CA1107232A - Use of autoionization transitions in isotopically selective photoexcitation - Google Patents
Use of autoionization transitions in isotopically selective photoexcitationInfo
- Publication number
- CA1107232A CA1107232A CA318,103A CA318103A CA1107232A CA 1107232 A CA1107232 A CA 1107232A CA 318103 A CA318103 A CA 318103A CA 1107232 A CA1107232 A CA 1107232A
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- particles
- energy
- ionization
- excited
- isotope
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D59/00—Separation of different isotopes of the same chemical element
- B01D59/34—Separation by photochemical methods
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Abstract
A B S T R A C T
Apparatus and process for isotopically selective multi-step photoionization in which the final or ionizing step is tuned to produce a specific transition to an excited state above ionization for which the ionization cross-section is substantially greater than for ionization transitions in general. The autoionization transition to an ionized state is typically made from a highly excited bound state which is reached in one or two isotopically selective energy jumps from the ground state or other low-lying levels. The isotope shift for the ionization transition is typically small compared to the bandwidth of the ionization transition and relatively broad band photoionization radiation covering the entire absorption line can be employed. Broad band radiation is more economic and is preferable for use whereever possible.
A technique is also shown for identifying the ionization transitions of augmented cross-section.
Apparatus and process for isotopically selective multi-step photoionization in which the final or ionizing step is tuned to produce a specific transition to an excited state above ionization for which the ionization cross-section is substantially greater than for ionization transitions in general. The autoionization transition to an ionized state is typically made from a highly excited bound state which is reached in one or two isotopically selective energy jumps from the ground state or other low-lying levels. The isotope shift for the ionization transition is typically small compared to the bandwidth of the ionization transition and relatively broad band photoionization radiation covering the entire absorption line can be employed. Broad band radiation is more economic and is preferable for use whereever possible.
A technique is also shown for identifying the ionization transitions of augmented cross-section.
Description
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Il FIELD OF THE INVENTION
1 j The present invention relates to photoionization methods
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Il FIELD OF THE INVENTION
1 j The present invention relates to photoionization methods
2 !!and apparatus and in particular to a photoionization transi-
3 ''tion of increased cross-section.
. I ' ' .
I BACKGROUND OF THE INVENTION . -. i''
. I ' ' .
I BACKGROUND OF THE INVENTION . -. i''
4 I By way of background, reference is made to the prior disclosures of techniques for isotopically selective photo-6 ¦l excitation and ionization of uranium as represented in 7 commonly assigned U. S. Patents 3,772,519, 3,944,947 and 8 ¦Belgian Patent 807.118, and additionally to German patent 9 publication 2,312,194. The techniques there presented ¦linclude photoexcitation in one or more steps along witk 11 jphotoionization which may be via an autoionization transi-12 ¦I tion. As known in the art, autoionization is a transition 13 llwhich excites particles to an energy level above ionization 14 ¦ifrom which it degenerates into an ion and a released elec-¦tron. In application to uranium enrichment the suggested 16 lutilization of an autoionization transition would have the 17 ladvantage of increasing the normally low cross-section for 1~ lionization, thereby reducing the requirements on the ionization 19 ,excitation source or laser.
i Until the present invention, however, efficient utiliza-21 ''tion of autoionization in an isotope separation technique 22 i~has not been identified.
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1 ~ The present invention contemplates an improvement in 2 , the isotopi'cally selective ionization process making use of 3 I characteristics of the autoionization transition. The 4 ll improvement more efficiently achieves the autoionization !, advantages of ionization from an excited state through a 6 ,j transition having an enhanced cross-section as opposed to 7 I ionization transitions in general.
8 ¦ The improvement of the present invention is made possible 9 ! by the development of a technique for detecting the relative' 1I cross-sections for photoionization transitions from a selected ll , excited state. The technique employs frequency scanning of 12 ~, time-sequenced lasers in order to monitor ionization over a 13 ¦, spectrum of absorption lines to the continuum from a selected 14 excited state. In this manner transitions to the continuum '! of increased cross-sections, autoionization transitions as 16 , used herein, are identified.
17 i, The process of photoionization with enhanced ionization 18 i cross-sections as thus defined h,as led to a system for ~ producing a photoionization transition with radiation of ,i identified properties such as spectral width and frequency 21 !~ at which the enhanced cross-sections occur and may be effi-22 !I ciently utilized. The process typically employs two, three 23 , or four energy steps, the first being isotopically selective 24 and the last being the autoionization step. This latter ! step proceeds from a highly excited state to the ionization t ' 11~7Z32 continuum and typically to a lcvel within an identified energy range above the ioniæation level. secause the auto-ionized bound state has been found to be very short lived~ the absorption line for the photoionization process with enhanced cross-section tends to be relatively broad in frequency as compared to absorption lines between excited states. The isotope shift between the desired and undesired isotopes is a fraction of this bandwidth.
As a result, the preferred embodiment employs moderately broad banded, less intense photoionization radiation, there being no reason to limit bandwidth for isotopic selectivity. The invention thus achieves the advantage of more efficient generation of laser ionization radiation and reduces the chance for non-selectivity from the use of intense ionizing radiation.
The ionized particles may then be separated for collection on pre-determined surfaces in response to forces applied to them. These forces are preferably produced by crossed magnetic and electric fields~ in the nature ~ of an MHD accelerator, to direct the ionized particles away from an original - flow direction.
According to a first aspect of the present invention, there is provided a method for ionizing particles of a selected isotope type in an environment of plural isotope types comprising the steps of: selectively exciting the particles of said selected isotope type in one or more energy steps to an excited state; applying electromagnetic radiation of a predeter-mined spectral width and frequency and tuned to an absorption line for the particles in said excited state which both excites the particles from the excited state to a state within a predetermined energy range above the ioniz-ation level and excites the particles with a predetermined frequency which is absorbed more strongly than absorption at immediately adjacent frequencies by at least an order of magnitude.
