Monte Carlo

  • The MaGe framework:

  • The GERDA and Majorana collaborations are developing a GEANT-4 based framework for the simulation of low-energy processes named MaGe [1]. It is ment as a tool to estimate the background in low-energy and low-background experiments with a focus on neutrinoless double beta-decay. The main effort in the development of the framework is to provide a flexible tool which can be applied not only to simulate the signal and background processes in the GERDA and Majorana experiments, but to also simulate test stands and auxilliary experiments. A verification of the Monte Carlo code is constantly being performed.

    [1] M. Bauer, Journal of Physics, Conf. Series. 39 (2006) 362.

  • Background estimate:

  • Physics processes likely to occur in the GERDA experiment were simulated and their contribution to the overall background were estimated (Fig. 1-4). The simulation was done for a nominal Phase II setup with 21 detectors. The impact of segmented germanium detectors on the reduction of background was studied [1]. A compilation of the most important background contributions is currently being performed.

    Figure 1: Simulated spectrum of neutrinoless double beta-decay (.eps) (.jpg)
    Figure 2: Simulated spectrum of Co-60 (.eps) (.jpg)
    Figure 3: Simulated spectrum of Ge-68 (.eps) (.jpg)
    Figure 4: Simulated spectrum of Tl-208 (.eps) (.jpg)

    [1] I. Abt et al., Nucl. Instr. and Meth. A 570 (2007) 479.

  • Monte Carlo validation:

  • The first GERDA Phase II prototype detector was irradiated with different gamma-sources: Co-60, Eu-152, Th-228. The data was compared with the results of an accompanying Monte Carlo simulation. The agreement was found to be good, with deviations of the order of 5-10% (Fig. 1-2). It was shown that the prediction for the rejection of photon induced events is better than 5%. The Monte Carlo framework MaGe is thus reliable with respect to background estimates [1].

    Figure 1: Spectrum of Co-60 for data and Monte Carlo (.eps) (.jpg)
    Figure 2: Segment occupancy for data and Monte Carlo (.eps) (.jpg)

    [1] I. Abt et al., arxiv:nucl-ex/0701005.

  • Pulse Shape Simulation

  • Neutrinoless double beta decays in general deposit energy locally. i.e. they are single-site events. Most background events have photons in their final state which interact through Compton scattering causing energy deposition at different places and thus creating multi-site events. A natural idea is to segment the detector according to the different ranges of the signal and background processes. If an event has energy deposited in several segments, it is rejected as background.

    However, (1) there are some multi-site events which are confined to one segment and (2) there are some single-site events that happen on the boundary between two segments. The boundary events are rejected erroneously, because the energy deposited is shared between segments. The analysis of the electrical pulses associated with the events can help with both problem categories. For events in category (1) the time development of the pulse can reveal a multi-site event; while for events in category (2) the relative strength of the two pulses plus the additional mirror pulses can reveal whether the interaction position is near the boundary or not.

    Pulse shape analysis can also help with several other aspects: rejection of background from α-particle and neutron interactions with detectors, Compton continuum suppression, detection of crystal structure, etc. In combination with crystal segmentation, pulse shape analysis plays a crucial role to reach an extremely low background count rate.

    Several data samples were recorded by an 18-fold segmented n-type prototype named Siegfried for the pulse shape analysis. The single-site events include: (1) the double escape peak at 1.592 MeV derived from the 2.614 MeV γ ray from Tl208, and (2) the events with energy deposits around 2 MeV left by photons interacting once with detector and then captured by a second detector outside.

    There are several caveats to the pulse shape analysis by only using the real data samples. For instance, the double escape peak mentioned before is not located near the Q-value for double beta decay of Ge76. In addition, the events from the peak are not uniformly distributed throughout the detector crystal. The second sample of single-site events mentioned before are collected with a very low event rate (~1Hz). The low rate is intrinsic to the measurement and makes it difficult and time consuming to collect samples of satisfactory size. Therefore the data have to be complemented by reliable pulse shape simulation.

    There are a number of experiments, such as Majorana, AGATA, GRETA, Heidelberg-Moscow, IGEX, MINIBALL, etc., which have already developed packages for the pulse shape simulations. Some of them are very mature, e.g., APP and MGS used by AGATA. A preliminary simulation package is also available in our group. Currently, we are developing a framework of pulse shape simulation and analysis together with Majorana people. New framework is coming up later this year.

  • Sensitivity studies:

  • A study of the sensitivity of GERDA to the observation of neutrino accompanied double beta-decay of Ge-76 into the excited states of Se-76 was performed using the MaGe framework [1]. The simulation assumed the nominal Phase II setup with 21 segmented germanium detectors. The impact on the sensitivity for segmented and unsegmented detectors was studied.

    It was shown that the current limits on this process can be improved by about two orders of magnitude. The sensitivity is improved by a factor of about 2.5 using segmented germanium detectors. The discovery potential allows a discovery within the range of current nuclear models.

    [1] K. Kröninger, L. Pandola, V. Tretyak, arxiv:nucl-ex/0702030.