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EMF-BIAS


     Sets electron and photon special biasing parameters, including leading
     particle biasing region by region, and mean free path biasing material by
     material

     See also EMF, EMFCUT, LAM-BIAS

     For 
SDUM
= LPBEMF (default):
WHAT(1)
> 0.0: leading particle biasing (LPB) is activated. Which combination of leading particle biasing is actually set up depends on the bit pattern of
WHAT(1)
Let
WHAT(1)
be represented as: 2^0xb0 + 2^1xb1 + 2^2xb2 + 2^3xb3 + 2^4xb4 + 2^5xb5 + 2^6xb6 + 2^7xb7 + 2^8xb8 + 2^9xb9 then the meaning of the ten bits is the following: b0 = 1 --> LPB activated for bremsstrahlung and pair production (old default) b1 = 1 --> LPB activated for bremsstrahlung b2 = 1 --> LPB activated for pair production b3 = 1 --> LPB activated for positron annihilation at rest b4 = 1 --> LPB activated for Compton scattering b5 = 1 --> LPB activated for Bhabha & Moller scattering b6 = 1 --> LPB activated for photoelectric effect b7 = 1 --> LPB activated for positron annihilation in flight b8 = 1 --> not used b9 = 1 --> not used Note that
WHAT(1)
= 1022 activates LPB for all physical effects (values larger than 1022 are converted to 1022) < 0.0: leading particle biasing is switched off = 0.0: ignored
WHAT(2)
> 0.0: energy threshold below which leading particle biasing is played for electrons and positrons (for electrons, such threshold refers to kinetic energy; for positrons, to total energy plus rest mass energy) < 0.0: resets any previously defined threshold to infinity (i.e., leading particle biasing is played at all energies) = 0.0: ignored This value can be overridden in the user routine UBSSET (see 13}) by assigning a value to variable ELPEMF
Default
: leading particle biasing is played at all energies for electrons and positrons
WHAT(3)
> 0.0: energy threshold below which leading particle biasing is played for photons < 0.0: resets any previously defined threshold to infinity (i.e., leading particle biasing is played at all energies) = 0.0: ignored This value can be overridden in the user routine UBSSET by assigning a value to variable PLPEMF.
Default
: leading particle biasing is played at all energies for photons
WHAT(4)
= lower bound (or corresponding name) of the region indices where the selected leading particle biasing has to be played ("From region
WHAT(4)
...")
Default
= 2.0
WHAT(5)
= upper bound (or corresponding name) of the region indices where the selected leading particle biasing has to be played ("...to region
WHAT(5)
...")
Default
=
WHAT(4)
WHAT(6)
= step length in assigning indices ("...in steps of
WHAT(6)
")
Default
= 1.0
SDUM
= LPBEMF (Leading Particle Biasing for EMF). This is the default, for other values of
SDUM
see below. This value can be overridden in the user routine UBSSET by assigning a value to variable LPEMF For
SDUM
= LAMBEMF, LAMBCOMP, LAMBBREM, LBRREMF, LBRRCOMP, LBRRBREM: (not yet implemented for photons)!
WHAT(1)
> 0.0 and < 1.0: the interaction mean free paths for all electron and positron electromagnetic interactions (
SDUM
= LAMBEMF), or for electron/positron bremsstrahlung only (
SDUM
= LAMBBREM) are reduced by a multiplying factor =
WHAT(1)
= 0.0: ignored < 0.0 or >= 1: resets to default (no mean free path biasing for electrons and positrons)
WHAT(2)
> 0.0 and < 1.0: the interaction mean free paths for all photon electromagnetic interactions (
SDUM
= LAMBEMF), or for Compton scattering only (
SDUM
= LAMBCOMP) are reduced by a multiplying factor =
WHAT(2)
= 0.0: ignored < 0.0 or >= 1: resets to default (no mean free path biasing for photons)
WHAT(3)
= generation up to which the biasing has to be applied
Default
: biasing is applied only the first generation (i.e., the primary BEAM or SOURCE particles)
WHAT(4)
= lower bound (or corresponding name) of the indices of materials in which the indicated mean free path biasing has to be applied ("From material
WHAT(4)
...")
Default
= 3.0
WHAT(5)
= upper bound (or corresponding name) of the indices of materials in which the indicated mean free path biasing has to be applied ("... to material
WHAT(5)
...")
