GB2038075A - A method of and apparatus for isotope separation - Google Patents

Abstract

A method of and apparatus for isotope separation wherein an isotopic mixture is diluted by a mixture of diluent gases in a gas reservoir 22 and then solidified on a low temperature retaining plate 2 secured inside an isotope separation chamber 1. The composition of the mixture of diluent gases is selected to shift the absorption wavenumber of a given isotope to be separated to coincide with the wavenumber of an infra red laser beam 1 directed at the plate 2. The given isotope absorbs the radiation raising the temperature of the plate 2. The given isotope is then sublimed off and crystals rich in the given isotope are collected in cold traps 24, 25.

Description

SPECIFICATION A method of and apparatus for isotope separation using a plurality of diluent gases This invention relates to a method of an apparatus for separating isotopes (which term is used herein to include isotopic compounds) and concerns a method of and apparatus for separating isotopes by the use of a plurality of diluent gases and an infrared laser beam.

In general, problems are encountered in using infrared beams for laser isotope separation because existing infrared lasers are not capable of continuous wavelength variation of the infra red beam. This is a disadvantage especially when it is desired to separate isotopes having similar absorption wavelengths, for example, uranium compounds in which the isotopic shift is small.

For instance, UF6 (uranium hexafluoride) intensely absorbs infra red radiation of a wavelength close to 16 ym (620 cm - 1 in terms of wave number). However the uranium 238 isotopic compound of uranium hexafluoride (238us6) which is gaseous at a normal temperature absorbs infra red radiation by assymetric bond stretching (V3 vibration) at 625.5 cm-' whereas the uranium 235 isotopic compound of uranium hexafluoride (235us) absorbs at 625.15cm-1, an isotopic shift to a higher wave number of 0.65 cm-l.

Attention has been drawn to the carbon dioxide laser as a useful source for the excitation of uranium hexafluoride because of the operating wavelengths of the laser, and carbon tetrafluoride (CF4) photo-exciting lasers pumped by pulsed carbon dioxide lasers have been found to provide 12 lines in a spectrum band at 161bum. However none of these emission lines coincides with the absorption lines of gaseous UF. Also, the emission lines are, in some cases, up to ten wave numbers apart, making it impossible either to distinguish between 235UF6 and 238UF6 and or to selectively excite one of them. Furthermore, the absorpiton lines produced by gaseous molecules at normal temperatures have a certain finite width partly due to the natural line-width of the vibration state and also to rotation effects.

For heavy molecules, for example UF6, the broadening of the line-width by these means is greater than the isotope shift, hence in order to selectively excite either 235UF6 or 238UF6 the broadening of the absorption line must first be eliminated.

To reduce the broadening of the absorpiton line it is sufficient to reduce the temperature of the sample to such a level that the rotation and natural vibration effects become minimal with respect to the absorption peak.

Several methods of lowering the temperature of sample are known including: a supersonic expansion cooling method in which a highly pressurized gas sample is jetted from a nozzle into a vacuum at supersonic speed; and a matrix isolation method (an isolation method using low temperature diluent molecules, hereinafter referred to as "dilution solidification method") in which the molecules to be isotopically separated are surrounded and fixed by low temperature inert gas molecules for example Xe (xenon) Ar (argon), N2 (nitrogen) or the like.

In the supersonic expansion cooling method, the line width of the absorption line is decreased in comparison to the line width at normal temperatures and the line is also shifted a certain distance towards the short wave length end (or high wave number end) of the spectrum.

Alternatively, the line width may be decreased whilst the absorption line is shifted towards the high wavelength (low wave number) end of the spectrum using the dilution solidification method, the degree of the shift depending on the type of gas used as a diluent. For example when UF6 is diluted with Ar, Xe or CO (carbon monoxide), the wave number of the y3 absorption line of 236UF6 shifts to: 619.3 cm-' for Ar; to 617.0 cm-' for Xe; and to 618.4 cm-l for CO.

