GSI On-line Mass Separator Research Articles

Beta decay of 101Sn

The β decay of the very neutron-deficient isotope 101Sn was studied at the GSI on-line mass separator using silicon detectors for recording charged particles and germanium detectors for γ-ray spectroscopy. Based on the β-delayed proton data the production cross-section of 101Sn in the 50Cr + 58Ni fusion-evaporation reaction was determined to be about 60nb. The half-life of 101Sn was measured to be 1.9(3)s. For the first time β-delayed γ-rays of 101Sn were tentatively identified, yielding weak evidence for a cascade of 352 and 1065keV transitions in 101In. The results for the 101Sn decay as well as those from previous work on the 103Sn decay are discussed by comparing them to predictions obtained from shell model calculations employing a new interaction in the 88Sr to 132Sn model space.

Decay Q-value of 105Sn and of other nuclei near 100Sn, measured at the GSI on-line mass separator

Within a programme of measuring Q EC, Q EC − S p and S p values of nuclei in the 100Sn region, the Q EC value of 105Sn was determined to be 6230 (80) keV. This result was obtained by means of the total-absorption spectrometer at the On-line Mass Separator of GSI Darmstadt. Other decay Q-values of nuclei near 100Sn, measured at this facility by investigating positron, electron-capture, β-delayed proton as well as direct proton and α-decay, are also presented. The accuracy of these data ranges from 30 to 260 keV, is thus lower than that obtained by other methods but still sufficient concerning most of the current nuclear-structure studies. Moreover, the determination of Q-values by means of decay measurements has an unrivalled sensitivity.

Reinvestigation of direct proton decay ofSb105

Proton radioactivity from $^{105}\mathrm{Sb}$ has been reinvestigated at the GSI on-line mass separator. The nucleus was produced in the reaction $^{50}\mathrm{Cr}$($^{58}\mathrm{Ni}$,$1p2n$), and collected with a tape transport system. A double-sided Si strip detector was used for proton spectroscopy. The direct proton decay of $^{105}\mathrm{Sb}$ reported in an earlier experiment at Berkeley was not observed. The present data imply an upper limit $~{10}^{\ensuremath{-}3}$ for the ground-state proton decay branching ratio for proton energies higher than 430 keV.

β decay ofAg95

We studied the {beta}-decay properties of the N=Z+1 nucleus {sup 95}Ag by measuring {beta}-delayed {gamma} rays and {beta}-{gamma}-{gamma} coincidences with a plastic scintillator as {beta} detector and a Ge-detector array. The {sup 95}Ag nuclei were produced by means of the {sup 58}Ni({sup 40}Ca,p2n) reaction and separated with the GSI online mass separator. The previously reported level scheme of the {sup 95}Pd daughter nucleus was extended considerably. The deduced level scheme is compared with different shell-model calculations with or without breaking the {sup 100}Sn core.

Beta decay of 103Sn

The β decay of 103Sn, a three-neutron-particle nucleus with respect to the 100Sn core, was investigated at the GSI on-line mass separator using an array of 17 germanium crystals and a total absorption spectrometer. A total of 31 β-delayed γ-rays (29 new) of the 103Sn →103 In decay were observed and, on the basis of β-γ-γ coincidences, the 103Sn decay scheme was established for the first time. By means of total absorption spectroscopy, β intensities, the Gamow-Teller strength distribution and the summed Gamow-Teller strength value of 3.5±0.5 were determined for this decay. Its half-life and QEC value were found to be 7.0±0.2 s and 7.64±0.7 MeV, respectively. The β-delayed proton branching ratio was measured to be 1.2±0.1%. The results are discussed in comparison with shell-model predictions based on realistic and empirical interactions.

Measurement of the nuclear charge radii of 8, 9Li

Nuclear charge radii of 6, 7, 8, 9Li have recently been measured at the GSI on-line mass separator using high-resolution resonance ionization mass spectroscopy. We give a brief description of the experimental method. The results for the charge radii are compared with different theoretical predictions.

