ISOLDE Research Articles

Alpha -decay rates for 181-186Au and 181-185Pt isotopes.

A systematic study of the \ensuremath{\alpha} decay of $^{181\mathrm{\ensuremath{-}}186}\mathrm{Au}$ utilizing an on-line isotope separator has revealed several \ensuremath{\alpha} groups and resulted in the observation of the \ensuremath{\alpha} decay of $^{186}\mathrm{Au}$ and $^{185}\mathrm{Pt}$. By simultaneous measurement of \ensuremath{\gamma}-ray and x-ray spectra and use of known EC/${\mathrm{\ensuremath{\beta}}}^{+}$ decay schemes, \ensuremath{\alpha}-decay branching ratios and hindrance factors were deduced for each \ensuremath{\alpha} transition. In $^{185}\mathrm{Au}$ and $^{183}\mathrm{Au}$ the ground state is known to be dominated by the 5/${2}^{\mathrm{\ensuremath{-}}}$,1/2[541] configuration associated with the ${\mathit{h}}_{9/2}$ intruder from above the Z=82 closed shell. Thus, the unhindered \ensuremath{\alpha} decay of these parents identify levels in the Ir daughters which also are dominated by the ${\mathit{h}}_{9/2}$ orbital. Several previously unknown low-lying \ensuremath{\gamma}-ray transitions were observed in the Ir daughters by means of \ensuremath{\alpha}-\ensuremath{\gamma} coincidence measurements. The hindrance factor for the \ensuremath{\alpha} decay of $^{181}\mathrm{Au}$ to the 5/${2}^{\mathrm{\ensuremath{-}}}$,1/2[541] ground state of $^{177}\mathrm{Ir}$ suggests that the ground state of $^{181}\mathrm{Au}$ has a different configuration than those of $^{183}\mathrm{Au}$ and $^{185}\mathrm{Au}$.

The use of a radiactive beam of fission products for TDPAC experiments

In order to use a wide variety of radioactive nuclei as probes in material research, an on-line isotope separator (ISOL) for fission products has been utilized to produce and accelerate short-lived nuclei suitable for TDPAC experiments. The140Cs isotope (T 1/2=63.7 s) from ISOL has been implanted into YBaCuO compounds to investigate the hyperfine field around a Ba atom. The140Ba-140La source of 3×104 decays/s is obtained for TDPAC experiments on the 329–487 keV γ-γ cascade in140Ce. Another candidate was examined to use the 345–94 keV cascade in the91Rb (T 1/2=58 s) decay. The angular correlation coefficients, however, unfortunately turned out to be fairly small. A tentative result of theg-factor measurement has been given. The details of the present method have been described with emphasis on the application to material research.

Methodology for the measurement of β−-decay intensity distributions from the analysis of total absorption γ-ray spectra

A total absorption γ-ray spectrometer (TAGS), based on a 25.4-cm diameter × 30.5-cm long NaI(Tl) well detector, has been developed at the INEL on-line isotope separator facility. A Si detector in the well of the NaI(Tl) detector allows one to collect β-particle-gated coincidence γ spectra in addition to singles γ spectra. From the analysis of these total absorption γ-ray spectra, β−-decay intensity distributions are obtained. The analysis methodology is based on response functions that simulate the response of the NaI(Tl) detector to the γ-rays and electrons (including external bremsstrahlung) from a radioactive source in the detector well. These response functions are computed with a Monte Carlo photon and electron transport code. This methodology has been completely upgraded since it was first developed and now provides very good simulations of both singles and coincidence spectra. As an example of the application of this methodology, the Iβ distribution for the decay of 141Cs is determined from the TAGS spectra.

