accurate simulations of $$^{206}$$ 206 pb recoils in supercdms

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J Low Temp Phys DOI 10.1007/s10909-014-1102-z Accurate Simulations of 206 Pb Recoils in SuperCDMS P. Redl Received: 15 July 2013 / Accepted: 23 January 2014 © Springer Science+Business Media New York 2014 Abstract SuperCDMS is a direct detection search for WIMPs, currently operating a 9 kg array of germanium detectors in the Soudan Underground Laboratory. The detectors, known as iZIPs, are cylindrical in shape and each flat surface is instru- mented with both ionization and phonon sensors. Charge and phonon information is collected for each event, and comparing the energy collected in the phonon sensors to the charge sensors gives excellent discrimination power between nuclear recoil and electron recoil events. Furthermore, this technology provides excellent discrimination between surface and bulk events. In order to show the surface event rejection capa- bility of these detectors, two 210 Pb sources were installed facing two of the detectors currently operating in the Soudan experimental run. The 210 Pb decays to 210 Bi, which in turn decays to 210 Po. The 210 Po decays by alpha emission, yielding a recoiling 206 Pb ion with 103 keV kinetic energy and an alpha particle with 5.4 MeV kinetic energy. We used the non-standard Screened Nuclear Recoil Physics List (Mendenhall and Weller, Nucl. Instrum. Methods Phys. Res. B 227:420–430, 2005) in Geant4 (Agostinelli et al., Nucl. Instrum. Methods Phys. Res. Sect. A 506:250–303, 2003) to simulate all of the above decays and achieve excellent agreement with experiment. The focus of this paper is the simulation of the 210 Po decay. Keywords Geant4 · Simulation · Pb210 · Pb206 · SuperCDMS · CDMS 1 Introduction One of the dominant backgrounds in dark matter searches is events caused by radon daughters. Events from the 210 Pb decay chain (Fig. 1) are especially troublesome, For SuperCDMS Collaboration. P. Redl (B ) Department of Physics, Stanford University, 382 Via Pueblo, Stanford, CA 94305, USA e-mail: [email protected] 123

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Page 1: Accurate Simulations of $$^{206}$$ 206 Pb Recoils in SuperCDMS

J Low Temp PhysDOI 10.1007/s10909-014-1102-z

Accurate Simulations of 206Pb Recoils in SuperCDMS

P. Redl

Received: 15 July 2013 / Accepted: 23 January 2014© Springer Science+Business Media New York 2014

Abstract SuperCDMS is a direct detection search for WIMPs, currently operatinga 9 kg array of germanium detectors in the Soudan Underground Laboratory. Thedetectors, known as iZIPs, are cylindrical in shape and each flat surface is instru-mented with both ionization and phonon sensors. Charge and phonon information iscollected for each event, and comparing the energy collected in the phonon sensorsto the charge sensors gives excellent discrimination power between nuclear recoil andelectron recoil events. Furthermore, this technology provides excellent discriminationbetween surface and bulk events. In order to show the surface event rejection capa-bility of these detectors, two 210Pb sources were installed facing two of the detectorscurrently operating in the Soudan experimental run. The 210Pb decays to 210Bi, whichin turn decays to 210Po. The 210Po decays by alpha emission, yielding a recoiling 206Pbion with 103 keV kinetic energy and an alpha particle with 5.4 MeV kinetic energy. Weused the non-standard Screened Nuclear Recoil Physics List (Mendenhall and Weller,Nucl. Instrum. Methods Phys. Res. B 227:420–430, 2005) in Geant4 (Agostinelli etal., Nucl. Instrum. Methods Phys. Res. Sect. A 506:250–303, 2003) to simulate all ofthe above decays and achieve excellent agreement with experiment. The focus of thispaper is the simulation of the 210Po decay.

Keywords Geant4 · Simulation · Pb210 · Pb206 · SuperCDMS · CDMS

1 Introduction

One of the dominant backgrounds in dark matter searches is events caused by radondaughters. Events from the 210Pb decay chain (Fig. 1) are especially troublesome,

For SuperCDMS Collaboration.

P. Redl (B)Department of Physics, Stanford University, 382 Via Pueblo, Stanford, CA 94305, USAe-mail: [email protected]

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210Pb

210Po

206Pb

210Bi

22.3 y

5.01 d

138.4 d

80%: 17.0 keV

20%: 63.5 keV

100%: 1161.5 keV

100%: 5.3 MeV

13.7%: conv. e 42.5 keV + Auger e3.5%: conv. e 45.6 keV + Auger e4.3%: 46.5 keV

103 keV

58.1%:conv. e 30.2 keV + Auger e+ 22.0%: x-rays 9.4-15.7 keV

Fig. 1 This figure shows the 210Pb decay chain. Only dominant decay modes of 210Pb are shown forsimplicity (Color figure online)