According to a second aspect of the present invention, there is provided a process for identifying ionization peaks comprising: exciting particles to an excited level below the ionization level therefor; applying to said particles photons of energy sufficient to ionize the excited particles;
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varying the energy of the applied radiation over a range of energies to excite said particles to above ionization; and detecting the absorption rate for the applied pllotons as a funetion of energy.
According to a third aspect of the present invention, there is provided apparatus for ionizing particles of a selected isotope type in an environment of plural isotope types comprising: means for selectively exciting the particles of said selected isotope type in one or more energy steps to an excited state; means for applying electromagnetic radiation of a predetermined spectral width and frequency and tuned to an absorption line for the particles in said excited state which both excites the particles from the excited state to a state within a predetermined energy range above the ionization level and excites the particles with a predetermined frequency which is absorbed more strongly than absorption at immediately adjacent frequencies by at least an order of magnitude.
According to a fourth aspect of the present invention, there is provided apparatus for identifying ionization peaks comprising: means for exciting particles to an excited level below the ionization level therefor;
means for applying to said particles photons of energy sufficient to ionize the excited particles; means for varying the energy of the applied radiation over a range of energies to excite said particles to above ionization; and means for detecting the absorption rate for the applied photons as a function of energy.
BRIEF DESCRIPTION OF THE DRAWING
. .
These and other features of the present invention are more fully set forth below in the detailed description of the preferred embodiment, presented for purposes of illustration -4a-~10723~ ` I
l and not by way of limit~tion, and in the accompanying drawing, 2 of which: ¦
3 Fig. 1 is an energy step diagram useful in illustrating 4 the methcid of the present invention;
¦ Fig. 2 is a view of simplified apparatuis for use in 6 Ipracticing the present invention; -7 1 Fig. 3 is an interior view of the apparatus of Fig. 2, 8 ?1 and ~ !I Fig. 4 is a view of laser apparatus for generating the ¦,'excitationjand ionization radiation for use in the present invention, and further includes apparatus for use in identify-12 ¦jing autoionization transitions, photoionization transitions 13 with improved cross-sections.
:!
- DETAILED DESCRIPTION OF THE INVENTION
14 ' The present invention contemplates a technique for ,producing isotopically selective multistep photoionization 16 li with improved efficiency in the use of autoionization transitions 17 j! ~ enhanced cross-section. Isotopic selectivity is achieved 18 ~,with appropriate tuning of transitions between bound states, 19 I while the transition to the ionization continuum is achieved 1 in a manner which makes optimal use of the characteristic of 21 ,~he enhanced cross-section for autoionization.
22 A transition from a highly excited, but bound state to 23 the ionized states is defined as t~e iOniZatiOD transition ,.
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1 ¦! This transition typically employs radiation energy just 2 !I sufficient to reach the ionized level, more being unnecessary.
3 i` Such an ionization transition is normally much less probable 4 11 of occurrence, of low cross-section, and is thus of low i efficiency. Apparatus has now been devised which confirms - --6 l the existence and identifies the location of numerous peaks - 7 ¦ in the ionizatiQn cross-section over a range of energies 8 ~ well above the ionization level. The peaks exhibit very 9 significant enhancement in the cross-section over ionization transitions in general. These peaks make it advantageous to 11 ionize excited particles by excitation to one of the elevated 12 levels above ionization on a transition of enhanced cross-13 I section.
14 The upper state reached by the enhanced cross-section, , autoionization transition is sometimes interpreted as con-16 1l sisting of an electronically excited ion core to which is 17 ll attached a loosely bound electron wherein the whole ensemble contains sufficient energy so that when the excited ion core 19 ¦ decays to the ion ground state, the accompanying energy I release is sufficient to eject the electron. This results 21 ¦ in ionization. These decays typically occur in times comparable 22 with 10 11 seconds, and will in accordance with uncertainty 23 principles of physics exhibit spectral line widths on the 24 ! order of 100 GHZ. These widths are large relative to any 1! Doppler or isotope shifts, but sufficiently narrow to exhibit 26 1ll significant cross-section peaks.
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' 1 , This situation is advantageously e~ploited by the use 2 of moderately broad banded radiation tuned for an auto-3 I ionization transition of enhanced cross-section. The broad 4 l,banded radiation is more efficiently generated, but be-cause li of the large width of the peak is kept within the range of 6 jenhanced cross-section. With autoionization radiation 7 l spread over this relatively wide peak, a ~reater ionization 8 cross-section is achieved and less intense radiation is g required. With less intense radiation, the probability of ¦non-selective ionization by multiphoton absorption from the 11 ground state is reduced.
12 The invention is more particularly illustrated with 13 reference to Fig. 1 of the drawing in which an energy step 14 Idiagram is illustrated for the preferred form of isotopically ¦selective photoionization in accordance with the present 16 invention. As shown there, a first transition 12 is pro-17 jduced from the ground state 14, or possibly low-lyin~ thermally 18 excited states, to a first excited state 16 in response to 19 ~ isotopically selective laser radiation from a first laser.
¦Excitation from state 16 occurs in a second energy step 18 21 ¦to a second excited state 20. The radiation producing the 22 transition 18 may or may not be tuned for isotopic selectivity -23 as desired. From the second excited state 20 a final photo-24 ionization transition 22 is produced to a third excited Ii. state 24 which is above the ionization level 26. From the 26 lexcited state 24, the excited atom automatically decays into 27 an ion 28 and a released electron 30. Instead of two steps 12 1 !
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1 1 and 14, one, three or more steps may be used. The intermediate - - 2 energy levels may be selected from known tabulations such ---3 as: LA-4501, Present Status of the_Analyses of the First and 4 li Second-Spectra of Uranium (U I and U II) as Derived from -, i Measurement-s of Optical Spectra, issued October l-g71.
