B. Feinberg

Measurement of synchrotron pulse durations using surface photovoltage transients

MEASUREMENT OF SYNCHROTRON PULSE DURATIONS USING SURFACE PHOTOVOLTAGE TRANSIENTS T.E. Glover 1 , G.D. Ackermann 1 , A. Belkacem 2 , B. Feinberg 1 , P.A. Heimann 1 , Z. Hussain 1 , H.A. Padmore 1 , C. Ray 2 , R.W. Schoenlein 3 , W.F. Steele 1 . Advanced Light Source Division, 2 Chemical Sciences Division, 3 Materials Sciences Division Lawrence Berkeley National Laboratory 1 Cyclotron Road, MS 2-345 Berkeley, California 94720, USA Abstract We report results on experiments using combined laser and synchrotron radiation. Picosecond laser pulses at 800 nm are used to induce surface photovoltage transients in p-type Si samples. A two-component decay is observed. The fast component of decay provides a direct measure of synchrotron soft x-ray pulse durations. PACS : 42.50.Hz, 32.80.Wr Keywords : Combined laser-synchrotron spectroscopy; x-ray pulse measurement. Correspondence author : T. Ernest Glover MS 2-345, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA. 94720, USA. phone : 510-486-6556 fax : 510-486-5530 email : teglover @lbl.gov

Placed in a steady magnetic field, the flux density inside a permalloy-shielded volume decreases over hours and days

Following the application of an external magnetic field to a thin-walled demagnetized Permalloy cylinder, the magnetic flux density at the center of the shielded volume decreases by roughly 20% over periods of hours to days. We measured this effect for applied magnetic fields from 0.48 A/m to 16 A/m, the latter being comparable to the Earths magnetic field at its weakest point. Delayed changes in magnetic flux density are also observed following alternating current demagnetization. We attribute these effects to delayed changes in magnetization, which have previously been observed in thin Permalloy films and small bulk samples of ferromagnetic materials. Phenomenological models of thermal activation are discussed. Some possible effects on experiments that rely on static shielding are noted.

Measurement of electron capture from e+‐e− pair production by 0.956 GeV/u U92+ on Au, Ag, Cu, and Mylar targets

We describe the first experimental observation of electron capture from electron‐positron pair production in relativistic heavy ion collisions. We have used a novel new spectrometer to make the measurement of the cross section for a 0.956 GeV/u U92+ beam produced at the BEVALAC facility at LBL on Au, Ag, Cu, and Mylar targets. We also measured the energy and angular distribution of the positrons for the Au target. The total cross section for a Au target is measured to be 2.19 (0.25) barns for capture from pair production and 3.30 (0.65) barns for pair production without capture.

Measurement of the lamb shift in lithiumlike uranium (U89

We have measured the 1S22p22P1/2−1s22s22S1/2 (first‐excited state to ground state) transition energy in lithiumlike uranium (U89+) to be 280.58±0.13 eV using Doppler‐tuned spectroscopy. This is a test of the calculation of the Lamb shift in the presence of more than one electron.

K-shell ionization and double-ionization of Au atoms with 1.33 MeV photons

At relativistic energies, the cross section for the atomic photoelectric effect drops off as does the cross section for liberating any bound electron through Compton scattering. However, when the photon energy exceeds twice the rest mass of the electron, ionization may proceed via electron-positron pair creation. We used 1.33 MeV photons impinging on Au thin foils to study double K-shell ionization and vacuum-assisted photoionization. The preliminary results yield a ratio of vacuum-assisted photoionization and pair creation of 2x10{sup -3}, a value that is substantially higher than the ratio of photo double ionization to single photoionization that is found to be 0.5-1x10{sup -4}. Because of the difficulties and large error bars associated with the small cross sections additional measurements are needed to minimize systematic errors.

The Bevalac long spill

The Bevalac extraction time was increased from 1 to a maximum of 9.5 seconds, thus increasing the synchrotron duty factor and the data rate for experiments by a factor of 2-3, depending on the magnetic field. This slow rate of extraction required improved control of beam time structure, since magnet ripple remained approximately constant while the spill rate was decreased. Measurements of spill structure for the long spill are presented. Changes made to the accelerator systems are described, as well as tuning procedures found to be necessary, and the ultimate hardware limits found for the spill length.<<ETX>>

Capture from pair production as a beam loss mechanism for heavy ions at RHIC

Electron capture from electron-positron pair production is predicted to be a major source of beam loss for the heaviest ions at RHIC. Achieving the highest luminosity thus requires an understanding of the capture process. We report measurements of this process at Brookhaven National Laboratorys AGS using 10.8 GeV/nucleon Au/sup 79+/ projectiles on Au targets. Capture from pair production is a process in which the very high electromagnetic field involved in the collision of two relativistic heavy ions results in the production of an electron-positron pair with the capture of the electron by one of the ions. There are many theoretical papers published on capture from pair production with discrepancies between predicted cross sections. The experimental results are compared to theory and to previous experiments at 1 GeV/nucleon. The implications of extrapolations to RHIC energies are presented.