Γ-ray Multiplicity Research Articles

Spin distributions at the Coulomb barrier in the 58Ni+ 60Ni fusion reaction from gamma-ray multiplicity measurements

Heavy-ion fusion barrier distributions are now routinely obtained directly from experimental data. Measurements of the total γ-ray multiplicity for the fusion channels of the 58Ni + 60Ni system, which has striking yet well understood barrier structures, confirm the theoretical predictions that very high spins can be populated at energies close to (and even below) the nominal Coulomb barrier. The mapping from multiplicities to spin populations shows that structures in the barrier distribution are still evident in the γ-ray results.

Influence of fusion barrier distributions on spin populations

Heavy-ion fusion barrier distributions are now routinely obtained directly from experimental data. By measuring the total γ-ray multiplicity for the 58 Ni+ 60 Ni system, which has a striking yet well understood barrier distribution, we show that some evidence of the barrier structures is present in the derived spin populations. In particular, very high spins can be populated at energies rather close to (and even below) the ‘nominal’ Coulomb barrier.

Prompt dipole γ-ray emission in fusionlike heavy-ion reactions

The 32S + 100Mo and 36S + 96Mo fusionlike reactions were studied at incident energy of Elab = 298 MeV and 320 MeV, respectively, with the aim of probing the influence of the entrance channel charge asymmetry on the dipole γ-ray emission. The excitation energy and spin distribution of the compound nucleus created in these reactions were identical, the only difference being associated with the unequal charge asymmetry of the two entrance channels. High-energy γ-rays were detected in an array of 9 seven-pack BaF2 clusters. Coincidence with fusionlike residues detected in four PPAC ensured the selection of central reaction events. By studying the differential γ-ray multiplicity associated with the two reactions it was shown that the dipole strength excited in the compound nucleus increases with the entrance channel charge asymmetry. From the linearized spectra, the increase of the GDR γ-ray intensity was found to be ∼ 25% for the more charge asymmetric system. The results are discussed and compared with those of previous data obtained at different incident energies.

Pre-equilibrium dipole strength in charge asymmetric peripheral heavy-ion reactions

We report on the results obtained from the study of the 32S + 64Ni and 32S + 58Ni peripheral reactions at incident energies E lab = 288 MeV and E lab = 320 MeV, respectively. High-energy γ-rays were detected in an array of 8 seven-pack BaF2 clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. All of the relevant reaction parameters were kept constant with the exception of the different initial dipole moment caused by the different entrance channel charge asymmetry. While for quasi-elastic events no N/Z effect was observed in the differential γ-ray multiplicities of the two reactions, for deep-inelastic events a larger dipole γ-ray emission occurs during the more N/Z asymmetric reaction. A theoretical interpretation based on a collective Bremsstrahlung analysis of the reaction dynamics is presented.

Gamma-ray multiplicity distribution in ternary fission of 252Cf

From multiparameter data obtained at Lawrence Berkeley National Laboratory, the integral characteristics of the prompt γ-ray emission were extracted for tripartition of 252Cf with He, Be and C being the third light charged particle. We used multifold γ-ray coincidence spectra for the determination of γ-ray multiplicities assuming a Gaussian distribution for γ-ray multiplicity. The multiplicity distribution characteristics, i.e. mean multiplicity and its dispersion were obtained by minimizing with respect to the calculated values of probabilities of multifold γ-ray coincidences using a combinatoric method. Comparison with the known experimental data from binary fission was made. Further, we investigated dependencies of the mean γ-ray multiplicity on the kinetic energy of the light charged particle. The mean γ-ray multiplicity for He ternary fission is found to increase rapidly with increasing kinetic energy of He in the region less than 11 MeV and then decrease slowly with increasing kinetic energy of He. The anomalous behaviour of γ-ray emission is discussed. The mean γ-ray multiplicity was determined for the first time for Be and C ternary fission. For Be, the γ-ray multiplicity as a function of kinetic energy was obtained as well.

The MINIBALL array

MINIBALL a highly efficient, position sensitive γ-spectrometer is under construction for upcoming radioactive-beam experiments. In contrast to high spin arrays the total full energy peak efficiency and not the resolving power is optimized with a factor of two larger efficiency for γ-ray energies up to 10 MeV and for low to medium γ-ray multiplicity events. Cluster detectors, incorporating three or four Ge crystals, will be employed to provide high flexibility and to accomodate the various demands of the experiments.

