Abstract:
A method of separating boron isotopes  10  B and  11  B by laser-induced selective excitation and photodissociation of BCl 3  molecules containing a particular boron isotope. The photodissociation products react with an appropriate chemical scavenger and the reaction products may readily be separated from undissociated BCl 3 , thus effecting the desired separation of the boron isotopes.

Description:
BACKGROUND OF THE INVENTION 
     The invention described herein was made in the course of, or under, a contract with the U. S. ATOMIC ENERGY COMMISSION. 
    
    
     It relates to a method for separating boron isotopes and more particularly to a method based on laser-induced selective excitation and photodissociation of BCl 3  molecules containing a particular boron isotope. 
     Boron highly enriched in  10  B has substantial utility as a neutronic poison in nuclear reactors. The art discloses two methods for separation of  10  B and  11  B. The first consists of the fractional distillation of the dimethyl- or diethyl-ether complex of BF 3 . The second involves the low temperature fractional distillation of BF 3  itself. Heretofore there has been no known technique for separating these isotopes by selective excitation and photodissociation of a boron-containing compound. 
     SUMMARY OF THE INVENTION 
     The isotopes  11  B and  10  B may readily be separated by irradiating gaseous BCl 3  containing both isotopes, selectively exciting those BCl 3  molecules containing the desired B isotope, photodissociating the excited BCl 3 , reacting the photodissociation products with an appropriate chemical scavenger, and separating the undissociated BCl 3  from the reaction products. 
     The necessary selective excitation is accomplished by irradiating a gaseous mixture of the BCl 3  and the scavenger with light from the P or R branch of a CO 2  laser. Simultaneous irradiation of the mixture with ultraviolet light at 213 to 215 nm then results in photodissociation of selectively excited BCl 3  and prompts a reaction of the photodissociation products with the scavenger. 
     Preferably the CO 2  laser radiation is from the P branch which selectively or at least preferentially excites those BCl 3  molecules containing  11  B. The chemical scavenger may be any material that is gaseous, does not substantially absorb either the infrared or the ultraviolet radiation, and is reactive with the photodissociation products of BCl 3  but substantially non-reactive with BCl 3  itself. Oxygen and various alkenes are suitable for this purpose. The requisite ultraviolet radiation is readily obtained from the output of Xe or D 2  flashlamps filtered through unexcited BCl 3 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is the infrared absorption spectrum of BCl 3  containing a natural abundance of boron isotopes. 
     FIG. 2 displays ultraviolet absorption spectra of BCl 3  containing a natural abundance of boron isotopes. 
     FIG. 3 shows calculated ultraviolet absorption spectra of ground state and excited BCl 3 . 
     FIG. 4 shows a reaction vessel and ultraviolet filter used in an actual reduction to practice of the invention. 
     FIG. 5 shows the placement of the ultraviolet source and the reaction vessel within the flashlamp cavity. 
     FIG. 6 is a plot of measured enrichment versus pressure of BCl 3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The isotopic abundance of natural boron is 19.8 at %  10  B with the remainder being  11  B. The compound BCl 3  has a boiling point at 1 atm of 12.5° C and absorbs the radiation of a CO 2  laser. This absorption takes place in the ν 3  mode which for  10  BCl 3  has a frequency ν 3  = 995 cm -1  and for  11  BCl 3  a frequency ν 3  = 956 cm -1 . The infrared absorption spectrum of BCl 3  at 0.5 torr and containing a natural ratio of boron isotopes is shown in FIG. 1. The isotope shift of 39 cm -1  is a very large one and is located within the manifold of rotational lines available from the CO 2  laser. The CO 2  P branch lines largely coincide with the  11  BCl 3  absorption peak, whereas the R branch lines are matched to a large degree by the  10  BCl 3  absorption peak. As shown in FIG. 1, the CO 2  P(20) line which is one of the strongest P branch lines is well within the  11  BCl 3  absorption peak. 
     Boron trichloride dissociates in the vicinity of 3.8 × 10 4  cm -1 . The products of dissociation are not accurately known but are quite probably the free radicals Cl and BCl 2 . As free radicals, both are highly reactive and readily consumed by an appropriate chemical scavenger. In accordance with the preferred embodiment, BCl 3  molecules containing a particular boron isotope are excited with the appropriate line of a CO 2  laser, the excited molecules are preferentially dissociated by ultraviolet radiation, and the dissociation products containing the particular isotope react with a chemical scavenger. 