Thus, in the dilution solidification method, the absorption line can be slightly shifted in wave number by proper use of the diluent and it is therefore quite simple to make the absorption line coincide with the exciting line from the pulsed infrared laser beam irradiator. However, both the supersonic expansion method and the dilution solidification method have disadvantages.

Although the line width of the absorption line is decreased in both methods, unless the shifted wave number position of the absorption line accidently coincides with that of the laser exciting line, it will not be possible to selectively excite the required UF6 isotope.

It is therefore an object of the present invention to provide a method of and apparatus for isotope separation in which the infrared absorption lines of different isotopes of an isotopic mixture are artificially shifted in wave number so that the absorption line of a particular species of isotope coincides with the wave number of the laser exciting line allowing the required isotope to be selectively excite and thus separated.

According to one aspect of the present invention there is provided a method of separating a given isotope from an isotopic mixture using an infrared laser beam, comprising diluting the isotopic mixture with a diluent gas which is chemically inert with respect to the isotopic mixture and is transparent to the infrared radiation of the laser beam, solidifying the diluted isotopic mixture on a low temperature retaining device, and irradiating the solidified matter with the infrared laser beam to selectively excite the given isotope to absorb the radiation, in which method the diluent gas is composed of a mixture of gases and the composition of the diluent gas is selected to shift the wavenumber at which the given isotope in the solidified matter absorbs radiation to coincide with the wavenumber of the radiation of the irradiating laser beam.

According to a further aspect of the present invention there is provided apparatus for separating a given isotope from an isotopic mixture, comprising: an isotope separation chamber including a low-temperature retaining device; guide means for directing onto the low-temperature retaining device the isotopic mixture diluted by a mixture of gases which is chemically inert with respect to the isotopic mixture and is transparent to infrared radiation, whereby the diluted isotopic mixture is solidified on the retaining device; infrared laser beam producing means for irradiating the solidified matter to selectively excite the given isotope to absorb the radiation; and gas regulating means for adjusting the composition of the diluent gas mixture to shift the wavenumber at which the given isotope in the solidified mixture absorbs radiation to coincide with the wavenumber of the irradiating laser radiation.

Embodiments of apparatus for performing the method of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic view of one embodiment of the apparatus; Figure 2 is a schematical view of an oven for gasifying an isotope mixture in the form of a solid or liquid according to a second embodiment of the apparatus; Figure 3 is a graph illustrating the effect of altering the composition of the diluent gas mixture; and Figure 4 is a schematical view showing another embodiment of the apparatus for carrying out the isotope separation method according to the invention.

Referring now to Fig. 1, there is shown an isotope separation chamber 1 having a window 1 a to admit an infrared laser beam, and windows 1 b, 1 cto allow the beam to be measured by infrared spectro-analysis. Within the isotope separation chamber 1 there is provided a retaining plate 2 having a controlled temperature, a face of which is directed towards the irradiation window 1 a.

The isotope separation chamber is of such a construction that the interior of the chamber may be subjected to vacuum conditions and can be cooled to super low temperature for example the temperature of liquid nitrogen. A suitable chamber would be a low temperature cryostat.

The composition of the retaining plate 2 depends on the type of isotopes or the analysing technique being used and, for example, BaF2 (barium difluoride) crystals, copper, aluminium, stainless steel and the like may be used. The retaining plate 2 may be square, rectangular or of any other suitable shape.

The temperature of the retaining plate 2 may either be regulated by indirect means which control the temperature within the isotope separation chamber or by direct means controlling temperature of the retaining plate 2.

An infra red laser 3 is placed adjacent to the isotope separation chamber 1 so that the output end of the laser is lined up with the window, thus exposing the retaining plate 2 to the infra red laser beam L.

The infra red laser may be for example a fixed or variable wavelength carbon dioxide laser.

One end of a main pipe 5 extends through a wall of the isotope separation chamber and terminates in a gas injection nozzle 4 directed towards the retaining plate 2.

