^:y/j/ ^-^^y \ ^ MDDC - 867 ■^ UNITED STATES ATOMIC ENERGY COMMISSION PRODUCTION AND PROPERTIES OF 50-YEAR ELEMENT 94 by G. T. Seaborg A. C. Wahl J. W. Kennedy University of California U This document consists of 5 pages. Date of Manuscript: March 20, 1942 Date Declassified: March 5, 1947 This document is issued for official use. Its issuance does not constitute authority to declassify copies or versions of the same or similar content and title and by the same author(s) Technical Information Division, Oak Ridge Directed Operations Oak Ridge, Tennessee PRODUCTION AND PROPERTIES OF 50-YEAR ELEMENT 94 Py G. T. Seaborg, A. C. Wahl, and J. W. Kennedy The bombardment of uranium with deuterons in the 60-inch Berkeley cyclotron produces, in addi- tion to the 2.3-day' 93"°, a new isotope of element 93, which has a half -life of 2.0 days, and beta and gamma-radiation distinctly different from those of 93"°. Observation of the growth of alpha-particles from this 2.0-day 93 led us to the discovery^ of element 94, and this report describes the preparation of and the measurement of the physical properties of this isotope of 94. It is this isotope, which has a half-life of about 50 years and 4.1 cm alpha-particles, which we have used for a tracer to study the chemical properties of element 94 as described in another report.^ It is not possible at present to make a definite isotopic assignment for this activity, but the best assignment from the evidence at hand seems to be 94"^ or 94"°, and, in order to facilitate the discussion, we will refer to it as 94"°. In these experiments the uranium has been bombarded, sometimes in the form of black oxide (UjOj., and sometimes in the form of an alloy of uranium. In a typical experiment which demonstrates the pres- ence of element 94 in this bombarded uranium, the uranium is dissolved in nitric acid, and element 93 is isolated in a thin sample (approximately 0.3 mg/cm^) with the help of lanthanum or cerium carrier by the method of precipitatmg the fluoride in oxidizing and reducing media. There is found to be pres- ent in this 93 fraction, besides the 2.3 -day 93='^ formed by U^°°(d,p)U"° (followed by U"° 23 min. 93"«; and perhaps also by U"°(d,p)93"°, a new radioactive isotope of element 93 with a half-life of 2.0 days (93"8). Absorption measurements with aluminum show that the beta-particles of this 2.0-day 93"° have a range of 0.38 gm/cm^, corresponding to an energy of 1.0 Mev, and lead absorption measurements show the presence of 1.1 Mev gamma-rays. The absorption measurements were taken with an air-filled ionization chamber connected to an FP-54 electrometer tube. The arrangement for the aluminum ab- sorption measurements is shown in Figure 1, and the absorption curve for the beta-particles of the mixture of 2.0-day 93"° and 2.3-day 93"° is shown in Figure 3. The absorption curve for the beta- particles of the 2.3-day 93"°, formed in the bombardment of uranium with neutrons, is shown, for the purpose of comparison, in Figure 2. Both samples were very thin ( < 0.3 mg/cm^) and mounted on cellophane. By following the decay of the higher energy beta-particles and of the 1.1 Mev gamma-rays, neither of which are present with the 2.3-day 93"°, it is possible to establish the half-life (2.0 days) of radio- active 93"°, in spite of the large amount of 2.3-day 93"° present with it. The growth of alpha-particles, which are due to 94^'°, the daughter of the 2.0-day 93"°, is observed in such a sample, using an ioniza- tion chamber and linear pulse amplifier together with a magnetic field to bend out the very strong beta- particle background radiation. The growth curve indicates a half-life of 2.0 days for the parent activ- ity as is to be expected. * Note added at time of publication. This alpha-radioactivity, referred to in the original report as 94"^, was later showti to be due to the isotope 94^'° with a slight admixture of alpha radioactivity due to the isotope 94^'^. In order to avoid confusion this paper has been edited so that it always refers to this radioactivity as due to the isotope 94"°. MDDC - 867 [1 2] MDDC - 867 To electrometer \ - 320 volts Insulator 12 Aluminum rods form cylinder cage Aluminum Windows 0.18 mg/cm^ '^^^ m^m:<^4 — 2^- — 4^s^ s ^ ^ Absorbers Sample Figure 1. Arrangement for absorption measurements. That these alpha-particles are due to element 94 finds complete confirmation in our chemical ex- periments. These experiments have shown that the 94"° activity is separable from all the elements, including element 93, and a description of these experiments is given in another report.' Thus, in ad- dition to the method of allowing 94 to grow in a chemically isolated 93 fraction, 94 can be isolated by a direct chemical separation, after the decay of the 2.0-day 93"", from the deuteron activated uranium. The range of the 94^'° alpha-particles has been measured rather roughly by two methods. (1) The alpha-particles were absorbed by air. In this method, a plot of the counting rate times the distance from the ionization chamber squared vs. the distance from the chamber was compared with a similar plot for the alpha-particles from polonium. (2) The alpha-particles were absorbed by aluminxm^ and a comparison made with the absorption of poloniimi alpha-particles by aluminum. Both methods gave as the (extrapolated) range in air (15°C and 760 mm) of the 94"° alpha-particles an average value of 4.1 ±0.1 cm. No decay has been observed in the alpha -activity of M"' over a period of many months. In prin- ciple, one could evaluate the half -life of 94^" by using the measured value of the intensity of the beta- MDDC - 867 [3 ( 1000 L 1 -< — — HALF THICKNESS = 8.7 MG/CM* 1 i 100 A > 1- V) z UJ t- z :\ 10 — \ - \j^^ END POINT, 260 