Search

A ~10 µm thick liquid lithium film, flowing at ~50 m s−1 bombarded by a 65 keV, 300 W proton beam was successfully tested at Argonne National Laboratory for the Facility for Rare Isotope Beams (FRIB). An ion source, originally developed for the low energy demonstration accelerator, with a new beam transport system built at FRIB, deposited the proton beam in the lithium film at 43 % of the beam power expected at FRIB when accelerating 400 kW of uranium beam at 200 MeV per nucleon. This technology may also be applicable to other high power target areas.

Reactions with a heavy projectile incident on a light target can be used for the efficient in-flight production of secondary radioactive beams. An overview of this technique is given using data on 17F beams produced via the p(17O, 17F)n and d(16O, 17F)n reactions. With primary 16,17O beam currents of 100 pnA, intensities of up to 2×106 17F/s on target were achieved. Using this beam, the p(17F, α) 14O reaction was measured.

With 600 meters of superconducting accelerator, the Rare Isotope Accelerator (RIA) facility will have an extensive cryogenic system operating at both 2.0 K and 4.4 K. Approximately 250 4.4 K resonators, 200 2.0 K resonators, 160 4.4 K magnets, and several 4.4 K bunchers will be used in a 1.4-GV superconducting CW driver linac. Ion beams ranging from protons (up to 900 MeV) to uranium (up to 400 MeV per nucleon) at beam powers up to 400 kW will be produced. The facility will also have a superconducting linac to accelerate short-lived rare isotopes produced by the driver. This post accelerator is composed of another 100 superconducting resonators and associated superconducting focussing magnets. Liquid helium will also be provided to a variety of experimental instruments including, for example, large superconducting magnetic spectrographs. Overall, the liquid helium refrigerator will need to provide approximately 8.6 kW of cooling at 2.0 K, 4.8 kW at 4.4 K, and 15.3 kW at 35 K for shield cooling. A review of the various loads, cryostats, distribution system, and refrigeration schemes will be presented along with some special needs for reliable operation.

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.

Beam currents of radioactive {sup 17}F(T{sub 1/2} = 65s) as high as 2 x 10{sup 6} s{sup {minus}1} have been produced at the ATLAS facility and delivered to target for nuclear physics research. The d({sup 16}O, {sup 17}F)n and p({sup 17}O,{sup 17}F)n reaction were used to produce the {sup 17}F in the energy range of 65-110 MeV with {sup 17}F intensities of up to 250 pnA. The target employed is a liquid nitrogen cooled H{sub 2} gas cell, with HAVAR windows, operating at up to 8 x 10{sup 4} Pa pressure. A new beam optics geometry consisting of a superconducting solenoid immediately after the production target followed by a single superconducting resonator has significantly improved the total capture efficiency of the transport system. The superconducting solenoid captures the highly divergent secondary beam and refocuses it to improve the beam match into the remainder of the transport system. A single superconducting resonator then ''debunches'' the beam, reducing the energy spread by a factor of four. The beam energy can also be varied, using the resonant cavity, without changing the primary beam energy. Detailed discussion of the results, comparison to calculations, and further possible improvements will be presented.

A direct experimental determination has been made of the ratio of the forward-angle positive-pion electroproduction cross section for a proton bound in the deuteron with that of a free proton for invariant masses of 1160 and 1232 MeV. A significant quenching of the reaction in the deuteron is observed.

Experiments were conducted to demonstrate the stable operation of the windowless liquid lithium target under extreme thermal loads that are equivalent to uranium beams from the proposed Rare Isotope Accelerator (RIA) driver linac. The cross section of the windowless liquid lithium target was 5mm×10mm and the velocity of the liquid lithium was varied up to 6m∕s. Thermal loads up to 20 kW within a beam spot of 1 mm in diameter were applied on the windowless liquid lithium target by 1 MeV electron beams. The maximum power density and total power deposited within the target was equivalent to that of a 200 kW, 400MeV∕u uranium beam. It was demonstrated that the windowless liquid lithium target flowing at as low as 1.8m∕s stably operates at a beam energy deposition up to 20 kW without disruption or excess vaporization.

