Nuclear Properties of Some of l000- , Elements 99 anII 100$ G. R. CHOPPIN, S. G, THOMPSON, A. GHIORSO, AND B. G. HARVEY Radiation I.aboratory and Department of , University of Berkeley, California (Received March 19, 1954) ' N recent publications' ' we reported the formation of the 99 5', 99 ', and 10025 by intense bombard- ment of and higher- nuclides. Further observa- 100 tions of the nuclear properties of these nuclides have now been made, using, in some cases, samples of higher activity prepared I- in new neutron bombardments. ln addition, a 15-hour alpha- emitting of element 100 has been observed in the products C9 of the neutron bombardment of plutonium. Cf ' amd 99 '.—The Cf " decays by P emission to 99'". From the rate of growth of the 99'" alpha activity in a K V) purified Cf " sample and the rate of decay of separated 99'", the half-life of both these nuclides was found to be approximately 10 20 days. Because of the high rate in californium (due mainly to Cf'5s), the P emitting fission fragments prevented any simple measurement of the P-particle energy of Cf253. By comparison with Bi"' and Po"4 alpha-energy standards, the energy of the 99'"alpha particles was found to be 6.63~0.02 Mev. 99254, 100254, aed 100"'.—Pure samples of element 100 were

I I I I I lo 200 400 600 800 1000 1200 10000- TI ME t FIG. 2. Decay curve of 100»4.

The 7.22-Mev alpha activity of 100"4 was observed to grow gg253 into a separated 99 fraction, and then to decay with a 36-hour half-life, that of its 99"4 emitting parent (These 1000- P (Fig. 3). curves are from the first experiments performed early in January and have since been repeated with substantially the same results using orders of magnitude more activity. ) The P particles of 99'" were found to have a maximum energy of 1.1&0.1 Mev as deter- mined by an anthracene scintillation spectrometer. The 7.1-Mev alpha activity was also observed in purified 100- element-99 fractions, and thus it was deduced that it also has a O 100254 P emitting parent. Rough decay measurements of this activity C3 in the element-99 fraction indicate that the half-life of this parent, presumably 99255, is approximately one month. Spontaneous fissions, decaying with a half-life of about 3 (A were observed in element-100 fractions. These are V) hours, pure O CL presumably due to 100"4. From the ratio of fission to alpha- 10- emitting events ( 0.001), the partial half-life of 100ssi for spon- taneous fission was found to be approximately 200 days. Spon- taneous fissions were not observed in purified element-99 samples, but appeared as the 100"4 grew from the 99"4 P decay.


I I I i I I I 0 10 15 20 25 30 35 DROP NUMBER

FIG. 1. Elution of elements 98, 99, and 100 from Dowex-50 I.P column with 0.04 M ammonium lactate, pH 4.50.

separated from 99 and californium by elution through columns of P. I Dowex-50 -exchange resin. A typical elution curve is shown in

Fig. 1.For this run the bulk of the californium had been removed I— C3 in a previous ion-exchange column. cl Two alpha activities assigned to element 100 have been ob- &10 served. The more abundant, probably due to 1002'4, decays with a half-life of 3.2 hours (Fig. 2) by emission of 7.22&0.03-Mev alpha-particles, as previously reported. 2 The other, probably due IP 2 3 4 5 6 7 8 9 IP I I I 2 I 3 I4 to 1002'5, decays with a half-life of about 15 hours by emission TIME, DAYS of 7.1-Mev alpha particles. FIG, 3. Decay curve of 99»4. LE TTE RS TO TH E E D I TOR i08i

