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Manuscript Title: PHOCHA: a Monte Carlo program to calculate the characteristics of a beam of photons produced by annihilation and bremsstrahlung of relativistic positrons.
Authors: E. De Sanctis, V. Lucherini, V. Bellini
Program title: PHOCHA
Catalogue identifier: AAVN_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 30(1983)71
Programming language: Fortran.
Computer: CDC 7600.
Operating system: CDC SCOPE 2.1.5, NOS/BE.
RAM: 3000K words
Word size: 60
Peripherals: disc.
Keywords: Nuclear physics, Cyclotron, Intermediate energy Photon beam, In flight positron Annihilation and Bremsstrahlung, Monte carlo calculation.
Classification: 17.1.

Nature of problem:
The program PHOCHA calculates the absolute energy spectra, the radial and vertical profiles and the angular distributions of annihilation and bremsstrahlung photons from intermediate energy positrons. The energy spread, the energy loss and the multiple scattering of the positrons in the annihilation target are taken into account. Moreover the positron emittance, the positron incidence angle and the finite angular acceptance of the photon collimation channel are explicitly considered.

Solution method:
A positron, whose energy is extracted according to a given energy distribution, impinges on the target, flying in a given direction. The radiation point is determined according to the positron beam emittance and by a uniform extraction in the effective target thickness. Positron multiple scattering and energy loss are then sampled according to the appropriate laws. In order to minimize photon losses, the photon emission angle is extracted, for each positron, within a variable solid angle whose size and orientation are calculated by taking into account the positron flight direction and the photon production point in the target; moreover a number of photons proportional to the selected solid angle is always sampled.

Restrictions:
a) the equations of the radial and vertical phase-space ellipses of the positron beam are used in the canonical form;
b) the incoming and outcoming target surfaces are assumed to be plane;
c) bremsstrahlung straggling is not considered.

Unusual features:
Use of the CERN program library: subroutines: MXMPY, POLROT, VMATR; functions: RNDM.

Running time:
In the test program set-up (positron energy 200 MeV, positron beam incidence angle on the target 0 degrees, no. of collimators 8, about 25 photon histories for each of the 5000 sampled positrons) 271 s of CDC- 7600 CPU time are required to reach a statistical accuracy better than 2% on the photon spectra.