| The figure shows a neutrino vertex in the CHORUS
nuclear emulsion, as it appears under a microscope of 50x magnification.
The size of the image shown here is about 100 × 100µm. Nuclear
emulsion is a solution of AgBr crystals in a gelatinous medium. Ionizing
particles traversing the emulsion leave a trail of "excited" crystals.
The photographic development procedure transforms these "excited" crystals
in spheres of metallic silver, less than a micron in diameter, whilst at
the same time removing the AgBr crystals that have not been ionized. The
silver grains are fully reflecting and they appear as tiny, black spots
on the white background of the transmitted light. The brightness variations
in the image can be ascribed to varying transmission of the emulsion, both
below and above the slice imaged. The thickness of the imaged slice corresponds
to the depth of focus of the optical system, which is a few microns.
The image exhibits essentially all features of the CHORUS nuclear emulsion.
a) Black tracks. The most conspicuous feature of the image is the series of dark lines emerging from the center. These correspond to a series of silver grains, so closely spaced that they appear as a single trait. A large density of grains indicates a highly ionizing track, typical of low energy charged particles. In this case, the tracks arise from the passage of nuclear fragments emerging from the nucleus with which the neutrino has interacted. The neutrino direction is perpendicular to the image and the nuclear fragments are seen to come out at large angles with respect to the neutrino direction. The tracks in the lower half of the image emerge at a smaller angle. Thus, within the depth of focus of a few microns, only a relatively short section of the track is in focus.
b) Cosmic ray tracks. Apart from the dark tracks emerging from the center of the image, a number of low ionization tracks can be discerned: series of aligned grains, at distances of the same order as the grain size. Since the emulsions have been exposed for almost two years, they have integrated a large number of tracks from charged cosmic ray particles. Most of these are high-energy particles, in this context referred to as minimum ionizing particles. The CHORUS emulsion sensitivity corresponds to about 30 developed grains along the track of a minimum ionizing particle. A relatively large section of these tracks appears in a single slice since the particles have traversed the emulsion in the vertical plane, the orientation in which the plates have been exposed.
c) Low energy electrons. Here and there, one can see very short, curved tracks. These mark the passage and absorption of low energy electrons. Because of their small mass and low energy, they are subject to a large series of scatters, which explains their curly appearance. Low energy electrons arise either from nuclear decays in the emulsion at the level of natural radioactivity or from delta rays, electrons knocked out of their atomic orbit by passing, high energy particles.
d) Fog. Apart from the above-mentioned sources
of silver grains in the developed image, there is some level of random
background. These are grains not induced by ionization of charged particles,
but rather arising "spontaneously" in the development process. The density
of fog grains is strongly dependent on the type of emulsion, as well as
on the details of the development procedure. They constitute an important
background that needs to be taken into account in any pattern recognition
However, note that the main interest of emulsion scanning in the CHORUS experiment can not be shown in this single image. The tracks of interest to the CHORUS analysis are the high energy, minimum-ionizing particles emerging from the neutrino interaction. These are at relatively small angles from the neutrino direction and thus perpendicular to the image. In any given image one, at most two, grains of such tracks will be within the depth of field. In fact, in the image shown, a single grain of one such track can be seen at the position, which appears as the neutrino vertex. The vertex point itself is actually a few microns below the image shown.