Search

Yields of fission products are essential for designing Radioactive Ion Beam facilities and extracting cross section data to refine reaction models. This work reports the results of irradiations of natural uranium targets with a continuous spectrum of fast neutrons. A rapid transport system has been used to bring the targets in front of two segmented Clover detectors operated in coincidence. Decay γ -ray spectroscopy of the fission products has allowed for cross sections to be obtained in 61 mass chains, either as independent or as cumulative for the chain. Interpolation versus mass number together with the assumption of a Gaussian distribution of isobars enable cross sections to be evaluated in nearly 80 mass chains. The measurement demonstrates the power of the detectors for twofold γ – γ events following β -decay and indicates that new fission product decay data could be obtained with a dedicated set up. [ABSTRACT FROM AUTHOR]

We review structure data obtained by decay spectroscopy of neutron-rich nuclei of mass close to 100. Emphasis is put on the contribution of experiments at IGISOL in the nineties. They confirmed the earlier postulated shape coexistence in the fast shape-transition region between N = 58 (spherical ground states and low collectivity) and N = 60 (strong axial deformation). A detailed spectroscopic study of the A = 99 chain established the upper-Z limit of the N = 56 shell closure region with 99Nb, owing to striking similarities with 97Y. A consequence of the N = 56 closure is that the s1/2 odd-neutron becomes the ground state of the most neutron-rich N = 57 isotones, starting with 99Mo, instead of the degenerated d5/2 and g7/2 subshells familiar in the tin region. Consequences on the change of spin on astrophysical r-process calculations are briefly discussed. Finally, we say a few words about neutron-rich rhodium and palladium isotopes near the neutron midshell where regular and intruder states coexist very close to each other. [ABSTRACT FROM AUTHOR]

The study of β-delayed neutron emission plays a major role in different fields such as nuclear technology, nuclear astrophysics and nuclear structure. However the quality of the existing experimental data nowadays is not sufficient for the various technical and scientific applications and new high precision measurements are necessary to improve the data bases. One key aspect to the success of these high precission measurements is the use of a very pure ion beam that ensures that only the ion of interest is produced. The combination of the IGISOL mass separator with the JYFLTRAP Penning trap is an excellent tool for this type of measurement because of the ability to deliver isobarically and even isomerically clean beams. Another key feature of the installation is the non-chemical selectivity of the IGISOL ion source which allows measurements in the important region of refractory elements. This paper summarises the β-delayed neutron emission studies that have been carried out at the IGISOL facility with two different neutron detectors based on 3He counters in a polyethylene moderator: the Mainz neutron detector and the BEta deLayEd Neutron detector. [ABSTRACT FROM AUTHOR]

Cross-sections for nuclide production in fast-neutron induced fission of natural uranium are part of the input for predictions of yields of neutron-rich nuclides obtainable at Radioactive Ion Beam facilities. We first describe the neutron spectra produced according to the scheme once envisaged for SPES (protons on an enriched 13C target) and the one adopted for SPIRAL2 (deuterons on natural carbon), which both have been measured at JYFL. We then present the measurements of Z-splits in isobaric chains performed at IGISOL. When coupled with the fission cross-section and A-splits for the relevant neutron spectrum, they allow estimates of nuclide cross-sections. It looks that calculations, even those based on modern libraries, are too optimistic by about a factor of two. [ABSTRACT FROM AUTHOR]

Neutron production by stopping 55 MeV deuterons in thick carbon and heavy-water targets has been measured by the activation method. The geometry was close to the one defined for the SPIRAL2 uranium-carbide target in the initial phase. A comparative method for obtaining the neutron flux has been used and is presented in detail. The neutron flux generated by 55 MeV deuterons on carbon is 2.3 times the flux at the deuteron energy of 40 MeV. The flux further increases by a factor 1.4 when using a heavy-water target. These results are discussed in the context of an energy upgrade of the SPIRAL2 driver accelerator. [ABSTRACT FROM AUTHOR]

