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Manuscript Title: OMEGA code: calculation of the geometrical parameters of detection systems.
Authors: V.P. Likhachev, J.D.T. Arruda-Neto, M.N. Martins, F. Garcia, J. Mesa, O. Rodriguez, V.B. Shostak, I. Evseev, S.A. Paschuk, H.R. Schelin, V.V. Denyak
Program title: OMEGA
Catalogue identifier: ADNJ_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 135(2001)190
Programming language: Fortran.
Computer: Micro Intel 80386+80387, Intel 80486+80487, Pentium Intel 233.
Operating system: MS-DOS 6.00, Windows 95, Windows NT 4.0.
RAM: 108M words
Word size: 16
Keywords: Nuclear physics, Particle detection, Photonuclear reactions, Nuclear disintegration, Relativistic kinematics, Monte Carlo method.
Classification: 17.5.

Nature of problem:
The two-particle disintegration of light nuclei by linearly polarized photons is an intensively developing field of Nuclear Physics. Such kind of experiments involves sophisticated processing of the experimental data by software. In particular, the procedure for the extraction of the differential cross section, dsigma/dOmega, from the measured yields of two-particle photodisintegration, involves, in the case of extended targets, a complex evaluation of the average solid angles [1-5], even for single arm measurements. Two-arm measurements make the problem yet more complicated because it is necessary to take into account the reaction kinematics, that may drastically change the efficiency of the detection system. Moreover, the theoretical analysis of the data is usually done in the center of mass system (CM) and one must extract from the experimental data just the CM-cross section. It is clear that the space extended character of the target restricts the angular definition that can be achieved in a single-arm angular distribution experiment. For a reaction with a well defined kinematics, it is useful to install a second detection arm (channel) and perform coincidence measurements. In this case it is possible to trace back the reaction position in the target and to define which angles (theta,phi) contribute to the cross section measured by each detector pair. On the other hand, the extraction of the reaction cross section is more complicated in this case. The geometry optimization of a detection/target system presents a wide choice of geometrical parameters to be changed in order to minimize the uncertainties of the measured quantities (like cross section, Sigma-asymmetry, etc.), obtained for a fixed acquisition time, taking into account the physical requirements and characteristics of the experiment (angular resolution, beam spot size, target and detectors dimensions, etc.). The aim of the described code is to obtain (i) polar and azimuth angular distributions on the phase plane and (ii) the average dynamic coincidence solid angle in CM system for each detector pair.
The main characteristics of the detectors were presented in [6]. The method used in the calculations was described in a previous work [7]. Here we will be concerned with the relevant geometrical parameters of the arrangement as a whole and with the description of the calculation procedure and program organization. To ease the understanding of the reader, in Fig. 1 we present a schematic view of a test-case target/ detector configuration, in which a tilted film target is placed between two sets of planar silicon strip detectors.

Solution method:
The most reasonable way to avoid the problems described above and to obtain the required data is the Monte Carlo (MC) simulation of the two-particle photodisintegration events. The attempt to calculate analytically the average dynamic coincidence solid angles leads to ambiguities in the solutions and huge difficulties, which arise from multiple integration over the spatial boundaries of the target and detectors. The MC approach for the evaluation of dynamic coincidence solid angles for a 2pi-detector arrangement is described in our previous work [7]. The computer code OMEGA involves not only the experimental setup geometry, but also the physical parameters of the reaction and allows to obtain the average dynamic coincidence solid angles in the CM system, even for complicated detector arrangements. This method allowed the evaluation of the dynamic coincidence solid angles for a 2pi-detector arrangement not only for large illuminated area plane targets but also for 3D gaseous targets.

Running time:
Depends on the desirable statistical accuracy. For the example set of initial parameters for beam, target and detector geometries, the running time is approximately 10 seconds.

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