The Discovery of Protactinium Ruth Lewin Slme Sacramento City College, Sacramento, CA 95822
Most scientific discoveries are described only after they are completed, in articles, memoirs, or biographies that are unavoidably colored by hindsight. An exception which has recently come to light is the discovery of protactinium, vividly described inletterslwritten by Lise Meitner to Otto Hahn between 1916 and 1918, when Hahn was in the army and Meitner sustained their collaboration alone. Hahn's part of the corres~ondenceisnot extant, but Meitner's letters clearly reveal that she did nearly all the work: chemical separations, complex indicator experiments, exacting measurements withprimitive electro~co~eli, procurementof material and equipment despite wartime shortages. (The lrtters also reveal-the formalities of an earlier era: after 10 vears of friendship Meitner addressed Hahn with the formal "Sie" and beean everv letter "Dear Herr Hahn!") From her letters we also sense ~ e i t n e r ' sfrankness and independence, her concern with com~etition.her doubts, and later her . joy as the existence of prkactini'um became certain. Unlike most other radioactive substances, protactinium was found not by chance detection of its radiation but by a painstaking search for a "missing" element. The search began early in 1913,just after Otto Hahn and Lise Meitner left their old laboratory in the University of Berlin for the newly founded~~~~-~~ Kaiser Wilhelm Institute (KWI) fiir Chemie in Berlin-Dahlem. Meitner and Hahn we're by.then experts in the field of radioactivity; physicist and chemist, they had collaborated for over five years. But their old laboratory had become so thoroughly contaminated with radiation that they no longer could investigate weakly radioactive substances. In their clean new laboratories Lise Meitner and Otto Hahn decided upon an intriguing new problem, made difficult by its complexitv and the absence of strong radioactivity. They immedidecided to search for the "mother suhs&ce"-the ate precursor-of actinium. Actinium had always been one of the least well understood of the radioactive elements. Extremely scarce, its atomic weight was unknown, its chemistry uncertain. A beta emitter, it headed a decay series distinct from the uraniumradium series, yet it appeared that actinium was itself derived from uranium: it was always-and only-found in uranium-bearine minerals. alwavs in amounts ~roportionalto the uraniumUpresent. But tke link between uranium and actinium was a mvsterv. The problem, as Meitner and Hahn expressed it, was "to fibd thatsubst&ce which.. . forms the starting point for the actinium series, and to determine whether and through which intermediates actinium is derived" ( I ) . This was not a simple task. Purified uranium had never been found to generate actinium, even though actinium, with a half-life of about 25 years, evidently required constant replenishment to prevent its complete disappearance. For several years scientists were "compelled t o assume a long-lived mother substance for actinium without, however, being able to say anything about its chemical or radioactive properties" (2). In 1913 two major new principles clarified the entire field ~
-
~
~
~
~~
~
~
'The letters were in Ono Hahn's possession for many years, disappearedduring World War 11, and surfacedIn 1983 in a coflection of Hahn's personal papers which was acquired by the Bibliothek und Archiv zur Geschichte der Max-Planck-Gesellschaft. Berlin-Dahlem. The author is responsible for the translations.
Lise Meitner and Olto Hahn in
their laboratory, ca.
1920.