     The ultraviolet absorption spectra of natural abundance BCl 3  at pressures of 4 and 2.1 torr are given in FIG. 2. The absorption is centered at 207.6 nm and free of any visible structure. It has a peak attenuation coefficient of α = 0.014 cm -1  torr -1 . FIG. 3 displays the 207.6 nm ultraviolet absorption fitted by a 5.0 nm FWHM Gaussian. This absorption leads to dissociation of the BCl 3 . These conditions are appropriate to a BCl 3  pressure of about 2 torr. Curve (a) depicts the relative magnitude and location of absorption by 50% of the naturally available  10  BCl 3  molecules which have been promoted to the ν 3  mode by an infrared pulse of appropriate wavelength, while curve (b) displays the same information for excited  11  BCl 3 . Since the strongest lines of the CO 2  laser are absorbed by  11  BCl 3 , it is preferable to preferentially dissociate the  11  BCl 3  and scavenge the dissociation products, leaving the gas enriched in  10  BCl 3 . The  10  B can then be readily recovered by conventional reduction of BCl 3  with H 2 . Alternatively, if desired, the  10  BCl 3  can be preferentially dissociated in accordance with this method. 
     The ultraviolet wavelength necessary to dissociate excited BCl 3  is dependent on the wavelength of the infrared radiation used to produce the excitation. It is in the spectral region of 213 to 215 nm. There are presently no lasers available having outputs in this wavelength; however, an intense continuum source such as Xe or D 2  lamps can readily be used if first filtered by being passed through a quartz cell containing BCl 3 . The quartz passes little radiation below ˜ 190.0 nm and the BCl 3 , being unexcited and in the ground state, strongly attenuates the radiation centered around 207.6 nm indicated by the solid curve of FIG. 3. As a consequence, radiation filtered by this cell can only dissociate excited BCl 3 . Although the filtering action will result in dissociation of BCl 3  in the filter cell, recombination will rather quickly occur, so that the filter cell will exhibit little if any aging. 
     A critical feature of the method of this invention is the presence of an appropriate chemical scavenger to react with the dissociation products of the selectively excited BCl 3 . An appropriate scavenger should be gaseous, absorb little or none of the ultraviolet or infrared radiation, and react readily with the dissociation products of BCl 3 , but not the BCl 3  itself. Suitable scavengers include O 2  and various alkenes. 
     Using the apparatus shown in FIGS. 4 and 5, the method of the invention was actually reduced to practice in the following manner. Reaction vessel 1 3 mm in diameter and having Brewster angle windows 2, 2&#39; contained mixtures of 1 to 6 torr BCl 3  and 20 to 25 torr O 2 . Surrounding reaction vessel 1 is filter vessel 3 25 cm long and 24 mm in diameter which contained BCl 3  at 600 torr. Reaction vessel 1 and filter vessel 3 were placed at one focus of elliptical flashlamp cavity 4, while a Xe flashlamp 5 was placed at the other focus. Flashlamp 5 discharged 750 joules of electrical energy in 250 μs. The ultraviolet filter 8 provided a 12 to 1 contrast ratio between 207.6 nm and 215.0 nm. Infrared radiation 6 at 4 × 10 4  W/cm 2  was provided from a CO 2  laser (not shown) operating on the P(20) line. 
     The infrared radiation 6 was propagated axially through reaction vessel 1 containing a desired (BCl 3 ,O 2 ) mixture at the peak of the ultraviolet pulse. Five pulses of combined ultraviolet and infrared radiation dissociated about half of the initial amount of BCl 3  present in reaction vessel 1. Infrared radiation 6 alone would not dissociate any BCl 3 , whereas the filtered ultraviolet radiation required about 15 pulses to induce the same reaction as a single combined ultraviolet and infrared pulse. The product of the reaction between the scavenger O 2  and the dissociation products is thought to be (BOCl) 3 . 
     The results of mass spectrometer analysis of the residual BCl 3  in irradiated mixtures of 1 to 6 torr of BCl 3  in 20 to 25 torr O 2  are shown in FIG. 6. The enrichment factors indicated in FIG. 6 can be increased by increasing the power level of the ultraviolet source.