R.L. Hahn

New neptinium isotopes, 230Np and 229Np

Abstract Two new neptunium isotopes 230 Np and 229 Np were produced by bombarding enriched 233 U with protons at the Oak Ridge Isochronous Cyclotron. In the experiments, recoil nuclei ejected from the target were collected and then assayed with an α-particle spectrometer. The 230 Np and 229 Np isotopes were found to have respective α-particle energies and half-lives of 6.66±0.02 MeV and 4.6±0.3 min and 6.89±0.02 MeV and 4.0±0.2 min. The 226 Pa and 225 Pa collateral series from the decay of the neptunium parents were also seen. The α-particle energies found for the 225 Pa series are more precise than previously available values: 225 Pa, 7.25±0.02 MeV (new value); 221 Ac, 7.63±0.02 MeV; 217 Fr, 8.31±0.02 MeV.and 213 At, 9.06±0.02 MeV. Evidence for the possible decay of 221 Ac via emission of a 7.42 MeV α-particle to an excited state of 217 Fr is presented. The α-particle energies measured for 230 Np, 229 Np and 225 Pa verify the predictions of Viola and Seaborg from α-decay systematics.

Hydration of the Dy3+ ion in dysprosium chloride solutions determined by neutron diffractiona)

The coordination of water molecules about the Dy3+ ion in a 2.38 m DyCl3 in D2O solution has been determined by neutron scattering. The information was obtained using samples that were identical except for the isotopic species of the Dy3+ ions. The experiment yields the distribution of the deuterium and oxygen atoms in the first hydration shell. Each Dy3+ ion is surrounded by 7.4±0.5 water molecules. The dysprosium–oxygen separation is 2.37 A and the dysprosium–deuterium separation is 3.04 A. The cation–water molecule orientation is closer to planar than that which has generally been observed in other aqueous solutions.

Chemical Effects of Isomeric Transitions: Separation of the Isomers of Te127, Te129, and Te121

The problem of the chemical separation of radioactive tellurium isomers has been reinvestigated. The separation efficiencies obtained with end‐window β‐particle detectors for Te127 and Te129 are significantly less than unity and are in agreement with the values determined by Williams. It is demonstrated, however, that these values are not proper in that they have not been corrected for differences in detection efficiencies for the different β particles which were observed. Use of γ‐ray spectrometry has overcome this difficulty; measurement of the relative rate of growth of an individual γ ray, from a sample in which the equilibrium between metastable‐ and ground‐state isomers has been disturbed by the chemical separation, determines the percent of isomer separation, independent of detection efficiency. The separation efficiencies for Te127 and Te121 were found to 0.984±0.022 and 1.0±∼0.05, respectively. These results also were used in support of the concept that the chemical properties of the unstable Te ...

Interactions of 3He particles with B, N, Na and Be

Abstract Thick-target yields for reactions of 3 He particles with B, N, Be and Na were determined by irradiating boron, beryllium nitride and sodium iodide with 3 He ions of energies from 6 to 18 MeV. Excitation functions for the reactions 11 B( 3 He, n) 13 N, 10 B( 3 He, d) 11 C, 14 N( 3 He, α) 13 N, 14 N( 3 He, d) 15 O, 9 Be( 3 He, n) 11 C and 23 Na( 3 He, 2p) 24 Na were obtained by numerical differentiation of the smoothed yield versus energy curves. The distorted-wave theory of nuclear reactions was used to compute excitation functions for comparison with the data for the 14 N( 3 He, α) 13 N reaction and for the ( 3 He, d) reactions on 14 N and 10 B. Optical-model potentials, not for the particular nucleides observed, but determined for neighbouring light nuclei, were used in the calculations. The results of this comparison of theory with experiment indicate that the 14 N( 3 He, α) and 10 B( 3 He, d) reactions appear to proceed mainly by direct pick-up and stripping processes, respectively. For the 14 N( 3 He, d) reaction, a stripping calculation that considered only those 15 O states with known spins and parities yielded a lower limit of ≈30% of the observed excitation function.

The discovery of 260Md and the decay properties of 258Fm, 258m,gMd and 259Md☆

Abstract We have discovered a new neutron-rich isotope, 260Md, from 18O and 22Ne bombardments of 254Es. We observed a spontaneous-fission (SF) activity with a half-life of 32 days in electromagnetically separated fractions with mass number 260 from these bombardments and we measured the mass and kinetic energy distributions of this SF activity. The mass distribution was symmetric with the principal energy peak at a total kinetic energy (TKE) of 234 MeV, similar to previous observations for heavy fermium isotopes. Surprisingly, we also observed a smaller symmetric component with a TKE of 195 MeV. We interpret these two peaks in the TKE distribution as arising from two types of fission in the same nucleus, or bimodal fission. The observed fission activity may be either from the SF decay of 260Md or from 260Fm which would arise from electron-capture (EC) decay of 260Md. We have eliminated the possible β− decay of 260Md by measuring β−-SF time correlations for the decay of 260Md and we plan to determine whether 260Md decays by EC by measuring time correlations between fermium X-rays and SF events. We also measured various properties of the heavy fermium and mendelevium isotopes and obtained 1. (1) more accurate cross-sections for the neutron-rich mendelevium isotopes which we use to predict the production rates of yet undiscovered nuclides, 2. (2) improved half-Me measurements for 258m,gMd and 259Md, 3. (3) confirmation of the EC decay of 258mMd by measurement of the fermium X-rays preceding the SF decay of 258Fm and 4. (4) very substantially improved mass and TKE distributions for the SF decay of 258Fm and 259Md.