A UC target material of 11.3±0.5 g/cm^3 uranium density with the grain size of 20 and 5μm manufactured in a form of pills by the method of powder metallurgy has been tested on-line within the temperature range of 1800-2100 ° C . The mass of uranium exposed to the beam was 4-7g. The yields and release rates of Cs and Fr isotopes produced by fission and spallation reactions of 238U by 1GeV protons have been measured. The yields of Cs and Fr isotopes obtained from the tested target materials have been compared, including yields of very short-lived Fr isotopes with half-lives down to 1ms. Temperature-resistant materials (porous graphite and tantalum foil) have been used for the internal-container construction, which holds the UC target pills inside a tungsten external container heated by the resistant heating. The fastest release and the highest efficiency for short-lived isotopes have been obtained for the targets with the internal container manufactured from the tantalum foil. Results of on-line tests of a big mass target (730g of 5μm grain UC target material) have been discussed. [ABSTRACT FROM AUTHOR]

Various alternative models were used to describe the structure of 106Ru . For example, the General Collective Model (GCM) predicts shape-coexistence for 106Ru with a spherical and a triaxial minimum and strongly mixed structures, while in the IBA-2 calculations, where 106Ru was considered as transitional from vibrational U(5) to γ -soft O(6) , no need was found to include the shape-coexisting configurations. In order to provide additional constraints on the model interpretations, we have applied the Advanced Time-Delayed (ATD) βγγ( t) method to measure the level lifetimes of the excited levels in 106Ru . The new results include the half-lives of T 1/2 = 183(3) ps and 7.5(30)ps for the 2+ 1 and 2+ 2 states, respectively. [ABSTRACT FROM AUTHOR]

The 113Ru and 113Rh nuclei, produced in the spontaneous fission of 248Cm and in proton-induced fission of 238U , were studied by means of prompt gamma spectroscopy using the EUROGAM 2 array of Anti-Compton spectrometers and the IGISOL mass separator, respectively. For the 0.5s isomer, found in 113Ru in our recent study we propose the 7/2-[523] configuration. The present data indicate spin and parity 1/2+ for the ground state in 113Ru . This and some new data on the excited levels in 113Rh allows reinterpretation and a better understanding of the β- {} decay scheme of 113Ru . The properties of 113Ru were studied in the frame of the Core-Quasiparticle Coupling model, which has well reproduced the experimental data. [ABSTRACT FROM AUTHOR]

A combined target-ion source unit (ionizing target) has been developed for the on-line production of radioactive single-charged ions. The target is able to withstand temperatures of up to 2500 °C and also acts as an ion source of surface, electron-beam, and laser ionizations. Using tantalum foil as a target material, the on-line combined target-ion source unit efficiency, which is the product of the ionization and release efficiencies, has been obtained for neutron-deficient isotopes of Eu, Gd, and Yb. These nuclides were ionized by the surface ionization inside a hot tungsten target container, holding tantalum foils as a target material. The results of the combined target-ion source unit use for on-line laser resonant ionization spectroscopy investigation of neutron-deficient Gd isotopes have been also presented. For neutron-rich isotopes produced from a high-density UC target, which were ionized by the surface ionization inside the target container, the ionization efficiency values have been obtained close to 100% for Rb and Cs and 10% for In. For Ag and Sn neutron-richisotopes, the values of the electron-beam ionization efficiency in the volume of the targetcontainer, correspondingly equal to 4% and 2%, have been obtained. [ABSTRACT FROM AUTHOR]

A series of on-line mass separation experiments have been performed at the IRIS facility to measure the yield and release of Rb and Cs neutron-rich isotopes produced by fission reaction of238U. A 1 GeV proton beam was used to bombard uranium carbide targets with the densities of 11 g/cm3 and 1.5 g/cm3 held at temperatures in the range (2000-2230)°C. The release curves of Rb and Cs long-lived isotopes were measured from both kinds of targets. The overall production efficiency was determined making use of experimentally measured cross-sections of that isotope production. Comparison of the experimental yields of Rb and Cs isotopes with the calculated ones after corrections for losses due to finite release times suggests that the diffusion is the dominating process reducing the efficiency for short-lived isotopes. When normalized to the same thickness, an enhancement for the high-density rod target of the measured isotope yields is observed when going far from stability. This is possibly explained by the reactions induced by secondary neutrons. A significant odd-even effect with higher yields of Cs even neutron isotopes has been observed, confirming a similar effect obtained in earlier experiments. [ABSTRACT FROM AUTHOR]

