1. Overview of GERDA simulation activities with MaGe
Luciano Pandola
INFN, Laboratori del Gran Sasso
MaGe Joint Workshop, Munich, January 2010

2. Main activities in GERDA that are using MaGe
Simulation of the main GERDA set-up (Phase I, Phase II, with segmented/BEGes)
Total expected background rate and spectrum
Many contributions to be taken into account (require a coherent approach and infrastructure)
Calibration system
Activity and position of calibration sources
Pulse shape simulation
Study expected background rejection performances, provide realistic “data” to test reconstruction algorithms
Test stands and prototypes
Interpretation of data and validation of simulations
BeGes (Hd, LNGS, Zurich) and segmented (Munich)
Some topics will be described in dedicated talks

3. GERDA geometries selectable in MaGe - 1
Top muon veto
Neck
Cryostat
Water tank
Detector array
Directory /gerdageometry
Used to simulate the main GERDA setup
all infrastructure in place
water tank, cryostat, detector array, holders
Detector array can be customized in number, type and size of crystals (can be set individually) within the 95 positions available: any combination of segmented, BeGes, non-coax, natGe, enrGe and dimensions (provide they are not too big!)

4. GERDA geometries selectable in MaGe - 2
Directory /munichteststand
Contains about 20 geometries for test stands and screening facilities at Munich, LNGS, Heidelberg and Hades
Work performed for data interpretation (e.g. with BEGes) and Monte Carlo validation (e.g. Munich prototypes)

5. GERDA output schemes
The directory /gerdaio contains 12 GERDA specific output schemes
Most of them are ROOT-based (+ possibly ASCII outputs)
3 output schemes are related to the main GERDA setup
change for the amount/type information they store (radioactive isotopes, neutrons, all hits)
Other schemes to be used with tests stands and other GERDA geometries
some of them are general enough to be used with other geometries, e.g. file-based geometries
tools to dump hit information on ASCII files  interface to external software for pulse shape simulation

6. MCC2 Campaign - 1
Simulations run for Phase I and Phase II arrangements
Phase II array contains: 8 enrGe unsegmented detectors (HdM-IGEX), 14 enrGe 18-fold segmented detectors and 8 natGe unsegmented detectors (GTF)
Phase I array has HdM-IGEX-GTF. Individual dimensions considered.
Main contributions of energy depositions in crystals:
Contamination of materials
Neutrons-induced
Muon-induced
Various physics processes contributing to the energy spectrum are simulated using MaGe

7. MCC2 Campaign - 2
Final goal: get expectation of full energy spectrum of both phases of GERDA (rate & shape)
can also be used “a posteriori” (with a fit) to evaluate contaminations of materials
Assumptions:
Core and segment thresholds = 10 keV
Resolution: 2.5 keV FWHM at MeV
GTF detectors are considered only for anti-coincidence (not for the energy spectrum)
Need to keep track of (many) individual contributions, each with parameters, as half-lifes, activities, etc. ( NEST)

8. Example: 0nbb decay
0nbb decay of 76Ge in the active volume and dead layer for Phase I and II arrays. Spectrum in the assumption of DBD mediated by massive Majorana neutrinos.
Dead layer: 0.8 mm for p-type detectors and negligible for n-type
For T1/2 = 1.2 · 1025 y (KK)
active volume
dead layer
bck goal
Signal efficiency vs. anti-coincidence cuts (Phase II array)
No cuts: 92.0%
Det anti-coinc: 91.3%
Seg anti-coinc: 87.2%

9. Example: Prompt m-induced background
Prompt m-induced background simulated again with MaGe in the MCC2 framework (Phase I & II). Derived info for new estimate of m-induced delayed background (e.g. 77mGe, 38Cl)
Notice: the previous simulation [NIM A 570 (2007) 149] run with different geometry. Used MUSUN to simulate explicitely energy-angle correlation
No cut
Segment anti-c
Results qualititatively consistent with the previous work. For Phase II (at Qbb):
9·10-3 counts/(keV·kg·y) without cuts and 4·10-4 counts/(keV·kg·y) with segment anti-coincidence.
Cherenkov veto needed and allows for < 10-5 counts/(keV·kg·y)

10. Example: n-induced background
Water buffer absorbs effectively all external neutrons: main contribution comes from neutrons produced in the setup (specifically, the stainless-steel cryostat!).
Global limit to bck from external neutrons: 10-7 cts/(keV·kg·y)
nuclear recoils
(n,n’g)
1000 kg·y exposure
Spectrum due to interactions of n from cryostat
Prompt background at Qbb: 7·10-6 cts/(keV·kg·y) with no cut. Reduction by factor of 4 by segment anti-coincidence

11. Other applications: background from 222Rn in LAr
Measurements of the 222Rn cryostat emanation triggered a new Monte Carlo campaign for the background estimate  simulated effect of 222Rn daughters (214Bi and 214Pb) for the Phase I & II arrays
Phase II array [uniform 222Rn distribution]:
214Bi
no cut
214Pb
no cut
segmentation
segmentation
Segmentation cut: factor ~2 at Qbb

12. Conclusions
GERDA Monte Carlo group (aka TG10) uses/develops MaGe as the main simulation tool
main GERDA set-up precisely coded, with proper shapes and dimensions
customizable in terms of shape, number and dimensions of detectors
also many test-stands and prototypes available in MaGe
used for data analysis and MC validation
other home-made codes used for cross-check and/or as quicker solutions for intensive jobs
Ongoing simulations to produce:
full background spectrum (not only at Qbb)
requires infrastructure to keep track of individual contribution
pulse shapes for candidate Phase II detectors