9 Nuclear Emulsions
385
electronic and ionic processes are repeated by receiving electrons from the reducer
through the latent image centre because it is a deep electron trap. This repetition
lasts until all the crystals are reduced. The reaction is expressed as follows:
Red + nAg
+
nAg + Ox + mH
+ ,
where Red and Ox are the developing agent and the oxidized developing agent,
respectively; n is the number of ions and m is the number of protons produced. Thus,
a metallic silver filament remains at the position of the crystal with a latent image
centre, whereas crystals without latent image centres remain unchanged. The gain
of this amplification is very high, O(10 8 ). After washing out the remaining AgBr
crystals via the fixing procedure, particle tracks are ready to be observed under the
microscope, as shown in the right image of Fig. 9.1.
The detection efficiency of a single crystal for minimum ionizing particles (MIP)
is about 0.17 [2]. The sensitivity of nuclear emulsions is translated into the number
of grains per unit length. A typical emulsion has a sensitivity of 30–50 grains per
100 µm along the particle trajectory for minimum ionizing particles. Apart from
the crystal size and chemical sensitisation, the sensitivity scales with the volume
occupancy of the AgBr crystals with respect to the total volume of the emulsion
layer, which ranges from 30 to 55%. The number of grains is proportional to the
ionization power of the particle, which allows the measurement of local energy
deposition (dE/dx) of each track. The random noise, so called “fog”, is due to
several reasons, such as thermal noise, gelatine impurity and over-sensitisation.
In general, a fog density of <5 grains/10-µm-cubic is considered acceptable. In
the process of producing nuclear emulsion as detectors, emulsion layers with
thicknesses of 10–300 µm are formed on a glass or plastic base. To track high-energy
particles (>100 MeV), a double-side coated emulsion film with a 50-µm-thick
emulsion layer on either side of a 200-µm-thick plastic base is often employed.
To observe both emulsion layers across the plastic base with optical microscopes,
the plastic base material should not have double refraction; e.g. triacetyl cellulose
and polymethylmethacrylate are appropriate.
The RMS resolution of a one-dimensional detector with a segmentation pitch
of D is D/
√
12. Assuming that the silver halide crystal shape is approximately
spherical, the resolution with a crystal diameter D is
√
πD/8 (RMS). For example,
this gives 44 nm for an emulsion with 200-nm-diameter crystals. In reality, these
values are slightly larger owing to the delta-ray component. A measured resolution
of 50 nm (RMS) was reported for an emulsion film with a 200-nm crystal size by
using high-energy particles [3], as shown in Fig. 9.2. The one-dimensional intrinsic
angular resolution of a double-sided emulsion film with 200-nm-diameter crystals
and a base thickness of 200 µm is therefore 0.35 mrad. Owing to the excellent
position and angular resolution, one can build a vertex detector, while using a
sampling calorimeter to reconstruct electromagnetic showers and also measuring
the momentum of particles by the multiple Coulomb scattering, which will be
discussed in Sect. 9.2. Nuclear emulsion detectors may be coupled with electronic
detectors to add timing information and/or muon identification. Since emulsion
385
electronic and ionic processes are repeated by receiving electrons from the reducer
through the latent image centre because it is a deep electron trap. This repetition
lasts until all the crystals are reduced. The reaction is expressed as follows:
Red + nAg
+
nAg + Ox + mH
+ ,
where Red and Ox are the developing agent and the oxidized developing agent,
respectively; n is the number of ions and m is the number of protons produced. Thus,
a metallic silver filament remains at the position of the crystal with a latent image
centre, whereas crystals without latent image centres remain unchanged. The gain
of this amplification is very high, O(10 8 ). After washing out the remaining AgBr
crystals via the fixing procedure, particle tracks are ready to be observed under the
microscope, as shown in the right image of Fig. 9.1.
The detection efficiency of a single crystal for minimum ionizing particles (MIP)
is about 0.17 [2]. The sensitivity of nuclear emulsions is translated into the number
of grains per unit length. A typical emulsion has a sensitivity of 30–50 grains per
100 µm along the particle trajectory for minimum ionizing particles. Apart from
the crystal size and chemical sensitisation, the sensitivity scales with the volume
occupancy of the AgBr crystals with respect to the total volume of the emulsion
layer, which ranges from 30 to 55%. The number of grains is proportional to the
ionization power of the particle, which allows the measurement of local energy
deposition (dE/dx) of each track. The random noise, so called “fog”, is due to
several reasons, such as thermal noise, gelatine impurity and over-sensitisation.
In general, a fog density of <5 grains/10-µm-cubic is considered acceptable. In
the process of producing nuclear emulsion as detectors, emulsion layers with
thicknesses of 10–300 µm are formed on a glass or plastic base. To track high-energy
particles (>100 MeV), a double-side coated emulsion film with a 50-µm-thick
emulsion layer on either side of a 200-µm-thick plastic base is often employed.
To observe both emulsion layers across the plastic base with optical microscopes,
the plastic base material should not have double refraction; e.g. triacetyl cellulose
and polymethylmethacrylate are appropriate.
The RMS resolution of a one-dimensional detector with a segmentation pitch
of D is D/
√
12. Assuming that the silver halide crystal shape is approximately
spherical, the resolution with a crystal diameter D is
√
πD/8 (RMS). For example,
this gives 44 nm for an emulsion with 200-nm-diameter crystals. In reality, these
values are slightly larger owing to the delta-ray component. A measured resolution
of 50 nm (RMS) was reported for an emulsion film with a 200-nm crystal size by
using high-energy particles [3], as shown in Fig. 9.2. The one-dimensional intrinsic
angular resolution of a double-sided emulsion film with 200-nm-diameter crystals
and a base thickness of 200 µm is therefore 0.35 mrad. Owing to the excellent
position and angular resolution, one can build a vertex detector, while using a
sampling calorimeter to reconstruct electromagnetic showers and also measuring
the momentum of particles by the multiple Coulomb scattering, which will be
discussed in Sect. 9.2. Nuclear emulsion detectors may be coupled with electronic
detectors to add timing information and/or muon identification. Since emulsion