6 ¦ The ionization level 26 for uranium in elemental form ~ I as a vapor of atomic particles is approximately 6.18 ev or 8 just below 50,000.0 wave numbers. The frequency for the - 9 transition 22 is typi.cally in the range of 5,800 - 6,000 angstroms (16,667 - 17,241 wave numbers) and the third 11 excited state 24 is typically in the range of 50 to 300 wave ~12 numbers above the ionization level 26. In that form, each ; 13 of the transitions 12, 18 and 22 is approximately 1l3 of the 14 total ionization potential for the uranium atom, as a I typical relationship. The radiations for the transitions 12, 16 ¦, 18 and 22 are typically applied simultaneously but may be 17 ll slightly staggered in the time sequence of occurrence of 18 ¦ each transition as described below.
19 Lasers employed for each of the transitions 12, 18 and 22 are typically tunable dye lasers, one example of 21 which is the Dial-A-Line laser, where Dial-A-Line is a 22 registered trademark of the Avco Everett Research Laboratory, 23 Everett, Massachusetts. The final laser for the transition 22 24 ¦ is preferably tuned to encompass a broader bandwidth than ¦Ithe bandwidth used for transitions between excited states.
26 In particular, due to the uncertainty principle discussed 27 Il above, the radiation absorption bandwidth for photoionization ., jj .
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11(~7Z32 is on the order of 0.5 angstroms or approximately 1.5 wave numbers at the typical frequencies used. This is over an order of magnitude wider than typically encountered differences between absorption lines for different isotopes at the same transition.
As a result, isotopic selectivity must be produced by selective excitation in lower transitions such as transitions 12 and 18. Th~s permits th'e translt~on 22 to be achie~ed with a broad band laser approximately 1/2 angstroms in breadth which is capable of more efficient lasing by utilizing less fre-quency limiting in the dye laser as opposed to other transi-tions.
In accordance with the technique for identifying specific ionization transitions for excitation to the third excited state 24, as described below, a transition may be selected having an improvement in cross-section of one, two or more orders of magnitude greatly overcoming the previous difficulty - encountered in achieving ionization due to the relatively small cross-section.
With respect to Figures 2 and 3, apparatus is shown by which the present invention may be practiced, in the main, the apparatus conforms to that shown in United States Patent 3,939,354, commonly assigned.
The apparatus includes a chamber 40 which is surrounded by a set of coils 42 carrying a current for producing a mag-netic field 44 axially within the chamber 40.
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, ji ! ` -1 ~I Radiation is applied in a laser beam 46 through a 2 ¦ window-48-on an extension pipe 50 to the interior of the ~ .
3 .I chamber 40, and in particular between the plates of an - 4 ¦¦ accelerator 52 functioning as an ion extractor. The accel- .
¦ erator 52 is placed above a vapor source 54 which is typicalLy. ..
- 6 --- -an--electron beam evaporator- f~ uranium contained within a ~ -. ..
7 reservoir.. ~- The evapQrator- provides a radially expanding .
. . 8 vapor flow into the accelerator 52 over the axial length of 9 ¦ the chamber 40. .
¦ This is more clearly illustrated in Fig. 3, where the .
11 vapor source is shown to include a crucible 56 having a 12 supply 58 of uranium which is evaporated by the impact of a .
13 linear magnetically focused electron beam 60 to expand into 14 ! the region of the accelerator 52. The accelerator 52 is defined as a set of chambers 62 bounded by collection plates.64 i 16 and having therein a central anode electrode ~6. Between 17 I the anode 66 and collection plates 64 within the chamber 62 -18 are regions 68 of laser illumination. The pattern of regions 68 ].9 may be achieved by producing multiple reflections o the beam 46 and/or by optical beam shaping to the approximate 21 desired cross-section. Other techniques may also be employed 22 for this purpose.-23 The laser illuminated regions 68 are illuminated in 24 pulses which are directly followed by a pulse of electric I field between the anodes 66 and, typically, the collection 26 plates 64 or plasma environment generally through a pulsed 27 ¦ivoltage source 70.
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1 ll The beam 46 is typically a composite beam containing --2- the three colors for the transitions 12, 18 and 22 and pro-3 1 vided as illustrated in Fig. 4. Typically, a set of three ~4 .~ i dye lasers 80, 82 and 84 are provided with their output ~ -
i Until the present invention, however, efficient utiliza-21 ''tion of autoionization in an isotope separation technique 22 i~has not been identified.
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.' ' ` ,,'' BRIEF S~RY OF THE INVENTION ' , .. . .
1 ~ The present invention contemplates an improvement in 2 , the isotopi'cally selective ionization process making use of 3 I characteristics of the autoionization transition. The 4 ll improvement more efficiently achieves the autoionization !, advantages of ionization from an excited state through a 6 ,j transition having an enhanced cross-section as opposed to 7 I ionization transitions in general.
8 ¦ The improvement of the present invention is made possible 9 ! by the development of a technique for detecting the relative' 1I cross-sections for photoionization transitions from a selected ll , excited state. The technique employs frequency scanning of 12 ~, time-sequenced lasers in order to monitor ionization over a 13 ¦, spectrum of absorption lines to the continuum from a selected 14 excited state. In this manner transitions to the continuum '! of increased cross-sections, autoionization transitions as 16 , used herein, are identified.
17 i, The process of photoionization with enhanced ionization 18 i cross-sections as thus defined h,as led to a system for ~ producing a photoionization transition with radiation of ,i identified properties such as spectral width and frequency 21 !~ at which the enhanced cross-sections occur and may be effi-22 !I ciently utilized. The process typically employs two, three 23 , or four energy steps, the first being isotopically selective 24 and the last being the autoionization step. This latter ! step proceeds from a highly excited state to the ionization t ' 11~7Z32 continuum and typically to a lcvel within an identified energy range above the ioniæation level. secause the auto-ionized bound state has been found to be very short lived~ the absorption line for the photoionization process with enhanced cross-section tends to be relatively broad in frequency as compared to absorption lines between excited states. The isotope shift between the desired and undesired isotopes is a fraction of this bandwidth.
