Laser isotope separation apparatus

The laser isotope separation apparatus utilizes the circularly-polarized light to selectively excite the isotopes to the first or the second excited level. In case where the isotope shift is equal to or smaller than the line width of the laser light, it can be selectively ionized and separated because the isotopes absorb the circularly-polarized light based on the angular momentum selection rule and are excited and the isotopes not excited according to the mass number of isotopes (or those having nuclear spin not equal to zero and equal to zero). The selective excitation by the angular momentum selection rule and the ionization are performed by 3 steps of excitation. Three excitation wavelengths are generated from two or more lasers, and the optical paths of these light beams are made to be in same length by optical delay circuit. Accordingly, even when the laser light source is pulse-operating, selective excitation and ionization can be performed. Also, because the first circularly-polarized laser light and the second circularly-polarized laser light are irradiated on the substance containing the gasified isotopes from opposite directions, Doppler effects of the moving atoms can be canceled each other vector-wise, and the separation efficiency is increased.

BACKGROUND OF THE INVENTION 
The present invention relates to a separation apparatus of the isotopes 
with different mass numbers, and more particularly to a laser isotbpe 
separation apparatus. 
For example, natural palladium contains the isotopes with mass numbers 102 
(1%), 104 (11%), 105 (22.2%), 106 (27.3%), 108 (26.7) and 110 (11.8%). The 
palladium in the insoluble residue from the reprocessing of spent fuel 
contains the isotopes of the mass number 107 by 18%, for example, in 
addition to the isotopes of the above mass numbers. There is strong demand 
on the effective method to selectively separate the isotopes of platinum 
group elements. (For example, only the palladium with mass number 107 is 
radioactive, and if this can be selectively removed, the palladium in the 
insoluble residue from the reprocessing of spent fuel can be effectively 
utilized as valuable resources.) Laser isotope separation method is known 
as a means to separate isotopes. By this method, the substance containing 
isotopes is gasified, and linearly-polarized laser light is applied to 
selectively excite only the desired isotopes by isotope shift. Further, 
laser beam is applied on the atoms selectively excited, and the ionized 
atoms are separated from the other neutral isotopes by means of the 
electrode applied with electric field or magnetic field. 
Taking an example in uranium, the principle of the conventional laser 
isotope separation method is described below. In the schematical 
illustration of FIG. 8, when linearly-polarized laser light of 
.lambda..sub.1 (.about.591 nm) is applied to the uraniums 235 and 238 on 
ground level, only uranium 235 absorbs light because of the isotope shift 
(280 milli cm.sup.-1) and is excited to the first excited level, while 
uranium 238 does not absorb light and is not excited. When the 
linearly-polarized laser light of .lambda..sub.2 (.about.563 nm) is 
applied on uranium 235 it is excited from the above excited level to the 
intermediate excited level. It is further excited to higher than the 
ionized potential (I.P.) by the third light (.lambda..sub.3 =625 nm) and 
is ionized. On the other hand, uranium 238 is not excited at all. 
Accordingly, in the gas containing ionized uranium 235 and neutral uranium 
238, the former can be separated from the latter by means of the electrode 
applied with electric field or magnetic field. 
Meanwhile, it is known that the isotope shift depends upon the mass number 
of the element as shown in the graph of FIG. 5. From the mass number of 
about 100 up, isotope shift increases with the increase of mass number due 
to volume effect of atoms, while the isotope shift increases due to mass 
effect of atoms when the mass number decreases. As it is evident from this 
graph, the isotope shift is very small near the palladium with mass number 
of 102 to 110. Actually, it is about 8 milli cm.sup.-1. In contrast, the 
line width of laser light is about 30 milli cm.sup.-1. Thus, when the 
method of FIG. 8 is applied on the element with mass number of about 100, 
the isotopes cannot be selectively excited because the isotope shift is 
smaller than the line width of laser light. 
