Deep Inelastic Reactions Research Articles

Fusion-fission studies in theNe20+Ta181reaction atE/A=7.5and 9 MeV/nucleon

Mass and mass resolved angular distribution of the fission products have been measured in the $^{20}\mathrm{Ne}+^{181}\mathrm{Ta}$ reaction at ${E}_{\mathrm{lab}}=150$ and 180 MeV using recoil catcher technique followed by off-line $\ensuremath{\gamma}$-ray spectrometry. Recoil range distribution (RRD) of the evaporation residues (ERs) have been measured at ${E}_{\mathrm{lab}}=180$ MeV. Due to high angular momentum of the fissioning nucleus, variances of the mass distribution in the present system have been found to be higher than those in similar systems involving comparatively less angular momentum. Significant contribution from noncompound nucleus processes such as deep inelastic collisions (DIC) and incomplete fusion reaction (ICF) is expected in the present system due to high angular momentum. The code HICOL predicts that the noncompound nucleus process would result in fission like events. In the present study, experimental fission cross sections are in reasonably good agreement with the calculation of statistical model code PACE2. Measurement of the ER cross section indicates significant contribution from the ICF reaction. The recoil range distributions of the ERs could resolve the complete fusion (CF) and ICF channels and were used to obtain ICF cross section. The experimentally measured ICF cross section accounts for almost entire cross section between ${l}_{\mathrm{crit}}$ and ${l}_{\mathrm{max}}$, indicating ICF reaction to be the dominant non-compound nucleus process at beam energies of the present study. DIC products could not be detected at both the beam energies. At these beam energies, the entrance channel pocket configuration for many collision trajectories is expected to be more elongated compared to the unconditional saddle point and, therefore, significant contribution form noncompound nucleus fission (i.e., fission without passing through the unconditional saddle point) is expected. However, the experimental anisotropies of fission products could be reproduced by the statistical theory calculation after considering the change in the saddle point shape with angular momentum of the fissioning nucleus. No correlation between the angular anisotropy and mass asymmetry of the fission products was observed in the present study.

Yrast structure of neutron-richTi53

{gamma} rays from neutron-rich Ti nuclei in the vicinity of N=32 have been studied at Gammasphere using deep-inelastic reactions induced by {sup 48}Ca beams on thick {sup 208}Pb and {sup 238}U targets. The {sup 53}Ti yrast cascade was identified for the first time, and the location in energy of states with spin up to I = 21/2 was determined. The excitations of {sup 53}Ti provide new tests of effective interactions for full-pf-shell calculations. Comparisons between theory and experiment for the highest spin states in {sup 53}Ti favor the recently proposed GXPF1A interaction, which accounts for the appearance of a neutron subshell closure at N=32 in Ti isotopes as well as for the lack of a similar subshell closure at N=34, as predicted by earlier calculations with another interaction.

Population of yrast states in 191Os using deep-inelastic reactions

Several nuclei in the A ∼ 190 region have been studied following deep-inelastic reactions using a 460 MeV 82Se projectile impinging upon a thick 192Os target. The GASP array (at the Legnaro National Laboratory in Italy) was used to measure the resulting γ-decays. The previously reported near-yrast structure of 191Os is extended to a isomer, at an energy of 2640 keV. Branching ratios for ΔI = 1 and ΔI = 2 transitions in the band have been measured, giving |(gK − gR)/Q0| = 0.022(3) and 0.024(7) for transitions from the and states respectively. These are consistent with the theoretical calculation for the proposed ν11/2+[615] configuration of the band. Nilsson plus BCS calculations reveal that the isomer is likely to have a {ν11/2+[615] ⊗ π11/2−[505] ⊗ π9/2−[514]} configuration with . This yields an implied reduced hindrance of fν= 1.9, in accordance with empirical systematics of K isomers in the A ∼ 180–190 region.

Spectroscopy of Ne, Na and Mg isotopes approaching the Island of Inversion

A beam of 26Mg at 160 MeV incident on a thin target of 150Nd was used to initiate deep-inelastic and multi-nucleon transfer reactions. The gamma-decay of the fragments produced in the reaction was studied using the EUROBALL IV array of escape suppressed Ge detectors at Strasbourg combined with the binary reaction spectrometer used to detect the projectile-like fragments. Preliminary results from the experiment are presented.

