Search

We have studied the properties of low-lying levels in {sup 7}He using the {sup 2}H({sup 6}He,p){sup 7}He reaction at 11.5 MeV/u. This reaction probes the {sup 6}He{sub g.s.}+n character of states in {sup 7}He. The ground state was populated with a spectroscopic factor comparable to that obtained from ab initio calculations, supporting the tentative spin-parity assignment of 3/2{sup -} in the literature. In addition to the ground state, a broad structure is observed between E{sub X}=2-3 MeV, the excitation-energy range expected for the 1/2{sup -} state in {sup 7}He. No evidence was found for a lower-lying, first-excited state reported recently.

For measurements of stellar reaction rates of proton rich nuclei involving resonance levels just above threshold, targets of Si-28 were used in studies of the Si-21(He-4, He-6)Si-26 reaction using the Research Center for Nuclear Physics (RCNP) Ring Cyclotron at Osaka University. Resonance structure observed in Si-26 above the 5.5 MeV proton threshold identified levels populated in the astrophysically important reaction Al-26(p,gamma) Si-26 and help to determine the yield of Al-26 gamma-rays seen in Novae. A few isolated states were also observed above the 9.2 MeV alpha threshold in the (He-4, He-6) reaction which can be used to improve the present stellar rate calculations. We describe here the involved preparation of large area Si-28 metal targets used for these experiments produced by the method of electron beam evaporation. The parting agent used was betaine. To obtain the 0.7 Mg/cm(2) Si-28 target thickness needed for the experiment, several mounted layers were assembled together by stacking. This formidable approach therefore required many foils. Two targets stacks of 0.7 Mg/cm(2) and 0.5 mg/cm(2) Si-28 were delivered to Osaka for the run. Published by Elsevier B.V.

Gamma-ray singles spectra of extremely pure (chemically and isotopically) samples of $^{255}\mathrm{Fm}$, with strengths of $~1$ mCi, have been measured with a high-resolution 2-cm${}^{2}\ifmmode\times\else\texttimes\fi{}$10-mm germanium LEPS detector and with a 25% Ge spectrometer. Gamma rays with intensities as low as $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$% per $^{255}\mathrm{Fm}$ \ensuremath{\alpha} decay have been identified. The electron spectrum of a mass-separated $^{251}\mathrm{Es}$ source was measured with a cooled Si(Li) electron spectrometer. The spectrum provided the conversion coefficients of low-energy transitions in $^{251}\mathrm{Cf}$ and thereby their multipolarities. The present measurements confirm the previous assignments of single-particle states in $^{251}\mathrm{Cf}$. These include $1/{2}^{+}$[620], 0.0 keV; $7/{2}^{+}$[613], 106.30 keV; $3/{2}^{+}$[622], 177.59 keV; $11/{2}^{\ensuremath{-}}$[725], 370.47 keV; $9/{2}^{\ensuremath{-}}$[734], 433.91 keV; $5/{2}^{+}$[622], 543.98 keV; $1/{2}^{\ensuremath{-}}$[750], 632.0 keV; $9/{2}^{+}$[615], 683 keV; and $9/{2}^{+}$[604], 974.0 keV. A vibrational band was identified in previous studies at 981.4 keV and given an assignment of ${7/{2}^{+}[613]\ensuremath{\bigotimes}{2}^{\ensuremath{-}}}3/{2}^{\ensuremath{-}}$. Three new vibrational bands are identified in the present work at 942.5, 1086.5, and 1250.0 keV with tentative assignments ${7/{2}^{+}[613]\ensuremath{\bigotimes}{1}^{\ensuremath{-}}}5/{2}^{\ensuremath{-}},{7/{2}^{+}[613]\ensuremath{\bigotimes}{1}^{\ensuremath{-}}}9/{2}^{\ensuremath{-}}$, and ${7/{2}^{+}[613]\ensuremath{\bigotimes}{0}^{+}}7/{2}^{+}$, respectively. A level was identified at 1185.5 keV with spin of 5/2 or 7/2 but it was not given any configuration assignment. Another level was identified at 1077.5 keV and given a spin of 9/2. Again, no configuration could be assigned to this level.

In-beam {gamma}-ray spectroscopic measurements have been made on {sub 102}{sup 253}No. A single rotational band was identified up to a probable spin of 39/2({Dirac_h}/2{pi}), which is assigned to the 7/2{sup +}[624] Nilsson configuration. The bandhead energy and the moment of inertia provide discriminating tests of contemporary models of the heaviest nuclei. Novel methods were required to interpret the sparse data set associated with cross sections of around 50 nb. These methods included comparisons of experimental and simulated spectra, as well as testing for evidence of a rotational band in the {gamma}{gamma} matrix.