The most stringent test to date of the concept of isospin splitting of the giant dipole resonance in a medium-weight nucleus has been performed by a study of the (γ, n0), (γ, p0), and (γ, γ) reaction channels for 60Ni. The ground-state photoneutron cross section for 60Ni was measured and compared with the already known (γ, p0) reaction cross section in order to demonstrate isospin splitting. The relative strength and separation of the isospin-dependent components of the resonance were estimated from an analysis of photon scattering data.

This Letter reports the first measurement of tensor polarization (t/sub 20/) in ..pi..-d elastic scattering and also measurements of the ..pi..-d elastic-scattering cross section at 180/sup 0/. The measurements were performed using the low-energy pion channel at Clinton P. Anderson Meson Physics Facility and a novel high-efficiency deuteron polarimeter. The result of the polarization measurement, t/sub 20/=-0.23 +- 0.15 at T/sub ..pi../+=140 MeV, is in disagreement with all theoretical predictions.

The angular distribution for the /sup 13/C(..gamma..,n/sub 0/)/sup 12/C reaction was observed in the energy region 6.5 to 9.3 MeV and at angles of 90/sup 0/ and 135/sup 0/. The photoneutron measurements were analyzed in terms of a multilevel R-matrix formalism. The /sup 12/C(n,n)/sup 12/C reaction channel was explicitly included in this analysis. The effects of potential capture were directly observed in the photoneutron spectra. The ground-state radiative widths for resonances in this energy region were deduced from the R-matrix interpretation of the results. The ground-state transition probabilities for E1 excitations at 7.69 and 8.19 MeV were found to be in good agreement with the predictions of the weak-coupling model.

The photoneutron cross section of $^{208}\mathrm{Pb}$ was observed using a very high resolution time-of-flight spectrometer. The cross sections were observed in the photoneutron energy range 16 to 1000 keV and at reaction angles of 90\ifmmode^\circ\else\textdegree\fi{} and 135\ifmmode^\circ\else\textdegree\fi{}. The deduced ground-state radiative widths were calibrated to the well-known $^{2}\mathrm{H}$($\ensuremath{\gamma}$,$n$)$^{1}\mathrm{H}$ reaction cross section. The 7.99-MeV resonance, previously believed to be an $M1$ excitation, is shown to be an $E1$ resonance. Current progress in the search for the collective $M1$ resonance in $^{208}\mathrm{Pb}$ is reviewed. In addition, recent theoretical predictions of the properties of the giant $M1$ resonance are reviewed. Finally, the present high resolution observations in conjunction with previous photoneutron polarization measurements were employed in order to deduce the $s\ensuremath{-}d$-wave admixtures for eight $E1$ excitations.NUCLEAR REACTIONS $^{208}\mathrm{Pb}$($\ensuremath{\gamma}$,$n$)$^{207}\mathrm{Pb}$, ${E}_{\mathrm{exc}}=7.4\ensuremath{-}8.4$ MeV, observed $\ensuremath{\sigma}(\ensuremath{\theta})$, $\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}, 135\ifmmode^\circ\else\textdegree\fi{}$; deduced ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}0}$, ${J}^{\ensuremath{\pi}}$.

A systematic set of measurements of pion and proton yields from pion reactions on nuclei 4≤A≤180 has been performed for incident pions in the Δ(3,3) energy region. Detailed energy spectra for protons at many angles were obtained along with inclusive pion yields. The implications of general features of these data for the pion absorption and scattering processes are discussed.

The proton yields from the interaction of pions on nuclear targets with 12

Nuclear pion single-charge exchange has been studied using a method of ..pi../sup 0/ detection based on the spectroscopy of the back-angle ..gamma..-ray decay. Charge-exchange scattering from nuclei ranging from Be to Pb was studied for incident pion energies of 50 and 100 MeV. The angular distributions and ..pi../sup 0/ energy spectra beyond 60/sup 0/ suggest effects characteristic of quasifree scattering. Total charge-exchange cross sections are larger than values suggested by optical-model calculations.

