Anomalies of neutron and γ-ray emission in light charged particle accompanied fission of 252Cf

Correlations between the neutron and γ-ray emission and the kinetic energy of light charged particles (LCPs), such as protons, deuterons, tritons and alphas, from the fission of ²⁵²Cf were studied in a four parameter experiment. A ⊘ 16 cm × 5 cm liquid scintillator with n-γ discrimination and a ⊘ 10 cm × 10 cm NaI(Tl) crystal were employed to detect the neutrons and the γ rays, respectively, while a thin CsI(Tl) crystal, which had the ability to separate the LCPs, was used to determine the LCP energy. The experimental results show that for the α particle accompanied fission, the average total number of neutrons emitted per fission increases in the case of varying the alpha particle energy E_α from 7 to 11 MeV and then falls off linearly in a first approximation with increasing E_α, and the average total energy and the average total number of γ-rays per fission as well as the average γ-ray energy as functions of E_α show similar correlation features. For the hydrogen-ion accompanied fission, the correlations of the neutron and γ-ray emission with hydrogen ion (t, d and p) energy are very similar to those for the alpha-particle accompanied fission. The correlations of the neutron and γ-ray emission in the low LCP energy region, in contrast to that observed in the high LCP energy range, are called anomalies. On the basis of the results obtained by the three-point charge model and the liquid drop model calculation with shell and pairing correction, the anomalous behavior of the neutron and γ-ray emission is explained tentatively.     

Time distribution of γ-rays from spontaneous fission of 252Cf at short times

The gross time distribution of γ-rays from spontaneous fission of ²⁵²Cf has been measured in the time range 10^?14?10^?10 s after fission for γ-ray energies greater than 0.15 MeV. The measurements have been made by a new method based upon the solid angle aberration. From the measured correlation between half-lives and γ-ray transition energies it is concluded that the transitions are predominantly E2, single-particle transitions at high energies, and mostly vibrational and rotational transitions at lower energies. Some contribution of E1 transitions and M1-E2 mixtures cannot be excluded, however. It was found that about 30% ofthe γ-rays emitted within 12 ns with energies greater than 0.10 MeV, have half-lives shorter than 1 ps and about 52 % half-lives between 1 and 100 ps. The relative yield of γ -rays and γ-ray energy in the time range 10^?14 to 1.2 × 10^?7 s ater fission is given.     

Determination of fragment isotopic yields in the fission of 252Cf accompanied by light charged particles

High resolution measurements of prompt γ-rays emitted from fission fragments accompanied by light charged particles (LCP) and those emitted from normal binary fission fragments in ²⁵²Cf fission were simultaneously carried out using a 30 cm³ Ge(Li) detector. A “sandwich” type of arrangement in the source-detector assembly was used to eliminate Doppler broadening of the γ-ray lines in both cases. About thirty γ-ray lines were assigned to specific fission fragments by comparison with the published results for binary fission, and the ratios of intensities of these γ-ray lines in LCP fission and binary fission were determined. Assuming that 2⁺ → 0⁺ transition intensities of doubly even isotopes are a measure of their yields, the yields of several such isotopes in LCP accompanied fission relative to that in binary fission were obtained.     

Prompt γ-rays emitted in the thermal-neutron induced fission of 233U and 239Pu

The average number and average energy of γ-rays emitted within ≈ 5 nsec after fission have been determined as functions of fragment mass and as functions of total kinetic energy. They were obtained from a four-parameter experiment that recorded, event-by-event, correlated of γ-rays and of fission-fragment pairs and the time, relative to fission, at which a γ-ray was detected. For ²³³U(n_th, f) the average total number and energy emitted per fission were found to be 6.31 ± 0.3 and 6.69 ± 0.3 MeV, respectively, giving an average photon energy of 1.06 ± 0.07 MeV. The results for ²³⁹Pu(n_th, f) given in the same order, are 6.88 ± 0.35,6.73 ± 0.35 MeV, and 0.98 ± 0.07 MeV.     

