Gamma-gamma Angular Correlation Research Articles

Nuclear Structure Studied with Direct Reactions for Fundamental Symmetry Tests

Direct reaction studies provide crucial information on nuclear structure for nuclei that are used in weak interaction and fundamental symmetry tests. Motivated by the superallowed Fermi beta-decay studies, we have investigated the structure of Cr-50 and Zn-62, the daughters of the superallowed beta(+)/EC emitters Mn-50 and Ga-62, with (p,t) reactions. For both nuclei, we reassigned the location of the first excited 0(+) state. The data for these reassignments are reviewed and limits on the possible L = 0 transfer to the previously assigned 0(2)(+) states are determined. For Cr-50, we find no evidence for the presence of a 0(+) state at 3695 keV. For Zn-62, the data allow for a possible contribution of L = 0 and L = 2, indicating a possible 0(+)/2(+) doublet of states at 2342 keV. However, this relies on an enhanced cross section at only one data point, and thus the existence of a 0(+) state at this energy is not clear. A preliminary result from a gamma-gamma angular correlation measurement following the beta(+)/EC decay of Ga-62 weakly favours the existence of a 0(+) state, but requires further study with increased statistics.

Cd and In-doping in thin film SnO2

In this paper, we investigate the effects of doping in the local structure of SnO2 by measuring the hyperfine interactions at impurity nuclei using the Time Differential Perturbed Gamma-Gamma Angular Correlation (TDPAC) method in addition to density functional theory simulations. The hyperfine field parameters have been probed as a function of the temperature in thin film samples. The experimental results reveal that 117Cd/In and 111In/Cd are incorporated and stabilized in the SnO2 lattice replacing the cationic site. Significant differences in the electric field gradient were observed from TDPAC measurements with both the probe nuclei. Furthermore, the absence of strongly damped spectra further indicates that implanted Cd atoms (for 117Cd/In probe nuclei measurements) easily occupy regular substitutional Sn sites with good stability. The simulated value for the electric field gradient obtained with the first oxygen neighbor removed is closer to the experimental value observed for 117Cd, which also indicates this configuration as stable and present in the sample.

High-spin structures of 124−131Te: Competition of proton- and neutron-pair breakings

The 124-131Te nuclei have been produced as fission fragments in two fusion reactions induced by heavy-ions (12C + 238U at 90 MeV bombarding energy and 18O + 208Pb at 85 MeV) and studied with the Euroball array. Their high-spin level schemes have been extended to higher excitation energy from the triple gamma-ray coincidence data. The gamma-gamma angular correlations have been analyzed in order to assign spin and parity values to many observed states. Moreover the half-lives of isomeric states have been measured from the delayed coincidences between the fission-fragment detector SAPhIR and Euroball, as well as from the timing information of the Ge detectors. The behaviors of the yrast structures identified in the present work are first discussed in comparison with the general features known in the mass region, particularly the breakings of neutron pairs occupying the nuh11/2 orbit identified in the neighboring Sn nuclei. The experimental level schemes are then compared to shell-model calculations performed in this work. The analysis of the wave functions shows the effects of the proton-pair breaking along the yrast lines of the heavy Te isotopes.

High-spin structures of3688Kr52and3789Rb52: Evolution from collective to single-particle behavior

The high-spin states of the two neutron-rich nuclei, 88Kr and 89R have been studied from the 18O + 208Pb fusion-fission reaction. Their level schemes were built from triple gamma-ray coincidence data and gamma-gamma angular correlations were analyzed in order to assign spin and parity values to most of the observed states. The two levels schemes evolve from collective structures to single-particle excitations as a function of the excitation energy. Comparison with results of shell-model calculations gives the specific proton and neutron configurations which are involved to generate the angular momentum along the yrast lines.