A gas bomb 17 containing the isotope mixture is connected via a gas feed pipe 6 having a valve 6ato the main pipe 5. In parallel with bomb 17 are connected gas feed pipes 7, 8 having valves 7a, 8a, leading to gas bombs 18 and 19 respectively. Each bomb 18, 19 contains a respective dilution gas (for example a noble gas such as xenon or argon) to dilute the isotope mixture.

A pressure gauge 15 and a fine pressure detector 16 are connected to the pipe 5 between feed pipes 7 and 8, and 7 and 6 respectively to measure the pressure of the gas flowing through the pipe.

A valve 11, together with an exhaust gas valve provided in the other end of pipe 5, both normally being kept closed to isolate the gas bombs 17. 18, 19 from a gas reservoir 22 in which uniform dilution of the isotope mixture by the dilution gases occurs.

The gas reservoir 22 is normally separated from a flow regulator 13 and a flow meter 14 provided in the pipe 5 by a closed valve 10.

A normally closed valve 9 separates the nozzle 4 from the flow regulator 13 and flow meter 14.

An oven 20 as shown in Fig. 2 may be used in place of the gas bomb 17 shown in Fig. 1 as a feed source of the gaseous isotope mixture. In the oven 20 a solid or liquid isotope mixture may be heated, as required, by a heating means for example a heater 21 having a temperature control system 21 a, thereby producing a gaseous isotope mixture.

In which case, the gaseous isotope mixture may be fed to the main pipe 5 through the feed pipe 6 or may be directly injected into the isotope separation chamber 1.

A pipe 23 is connected to the isotope separation chamber 1 by means of a discharge port 1 d to allow the product enriched in the selected isotope to be drawn off. The pipe 23 is provided with cold traps 24, 25 having valves 24a, 25ato collect the enriched product.

A mass spectrograph of the product may be obtained by connecting, for example, a quadruple mass spectrometer to the pipe 23.

Examples of dilution gases are those gases which are transparent to the infra red radiation employed and chemically inert with respect to the isotopes to be separated, i.e. inert gases, hydrogen, nitrogen, oxygen, carbon monoxide, carbon dioxide and similar gases. In order to make the half-width as small as possible, it is desirable to use inert gases such as neon, argon, krypton, xenon and the like.

The temperature of the retaining plate 2 should be at below at least the temperature of liquid nitrogen, and, through dependent on the type of diluent gas employed, it is preferable that the temperature be below 10 K when using neon and below 30 K, 45 K and 60 K when using argon, krypton and xenon, respectively.

It is sufficient for the isotopic mixture to be diluted 10-2000 times by volume, at which dilution, the molecules of the respective isotopes to be separated are isolated from each other.

Preferably, for compounds in which the different isotopes show a great difference in properties the dilution is about 100 times by volume and approximately 1000 times for compounds in which the differences between isotopes are small.

The reaction occuring during the laser isotope separation is a photo-ionization, photodissociation, photo-chemicai reaction or the like.

As an example, the method of separating a sulphur 32 isotopic compound (32SF6) and a sulphur 34 isotopic compound (34SF6) of SF6 (sulphur hexafluoride) using the apparatus shown in Fig. 1 will be described. In this method, xenon and argon are used as the diluent gases.

The retaining plate 2 is secured on the isotope separation chamber 1 and the chamber then evacuated, valve 9 being closed. Meanwhile the retaining plate 2 is cooled to below 50 K.

Then, having sealed off the gas bombs 17, 18, 19 by ensuring that valves 6a, 7a and 8a respectively are closed, the gas-feed system, that is, the main pipe 5 and the feed pipes 6-8, is evacuated and the valves 9-12 are closed. By use of the pressure gauges 15, 16 a predetermined amount of SF6 is introduced through the valve 6a into the portion of pipe 5 which is isolated by valves 11 and 12, to which xenon and argon are then added in predetermined quantities through the valves 7a and 8a respectively so that a predetermined mixing ratio and dilution (usually about 100 times by volume) are attained.

