Search Results (26 total)

The Advanced Photon Source (APS) injector complex includes an option for rf photocathode (PC) gun beam injection into the 450-MeV S-band linac. At the 150-MeV point, a four-dipole chicane was used to compress the micropulse bunch length from a few ps to sub-0.5 ps (FWHM). Noticeable enhancements of the optical transition radiation (OTR) signal sampled after the APS chicane were then observed as has been reported in the Linac Coherent Light Source (LCLS) injector commissioning. A far-infrared (FIR) coherent transition radiation detector and interferometer were used to monitor the bunch compression process and correlate the appearance of localized spikes of OTR signal (5 to 10 times brighter than adjacent areas) within the beam-image footprint. We have performed spectral-dependency measurements at 375 MeV with a series of bandpass filters centered in 50-nm increments from 400 to 700 nm and with an imaging spectrometer and observed a broadband enhancement in these spikes. Mitigation concepts of the observed coherent OTR, which exhibits an intensity enhancement in the red part of the visible spectrum as compared to incoherent OTR, are described.

Observations of strongly enhanced optical transition radiation (OTR) following significant bunch compression of photoinjector beams by a chicane have been reported during the commissioning of the Linac Coherent Light Source accelerator and recently at the Advanced Photon Source (APS) linac. These localized transverse spatial features involve signal enhancements of nearly a factor of 10 and 100 in the APS case at the 150-MeV and 375-MeV OTR stations, respectively. They are consistent with a coherent process seeded by noise and may be evidence of a longitudinal space charge microbunching instability which leads to coherent OTR emissions. Additionally, we suggest that localized transverse structure in the previous self-amplified spontaneous emission (SASE) free-electron laser (FEL) data at APS in the visible regime as reported at FEL02 may be attributed to such beam structure entering the FEL undulators and inducing the SASE startup at those “prebunched” structures. Separate beam structures 120 microns apart in x and 2.9 nm apart in wavelength were reported. The details of these observations and operational parameters will be presented.

We report the first unambiguous demonstration of near-field imaging of optical diffraction radiation (ODR). The source of the ODR was an aluminum metal reflective surface with a 7-GeV electron beam passing nearby its single edge. Because of the high Lorentz factor γ involved, appreciable ODR is emitted at visible wavelengths even for impact parameters of 1 to 2 mm, so standard imaging techniques were employed. The experimental results are compared to a simple near-field model. We show that the ODR signals are sensitive to both beam size and position. Applications to multi-GeV beams in transport lines in the major synchrotron radiation facilities, x-ray free-electron lasers, energy recovering linacs, and the International Linear Collider are possible.

A method for producing short electron bunches in an electron storage ring using pulsed phase modulation has been demonstrated. A simple theoretical model was validated using the particle tracking code elegant, and the bunch compression process was observed experimentally in the Advanced Photon Source storage ring using a visible light streak camera. Compression to 54% of the initial bunch length was achieved.

We report the first measurements of z-dependent coherent optical transition radiation (COTR) due to electron-beam microbunching at high gains ( >10⁴) including saturation of a self-amplified spontaneous emission free-electron laser (FEL). In these experiments the fundamental wavelength was near 530 nm, and the COTR spectra exhibit the transition from simple spectra to complex spectra ( 5% spectral width) after saturation. The COTR intensity growth and angular distribution data are reported as well as the evidence for transverse spectral dependencies and an “effective” core of the beam being involved in microbunching.

We report the first measurements of the electron-beam microbunching z dependence in a self-amplified spontaneous-emission (SASE) free-electron laser (FEL) experiment by the observation of visible wavelength coherent transition radiation (CTR). In this case the fundamental SASE wavelength was at 537 nm, and the CTR exhibited an exponential intensity growth similar to the SASE radiation. In addition, we observed for the first time structure in the CTR angular distribution patterns that may be useful for optimizing SASE FEL performance.

Experimental evidence for self-amplified spontaneous emission (SASE) at 530 nm is reported. The measurements were made at the low-energy undulator test line facility at the Advanced Photon Source, Argonne National Laboratory. The experimental setup and details of the experimental results are presented, as well as preliminary analysis. This experiment extends to shorter wavelengths the operational knowledge of a linac-based SASE free-electron laser and explicitly shows the predicted exponential growth in intensity of the optical pulse as a function of length along the undulator.