Investigation of the magnetic hyperfine field at R and Zn sites in RZn (R = Gd, Tb, Dy) compounds using perturbed gamma-gamma angular correlation spectroscopy with 140Ce and 111Cd as probe nuclei

The magnetic hyperfine field (Bhf) in RZn compounds (R = Gd, Tb, Dy) has been investigated by perturbed angular correlation spectroscopy using 140Ce and 111Cd as probe nuclei, respectively, at R and Zn sites, in order to study the origin of the magnetic coupling in these compounds. The results for 111Cd probe showed that the temperature dependence of Bhf roughly follows the Brillouin function for the R total angular momentum J of each compound. The temperature dependence of Bhf measured with 140Ce probes showed, however, a sharp deviation from the Brillouin curve for all compounds, which was ascribed to the contribution of the 4f-electron of Ce3+ to Bhf.

Study of electric quadrupole interactions at 111Cd on Zn sites in RZn (R = Ce, Gd, Tb, Dy) compounds using the PAC spectroscopy

Nuclear quadrupole interactions at Zn sites in the intermetallic compounds RZn (R = Ce, Gd, Tb, Dy) have been investigated by perturbed gamma-gamma angular correlation (PAC) spectroscopy using 111In(111Cd) as probe nuclei. Measurements were carried out in the temperature range of 10–295 K. These compounds exhibit CsCl type cubic structure and while CeZn shows antiferromagnetic behaviour, the compounds GdZn, TbZn, DyZn are ferromagnetic. The results show that the EFG in these compounds is sensitive to the distribution of rare-earth 4f-electron charges.

A weak magnetism observed in SnO2 doped with Fe by means of Perturbed Gamma-Gamma Angular Correlation and Mössbauer Spectroscopy

Abstract Nano-structured samples of SnO2 doped with Fe prepared by the sol-gel method were studied by the Perturbed Gamma-Gamma Angular Correlation (PAC) Spectroscopy using 111 In ( 111 Cd) probe nuclei as well as by 57Fe Msssbauer spectroscopy. The samples were prepared from very pure metallic Sn and Fe. Carrier-free 111 In nuclei were introduced during the sol-gel process of sample preparation for PAC measurements. The PAC measurements were carried out after annealing the samples at different temperatures and the results show a combined electric quadrupole and magnetic dipole interaction for probe nuclei that do not occupy the regular Sn sites. The hyperfine parameters revealed weak magnetic interactions.

Impurities in Magnetic Materials Studied by PAC Spectroscopy

Perturbed gamma-gamma angular correlation (PAC) spectroscopy is a precise and highly efficient tool to follow the temperature dependence of local magnetic fields in any material. Its resolution and efficiency does not depend on temperature and therefore can measure local fields at low as well as high temperature with the same accuracy. Due its versatility in using different probe nuclei it can sense the local magnetic fields at different sites in the crystalline structure of materials. In this review, important results obtained with PAC spectroscopy are shown in two classes of materials: transition metal and transition-metal based compounds and rare earth elements and rare-earth-element based compounds using mainly three different probe nuclei: 111Cd, 181Ta and 140Ce. PAC spectroscopy has contributed to the systematic study of the magnetic hyperfine field in impurities in matrices of Fe, Co and Ni as well as in transition-metal based Heusler alloys. It has also provided important contribution to the investigation of magnetism in rare-earth elements and intermetallic compounds. An still open issue concerning the local fields in metallic magnetic compounds and elements is the exchange interaction between the magnetic ions of the host and a dilute magnetic impurity, which acts as a defect in the magnetic lattice. PAC spectroscopy has been contributing to study this problem with success. Also shown in this review is the crucial role of ab-initio first principle calculations in the interpretation of PAC results.

Local investigation of magnetism at R and In sites in RNiIn (R=Gd, Tb, Dy, Ho) compounds

The local magnetism at R and In sites in RNiIn (R=Gd, Tb, Dy, Ho) compounds was investigated by measuring hyperfine interactions via perturbed gamma-gamma angular correlation (PAC) technique using La140–Ce140 and In111–Cd111 probe nuclei. PAC measurements carried out below respective TC for each compound showed a combined electric quadrupole plus magnetic dipole interaction for Cd111 probe at In sites and a pure magnetic interaction for Ce140 at R sites. The temperature dependence of the magnetic hyperfine field (Bhf) shows that, whereas the Bhf values at rare-earth sites measured with Ce140 drop to zero at temperatures around the expected TC for each compound, the respective fields at In sites measured with Cd111 drop to zero at lower temperatures. The difference between the temperatures at which Bhf is zero for Ce140 and Cd111 probes when compared with TC of each compound shows that this difference decreases with decreasing TC values. The results are discussed in terms of the Ruderman-Kittel-Kasuya-Yosida model for magnetic interactions and the crystalline electric field.