Using a 33 MeV proton beam with our Enge split-pole spectrograph, we measured the ${Q}_{0}$ values of the $^{24}\mathrm{Mg}(p,t)^{22}\mathrm{Mg}$ and $^{28}\mathrm{Si}(p,t)^{26}\mathrm{Si}$ reactions relative to that of the $^{16}\mathrm{O}(p,t)^{14}\mathrm{O}$ reaction. Data were obtained at scattering angles of ${15}^{\ifmmode^\circ\else\textdegree\fi{}}$ and ${25}^{\ifmmode^\circ\else\textdegree\fi{}}$. Based on the masses of $^{24}\mathrm{Mg}$, $^{28}\mathrm{Si}$, $^{16}\mathrm{O}$, ${}^{14}O,p$, and t, we determined the mass excesses of $^{22}\mathrm{Mg}$ and $^{26}\mathrm{Si}$ to be \ensuremath{-}400.5(1.0) and \ensuremath{-}7139.5(1.0) keV, respectively. Our result for the mass of $^{22}\mathrm{Mg}$ is in excellent agreement with recent high-precision measurements.

The level structure of $^{249}\mathrm{Bk}$ has been investigated by measuring the \ensuremath{\gamma}-ray spectra of an extremely pure $^{253}\mathrm{Es}$ sample obtained by milking this nuclide from $^{253}\mathrm{Cf}$ source material produced in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Additional information on the $^{249}\mathrm{Bk}$ levels was obtained from the ${\ensuremath{\beta}}^{\ensuremath{-}}$-decay study of $^{249}\mathrm{Cm}$, produced by neutron irradiation of $^{248}\mathrm{Cm}$. Using the results of the present study together with the data from previous $^{248}\mathrm{Cm}$($\ensuremath{\alpha},t$) and $^{248}\mathrm{Cm}$($^{3}\mathrm{He},d$) reactions, the following single-particle states have been identified in $^{249}\mathrm{Bk}$: 7/2${}^{+}$[633], 0.0 keV; 3/2${}^{\ensuremath{-}}$[521], 8.78 keV; 1/2${}^{+}$[400], 377.55 keV; 5/2${}^{+}$[642], 389.17 keV; 1/2${}^{\ensuremath{-}}$[530], 569.20 keV; 1/2${}^{\ensuremath{-}}$[521], 643.0 keV; 5/2${}^{\ensuremath{-}}$[523], 672.9 keV; and 9/2${}^{+}$[624], 1075.1 keV. Four vibrational bands were identified at 767.9, 932.2, 1150.7, and 1223.0 keV with tentative assignments of {7/2${}^{+}$ [633] $\ensuremath{\bigotimes}{1}^{\ensuremath{-}}$}9/2${}^{\ensuremath{-}}$, {7/2${}^{+}$ [633] $\ensuremath{\bigotimes} {0}^{\ensuremath{-}}$}7/2${}^{\ensuremath{-}}$, {7/2${}^{+}$ [633] $\ensuremath{\bigotimes} {1}^{\ensuremath{-}}$}5/2${}^{\ensuremath{-}}$, and {7/2${}^{+}$ [633] $\ensuremath{\bigotimes} {0}^{+}$}7/2${}^{+}$, respectively. A band at 899.9 keV was observed in $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence measurements and given a tentative spin assignment of 3/2. It is possibly associated with a 2${}^{\ensuremath{-}}$ phonon coupled to the ground state, with configuration {7/2${}^{+}$ [633] $\ensuremath{\bigotimes} {2}^{\ensuremath{-}}$}3/2${}^{\ensuremath{-}}$. Three levels at 624.3, 703.5, and 769.1 keV were assigned spins of 5/2, 7/2, and 9/2, respectively. These could be the members of the 3/2${}^{+}$ [651] band, expected in this energy region.