Identification of Excited States in 117Xe

Excited states of ¹¹⁷Xe were populated via the reaction ⁹²Mo(²⁸Si,2pn)at 115MeV.The emitted promt γ-rays were measured by using in beam γ-ray experimental techniques.Five bands in ¹¹⁷Xe have been identified,two of them are observed for the first time,and the three known bands are extended to higher spins.     

Fragment dependence of high energy γ-ray emission in the spontaneous fission of 252Cf

The high energy γ-ray emission accompanying the spontaneous fission of ²⁵²Cf has been measured in coincidence with individual fission fragments selected by discrete γ-ray transitions. The enhancement of the γ-ray emission probability in the energy range E_γ= 3–8 MeV has been observed for the fission fragments in the region of nearly symmetric mass splitting, confirming results reported in previous investigations. The γ-γ coincidence technique employed in the present work clearly demonstrate that the major contribution to this enhancement is caused by the fission channels where one fragment is near to the N= 82 or Z= 50 shell closures. The high energy γ-ray emission probability does not show any significant dependence on the number of neutrons emitted in the fission process, supporting the hypothesis that high energy γ-rays are mainly emitted from the fragments after the neutron evaporation. Received: 22 December 1998     

Decay of 59.3 min 128Sn to levels of 128Sb

The γ-ray spectra associated with the decay of 59.3 min ¹²⁸Sn have been measured with Ge(Li) detectors. In order to recognize γ-rays of ¹²⁸Sn and ^128mSb, the decay and/or growth of γ-rays emitted from a tin sample separated chemically from fission products were measured. The decay of ¹²⁸Sn is followed by the intense Sb X-rays and 32.1, 45.8, 75.1, 80.9, 115.9, 152.6, 404.4, 482.3, 557.3 and 680.2 keV γ-rays. On the basis of the measured singles and γ-γ coincidence spectra and the analysis of intensities of true sum peaks, a new decay scheme has been constructed. The 10.0 min isomer in ¹²⁸Sb decays by β-emission (96.4%) to excited levels in ¹²⁸Te and by an isomeric transition (3.6 %) to the 9.1 h ground state.     

Study of the Low Excited States of 72Ge

The radioactive decay of ⁷²Ga has been investigated by means of NaI-HpGe Compton-suppress spectrometer,and γ-γ coincidence measurements Performed with two HpGe detectors coupled to three-parameter(γ-γ-T) data acquisition system. 2.3×10⁷ coincidence events are collected. A new decay scheme of 72Ga including 26 levels,87 γ-rays is established. The extensive coincidence relationships are used to identify the γrays and levels of 72 Ge, such as new γ-rays of 225. 92, 826.97, 1349.71,1475. 32, 1667.91, 2105.28, 2247.39keV and new levels of 3248.01, 3396.27, 3806.10,3864.56keV. The unsure transitions of 112.59, 937.97, 2402.25, 2939.95keV and level of 2939.83keV are confirmed in thes work. No evidence has been observed for the existences of 113.5, 1155.7, 1192.4keV γ-rays and level of 3707.1keV. The transitiou of 317.72keV γ-ray is moved from 3757.26-3439. 51keV level to 3565.85-3248.01keV level.     

<Emphasis Type="Italic">T</Emphasis>-odd asymmetries for evaporation γ-rays in nuclear fission

T-odd asymmetries in angular distributions of evaporation γ-rays emitted by thermalized fragments resulting from the fission of axially symmetric deformed nuclei induced by cold polarized neutrons are investigated within the quantum theory of fission. The asymmetries in question are due to the anisotropy of the angular distributions of evaporation γ-rays, caused by zero wriggling vibrations of the fissioning nucleus and associated with the orientation of large fission fragment spins in the direction perpendicular to the direction n ₀ of the symmetry axis of the fissioning nucleus at the time of its separation into fragments. The angle of rotation of the vector n ₀ with respect to the asymptotic direction of the light fission fragment emission is calculated with allowance for the interference of fission amplitudes of neutron resonances excited in the fissioning nucleus as it captures the incident neutron. It is shown that the angular and energy characteristics of the T-odd asymmetry calculated for evaporation γ-rays agree with the characteristics of the experimentally investigated T-odd asymmetry in angular distributions of all γ-rays emitted by a fissioning ²³⁶U nucleus.     

