Compilation of giant electric dipole resonances built on excited states

Over the last several decades, extensive experimental and theoretical work has been done on the giant dipole resonance (GDR) in excited nuclei covering a wide range of temperature (T), angular momentum (J) and nuclear mass. A reasonable stability of the GDR centroid energy and an increase of the GDR width with T (in the range $ \sim 1 - 3$
 MeV) and J are the two well-established results. Some experiments have indicated the saturation of the GDR width at high T . The gradual disappearance of the GDR vibration at much higher T has been observed. Experiments on the Jacobi transition and the GDR built on superdeformed shapes at high rotational frequencies have been reported in a few cases. Theoretical calculations on the damping of the collective dipole vibration, characterised by the GDR width, have been carried out within various models such as the thermal shape fluctuation model and the phonon damping model. These models offer different interpretations of the variation of the GDR width with T and J and have met with varying degrees of success in explaining the experimental data. In this review, the present experimental and theoretical status in this field will be discussed along with the future outlook. The interesting phenomenon of the pre-equilibrium GDR excitation in nuclear reactions will be briefly addressed.

Giant dipole resonance in hot rotating nuclei

High-energy $\ensuremath{\gamma}$ rays in the energy range of $\ensuremath{\sim}4--35\mathrm{MeV}$ were measured in coincidence with a $\ensuremath{\gamma}$-ray multiplicity detector array in the reaction ${}^{28}\mathrm{Si}{+}^{58}\mathrm{Ni}$ at ${E(}^{28}\mathrm{Si})=100$ and 125 MeV. The data were analyzed using the statistical model, and the giant dipole resonance parameters were extracted for various multiplicity windows. The exclusive temperature dependence of the $\ensuremath{\gamma}$-ray strength function has been obtained for a wide range of angular momentum, while the angular momentum dependence has been derived over a restricted range of temperature. The observed increase in the resonance width can be attributed to an increase in temperature. No dependence on angular momentum is observed. The last observation is at variance with the results in the tin region for similar rotational frequencies.

Angular momentum gated giant dipole resonance measurements in the reaction 28 Si + 58 Ni at E ( 28 Si ) = 100 and 125 MeV

A large NaI HIgh energy Gamma Ray Spectrometer (HIGRASP) has been designed and constructed at NSC, New Delhi. The spectrometer has been working quite satisfactorily for the last two years and has achieved the set goals. Detailed design considerations have resulted in very good energy resolution (2.5% at 22.5 MeV) and line shape of the gamma-ray spectrum. Experiments to study giant dipole resonance decay from hot rotating nuclei and nuclear Bremsstrahlung gamma emission have been successfully carried out using this spectrometer. The design, construction and response of the detector is discussed in detail.

A large high-energy gamma-ray spectrometer at NSC

LaBr3:Ce scintillators have recently become commercially available in sizes large enough for measurements of high energy gamma-rays. In this communication, we report our studies on properties and response of large volume square bars ( 2 ′ ′ × 2 ′ ′ × 8 ′ ′ ) of LaBr3:Ce detectors, individually, and in a compact array of four square bars, with gamma-rays up to 22.5 MeV. The properties studied are, uniformity of the crystal, internal radioactivity, energy resolution, timing resolution, linearity of the response and detection efficiencies. The response of the detectors for 22.5 MeV γ -rays produced from  11B( p , γ )12C capture reaction and for 15.1 MeV γ -rays produced from  12C( p , p ′ γ )12C inelastic scattering reaction are studied in detail. The measured absolute efficiencies (both total detection and photo-peak) for 662 keV gamma-rays from  137Cs are compared to those obtained using realistic GEANT4 simulations. The primary aim of the array is to measure high energy gamma-rays (5–50 MeV) produced from the de-excitation of excited Giant Dipole Resonance (GDR) states, radiative capture reactions, nuclear Bremsstrahlung process and inelastic scattering process. The highly satisfactory performance of the array provides the impetus for future efforts toward building a bigger array.

Characterization of a 2 × 2 array of large square bars of LaBr 3 :Ce detectors with γ-rays up to 22.5 MeV

The present paper describes the measurement of high energy gamma rays ({approximately}5{endash}22 MeV) in the reaction {sup 16}O+{sup 181}Ta. Singles measurements were performed at {ital E}({sup 16}O)=84, 89, and 107 MeV and the multiplicity gated measurements along with the angular distributions were made at {ital E}({sup 16}O)=94 MeV. It is seen that the Tl nuclei are nonspherical at the excitation energies and spins relevant for the present work. The angular distribution data can be explained only by incorporating large orientation fluctuations. An attempt has been made to extract information on shape fluctuations directly from the {gamma} spectra. Under the restriction of using a common {open_quote}{open_quote}intrinsic{close_quote}{close_quote} strength function characterized by {ital E}{sub 0} and {Gamma}{sub 0} for the multiplicity gated data sets, a small increase of the effective mean deformation and a drive towards triaxiality ({gamma}{approximately} 30{degree}) with angular momentum is brought out. If this restriction is relaxed, however, the data can be understood also with a smaller value of {gamma}. The singles data can be understood within the same framework with a higher value of {Gamma}{sub 0}. {copyright} {ital 1996 The American Physical Society.}

Study of nuclear shapes in Tl isotopes via excited state giant dipole resonance studies in the reaction 16O+181Ta.

An array of seven hexagonal NaI(Tl) detectors has been set up for measuringγ-ray spectra in the energy region 5 MeV ≤E
 γ ≤ 40 MeV with good accuracy. This is in contrast to earlier set ups which mostly used one large sized (about 10 inchesφ × 15 inches long)NaI(Tl) detector. This set up has been made for the study ofγ decay of GDR based on high spin states and ultra-dipole radiations. The array has been provided with the following features: a) TOF discrimination against neutrons, b) pile up detection and elimination, c) active and passive shielding to cut down background and d) an array of trigger counters for multiplicity dependence measurements. The well known program EGS4 has been used to determine the response of the array forγ-rays in the energy region 5–40 MeV and several test measurements have been carried out to confirm the validity of the calculated response functions. Some typicalγ-ray spectra fromα and16O induced reactions measured at VECC, Calcutta and Pelletron accelerator at TIFR are also shown.

A multi NaI(Tl) detector array for medium energyγ-ray spectroscopy

[gamma]-ray yields in the energy range [similar to]5--35 MeV have been measured in the reactions [alpha]+[sup 197]Au and [sup 159]Tb at [ital E][sub [alpha]]=40 MeV and [alpha]+[sup 197]Au, [sup 116]Sn, [sup nat]Ag, and [sup 59]Co at [ital E][sub [alpha]]=50 MeV at [theta][sub [gamma]]=90[degree]. Angular distribution measurements have been carried out for the heaviest and the lightest target at [ital E][sub [alpha]]=50 MeV. A reasonably good agreement with the statistical model calculations is obtained for photon energies up to the giant dipole resonance region. A nonstatistical component is observed at higher [gamma]-ray energies for all the systems studied. An attempt to understand the production mechanism of the nonstatistical part is made in terms of the potential bremsstrahlung and the incoherent nucleon-nucleon bremsstrahlung processes. While the calculations give the right order of magnitude of the measured cross sections for the highest [gamma]-ray energies, they fail to reproduce the measured angular distributions and the cross sections in the entire [gamma]-ray energy range.

M. K. Sharan

Papers

Study of nuclear shapes in Tl isotopes via excited state giant dipole resonance studies in the reaction 16O+181Ta.

A multi NaI(Tl) detector array for medium energyγ-ray spectroscopy

Ultradipole photon production in 40 and 50 MeV alpha -nucleus collisions.