FN Tandem Accelerator Research Articles

Proton–neutron structure of low lying collective quadrupole excitations in 126Xe

Low spin states of the nucleus 126Xe were studied by means of γ-spectroscopy using the fusion evaporation reaction 123Te( α,n) 126Xe performed at the Cologne fn tandem accelerator. The γγ coincidence and singles spectra were measured with the osiris-cube spectrometer. Beside the ground and quasi-gamma bands many other low lying states were observed. γγ directional correlations from oriented states (DCO) were analyzed to determine the multipolarities of the γ-transitions. We observed dominant E2-transitions in the quasi-gamma band and from the gamma- to the ground band but also a large M1-fraction in the 6 + 2→6 1 + transition. We additionally found a strong M1-decay depopulating the 2 + 4-state at 2064 keV. The lifetime of this state was measured using the Doppler shift attenuation technique. The experimental data are compared with a calculation using the Proton–Neutron Interacting Boson Model (IBM-2). Good agreement is reached in the vicinity of the O(6)-limit. The observed M1-transition of the 2 + 4 state is interpreted as due to a fragment of the lowest 2 + mixed-symmetry state predicted by the IBM-2.

Low spin structure of theN=Zodd-odd nucleus2346V23

Low spin states in the odd-odd nucleus ${}^{54}\mathrm{Co}$ were investigated with the ${}^{54}\mathrm{Fe}(p,n\ensuremath{\gamma}{)}^{54}\mathrm{Co}$ fusion evaporation reaction at the FN-TANDEM accelerator in Cologne. $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidences, $\ensuremath{\gamma}\ensuremath{\gamma}$-angular correlations, and Compton asymmetries were measured. 28 low spin states were observed. 19 for the first time, eight new spin assignments and six new parity assignments were made, and seven $\ensuremath{\gamma}$-branching ratios were measured for the first time. $M1$ and $E2$ transition strengths between low-lying states in ${}^{54}\mathrm{Co}$ were calculated in the shell-model framework. The experimental branching ratios and multipole mixing ratios indicate the existence of very strong isovector $M1$ transitions.

Non-yrast states of 132Ce polutated in β-decay

Non-yrast low spin states of the nucleus 132Ce were studied by means of γ-spectroscopy following the β-decay of the ground and medium spin isomeric states of 132Pr. The activity was produced with the reaction 117Sn( 19F,4n) 132Pr at the Cologne FN TANDEM accelerator. The γγ coincidences and singles spectra were measured with the OSIRIS cube spectrometer. The β-decay of 132Pr populates states with spins up to 6ħ and excitation energies up to 4.4 MeV in 132Ce. Besides ground and quasi-gamma bands, an excited band based on the 0 2 + state and many other low-lying states were observed. The γγ angular correlations were analyzed to assign spins and parities to the excited states, and to determine the multipolarities of the γ-transitions. We found dominant E2 transitions in the quasi-gamma band and from the quasi-gamma band to the ground band. The experimental data are compared with calculations using the Interacting Boson Model (IBM). Good agreement is reached in the vicinity of the O(6) limit.

Intruder bands in 113Sn

The level structure of 113Sn has been investigated. The data were obtained by in-beam γ-ray spectroscopy using the reaction 100Mo(18O,5n)113Sn at 70 MeV performed at the OSIRIS gamma array in the Institut fur Kernphysik FN Tandem accelerator. We report in this short note the observation of two intruder type rotational bands in this nucleus.

The γ-decay of single-particle states in 205Pb and 207Pb using an Euroball Cluster detector

Single-particle states in the odd lead isotopes 205Pb and 207Pb were investigated using an Euroball Cluster detector and five usual Ge detectors at the Osiris cube spectrometer of the Cologne FN-Tandem accelerator. The states were populated by the subCoulomb neutron stripping reaction ( d, pγ) at 10 MeV. About 102 γ-transitions could be observed for the first time. The energy of most of the observed states could be determined to an accuracy of about 0.2 keV. The ground state wave function of 206Pb could be determined to contain: 66(5) % ν3 p 1 2 −2 , 19(3) % ν2 f 5 2 −2 , 10(3) % ν3 p 3 2 −2 , and 2(1) % ν2 f 7 2 −2 .