of radioactivitv. Frederic Soddv's c o n c e ~ of t i s o t o ~ voreat a b i f o r ;he nized the radioactive species into the first time, and the group displacement laws, stated first by Kasimir Fajans and then by Soddy, stipulated that an element was displaced two places to the left by alpha emission, one place to the right by betaemission. Alsoin 1913actinium was finallv assianed erouv. 111 in the periodic table. With - to radium in group 11, thorium in group IV, and uranium in group VI, only the position between thorium and uranium remained empty. The missing element, presumably pentavalent, was expected to resemble tantalum in its chemical behavior. According to the group displacement laws only two choices were oossible for a Drecursor of actinium: a betaemitting radium isotope, or an alpha-emitting isotope of the unknown pentavalent element. The first possibility was ruled out: no actinium had ever been found in the presence of a radium preparation. In 1913Frederick Soddy concluded that the mother substance of actinium must be an alphaemittine isotope of the missing pentavalent element, which he desi&ated"eka-tantalum%). Not long afterwards, Kasimir Fajans and Oswald G6hring discovered UX2,the decay product of the beta-emitting thorium isotope UXI [Th-2341. Applying his group displacement laws, Fajans assumed UX2 to be an isotope of the missing group V element and achieved a partial separation hv takine advantaee of its chemical resemblance to tantah;m. ~ a j i n s a n dG(yhringnamed their new element brevium, for its verv short half-life of nbout one minute. T h ~ vrecoenized, however, that brevium, a short-lived beta ;mitt&, could not be the mother substance of actinium. Faians and Giihring searched for a long-lived alpha-emitting isotope of US, but found nothing. Meanwhile Meitner and Hahn impro;ed Fajans' technique somewhat t o obtain better separation of the isotope (4). Encouraged, Hahn and Meitner began their search for the actinium precursor whose chemistry, they were sure, would resemble tantalum. They explored two sources: uranium salts, which had been extracted from their ore 25 years hefore, and the uranium ore pitchblende. The uranium salts Volume 63 Number 8 August 1986
653
contained fewer extraneous radioactive suhstances and much less mother suhstance than the cruder pitchblende. In hoth cases. however. extracts of the groun chemically similar to tantalum contai"ed polonium, tKori;m, and many other highly radioactive suhstances which obscured the weak radiation from the mother suhstance. No matter what the nrocedure, the inherent difficulty w a that the mother substance, with its slow rate of decay and correspondingly low energy alpha particles, was impossible to detect in the presence of stronger radioactivity. All that could he done was to monitor the preparations for several years, in the hope that when the short-lived contaminants were gone the weak hut steady activity of the mother suhstance could he detected. This was not enoueh. however. T o prove they had mother substance Hahn anihieitner would have to show that i t eenerated actinium. But weak prenarations of actinium coild not he detected directly, since-beta electrosco~eswere relativelv insensitive. I t was necessary to monitor aipha radiation from the decay products of akinium. actinium emanation IRn-2191, and the active deposit, which included Po-215 and ~ i - 2 1 1 : ~ensure o constant conditions for long-term measurements, Hahn and Meitner mounted various preparations in place under fixed electroscopes. They knew that the investigation would take years. In the spring of 1914 Hahn and Meitner discovered a new method for separating the tantalum group from pitchhlende that minimized contamination from other radioactive suhstances. They found that pitchhlende treated with nitric acid left an insoluhle residue of silicon dioxide "which retained only very small quantities of radium, radiolead, and ionium: i t contained. in contrast. nearly all of the elements similar'to tantalum.' Thus the sought:after [mother] suhstance must here he enriched relative t o radium, etc." (5). Aware that Soddy, Fajans, and others were also looking for the mother suhstance. Meitner and Hahn kept this promising new method to themselves. Before they-could proceed World War I hegan. Otto Hahn was conscripted a t once. He served for the duration of the War in the special gas warfare unit organized hv Fritz- -Haher (6). Lise Meitner remained in Berlin and k;pt busy with herown projects and the joint work on actiuium. All Germanv and Austria were confident that the war would he won in few months. But the war dragged on and Meitner became restless. In the summer of 1915 she returned t o her native Vienna and volunteered as an X-ray nurse in Austrian army hospitals. The experience was not what she expected-she was idle much of the time--and in October 1916 she returned t o Berlin, eager for physics and hrimming with new ideas (7). Meitner was appalled to find that Fritz Haher had requisitioned much of the KWI fiir Chemie for war work. "I hope that the Lion [i.e., Haber] will not get his claws into our modest section, especially