Scorpion,the Stuttgart scattering facility for fast polarized neutrons

The scattering facility SCORPION has been designed for simultaneous measurement of neutron differential cross sections and analyzing powers in the energy range of 4–16 MeV. This facility is constructed in close double shielded geometry and utilizes the high neutron fluxes produced with 0.5–1 mA currents of the 4 MV Dynamitron accelerator. Neutron spectroscopy is performed by pulse height analysis and by unfolding of the proton recoil spectra, using the new code FANTI, developed for this experiment. Results are given for the nucleus ¹²C as an example. As measured simultaneously in the same setup, consistent evaluation of cross section and polarization data reveal new and accurate information on optical model parameters in particular for the spin orbit term.     

A compton polarimeter for synchrotron radiation in the x-ray energy range

A polarimeter for synchrotron radiation in the energy range E ? 15 keV is described. It utilizes 90° Compton scattering by low Z atoms in a thin organic foil. The effect of double scattering on the analyzing power is calculated. The instrument can be used for continuously monitoring the intensity and the polarization of a photon beam during an experiment.     

Design and investigation of gas cluster ion accelerator

A gas cluster ion accelerator with energies up to 20 keV is described. Parameters of the main components of the equipment were determined. It was demonstrated that a pulse regime of the accelerator can be realized with a relatively low (500 l/sec) pumping speed of the cluster ion source. Conditions for stable cluster beam generation were found. In pulse mode ion clusters appear in the accelerated beam in the first stages of ion beam evolution. A model describing the behavior of the ion beam in the pulse regime is suggested. To analyze the beam mass composition two systems were constructed: a magnetic mass-separation system and a time-of-flight system. Cluster ions Ar_n⁺ with number of atoms/charge n ≥ 500 were detected in the beam by using deflection in a homogeneous magnetic field. Clusters up to 3000 atoms/charge were detected by using time-of-flight measurements.     

Development of a momentum determined electron beam in the 1–45 GeV range

A beam line for electrons with energies in the range of 1–45 GeV, low contamination of hadrons and muons and high intensity up to 10⁶ per accelerator spill at 27 GeV was setup at U70 accelerator in Protvino, Russia. A beam tagging system based on drift chambers with 160 μm resolution was able to measure relative electron beam momentum precisely. The resolution σ_p/p was 0.13% at 45 GeV where multiple scattering is negligible. This test beam setup provided a possibility to study properties of lead tungstate crystals (PbWO₄) for the BTeV experiment at Fermilab.     

A three-dimensional degree of polarization based on Rayleigh scattering

A measure of the degree of polarization for the three-dimensional polarization matrix (coherence matrix) of an electromagnetic field is proposed, based on Rayleigh scattering. The degree of polarization at a point is defined as an average, over all scattering directions, of an imagined dipole scattering of the three-dimensional state of polarization. This gives a well-defined purity measure, which, unlike other proposed measures of the three-dimensional degree of polarization, is not a unitary invariant of the matrix. This is demonstrated and discussed for several examples, including a partially polarized transverse beam.     

A new source for polarized 6,7Li negative ions

Thermal polarized ^6,7Li atoms, produced by an atomic-beam source, were bombarded by a beam of fast Cs⁰ atoms in order to form polarized negative ^6,7Li ions for injection into a tandem accelerator. A beam intensity of 0.28 μA has been achieved. The measured tensor polarization of the ⁶Li beam was P_ζζ = 0.88. The beam intensity agrees well with the value calculated using the known charge exchange cross section.     

Subnanosecond 1-GW pulsed 38-GHz radiation source

The regime of excitation of subnanosecond high-power microwave pulses has been studied in a Cherenkov device with an extended periodic slow-wave structure, using an electron beam from a compact pulsed high-current electron accelerator (290 keV, 2.3 kA, 1 ns). Conditions are established for which the power conversion coefficient can reach up to 1.5 at an output pulse power of 1.2 GW and a pulse duration of 200 ps.     

Towards a plasma wake-field acceleration-based linear collider

A proposal for a linear collider based on an advanced accelerator scheme, plasma wake-field acceleration in the extremely nonlinear regime, is discussed. In this regime, many of the drawbacks associated with preservation of beam quality during acceleration in plasma are mitigated. The scaling of all beam and wake parameters with respect to plasma wavelength is examined. Experimental progress towards high-gradient acceleration in this scheme is reviewed. We then examine a linear collider based on staging of many modules of plasma wake-field accelerator, all driven by a high average current, pulse compressed, RF photoinjector-fed linac. Issue of beam loading, efficiency, optimized stage length, and power efficiency are discussed. A proof-of-principle experimental test of the staging concept at the Fermilab test facility is discussed.     

