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Abstract: An interlock system is described that allows volatile targets to be prepared in an evaporator, sealed under vacuum, transported to the -array Gammasphere, and inserted into the reaction chamber for experiments, while maintaining a vacuum better than . The performance of the system is demonstrated for lithium metal targets. A comparison is made between lithium targets covered by a gold evaporation to reduce oxidation and open metal targets transferred in the interlock. [Copyright &y& Elsevier]

Abstract: A new focal plane detector arrangement for the Fragment Mass Analyzer (FMA) has been built and tested at Argonne National Laboratory. This set-up is particularly sensitive for performing Recoil-Isomer Tagging on nuclei with isomeric states with lifetimes in the microsecond range. Recoiling nuclei from fusion–evaporation reactions at the target position are dispersed by their ratio of mass to charge () by the FMA and stopped in low pressure gas (air) at the focal plane. Subsequent gamma decays from isomeric states in the reaction products are observed using Ge detectors. A constant gas flow through the focal plane chamber efficiently removes longer-lived beta-decaying species from sight of the detectors. This set-up has been commissioned successfully with the microsecond isomer in 80Rb, populated via the 52Cr(32S, 3pn) reaction at 135MeV. [Copyright &y& Elsevier]

The segmentation of the electrodes of germanium detectors facilitates gamma-ray imaging and tracking. Replacing the traditional n-type lithium drifted contact is a key to finer segmentation. Amorphous-germanium is a promising alternative contact technology, and large, highly segmented detectors have been fabricated. One factor in adopting any new detector technology is its robustness in hostile environments. Therefore, to explore the effects of neutron damage on position sensitive amorphous-contact germanium gamma-ray detectors and investigate methods for mitigation and repair of damage, two detectors were intentionally exposed to a non-uniform neutron fluence of greater than 4(1) ×10 9 n/cm 2 produced in the 7 Li(p, n) 7 Be reaction at the UMass Lowell Van-de-Graaff accelerator. Post-irradiation tests were made on the counters by varying the electric field, the charge deposition rate, the operating temperature, and utilizing various annealing cycles in order to ascertain the robustness of their performance after irradiation. [ABSTRACT FROM AUTHOR]

Abstract: Cs2LiYCl6:Ce (CLYC) has been demonstrated to be sensitive to thermal neutrons via the 6Li(n, )t reaction, and recently to fast neutrons via the 35Cl(n,p) reaction. The scintillation properties of CLYC have been investigated in more detail to further understand its capabilities. Pulses from thermal neutron, fast neutron, and -ray induced excitations were captured, digitized over a time range, and analyzed to identify the scintillation mechanisms responsible for the observed shapes. Additionally, the timing resolutions of CLYC crystals of different sizes were measured in coincidence with a fast CeBr3 scintillator. The effect of high count rates on fast neutron energy resolution and pulse-shape discrimination was investigated up to 45kHz. [Copyright &y& Elsevier]

Abstract: Large area position-sensitive planar germanium wafers are increasingly being used for a variety of gamma-ray imaging and tracking tasks. Position sensitivity can be achieved through measuring charge collected on orthogonal strip electrodes, and by digital pulse shape analysis. However, the development of this detector technology has been slow, and criteria to measure improved performance have not been well established. We have studied 93 and 88mm square segmented detectors of 20mm thickness made with conventional boron and lithium electrodes, and two 100mm circular segmented detectors of 14mm thickness with amorphous germanium contacts. We have compared these detectors with a planar detector of 15mm thickness. Conventional energy resolution tests of individual strips are insufficient to fully categorize the performance of the position sensitive detectors. We propose some basic tests which can rapidly quantify any segmented detector characteristics, and show potential inadequacies which become important when imaging or tracking is attempted. [Copyright &y& Elsevier]

Abstract: Tracking of gamma-rays in germanium detectors can allow the full reconstruction of interactions, a feature that is useful in many applications. Scrutiny of the kinematics and geometry of gamma-rays which are Compton scattered only once prior to full absorption reveals that there are special cases where even perfect spatial and energy resolution cannot resolve the true interaction sequence and consequently the gamma-ray tracks cannot be unambiguously reconstructed. The photon energy range where this ambiguity exists is from 255keV to about 700keV. This is an energy region of importance for nuclear structure research and a domain where two-point interactions are probable. [Copyright &y& Elsevier]

