مقاله انگلیسی رایگان در مورد توسعه یک مانیتور انتقال برای پرتوهای فوتون – الزویر 2018

ABSTRACT

A transmittance monitor has been developed for the second tagged-photon beamline at the Research Center for Electron Photon Science, Tohoku University, Japan. In this beamline, an internal radiator is employed to produce the bremsstrahlung photon beam out of circulating electrons in a synchrotron. The transmittance, which is defined as the probability of finding a photon coming to the target position when an electron is detected with a photon-tagging counter, should be determined to deduce cross sections for photo-induced reactions. The developed monitor consists of a telescope of thin plastic scintillators with a positron and electron converter, and a dedicated circuit implemented in a field-programmable gate array chip. The transmittance can be measured with this monitor for high-intensity photon beams corresponding to 20 MHz tagging signals. The measured transmittance is found to be constant with respect to the photon intensity for each photon-tagging channel.

Introduction

Photo-induced reactions with the incident energy ranging from several hundred MeV to a few GeV are important to study the structure of hadrons in detail. A hadron is a color-neutral object, and described in principle by quark and gluon dynamics as a solution of the fundamental theory of the strong interaction, quantum chromodynamics (QCD). At low energies, little is known for the solution owing to the large running QCD coupling constant. What are the effective degrees of freedom describing hadrons is a subject of interest associated with the color confinement problem. Meson photoproduction experiments have been conducted [1–5] to give information on the subject with an electromagnetic-calorimeter (EMC) system, FOREST [6], on the second photon beamline [7] at the Research Center for Electron Photon Science (ELPH), Tohoku University, Japan. Fig. 1 shows a schematic view of the second photon beamline at ELPH. Bremsstrahlung photons are generated by inserting a thin carbon fiber (radiator) with a diameter of 11 μm into circulating electrons [7] in the electron synchrotron called Booster STorage (BST) ring (previously known as the STB ring) [8]. The energy of each photon is determined by measuring the momentum of its corresponding post-bremsstrahlung electron with a tagging detector system, STB-Tagger II [7], having 116 photon-tagging channels. Each tagging signal is produced from a telescope of two scintillating fibers with a cross section of 4 × 4 mm2 , covering a photon energy with a width of 0.5–2.8 MeV (?). The tagging signal does not always correspond to a photon arriving at the target. Some of the tagged photons may be converted into a positron and electron (? +? −) pair owing to the material on the photon beamline. These undesirable ? +? − pairs are swept out with a dipole magnet, TAGX [9], which is placed in front of FOREST. Furthermore, a photon is not generated when Møller scattering or Coulomb multiple scattering takes place at the radiator, but the scattered electron might hit a tagging detector. Therefore, the photon transmittance (so called the tagging efficiency), which is defined as the probability of finding a photon coming to the target position when an electron is detected with a tagging detector, should be determined to deduce cross sections for photoinduced reactions. The transmittance is experimentally measured since it is difficult to incorporate all the conditions of circulating electrons in the calculation. So far, we have measured the transmittance, ?? , using an EMC module on the photon beamline. The energy of a photon is measured with the module in response to the tagging signal. The singles rate of the module should be reduced so that it could work, namely the photonbeam intensity should be much lowered. Thus, we make the circulating current in the BST ring very low during the transmittance measurement. We describe a photon beam in this condition as a faint beam.