In-beam Gamma-ray Spectroscopy Research Articles

Dissipative Reactions with Intermediate-Energy Beams: A Novel Approach to Populate Complex-Structure States in Rare Isotopes.

A novel pathway for the formation of multiparticle-multihole excited states in rare isotopes is reported from highly energy- and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of ^{38}Ca on a Be target. The negative-parity, complex-structure final states in ^{38}Ca are observed following the in-beam γ-ray spectroscopy of events in the ^{9}Be(^{38}Ca,^{38}Ca+γ)X reaction in which the scattered projectile loses longitudinal momentum of order Δp_{||}=700 MeV/c. The characteristics of the observed final states are discussed and found to be consistent with the formation of excited states involving the rearrangement of multiple nucleons in a single, highly energetic projectile-target collision. Unlike the far-less-dissipative, surface-grazing reactions usually exploited for the in-beam γ-ray spectroscopy of rare isotopes, these more energetic collisions appear to offer a practical pathway to nuclear-structure studies of more complex multiparticle configurations in rare isotopes-final states conventionally thought to be out of reach with high-luminosity fast-beam-induced reactions.

Spectroscopy of Cd98 by two-nucleon removal from In100

Low-lying states of Cd-98 have been populated by the two-nucleon removal reaction (In-100, Cd-98+gamma) and studied using in-beam gamma-ray spectroscopy at the Radioactive Isotope Beam Factory at RIKEN. Two new gamma transitions were identified and assigned as decays from a previously unknown state. This state is suggested to be based on a pi 1g(/9/2)(-1)2p(1/2)(-2) configuration with J(pi) = 5(-). The present observation extends the systematics of the excitation energies of the first 5(-) state in N = 50 isotones toward Sn-100. The determined energy of the 5(- )state in Cd-98 continues a smooth trend along the N = 50 isotones. The systematics are compared with shell-model calculations in different model spaces. Good agreement is achieved when considering a model space consisting of the pi(1f(5/2), 2p(3/2), 2p(1/2), 1g(9/2)) orbitals. The calculations with a smaller model space omitting the orbitals below the Z = 38 subshell could not reproduce the experimental energy difference between the ground and first 5(-) states in N = 50 isotones, because proton excitations across Z = 38 subshell yield a large amount of correlation energy that lowers the ground states. (Less)

UCGretina geant4 simulation of the GRETINA Gamma-Ray Energy Tracking Array

UCGretina, a geant4 simulation of the GRETINA gamma-ray tracking array of highly-segmented high-purity germanium detectors is described. We have developed a model of the array, in particular of the Quad Module and the capsules, that gives good agreement between simulated and measured photopeak efficiencies over a broad range of gamma-ray energies and reproduces the shape of the measured Compton continuum. Both of these features are needed in order to accurately extract gamma-ray yields from spectra collected in in-beam gamma-ray spectroscopy measurements with beams traveling at v∕c≳0.3 at the National Superconducting Cyclotron Laboratory and the Facility for Rare Isotope Beams. In the process of developing the model, we determined that millimeter-scale layers of passive germanium surrounding the active volumes of the simulated crystals must be included in order to reproduce measured photopeak efficiencies. We adopted a simple model of effective passive layers and developed heuristic methods of determining passive-layer thicknesses by comparison of simulations and measurements for a single crystal and for the full array. Prospects for future development of the model are discussed.

Fabrication of the gold (Au) backed [formula omitted] targets for in-beam gamma-ray spectroscopy

The isotopically enriched T130e targets of ~ 1.2 mg/cm2 thickness were fabricated on 4–6 mg/cm2 gold (Au) backing. The vapor deposition method was employed using the resistive heating technique. To minimize the material consumption and increase the deposition efficiency, an indigenously developed graphite crucible with 1 mm opening was used to deposit T130e metal powder at a minimum source to substrate distance of 2 cm. A heat shield was placed at 1 cm above the graphite crucible to dissipate the heat. The fabrication of these targets were successfully achieved by using 10 mg of metal powder in each evaporation. The fabricated targets were characterized by using X-ray Fluorescence (XRF) measurements, Energy Dispersive X-ray Spectroscopy (EDS) and Scanning Electron Microscope (SEM). An experiment to study the nuclear structure via in-beam γ-ray spectroscopy was preformed using one of the fabricated T130e targets. The T130e(B9e,xn) reaction was used to populate the intermediate and high-spin states of Ba isotopes with B9e beam at 36- and 48 MeV lab energy. The relative percentage (%) cross-sections were measured for the B134−136a nuclei and found to be in agreement with the PACE4 calculations.

