ACCAPP 09 PhotFis Poster
File:ACCAPP 09 PhotoFisPoster V1.pdf
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 = 1, 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.