Difference between revisions of "ACCAPP 09 PhotFis Poster"

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-Introduction
-It has long been known that the nuclear fragments resulting fromphoton induced fission of heavy nuclei are emitted anisotropically when measured with respect to the incident photon
-direction. The first such measurement, performed with unpolarized photons was done in 1956 by Sommerfeld[1] who showed that the angular distribution depends on the polar angle
-theta. The introduction of linear photon polarization breaks theazimuthal symmetry by imposing a preferred direction in space perpendicular to the incident photon beam, which was
-shown in 1982 by Winhold [2] with the measurement of fission fragments from the polarized photofission of thorium.
-For linearly polarized photons and considering only electric dipole (E1) transitions, the photofission of an even-even
-nucleus gives the angular distribution of the fission fragments
-The angular distribution coefficients A0and A2depend on the transition state (J,K), where Kis the projection of the
-total spin  J on the symmetry axis of the deformed nucleus. For J = 1, K = 0, we have A0= 1/2, A2= -1/2 and for J =
-, K = 1, we have A0= 1/2, A2= 1/4. P
-γ is the photon polarization, and f2(1,1) = 3 sin2θ. θis the polar angle with
-respect to the beam and φisthe azimuthal angle (φ = 0 parallel to the electric field vector and φ = π/2perpendicular
-to E). The asymmetries for fragments emitted parallel and perpendicular to the polarization vector are large even for
-relatively low polarization.
-For any target thicker than a few mg/cm2, of course, the fission fragments are not detectable. The question we wish
-to address concerns whether or not the angular asymmetry in the fission fragments is manifest in the angular
-distribution of the prompt neutrons which they emit, thus providing a possible signature for the presence of
-photofission. Such a technique exploits the unique kinematics of the fission process in conjunction with the relative
-penetrability of the fission neutrons.
-Preliminary Measurements
-A low energy electron accelerator can produce 80 mApeak currents which can intersect a radiator target to produce bremsstrahlung photons. The bremsstrahlung photons enter an
-experimental cell through a collimator configured to select off axis polarized photons. A target (H2O/D2O), in the experimental cell, is positioned in the path of the polarized photons.
-Two neutron sensitive scintillatorsare placed at 90 degrees to the incident photon beam and in thesame plane as a 20 cm long target. A NaI crystal optically coupled to a
-photomultiplier tube (PMT) was placed off the beamline axis at the end of the experimental cell to monitor the incident photon flux.
-After polarized photons hit the target, both neutrons and photons are
-emitted and detected by the scintillator detectors. A typical time of flight
-spectrum is shown on the right with green line representing D2O (both
-photon and neutron peaks are present) and blue one representing H2O
-(virtually no neutron are detected). Preliminary results demonstrated the
-possibility of measuring the asymmetry?
-Conclusions and Outlook
-The possibility of obtaining polarized photons and measuring asymmetries was proved in the preliminary measurements. We propose to optimize our polarized beamline and
-construct a polarimeter to measure its performance. It consists of a pulsed electron accelerator delivering electrons which are bent 90 degrees in the horizontal plane. In order to
-change the photon polarization state, the electrons are then bent up or down in the vertical plane before they strike a Ti bremsstrahlung converter. The bremsstrahlung photons
-propagate down the beamline in a cone with a characteristic opening angle of me/Ebeamradians. A fixed collimator is placed downstream of the bremsstrahlung radiator and is offset
-in the horizontal plane. This collimator selects bremsstrahlung photons which are off axis from the primary electron beam, and therefore are linearly polarized 45 degrees with respect
-to the horizontal, with an orientation depending upon the angle of incidence of the electron beam on the bremsstrahlung radiator. Between the radiator and the collimator, a magnet
-deflects charged particles in the horizontal plane away from thephoton collimator. The photons exit the vacuum via a thin window at the downstream edge of the collimator on the
-downstream side of a concrete wall. This window serves as a thinconverter for a pair spectrometer luminosity monitor which is used for relative normalization of the photon flux in the
-two polarization states. The yields of the electron-positron pairs will be directly proportional to the photon flux at a photon energy given by the position of the plastic scintillator
-detectors and the pair spectrometer magnetic field setting, thusproviding a relative flux normalization between the two polarizations states. The pair converter thickness, electron and
-positron detector size and position, and magnetic field setting are chosen to provide electron-positron coincidences for about one out of every ten beam pulsesin order to minimize
-accidental coincidences. Electron-positron timing distributions will be recorded in a time to digital converter.
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Difference between revisions of "ACCAPP 09 PhotFis Poster"

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