Understanding the processes which create and destroy 22Na is important for diagnosing classical nova outbursts. Conventional 22Na(p,γ) studies are complicated by the need to employ radioactive targets. In contrast, we have formed the particle-unbound states of interest through the heavy-ion fusion reaction, 12C(12C,n)23Mg and used the Gamma-sphere array to investigate their radiative decay branches. Detailed spectroscopy was possible and the 22Na(p,γ) reaction rate has been re-evaluated. Hydrodynamical calculations incorporating the revised rate indicate a reduction in the yield of 22Na by a factor of four from previous estimates implying a reduction by a factor of two for the maximum detectability distance for 22Na γ rays from novae. [Copyright &y& Elsevier]

Large area, position sensitive planar germanium detectors offer new opportunities in γ -ray spectroscopy. For in-beam studies remediating neutron damage is important. This work studied a mechanically cooled circular planar 9 × 1 cm wafer with orthogonal 16 × 16 amorphous germanium strip electrodes. Before neutron irradiation the wafer was heated in-cryostat to above 373 K for more than 24 h in order to test the robustness of the contacts and the mechanical cooler. No deterioration in performance was observed. The detector was then exposed to controlled doses of 2.2 MeV neutrons, produced from the 7 Li(p,n) 7 Be reaction until substantial damage was observed. Averaged over the surface of the wafer, a flux of 1. 9 (1) × 1 0 9 n/cm 2 was delivered. The detector then survived thermal cycling and 350 K (∼ 77 ° C) annealing for 70 h which substantially reduced hole trapping. pulse-height correction is investigated to further mitigate the neutron damage. Higher temperature annealing at 365 K was not successful. [ABSTRACT FROM AUTHOR]

Electromagnetic quadrupole transition strength is a sensitive probe of the evolution of the structure of nuclei, particularly the competition between collectivity and magicity. We have performed a new lifetime measurement of the 2 1 + state of 12 Be to study the interplay of these phenomena. The lifetime was measured with the Doppler Shift Attenuation Method using the γ -ray detector GRETINA. Excited states of 12 Be were produced via inelastic scattering at 55 MeV/nucleon, using several different targets to control for systematic uncertainties in the stopping powers. The lifetime is determined to be τ = 1.38 ± 0.10 (stat) ± 0.19 (sys) ps, which is about half the previously reported value at twice the precision. The reduced transition strength deduced from this result is B ( E 2 ; 2 1 + → 0 1 + ) = 14.2 ± 1.0 (stat) ± 2.0 (sys) e 2 fm 4 , which supports the quenching of the N = 8 shell gap in 12 Be. [ABSTRACT FROM AUTHOR]

The sensitivity of the position resolution of the gamma-ray tracking array GRETINA to the hole charge-carrier mobility parameter is investigated. The χ 2 results from a fit of averaged signal (“superpulse”) data exhibit a shallow minimum for hole mobilities 15% lower than the currently adopted values. Calibration data on position resolution is analyzed, together with simulations that isolate the hole mobility dependence of signal decomposition from other effects such as electronics cross-talk. The results effectively exclude hole mobility as a dominant parameter for improving the position resolution for reconstruction of gamma-ray interaction points in GRETINA. [ABSTRACT FROM AUTHOR]

A number of 3-quasiparticle isomers have been found and characterised in the odd-mass, neutron-rich, 187 Re, 189 Re and 191 Re nuclei, the latter being four neutrons beyond stability. The decay of the isomers populates states in the rotational bands built upon the 9 / 2 − [ 514 ] Nilsson orbital. These bands exhibit a degree of signature splitting that increases with neutron number. This splitting taken together with measurements of the M 1 / E 2 mixing ratios and with the changes observed in the energy of the gamma-vibrational band coupled to the 9 / 2 − [ 514 ] state, suggests an increase in triaxiality, with γ values of 5°, 18° and 25° deduced in the framework of a particle-rotor model. [ABSTRACT FROM AUTHOR]