As a result, the preferred embodiment employs moderately broad banded, less intense photoionization radiation, there being no reason to limit bandwidth for isotopic selectivity. The invention thus achieves the advantage of more efficient generation of laser ionization radiation and reduces the chance for non-selectivity from the use of intense ionizing radiation.
The ionized particles may then be separated for collection on pre-determined surfaces in response to forces applied to them. These forces are preferably produced by crossed magnetic and electric fields~ in the nature ~ of an MHD accelerator, to direct the ionized particles away from an original - flow direction.
According to a first aspect of the present invention, there is provided a method for ionizing particles of a selected isotope type in an environment of plural isotope types comprising the steps of: selectively exciting the particles of said selected isotope type in one or more energy steps to an excited state; applying electromagnetic radiation of a predeter-mined spectral width and frequency and tuned to an absorption line for the particles in said excited state which both excites the particles from the excited state to a state within a predetermined energy range above the ioniz-ation level and excites the particles with a predetermined frequency which is absorbed more strongly than absorption at immediately adjacent frequencies by at least an order of magnitude.
According to a second aspect of the present invention, there is provided a process for identifying ionization peaks comprising: exciting particles to an excited level below the ionization level therefor; applying to said particles photons of energy sufficient to ionize the excited particles;
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varying the energy of the applied radiation over a range of energies to excite said particles to above ionization; and detecting the absorption rate for the applied pllotons as a funetion of energy.
According to a third aspect of the present invention, there is provided apparatus for ionizing particles of a selected isotope type in an environment of plural isotope types comprising: means for selectively exciting the particles of said selected isotope type in one or more energy steps to an excited state; means for applying electromagnetic radiation of a predetermined spectral width and frequency and tuned to an absorption line for the particles in said excited state which both excites the particles from the excited state to a state within a predetermined energy range above the ionization level and excites the particles with a predetermined frequency which is absorbed more strongly than absorption at immediately adjacent frequencies by at least an order of magnitude.
According to a fourth aspect of the present invention, there is provided apparatus for identifying ionization peaks comprising: means for exciting particles to an excited level below the ionization level therefor;
means for applying to said particles photons of energy sufficient to ionize the excited particles; means for varying the energy of the applied radiation over a range of energies to excite said particles to above ionization; and means for detecting the absorption rate for the applied photons as a function of energy.
BRIEF DESCRIPTION OF THE DRAWING
. .
These and other features of the present invention are more fully set forth below in the detailed description of the preferred embodiment, presented for purposes of illustration -4a-~10723~ ` I
l and not by way of limit~tion, and in the accompanying drawing, 2 of which: ¦
3 Fig. 1 is an energy step diagram useful in illustrating 4 the methcid of the present invention;
¦ Fig. 2 is a view of simplified apparatuis for use in 6 Ipracticing the present invention; -7 1 Fig. 3 is an interior view of the apparatus of Fig. 2, 8 ?1 and ~ !I Fig. 4 is a view of laser apparatus for generating the ¦,'excitationjand ionization radiation for use in the present invention, and further includes apparatus for use in identify-12 ¦jing autoionization transitions, photoionization transitions 13 with improved cross-sections.
:!
- DETAILED DESCRIPTION OF THE INVENTION
14 ' The present invention contemplates a technique for ,producing isotopically selective multistep photoionization 16 li with improved efficiency in the use of autoionization transitions 17 j! ~ enhanced cross-section. Isotopic selectivity is achieved 18 ~,with appropriate tuning of transitions between bound states, 19 I while the transition to the ionization continuum is achieved 1 in a manner which makes optimal use of the characteristic of 21 ,~he enhanced cross-section for autoionization.
22 A transition from a highly excited, but bound state to 23 the ionized states is defined as t~e iOniZatiOD transition ,.
- , . .
.
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Il ( 1107Z32( .. 11 '`I ' ` .
1 ¦! This transition typically employs radiation energy just 2 !I sufficient to reach the ionized level, more being unnecessary.
3 i` Such an ionization transition is normally much less probable 4 11 of occurrence, of low cross-section, and is thus of low i efficiency. Apparatus has now been devised which confirms - --6 l the existence and identifies the location of numerous peaks - 7 ¦ in the ionizatiQn cross-section over a range of energies 8 ~ well above the ionization level. The peaks exhibit very 9 significant enhancement in the cross-section over ionization transitions in general. These peaks make it advantageous to 11 ionize excited particles by excitation to one of the elevated 12 levels above ionization on a transition of enhanced cross-13 I section.
14 The upper state reached by the enhanced cross-section, , autoionization transition is sometimes interpreted as con-16 1l sisting of an electronically excited ion core to which is 17 ll attached a loosely bound electron wherein the whole ensemble contains sufficient energy so that when the excited ion core 19 ¦ decays to the ion ground state, the accompanying energy I release is sufficient to eject the electron. This results 21 ¦ in ionization. These decays typically occur in times comparable 22 with 10 11 seconds, and will in accordance with uncertainty 23 principles of physics exhibit spectral line widths on the 24 ! order of 100 GHZ. These widths are large relative to any 1! Doppler or isotope shifts, but sufficiently narrow to exhibit 26 1ll significant cross-section peaks.
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' 1 , This situation is advantageously e~ploited by the use 2 of moderately broad banded radiation tuned for an auto-3 I ionization transition of enhanced cross-section. The broad 4 l,banded radiation is more efficiently generated, but be-cause li of the large width of the peak is kept within the range of 6 jenhanced cross-section. With autoionization radiation 7 l spread over this relatively wide peak, a ~reater ionization 8 cross-section is achieved and less intense radiation is g required. With less intense radiation, the probability of ¦non-selective ionization by multiphoton absorption from the 11 ground state is reduced.