Towards a plasma wake-field acceleration-based linear collider

A proposal for a linear collider based on an advanced accelerator scheme, plasma wake-field acceleration in the extremely nonlinear regime, is discussed. In this regime, many of the drawbacks associated with preservation of beam quality during acceleration in plasma are mitigated. The scaling of all beam and wake parameters with respect to plasma wavelength is examined. Experimental progress towards high-gradient acceleration in this scheme is reviewed. We then examine a linear collider based on staging of many modules of plasma wake-field accelerator, all driven by a high average current, pulse compressed, RF photoinjector-fed linac. Issue of beam loading, efficiency, optimized stage length, and power efficiency are discussed. A proof-of-principle experimental test of the staging concept at the Fermilab test facility is discussed.     

Polarized light-pulse transport through scattering media

The propagation of a polarized pulse in random media is investigated using the discrete-ordinates method to solve the transient vector radiative transfer. The angular analysis of the transient polarized features of scattering fluxes makes it possible to investigate subtle details of the polarization flip encountered for circularly polarized waves. We found that, depending on the geometry, the state of polarization, and the angle of detection, the degree of polarization decays at either a slower or faster rate when the beam is impinging at an angle far from the normal incidence. At normal incidence, our results confirm that, for large particles, the circular polarization maintains a greater degree of polarization.     

Demonstrating gain in a dielectric Cherenkov maser with a rod slow-wave system

We report the first results of experiments that demonstrate the amplification of megawatt nanosecond microwave pulses in a Cherenkov maser with a dielectric rod and moderately relativistic annular electric beam generated in a compact linear induction accelerator module. The input signal was generated by a resonant microwave compressor operating in a 3-cm wavelength range. A maximum gain of ∼12.5 dB and a maximum output power of ∼16 MW for a pulse duration of ∼4 ns at a frequency of 9.388 GHz were obtained with a quartz rod. The dependence of the gain on the compressor power was determined for various values of the accelerating voltage and beam current.     

The design and performance of the FNAL high-energy polarized-beam facility

A new polarized-proton and -antiproton beam with 185 GeV/c momentum has been built at Fermilab. The design uses the parity-nonconserving decays of lambda and antilambda hyperons to produce polarized protons and antiprotons, respectively, a beam-transport system that minimizes depolarization effects, and a set of twelve dipole magnets that rotate the beam-particle spin direction. A beam-tagging system determines the momentum and polarization of individual beam particles. This allows a selection of particles in definite intervals of momentum and polarization. Measurements performed by two different polarimeters showed that the beam is polarized and the determination of polarization by beam-particle tagging is verified. A new measurement of the analyzing power of large-x_F π⁰ production may lead to another beam polarimeter.     

A high-efficiency polarimeter for the measurement of the deuteron tensor polarization t20inπ+-d scattering

We describe the design and the calibration of a high-efficiency polarimeter which is sensitive to the deuteron tensor polarization t₂₀ and capable of working in a high background. The polarimeter is based on the ³He(d,p)⁴He reaction, calibrated in the energy range between 10 and 36 MeV; it has an average efficiency of 0.7 × 10^?4 and an average analyzing power of T₂₀ = ?0.5. The use of this polarimeter to measure the tensor polarization t₂₀ of the recoil deuterons in π⁺-d elastic scattering is described.     

Determination of proton beam polarization at high energies by measurements after deceleration

We are investigating a new method to determine the polarization of proton beams accelerated to high energies by measurements after deceleration to low energies, where simple and precise techniques can be used based on the large and well known analyzing power of pp elastic scattering. The polarized proton beam of SATURNE II was accelerated to 520 MeV and its polarization was measured by extracting the beam onto the NN beam line polarimeter.The beam was then accelerated to 800 MeV, decelerated to 520 MeV and again extracted. The loss in polarization is due to crossing twice the intrinsic depolarizing resonance γG = 3 at 631 MeV with adiabatic spin flip, once during acceleration and once during deceleration. The depolarization was intentionally increased by partially correcting the resonance, thus making the adiabatic flip less complete. The correction was introduced either at the rise or at the descent. The final polarization was the same in both cases showing that the depolarization was, as expected, the same during acceleration and deceleration. Another measurement was performed between 880 and 1200 MeV crossing successively two intrinsic resonances $\text{γG = ν}{}_{\mathrm{z}}\text{at}\text{?900}\text{Mev}$ and γG = 4 at 1145 MeV. Here the polarization at 1200 MeV was measured directly and is compared to the value calculated from the measurements at 880 MeV before accelerating to 1200 MeV and after decelerating from 1200 MeV, assuming symmetric depolarization. The measured and the calculated values agree with ΔP_B = 0.03 at $\text{P}{}_{\mathrm{B}}\text{? 0.75}$.     

