PAIRBREM

.98

Summary

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 (see Note 1).

Default
= 3.0  (both pair production and bremsstrahlung are activated).
However, if option 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 (see Note 2).

< 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​ (see Note 3).

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 (see Note 2).

≤ 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​ (see Note 3).

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 to which the choices expressed by WHAT(1)…WHAT(3) apply
(“From material ``WHAT(4)``…”)
Default

= 3.0

WHAT(5) =
upper bound of the indices of materials to which the choices expressed 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 option 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 mouns, charged particles 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 energies. 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)

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

    • photon production threshold (WHAT(3)) must of course not be 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. In this way, 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:

    • 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.

    • EET/TRANSMUT, EM-CASCADE, NEUTRONS, SHIELDING: pair production and bremsstrahlung are inhibited.

    • 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. For muons, explicit nuclear interactions are also requested.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+...
PAIRBREM         3.0       0.0    0.0012       4.0      10.0       3.0
MUPHOTON         1.0       0.0       0.0       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....+...
PAIRBREM         3.0      -1.0      -1.0       6.0       7.0