Lawrence Berkeley National Laboratory Recent Work
Title NEW ISOTOPES OF EINSTEINIUM
Permalink https://escholarship.org/uc/item/462945g3
Authors Harvey, Bernard G. Chetham-Strode, Alfred Ghiorso, Albert et al.
Publication Date 1956-06-01
eScholarship.org Powered by the California Digital Library University of California UCRL 3442
' -:\·.l..l UNIVERSITY OF CALIFORNIA
,.. ,·,
TWO-WEEK LOAN COPY This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Dioision, Ext. 5545
BERKELEY. CALIFORNIA DISCLAIMER
This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal 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 its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. UCRL-3442
UNIVERSITY OF CALIFORNIA Radiation Laboratory Berkeley, California Contract No. W·7405-eng-48
•
NEW ISOTOPES OF EINSTEINIUM
Bernard G. Harvey, Alfred Chetham-Strode, Jr., Albert Ghiorso, Gregory R. Choppin, and Stanley G. Thompson
June 1956
,;{'f-'rl' ~;J.:o·.;
~ ~ Printed for the U. S. Atomic Energy Commission
-~ _... .. ~,.. "\" ,'~
,. UCR.t-J«Z
~ ... NEW ISOTOPES. OF EINSTElNlt;rM Bernard G. Harvey, Allred Chetham•Strocle, Jr .• Albert ·Radiation l..abor•tory and Depa~tm·ent of Cb'emiatry · University of Califo·rllia, Bttrkeley, ·California . ' . . ' . ... . A.SS1'llACT . . . 24 . . ·The new isot~pes E 9, .E250, ~t 251 an4 EZSZ were produced by bom ... bardment of BkZ49 with. bellum iona."· A ·rec:oll method was used to eeP'lrate the bombardment products from. the target material, so that the·target coUld ~e used repeatedly. · ' , · Appro~imate excitation tul\c:tione for the fi:Jrmati.oft of t.he einsteinium isotopes were measttred as well as. their half Uvea, .,,A .. particle energies., and elecb:on.. capture bran.chln.i ratios ... · · .. ' ,_ ~ > .. ,. . . ,. . . .. . . -3- UCRL-3442 NEW ISOTOPES OF EINSTEINIUM* Bernard G. Harvey, Alfred Chetham-Strode, Jr., Albert Ghiorso, Gregory R. Choppin, and Stanley G. Thompson Radiation Laboratory and Department of Chemistry University of California, Berkeley, California June 1956 INTRODUCTION The bombardment of heavy-element nuclei with charged particles leads 1 mainly to fission. The cross sections of reactions such as (o., xn) are there fore mach lower for target nuclei of Z > 90 than they are for lighter elements. The excitation functions of several reactions of the (o., xn) and (a., xpyn) types 244 1 2 have been studied for target nuclei up to Cm . • It is of some interest to investigate similar reactions with even heavier targets to see whether the same pattern of competition between fission and spallation is maintained. In the work described herein, the 280-day fS. -emitting Bk249 was bom barded with helium ions of from 20 to 40 Mev, and reactions of the type (c, xn) were studied radiochemically. Such reactions can produce four previously un observed isotopes of einsteinium (symbol E, atomic number 99) with mass 248 numbers from 249 to 252. E could presumably be produced by (a., 5n) re action at the highest energies, but in fact it was not observed. . . . 249 - The production of Bk by multiple neutron captures and~ decays, 239 3 249 starting wit)} Pu , ~s already been described. The amoant of Bk avail- -8 13 able was only 1. Z x 10 g. or about 3 x 10 atoms. Hence it was necessary to use all of it in each bombardment, and to use the highest available helium ion beam density in the Crocker Laboratory 60-in.ch cyclotron. These requirements were met by using a deflector-channel target assembly preYiously developed for the production of the new element mendelevium (atomic . 4 number 101 ). The target material was deposited as a very thin layer over a This work was done under the auspices of the U. S. Atomic Energy Commission. ----·------... -:-·-~··r " ( ;o, '< . 