Emulsion Techniques

The EMU01 collaboration uses nuclear emulsion to detect and measure the production angles of charged particles produced in nucleus-nucleus collisions. Two complementary exposure techniques are employed. They are the conventional emulsion stack technique and the emulsion chamber technique.

In the emulsion stack technique, stacks of 30 NIKFI BR-2 type emulsion pellicles are exposed with the beam parallel to the emulsion plane (horizontally). Two different size stacks were employed, one with pellicles of dimensions 20 x 10 x 0.06 and the other 10 x 10 x 0.06cm^3. Their sensitivity varies between 20 and 30 grains per 100 microns for minimum ionizing particles. The exposure densities were about 5 x 10^3 nuclei cm^2.

The emulsion chamber technique uses chambers of emulsion plates and spacers which are exposed perpendicular to the beam (vertically). The emulsion plates consist of 10 x 10 cm^2 by 780 microns thick polystyrene plates coated on both sides by Fuji ET7B emulsion. The chambers consist of one or two target, or ``thick'', plates with 300 microns emulsion for the oxygen exposures and 220 microns emulsion for the sulfur exposure. The rest of the plates were ``thin'' plates with 100 microns emulsion for the oxygen and 90 microns for the sulfur exposures. Some chambers for the sulfur exposure also contained 250 microns gold foil targets upstream of the thick plates. The chambers for the 1990 sulfur exposure contained 500microns thick acrylic base plates with 185 microns and 120 microns thick emulsion for the thick and thin plates respectively.

Except for two test chambers, all of the exposures were done without the benefit of a magnetic field. Therefore only the charged particle production angles and in the case of the pellicles, absolute charge, could be measured. No momentum measurements and particle identification were possible.

These two methods each have their advantages and disadvantages so they are complementary to each other. The pellicle stacks provide a good measure of minimum bias cross-sections because ``along the track'' scanning can determine the fate of all of the projectile ions that are observed. The emulsion itself serves as a track sensitive target. They also provide a good way to determine the charge of projectile fragments which travel horizontally through the plates allowing the application of grain counting and delta ray counting techniques. Thirdly, the pellicles allow complete 4pi coverage of the interactions because the interactions occur inside the emulsion stack allowing the observation of all outgoing tracks.

The disadvantage of the stacks is that distortions limit the accuracy with which production angles can be measured. The emulsion can experience distortions during processing, especially in the vertical direction because of its shrinkage during processing. The processed emulsion has less than half the thickness that it had during the exposure. Also, the particles travel a long distance through the emulsion so that multiple Coulomb scatterings can significantly alter their direction of travel. These distortions and scatterings limit the angular resolution to about 0.1 units of pseudorapidity. (Pseudorapidity, is a function of the angle with respect to the beam direction which provides a good approximation to the true rapidity.

The emulsion chambers are designed to overcome these problems. Because the plates in the chambers are separated by honeycomb paper spacers, the particles travel through mostly empty space. The total thickness of a typical chamber in the beam direction is about 1.25gr/cm^2, or 1.73gr/cm^2 with a gold target. This greatly reduces the deviations caused by multiple Coulomb scatterings. Secondly, the tracks are measured at the emulsion-air and emulsion-plastic interfaces, and their locations are measured relative to other beam tracks, thus removing the emulsion distortion effects. This allows for a more accurate measurement of production angles, to \sim 0.01 units of pseudorapidity in the central region, and it allows a very accurate determination of projectile fragment angles up to a pseudorapidity of \sim 10. The other design advantage of the chambers is that by allowing the track patterns to spread out over a long distance, \sim 5 cm, it is possible to separate out all of the tracks within the dense cores of high multiplicity S + Au interactions. These interactions, which have multiplicities as large as 600 prongs, can be easily measured using the chamber technique.

One disadvantage with the chambers is that along the track scanning is not possible so that minimum bias cross-sections cannot be determined. The events in chambers are found by ``area'' scanning which is inefficient for finding the smallest events. A second disadvantage is that it is difficult, and for the gold interactions impossible, to measure the large angle and backward moving particles. Therefore, only particles produced within 30 degrees of the beam direction can be measured with nearly 100% efficiency. This limit corresponds to 1.32 units of psuedorapidity.(Pseudorapidity, is a function of the angle with respect to the beam direction which provides a good approximation to the true rapidity.

The emulsion chambers are designed to overcome these problems. Because the plates in the chambers are separated by honeycomb paper spacers, the particles travel through mostly empty space. The total thickness of a typical chamber in the beam direction is about 1.25gr/cm^2, or 1.73gr/cm^2 with a gold target. This greatly reduces the deviations caused by multiple Coulomb scatterings. Secondly, the tracks are measured at the emulsion-air and emulsion-plastic interfaces, and their locations are measured relative to other beam tracks, thus removing the emulsion distortion effects. This allows for a more accurate measurement of production angles, to about 0.01 units of pseudorapidity in the central region, and it allows a very accurate determination of projectile fragment angles up to a pseudorapidity of about 10. The other design advantage of the chambers is that by allowing the track patterns to spread out over a long distance, around 5 cm, it is possible to separate out all of the tracks within the dense cores of high multiplicity S + Au interactions. These interactions, which have multiplicities as large as 600 prongs, can be easily measured using the chamber technique.

One disadvantage with the chambers is that along the track scanning is not possible so that minimum bias cross-sections cannot be determined. The events in chambers are found by ``area'' scanning which is inefficient for finding the smallest events. A second disadvantage is that it is difficult, and for the gold interactions impossible, to measure the large angle and backward moving particles. Therefore, only particles produced within 30 degrees of the beam direction can be measured with nearly 100% efficiency. This limit corresponds to 1.32 units of pseudorapidity so that the central production region for 200 A GeV projectiles is easily covered.

J.G.