because of the relatively long half-life of 210Pb and the 206Pb ion emitted by the 210Podecay. Betas from the 210Pb decay are generally below ∼65 keV in kinetic energy;several gamma lines are also present in the 10–65 keV region. The problem with eventsin that energy range is that the betas don’t penetrate the detectors deep enough to allowfor complete charge collection, so have a chance to leak into the nuclear recoil bandand mimic a WIMP signal [3]. The 210Bi ion undergoes simple beta decay with a betaendpoint at 1.2 MeV and the lower spectral density of events in the low energy WIMPsearch region makes this decay less of a problem. Finally, the 210Po alpha decay to206Pb causes nuclear recoil events inside a detector directly. The alpha in this casehas energy of ∼5.4 MeV and rejecting such an alpha is straight forward, however,the recoiling 206Pb nucleus has 103 keV of kinetic energy and can therefore fake aWIMP signal quite readily. This problem would be mitigated if all of the 206Pb eventshad an energy 103 keV, however, if the decaying 210Po nucleus is located inside thesurrounding material the nucleus will lose some energy before hitting a detector andtherefore no spectral line or a much smaller spectral line at 103 keV will be seen. Thelow energy of the nucleus means that it will not penetrate very far into a germaniumdetector and therefore rejecting surface events will eliminate this class of events [3].

SuperCDMS iZIP detectors show excellent rejection capabilities for surface eventsand demonstrate that this technology gives sufficient rejection capabilities for thisbackground to make a 200 kg scale experiment planned for the SNOLAB laboratoryviable [3]. Our Geant4 background simulations with the iZIP detectors, shown in Fig.2, give good agreement between data and Monte Carlo, thereby demonstrating goodunderstanding of the physics involved.

2 The Source Plate

The planar 210Pb sources used for this study were manufactured by the Stanfordgroup using silicon wafers sealed for 12 days in an aluminum box with 222Rn gasproduced by a 5 kBq 226Ra source. The silicon wafers then underwent a standardwafer cleaning process to remove any dust particles containing radionuclides, removethe native oxide, and chemically regrow a 2 nm thick clean oxide. This cleaning processreduced the alpha rate measured from the wafers by a factor of ∼2. The wafers were

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Fig. 2 a Phonon and ionization sensor layout for iZIP detectors deployed at Soudan. The Ge crystalis 76 mm in diameter and 25 mm thick. b Magnified cross section view of electric field lines (red) andequipotential contours (blue) near the bottom face of a SuperCDMS iZIP detector. c Fabricated iZIP detectorin its housing (Color figure online)

Fig. 3 This figure shows a cartoon of the experimental setup. The silicon plate is mounted above a germa-nium iZIP detector in this case. The 210Pb is implanted to a depth of upto ∼58 nm (∼30 nm median) andeventually decays to 210Po. Considering that the decays leading to the 210Po are all beta decays, the 210Ponucleus is located in the same position as the original 210Pb nucleus. The alpha decay of the 210Po nucleusproduces a 103 keV 206Pb ion that has a chance of being observed by the iZIP detector (Color figure online)

mounted above detector T3Z1 and below detector T3Z3 in the SuperCDMS Soudanexperiment. The two deployed sources were implanted nearly uniformly with 210Pb toa depth of ∼58 nm [3]. From the known decay chain shown in Fig. 1, we observe a totalinteraction rate of ∼130 events per hour. Figure 3 shows a cartoon of the experimentalsetup.

3 Simulation Setup

In order to gain confidence in our understanding of this experiment we performeda full simulation of the setup. In the simulation we assumed that 214Po, 214Pb and214Bi ions will plate out onto our silicon wafers, with the 214Po alpha decay actuallyimplanting 210Pb nuclei into the wafers. We realize that some 218Po will plate out ontoour wafers as well, but since the 218Po half life is much shorter than the combined

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Fig. 4 Comparison of data from the two Ge iZIP detectors with a Geant4 simulation. (top panel) Gammaelectron recoil band data (defined as ionization yield > 0.8) includes in the data the continuous spectrumfrom bulk gamma interactions (not simulated), and the simulated X-ray peaks at ∼12 and 46.5 keV. (middlepanel) Beta band data (yield 0.4–0.8) includes the simulated beta decays from 210Pb and 210Bi. (bottompanel) Recoiling 206Pb band data (yield 0.1–0.4) include the simulated 206Pb recoils from 210Po decay.The drop in T3Z3 relative to T3Z1 below ∼15 keV is due to differing cut efficiencies. Normalizations ofthe simulations in the top panel and the middle panel were fixed by the normalization needed to match dataand simulation in the bottom panel. Unlike the perfect simulation classifications, there is significant mixingin the data yield-based classification between “gammas” and “betas,” and between “betas” and “recoils of206Pb” at energies below ∼20 keV. Unsimulated Frenkel defects[4] can account for the difference near103 keV (Color figure online)