Rotation-aligned isomeric states and associated oblate collective sequences are established in even Pt isotopes. Reduced E 2 transition probabilities for the deexcitation of the 12 + isomers indicate an abrupt and unexpected quenching of oblate collectivity around neutron number N = 120 . Structure and shape evolution at high spin in the heaviest stable isotopes is found to be markedly different from observations in the lighter ones. [ABSTRACT FROM AUTHOR]

A new decay-spectroscopy station has been commissioned for experiments with low-energy, fission-fragment radioactive beams from the CARIBU ion source. The new set-up consists of the ‘X-Array’, a highly-efficient array of HPGe clover detectors, and ‘SATURN’ (Scintillator And Tape Using Radioactive Nuclei), a plastic scintillator detector combined with a tape-transport system for detection of β particles and removal of long-lived isobaric decay products. [ABSTRACT FROM AUTHOR]

Abstract: New spectroscopic results for high-spin states in 192Os populated in deep-inelastic reactions include the identification of a 2-ns, 12+ isomeric state at 2865 keV and a 295-ns, 20+ state at 4580 keV and their associated sequences. The structures are interpreted as manifestations of maximal rotation alignment within the neutron and proton shells at oblate deformation. Rotational band members based on the long-lived, isomer are also identified for the first time. Configuration-constrained, potential-energy-surface calculations predict that other prolate multi-quasiparticle high-K states should exist at low energy. [Copyright &y& Elsevier]

Abstract: Cs2LiYCl6 (CLYC) has generated recent interest as a thermal neutron detector due to its excellent pulse-shape discrimination and energy resolution. Here, the capabilities of CLYC as a fast neutron detector and spectrometer are reported. A 1in.×1in. CLYC detector was used to measure the response of mono-energetic neutrons over a range of 0.8–2.0MeV produced via the 7Li(p,n) reaction at the University of Massachusetts Lowell 5.5MV Van de Graaff accelerator. A broad continuum from the 6Li(n, ) reaction was observed, as well as additional peaks below the thermal capture peak. Based on possible reactions in CLYC, the additional peaks are determined to be due to the 35Cl(n,p)35S reaction, with a Q-value of +615keV, and corroborated in simulations using MCNPX. The average resolution of 9% for these peaks makes CLYC a promising candidate for a fast neutron spectrometer. [Copyright &y& Elsevier]

Abstract: A particle- coincidence technique has been used to measure the fusion cross-section in the system 12C+12C at low beam energies. Based on these results, it should be possible to measure this important fusion cross-section down to the 10pb level within a week-long run. [Copyright &y& Elsevier]

Abstract: Deep-inelastic reactions have been used to populate high-spin states in the iridium isotopes. New results include the identification of particularly long-lived three-quasiparticle isomers in 191Ir and 193Ir, with mean-lives of 8.2(7) s and 180(3) μs respectively, decaying into newly identified states of the proton bands and into other structures. Spins and parities of are suggested for both, consistent with coupling of the proton to the two-neutron excitations in the cores. These and other configurations are discussed in the context of configuration constrained potential-energy-surface calculations. All calculated intrinsic states are expected to be associated with triaxial shapes and the extreme isomerism observed is attributed to spin-trapping rather than K-hindrance. [Copyright &y& Elsevier]

Abstract: Yrast and near-yrast states in 30Al and 30Si have been populated to high spin with the fusion–evaporation reaction in inverse kinematics. The level schemes for these two isobars have been extended up to at 9.4 and 15.5 MeV, respectively. Their decay schemes indicate that cross-shell excitations dominate at high spin, where negative-parity structures exist. Positive-parity states are compared to the results of shell-model calculations using the USD, USDA, and USDB effective interactions. The negative-parity levels are compared to predictions of the WBP interaction and the recently-developed WBP-a Hamiltonian, by allowing 1p–1h excitations to fp-shell orbitals. The results suggest that single-neutron excitations to the orbital play a significant role at high spin. [ABSTRACT FROM AUTHOR]