Magnetic hyperfine fields at Gd and in sites in GdPdIn compound

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the hyperfine interactions in the intermetallic compound GdPdIn using 111In→111Cd and 140La→140Ce probe nuclei at the In and Gd sites, respectively. The PAC results for 111Cd show two well-defined electric quadrupole frequencies above T C assigned to probes occupying Gd and In sites, with ∼ 50% of site occupation each. The fraction corresponding to In sites increases with temperature reaching 95% around 500 K. Below Tc the measurements for 111Cd probe showed combined electric quadrupole plus magnetic dipole interaction with sharp increase in the magnetic field below around 80 K. A pure magnetic interaction with lower hyperfine field values was observed at the Gd sites occupied by 140Ce below 100K.

Hyperfine interactions at R and In sites in RNiIn (R = Gd, Tb, Dy, Ho) compounds measured by perturbed angular correlation spectroscopy

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (mhf) in RNiIn (R=Gd, Dy, Tb, Ho) intermetallic compounds using the 111 In → 111Cd and 140La → 140Ce probe nuclei. The PAC spectra for 111Cd measured above magnetic transition temperature show a major fraction with a well defined quadrupole interaction for all compounds except GdNiIn where a single frequency was observed. PAC measurements below Te showed a combined electric quadrupole plus magnetic dipole interaction for 111Cd probe at In sites, and a pure magnetic interaction for 140Ce at R sites. The temperature dependence of mhf measured with 140Ce at R sites shows that the values of fields drop to zero at temperatures around the expected Te for each compound. However, in the measurements with 111Cd at In sites, the mhf values become zero at temperatures which are smaller than TC. The difference between the temperatures at which mhf is zero for 140Ce and 111Cd probes correlates with TC. For each compound this difference decreases with TC. The results are discussed in terms of the RKKY model for magnetic interactions and the existence of two magnetic systems, with distinct exchange interaction energies due to different types of atomic layers in these compounds.

Temperature dependence of the magnetic hyperfine field at cerium impurity in Co

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (B hf ) at Ce impurity in Co using 140La→140Ce probe. The radioactive 140La produced by neutron irradiation of lanthanum metal with thermal neutrons was introduced in Co by arc melting in argon atmosphere. The present measurements cover the temperature range from 4.2–1300 K. Two pure magnetic interactions were observed at impurity sites, corresponding to a ferromagnetic ordering of Co moments in hcp and fcc phases. The temperature dependence of B hf for both phases, however, shows a sharp deviation from an expected standard Brillouin-like behavior for the host magnetization. The results are discussed in terms of a simple molecular-field model where the localized moment at impurity ions as well as the conduction electron contributions to the hyperfine field are taken into account.

Temperature Dependence of the Magnetic Hyperfine Field at 140Ce on Gd Sites in GdAg Compound

Perturbed gamma-gamma angular correlation technique was used to measure the magnetic hyperfine field at Gd sites in the intermetallic compound GdAg using the 140La→140Ce nuclear probe. A major and well-defined magnetic interaction is observed at 140Ce substituting Gd sites in GdAg below 130 K, corresponding to a ferromagnetic ordering of Gd moments. The temperature dependence of magnetic hyperfine field, however, shows a sharp deviation from an expected Brillouin-like behavior for temperatures below 75 K. This additional magnetic interaction is believed to result from the polarization of Ce spin moments induced by the magnetic field from Gd atoms.

