Search Results (183 total)

The low-energy structure of the proton dripline nucleus ²⁰Na has been studied using Coulomb excitation at the TRIUMF-ISAC radioactive ion beam facility. A 1.7-MeV/nucleon ²⁰Na beam of ~5×10⁶ ions/s was Coulomb excited by a 0.5-mg/cm^2 natTi target. Scattered beam and target particles were detected by the BAMBINO segmented Si detector while γ rays were detected by two TIGRESS HPGe clover detectors set perpendicular to the beam axis. Coulomb excitation from the 2⁺ ground state to the first excited 3⁺ and 4⁺ states was observed, and B(λL) values were determined using the 2⁺→0⁺ de-excitation in ⁴⁸Ti as a reference. The resulting B(λL)↓ values are B(E2;3⁺→2⁺)=55±6 e²  fm⁴ (17.0±1.9 W.u.), B(E2;4⁺→2⁺)=35.7±5.7 e²  fm⁴ (11.1±1.8 W.u.), and B(M1;4⁺→3⁺)=0.154±0.030 μ_N² (0.086±0.017 W.u.). These measurements provide the first experimental determination of B(λL) values for this proton dripline nucleus of astrophysical interest.

We report on experimental evidence for collective oblate rotation becoming favored at high spins in a rigid, well-deformed, axially symmetric nucleus. Excited states established up to spin 20ℏ in ¹⁸⁰Hf are consistent with predictions that nucleon alignments would favor oblate over prolate shapes at high spins in neutron-rich Hf isotopes. The results highlight the influence of valence orbitals on the interplay between nucleon alignments and nuclear shapes and provide a rare example of independent particle dynamics in competing potential wells.

The low-energy structures of the mirror nuclei ²¹Ne and radioactive ²¹Na have been examined by using Coulomb excitation at the TRIUMF-ISAC radioactive ion beam facility. Beams of ~5×10⁶ ions/s were accelerated to 1.7 MeV/A and Coulomb excited in a 0.5 mg/cm^{2 nat}Ti target. Scattered beam and target particles were detected by the segmented Si detector BAMBINO, while γ rays were observed by using two TIGRESS HPGe clover detectors perpendicular to the beam axis. For each isobar, Coulomb excitation from the 3 / 2⁺ ground state to the first excited 5 / 2⁺ state was observed and B(E2) values were determined by using the 2⁺→0⁺ de-excitation in ⁴⁸Ti as a reference. The ϕ segmentation of BAMBINO was used to deduce tentative assignments for the signs of the mixing ratios between the E2 and M1 components of the transitions. The resulting B(E2)↑ values are 131±9 e² fm⁴ (25.4±1.7 W.u.) for ²¹Ne and 205±14 e² fm⁴ (39.7±2.7 W.u.) for ²¹Na. The fit to the present data and the known lifetimes determined E2/M1 mixing ratios and B(M1)↓ values of δ=(-)0.0767±0.0027 and 0.1274±0.0025 μ_N² and δ=(+)0.0832±0.0028 and 0.1513±0.0017 μ_N² for ²¹Ne and ²¹Na, respectively (with Krane and Steffen sign convention). By using the effective charges e_p=1.5e and e_n=0.5e, the B(E2) values produced by the p-sd shell model are 30.7 and 36.4 W.u. for ²¹Ne and ²¹Na, respectively. This analysis resolves a significant discrepancy between a previous experimental result for ²¹Na and shell-model calculations.

The structure of the neutron-rich N=20 nucleus ³⁵P has been investigated through nucleon transfer experiments using the ²⁰⁸Pb(³⁶,Xγ) reaction at 230 MeV. The level structure, of mainly 1ℏω excitations in ³⁵P, has been significantly expanded. The measurements are compared with shell model calculations. Experimental branching ratio limits are reported for predicted transitions to the 2ℏω bandheads in ³⁵P and ³⁴Si.