The valve 11 is then open to allow the mixed gas to pass into the gas reservoir 22, in which it is uniformly mixed.

The flow rate of the mixture through valve 10 being measured by means of the flow meter 14 and regulated by means of the flow regulator 13, valve 9 is opened to allow the mixed dilution gas (sulfur hexafluoride + xenon + argon) to be forced through the nozzle 4 and on to the low temperature retaining plate 2 where the mixture solidifies. The solidified mixture is irradiated through the window 1 a with an infrared laser beam of a predetermined wavelength generated by the infrared laser 3.

If necessary infrared spectroscopic analysis of the absorption spectra of the solidified phase of the required isotopic compound (32SF6 or 34SF6) diluted with xenon and argon can be performed through the windows 1 b, 1 c.

Irradiation with the correct wavelength of infra red laser radiation causes the selected isotopic compound to absorb the radiation, raising the temperature of the retaining plate 2 so that the selected compound is sublimed, thus producing a product enriched in the selected compound which is then collected in the cold traps 24, 25.

Isotopic compounds of SF6 may be selectively excited by means of the P branch emission lines of the CO, laser in the spectrum band of 10.61tom.

The P branch emission lines are defined as the emission lines for which the selection rule for the rotational quantum number, is AJ = 1 ie the value of J decreases by 1 for each transition.

According to the experimental results, the 32SF6 isotopic compound of SF6 isotopic compound of SF6 diluted by pure xenon 200 times by volume, and solidified absorbs infra red radiation by v3 vibration at 931.3 i 0.5 cm-' and 34SF6 absorbs by the same mechanism 914.1 + 0.5 cam~'. The difference between these wave number is 17.2 cm-1 the CO, laser has nine P branch emission lines between these absorption lines as shown in Table 1.

The half width of the y2 absorption line for 32SF6 diluted with xenon is 3.7 cm- so that the line is sufficiently distant from the absorption line of 34SF6. The laser emission lines are about 2 cm-' apart and are thus relatively dense. Therefore, any of the emission line occuring at a wavenumber near the respective absorption line wavenumbers may be used to selectively excite the to some extent: that is for example, the line P(32) (that is the P branch lie (P(34) is suitable for 32SF6 for which the rotation quantum number of the upper state is J = 32) or the diluted with xenon and P(48) or P (50) for 34SF6 similarly diluted.

The SF6 may also be diluted by argon, in which case the isotopic compound 32SF6 may be selectively excited to some extent by the use of P (26) or P (28) and 24SF6 by the use of P (44) or P (46). However, as the wavenumber of the absorption line cannot be exactly matched to the wavenumber of the emission line using either argon or xenon neither method is efficiently feasible.

Table 1 CO2 Laser Emission Lines and SF6 Absorption Lines in the Spectrum Band at 10.6 jum Laser Emission Line Position of Half Remarks Absorption Width Line v Av P(J =) No. cm-' cm-' cm - ' P(22) 942.35 P(24) 940.52 P(26) 938.62 937.7 3.7 32so6 diluted P(28) 936.78 with Ar.

P(30) 934.88 P(32) 932.89 931.3 3.7 32so6 diluted P(34) 930.96 with Xe.

P(36) 928.95 P(38) 926.95 P(40) 924.90 P(42) 922.85 P(44) 920.77 920.5 3 34so6 diluted P(46) 918.65 with Ar.

P(48) 916.51 P(50) 914.41 914.1 3.4 34sis diluted with Xe.

Using a mixture of xenon and argon as the diluent gas to observe shifting of the absorption line in relation to variation in the composition of the diluent gas the results shown in Fig. 3 were obtained.

The top graph (a) in Fig. 3, shows the wave number at which 22SF6 absorbs infra red radiation by v3 vibration plotted against the percentage concentration of argon, in the mixed diluent gas of xenon and argon for a dilution of 200 times by volume of SF6. From this it can be seen that the shift in the position of the peak of the absorption line is substantially linearly related to the concentration of argon.