Default
=
WHAT(4)
WHAT(6)
= step length in assigning indices ("...in steps of
WHAT(6)
")
Default
= 1.0
SDUM
= LAMBEMF (LAMbda Biasing for ElectroMagnetic FLUKA): mean free path biasing is applied to all electron, positron and photon interactions, and both the incident and the secondary particle are assigned a reduced weight LAMBCOMP (LAMbda Biasing for Compton interactions): mean free path biasing is applied only to photon Compton effect, and both the incident photon and the secondary electron are assigned a reduced weight LAMBBREM (LAMbda Biasing for BREMsstrahlung interactions): mean free path biasing is applied only to electron and positron bremsstrahlung, and both the incident electron/positron and the secondary photon are assigned a reduced weight LBRREMF (Lambda Biasing with Russian Roulette for ElectroMagnetic FLUKA): mean free path biasing is applied to all electron, positron and photon interactions, and the incident particle either is suppressed or survives with the same weight it had before collision, depending on a random choice LBRRCOMP (Lambda Biasing with Russian Roulette for Compton interactions): mean free path biasing is applied only to photon Compton effect, and the incident photon either is suppressed or survives with the same weight it had before collision, depending on a random choice LBRRBREM (Lambda Biasing with Russian Roulette for BREMsstrahlung interactions): mean free path biasing is applied only to electron and positron bremsstrahlung, and the incident electron/positron either is suppressed or survives with the same weight it had before collision, depending on a random choice
Default
: LPBEMF (see above)
Default
(option not requested): none of the above biasings apply. Note, however, that leading particle biasing can also be requested by option EMFCUT (not recommended).
Notes:
1) Depending on the
SDUM
value, different kinds of biasing are applied to the secondary particles issued from the reaction. 2) If
SDUM
= LPBEMF, the interaction point of electrons, positrons and photons is sampled analogically and Leading Particle Biasing is applied to the secondary particles, in a manner similar to that provided by option EMFCUT. However, Leading Particle Biasing with EMFCUT applies to all electromagnetic effects, while EMF-BIAS can be tuned in detail for each type of electron and photon interactions. 3) With all other values of
SDUM
, the interaction point is sampled from an imposed (biased) exponential distribution, in a manner similar to that provided by option LAM-BIAS for hadrons and muons. Further differences in
SDUM
values allow to restrict biasing to one specific type of interaction and/or to select different treatments of the incident particle. 4) If
SDUM
= LAMBEMF, LAMBCOM, LAMBBREM, the weights of both the incident and the secondary particle are adjusted according to the ratio between the biased and the physical interaction probability at the sampled distance. 5) If
SDUM
= LBRREMF, LBRRCOM, LBRRBREM, the suppression or survival of the incident particle (with unchanged weight) is decided by Russian Roulette with a probability equal to the ratio between the biased and the physical interaction probability at the sampled distance. The weight of the secondary particle is adjusted by the same ratio. 6) When using option EMF-BIAS, and in particular when choosing the Russian Roulette alternative, it is suggested to set also a weight window (cards WW-FACTOR and WW-THRESh) in order to avoid too large weight fluctuations. 7) LAMBCOMP (LBRRCOMP) and LAMBBREM (LBRRBREM) are synonyms: i.e., input concerning photon interaction biasing given with
SDUM
= LAMBBREM (LBRRBREM) is accepted and treated in the same way as with
SDUM
= LAMBCOMP (LBRRCOMP); and input concerning electron/positron interaction biasing with
SDUM
= LAMBCOMP (LBRRCOMP) is the same as with LAMBBREM (LBRRBREM). This allows to issue just a single EMF-BIAS card requesting both electron and photon interaction biasing at the same time. 8) Option EMF-BIAS concerns only electromagnetic interactions; photonuclear interaction biasing is provided by option LAM-BIAS. 9) Leading particle biasing (LPB): Leading particle biasing (available only for electrons, positrons and photons) is generally used to avoid the geometrical increase with energy of the number of particles in an electromagnetic shower. It is characteristic of all electromagnetic interactions that two particles are present in the final state: when this option is selected, only one of them (with a probability proportional