Radioactive Decay Process and Data

1
Radioactive Decay Process and Data

• P Truscott and F Lei
• Space Department
• DERA, Farnborough UK

2
Objectives

• Allow simulation by GEANT4 of
• Nuclear radioactive decay, i.e. ?, ?-, ?,
  electron capture (EC), and isomeric transition
  (IT) or long-lived meta-stable states, the
  latter through the existing photo-evaporation
  code
• Neutron decay (?- emission).
• Simulation to be applicable to
• nuclei at rest or in motion
• nuclei specified explicitly as primary particles
  or the product of nuclear interactions

3
Functionality

• All nuclear/neutron decay products to be
  submitted back to the tracking process, including
• daughter nucleus (tracking interactions and
  radioactive decay through multiple generations)
• ?-rays from prompt de-excitation
• ? anti-?
• details of the atomic excitation state (EM atomic
  relaxation model)
• Need to control the scope of the simulation
• range of nuclei for which process is applicable
• volumes for which process is applicable

4
Functionality (concluded)

• Application of variance reduction techniques
• bias decays to occur within user-defined times of
  observations
• split radionuclei to increase sampling
• apply minimum bias limit to ensure adequate
  sampling of low-probability channels which have
  high impact
• apply a source particle flux-versus-time profile
• Default operation is analogue Monte Carlo

5
Logical Design
6
Class Design

• Process
• G4RadioactiveDecay process derived from
  G4VRestDiscreteProcess
• Decay channel data
• G4NuclearDecayChannel derived from
  G4GeneralPhaseSpaceDecay
• G4AlphaDecayChannel, G4BetaMinusDecayChannel,
  G4BetaPlusDecayChannel, G4KshellECDecayChannel,
  G4LshellECDecayChannel, G4ITDecayChannel derived
  from G4NuclearDecayChannel
• The decay channel data are loaded whenever a
  DecayIt is requested for a new radionuclear
  species
• Control
• G4RadioactiveDecayMessenger derived from
  G4UImessenger to allow control of process through
  UI

7
Data Format and Sources

• Radioactivity data to be distributed in ASCII
  text files
• Filename format consistent with PhotoEvaporation
• Tabulated as a function of parent nucleus
  excitation energy and mean-lifetime
• Data include decay mode, daughter nucleus
  excitation level, branching ratio, emitted
  particle end-point energy
• Radioactivity data need to be consistent with
  PhotoEvaporation (cross-reference of excitation
  energy)
• Possible methods of generating decay data
• Derive decay and nuclear level data at the same
  time from ENSDF to ensure consistency - preferred
  option, but significant human intervention
  required due to irregular format (discussions
  with A Brunengo and H-P Wellisch continuing)
• Derive only decay data from current ENSDF, and
  check consistency with current nuclear level data
  used by G4PhotoEvaporation
• Use decay data from DERA/UoS (derived from
  previous ENSDF)

8
Current Status

• Technical Note covering much of the theory behind
  application of variance reduction to radioactive
  decay
• G4Ion and G4IonTable have been modified to permit
  adequate description of nuclear and atomic state
  (Hisaya Kurashige)
• Analogue Monte Carlo decay mostly implemented,
  with all decay modes possible
• G4RadioactiveDecayMessenger allows restriction of
  range of radionuclei
• Radioactivity database for the moment contains
  just a few example nuclei (for the purposes of
  testing)

9
Future Work

• Complete G4RadioactiveDecay process for analogue
  Monte Carlo only
• Coulomb correction for ?
• Modification of G4DynamicParticle properties to
  account for changes to atomic state following
  EC-decay
• Distinction between long-lived and
  short-lived nuclear species (G4RadioactiveDecay
  including IT, or IT only, i.e. photo-evaporation)
• G4PhotoEvaporation
• does not simulate internal conversion electron
  emission (planned enhancement in 1 year)
• Problems with DoChain for G4VDiscreteGammaDeexci
  tation?
• Develop a consistent data-base(s) for radioactive
  decay and nuclear de-excitation
• Implement variance reduction schemes

10
Documentation Status

• User Requirements Document issued at v1.0
• Software Specification Document issued at v0.c
• Technical Note issued at v0.b
• (Ref DERA/CIS/CIS2/7/36/4/9 and WWW page
  http//www.space.dera.gov.uk/space_env/geant_docs/
  geant_docs.html)