O. Konno

THE LARGE-ACCEPTANCE SPECTROMETER TAGX FOR PHOTOREACTION STUDIES AT THE 1.3-GEV TOKYO ELECTRON SYNCHROTRON

Abstract The study of photoreactions on light nuclei with the TAGX spectrometer started in 1987 using the 20% duty-cycle tagged-photon beam at the 1.3-GeV Tokyo electron synchrotron. TAGX is comprised of a π-sr magnetic spectrometer for detection of charged pions, kaons, and protons and a 0.85-sr time-of-flight spectrometer for neutrons. It has served in the past eight years as a unique medium-energy-resolution multi-particle spectrometer for coincidence experiments to detect such final states as pn, pp, π + π − , ppn, pp − , and pn π + π − : some of which were kinematically-complete measurements of three-particle and four-particle final states. Details of the detector components, their performance, data acquisition, event reconstruction analyses, and detector-acceptance calculations are described together with the results of experience acquired in those experiments. A TAGX improvement in the momentum resolution required for charged particle measurements in the 1-GeV photon energy region is also reported.

The 150-MeV photon tagging system at Sendai

Abstract A description is given of the 32-channel photon tagging system recently constructed at the Laboratory of Nuclear Science. Tohoku University. Some results of photonuclear experiments performed using this system in conjunction with the high duty factor electron beam from the pulsed beam stretcher are presented.

A large-volume liquid-scintillator neutron detector

Abstract A large-volume (13.3 l ) liquid-scintillator (NE213) neutron detector has been developed. It is constructed from a long aluminum cylinder coated internally with a stable and inexpensive specular reflector, and provides a rugged, highly efficient neutron detector, suitable for TOF measurements. It has good n−γ discrimination and a time resolution of 0.72 ns FWHM in a photon tagging experiment. The absolute neutron detection efficiencies for E n = 15, 21 and 27 MeV and a detection threshold of 6 MeVee were 6.8, 7.4 and 8.5%, respectively.

A new type of liquid-3He target system using small mechanical refrigerators

Abstract A new type of liquid- 3 He target has been developed for photoabsorption experiments at intermediate energies. Using the cooling power of liquid 4 He at reduced vapour pressure, 3 He gas is liquefied into a cylindrical target cell of 180 ml and is maintained at 2.0 K during the experiment. Evaporated 4 He gas is evacuated by a rotary pump and returned into the 4 He bath in the cryostat, where two small mechanical refrigerators with cooling capacities of 3 W at 4.3 K and 10 W at 20 K are operated for the purpose of 4 He recondensation. A maintenance-free operation of more than 1000 hours has become possible by adopting the 4 He circulation system.

A STRAW DRIFT CHAMBER FOR THE TAGX SPECTROMETER

Abstract A new detector element, the straw drift chamber (SDC), has been added to the TAGX spectrometer to improve the momentum resolution. The construction of the chamber is described, and the procedures for analysis of the SDC data, including the determination of the drift time to drift distance relation and track reconstruction, are summarized. With the SDC, the momentum resolution of TAGX improves by approximately 50%.

An electron-beam transport system for parasitic experiments

Abstract A new beam line to allow parasitic experiments in two separate experimental halls has been installed. The total length of the beam line is about 80 m. Two achromatic 90° deflecting systems have been incorporated. For the beam delivered to the second experiment, the beam diameter was 2 mm and the emittance was 1 mm mrad. The duty factor was reduced to an average of 50% from 80%. This decrease was satisfactorily recovered using a deflector system synchronized with the beam pulse. The tagged photon beam, and experimental conditions were significantly improved when the transported beam was used. Successful parasitic experiments involving electron scattering and tagged photon experiments have now been demonstrated.

Observation of fine thermalization effects in the electrofission of preactinide nuclei

The absolute electrofission cross sections of Au and Ta were measured in the energy interval 40-250 MeV. Pronounced inflexions of the (e,f) curves are observed for both Au and Ta around 200-220 MeV, which are signatures of structures in the corresponding photofission cross-section curves. We show that these ( gamma ,f) structures are related to peculiarities of the thermalization process.