MDDC - 940 ^ ^ ^ UNITED STATES ATOMIC ENERGY COMMISSION STUDIES OF THE DELAYED NEUTRONS n. CHEMICAL ISOLATION OF THE 56-SECOND AND THE 23-SECOND ACTIVmES by Arthur H. Snell J. S. Levinger E. P. Meiners, Jr. M. B. Sampson R. G. Wilkinson University of Chicago This document is reproduced as a project report and is without editorial preparation. The manuscript has been submitted to The Physical Review for possible publication. Date Declassified: May 9, 1947 ■ Issuance of this document does not constitute authority for declassification of classified copies of the same or similar content and title and by the same authors. Technical Information Branch, Oak Ridge, Tennessee AEC, Oak Ridge, Tenn., 5-2-49--850-A4417 Printed in U.S.A. PRICE 10 CENTS !2snB^ Digitized by tlie Internet Arcliive in 2011 witli funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/studiesofdelayedOOuniv STUDIES OF THE DELAYED NEUTRONS 11. CHEMICAL ISOLATION OF THE 56-SECOND AND THE 23-SECOND ACTIVITIES By Arthur H. Snell, J. S. Levinger, E. F. Meiners, Jr., M. B. Sampson, aiid R. G. Wilkinson ABSTRACT The 23-second delayed neutron activity is found to follow the chemistry of iodine, and the 56- second delayed neutron activity is found to follow the chemistry of bromine. Comparison with known beta emitters of like half-lives suggests that the neutron-emitting nuclei may be Xel37 and Kr^''. In the preceding paper, resolution of the decay curve of ^^he delayed neutrons resulting from the fission of uranium is described. Activities with half-lives of 0.4, 1.8, 4.4, 23, and 56 seconds were found. The fact that discrete decay periods are present is good evidence in favor of the interpre- tation put forward by Bohr and Wheeler' of the existence of delayed neutrons, namely that they originate in the fission products, and are emitted when the beta-decay of a fragment leaves the nucleus in a state of excitation higher than the binding energy of a neutron in that nucleus. The neutron is then immediately emitted, and the rate of decay of the neutron-emitting activity observed is just that of the preceding beta-activity. In this paper we shall describe successful chemical iso- lation of the 56-second and the 23-second activities in the fission products. EXPERIMENTAL PROCEDURE AND RESULTS Most of the irradiations were made with the University of Chicago cyclotron, using 7.3 Mev deuterons on beryllium as a neutron source. Beam currents ranged up to 100 microamperes, giving in the paraffin-surrounded sample a slow neutron flux of about 109 neutrons per cm^ per sec. The samples were usually a few hundred milliliters of aqueous solution of uranyl nitrate. The counting was done with a boron trifluoride proportional counter having a total boron cross section of about 2 cm2^ surrounded with several inches of paraffin. The counting was done by three workers; one called the time, the second read the scaler, and the third recorded the readings. During the early part of the decay the accumulating count was recorded every 5 seconds, but as the activity became weaker this interval was lengthened to 10 seconds and finally to 30 seconds. The problem on first approach looked like a difficult one, namely, to identify within the time limit for chemistry of about one minute, one or two of the thirty-odd fission product elements. With the idea of at least narrowing down on the possibilities, we made a number of experiments which led to negative results. It seems, nevertheless, worth while to mention them briefly because some of them rule out possibilities for the still unidentified shorter activities, and others give evidence against the odd chance of the presence in the elements concerned of activities with about the same decay periods as those which we have been able to extract. MDDC - 940 1 MDDC - 940 1) Sulphate precipitation: We irradiated 250 cc aqueous solution containing 40 g uranyl nitrate, fission product carrier, and barium nitrate. After irradiation we added sulphuric acid, filtered, and examined the precipitate. Only a very weak neutron-emitting activity was found in the precipitate 30 seconds after the stop of the activation. 2) Water extraction from ether solution: We irradiated 80 g of uranyl nitrate dissolved in 500 cc ethyl ether in a separating funnel, with 10 cc water and fission product carrier. After irradiation, the liquid was shaken, allowed to settle, and the water was drained off. Most of the neutron-emitting activity remained in the ether subsequent to 40 seconds after stop of activation. 3) Barium: A barium chloride precipitate was taken from the water layer following an ether extraction. Only very weak activity was found in the precipitate 60 seconds after stop of activation. In other experiments, aqueous solutions were activated and barium chloride precipitates were taken out after irradiation. The precipitates had no neutron- emitting activity 40 seconds after the stop of activation. 4) Bare gases: (a) A flask containing an aqueous solution of uranyl nitrate was boiled under reduced pressure while under irradiation. An air stream was led through a flask to a NaOH trap, and thence to a charcoal trap. A weak neutron-emitting activity built up in the NaOH trap, but none in the charcoal, although the latter became beta active, (b) 1400 cc of uranyl nitrate solution con- taining 100 grams of the nitrate were irradiated, and after irradiation allowed to pour through a pipe into another vessel in which the boron trifluoride counter was set. A decay curve was taken. Then the experiment was repeated except that the solution was kept boiling during activation. The boiling should have greatly weakened any rare gas activities, but the decay curves were of the same shape and the activities were of about the same intensity subsequent to 10 seconds after stop of activation. Experiments which gave positive results started after we tried a silver halide precipitation, and found that both the 23-second and the 56-second activities came down very strongly. Since silver selenite and presumably tellurite also would have prtcipitated from the solution, we made separa- tions from solutions which had been made strongly acid with HNO3, under conditions such that (as we