Level Scheme of High-Spin States in 145Tb

High-spin states of ¹⁴³Tb have been studied via the ¹¹⁸Sn(³²S,1p4n)¹⁴⁵Tb reaction by means of in-beam γ-ray spectroscopy. Measurements of γ-ray excitation functions and γ-γ-t coincidences were performed with 12 BGO(AC)HPGe detectors. A level scheme for ¹⁴⁵Tb including 41 γ-rays which come from the deexcitations of the 24 levels, has been established for the first time based on an analysis of the γ-ray excitation functions, γ-γ coincidence relationships, intensity balances and cross-over transitions. The spins for most of the levels in the scheme are assigned according to the results of the measured γ-ray anisotropies. Considering the structure similarity between ¹⁴³Eu and ¹⁴⁵Tb, the low-lying levels in 145Tb are interpreted by coupling a h_9/2 proton to the excited states in ¹⁴⁴Gd core.     

High-Spin Level Scheme of Odd-Odd 140Pr Nucleus

High spin level structure of doubly odd nucleus ¹⁴⁰Pr has been investigated using in beam γ spectroscopy techniques by means of the ¹³⁰Te(¹⁴N,4n)¹⁴⁰Pr reaction at beam energy of 58MeV.γ-ray excitation functions,γ-ray singles and γ-γ-t coincidences were measured experimentally.The level scheme of ¹⁴⁰Pr,including 27 new levels and 42 new γ-rays,was established for the first time up to 4717.3keV in excitation energy.Based on the measured γ-ray anisotropies,spin values were suggested to the levels in ¹⁴⁰Pr.Several quasiparticle configurations are discussed on the basis of systematics in the neighboring odd odd nuclei,and interpreted qualitatively.     

Collective Bands in Neutron-Rich 107Ru Nucleus

Through measuring prompt γ-rays emitted in the spontaneous fission of ²⁵²Cf with higher statistic data,level structures in the neutron rich,odd A¹⁰⁷Ru nucleus have been re investigated. New level scheme of ¹⁰⁷Ru has been established. The ground state band was extended with spin up to 27 2h. The νh_11/2 band structure observed in our previous publication has been confirmed and extended. These results clear up the inconsistence between our earlier work and results from another experiment published recently. A collective band built on (9/2^－) level has been observed for the first time. Some important features of the level scheme have been discussed.     

The absorption of π− at rest on complex nuclei

The energy, width and intensity of prompt nuclear γ-rays following capture of π^? at rest by ⁹Be, ¹⁰B, ¹⁶O, ¹⁹F, ³¹P, Ca and ⁹³Nb were measured with a Ge(Li) detector. In most cases it was possible to identify the final nucleus from the energy of the observed γ-ray. Using the measured Doppler broadening of γ-rays from short-lived states, the vector sum momentum distribution of the emitted nucleons was calculated. From the measured γ-ray intensities, isotopic yields were deduced corresponding to the removal of from 1 to 12 nucleons. The average number of removed nucleons changes from 3.0 to 5.5 as the target varies from ¹⁶O to ⁴⁰Ca. A comparison was made with data from both spallation reactions and multi-nucleon pickup reactions.     

