Performance of Geant4 in simulating semiconductor particle detector response in the energy range below 1 MeV

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Abstract

Geant4 simulations play a crucial role in the analysis and interpretation of experiments providing low energy precision tests of the Standard Model. This paper focuses on the accuracy of the description of the electron processes in the energy range between 100 and 1000 keV. The effect of the different simulation parameters and multiple scattering models on the backscattering coefficients is investigated. Simulations of the response of HPGe and passivated implanted planar Si detectors to β particles are compared to experimental results. An overall good agreement is found between Geant4 simulations and experimental data.

Introduction

The search for physics beyond the Standard Model takes many forms. At the high energy frontier accelerators such as the LHC are able to produce new particles which could point toward new physics. The other, precision frontier relies on measurements of different observables, as e.g. in neutron and nuclear β decay, where a deviation from the Standard Model value is an unambiguous and model independent sign of new physics [1], [2], [3], [4], [5], [6]. In order to further increase the precision of such measurements all possible systematic effects need to be evaluated, which often include Monte Carlo simulations such as the Geant4 simulation toolkit [7]. Among others it is widely used in neutron and nuclear correlation measurements [8], [9], [10], [11] and in searches for neutrinoless double-β decay [12], [13].

The majority of these experiments are focusing on tracking and detection of electrons with typical β decay energies (100 keV–1 MeV) where one of the dominant systematic effects is the electron scattering from energy sensitive detectors. With the relative precision of the Standard Model tests in neutron and nuclear β decay reaching the sub-percent level the accuracy of the Geant4 models needs to be re-evaluated and compared to new, high precision and high quality experimental data.

This work focuses on the influence of the various Geant4 models and their parameters on the simulated values of the backscattering coefficients. It also investigates the quality with which experimental spectra of different β decaying isotopes are reproduced. The results can be used to assign systematic errors to the simulations and also to estimate the systematic difference between simulated and experimental spectra.

Section snippets

Relevant Geant4 processes

Geant4 [7] is a toolkit for simulating the passage of particles through matter. It was developed with the experiments at the LHC accelerator in mind and is therefore tuned to simulate high energy physics experiments. However, low-energy weak interaction experiments in neutron and nuclear β decay that are dealing with β particles of around 1 MeV kinetic energy typically, can also benefit from information provided by Geant4. At these low energies the physical processes involved are greatly reduced

Introduction and literature review

In order to validate the Geant4 electron processes one needs simple experiments (both in terms of geometry and of the physics involved) and high-quality data. A rather simple experimental observable related to electron processes is the backscattering coefficient. When an electron backscatters from the detector it deposits only part of its energy and then escapes from the detector, thereby distorting the shape of the measured electron spectrum. By simulating such a rather straightforward

Simulation of detector response

A Geant4 simulation records the energy deposited in a specified geometrical volume. Such a simulated spectrum, however, cannot be directly compared to the measured spectrum since several instrumental effects as well as the decay scheme of the isotope considered still have to be taken into account.

Geant4 performance for planar HPGe detectors

Custom made planar HPGe detectors were developed [45], [46] for low temperature nuclear orientation β-asymmetry measurements. They were used in an experiment with 114In [38] and in the 67/68Cu experiment at ISOLDE, CERN [47]. All of them were extensively tested [48], and in this paper we will focus on the 15/4 detector. Fig. 8 shows a sketch of the detector with all its dimensions noted. The detector has its front electrode made with boron implantation and the thickness of this dead layer is

Geant4 performance for PIPS detectors

A 1.5 mm thick, fully depleted pure PIPS detector (MSX03-1500, from Micron Semiconductor) was recently tested by our group, in part to replace the Hamamatsu 0.5 mm thick PIN diode detectors [8], [38]. The front dead layer of this detector is 100 nm thick. The entrance window consists of a 300 nm thick Al grid with a 3% coverage. This detector is well suited for precision β spectroscopy because the low Z values of Al and Si limit the probability for electron backscattering from the entrance window.

Conclusions and outlook

The influence of the various Geant4 physics lists, MSC models and parameters on simulated electron backscattering coefficients from Si for energies in the range of nuclear β decay was investigated. It was found that for the energy region of typical low energy experiments in neutron and nuclear β decay the usage of the low energy physics lists-Livermore and Penelope-is not absolutely required. Best agreement between the simulated results and experimental data is found for the Single Scattering

Acknowledgments

This work was supported by the Fund for Scientific Research Flanders (FWO), Projects GOA/2004/03 and GOA/2010/10 of the K. U. Leuven, the Interuniversity Attraction Poles Programme, Belgian State Belgian Science Policy (BriX network P6/23), and Grant LA08015 of the Ministry of Education of the Czech Republic.

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    Current address: Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, Washington 98195, USA.

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