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Manuscript Title: ASYM: a program to examine fission fragment mass asymmetry in hemispherical chambers.
Authors: R.S. Tanczyn, G.P. Couchell, W.A. Schier
Program title: ASYM
Catalogue identifier: AADD_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 38(1985)61
Programming language: Fortran.
Computer: CDC CYBER 170-825.
Operating system: NOS 2.3.
RAM: 28K words
Word size: 60
Keywords: Nuclear physics, Fragment-fission, Hemispherical fission Chamber, Heavy ion.
Classification: 17.7.

Nature of problem:
The program ASYM was written to assist in the design of a fission chamber for an experiment to measure delayed neutron spectra following fission of U-235. Fission takes place in a U-235 lined hemispherical fission chamber. Fission fragments emitted from the infinitely thick foil are trapped by helium gas in the chamber and are transferred to a counting area by a helium-jet system. To avoid a possible distortion in the spectra one would like fragments fo all masses to be stopped with equal probability in the gas. Since less massive fragments are emitted preferentially from the foil, a potential mass asymmetry exists for fragments stopped in the helium. Fortunately the lighter, more energetic fission fragments also have a higher probability of reimbedding themselves in the chamber wall. The problem is further complicated by a possible dead layer of oil on the fission foil surface. ASYM examines the compensating effect of chamber size and helium pressure on the asymmetry and studies the effect of a dead layer.

Solution method:
For fragments originating at a given depth in the foil the number of each mass stopped in the helium is propotional to the solid angle into which the fragments are both emitted from the foil and stopped in the gas. Using parameterized expressions for the fragment ranges to determine the distance a fragment can travel in a particular direction, ASYM calculates this solid angle for a discrete number of points on the hemisphere. The sum of these solid angles gives an effective solid angle which when properly weighted gives a value proportional to the number of stopped fragments.

The program is restricted by the mesh size chosen for three quantities:
1) An energy mesh is defined such that fragments are assumed to be emitted from the fission foil having energies of a multiple of 0.05 MeV.
2) The solid angle of stopped fragments is calculated for points along a semicircle in one degree steps.
3) The fission foil is assumed to consist of ten layers, each layer one tenth of a fragment range thick. An effective solid angle of stopped fragments is calculated for the mean depth of each layer.
In addition, two simplifying assumptions are made:
1) Fission fragment emission is isotropic.
2) The fission foil is uniformly bombarded with incident neutrons.
Both conditions apply when the fission foils are positioned in the thermal column of our 1 MW reactor.

Running time:
The program is user interactive. Actual CP time ranges from 300 to 1400 s, depending on the maximum energy of the fragment and the dead layer thickness.