Gamma decay of the giant dipole resonance and feeding of superdeformed states in 143Eu

The γ decay of the giant dipole resonance (GDR) built on excited nuclear states has been measured in coincidence with the low-energy γ discrete transitions for the nucleus ¹⁴³Eu. The reaction used was ¹¹⁰Pd(³⁷Cl, 4n)¹⁴³Eu at a beam energy of 165 MeV. The EUROBALL spectrometer (for the measurement of discrete γ transitions) coupled with the HECTOR array (for high-energy γ-ray detection) has been used. The high-energy γ-ray spectrum in coincidence with superdeformed (SD) discrete transitions of ¹⁴³Eu shows an “excess” between 9–12 MeV if compared with the one associated to cascades which do not pass through the SD configurations. Such an “excess” is in the energy region where one expects the low-energy component of the GDR strength function built on a SD state. The measured intensity can be reproduced by the statistical model assuming that the superdeformation survives only few MeV above the yrast line. A similar and consistent scenario has also been obtained by comparing the high-energy γ-ray spectra of ¹⁴³Eu in coincidence with its spherical (which is fed by the SD configuration) and its triaxial configuration (which is bypassed by the decay of the SD states).     

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.     

X-ray generation from slanting laser–Compton scattering for future energy-tunable Shanghai Laser Electron Gamma Source

There is great interest in the generation of energy-tunable, bright, short-pulse X/γ-ray sources, which are required in various research fields. Laser–Compton scattering (LCS) is considered to be one of the most promising methods to implement this kind of X/γ-ray source. At the 100-MeV LINAC of the Shanghai Institute of Applied Physics, a 2-J, 8-ns, 1064-nm, Q-switched Nd:YAG laser is brought to a slanting collision at 40° (44°) with an 112-MeV, 0.9-ns (rms) relativistic electron beam. We measured the LCS X-ray energy spectrum with a peak energy of 31.73±0.22_stat±1.64_syst keV and a peak width (rms) of 0.74±0.26_stat±0.03_syst keV. This preliminary investigation was carried out to understand the feasibility of developing an energy-tunable X/γ-ray source. Based on this study, the future Shanghai Laser Electron Gamma Source (SLEGS) at the Shanghai Synchrotron Radiation Facility (SSRF) can be constructed to be not only an energy-tunable γ-ray source by guiding the laser incident angle from laser–Compton scattering, but also a high flux (～10¹⁰ photons/s or even higher) γ-ray source by adding a laser super-cavity.     

Evidence for neutron capture through doorway states in29Si

Theγ-ray spectra following neutron capture in silicon have teen recorded in the neutron energy range 2.7–6.2 MeV and partial cross sections forγ-rays to the 2s_1/2 ground state and 1d_3/2 first excited states in²⁹Si determined. The results indicate considerable fluctuations with neutron energy with a prominent resonance peak at 4.6 MeV in the (n,γ _o) cross section. The existence of fluctuations is predicted in a recent theoretical calculation based on a model designed to include single-particle resonances in nuclear reaction processes.     

Spectroscopy of highly excited states in29Si

Particleγ-ray coincidences have been measured in the²⁸Si (d,pγ) reaction at 6.5 and 7 MeV bombarding energy, in the²⁶Mg (α,nγ) reaction at 12, 14 and 15 MeV, and in the²⁷A1 (τ,pγ) reaction at 9 MeV. Theγ-decay has been observed for all bound states of²⁹Si and for 56 unbound states up to 12,960 KeV excitation energy. Particleγ-ray angular correlations were measured in the²⁸Si (d,pγ) reaction at 6.5 MeV and in the²⁶Mg (α,nγ) reaction at 12 MeV. Spin (-parity) assignments or restrictions were obtained for nearly all bound states and some high-spin states above the binding energy. The assignment of mirror levels in²⁹Si and²⁹P has been extended to 8.2 MeV excitation energy. The excitation energies of 41 positive-parity states are reproduced by shell model calculations. The possible existence of aK ^π=5/2⁺ band with prolate deformation is discussed.     

Invariant Mass Spectroscopy of 23O via the (p, p′) Reaction in Inverse Kinematics

The neutron-rich oxygen isotope ²³O has been investigated via proton inelastic scattering on a liquid hydrogen target at 63.5 MeV/nucleon. The invariant mass method in inverse kinematics was employed to reconstruct the energy spectrum. A sharp resonance state was observed at 42(5) keV above the neutron threshold. A comparison of the differential cross section leading to this state with a microscopic DWBA calculation allowed a J ^π assignment of 5/2⁺ for this state.     

