Gamma-ray Multiplicity Research Articles

Angular-momentum limitations in the high-spin gamma-decay of medium-mass evaporation residues

Gamma-ray multiplicity measurements in coincidence with mass-separated evaporation residues are reported for the reaction 28Si+54Fe. The bombarding energy dependence [75 ⩽ El (28Si) ⩽ 145MeV] of the average gamma-ray multiplicity shows a saturation for the heaviest observed masses (80 and 79). Complementary information from partial-cross-section measurements, performed in the same energy range, is analyzed in the framework of the statistical model. A consistent description of both data sets suggests an enhanced alpha competition in the decay of the compound nucleus 28Zr. The use of deformation-dependent optical-model transmission coefficients in statistical-model calculations at very high angular momenta is considered.

Angular Momentum Transfer in Reactions Induced by 14N Ions on 154Sm

The angular momentum input in reactions induced by 151 MeV 14N ions on 154Sm has been investigated by measuring the gamma-ray multiplicity in coincidence with emitted charged particles. The average multiplicity of gamma-ray cascades following the emission of protons, deuterons and tritons in reactions leading to 158Er amounts to 17.5 which is far below the value 31, earlier reported for a bombarding energy of 167 MeV. Predictions by Wilczynski's sum-rule model for complete and incomplete fusion reactions combined with statistical evaporation of particles are in good agreement with the present results.

Determination of an effective radius from the gamma-ray multiplicities in fusion reactions.

The gamma-ray multiplicity and the fusion cross-section data, together, are used to determine the effective radius employed in a previously developed inversion procedure. Implications on the relationship between the gamma-ray multiplicity and the average angular momentum are pointed out.

Gamma-ray multiplicity distribution associated with massive transfer.

The first three moments of the \ensuremath{\gamma}-ray multiplicity distribution were measured for individual \ensuremath{\alpha}xn exit channels of the reactions 85-MeV $^{12}$C${+\mathrm{}}^{154}$Sm, 109-MeV $^{12}$C${+\mathrm{}}^{154}$Sm, and 115-MeV $^{14}$N${+\mathrm{}}^{159}$Tb, in which massive transfer had been studied previously. The average \ensuremath{\gamma} multiplicity (first moment) was found to be almost constant, independent of yrast spin for fast \ensuremath{\alpha}-particle emission, while it increases with yrast spin for slow \ensuremath{\alpha}-particle emission as is common in complete fusion. The variance (second moment) is also independent of yrast spin. For fast \ensuremath{\alpha}-particle emission, the average \ensuremath{\gamma} multiplicity decreases monotonically with increasing \ensuremath{\alpha}-particle energy, and simultaneously the skewness (third moment) tends to be positive. This indicates that massive transfer accompanied by fast \ensuremath{\alpha}-particle emission is similar to a quasielastic process in heavy-ion reactions. A statistical model calculation has been made to study the dependence of the \ensuremath{\gamma}-ray multiplicity distribution on angular momentum and excitation energy. The results are consistent with the experimental data. It is found that the sign of skewness depends on the widths of angular-momentum and excitation-energy distributions.

Angular momentum partition in heavy ion induced fission and the effects of shells on gamma-ray multiplicities

Gamma-ray multiplicities have been measured following fission of nuclei with a wide range of mass and angular momentum. The average multiplicity reflects the total angular momentum of the fragments, but the observed variation of multiplicity with fragment mass asymmetry is dominated by shell effects. The highest average multiplicity arises fission of the heaviest compound system, produced with the lowest angular moméntum. This behaviour is well described by spin enhancement through statistical excitation.

Temperature dependent effects and statistical gamma-ray multiplicities in the147Sm(3He, ?)146Sm reaction

Gamma-ray multiplicities associated with the statistical decay from highly excited states in146Sm are calculated. The formalism, which accounts for the temperature dependence of the level density parameter, is applied to the analysis of the statistical gamma decay from states populated by the (3He, α) reaction. The results are found to be in a reasonable agreement with the experimental data.

Measurement of Gamma-Ray Multiplicity Emitted in the Alpha-Induced Fission of232Th

Multiplicity of gamma-rays from the fission fragments was measured in the α-induced fission of 232 Th, at the beam energy of 30 to 65 MeV. Incident energy dependence of γ-ray multiplicity as a function of fragment mass was obtained at each incident beam energy. Dependence of γ-ray multiplicity on the beam energy was compared with a classical calculation of transfer of angular momentum. Possible causes of the difference of the multiplicity between the symmetric and asymmetric mass region are discussed.

