9 Nuclear Emulsions
415
Particles (WIMPs) with target nuclei. The measurement of the direction of WIMPinduced nuclear recoils is a challenging strategy to extend the sensitivity of dark
matter searches beyond the neutrino-induced background event rate and provide
an unambiguous signature of the detection of Galactic dark matter [139]. Current
directional experiments are based on the use of gas TPC whose sensitivity is strongly
limited by the small achievable detector mass. Nuclear Emulsions for WIMP Search
with directional measurement, NEWSdm, is an innovative directional experiment
proposal based on the use of a solid target made by newly developed nuclear
emulsion films and read-out systems capable to detect nanometric trajectories.
The approach proposed by the NEWSdm Collaboration [140] consists of using a
nuclear emulsion-based detector acting both as target and as nanometric tracking
device. The NEWSdm project foresees the employment of NIT. The detector
is conceived as a bulk of NIT surrounded by a shield to reduce the external
background. The detector is then placed on an equatorial telescope in order to
absorb the Earth rotation, thus keeping fixed the detector orientation with respect to
the incoming apparent WIMP flux. The angular distribution of the WIMP-scattered
nuclei is therefore expected to be strongly anisotropic with a peak centred in the
forward direction.
NIT have a linear density of about 11 crystals/μm [124], thus making the
reconstruction of trajectories with path lengths as short as 100 nm possible, if
analysed by means of microscopes with enough resolution. The presence in the
emulsion gel of lighter nuclei such as carbon, oxygen and nitrogen, in addition
to the heavier nuclei of silver and bromine, is a key feature of the NEWSdm
project, resulting in a good sensitivity to WIMPs in the mass range between 10
to 100 GeV/c 2 .
In the NEWSdm experiment a WIMP signal consists of short-path, anisotropically distributed, nuclear recoils over an isotropically distributed background. The
search for signal candidates requires the scanning of the whole emulsion volume.
The read-out system has therefore to fulfil two main requirements: a fast, completely
automated, scanning system is needed to analyse the target volume over a time
scale comparable with the exposure; the spatial resolution achievement has to go
well beyond the diffraction limit, in such a way to ensure high efficiency and purity
in the selection of signal candidates. The analysis of NIT emulsions is performed
with a two-step approach: a fast scanning with a state-of-the-art resolution for the
signal pre-selection followed by a pin-point check of preselected candidates with
unprecedented nanometric resolution to further enhance the signal to noise ratio.
In the first analysis phase, a fast scanning is performed by means of an improved
version of the optical microscope used for the scanning of the OPERA films [141].
An R&D program has achieved a speed of about 200 cm 2 /h [123, 142].
The starting point of the emulsion scanning is the image analysis to collect
clusters making up silver grains. Given the intrinsic resolution of the optical
microscope (∼200 nm), the sequence of several grains making a track of a few
hundred nanometers may appear as a single cluster. Nevertheless, a cluster made
of several grains tends to have an elliptical shape with the major axis along the
direction of the trajectory, while a cluster produced by a single grain tends to have a
415
Particles (WIMPs) with target nuclei. The measurement of the direction of WIMPinduced nuclear recoils is a challenging strategy to extend the sensitivity of dark
matter searches beyond the neutrino-induced background event rate and provide
an unambiguous signature of the detection of Galactic dark matter [139]. Current
directional experiments are based on the use of gas TPC whose sensitivity is strongly
limited by the small achievable detector mass. Nuclear Emulsions for WIMP Search
with directional measurement, NEWSdm, is an innovative directional experiment
proposal based on the use of a solid target made by newly developed nuclear
emulsion films and read-out systems capable to detect nanometric trajectories.
The approach proposed by the NEWSdm Collaboration [140] consists of using a
nuclear emulsion-based detector acting both as target and as nanometric tracking
device. The NEWSdm project foresees the employment of NIT. The detector
is conceived as a bulk of NIT surrounded by a shield to reduce the external
background. The detector is then placed on an equatorial telescope in order to
absorb the Earth rotation, thus keeping fixed the detector orientation with respect to
the incoming apparent WIMP flux. The angular distribution of the WIMP-scattered
nuclei is therefore expected to be strongly anisotropic with a peak centred in the
forward direction.
NIT have a linear density of about 11 crystals/μm [124], thus making the
reconstruction of trajectories with path lengths as short as 100 nm possible, if
analysed by means of microscopes with enough resolution. The presence in the
emulsion gel of lighter nuclei such as carbon, oxygen and nitrogen, in addition
to the heavier nuclei of silver and bromine, is a key feature of the NEWSdm
project, resulting in a good sensitivity to WIMPs in the mass range between 10
to 100 GeV/c 2 .
In the NEWSdm experiment a WIMP signal consists of short-path, anisotropically distributed, nuclear recoils over an isotropically distributed background. The
search for signal candidates requires the scanning of the whole emulsion volume.
The read-out system has therefore to fulfil two main requirements: a fast, completely
automated, scanning system is needed to analyse the target volume over a time
scale comparable with the exposure; the spatial resolution achievement has to go
well beyond the diffraction limit, in such a way to ensure high efficiency and purity
in the selection of signal candidates. The analysis of NIT emulsions is performed
with a two-step approach: a fast scanning with a state-of-the-art resolution for the
signal pre-selection followed by a pin-point check of preselected candidates with
unprecedented nanometric resolution to further enhance the signal to noise ratio.
In the first analysis phase, a fast scanning is performed by means of an improved
version of the optical microscope used for the scanning of the OPERA films [141].
An R&D program has achieved a speed of about 200 cm 2 /h [123, 142].
The starting point of the emulsion scanning is the image analysis to collect
clusters making up silver grains. Given the intrinsic resolution of the optical
microscope (∼200 nm), the sequence of several grains making a track of a few
hundred nanometers may appear as a single cluster. Nevertheless, a cluster made
of several grains tends to have an elliptical shape with the major axis along the
direction of the trajectory, while a cluster produced by a single grain tends to have a