Intruder configurations in neutron-rich P and S isotopes

Neutron-rich sdf-shell nuclei have attracted a great deal of international interest, both theoretically and experimentally. There is an increasing interest in using deep-inelastic reactions to obtain spectroscopic information about these exotic nuclei. New results obtained from two such experiments are presented here. The highly sensitive GASP array was used to measure the γ decays of previously unobserved yrast and near yrast states of 34P which were populated when 230 MeV 36S ions were incident on a thick target of 176Yb. In addition, a recent investigation of the spectroscopy of neutron-rich P and S isotopes has been undertaken with 215 MeV 36S ions incident on a thin target of 208Pb using the CLARA Ge detector array in conjunction with the PRISMA magnetic spectrometer. Preliminary results from this experiment are presented here.

Production of beams of neutron-rich nuclei between Ca and Ni using the ion-guide technique

It was shown for the first time that quasi- and deep-inelastic reactions can be successfully incorporated into the conventional Ion-Guide Isotope Separator On-Line (IGISOL) technique. Yields of radioactive projectile-like species such as 62,63Co are about 0.8 ions/s/pnA corresponding to a total IGISOL efficiency of about 0.06%.

Structure of two-, four-, and six-quasiparticle isomers inYb174andK-forbidden decays

The stable nucleus Yb-174 has been studied using deep-inelastic reactions and time-correlated gamma-ray spectroscopy. New intrinsic states assigned include a 370-ns isomer at 1765 keV, which we associate with a predicted K-pi=7(-) two-quasineutron configuration. Analysis of the alignment and in-band properties of its rotational band, identified using time-correlated coincidences, allows characterization of the configuration. The properties of a newly identified rotational band built on the known 830-mu s isomer at 1518 keV support the 6(+), 2-quasineutron configuration assignment proposed previously. The 6(+) band is fed by a four-quasiparticle, K-pi=14(+) isomer at 3699 keV and several higher multiquasiparticle states, including a six-quasiparticle isomer at 6147 keV with K=(22,23). The results are discussed in terms of the states predicted on the basis of multiquasiparticle calculations. The anomalously fast K-forbidden transition strengths from the 14(+) isomer are attributed to either K mixing in the neutron configuration or to random mixing in the high-level-density region. The 7(-) isomer decays are not abnormal, whereas the very hindered E2 transition from the 6(+) isomer to the ground-state band remains unexplained.

New ion-guide for the production of beams of neutron-rich nuclei between Z=20–28

It has been shown for the first time that quasi- and deep-inelastic reactions can be successfully incorporated into the conventional Ion-Guide Isotope Separator On-Line (IGISOL) technique. This is of particular interest for characterizing the decay properties of refractory elements and is applied to neutron-rich nuclei between Z = 20 – 28 . As a first step of this project, the kinematics of quasi- and deep-inelastic reactions, such as 197Au( 65Cu,X)Y, were studied. Based on these studies, a specialized IGISOL target chamber was designed and built. This chamber was tested in on- and off-line conditions at the Jyväskylä IGISOL facility. Yields of radioactive, projectile-like species such as 62, 63Co are about 0.8 ions/s/pnA corresponding to a total IGISOL efficiency of about 0.06%.

Yrast states in N = 3050Ca and51Sc isotones studied with deep-inelastic heavy ion reactions

Yrast states in N = 3050Ca and51Sc isotones studied with deep-inelastic heavy ion reactions

High- K Isomerism

Recent results on the identification of high- K isomers in the Yb-Lu-Hf-Ta region populated using deep-inelastic reactions are presented in the context of the expectations for the formation of multi-quasiparticle states close to and to the right of the stability line. Factors that affect the purity of the K -quantum number include chance degeneracies, which provide an opportunity to extract mixing-matrix elements, and statistical mixing due to the presence of high level densities above the yrast line.