In the frame of the R&D program for the SPES project of the Legnaro Laboratories, the production yields of Rb, In and Cs isotopes have bean measured at the synchrocyclotron of the Petersburg Nuclear Physics Institute (PNPI) of Gatchina (Russia). Production yields for direct proton fission and neutron-induced fission on a high-density UC2 powder target are compared. The overall efficiencies as a function of the target temperature have been measured. [ABSTRACT FROM AUTHOR]

The beta-decay of the neutron-rich nucleus 110Mo, separated by the IGISOL on-line mass separator from other fission products, has been investigated by using beta-gamma and gamma-gamma coincidence techniques. The decay scheme of 110Mo has been revised, including 3 new excited states and 7 new γ transitions in 110Tc. The β-feedings were measured and log ft values and B(GT) values were deduced based on a Qβ-value from systematics. Three excited 1+ states in 110Tc fed by spin-flip allowed-unhindered beta transitions were identified. The deduced beta-decay strengths are compared with the Gamow-Teller strength distribution obtained from a macroscopic-microscopic calculation. The role of the asymptotic quantum numbers in the context of the allowed beta-decay is discussed. [ABSTRACT FROM AUTHOR]

: Cross-sections for the production of neutron-rich nuclei obtained by neutron-induced fission of natural uranium have been measured. The neutrons were generated by bombarding a 13C target with 55 MeV protons. The results, position of the maximum in the (Z, A)-plane, width and magnitude, are very comparable with those where the neutrons are generated by bombardment of natural 12C graphite with 50 MeV deuterons. Depending on the geometry of the converter/target assembly the isotope yields, however, are a factor of 2-3 lower due to less efficient production of neutrons per primary projectile, especially at small forward angles. [ABSTRACT FROM AUTHOR]

The decay of neutron-rich isotope 113Ru obtained as on-line mass separated product of proton-induced fission has been investigated by γγ coincidence and spectrum multiscaling measurements. Decay schemes for both low- and high-spin isomers of 113Ru have been constructed. The level scheme of 113Rh is considerably extended. Systematics of the lowest-lying rhodium levels is smooth. The picture of shape coexistence established for neutron-rich Rh isotopes near-neutron midshell is confirmed with the observation of a K = 1/2 deformed band, with its 3/2+ state at 600 keV being the lowest-lying level and of probable 7/2+ and 5/2+ band members. A large fraction of β feeding is found to populate high-lying levels in 113Rh. The GT strength in 113Rum decay is significantly larger than for the decay of 113Rug and of lighter rhodium isotopes. [ABSTRACT FROM AUTHOR]

In the context of a parameter study conducted by several laboratories for future European radioactive beam facilities based on fast-neutron induced fission, in particular for the SPIRAL-II project at GANIL, we have measured the yields of neutron-rich isotopes in the mass range of 88 to 144. These nuclei were obtained as fission products of natural uranium bombarded by neutrons of 20 MeV average energy emitted by a thick carbon target irradiated by 50 MeV deuterons. Yields have been measured using on-line mass separation with the ion-guide method. Compared with proton-induced fission at 25 MeV the magnitude of cross-sections, except for the symmetric region, is similar. Z-distributions of isobars have the same width, 0.7 charge units, but their maxima are shifted by about 0.8 charge units, favouring production of the neutron-richer isobars. Our data allow calculations of absolute cross-sections for fission of natural uranium induced by neutrons of about 20 MeV. [ABSTRACT FROM AUTHOR]

The low-energy level scheme of the N=63 nucleusSr has been obtained from a β-decay study ofRb. The ν[532]5/2 and ν[411]3/2 orbitals are well established as the ground state and a band head at 271.2keV, respectively. The general properties of the level scheme indicate a quadrupole deformation of β ≃ 0.4, confirming the unique feature of saturation of deformation in N ≥ 60 Sr isotopes. The energies of the K=3/2 intraband transitions in the odd-neutron neighboursSr andSr are very similar. More generally, the identical bands at low spin inSr nuclei are correlated with the mass-independent moments of inertia in this region. [ABSTRACT FROM AUTHOR]