Terminal ion source for an FN tandem

An RF discharge source assembly has been developed for use in the terminal of the FN tandem Van de Graaff accelerator at the Nuclear Physics Laboratory of the University of Washington. The primary motivation for developing the source was to provide a high intensity beam of 3H + to produce 8B from the reaction 6Li( 3He,n) 8B. The design of the optics and the performance of the source are described here.

The response of the Euroball Cluster detector to γ-radiation up to 10 MeV

The Euroball Cluster detector consists of seven closely packed hexagonal encapsulated germanium detectors which are mounted in a common cryostat and surrounded by a BGO suppression shield. By adding back the Compton scattered events between the seven germanium detectors the detector system behaves similar as a single germanium detector with a volume of 2000 cm 3. The response of this very large germanium detector to γ-rays up to 10 MeV has been measured using a short lived 24Al source produced at the Cologne FN Tandem accelerator. At 10 MeV the efficiency of the Cluster detector is nearly one order of magnitude larger than that of a 30% germanium detector.

FN-tandem accelerator in Bucharest after the seismic protection system installation

Subsequent to the installation of the seismic protection system in the HVEC FN-tandem accelerator in Bucharest, observations have been made of the efficiency of the protection devices. Performance and improvements of the new voltage divider are discussed. The SF 6 content of the insulating gas is shown to be conveniently measured using the elastic recoil technique and a robust ioniser heater is described for the HICONEX source.

Coincidence and correlation analysis for low spin states in 128Xe

Abstract Using the fusion evaporation reaction 125Te(α,n) 128Xe, low spin states of 128Xe have been investigated. The alpha beam at a beam energy of 16 MeV was supplied by the Cologne FN-tandem accelerator. Using the OSIRIS-6-Cube spectrometer we obtained both coincidence and singles data. We evaluated the γγ coincidence data, angular distributions and angular correlations. The technique of spin assignment by angular correlation analysis is discussed in some detail. We present an extended level scheme with 154 levels in the energy range up to 4 MeV and a spin range up to 10h. Two collective bands build up upon bandheads with spin I = 0 and I = 4, respectively, are established. Level energies and B(E2) values are compared to predictions of the IBM-1 near the O(6) dynamical symmetry and interpreted in the framework of the new Q phonon scheme.

Operation of a radio-frequency ion source in a tandem electrostatic accelerator

A radio-frequency ion source using electrostatic inflection has been operated in the terminal of a model FN tandem Van de Graaff accelerator. This arrangement is useful for high resolution resonance experiments in atomic and low energy nuclear physics and for hydrogen profiling using the the 6.4 MeV 1 H( 15 N, αγ) 12 C reaction.

Duoplasmatron ion source for proton and deuteron intense beams at low energy (abstract)

The duoplasmatron source was specially designed as a component of the hydrogen and deuterium negative polarized ion source, of Lamb–Shift type, for equipping the FN–Tandem Accelerator in Bucharest. The source can operate long-term at an arc power of 2.5 kW with an intensity of the arc current of 10–11 A. The duoplasmatron source anode is of a special construction in order to allow a strong cooling and a quick adaptation of some cups for the plasma expansion. Three variants of the extraction ion-optics configuration have been analyzed by computer and then experimentally carried out and tested. All the extraction ion-optics configurations employ plasma expansion cups and the process of deacceleration of the beam down to energies of 500–1000 eV is taking place inside of a magnetic lens. The proton and deuterium beam intensity obtained beyond the magnetic lens may exceed 10 mA, function of the discharge conditions in the arc and the ion-optics configuration employed. The ion composition of the beam (H1, H2, H3) function of the discharge parameters in the arc, has also been analyzed and the results showed that the percentage of the atomic ions is higher than 70%.