1 ~ ••.•'" UCR.I,i- 344Z . . .... • ' ~R, l • .. ~ i . ' . •' '• . . . . • ' . . ·. ' . ' . '" ~~;· ~ .· . ; ~~~ rectangular ·a.rea 0.6 x 6 mm on a o.oos .. c:m-tbi~k gold foil. (This smaU target< ~~·:>·,-. ,) _. a~ea waa chosen to match the area: of t)le moiSt tntense.heU~•iC)n beam.·; Th~ ':· .· f..~::·~<~ • •• • • • • • • • • • • • • • • • •••• ,• .", .: ... §' ,; •...... ~ ...... t~rget area was expa.eed to a coltimatecl belhll\ft•iO"' bea~ wboee ~ntensity ·vuied ~ ·~·-~·· · .:· . . <:,::·.:;\~~om about. ZO~~~~ 75:.~mp /er.r/·; d~pen~~g·on how we·u the eyclotron wis ad·.,·~>:'~~~:~_,,~~:~. . ·:. ~_·.'·stl~ted. The b~am pa.iaeci.fb:efthr~~~bth~.g~ld foil ancl then thro~ab the Bti~f.9 ,;_ )-:;·:.\_. I' . ' ~ .. ; . ' ' . • . ' . ' • . . ~ .. . ~,· . ' '! . ' .:- >-r~#.et, so that nuclear reaction pr(Hlucta were ejected £-rom the target in .the·,:·-.,.'·;:\~'\;;~ -· ·.. · f~r~rd directioa. They:wej'e caught on :a 0.00025-cm~thiek gold ttcatcheri!·-' :. ..~: ,\;;\ ::: . . ( ' ·~ • . ,. • ' . . . .. l ·... '· ., ' f ' ·· )\:. loll 'placed about S.S min from th.e tuJet toij.. · ~y this methocl}t was possible. _\i(#)~i;j.:, " , .., ,, . . ; ia49 , ·. ' '.. ,._, '1 I., \ ,L,, ...... >- .. "to'b-=bard the Bk target a targ6 number of timee Witho~t the n:·eceasity oi .--~~~~/:;i;J~*:b~ ·.. · -~.<> ·. ~ .'.~ ~ ' . . . . . ·~.... . " • ' ·. !>of~~... :. '~t*.fi'l:~~; ..~~~ , _ ·.. >~:··;, di~a_ol-vi!ll it. · So~~ 45 ~omba.r~erds~ _: ~~y f)f ·them for as tot1~ ~~ 9 h4>ura .• · .···12~·~~};~~t~·· l k·:··. ,,,·~,/· . ·,were made with no 1oas of tarjet material.. · · · . · ;' · . . . lr;. "\ , . , I .. . • , ' . • • ~ • • ... ( ' ~- ~- • ~ ~·... • . ·. '• :· . .'· i Tb'e 'eaergy o£ the helium toae. varf.ed by inserting al'W11inum. degra~-..)>\;;:~t,;f.. :.~ . ' . . wa• . . . . . ,') .! ~i if• ~ ,. - ~ _ . ··:~ ·; ~os foils into the beam ahea_d of·. thi U:~g,t: .. · .The helium.. io\'1:. ~eam ·intensity· ~a~ -; ·; ~;/ ·. ·;_ . ;, .., / <,.in~asured with a J"ara~y cup ~eli behbut 'the thia aotd catcher foil.. The .. _:. ~.z;:r,;/:t>·:;;~ 244 246 : :-,~;:·:.' _.1~~ 249 targ41t also c:~nta.lned a ltn~wn -~~t.. of cm • tro~· ~hic:h C£ w~s :}~,1:;':: >:"~ •.. '·.,__ ~ . -. . • • ' '. ' ~· ,1.•;.\;J~·:.-- !"~1 ~l :-,<·.. produced by the (4. Zn) reaction. From the-known crosa section for this re... \;, -:"Y·.: .. ;.· .: . "' ,''action 5 and the measured amount of cr246 ti~ally separated fr,otri the catch~~_. - 7' ,>:?~{/;:_ ... ~:; • ) 't· • ~ ·.- ·..• . . '. . . . . ' • . . ~ • • . • ~ . 1 ... ~-"". . t:...... _,_ ,. foil, the over-all yield was ealculate4 £or_-.eac;h bombardn\e~t. The yield val~e ·. ~ ·.~:·h:.·: \ ) ' .. ~ ' : • ' ' . . '· • I • ·... ' ' ' • ' '.··,,I • ' ·' ... .-.·!-._.'!(.._ _: ~ .. ~ ' : . 1i. th~ collee~tion <~:. •: . l:' . . · ·obtained. in this way iru;:luded both tecoU effiCiency and the r.;:- ... -~ . .. ·. .··- ~ · · ., . ~ r~ :chemical handling loas.. tn almost ~~ery· case. yields between 60~ 90% w&~e ;_-. ·,.. ·. ··. · -~ :;, ~ • l '· ' • • :. -..~. :., .... obtained. .~ . . . . •., ...'' • .,:._ ;. -~ EXf'ERIMEMTAL ..... ,;. [ ' .~ . ~-~= -~. .\ :··, ·.. ·-~ ·:· . ' ~' . 1..' · Tar1et Preparation · " .~· ._. . . . . 209 t . ~- ' . The recoil o£ AtZll atoms produced by bthnbar.d.n1ent of Bi with h;li~··:t· ·' . ~;-,-. 