Angular distributions of the deuteron tensor polarization, t/sub 20/, in ..pi..-d elastic scattering at pion energies of 180, 220, and 256 MeV and excitation function at theta/sub c.m.//sup ..pi../roughly-equal144/sup 0/ have been measured. The results suggest that all published calculations fail to include true pion absorption properly in the treatment of the ..pi..NN system. No rapid angular or energy dependence was found near pion energies of either 134 or 256 MeV, where other experiments have suggested the existence of dibaryons.

We report on the detailed design and calibration of a novel, high-efficiency polarimeter for tensor polarized deuterons. This device measures the t 20 moment of the deutron, and was built to observe the polarization of recoil deutrons from 180° π−d elastic scattering. Based on the 3 He(d, p) 4 He reaction at low energies and forward angles, the polarimeter detects 1.8×10 −4 protons/incident deuteron with an average analyzing power of T 20 = −0.6.. The performance of the polarimeter in high background fluxes is discussed.

The angular distribution for the $^{17}\mathrm{O}(\ensuremath{\gamma}, {n}_{0})^{16}\mathrm{O}$ reaction was observed throughout the excitation energy region 4.3-7 MeV and at angles of 90\ifmmode^\circ\else\textdegree\fi{} and 135\ifmmode^\circ\else\textdegree\fi{}. The ground-state radiation widths for resonances in this energy region were extracted from the data. The value of the radiation width for the ${d}_{\frac{5}{2}}\ensuremath{\rightarrow}{d}_{\frac{3}{2}}$ spin-flip transition at 5.08 MeV was found to be approximately $\frac{1}{3}$ of the value expected for a pure single-particle transition. The implications that this result has for the nuclear structure of $^{17}\mathrm{O}$ is discussed. The effects of potential radiative capture were observed directly in a photoneutron reaction for the first time. At the location of the 5.38-MeV, $\frac{3}{{2}^{\ensuremath{-}}}$ resonance in $^{17}\mathrm{O}$, an anomalous symmetric dip was observed in the cross section at both reaction angles. The data were interpreted in terms of a general $R$-matrix reaction theory which includes the effects of internal, channel, and potential radiative capture in a self-consistent manner. The neutron channel was defined by incorporating an $R$-matrix analysis of the $^{16}\mathrm{O}(n, n)^{16}\mathrm{O}$ reaction into the present interpretation. The anomalous minimum at 5.38 MeV was found to be due to a unique feature of channel capture. The $R$-matrix prediction for the total cross section was extrapolated into the keV region and the significance that this cross section has for stellar nucleosynthesis is discussed.NUCLEAR REACTIONS $^{17}\mathrm{O}(\ensuremath{\gamma}, {n}_{0})^{16}\mathrm{O}$; observed angular distribution; ${E}_{\ensuremath{\gamma}}=4.3\ensuremath{-}7$ MeV; $\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}, 135\ifmmode^\circ\else\textdegree\fi{}$; $R$-matrix analysis; measured ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}0}$ for $E1$ and $M1$ resonances.

This paper reports the first measurement of the tensor polarization t/sub 20/ in e-d elastic scattering. The polarization of the recoil deuterons was measured for two values of momentum transfer, q = 1.74 and 2.03 fm/sup -1/, with a high-efficiency polarimeter. The results are in good agreement with reasonable models for the deuteron.

The (..pi../sup +/,..pi../sup 0/) and (..pi../sup -/,..pi../sup 0/) reactions were studied on a variety of nuclei at a bombarding energy of 160 MeV. The full ..pi../sup 0/ energy spectra were measured with a ..pi../sup 0/ spectrometer at several detection angles. The energy spectra and angular distributions are characteristic of quasifree reactions. Effects due to multiple scattering, neutron screening, and Pauli blocking are clearly observed.

High-accuracy angular distribution measurements relative to that at 90° were performed for the 2(γ,n)H reaction throughout the energy range of 4–18 MeV and for reaction angles of 45°, 135°, and 155°. The results indicate a significant departure from existing theoretical calculations below a photon energy of 10 MeV.