The 685 keV excitation energy of the first excited 0⁺ state in ¹⁵²Sm makes it an attractive candidate to explore expected two-phonon excitations at low energy. Multiple-step Coulomb excitation and inelastic neutron scattering studies of ¹⁵²Sm are used to probe the E2 collectivity of excited 0⁺ states in this “soft” nucleus and the results are compared with model predictions. No candidates for two-phonon K^π=0⁺quadrupole vibrational states are found. A 2⁺,K=2 state with strong E2 decay to the first excited K^π=0⁺ band and a probable 3⁺ band member are established.

The combined data of two Coulomb excitation experiments has verified the purely electromagnetic population of the K^π=4⁺,6⁺,8^-, and 16⁺ rotational bands in ¹⁷⁸Hf via 2≤ν≤14 K-forbidden transitions, quantifying the breakdown of the K-selection rule with increasing spin in the low-K bands. The γ-, 4⁺, and 6⁺ bands were extended, and four new states in a rotational band were tentatively assigned to a previously known K^π=0⁺ band. The quasiparticle structure of the 6⁺ (t_{1 / 2}=77 ns) and 8^- (t_{1 / 2}=4 s) isomer bands were evaluated, showing that the gyromagnetic ratios of the 6⁺ isomer band are consistent with a pure π7 / 2⁺[404],π5 / 2⁺[402] structure. The 8^- isomer band at 1147 keV and the second 8^- band at 1479 keV, thought to be predominantly ν7 / 2^-[514],ν9 / 2⁺[624] and π9 / 2^-[514],π7 / 2⁺[404], respectively, are mixed to a degree approaching the strong-mixing limit. Based on measured 〈K^π=16⁺‖E2‖K^π=0⁺〉 matrix elements, it was shown that heavy-ion bombardment could depopulate the 16⁺ isomer at the ~1% level, although no states were found that would mediate photodeexcitation of the isomer via low-energy x-ray absorption.

High-spin states in neutron-rich ¹⁸⁰Hf and ¹⁸²Hf nuclei were populated through inelastic and transfer reactions with a ¹³⁶Xe beam incident on a thin ¹⁸⁰Hf target, and investigated using particle-γ coincidence techniques. New collective band structures were observed, and previously known rotational and vibrational bands in these nuclei were extended to higher angular momenta. No obvious nucleon alignment was observed in the ground state band of either nucleus up to ℏω=0.43 MeV, a significant delay compared to lighter even-even Hf isotopes. Woods-Saxon cranking calculations were performed to predict the nature of the first band crossings and shape evolution in ^180,182Hf.

We present the results of the first exposure of a Liquid Argon TPC to a multi-GeV neutrino beam. The data have been collected with a 50 liters ICARUS-like chamber located between the CHORUS and NOMAD experiments at the CERN West Area Neutrino Facility (WANF). We discuss both the instrumental performance of the detector and its capability to identify and reconstruct low-multiplicity neutrino interactions.

Space charge and coherent synchrotron radiation may deteriorate electron beam quality when the beam passes through a magnetic bunch compressor. This paper presents the transverse phase-space tomographic measurements for a compressed beam at 60 MeV, around which energy the first stage of magnetic bunch compression takes place in most advanced linacs. Transverse phase-space bifurcation of a compressed beam is observed at that energy, but the degree of the space charge-induced bifurcation is appreciably lower than the one observed at 12 MeV.

A medium-spin isomer in ¹⁹⁷Au is identified with t_1/2=150(5) ns following a multinucleon transfer reaction between an 850-MeV ¹³⁶Xe beam and a ¹⁹⁸Pt target. The transitions identified here are considered and possible configurations for the associated levels discussed. In addition, a newly observed out-of-beam transition in ¹⁹⁵Au is briefly reported.