SUMMARY OF THE INVENTION 
Therefore, it is the object of this invention to provide a new isotope 
separation apparatus, which eliminates the disadvantages of the 
conventional laser isotope separation method as described above. More 
particularly, it is to provide a laser isotope separation apparatus for 
the isotope, in which isotope shift is equal to or smaller than the line 
width of laser light. 
To attain such object, the laser isotope separation apparatus according to 
the present invention is an apparatus, in which the isotope shift is equal 
to or smaller than the line width of laser light, the substance containing 
isotopes is gasified and laser light with the first wavelength is 
irradiated to excite only the isotopes with specific mass number or all 
isotopes to the first excited level, the laser light with the second 
wavelength is irradiated to excite all isotopes excited by the laser light 
with the first wavelength or only the isotopes with specific mass number 
to the second excited level, the laser light with the third wavelength is 
irradiated to ionize the isotopes on the second excited level, and the 
ionized isotopes are separated from the other neutral isotopes by the 
electrode applied with electric field or magnetic field, characterized in 
that two or more different lasers are pumped by the same pumping light 
source, that the wavelength of the laser light from at least one of the 
lasers is converted by non-linear optical effect, that the laser light 
with said first wavelength of the light beams having two or more 
wavelengths thus obtained is converted to the right-handed or left-handed 
circularly-polarized light by circular polarization converter, that the 
laser light with said second wavelength is converted to the right-handed 
or the left-handed circularly polarized light by a circular polarization 
converter, that the optical paths of these two are made to be in same 
length by optical delay circuit, and that said first circularly-polarized 
laser light and said second circularly-polarized laser light are 
irradiated on the substance, which contains gasified isotopes, from the 
opposite directions. 
This apparatus is preferably arranged in such manner that the laser light 
with the third wavelength is generated by a third laser, which is 
different from the first and the second lasers pumped by the light from 
the same pumping light source. 
In case the isotopes with different mass numbers in the isotopes to be 
separated are further separated, the laser light from ring dye laser with 
narrower line width than the isotope shift between these isotopes is 
amplified by one of the above two or more lasers, and the wavelength of 
this light is preferably converted by non-linear optical effect or it is 
used as the laser light with the first or the second wavelength without 
converting. 
The apparatus can be arranged in such manner that it can be applied to 
palladium as the substance containing isotopes, whereby the laser light 
with the second wavelength can also be used as the laser light with the 
third wavelength, that the same pumping the light with wavelength of 1064 
nm, that it is converted to the one-half of the wavelength, i.e. to 532 
nm, by non-linear optical element and the first laser is pumped, that the 
light with wavelength of 552.6 nm is oscillated from the first laser, and 
this light is converted to one-half of the wavelength, i.e. to 276.3 nm, 
by non-linear optical effect and this is used as the laser light with the 
first wavelength, that the laser light from said pulse operating YAG laser 
is converted to 1/3 of the wavelength, i.e. to 355 nm, by non-linear 
optical element and the second laser is pumped, and that the light with 
wavelength of 521 nm is oscillated from the second laser and this is used 
as the laser light with the second and the third wavelengths. 
The laser isotope separation apparatus according to this invention can also 
be applied in the cases where the substance containing the isotopes is Ca, 
Zn, Sr, Cd, Ba, Hg, Yb, C, Si, Ge, Sn, Sn, Sm, Pb or Pu. 
The laser isotope separation apparatus according to this invention utilizes 
the circularly-polarized light to selectively excite the isotopes to the 
first or the second excited level. Even in case of the isotope, in which 
the isotope shift is equal to or smaller than the line width of the laser 
light, it can be selectively ionized and separated because there are the 
isotopes absorbing the circularly-polarized light based on the angular 
momentum selection rule and being excited and the isotopes not excited 
according to the mass number of isotopes (or those having nuclear spin not 
equal to zero and equal to zero). The selective excitation by the angular 
momentum selection rule and the ionization are performed by 3 steps of 
excitation. Three excitation wavelengths are generated from two or more 
lasers pumped by the light from the same pumping light source, and the 
optical paths of these light beams are made to be in same length by 
optical delay circuit. Accordingly, even when the laser light source is 
pulse-operating, selective excitation and ionization can be performed. 
