Nuclear wave functions of microscopic a-particle model, which were obtained in the generator coordinate calculation by the present authors, are found to reproduce properties of low energy levels of 12 C nucleus, i.e., a-decay widths, electric transitions and charge form factors for electron scattering. Especially calculated widths of the Oz +and 10.3 MeV levels are found to be in excellent agreement with experiment, assuming that the spin of the latter is 2. The monopole transition from the o, + state to the ground state is also well reproduced as well as the E2 transition to the 2, + state. Furthermore analysis of the reduced width amplitudes is made which clarifies characters of the levels. It is found that in the o, + and 10.3 MeV states an a-particle is coupled mainly to the 'Be ground state with orbital angular momenta l equal to 0 and 2 respectively. In the predicted higher positive-parity states O, +, 2, + and 4, +, an a-particle is coupled mainly to the first excited state of 'Be with l equal to 2. § 1. IntroductionIn 12 C nucleus the 0/ level at 7.7 MeV and the next positive-parity level at 10.3 MeV are well known as anomalous states due to their low excitation energies and extremely large a-reduced widths. v From the viewpoint of nuclear shell model, low-lying o+ states in 12 C and 16 0 are considered to come from four-particle excitations. But the experimental excitation energies are too low to be reproduced by simple four-particle four-hole shell-model configurations. Morinaga suggested that the 0, + and 10.3 MeV levels form a rotational band with a linear chain structure of 3 a-particles.') According to Ikeda's diagram,') we can expect the appearance of nuclear states with some cluster structure near the threshold energies for decay into a-particles. The above-noted excited states in 12 C are expected to be such states, that is, to have distinct cluster structure which is associated with the 8 Be+a channel open at 7.4 MeV. Ikeda et al. 4 l analyzed a-decay widths of 3 a-cluster configurations by using the Bloch-Brink wave function 5 J and suggested a structure of loosely coupled 3 a-particles for the excited states. Horiuchi succeeded in obtaining a shell-like ground state and an excited state with a well-developed cluster structure by extending the orthogonality condition model to three-cluster system.At the same time, full microscopic calculations were made on this nucleus by several authors. 6 l, 7 l They, however, restricted freedom of 3 a-particle motion to by guest on April 10, 2015 http://ptp.oxfordjournals.org/ Downloaded from 1622 E. Uegaki, Y. Abe, S. Okabe and I-I. Tanalw the linear chain configuration 61 or to the hinge-angle vibration.n As a result they obtained only moderate success for the excited states.We recently investigated the structure of 12 C nucleus by using the microscopic ct-particle model of Bloch-Brink and by taking full account of motion of :1 a-clusters (Ref. 8), hereafter referred to as I). vVe succeeded in reproducing all the low energy levels of 12 C nucleus (except f...

An integrating x-ray area detector that operates on the basis of laser-stimulated luminescence was used in a diffraction study of muscle contraction. The area detector has a dynamic range of 1 to 10(5), a sensitivity about 60 times greater with approximately 1/300 as much fog background as x-ray film. It is erasable and reusable but, like film, can integrate at a practically unlimited counting rate. The high sensitivity and wide dynamic range of the detector resulted in a sufficient reduction in the exposure time to make possible the recording of a clear x-ray diffraction pattern, with up to 2.0-nanometer axial spacing, from a contracting frog skeletal muscle in as little as 10 seconds with synchrotron radiation. During the isometric contraction of the muscle, most of the actin diffraction lines increased in intensity without noticeable changes in their peak positions. Changes also occurred in diffraction intensities from the myosin heads. The results indicate that during contraction the structure of the actin filaments differs from that in the rigor state, suggesting a possible structural change in the actin subunits themselves; the myosin heads during contraction retain the axial periodicity of the myosin filament and become aligned in a more perpendicular manner to the actin filaments.

Time-resolved x-ray diffraction studies have been made on the 5.9- and 5.1-nm actin layer lines from frog skeletal muscles during an isometric tetanus at 6 degrees C, using synchrotron radiation. The integrated intensities of these actin layer lines were found to increase during a tetanus by 30-50% for the 5.9-nm reflection and approximately 70% for the 5.1-nm reflection of the resting values. The intensity increase of both reflections was greater than that taking place in the transition from rest to rigor state. The intensity change of the 5.9-nm reflection preceded those of the myosin 42.9-nm off-meridional reflection and of the equatorial reflections, as well as the isometric tension development. The intensity profile of the 5.9-nm layer line during contraction was found to be different from that observed in the rigor state.

SUMMARY1. The mechanism of the enhancement and the deficit of isometric force by slow length changes in frog skeletal muscle was studied with the time-resolved X-ray diffraction technique, using intense X-rays of synchrotron radiation.2. When a tetanized muscle was slowly stretched by 4 % from sarcomere lengths 2-3-2-4 ,um, the force rose to a peak during stretch and then decreased to a steady level 10-15 % higher than that immediately before stretch.3. The intensity of the 1,1 equatorial reflection decreased nearly linearly during stretch and then again increased after the completion of stretch, reaching a steady level 12+5% (mean + S.D., n = 11) lower than that immediately before stretch. The above 1,1 intensity change was roughly a mirror image of the force change.4. The intensity of the 1,0 equatorial reflection showed no marked changes in response to a slow stretch, except for an initial transient increase observed occasionally.5. If a tetanized muscle was slowly released by 4 % from sarcomere lengths 2-3-2'4 ,sm, the steady force attained after the completion of release was lower than that immediately before release. 6. The 1,1 intensity increased slightly during release, while the 1,0 intensity did not change significantly.7. The half-width of both the 1,0 and the 1,1 reflections did not change appreciably in response to slow length changes.8. Slow length changes always produced changes in the spacing between the reflections as expected from the constant-volume behaviour of the myofilament lattice.9. These results indicate that a slow stretch produces disordering of the myofilament lattice in such a way that the thin filaments are displaced from trigonal positions in the thick filament lattice. The resulting increase in the overall repulsion * Where this work was carried out. t To whom correspondence should be addressed.

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