Future Perspectives in Nuclear Structure: From high intensity stable to radioactive nuclear beams

Future perspectives in nuclear structure rely on radioactive nuclear beams as well as on high intensity beams of stable ions. Especially for neutron rich nuclei, deep-inelastic and multi-nucleon transfer reactions can be used to populate yrast and non yrast states. Particularly powerful is here the combination of large acceptance spectrometers with highly segmented γ-detector arrays. Such devices, eventually complemented by large coverage particle detectors, can provide the necessary channel selectivity to identify very rare signals. An example is the CLARA γ-ray detector array coupled with the PRISMA spectrometer at the Legnaro National Laboratories (LNL). The physics aims achievable with this setup will complement studies performed with current radioactive beam (RIB) facilities. With such set-up we have recently investigated the stability of the N=50 shell closure when moving towards more exotic systems. Here the comparison of the experimental data with shell model calculations seems to indicate a persistence of the N=50 shell gap down to Z=32. Future perspectives at LNL are based on an increase in intensity as well as on the availability of heavy ion species. Beams like 136 Xe or 208 Pb, which will be provided by the new PIAVE injector, can be used to drive the multinucleon flux toward the more exotic regions. Moreover a new ISOL facility (SPES) dedicated to the production and acceleration of radioactive neutron rich species is now under development at LNL. It will be based on an high intensity proton and deuteron LINAC. Induced fission fragments will be ionized and then accelerated using the presently existing superconductive LINAC (ALPI). Among the new instrumentation the concept of γ-ray tracking has been recently introduced in nuclear spectroscopy. Detectors based on γ-ray tracking have position resolution capabilities with excellent performances both in efficiency and in achievable Doppler correction. A new γ-ray detector array (AGATA) based on such technique is now under study in a wide european collaboration. 1. Probing Nuclear Structure with Radioactive and Stable ion beams

A SEMI-CLASSICAL ANALYSIS OF THE PROTON SEQUENTIAL EMISSION IN 16O+58Ni DEEP INELASTIC COLLISIONS

We study the 16 O +58 Ni deep inelastic reaction by using coincident charged techniques. Inclusive as well as exclusive data of the C , N , and O fully-damped fragments and their associated light charged particles ( p , d , t , and α-particles) have been collected at the IReS Strasbourg VIVITRON Tandem facility. The velocity distributions of the emitted protons and the associated multiplicity polar plots are analyzed by means of a model which describes simultaneously the nonequilibrium and the evaporative (equilibrated) components of a deep inelastic reaction mechanism. Estimates on polarization phenomena as well as the associated "decay times" of the reaction have been obtained. The hypothesis of a new "fan effect" is proposed for the proton sequential emission in the deep inelastic scattering of 16 O +58 Ni at 8.25 MeV/nucleon.

Development of shell closures atN=32,34. II. Lowest yrast excitations in even-evenTiisotopes from deep-inelastic heavy-ion collisions

Gamma rays from neutron-rich nuclei in the vicinity of $Z=20$, $N=28$ have been studied at Gammasphere using deep-inelastic reactions induced by a $330\text{\ensuremath{-}}\mathrm{MeV}$ $^{48}\mathrm{Ca}$ beam on a thick $^{238}\mathrm{U}$ target. The yrast $\ensuremath{\gamma}$-ray cascade of $^{56}\mathrm{Ti}$ was identified for the first time and the location in energy of the ${2}^{+}$, ${4}^{+}$, and ${6}^{+}$ states was determined. The low-spin $^{56}\mathrm{Ti}$ yrast structure does not support the presence of a subshell closure at $N=34$ as suggested on the basis of other data on nuclei in the region as well as shell model calculations with a recently proposed interaction.

High-spin isomers and three-neutron valence configurations in 211Pb

Deep-inelastic reactions between a beam of 1360 MeV 208Pb ions and a thick 238U target have been used to populate the neutron-rich nucleus 211Pb. The observation of its γ decay has allowed identification of excited states up to the highest spin which can be formed from the three valence neutrons, including identification of three high-spin isomers. Level energies and transition strengths are compared to shell-model calculations with empirical interactions and predictions are made for the expected behaviour of more neutron-rich lead isotopes. The evidence for a possible increase in the neutron effective charge moving away from the N=126 shell gap is evaluated.

High-spin states populated in deep-inelastic reactions

High spin states in the neutron rich 188Os and 190Os nuclei have been populated using the 82Se + 192Os deepinelastic reaction. The level schemes are extended up to spin I <FONT FACE=Symbol>»</FONT>21. The observed new structures are tentatively interpreted as fragments of rotational bands built on multi-quasiparticle configurations.