Spectroscopy and lifetime measurements near the proton drip line: P26,27,28

We report on the observation of excited states in the neutron-deficient phosphorus isotopes $^{26,27,28}$P via in-beam gamma-ray spectroscopy with both high-efficiency and high-resolution detector arrays. In $^{26}$P, a previously-unobserved level has been identified at 244(3) keV, two new measurements of the astrophysically-important 3/2$^+$ resonance in $^{27}$P have been performed, gamma decays have been assigned to the proton-unbound levels at 2216 keV and 2483 keV in $^{28}$P, and the gamma-ray lineshape method has been used to make the first determination of the lifetimes of the two lowest-lying excited states in $^{28}$P. The expected Thomas-Ehrman shifts were calculated and applied to levels in the mirror nuclei. The resulting level energies from this procedure were then compared with the energies of known states in $^{26,27,28}$P.

In-beam study of 253No using the SAGE spectrometer

.The heavy actinide nucleus 253No (Z = 102) was studied using the (S)ilicon (A)nd (Ge)rmanium (SAGE) spectrometer allowing simultaneous in-beam gamma-ray and conversion electron spectroscopy at the accelerator laboratory of the University of Jyväskylä. Using the recoil-tagging technique, gamma-electron coincidences have allowed for the extension of the level scheme in the lower-spin region of the yrast band. In addition, internal conversion coefficient (ICC) measurements to establish the multipolarity of transitions have been performed. Measurement of the interband-intraband branching ratios supports the assignment of the Nilsson band-head configuration 9/2^{-}[734] assigned in previous studies. The study shows the viability of combined in-beam electron and gamma-ray spectroscopy down to μb cross sections.

Observation of aγband based on a two-quasiparticle configuration inGe70

The structure of $^{70}$Ge has been studied through in-beam gamma ray spectroscopy. A new band structure is identified that leads to forking of the ground-state band into two excited bands. Band structures have been investigated using the microscopic triaxial projected shell model approach. The observed forking is demonstrated to result from almost simultaneous band crossing of the two neutron aligned and the \gamma-band built on this two-quasiparticle configuration with the ground-state band.

Spectroscopy of Actinide Nuclei – Perspectives with Position Sensitive HPGe Detectors

Recent advances in in-beam gamma-ray spectroscopy of actinide nuclei are based on highly efficient arrays of escape-suppressed spectrometers. The sensitivity of these detector arrays is greatly enhanced by the combination with powerful mass separators or particle detector systems. This technique is demonstrated by an experiment to investigate excited states in 234U after the one-neutron-transfer reaction 235U(d,t). In coincidence with the outgoing tritons, γ-rays were detected with the highly efficient MINIBALL spectrometer. In the near future an even enhanced sensitivity will be achieved by utilizing position sensitive HPGe detectors which will exploit the novel detection method of gamma-ray energy tracking in electrically segmented germanium detectors. An example for this novel approach is the investigation neutron-rich actinide Th and U nuclei after multi nucleon transfer reactions employing the AGATA demonstrator and PRISMA setup at LNL, Italy. A primary 136Xe beam hitting a 238U target was used to produce the nuclei of interest. Beam-like reaction products after neutron transfer were selected by the PRISMA spectrometer. Coincident γ-rays from excited states in beam and target like particles were measured with the position sensitive AGATA HPGe detectors. Improved Doppler correction and quality of the γ-spectra is based on the novel γ-ray tracking technique, which was successfully exploited in this region.

Collectivity evolution in the neutron-rich Pd isotopes toward theN=82shell closure

The neutron-rich, even-even 122,124,126Pd isotopes has been studied via in-beam gamma-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Excited states at 499(9), 590(11), and 686(17) keV were found in the three isotopes, which we assign to the respective 2+ -> 0+ decays. In addition, a candidate for the 4+ state at 1164(20) keV was observed in 122Pd. The resulting Ex(2+) systematics are essentially similar to those of the Xe (Z=54) isotopic chain and theoretical prediction by IBM-2, suggesting no serious shell quenching in the Pd isotopes in the vicinity of N=82.