since our private laboratory with its physical apparatus would hardly he useful for the chemical studies in question", she wrote t o Otto Hahn (7).". . .The Haher people treat us, of course, like captured territorythey don't take what they need, hut what they like.. . Who will guarantee that they won't come over here, and then evervthine will reallv he lost." Meitner feared that three . years of work on the actinium project would he destroyed if the fixed electroscopes were moved. "1 shall do everything to prevent it, we havebad measurements going for such a long time . . . hut they have the arrogance of victory" (8).Hurriedly Lise Meitner measured the activities of the old samples, and prepared to test for actinium emanation (Rn-219). "Working in the Institute is like running hurdles" (8). In January 1917 Otto Hahn spent several weeks in Berlin. With Lise Meitner permanently hack in the lahoratory they could plan ahead for the first time since the start of the war (9). They abandoned the uranium s a l t e t h e y showed no signs of mother substance. But the silica residue extracted
a
from pitchhlende over two years before displayed a small hut unmistakable amount of actinium emanation (5). Here a t last was the first glimmer of success. Now Hahn and Meitner were anxious to move quickly. They no doubt hoped that a major project would protect their lahoratory to some extent, and they also feared that Fajans and Soddy were on the trail of the mother substame. But research during the war was exceptionally difficult. Equipment was scarce and expensive. Pitchblende was no loneer readilv available. Bv the third vear of the war food a n i f u e l were scarce, and &vilians in dermany and Austria suffered their first "turnin winter". But the worst nrohlem was that the students, ali;istants, and technicians had long disappeared into the military. Lise Meitner worked entirely alone:~nFehruary Hahn returned to the front, impatient fo> news. Dear Herr Hahn!The pitchblende experimentis of course important and interestine hut vou m if I cannot do it rieht - must not he a n-. " now. .. I have ordered the vessels for our actinium experiments, will get them in a few days and will begin right away .. . be well and please don't be angry about delays with the pitchhlende. Believe me, it is not because of lack of will, hut because of lackof time. I can't very well do as much work alone as the three of usZdid together. Yesterday I bought 3 meters of rubber tubing for 22M!! I got quite a shock when I saw the bill (10).
-
By the end of February Meitner began the separations. From 21 e - of -pitchblende, she ohtained 2 e of SiOs, of which 1.5 g was set aside. After adding potassiuktantalu& fluoride (a carrier for the mother suhstance) to the remaining 0.5 g SiOz, she dissolved the mixture in HF, boiled it in concentrated sulfuric acid, and ohtained an insoluhle residue containing all the tantalum and with it, presumably, the mother suhstance of actinium. As the mother substance emits alpha radiation (presumably of shorter range than its decay products), our preparations ought to show a strone.. aloha . activitv... which in view of the half-life of actinium ought to increase only very slowly, despite i t s 5 alpha emitting decay products. However. if one fikQrd out the ~ l n walpha radiation from t h e mother substanceofactinrum,oneahuuldohserve an alpha activity [from the decay products of actinium] which rises from almost zero at a much faster rate. (One cannot think of measuring the [actinium]beta activity of such a weak preparation.) (5). The alpha activity of the new preparation was monitored; exactly as expected, "the original suhstance possesses a short-range alpha radiation, and decays to products with faster alpha radiation. . .whose intensity climbed to double the initial intensity in several weeks." The untreated 1.5 g SiOn showed exactly the same behavior, "proof that the SiOn residue of pitchblende contained the new substance and was indeed a suitable starting material for its extraction" (11). The results could not have been better, hut they did not prove that actinium or its mother suhstance was the source of the alpha activity. The most eonvincine evidence that ~-~the increase ~~-~in activitv ,is due to the accumulation of artinrum would of course be the measurement of its active deposit or emanation As the preparations we had available were too weak, we chose an indirect method, that is, the exclusion of all other possible radioactive products (11). ~n
~~
~
~
~~~
~~
~~~~
u
854
Journal of Chemlcal Education
In Anrill917 Otto Hahn was in Berlin again for a short period:and with Lise Meitner devised a series of indicator tests for other radioactive snecies which mieht he resnonsihle for the observed alpha radiation. "If the radiation were due to previously known substances, only radium D [Pb-2101or ionium [Th-2301 exhibit the chemical hehavior and the short range of the original It is not clear whether Mekner is referrina to Otto von Baever. wkh on the magr;etic line whom she and~tiahnworked for several spectra of beta emitters, or to a former student in their laboratory. ~
