Search

The effects of fine metal oxide particles, particularly those of high-density elements (7.7 to 9.7 x 10(3) kg/m3), on the acoustic properties of silicone rubber have been investigated in order to develop an acoustic lens with a low acoustic attenuation. Silicone rubber doped with Yb2O3 powder having nanoparticle size of 16 nm showed a lower acoustic attenuation than silicone rubber doped with powders of CeO2, Bi2O3, Lu2O3 and HfO2. The silicone rubber doped with Yb2O3 powder showed a sound speed of 0.88 km/s, an acoustic impedance of 1.35 x 10(6) kg/m2s, an acoustic attenuation of 0.93 dB/mmMHz, and a Shore A hardness of 55 at 37 degrees C. Although typical silicone rubber doped with SiO2 (2.6 x 10(3) kg/m3) shows a sound speed of about 1.00 km/s, heavy metal oxide particles decreased the sound velocities to lower than 0.93 km/s. Therefore, an acoustic lens of silicone rubber doped with Yb2O3 powder provides increased sensitivity because it realizes a thinner acoustic lens than is conventionally used due to its low sound speed. Moreover, it has an advantage in that a focus point is not changed when the acoustic lens is pressed to a human body due to its reasonable hardness.

The energy spectra (40 to 300 MeV/n) of galactic cosmic ray particles (C, N, O, Ne, Mg, Si, and Fe) observed by the HEP-HI silicon detector telescope onboard the GEOTAIL satellite were investigated over the period from January 1993 to May 1996, from the viewpoint of solar modulation. From the GEOTAIL data and those of relativistic heavy particles observed by HEAO-3, the modulation potential φ was obtained by using a solution of the Fokker-Plank equation with the “force-field” approximation. In the equation the diffusion coefficient was used as adjustable parameter, under the assumptions of a fixed velocity of solar wind (400km/sec) and the radius of the heliosphere of 50AU. The time variation of the modulation potential thus obtained is compared with that obtained from the CLIMAX neutron monitor. A reasonable correlation between both time variations is found when five months delay is applied to the results obtained by the HEP-HI telescope.

Using 5-in.-diameter Si(Li) detectors with a detector thickness of 1, 2 and 3 mm combined with 62 mm × 62 mm position-sensitive Si detectors, an excellent isotope telescope has been constructed for the Geotail mission. The performance of the telescope and the individual detectors has been investigated using 56Fe beams with energies between 430 and 696 MeV/n. The nonuniformity of the detector thickness as measured by exposing the detectors to a monochromatic Fe beam is 0.44% for 1 mm thick detectors, 0.54% for 2 mm thick detectors and 0.15% for 3 mm thick detectors. The secondary isotopes of Al through Fe produced by 430 MeV/n 56Fe in a polyethylene target were clearly resolved by the telescope. The measured mass resolution for Si, Ca and Mn were 0.36 amu, 0.42 amu and 0.44 amu in FWHM, respectively. The telescope has the following two key features: an excellent resolution (better than 0.5 amu) and an exceptionally high sensitivity ( SΩ = 43 cm 2 sr ) for a broad range of species from He through Ni with energies between 2.4 and 210 MeV/n.

35 × 10 6 kg/m 2 sv s.Ztissue = 1.55 × 10 6 kg/m 2 s. In general, the ultrasound is reflected at the boundary with different Z. Moreover, a convex-shaped acoustic lens is attached on top of the acoustic matching layers to focus the ultrasound beam and to ensure good contact to the human body. Specifically, the acoustic lens is used for focusing of the short axis; the elevation dimension determines the effective slice thickness of the image plane. The ultrasound transmission is attenuated in acoustic matching layers, acoustic lens, and human tissue. The acoustic attenuation (α) of the acoustic lens is very important because it is usually thicker than the matching layers. Moreover, α becomes large with increasing frequency; the α of the acoustic lens has a great effect on the sensitivity of a medical ultrasonic probe as well. Silicone rubber has been the predominant acoustic lens material for external focusing since 1974 (Iinuma and Hashiguchi 1974, 1978) because it is safe to use

Effects of fine nano-powder ceramic powders on the acoustic properties of silicone rubber have been investigated, in order to develop an acoustic lens material with a low sound velocity (c) and a low acoustic attenuation (alpha). Silicone rubber doped with Yb2O3 powders having nanopowder size of 16 nm showed lower alpha than silicone rubber doped with other powders. The silicone rubber doped with Yb2O3 powder showed a density of 1.54 times 103 kg/m3, c=919 m/s, an acoustic impedance Z= 1.42 times 106 kg/m2s, alpha = 1.08 dB/mm/MHz at 25degC. The low c and alpha of the lens material contributes to realize a low loss acoustic lens for medical array probes for diagnostic application.