We have used the 7-GeV stored positron beam at the Advanced Photon Source to approximate relevant particle beam parameters of a diffraction-limited x-ray source and to perform measurements of few- to sub-angstrom radiation. Results are based on the 198-period diagnostics undulator optimized for low-divergence radiation cones (∼2.6 μrad) at λ∼0.5 Å on the fundamental and on a unique synchroscan and dual-sweep x-ray streak camera (with a few picosecond resolution) that was phase locked to the accelerator's master oscillator subharmonic. Scaling of these techniques to future sources is presented.

A nearly monochromatic beam of 100% linearly polarized γ rays has been produced via Compton backscattering inside a free electron laser optical cavity. The beam of 12.2 MeV γ rays was obtained by backscattering 379.4 nm free-electron laser photons from 500 MeV electrons circulating in a storage ring. A detailed description of the γ-ray beam and the outlook for future improvements are presented.

High-spin states have been populated in ¹⁰²Pd by ⁹²Zr(¹³C,3nγ)¹⁰²Pd with E_lab=56 MeV and in ¹⁰⁰Pd by ⁹¹Zr(¹²C,3nγ)¹⁰⁰Pd with E_lab=63 MeV. We find in each nuclide a ground state band ΔJ=2 of positive parity extending to J^π=14⁺, as well as two negative parity bands ΔJ=2, one of odd spin and one of even spin. For ¹⁰⁰Pd, the present level scheme represents the first definitive study of excited states in this nuclide. For ¹⁰²Pd the present data include the first measurement of the γ-ray linear polarizations. The level energies are compared to the variable moment of inertia model, the interacting boson approximation model, and to the results of a quasiparticle calculation. NUCLEAR REACTIONS, NUCLEAR STRUCTURE ⁹²Zr(¹³C,3nγ)¹⁰²Pd, E=56 MeV, ⁹⁴Zr(¹²C,4nγ)¹⁰²Pd, E=69 MeV, ⁹¹Zr(¹²C,3nγ)¹⁰⁰Pd, E=56, 63 MeV; measured E_γ, I_γ, γ(θ), γ(t), γ(E), γ-γ coin, linear polarizations. ^{102,  100}Pd deduced levels, γ branching, J, π, δ, t_{1 / 2} limits. Enriched ^{91,  92,  94}Zr targets, Ge(Li) detectors, Ge(Li) Compton polarimeter. Variable moment of inertia model, interacting boson approximation model, rotation alignment model.

The enhanced yields of high spin states populated in the p and d exit channels for ¹²C+¹²C reactions are utilized for particle-γ coincidence measurements at E_c.m.≃14.3 and 25.2 MeV. The (¹²C,pγ) results at E_c.m.≃14.3 MeV support the 15 / 2⁺ assignment to the E_x=9.80 MeV state. The (¹²C,dγ) results at E_c.m.≃25.2 MeV show an enhanced 9.77 MeV (9⁺) state to have a strong γ-ray branch to the E_x=4.52 MeV (7⁺) state.

NUCLEAR REACTIONS ¹²C(¹²C,pγ), E_c.m.≃14.3 MeV; measured E_p, E_γ, p-γ coin; ¹²C(¹²C,dγ), E_c.m.≈25.2 MeV measured E_d, E_γ, d-γ coin.

The lifetime of the 9.155 MeV J=5 / 2 state in ¹⁵N has been measured to be < 13 fsec from a study of the ¹²C(⁷Li, αγ)¹⁵N reaction at E_⁷Li=28 MeV. The γ decay from the 9.155 MeV state to the 5.30 MeV 1 / 2⁺ state is established as E2 and thereby determines the parity of the 9.155 MeV state to be positive. Particle-gated γ-ray spectra from ¹²C(⁷Li, αγ) and ¹²C(⁶Li, ³Heγ) reactions show that the 9.155 MeV state has a large triton cluster parentage. The other member of the unresolved doublet, the 9.152 MeV 3 / 2^- state, has a very small cluster parentage. These results are in disagreement with simple ¹²C+t interpretations of ¹⁵N but agree with weak coupling model calculations.

NUCLEAR STRUCTURE ¹²C(⁶Li, ³Heγ), E=34 MeV; ¹²C(⁷Li, αγ), E=34 MeV, measured cross section to 9.155 and 9.152 MeV ¹⁵N states; deduced lifetime and parity of 9.155 MeV state, compared to cluster and shell models.