4 ,, ' io~s- waa iavestipted by Chetham .. Str~de. For helium ion energies of ZO to 40, ·'. ~- ,.. ~I ''. ·~· • .' Mev, he found that high ejectiQn yields of nuclear reaction prod\lcts shoUld be 2 t ,·,' · obt~ined if the density of the target d.epoeit waa less than about 30 f48 /em .: . : : /-':-, · · l ~ ... ·'~· 2 ·.Since the. tarset area .wa!J only 0.036 em , the total weight of target and im-. . , . _.. -,.. ·p~ities could not be allowed to exceed 10·.6 g. Preliminary experiments with ·: >;- :. ia~gets of Cmz44 showed that it wae possible tn practice~ obtain ejection }'ield_s"~~;;. .. ~ 46 ~ery close to 1001?/o for the (a., 2.n.) reaction. ·The dt~ produced in t~s ~a.ee W:.~~<· .-: ' ... · found to be projected forward i1).t~ a rather narrow con.~... ;-: · -5- UCRL.. 344Z Radtochemica.lly pure Bk249 was isolated by precipitation and ioa exchaage methods, which havf been described earlier. 6 At this point, a kftOWQ ~mount of -em Z4-4 was added. Final chemical purity was obtained by elution of the berkelium and curium. with pure 6M,_,. hydrochloric acid through a bed, 4 em by 0.3 em in diameter, of purified Dowex·50 cation-exchange . resin (4o/o cross-linked). By use of a very low flew rate of eluant, the "I' .berkelium anci curium were eluted as a very sharp band in a volume of only about 0.1 ml. The solution was then evaporated to dryness. .,' 249 244 The deposition of the mtncture of Bk and Cm onto the gold target foil vias made electrolytically.' The Bk and Cm were dissolved in 6M ammo- _nium chloride of high purity at a pH of about 2, and transferred to a cell in I which the annealed gold targ'et foil was the cathode. The bottom of the cell l. (Fig~ 1) defined the area over which the target materials would deposit. A carefully cleaned platinum. wire anode was used. A current density of aittout 2 1 a:inp/cm of cathode surface; maintained for about 40 min, brought about the deposition of 80 to 90o/o o£ the target material onto tht!! gold foil. After _ the. ammonium chloride was rinsed off, the foil was washed with acetone t1· . \,,,. l and. heated to a dull red to expel volatile substances. f'inally it was fastened into a stainless steel holder (v. infra) with Epon* resin. 249 The amount of Bk in the target was estimated by counting aliquots of the cell solution before and after the deposition. A Wind.owless proportional counter was assumed to give a counting efficiency of 90% for the ~- particles 249 from Bk mounted on thick platinum plates. Z. Cyclotron Probe An expanded schematic view of the deflector-channel probe is shown in Fig. 2. The helium-ion beam was first collimated by means of the water-cooled copper and graphite collimator A. The second collimator B reduced the beam to the required size. Stainless st~l holder C supported to 0.04- em-thick An epoxy resin supplied by ~l(~:tiJi Development Company, Emeryville, California ,• , -6- UCRL•344Z Holder D supported the target. The foils C and D were cooled by means of rapid circulation of helium between them. The gold "catcher"foil E was mounted on the end of rod F with Scotch tape. The rod F terminated in a water-cooled aluminw:n block which served as the Faraday cup. The space between the target and the catcher foil was evacuated. The several cooling ~. and vacuum systems were interlocked so that the cyclotron could not be operated until proper conditions were obtained. The rod F could be very rapidly withdrawn after completion of a bombard- · ment, so that no time was lost in examining the products. Holders C and D sealed into the assembly with 0-rings so that they too could be readily withdrawn to permit a change of target or absorber. The target and collimator B assembly could be moved vertically by remote control so that the most intense beam could be located. Cooling of the foils was adequate to permit a beam as large as ZOO JJ.&mp /em Z to be used over prolonged periods of time, but such intense beams were not commonly available. 