Summary of Work Zhang Qiwei INFN - CIAE. Validation of Geant4 EM physics for gamma rays against the SANDIA, EPDL97 and NIST databases

Summary of Work

Zhang Qiwei

INFN - CIAE

Validation of Geant4 EM physics for gamma rays against the SANDIA,

EPDL97 and NIST databases

Migration of low energy EM processes From version 9.3 of Geant4, the low energy EM

processes and models (Livermore & Penelope) have been migrated to standard EM software design

A physical process can be simulated according to several models

models can be alternative and/or complementary on certain energy ranges

G4PhotoElectricEffect (standard default)

G4LivermorePhotoElectricModel

G4PenelopePhotoElectricModel

G4PhotoElectricEffect

Verification for migrated Livermore & Penelope processes: migrated vs non migrated

Comparison of Mean Free Path (MFP) for migrated and non migrated Livermore models

Electromagnetic processes of photon: photoelectric effect, compton scattering,

gamma conversion, rayleigh scattering Materials: Ne Pb Water Energy range: 250 eV ~ 105 MeV Versions: Geant4 9.2 tag3

Detail: the slides


Comparison of Mean Free Path (MFP) for migrated and non migrated Penelope models



Detail: the slides


In the original approach of the LowEnergy package, the process itself takes care of building the cross section vs energy table, which is loaded in memory and internally used for tracking. The table is made of 200 points with uniform logarithmic spacing in energy. Cross section for energies not included in the grid are calculated by log-log interpolation on the existing table.

For the migrated models, an interpolation is performed on an external cross section database (instead of an analytical approach). However, the number of points in the energy grid of the original datasets is usually much larger than the one implemented in the Geant4 internal tables.


For the above reason, we can see some shell-structures in the mean free path (MFP) profile of photoelectric effect that are present in the migrated model but not visible in the original process

There is a difference in migrated vs. non migrated Penelope Compton scattering models for very low energies. The change is due to a recent bug fix in the migrated Penelope model,which is giving now the same results as the original Penelope 2001 FOR-TRAN code.

Verification for migrated Livermore & Penelope processes: comparison with Standard processes

Mean Free Path (MFP) for migrated Livermore and Penelope models compared to Standard models



Detail: the slides

Identification of data libraries for validation SANIA National Laboratories provide analytical approximations to

cross sections for both photoelectric absorption of photons by atoms and incoherent scattering of photons by atoms

Atoms and incoherent scattering of photons by atoms

EPDL97 (Evaluated Photon Interaction Data Library) designed to meet the needs of users at Lawrence Livermore National Laboratory for use in photon transport calculations

http://www-nds.iaea.org/epdl97/

The National Institute of Standards and Technology (NIST) maintains a large number of databases about photon-atom interactions ( Form Factors, Attenuation & Scattering Tables, NIST X-Ray and Gamma-Ray Attenuation Coefficients and Cross Section)

http://physics.nist.gov/PhysRefData/contents.html

Identification of data libraries for validation Detail: the slides the photoelectric cross sections reported by the three

databases are not mutually consistent close to the absorption edge energies.

Furthermore, the Compton cross sections from the SANDIA library are substantially higher than the other two databases at low energy (< 1 keV).

The cross section data of EPDL97 for gamma conversion are very close to NIST, because the two datasets are based on the same data source.

Rayleigh scattering cross sections reported by the EPDL97 library are not consistent with NIST database for energies close to the absorption edges.

Validation of the Geant4 photon processes with respect to the libraries based on experimental data

Comparison of cross section for Geant4 photon models (Livermore Penelope and Standard) and experimental data


gamma conversion, rayleigh scattering

Materials: Compound- Air Water NaI Element(15) Energy range: 250 eV ~ 105 MeV Versions: Geant4 9.2 tag3


Experimental libraries: SANDIA (result) EPDL97 (result)

NIST (result)

Statistic analysis:

Chi-square test (result)


The Livermore models are actually based upon the EPDL97 data libraries; therefore, the agreement of the Livermore models with the EPDL97 data is simply a software verification. In fact, the deviation between the EPDL97 library and the Livermore cross sections is below 0.2% for all models and all materials.

It is shown that all the Geant4 photon models are in statistical agreement with the NIST datasets, with the exception of Rayleigh scattering. The disagreement for G4LivermoreRayleighModel is due to the fact that the NIST and the EPDL97 libraries are mutually inconsistent at low energy.

Test of the new interpolation classes for photon processes


comparison of pre-modification and post-modification using photon Livermore and Penelope models


gamma conversion, rayleigh scattering

Materials: Compound- Air Water NaI Element(15) Energy range: 250 eV ~ 105 MeV Versions: Geant4 9.2 tag7

Result of comparison

detail

Statistic analysis Chi-square test

The deviation of cross section for pre-modification and post-modification models < 10-6

The new interpolation raise the compute time


The work plan in China Institute of Atomic Energy

The work plan in CIAE My work group in CIAE

Name: Gamma Total Absorption Facility --- GTAF

Aim: measure the neutron capture cross section

Component: 4π BaF2 scintillator detector

(40 BaF2 detectors)

The work plan in CIAE

Simulation of GTAF with Geant4 • Simulation of reaction (n , γ)• Simulation of single BaF2 detector: Gamma ray

• Neutron source: 7Li (p , n) 7Be• Neutron collimator• Li-glass detector: neutron• Facility of neutron capture • Total simulation of GTAF

Geant4 Workshop in China

Documents

Summary of Work Zhang Qiwei INFN - CIAE. Validation of Geant4 EM physics for gamma rays against the SANDIA, EPDL97 and NIST databases