~~
radiation which this substance shows" (11). (Pb-210, a heta emitter. decavs t o Bi-210. and then t o Po-210. a n aloha emitter'of faihy long halglife. Th-230 is also along-l&ed alpha emitter.) T o determine whether radium D or ionium were carried along with the tantalum group, known quantities of their respective isorrwes ThH [l'h-2121 and UX [Th2341, both beta-emitters, were a d d e d a s indicators a n d t h e silica processed a s before. Dear Herr Hahn! Preparation #9 seems very promising . . . the strong alpha activity cannot under any circumstances arise entirely from ionium . . . In my opinion, ruling out RaD [Ph-2101 is not as certain, since at first #9 had some ThB [Ph-2121.. . Possibly the alpha increase is still masked by the decay of ThB. I am also disturbed by # 5 ; its alpha activity has been constant since April 20, its heta activity, however, has continuously increased. According to your experiments this cannot he due to uranium X [Th-2341 (you tested for uranium and found none) so maybe it is due to the accumulation of RaE [Bi-2101 .. . At any rate I completely agree with you that these tests must he conscientiously pursued, hut if possible a direct determination of actinium should also he made. The question is only whether the emanation method is most practical, or whether it would be better to try the active deposit of actinium, especially if we use large quantities. What do you think? 1 will do a trial in any case (12). For the next phase-direct evidence of actinium-larger amounts of starting material were required. In May Lise Meitner went to Vienna t o ask Stefan Meyer, head of the Radium Institute, for a supply of pitchhlende. Meyer was Lise Meitner's lone-standine collearme a n d friend. with whom she began ra&oactivit;researcTh years before. w e see t o Hahn t h a t wartime conditions from Meitner's report . (12) . created difficulties, even between German and Austrian allies. One cannot buy pitchblende [in Austria] because its export is prohibited. I spoke to Prof. Meyer without telling him any more than was necessary about our experiments. He offered to give me 1 kg of uranium residue, that is, whet is left over after uranium bas been removed. It thus contains all the radium! I wanted to take it withme, went to the Finance Ministry for apermit, but they wanted assurances from the Radium Institute. So I had to rely on Meyer's promise that he would take care of it as soon as possible. Even without radium one needs a permit from the Austrian ministry to ship the residues. But first I want to wait for the results of Meyer's efforts. . . Export is strictly forbidden, a consequence of the emhargo initiated by Germany. .. . A week later Meitner reported (13) on t h e progress of the indicator experiments, . .. I assume you cannot remember all the preparations and their numbers in your head, so I will quickly remind you of the separation process. Preparation # 8 is the one not dissolved in HF. #9 [contains] Ta after HzS04evaporation, which was later heated in HN08. #10 is the NH3 precipitate from the nitric acid solution. [Of the three, only #9 was expected to contain mother substance; # 8 and # 10were controls. Known quantities of the indicators ThB and UX (both heta emitters) had heen added to all three preparations.] #8 originally contained most of the ThB [Ph-2121 and UX [Th234). Since April 30 the @-activityhas decayed exactly like UX . . . The alpha activity decayed faster than UX but slower than the 0 activity. That indicates that alpha radiation of short range is still present, in any case ionium, could also include some RaD + polanium. #9also must have sumr ThR and a little UX. From IMayl 5th to the 14th ita hem activity dcrlined exactly as L'X [indication of roniuml, itsalphaactivity showsmuch lessdeerease l'hrrsra,therefore, a constant alphaactivity, which inany case cannot he due only to iunium, ns m e ran see from the rrlati\,e amount oi ionium and uranium X in # X and q 1 0 . . . In addition there is also a not very wnrtrntine heta radiatiun. nossilrlv Hal) or RaE.. . 6 10 had (contrary to what you enpectea only ;cry little ThB add almostonly UX, the ratio of these, as seen by decay measurements, is the same in #9,only #lohas muchlesseonstantalpha.. . #9and #lOmust have the same ratio of ThB to UX, hut since #10 is much more strongly active than #9, it must have much more RaD than #9, if RaD is there at all. The fact that #10 has a negligible amount of alpha activity compared to #9, speaks against any RaD whatever in