12 The invention is more particularly illustrated with 13 reference to Fig. 1 of the drawing in which an energy step 14 Idiagram is illustrated for the preferred form of isotopically ¦selective photoionization in accordance with the present 16 invention. As shown there, a first transition 12 is pro-17 jduced from the ground state 14, or possibly low-lyin~ thermally 18 excited states, to a first excited state 16 in response to 19 ~ isotopically selective laser radiation from a first laser.
¦Excitation from state 16 occurs in a second energy step 18 21 ¦to a second excited state 20. The radiation producing the 22 transition 18 may or may not be tuned for isotopic selectivity -23 as desired. From the second excited state 20 a final photo-24 ionization transition 22 is produced to a third excited Ii. state 24 which is above the ionization level 26. From the 26 lexcited state 24, the excited atom automatically decays into 27 an ion 28 and a released electron 30. Instead of two steps 12 1 !
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1 1 and 14, one, three or more steps may be used. The intermediate - - 2 energy levels may be selected from known tabulations such ---3 as: LA-4501, Present Status of the_Analyses of the First and 4 li Second-Spectra of Uranium (U I and U II) as Derived from -, i Measurement-s of Optical Spectra, issued October l-g71.
6 ¦ The ionization level 26 for uranium in elemental form ~ I as a vapor of atomic particles is approximately 6.18 ev or 8 just below 50,000.0 wave numbers. The frequency for the - 9 transition 22 is typi.cally in the range of 5,800 - 6,000 angstroms (16,667 - 17,241 wave numbers) and the third 11 excited state 24 is typically in the range of 50 to 300 wave ~12 numbers above the ionization level 26. In that form, each ; 13 of the transitions 12, 18 and 22 is approximately 1l3 of the 14 total ionization potential for the uranium atom, as a I typical relationship. The radiations for the transitions 12, 16 ¦, 18 and 22 are typically applied simultaneously but may be 17 ll slightly staggered in the time sequence of occurrence of 18 ¦ each transition as described below.
19 Lasers employed for each of the transitions 12, 18 and 22 are typically tunable dye lasers, one example of 21 which is the Dial-A-Line laser, where Dial-A-Line is a 22 registered trademark of the Avco Everett Research Laboratory, 23 Everett, Massachusetts. The final laser for the transition 22 24 ¦ is preferably tuned to encompass a broader bandwidth than ¦Ithe bandwidth used for transitions between excited states.
26 In particular, due to the uncertainty principle discussed 27 Il above, the radiation absorption bandwidth for photoionization ., jj .
, !
11(~7Z32 is on the order of 0.5 angstroms or approximately 1.5 wave numbers at the typical frequencies used. This is over an order of magnitude wider than typically encountered differences between absorption lines for different isotopes at the same transition.
As a result, isotopic selectivity must be produced by selective excitation in lower transitions such as transitions 12 and 18. Th~s permits th'e translt~on 22 to be achie~ed with a broad band laser approximately 1/2 angstroms in breadth which is capable of more efficient lasing by utilizing less fre-quency limiting in the dye laser as opposed to other transi-tions.
In accordance with the technique for identifying specific ionization transitions for excitation to the third excited state 24, as described below, a transition may be selected having an improvement in cross-section of one, two or more orders of magnitude greatly overcoming the previous difficulty - encountered in achieving ionization due to the relatively small cross-section.
With respect to Figures 2 and 3, apparatus is shown by which the present invention may be practiced, in the main, the apparatus conforms to that shown in United States Patent 3,939,354, commonly assigned.
The apparatus includes a chamber 40 which is surrounded by a set of coils 42 carrying a current for producing a mag-netic field 44 axially within the chamber 40.
X
.
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, ji ! ` -1 ~I Radiation is applied in a laser beam 46 through a 2 ¦ window-48-on an extension pipe 50 to the interior of the ~ .
3 .I chamber 40, and in particular between the plates of an - 4 ¦¦ accelerator 52 functioning as an ion extractor. The accel- .
¦ erator 52 is placed above a vapor source 54 which is typicalLy. ..
- 6 --- -an--electron beam evaporator- f~ uranium contained within a ~ -. ..
7 reservoir.. ~- The evapQrator- provides a radially expanding .
. . 8 vapor flow into the accelerator 52 over the axial length of 9 ¦ the chamber 40. .
¦ This is more clearly illustrated in Fig. 3, where the .
11 vapor source is shown to include a crucible 56 having a 12 supply 58 of uranium which is evaporated by the impact of a .
13 linear magnetically focused electron beam 60 to expand into 14 ! the region of the accelerator 52. The accelerator 52 is defined as a set of chambers 62 bounded by collection plates.64 i 16 and having therein a central anode electrode ~6. Between 17 I the anode 66 and collection plates 64 within the chamber 62 -18 are regions 68 of laser illumination. The pattern of regions 68 ].9 may be achieved by producing multiple reflections o the beam 46 and/or by optical beam shaping to the approximate 21 desired cross-section. Other techniques may also be employed 22 for this purpose.-23 The laser illuminated regions 68 are illuminated in 24 pulses which are directly followed by a pulse of electric I field between the anodes 66 and, typically, the collection 26 plates 64 or plasma environment generally through a pulsed 27 ¦ivoltage source 70.
. . I
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1 ll The beam 46 is typically a composite beam containing --2- the three colors for the transitions 12, 18 and 22 and pro-3 1 vided as illustrated in Fig. 4. Typically, a set of three ~4 .~ i dye lasers 80, 82 and 84 are provided with their output ~ -
5 - ¦I heams applied to a beam combiner 86 which may be a prism or _
-6 .~ dichroic element. The beam combiner 86 provides the-composite.~.
7 . ~ ~output.beam 46 applied to the chamber 40 in Fig. 2.