Quadrupole moments and nuclear shapes of neutron-deficient gold isotopes

The hyperfine structure splitting and the isotope shift of the neutron-deficient gold isotopes 194−191Au in the optical transition 5 d 9 6 s 2 2 D 3 2 → 5 d 10 6 p 2 P 1 2 (627.8 nm) have been determined by collinear laser spectroscopy at the on-line isotope separator ISOLDE. The nuclear magnetic moments, spectroscopic quadrupole moments and changes in the mean square charge radii are deduced. Experimental electromagnetic moments and deformation parameters are compared with results of a particle-triaxial-rotor model calculation for both odd- A and odd-odd gold nuclei. Very good overall agreement between calculation and experimental data has been achieved. We find, for the first time, consistent evidence for a relatively large contribution from hexadecapole deformation ( ϵ 4 = 0.06, β 4 = −0.05) to odd- A gold isotopes ground states in the region of 183 ≤ A ≤ 197. Calculated values of quadrupole deformation are consistent with the prolate-to-oblate shape change between ground states of 186Au and 187Au. In addition, our results suggest another shape change between 190Au (oblate) and 191Au (triaxial). The former change is accompanied by a considerable change in quadrupole deformation, the latter occurs at almost constant deformation.

Beta Decay of151Pr into Levels in151Nd

The β-decay of 151 59 Pr 92 to the levels in 151 60 Nd 91 has been studied from the fission products of 235 U using an on-line isotope separator (KUR-ISOL). Gamma-rays and conversion electrons have...

Decay of the new isotope127Pr

A new isotope,127Pr, has been identified and its decay has been studied using on-line isotope separation of94Mo (36Ar,p2n) reaction products. The half-life of the isotope was determined to be 7.7(6) s, and a γ/X-ray spectrum has been obtained. A partial level scheme is proposed for127Ce on the basis ofγ-singles,β-gatedγ andγγ-coincidence data.

First observation of low lying states in the doubly odd nucleus132Pr from beta decay of132Nd

The lowlying level scheme of the oddodd nucleus132Pr has been investigated for the first time. The radioactive nuclei were produced by bombarding96Mo targets with 225 MeV40Ca beam. Levels have been populated from the132Nd β- decay. A new value of its halflife has been measured: T1/2=88±7 s. The mass identification has been made with an ionguide coupled with an online isotope separator. γ- γ- t, X-γ-t and e−-γ-t coincidence measurements have been performed. The observed distribution of the level feedings is similar to that of the neighbouring odd-odd nuclei.

The production of accelerated radioactive ion beams

During the last few years, substantial work has been done and interest developed in the scientific opportunities available with accelerated radioactive ion beams (RIBs) for nuclear physics, astrophysics, and applied research. This interest has led to the construction, development, and proposed development of both first- and second-generation RIB facilities in Asia, North America, and Europe; international conferences on RIBs at Berkeley and Louvain-la-Neuve; and many workshops on specific aspects of RIB production and science. This paper provides a discussion of both the projectile fragmentation, PF, and isotope separator on-line, ISOL, approach to RIB production with particular emphasis on the latter approach, which employs a postaccelerator and is most suitable for nuclear structure physics. The existing, under construction, and proposed facilities worldwide are discussed. The paper draws heavily from the CERN ISOLDE work, the North American IsoSpin Laboratory (ISL) study, and the operating first-generation RIB facility at Louvain-la-Neuve, and the first-generation RIB project currently being constructed at ORNL.

Quadrupole Interaction at Surfaces -Nuclear Methods

Abstract The measurement of the electric quadrupole interaction with nuclear techniques allows the study of monolayer coverages or even isolated atoms at surfaces and interfaces due to the inherent sensitivity of these methods. Recently electric field gradients at surface atoms have been investigated with Mößbauer Spectroscopy, Polarized Beam NMR , and Perturbed Angular Correlation (PAC). The highly sensitive PAC technique with 111In and 111mCd sources, in particular, has been applied to several metal and semiconductor surfaces. The use of flat as well as stepped surfaces has allowed the assignment of the measured quadrupole interaction frequencies to specific surface sites. For the Pd(111) surface a particularly complete description is possible, since for five different sites (terrace adatoms, ledge adatoms, kink adatoms, ledge and terrace atoms) the field gradients and their orientation could be determined. They represent a challenging problem for understanding the electronic structure of metallic surfaces. Through the study of the temperature dependence of the different components the diffusion of the radioactive probe atoms can be followed. The data from 57Fe Mößbauer spectroscopy on iron surfaces with monolayer coverage and NMR surface studies of 7Li and 23Na at high temperatures are reviewed. An outlook for further developments of the nuclear techniques is presented, particularly in view of the variety of radioactive isotope beams available from on-line isotope separators like the ISOLDE facility at CERN.