M6/GM* - \^^^^ / (0.78 Mev) ^ ® o — <» ©— 1 1 1 1 1 1 1 1 1 1 1 100 200 300 MG Al /cm' 400 500 600 Figure 2. Absorption of 93*™ radiation in aluminum. Radiation consists of approximately equal number conversion electrons and disintegration beta-particles. particles of the 2.0-day 93'''° together with the corresponding measured value of the intensity of alpha- particles from the daughter 94^'°. However, it is difficult to make a reliable estimate of the intensity of the beta-particles from 93"°, because, as mentioned above, the deuteron bombardment of uranium produces a mixture of 2.0-day 93^'° and 2.3 -day 93"', and the similar half-lives make it difficult tc resolve accurately the decay curve. There are three methods which can be used to estimate the fraction of 93"° which is present in the mixture of 93^'° + 93"°, and, although they are all rather rough, they all give approximately the same result. (1) The rough resolution of the decay curve (taken without absorber) into its 2.0-day and 2.3- day components. (2) The extrapolation to zero absorber of the high energy portion (the portion due to 93"° of the complex (93"° i 93"°) beta-absorption curve. The complex beta (93"° + 93"°) absorption 4] MDDC - 867 curve (Figure 3) was resolved into its two components with the help of the 93«» beta-absorption curve (Figure 2), after the subtraction of the gamma-rays from each of the two curves. The dotted line in Figure 3 represents the 93^=» beta components of the absorption curve! (S) The use of the intensity of the ionization of the 11 Mev gamma-ray (due to 93"») to estimate the intensity of the corresponding be a-particles due to 93-, using the usual ratio of efficiency between beta- and gamma-ionizationls determined with other well-known beta-gamma emitters. J II I I ■ I HOO 200 300 400 500 600 MG Al /CM* Figure 3. Absorption of mixture of 93'^'' and 93"« r.diaticr in aluaJroi ion MDDC - 867 [5 All of these methods indicate, after correcting for the relative ionizing efficiencies of the 93^" and 93"» beta-radiation* that about 12 per cent of the initial beta-disintegration rate of the 93 fra^: IS due to 93"». This means that in every millicurie of total 93 (93"» + 93"») formed in the deuteron bombardment of uranium there is present 0.12 millicurie of 93^'«, and the alpha-counting experiments show that 0.014 microcuries of 94"« grow from a sample of this strength. (The amount of 94^'' alpha- ^*^*'q"4'^*'''''' ^""^^ ^'"°'" ""^ ^^"^ ""^'^^ '^ present is entirely negligible compared with that from the 94- .) These data lead to a value of about 50 years for the half -life of 94"=. We believe this va'uc to be accurate to within a factor of 2. The relative yields of the 2.0-day 93 and the 2.3 -day 9Z"^ activities favor the assignment of the 2.0 -day activity to the mass number 235 or 238 rather than 236. Since the abundence of U"^ is 0.7 per cent of that of U"«, an assignment of the activity to the mass number 236 would mean that the yield of the reaction U"^(d,n)93"« (plus that of U"=(d,p)U"= if U"^ should be a short-lived beta-emitter) must be of the order of 15 times as great as the sum of the yields of U"''(d,n)93"'' and U^^»(d,p)U"' and thia does not seem to be very likely. If the activity is assigned the mass number 238, the yield of' the reaction U^^»(d,2n)93"« must be only 12 per cent as great as that of the sum of the reactions U"«(d,n) 93- - and \r-^Hd,p)V^^^ while, if the activity is given the mass number 235, the yield of the reaction U- ■(d,^2n)93"'^ must be about 1 5 times greater than the sum of the yields of the reactions U"«(d,n)93"^ and U (d,p)U"^ However, the competition of the fission reactions may considerably alter the ordi- narily expected relative yields, and for this reason a choice between the mass numbers 235 and 238 cannot be made on the basis of yield arguments. Whether the activity is formed in a d,2n reaction can probably eventually be settled by observing how the yield varies with the energy of the deuterons. If the activity actually is due to an isotope of mass number 235, then there probably should be formed about 140 times as much 93"° as 93=" s^r-^e If'' is 140 times as abundant as U"^ This would mean that a thousand-micro-ampere-hour bombard- ment of uranium would form of the order of micrograms of 93=^\ Such an amount of 93"« could prob- ably be detected and the amount estimated by neutron fission tests, and since this might afford a better means of making a choice between 94"= and 94"» for the isotopic assignment of the 50-year 9-! activity, we propose to perform this experiment in the near future. Following are the approximate yields per thousand micro-ampere-hours of deuterons in the 60- mch cyclotron on a pure thick uranium target: about 10 miUicuries of 2.0-day 93"» and, from the de- cay of this, about one microcurie of 50-year 94"«; about 80 miUicuries of 2.3-day 93"« and from the decay of this, about 0.02 microcuries of 30,000-year 94"«. For purpose of comparison it may be stated that the bombardment of 2 kg of uranyl nitrate hexahydrate {UO^^h^W.m^O) with the neutrons from one thousand micro-ampere-hours of deuterons on beryllium in the 60-inch cyclotron produces about 80 miUicuries of 2.3-day 93-'^^ and, from the decay of this, about 0.02 microcuries of 30,000-year REFERENCES 1. McMillan, E.M., and P.H. Abelson, Phys. Rev. 57: 1185 (1940). 2. Seaborg, G.T., A.C. Wahl, and J.W. Kennedy, Phys. Rev. (In publication submitted March 7, 1941). 3. Seaborg, G. T., and A.C. Wahl, J. Am. Chem. Soc. (Paper submitted). * An important factor is the fact that the 2.3-day 93-^ radiation contains, in addition to the dismto- gration beta-particles, about an equal number of conversion electrons. UNIVERSITY OF FLORIDA 3 1262 08910 5315 ■