Is Geant4 a good tool to simulate the ionization effect in the CDMS experiment?

Xinjie Qiu, Dec 2, 2005

abstract: Not sure yet, might be wrong, but I strongly doubt that Geant4 is a good tool to simulation the ionization effects in crystals, in which atomic shell structure and band gap structure are more important than the nuclear structure. A few pieces of information are collected and elaborated below. We need to talk with an expert in Geant4 to confirm or unconfirm this.

Structure of this note:

A few pieces of evidence
1. The scope of Geant4
2. Low Energy Extentions of Geant4
3. Penelope physics
4. Expert answered a related question in Geant4 user forum
5. How Geant4 specifies geometry and materials in the detector?
6. What we supposed Geant4 should do but it does not do?
7. How did other people in our Collaboration handle the ioniaztion before?
Conclusion

A few pieces of evidence:

1. The scope of Geant4

Although Geant4 is a software package to accurately simulate the passage of particles through matter, it has an abundant set of physics models to handle the interactions of particles with matter across a very wide energy range, its major application is limited to the very high energy range, like high energy physics and nuclear experiments, medical, accelerator and space physics.

2. Low Energy Extentions of Geant4

The following is found in the geant4 Physics Reference Manual.

Additional electromagnetic physics processes for photons, electrons, hadrons and ions have been implemented in Geant4 in order to extend the validity range of particle interactions to lower energies than those available in the standard Geant4 electromagnetic processes.

Because atomic shell structure is more important in most cases at low energies than it is at higher energies, the low energy processes make direct use of shell cross section data. The standard processes, which are optimized for high energy physics applications, rely on parameterizations of these data.

The low energy processes of Geant4 represent electromagnetic interactions at lower energies than those covered by the equivalent Geant4 standard electromagnetic processes. The current implementation of low energy processes is valid for energies down to 250 eV (and can be used up to approximately 100 GeV).

3. Penelope physics

Penelope is a new set of physics processes for photons, electrons and positrons is implemented in Geant4: it includes Compton scattering, photoelectric effect, Rayleigh scattering, gamma conversion, bremsstrahlung, ionization (to be released) and positron annihilation (to be released). These processes are the Geant4 implementation of the physics models developed for the PENELOPE code (PENetration and Energy LOss of Positrons and Electrons). The Penelope models have been specifically developed for Monte Carlo simulation and great care was given to the low energy description (i.e. atomic effects, etc.). Hence, these implementations provide reliable results for energies down to a few hundred eV and can be used up to ~1 GeV. For this reason, they may be used in Geant4 as an alternative to the Low Energy processes.

4. Expert answered a related question in Geant4 user forum

A user asked about the Low Energy Electron Ionization Energy Deposit Question in the Geant4 user forum,

In my current simulation I have been trying to simulate low energy ionization processes. I am curious as to what other processes are "lumped" into this process. In the physics manual it is mentioned that this excess energy is deposited locally. What is causing this local energy deposit and how is it calculated? I believe that this is not covered in your physics manual.

and an expert anwsered as following:

The delta-rays are generated only above a given threshold, i.e. only for "hard" ionization events. In reality, there is a large number of "soft" ionization events, that produce very small-range electrons. In the simulation, those soft particles are not tracked explicitely and their energy is deposited locally.

Luciano

5. How Geant4 specifies geometry and materials in the detector?

In Geant4, the geometry is defined by a solid(shape, size, position), and a Logical volume( using solid, add material attributes). the material of the detector describes the macroscopic properties of matter: density, state, temperature, pressure. Materials are made of elements, Geant4 describes the properties of the atoms: atomic number, number of nucleons, atomic mass, shell energy, as well as quantities such as cross sections per atom, etc.

But it's clear that Geant4 don't have the structure information of the crystal, it doesn't know what's the posioin of the nearest atom other than the information of the densitiy of the atom. In condensed matter, the effect of other atoms/electons to the one we study on is crucial important, it determined the band gap structure of electron in the crystal. Without this, we are hardly to study the electron-hole pairs ionizion effect in the detector.

6. What we supposed Geant4 should do but it does not do?

From the description in the Geant4 Maunal, conclusion might already been drawn that Geant4 is not good for our study of ionization effect in the Ge crystal detector in the CDMS experiment. But We continue the simulation to give a try.

What we supposed to see is the elastic interaction of neutrons with Ge atoms, some kinetic energy is transfered from neutrons to Ge crystal. Because these energies are much higher than the band gap in the Ge crystal, electron-hole pairs will be produced to carry away the extra energy, all other energy will be the phonon energy depostied in the crystal and dissipated into the phonon sensor on the surface of the detector.

During the simulation, intead of G4ionIonization(the Geant4 standard electromagnetic process), G4hLowEnergyIonization(Geant4 low engery extension) class is used in the physics list to better simulate the ionization process in the low energy range.

The incident nuetron with 3 MeV Kinectic Energy hits on a Ge atom, elestically interact with it. The escape neutron carries 2.8686 MeV energy awasy, leaves 131.4 keV to the ⁷⁴Ge atom. Such high energy does not excite any electron-hole pairs in the crystal, but all deposit into the crystal locally, results no inization detection on the charge channel of the detector for this events.

In the simulation, we does see a lot of electrons if the neutron energy is high enough( greater than 1eV) and the size of the detector is large. But these electrons are not due to the ionized eletron-hole pairs, but due to the nuclear interaction( e.g. n + ⁷⁴Ge -> ⁷⁴Ga + p + n). These are not the ionization charge signal we expected to see.

7. How did other people in our Collaboration handle the ioniaztion before?

In the Monto Carlo simulation notebook, I did not find anyone in our collaboration did the ioniaztion study using Geant4 simulation. Sharmila did the simulation on neutron Monte Carlo simulations, But she did not actully simulate the ionization, but calculated it."The corresponding ionisation energies were found using Don's function for the centroids of the 6V Nuclear Recoil Bands. Noise was added to the ionisation energies."

Conclusion: We might need to give up Geant4 and find another tools to simulate the ionization effect of the nuclear recoil in the CDMS detector, or continue using Geant4 to do study things other than that.