PROGRAM SUMMARY
Title of program:
OMEGA
Catalogue identifier:
ADNJ
Ref. in CPC:
135(2001)190
Distribution format: tar gzip file
Operating system: MS-DOS 6.00, Windows 95, Windows NT 4.0
High speed store required:
108MK words
Number of bits in a word:
16
Number of lines in distributed program, including test data, etc:
1027
Keywords:
Nuclear physics, Particle detection, Photonuclear reactions,
Nuclear disintegration, Relativistic kinematics, Monte Carlo method.
Programming language used: Fortran
Computer:
Micro Intel 80386+80387 ,
Intel 80486+80487 ,
Pentium Intel 233 .
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.
Method of solution:
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.
Typical 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.
References:
[1] R.P. Gardner, K. Verghese, H.M. Lee, Nucl. Instr. and Meth.
176 (1980) 615.
[2] R.P. Gardner, K. Verghese, Nucl. Instr. and Meth. 93 (1971) 163.
[3] K. Verghese, R.M. Felder, R.P. Gardner, Nucl. Instr. and Meth.
101 (1972) 391.
[4] M. Bellusci, R. Deleo, A. Pantaleo, A. Vox, Nucl. Instr. and Meth.
114 (1974) 145.
[5] M.V. Green, R.L. Aamodt, G.S. Johnston, Nucl. Instr. and Meth.
117 (1974) 409.
[6] V.P. Likhachev, J.F. Dias, M.-L. Yoneama, M.N. Martins,
J.D.T. Arruda-Neto, C.C. Bueno, V. Perevertailo, O. Frolov, Nucl.
Instr. and Meth. A 376 (1996) 455.
[7] V.P. Likhachev, M.N. Martins, J.D.T. Arruda-Neto, C.C. Bueno,
M. Damy de S. Santos, I.G. Evseev, J.A.C. Goncalves, O.A.M. Helene,
S.A. Paschuk, H.R. Schelin, Nucl. Instr. and Meth. A 390 (1997) 251.