Enhanced population of ²⁴Mg states at E_x=20.1, 21.3, 23.5, and 28.2 MeV is observed in the reaction ¹²C(¹⁶O, α)²⁴Mg at E_c.m.=26.7 MeV, an energy where the ¹²C + ¹⁶O inelastic channel also exhibits structure. It is suggested that the 28.2-MeV state may be identified with the 10⁺ member of a postulated excited collective band. J^π=10⁺ assignments for the other states are consistent with their decay modes observed in a particle-γ coincidence experiment and their excitation energies are suggestively close to recent shell-model calculations.

A γ-ray cascade with side chains previously assigned to ⁹⁹Pd and recently shown to occur in a nuclide with Z≤45, is unambiguously assigned to ₄₅Rh. Particle-γ coincidence measurements on the ¹²C + ⁹⁰Zr and ¹¹B + ⁹¹Zr reactions at 56 and 40 MeV, respectively, identify transitions in coincidence with charged particles to allow the elemental assignment. Systematics indicate the probable isotope is ⁹⁹Rh.

[NUCLEAR REACTIONS ¹²C + ⁹⁰Zr, E_lab=56 MeV; measured E_γ, particle-γ coin.; ¹¹B + ⁹¹Zr, E_lab=40 MeV; measured E_γ, particle-γ coin. ⁹⁹Pd, ⁹⁹Rh, enriched targets, Ge(Li), Si detectors.]

Enhancements are observed in the ¹²C(¹²C,d)²²Na reaction for the population of the J^π=5⁺ and 7⁺ states at E_x=1.53 and 4.52 MeV in the energy region from 13.0 to 16.8 MeV (c.m.). A comparison of ¹²C(¹²C,⁸Be) and ¹²C(¹²C,d) suggests that the selective population of the states in ²²Na arises from the deuteron exit channel penetrabilities for grazing partial waves rather than nuclear structure similarities between the ¹²C + ¹²C and d+²²Na systems.

[NUCLEAR REACTIONS ¹²C(¹²C,d), E_c.m.=13.0 to 16.8 MeV; measured E_d and dσ / dΩ at θ_1ab=10^∘ and 20^∘.]

The enhanced population of the 4.522 MeV state in ²²Na at E_c.m.=19.4 MeV in the ¹²C(¹²C, d)²²Na reaction is confirmed with the particle-γ coincidence technique. Deuteron-γ angular correlation results, lifetime measurements, and transmission coefficient calculations combine to allow only a J^π(4.522)=7⁺ assignment.

NUCLEAR REACTIONS ¹²C(¹²C, dγ), E_c.m.=19.41 MeV; measured E_d, E_γ, d-γ coincidence; ²²Na, J^π for state at 4.522 MeV.

Particle-γ-ray angular correlations have been performed using the ¹²C(¹²C, pγ)²³Na reaction at the resonant laboratory energy of 38.82 MeV. Branching ratios, mixing ratios, and lifetimes are determined for the γ decay of states up to 13 MeV excitation energy in ²³Na. These results are combined with the ¹²C(¹²C, ⁸Be)¹⁶O, ¹²C(¹²C, d)²²Na, and ¹²C(¹²C, p)²³Na reaction analyses to produce the following spin and parity assignments: 6.114 MeV (11 / 2⁺), 6.235 MeV (13 / 2⁺), 7.267 MeV (13 / 2⁺), 9.038 MeV (15 / 2⁺), and 9.803 MeV (11 / 2⁺). Shell-model calculations are superior to Nilsson model calculations in describing these results.

NUCLEAR REACTIONS ¹²C(¹²C, pγ), E_¹²C=38.82 MeV, measured lifetimes, branching ratios, and p-γ angular correlations. Deduced J^π of ²³Na levels. Compared results with shell and Nilsson models.

We report on γ rays which could interfere with the observation of x rays from the superheavy element 126.

The excitation function for the J^π=11 / 2⁺ state at E_x=4.51 MeV in the ¹²C(¹⁶O,p)²⁷Al reaction exhibits resonance-like behavior (Γ≈300 keV) at E_c.m.=11.83 and 14.36 MeV. Proton-γ coincidence measurements performed at these energies verify that the 4.510 MeV state is preferentially populated instead of lower spin states at this excitation energy. The "anomalies" occur at center-of-mass energies similar to those of reported anomalies for high spin states in the ¹²C(¹²C,p)²³Na reaction.

NUCLEAR REACTIONS ¹²C(¹⁶O,p), E_c.m.=11.0-15.0 MeV; measured E_p, σ(p); ¹²C(¹⁶O,pγ), E_c.m.=11.83 and 14.36 MeV; measured E_p, E_γ, p γ coin ²⁷A1, spin limits for states at 7.285, 8.525, and 8.698 MeV.

