Research proposed in the following areas are briefly described: the measurement of photoionization cross sections near threshold for the excited states of uranium; measurement of the symmetric charge exchange cross section; and measurement of chemiionization cross sections for electronically excited uranium atoms

Dubrin, J.

Duewer, W.

UNT Digital Library

M A  UCRL-ID- 126662 1 Research Proposal on Certain Atomic Physics Measurements Associated with the Laser Isotope Separation Method J. Dubrin W. Duewer March 6,1973 DISCLAIMER This document was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or imp l i e  or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not n d y  constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not n-y state or reflect these of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. This report has been reproduced directly from the best available copy. Available to DOE and DOE contractors from the Office of Scientific and Technical Information P.O. Box 62, Oak Ridge, TN 37831 Prices available from (615) 57&8401, Fl'S 626-8401 Available to the public from the National Technical Information senice U.S. Department of Commerce 5285 Port Royal Rd., Springfield, VA 22161 I DISCLAIMER Portions of this document may be illegible in electronic jmqge products. Images are produced from the best available original dOCUment. DISCLAIMEX This report was prepared as an account of work spomored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabili- ty or responsibility for the accuracy, completeness, or usefulness of any information, appa- ratus, product, or process disdosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commerdal product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily comtitute or imply its endorsement, recommendation, or favoring by the United States Government of any agency thereof. The views and opinions of authors expressed herein do not uecessar- ily state or reflect those of the United States Government or any agency thereof. T 0: FROM: James Dubrin and W i l l i a m  Duewer James  Kane and John Emmet t  . SUBJECT: Resea rch  proposal on cer ta in  a tomic physics measurements  associated with the l a se r  isotope separa t ion  method as applied to U-metal vapor. The select ive photoexcitation of U235, e i ther  in the elemental  o r  U F  heavier ,  m o r e  abundant isotope. In a second step,  the electronical ly  exci ted uranium a tom (U') is photoionized, while the vibrationally excited U F  is photodissociated to  give ei ther  U F  o r  U F  Although photoionization is present ly  the mos t  promising second s t ep  in the meta l -a tom scheme,  other  methods including selective sur face  ionization and chemiionization form,  has been proposed a s  a f i r s t  s t ep  in its separa t ion  f rom the 6 4. 6 5 4' must a l s o  be considered. problems and  uncertainties associated with the two-photon approach, in general .  Our en t i r e  attention in this proposal is d i rec ted  toward the technique employing U -metal as the s tar t ing material. Needless to  s a y  there  are many fundamental We have little doubt that  the two photon approach  will  work on the r e s e a r c h  level. The real question, though, is whether i t  can  be c o m e r -  c ia l ly  competit ive with the existing methods, and the answer  to  this is totally unclear  to  us at this stage.  the fu tu re  abil i ty to  fabricate powerful, bu t  highly efficient and stable,  Its success  m u s t  cer ta in ly  depend on Universityof California IAWRENCE LIVERMORE IABORATORY . Memo to J. Kane/J .  Emmett  - 2 -  March 6, 1 9 7 3  "! tunable visible l a s e r s  wi th  narrow line widths. of importance is the need to know the radiative l ifetimes of a number of the electronical ly  excited s ta tes  of uranium. At nearly the same  level The studies proposed here are p r imar i ly  concerned with the second s t ep  of the  process .  to  m e a s u r e  m a y  be useful to predict genera l  t rends,  but we are  r a the r  skept ica l  of their  utility beyond that. Theoretical  calculations on the c r o s s  sections we wish The modest  program outlined below would generate a n  important f rac t ion  of the input data necessary  to  c l ea r ly  differentiate the many possible variations of the two s tep process  f r o m  both a standpoint of feasibility and cost. . Proposed Work Three  major  experimental  investigations are planned. Common to  all of these is a standard, differentially pumped molecular  beam apparatus consisting of a main  scattering chamber ,  sou rce  chambers  housing the neu t r a l  and ion beam sources ,  and finally a detector chamber enclosing a medium resolut ion-mass spectrometer .  min ima l  (1 0 equipment are  readily available. r e s u l t  of requir ing only i o n s  ignal measurements  for  the c r o s s  section determinations.  A significant problem will be the production and use of high tempera ture  U-vapor sources .  Fortunately,  exper t i se  in  this area a l r eady  ex is t s  at the Laboratory. The vacuum requirements  a r e  - 6  t o r r )  and fur thermore the necessa ry  electronics  and other This very happy circumstance is a d i rec t  A .  Measurement  of Photoionization C r o s s  Sections Near  Threshold F o r  The Excited States of U. Photoionization c r o s s  sections of ground state atoms a r e  typically two to  three  o rde r s  of magnitude less than the charac te r i s t ic  gas  . Memo to J .  Kane/J .  