Abstract: Excited states in 124Xe have been studied via the 12C(124Xe, 124Xe∗) Coulomb excitation reaction. Their population cross-sections relative to the state have been determined from the γ-ray yields observed with Gammasphere. More than twenty absolute E2 strengths for seven off-yrast, low-spin states of 124Xe have been deduced for the first time. The absolute values indicate pronounced symmetry, even for the off-yrast states with high quantum number τ, while the symmetry is substantially broken. [Copyright &y& Elsevier]

Abstract: An inverse kinematics Coulomb excitation experiment was performed to obtain absolute E2 and M1 transition strengths in 134Xe. The measured transition strengths indicate that the state of 134Xe is the dominant fragment of the one-phonon mixed-symmetry state. Comparing the energy of the mixed-symmetry state in 134Xe to that of the levels in the isotonic chain indicates that the separation in energy between the fully-symmetric state and the level increases as a function of the number of proton pairs outside the shell closure. This behavior can be understood as resulting from the mixing of the basic components of a two-fluid quantum system. A phenomenological fit based on this concept was performed. It provides the first experimental estimate of the strength of the proton–neutron quadrupole interaction derived from nuclear collective states with symmetric and antisymmetric nature. [Copyright &y& Elsevier]

Abstract: A new type of spectrometer for the study of reactions in inverse kinematics is proposed, in which the target and detector are placed inside a uniform magnetic field. This device has significant advantages over more conventional approaches to measurements of key reactions, and should prove useful in the study of short-lived nuclei. The technique is directly applicable to existing research programs at present accelerators with secondary beams, and the technique is likely to be important at future radioactive beam facilities. [Copyright &y& Elsevier]

Abstract: The level structures of 55V and 55Ti, the two nuclei with a single nucleon outside the semi-magic 54 22Ti32 core, have been investigated in order to provide new tests of full pf-shell calculations with the GXPF1A interaction. The addition of a proton does not appear to affect the shell gap significantly, although comparisons between calculations and experiment at high spins () indicate the need for a larger model space for an accurate description of the data in this regime. The energy separation between the and orbitals in neutron-rich Ti isotopes is not large enough to result in an shell gap. However, comparisons between the 55Ti data and the calculations argue for the presence of a sizable gap in 54Ca. [Copyright &y& Elsevier]

Abstract: The excitation function for the fusion–evaporation reaction has been measured down to a cross section of 25 nb. This is the first observation of fusion hindrance at extreme sub-barrier energies for a system with a small, negative Q-value (). This result is further proof that heavy-ion fusion hindrance, reported earlier only for systems with large, negative Q-values, is a general phenomenon. The measured behavior can be reproduced by coupled-channels calculations with a modified ion–ion potential incorporating the effects of nuclear incompressibility. [Copyright &y& Elsevier]

Abstract: A Transmission Ion Chamber (TIC) was constructed for use at the focal plane of the Fragment Mass Analyzer (FMA). The motivation for the construction of the TIC was to allow energy loss information to be added to the focal plane identification data. Three experiments immediately benefited form this information. First, an astrophysics experiment studying 69Br via the β decay of 69Kr would benefit greatly by adding a TIC to the existing implantation station. Second the study of extreme sub-barrier heavy-ion fusion evaporation reaction could be made more sensitive. Third, a study was performed using a PPAC–TIC–DSSD configuration to separate 101Sn from the other evaporation products of highly excited 108Te. [Copyright &y& Elsevier]

Abstract: A new focal-plane detector system has been designed and constructed for measuring very small heavy-ion induced fusion-evaporation cross sections with the Fragment Mass Analyzer (FMA) at Argonne National Laboratory. The system has been tested and successfully used in a fusion-evaporation excitation function measurement for down to the 5nb level. The design and performance of the detector system are described in detail with some examples from the latest data. Besides excellent tracking capabilities for each event measured in the detector, suppression of scattered beam background of the order of has been demonstrated. [Copyright &y& Elsevier]