The ratio of nuclear quadrupole moments of the first excited state with respect to the ground state in ⁹⁹Ru has been determined from Mössbauer effect measurements with single crystal absorbers of ruthenocene. The result, Q₁ / Q₀=2.93±0.07, when combined with a recent measurement of Q₀, gives the value Q₁=0.23×10^-24 cm₂. An improved value for the E2 / M1 amplitude mixing ratio δ=-1.56±0.02 is determined from magnetic hyperfine spectra obtained with an absorber of ruthenium dissolved in iron. A determination of the angle between the axis of the electric field gradient and each of the three crystal axes of ruthenocene is in good agreement with the crystal structure reported for this compound. The recoil-free fraction was found to vary with respect to orientation of the crystal axes by a factor of greater than 2.

NUCLEAR REACTIONS ⁹⁹Ru(γ ,γ), E=89.4 keV; measured Mössbauer effect ruthenium iron, single crystal ruthenocene. Deduced δ, q₁ / q₀, EFG axis, recoilless fraction anisotropy.

¹²C(¹²C, pγ)²³Na proton-γ angular correlations were measured at E_B=38.82 MeV for the 9.042- and 9.805-MeV states in ²³Na which decay to levels at 7.270, 6.235, and 5.534 MeV with branching ratios of 20%, 60%, and 20%, respectively, for the 9.042-MeV state, and 30%, 50%, and 20%, respectively, for the 9.805-MeV state. Angular correlations and lifetime data eliminate 17 / 2 assignments for either state.

As part of a program utilizing the fusion-evaporation reaction to study high-spin states in A≃40 nuclei, linear polarization measurements were made on the γ rays emitted following fusion of the six projectile-target combinations: ¹⁴N + ²⁶Mg, ¹⁸O + ²⁴Mg, ¹⁸O + ²⁶Mg, ¹⁸O + ²⁷Al, ¹⁹F + ²⁷Al, and ¹⁹F + ²⁸Si. All beam energies were 40 MeV except the last, which was 45 MeV. The linear polarization of about 50 γ rays from the highly aligned states were measured in each experiment using a two-crystal Ge(Li) Compton polarimeter. The results are tabulated. As an example of the application of these results, the polarization data in conjunction with previous γ-γ coincidence and γ-ray angular distribution measurements suggest E2 character for the γ cascades between the ⁴¹K levels at 4983, 4275, 2762, and 1294 keV and thus support the spin-parity sequence 19 / 2^-, 15 / 2^-, 11 / 2^-, and 7 / 2^- for these states. Applications to the spectroscopy of other levels in ⁴¹K, ⁴¹Ca, and ³⁹K are discussed.

NUCLEAR REACTIONS (HI, xn, yp, zα) studies for A∼40 nuclei, ¹⁴N, ¹⁸O, ¹⁹F projectiles, ^24,26Mg, ²⁷Al, ²⁸Si targets, E=40-45 MeV; measured P(E_γ, 90°), deduced Λ for transitions, and J^π, δ for high-spin states in ^39,41K and ⁴¹Ca.

The strong linear polarizations of the deexcitation γ rays from the ⁹⁶Zr(¹⁸O,4nγ)¹¹⁰Cd reaction at 60 MeV were measured using a Ge(Li) two-crystal Compton polarimeter. Previous experiments with this reaction resulted in the identification of a side band feeding into the known 4⁺ and 6⁺ states via two dipolar γ rays from states with spins 5 and 7 and with energies of 996 and 399 keV, respectively. These two dipole transitions were found to be E1. Several other known quadrupole transitions were found to be E2. Thus we assign odd parity to the levels at 2538 keV (5^-), 2878 keV (7^-), and 3344 keV (9^-). The feasibility of such measurements with this polarimeter is discussed.

[NUCLEAR REACTIONS ⁹⁶Zr(¹⁸O,4nγ), E=60 MeV; enriched ⁹⁶Zr target. Linear polarizations of ¹¹⁰Cd γ rays measured.]

The linear polarization has been measured for the 2.21-, 2.73-, 2.98-, and 3.00-MeV transitions in ²⁷Al. The mixing ratios δ(E2 / M1) for these transitions were determined to be δ(2.21)=-0.57±0.08, δ(2.73)=0.22±0.09, and δ(2.98)=0.05±0.03. The 3.00-MeV transition was found to be consistent with a pure E2 transition. The results are compared with the predictions of the rotational-vibrational model of Röpke.