Fast automated scanning of emulsion films for the OPERA (CNGS1) experiment

1. Nuclear Emulsion

Nuclear emulsions are made of microcrystals of silver halides (AgBr) dispersed in a gelatin layer.  The energy released by ionizing particles to the crystals, produces a latent image which is almost stable in time.

A chemical development process reduces irradiated grains to metallic Ag. After fixing and washing (to remove undeveloped crystals) the gelatin is transparent; the paths of an ionizing particle is visible as sequence of black silver grains about 0.5 μm size. Ideal to detect short-lived particles (τ decay): - Three-dimensional spatial information. - Excellent Resolution: < 1 μm. - High hit density: ~ 300 hits/mm.

2. OPERA Experiment

OPERA is a long baseline experiment (located in the INFN Gran Sasso Underground Laboratories) which will investigate for νμ → ντ oscillations in the parameter range suggested by atmospheric neutrino experiments. The goal is to observe the appearance of τ leptons in a pure νμ beam produced at the CERN-SPS (the CNGS neutrino beam). The leptons are identified through the direct detection of their decays that, at the CNGS energies, are at distances of 1 mm from the production point. Therefore, a high precision detector is needed.

3. OPERA Lead-Emulsion Target

The OPERA detector is a hybrid system consisting of electronic detectors and a massive lead-emulsion target segmented into ECC bricks. The EmulsionCloud Chamber (ECC), a sequence of dense material (Lead) sheets, acting as target, interleaved with emulsion sheets, acting as high precision trackers, satisfies the need of both a large mass and a high precision tracking capability. Industrial mass production of 13600000 films by Fuji Co in 2 years. Real time analysis: • ~ 30 neutrino selected interactions per day; • ~ 6000 cm² per day have to be analyzed with a submicrometric precision during 5 years data taking.

4. Automatic Emulsion Scanning System

New automatic fast automatic scanning systems have been developed: the European Scanning System (ESS) and the S-UTS in Japan. The ESS is based on the use of commercial hardware components or developed in collaboration with specialized companies. The ESS reaches the speed of 20 cm²/h in an emulsion volume of 44 μm thickness. This represents an improvement of more than an order of magnitude with respect to the systems developed in the past. The S-UTS uses a dedicated hardware suitable for point scanning with a speed of 1.2 s/prediction (~ 15 min/brick).

5. Emulsion Readout

By adjusting the focal plane of the objective, the whole 44 μm emulsion thickness is spanned and a sequence of 15 tomographic images of each field of view, taken at equally spaced depth levels (3 μm), is obtained. Emulsion images are digitized, converted into a grey scale of 256 levels, sent to a vision processor board, hosted in the control workstation, and analyzed to recognize sequences of aligned grains (clusters of dark pixels of given shape and size). The three-dimensional structure of a track in an emulsion layer is reconstructed by combining clusters belonging to images at different levels and searching for geometrical alignments. A linear fit to these clusters allows the determination of the track position and angle. After emulsion sheets alignment, tracks are reconstructed in the entire brick.

6. Scanning Performances

Several test exposures at pions beams were performed for estimating the scanning performances. The scanning systems are successfully running with high efficiency (>90%), good signal/background ratio (~2 tracks/cm² /[angle<0.4rad]) and the design speed of 20 cm²/h. The position and angular resolutions at small incident angles are 1 µm and 2 mrad.