The properties of the triplet of low-lying states inMo have been studied through spectroscopy of the γ radiation following thermal neutron capture inMo and β decay ofNb and through a measurement of the proton angular distributions in theMo( d,p) reaction with 14 MeV deuteron energy. The half-lives of the 13.5 keV state and the 57.0 keV 5/2 state have been measured as 226(7) and 133(7)ns, respectively. These values and the quadrupole/dipole mixing ratios of the 13.5 keV and 43.5 keV transitions yield spin and parity 3/2 for the 13.5 keV level. The E2 components in the 13.5 (3/2 →1/2) and 43.5 keV (5/2→3/2) transitions are ≦ 8·10 and 54(9)%, respectively. The possibility of an additional state near to the 57.0 keV level is discussed. IBFM/PTQM calculations, taking into consideration the transitional character of theMo boson core, account for the electromagnetic-transition and transfer-reaction pattern of the triplet of states. [ABSTRACT FROM AUTHOR]

Beta-decays ofAl andTi have been studied by applying ion guide techniques, gamma detection and a gas-Si telescope for charged-particle detection. The experimental beta-decay strength ofTi was found to be quenched by a factor of q = 0.64 compared to our shell model calculations below 8 MeV excitation energy inSc. This result is in line with the generally accepted value of q = 0.77. The isobaric analogue state of theTi ground state was measured to contain 10(8)% isospin impurity. This relatively high value could be well reproduced by the shell model calculation in the sdfp space. Gamma-decay of the T = 3/2 isobaric analogue state inMg was observed together with its decay by proton emission. The resonance strength from the ground state ofNa to the isobaric analogue state inMg was determined as 5.4(17) meV. The experimental proton width, 13(4) meV, agrees with the calculated value of 21 meV. [ABSTRACT FROM AUTHOR]

From the halflife of t=3.91 (16) ns for the 2 state inSr a deformation parameter of β=0.40(1) has been derived. Within the given uncertainties, this value is equal to the recently determined β-parameters forSr andSr, indicating that in the Sr isotopes saturation of deformation is reached immediately at its onset well before neutron midshell. This behaviour is reflected by the 'twin' character ofSr andSr with identical features of their unperturbed ground-state rotational bands. These observations are discussed in terms of strong two-nucleon interactions. [ABSTRACT FROM AUTHOR]

A band with a rotational pattern based on a state at 585.1 keV has been identified in the N=59 neutron-rich nucleusSr. Its properties lead to the [422] 3/2 Nilsson-orbital assignment for the band head. There is evidence for a second band with the head at 644.7 keV and the configuration [541] 3/2. Since the ground state and the lowest excited levels are spherical, shape coexistence is established forSr. A deformed nature of several levels at 500-600 keV results also from QRPA-model calculations. The structure of the low-lying spherical levels has been studied in the frame of the IBF model. The results of the present investigations lead to a better understanding of the N=59 isotones which constitute the link between the spherical and deformed nuclei at A∼100 as a species with shape coexistence but without any indications of particular softness. [ABSTRACT FROM AUTHOR]

A level scheme has been established forRb based on studies at the fission-product separator JOSEF of γγ coincidences following the β- decay ofKr. For the 190.7 keV level a half-life of 2.5(8) ns has been determined through the centroid-shift method. The major β-decay branch ofKr proceeds with log ft=3.7 into the 988.4 keV state inRb. This transition must be of the 0→1 GT type involving the transition νg→ πg. IBFFM calculations have been performed forRb, starting from the parametrization used in the previous IBM investigations for the isotopeRb and the isotoneY. A theoretical interpretation of the low-lying levels including propositions for the spins is given in a comparison to the available data. [ABSTRACT FROM AUTHOR]