The excitation function for the fusion-evaporation reaction 64Ni+100Mo has been measured down to a cross-section of ~5 nb. Extensive coupled-channels calculations have been performed, which cannot reproduce the steep fall-off of the excitation function at extreme sub-barrier energies. Thus, this system exhibits a hindrance for fusion, a phenomenon that has been discovered only recently. In the S-factor representation introduced to quantify the hindrance, a maximum is observed at E_s=120.6 MeV, which corresponds to 90% of the reference energy E_s^ref, a value expected from systematics of closed-shell systems. A systematic analysis of Ni-induced fusion reactions leading to compound nuclei with mass A=100-200 is presented in order to explore a possible dependence of the fusion hindrance on nuclear structure., 10 pages, 9 figures, Submitted to Phys. Rev. C

We have studied the 2H(8Li,p)9Li reaction to obtain information on the spins, parities, and single-neutron spectroscopic factors for states in 9Li, using a radioactive 8Li beam. The deduced properties of the lowest three states are compared to the predictions of a number of calculations for the structure of 9Li. The results of ab initio quantum Monte Carlo calculations are in good agreement with the observed properties.

The magnetic moments of I$^\pi = 2^+_1$ states in even–even A $\sim$ 100 fission fragments have been measured using the Gammasphere array, using the technique of time-integral perturbed angular correlations. The collective (core) g factors of several odd nuclei have also been determined. The data are interpreted within the context of the interacting boson model (IBA2).

The low intensity of the RIBS presently available at the first generation production facilities (10(5)-10(6) pps) and the necessity to reconstruct the event kinematics in RIB measurements require detection systems having both a large solid-angle coverage and a high granularity. The EXODET (EXOtic DETector) apparatus has been accomplished to respond to these requirements and the first experiment has been successfully performed studying the F-17 scattering on Pb-208 at 90.4 MeV.

We have detected the electrons accompanying the decay of a long‐lived isomer in 254No, which was produced with the 208Pb(48Ca,2n) reaction. Time and spatial correlations of A = 254 evaporation residues, isomeric electrons and ground‐state α‐particles confirmed the existence of the isomer. The measured electron sum‐energy spectrum favors a Kπ = 7− assignment for the isomer.

International audience; The Canadian Penning trap (CPT) mass spectrometer at the Argonne National Laboratory makes precise mass measurements of nuclides with short half-lives. Since the previous ENAM conference, many significant modifications to the apparatus were implemented to improve both the precision and efficiency of measurement, and now more than 60 radioactive isotopes have been measured with half-lives as short as one second and with a precision ( Δm/m) approaching 10-8. The CPT mass measurement program has concentrated so far on nuclides of importance to astrophysics. In particular, measurements have been obtained of isotopes along the rp-process path, in which energy is released from a series of rapid proton-capture reactions. An X-ray burst is one possible site for the rp-process mechanism which involves the accretion of hydrogen and helium from one star onto the surface of its neutron star binary companion. Mass measurements are required as key inputs to network calculations used to describe the rp-process in terms of the abundances of the nuclides produced, the light-curve profile of the X-ray bursts, and the energy produced. This paper will present the precise mass measurements made along the rp-process path with particular emphasis on the "waiting-point" nuclides 68Se and 64Ge.

We have studied the properties of low‐lying levels in 7He using the d(6He,p)7He reaction. Recent measurements of 8He breakup and the p(8He,d)7He reaction present conflicting results on 7He first excited state A broad resonance observed in the present work at EX = 2.2−0.1+0.2 MeV is identified with the expected 1/2− spin‐orbit partner to the 7He ground state. No evidence is seen of a lower first‐excited state that was recently reported.

Targets employing refractory compounds are being developed for the rare isotope accelerator (RIA) facility to produce ion species far from stability. With the 100 kW beams proposed for the production targets, dissipation of heat becomes a challenging issue. In our two-step target design, neutrons are generated in a refractory primary target, inducing fission in the surrounding uranium carbide. The interplay of density, grain size, thermal conductivity and diffusion properties of the UC 2 needs to be well understood before fabrication. Thin samples of uranium carbide were prepared for thermal conductivity measurements using an electron beam to heat the sample and an optical pyrometer to observe the thermal radiation. Release efficiencies and independent thermal analysis on these samples are being undertaken at Oak Ridge National Laboratory (ORNL). An alternate target concept for RIA, the tilted slab approach promises to be simple with fast ion release and capable of withstanding high beam intensities while providing considerable yields via spallation. A proposed small business innovative research (SBIR) project will design a prototype tilted target, exploring the materials needed for fabrication and testing at an irradiation facility to address issues of heat transfer and stresses within the target.

