The Giant Dipole Resonance in the superdeformed143Eu nucleus

The γ-decay of the Giant Dipole Resonance (GDR) built on excited nuclear states has been measured for the nucleus¹⁴³Eu. The reaction¹¹⁰Pd(³⁷Cl, 4n)¹⁴³Eu at a beam energy of 165 MeV has been employed. This experiment aimed at searching the γ-decay of the GDR built on the superdeformed¹⁴³Eu states, populated at high spins. High-energy γ-rays were detected in 8 large BaF₂ scintillators in coincidence with discrete transitions measured with the NORDBALL array (in the configuration consisting of 17 HPGe detectors and a 2π multiplicity filter). The spectrum of high-energy γ-rays gated by low-energy transitions between states fed by the superdeformed states shows some excess of yield in the 7–10 MeV region with respect to that gated by transitions between states not populated by the superdeformed states. This excess should be due to the γ-decay of the the giant dipole oscillation along the superdeformed axis of the nucleus that is expected to have a frequency corresponding to ≈9–10 MeV (low-energy component of the GDR strength function). The measured excess, in spite of the large error bars, is found to be of the same order as predicted statistical model values.     

Effect of E1 decay in the population of superdeformed structures

Spectra of the yrast and excited superdeformed bands, forming the E2 quasi-continuum, are measured with the EUROBALL array for the nucleus ¹⁴³Eu, in coincidence with high-energy γ-rays (E_γ>3 MeV). It is found that the intensity population of the superdeformed states is enhanced by a factor of ≈1.6 when a coincidence with a γ-ray with energy >6 MeV is required, in reasonable agreement with the increase of the line shape of the Giant Dipole Resonance built on a superdeformed configuration. This result shows that when an high energy E1 γ-ray is involved in the decay it is more likely connected with a SD rather than a ND nucleus. In addition, the analysis of the rotational quasi-continuum suggests the presence of a superdeformed component. The data are also compared and found consistent with simulation calculations of the relative intensities of the SD states, including the E1 decay of superdeformed nature.     

High-energy gamma rays emission in coincidence with light charged particles from the32S +74Ge reaction at 210 MeV

High-energyγ-rays from the³²S+⁷⁴Ge reaction at 210 MeV bombarding energy were measured in coincidence with light charged particles detected in a large area hodoscope. Experimental results show that energeticγ-rays in coincidence with light charged particles are essentially emitted in the compound nucleus decay. The parameters of the giant dipole resonance (GDR) have been extracted from alineshape analysis of the experimentalγ-ray spectrum. The derived values of mean energyE _D, widthΓ and strengthS are in good agreement with results from previous experiments on Sn isotopes obtained by using different experimental techniques.     

The decay of the 560 ns Isomer at 8.6 MeV in 64 147 Gd83

In (α,xn) and (¹²C, 5n) in-beamγ-ray experiments with pulsed beams the decay of the 560 ns high-spin isomer in¹⁴⁷Gd has been investigated. Extensive coincidence measurements identified in the isomeric decay more than eightyγ-rays which almost all were placed in the level scheme. Of crucial importance for establishing the highly complex decay was the comparison of data from the different reactions and the identification of transitions which bypass the 27 ns isomer at 3.58 MeV. Our data determine the excitation of the 560 ns isomer as 8.588 MeV and suggest 47/2 as its most probable spin.     

Gamma-gamma directional correlations and coincidence studies in154Gd

The intensities, placements andE2/M1 mixing ratios of transitions in the decay of¹⁵⁴Eu have been carefully studied to provide accurate data for microscopic calculations. Coincidence relationships in the decay of¹⁵⁴Eu have been studied extensively with a multiparameterγ-γ coincidence system with two large volume Ge(Li) detectors. Spectra in coincidence with twenty energy gates were analyzed. Twenty-nine new coincidence relationships were established and confirmed most, but not all, of several levels previously assigned by energy fits only. From an analysis of coincidence spectra and singles spectra with a 18% efficiency Ge(Li) detector new information on the gamma-ray intensities were obtained. Precise values of theE2/M1 mixing ratios of transitions from the gamma- and beta-vibrational bands to the g.s. band have been determined fromγ-γ directional correlation measurements with a NaI(Tl)-Ge(Li) detector coincidence system. Mixing ratios were obtained for a number of other transitions including those fromK ^π =0^? and 2⁺ bands from direct and skipped cascade correlations.     

