PAIRBREM

controls simulation of pair production and bremsstrahlung by high-energy muons, charged hadrons and light ions (up to alpha's)

See also MUPHOTON

     WHAT(1) =  0.0 : ignored
             =  1.0 : pair production by muons and charged hadrons is
                      activated
             =  2.0 : bremsstrahlung by muons and charged hadrons is
                      activated
             =  3.0 : pair production and bremsstrahlung are both
                      activated
             = -1.0 : pair production is inhibited
             = -2.0 : bremsstrahlung is inhibited
             = -3.0 : pair production and bremsstrahlung are both
                      inhibited
             Default = 3.0 (both pair production and bremsstrahlung are
                      activated).
                      However, if DEFAULTS is present with SDUM = EET/TRANsmut,
                      EM-CASCAde, NEUTRONS or SHIELDINg, the default is = -3.0
                      (pair production and bremsstrahlung inhibited)

     WHAT(2) >= 0.0 : e+, e- kinetic energy threshold (in GeV) for
                      explicit pair production. A value of 0.0 is meaningful
                      (it does mean zero energy) and is even recommended.
              < 0.0 : no explicit pair production (overrides a possible
                      non-negative value set in a previous PAIRBREM
                      card), but the energy loss due to pair production is
                      taken into account in a continuous approximation.
              Default = -1.0 (no explicit pair production).
                      However, if DEFAULTS is not present, or is present
                      with SDUM = CALORIMEtry, ICARUS, NEW-DEFAults or
                      PRECISIOn, the default is 0.0 (explicit pair production
                      with zero threshold).

     WHAT(3) >  0.0 : photon energy threshold (GeV) for explicit
                      bremsstrahlung production
             =< 0.0 : no explicit bremsstrahlung production is simulated
                      (it overrides a possible positive value set in a
                      previous PAIRBREM card), but the energy loss due to
                      bremsstrahlung is taken into account in a
                      continuous approximation
              Default = -1.0 (no explicit bremsstrahlung production)
                      However, if DEFAULTS is not present, or is present
                      with SDUM = NEW-DEFAults, the default is 0.001 (explicit
                      bremsstrahlung production with threshold 1 MeV).
                      If DEFAULTS has SDUM = CALORIMEtry, ICARUS or PRECISIOn,
                      the default is 0.0003 (explicit bremsstrahlung production
                      with threshold 300 keV).

     WHAT(4) = lower bound of the indices of the materials in which the
               indications given by WHAT(1)...WHAT(3) apply
               ("From material WHAT(4)...")
               Default = 3.0

     WHAT(5) = upper bound of the indices of the materials in which the
               indications given by WHAT(1)...WHAT(3) apply
               ("... to material WHAT(5)...")
               Default = WHAT(4)

     WHAT(6) = step length in assigning indices
               ("...in steps of WHAT(6)")
               Default = 1.0

     SDUM    = reserved for program development

     Default (option PAIRBREM not given): if DEFAULTS is not present, or is
             present with SDUM = NEW-DEFAults, both pair production and
             bremsstrahlung are activated in all materials, with explicit
             generation of secondaries of energy >= 0 and >= 1 MeV, respectively.
             If DEFAULTS is present with SDUM = CALORIMEtry, ICARUS or
             PRECISIOn, both pair production and bremsstrahlung are activated in
             all materials, with explicit generation of secondaries of energy
             >= 0 and >= 300 keV, respectively.
             If DEFAULTS is present with SDUM = EET/TRANsmut, EM-CASCAde,
             NEUTRONS or SHIELDINg, pair production and bremsstrahlung are
             inhibited.
             In any other case, both pair production and bremsstrahlung are
             activated in all materials, without explicit generation of
             secondaries (continuous loss approximation)

Notes:

1) Initialisation of bremsstrahlung and pair production by muons, charged hadrons and light ions (up to alpha's) is very demanding in computer time. On the other hand, these effects must be taken into account for a correct simulation at high energy. It is suggested to inhibit them in the phase of input preparation and debugging, but to activate them again in production runs (in long runs the time taken by initialisation is of course a smaller fraction of total time). In pure electron-photon problems and in low-energy hadron problems, the effects should be inhibited (WHAT(3) = -3.0), unless this is already ensured by the chosen DEFAULTS option.

2) When setting a threshold for pair and bremsstrahlung production by muons, charged hadrons and light ions (up to alphas), the following considerations should be taken into account:

photon production threshold (WHAT(3)) must be of course not lower than the photon transport cutoff as set by EMFCUT or by the chosen default. (In general it will be reasonable to set it equal to it)
on the contrary, the electron and positron production threshold (WHAT(2)) should in general be set = 0.0, whatever the electron transport cutoff, unless PHOTON transport cutoff is set higher than 511 keV. If the positron is produced with an energy lower than electron transport cutoff, it will be forced to annihilate at the point of production, but the two 511 keV annihilation photons will be generated correctly.

3) If option PAIRBREM is not activated, by default FLUKA treats both bremsstrahlung and pair production by muons and charged hadrons as continuous energy losses (i.e., without generating secondaries and depositing their energy at the point of production). This will reproduce correctly the average ranges but not the straggling and the dose distributions. A similar approach is found in A. Van Ginneken's code CASIM [Van75,Van86,Qia87].

4) Virtual photonuclear interactions by high-energy muons are controlled by option MUPHOTON.

5) Here are the settings for pair and bremsstrahlung production by high-energy muons and charged hadrons, corresponding to available DEFAULTS options:

         6) CALORIMEtry, ICARUS, PRECISIOn: pair production is activated in all
              materials, with explicit generation of secondaries of any energy;
              bremsstrahlung is also activated in all materials, with explicit
              generation of photons having energy >= 300 keV
            NEW-DEFAults, or DEFAULTS missing: pair production is activated in
              all materials, with explicit generation of secondaries of any
              energy; bremsstrahlung is also activated in all materials, with
              explicit generation of photons having energy >= 1 MeV.
            Any other SDUM value: both pair and bremsstrahlung production are
              activated in all materials, without explicit generation of
              secondaries (continuous loss approximation)

Example 1:

 * Explicit pair production and bremsstrahlung requested for muons and charged
 * hadrons in materials 4, 7 and 10, provided the energy of the secondary
 * electrons and positrons is > 1.2 MeV. No threshold is requested for photon
 * production.
 *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
 PAIRBREM         3.0       0.0    0.0012       4.0      10.0       3.0

Example 2:

 * Energy loss due pair production and bremsstrahlung by muons and charged
 * hadrons accounted for in materials 6, and 7, without explicit generation
 * of secondaries
 *...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
 PAIRBREM         3.0      -1.0      -1.0       6.0       7.0

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