We have performed precision laser spectroscopy on individual $^{6}\mathrm{H}\mathrm{e}$ (${t}_{1/2}=0.8\text{ }\text{ }\mathrm{s}$) atoms confined and cooled in a magneto-optical trap, and measured the isotope shift between $^{6}\mathrm{H}\mathrm{e}$ and $^{4}\mathrm{H}\mathrm{e}$ to be $43\text{ }\text{ }194.772\ifmmode\pm\else\textpm\fi{}0.056\text{ }\text{ }\mathrm{M}\mathrm{H}\mathrm{z}$ for the ${2}^{3}{S}_{1}\ensuremath{-}{3}^{3}{P}_{2}$ transition. Based on this measurement and atomic theory, the nuclear charge radius of $^{6}\mathrm{H}\mathrm{e}$ is determined for the first time in a method independent of nuclear models to be $2.054\ifmmode\pm\else\textpm\fi{}0.014\text{ }\text{ }\mathrm{f}\mathrm{m}$. The result is compared with the values predicted by a number of nuclear structure calculations and tests their ability to characterize this loosely bound halo nucleus.

Fusion-evaporation cross sections for $^{64}\mathrm{N}\mathrm{i}+^{64}\mathrm{N}\mathrm{i}$ have been measured down to the 10 nb level. For fusion between two open-shell nuclei, this is the first observation of a maximum in the $S$-factor, which signals a strong sub-barrier hindrance. A comparison with the $^{58}\mathrm{N}\mathrm{i}+^{58}\mathrm{N}\mathrm{i}$, $^{58}\mathrm{N}\mathrm{i}+^{60}\mathrm{N}\mathrm{i}$, and $^{58}\mathrm{N}\mathrm{i}+^{64}\mathrm{N}\mathrm{i}$ systems indicates a strong dependence of the energy where the hindrance occurs on the stiffness of the interacting nuclei.

A new pixel-structure detector array with a large solid angle coverage has been used for the first time to study the elastic scattering of exotic $^{17}\mathrm{F}$ nuclei from a $^{208}\mathrm{Pb}$ target at $90.4\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. The experimental data have been analyzed in the framework of the optical model potential and the real and imaginary strong absorption radii have been evaluated. These quantities have been compared with those obtained for the system $^{19}\mathrm{F}+^{208}\mathrm{Pb}$ at the same energy in the center of mass frame. The $^{17}\mathrm{F}+^{208}\mathrm{Pb}$ reaction cross section is more similar to those of the systems $^{16,17}\mathrm{O}+^{208}\mathrm{Pb}$ rather than to the one of the system $^{19}\mathrm{F}+^{208}\mathrm{Pb}$ at similar energies: this indicates that in the energy range around the Coulomb barrier the breakup channel is still weak. The exclusive breakup cross section $^{17}\mathrm{F}\ensuremath{\rightarrow}^{16}\mathrm{O}+p$ has been measured for the first time at energy below the Coulomb barrier.

Atom trap trace analysis, a novel method based upon laser trapping and cooling, is used to count individual atoms of $^{41}\mathrm{Ca}$ present in biomedical samples with isotopic abundance levels between ${10}^{\ensuremath{-}8}$ and ${10}^{\ensuremath{-}10}$. The method is calibrated against resonance ionization mass spectrometry, demonstrating good agreement between the two methods. The present system has a counting efficiency of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$. Within 1 h of observation time, its $3\mathrm{\text{\ensuremath{-}}}\ensuremath{\sigma}$ detection limit on the isotopic abundance of $^{41}\mathrm{Ca}$ reaches $4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$.

The excitation energy, spin and parity of the yrast superdeformed (SD) band in 152Dy have been established. The excitation energy of the lowest observed level in this band is found to be 10,644 keV with an assigned spin and parity of 24+. In addition, nine transitions of dipole character have been identified, which connect the excited superdeformed band 6 in 152Dy to the yrast SD band. The excitation energy of the lowest level in the excited SD band 6 is 14,239 keV. The spin and parity of this state has been determined to be either 29− or 31−. The measured properties of SD band 6 are consistent with an interpretation in terms of a rotational band built on an octupole vibration. A comparison with an RPA calculation by Nakatsukasa et al. suggests that the spin of the lowest SD band 6 level is 31−.

A new detector apparatus has been designed and developed to be used in experiments performed with radioactive ion beams. It consists of 16 highly segmented silicon strip detectors arranged in two‐layer telescopes and subtending a large solid angle (about 70% of 4π sr). An innovative readout system for the position information that uses highly integrated electronics (ASIC chips) has been implemented. A first successful experiment has been performed at the Argonne National Laboratory (USA) to study the 17F scattering by a 208Pb target at 90.4 MeV of incident energy. The 17F angular distribution has been analyzed and the optical model potential best‐fit parameters determined. The same analysis performed on 17F data taken at higher incident energy, in completely different experimental conditions, gives consistent results. The comparison with experiments performed with stable beams (19F, 16O, 17O) indicates a behavior for the 17F more similar to that of the Oxygen isotopes than to the 19F one. Despite of the short data collection time, also the cross section for the 17F →16O + p break‐up process has been estimated.