Romanian ECR ion source project RECRIS

A 14.5 GHz ECR ion source is in an advanced stage of construction at the Institute of Atomic Physics of Bucharest, Romania. This ECR multicharge heavy ion source (RECRIS) is designed to be independently used in atomic physics research as well as an injector into the existing rf heavy ion postaccelerator of the Bucharest FN tandem accelerator. The general design and main characteristics of RECRIS are presented as well as the present status and the schedule of this project. A research program and facility was started in order to study physical ways to improve the ECR ion source performances.

Non-yrast states in 132Ba

Abstract The low spin structure of 132 Ba has been investigated following the β + -decay of ground and medium spin isomeric states of 132 La. The lanthanum nuclei were produced at the Cologne FN-tandem accelerator using the reaction 122 Sn( 14 N, 4n) 132 La. γγ-coincidence data was taken by means of the OSIRIS 6-cube spectrometer. Many new low lying levels were established. We report the level and transition energies, spins, parities and relative branching ratios obtained in this measurement. The low lying states of positive parity are discussed within the framework of the IBA-1 model and compared with results for the F-spin partner 128 Xe. A good description is obtained in terms of the Consistent-Q formalism near the dynamical O(6)-symmetry.

Analogous negative parity spectra of125Xe and127Xe

The low spin states of the nucleus127Xe have been investigated with the (α, 2n) reaction by means of in-beam-γ-spectroscopy, using theosiris-cube-spectrometer at the FNTandem accelerator facility of the University of Cologne. In the negative parity system, four excited states for each of the spins 11/2−.15/2− and two 5/2−.9/2− — states could be established at excitation energies up to ≈ 1MeV above the Yrast line. With this new data, we are able to compare the non-Yrast states and their decay properties to the corresponding states of125Xe, which had been studied with similar methods in Cologne. The almost one-to-one correspondence between the states of these neighboring neutron-odd nuclei is a clear indication that their excitations at low spin remain collective well above the Yrast line.

Low spin states in 130Ba

Abstract The low spin structure of 130 Ba has been investigated following the β + -decay of ground and medium spin isomeric states of 130 La. The lanthanum nuclei were produced at the Cologne FN-tandem accelerator using the reaction 120 Sn( 14 N,4n) 130 La. γγ-coincidence data was taken by means of the OSIRIS 6-cube spectrometer. Many new low lying levels were established. Of special interest are new “ K = 0”- and “K = 4”- bands . We give the level and transition energies, spins and parities, multipole mixing ratios and B (E2)-ratios obtained from this measurement. The low lying states of positive parity are discussed in the framework of the IBA-1 model and compared with results for the F-spin partner 126 Xe. The description is given in terms of a consistent-Q formalism near the dynamical O(6)-symmetry. A new scheme which gives analytical expressions for the O(6)-wave functions is presented. The scheme is still valid approximately in vicinity of O(6).

Particle excitations and collectivity in the N = 48 nuclei 83Br and 85Rb

Excited states of the nuclei 83Br and 85Rb were populated in the reactions 82Se( 7Liα2n) and 82Se( 7Li,4n), respectively, using the 7Li beams of the FN tandem accelerator in Cologne ( E = 32 MeV) and of the 120 cm cyclotron in Rossendorf ( E = 35 MeV). Gamma-gamma-particle coincidences were measured with six Ge and fourteen Si detectors installed at the detector array Cologne OSIRIS CUBE. This technique enabled a reaction channel selection. Multipole orders of the γ-rays were derived from directional correlations of coincident γ-rays and from angular distributions. Mean lifetimes were deduced for seven levels in 83Br and two levels in 85Rb using the Doppler-shift-attenuation method. The level scheme of 83Br has been extended up to ( 21 2 + ) and 15 2 − states. In 85Rb a level sequence built on the ground state has been established up to 15 2 ( −) and a new cascade of fast M1 transitions starting with a 17 2 ( −) state at 3198.2 keV has been found. Furthermore, several medium-spin states have been newly introduced. The level sequences in both nuclei have been interpreted in terms of the shell model. The calculations performed in the model space π(0 g 9 2 , 1 p 1 2 , 1 p 3 2 , 0 f 5 2 ) ν(0 g 9 2 , 1 p 1 2 ) reproduce single-particle characteristics as well as collective properties of the level sequences.

