1. Surface Contamination Simulations with MaGe Rob Johnson Center for Experimental Nuclear Physics and Astrophysics University of Washington 15 February, 2007 MaGe MC workshop Max-Planck-Institut für Physik, München, Deutschland Surface contamination Additions to MaGe Dead Layer Dependencies Validation: WIPP-n Majorana Reference Design Outline

2. 15 FebruaryMaGe Meeting - MPI München Surface Contamination of HPGe detectors Important background Any surface  from U and Th decay chains can deposit energy around 2039 keV Energy deposits are single-site (no help from granularity / segmentation)

3. 15 FebruaryMaGe Meeting - MPI München Surface Contamination of HPGe detectors Surface contamination can arise from 2 mechanisms Solid contamination (dust) Contains some amount of 232 Th and 238 U decay chains May or may not be in equilibrium Radon daughters 222 Rn decays can leave daughters implanted in surfaces 22 year half-life of 210 Pb 210 Pb  210 Bi  210 Po  206 Pb +  (5.3 MeV)

4. 15 FebruaryMaGe Meeting - MPI München Surface Simulations Would like to predict effects of surface contamination Monte-Carlo can determine efficiencies for decays to land in the ROI Can see effects of analysis cuts Using efficiencies and desired sensitivity, can determine allowable concentration of contaminants Some tricky things to work out for surface simulations How to effectively sample a complicated surface? How to deal with radioactive decay chains?

5. 15 FebruaryMaGe Meeting - MPI München Additions to MaGe: General Surface Sampler Geant4 has no method to sample complex surfaces (Boolean volumes) We have complex geometries with surfaces that need sampling, however The General Surface Sampler (GSS) was born out of desperation/inspiration The algorithm: A chosen volume is enclosed by a bounding sphere of radius R Random direction in 4π is chosen Projection of sphere in random direction gives a disc of radius R, tangent to sphere. A random impact parameter is chosen from within this disc (b in picture) A geantino is shot from this point orthogonal to disc r r + b b

6. 15 FebruaryMaGe Meeting - MPI München Additions to MaGe: General Surface Sampler The algorithm, continued: The geantino picks out where it crosses a volume boundary The GSS determines whether a boundary is interesting (belongs to a volume that you want to sample) At the end of an event, there’s an array of G4ThreeVectors corresponding to points along the geantino’s track that crossed a volume Let N be the maximum number of intersections the geantino can make with the sampled volumes Pick a random integer between 1 and N. If it’s > the size of the array of points, throw the event away and on to the next. If it’s ≤ the size of the array of points, pick one of the position points at random and write it to a ROOT file (GSSTree). At the end of a run, there’s a ROOT file with many randomly sampled points on your surface. r r + b b

7. 15 FebruaryMaGe Meeting - MPI München Additions to MaGe: General Surface Sampler The Implementation (by macro): /MG/eventaction/rootschema GSS /MG/eventaction/rootfilename GSSOut.root /MG/generator/gss/boundvol InnerCryostatPhysical /MG/io/gss/addVolume InnerCryostatPhysical /MG/io/gss/addVolume Crystal1CrystalColumn2 /MG/io/gss/addVolume Crystal0CrystalColumn13 /MG/io/gss/setMaxIntersections 10

8. 15 FebruaryMaGe Meeting - MPI München Additions to MaGe: General Surface Sampler The Implementation (by macro): /MG/eventaction/rootschema GSS /MG/eventaction/rootfilename GSSOut.root /MG/generator/gss/boundvol InnerCryostatPhysical /MG/io/gss/addVolume InnerCryostatPhysical /MG/io/gss/addVolume Crystal1CrystalColumn2 /MG/io/gss/addVolume Crystal0CrystalColumn13 /MG/io/gss/setMaxIntersections 10 # every surface (I think) inside the RD cryostat #/MG/io/gss/addVolume StringerMountPlate #/MG/io/gss/addVolume SupportRod*CrystalColumn* #/MG/io/gss/addVolume ContactRing*CrystalColumn* #/MG/io/gss/addVolume GeTray*CrystalColumn* #/MG/io/gss/addVolume StringerLidCrystalColumn* #/MG/io/gss/addVolume LFEP*CrystalColumn* #/MG/io/gss/addVolume Crystal?CrystalColumn* #/MG/io/gss/addVolume CrystalColumn* #/MG/io/gss/addVolume InnerCryostatPhysical #/MG/io/gss/setMaxIntersections 70 #/MG/generator/gss/boundvol InternalVolume