Emmett  - 3 -  March 6, 1973 -15  2 kinetic value, 3 x 10 cm . However, a toms populated in cer ta in  excited s ta tes  c a n  have c ros s  sections considerably la rger  than the ground state values. Moreover,  near threshold, these c ross  sections c a n  be  expected to display r a the r  s h a r p  resonances corresponding to absorpt ion into autoionizing s ta tes ,  and the maximum c r o s s  section for such a transit ion may  be o r d e r s  of magnitude la rger  than the off- re s onant background (continuum absorption) . To establish these c r o s s  sections selected,  excited states of uranium, formed by pumping a U-beam with a dye laser, are subjected to variable monochromatic radiation (bandpass - 0. lev) f r o m  a high intensity lamp. * the decay of U , and using the radiative l i fe t imes  of this s ta te  f r o m  sepa ra t e  measurements ,  the photoionization c r o s s  sect ion follows directly.  . By simultaneously monitoring the visible photons f r o m  Quite conservative feasibil i ty calculations indicate that c r o s s  * sections ca. cm2 can  be measu red  without difficulty. + B. Measurement  of the Symmetr ic  Charge Exchange Cross  Section: U -U. At  low relative kinetic energies ,  charge t ransfer  c r o s s  sections of the above type are far grea te r  than the geometr ic  or  gas kinetic value, especially if the atom-ionization potential is small a s  i s  the case  for  U. photon method as applied to U vapor, for its occurence will effectively des t roy  the selectivity achieved in the f i r s t  step.  T o  illustrate: If the charge  exchange c ross  section were 5 x 10 cm at 0. SeV(ca. 20OO0C), a value which would not s u r p r i s e  us,  about 98% of the U exchange with the U238 in t ravers ing  a 2 cm-path of U vapor at a p r e s s u r e  of 10 t o r r !  The  experimental  determination-of the charge t r ans fe r  c r o s s  sections will be achieved using the conventional technique This naturally represents  a ser ious  problem in the two -14 2 2 3  5+ would -2 Memo to  J. Kane/J .  Emmett  -4 - March 6, 1973 of counting the s low ions produced in the encounter. beam intensity in all likelihood will be  monitored with a surface ionization detector, consisting of a n  "oxygenated" tungsten wire.  A l imited amount of time wil l  be devoted to calculating the c ros s  sections with the aid of simple theoret ical  models. The neutral  C. Measurements  of Chemiionization C r o s s  Sections for  Electronically Excited U Atoms. A n  alternative to photoionization as the second s tep  would be 23 5:g $c t t O2 -+ U 0 2  the removal  of the U g ive  a n  ion, e. g., U chemiionization of the ground state uranium m u s t  be much slower.  The re  a r e  three advantages that  this chemica l  react ion scheme holds over photoionization: (1) The cos t  pe r  "particle" is considerably less for  molecules (e. g . ,  0 - )  than photons; (2) Asymmetr ic  charge t ransfer  c r o s s  sections normally are markedly  smaller than the  i symmet r i c  cases  (UO vs. Ut); (3) Chemiionization c r e s s  sections for  exoergic  reactions can be a s  high a s  2 x 10 major  disadvantage is that  the upper l imit  on  the molecular f lux,  set by vacuum requirements,  is substantially l e s s  than the photon f lux  attainable by present  day lasers. through react ion with another species  t o  t e-. Of course  the rate of 2 2 -14 2 cm The very Chemiionization c r o s s  sections for  both the ground and various excited s ta tes  of U will be measured  employing a lmos t  the exact same procedure and analysis as outlined under A. fication is the substitution of the photoionizing light beam by a molecular  beam. The t r iv ia l  modi- Since the re  is  documented evidence?,  that uranium t W. L .Fi te  and P. Irving, J. Chem. Phys. ,  56, 4227 (1972). P o  absolute * -c r o s s  sections were obtained, and in fact it is unclear whether U or U was the ma jo r  precursor  to the observed product ion, U 0 2  3 t Memo to  J. Kane/J .  Emmet t  - 5 -  March 6, 1973 a toms  undergo chemiionization with molecular oxygen, we would initially concentrate on the U ( U " ) - 0  collision sys tems.  Other candidates such  as N 0, NO and NO would a l so  be examined. 2 2 2 Support and Per s onne 1 On the basis  of past  experience in molecular beams,  we es t imate  that 757'0 of the overa l l  effort  will be devoted to the construction of the appara tus  and  the remainder  to data acquisition. elimination of single study (A, B, o r  C)  will  not mater ia l ly  reduce  the cos t  o r  t ime).  (Thus fo r  example the 3k Personne l  1 .  J. Dubrin 1 m a n  yea r s  11 2. W. Duewer 1 a 3. Technician 4. Exper t  1 5. Expe r t  ZC 1 / 4  6 Shop Support  I 1  1 I '  1/8 I '  b 1-1/8 d a. To a id  in: Apparatus assembly,  Major component testing, Data acquisition. b. U-beam sources  c. Dye l a s e r s  d.  Machine, glass,  and electronic * Much of the same apparatus can be used in a measu remen t  of the radiat ive l ifetimes.  Mcrno to J. K a n e / J .  Emmett -6- Estimated Costs  of Major Components 1 .  2.  3. 4. 5. 6. Vacuum sys t em Laser and beam monitor High intensity light sources ,  filtering sys t ems  and detectors  Fluorescence monitoring system, including counting sys tern Mas s spec t rometer ,  including counting s y s t e m  Signal averaging electronics  :< * M a r c h  6 ,  1973 $ 5 , 0 0 0  25,000 4,000 5,000 20,000 10,000 * M a y  be  available J W  D: WHD: ob Distribution J. Kane J. Emmett R. Batzel  E. Catalan0 J. F r a z e r  C.. Haus smann R. Kidder W. Krupke E. Teller E. Worden File Copy 1/A Copy 2/A Copy 3/A . Copy 4/A Copy 5/A Copy 6/A Copy 7/A Copy 8/A Copy 9/A Copy 10/A Copy 1 1  /A 

Research proposal on certain atomic physics measurements associated with the laser isotope separation method

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Research proposal on certain atomic physics measurements associated with the laser isotope separation method

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