Der zerfall des J121 (T 1/2=2,12 h)

I¹²¹ was produced by bombarding a target of enriched Te¹²² with deuterons of 25 MeV. Theγ rays associated with the decay of I¹²¹ have been investigated using a Ge(Li) detector and a Ge(Li)-NaI coincidence set up. 58γ transitions ranging from 212.5 to 1841.8 keV have been identified. A decay scheme for the I¹²¹→Te¹²¹ decay which accounts for 50 of the observedγ transitions is proposed.     

The decay of 8.7 min237Pa

The beta and gamma radiations of²³⁷Pa have been investigated employing semiconductor and scintillation spectrometers and coincidence techniques. Sources of²³⁷Pa were obtained after bombardments of²³⁸U with bremsstrahlung and 14-MeV neutrons and subsequent chemical separation. From the total of 18γ-rays following the decay of 8.7±0.2 min²³⁷Pa 17 transitions, representing 99.9% of theγ-ray intensity, could be placed in a level scheme of²³⁷U. AQ _β- value of 2.25±0.1 MeV has been determined.     

Preequilibrium GDR excitation and entrance channel angular momentum effects

The energy spectra of the γ-rays emitted in the ³⁵Cl + ⁹²Mo reaction at incident energy E = 260 MeV were measured in coincidence with the ejectiles produced in dissipative reaction events. The cumulative energy spectrum of the γ-rays coming from the decay of the ejectiles was calculated within the statistical model and its comparison to the experimental spectrum evidences an excess in the data for E _γ = 8 to 12 MeV. Such an excess, fitted with a Lorentz curve, is attributed to the preequilibrium GDR γ-decay of the intermediate dinuclear system. The centroid energy of the Lorentz curve corresponds to a dipole oscillation along the symmetry axis of the system and its width is found to be comparable to that of the ground state GDR low energy component of the deformed dinucleus. The small quantal dispersion Δ? = (10.3 ± 0.1)? of the entrance channel angular momentum, determined by analysing the dissipative fragment angular distribution in the framework of the Strutinsky model, is suggested to limit the broadening of the preequilibrium GDR width.     

Nuclear structure of194Pt

The decay of¹⁹⁴Ir has been thoroughly studied resulting in the construction of a decay scheme consisting of 26 excited states and 69 transitions. Nine new levels and 29 new transitions were added to the previously known scheme. Of these, levels at 1,893.6 and 2,053.0 keV as well as 9γ-ray transitions are new to the¹⁹⁴Pt level scheme. Preciseγ-ray energy and intensity measurements as well as quantitative coincidence measurements were performed, and the internal conversion spectrum was investigated with a Si(Li) detector. Directional correlation measurements were performed for selected cascades with the major result being the unique assignment of 0⁺ to the new level at 1,893.6 keV. The structure was interpreted within the framework of the PPQ model as well as the effective-core picture including nuclear triaxiality.     

Discovery of the First 2- State in 16C via Neutron Knockout Reaction

The ¹⁶C nucleus has been investigated by the neutron knockout reaction of ¹⁷C on a liquid hydrogen target. Applying the invariant mass method in inverse kinematics and γ-ray spectroscopy, the energy spectrum was reconstructed by triple-coincidence measurement, in which neutrons, charged fragments, and γ rays from the decay of the reaction residue (¹⁶C*) were detected. A peak at 0.47 MeV was observed in the invariant mass spectrum in coincidence with a peak at 0.74 MeV in the γ-ray spectrum, which indicates the presence of an unbound state with an excitation energy of 5.46 MeV. Comparison of the experimental cross section with the value derived by a theoretical calculation provided evidence that the spin-parity of this state is 2^?.     

