elements with a rather poor volatility provided their ionization potential is not too high. In combination with a mass separator it may provide mass-separated elementally pure beams with both good efficiency and selectivity.
References [1] S.V. Andreev, V.I. Mishin and S.K. Sekatsky, Kvant. Elektron. 12 (1985) 611.
143
[2] S.V.Andreev, V.I. Mishin and V.S. Letokhov, Opt. Comm. 57 (1986) 317. [3] G.D. Alkhazov, E.Ye. Berlovich and V.N. Panteleyev, Zh. Tekkn. Fiz. 14 (1988) 1109. [4] G.D. Alkhazov, E.Ye. Berlovich and V.N. Panteleyev, LNPI preprint N 1365, Leningrad (1988). [5] R. Kirchner, Nucl. Instr. and Meth. 186 (1981) 275. [6] G.D. Alkhazov et al., Pism. Zh. Teor. Eksp. Fiz., to be published. [7] E.H. Dilzer and C. Engler, Nucl. Instr. and Meth. B26 (1987) 218. [8] F. Touchard et al., Nucl. Instr. and Meth. 186 (1981) 329.
141
Letter to the Editor A NEW HIGHLY EFFICIENT SELECTIVE LASER ION SOURCE G . D . A L K H A Z O V , E.Ye. B E R L O V I C H a n d V.N. P A N T E L E Y E V
Leningrad Nuclear Physics Institute, 188350, Gatchina, Leningrad district, USSR Received 17 March 1989
A new high-temperature resonant laser ion source is described to be used at on-line mass separators.
For studies of nuclei far from beta-stability and for some other applications it is necessary to produce intense mass-separated ion beams uncontaminated by isobars from neighbouring Z elements. This Z-selectivity may be easily achieved for some elements making use of the appropriate combination of the physical and chemical properties of the given element and the target material. For example, very pure atomic beams may be produced for alkali metals and halogens due to chemical selectivity of positive and negative ion formation by surface ionization. A more universal method of selective ion production may be based on the resonant laser ionization. Here a high element selectivity is achieved by tuning the laser frequencies in resonance with the frequencies of the atomic transitions. Such an ion source was proposed by Andreev et al. and is described in refs. [1,2]. This source provides a rather good efficiency of ionization; however, its application is restricted only to elements with sufficiently high volatility at temperatures lower than 1800-2000 ° C since the enclosure of the ion source contains insulators. In ref. [3] we have proposed a new version of a highly efficient selective laser ion source which is free of the above limitation. Here we describe this ion source in more detail. The ion source (fig. 1) consists of a vessel fabricated from refractory metal. The atoms to be ionized are fed from the target into the ion source through transfer tube 3. To reduce the time of sticking of the atoms to the surface, the vessel is kept at a high enough temperature (2200-2500 ° C) by heating it with a d c current flowing in the axial direction. This way of heating produces an axial electric field in the ion source which pushes the ionized atoms to exit opening 2. Two or three laser rays tuned to appropriate transition frequencies merged together in one ray are fed into the ion source through inlet opening 1. The laser system consists of pulsed pumping laser L1-L 3 with a rather high repetition rate (in our experiments we use copper-vapour lasers with a repetition rate f = 10 kHz and an average power - 8 0168-9002/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
W) and tunable dye lasers L'I-L 3. The atoms are excited to intermediate states and finally to some autoionization state which decays producing a free electron and a positive ion. The probability Pion of ionization of the atom, provided it is exposed to the resonant laser radiation, is very high. It depends on the laser power and the excitation scheme of the given element and may be from 0.1 to 1. To estimate the total efficiency of the ion source one has to take into account that atoms may in principle escape laser irradiation in case the repetition rate of laser pulses is too low and that the ionized atoms may PUMPING LASERS
E H TUNABLE LASE~tS
HOT CAVITY
",,i
~
l/1//H/I/1////_ 3
bI
=
B~'"
EXTRACTION ,ELECTRODE
Fig. 1. Schematic drawing of the high-temperature resonant laser ion source. The dashed area is the region of ionization.
142
G.D. Alkhazov et a L / A new highly efficient selective laser ion source
be lost due to their neutralization on the surface. The rather high efficiency of the described ion source is due to the fact that the atoms before they leave the ion source through the holes are exposed to the laser radiation several times and that the probability of neutralization of the ionized atoms, as will be discussed later, is very low. The total efficiency of the ion source may be estimated by the formula P =Pexit [1
--
(1
-- XpionPgeom)
,,,/xl ] .