The spectroscopy of neutron-rich ^{109,110,111,112}Ru nuclei was studied by measuring the prompt γ rays that originate from fission fragments, produced by the ²³⁸U(α,f) fusion-fission reaction, in coincidence with the detection of both fragments. For ^109,111Ru, both the negative-parity (h_11/2 orbitals) and K=5/2 positive-parity (mainly g_7/2 and d_5/2 orbitals) bands were extended to substantially higher spin and excitation energy than known previously. The ground-state and γ-vibrational bands of ^110,112Ru also were extended to higher spin, allowing observation of the second band crossing at the rotational frequency of ≈450 keV in ¹¹²Ru, which is ≈50 keV above the first band crossing. At a similar rotational frequency, the first band crossing for the h_11/2 band in ¹¹¹Ru was observed, which is absent in ¹⁰⁹Ru. These band crossings most likely are caused by the alignment of the g_9/2 proton pair. This early onset of the band crossing for the aligned πg_9/2 orbitals may be evidence of a triaxial shape transition from prolate to oblate occurring in ¹¹¹Ru. The data together with a comparison of cranked shell‐model predictions are presented.

A free relativistic electron in an electromagnetic field is a pure case of a light-matter interaction. In the laboratory environment, this interaction can be realized by colliding laser pulses with electron beams produced from particle accelerators. The process of single photon absorption and reemission by the electron, so-called linear Thomson scattering, results in radiation that is Doppler shifted into the x-ray and γ-ray regions. At elevated laser intensity, nonlinear effects should come into play when the transverse motion of the electrons induced by the laser beam is relativistic. In the present experiment, we achieved this condition and characterized the second harmonic of Thomson x-ray scattering using the counterpropagation of a 60 MeV electron beam and a subterawatt CO₂ laser beam.

Coulomb activation of the four quasiparticle K^π=16^{+ 178}Hf isomer (t_1/2=31  y) has led to the measurement of a set of Eλ matrix elements coupling the isomer band to the ground band. The present data combined with earlier ¹⁷⁸Hf Coulomb excitation data have probed the K components in the wave functions and revealed the onset and saturation of K mixing in low-K bands, whereas the mixing is negligible in the high-K bands. The implications can be applied to other quadrupole-deformed nuclei.

A coexisting band structure is identified in ¹⁵²Sm through γ-ray coincidence spectroscopy following β decay of ^152m,gEu and following multistep Coulomb excitation. This structure is interpreted as a pairing isomer analogous to a similar band identified in ¹⁵⁴Gd, based on relative B(E2) values for transitions out of the band and two-neutron transfer reaction population of the 0⁺ and 2⁺ band members. Systematics for odd-A isotopes near N=90 suggest that there should be a low-lying pairing isomer in ¹⁵⁶Dy and similar structures at higher energy in ¹⁵⁰Nd and ¹⁵⁸Er.

Neutron-rich ^96,97Sr and ^98,99Zr nuclei were populated as fission fragments produced by the ²³⁸U(α,f) fusion-fission reaction. The yrast states of these nuclei have been extended up to ≈20ℏ, which is about 6ℏ on average beyond the previously known spin, by studying the prompt γ rays in coincidence with the detection of both fission fragments. This extension allows the observation of yrast states with spins beyond ≈4⁺ in ⁹⁶Sr and ⁹⁸Zr evolving from vibrationlike states to rotationlike states. With an additional neutron, the yrast states with excitation energies above ≈600  keV in ⁹⁷Sr and ⁹⁹Zr have the characteristics of members of rotationlike bands for both positive- and negative-parity states. However, their underlying single-particle configurations cannot be determined uniquely by the measured intensity ratios of ΔI=1 to ΔI=2 transitions because of possible configuration mixing. The sharp variations in the yrast structure of these nuclei are discussed in terms of the gradualness of the onset of quadrupole deformation.