verified by trial) silver selenite would not precipitate. The 23-second and 56-second activities still came down with the silver halide. This evidence that these activities were distributed between bromine and iodine seemed not to be in disagreement with the results of experiments described in the preceding paragraph. THE IODINE ACTIVITY About 150 cc of an aqueous solution of uranyl nitrate were irradiated for 2 minutes in a sepa- rating funnel. A few milligrams of KI and KBr were present to act as carrier, and a few cc of carbon tetrachloride were present. The concentration of the uranyl nitrate solution was held down so that the carbon tetrachloride would settle promptly after the funnel had been shaken. One cc of concentrated HCl was present in the solution, and after irradiation, 10 cc of 5% sodium nitrite solution were added. Following shaking and settling, the carbon tetrachloride layer (colored violet by the iodine) was drawn off and counted. The result was a single exponential decay with a half- life of 24 seconds, covering the time interval 30 seconds to 200 seconds after stop of irradiation. Later we found that the sodium nitrite could just as well be present during the activation, and that repeated extractions could be made from one batch of solution. One added new carrier and new carbon tetrachloride before each activation, and did not shake until after the activation. By standard- izing the procedure so that counts were always recorded at the same time after the stop of activa- tion, we ran through eight extractions and by averaging the resulting readings we obtained the curve reproduced in Figure 1 and labeled "Iodine". It will be noticed that the first point comes at 28 seconds after the stop of activation, and that the curve is a simple exponential over an intensity factor of about 100, and that the half-life is 23.8 iO.7 seconds. The limits of error for this figure are those allowed by the scatter of the points. MDDC - 940 THE BROMINE ACTIVITY Since the 23-second delayed neutron activity appeared to follow the chemistry of iodine, it seemed very probable that the 56-second activity would follow that of bromine. A positive experiment required a specific separation for bromine in an attempt to get the longer-lived delayed neutron emitter clear of the others. This was accomplished at the Clinton Laboratories, and the experiment was as follows: About 1 cc of uranyl nitrate solution was irradiated for 2 minutes in the Clinton pile. The trans- fer in and out of the pile was accomplished with the pneumatic tube arrangement. After irradiation, the sample was allowed to stand for 16 seconds to permit decay to a moderately safe level of a strong 8-second nitrogen 1'' activity which was induced in the lucite container of the solution. Then the uranyl nitrate was poured into a separating funnel containing 30 ml of 8N nitric acid which was saturated with potassium chlorate. Br- and I-carriers were added and Br2 and I2 extracted with carbon tetrachloride. The carbon tetrachloride was run into a second separating funnel which con- tained a potassium nitrite solution made slightly acid with nitric acid. Here the bromine was re- duced to bromide, the iodine being at the same time kept oxidized. The carbon tetrachloride was drained off, and the water layer counted. As in the case of iodine, we standardized times and pro- cedures so that five good runs could be averaged. The resulting decay curve is given in Figure 1, labeled "Bromine". For comparison, we took readings on samples of uranium nitrate irradiated for 2 min but followed by no chemical extractions. These gave the curve labeled "unseparated". The presence in the bromine of a small amount of shorter activity as indicated by the earliest two points on the bromine curve is explicable on the basis of imperfect chemical separation in the somewhat hasty manipulations involved. The isolated bromine activity seems to have a half-life of 54 ± 1 seconds, where again the limits of error are those permitted by the scatter of the points. To compare the decay of the separated bromine with that of an unseparated sample in which other activities have fully decayed, we activated several grams of uranyl nitrate, waiting 6 minutes before starting the count. The resulting curve is labeled "Larger Sample, Unseparated" in the figure. Between 6 and 12 minutes after stop of irradiation, the slope corresponds to a half-life of 57 1 1 seconds. This is a little longer than that of the extracted bromine, but the difference is probably attributable to experimental effects such as the presence of a little iodine in the bromine samples. DISCUSSION Probably the bromine and iodine activities account for all of the 23-second and 56-second activity observed in the total delayed neutron decay curve. We are not able to set very low experi- mental limits on the presence in other fission products of activities having about the same decay periods, but in one set of experiments we compared the intensities of silver halide precipitates with those of their filtrates. The filtrates were 5 to 10 times weaker, and this residual activity is at- tributable to the imperfection of the fast filtering, and to the probable presence of some activity in the form of bromates and iodates which did not precipitate. The filtrate decay curves were rough but did not require new decay periods for their description. It is tempting to identify these two delayed neutron activities respectively with the 30 - 6-second iodine beta activity and the 50 ± 10-second bromine beta activity found by Strassmann and Hahn.^ Seelmann-Eggebert and Born^ subsequently found that a 3.8-minute xenon grows from the 30 ± 6- second iodine, and a 75-minute krypton grows from the 50 1 10-second bromine. In other work,* the 75-minute krypton was identified as krypton87 and the 3.8-minute xenon identified as probably xenonl3'^. If the possible existence of conflicting bromine and iodine beta activities had been ruled out, it would seem that the mass assignments of the delayed neutron emitters would be settled. MDDC - 940 *^ T! E CD o rt u b, •4-) ra T3 ^ ■a 01 « r: Oi T3 (U o c *-' T3 a o (1> c o J3 J3 a (4 T3 V 3 C XI o a n >. tD -*-* rt C a) Q O