Also, because the first circularly-polarized laser light and the second 
circularly-polarized laser light are irradiated on the substance 
containing the gasified isotopes from opposite directions, Doppler effects 
of the moving atoms can be canceled each other vector-wise, and the 
separation efficiency is increased. It is not necessary to optically 
overlap both light beams in advance, and the circularly-polarized light 
can be efficiently utilized.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Before explaining the embodiments of this invention, the principle of the 
isotope separation of the laser isotope separation apparatus according to 
this invention is described. The principle used in this invention is the 
laser isotope separation method utilizing the angular momentum selection 
rule, and it is based on the angular momentum selection rule of the atoms 
to the absorption of the circularly-polarized light. For example, if 
palladium is taken as an example, in FIG. 2 where the status of angular 
momentum quantum number on its energy level is schematically illustrated, 
nuclear spin I is zero in the palladiums having even mass numbers 102, 
104, 106, 108 and 110. Accordingly, there is no hyperfine structure at 
energy level. The total angular momentum quantum number M at the first 
intermediate excited level is 1, and total angular momentum quantum number 
M at the second intermediate excited level is also 1. Between these 
levels, transition does not occur when the circularly-polarized light 
enters. Namely, it is known from the angular momentum selection rule that 
transition occurs only between the energy level where the variation 
.DELTA.M of total angular momentum quantum number when the left-handed 
circularly-polarized light enters is +b 1 and the variation .DELTA.M of 
total angular momentum quantum number when the right-handed 
circularly-polarized light enters is -1. However, because there is no 
variation of total angular momentum quantum number between the levels of 
M=1 and M=1, no excitation occurs by absorbing the circularly-polarized 
light. On the other hand, nuclear spin I is 5/2 and is not zero in 
palladiums having odd mass number of 105 and 107. Therefore, nuclear spin 
exerts influence on energy condition of the atoms, and the complicated 
hyperfine structure as shown is generated on energy level. When 
left-handed circularly-polarized light enters, the variation .DELTA.M=+1 
of total angular momentum quantum number occurs between the levels shown 
by arrow in the figures. Transition occurs and it is excited. Therefore, 
by irradiating the left-handed circularly-polarized light to the gas 
containing the palladiums 102, 104, 105, 106, 107, 108 and 110, the 
palladium 105 and 107 can be selectively ionized, and the ionized isotopes 
can be separated by the electrode applied with electric field or magnetic 
field. 
The laser isotope separation method of palladium according to the above 
principle is as schematically given in FIG. 3. Namely, when the laser 
light for the first selective excitation of the left-handed 
circularly-polarized light having wavelength .lambda..sub.1 (.about.276 
nm) is applied to the palladium group having odd mass numbers of 105 and 
107 at ground level and to the palladium groups having even mass numbers 
of 102, 104 106, 108 and 110, all atoms of two groups are excited to the 
first excited level. When the laser light for the second selective 
excitation of the left-handed circularly-polarized light having wavelength 
.lambda..sub.2 (.about.521 nm) is applied to the atoms of these two groups 
thus excited, only the group with odd mass numbers absorbs the light 
according to the principle of FIG. 2 and is excited to the intermediate 
excited level, and the palladium group with even mass number does not 
absorb the light and is not excited to the intermediate excited level. 
Further, the laser light for ionization of the left-handed 
circularly-polarized light of .lambda..sub.3 (.about.521 nm) (which is not 
necessarily the circularly-polarized light) is applied to the palladium 
group having odd mass number, and it is excited to higher than the 
ionization potential (I.P.) and ionized. On the other hand, the palladium 
group with even mass number is not excited. Thus, in the gas containing 
the ionized palladiums 105 and 107 and also neutral palladiums of 102, 
104, 106, 108 and 110, the former can be separated from the latter by the 
electrode applied with electric field or magnetic field. 