to its energy) is randomly retained and its weight is adjusted accordingly. Derived from the EGS4 implementation [Nel85], it has been modified to account for the indirectly enhanced penetration potential of positrons due to the emission of annihilation photons. The probability of each of the two particles to be selected is therefore not proportional to their kinetic energy but rather to their "useful" energy (kinetic plus - in the case of positrons only - twice the mass energy). The weight of the particle selected is adjusted multiplying it by the inverse of the selection probability. This kind of biasing is aimed at reducing the mean computing time per history rather than the variance of the scored quantities (computer cost is defined as the product of variance times the computing time per primary particle). It is mainly used to estimate shower punchthrough (but comparable and even better efficiency can be obtained with importance splitting, see BIASING), or to reduce the time spent in simulating secondary electromagnetic showers produced by pi0 in hadronic cascades. As any other kind of biasing, leading particle biasing must be used with judgement, since it may lead to a slower convergence of dose estimation in some regions of phase space (see Note 5 to option BIASING). In particular, the fact that the particle of highest energy is selected preferentially can have the following effects: a - the radial profile of the electromagnetic shower might be reproduced less efficiently. This is in general not very important for showers generated inside hadronic cascades, since the overall lateral spread is governed essentially by hadrons. b - a few low-energy particles might result with a very large weight giving rise to strong energy deposition fluctuations (this inconvenience can be limited to some extent by the use of a weight window). Therefore, biasing should be avoided in scoring regions and in adjacent ones, especially when using energy deposition bins of very small volume. When applied in energy deposition calculations, the use of weight windows is recommended in order to avoid large local dose fluctuations (see WW-FACTOR and WW-THRESh). 10) Option EMFCUT provides an alternative way to request LPB, but without the possibility to set an energy threshold or to limit biasing to a specified number of generations. Example 1 (number-based):
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7...+...8
EMF-BIAS 152. 0. 5.E-4 16. 20. 2.LPBEMF
* LPB is applied in regions 16, 18 and 20 as regards Compton scattering
* below 0.5 MeV and positron annihilation in flight and at rest.
* Code 152 = 2^3 (annihilation at rest) + 2^4 (Compton) + 2^7
* (annihilation in flight).
The same example, name-based: EMF-BIAS 152. 0. 5.E-4 Rsixteen Rtwenty 2.LPBEMF Example 2 (number-based):
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7...+...8
EMF-BIAS 1022. 0.0 0.0 3.0 8.0
* LPB is applied in regions 3, 4, 5, 6, 7 and 8 for all electron and photon
* interactions at all energies
The same example, name-based: EMF-BIAS 1022. 0.0 0.0 thirdReg eighthRg Example 3 (number-based):
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7...+...8
EMF-BIAS 1022. 0.0 0.0 1.0 15.0 EMF-BIAS -1. 0.0 0.0 7.0 11.0 2.0 WW-FACTOR 0.5 5.0 1.0 1.0 15.0 WW-FACTOR 0.5 5.0 0.2 7.0 11.0 2.0 WW-THRESH 1.0 0.001 20.0 3.0 4.0 WW-THRESH 1.0 1.E-4 20.0 7.0
* The above example illustrates the combined use of leading particle biasing
* and a region-dependent weight-window. Leading particle biasing is requested
* in all regions from 1 to 15, except 7, 9 and 11. To avoid too large weight
* fluctuations, a weight window is defined such that at the lowest energies
* (=< 20 keV for photons and =< 200 keV for electrons in regions 7, 9, 11;
* =< 100 keV for photons and <= 1 MeV for electrons in the other regions),
* Russian Roulette will be played for particles with a weight =< 0.5 and
* those with weight larger than 5.0 will be splitted. The size of this window
* (a factor 10) is progressively increased up to 20 at the higher threshold
* (200 MeV for both electrons and photons in regions 7, 9 and 11, 1 GeV in
* the other regions).
The same example, name-based, assuming that the 15 regions are all the regions of the problem: EMF-BIAS 1022. 0.0 0.0 first @LASTREG EMF-BIAS -1. 0.0 0.0 seventh eleventh 2.0 WW-FACTOR 0.5 5.0 1.0 1.0 15.0 WW-FACTOR 0.5 5.0 0.2 seventh eleventh 2.0 WW-THRESH 1.0 0.001 20.0 ELECTRON POSITRON WW-THRESH 1.0 1.E-4 20.0 PHOTON

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