Observation of a spin 4 neutral meson with 2 GeV mass decaying in π° π°

The invariant mass spectrum of neutral meson states from π^?p interactions at 40 GeV/c incident momentum has been investigated in a high statistics experiment performed at the 70 GeV IHEP accelerator. To detect the high energy photons coming from the produced neutral states, a hodoscope spectrometer with a computer on-line was used. A clear structure on the mass spectrum of dipions produced in the reaction π^?p→π°π°n is observed at 2 GeV. The decay angular distributions show in this mass region the variation with mass typical of a state with a spin J = 4. The mass of the observed meson is found to be M = (2020±30)MeV and the estimate of the full width is (180±60) MeV.     

Competition between few nucleon emission andγ-ray deexcitation for the reaction systems65Cu+87Rb and40Ar+110Pd

Bothγ-ray and neutron emission have been studied for the reaction systems⁶⁵Cu(237MeV) +⁸⁷Rb→¹⁵²Dy^* and⁴⁰Ar(158MeV)+¹¹⁰Pd→¹⁵⁰Gd^*. By using a sum spectrometer in coincidence with neutron counters, Ge(Li) or Nal detectors, we have measured the totalγ-ray energy and the average totalγ-multiplicity distributions as well as the neutron spectra for various exit channels. These measurements provide strong evidence for thermal equilibrium in reactions involving a small number of emitted neutrons (i.e.⁸⁷Rb(⁶⁵Cu,n or 2n)) at rather high excitation energy (～54MeV). This statistical emission of only a few neutrons is controlled by very strong y-ray competition: theγ-entry line is found not to be parallel to the yrast line. Instead the energy gap is about 8MeV for J～27? and rises to at least 13MeV for J～36?. There are some indications that the main part of the energy from this gap is removed by statisticalγ-ray cascades. The main features of the experimental data for both entrance channels are well reproduced by statistical model calculations with proper attention to the yrast line position and an adjustement of the dipoleγ-ray normalization coefficient. It is conceivable that the y-ray enhancement that we introduce may be related to a lack of knowledge of the absolute level densities at high energy and spin, or possibly to the presence of new or additional degrees of freedom that may enter into the competition between neutron andγ-ray emission.     

Energy levels,decay scheme, γ-ray branching ratios and lifetimes in 55Fe from the 55Mn(p,nγ)55Fe reaction

The reaction ⁵⁵Mn(p, nγ)⁵⁵Fe has been studied at E_p = 4.0 and 6.0 MeV using a pulsed beam. From the experiments at 6.0 MeV, the energy levels of ⁵⁵Fe up to an excitation of 3810 keV, their decay scheme and the γ-ray branching ratios have been determined. Levels have been identified for the first time in the (p, nγ) reaction. The results have been compared with those available from the literature. From the 4.0 MeV experiment, the mean lifetime of the 1408 keV level has been determined to be 142.7 ± 6.6 ps by direct timing techniques.     

Die Spektren der Neutronen aus den Reaktionen C12(γ,n)C11 und O16(γ,n)O15

The (γ, n)-reactions on carbon and oxygen were studied using the 30,5 MeV brems-strahlung beam of the Heidelberg betatron. The photoneutrons were detected via the recoil protons in a stilbene scintillator. The pulse height spectrum of scintillations produced in the stilbene crystal by the recoil protons is analyzed to yield the incident neutron energy distribution. Theγ-ray and electron background was strongly reduced through pulse shape discrimination. To test the apparatus the spectrum of the neutrons from a Po-α-Be source was measured. The energy spectrum of the photo-neutrons from O¹⁶ shows clearly two peaks at excitation energies of 22,4 and 24,4 MeV also seen in the photo-proton spectra. The energy distribution of the neutrons from C¹² (γ, n) too contains indication of structure already known from the proton spectra from C¹² (γ, p). The derived cross sections of C¹² (γ, n ₀) and O¹⁶ (γ, n ₀) coincide as well in their shape as in their absolute magnitude with the corresponding (γ, p ₀)-cross sections. In both C¹² (γ, n) and O¹⁶ (γ, n)-reaction the neutrons seem to leave the final nucleus mainly in its ground state.     

