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NEA-0643 DISCUS.

DISCUS, Neutron Single to Double Scattering Ratio in Inelastic Scattering Experiment by Monte-Carlo

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1. NAME OR DESIGNATION OF PROGRAM:  DISCUS: A computer program for the calculation of multiple scattering effects in inelastic neutron  scattering experiments.
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2. COMPUTERS
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Program name Package id Status Status date
DISCUS NEA-0643/01 Report 10-FEB-1993

Machines used:

No item found

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3. DESCRIPTION OF PROBLEM OR FUNCTION

DISCUS calculates the ratio of  once-scattered to twice-scattered neutrons detected in an inelastic  neutron scattering experiment. DISCUS also calculates the flux of once-scattered neutrons that would have been observed if there were  no absorption in the sample and if, once scattered, the neutron would emerge without further re-scattering or absorption. Three types of sample geometry are used: an infinite flat plate, a finite  flat plate or a finite length cylinder. (The infinite flat plate is  included for comparison with other multiple scattering programs.) The program may be used for any sample for which the scattering law  is of the form S(/Q/, omega).
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4. METHOD OF SOLUTION

Monte Carlo with importance sampling is used.
Neutrons are "forced" both into useful angular trajectories, and useful energy bins. Biasing of the collision point according to the  point of entry of the neutron into the sample is also utilised. The  first and second order scattered neutron fluxes are calculated in independent histories. For twice-scattered neutron histories a square distribution in Q-omega space is used to sample the neutron coming from the first scattering event, whilst biasing is used for the second scattering event. (A square distribution is used so as to obtain reasonable inelastic-inelastic statistics.)
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5. RESTRICTIONS ON THE COMPLEXITY OF THE PROBLEM

Unlimited number of  detectors. Max. size of (Q, omega) matrix is 39*149. Max. number of points in momentum space for the scattering cross section is 199.
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6. TYPICAL RUNNING TIME

Typically 0.42 secs. per energy bin per 1,000 neutron histories on the Harwell IBM 370/165.
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7. UNUSUAL FEATURES OF THE PROGRAM

An essential feature of the code is its calculation speed due to consideration of only single and double scattering effects, and sophisticated importance sampling.
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8. RELATED AND AUXILIARY PROGRAMS

A comparable program is the Monte Carlo code 'MSC' which included up to 10th order scattering effects.
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9. STATUS
Package ID Status date Status
NEA-0643/01 10-FEB-1993 Report Only
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10. REFERENCES

- M.W. Johnson:
  "DISCUS - A Computer Program for the Calculation of Multiple
  Scattering Effects in Inelastic Neutron Scattering Experiments"
  AERE-R7682 (1974).
- F.G. Bischoff et al.:
  (MSC)
  Nucl. Sci. Eng. 48, 266 (1972).
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11. MACHINE REQUIREMENTS:  120 K bytes of core storage are required by the program.
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12. PROGRAMMING LANGUAGE(S) USED

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13. OPERATING SYSTEM UNDER WHICH PROGRAM IS EXECUTED:
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14. OTHER PROGRAMMING OR OPERATING INFORMATION OR RESTRICTIONS

In ad-  dition to the four subroutines in DISCUS, the user must supply the following subroutines which are required in connection with the gen- eration of the pseudorandom number sequence. (These are all stan- dard HARWELL Subroutine Library Programs.):

REAL FUNCTION FAO1AS (I)
The function value returned by FAO1AS is a real pseudorandom  number in the range (0.0 to 1.0) if I is positive or zero;    or in the range (-1.00 to 1.00) if I is negative.

SUBROUTINE FAO1BS (MAX, NVALUE)
This subroutine will set NVALUE to a pseudorandom integer in the  interval (1 to MAX) inclusive. MAX should always be positive    otherwise there will be a bias towards zero.

SUBROUTINE FAO1DS (I,J)
Resets the value of the generator words used in FAO1AS or FAO1BS  according to the input values of I,J. J must always be odd if    one uses the Harwell subroutine library programs.

SUBROUTINE FAO1CS (I,J)
Passes back the current contents of the generator words via the    variables I,J.
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15. NAME AND ESTABLISHMENT OF AUTHOR

          M. W. Johnson
          Materials Physics Division
          AERE Harwell
          Oxfordshire, United Kingdom.
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16. MATERIAL AVAILABLE
NEA-0643/01
report                                                              REPPT
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17. CATEGORIES
  • O. Experimental Data Processing

Keywords: Monte Carlo method, inelastic scattering, neutron flux.