Presented are details of the staged electron laser acceleration (STELLA) experiment, which demonstrated high-trapping efficiency and narrow energy spread in a staged laser-driven accelerator. Trapping efficiencies of up to 80% and energy spreads down to 0.36% (1σ) were demonstrated. The experiment validated an approach that may be suitable for the basic design of a laser-driven accelerator system. In this approach, a laser-driven modulator together with a chicane creates a train of microbunches spaced apart by the laser wavelength. These microbunches are sent into a second laser-driven accelerator designed to efficiently trap the microbunches in the ponderomotive potential well of the laser electric field while maintaining a narrow energy spread. The STELLA scientific apparatus and procedures are described in detail. In-depth comparisons between the data and model are given including the predicted energy spectrum, energy-phase plot, and microbunch length profile. Data and model comparisons as a function of the phase delay between the microbunches and the accelerating wave are presented. The model is exercised to reveal how the high-trapping efficiency process evolves during the acceleration process.

High spin states in ²³⁷Np and ²⁴¹Am have been studied with the “unsafe” Coulomb excitation technique. In each nucleus, signature partner rotational bands built on the [523]5∕2⁻ and [642]5∕2⁺ orbitals of respective h_9∕2 and i_13∕2 parentage have been delineated. An additional pair of bands based on the [521]3∕2⁻ (f_7∕2) state was also observed in ²⁴¹Am. New information on the even-even ²³⁶Pu and ²⁴²Cm transfer products is also presented. From the present data, the role of i_13∕2 protons in generating angular momentum in the even-even nuclei of the region is documented. A satisfactory description of the evolution of the rotational sequences with spin is achieved within the framework of the cranked shell model. Nevertheless, when combined with information on odd-neutron nuclei available from elsewhere, the data highlight significant shortcomings of the available theoretical predictions.

The neutron-rich ^169,171,172Er nuclei were populated by few-neutron transfer reactions between ¹⁷⁰Er and ²³⁸U at a near barrier energy. The spectroscopy of these Er isotopes was studied using prompt γ rays correlated with delayed transitions or events involving at least three prompt transitions. The ground-state band of ¹⁷²Er was populated up to spin 22⁺ at an excitation energy of 5528  keV. Rotational bands built on the 1∕2⁻[521], 5∕2⁻[512], and 7∕2⁺[633] neutron configurations in ^169,171Er were extended to substantially higher spins than previously known. The signature splitting observed in these rotational bands is addressed within the framework of the particle-rotor model in terms of triaxiality and Coriolis attenuation. The signature inversion observed in the 5∕2⁻[512] band is well reproduced by including the triaxial degree of freedom in the calculation. Attenuating the Coriolis interaction in the calculation is found to be necessary to reproduce the signature splitting observed in the 7∕2⁺[633] band. A similar Coriolis attenuation also is needed to account for the signature splitting as well as the B(M1)∕B(E2) ratios in the 7∕2⁺[633] ground-state band in the neighboring N=99 isotones, ¹⁶⁷Er and ¹⁶⁹Yb.

A multinucleon transfer reaction between a thin self-supporting  78198Pt target and an 850  MeV  54136Xe beam has been used to populate and study the structure of the N=80 isotone  56136Ba. Making use of time-correlated γ-ray spectroscopy, evidence for an Iπ=(10+) isomeric state has been found with a measured half-life of 91±2  ns. Prompt-delayed correlations have also enabled the tentative measurement of the near-yrast states which lie above the isomer. Shell-model calculations suggest that the isomer has a structure which can be assigned predominantly as (νh11∕2)10+−2. The results are discussed in terms of standard and pair-truncated shell-model calculations, and compared to the even-Z N=80 isotones ranging from  50130Sn to  68148Er. A qualitative explanation of the observed dramatic decrease in the B(E2:10+→8+) value for the N=80 isotones at 136Ba is given in terms of the increasing single-hole energy of the h11∕2 neutron configuration as the proton subshell is filled. The angular momentum transfer to the binary fragments in the reaction has also been investigated in terms of the average total γ-ray fold versus the scattering angle of the recoils.

