Part 1: The Emulsion Stacks and Scintillating Fiber Planes
Arguably the most important piece to the DONUT experiment, the emulsion target is where all the action takes place. The idea is very simple. Neutrinos enter the target and interact with silver bromide, AgBr, crystals suspended in gelatin solution that compose the emulsion sheets. These interactions produce a number of products, most importantly charged particles, which ionize the Ag and Br atoms as they pass through the target. The ionization process is the same as that of light used in conventional photography. (In fact, consumer film is often made of AgBr, similar to our detector). By looking at the trails left by the charged particles, physicists can determine the type, charge and momentum of passing particles. The emulsion sheets form an active detector in the sense that they record everything that passes through them, however, they are passive in the sense that they have no time relations between any of the recorded events.
The targets measure 50x50x6 cm3 and consist of pizza-box shaped steel containers filled with emulsion sheets. There are three different types of modules used in the DONUT detector. The first type, bulk emulsion module, is 95 % emulsion material by mass and contains 15L and 84 sheets of emulsion layered 360 mm thick per side on an 80 mm sheet of inert plastic for support. This type uses the most emulsion material by far, and consequently is the most expensive.
DONUT scientists decided to experiment with other modules such as the Emulsion Cloud Chamber (ECC) and the Hybrid modules which use less emulsion materials but compensate for the lack target density by integrating steel sheets into the design. The ECC modules, reminiscent of the early 20th century cloud chambers, work well and consist of 1000 mm of steel as a target, followed by 100 mm of emulsion, then 800 mm of plastic, followed by another 100 mm of emulsion. This type of detector employs a different principle than the bulk modules. Instead of using volume tracking, as the bulk does, the ECC is uses sampling detector planes, identifying the particle at discrete points then interpolating to reproduce the entire trajectory. This limits the costs of the experiments and also the volume to be scanned. *Additionally, it picks up less of the background due to coulomb scattering and electromagnetic showering. The hybrid is a compromise between the two previous designs.
In addition to the modules, several changeable emulsion sheets are placed after the modules to aid in track identification and alignment. These layers are replaced once a week to lessen exposure time and achieve lower track density so that interesting tracks are more easily recognized.
A total of seven modules were exposed, four at a time. The sheets are precisely aligned using 55Fe radioactive sources to leave x-ray marks on the changeable sheets and targets, which are used to match up the tracks during analysis. Additionally, muon tracks from the original proton beam and from cosmic rays are used to line up the sheets.
Scintillating Fiber Trackers:
The emulsion sheets contain a wealth of data about collisions and particles; however, none of it is accessible until development is complete. Additionally, the emulsion sheets are indiscriminate; they record everything that passes through them, so isolating interesting tracks was necessary. Physicists designed the scintillating fiber trackers to do just that. In general, scintillators are materials that emit light when charged particles pass through them. If you can determine where a charged particle passed through, you can extrapolate back to its vertex and therefore determine where the particle originated. In DONUT the trackers are made of a polystyrene core surrounded by a PMMA (polymethyl methacrylate) outer layer to induce total internal reflection so that all of the light emitted would remain in the fiber. In total, there were 6000 scintillating fibers arranged in layers of 1200 fibers in the u, v, (u and v are at 45 degrees from the normal x and y axis) and x directions. When a charged particle passes through the trackers it causes the emission of light in the three planes which are used as coordinates to determine its position. This light travels through the fibers to an image intensifier, which uses the light to create electrons, which are then used as data.
Last updated: 6/29/01 comments