Abstract: Excited states in 57Cr have been populated to high spin in the reaction at a beam energy of 130 MeV. A regular sequence of stretched quadrupole transitions has been established above the yrast level. This sequence is interpreted as a rotational band associated with prolate deformation induced by the excitation of the odd neutron into the orbital. Total Routhian surface calculations, which follow this configuration to high spin, reproduce the observed features of the band. They are also able to account for a similar, but less well developed structure in 55Cr. In contrast, the isomeric yrast state in 59Cr appears to be a band-head state dominated by the configuration, which is favoured at oblate deformations. Such a marked difference presents a significant challenge for theoretical descriptions, but is consistent with features exhibited by the low-lying negative-parity states in 59Cr. [Copyright &y& Elsevier]

Abstract: A study of high spin states in the odd-neutron isotones 239Pu and 237U is reported. Striking differences were found in the high-spin properties of rotational bands built on the ground states in these two nuclei. These differences mirror those observed in the even–even Pu and U immediate neighbors and appear to be related to the strength of octupole correlations. [Copyright &y& Elsevier]

The magnetic moments of Iπ=21+ states in even–even A∼100 fission fragments have been measured using the Gammasphere array, using the technique of time-integral perturbed angular correlations. The data are interpreted within the context of the interacting boson model (IBA2) leading to the suggestion of a strong νh11/2 component in the deformed 21+ states of this region. [Copyright &y& Elsevier]

Long-lived high-K states have been identified in 177Lu, including an isomer with τ>10 μs, placed at 3530 keV and associated with the yrast Kπ=39/2−, 5-quasiparticle state predicted in multi-quasiparticle-state calculations. The γ-decay of the isomer exposes the rotational band based on the 160-day 23/2− isomer, 177mLu, with band properties which support its proposed configuration. A 90-ns isomer at 1325 keV is associated with the predicted 3-quasiparticle Kπ=25/2+ state while a 5-quasiparticle 33/2+, 902-ns isomer with highly-hindered decays is identified at 2771 keV. Both exhibit rotational bands whose properties are used to support the assigned configurations. The possibility that the Kπ=39/2− isomer found in this work should be identified with a β-decaying isomer, proposed recently to populate the 51-min 37/2− isomer in 177Hf, is discussed in terms of the implied logft values, configuration changes, and hindrances for K-forbidden γ-decays. [Copyright &y& Elsevier]

Exclusive measurements of high energy γ-rays in the decay of 164Er, formed in the reaction 40Ar+124Sn, have been performed to study angular momentum dependence of the giant dipole resonance (GDR) strength function. The γ-ray spectra were measured in coincidence with A=160 evaporation residues (4n channel at Ebeam = 163 MeV and 187 MeV. A statistical model analysis, incorporating a two-component GDR strength function, shows increase in the GDR width as the average spin of the compound nucleus increases from 26 h̵, (at 163 MeV to 54 h̵ (at 187 MeV). This is consistent with the predictions of a thermal shape fluctuation model. [Copyright &y& Elsevier]

Isomer decays, β–γ and electron–γ spectroscopy, α–γ fine structure studies, and Coulomb excitation are cases of nuclear structure experiments where the ideal detector is a compact, highly segmented, very efficient germanium (Ge) detector box. In order to develop such a structure, we are working on the R&D of large, segmented, high-purity planar Ge strip detectors (HPGeDSSD) which form the walls of such a box. We have developed a 92 mm×92 mm×20 mm HPGeDSSD, which has 16×16 orthogonal strips of 5 mm width. We are in the process of designing a focal plane detector for the Argonne fragment mass analyzer which consists of a five-sided box, each side having a HPGeDSSD backed by a large segmented clover HPGe detector. MCNP simulations indicate this detector would have an efficiency of ∼60% for 122 keV gamma rays and ∼15% for 1.33 MeV radiation, which is ideal for studying the decays of nuclei far from stability that are usually produced with very low cross-sections. [Copyright &y& Elsevier]

The spectra of high-energy γ rays emitted by the Giant Dipole Resonance (GDR) built on moderately excited states associated with the evaporation of 0, 1 and 2 nucleons were measured in the 90Zr + 89Y symmetric fusion reaction. The radiative fusion data suggest statistical emission from the compound nucleus. In addition, the analysis of the high-energy γ-ray spectra associated with the different evaporation channels at the present temperature of 0.7 MeV and spin range 15–20 ℎ show a fairly narrow width of 5.0±0.35 MeV. This value is smaller than what would be expected in a nucleus where shell effects do not play a role. [Copyright &y& Elsevier]