We have measured the total energy of the alpha particles following the beta decay of 8B by implanting 8B into a planar silicon surface barrier detector. Calibration was performed using alpha particles following the beta decay of 20Na, similarly implanted. The alpha spectrum is used to infer the 8B neutrino spectrum which is an important input in the interpretation of experiments that detect energetic neutrinos from the Sun. The alpha spectrum reported here is in disagreement with the previous best measurement which used two detectors in coincidence.

$\ensuremath{\alpha}$-Particle, conversion-electron, and $\ensuremath{\gamma}$-ray singles spectra of $^{251}\mathrm{Cf}$ sources containing few Bq activities were measured with high-resolution semiconductor detectors. $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence and level half-life measurements were also performed. On the basis of these measurements the following single-particle states have been identified in $^{247}\mathrm{Cm}$: $9∕{2}^{\ensuremath{-}}[734]$, 0 keV; $5∕{2}^{+}[622]$, 227.4 keV; $7∕{2}^{+}[624]$, 285.4 keV; $1∕{2}^{+}[620]$, 404.9 keV; $1∕{2}^{+}[631]$, $518.6\phantom{\rule{0.3em}{0ex}}\text{keV}$. The measured single-particle energies are in good agreement with energies calculated with a Woods-Saxon single-particle potential. From the measured half-life of the $227.4\text{\ensuremath{-}}\text{keV}$ level and the $E3$ branching ratio, a $B(E3)$ value of $5\phantom{\rule{0.3em}{0ex}}\text{W.u.}$ has been deduced between the $5∕{2}^{+}[622]$ and $9∕{2}^{\ensuremath{-}}[734]$ bands. The large $B(E3)$ value is an indication of octupole mixing in the $227.4\text{\ensuremath{-}}\text{keV}$ level.

The branching ratio ${\ensuremath{\Gamma}}_{\ensuremath{\alpha}}/{\ensuremath{\Gamma}}_{\ensuremath{\gamma}}$ of the 4.033 MeV ${3/2}^{+}$ state in ${}^{19}\mathrm{Ne}$ plays a crucial role in the breakout from the hot CNO cycle into the rapid proton capture process. This ratio has been studied by making use of the advantages of inverse kinematics. The state was populated via the ${}^{3}\mathrm{He}{(}^{20}\mathrm{Ne},\ensuremath{\alpha}{)}^{19}{\mathrm{Ne}}^{*}$ reaction and its decay via $\ensuremath{\gamma}$ or $\ensuremath{\alpha}$ emission was measured by detecting the heavy reaction products ${(}^{19}\mathrm{Ne}$ or ${}^{15}\mathrm{O})$ in coincidence in a magnetic spectrograph. An upper limit ${\ensuremath{\Gamma}}_{\ensuremath{\alpha}}/{\ensuremath{\Gamma}}_{\ensuremath{\gamma}}l~6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ has been obtained. With these results, the astrophysical reaction rate for the ${}^{15}\mathrm{O}$ $(\ensuremath{\alpha},\ensuremath{\gamma})$ ${}^{19}\mathrm{Ne}$ reaction has been calculated. Its influence on the breakout at various astrophysical sites, novas, x-ray bursts, and supermassive stars, is discussed.

In the production of the heaviest elements, the cross‐sections for evaporation residues are very small, which, in turn, requires the usage of intense beams. Hence, the targets used tend to exhibit shortened lifetimes as beam currents are raised. Tightly focused beams on stationary targets of modest melting point and/or high sputtering yield material will eventually melt or destroy the target. Defocused or “wobbled” beams enhance target survival only to a modest degree. Rotating the target on a wheel can overcome target melting, and using, in addition, a low sputtering rate material as a covering layer can address this issue and allow for higher beam currents to be used for experiments. The purpose of the calculations done for this work is to attempt to predict the safe range of beam currents allowable, i.e. currents which produce heat loads below the melting point of the target. Materials with favorable sputtering rates and thermal properties are also examined. Calculations of the heating and sputtering t...

