JPS6325812B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to methods of photoionization, and in particular to large cross-section photoionization transitions.

References that form the background of the present invention include US Pat. No. 3,772,519 and US Pat.
No. 3944947 and Belgian patent No. 807118,
Furthermore, there is German Patent No. 2312194. The techniques disclosed therein include one or several steps of photoexcitation in addition to photoionization via self-ionization transitions. Self-ionization is a transition that excites a particle to an energy level above its ionization level and then separates into ions and electrons. Utilizing self-ionization transitions for uranium enrichment typically has the advantage of increasing the cross section for small ionizations, thus reducing the demands on the ionization excitation source or laser.

However, prior to the present invention, effective use of self-ionization in isotope separation technology had not been established.

The present invention seeks to improve the selective ionization process of isotopes using the properties of self-ionization transitions. The improvement more effectively takes advantage of self-ionization of ionization from excited states through transitions with increased cross-sections that are opposite to the general ionization transitions.

The improvements of the present invention are made possible by the development of techniques for finding the relative cross sections for photoionization transitions from selected excited states. This technique uses frequency scanning of a successive laser to monitor ionization in the spectrum of absorption lines as it is pumped from a selected excited state to a continuous energy state. It can be seen that there is a transition to a continuous energy state of the increased cross section, ie, a self-ionization transition.

Such a photoionization process with an increased ionization cross section creates a system in which photoionization transfer occurs with light of a spectral width and frequency such that an increased cross section reaction occurs. This process can be used effectively. This process typically has two, three or four energy stages, with the first
The steps are selective excitation of isotopes and the final step is a self-ionization step. In this latter stage, the particle progresses from a highly excited state to a continuum of energy states of ionization, typically one level within a defined energy range above the ionization level. Since the lifetime of a self-ionizing bond state is very short, the frequency width of absorption lines for ionization processes with increased cross-sections is wide compared to the width of absorption lines for transitions between excited states. The isotopic shift between the desired isotope and other isotopes is a fraction of this bandwidth. As a result, the preferred embodiment uses a somewhat broad banded, less intense photoionizing radiation, and there is no reason to limit the bandwidth for isotope selection. The present invention utilizes the high efficiency of the ionization effect caused by laser light irradiation to reduce the occurrence of non-selectivity caused by strong ionizing light irradiation.

The ionized particles are separated because they are collected on a predetermined surface by the force applied to them. This force is preferably created by crossed electric and magnetic fields, in the nature of an MHD accelerator, to deflect the ionized particles from their original flow direction.

These and other features of the invention are more fully illustrated by the detailed description, which is given by way of illustration but not limitation, and the accompanying drawings, in which: FIG.

The present invention contemplates a technique for efficiently performing selective multi-step photoionization of isotopes using self-ionizing transitions with increased cross-sections. Isotopic selection is obtained by appropriate tuning of the transitions between bonded states, and transitions to ionizing continuous energy states are accomplished in a manner that optimally exploits the properties of increased cross sections for self-ionization. .

A transition from a highly excited but bound state to an ionized state is defined as an ionizing transition. This transition typically uses radiation of just enough energy to reach the ionization level, and no more energy is required. Such ionization transitions usually occur infrequently, have a small cross section, and are therefore inefficient. Apparatus has now been devised to confirm the presence and location of numerous peaks in the ionization cross section over the energy range above the ionization level. This peak generally represents a very significant increase in the cross section of the ionization transition. These peaks favor the ionization of the excited particles by exciting them one level above the ionization level with a transition of increased cross section.

high state reached by increased cross-sectional area,
That is, self-ionized states often consist of an electronically excited ion core with electrons loosely bound to it, such that when the excited ion core falls to the ground state, the energy released is sufficient to eject the electrons. It is interpreted as having a certain amount of energy. This elevated state causes ionization.
This collapse typically occurs within approximately 10 ^-11 seconds,
Due to the uncertainty principle of physics, it exhibits a spectral linewidth of about 100 GHz. This width is larger than any Doppler shift or isotopic shift, but much narrower than the width of the important cross-sectional peak.

This situation was exploited by using a relatively broad band illumination tuned to self-ionizing transitions with increased cross-sections. Broadband illumination is efficiently generated when its width is within the increased cross-sectional area. If we use self-ionizing radiation that spreads into this relatively wide peak,
A larger ionization cross section can be utilized and there is no need for intense irradiation light. Using less intense illumination light reduces the probability of non-selective ionization due to multiphoton absorption from the ground state.

Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram illustrating the energy steps of a preferred form of selective photoionization of isotopes according to the present invention. As shown, the first transition 12 is from the ground state 14, or in some cases a thermally excited lower excited state, to a first excited state 16 from the isotopic selection from the first laser. This is done by irradiating with laser light. Excitation from state 16 is carried out by a second transition 18 to a second excited state 20. The illumination light that causes transition 18 may or may not be tuned to the desired isotope selection. From the second excited state 20 a final photoionization transition 22 takes place to a third excited state 24 which is higher than the ionization level 26 . Excited atoms in the excited state 24 automatically form ions 28 and emitted electrons 30.
It collapses. Instead of two-step transitions 12 and 18,
One, three or more stages may be used. The intermediate energy levels can be selected from the following known tables: LA-4501, Current status of analysis of primary and secondary spectra of uranium (U and U) derived from measurements of optical spectra, 1971. year 10
Published monthly.

The ionization level 26 of uranium in the elemental state, such as the vapor of atomic particles, is approximately 6.18ev in wave number.
A little smaller than 50000.0cm ^-1 . The frequency for transition 22 is 5800-6000 angstroms (wavenumber
16667−17241cm ^-1 ), and the third excited state 24
is in the range of 50 to 300 cm ^-1 as a wave number above the ionization level 26. In this format, transitions 12, 18
and 22 are approximately 1/3 of the total ionization potential, and
They have a typical relationship. Irradiation of transitions 12, 18, and 22 is typically simultaneous, but may be staggered in time, as described below.

The laser used for each of transitions 12, 18 and 22 is typically a tunable dye laser, one of which
An example is Dial-A-line (Dial-A-line).
Dial-A-Line is a registered trademark of Avco Everett Research Laboratory, Inc., Everett, Massachusetts.
Preferably, the laser for the final transition 22 can be tuned to a range that includes a wider bandwidth than that used for the transition between excited states. especially,
From the above-mentioned uncertainty principle, the irradiation light absorption band width for photoionization is about 0.5 angstroms,
The wave number is approximately 1.5 cm ^-1 at the typical frequency used. This value is larger than the width of the difference that exists between the absorption lines of different isotopes at the same transition.

Therefore, isotopic selection must be made with excitations at lower transitions such as transitions 12 and 18. In this way, the final transition 22
The irradiation is performed with a laser having a wide band width of approximately 1/2 angstrom, and unlike the case of other transitions, more effective laser irradiation can be achieved by using a dye laser with a looser frequency limit.

According to the technology for identifying the ionization transition excited to the third excited state 24, as will be described later, the cross-sectional area 1,
It is possible to select transitions that are two or several orders of magnitude larger, thereby largely overcoming the conventional difficulties encountered in performing ionization due to relatively small cross-sectional areas.

FIGS. 2 and 3 show the main parts of an apparatus in which the present invention is implemented. This device is identical to and designated in US Pat. No. 3,939,354 and is incorporated herein by reference.

The device includes a chamber 40 having a set of coils 42 wound thereon, through which current is passed to create an axial magnetic field 44 within the chamber 40.

The laser beam 46 is directed to the window 4 at the end of the extension pipe 50.
8 is used to irradiate the inside of the chamber 40, particularly between the electrode plates of the accelerator 52 which serves to extract ions. Accelerator 52 is typically placed above a vapor source 54 that vaporizes uranium contained in a container with an electron beam. The vapor flow evaporated by the electron beam spreads radially throughout the chamber 40 of the accelerator 52 in the axial direction.

This situation is clearly shown in Figure 3. In FIG. 3, a crucible 56 containing supplied uranium 58 is shown as a vapor source, and the uranium 58 is bombarded by the electron beam focused by the magnetic field 44 and spreads into the region of the accelerator 52. The accelerator 52 has one of the chambers 62 separated by a collection plate 64 and having an anode 66 in the center thereof.
It consists of a group. A laser beam irradiation area 68 is located between the anode 66 and the collection electrode plate 64 in the chamber 62 . Irradiation in region 68 is achieved by multiple reflections of beam 46 and/or by a beam having approximately the desired cross-sectional shape. Other techniques can also be used for this purpose.