Gamma deexcitation of the 180m Ta isomer

Vibrational states built on the K ^π = 9^? isomer and on the ground state (K ^π = 1⁺) in ¹⁸⁰Ta are calculated within the quasiparticle-phonon nuclear model using the ¹⁷⁸Hf nucleus as a core. A procedure for calculating the rates of K-allowed γ-ray transitions from vibrational states built on the isomer to those built on the ground state is presented. The probabilities of two-step processes consisting of a dipole excitation of the isomer and successive E1 and E2 transitions from them to vibrational states built on the ground state of the ¹⁸⁰Ta nucleus are calculated. Two-step transitions from the isomer to vibrational states below 2.7 MeV and to the vibrational states built on the ground state appear to be very weak. There are many E1 transitions from the vibrational states built on the isomer to the vibrational states built on the ground state. They are weak and cannot be responsible for the strong deexcitation of ^180m Ta in the relevant (γ, γ′) reaction. A decisive role is played by collective E2 transitions from dipole excitations in several excitation energy intervals ranging between 2.7 and 4.0 MeV. These highly intense K-allowed two-step γ-ray transitions can be responsible for the strong deexcitation of the ^180m Ta state in the (γ, γ′) reactions.     

Neutron and high-energy gamma-ray emission in 12C Induced reactions on 159Tb

The gamma-ray multiplicity associated with the emission of high-energy gamma rays and neutrons following ¹²C+¹⁵⁹Tb reactions at E_lab= 100 MeV has been measured. The spin decrease deduced for coincident gamma rays within the GDR energy region is found to be in agreement with an estimate based on a statistical model. Neutron spectra measured in coincidence with high-energy gamma rays are explained in terms of two components. The cooler component corresponds to neutrons evaporated from the compound nucleus. The hotter component emanates from a precompound source moving in the center-of-mass system along the beam direction. The multiplicity of gamma-ray cascades, coincident with neutrons from the two different sources, has been deduced. The results show that beside the GDR there appears to be a second source of gamma rays with E_γ≈10MeV. These gamma rays are emitted in more peripheral reactions and before neutron evaporation.     

Response of cooled PWO scintillators to low-energy gamma-rays and its importance in studies of the h c →η c + γ transition in charmonium

Suppression of non-resonant background due to emission of π ⁰ increases signal to background ratio in γ-ray emission studies between the discrete states of charmonium. We demonstrate with Monte-Carlo simulations how π ⁰ identification capability increases with decreasing threshold for γ-ray detection in π ⁰ decay. An apparatus has been designed and measurements of the FWHM resolution performed for the three energies 6.13, 12.7 and 16.9 MeV. The trend established in the 50–650 MeV energy range is corroborated.     

Quantitative study of EγEγ correlations in the 122Sn + 106 MeV 12C compound system

The E_γE_γ correlations in the ¹²²Sn + 106 MeV ¹²C compound system have been measured by using the techniques of time of flight to reject events due to neutrons and of unfolding to remove events due to Compton escape. The rotational correlations in the measured matrix can be followed up to E_γ = 1.2 MeV which is shown to correspond to a collective spin of 15. An attempt to estimate the amount of aligned angular momentum by utilizing the intensities in the unfolded matrix failed mainly because at least half of all E2 γ-rays showed no rotational correlations. An independent evidence for this background of uncorrelated events is obtained by comparing the rotational correlations in the data with the rotational correlations obtained by simulating the decay of the known discrete levels in ^{127, 128}Ba on a computer. The comparison indicates that the observed rotational correlations are mainly due to the known discrete transitions. While the moment of inertia obtained from the correlation pattern is less than 80 % of the rigid-body value the high-energy end of the spectrum of E2 radiation can only be explained if γ-ray energy and spin are related by the rigid-body moment of inertia. The area of positive correlations that occurs in most published correlation plots at E₁ = E₂ and at the high-energy end of the E2 radiation is understood from the unfolding to be an artifact of the subtraction procedure.     