Total absorption spectroscopy of the β-delayed proton emitter 117Ba

The very neutron-deficient 117Ba nuclei were produced in 58Ni-induced reactions on a 63Cu target and selected for spectroscopic studies by using BaF+ molecules formed in the ion source of the GSI on-line mass separator. The β-decay of 117Ba was investigated by means of the total absorption γ-ray spectrometer and a telescope for β-delayed particle detection. In the analysis combining the β-delayed γ-ray and proton data the energy window available for β-delayed proton emission, the branching ratios for proton transitions to the 116Xe levels and the β-feeding of the γ-ray and proton-emitting 117Cs states were determined. The β-strength function for 117Ba derived from the measured β-feeding distribution revealed the existence of a broad resonance structure at 117Cs excitation energy of about 4-5 MeV. The results of the β-delayed proton studies and β-strength measurements are confronted with theoretical predictions.

Measurements of 110Xe and 106Te decay half-lives

The α-decays of 110Xe and 106Te were studied at the GSI on-line mass separator. By using the grow-in and decay of the α activity, observed in pulsed-beam measurement, the half-life of 110Xe was found to be T1/2 = 105+35-25ms. The lifetime of 106Te was determined to be T1/2 = 70+20-10μs by measuring the time between two successive 110Xe →{106} Te →{102} Sn α-decays. The newly determined half-lives were used to calculate reduced α-decay widths for 106Te and 110Xe. Universal systematics of reduced α widths are proposed to search for the evidence of enhancement of the α formation amplitude in the emitters above 100Sn.

β-delayed proton decay of a high-spin isomer inA94g

The decay of the (7{sup +}) and (21{sup +}) isomers of the N=Z isotope {sup 94}Ag was studied at the GSI on-line mass separator by measuring {beta}-delayed protons, {gamma} rays, proton-{gamma} and proton-{gamma}-{gamma} coincidences as well as the {beta}-strength distribution. We have observed high-spin (up to 39/2) states in {sup 93}Rh populated by proton emission following the {beta} decay of the {sup 94}Ag isomers. The major part of the population is related to the {beta} decay of the known (7{sup +}) isomer whose half-life is 0.61(2) s. The assignment of the high-spin (21{sup +}) isomer in {sup 94}Ag with a half-life of 0.39(4) s has been confirmed. The excitation energy and {beta}-decay energy of the (21{sup +}) isomer were measured to be at least 5.4 and 17.7 MeV, respectively. At this excitation energy, the (21{sup +}) isomer is expected to be unbound to direct one-proton, two-proton, or {alpha} decays. The remarkably long half-life of the (21{sup +}) isomer with the highest spin and excitation energy ever observed for {beta}-decaying nuclei makes a new textbook example of a nuclear high-spin trap. The branching ratios for {beta}-delayed proton emission are about 20% and 27% for the decays of the (7{sup +}) andmore » (21{sup +}) isomers, respectively. The properties of the experimentally identified {sup 93}Rh levels are discussed in comparison to shell-model predictions.« less

Excitation energy of the T=0 β-decaying 9+ isomer in Br70

The total absorption spectrometer installed at the GSI on-line mass separator was used to study the $\ensuremath{\beta}$-decay of the ${J}^{\ensuremath{\pi}}={9}^{+}$ isomer in $^{70}\mathrm{Br}$. The decay energy ${Q}_{EC}$ of the isomer was found to be $12.19(\ifmmode\pm\else\textpm\fi{}{7}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{4}_{\mathrm{sys}})\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. By comparing this value with the estimated ${Q}_{EC}$ value for the ground state of $^{70}\mathrm{Br}$, the excitation energy of isomer is found to be $2.23(9)\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. The latter result removes ambiguities that can be found in literature.