The excitation energy, spin and parity of the yrast superdeformed band 1 in 152Dy have been established. The evidence comes mainly from the measured properties of a 4011 keV single‐step transition connecting the yrast superdeformed level fed by the 693 keV transition to the 27− yrast state. Four additional, weaker, linking γ rays have been placed as well. The excitation energy of the lowest superdeformed band 1 member is 10,644 keV and its spin and parity are determined to be 24+. A further nine transitions of dipole character have been identified, which connect the excited superdeformed band 6 in 152Dy to the yrast SD band 1. The excitation energy of the lowest level in the excited SD band 1 is 14,239 keV. The lowest state in band 6 has been determined to be of negative parity and odd spin (29− or 31−). The measured properties of band 6 are consistent with an interpretation in terms of an octupole vibrational band. A comparison with an RPA calculation by Nakatsukasa et al. suggests that the spin of the lowest SD band 6 level is 31−.

In an effort to explore new methods of producing ion beams from solid materials, a laser‐ablation technique for evaporating materials directly into an electron cyclotron resonance (ECR) ion source plasma was developed. A pulsed NdYaG laser with approximately 25 W average power and peak power density on the order of 107 W/cm2 has been used off‐line to measure ablation rates of various materials as a function of peak laser power. The benefits anticipated from the successful demonstration of this technique include the ability to use very small quantities of materials efficiently, improved material efficiency of incorporation into the ECR plasma, and decoupling of the material evaporation process from the ECR source tuning operation. The results of these tests are reported herein and the design is described for incorporating such a system directly with the Argonne Tandem Linac Accelerator System Positive Ion Injector ECR (ATLAS PII‐ECR) ion source.

Scattering of alpha particles from 44Ti, the lightest unstable alpha-particle nucleus above A=40, has been measured at backward angles. The "anomalous" order-of-magnitude enhancement that is characteristic of 40Ca and other light alpha-particle nuclei is not observed. Instead, the backward yield is similar to that observed for other nuclei heavier than 40Ca, and is well described with average optical model parameters.

A high-precision measurement of excitation energies in ${}^{22}\mathrm{Mg}$ was performed using the ${}^{25}\mathrm{Mg}{(}^{3}\mathrm{He}{,}^{6}\mathrm{He}{)}^{22}\mathrm{Mg}$ reaction as a different approach to study this proton-rich, astrophysically interesting nucleus. The reaction was studied at 51.0 MeV with the Enge split-pole spectrograph at Yale. Proton-unbound states at 6.051(4) and 6.329(6) MeV were observed, confirming a recent identification of these new states with the ${}^{24}\mathrm{Mg}{(p,t)}^{22}\mathrm{Mg}$ reaction. There is no evidence in our data of a previously reported state at 5.837 MeV.

The excitation function for fusion evaporation in the $\text{ }{\text{ }}^{60}\text{Ni}+\text{ }{\text{ }}^{89}\text{Y}$ system was measured over a range in cross section covering 6 orders of magnitude. The cross section exhibits an abrupt decrease at extreme sub-barrier energies. This behavior, which is also present in a few other systems found in the literature, cannot be reproduced with present models, including those based on a coupled-channels approach. Possible causes are discussed, including a dependence on the intrinsic structure of the participants.

We have studied the ${}^{29}\mathrm{Si}{(}^{3}\mathrm{He}{,}^{6}\mathrm{He}{)}^{26}\mathrm{Si}$ reaction and have identified new states in ${}^{26}\mathrm{Si}$ at ${E}_{x}=5.140(10),$ ${E}_{x}=5.678(8) \mathrm{MeV},$ and ${E}_{x}=5.945(8).$ Based on these measurements and other recent evidence, we suggest spin-parity assignments of ${1}^{+}$ for the 5.678 MeV state and ${3}^{+}$ for the 5.945 MeV state, which would account for all the ``missing'' unnatural parity states in ${}^{26}\mathrm{Si}$ in the excitation energy region important to hydrogen burning in novae. New reaction rates are presented for the ${}^{25}\mathrm{Al}(p,\ensuremath{\gamma}{)}^{26}\mathrm{Si}$ reaction based on this possible assignment of states.

In 1998, we reported the three-laboratory measurements of the 44Ti half-life which was determined relative to the well known value (5.2714 ± 0.0005 yr) of the 60Co half-life. We have continued the measurement at Argonne and Jerusalem and inclusion of data points for additional two years does not change our published value of 59.0 ± 0.6 yr.