Nuclear Data Sheets for A = 175

Evaluated spectroscopic data and level schemes from radioactive decay and nuclear reaction studies are presented for all nuclei with mass number A = 175. This evaluation for A = 175 supersedes the earlier one by A.O. Macchiavelli and E. Browne (1993Ma79), published in Nuclear Data Sheets 69, 903 (1993). Highlights of this publication are the following: This evaluation includes the first study (1996Zh03) of 175Er β − decay, which has been used to interpret a predicted ground state doublet in 175Tm. The deep-inelastic reaction studies (1999As05, 1997Le11, and 1997Le10) extended the yrast band of 175Yb to the J π = 41 / 2 − level at 4426 keV. New studies (1994Mi04) of 175Yb β − decay have provided significantly more precise γ-ray intensities. New data (2002Ro17 and 1998To14) from the alpha decay of 179Tl have been used for tentatively assigning spin and parity to 175Au ground and first-excited states. Tentative assignment of 175Hg g.s. is adopted from 176Tl proton decay (2004Ke06).

Evidence for a high-spinβ-decaying isomer inLu177

Nuclei in the $A\ensuremath{\sim}180$ region have been populated and investigated in a series of multinucleon transfer and deep-inelastic reactions involving an $11.4\phantom{\rule{0.3em}{0ex}}\text{MeV}$ per nucleon $^{136}\mathrm{Xe}$ beam produced by the GSI UNILAC accelerator, impinging on a selection of tantalum and tungsten targets. The reaction products were released from a thermal ion source and subsequently mass selected using the GSI on-line mass separator. The unexpectedly high yield of $\ensuremath{\gamma}$ rays associated with the decay of the well established ${K}^{\ensuremath{\pi}}=37∕{2}^{\ensuremath{-}}$,${t}_{1∕2}=51.4\phantom{\rule{0.3em}{0ex}}\mathrm{min}$ isomer in $_{\phantom{\rule{0.3em}{0ex}}72}^{177}\mathrm{Hf}$ and anomalous half-life characteristics associated with this decay lead to these data being interpreted as the ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay of a high-$K$ isomer in the mother nucleus, $^{177}\mathrm{Lu}$. By comparing the experimental findings with the predictions obtained from multi-quasiparticle blocked-BCS-Nilsson calculations, the proposed decay is suggested to be from a ${K}^{\ensuremath{\pi}}=39∕{2}^{\ensuremath{-}}$ five-quasiparticle state in $_{\phantom{\rule{0.3em}{0ex}}71}^{177}\mathrm{Lu}$. A half-life of $7\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{min}$ is determined for this $\ensuremath{\beta}$-decay path which is estimated to have an excitation energy of $\ensuremath{\approx}3.9\phantom{\rule{0.3em}{0ex}}\text{MeV}$ above the $^{177}\mathrm{Lu}$ ground state.

STRUCTURE OF NEUTRON–RICH NUCLEI FROM DEEP–INELASTIC REACTIONS

Neutron rich nuclei have been populated using the 82 Se +192 Os deep-inelastic reaction. New experimental results on 188 W , 188,190 Os target-like nuclei, as well as 74,76,78 Ge beam-like nuclei are presented.

Coupling a CLOVER detector array with the PRISMA magnetic spectrometer

Following the commissioning of the PRISMA large-acceptance spectrometer, installed at the Laboratori Nazionali di Legnaro (LNL), an international nuclear-structure collaboration has started to develop a large $\gamma$ -ray setup to be installed in the target position of the spectrometer. The array is based on the EUROBALL composite CLOVER detectors. In this contribution the CLOVER detector array is described and its expected performance figures discussed. This new setup, by using the high-intensity heavy-ion beams provided by the LNL ALPI linac, will push the study of nuclear structure towards moderately neutron-rich nuclei by means of quasi-elastic and deep inelastic reactions.

Hard-to-reach nuclei studied with deep-inelastic heavy-ion reactions

Spectroscopic studies performed with deep-inelastic heavy-ion reactions are reviewed for two regions of neutron-rich nuclei. The identification of $\nu(h_{11/2})^n$ isomers in nearly complete series of Sn isotopes and the resulting systematic of B(E2) values for isomeric transitions is presented and followed by the discussion of shell model states studied in neutron-rich Te isotopes including the new four neutron-hole isomers identified in 130 Te. Yrast spectroscopy studies of nuclei from the doubly magic 208 Pb region are described by outlining the highest spin states observed in the 208 Pb core nucleus. The E3 transitions observed abundantly in yrast decays are discussed within the framework of particle-octupole vibration coupling and the validity of a simple rule connecting energy shifts of octupole states built on one-particle states with the ones observed for two-particle states is demonstrated.