Shell-model states and collectivity in 83Br48 and 85Rb48

Excited states of the nuclei 83Br and 85Rb were populated in the reactions 82Se(7Li,α2n) and 82Se(7Li,4n), respectively, using the 7Li beams of the FN tandem accelerator in Cologne (E = 32 MeV) and of the 120 cm cyclotron in Rossendorf (E = 35 MeV). Gamma-gamma-particle coincidences were measured with six Ge and fourteen Si detectors installed at the detector array Cologne OSIRIS CUBE. This technique enabled a reaction channel selection. Multipole orders of the γ rays were derived from directional correlations of coincident γ rays and from angular distributions. Mean lifetimes were deduced for seven levels in 83Br and two levels in 85Rb using the Doppler-shift-attenuation method. The level scheme of 83Br has been extended up to (21/2+) or 15/2− states. In 85Rb a level sequence built on the ground state has been established up to 15/2(−) and a new cascade of fast M1 transitions starting with a 17/2(−) state at 3198.2keV has been found. Furthermore, several medium-spin states have been newly introduced. The level sequences in both nuclei have been interpreted in terms of the shell model. The calculations performed in the model space π(0g9/2, 1p½, 1p3/2, 0f5/2)v(0g9/2, 1p½) reproduce single-particle characteristics as well as collective properties of the level sequences.

Equipment and methodology for high precision, high throughput 14C AMS analyses at ANTARES

The original Rutgers FN tandem accelerator has been modified for high-precision high-throughput AMS analyses at the ANTARES AMS Center. The status of this work and future plans are discussed, with emphasis on 14C. Isotopic ratios are measured at ANTARES by sequentially injecting stable isotopes and the radioisotope, and the systems developed for this purpose are described. Progress with a new injection platform and high intensity, multi-sample ion source is also given.

RBS and recoil spectrometry analysis of CoSi2 formation on GaAs

Mass and energy-dispersive recoil spectrometry has recently reached the state of development where it is possible to separately characterise Ga and As in GaAs samples. Since it is possible to simultaneously characterise several elements (light as well as heavy), e.g. C, O, Si, Co, Ga and As, the technique is suited for examining the depth distribution of metallisation contacts on GaAs. In a Swedish-Australian collaboration a recoil detector telescope was attached to a beamline of the FN tandem accelerator “ANTARES”, at Lucas Heights Research Laboratories, Australia. In the measurements presented here, 127I10+ at an energy of 77 MeV was employed to analyse GaAs samples with thin film overlayers — Si(220 nm)/Co(50 nm)/〈100〉-GaAs. A reference sample and samples annealed at 300 to 600°C were analysed. The measurements showed that CoSi2 is formed during annealing at and above 500°C with no detectable reaction between the GaAs-substrate and the CoSi2 overlayer.

Transition strengths in 79Rb.

High angular momentum states in $^{79}\mathrm{Rb}$ were populated via the $^{63}\mathrm{Cu}$${(}^{19}$F,2pn${)}^{79}$Rb and $^{65}\mathrm{Cu}$${(}^{18}$O,4n${)}^{79}$Rb reactions. The 65 MeV $^{19}\mathrm{F}$ and 65 MeV $^{18}\mathrm{O}$ beams were provided by the Florida State University FN Tandem accelerator. Prompt \ensuremath{\gamma}-\ensuremath{\gamma} coincidences were measured for both reactions. Two additional \ensuremath{\gamma} cascades have been assigned to $^{79}\mathrm{Rb}$, including a new high-excitation structure dominated by dipole transitions. Firm spin assignments were made for the first time for a number of states by means of directional correlation ratios and an angular distribution measurement using the $^{63}\mathrm{Cu}$${(}^{19}$F,2pn) reaction. The Doppler-shift attenuation method was used to determine lifetimes for states and their respective side-feeding times in both the yrast positive-parity and yrast negative-parity states. Transition quadrupole moments were deduced from the transition strengths and compared with those predicted by Woods-Saxon cranking calculations. The energy spacings and mixing ratios were compared with particle-rotor calculations.