8 , The lasers 80, 82 and 84 are controlled by a timer 88
9 Itypically to produce coincident pulses of at least a substantial Ifraction of a microsecond duration for application as the 11 ¦composite beam 46 to the chambers.
12 ¦ For the purposes of identifying the frequencies of the 13 desired transitions 22 of augmented cross-section, a motor 90 14 ¦may be provided to control the tuning of an element,such as ilgrating 92, within a cavity 94 defined by mirrors 95 and 97 16 ,of the laser 84 to thereby scan the frequency of an output 17 Ibeam 96 from the laser 84. By thus slowly varying the 18 ~radiation frequency, different amounts of ionization are 19 produced within the chamber 62 and in particular the region 68 of application of the laser beam. The rate of ionization at 21 any isotope number may then be detected by the use of a 22 conventional mass spectrometer 98 having a probe 100 within 23 the chamber 40 to detect ions resulting from photoionization 24 l and/or acceleration as is known in the art.
ll For the purposes of production level laser enrichment, 26 ' the lasers ~0, 82 and 84 are preferably augmented with i, . .
27 ' several stages of amplification. And in addition, to achieve ' -11- I
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., , . ~:
1()7232 1!
l l'desired pulse repetition rates, it may be desired to employ -2 a multiplicity of laser'systems for each laser 80, 82 and 84, 3 'each fired sequen~ially and pulse-combined to produce an 4 I~augmented pulse rate through the mechanism of rotating ,optical elements as is described in United States Patent 6 l~3,924,937. - ,, 7 ~, Having desc~ibed ab~ve the details of the method and 8 ,¦apparatus of the present in~ention, it will occur to those 9 ¦skilled in the art that various modifications,and alterations ImaY be made to the invention without departing from its 11 ¦scope. It is accordingly intended to define the invention 12 ~only as indicated in the following claims., .
.. . i ', .
'. .
I .
12 ¦ For the purposes of identifying the frequencies of the 13 desired transitions 22 of augmented cross-section, a motor 90 14 ¦may be provided to control the tuning of an element,such as ilgrating 92, within a cavity 94 defined by mirrors 95 and 97 16 ,of the laser 84 to thereby scan the frequency of an output 17 Ibeam 96 from the laser 84. By thus slowly varying the 18 ~radiation frequency, different amounts of ionization are 19 produced within the chamber 62 and in particular the region 68 of application of the laser beam. The rate of ionization at 21 any isotope number may then be detected by the use of a 22 conventional mass spectrometer 98 having a probe 100 within 23 the chamber 40 to detect ions resulting from photoionization 24 l and/or acceleration as is known in the art.
ll For the purposes of production level laser enrichment, 26 ' the lasers ~0, 82 and 84 are preferably augmented with i, . .
27 ' several stages of amplification. And in addition, to achieve ' -11- I
1,, 1 ., ' . . , ! I
., , . ~:
1()7232 1!
l l'desired pulse repetition rates, it may be desired to employ -2 a multiplicity of laser'systems for each laser 80, 82 and 84, 3 'each fired sequen~ially and pulse-combined to produce an 4 I~augmented pulse rate through the mechanism of rotating ,optical elements as is described in United States Patent 6 l~3,924,937. - ,, 7 ~, Having desc~ibed ab~ve the details of the method and 8 ,¦apparatus of the present in~ention, it will occur to those 9 ¦skilled in the art that various modifications,and alterations ImaY be made to the invention without departing from its 11 ¦scope. It is accordingly intended to define the invention 12 ~only as indicated in the following claims., .
.. . i ', .
'. .
I .
Claims (40)
1. A method for ionizing particles of a selected isotope type in an environment of plural isotope types comprising the steps of:
selectively exciting the particles of said selected isotope type in one or more energy steps to an excited state;
applying electromagnetic radiation of a predetermined spectral width and frequency and tuned to an absorption line for the particles in said excited state which both excites the particles from the excited state to a state within a predetermined energy range above the ionization level and excites the particles with a predetermined frequency which is absorbed more strongly than absorption at immediately adjacent frequencies by at least an order of magnitude.
selectively exciting the particles of said selected isotope type in one or more energy steps to an excited state;
applying electromagnetic radiation of a predetermined spectral width and frequency and tuned to an absorption line for the particles in said excited state which both excites the particles from the excited state to a state within a predetermined energy range above the ionization level and excites the particles with a predetermined frequency which is absorbed more strongly than absorption at immediately adjacent frequencies by at least an order of magnitude.
2. The method of claim 1 wherein the step of selectively exciting includes the step of exciting in at least three energy steps to said excited state.
3. The method of claim 1 wherein said electromagnetic radiation is tuned within a range of approximately 5,800 to 6,000 angstroms.
4. The method of claim 1 wherein said predetermined energy range is approximately 50 to 300 cm-1 above the ionization level.
5. The method of claim 1 wherein:
the step of selectively exciting includes applying at least one frequency of narrow band radiation tuned selec-tively for an absorption line of said selected isotope; and said predetermined spectral width is substantially greater than the spectral width of said narrow band radia-tion.
the step of selectively exciting includes applying at least one frequency of narrow band radiation tuned selec-tively for an absorption line of said selected isotope; and said predetermined spectral width is substantially greater than the spectral width of said narrow band radia-tion.
6. The method of claim 5 wherein said predetermined spectral width is on the order of magnitude of approximately 0.5 angstroms.
7. The method of claim 1 further including the steps of:
varying the frequency of said electromagnetic radiation over a range including said predetermined frequency and frequencies either side thereof which are less strongly absorbed by particles in said excited state; and detecting the degree of absorption by the particles in said excited state over the range of frequency variation.
varying the frequency of said electromagnetic radiation over a range including said predetermined frequency and frequencies either side thereof which are less strongly absorbed by particles in said excited state; and detecting the degree of absorption by the particles in said excited state over the range of frequency variation.