Laser-driven electron accelerators (laser linacs) offer the potential for enabling much more economical and compact devices. However, the development of practical and efficient laser linacs requires accelerating a large ensemble of electrons together (“trapping”) while keeping their energy spread small. This has never been realized before for any laser acceleration system. We present here the first demonstration of high-trapping efficiency and narrow energy spread via laser acceleration. Trapping efficiencies of up to 80% and energy spreads down to 0.36% (1σ) were demonstrated.

Neutron-rich Zr and Mo isotopes were populated as fission fragments produced by the ²³⁸U(α,f) fusion-fission reaction. The level schemes of these nuclei have been extended beyond the first band crossing region, which can be ascribed to the h_11∕2 neutron pair alignment. The spin alignment and signature splitting for the νh_11∕2 orbitals in term of triaxiality is addressed. The crossing frequency of the aligned bands in even Zr and Mo isotopes can be reproduced well by calculations using the cranked shell model. Compared to the Zr isotopes, band crossings in Mo isotopes shift to a lower rotational frequency due to the emerging importance of the γ degree of freedom. Within the framework of particle-rotor model, the difference in the signature splitting observed for the νh_11∕2 bands between the odd Zr and Mo isotopes can be attributed to the triaxial degree of freedom in the Mo isotopes. The surprising shift of the band crossing to a higher rotational frequency in ¹⁰⁶Mo is interpreted as a manifestation of the deformed subshell closure at N=64.

The near-yrast states of ^99,100Mo have been studied following their population via a binary reaction between a ¹³⁶Xe beam and a thin, self-supporting ¹⁰⁰Mo target. The yrast sequence in ¹⁰⁰Mo has been extended to a tentative spin∕parity (20⁺), while the decoupled band built on the I^π=11 / 2⁻ isomeric state in ⁹⁹Mo has been extended through the first alignment up to a tentative spin∕parity of (43 / 2⁻). The results are compared with self-consistent, cranked-mean-field calculations using a Woods-Saxon potential. The alignment systematics of the intruder h_11∕2 bands in the N=57 isotones from Mo (Z=42) to Cd (Z=48) and the yrast sequences in their N=58 even-even neighbors are discussed. An overall picture emerges, where the alignment properties evolve from being due to positive-parity neutrons in the ₄₈¹⁰⁵Cd to predominantly (g_9∕2)² proton crossings closer to the Z=40 subshell. Qualitatively, this can be explained by an increase in the quadrupole deformation and a simultaneous lowering of the proton Fermi surface in the g_9∕2 shell with decreasing proton number. These data provide excellent examples of rotational-alignment phenomena in weakly deformed nuclei.

The half-life of the known I,K^π=4⁻,4⁻ state in ¹⁷⁰Er has been measured to be 42.8±1.7  ns. The rotational band built on this isomer was excited inelastically up to spin 18⁻ by a ²³⁸U beam at E_lab=1358  MeV. A similar band in ¹⁶⁸Er was extended to spin 15⁻. The wave function of the isomeric state in ¹⁷⁰Er has been determined from the measured ∣g_K−g_R∣ values, which were deduced from the intensity ratios of the ΔI=1 to ΔI=2 transitions within the band. The dominant component consists of a two-quasiproton configuration involving the Nilsson orbits 7∕2⁻[523] and 1∕2⁺[411]. In contrast, the two-quasineutron configuration involving the 7∕2⁺[633] and 1∕2⁻[521] Nilsson orbits constitutes the major component for the wave function of the K^π=4⁻ isomer in ¹⁶⁸Er.

The configuration mixing between the first two 0⁺ states in ⁹⁸Sr, which have very different deformation, has been reanalyzed by introducing the term involving the intrinsic E2 matrix element between them in a two-state-mixing model calculation. A mixing strength of ≈2.6% was determined using the known E0 strength and the intrinsic E2 matrix elements for those two 0⁺ states. This mixing strength is nearly a factor of 4 weaker than that of the early analysis. Comparison is made to a similar case of configuration mixing in ¹⁰⁰Zr.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.