Short-lived Nuclei Research Articles

Fission of nuclei far from stability

The secondary-beam facility of GSI provided the technical equipment for a new kind of fission experiment. Fission properties of short-lived neutron-deficient nuclei have been investigated in inverse kinematics. The measured element distributions reveal new kinds of systematics on shell structure and even–odd effects and lead to an improved understanding of structure effects in nuclear fission. The relevance of these studies for some presently considered applications is described. Prospects for future experiments are discussed.

Evolution of nuclear spectroscopy at Saha Institute of Nuclear Physics

Experimental studies of nuclear excitations have been an important subject from the earliest days when the institute was established. The construction of 4 MeV proton cyclotron was mainly aimed to achieve this goal. Early experiments in nuclear spectroscopy were done with radioactive nuclei with the help of beta and gamma ray spectrometers. Small NaI(Tl) detectors were used for gamma-gamma coincidence, angular correlation and life time measurements. The excited states nuclear magnetic moments were measured in perturbed gamma-gamma angular correlation experiments. A high transmission magnetic beta ray spectrometer was used to measure internal conversion coefficients and beta-gamma coincidence studies. A large number of significant contributions were made during 1950–59 using these facilities. Proton beam in the cyclotron was made available in the late 1950’s and together with 14 MeV neutrons obtained from a C-W generator a large number of short-lived nuclei were investigated during 1960’s and 1970’s. The introduction of high resolution Ge gamma detectors and the improved electronics helped to extend the spectroscopic work which include on-line (p 7 p′γ) and (p 7 nγ) reaction studies. Nuclear spectroscopic studies entered a new phase in the 1980’s with the availability of 40–80 MeV alpha beam from the variable energy cyclotron at VECC, Calcutta. A number of experimental groups were formed in the institute to study nuclear level schemes with (α 7 xnγ) reactions. Initially only two unsuppressed Ge detectors were used for coincidence studies. Later in 1989 five Ge detectors with a large six segmented NaI(Tl) multiplicitysum detector system were successfully used to select various channels in (α 7 xnγ) reactions. From 1990 to date a variety of medium energy heavy ions were made available from the BARC-TIFR Pelletron and the Nuclear Science Centre Pelletron. The state of the art gamma detector arrays in these centres enabled the Saha Institute groups to undertake more sophisticated experiments. Front line nuclear spectroscopy works are now being done and new informations are obtained for a large number of nuclei over a wide mass range. Currently Saha Institute is building a multi-element gamma heavy ion neutron array detector (MEGHNAD), which will have six high efficiency clover Ge detector together with charged particle ball and other accessories. The system is expected to be usable in 2002 and will be used in experiments using high energy heavy ions from VECC.

Measurement of activation cross-sections for (n,2n) reactions producing short-lived nuclei in the energy range between 13.4 and 14.9 MeV

Activation cross-sections for 18 (n,2n) reactions producing short-lived nuclei with half-lives between 20 s and 38 min were measured in the energy range between 13.4 and 14.9 MeV by an activation method. The measured target isotopes were 14N, 31P, 54Fe, 63Cu, 79Br, 87Rb, 92Mo, 108Pd, 113In, 132Ba, 138Ba, 140Ce, 141Pr, 144Sm and 165Ho. Four cross-sections for 108Pd, 132Ba, 144Sm and 165Ho were obtained at six-point energies for the first time. The intense 14 MeV neutron source facility (OKTAVIAN) at Osaka University was used for irradiation. All cross-section values were relatively obtained on the basis of the standard cross-section of 27Al(n, α) 24Na reaction (ENDF/B-VI). To obtain reliable neutron activation cross-sections, careful attention was paid to neutron irradiation and induced activities measurement. The present results were compared with previous data and the evaluated data in JENDL-3.2, JENDL-Activation File and ENDF/B-VI. The previous data measured at multi-point energies show reasonable agreement with the present results in comparison with those at one-point energy. There are overestimations of the cross-section for 31P in JENDL-3.2 and ENDF/B-VI, and for 132Ba and 165Ho in JENDL-Activation File.