Investigation of Hyperfine Interactions in GdNiIn Compound

Perturbed gamma-gamma angular correlation technique was used to measure the hyperfine interactions in the compound GdNiIn using the 111In→111Cd and 140La→140Ce probe nuclei at the In and Gd sites, respectively. A unique quadrupole frequency with asymmetry parameter ν = 0.78 was observed for 111Cd probe at In sites for the measurements above Curie temperature. Below TC, the spectra for 111Cd show combined magnetic dipole and electric quadrupole interaction. Below 85 K, a unique magnetic interaction is observed at 140Ce. A linear relationship between the saturated magnetic hyperfine field and the magnetic transition temperature was observed for both probes, indicating that the main contribution to the mhf comes from the conduction electron polarization.

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.

Band structure of 146Ce studied through γ-γ angular correlation measurements

The β-decay of 146La was studied using the on-line isotope separator KUR-ISOL. Gamma-gamma angular correlation measurements were performed with a 4-Ge detectors system. Spin assignments of three levels were made: 3+ for the 1576.5 keV level, 4+ for the 1627.1 keV level and 5+ for the 1810.2 keV level. The mixing ratios (E2/M1) were deduced to be δ183.2= 0.25 ± 0.08, δ638.9= 0.33 ± 0.05, δ959.0= 1.19+0.16 −0.14, δ1015.9= 5.4+3.1 −1.5 and δ1318.1= 6.5+1.7 −1.1. These were compared to the calculated values obtained in three cases involving different Majorana force parameter values. The band structure of 146Ce is discussed based on the results of calculation using the IBM-2 theory.

Intruder states in even-even neutron-rich Pd isotopes

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

Evidence for abnormal M1 transition in 187Pt

Gamma-gamma angular correlation measurements have been performed in 187Pt in order to determine the spin values of the states connected by the highly converted transitions. For one of these transitions, we have determined that the spin values of the initial and final states are different, which implies abnormal M1 multipolarity for this highly converted transition. In that case the deduced penetration parameters have especially high absolute values. Moreover we have determined uniquely the spin value of most of the excited states below 800 keV. The occurence of highly converted transitions in 187Pt is examined and the low-spin negative-parity levels are discussed in terms of shape coexistence.

Coordination Geometry of Cadmium at the Zinc and Copper Sites of Superoxide Dismutases: a Study Using Perturbed Angular Correlation of gamma-Rays from Excited 111Cd.

111Cd time-differential perturbed gamma-gamma angular correlation (PAC) has been used to investigate the Zn site in yeast and bovine copper and zinc-containing superoxide dismutases by substitution of the zinc ions with excited 111Cd(2+) ions. The PAC spectra obtained from the enzymes in aqueous solution reveal a single coordination geometry of 111Cd(2+) showing that the coordination of 111Cd(2+) to the Zn site in the two subunits is identical. Furthermore, the PAC spectra of the yeast and bovine enzymes show that the Zn sites are very similar in the two enzymes. The PAC experiments show a clear difference depending on whether the copper ion is in the oxidized or the reduced state. In the latter case the results resemble those obtained for derivatives with no metal ion at the Cu site. Hence the coordination geometry of the Zn site in these two situations must be similar, and it is very unlikely that the imidazole ring of His61 bridges the two metal ions in the reduced enzyme. The PAC spectrum of 111Cd(2+) ions at the Zn site with copper(II) ions at the Cu site is in agreement with that predicted by applying the angular overlap model (AOM) to the known crystal structure of the bovine enzyme, with known nuclear quadrupole interactions for the ligands involved. Furthermore results from experiments with copper in the reduced state show that reduction of the copper ion causes a significant change at the Zn site. An explanation for this conformational change has been proposed by computer modelling. The PAC experiments also show that it is possible to incorporate cadmium ions into the Cu site in the absence of copper ions, and the result has also been interpreted in terms of the AOM.

Low-energy excited states in124Te nucleus

The energies and intensities of the gamma transitions in the decay of124Sb are measured using a large-volume HPGe detector, with by far the best resolution, in singles and in coincidence with a NaI(Tl) detector. Gamma-gamma angular correlations are performed on six cascades. The results not only resolve several existing controversies about the level structure of124Te but also detect for the first time four new gamma transitions. Relative intensities, branching ratios, α(K) and logft values are calculated and multipolarities, spins and parities deduced. Comparison with the predictions ofSU(5) andO(6) limits of interacting boson approximation model seemed to favour the former approximation.