8. The method of claim 1 wherein said particles include uranium atoms.
9. The method of claim 8 wherein the selected isotope is the U-235 isotope.
10. The method of claim 1 further including the step of separating the particles excited to said state above the ionization level from said environment.
11. The method of claim 10 wherein said separating step includes applying crossed electric and magnetic fields.
12. The method of claim 1 wherein said electromagnetic radiation includes laser radiation.
13. The method of claim 1 wherein said step of selectively exciting includes exciting in two energy steps.
14. The method of claim 1 wherein the selectively exciting step includes the step of applying laser radiation.
15. The method of claim 1 wherein the step of applying electromagnetic radiation includes producing autoionization of particles in said excited state.
16. The method of claim 15 wherein the autoionization step is generally produced without isotopic selectivity with respect to particles in said excited state.
17. A method for ionizing particles of a selected isotope type in an environment of plural isotope types including the steps of:
selectively exciting particles of said selected isotope to a level below ionization therefor;
applying to the excited particles photons of energy sufficient to further excite the particles to a bound energy state substantially above the ionized level, and which bound state has a short but finite life time, substantially less than a nanosecond before degenerating to an ion and released electron;
the energy of said photons spanning an energy range sub-stantially greater than the isotope shift for isotopes of said plural isotope types including the selected isotope type and centered on an absorption peak for said particles which is greater by at least an order of magnitude than the absorption for immediately neighboring photon energies.
selectively exciting particles of said selected isotope to a level below ionization therefor;
applying to the excited particles photons of energy sufficient to further excite the particles to a bound energy state substantially above the ionized level, and which bound state has a short but finite life time, substantially less than a nanosecond before degenerating to an ion and released electron;
the energy of said photons spanning an energy range sub-stantially greater than the isotope shift for isotopes of said plural isotope types including the selected isotope type and centered on an absorption peak for said particles which is greater by at least an order of magnitude than the absorption for immediately neighboring photon energies.
18. The method of claim 17 wherein said energy range is at least approximately 1.5 cm-1.
19. The method of claim 17 wherein said excited state life time is approximately 10-11 seconds and said energy range is at least approximately 100 GHZ.
20. The method of claim 17 wherein said bound excited state is within the range of approximately 50 to 300 cm-1 above the ionization level for said particles.
21. The method of claim 17 wherein the energy of the applied photons corresponds approximately to 5800 to 6000 angstroms.
22. A process for identifying ionization peaks comprising:
exciting particles to an excited level below the ioniza-tion level therefor;
applying to said particles photons of energy sufficient to ionize the excited particles;
varying the energy of the applied radiation over a range of energies to excite said particles to above ionization;
and detecting the absorption rate for the applied photons as a function of energy.
exciting particles to an excited level below the ioniza-tion level therefor;
applying to said particles photons of energy sufficient to ionize the excited particles;
varying the energy of the applied radiation over a range of energies to excite said particles to above ionization;
and detecting the absorption rate for the applied photons as a function of energy.
23. Apparatus for ionizing particles of a selected isotope type in an environment of plural isotope types comprising:
means for selectively exciting the particles of said selected isotope type in one or more energy steps to an excited state;
means for applying electromagnetic radiation of a pre-determined spectral width and frequency and tuned to an absorption line for the particles in said excited state which both excites the particles from the excited state to a state within a predetermined energy range above the ionization level and excites the particles with a predetermined fre-quency which is absorbed more strongly than absorption at immediately adjacent frequencies by at least an order of magnitude.
means for selectively exciting the particles of said selected isotope type in one or more energy steps to an excited state;
means for applying electromagnetic radiation of a pre-determined spectral width and frequency and tuned to an absorption line for the particles in said excited state which both excites the particles from the excited state to a state within a predetermined energy range above the ionization level and excites the particles with a predetermined fre-quency which is absorbed more strongly than absorption at immediately adjacent frequencies by at least an order of magnitude.
24. The apparatus of claim 23 wherein the means for selec-tively exciting includes means for exciting in at least three energy steps to said excited state.
25. The apparatus of claim 23 wherein said electromagnetic radiation is tuned within a range of approximately 5,800 to 6,000 angstroms.
26. The apparatus of claim 23 wherein said predetermined energy range is approximately 50 to 300 cm-1 above the ionization level.
27. The apparatus of claim 23 wherein:
the means for selectively exciting includes means for applying at least one frequency of narrow band radiation tuned selectively for an absorption line of said selected isotope; and said predetermined spectral width is substantially greater than the spectral width of said narrow band radia-tion.
the means for selectively exciting includes means for applying at least one frequency of narrow band radiation tuned selectively for an absorption line of said selected isotope; and said predetermined spectral width is substantially greater than the spectral width of said narrow band radia-tion.
28. The apparatus of claim 27 wherein said predetermined spectral width is on the order of magnitude of approximately 0.5 angstroms.
29. The apparatus of claim 23 wherein said particles include uranium atoms.
30. The apparatus of claim 29 wherein the selected isotope is the U-235 isotope.
31. The apparatus of claim 23 further including means for separating the particles excited to said state above the ionization level from said environment.
32. The apparatus of claim 31 wherein said separating means includes means for applying crossed electric and magnetic fields.
33. The apparatus of claim 23 wherein said electromagnetic radiation includes laser radiation.
34. The apparatus of claim 23 wherein said means for selec-tively exciting includes means for exciting in two energy steps.