9. 15 FebruaryMaGe Meeting - MPI München Additions to MaGe: a Time Windower The problem: Geant4/Radioactive Decay Module treats an entire decay chain as 1 event It’s easy to record aTrack->GetGlobalTime() for each step, allowing one to separate out the different parts that correspond to an actual physical event. Record events that satisfy abs(t 2 -t 1 )<fTimeWindow Works fine for “short” global times A typical global time for a 234 Th isotope is ~ 10 17 s My computer can’t distinguish between two numbers that differ by less than ~ 1 part in 10 15 Yours probably can’t either (10 17 seconds + 1 second) - (10 17 seconds) = 0 The moral: If you try this out with typical decay chains, you’ll get mucho unphysical “pileup”

10. 15 FebruaryMaGe Meeting - MPI München Additions to MaGe: a Time Windower The solution: When a particle(a track) is created, check to see Was it created by radioactive decay? Is its global time > some typical event time? If so, the track is placed in the “waiting” stack, it’s globaltime is saved and then reset to zero. The track is then processed after all tracks in the “processing” stack have been processed This allows the use of the full power of the radioactive decay manager Virtual methods have been implemented into MGVOutputManager Since each output class has it’s own ways of handling and recording data, each output class needs to have it’s own method Has been implemented for MGOutputG4Steps

11. 15 FebruaryMaGe Meeting - MPI München Dead Layer Dependencies N-type crystals have “thin” outer dead layers (0.3-1.0 µm) A 5.3 MeV alpha has a range of ~20 µm in Ge - easily penetrates thin dead layers Using MaGe/GSS, 210 Po was scattered on surface of Ge detector and allowed to decay A range of dead layers was used A thicker dead layer Pushes  peak lower in energy Puts more counts in ROI near 2039 keV Thickness of dead layer can have a strong effect on backgrounds from surface contamination

12. 15 FebruaryMaGe Meeting - MPI München The Waste Isolation Pilot Plant (WIPP) 655 meters (2150 feet, 3.25 furlongs) below the surface (1600 m.w.e.) Located near Carlsbad, NM Department of Energy operated Current science activities include lab space for Majorana collaborators and EXO-200 Also happens to be used as a pilot project to study storage of transuranic waste

13. 15 FebruaryMaGe Meeting - MPI München The WIPP-n Detector WIPP-n Detector N-type, “low background” packaged Ge detector 82 cm 3 active volume Cu housing, Be window Made in 1985, underground at WIPP since 1998 1.5 60 Co halflives >10 68 Ge halflives Hope to use it as a screening detector Shield 2” Cu 4” Pb on sides and top, 8” Pb on bottom Stainless steel enclosure, can be purged with N 2 for radon exclusion.

14. 15 FebruaryMaGe Meeting - MPI München The WIPP-n Detector Definite peak stucture around 5.3 MeV Simulation of 210Po with dead layer of 0.3 µm Captures peak position well Still not certain of continuum between 208Tl and ~ 5 MeV

15. 15 FebruaryMaGe Meeting - MPI München The Majorana Reference Design The Majorana experiment design is driven by the need to reduce backgrounds 1.1 kg, segmented Ge detectors enriched to 86% 76 Ge, Deep underground (4500-6000 m.w.e.) 2 x 57-crystal modules, 120 kg of detector 57 in a close-packed array, Housed in ultra-clean, electroformed Cu cryostat, Encased within active and passive shielding

16. 15 FebruaryMaGe Meeting - MPI München Majorana RD Surface Simulations MaGe/GSS used to sample surfaces of: Crystals Cryostat Cold Plate Support Rods Support Trays Contact Rings 210 Po Supported by 210 Pb 238 U and 232 Th decay chains Chains in equilibrium 4:1 ratio of Th to U (elemental ratio in crust)

17. 15 FebruaryMaGe Meeting - MPI München Conclusions Majorana has a background goal of 1 count/tonne-year in a 4 keV region of interest around 2039 keV Dead layer thickness has a large effect on the counts in ROI Crystal surfaces are by far the most important surface to keep clean For 0.1 cts/tonne-year in the ROI 5x10 -10 Bq/cm 2 of 210 Po Similar for U/Th alphas For comparison, the SNO neutral current detectors One of the cleanest surfaces ever 2x10 -9 Bq/cm 2 of 210 Po It is important to understand the effects of surface contamination!

18. 15 FebruaryMaGe Meeting - MPI München Outlook Attempting to get waveform data of alpha events from WIPP-n Current “Radial Cut” not at all useful for surface alphas Actual PSA simulation Accounting for incomplete charge collection