The bottom graph (b) in Fig. 3 plots the half-width of the absorption line for the percentage composition of argon used in graph (a) and as shown so that, the use of such mixed diluent compound does not seem to increase the half-width of the absorption line.

These experimental results demonstrate that the absorption line of 32SF6 or 34so6 can be made to coincide precisely with a laser emission line. There exist three laser oscillation lines P (28), P (30) and P (32), as shown in Table 1, between the v3 absorption line of 32so6 diluted with xenon and the V3 absorption line of 32SF6 diluted with argon. Similarly for 34so6 there are three laser emission oscillation lines P (46), P (48) and P (50).Therefore, in order to make coincide the absorption line of 22SF6 coincide with one of the emission lines P (28), P (30) and P (32) by the use of a mixed diluent gas of xenon and argon, it is necessary to have a percentage concentration of argon in the mixed diluent of 86, 56 and 25% respectively. To make the absorption line of 34SF6 coincide with an emission oscillation line P (46), P (48) or P (50) the percentage concentration of argon must be 71, 38 or 5% respectively.

When the isotopic compound in the solidified diluent gas is vibrated and excited by the infrared laser beam, fine crystals of the excited isotopic compound are inclined to grow on the low temperature retaining plate 2. When the absorption line and the laser emission line do not coincide exactly, the absorption coefficient of the 32so6 is reduced slowing crystal growth, as for example when xenon is used as a single diluent gas, with respect to the absorption coefficient using a mixed diluent of xenon and argon of the correct proportions as the results in Table 2 show.

According to the results of Table 2, the V3 vibration of 32SF6 diluted with xenon absorbs at 931.3 cm-'. When this vibration is excited by the P (32) line of the CO2 laser which has a wavenumber of 932.89 cm-', the absorption line and the emission line differ by 1.59 cm-l and the height of the absorption line 22SF6 crystals produced is reduced to 0. 12 in terms of the absorption coefficient.

On the other hand, when a mixed diluent gas of xenon and argon having an argon concentration of 25% is used, the r3 absorption line of 32so6 just coincides with the P (32) laser emission line, and the height of the absorption line of the 32SF6 crystals is found to be 0. 15.

Thus by using a mixed diluent gas to make the absorption line of the isotopic compound coincide with the laser emission line large amounts of crystals are produced more efficiently.

Table 2 Comparison of the Method of the invention with the Prior-art Method as regards the Absorption Coefficient of 32SF6 Isotopic Kind of Absorption Laser Peak value compound diluent line oscillation of crystals to be gas (cm-1) line (absorption) excited (cm-l) coefficient) Prior art 32SF6 xenon 931.3 932.89 0.12 Method 100% Inventive 32so6 mixed 932.9 932.89 0.15 Method diluent gas of 25% argon and xenon As previously mentioned the difference in wavenumber of the absorption lines of the isotopes of UF6 is as small as 0.65 cm-' in relation to which the spacing of the laser emission lines is so great that the technique of shifting the absorption line becomes more important.

The relationship between the wavenumbers of the emission lines of a photo-excitation CF4 laser and the v3 absorption lines of 2,38UF6 measured by the dilution soilidification method is shown in Table 3.

Table 3 CF4 Laser Emission Lines and UF6 Absorption Lines in a Spectrum Band of 16 ,um Laser emission line Position of Remarks absorption line No. cm-' cm-' (1) 653.32 (2) 649.3 (3) 646.1 (4) 643.23 (5) 642.4 (6) 640.73 (7) 631.15 (8) 631.05 (9) 631.12 619.3 238U F6 diluted with Ar.

618.4 238UF6 diluted with CO.