3. Chemical Separations At the end of a bombardment, the Faraday cup rod was removed and the catcher foil torn off. The foil was dissolved in a few drops of 8~ hydrochloric add containing a little nitric acid. The solution was then forced rapidly through a 5-byM0.3-cm bed of Oowex-1 anion-exchange resin. The resin bed was heated to 8_7°C by means of an outer jacket through which trichloroethylene vapor was circulated. Under these conditions, gold was very completely retained by the resin, while the tripositive actinide elements passed through. The resin bed was washed with a few drops of S_M hydrochloric acid. The solution thus obtained was evaporated to dryness. The several actinide elements were then separated from one another by means of elution through a bed of Dowex- SO cation-exchange resin, with a solution of ammonium CL-hydroxy isobutyrate as eluant. This, and the other chemical procedures, have been fully described el~ewhere. 6 The CL- radioactivity of the separated actinide ele ment fractions was then examined with a gridded ionization chamber and pulse height analyzer. T' -7- UCRL-3442 In experiments where the electron- capture activities were to be examined, further decontamination from radioactive impurities was required. The actinide element fraction from the Dowex- 1 anion-exchange bed was purified by co precip itation of the actinide elements with a carrier of lanthanum fluoride. The pre cipitate was redissolved in a saturated solution of boric acid. The actinide elements were further purified (and separated from the lanthanum carrier) by elution through a bed of Dowex-50 cation-exchange resin with saturated hydro 6 chloric acid containing ZOo/o ethyl alcohol. The final separation of the actinide elements from one another was then made as described above. The full chemical procedure required about Z to 2.5 hours. NUCLEAR PROPERTIES OF EINSTEINIUM ISOTOPES Four previously unobserved isotopes of einsteinium were produced in this work. Their mass assignments were made initially from the systematics of 7 decay and confirmed by means of excitation functions. Their properties are summarized in Table I. Table I. Nuclear Properties of Einsteinium Isotopes · lsotope Type of Half o.- Particle Ratio of electrons decay life energy capture to branching (Mev EZ49 EC. 0. Z hr 6.76 760 Ezso EC, o.? 8 hr Not observed a Not observed - 251 E EC, 0. 1.5 days 6.48 190 EZ52 o., EC? -140 days 6.64 EC Not observed t l/ 2 !3- > 20 yr The electron-capture decays were observed in a windowless proportional counter, for which a counting efficiency of 80o/o was assumed for thin samples -8- UCRL-3442 mounted on platinum plates. It was not possible to measure the counting efficiency .for any of the nuclides involved, but the assumed counting effi ciency is based on the general experience of this labo.ratory. The observed a-particle energies (not necessarily g~:ound-state transi tions) of the known isotopes of californium and e~nsteinium are plotted against their mass numbers in Fig. 3. The einstei.nium isotopes show a. break associ ated with a neutron number of 152, which was first observed for the californium 8 isotopes. A similar break has also been observed for the isotopes of fermium. 9 248 (In Fig. 3, data are included for the recently discovered nuclides E and Fm2Sl.IO, 11) The half-life measurements were most accurately made by resolution of proportional-counter decay curves such as that shown in .Fig. 4. The decay of E 252, however, was observ~d only by means of a.