polystyrene sphere in water) and a diameter much larger than the wavelength of the
light beam. The light beam is converged by the lens with high numerical aperture
(a microscope objective lens with NA ¼ 1.3 is often used). When the center of the
sphere moves on the optical axis off the focal point of the lens, the ray refracted by
the sphere will exert a force on the sphere that is toward the focal point; when the
center of the sphere becomes off-axis, the force is also directed toward the focal
point. Hence, the force exerted by the light beam is a restoring force that pulls the
sphere to the focal point; the magnitude of the force is approximately proportional to
Detector ring
Pre-amplifier
Detector
address
timing
Circuit for same time
Computer
Pre-amplifier
Detector
address
timing
pair annihilation
g ray
g ray
Man
Fig. 3.24 Scheme of PET
Human body injected by positron detection nuclide is placed within detector ring. PET has detectors
arranged on circle for γ-ray from the pair annihilation. When two detectors detect γ-ray at the same
time, and position of the pair annihilation is identified as middle point of both detectors
180°
+ -
Electron
Positron
11
C
11
B
Nucleus specie of positron relese
Flight distance
Radiation of pair annihilation
(511keV)
(511keV)
Radiation of pair annihilation
Fig. 3.23 Process of positron decay and pair annihilation
Water molecules occupy more than 60% in total weight of human body and pair annihilation occurs
between positron and electron of water molecule
3.9 Optical trapping
55