35. Apparatus for ionizing particles of a selected isotope type in an environment of plural isotope types including:
means for selectively exciting particles of said selected isotope to a level below ionization therefor;
means for applying to the excited particles photons of energy sufficient to further excited the particles to a bound energy state substantially above the ionized level, and which bound state has a short but finite life time, substantially less than a nanosecond before degenerating to an ion and released electron;
the energy of said photons spanning an energy range substantially greater than the isotope shift for isotopes of said plural isotope types including the selected isotope type and centered on an absorption peak for said particles which is greater by at least an order of magnitude than the absorption for immediately neighboring photon energies.
means for selectively exciting particles of said selected isotope to a level below ionization therefor;
means for applying to the excited particles photons of energy sufficient to further excited the particles to a bound energy state substantially above the ionized level, and which bound state has a short but finite life time, substantially less than a nanosecond before degenerating to an ion and released electron;
the energy of said photons spanning an energy range substantially greater than the isotope shift for isotopes of said plural isotope types including the selected isotope type and centered on an absorption peak for said particles which is greater by at least an order of magnitude than the absorption for immediately neighboring photon energies.
36. The apparatus of claim 35 wherein said energy range is at least approximately 1.5 cm-1.
37. The apparatus of claim 35 wherein said excited state life time is approximately 10-11 seconds and said energy range is at least approximately 100 GHZ.
38. The apparatus of claim 35 wherein said bound excited state is within the range of approximately 50 to 300 cm-1 above the ionization level for said particles.
39. The apparatus of claim 35 wherein the energy of the applied photons corresponds approximately to 5800 to 6000 angstroms.
40. Apparatus for identifying ionization peaks comprising:
means for exciting particles to an excited level below the ionization level therefor;
means for applying to said particles photons of energy sufficient to ionize the excited particles;
means for varying the energy of the applied radiation over a range of energies to excite said particles to above ionization; and means for detecting the absorption rate for the applied photons as a function of energy.
means for exciting particles to an excited level below the ionization level therefor;
means for applying to said particles photons of energy sufficient to ionize the excited particles;
means for varying the energy of the applied radiation over a range of energies to excite said particles to above ionization; and means for detecting the absorption rate for the applied photons as a function of energy.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US86170077A | 1977-12-19 | 1977-12-19 | |
| US861,700 | 1977-12-19 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1107232A true CA1107232A (en) | 1981-08-18 |
Family
ID=25336528
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA318,103A Expired CA1107232A (en) | 1977-12-19 | 1978-12-18 | Use of autoionization transitions in isotopically selective photoexcitation |
Country Status (11)
| Country | Link |
|---|---|
| JP (1) | JPS5499898A (en) |
| AU (1) | AU524246B2 (en) |
| BE (1) | BE872873A (en) |
| CA (1) | CA1107232A (en) |
| DE (1) | DE2854909A1 (en) |
| ES (2) | ES476145A1 (en) |
| FR (1) | FR2412341A1 (en) |
| GB (1) | GB2011160A (en) |
| IT (1) | IT1106833B (en) |
| NL (1) | NL7812326A (en) |
| SE (1) | SE7813055L (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5316635A (en) * | 1992-05-22 | 1994-05-31 | Atomic Energy Of Canada Limited/Energie Atomique Du Canada Limitee | Zirconium isotope separation using tuned laser beams |
| US5527437A (en) * | 1994-08-19 | 1996-06-18 | Atomic Energy Of Canada Limited/Energie Atomique Du Canada Limitee | Selective two-color resonant ionization of zirconium-91 |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4320300A (en) * | 1979-09-28 | 1982-03-16 | Allied Chemical Corporation | Isotope separation by solar photoionization |
| JPS58153526A (en) * | 1982-03-10 | 1983-09-12 | Japan Atom Energy Res Inst | Separation of isotope by 3 photon absorption |
| EP3391956B1 (en) * | 2015-12-17 | 2023-07-26 | Riken | Device and method for selective ionization of palladium isotopes |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5224699A (en) * | 1975-08-18 | 1977-02-24 | Hitachi Ltd | Uranium isotopic separation method |
-
1978
- 1978-12-18 AU AU42621/78A patent/AU524246B2/en not_active Expired
- 1978-12-18 CA CA318,103A patent/CA1107232A/en not_active Expired
- 1978-12-19 JP JP15841878A patent/JPS5499898A/en active Granted
- 1978-12-19 SE SE7813055A patent/SE7813055L/en unknown
- 1978-12-19 DE DE19782854909 patent/DE2854909A1/en not_active Withdrawn
- 1978-12-19 ES ES476145A patent/ES476145A1/en not_active Expired
- 1978-12-19 GB GB7849039A patent/GB2011160A/en not_active Withdrawn
- 1978-12-19 NL NL7812326A patent/NL7812326A/en not_active Application Discontinuation
- 1978-12-19 BE BE192403A patent/BE872873A/en unknown
- 1978-12-19 IT IT52351/78A patent/IT1106833B/en active
- 1978-12-19 FR FR7835704A patent/FR2412341A1/en not_active Withdrawn
-
1979
- 1979-08-22 ES ES483579A patent/ES483579A1/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5316635A (en) * | 1992-05-22 | 1994-05-31 | Atomic Energy Of Canada Limited/Energie Atomique Du Canada Limitee | Zirconium isotope separation using tuned laser beams |
| US5527437A (en) * | 1994-08-19 | 1996-06-18 | Atomic Energy Of Canada Limited/Energie Atomique Du Canada Limitee | Selective two-color resonant ionization of zirconium-91 |
Also Published As
| Publication number | Publication date |
|---|---|
| IT1106833B (en) | 1985-11-18 |
| DE2854909A1 (en) | 1979-07-05 |
| ES483579A1 (en) | 1980-04-16 |
| FR2412341A1 (en) | 1979-07-20 |
| AU524246B2 (en) | 1982-09-09 |
| JPS5499898A (en) | 1979-08-07 |
| BE872873A (en) | 1979-04-17 |
| SE7813055L (en) | 1979-06-20 |
| IT7852351A0 (en) | 1978-12-19 |
| ES476145A1 (en) | 1979-11-16 |
| GB2011160A (en) | 1979-07-04 |
| AU4262178A (en) | 1979-06-28 |
| NL7812326A (en) | 1979-06-21 |
| JPS6325812B2 (en) | 1988-05-26 |
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