Muon physics possibilities at a muon-neutrino factory

New intense proton accelerators with above GeV energies and MW beam power, such as they are discussed in connection with neutrino factories, appear to be excellently suited for feeding bright muon sources for low-energy muon science. Muon rates with several orders of magnitude increased flux compared to present facilities will become available. This will allow higher precision in experiments which were statistics limited so far such as searches for rare decays, muonium spectroscopy, muon capture, muon catalyzed fusion, muon decay studies and measurements muon moments and parameters. Novel and most important experiments will become possible. For example a permanent electric dipole moment (edmμ) of a muon could be searched with by far unprecedented accuracy and with a physics potential well beyond the possibilities of present electron, neutron and nuclear edm searches. Investigations of short lived radioactive nuclei using muonic atom spectroscopy would become feasible.

Changes in neutron shell closures of light very neutron-rich nuclei

We present a review of experimental studies on the properties of extremely neutronrich nuclei (Z=6–20, A=20–60) near the neutron drip line. Recent data on mass measurements of neutron-rich nuclei at GANIL (Caen, France) and some characteristics of binding energies in this region are discussed. Being sensitive to the existence of nuclear shells separation energies S2n together with the evidence of particle instability of doubly magic nuclei 10He and 28O could provide information on changes in shell closures. Particularly opening of an N=16 shell and vanishing of the N=20 shell for very neutron-rich isotopes is under discussion. Decay studies of these hardly accessible short-lived neutronrich nuclei from oxygen to silicon using the in-beam γ-ray spectroscopy are also reported.

The Use of Multi-Pass Time-of-Flight Mass Analyzers for Nuclear-Decay Spectroscopy of Mass Identified Nuclei

The masses of ions of some keV can be determined in multi-pass time-of-flight mass analyzers [1,2] with high precision. The mass accuracies thus achieved are sufficient to determine the proton and neutron numbers for most short-lived and stable nuclei [3,5]. Recording α- or γ-radiation of the investigated nuclei in delayed coincidence to the ion arrival, one thus can perform nuclear spectroscopy of selected nuclei.

Weak interaction rates of neutron-rich nuclei and ther-process nucleosynthesis

Weak interaction rates for very short-lived neutron-rich nuclides involved in the r process are mostly beyond the experimental reach at the present time. Accurate theoretical predictions demand a coherent extrapolation of different nuclear properties away from the experimentally known regions. The b-decay and neutrino-capture rates are estimated within the extended Thomas-Fermi plus Strutinsky integral method and the continuum quasiparticle random phase approximation. This approximation on the self-consistent calculation of the ground-state and b-decay properties is described. The large-scale calculations of the b-strength function for stable and short-lived nuclei are performed to estimate, simultaneously, the b-decay and electron neutrino capture rates for nuclides of relevance to the r-process nucleosynthesis. A comparison of the results with existing experimental data, global calculations, and self-consistent microscopic predictions is presented.

First isochronous mass spectrometry at the experimental storage ring ESR

Short-lived exotic nuclei can be produced and separated with the high-energy secondary nuclear beam facility FRS at GSI. These nuclides can be injected and stored in the storage ring ESR. The lower lifetime limit of the presently existing methods for mass measurements on these nuclides at the ESR is about a few seconds. We have developed and investigated an isochronous operational mode of the ESR, that makes mass measurements of nuclides with lifetimes down to a few μs feasible. It has been commissioned in experiments using long-lived nuclides with known masses. A mass resolving power of about 150 000 has been achieved in a first pilot experiment. A suitable detector system has been implemented and commissioned at the ESR that allows measurements of the revolution period of single particles within a few turns in the ESR.

Comment on "Stellar reactions with short-lived nuclei: 17F(p, alpha)14O"

Comment on "Stellar reactions with short-lived nuclei: 17F(p, alpha)14O"

Relativistic radioactive beams: A new access to nuclear-fission studies

The secondary-beam facility of GSI Darmstadt was used to study the fission properties of 70 short-lived radioactive nuclei. Most of them have not been accessible so far in conventional fission experiments. Relativistic secondary projectiles were produced by fragmentation of a 1 A GeV 238 U primary beam and identified in nuclear charge and mass number. Using these reaction products as secondary beams, the giant resonances, mostly the giant dipole resonance, were excited by electromagnetic interactions in a secondary lead target, and fission from excitation energies around 11 MeV was induced. The fission fragments were identified in nuclear charge, and their velocity vectors were determined. Elemental yields and total kinetic energies have been obtained for a number of neutron-deficient actinides and preactinides. The characteristics of multimodal fission of nuclei around 227 Th were systematically investigated. The proton even–odd effect was determined for all systems.