Bunching of radioactive ion beam in the millisecond region — a pulsed gating-potential method in an ion source for on-line isotope separation

A simple method of bunching radioactive ion beam in the millisecond region has been studied using an isotope separator on-line (ISOL). By applying positive and negative potential alternately between the exit hole of a surface ionizer of the ISOL and an electrode set 1 mm apart from it, we have observed clear bunching effects for alkali elements having relatively high ionization efficiencies. The bunching efficiency defined as intensity ratio of the bunched beam to the usual DC beam was found to increase with the increase of repetition rate of the applied alternate potential; for example, at the duty factor of 20%, the bunching efficiencies for 39 K, 23 Na and 7 Li ions were respectively 0.45, 0.25 and 0.23 at 50 Hz, 0.78, 0.32 and 0.27 at 200 Hz and 1, 0.59 and 0.42 at 1 kHz. The in-beam measurement using the 40 Ca ( 3 He, αp) 38 K reaction showed nearly the same magnitude of the bunching effects as in the case of 39 K.

Levels of146Ce studied through the decay of high-spin isomer of146La

The decay of 10.0 s high-spin isomer of146La to levels of146Ce has been studied using the on-line isotope separator KUR-ISOL. Gamma-ray singles,γ -γ coincidence and conversion-electron spectra have been measured. A level scheme containing 90 levels and 206γ-ray transitions has been constructed for146Ce, populated in the decay of the high-spin isomer of146La. Among these, 39 levels are reported for the first time. Spins and parities for some levels are proposed on the basis ofγ-decay mode, logft values and conversion-electron measurements. The band structure of the low-lying levels in146Ce is discussed.

Picosecond lifetime measurement systems for levels populated by electron capture and for levels decaying by E0 transitions

Two new systems, to measure lifetimes of nuclear levels in the picosecond range that decay by E0 transitions, have been developed at the UNISOR on-line isotope separator facility. These systems make use, for the first time, of fast coincidences involving characteristic X-rays. The first of the two systems involves specifically the X-rays originating from the internal conversion process, while the second involves X-rays resulting from both the internal conversion and electron capture processes.

A new cooling and focusing device for ion guide

A highly efficient focusing device which uses a RF multipole field (sextupole-ion-beam guide (SPIG)) has been developed for the ion guide isotope separator on-line (IGISOL). The SPIG, placed after the nozzle, consists of six circular rods uniformly distributed on a circle in the plane perpendicular to the axis of symmetry. Under the combined action of the sextupole RF electric field produced by the rods and helium gas flow, well focused ion beams with low energy spread can be transmitted efficiently by use of the SPIG. The experimental results, using a discharge ion source, show that almost nearly 90% of the ions which leave the nozzle can be transported through the SPIG, and that the width of the kinetic energy distribution after the SPIG is about 0.8 eV (FWHM). These experimental results agree with the results of Monte Carlo simulations; the simulation studies also indicate that the size of the spatial distribution in the plane perpendicular to the symmetry axis is less than 1 mm in diameter at the end of the SPIG.

Decay scheme of mass-separated 152Nd

Gamma-rays and internal conversion electrons in the decay of 11.6-min 152Nd were measured. The radioactive sources were separated from the fission products of 235U(n,f) with an on-line isotope separator (KUR-ISOL). A precise decay scheme of 152Nd involving 35 γ-rays and 12 levels has been established. The 25 γ-rays and 7 levels were observed for the first time.