-pulse analysis, which yielded th~ decay curve shown in Fig. 5. The decay of the other nuclides was also studied by a-pulse analysis to check the half-life values obtained in the propor tional counter. REACTION CROSS SECTIONS r ' The yields of the four new isotopes of einsteinium were measured in • some experiments with a windowless proportional counter and in others by e1-pulse analysis. The accuracy of the cross- section measurements was 249 limited to about :1: 30% by ( 1) uncertainty in the amount of Bk in the target - 249 (the very soft~ activity of Bk is not easy to count accurately, since the . counting yield is very sensitive to the thickness of the sample), and (Z) count ing statistics in those experiments in which yields were measured by a-pulse . analysis. For example, less than 20 events of the 6. 76-Mev a-particle of E249 were usually observed. 249 249 The~- decay of the Bk produced C£ , so that bombardment of the target yielded progressively larger amount$ o£ Fm isotopes whose electron-: capture decay produced further amounts of E 250, E 251, and possibl-, E 249. Z49 250 , . Reactions such as Cf (4, p2n.) E presumably also occurred . .. • -9- UCRL-3442 249 The Bk (4, n) and (~, 2n) reac~ions wete studied-at a time when the 249 250 amount of C£ in the target was negligible. The E was produced by 249 250 250 · s:k249 (a, ln), by C£249 (4. p2n), and by c£ (a, 3n) Fm ~ E . 249 249 Although the atom J"atio of Bk to C£ in the target was alway~ larger . . . '250 . . 249 . than 2: l, a large fract1oon of the E ·was probably produced by the Cf (o, p2n) and: (a, t) reactions, since reactions of this type appear to have unexpectedly . 12 .· " large cross sections.. It was not possible to distinguish between the various 250 mode.s of production of E , so that the excitation curve shown in Fig. 6 is a ·sum of the cross sections and serves mainly to confirm the mass assignment 250 of E . The cross sections gi~en for the (J:a., n) curve are based on the assump tion ~at E 252 is ~-stable. This is: almost certainly not true. Hence the cross· 249 252 s~ctlons for Bk (4, n) E represent lower limits, but probably fa~ly close· . ' ~~ ones . .;-.·· . '· t . f . : ~ : ' . . . The (B, n), (a., Zn), and (n. 4n) curves in Fig. 6 are very similar in shape . ' · ·•. 1 ·and magnitude to th()se obtained for other somewhat lighter nuclides such as · · . ·. 238 . Z39 •+ . Pu or Pu . The cross sections are comparable in magnitude, showing ·~l that fission is not rapidly superseding spallation as the Z and A ol the target k:. a.-e increased, (For a more detailed discussion, see Ref. 1.) The flatness of both the (a, n) and (a, 2n) curves is not readily to be explained by the usual compound·nucleus mechanism. To avoid fission or further particle evapora tion, the residual nucleus must be left with less than about 6 Mev of excitation . . This implies that much more energy is carried off by the neutrons in the (4, n) (·.. " and· {a., Zn) reactions tha.n is required by an evaporation mechanism involving commonly accepted values for nuclear temperatures. The small peak in the 1 . 1 (.4. 2n) curve might then be attributed to a compound-nucleus-evaporation meth.. '•' atdsm, while the long "tail11 at higher energies must be due to a different mechanism- - perhaps a direct interactioa of the helhtm ion with one or two neutrons. Such a non.·compound-n.ucleus m·echanism implies the transfer of lese momentum to the reaction prGduct, and hence that the recoil collection effi ciency might be less than expected. However, the Bk249 target depo,sit was sufficiently thih that recoils with much lees than the full momentum would -10- UCRL-3442 escape. (This was shown by measuring in several bombardments the small amount of Bk249 ejected from the target by elastic and inelastic scattering processes, which gives a very sensitive measure of the thickness of the 13 246 target. ) In addition, the measurement of the Y,ield of C£ from the Cm244 ad<,ied to the target automatically compensated for any such effects insofar as they occurred equally for nuclear reactions o£ Bk249 and for the .