The laser beam irradiation area 68 typically includes an anode 66 and a collector plate 64 generated by a pulsed power source 70.
irradiated by a pulse immediately following an electric field pulse between

Beam 46 typically includes transitions 12, 18 and 2.
A composite beam containing three colors corresponding to
It is configured as shown in the figure. A set of three dye lasers 80, 82 and 84 is typically provided and their output beams are fed to a beam combiner 86 made of pribum or dichroic elements.
Beam combiner 86 provides a combined output beam 46 that feeds chamber 40 in FIG.

Lasers 80, 82, and 84 are controlled by a timer 88 to produce simultaneous pulses with a duration not significantly less than at least 1 microsecond in the combined beam 4.
6 and send it to room 40.

To ensure the desired frequency of transition with increased cross-sectional area, a motor 90 is provided to drive the diffraction grating in the cavity 94 formed by mirrors 95 and 97 of the laser 84. The tuning of elements such as 92 is controlled thereby scanning the frequency of output beam 96 from laser 84. By gradually changing the frequency of the irradiated light in this manner, the amount of ionization created within the chamber 62, particularly within the laser beam irradiation area 68, changes. The production rate of ionization of an isotope of any mass number is detected using a conventional mass spectrometer 98 with a probe 100 inside the chamber 40, and is determined by photoionization and/or as is well known in the art. Detects ions that result from acceleration.

To increase the intensity of the laser beam, use the laser 8
It is preferable to intensify 0, 82 and 84 in several steps. Additionally, to obtain the desired pulse repetition rate, lasers 80, 82, and 84 may each be multiplied to fire sequential pulses, as described in U.S. Pat.
A rotating optical element mechanism, such as that described in US Pat. No. 3,924,937, is used to combine the sequentially fired pulses and increase the pulse rate.

Although the method of the present invention and the apparatus used have been described in detail, it will be apparent to those skilled in the art that various modifications and changes may be made to the present invention without departing from the scope thereof. There will be. It is believed, therefore, that the invention be limited only as indicated by the scope of the claims below.

[Brief explanation of the drawing]

1 is an energy phase diagram useful in explaining the method of the invention; FIG. 2 is a simplified illustration of the apparatus used to carry out the invention; and FIG. 3 shows the interior of the apparatus of FIG. The explanatory diagram, FIG. 4, shows the laser device for generating the excitation and ionizing radiation used in the present invention, and also shows the device used to confirm the self-ionization transition, that is, the photoionization transition with increased cross section. FIG. 12...first transition, 14...ground state, 16
...First excited state, 18...Second transition, 20
... second excited state, 22 ... final photoionization transition, 24 ... second excited state, 26 ... ionization level, 28 ... ion, 30 ... emitted electron, 40 ...
...Chamber, 42...Coil, 44...Magnetic field, 46...
Laser beam, 48...Window, 50...Extension pipe, 52...Accelerator, 54...Steam source, 56...
Crucible, 58... Uranium supplied, 60... Electron beam, 62... Chamber, 64... Collection plate, 66
... Anode, 68 ... Laser beam irradiation area, 70
... Pulse power supply, 80, 82, 84 ... Dye laser, 86 ... Beam combiner, 88 ... Timer, 9
0... Motor, 92... Diffraction grating, 94... Cavity, 95, 97... Mirror, 96... Output beam, 9
8...Mass spectrometer, 100...Probe.

Claims (1)

[Claims] 1. Selectively excite particles of an isotope selected from a plurality of isotope sets to an excited state through one or more energy levels, and bring the particles in the excited state into the excited state. A method for ionizing an isotopic particle, which consists of excitation to a state within a predetermined energy range above the ionization level of the particle, the excitation having a predetermined frequency tuned to the absorption line of the particle in the excited state; irradiation with an electromagnetic wave having a predetermined spectral width larger than the spectral absorption line of the state, such that the predetermined frequency is absorbed by the particle at least one order of magnitude stronger than the absorption of electromagnetic waves of frequencies very close to the predetermined frequency. A method characterized by being a frequency. 2. The method according to claim 1, wherein the isotope is uranium. 3. The method according to claim 2, wherein the electromagnetic wave has a wave number of 16667 to 17241/cm and a predetermined spectral width of 5800 to 6000 angstroms.