#9, or else one must assume that the polonium all goes with #9. If so then #9 must decrease for a long time, while # 10 will soon begin to increase. Our measurements will s w n make it possible to distinguish [these possihiliies.] # 5 is the part we worked up (dissolved in HF, evaporated with HzS03 [with no added indicators] from the 2 g of SiOz out of 21 g pitchhlende. It shows, in addition toa quite constant alphaactivity, a markedly increasing beta activity [possibly from actinium] . . . You must only give me a little time--I mean you must not get impatient. The measurements all take a long time-I must also standardize [the electroscopes] exactly, very necessary with the weather we are now having. I shall also measure range [of the alpha particles] as soon as I have a stronger preparation. . .Last week I again asked Meyer about the U residue, and today I got a card from him ... He sent 1 kg of U residue to me. Unfortunately it is the uranium residue, 1 kg therefore contains about 1mg Ra. I am quite worried about contamination [from Ra and its decay products.] On the other hand, perhaps one can he more certain that tantalum is still there, than with the radium residue . . . P.S. You don't need to worry that others are trying to find eka-tantalum. I know for certain that it is not being done in Vienna, and I heard last winter that Fajans is in the military. T h e indicator experiments were a complete success.They showed t h a t t h e tantalum extract in . urenaration 9 emitted .-a steady alpha radiation that was not due toHnD or ionium. In June T h e Meitner wrote h a ~ ~ i to l vHahn (1.1). ". . . I have good news about our work.. #9 really does &em t o have something, the alpha activitv is a ~ ~ a r e n treallv lv constant . . . I think wecan count on having ihesub&ce i n h a n d . . ." For direct e\,idence nfactinium Meirner and Hahn needed to obtain larger amounts pitchblende. They preferred the "double residue"-pitchblende without uranium and radium. When Stefan Meyer wrote (15) t h a t t h e Joachimstal mines could not ship pitchhlende until t h e end of t h e war Meitner turned t o Friedrich Giesel of Buchler & Co., a German producer of radium. With considerable difficulty she obtained 100 a of "double residue." and immediatelv. heean "~~~ treating 43 g "f pitchblende, twicetheamount of their previous trial. But the comoosition of Ciesel's ~ i t c h b l e n d ewas different from their previous samples, and their earlier method did not work. By t h e end of July Lise Meitner was exasperated (16).
.~~
~
~~
~
~
Dear Herr Hahn! With all due respect for your reasons far not writing, don't you think that others besides yourself are also pressed for time? You are thrifty with everything, even with friendly words. Unfortunately as you can see I am not at the exalted place where Ido not write to you. The extenuating circumstences are that I am not just writing to he friendly hut also have some news to report. And so as not tooffend the practical God of time I shallget to it right away. . . .With somewhat unjustified optimism I attempted to separate 43 g of Giesel's preparation A. It appeared to go quite well, hut the final result was not very satisfactory. . .the preparation is much t w weak.. . After a detailed description of her attempts, Lise Meitner interrupted t h e letter.. 6 August 17. I got this far with witing when your wife visited me and invited me to go swimming with her in the Krumme Lanke. When I came home that evening I found in addition to hoth of your letters (what I wrote at the beginning of my letter is still justified since youonly wrote about radioactivity) a telegram from my hrother telling me he would arrive the next morning.. . Now that short pleasure is over.. . Be well, and write, at Least ahout the radioactive things. I remember a time very long ago when you would once in a while send a line even without radioactivity.
B y the end of August Lise Meitner succeeded with Giesel's pitchblende and set u p samples to be monitored. Dear Herr Hahn! Well this time you cannot complain that I haven't written. Concerning our small personal misunderstanding, I an uneonvineed by your objections. I am still of the opinion that from time to time a friendly line is not a great sacrifice to friends h i p b u t I do not want to lead you to such old-fashioned excesses if you yourself do not feel a need for them. Anyway I am not angry now, nor was I before. Volume 63
Number 8
August 1986
655
Concerningthe ahrakadahra,3the only news I can tell you is that I just prepared 10 g of Giesel's B-preparation . . . I hope you will not he angry with me if I do not try any further separations. First I really believe it is of no use as long as the preparations are decreasing, and second, totell the truth I would like finally to have avacation. With all the things left tomeasure, I will not finish until the 31st, and then I would like to go to Vienna. Before I go I will set up some kind of system for the collection of active deposits. As long as the strong preparations are still decreasing, it is difficult to take convincing measurement s . . . . Faian~ In the last issue of Electrochemlischel. Zleikhriftl , ~, . .. defends his hrain-child "hrevrum" and defends himself against the puniblity that Soddy.3 "eka-tantalum" might have priority over hi* "brtvium". life says that1 & d d y propo4t.d that ekn-tantalum is a long-lived mother substance of actinium, that has neuer been found and for that reason has never been named. So you see, Fajans does not have eka-tantalum! . . .Now be well. Your wife told me that you have obtained a bottle of schnaoos far mv "sdritual" needs. Manv thanks! It will come in handy a s k y last hottie disappeared alm& entirely into my brother's travel flask ~~
~
~
~
.~~~~
.