Beams of short-lived nuclei by selective laser ionization in a gas cell

Resonant laser ionization in a gas cell has been implemented at the Leuven Isotope Separator On Line facility to produce beams of short lived radioactive nuclei and to study their decay characteristics. A description of the laser ion source is presented together with technical details on the laser system, the gas cell and the gas purification system. Results from off line as well as on line measurements are discussed. The new laser ion source has allowed β decay studies of neutron deficient (54Ni, 92Rh) as well as neutron rich (up to 74Ni and 70Co) isotopes.

Composition of the nuclear periphery from antiproton absorption using short-lived residual nuclei

The neutron-to-proton density ratio at the nuclear periphery was determined for six targets from the yield of nuclei with mass ${(A}_{t}\ensuremath{-}1)$ generated by antiproton annihilation on targets with mass number ${A}_{t}.$ A new setup allowed us to measure this yield for nuclei with half-lives down to 6 s. The experiment confirmed the negative correlation between the peripheral neutron-to-proton density ratio and the neutron binding energy, which had been previously found. Results of Hartree-Fock-Bogoliubov and relativistic Hartree-Bogoliubov calculations show good agreement for a large number of nuclei. By comparing the measured values with those derived from Hartree-Fock-Bogoliubov calculations, the ratio R of the imaginary parts of the effective scattering lengths of antiprotons on bound neutrons and protons was deduced to be $\mathcal{R}=0.69(4).$ For the first time yields of several excited states of some ${(A}_{t}\ensuremath{-}1)$ nuclei have been investigated.

A 25.5 MHz double-coaxial λ/4-resonator as a rebuncher in heavy ion linac system

A 25.5 MHz double-coaxial λ/4-resonator has been constructed to be used as a rebuncher between a 25.5 MHz RFQ linac and a 51 MHz interdigital-H linac for the acceleration of short-lived nuclei. By employing a double coaxial structure, the resonator length is only 130 cm; the length of the natural λ/4-resonator is 294 cm. The resonator, 69 cm in inner diameter, has six acceleration gaps. The bare shunt impedance is 40.6 MΩ/m. The ions with a charge-to-mass ratio 1/10 can be rebunched by feeding an rf power of 1.4 kW.

Stellar Reactions with Short-Lived Nuclei:17F(p,α)14O

A method has been developed that can provide beams of many short-lived nuclei of interest in nucleosynthesis along the $\mathrm{rp}$ process path. With a ${}^{17}\mathrm{F}$ beam $({T}_{1/2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}64\mathrm{s}$) the excitation function of the ${}^{17}\mathrm{F}(p,\ensuremath{\alpha}{)}^{14}\mathrm{O}$ reaction was measured to determine properties of excited states in ${}^{18}\mathrm{Ne}$. These states influence the rate of the ${}^{14}\mathrm{O}(\ensuremath{\alpha},p{)}^{17}\mathrm{F}$ reaction which is important for understanding energy generation and nucleosynthesis in x-ray bursts. The present direct measurements yield a pattern of resonances and cross sections which differ substantially from previous estimates.

Radioactive ion beam line in Lanzhou

Radioactive ion beam line in Lanzhou (RIBLL) has been constructed for the production of short-lived radioactive nuclei and studies of exotic nuclei far from the β-stability line. It has been put into operation recently at the National Laboratory of Heavy Ion Accelerator Lanzhou. RIBLL consists of two doubly achromatic parts with a solid acceptance ΔΩ⩾6.5 msr, momentum acceptance Δp/p=±5% and maximum magnetic rigidityBp max=4.2 Tm. The second part of RIBLL serving as a spectrometer gives an element resolutionZ/ΔZ>150 and mass resolutionA/ΔA>300. The polarized secondary beams can be obtained by using a swinger dipole magnet to change the incident direction of primary projectile from 0° to 5°. The shortest lift time for secondary beams on RIBLL is less than 1 μs. First experiments were performed with neutron rich nuclei for understanding the properties of halo nuclei and exotic nuclear reactions.

