RADDECAY¶

Summary

requests simulation of radioactive decays and sets the corresponding biasing and transport conditions

See also DCYTIMES, DCYSCORE, IRRPROFIle, RESNUCLEi

WHAT(1) :

flag for activating radioactive decays

= 1.0:

radioactive decays activated for requested cooling times

= 2.0:

radioactive decays activated in semi-analogue mode

= 0.0:

ignored

= -1.0:

reset to default

Default

: no radioactive decays

WHAT(2) :

flag for “patching” isomer production, while waiting for a better production model

= 1.0:

isomer production “patching” activated

= -1.0:

isomer production “patching” disabled

= 0.0:

ignored

Default

: activated if non-analogue radioactive decays are requested, disabled otherwise

WHAT(3) :

number of “replicas” of the decay of each individual residual

= 0.0:

ignored

< 0.0:

reset to default

Default

= 1.0 for analogue decays, 3.0 otherwise

WHAT(4)

: switch for applying various biasing features only to prompt particles, or only to particles originated in radioactive decays, or to both.

> 0.0:

a 9-digit number abcdefghi, where each a-i digit is interpreted as follows (but see Note 4 below):

0.0 = ignored

1.0 = the corresponding biasing is applied to prompt radiation only

2.0 = applied to decay radiation only

3.0 = applied to both prompt and decay radiation

and the digit position is interpreted as follows:

a = hadron/muon interaction length or decay biasing, as defined by command

LAM–BIAS

b = hadron/muon leading particle biasing (not defined at the moment)

c = hadron/muon importance and weight window biasing, as defined by commands

BIASING and WW–FACTOr

d = e\(^\pm\)/\(\gamma\) interaction length biasing, as defined by command EMF–BIAS

e = e\(^\pm\)/\(\gamma\) leading particle biasing, as defined by command EMF–BIAS

f = e\(^\pm\)/\(\gamma\) importance and Weight Window biasing, as defined by commands BIASING and WW–FACTOr

g = low-energy neutron biased downscattering, as defined by command

LOW–DOWN, and non-analogue absorption, as defined by LOW–BIAS

h = no meaning for the time being

i = low-energy neutron importance and Weight Window biasing, as defined

by commands BIASING, WW–FACTOr and WW–PROFIle

= 0.0:

ignored

< 0.0:

reset to default

Default

: all biasing is applied to prompt showers only (equivalent to 111111111.)

WHAT(5) :

multiplication factors to be applied to e\(^\pm\)/\(\gamma\) transport energy cutoffs, respectively for prompt and decay radiation

> 0.0:

a 10-digit number xxxxxyyyyy, where the first and the last 5 digits are interpreted as follows (see Note 5 below):

xxxxx\(\times\)

0.1 = transport energy cutoff multiplication factor for \(\beta^\pm\) and \(\gamma\) decay radiation

yyyyy\(\times\)

0.1 = transport energy cutoff multiplication factor for prompt e\(^\pm\) and \(\gamma\) radiation

= 0.0:

ignored

< 0.0:

reset to default

Default

: e\(^\pm\) and \(\gamma\) transport energy cutoffs are unchanged: the multiplication factors are set = 1.0 for both prompt and decay radiation (equivalent to 0001000010.)

WHAT(6)

flag for generating \(\beta^+/\beta^-\) spectra with Coulomb and screening corrections

> 0.0:

Coulomb and screening corrections are activated

= 0.0:

ignored

< 0.0:

Coulomb and screening corrections are ignored

Default

= 1.0 Coulomb and screening corrections are activated

SDUM:

not used

Default

(option RADDECAY not given): no radioactive decay is activated, and no multiplication factors are applied to transport energy cutoffs

Notes

FLUKA allows for two different ways of simulating radioactive decay. In the semi-analogue mode, (WHAT(1) > 1) each single radioactive nucleus is treated in a Monte Carlo way like all other unstable particles: a random decay time, random daughters, random radiation are selected and tracked. This allows for event-by-event analysis, with the time structure recorded in the particles age variable. It is called semi-analogue because the radiation spectra are inclusive (i.e. no correlated \(\gamma\) cascade is reproduced, etc.).

In the “activation study” mode (WHAT(1) = 1) the time evolution is calculated analytically and all daughter nuclei and all associated radiation are considered, but at fixed times. (See Note 6 below).

In both cases, the emitted particles are transported like all other secondaries, within the same run.
In the analytical evolution, each radioactive nucleus can be “decayed” several times, in order to improve statistics on, for instance, energy deposition, as set by WHAT(3).
Although FLUKA allows to simulate in a same run the transport of cascade particles and that of particles generated by decay of the produced residual nuclei, transport and biasing need in general to be set at very different levels. For instance, in a study of induced activity due to photonuclear reactions, it is recommended to set the photon transport threshold not lower than the photonuclear reaction threshold. However, gammas produced in the decay of those residual nuclei have in general lower energies and need to be transported with much lower energy cutoffs (see Note 5 below).
Biasing can be applied to radiation products. At present, for the biasing switch represented by WHAT(4), only the d, e and f choices are relevant since only \(\beta^\pm\) and \(\gamma\) decays are considered for the time being.
Both multiplication factors imbedded in WHAT(5) must be ≥ 1.0. If any of the multiplication factors is set to a value larger than 9999.0, it is effectively considered as \(\infty\), i.e. WHAT(5)= 0000099999. will kill the electromagnetic cascade in the prompt part, while leaving it untouched in the decay part.

WHAT(5)= 9999900000. will do the opposite.
It is possible to perform on-line time evolution of decay radiation, and to score all standard quantities (energy deposition, residuals…) according to a user-defined irradiation profile (IRRPROFIle command, see) and one or more user-defined decay times (DCYTIMES command). Radiation transport will be performed only once, and the evolution will be applied as a weight depending on the setting of the estimator, to be defined with the DCYSCORE command.
If decays are simulated in semi-analogue mode, detector results are expressed per unit primary weight (possibly scoring together prompt and decay particles, if requested by DCYSCORE with WHAT(1) = –1.0). If decays are instead calculated for requested cooling times, the results are expressed per unit time (pSv/s, cm\(^{-2}\)/s, Bq, etc.)
When the source is a radioactive isotope (defined by command BEAM with SDUM = ISOTOPE and by command HI–PROPErt), RADDECAY must be used in semi-analogue mode (WHAT(1) > 1). The detector results are then expressed per isotope decay. Note that command DCYSCORE must be issued with WHAT(1) = –1, and must be applying to all relevant estimators and detectors. Without DCYSCORE, no scoring will occur (see Note 8 to command BEAM).
Different transport cutoffs for prompt and decay e\(^+\)/e\(^-\)/\(\gamma\) can be set also differently for each region by command EMFCUT with SDUM = PROMPT or DELAYED. With WHAT(5) of RADDECAY it is possible only to define a ratio, identical for all regions.

Example:

* In this example, radioactive decays are activated for requested cooling
* times, with an approximated isomer production. Each radioactive nucleus
* produced will be duplicated.
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7...
RADDECAY         1.0       1.0       2.0   111000.      200.
* Any biasing of electrons, positrons and photons is applied only to
* prompt particles in the electromagnetic shower, and not to beta and
* gamma particles from radioactive decay.
* The transport energy cutoffs set by EMFCUT (or by DEFAULTS) are
* applied as such to decay betas and gammas, but are multiplied by a
* factor 20 when applied to prompt particles.