(In.
When she returned to Berlin, Lise Meitner monitored the old preparations. "I measured the abrakadabra. They are increasing [in alpha activity] entirely on schedule", she wrote in November (18). "For six months now the increase has been linear; that is very comforting [because if the alpha radiation arose from nolonium i t would have levelled off noticeably by then.]" T h e larger samples from Giesel's nitchblende also showed increasine alnha activitv. - . indicating the accumulation of actinium emanation and active denosit. "I have made a n actinium emanation standard of mod consistency. . .Shall1test # 25 for emanation? . . .Its activit y has increased quite strongly.. . Preparations 21 and 22, which I made from Giesel's residues A and B, are increasing well" (19). For Lise Meitner all doubts were gone; every experiment afforded unequivocal evidence of the existence of the mother substance, ofactinium itself, its emanation, and its active deposit. What remained was to determine the decay constant of the mother substance by measuring the range of its alpha particles, and the rate a t which emanation and active d e ~ o s iwere t eenerated. " The quantitative measurements required much stronger preparations than before. Again Lise Meitner turned to Giesel, "as we did not have available the necessary starting material, and in addition prevailing [wartime] conditions would have made the processing of larger quantities in our laboratory . very . difficult" (20). In November 1917 Lise Meitner went to Braunschweig to meet Giesel in his laboratory. The 65-year-old industrial chemist had been one of the first t o produce radium, and in 1902 had discovered actinium independently of Andre Debierne, but several years later.
. .. Tuesday I went to Braunsehweig. The connections are very poor.. . hut I had a gaod lunch, which really counts for a lot these days . . . According to Giesel, there are available a few hundred pounds each of residues A and B, which contain a high percentage of pitchblende. Since the factory stopped radium production during the war, those are the only residues available. In addition, they had to give up their platinum dishes, and of course one cannot boil sulfuric acid in Lead dishes. My discussion with Giesel was not very easy because he is a somewhat ahsent-mindedolder gentleman, who finds it hard to follow someone else's thoughts, and in addition seems to be somewhat impractical,but we agreed on the following: Giesel will treat 1 to 2 kg of the residue with acid and HF without evaporating the sulfuric acid.. . I think the quantity then will be small enough for me to treat with HF and HzSOd again, and evaporate the H2S04in our large platinum dish. I also asked him not to filter off the undissolved rare earths, etc. since they might again contain the eka-tantalum, hut to send me the entire HF mixture as soon as the silica is dissolved.. . Of course I told him that the KWI would pay all costs. At the end he was quite interested in it. I wrote the entire procedure down for him, as be is terribly forgetfuk for instance he asked me three times why we did not do the first acid extraction with sulfuric acid. Each time I asked him if concentrated 656
Journal of Chemical Education
sulfuricacid can he filtered;when he said no, I told him that radium would precipitate in dilute sulfuric acid. He forgot this several times. He certainly has gotten old, completely white and a little unsteady, but still he is quite a character. The death of his only son apparently dealt him a terrible blow. Personally he was very kind to me, insisted that I come to his house one evening where he and his intelligent and charming wife outdid themselves on my behalf.. . (21).