Angular momentum limit of Hf isotopes produced in three fusion–evaporation reactions

The compound nucleus 168 Hf was populated in three fusion–evaporation reactions with different beam–target mass asymmetries: 50Ti + 118Sn, 64Ni + 104Ru and 74Ge + 94Zr. Due to the large negative Q values of these reactions the compound nucleus is formed at low excitation energy. At three or four excitation energies for each reaction γ-ray spectra of the evaporation residues 166Hf to 163Hf, corresponding to the 2n to 5n exit channels, respectively, were recorded with the Ge detectors of the 8 π-spectrometer array. The γ-ray multiplicity and total energy were measured using the inner ball of BGO detectors. This data was used to determine the maximum angular momentum transferred to each evaporation channel, the γ-ray decay entry region and the relative cross sections. No differences are observed between the three reactions. This is explained by the very similar dependence of the excitation energy on the angular momentum of the compound nucleus. The relative cross sections for the 2n and 3n channels are observed to be significantly larger than predicted by statistical model calculations.

MINIBALL A Ge detector array for radioactive ion beam facilities

Dedicated Ge-detector arrays are being developed for the investigation of rare γ decays with low γ-ray multiplicity at the upcoming radioactive ion beam facilities. These arrays are optimized for the high full-energy peak efficiency and angular resolution of the γ-ray detection needed for a proper Doppler correction of the γ-rays emitted by fast recoiling nuclei. MINIBALL will consist of 40 six-fold segmented, encapsulated Ge detectors which are clustered in eight cryostats with three detectors each and four cryostats with four detectors, respectively. The individual components - the six-fold segmented Ge detector, the cryostats, the fast preamplifier, the digital pulse-processing electronics and the mechanical frame - and their properties are described. The results of test measurements with the first MINIBALL cluster detector using a 137Cs source and the in-beam reaction D( 37Cl, n) 38Ar are presented. It is shown that from pulse-shape analysis of the events within a detector segment the effective granularity of the MINIBALL array can be enhanced from 240 to ∼ 4000. The specifications of MINIBALL are compiled on the basis of experimental data. First results with a 12-fold segmented, encapsulated detector are discussed with respect to the feasibility of future γ-ray tracking arrays.

Study of the Mo-Ba partition in 252Cf spontaneous fission

Measurements of fission fragment yields and neutron multiplicities have been carried out for the Mo-Ba fragment pairs in the spontaneous fission of 252Cf, using the γ-ray spectroscopy technique to analyze γ-γ-γ coincidence data. Prompt γ -ray multiplicities were also measured as a function of the number of neutrons emitted in the fission process leading to the Mo-Ba partition. We do not observe the enhancement in the yields of events with high neutron emission multiplicity (νn > 7) that has been associated to a second fission mode leading to the production of hyperdeformed Ba fragments, as reported in some earlier studies. The average γ-ray multiplicity is found to be rather weakly dependent on the number of neutrons emitted in the fission process.

Impact parameter dependence of linear momentum transfer and the role of two-body dissipation mechanisms in heavy ion collisions around the Fermi energy

High energy γ-rays ( E γ >35 MeV) have been measured in coincidence with heavy residues emitted in reactions induced by a 37 MeV/u 36Ar beam on a 98Mo target. The γ-ray yield increases strongly with increasing linear momentum transfer indicating the importance of two-body collisions in the transfer mechanism. The high energy γ-ray multiplicity has been used to correlate the linear momentum transfer to the impact parameter. This correlation is compared to dynamical BNV simulations to show the essential role of two body nucleon-nucleon collisions at these bombarding energies.

Influence of the entrance channel in the fusion reaction 318 MeV 74Ge+ 74Ge

Entrance channel effects in the fusion of heavy ions have been studied by using the 74Ge+ 74Ge reaction at 318 MeV. The population of the yrast superdeformed band in 144Gd shows an increase when compared with the results obtained in the more asymmetric 48Ti+ 100Mo reaction at 215 MeV. The relative yields of the different evaporation residues produced in the 74Ge+ 74Ge and in the 48Ti+ 100Mo reactions are very similar, with the exception of the 145,144Gd residual nuclei (3 n and 4 n decay channels) which are populated with a larger yield in the symmetric reaction. Statistical model calculations reproduce qualitatively such effect if a fission delay is explicitly taken into account. Effects related to fusion barrier fluctuations seem to be important in determining the spin distributions of the compound nucleus. The spectra of the high energy γ-rays emitted in the 74Ge+ 74Ge reaction have been measured as a function of the γ-ray multiplicity as well as in coincidence with selected evaporation residues. They are reproduced by standard statistical model calculations with GDR parameters taken from systematics, demonstrating that, in agreement with dynamical model prediction, the emission of γ-rays from the dinucleus formed in the earlier stage of the collision is unimportant.

Concluding remarks

Concluding remarks

High-energy γ-rays in α-accompanied spontaneous fission of 252 C f

An experiment was performed on prompt γ-ray emission in binary and α-particle accompanied spontaneous fission of 252C f using the Darmstadt-Heidelberg 4π NaI Crystal Ball spectrometer. The enhancement in γ-ray yield, denoted as the “high-energy component”, which appears between 3.5 and 8 MeV and in the region of near-symmetric fragment mass splits, was observed to be equally pronounced in both fission modes. Analyzing the fragment mass dependence of the mean γ-ray multiplicity in both fission modes clearly identifies the disintegration of equilibrated fission fragments in a narrow mass range around the double-magic 132Sn as the source of these γ-rays.