Neutron Scattering Facility for the Measurement of Light Quenching Factors of Dark Matter Detectors at Low Temperatures

The light yield of scintillating crystals which is quantified by light Quenching Factors (QFs) strongly depends on the kind of interaction in the crystal. For Dark Matter experiments like CRESST the precise knowledge of QFs is crucial for the discrimination of background events from possible WIMP signals. At the tandem accelerator of the Maier-Leibnitz-Laboratorium (MLL) in Garching a low-temperature scattering facility was set up, which in its current phase aims at the determination of the QFs of O, Ca, and W in CaWO₄ crystals as used in the CRESST experiment. A CRESST detector module consists of a 300?g CaWO₄ target crystal operated as a phonon detector and a separate silicon-on-sapphire (SOS) light detector to detect the corresponding scintillation light simultaneously. In order to disentangle the light yield corresponding to O, Ca and W recoils, monoenergetic neutrons (11 MeV) produced by the accelerator are scattered off an especially developed CRESST-like detector module, which is operated at mK temperatures in a dilution refrigerator. Arrays of liquid-scintillator detectors placed at fixed scattering angles allow one to identify the recoiling nuclei by a neutron time-of-flight measurement. The unique facility is suited for the characterization of different detector materials and will be a powerful tool also for the future multi-material experiment EURECA. We report on the experimental approach, the low-temperature setup and present first results.     

National Ignition Facility laser performance status

The National Ignition Facility (NIF) is the world's largest laser system. It contains a 192 beam neodymium glass laser that is designed to deliver 1.8 MJ at 500 TW at 351 nm in order to achieve energy gain (ignition) in a deuterium-tritium nuclear fusion target. To meet this goal, laser design criteria include the ability to generate pulses of up to 1.8 MJ total energy, with peak power of 500 TW and temporal pulse shapes spanning 2 orders of magnitude at the third harmonic (351 nm or 3omega) of the laser wavelength. The focal-spot fluence distribution of these pulses is carefully controlled, through a combination of special optics in the 1omega (1053 nm) portion of the laser (continuous phase plates), smoothing by spectral dispersion, and the overlapping of multiple beams with orthogonal polarization (polarization smoothing). We report performance qualification tests of the first eight beams of the NIF laser. Measurements are reported at both 1omega and 3omega, both with and without focal-spot conditioning. When scaled to full 192 beam operation, these results demonstrate, to the best of our knowledge for the first time, that the NIF will meet its laser performance design criteria, and that the NIF can simultaneously meet the temporal pulse shaping, focal-spot conditioning, and peak power requirements for two candidate indirect drive ignition designs.     

Relationship between photorefractive activity and Raman scattering in lithium niobate crystals

We performed Raman scattering experiments in pure, Mg-doped and Fe-doped single crystals of lithium niobate. The measurements were carried out in 90° geometry at several laser beam intensities, and in several polarization arrangements. The possible occurrence of light-induced polarization-anisotropic scattering, and its effects on Raman spectra, were checked by simultaneously measuring the time dependence of the laser beam intensity transmitted by the sample with unchanged polarization, and the time dependence of the intensity of Raman lines from vibrational modes of known symmetry. The results give no indication of any effect of photorefractive activity on Raman observations in our pure and Mg-doped samples. On the contrary, in Fe-doped lithium niobate the build-up of light-induced o–e scattering strongly affected the relative intensities of Raman lines in specific experimental arrangements. We discuss the possible application of these effects to the characterization of lithium-niobate-based materials with strong photovoltaic and photorefractive activity.     

Stress measurements in glass by use of double thermal gratings

We developed a nondestructive and noncontact method for measuring stress at the midplane of tempered glass plates that uses Bragg scattering from a pair of thermal gratings. These gratings are formed by 1064-nm beams from a seeded Nd:YAG laser, and we measure the polarization state of light from a 532-nm beam that scatters from both thermal gratings. The change in polarization of the doubly scattered light with separation between the two gratings allows measurement of the in-plane stress. A model of the Bragg scattering efficiency, experimental investigations of the scattered beams, and stress measurements are reported.     

Anisotropy and multiple scattering in thick mammalian tissues

A dual-channel Mach-Zehnder interferometer using heterodyne detection allowed us to measure simultaneously parallel and perpendicular polarization components through various mammalian tissues at a wavelength of lambda = 633 nm. By contrast with liver tissue, squeletic muscles of a few millimeters thickness exhibit strong anisotropic properties that change the direction of the linear polarization of the light. This rotation of the initial plane of polarization is to be distinguished from the depolarization that is due to the multiple light scattering that goes along with large temporal fluctuations. Complementary photos under linearly polarized light illustrate the behavior difference between liver (isotropic medium) and muscle (anisotropic medium).