FIG. 1 shows an optical path of the Embodiment 1 of the laser isotope 
separation apparatus according to this invention to execute the laser 
palladium isotope separation method of FIG. 3. This apparatus primarily 
consists of vacuum chamber 1, crucible 2, exhaust apparatus 3, entrance 
window 4, YAG laser 5, dye laser 6, second higher harmonics generating 
crystal 7, circularly-polarized light generator 9, dye laser 10, 
circularly-polarized light generator 11, and optical delay circuit 12. 
Natural palladium specimen or the palladium specimen collected from the 
insoluble residue of the reprocessing of spent fuel is placed into a 
crucible 2 in a vacuum chamber 1, and the vacuum chamber 1 is turned to 
vacuum by the exhaust apparatus 3. Electron beam is applied on the 
specimen obliquely from above by an electron gun (not shown), and 
palladium vapor is generated inside the vacuum chamber 1. In the vacuum 
chamber 1, the entrance windows 4 and 4' are provided at the opposite 
positions to let the laser light enter. As described later, the 
circularly-polarized light is introduced through the entrance windows 4 
and 4', and these windows 4 and 4' are arranged at an angle of 80.degree. 
to the entering direction of the laser light in order to introduce the 
circularly-polarized light efficiently into the vacuum chamber 1. Laser 
light is generated by synchronizing the laser light for the first 
selective excitation and the laser light for the second selective 
excitation through pumping of the dye lasers 6 and 10 by the pulse 
operating YAG laser 5 for pumping. First, to generate the laser light with 
wavelength .lambda..sub.1 of FIG. 3, the light with wavelength 1064 nm 
coming from YAG laser 5 is turned to the light with 1/2 of the wavelength, 
i.e. 532 nm, by non-linear optical element (not shown). Pumping the dye 
laser 6 by this light, the light with wavelength 552.6 nm is emitted. This 
light is further applied on the second higher harmonics generating crystal 
7, and the light having 1/2 of the wavelength, i.e. 276.3 nm 
(.lambda..sub.1) is emitted. The light with unnecessary wavelength is 
removed by spectral prism 8, and it is turned to the left-handed 
circularly-polarized light by the circularly-polarized light generator 9 
comprising 1/4 wave plate. This is irradiated on the specimen vapor 
through the entrance window 4 as the laser light for the first selective 
excitation and it is excited to the first intermediate excited level. To 
generate the laser light with wavelength of .lambda..sub.3 of FIG. 3, the 
light with wavelength 1064 nm coming from YAG laser is turned to 1/3 of 
the wavelength, i.e. to 355 nm, by non-linear optical element (not shown). 
(Frequency is tripled.) After pumping dye laser 10 by this light, the 
light with wavelength 521 nm is emitted. This light is turned to the 
left-handed circularly-polarized light by the circularly-polarized light 
generator 11 comprising 1/4 wave plate. Passing through the optical delay 
12 consisting of the reflecting mirrors M.sub.14 -M.sub.17 with M.sub.15 
and M.sub.16 adjustable to the arrow directions, the optical path is 
aligned to irradiate the specimen at the same time with the laser light 
for the first selective excitation (.lambda..sub.1). (It is necessary to 
bring to the higher excitation status while it is excited.) It is then 
irradiated on the specimen vapor as the laser light for the second 
selective excitation through the entrance window 4', and only the 
palladiums having odd mass numbers of 105 and 107 are selectively excited 
to the second intermediate excited level. The laser light with this 
wavelength .lambda..sub.2 also serves as the laser light for ionization 
with wavelength .lambda..sub.3 of FIG. 3. Palladiums with odd mass number 
excited to the second intermediate excited level are ionized at the same 
time following to this excitation. The ionized palladiums having mass 
numbers of 105 and 107 are separated from the non-ionized palladiums with 
even odd mass numbers by the electrode (not shown) applied with electric 
field or magnetic field. 