Wide aperture kinematic separator COMBAS realized on the strong focusing principle

The COMBAS large solid angle and high momentum acceptance and high-resolving kinematic separator has been created at the Flerov Laboratory of Nuclear Reactions, JINR, to efficiently collect extremely short-lived nuclei near the zero angle which are produced in intermediate energy fragmentation reactions with wide momentum and wide angular distributions. For the first time the M 1M 2M 3M 4F dM 5M 6M 7M 8F a magneto-optical configuration of the COMBAS separator has been realized on the strong focussing principle. The separation and trajectory analysis of particles by the separator are carried out by three parameters: the magnetic rigidity (B· ρ), the energy loss difference in the degrader (Δ E/Δ x) and the time-of-flight (Δ T) of the analyzed particles. The COMBAS separator can be used efficiently both in the mode of a high- resolving spectrometer to study reaction mechanisms and in the mode of an in-flight separator in experiments on the synthesis and study of properties of short-lived exotic nuclei near the drip-lines.

Stellar reactions with short-lived nuclei: 17F(p,α)14O

Stellar reactions with short-lived nuclei: 17F(p,α)14O

Hyperfine interactions of 19 O in TiO 2 and quadrupole moments of 13,19 O

The electric quadrupole interaction of 19O(Iπ=(5/2)+, T1/2=27.0 s) in TiO2 single crystal was studied in detail by means of the β-NQR to determine the electric quadrupole moments Q of short-lived β-emitting nuclei 19O and 13O(Iπ=(3/2)-, T1/2=8.6 ms). Two implantation sites were found for the implanted O nucleus and the quadrupole coupling constants of 19O at these sites were determined. We observed FT-NMR of the enriched stable isotope 17O in TiO2 and obtained the electric field gradient (EFG) at the oxygen substitutional site. With this knowledge, we have determined Q(13O)=11.0 ± 1.3 mb and Q(19O)=3.7 ± 0.4 mb. The present results are compared with the theoretical values calculated by the shell model code, OXBASH and by the Hartree–Fock calculation with the realistic potential.

Determination of the proton and neutron densities at the nuclear periphery with antiprotonic X-rays and overline p -nucleus reactions

The nucleon density at the nuclear periphery has been investigated with two different methods. The first one is the measurement of the energy shift and the width of antiprotonic X-ray lines. Besides various other nuclides the X-rays from the isotopes 172Yb and 176Yb have been measured very extensively. From these data the absorption widths of six antiprotonic levels and the shifts of the lowest visible transitions could be determined. The comparison of the experimental intensities of the transitions with results from calculations of the antiprotonic cascade showed a good agreement. The second method is the determination of the yield of the annihilation products from the reaction of the antiprotons with nuclei. From this the neutron-to-proton density ratio at a distance of about 3 fm outside the half-density radius can be determined. This method was extended to short-lived residual nuclei with a half-life down to 5 s. The experiment confirmed the previously found negative correlation between the size of the neutron skin and the binding energy of the last neutron. The results have been compared with calculations. For many nuclei a good agreement was found between experiment and calculation, however for some nuclei measured and calculated values are at variance.

Measurement of (n, α) cross-sections for short-lived products by 13.4–14.9 MEV neutrons

Twelve (n,α) activation cross-sections leading to short-lived nuclei with half-lives between 37s and 15 min were measured in the energy range between 13.4 and 14.9 MeV by the activation method. The measured reactions involved the isotopes of 26Mg, 30Si, 31P, 54Cr, 63Cu, 64Ni, 69Ga, 71Ga, 87Rb, 89Y, 104Ru and 112Cd as the target. The intense 14 MeV neutron source facility (OKTAVIAN) at Osaka University was used for irradiation. The γ-rays emitted from the irradiated samples were measured with high-purity germanium detectors. All cross-section values were obtained relative to the standard cross-section of the 27Al(n,α) 24Na reaction. The cross-sections of three reactions were measured for the first time. The present experimental data were discussed by comparing them with previously reported experimental data and the evaluated data of JENDL-3 and ENDF/B-VI. Six evaluated items of data in JENDL-3 show reasonable agreement (within 7%), but the evaluation of the 26Mg(n,α) reaction is 25% higher than our data. Two items of evaluation data of ENDF/B-VI agree well, and one agrees within 25%.