(e~, Zn) reaction 244 of cm . ... ACKNOWLEDGMENTS it is a pleasure to thank the crew of the 60-inch cyclotron for their extremely careful and skUlful operation of the machine during some 200 hours of bombardments. We wish to thank Lennart Holm for participation in the later stages of this work, and Professor Qlenn T. Seaborg for his continued interest. We are especially indebted to Thomas C. Parsons for the production and 249 primary purification of the Bk taraet sample. .. ,... , .. -11- UCRL-3442 '. . '.• ·~.... : ...... REFERENCES . ..~. ' .. ,... l.. · Glass, Cobble, Carr, and Seaborg, Phys. Rev., ~o be pu~Ushed . . " .Z~- : Clu~tham-Strode, Choppiri, a:nd Harvey, Phy•. Rev. lOZ, 747 (1956). . , ...... · 3. ·.·ThOmpson#· Gb.iorso, Ha;rvey, and Choppitt, Phys. Rev. 93, 908 (1<)54). . ~ . . - ,..,..,. '·, 4. Gldorso, Harvey, Choppin, Thompsoll, and Seaborg, Phys. Rev...... 98, 15,18 (1955)~ , 5; · A.·. Chetham .. strode, Lilht Isotopes of Berkelium and Caliiornium, . UCRL- 33Z2, ·Feb. -1956. Unpublished. 6. ···Thompson, :Harvey, Choppin, and Seaborg, J. Am. Chem. Soc. 76, ...... 6ZZ9 (1954);' C:hoppin, Ha.rver. and Thompson, J. ~org. Nuc. Chem. £_, ' 66 (1956). 7. Glass, Thompson, and Seaborg, J. lnorg. Nuc. Chem.· 1. 3 (1955). - . . ._. 8. Ghiorao, Thompson, Higgins, Harvey,. and Seaborg, Phys. Rev. ~· 293 . (1954)~ ._. 9. A.- Ghiorso et al.. unpublished data: A. ·M. Frh_;dman et al., Phys. Rev. ·ioz~- sa~ (1956). - . lO,~· A. Chetham-Strode and L. W. Holm, to be piplisbed~ 11. !4rvey, Chetham-'Strode, Ho.lm, an.d. Thompson, to be published. ll.. W. A. Wade, private communication, May 1956. 13. · .A. Cbetham-Strode, unpublieh:ed data. " > ' I FIGURE LEGENDS Fig. 1. Cell for electrodeposition of target. Fig. 2. Cyclotron target assembly (schematic). Fig. 3. a.-particle energy vs neutron number for Z = 99 to 100. Fig. 4. Windowless proportional counter decay curve for sample contaimng. E 249 , E zso , and E 251 • 252 Fig. 5. Decay curve of 6.64·Mev o. activity of E • Fig. 6. Excitation functions for formation of einsteinium isotopes. key y Bk249 (a.,·n) E252 • Bk249 (a., 2~) E2S 1 • Bk249 (a., 3n) E250 + Cfl49 (a., 3n) Fm zso _ + Cf249 (a, pln) E2so + C£249 (a., t) E2so A Bk249 (a., 4n) E249 + C£249 (a., p 3n) EZ49. MU-11742 Figure 1 ABSORBER VACUUM GRAPH FARADAY CUP ¢Be ION BEAM F MU-11743 Figure 2 7.5---r------ro-2-...... -----ro---.. Fm 50 5 >- (!) ~7.0,~~---+~~~~~--~~ ~ E248 w lLJ _J .,_(.)- a:: ~ 0.:6.~-+----+---~~-~T-----t ~ 148 150 l52. 154 156 NEUTRON NUMBER MU-11744 Figure 3 . -~ .. • 3 10 9 8 7 End of Bombardm nt 2~15 6 5 4 ...... 3 ...... ·····:·.·· ...... 2 :, :::: ...... :.. "· •, (/) ... w ~102 z 9 8 ~ 7 N 6 (/)' 5 1- z 4 :::> 0 (.) 3 2 2400 2400 2400 2400 FRIDAY SATURDAY SUNDAY MONDAY TUESDAY MU-11747 Figure 4 C\1 10 C\1 1 w 0. I J'l I tT ~I w I ll • r- • / j :::::> 140 DAYS :I1 -z I ~ ...... (/) r- z :::::> 0 u I ~ 0.0 I 0 10 20 30 40 50 60 70 80 90 100 TIME (DAYS) MU-11745 Figure 5 . ) / ...... / -~ I ;• (/) 10 ~ .. z - ... c:: / - <( / " / ~· - co !.-'_ ~ \~ ...J I ~ ...J '/ ~f" ~ / --- I/~ .. z ~7 0 / 1- 1.0 I "!' (.) r/ / w /T -- (/) -- , -- r--... (/) if"' (/) I 0 ~49(cr Bk' 49(cr, c:: _/sk 3n) 4n) (.) v ( hresh pld T hresh old 0. I I 1 22 24 26 28 30 32 34 36 38 40 4 2 HELIUM ION ENERGY (LAB. SYSTEM) t.Aev. MU-11746 Figure 6 • t