The structure of the low-lying states with negative parity ofRb has been studied in the framework of the Interacting Boson-Fermion Model (IBFM) using the even-even neighbourSr as a core. A good description of the properties of the levels has been obtained. A sensitive test of the quality of the theoretical studies is the decay pattern of the second excited state at 267 keV with a half-life of 2.0(2) ns. The calculations give t=2.26 ns and they predict the existence of a 13 keV transition into the first excited state, the existence of which has been confirmed afterwards by experiments. [ABSTRACT FROM AUTHOR]

Unique-parity states ofY are studied in the framework of the interacting boson-fermion model (IBFM). Spin assignments for the levels of the unique-parity yrast band are proposed on the basis of nuclear systematics and the IBFM band pattern associated with coupling of a particle-type quasiparticle (g) to the SU(5) coreSr. The good fit of the experimental data with the IBFM calculations confirms the single particle character of this immediate neighbour of the doubly sub-magic nucleusZr. [ABSTRACT FROM AUTHOR]

The structure of the nucleusY is discussed in the framework of the parabolic rule for the splitting of members of proton-neutron coupled configurations and of the interacting boson-fermion-fermion model (IBFFM). A good description of the available experimental data including the existence of the β decaying 8 isomer with a half-life of 10 s has been obtained which demonstrates the influence of the subshell closures in the neighbourhood ofZr. [ABSTRACT FROM AUTHOR]

The β decay of the longer-lived isomer inNb has been studied at the fission-product separator JOSEF. Measurements of γ-ray singles spectra, of γ-γ coincidences and of γ-γ angular correlations have been performed. A value of t=2.99(11)s has been determined for the isomer which probably has I=4 or 5. A scheme of the levels ofMo which are populated in the decay of this isomer has been established. Information on the spins of several states ofMo has been obtained. Thus, I=0 levels have been identified at 1,505, 2,038 and 2,087 keV. The mixing ratios have been determined for the 2→2 and 2→2 transitions. The results provide evidence for a vibrational structure ofMo with separate bands based on the ground state and on the first excited 0 level. [ABSTRACT FROM AUTHOR]

Life-times of low-lying levels inRb andRb have been determined from β−γ coincidence measurements at the fission-product separator JOSEF. Values of t=17.0(8) ns and 2.0(2) ns have been obtained for the levels at 1,134 keV inRb and 267 keV inRb, respectively, and upper limits could be deduced for several other states. Calculations in the frame of IBFM/PTQM have been performed forRb, the results of which allow an interpretation of the low-lying levels of this nucleus. For the 1,134 keV level the configuration [ πg⊗2] 7/2 is suggested which lies close to or even below the 9/2 level. The calculated half-life of 14 ns for the 1,134 keV level reproduces well the experimental value. The half-life of the 267 keV level inRb favours the assignment of I=1/2 to this state over the alternative 3/2. [ABSTRACT FROM AUTHOR]

An isomeric state at 3,523 keV excitation energy inY with a half-life of 144(10) ms has been discovered with the fission-product separator JOSEF. This isomer is depopulated through a γ transition of 162 keV. An E3 multipolarity for this transition is consistent with the measured conversion coefficients of α=0.98(20) and α=1.00(19). Subsequent electromagnetic transitions populate several newY levels which have high spins. For the isomer the three-quasiparticle configuration [ πg⊗ν( h, g)]27/2 is proposed. The E3 transition with a strength about 2 single-particle units is supposed to be of the type h→d. These findings provide evidence for the rapid shape transition at A≃100 since they indicate shell-model character ofY even at high excitation energies while the immediate neighbourY contains a rotational band based on a level at 495 keV. [ABSTRACT FROM AUTHOR]

The g factors of the first excited 2 levels in the neutron-rich nucleiMo andMo have been studied through the measurement of the perturbed angular correlations for the γ-γ cascades between the 0-2-0 level sequences. The results of g=0.42±0.07 forMo and of g= forMo agree with the prediction of the vibrational-rotational model. In terms of IBA, with the assumption of N=3 for the Mo isotopes which takes into consideration a two particle, two-hole excitation across the Z=40 subshell, the proton-boson g factor is deduced to be g=1.00±0.23. It is shown that this value provides evidence for subshell effects inZr. [ABSTRACT FROM AUTHOR]