Evidence for a high-spinβ-decaying isomer inLu177

Nuclei in the $A\ensuremath{\sim}180$ region have been populated and investigated in a series of multinucleon transfer and deep-inelastic reactions involving an $11.4\phantom{\rule{0.3em}{0ex}}\text{MeV}$ per nucleon $^{136}\mathrm{Xe}$ beam produced by the GSI UNILAC accelerator, impinging on a selection of tantalum and tungsten targets. The reaction products were released from a thermal ion source and subsequently mass selected using the GSI on-line mass separator. The unexpectedly high yield of $\ensuremath{\gamma}$ rays associated with the decay of the well established ${K}^{\ensuremath{\pi}}=37∕{2}^{\ensuremath{-}}$,${t}_{1∕2}=51.4\phantom{\rule{0.3em}{0ex}}\mathrm{min}$ isomer in $_{\phantom{\rule{0.3em}{0ex}}72}^{177}\mathrm{Hf}$ and anomalous half-life characteristics associated with this decay lead to these data being interpreted as the ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay of a high-$K$ isomer in the mother nucleus, $^{177}\mathrm{Lu}$. By comparing the experimental findings with the predictions obtained from multi-quasiparticle blocked-BCS-Nilsson calculations, the proposed decay is suggested to be from a ${K}^{\ensuremath{\pi}}=39∕{2}^{\ensuremath{-}}$ five-quasiparticle state in $_{\phantom{\rule{0.3em}{0ex}}71}^{177}\mathrm{Lu}$. A half-life of $7\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{min}$ is determined for this $\ensuremath{\beta}$-decay path which is estimated to have an excitation energy of $\ensuremath{\approx}3.9\phantom{\rule{0.3em}{0ex}}\text{MeV}$ above the $^{177}\mathrm{Lu}$ ground state.

On the β-decaying (21 +) spin gap isomer in 94Ag

The β decay of 94Ag, a nucleus with three proton and three neutron holes with respect to 100Sn, was investigated at the GSI on-line mass separator by using an array of germanium and silicon detectors. On the basis of a βγγ coincidence measurement, the previously reported decay scheme has been considerably improved and evidence is presented for a spin–gap isomer with a spin-parity assignment of (21 +), i.e., the highest spin ever observed in β decay. The half-life of the low-spin isomer (7 +) was remeasured to be 0.59(2) s, whereas the half-life of the high-spin isomer was determined as 0.47(8) s. For the states in 94Ag and 94Pd, large-scale shell-model calculations that include up to 4p–4h excitations across the N= Z=50 shell gap and employ a realistic interaction were carried out. These core excitations play a crucial role in generating the E4 isomerism required for the interpretation of the long half-life of the (21 +) state in 94Ag.

Identification of isomers in theN=Z+1nucleus95Ag

Three $\ensuremath{\gamma}$-decaying isomers in the $N=Z+1$ nucleus ${}^{95}\mathrm{Ag}$ have been identified for the first time. The ${}^{95}\mathrm{Ag}$ nuclei were produced via the ${}^{58}\mathrm{Ni}{(}^{40}\mathrm{Ca},p2n)$ reaction and separated by using the GSI on-line mass separator. The emitted $\ensuremath{\gamma}$ rays were measured with a multi-Ge detector setup in anticoincidence with positrons. Three individual $\ensuremath{\gamma}$-decay cascades have been identified and assigned to ${}^{95}\mathrm{Ag},$ representing the depopulation of isomers at 344-, 2531-, and 4859-keV excitation energy with tentative spin-parity assignments of ${(1/2}^{\ensuremath{-}}),{(23/2}^{+}),$ and ${(37/2}^{+}),$ respectively. The spin and parity assignments are based on a comparison with shell-model predictions employing empirical interactions.

β decay of100In

The \ensuremath{\beta} decay of ${}^{100}\mathrm{In},$ the one proton hole and one neutron particle neighbor to ${}^{100}\mathrm{Sn},$ was investigated at the GSI on-line mass separator by using germanium detectors and a NaI total-absorption spectrometer. On the basis of \ensuremath{\beta}\ensuremath{\gamma}\ensuremath{\gamma} coincidences, the ${}^{100}\mathrm{In}$ decay scheme was established for the first time. The ground-state spin and parity for ${}^{100}\mathrm{In}$ are discussed by investigating \ensuremath{\beta} feeding of levels in ${}^{100}\mathrm{Cd}$ and $\ensuremath{\beta}$-delayed proton emission to ${}^{99}\mathrm{Ag}.$ The half-life was remeasured and found to be 5.9(2) s. The ${Q}_{\mathrm{EC}}$ value was determined from the measured $EC/{\ensuremath{\beta}}^{+}$ ratio for the \ensuremath{\beta}-delayed protons to be 10.08(23) MeV. The main fraction of the \ensuremath{\beta} feeding was established to populate the region of 6 MeV excitation energy, which corresponds to a total Gamow-Teller (GT) strength of 3.9(9) and a centroid at 6.4 MeV. Large-scale shell-model calculations employing a realistic interaction are used to assign configurations to states in ${}^{100}\mathrm{In}$ and ${}^{100}\mathrm{Cd}.$ The GT \ensuremath{\beta}-decay strength distribution measured in the total absorption experiment is compared to shell-model predictions. The deduced overall hindrance of the GT strength agrees with the values predicted for the ${}^{100}\mathrm{Sn}$ GT decay.