A study of the de-excitation of primary fission fragments from the neutron-induced fission of 235U

Two separate three-dimensional experiments have been performed in which the energies of coincident fragment pairs and γ-rays or internal conversion electrons, emitted within ≈ 1.6 nsec of the thermal-neutron-induced fission of ²³⁵U, were recorded event by event. The fragment kinetic energies were used for mass identification. The self-consistency of the values of electron energy, γ-ray energy and fragment charge, and its agreement with X-ray selection data, were used to identify the atomic numbers of the fragments. The analysis of the γ-ray and conversion electron spectra resulted in the assignment of many transitions to new isotopes as well as improvement in or confirmation of many assignments from the ²⁵²Cf spontaneous fission data. Limited information on the multipolarities of the transitions in even nuclei is presented. The relative yield of electrons per fragment indicates softness to deformation in the mass region 100–110. Data are presented supporting the assignment of a 193 keV transition as the 2⁺ → 0⁺ transition in ⁹⁸Sr. An examination of the 2⁺ → 0⁺ level systematics of neighbouring even nuclei suggests a transition from vibrational to rotational behaviour in the light fragments between neutron numbers 58 and 60.     

Evaluation of the 1077 keV γ-ray emission probability from 68Ga decay

⁶⁸Ga decays to the excited states of ⁶⁸Zn through the electron capture decay mode. New recommended values for the emission probability of 1077 keV γ-ray given by the ENSDF and DDEP databases all use data from absolute measurements. In 2011, JIANG Li-Yang deduced a new value for 1077 keV γ-ray emission probability by measuring the ⁶⁹Ga(n,2n) ⁶⁸Ga reaction cross section. The new value is about 20% lower than values obtained from previous absolute measurements and evaluations. In this paper, the discrepancies among the measurements and evaluations are analyzed carefully and the new values are re-recommended. Our recommended value for the emission probability of 1077 keV γ-ray is (2.72± 0.16)%.     

Identification of a new isotope154Pr

The heaviest isotope of praseodymium,¹⁵⁴Pr, has been identified for the first time by γ-ray measurements of mass-separated activities obtained from a He-jet type on-line isotope separator for fission products of²³⁵U. The Nd-K X-rays and 10 γ-rays have been assigned to be generated by the β-decay of¹⁵⁴Pr. The half-life has been determined to be 2.3(1) s, which is not far from the theoretical predictions of 1.5 s and 4.0 s.     

Gamma-rays following muon capture

The energy and intensity of γ-rays following capture of muons by Si, Ti, Mn, Fe, Co, Ni, Y, Ag and Au were measured with a Ge(Li) detector. In most cases it was possible to identify the final nucleus from the energy of the γ-ray observed. In general, this showed that one or more neutrons were emitted in the initial stages of de-excitation. In the case of capture by Fe and Ni the evidence pointed to a mode of de-excitation by the emission of a proton and a neutron and in Ti by γ-rays alone. In these measurements it was possible to account for as much as 60% of the muons captured in Co, less in the other elements, and only 15 % in Si.     

Evidence for intruder states in111Rh

Levels in¹¹¹Rh have been investigated via the γ -rays following the β^?-decay of 2.1 s¹¹¹Ru. The Ru activity was produced in the fission of²⁴⁹Cf and separated chemically from the fission product mixture. The emitted γ-rays were studied by γ singles and γ(t) coincidence measurements. Evidence for intruder states in¹¹¹Rh has been obtained. Their properties are discussed and compared with those in the lighter Rh isotopes.     

Mechanisms for the appearance of prescission γ-rays and their related T-odd asymmetries

Conditions for the appearance and observation of prescission γ-rays emitted by a fissioning nucleus before its separation into fission fragments were investigated within the quantum theory of fission. It was demonstrated that these conditions can be fulfilled in the γ-decay of giant electric isovector dipole resonances in a fissioning nucleus that become excited due to the nonadiabaticity of the collective deformation motion of the nucleus at the final stages of its prefission evolution. Angular and energy distributions of prescission γ-rays emitted by unpolarized fissioning nuclei were analyzed. Characteristics of T-odd asymmetries in angular distributions of prescission γ-rays were investigated for fission of unpolarized target nuclei induced by polarized cold neutrons, and these correlations were shown to be similar in nature to the T-odd ROT correlations earlier found for α-particles emitted in ternary nuclear fission.     

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 (3n and 4n 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.