(1)
Here x is the ratio of the atoms to be ionized in the volume of the source except those which are sitting on the surface of the cavity to the total number of these atoms, Pgeom is the geometrical factor which is equal to the ratio of the volume irradiated by the laser light in the ion source to the total volume of the cavity Pgeorn = ( d l / / D ) 2,
(2)
where d 1 is the diameter of the inlet opening 1 and D is the diameter of the ion source (we suppose the cross section of the laser beam in the source to be circular with the diameter of the inlet opening); Pexit is the probability of exit of the ionized atoms out of the ion source and n is the number of laser pulses at the mean time r which the atoms wander in the ion source before they leave it (this time does not include the time of sitting on the surface). The value of n may be found as follows: n =fr,
(3)
where r =
NI
--~-.
(4)
Here N is the mean number of flights of atoms from wall to wall in the cavity of the ion source, i is the corresponding mean length of flight and g is the effective mean velocity of the atoms. These values may be estimated as [4] N
4DL
(5)
d? +
(6)
i=D, and -1 6~-(v
-t)
1/'rrkT =V2M '
(7)
where d 2 is the diameter of the outlet hole 2, L is the length of the ion source cavity, M is the mass of the atom, k is Boltzmann's constant and T is the temperature of the ion source. Finally we obtain n
4 D 2L d? + d~
2~T
f"
(8)
An essential feature of this ion source is the extensive thermionic electron emission from the hot walls which causes space charges inside the source so that a sharp potential drop AU at the walls appears while the main part of the volume is free of electric field. This potential drop AU depends on the temperature and the wall material properties and may amount to 2.5 V [5] which is quite enough to imprison the positive ions inside the volume of the source. Due to this potential drop the ionized atoms cannot get on the walls of the source and thus they may not be neutralized on the surface. The axial electric field (due to heating of the source by the dc current) pushes the ions to the outlet hole from which they are extracted by the extraction field. Thus the efficiency of extraction of ionized atoms is high and we suppose the value of Pexit to be close to unity: Pexit = 1.
Usually the product X p i o n P g e o m is small and the value of n i x is large. Hence the formula for the total ion source efficiency may be written in the form
P=l-exp
4dr 2/--~ ] --Pio~d~lT'-d~fLV~ ].
(9)
N o t e that the value of P according to eq. (9) does not depend on the diameter D of the ion source and, for a given ratio of d~ and d 2 (for example for d I = d2) , depends only on the single geometrical parameter of the source its length L. However, since the laser power is limited, it is evident that to ensure a considerable probability of ionization Pion one has to use narrow laser beams and therefore an ion source with a relatively small diameter opening 1. Let us estimate the value of P for the case of d I = d2, L = 50 mm, f = 104 Hz, M = 150 au, T = 2500 K. For the value of Pion we take here Pion = 0.2 w h i c h is easily accessible in resonant laser ionization spectroscopy. Using eq. (9) we obtain P = 35%. Thus the efficiency of the ion source may be high enough. The concepts of the given work have been checked in a joint experiment performed with the laser group from the Moscow Institute of Spectroscopy of Academy of Sciences of the USSR. The ion source as described here has shown high efficiency in good agreement with the presented formulas [6]. As for the chemical selectivity, the proper resonant laser ionization selectivity is extremely high. However, the practical selectivity is limited by a contribution of nonresonant surface ionization of elements with relatively low ionization potential. To suppress this surface ionization one may use sources with the walls made of materials with low work functions (Nb, Hf, LaB 6, BaO the work function for the last case is ~ = 1.7 V). To reduce the work function, the surface of an ion source -
-
G.D. Alkhazov et aL / A new highly efficient selectioe laser ion source
fabricated from a refractory metal (for example, W) may be carbidized [7] or coated with a metal with low work function (for example, yttrium [8]). The ion source described may be employed for elements with a rather poor volatility provided their ionization potential is not too high. In combination with a mass separator it may provide mass-separated elementally pure beams with both good efficiency and selectivity.
References [1] S.V. Andreev, V.I. Mishin and S.K. Sekatsky, Kvant. Elektron. 12 (1985) 611.
143
[2] S.V.Andreev, V.I. Mishin and V.S. Letokhov, Opt. Comm. 57 (1986) 317. [3] G.D. Alkhazov, E.Ye. Berlovich and V.N. Panteleyev, Zh. Tekkn. Fiz. 14 (1988) 1109. [4] G.D. Alkhazov, E.Ye. Berlovich and V.N. Panteleyev, LNPI preprint N 1365, Leningrad (1988). [5] R. Kirchner, Nucl. Instr. and Meth. 186 (1981) 275. [6] G.D. Alkhazov et al., Pism. Zh. Teor. Eksp. Fiz., to be published. [7] E.H. Dilzer and C. Engler, Nucl. Instr. and Meth. B26 (1987) 218. [8] F. Touchard et al., Nucl. Instr. and Meth. 186 (1981) 329.