(10) 618.11 617.0 238U F6 diluted (11) 615.06 (12) 611.99 As will be clear from Table 3, the emission line (10) of the CF4 laser, of wavenumber 618.1 1 cm-', lies between the v3 absorption line, 617.0 cm-', of 238UF6 diluted with xenon and the v3 absorption line, 619.3 cm-', of 238UF6 diluted with argon. From the linear relationship between the composition of the mixed diluent gas and the degree of shifting of the absorption line, it will be understood that the' D3 absorption line of 236UF6 can be made to coincide with the laser emission line when the percentage concentration of argon in the mixed diluent gas of xenon and argon is 48%.Similarly, when the percentage concentration of argon is 29%, the V3 absorption line of 235UF6 can be made to coincide with the same laser emission line.

Thus, the selective excitation of UF6 isotopes is made possible by a dilution solidification method using a mixture of a plurality of diluent gases, allowing high efficiency laser isotope separation.

Alternatively the apparatus shown in Fig. 4 may be used, in which separate main pipes 31, 30 and 29, are connected to the gas bombs 19, 18 and 17 respectively. The isotopic compound mixture and the diluent gases are simultaneously blown against the low temperature.

retaining plate 2 from respective nozzles 26, 27 and 28 to provide a predetermined mixing ratioQ regulated by the flow regulator 13.

Different combinations of inert gases may be used as the deluent gas mixture, for example, argon and krypton; keypron and xenon; and the like. Additionally, a diluent gas mixture of three or more gases may be used.

Claims (16)

1. A method of separating a given isotope from an isotopic mixture using an infrared laser beam, comprising diluting the isotopic mixture with a diluent gas which is chemically inert with respect to the isotopic mixture and is transparent to the infrared radiation of the laser beam, solidifying the diluted isotopic mixture on a low temperature retaining device, and irradiating the solidified matter with the infrared laser beam to selectively excite the given isotope to absorb the radiation; in which method the diluent gas is composed of a mixture of gases and the composition of the diluent gas is selected to shift the wavenumber at which the given isotope in the solidified matter absorbs radiation to coincide with the wavenumber of the radiation of the irradiating laser beam.

2. A method according to claim 1, wherein the retaining device is cooled to the temperature of liquid nitrogen.

3. A method according to claim 1 or 2, wherein a mixture of two gases is used to dilute the isotopic mixture.

4. A method according to claim 3, wherein a mixture of two noble gases is used to dilute the isotopic mixture.

5. Apparatus for separating a given isotope from an isotopic mixture, comprising: an isotope separation chamber including a low-temperature retaining device; guide means for directing onto the low-temperature retaining device the isotopic mixture diluted by a mixture of gases which is chemically inert with respect to the isotopic mixture and is transparent to infrared radiation, whereby the diluted isotopic mixture is solidified on the retaining device; infrared laser beam producing means for irradiating the solidified matter to selectively excite the given isotope to absorb the radiation; and gas regulating means for adjusting the composition of the diluent gas mixture to shift the wavenumber at which the given isotope in the solidified mixture absorbs radiation to coincide with the wavenumber of the irradiating laser radiation.

6. Apparatus according to claim 5 wherein the retaining device is a retaining plate.

7. Apparatus according to claim 5 or 6 wherein the temperature of the retaining device is controllable.

8. Apparatus according to claim 4, 5, 6 or 7 wherein, the infrared laser beam producing means is a pulsed laser.

9. Apparatus according to any one of claims 5 to 8 wherein the infra red laser beam producing means is a carbon dioxide laser.

10. Apparatus according to any one of claims 5 to 9 wherein the guide means includes a pipe having a gas injecting nozzle extending into the chamber.

11. Apparatus according to any one of claims 5 to 9 wherein the guide means comprises three separate pipes each having a gas injecting nozzle extending into the chamber.

12. Apparatus according to any one of claims 5 to 10, wherein said guide means includes a gas reservoir in which the isotopic mixture is uniformly diluted with the diluent gases.

13. Apparatus according to any one of claims 5 to 12, wherein the gas regulating means are valve means.

14. A method of isotope separation substantially as hereinbefore described with reference to the accompanying drawings.

15. Apparatus for isotope separation substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

16. Any novel feature or combination of features described herein.