Pushing In-Beam Gamma-Ray Spectroscopy to the Shores of the Island of Stability

One hundred years after the discovery of the atomic nucleus by Ernest Rutherford, the nuclear chart has been extended far from stability, providing more and more stringent tests for nuclear models. Nuclei at the extremes of the chart are produced with extremely low production cross-sections and one has to ingeniously upgrade the best experimental devices in order to reduce the observational limits.

Nuclear structure of30S and its implications for nucleosynthesis in classical novae

The uncertainty in the 29P(p,gamma)30S reaction rate over the temperature range of 0.1 - 1.3 GK was previously determined to span ~4 orders of magnitude due to the uncertain location of two previously unobserved 3+ and 2+ resonances in the 4.7 - 4.8 MeV excitation region in 30S. Therefore, the abundances of silicon isotopes synthesized in novae, which are relevant for the identification of presolar grains of putative nova origin, were uncertain by a factor of 3. To investigate the level structure of 30S above the proton threshold (4394.9(7) keV), a charged-particle spectroscopy and an in-beam gamma-ray spectroscopy experiments were performed. Differential cross sections of the 32S(p,t)30S reaction were measured at 34.5 MeV. Distorted wave Born approximation calculations were performed to constrain the spin-parity assignments of the observed levels. An energy level scheme was deduced from gamma-gamma coincidence measurements using the 28Si(3He,n-gamma)30S reaction. Spin-parity assignments based on measurements of gamma-ray angular distributions and gamma-gamma directional correlation from oriented nuclei were made for most of the observed levels of 30S. As a result, the resonance energies corresponding to the excited states in 4.5 MeV - 6 MeV region, including the two astrophysically important states predicted previously, are measured with significantly better precision than before. The uncertainty in the rate of the 29P(p,gamma)30S reaction is substantially reduced over the temperature range of interest. Finally, the influence of this rate on the abundance ratios of silicon isotopes synthesized in novae are obtained via 1D hydrodynamic nova simulations.

High-Spin Level Scheme of 120I

Excited states in 120I are investigated utilizing in-beam gamma-ray spectroscopy following the 114Cd(11B,5n) reaction at 70 MeV bombarding energy. A level scheme is constructed with states up to 9 MeV. The levels are classified into several bands and their possible configurations at low spins are discussed briefly Based on the linking transitions between different bands at higher spins, the excitation energy of the previously reported T1/2 = 53 min isomer is ascertained to be 72 keV. Evidence showing the existence of an isomer at an excitation energy of 6.4 MeV is presented.

Combined in-beam gamma-ray and conversion electron spectroscopy with radioactive ion beams

In-beam gamma-ray and electron spectroscopy have been widely used as tools to study the broad variety of phenomena in nuclear structure. The SPEDE spectrometer is a new device to be used in conjunction with the MINIBALL germanium detector array to enable the detection of internal conversion electrons in coincidence with gamma rays from de-exciting nuclei in radioactive ion beam experiments at the upcoming HIE-ISOLDE facility at CERN, Switzerland. Geant4 simulations were carried out in order to optimise the design and segmentation of the silicon detector to achieve good energy resolution and performance.

In-beam gamma-ray spectroscopy at the RIBF

At the Radioactive Isotope Beam Factory stable primary beams are accelerated up to 345 MeV/u and incident on a primary target to produce secondary cocktail beams with the fragment separator BigRIPS ranging from the lightest nuclei up to the uranium region. For in-beam γ-ray spectroscopy, the secondary beam impinge on a reaction target at energies between 100 and 300 MeV/u. Reaction residues are identified with the ZeroDegree spectrometer and γ-rays detected with the NaI(Tl) based DALI2 array. This conference paper outlines the experimental setup and presents recent exemplary results.

Various Isomers in Doubly Odd I Isotopes

Isomeric states (Isomers) in odd-odd (120)I (122)I, (124)I nuclei have been investigated using in-beam gamma-ray spectroscopy with the (118)Sn((6)Li,4n), (120)Sn((7)Li,5n), and (122)Sn((7)Li,(5)n) reactions at E(lab) = 48, 58, and 54 MeV, respectively. Several isomers with half-lives ranging from a few nanoseconds to tens of microseconds have been identified for the first time and the absolute excited energies have been established by linking the low- and high-spin levels through the feeding and decay of the isomers in these nuclei. The properties of the isomers have been discussed in terms of spin traps and high-K isomerism based on two-quasiparticle configurations. The systematics for the absolute excitation energies of the J(pi) = 7(-) and 8(-) high-K isomers based on the pi g(9/2)circle times vh(11/2) configuration are presented for (116)I to (124)I and compared with the 9/2(+) states in the neighboring odd-A I isotopes.