Entry distributions in angular momentum and excitation energy have been measured for several very proton-rich isotopes of Pt and Au. This is the first systematic study of the energy-spin phase space for nuclei near and beyond the proton drip line. Comparisons are made between the distributions associated with proton-unbound Au nuclei and more stable Pt isobars. In 173Au the first evidence is seen for the limits of excitation energy and angular momentum which a nucleus beyond the proton drip line can sustain. [Copyright &y& Elsevier]

Excited structures in the proton-rich nucleus 140Dy have been established following the decay of an 8− isomer. The excitation energy of the isomer is established to be 2.16 MeV with a half-life of 7.3±1.5 μs. The isomer decays into the yrast line at the 8+ state, revealing a rotational band with a deduced deformation of β2=0.24(3). The isotope 140Dy is the daughter of the deformed proton emitter 141Ho. The new information obtained here supports the role of deformation in proton emission and the previous assignments of single-particle configurations to the two proton emitting states in 141Ho. In addition, the reduced hindrance factor measured for the isomer is consistent with the trend observed in the N=74 isotones. [Copyright &y& Elsevier]

Excited structures in the neutron-deficient nucleus 179Hg have been established for the first time using the Gammasphere spectrometer in conjunction with the fragment mass analyzer. Competing states originating from three different minima associated with nearly spherical, oblate, and prolate deformations were found. This result can be contrasted with the situation in heavier odd-mass Hg isotopes where only two minima (oblate and prolate) have been seen. The implications of these three shapes at low spin and excitation energy are discussed in the general context of shape coexistence in this mass region. [Copyright &y& Elsevier]

Compton polarimeters have played an important role in the study of nuclear structure physics, but have often been limited in their applications because of relatively low γ -ray detection efficiency. With the advent of γ -ray tracking detector arrays, which feature nearly 4 π solid angle coverage and the ability to identify the location of Compton-scattering events to within a few millimeters, this limitation can be overcome. Here we present a characterization of the performance of the Gamma Ray Energy Tracking In-beam Nuclear Array (GRETINA) as a Compton polarimeter using the 24Mg(p , p ′ ) reaction at 2.45 MeV proton energy. We also discuss a new capability added to the simulation package UCGretina to simulate the emission of polarized photons, and compare it to the measured data. Finally, we use these simulations to predict the performance of the Gamma Ray Energy Tracking Array (GRETA). [ABSTRACT FROM AUTHOR]

Two machine learning techniques, one supervised (Artificial Neural Network) and the other unsupervised (k -means + +) have been applied to the task of n/ γ discrimination in 7 Li-enriched CLYC detectors, and compared to traditional charge-comparison methods. The results show that a very basic artificial neural network can provide very good discrimination in the energy range investigated, and the k -means + + algorithm is capable of separating neutrons and gamma-rays in CLYC scintillators as well as suggesting reasonable window parameters for charge comparison methods. [ABSTRACT FROM AUTHOR]

The capabilities of 7 Li-enriched Cs 2 7 LiYCl 6 (C 7 LYC) scintillation detectors for fast neutron spectroscopy are explored in benchmark experiments that exploit its excellent pulse-shape discrimination between neutrons and γ rays, and its unprecedented ≈ 10% energy resolution for fast neutrons in the few MeV range, obtained through the 35Cl(n,p) reaction. Energy- and angle-resolved elastic and inelastic neutron scattering cross-section measurements of 56Fe(n,n') were performed at Los Alamos National Laboratory with a pulsed white neutron source and an array of 1 ″ × 1 ″ C 7 LYC crystals. The results convincingly establish the utility of this dual n/ γ scintillator for fast neutron spectroscopy. Intrinsic efficiency measurements of both 1 ″ × 1 ″ and the first ever 3 ″ × 3 ″ C 7 LYC crystal have been initiated, using mono-energetic fast neutron beams at UMass Lowell generated via the 7 Li(p,n) reaction. The spectroscopic capabilities and potential of C 7 LYC are discussed in the context of developing this emerging scintillator for targeted science applications. [ABSTRACT FROM AUTHOR]