A systematic study of inclusive pion scattering from targets of $^{4}\mathrm{He}$, $^{12}\mathrm{C}$, $^{58}\mathrm{Ni}$, and $^{208}\mathrm{Pb}$ has been performed at incident ${\ensuremath{\pi}}^{+}$ kinetic energies of ${T}_{{\ensuremath{\pi}}^{+}}=100, 160, 220, \mathrm{and} 300$ MeV. Particular attention was paid to the kinematic region where energy losses were greater than typical single-particle binding energies. The spectra of scattered pions were integrated to yield angular distributions and total inelastic-scattering cross sections. The spectra for all targets are similar, showing a peak near that expected for a single-step quasifree scattering process.NUCLEAR REACTIONS $^{4}\mathrm{He}$, $^{12}\mathrm{C}$, $^{58}\mathrm{Ni}$, $^{208}\mathrm{Pb}$, (${\ensuremath{\pi}}^{+}, {{\ensuremath{\pi}}^{+}}^{\ensuremath{'}}$), ${T}_{{\ensuremath{\pi}}^{+}}=100, 160, 220, 300$ MeV; measured $\frac{{d}^{2}\ensuremath{\sigma}}{d{\ensuremath{\Omega}}_{{{\ensuremath{\pi}}^{+}}^{\ensuremath{'}}}d{E}_{{{\ensuremath{\pi}}^{+}}^{\ensuremath{'}}}}$.

Angular distributions of the deuteron tensor polarization, ${t}_{20}$, in \ensuremath{\pi}-d elastic scattering have been measured at pion energies of 180, 220, and 256 MeV. The experiment and analysis are described in detail. Theoretical calculations in which the effects of pion absorption on the elastic channel are small seem to reproduce the data. An excitation function was measured in order to search for a rap- id energy dependence of ${t}_{20}$. No rapid angular or energy dependence was found near a pion energy of 134 MeV, where other experiments have suggested the existence of dibaryon resonances.

Quasimonochromatic photons have been used to measure elastic and inelastic photon scattering cross sections in the giant dipole resonance region of 52Cr, Fe, 60Ni, 92Mo, and 96Mo in an experiment in which the elastic and inelastic scattering are resolved. The elastic scattering cross sections show clear evidence for isospin splitting of the giant dipole resonance. The inelastic scattering to low-lying vibrational levels, which is a measure of the coupling between the giant dipole resonance and collective surface vibrations, is in qualitative agreement with the predictions of the dynamic collective model. However, when examined in detail, this model does not provide an adequate description of the scattering data.

The relative cross sections for the $2N+\ensuremath{\pi}\ensuremath{\rightarrow}2N$ reactions were determined where the initial two nucleons are in a relative $s$ state, and coupled either to isospin zero or to one. This information is extracted from two-nucleon coincidence measurements of the (${\ensuremath{\pi}}^{+}$, $2p$) and the (${\ensuremath{\pi}}^{\ensuremath{-}}$, $\mathrm{pn}$) reactions on targets of $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$.

A deuteron tensor polarimeter with the highest available figure-of-merit was developed in order to measure the tensor polarization t 20 in electron and pion elastic scattering from the deuteron. The polarimeter was calibrated at the Berkeley 88 cyclotron with the associated polarized ion source. The feasibility of the method was demonstrated by measuring t 20 in pion-deuteron elastic scattering at LAMPF. The application of this method to e-d elastic scattering will be discussed for the momentum transfer range 2 fm −2 2 −2 .

The proton spectra from pion and photon induced reactions on 12 C are found to be very similar in shape, and angular distribution. It appears that these protons arise from a common reaction mechanism (Δ production) for the two projectiles. Proton yields with pions and with photons are in approximately the same ratio as the total inelastic pion yield to the total photopion yield.

Very tightly bound, abnormally-high-density states of nuclei, suggested as a possibility by Bodmer and by Lee and Wick, have been searched for as an anomalous isotope of radon. This was done by looking for high-energy $gamma$ rays following thermal-neutron capture. The limit that may by set for the atomic concentration of such nuclides on Earth is approx.10$sup -29$ atom of Si. (AIP)