Spectroscopy of actinide and transactinide nuclei

Abstract The role of isomers for nuclei with Z ≥ 100 is discussed. Recent advances in experimental instrumentation leading to combined in-beam gamma ray and conversion electron spectroscopy are discussed. The rotational spectra of nuclei with Z ≥ 94 and their moments of inertia are discussed. Examples for in-beam spectroscopy leading to the discovery and identification of isomers are given in 248,250Fm. Here some attention is given to the assignment of nuclear configurations from g-factors measured via the branching ratios of coupled bands built on the isomers. A full list of the longest lived isomers in nuclei with Z ≥ 82 is given.

In-beam gamma-ray spectroscopy and shell-model description of 85,86Y isotopes

The nuclei 85Y and 86Y were studied in two series of experiments conducted by using the heavy-ion reactions 76Ge( 14N, 4n) 86Y and 52Cr( 37Cl, 2p xn) 85,86Y. The analysis of double and triple coincidences of γ rays along with the investigation of angular distributions led to the extension of the level schemes of the 85Y and 86Y nuclei to spins and excitation energies higher than observed in previous experiments. In addition, a few new levels with low spins and excitation energies were identified in both nuclei. In order to better understand the nuclear structure of these isotopes, shell-model calculations were carried out considering a valence space formed by the proton and neutron orbitals f 5 / 2 , p 3 / 2 , p 1 / 2 , and g 9 / 2 .

OBSERVATION OF FOUR-QUASIPARTICLE BAND AND SIGNATURE SPLITTING FEATURES IN 160Tm

High-spin states of 160 Tm are studied via in-beam gamma-ray spectroscopy. Previously known rotational bands are confirmed. These bands feed into low-lying states which are enriched in this work. A four-quasiparticle band with configuration of πg7/2 ⊗ νh9/2 ⊗ ν(i13/2)2 is established. Signature splitting features of πh11/2⊗νi13/2 and πh11/2⊗νh9/2 bands are presented.

LaBr 3:Ce scintillators for in-beam gamma-ray spectroscopy with fast beams of rare isotopes

The scintillator material LaBr 3:Ce, which was discovered in 2001, has grown in popularity for a variety of applications in homeland security and nuclear medicine. In this paper we investigate its use for nuclear science spectroscopy applications in the context of in-beam gamma-ray spectroscopy with fast ion beams. In such experiments, the Doppler broadening of the gamma-ray line measured with a finite-size detector in the laboratory fundamentally limits the achievable energy resolution. For a typical experiment this effect is of the order of 3%. With the intrinsic energy resolution of LaBr 3:Ce better than 3% FWHM at 662 keV, and its favorable stopping power, it could be a nearly ideal scintillator for gamma-ray spectroscopy applications with fast beams. Results from in-beam gamma-ray spectroscopy measurements using two LaBr 3:Ce scintillators are presented and compared to results from segmented germanium detectors. Results from these measurements suggest that LaBr 3:Ce based scintillation arrays can provide a viable tool for many experiments. Additionally, we show that the excellent timing properties of LaBr 3:Ce can significantly reduce background events in the gamma-ray spectra.

In-beam γ-ray spectroscopy and inclusive two-proton knockout cross section measurements atN≈40

In-beam gamma-ray spectroscopy of Fe-66,Fe-68 following intermediate-energy one- and two-proton knockout from cobalt and nickel secondary beams has been performed at the National Superconducting Cyclotron Laboratory. New transitions have been observed in Fe-66 and Fe-68. This is the first observation of gamma-ray transitions in Fe-68. In addition, Cr-64 was produced using the Be-9(Fe-66,Cr-64)X two-proton knockout reaction. An unexpectedly low inclusive cross section is observed for this reaction, an order of magnitude smaller than for the Be-9(Ni-68,Fe-66)X reaction. This observation is discussed in terms of a significant structural difference between Fe-66 and Cr-64 and considerable admixtures of nu(pf)(n-2) (g(9/2))(+2) configurations in the ground and excited states of Cr-64 at N = 40.