Beta decay of medium and high spin isomers in 94Ag

The very neutron-deficient isotope 94Ag was produced at the GSI on-line mass separator by using the reaction 58Ni( 40Ca, p3n). The β-decay properties of 94Ag were studied by detecting for the first time β-delayed γ rays and β– γ– γ coincidences. Both the population of excited levels in the daughter nucleus 94Pd and the β-decay half-life of 94Ag were investigated. The major part of the feeding was assigned to the decay of an I π =(7 +) isomer with a half-life of (0.36±0.03) s. A weak β-decay branch was found to populate high-spin levels in the 94Pd daughter with I⩾18. It is tentatively assigned to the decay of a high-spin parent state in 94Ag with I⩾17 and a half-life (0.3±0.2) s. The measured β-decay properties as well as the level structure of 94Ag and 94Pd are discussed in comparison with shell-model predictions.

Recent developments at the GSI Online Mass Separator

The research programme at the GSI Online Mass Separator focuses on the study of exotic nuclei far from the valley of stability. Special emphasis is placed on the investigation of decay properties of isotopes along the N= Z line between the double shell closures at 56Ni and 100Sn. In this contribution, the major recent achievements along with the corresponding experimental methods are reviewed.

Beta decay of 56Cu

The proton-rich isotope 56Cu was produced at the GSI On-Line Mass Separator by means of the 28Si( 32S, p3n) fusion–evaporation reaction. Its β-decay properties were studied by detecting β-delayed γ rays and protons. A half-life of 93±3 ms was determined for 56Cu. Compared to the previous work, six new γ rays and three new levels were assigned to the daughter nucleus 56Ni. The measured Gamow–Teller strength values for five 56Ni levels are compared to shell-model predictions.

Beta decay of 93Pd

The very neutron-deficient isotope 93Pd was produced in fusion-evaporation reactions of a 40Ca beam on a 58Ni target. The reaction products were separated at the GSI Online Mass Separator, using ion sources of the FEBIAD type. The β-decay properties of 93Pd were studied by detecting β-delayed protons and β-delayed γ-rays. The feeding of excited levels in the daughter nucleus 93Rh and the β-decay half-life of 93Pd were determined. The experimental results are discussed in comparison to shell model predictions.

Gamma-decay of the 27/2− 0. 6s E3-isomer in 15168Er83

The nucleus 151Er was investigated through γ-ray and conversion electron measurements from its 27/2− isomer decay produced in a HI reaction at the GSI on-line mass separator. Inaddition tothe known intense yrast cascade,a complex 8% decay branch to above-yrast states was identified. Among these, the 11/2+ member of the νf7/2 × 3− septet was unambiguously characterised. Its energy implies that the quadrupole moment of the 3− core has changed sign with respect to the lighter even-Z N=83 isotones.

Release studies of elementary and metal-fluoride ions at the GSI on-line mass separator

By using the techniques of implantation of heavy ions and of bunched-beam release at the GSI on-line mass separator, systematic release studies were performed. With the first technique the complete release process from the catcher-ionsource-system is analyzed in detail for 31 new tracer-host combinations. These include as new tracers the elements Ti, V, Zn, Sb, and Dy, and carbon foils of around 1 mg/cm 2 thickness as new catcher matrix. The carbon foils allow for a more reliable determination of rracer diffusion properties in carbon, avoiding the problems with porous sinter-graphites, where closed pores, inconsistencies in the manufacturer's data on grain size, or grain growth cause unacceptably large discrepancies within the derived diffusion coefficients. Measured with the bunched-beam-release technique, the mean adsorption times of the fluorides of group IIA and IIIA metals turned out to be orders of magnitude shorter than for the respective atoms, thus showing quantitatively the benefits of the CF 4-fluorination technique. The degree to which the respective alkali-fluoride ions are suppressed was determined to be < 10 −5, compatible with zero. The degree of fluorination is high in most cases, even for very low CF 4 partial pressures.