On the reactions of protons with 231Pa and 232Th

Abstract Yields of α-radioactive products from reactions induced in 231 Pa targets by protons from 35 to 63 MeV were measured by recoil-collection techniques. The results, as similarly observed by Lefort and co-workers for the reactions of 232 Th+p at energies ≦ 85 MeV, indicate that the yields of reactions involving charged-particle emission are comparable to or larger than those involving only neutron emission. In addition, the experimental yield curves exhibit high-energy tails. Results for both the 231 Pa and 232 Th targets are compared with the predictions of nuclearreaction calculations that take into account the competition between fission and particle emission. Bearing in mind the assumptions inherent in the calculations and the fact that no parameters were fitted to the data, we found that the compound-nucleus model, with fission, did not account for the experimentally observed trends. The intra-nuclear cascade model, including fission competition in the compound-nuclear de-excitation phase, on the other hand, predicted excitation functions that were reasonably consistent with the results for (p, x n) and (p, p x n) reactions. Neither model was successful in accounting for the (p, α3n) data. In the case of 231 Pa results, it was found that recoil-range effects had to be included in the nuclear-reaction calculations.

Heavy isotope production by multinucleon transfer reactions with 254Es

Abstract Fast automated on-line and quasi-on-line radiochemical techniques are applied to search for new isotopes, to measure their decay characteristics and to study the cross sections of the heaviest most neutron-rich actinide isotopes in reactions of 16,18O and 22Ne projectiles with 254Es as a target. The measured yields for isotopes up to lawrencium-260 are three or more orders of magnitude higher than in any other reaction used so far. A comparison with data for similar transfers from 248Cm targets is made. Transfer cross sections are extrapolated for the production of unknown, neutron-rich isotopes of elements 101 through 105, and the unique potential of 254Es as a target to make these exotic nuclei accessible is demonstrated.

Chemical Effects of Isomeric Transitions: The Separation of Tellurium Isomers in Different Chemical Media

The chemical separation of the nuclear isomers of 127Te and 129Te in different media has been studied. The variation of fj(127Te) with pH in aqueous solutions of HCl, HNO3, HClO4, and NaOH has been measured; fj is the probability that, if the parent metastable‐state activity is in the telluric acid form, the daughter ground‐state activity will be found as tellurous acid as a result of chemical interactions in a medium of substance j. The fjs for the different acids behave very similarly as the pH changes. In 6N acid solutions, fj is close to unity; it decreases monotonically with increasing pH and approaches the value for H2O, at pH of 6.0, of 0.760±0.017. In NaOH, fj increases with decreasing pH, and also approaches the value for H2O. The variation of fj with pH may be expressed by the relation, fj=exp(xj)/[1+exp(xj)], with xj=mj(pH)+bj; mj and bj are parameters which are obtained from the data. Also, relative fj values for 127Te and 129Te have been determined in solid telluric acid, H2O, and 6N solutio...

Inclusion of fission and charged-particle emission in calculations of nuclear reactions

Abstract The inclusion of the competition between particle emission and fission in existing intra- nuclear cascade calculations is discussed. It is demonstrated that the cross section for the evaporation from compound-nucleus c of any particles x , be they neutrons or charged particles, can be expressed as σ (c, x , f) = σ c P x Π x i = 1 [ Γ n /( Γ n + Γ f )] i . Here σ c is the compound-nucleus formation cross section, P x is the usual probability of emission of the x particles in the absence of fission, and the factor in brackets considers the competition between fission and particle emission at each step in the evaporation chain. This result is to be contrasted with most earlier treatments that assumed P x = 0 unless all of the emitted particles i were neutrons. The empirical formula of Sikkeland et al. , with its simple dependence upon atomic number Z and neutron number N for heavy-element nuclei, is shown to be an appropriate measure of [ Γ n / ( Γ n + Γ f )] i in cases where charged particles as well as neutrons compete with the fission process. Excitation functions were calculated for reactions of 231 Pa with ≦ 65 MeV protons (see the following paper for a detailed comparison with experiment) and are presented in this paper to illustrate effects due to the inclusion of charged-particle evaporation and fission-spallation competition.

Angular distributions of the recoil nuclei from the reactions 12C(3He, α)11C and 12C(3He, d)13N

Abstract Angular distributions of the 11 C and 13 N recoil nuclei from the reactions of 24.8 MeV 3 He particles with 12 C have been determined by assay of the radioactive nucleides after their collection in catcher foils. Data for laboratory angles from 7° to 170° are presented. Structure in the 11 C angular distribution is readily apparent. A calculated 11 C angular distribution, based upon the distorted-wave theory of direct reactions, can reproduce essentially all of the details of the experimental data. It is concluded from these data that the 12 C( 3 He, α) 11 C reaction proceeds predominantly by direct pick-up. Contrastingly, the 13 N recoil data, when compared with the data of Wegner and Hall for the 12 C( 3 He, d 0 ) 13 N reaction which can be fitted with the results of a direct stripping calculation, indicate that the ( 3 He, pn) reaction is a significant contributor (> 60 %) to the 12 C → 13 N cross section. Neither the shape nor the magnitude of the 13 N angular distribution can be explained on the basis of ( 3 He, d) stripping alone.