Chemically selective laser ion-source for the CERN-ISOLDE on-line mass separator facility

Radioactive atoms produced in proton-induced nuclear reactions and released from thick targets have been ionized resonantly by laser radiation in a hot tube connected to the target container. Pulsed tuneable lasers with a repetition rate as high as 10 kHz have been applied for stepwise resonant excitation and photoionization in the last step. In this way the efficiency and selectivity of the target and ion source system which serves as an injector to the on-line isotope separators at CERN-ISOLDE could be improved. In a series of off-line and on-line studies the ionization of Sn ( E i = 7.3 eV), Tm ( E i = 6.2 eV), Yb ( E i = 6.2 eV) and Li ( E i = 5.4 eV) was investigated. An ionization efficiency of up to 15% was obtained for Yb. The ratio of the laser-ionized and surface-ionized ion currents was measured as a function of temperature for different ionization cavity materials (W, Ta, Nb and TaC). It was shown that this ratio, i.e. the selectivity, rises for Tm from 10 to 10000 with falling temperature and is strongly dependent on the material. Since the lasers are pulsed the ion beam becomes bunched with a pulse width of about 10–50 μs. This width is strongly dependent on the potential drop along the tube (caused by the electric current used for heating the tube) and on the alignment of the laser beams with respect to the tube axis. The selectivity could be further improved by a factor of 10 using gated detection of the bunched ion beam.

Isomeric yield ratios of fission products in the system of 24 MeV proton-induced fission of238U

Isomeric yield ratios of 30 fission products in 24 MeV proton-induced fission of238U were measured by the use of the ion-guide isotope separator on-line. The obtained isomeric yield ratios were converted to the angular momenta of primary fission fragments based on the statistical model. The deduced angular momenta were examined from various aspects. It is found that in general the angular momentum continuously increases with the fragment mass number including the region of symmetric mass division. However, there are some exceptions. For Sn isotopes the deduced angular momenta are quite small due to the spherical shape of the nuclear shell configuration. It is also concluded from the consideration of the charge distribution that the angular momentum of fission product scatters considerably within the narrow range of mass division. The dependence of the angular momentum on the available energy of fragments at scission point indicates that the individual fragment possesses a characteristic deformation at scission and/or the deduced angular momentum is seriously affected by the particle excitation after scission.

Low-Lying Levels in147Nd in the Decay of147Pr

The level structure of 147 Nd has been studied from the decay of 147 Pr. The radioactive sources of 147 Pr were separated from the fission products of 235 U using the on-line isotope-separator KUR-ISOL and chemical separation techniques. A decay scheme has been constructed involving 93 γ-rays and a newly observed 1261.1 keV level. The half-lives of 49.9, 127.8 and 214.6 keV levels were determined to be 1.0 ±0.3 ns, 0.4 ±0.1 ns and 4.53 ±0.06 ns, respectively. Conversion coefficients of 15 transitions were determined. The parities for the levels at 49.9, 127.8, 214.6, 314.7 and 463.5 keV are deduced to be odd. Spins and parities for the 769.3 and 792.6 keV levels are deduced to be 3/2 + . The properties of transition probabilities between the low-lying triplet hole states are similar to that of three particle states in the N =85 isotones.

A precision measurement of the hyperfine structure of87Sr+

The ground state hyperfine splitting of87Sr+ was measured with a precision of 1×10−8. The experiments were performed with an RF ion trap connected to an ISOL (isotope separator on-line), where all the possible transitions between Zeeman sublevels were observed by a laser-microwave double resonance method. The magnetic dipole hyperfine constant was determined to beA=−1 000 473.673 (11) kHz.

Search for population of superdeformed states in 194Pb using 194Bi beta + decay.

A search for the population of superdeformed (SD) states in [sup 194]Pb following the [beta][sup +] decay of [sup 194]Bi was performed with the UNISOR on-line isotope separator at the Oak Ridge National Laboratory. The [beta] decay of [sup 194]Bi, with its large [ital Q][sub [beta]] value and high spins of the decaying isomeric states, provides a favorable opportunity to search for population of known SD states in the daughter nucleus, [sup 194]Pb. Both [gamma] rays and conversion electrons were measured in singles and coincidence; however, no evidence for the population of SD states was observed. Upper limits for the population of certain SD states are presented. A possible explanation for the lack of SD band population following this [beta] decay is discussed.

Towards a "perfect" Penning trap mass spectrometer for unstable isotopes

A Penning trap mass spectrometer has been set up at the on-line isotope separator ISOLDE/CERN for the mass determination of unstable heavy isotopes. The spectrometer should fulfil the following requirements: capture of external ions in high efficiency, high resolving power and accuracy, general applicability to all elements and isotopes available at the on-line facility.