Measurement of the average γ-ray multiplicity and the mean γ-energy in the photon cascades following the reaction154Sm(12C,4n)162Er

By unfolding theγ-γ coincidence spectra from the reaction¹⁵⁴Sm(¹²C, 4n)¹⁶²Er at a beam energy of 64 MeV the mean energy of the continuousγ-ray spectrum feeding the ground state rotational band was determined as about 1.2 MeV. The averageγ-ray multiplicity \(\bar v\) including the rotational band transitions was measured for each of the levelsJ through which the cascade passed. The values for \(\bar v\) were found to increase regularly from \(\bar v = 8\) forJ=4 to \(\bar v = 14\) forJ=18.     

Cross section of metastable state population in the photoneutron decay of the E1 giant resonance for 110Pd, 112Cd, and 113In nuclei

The cross sections of isomeric state excitation in the ¹¹⁰Pd(γ, n)^109m Pd, ¹¹²Cd(γ, n)^111m Cd, and ¹¹³In(γ, n)^112m In reactions have been investigated. The measurements were performed in the γ-ray energy range 9–18 MeV with a step ΔE = 0.5 MeV. The dependences of the isomeric ratios on the γ-ray energy are obtained. The experimental data are compared with the results of calculations within the cascade-evaporation model.     

Excitation of the dynamical dipole in the charge asymmetric reaction O + Sn

The γ-ray emission from the dynamical dipole formed in heavy-ion collisions during the process leading to fusion was measured for the N/Z asymmetric reaction ¹⁶O + ¹¹⁶Sn at beam energies of 8.1 and 15.6 MeV/nucleon. High-energy γ-rays and charged particles were measured in coincidence with the heavy recoiling residual nuclei. The data are compared with those from the N/Z symmetric reaction ⁶⁴Ni + ⁶⁸Zn at bombarding energies of 4.7 and 7.8 MeV/nucleon, leading to the same CN with the same excitation energies as calculated from kinematics. The measured yield of the high-energy γ-rays from the ¹⁶O-induced reaction is found to exceed that of the thermalized CN and the excess yield increases with bombarding energy. The data are in rather good agreement with the predictions for the dynamical dipole emission based on the Boltzmann–Nordheim–Vlasov model. In addition, a comparison with existing data in the same mass region is performed to extract information on the dipole moment dependence.     

In-beam study of127Xe

The level structure of the¹²⁷Xe nucleus has been studied through the¹²⁵Te(α, 2n)¹²⁷Xe reaction atE _α=27 MeV by in-beamγ-ray spectroscopic methods including three-parameterγ-γ-t coincidences, angular distribution and linear polarization as well as lifetime measurements. The results supplemented current information and, in particular, established the existence of a new isomeric state having a half-life of 25±3 ns.     

The Q-value and decay properties of114Ag

The decay of mass separated¹¹⁴Ag has been studied withβ- andγ-ray spectroscopy. The totalβ-decay energy has been accurately determined to (5.16±0.11) MeV using aβγ-coincidence technique. A detailed decay scheme has been constructed.     

Der β-Zerfall des208Tl

An iron-free intermediate-imageβ-ray spectrometer was used to investigate theβ-decay of²⁰⁸T1 in coincidence with the 2.615 MeVγ-ray of²⁰⁸Pb. Fourβ-groups were found, three of which were previously known. The fourth group has an endpoint energy of 0.64±0.03 MeV and an intensity of (4.5±1.5)%. It is supposed that this transition is still complex and feeds a group of levels around the known 4.3 MeV level of²⁰⁸Pb. Noβ-ray of maximum energy 1.032 MeV to the 3.961 MeV level could be found.     

Possible evidence for a narrow isovectorB=2 resonance below pion production threshold

Some evidence is presented for a narrow peak at 1969±2 MeV (FWHM=9±2 MeV) in the missing mass spectrum of the³He(p, d) reaction, with 3 standard deviations. The nature of this state, the mass of which is below the NN threshold, is discussed in connection with structures found in other experiments.     

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 ²⁵² C 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.     

Resonance states of superheavy hydrogen nuclei obtained in transfer reactions with exotic beams

A resonance state situated at 1.8±0.1 and, most likely, another state positioned at 2.7±0.1 MeV above the t+n+n decay threshold were observed in the missing mass energy spectrum of the ⁵H nucleus produced in the reaction ³H(t,p)⁵H. The peak located close to $E_{^5 H} = 1.8$ MeV also was seen in the ⁵H spectrum obtained from the energy distributions of ³H nuclei emitted in the reaction ²H(⁶He,⁵H)³He. The width (Γ_obs ≤ 0.5 MeV) obtained for the two ⁵H resonance states is surprisingly small. A state of ⁴H with E _res = 3.3 MeV and γ² = 2.3 MeV was obtained in the reaction ²H(t,p)⁴H from the proton spectrum.