Spin distribution of the compound nucleus in heavy ion reactions at near-barrier energies.

Gamma ray multiplicities and the (HI,xn) cross sections for the dominant reaction channel were measured at near-barrier bombarding energies for the systems \ensuremath{\alpha}${+\mathrm{}}^{154}$Sm and $^{12}$C${+\mathrm{}}^{154}$Sm. The mean value of the spin distribution of the compound nucleus was obtained for each of the systems studied. The results of these measurements together with the results of an earlier study on $^{16}$O${+\mathrm{}}^{154}$Sm indicate that at bombarding energies near and below the Coulomb barrier the spin distributions of the compound nucleus are broader than the triangular distributions expected from a sharp cutoff model, these deviations being larger for the more massive projectiles. The different behavior of the three systems clearly indicates the importance of the centrifugal barrier penetrability in determining the spin distribution of the compound nucleus. The absolute values of the mean angular momentum can be accounted for if one also includes deformation effects. A one-dimensional barrier-penetration model including the effect of averaging over the orientation of the deformed target can account for the experimental results.

Prompt proton emission in heavy-ion-induced fusion reactions below 10 MeV/nucleon

Pre-equilibrium emission of protons in fusion-like reactions has been observed for 135 MeV 16O + 93Mb, with the proton energy spectra exhibiting a forward-backward asymmetry in the exponential tails. Gamma-ray multiplicity data for 100–400 MeV 16O+ 96Mo reactions are consistent with prompt emission arising from a broad range of entrance-channel partial waves. The results are interpreted as favoring a central jetting mechanism.

New statistical and nonstatistical effects in deep inelastic heavy-ion collisions

The physics of deep inelastic heavy-ion collisions is briefly reviewed in the light of the experimental capabilities offered by large tandem accelerators. Three aspects are selected for illustration. The first is the problem posed by the angular distributions. The role of quantal fluctuations (diffraction) and of dynamical fluctuations is discussed. The possibility of extracting angular-momentum fluctuations from the angular-distribution width is pointed out, as well as the way to compare such results with those obtained from gamma-ray multiplicities or angular distributions. The likely sources of angular-momentum fluctuations are discussed. The second problem is that of energy dissipation and partition. The experimental evidence for thermal equilibrium is reviewed and experiments to test thermal equilibrium through the measurement of the variance in the energy distribution are suggested. The correlation between energy loss and the mass variance is reviewed in terms of the mass transfer mechanism. Possible ways to determine the mass of the transferred particle are illustrated. The third problem considered is the experimental lack of mass drift in the mass distributionsvs.Q-value when it is expected on the basis of potential-energy considerations. This problem is associated with the fast-fragment thermalization and explained in terms of a feedback effect associated with the temperature gradient that develops between two colliding nuclei of different mass.

Nuclear spectroscopic study of24Na

The nucleus24Na has been investigated by studying the gamma-rays emitted following thermal neutron capture in23Na, with curved crystal and Ge(Li) spectrometers. Of the 277 transitions assigned to24Na, 216 were placed in the24Na level scheme containing 45 levels, of which six (1,961, 1,977, 3,866, 5,810, 5,918, and 6,222 keV) are reported for the first time. An average gamma-ray multiplicity of 3.3 gammas per neutron capture was observed. The neutron binding energy was determined to be 6,959.73 (14) keV. The resulting level scheme is compared to shell and rotational model predictions.

The19F(n, ?)20F reaction

The gamma-ray spectrum emitted following thermal neutron capture in19F has been studied with curved crystal and Ge(Li) spectrometers. From the 109 transitions assigned to20F, 85 have been placed in a level scheme containing 26 levels. An average gammaray multiplicity of 2.8 gammas per neutron capture was observed. The neutron binding energy was found to be 6601.33(14) keV. The experimental level scheme is compared to rotational model predictions. In addition it is shown that the decay of the capture state is non-statistical and that there is a strong correlation between the strengths of excitation of levels by the (n, γ) and (d, p) reactions. Calculations of the partial cross-sections using the direct capture theory of Lane and Lynn give order of magnitude agreement with experiment.

Penetration of the centrifugal barrier in the fusion ofO16with heavy targets

Gamma ray multiplicities and rotational band transition intensities have been measured for the /sup 154/Sm(/sup 16/O, 4n) reaction for bombarding energies between 62.5 and 73.5 MeV. It is shown that centrifugal barrier penetration leads to a fairly broad spin distribution of the compound nucleus even at near-barrier energies. The results are discussed in terms of optical model absorption probabilities and parabolic barrier penetration probabilities.