The half-lives of the first excited levels in odd-neutron nucleiMo andMo have been measured. From these data, values of the deformation parameter of ɛ=0.31±0.05 and 0.29±0.06 forMo andMo, respectively, have been deduced assuming prolate spheroidal shapes. The present results in combination with previously determined properties of the ground-state bands of these nuclei are used to determine μ, the magnitude of the coefficient of the 1 term, in the Nilsson Hamiltonian, for the new region of deformation at A∼-100. [ABSTRACT FROM AUTHOR]

An extensive decay scheme ofIn, involving nearly 140 transitions and 35 levels inCd, as well as a tentative decay scheme ofIn are proposed. A number of previous inconsistencies between earlier decay scheme work ofIn and in-beam work onCd are removed. It is shown that the simple particle plus core treatment does not suffice to describe all states. A cluster plus core model seems to be an indispensable completion of this model, especially for the description of the low lying low spin states. Also the conversion coefficients of the internal transitions inIn andIn are measured. The results are well accounted for by the Moszkowski single-proton transition probabilities. [ABSTRACT FROM AUTHOR]

The decay of 111Ru obtained from fast on-line chemical and mass separation has been investigated by β-γ- t and γ-γ coincidence techniques. Earlier spin and parity assignments of 111Rh levels based on extrapolations of level systematics are confirmed. In particular, the K=1/2 intruder band is supported by the hindrance of E2 transitions between deformed and spherical states and enhancement of intraband E2 transitions. The excitation energies of intruder band members in Rh isotopes show a minimum at 109Rh64, with two neutrons less than 111Rh at the N=66 midshell. This trend, which differs from the one in the higher-Z neighbouring elements Ag and Cd with minima at N=66, follows the evolution of deformation observed in the lower-Z elements Ru and Mo. [ABSTRACT FROM AUTHOR]

In the framework of the production of radioactive ion beams by the isotope separator online method, a new system has been developed at GANIL/SPIRAL I to produce multicharged alkali ions. The principle, referred to as the “direct 1+/N+ method,” consists of a surface ionization source associated with a multicharged electron-cyclotron-resonance ion source without an intermediate mass separator. This new system has been tested online using a 48Ca primary beam at 60.3 A MeV. The experimental evidence of the direct 1+/N+ process has been obtained for a potential difference between the two sources of 11 V and with a 1+/N+ charge breeding efficiency of 0.04% for 47K5+. This value is significantly lower than the value of 6% obtained for stable K ions with the standard 1+/N+ method. A possible explanation is given in the text. [ABSTRACT FROM AUTHOR]

The nucleus Nb has been studied through the ß decay of the fission productZr at LOHENGRIN and JOSEF. The energies of the lowlying levels and the γ transitions indicate rotational bands based on the Nilsson proton configurations [422 5/2] [303 5/2] and [301 3/2] at 0, 164 and 248 keV, respectively. The measured half-lives of (4.7±0.5) ns and (2.0±0.6) ns of the levels at 164 and 248 keV are consistent with the Nilsson-model estimates including pairing. [ABSTRACT FROM AUTHOR]

Very neutron–rich fission products of the mass chain A=113 obtained from the IGISOL on–line mass separator have been investigated by γγ coincidence techniques and γ-spectra multiscaling. Gamma–rays following β–decay of 113Tc have been observed for the first time and a new 0.5 s isomeric state has been found in 113Ru. [ABSTRACT FROM AUTHOR]

The decays of 112Rh g and 112Rh m obtained as on-line mass separated fission products have been investigated by standard β and γ spectroscopic techniques and γ-γ angular correlations. Low-lying 0+ and 2+ states are identified in the daughter nucleus 112Pd. Systematics suggests the existence of an intruder band which is the lowest in 110Pd, i.e. two neutrons before the midshell. [ABSTRACT FROM AUTHOR]

The β-decay of the very neutron-rich 111Tc nucleus has been observed. A partial decay scheme of 111Ru is presented, complementing the high-spin levels earlier reported in spontaneous fission work. [ABSTRACT FROM AUTHOR]