9 Nuclear Emulsions
395
the hybrid design allowed the identification of the K + meson and the accumulation
of a large statistics. Emulsion stacks were exposed vertically, perpendicular to the
beam. Emulsion plates were of two types: 550 μm thick emulsion layers on both
sides of a 70 μm thick polystyrene base, and 70 μm layers on both sides of a 500 μm
lucite base. Thinner films with thicker base were used to avoid the degradation of the
angular resolution due to distortion effects. Three double- hypernuclei candidates
were observed [57, 59]. However, no conclusive answer was provided on the -
interaction. With this aim, the E373 experiment at KEK [60, 61] searched for S =
-2 nuclei in nuclear emulsion with higher statistics. The apparatus was based on an
emulsion-counter hybrid method, where a laser microscope performed the threedimension graphic processing of the emulsion images, scintillating fiber blocks
detected the decay products of strange particles, and a glass capillary tracker filled
with liquid scintillator provided precise predictions of the − emission angle and
position. The experiment reported the observation of double hypernuclei and the
- interaction was finally measured [62, 63]. A follow-up experiment is planned
for the new J-PARC hadron facility at Tokai, still employing the hybrid detector
technique with an emulsion plate stack [64].
9.4 Nuclear Emulsion Detectors with Digital Technology
9.4.1 Automated Scanning Systems and Analysis Methods
A major breakthrough in the emulsion technique occurred in 1974 when the idea
of a tomographic read out of the emulsion plates was introduced by the Nagoya
group [65]. In the case of an emulsion layer about 20 times thicker than the focal
plane depth, one can take multiple tomographic images by sampling the emulsion
layer. Those images can then be superimposed according to a given value of the
presumed track slope, looking for space coincidences of the grains. After applying
a detection threshold needed to remove the accidental background, a track can be
defined. A first implementation of this concept led to the development of a first
generation system [47] where 16 tomographic images were superimposed and a TV
tube used to grab the image. This concept was developed and successfully applied
to the CS emulsion scanning of the E653 experiment at Fermilab [51].
This technique was further developed by the Nagoya group and led to the socalled Track Selector [66]. The TV video was replaced by a CCD camera, yielding
to higher stability and better space resolution. An FPGA-based image processor
handled the 16 tomographic images of each emulsion plate. The scanning speed was
actually limited by the time required for the computer-controlled objective lenses to
move to the 16 different focal positions, since for each step some time was needed
to damp the stage vibrations. Another limiting factor was the size of the optics field
of view. A tracking efficiency as large as 90% was reached, with the main source
of noise given by short Compton electron tracks. A scanning system based on a
395
the hybrid design allowed the identification of the K + meson and the accumulation
of a large statistics. Emulsion stacks were exposed vertically, perpendicular to the
beam. Emulsion plates were of two types: 550 μm thick emulsion layers on both
sides of a 70 μm thick polystyrene base, and 70 μm layers on both sides of a 500 μm
lucite base. Thinner films with thicker base were used to avoid the degradation of the
angular resolution due to distortion effects. Three double- hypernuclei candidates
were observed [57, 59]. However, no conclusive answer was provided on the -
interaction. With this aim, the E373 experiment at KEK [60, 61] searched for S =
-2 nuclei in nuclear emulsion with higher statistics. The apparatus was based on an
emulsion-counter hybrid method, where a laser microscope performed the threedimension graphic processing of the emulsion images, scintillating fiber blocks
detected the decay products of strange particles, and a glass capillary tracker filled
with liquid scintillator provided precise predictions of the − emission angle and
position. The experiment reported the observation of double hypernuclei and the
- interaction was finally measured [62, 63]. A follow-up experiment is planned
for the new J-PARC hadron facility at Tokai, still employing the hybrid detector
technique with an emulsion plate stack [64].
9.4 Nuclear Emulsion Detectors with Digital Technology
9.4.1 Automated Scanning Systems and Analysis Methods
A major breakthrough in the emulsion technique occurred in 1974 when the idea
of a tomographic read out of the emulsion plates was introduced by the Nagoya
group [65]. In the case of an emulsion layer about 20 times thicker than the focal
plane depth, one can take multiple tomographic images by sampling the emulsion
layer. Those images can then be superimposed according to a given value of the
presumed track slope, looking for space coincidences of the grains. After applying
a detection threshold needed to remove the accidental background, a track can be
defined. A first implementation of this concept led to the development of a first
generation system [47] where 16 tomographic images were superimposed and a TV
tube used to grab the image. This concept was developed and successfully applied
to the CS emulsion scanning of the E653 experiment at Fermilab [51].
This technique was further developed by the Nagoya group and led to the socalled Track Selector [66]. The TV video was replaced by a CCD camera, yielding
to higher stability and better space resolution. An FPGA-based image processor
handled the 16 tomographic images of each emulsion plate. The scanning speed was
actually limited by the time required for the computer-controlled objective lenses to
move to the 16 different focal positions, since for each step some time was needed
to damp the stage vibrations. Another limiting factor was the size of the optics field
of view. A tracking efficiency as large as 90% was reached, with the main source
of noise given by short Compton electron tracks. A scanning system based on a