The excitation energy, spin, and parity of the yrast superdeformed band in 152Dy have been firmly established. The evidence comes mainly from the measured properties of a 4011 keV single-step transition connecting the yrast superdeformed level fed by the 693 keV transition to the 27- yrast state. Four additional, weaker, linking gamma rays have been placed as well. The excitation energy of the lowest superdeformed band member is 10 644 keV and its spin and parity are determined to be 24+.

In a series of experiments, 48Ca induced cold fusion reactions on Pb targets were employed to populate states in 253,254No isotopes. Evidence for decays from a rotational band built on a 7/2[624] configuration was obtained in 253No. The ground-state band of 254No is identified up to spin 20+. The deduced quadrupole deformation is β=0.27(2). The initial angular momentum and excitation energy of 253,254No residues, after neutron emission, was determined. States up to spin 22ℏ and excitation energy of E*=8.5 MeV were observed. The data provide direct information on the fission barrier at high spins Bf⩾5 MeV, and on the shell-correction energy Eshellexp⩾4.1 MeV.

Levels in the neutron-rich, odd-mass ${}^{107}\mathrm{Ru}$ nucleus have been reinvestigated with Gammasphere by measuring high-fold, prompt coincidence events following spontaneous fission of ${}^{252}\mathrm{Cf}.$ The ground state band has been extended up to $\frac{27}{2}\ensuremath{\Elzxh}.$ The structure associated with the ${h}_{11/2}$ excitation has been confirmed and extended to higher spin. The ${h}_{11/2}$ band head has been established to lie at 301.8 keV. These results clear up differences between our earlier work and results from another experiment published recently. A new collective band based on a ${9/2}^{\ensuremath{-}}$ level has been identified for the first time. Some distinct features of the level scheme are discussed.

The proton-rich nuclei ${}^{23}\mathrm{Al}$ and ${}^{27}\mathrm{P}$ were studied using the ${}^{24}\mathrm{Mg}{(}^{7}\mathrm{Li}{,}^{8}\mathrm{He}{)}^{23}\mathrm{Al}$ and ${}^{28}\mathrm{Si}{(}^{7}\mathrm{Li}{,}^{8}\mathrm{He}{)}^{27}\mathrm{P}$ reactions, respectively. Several energy levels are observed in each nucleus. The mass excess of ${}^{23}\mathrm{Al}$ was measured to be 6.773(28) MeV and the mass excess of ${}^{27}\mathrm{P}$ was measured to be $\ensuremath{-}0.670(41) \mathrm{MeV}.$ The first excited state of ${}^{23}\mathrm{Al}$ has been resolved from the ground state for the first time at an excitation energy of ${E}_{x}=0.550(20) \mathrm{MeV}.$ The first excited state of ${}^{27}\mathrm{P}$ has been measured for the first time at an excitation energy of ${E}_{x}=1.199(19) \mathrm{MeV}.$ The astrophysical implications of these results for the reaction rates of ${}^{22}\mathrm{Mg}(p,\ensuremath{\gamma}{)}^{23}\mathrm{Al}$ and ${}^{26}\mathrm{Si}(p,\ensuremath{\gamma}{)}^{27}\mathrm{P}$ for the nucleosynthesis of ${}^{22}\mathrm{Na}$ and ${}^{26}\mathrm{Al}$ in Ne novae will be discussed.

In heavy-ion physics, high beam currents can eventually melt or destroy the target. Tightly focused beams on stationary targets of modest melting point will exhibit short lifetimes. Defocused or “wobbled” beams are employed to enhance target survival. Rotating targets using large diameter wheels can help overcome target melting and allow for higher beam currents to be used in experiments. The purpose of the calculations in this work is to try and predict the safe maximum beam currents which produce heat loads below the melting point of the target material.

Various beams of short- and long-lived radioactive nuclei have recently been produced at the ATLAS accelerator at Argonne National Laboratory, using either the so-called In-Flight or the Two-Accelerator method. The production techniques, as well as recent results with 44Ti(T1/2=60y) and 17F(T1/2=64s) beams, which are of interest to nucleosynthesis in supernovae and X-ray bursts, are discussed.

Gamma-ray spectra of a 20-h {sup 255}Fm source containing {approx}1 mCi activity were measured with a 25% Ge detector and a low-energy photon spectrometer (LEPS). Gamma lines with intensities as low as 1.0x10{sup -6}% per {sup 255}Fm {alpha} decay were observed. Gamma-gamma coincidence spectra of a {sup 255}Fm sample were measured with the GAMMASPHERE array. A comparison of the {gamma}-ray spectrum gated by the Cf K{sub {alpha}} x-ray peak with the {gamma}-singles spectrum provided spins of the excited states in {sup 251}Cf. The {gamma}-ray data, in conjunction with previously measured {sup 250}Cf(d,p) reaction data, allowed us to characterize several single-particle and vibrational states above the N=152 subshell gap.