The optical path with wavelength .lambda..sub.2 passing through the 
removable reflecting mirrors M.sub.8 and ,.sub.7 is an experimental 
optical path to let the laser light for the second selective excitation 
from the same direction as the laser light for the first selective 
excitation with wavelength .lambda..sub.1, and it is not necessarily 
required. When the laser light for the first selective excitation and the 
laser light for the second selective excitation are irradiated from the 
same direction by selecting such optical path, Doppler effect of the 
moving atoms works vector-wise, and the separation efficiency is 
decreased. However, the laser light for the first selective excitation and 
the laser light for the second selective excitation are irradiated from 
entirely opposite directions, as in the case of the present invention. 
Doppler effects of the moving atoms can be canceled each other 
vector-wise, and the separation efficiency is increased. If the laser 
light for the first selective excitation and the laser light for the 
second selective excitation are irradiated from the opposite directions, 
it is very advantageous because there is no need to optically overlap both 
light beams in advance. It is practically impossible to overlap two 
circularly-polarized light beams efficiently without loss and without 
giving influence on the polarization condition, whereas such problem can 
be solved by the arrangement according to this invention. It is preferable 
to irradiate the laser light for the first selective excitation and the 
laser light for the second selective excitation at the crossing angle of 
0.degree., i.e. perfectly coaxially from the opposite directions, but it 
is necessary to adjust so that the crossing angle is 1.degree. or less. 
In FIG. 1, the bypass optical path 15 passing through the semi-transparent 
mirror M.sub.10, the fixed reflecting mirrors M.sub.11 and the M.sub.13 
and the semi-transparent mirror M.sub.12 is used when the laser light 
(.lambda..sub.3) for ionization is irradiated without 
circularly-polarizing, and this optical path is not necessarily required. 
Further, the optical path consisting of argon ion laser 16, ring dye laser 
17, removable reflecting mirrors M.sub.4 and M.sub.5, and the fixed 
reflecting mirrors M.sub.6 and M.sub.8 is for the experiment to study 
hyperfine structure on the excited level of FIG. 2, and this is not 
necessarily required. In the figure, M.sub.1, M.sub.2, M.sub.3 and 
M.sub.16 are the fixed or the removable reflecting mirrors, and the 
control unit of each laser is denoted by the number 13. 
FIG. 4 shows the results of the separation of natural palladium isotopes by 
such laser separation apparatus. The conditions for the separation are as 
follows: 
The volume of palladium crucible was 3 cc, the emission current of the 
heating electron gun was 100 mA, and the evaporation surface temperature 
was 1850.degree. K. The vapor of palladium was made parallel by passing 
through collimator hole of 5 mm.times.20 mm. Atom density at the portion 
irradiated by laser light was 6.3.times.10.sup.9 atoms/cm.sup.3. Further, 
the laser light with wavelength .lambda..sub.1 had wavelength of 276.3 nm, 
pulse width of 10 nsec, iteration of 10 Hz, beam diameter of 3 mm, and 
power density of 64 W/cm.sup.3. The laser light with wavelength 
.lambda..sub.2 had wavelength of 521 nm, pulse width of 10 nsec, iteration 
of 10 Hz, beam diameter of 3 mm, and power density of 375 kW/cm.sup.3. 
As it is evident from FIG. 4, the concentration of palladium 105 (Palladium 
107 does not exist in nature.) is extremely increased. By repeating this 
separation procedure, the concentration of palladium 105 is further 
increased. 
It is necessary in the apparatus of FIG. 1 to separate palladium 105 and 
107 from each other, which are contained in the palladium collected from 
insoluble residue of the reprocessing of spent fuel. The isotope shift 
between these isotopes is about 8 milli cm.sup.-1. On the other hand, the 
line width of the ring dye laser excited by argon ion laser is about 1 
milli cm.sup.-1, and this is narrower than the above isotope shift. Thus, 
it is possible to selectively excite only palladium 107 from the 
palladiums 105 and 107 having nuclear spin not at zero. Therefore, if ring 
dye laser 18 excited by argon ion laser is added to the apparatus of FIG. 