Possible evidence for the statistical excitation of collective modes in heavy ion induced fission

Angular momentum transfer in the fission of 194Hg has been studied utilizing gamma-ray multiplicity techniques. The results indicate that the fragment spins are considerably larger than expected from rigid rotation. The data are compared with the predictions of a simple model which assumes the statistical excitation of a variety of collective modes.

Angular momentum partition as a function of the mass asymmetry in heavy-ion induced fission

Gamma ray multiplicities and angular distributions have been measured as functions of the mass asymmetry of the fission fragments for the reactions of 150–210 MeV 137Cl beams with 107Ag, and 150–210MeV 32S beams with 112Cd. In all reactions, as the mass asymmetry increases the mean total gamma ray multiplicities decreases, the relative width of the multiplicity increases and the gamma ray anisotropy decreases. Thus the more asymmetric the mass splitting, the smaller is the total spin of the fission fragments, and, probably, the total angular momentum of the compound system.

Gamma ray multiplicities in deep inelastic collisions for the Cu + Au 400 MeV system

For the Cu + Au 400 MeV system γ-ray multiplicities have been measured. The mean value of the transferred angular momentum, the variances, and the repartition of angular momentum between the fragments have been deduced. It is shown that, early in the reaction the building up of angular momentum is a very fast process. For longer times, additional angular momentum is induced in the fragments. The excitation of collective modes can account for this effect. The properties of such modes have been analysed.

Collective modes associated with the proton-induced fission ofBi209

Gamma-ray multiplicity (first and second moments) has been measured in the 60 MeV proton-induced fission of $^{209}\mathrm{Bi}$. From this work we have evidence that shell effects play an effective role at the scission point, even at an excitation energy of 65 MeV. Collective modes seem to be more easily excited when the nascent fragments are far from spherical shapes. The $\ensuremath{\gamma}$-multiplicity moments seem to depend weakly on excitation energy of the fissioning nucleus and do not really reflect the angular momentum of the entrance channel. The good agreement of our results with the Moretto-Schmitt model highly supports the role played by collective modes (bending) in the heavy ion induced fission and deep inelastic collisions which are presently the field of many recent experimental and theoretical investigations.NUCLEAR REACTIONS $p$+$^{209}\mathrm{Bi}$; ${E}_{p}=60$ MeV; measured fission fragment mass distribution $\frac{\mathrm{dN}}{\mathrm{dM}}$; measured $\ensuremath{\gamma}$-ray multiplicity first and second moments $〈{M}_{\ensuremath{\gamma}}〉$ and ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ functions of fragment mass ratios $\ensuremath{\alpha}$ and ${E}_{\mathrm{TKE}}$;; measured average $\ensuremath{\gamma}$-ray energy $〈{E}_{\ensuremath{\gamma}}〉$ ($\ensuremath{\alpha}$); measured in-plane, out-of-plane ${〈{M}_{\ensuremath{\gamma}}〉}_{\ensuremath{\alpha}}(\ensuremath{\theta})$; comparison to s.f. and HI reaction mechanisms and to Moretto-Schmitt model; deduced effects of collective rotational modes (bending) on fragment spins.

Calculation of Gamma-Ray Multiplicity Moments Associated with Massive-Transfer Reactions

The first three moments of the γ-ray multiplicity distribution have been calculated for the massive-transfer residue. Importance of the sign of the third moment, i.e., skewness, is pointed out by showing that the skewness is likely to be positive in the case of massive transfer, being contrary to the case of complete fusion.

A multiplicity logic unit

The logic diagram principle of operation and some details of the design of the multiplicity logic unit are presented. This unit was specially designed to fulfil the requirements of a multidetector arrangement for gamma-ray multiplicity measurements. The unit is equipped with 16 inputs controlled by a common coincidence gate. It delivers a linear output pulse with the height proportional to the multiplicity of coincidences and logic pulses corresponding to 0, 1, … up to ⩾5-fold coincidences. These last outputs are used to steer the routing unit working with the multichannel analyser.

Information-theoretic analysis of gamma-ray multiplicities in deep-inelastic collisions

A constrained phase space approach accounts for the magnitude and the trends of the recently observed variance of gamma-ray multiplicity distributions in deep-inelastic collisions.