Geant4 has been used throughout the nuclear and high-energy physics community to simulate energy depositions in various detectors and materials. These simulations have mostly been run with a source beam outside the detector. In the case of low-background physics, however, a primary concern is the effect on the detector from radioactivity inherent in the detector parts themselves. From this standpoint, there is no single source or beam, but rather a collection of sources with potentially complicated spatial extent. LUXSim is a simulation framework used by the LUX collaboration that takes a component-centric approach to event generation and recording. A new set of classes allows for multiple radioactive sources to be set within any number of components at run time, with the entire collection of sources handled within a single simulation run. Various levels of information can also be recorded from the individual components, with these record levels also being set at runtime. This flexibility in both source generation and information recording is possible without the need to recompile, reducing the complexity of code management and the proliferation of versions. Within the code itself, casting geometry objects within this new set of classes rather than as the default Geant4 classes automatically extends this flexibility to every individual component. No additional work is required on the part of the developer, reducing development time and increasing confidence in the results. We describe the guiding principles behind LUXSim, detail some of its unique classes and methods, and give examples of usage. * Corresponding author, kareem@llnl.gov
Geant4 is a physics process simulation package developed at CERN, initially for highenergy physics simulations [1][2][3]. In the majority of high-energy experiments, the primary particles are generated separate from the active detector elements. This provided a clean distinction in the simulation between the machinery used to generate the beam and the hardware used to measure the beam's effects.
Over the years, Geant4 has been expanded to make it more useful for experiments at nuclear energies, including the category of low-background experiments such as neutrino research, searches for neutrinoless double-beta decay, and searches for WIMP Dark Matter. This expanded functionality included additional code to handle electromagnetic interactions down to 250 eV in energy, neutron interactions down to thermal energies, radioactive decays, and event generation from an arbitrary volume rather than a point or a beam.
Historically, a Geant4 simulation of a low-background experiment would be run, recording energy depositions only from the active detector components. Inevitably, unexpected phenomena required recording data from passive components as well, to account for all energy released in an event. Regardless of the time of an interaction, additional code had to be written into the simulation for every component that recorded data. It was rarely known a priori which parts were altering the observed energy depositions, so more and more components had to be included in the data record, leading to a large proliferation of additional code within the simulation.
In addition to data recording, low-background experiments must pay special attention to the energy sources of each individual component and material within the detector, support structure, shielding, and environment. These sources include cosmic ray spallation, intrinsic radioactivity, and surface contaminants, and multiple sources are frequently required for a single component. Although educated guesses could be made, it is difficult to know beforehand which sources in which components are the most relevant to the experiment.
Sources therefore have to be added to more and more components, with additional code required for each combination.
In the end, it is much easier to simply ensure that all parts have the ability to record data and carry multiple radioactive loads. The code to handle data recording and the code to handle intrinsic radioactivity is largely independent of the part itself. This implies the need for a set of classes that provide a consistent approach to both requirements. This paper includes details on such a new set of classes.
The new features described in this paper is useful across multiple current and future experiments involving nuclear-scale energies and low levels of background activity. They were therefore developed into a generalized code base called LUXSim. These features include creating multiple, simultaneous primary particle types and composite sources, as well as allowing those particles to be generated from multiple volumes of arbitrary spatial extent.
In addition to these physics-motivated features, LUXSim has a set of guiding principles to increase reliability and reproducibility, and to reduce the time and effort required to use or expand on the package.
In Section II, we briefly cover the LUX experiment to provide context for the simulation package, and in Section III we describe the guiding principles for LUXSim. In Section IV we discuss the details of the subsystems that make up the Geant4 user code within LUXSim.
In Section V we describe how the resulting data can be post-processed to make it similar to the data stream coming from the physical electronics. In Section VI we exercise the basic functionality of LUXSim, comparing simulation data with experimental data from a single-phase detector, as well as making preliminary predictions relevant for optical photon collection. In Section VII we describe how to use the LUXSim infrastructure for other experiments.
LUX is a search for WIMP Dark Matter based at the Sanford laboratory in Lead, South Dakota [4,5]. LUX utilizes a dual-phase detector with a 300-kg liquid xenon target (100 kg fiducial mass) to obtain a projected sensitivity in the WIMP-nucleon elastic scattering cross section of 7 × 10 -46 cm 2 for a 100-GeV WIMP. To attain this level of sensitivity, there can only be 2 background events in the 5-25 keV region of interest after 300 days of running, qualifying LUX as a low-background experiment.
The detector is comprised of a titanium cryostat inside a titanium vacuum vessel. The photomultiplier tubes used to detect the scintillation light resulting from charged particle interactions are housed in monolithic copper frames. The LUX detector will be installed in an 8-meter-diameter water tank to provide shielding from external gammas and neutrons. This water tank will be instrumented with photomultiplier tubes to create a tag for muons that pass close to the
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