1 as shown in FIG. and the light from this ring dye laser 18 is irradiated 
to the dye laser 6 as seed light, the light from the ring dye laser 18 is 
amplified. Because YAG laser 5 is oscillating pulse-wise and is pumping 
the dye laser 6, the light amplified by the dye laser 6 is pulsed. This 
light is then converted to the left-handed circularly-polarized light by 
the circularly-polarized light generator 9. By this light, palladium 107 
can be selectively excited, and it is possible to efficiently separate 
palladium 105 from palladium 107. 
In the apparatus of FIG. 1, 2-wavelength 3-step system is adopted where the 
laser light with the second wavelength also serves the laser light for 
ionization. In some cases, ionization efficiency is decreased in this 
system. In such case, the third laser 19 for ionization should be added as 
shown in FIG. 7. By pumping the third laser 19 through the pulse operating 
laser 5 for pumping, the laser light for ionization can be generated, in 
wavelength and intensity, independently from and in synchronization with 
the laser light with the second wavelength. In this case, the laser light 
for ionization does not have to be the circularly-polarized light. Before 
the laser light with the second wavelength and the laser light for 
ionization enter the circularly-polarized light generator 11, the 
linearily-polarized lights of both lights are bonded together with the 
polarization plane varying by 90.degree. from each other by polarization 
beam coupler 20. Then, only the laser light with the second wavelength 
should be converted to the circularly-polarized light by passing through 
the circularly-polarized light generator 11. (1/4 wave plate of the 
circularly-polarized light generator 11 can convert only one of two 
wavelengths to the circularly-polarized light if two wavelengths are 
separated from each other.) In the figure, M.sub.18 is a fixed 
semi-transparent mirror, and M.sub.20 and M.sub.21 are the fixed 
reflecting mirrors. 
In the above, description has been given on the laser isotope separation 
apparatus according to this invention where the isotopes of palladium with 
nuclear spin not at zero are separated from the other isotopes, whereas 
this apparatus can be applied for the separation of the isotopes of the 
elements other than palladium with nuclear spin not at zero such as Mg, 
Ca, Zn, Sr, Cd, Ba, Hg, Yb, C, Si, Ge, Sn, Sm, Pb, Pu, etc. from the other 
isotopes. 
As described already, the laser isotope separation apparatus based on this 
invention utilizes the circularly-polarized light to selectively excite 
the isotopes to the first or the second excited level. Because there are 
the isotopes absorbing the circularly-polarized light and excited 
according to the annular momentum selection rule of quantum mechanics and 
those isotopes not absorbing and not excited depending upon the mass 
number of the isotopes, it is possible by the present invention to 
selectively ionize and separate the isotopes, which have the isotope shift 
equal to or smaller than the line width of the laser light and which have 
been difficult to separate in the past. The selective excitation and 
ionization according to the angular momentum selection rule are carried 
out by 3-step excitation, and 3 excited wavelengths are generated by two 
or more lasers pumped by the light from the same pumping light source. 
Also, the optical paths for these lights are made to be in same length by 
optical delay circuit. Thus, it is possible to perform selective 
excitation and ionization even when laser light source is pulse operating. 
Because the laser light of the first circularly-polarized light and the 
laser light of the second circularly-polarized light are irradiated from 
opposite directions on the substance containing the gasified isotopes, 
Doppler effects of the moving atoms can be canceled each other 
vector-wise, and the separation efficiency is increased. There is no need 
to optically overlap both lights in advance, and the laser light 
circularly-polarized with high efficiency can be utilized. 
The present invention is particularly effective for the separation and 
purification of palladium. In the practical application, the only 
radioactive palladium isotope with mass number 107 can be separated from 
the palladium contained in the insoluble residue from the reprocessing of 
spent fuel, and the non-radioactive palladium with even mass number, which 
has been handled as waste in the past, can be separated and utilized as 
precious metal.