Physics with Ge-Miniball-arrays

For the measurement of rare γ decays with low γ-ray multiplicities ( M γ < 15) dedicated Ge-detector arrays are being developed. They are optimized with respect to the full-energy peak efficiency rather than the resolving power. These so-called miniball arrays will pre-dominantly be used at radioactive ion beam facilities. The detector arrays will have a high granularity using segmented Ge-detectors to provide a sufficient angular resolution of the γ rays which is needed for proper Doppler-shift corrections and small Doppler-broadening. We will describe shortly the Miniball presently developed by a German, Belgian, and Italian collaboration and the wide range of physics to be addressed with this new detector system.

Simulated response characteristics of Gammasphere

The response of the Ge detector array Gammasphere to γ-rays, both individually and in cascades, has been modeled with the Monte Carlo code GEANT 3. The effects on the performance of Gammasphere of including an auxiliary detector, the Microball, are investigated. The use of the existing Ge detectors and BGO suppression detectors as a γ-ray multiplicity and total γ-ray energy spectrometer is explored, and its total energy - multiplicity ( H, k) response is simulated in detail. The utility of the Hevimet collimators in spectroscopic studies, as opposed to the use of the available ( H, k) information, is discussed.

Study of Giant Dipole Resonance in Hot Rotating Nucleus 132Ce

High-energy γ-rays from the deexcitation of giant dipole resonance have been measured from the decay of compound nucleus 132Ce* formed by the 16O+116Sn reaction at an incident particle energy of 85.0 MeV of 16O. The γ-ray multiplicity distribution of this reaction is calibrated with a multiplicity filter composed of 14 hexagonal BGO crystals. The shape evolution of the 132Ce* has been studied as a function of spin. The results reveal that the deformation parameter of prolate 132Ce reduces when spin increases.

Statistical γ-ray multiplicity distributions in Dy and Yb nuclei

The statistical γ-ray multiplicity distributions following the reactions 163 Dy( 3He ,αx n) 162−x Dy and 173 Yb( 3He ,αx n) 172−x Yb have been studied. The mean value 〈 M〉 and standard deviation σ M have been extracted as functions of excitation energy. The method is based on the probability distribution of k-fold events, where an α-particle is observed in coincidence with signals in k γ-ray detectors. Techniques for isolating statistical γ-rays and subtracting random background, cross-talk and neutron contributions are discussed.

Gamma-ray production from52Cr(n, xγ) reactions at 14.6 MeV

We study γ-ray production from52Cr(n, xγ) reactions at 14.6 MeV neutron incident energy. Measured data relevant to production cross-sections for discrete γ-rays, total γ-ray spectrum and average γ-ray multiplicities, are analyzed in terms of the advanced statistical model of nuclear reactions, including pre-equilibrium emission of primary particles and γ-rays. The latter proves useful in rough description of high-energy part of the γ-ray spectrum. We find that integral quantities and data involving sufficient averaging (such as total γ-ray production, average γ-ray energies, average γ-ray multiplicities) can be predicted with a good precision. This is valid also for strong discrete γ lines. Generally, however, discrete γ-ray production cross-sections cannot be safely predicted better than within a factor of two. Things may be even worse for particular portions of the total γ-ray spectrum. Our findings set limits on predictability of many requested γ-ray production data even by most carefully applied theoretical methods.

Experiments with a 4π charged-particle detector and the 8π spectrometer

Experimental techniques for charged-particle detectors used with 4π γ-ray spectrometers are reviewed. As examples, recent experiments at the TASCC facility in Chalk River with a 4π steradians modular miniature array of CsI(Tl) crystals coupled to large-area photodiodes operating with the “8π” spectrometer, are briefly discussed. We point out the importance of charged-particle detection in channel selection for in-beam spectroscopic studies. The main emphasis of the initial experiments with this apparatus has been on a study of the charged particles associated with superdeformed bands, and on the search for nuclear-structure effects in the evaporation spectra. Charged-particle evaporation spectra tagged with γ-ray multiplicity, total γ-ray energy and nuclear-spin orientation have been measured for events selected by gates on discrete γ-ray transitions in the residues. This detector is also being used in the search for superdeformation and hyperdeformation.

Gamma-ray multiplicity distributions in 16O + 152Sm fusion near and below the Coulomb barrier

We have studied γ-ray multiplicity distributions for the system 16O + 152Sm at five beam energies in the range 60–80 MeV, near and below the Coulomb barrier. The data were obtained using a large array of γ-ray detectors and an efficient evaporation-residue detector system utilizing an electrostatic deflector. The data are in good agreement with predictions of angular momentum distributions calculated using a coupled-channels model incorporating the effects of deformation.