As promised, Giesel partially processed 1 kg of radium residues. and in December 1917t h e third and mostauantitative phase of the investigation began. Otto ~ a h n , - a ~ a in in Berlin for a brief leave, helped Lise Meitner with the difficult rvaporation of concent;ated sulfuric acid, repeated several times until they obtained a fairly pure tantalum product that was strnngly radioactive. After setting upsamples to be monitured, l.ise Meitner went to Vienna for Christmas and Otto Hahn returned to his unit. Three weeks later shc wrote from Berlin (22), Take a deep breath before you begin reading this, it will he a very long letter . .. I received your card of 29 Dee. the day I left Vienna. Once I gat here I wanted to finish some of the measurements so that I could tell what you want most to hear from me. And I shall tell you a variety of delightful things. Well, the strong Giesel preparations are quite all right. The small decrease, which you found, is, I think, due to the change in thickness after it was spread out. Because the three samples, A, B, C, which represent the same preparation, each decreased quite differently. B did not decrease at all, and all 3 have now increased strongly [in alpha activity]. In #34.. . you must have made a mistake in measurement, as I got 468" on 18. XII, yougot 545! on 23. XI1 and a day later an 24. XI1 you got440". [Thenumbers are the secondsrequired to discharge an electroscope; the shorter the time, the greater the activity.] That is impossible of course; therefore the decrease is not real, and the preparations are as good as they could possibly he. Also #6,9,21, and 22 [preparationsfrom the first andsecond trials] have increased nicely. I have also determined the range. The lead windows which you so kindly made for me were not usable in the original form but after several tries I altered them and got good measurements. . .. The measurements at the end of the range, which are so important, take hours, for 10 days I left the lab only at 830 ar 9 at night, hut at Least it was worth the effort. I will take at Least one range measurement with B or C. The relatively large value of 3.32 cm (Ra has 3.13 cm) should not worry you. For the actinium series the relationship4 Log X = A + B log R yields a straight line whose slope differs from that of the uranium-radium series . . . you can only compare within the same series. In the actinium series, though, there is the difficulty that either AcX or radioactinium doesn't correspond, so one gets two different slopes depending upon which one is irregular. That also gives two different values for the half-life. If one takes the value of AeX as determining [the slope], the half-life of our eka-tantalum would he 1200 years, the other one would give 150 000 years.S.. . In addition I have done the emanation measurements. The old preparation 25 [from the second trial] is mare than double as strong, the new Giesel preparation [from 1kg radium residues] is thirty times! stronger. The increase is so strong one notices it from one day to the next. That made me especially happy, I think you will also he glad. Any doubts, such as the possibility that radioactinium was there end the emanation might be due to AcX, have taken care of themselves by the increasing [overall] alpha activity.. . . Finally, I also collected actinium deposit from preparation 22 [second trial], and repeated the experiment 10 days later. Of course the activitv is verv weak. but it can he measured with comnlete certainty and can he verified by its rate of decay.. . In any case we can now think of publishingvery soon.. . On 16 March 1918 Lise Meitner and Otto Hahn submitted their paper "The Mother Substance of Actinium, a New
A ccde name for the actinium mother substance. 'This is tne Geiger-Nuttal equation, an empirical relation between A. the radnacl~veconstant (inversely propdona! to hall-life),and R, the measured range of alpha particles. The half-lifewas later determined to be 34,000 y.
Radioactive Element of Long Half-Life" to Physikalische Zeitschrift (1).With satisfaction they reported, The supposition that pitchblende was the suitable starting material was indeed iustified. We have succeeded in discoverine " a new radioactive element, and demonstrating that it is the mother s u b stance of actinium. We propos~.theref~re,the name protactinium. Remaining were the pleasant duties associated with the new discovery. At last Lise Meitner was free to discuss the discovery with Stefan Meyer. Concerning the symbol for protactinium, he responded happily (23),
. ..In your letter you pose a terribly difficult question about protactinium. I would prefer the names Lisonium, Lisottonium, etc., and I therefore propose the symbol Lo, but unfortunately these are unsuitable if one wants general acceptance . . . In Pn a completely unimportant letter is brought to the fore, which leaves me mast sympathetic to Pa: after all for same reason palladium is Pd . . . Although I still prefer Lisotto, it is much more significant to have discovered Pa or Pn than to came up with the most beautiful name ....