The relative {sup 4}He and {sup 5}He ternary fission yields were determined from a careful analysis of the energy distribution of {alpha} spectra from a new measurement with a {sup 252}Cf source and from published data on {sup 252}Cf and {sup 235}U(n,f). The kinetic energies of the {sup 5}He and {sup 4}He ternary particles were found to be approximately 11 and 16 MeV, respectively. {sup 5}He particles contribute 10-20 % to the total alpha yield with the remainder originating from {sup 4}He accompanied fission. (c) 2000 The American Physical Society.

Various changes have been made in two electron beam sources to make them more suitable for target preparation. The beam spot size has been made more flexible and the accessibility to some of the component parts has been improved.

Self-supporting targets of 92,98Mo, with thicknesses of 100 and 200 μg/cm2, were prepared by electron beam gun evaporation. Substrate heating proved crucial for the production of these foils. The numerous parting agents explored will be discussed. Targets of 92,98Mo were also prepared on carbon backings of various thicknesses. Problems encountered in the production of 100–200 μg/cm2 thick self-supporting and carbon-backed Mo targets for reactions with heavy ions are discussed.

A study of the $\ensuremath{\gamma}$-ray decay of low-lying excited states in fragments produced in the spontaneous fission of ${}^{252}\mathrm{Cf}$ has revealed a significant correlation between the angles of emission of the ${2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+}$ transitions of complementary fragment pairs. Calculations of the amount of dealignment that is needed to reproduce the measured ${a}_{2}$ values, and a comparison with the results of previous fragment-$\ensuremath{\gamma}$ angular distribution measurements, suggests that at scission there may be significant population of $m\ensuremath{\ne}0$ substates associated with the projection of the fragment spin vector on the fission axis. Fragments from the spontaneous fission of ${}^{248}\mathrm{Cm}$ emit ${2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+} \ensuremath{\gamma}$ rays that show markedly reduced interfragment correlations, suggesting that either a larger role is played by the relative angular momentum of the fragments, or that the dealignment introduced by the neutron emission and statistical $\ensuremath{\gamma}$ decay to the ${2}_{1}^{+}$ state is larger in ${}^{248}\mathrm{Cm}$ than ${}^{252}\mathrm{Cf}$ fission.

The Argonne concept for an accelerator complex for efficiently producing high-quality radioactive beams from ion source energy up to 6-15 MeV/u is described. The Isotope-Separator-On-Line (ISOL) method is used. A high-power driver accelerator produces radionuclides in a target that is closely coupled to an ion source and mass separator. By using a driver accelerator which can deliver a variety of beams and energies the radionuclide production mechanisms can be chosen to optimize yields for the species of interest. To effectively utilize the high beam power of the driver two-step target/ion source geometries are proposed (1) Neutron production with intermediate energy deuterons on a primary target to produce neutron-rich fission products in a secondary {sup 238}U target, and (2) Fragmentation of neutron-rich heavy ion rich fission products in a secondary beams such as {sup 18}O in a target/catcher geometry. Heavy ion beams with total energies in the 1-10 GcV range are also available for radionuclide production via high-energy spallation reactions. At the present time R and D is in progress to develop superconducting resonator structures for a driver linac to cover the energy range up to 100 MeV per nucleon for heavy ions and 200 MeV for protons. The post accelerator scheme is based on using existing ISOL-type 1+ ion source technology followed by CW Radio Frequency Quadruple (RFQ) accelerators and superconducting linacs including the present ATLAS accelerator. A full-scale prototype of the first-stage RFQ has been successfully tested with RF at full design voltage and tests with ion beams are in progress. A benchmark beam, {sup 132}Sn {at} 7 MeV/u, requires two stripping stages, one a gas stripper at very low velocity after the first RFQ section, and one a foil stripper at higher velocity after a superconducting-linac injector.

A description is given for a low-mass, rotating target wheel to be used within the Gammasphere target chamber. This system was developed for experiments employing high beam currents in order to extend lifetimes of targets using low-melting point target material. The design is based on a previously successful implementation of rotating target wheels for the Argonne Positron Experiment (APEX) as well as the Fragment Mass Analyser (FMA) at ATLAS (Argonne Tandem Linac Accelerator System). A brief history of these rotating target wheel systems is given as well as a discussion on target preparation and performance.