ACCAPP 09 PhotFis Poster
File:ACCAPP 09 PhotoFisPoster V1.pdf
Introduction
It has long been known that the nuclear fragments resulting from photon 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 . The introduction of linear photon polarization breaks the azimuthal 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 A0 and A2 depend on the transition state (J,K), where K is 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 high rep rate linac capable of producing electron peak currents of 80 mAs was used in conjunction with a radiator target to produce bremsstrahlung photons. The bremsstrahlung photons entered an experimental cell through a collimator configured to select off axis polarized photons. A target of H2O or D2O, in the experimental cell, was positioned in the path of the polarized photons. Two neutron sensitive scintillators were placed at 90 degrees to the incident photon beam and in the same plane as the 20 cm long target. A NaI crystal, optically coupled to a photomultiplier tube, 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 using a D2O target is shown in green on the right. Very few neutrons are observed using H20 as compared with the D20 target. Preliminary asymmetries have been observed but are not being shown at this time.
Conclusions and Outlook
Preliminary measurements of neutron rates has shown the presence of polarized photons from off axis bremsstrahlung. We propose to optimize the apparatus used to produce polarized photons and construct a polarimeter to measure its performance. Electrons from a high repetition rate linac (HRRL) are accelerated up to 15 MeV and deflected 90 degrees in the horizontal plane towards an experimental cell. The electrons are then bent up or down in the vertical plane before they strike a Titanium bremsstrahlung converter, in order to change the photon polarization state. The resulting bremsstrahlung photons propagate down the beampipe in a cone with a characteristic opening angle of \frac{m_e}{Ebeam} (radians). A fixed collimator is placed downstream of the bremsstrahlung radiator and is offset in the horizontal plane. An off-axis collimator will select bremsstrahlung photons which are off axis from the primary electron beam and 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. A magnet, located between the radiator and the collimator, deflects charged particles in the horizontal plane away from collimator. Photons enter an experimental cell and 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 thin converter for a pair spectrometer used for relative normalization of the photon flux in each polarization state. The electron-positron pair production rate 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. 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 pulses in order to minimize accidental coincidences. Electron-positron timing distributions will be recorded in a time to digital converter.