We wish to acknowledge the continued interest of Professor It is a pleasure to acknowledge the continued interest of Pro- G. T. Seaborg in this work. We are indebted to the staG of the fessor G. T. Seaborg in this work. We are indebted to the entire Materials Testing Reactor, and particularly to Dr. W. B. Lewis staG of the Materials Testing Reactor, and particularly to Dr. and Dr. R. R. Smith, for arranging the neutron bombardments. g W. B. Lewis and Dr. R. R. Smith. We wish to thank Almon E. Larsh for assistance with some of the experiments. f This work was performed under the auspices of the U. S. Atomic Energy Commission. I Thompson, Ghiorso, and *This work was performed under the auspices of the U. S. Atomic Harvey, Choppin, Phys. Rev. 93, 908 (1954) Commission. 2 Harvey, Thompson, Ghiorso, and Choppin, Phys. Rev. 93, 1129 (1954) Energy I Thompson, Ghiorso, Harvey, and Choppin, Phys. Rev. 93, 908 (1954). 2 See, e.g., G. T. Seaborg in The Elements (McGraw-Hill Book. Company, Inc. , New York, 1954), National Nuclear Energy Series, Plu- tonium Project Record, Vol. 14A, Div. IV, Chap. 17. 8 Street, Ghiorso, and Seaborg, Phys. Rev. 79, 530 (1950). New Isotopes of , and Californium* A. GHIQRso, S. G. THoMPsoN, G. R. CHQPPIN, AND B. G. HARvEY Radiation Laboratory and Department of Chemistry, University of California, Interaction of High-Energy Pions with Nuclei* Berkeley, California (Received March 19, 1954) E. D. CoURANT Brookhaven National Laboratory, Upton, Nevff York "N a recent publication, ' the prep@ration of new heavy isotopes (Received March 30, 1954) of berkelium ( probably 249) and californium ECENT experiments' ' have shown that the interaction (mass numbers greater than 248) by prolonged neutron irradia- cross sections of negative m mesons in the tion of Pu'" with thermal was described. As the result energy range of 0.6 to 1.0 Bev on are more than twice those of of neutron irradiation of Am" and Bk'", it has been possible to positive mesons of the same energy range; the cross section make new observations on the of Am'44, and x (v,N) ap- pears to be about equal to the (v+, cross section, as to prepare new berkelium and californium activities. p) expected from charge symmetry considerations. Am'44. —This nuclide was produced by short neutron bombard- This makes it attractive to m mesons as tools ments of americium, whose principal constituent was Am"'. After employ for the investigation of , in particular of the and bombardment the americium was rapidly purified by a combina- neutron distributions in nuclei. 4 Johnson Teller' tion of precipitation and ion-exchange procedures. ' The Am~~ and have sug- gested that the neutrons in a heavy nucleus extend out to a larger was found to decay by emission of P particles; its half-life was radius than the protons. If this is the case, the nuclear 26 minutes, in good agreement with a previously published value. apparent radius as measured with two diGerent will The and gamma radiations of Am'~ were studied with probe particles be greater for that probe particle which interacts more anthracene and iodide crystal spectrometers. Only one strongly with neutrons. P end point, at 1.5 Mev, could be resolved. There were no promi- The interaction cross sections of for positive and nent gamma rays. negative m mesons of 700 Mev have been computed Bk'".—A sample of Bk'" was subjected to a short neutron using the "optical model'" and assuming that the protons and neutrons are bombardment, followed by chemical purification, and a new (a) uniformly distributed in a sphere of radius R (uniform nucleus activity, presumably Bk'", was produced. It decayed by P model), and that the nucleus is composed of an inner zone of emission with a half-life of 3.13 hours. (b) radius EI containing Z protons and 2 neutrons, and an outer The P spectrum showed the existence of two groups, with end zone of radius R containing X—Z neutrons (Johnson-Teller points at 900 and 1900 kev. The lower-energy P was in model). In model the radii R& and R are chosen so that coincidence with a of about 900 kev. (b) the neutron is the same in both zones, i.e. R&/R= (Z/1V)&. Cf ".—Part of the neutron-irradiated berkelium, after chemical , In the optical model the nucleus is treated purification, was allowed to decay for five hours to produce the as a region with a complex refractive index. The total cross Cf'" daughter of the Bk'". The californium was then separated. section may be divided into the diffraction (elastic scattering) portion the inter- The Cf'" was found to emit 6.05-Mev alpha particles. From the ee and action (absorption) portion alpha disintegration rate and the beta disintegration rate of the a„where Bk'" parent, the alpha half-life of CP" was found to be about 12 B. ~1—e* '" ~'p&p . 0 Spontaneous fissions were observed in the samples containing Cf'~. ~, =2m (1—[e'stpl (')pdp. The ratio of alpha disintegrations to fissions was 400. The 0 (2) spontaneous fission half-life of CP" is therefore about 5000 years. Cf~'~.—Direct alpha-decay measurements performed over a Here &(p) is the (complex) shift of that part of an incident of several months on a sample consisting largely of Cf'" plane wave which passes through the nucleus along a line which is indicate a half-life for this nuclide of roughly two years. It was at a distance p from the center. Let k= p/k be the wave propaga- found that 26 percent of the alpha particles in the original cali- tion number of the incident particle outside the nucleus, and let fornium fraction produced from the highly irradiated plutonium its propagation number inside the nucleus at a distance r from the were due to CP'. Thus from the previously mentioned alpha-to- center be k+k~(r)+ —',fE(r). Then Cfss', fission ratio of 400 found for pure its contribution (approxi- ff(P) mately 3 percent) to the observed high spontaneous fission rate B(p) =2 /kg(r)+-', iK(r)gdx, in the californium fraction (alpha-to-fission ratio=42) can be calculated. The great bulk of the fission rate must therefore be where s(p) = (R' —p')&, and r = (p'+x')&. due to Cf'", and a calculation on this basis shows its spontaneous fission half-life to be about 100 years. For the purpose of this TABLE I. Interaction cross sections of 700-Mev pions on Pb. calculation it was assumed that in this case the correction for CP'4 spontaneous fission from would be small. Model R (10» cm) RI (10» cm) o.+ (mb) o —o+ (mb) Cf24'.—The Bk2'9 grows a californium daughter CP4' which emission of 6.0-Mev percent) and 5.82-Mev (90 8.295 1862 41 decays by (10 8,295 7.188 1827 —114 particles. From the amount of CP" alpha activity 7,110 percent) alpha 1430 —74 which grew from a known amount of Bk'4', the alpha half-life of 7.110 6.161 1405 51

, the Cf'4' was found to be about 400 years.