K. Niki

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.

Construction and beam tests of a 25.5 MHz split coaxial RFQ for radioactive nuclei

Abstract A 25.5 MHz split coaxial RFQ with modulated vanes has been constructed at INS. The RFQ will accelerate radioactive nuclei with a charge-to-mass ratio greater than 1 30 from 2 to 172 keV/u. The cavity, 0.9 m in inner diameter and 8.6 m in length, comprises four unit cavities, each of which comprises three module cavities further. The modulated vanes have been made by using both of a three-dimensional cutting technique and a two-dimensional one, the former for the first unit cavity, and the latter for the other ones. The transverse radius of curvature of the vane-tip is variable along the beam axis in a low-energy part, and constant in a high-energy part (the boundary at 1.1 m down from the RFQ entrance). The vanes have been aligned with an error less than ± 40 μm before installation in the unit cavities. After completion of the four unit cavities, they have been aligned with an error less than ± 50 μm on the floor of an accelerator room. Low-power tests after cavity-tuning show that the longitudinal voltage flatness and the azimuthal field balance are better than ± 1%, the resonant frequency is 25.46 MHz, and the unloaded Q-value is 5800, corresponding to a resonant resistance of 22 kΩ. We have achieved so far the goal intervane voltage of 109 kV at a duty factor of 15%. Through acceleration tests with N+ ions, we have obtained the following results: the output-beam emittances are well in the design ellipses of 0.06π cm mrad normalized; the data of transmission efficiency versus intervane voltage agree well with PARMTEQ results and the transmission at the nominal voltage is measured to be 90%.

Measurement of the 4He (γ, npp) n reaction in the Δ-resonance region

Abstract This is the first kinematically complete measurement of the 4He(γ, npp)n reaction in the energy range of 135–455 MeV. The ordered-momentum spectra and the double-differential cross sections are compared with the calculations which assume that photons are absorbed either on two- (np or pp), three- (npp), or four-nucleon systems. The ordered-momentum spectra below 400 MeV are reproduced well by pp and/or npp absorption but not by np absorption only.

New ISOL-based radioactive nuclear beam facility at INS

Abstract An ISOL-based radioactive nuclear beam facility is just about to come into operation at INS. The present status of the INS radioactive nuclear beam project is reported. The capability of the facility and possible experiments are also discussed, including research programs of nuclear physics and nuclear astrophysics.

The total cross section for the 4He (γ, npp)n reaction in the Δ-resonance region

Abstract This is the first measurement of the total cross section σ( 4 He (γ npp)n) in the range of 135–455 MeV. The σ, which cannot be reproduced by any model, has similar E γ dependence to γ T (γ 4 He → all channels). The averaged ratio σ( 4 He(γ, npp)n)/ σ T (γ 4 He → all channels) = (2.3 ± 0.3)% is compared with a cascade-model calculation in which quasi-free pion production followed by pion reabsorption is assumed.

Quasi-free K+ photo-production in 12C

Abstract Quasi-free K + photo-production in the 12 C ( γ , K + ) reaction has been investigated in a photon energy range of 0.7–1.1 GeV. Differential cross sections for the quasi-free process of the 12 C ( γ , K + ) reaction have been obtained and they are compared with a calculation of a quasi-free K + photo-production. The effective proton number Z eff = 4.2 ± 0.6 obtained from the experiment was in good agreement with a calculation of a semi-classical attenuation model.

A step-like rise in the 4He(γ,pn)2H cross section near the pion-production threshold

Abstract A kinematically complete 4He(γ,pn)2H measurement was carried out in the photon energy (Eγ) range 145–425 MeV. The total cross section (σα) forms a prominent structure whose peak lies at Eγ ∼ 245 MeV (the pnd invariant mass of 3960 MeV/c2). The ratio σ α σ d , where σd is the deuteron photodisintegration cross section, shows a step-like rise to a value of 6 at Eγ ∼ 140 MeV.

Beam test results of the INS RFQ/IH linac

A linac complex for radioactive beams has been constructed at INS, which comprises a 25.5-MHz split coaxial RFQ (SCRFQ) with modulated vanes and a 51-MHz interdigital-H (IH) linac . The SCRFQ accelerates ions with a charge-to-mass ratio (q/A) greater than 1/30 from 2 to 172 keV/u. The beam from the SCRFQ is charge-stripped by a carbon-foil, and is transported to the IH linac through two magnetic-quadrupole doublets and a 25.5-MHz rebuncher cavity. The IH linac accelerates ions with a q/A greater than 1/10, and the output energy is variable in the range of 0.17 through 1.05 MeV/u. Beam tests of the linac complex performed with N ions show that the output beam energy and transmission efficiency agree well with predictions.

Simultaneous measurement of 3He(γ, pπ±) yields and model implications

Abstract 3 He(γ, pπ ± ) reactions are simultaneously measured for tagged photon energies of 380 ⩽ Eγ ⩽ 700 MeV. The yield ratio for the two channels, under the same kinematical and dynamical conditions, is found to be 0.90±0.07. Implications of this ratio in terms of the delta component in the ground state of 3He are examined. An alternate interpretation of this ratio as due to an admixture of isospin T = 1 2 and 3 2 transition amplitudes in a pure isovector interaction is also presented.

Status of the RNB facility at INS

A Radioactive beam facility using a thick target, an ISOL and heavy ion linacs will soon come into operation at INS, University of Tokyo. The status of the project together with experimental programs are reported.