2 Performance Considerations

2.6 Acceptance

1. Setup I: Consists of two layers of aerogel separated by a 100 mm thick honeycomb support structure, but the upper layer phototubes are "face up", i.e. the aerogel volume is close to the TOF and the PMT's are inserted in specially machined holes in the support structure.

2. Setup II: Consists of two layers of aerogel, arranged as in setup I, but the thickness of the honeycomb support structure is reduced to 50 mm with the assumption that the stiffness of the structure will be improved by adding aluminum bars at the sides.

The dimensions of version A are: 110.76x87.76 cm2, and that of version B: 122.67x87.76 cm2. The rejection power in this region is the one given in table 2-5.

Furthermore, we investigated two versions of the ATC layers [2-9]. One which consists of 10 modules, five on each layer shifted in x, y by 5.5 cm, and one that consists of the same 10 modules but at the lower layer we have added a special module with width 113.5 mm (i.e. equal to the width of an aerogel block). Using the AMS Monte-Carlo program (version 207) we have simulated the geometry of the ATC and the Cerenkov light generation. Cerenkov photoelectrons were generated using the formula:

. . . 2-6

where, A is a conversion factor (=20), L is the particle trajectory in the aerogel (in cm), n is the aerogel index of refraction (=1.03) and b is the particle velocity. The number of generated photoelectrons is following a Poisson distribution. The study takes into account three different sensitivities of the ATC electronics (1, 2, or 3 photoelectron sensitivity). In addition, a cut on the particle reconstructed mass helps to eliminate most of the electron background. Table 2-7 summarizes the results of the study.

Geometric acceptance and rejection power of the ATC.

	Particle	Acceptance	Npe < 1/2/4 (%)	Mrec > 0.6 GeV
Setup I	e	73	0.2/0.25/0.6	0.03/0.04/0.13
		69	0.1/0.15/0.4	0.02/0.025/0.09
	p	73	70/70/71	66/67/68
		69	66/66/67	63/63/64
Setup II	e	83	0.12/0.15/0.4	0.02/0.03/0.08
	p	83	79/80/81	75/76/77

The proposed design has sufficient rejection power and adequate geometric acceptance for antiprotons (~75-80% of the total AMS acceptance, depending on the analysis cuts).