Decay of149Gd and directional correlations of gamma cascades in149Eu

The decay of¹⁴⁹Gd(9.4d) to the levels of¹⁴⁹Eu has been studied using large volume high resolution Ge(Li) and NaI(Tl) detectors,γ-γ coincidence and directional correlation experiments have been performed. Newγ-rays at energies 131.9, 278.6, 384.7, 415.7, 599.2, 726.5, 734.7, 956.4 and 1,231.1 keV in¹⁴⁹Eu have been observed. Three new levels in¹⁴⁹Eu have been placed at 956.4, 1,082.0 and 1,231.1 keV. Spin-parity assignments have been made to some of the levels in¹⁴⁹Eu. Multipolarity assignments have been made for most of the electromagnetic transitions from the conversion coefficients obtained through the knowledge of ourγ-transition intensities and the published conversion-electron intensities. Based on the newγ-transitions observed in the singles and coincidence spectra, a revised level scheme has been proposed for¹⁴⁹Eu and compared with previously published level schemes. Radioactivity¹⁴⁹Gd; measuredT _1/2,T _γ,γγ coin, γγ(θ), I.C.C.,¹⁴⁹Eu deduced levels,J,γ-mixing,π. Enriched targets, Ge(Li) detectors.     

Decay of193m Hg

The decay of^193m Hg to¹⁹³Au levels was investigated by means of Ge(Li) and Ge(Li)-NaI(Tl) coincidence spectrometers. In theγ-ray spectrum about 120 transitions were observed, about 80 of them being new. Forty coincidences betweenγ-transitions were observed. The level scheme of¹⁹³Au, including 12 earlier known levels as well as 17 new and corrected levels, has been constructed on the basis of the balance of energy sums and the observed coincidences. Possible multipolarities of 33 transitions were determined. Possible spins and parities of several levels were deduced from the determined multipolarities.     

The decay of 31 sec98Zr, 2.9 sec98Nb and 51 min98Nb

TheΒ- andγ-radiations of⁹⁸Zr and^{98m, g} Nb have been investigated employing scintillation and semiconductor spectrometers and coincidence techniques. Sources of⁹⁸Nb were produced by the⁹⁸Mo(n, p)⁹⁸Nb reaction, sources of⁹⁸Zr by fission of²³⁵U with thermal neutrons applying chemical separations. For⁹⁸Zr, a half-life of 30.7±0.4 sec and aQ _β-value of 2.3±0.2 MeV were obtained, for the⁹⁸Nb isomers, half-lives of 2.86±0.06 sec and 51.3±0.4 min, andQ _β-values of 4.8±0.2 MeV and 4.5±0.2 MeV, respectively. Noγ-rays were observed in the decay of⁹⁸Zr. The decay of 2.9 sec⁹⁸Nb was found to involve 11γ-ray transitions. In the decay of 51 min⁹⁸Nb, 54γ-transitions were detected. Spin and parity of 1⁺ and 4^? were deduced for the isomeric states of⁹⁸Nb.     

In-beam study of 63 143 Eu80

An in-beam γ-ray study with the¹⁴⁴Sm (α, p4n) reaction has established the high-spin states up to 5 MeV excitation and I=35/2 in the N=80 nucleus¹⁴³Eu. The observed yrast states show the typical irregular pattern of a spherical nucleus, with frequent decay branchings and often dominant dipole de-excitation. The level spectrum is in qualitative accord with the coupling of the h_11/2 proton particle to the known yrast levels of the¹⁴²Sm core nucleus. Only in a few cases specific configuration assignments are made.