There were also some delicate discussions with Kasimir Fajans. As the first to identify an isotope of the new element, he and Gohring had the right to name it, and they had chosen "brevium" for the short-lived UX2. Rut common usage required that the lonaest-lived and most abundant isotope he the chief representative of an element, and the name "hrevium" would hardly do for the mother substance of actinium. Fajans was known for his imperious and rather aggressive treatment of his colleagues-Stefan Meyer had taken to calling him "Kasimir the Greatest" (24)-but he agreed to the name protactinium. Nevertheless, Lise Meitner was apprehensive when Otto Hahn visited Fajans in Munich that June. "How are things going withFajans? Shall I write to him that we have chosen the svmbol Pa?" (25). Fajans had not pursued protactinium,-hut Frederic Soddy had. In June 1918 Soddv and John Cranston reported (26) that a heat treatment i f pitchblende yielded sublimate which gradually generated increasing quantities of actinium. They obtained very little protactinium and were unable to describe its decay characteristics. Acknowledging Hahn and Meitner's priority, Soddy and Cranston agreed t o the name protactinium (27). The discovery of protactinium was thus accepted. The invesiigation continued, however, for the origin of the actinium series and its link to uranium was as mysterious as ever. I t was impossible to know which uranium isotope began the series as long the atomic weiahts of the actinium series were unknown. he problem was intriguing because the two known uranium isotopes, U-234 and U-238, were members of
a
the uranium-radium series; if they were members of the actinium series as well new and unusual branching was required. Protactinium, the longest-lived and therefore the most abundant member of the actinium series, was the best candidate for atomic weieht determination. Otto Hahn and to sume extent l.ise Meitner attempted thiv for many years. without rurcpss.'l'he orieinoitheartinium series was finallv solved when the existence of U-235 was inferred from mass sDectroscoDvexoeriments in 1929. But for Lise Meitner and d t t o ~ a h pro&tinium n would later play a role in the discovery of fission. Acknowledgment I am grateful to Marie-l.uise Hehder, Giittingen, for calling my attention to the Mcitner letters, and 1 am indebted to the -Ribliothek und Arrhir zur Geschirhrp rler Max-PlanckGesellschaft, in particular Marion Kazemi, for making copies of the letters available to me. I wish to thank Marion M. Stewart of the Churchill College Archives, Cambridge, for conies of the Stefan Mever letters. S u o ~ o r of t this research by the National ~ c i e n f ~e o u n d a t i m a n dthe National Endowment for the Humanities is gratefully acknowledged. Literature Clted (11 Hahn, 0.; Meitnor, L. Phya. 2. 1918.19.208. I21 Ref I. p 209. 131 Fma mncise hackmound seeref1 and Badssh, L. "Radioactivityin America"; Johns P~OSS: Baltim~re.1979 p p 17G75.202.20748. H o ~ k i n University s 14) Hahn. 0,:Meitner.L. P h v s . 2 1913.1A 758.
~" ...,. .,
-..
16) Hahn, 0. "My Life, Autobiography of a Scientist": Kaiaer, E.; Wilkins, E.. ltana.: Herder and Herder: New York, 1970: Chap 7. (7) Letter from Meitner to Hahn. 25 Odober 1916 (unleas otbewise apecfied all lettern """ted are from the Otto Hahn Nachlass in the Bihliothek ""d Arehi" der Max-
I101 Mcitner to~ehn;22February1911. (111 R e f l , p 2 1 2 . 1121 Meitner toHahn. 7 Mau 1917. it^^^ to H& 15 M ~ 1917. Y (141 Meitner to Hahn, 19 June 1917. 1151 Card,Stefan Meyer foMeitner, 22June 1917 (Churchill CollegeArchiver,Cambridge Enelandl. (161 Meitner to Hahn. 27 July 1917. continued 6August (171 Meitner toHahn, 24Auguat 1917. 1181 Moitner to Hahn. 16 November 1917. I191 Meitner to Hahn. 28 November 1917. I201 Ref I, p 214. (211 Meitner to Hahn, 16 Novsmher 1917. (221 Meitncr toHehn, 17 January 1918. I231 Letter,StefanMeyertoMeimer,5June191S(ChuxhillCollegeArchives,Cambridge
i1.4
P"",O".,> u..e.
124)
Letter. Stefan Mever to Meitner. 23 November
1917 Churchill Colleee Archives. ' cambridge). (251 Mdfner to Hahn. 23 June 1918. I261 Soddy, F.; Cranston, J. Proe. Roy. Soc. Lond. 19t8,A%, 384. (271 Ref 9, p 93.
Volume 63 Number 8 August 1986
857