half lives of 214Po, 214Pb and 214Bi we simplified our simulation by neglecting 218Pocontamination. We used this as a starting point for our simulation and let the 214Podecay isotropically to 210Pb. The 210Pb ion recoils with 146 keV of kinetic energyand is implanted into the silicon wafer. We followed the decay chain of 210Pb allthe way to the stable 206Pb nucleus. The simulation approximates the 2 nm oxidelayer that is grown, by halving the density of 210Pb within the top 2 nm of the wafer.Considering the complexities involved in this simulation we decided to use Geant4with the Screened Nuclear Recoil Physics List, which allows for accurate simulationof low energy ion implantation and energy deposition [1].

4 Results

The results of this simulation are shown in Fig. 4, where we directly compare thesimulation to Soudan experiment data (T3Z1 and T3Z3). Figure 4 shows the resultfor the gamma band in the top panel (ionization yield > 0.8), betas in the middlepanel (0.4 < ionization yield < 0.8) and the lead recoil band in the bottom panel(0.1 < ionization yield < 0.4). The top panel shows that the simulation correctly

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Fig. 5 The figure shows a comparison between Standard Geant4, TRIM and Geant4 with the ScreenedNuclear Recoil Physics List. The top panel compares the implantation depth for a 103 keV lead ion shotstraight into a silicon wafer, while the bottom panel shows the spectrum recorded in the SuperCDMSexperiment compared to simulation. The triangular points show a Standard Geant4 simulation and theround points corresponding to Geant4 with the Screened Nuclear Recoil Physics List (Color figure online)

simulates the gamma lines, but misses the events between the gamma lines. This dis-agreement is caused by bulk events, which were not simulated, but were includedin the data sample in order to include the 46 keV gammas penetrating beyond thecrystal surface. The trade-off is that other background gammas coming from the cop-per detector housing and other surrounding materials are included as well, but werenot simulated. This changes the overall normalization and increases the size of the12 and 46 keV peaks in simulation compared to data. The middle panel shows betaevents, which exhibit good agreement between data and Monte Carlo. The simu-lated 30 keV peak is higher than the corresponding data peak because the simulationdoes not account for the degraded energy resolution of surface betas. Finally, thebottom panel shows good agreement between data and Monte Carlo in the regionbetween 20 and 100 keV. Below 20 keV the agreement is not as good because ofbetas leaking into the lead recoil band, while the endpoint discrepancy at 103 keVcan be explained by Frenkel defects, which cause a ∼ 3% phonon quenching factor[4]. The mis-matches seen between data and simulation do not hold up current Super-CDMS analysis, however, we are working on improving the agreement by, includ-ing Frenkel defects in our simulations, simulating bulk gamma events, as well asimproving our reconstruction algorithms to improve energy resolution for surfacebetas.

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4.1 The Screened Nuclear Recoil Physics List

Geant4 allows the use of different physics lists, which can be used to accuratelydescribe particle interactions in different energy regimes, materials, etc. Geant4 offersmany standard physics lists, but many of them have particle accelerator physics inmind, which means that low energy ion implantation is not part of standard Geant4physics. In order to accurately simulate low energy ions the user has to implement anon-standard list, which applies a similar physics framework to that used in TRIM [5].Figure 5 shows a comparison between TRIM, Standard Geant4 and Geant4 with theScreened Nuclear Recoil Physics List. It is clear that standard Geant4 physics doesnot accurately predict this data, nor does it agree with TRIM.

5 Conclusions

We show good agreement between data and simulation for the 210Pb sources installedfacing two SuperCDMS Soudan detectors. In our simulations we included knowledgeof how the source wafers were prepared and installed. The results validate our under-standing of the physics involved. We had to use Geant4 with the non-standard ScreenedNuclear Recoil Physics List, which allows Geant4 users to simulate low energy ionscorrectly. We showed that for a simple implantation depth simulation the Bragg peakof TRIM and Geant4 agree, although the straggle does not agree well. This differencedoes not significantly alter our results, and is due to the use of different screeningfunctions in Geant4 vs. TRIM.

Acknowledgments The SuperCDMS collaboration gratefully acknowledges the technical assistance fromJim Beaty and the staff of the Soudan Underground Laboratory and the Minnesota Department of NaturalResources. These iZIP detectors are fabricated in the Stanford Nanofabrication Facility which is a memberof the National Nanofabrication Infrastructure Network sponsored by NSF under Grant ECS-0335765.This work is supported in part by the National Science Foundation, by the Department of Energy, byNSERC Canada, and by MULTIDARK CSD2009-00064 and FPA2012-34694. Fermilab is operated byFermi Research Alliance, LLC under Contract No. De-AC02-07CH11359, while SLAC is operated underContract No. DE-AC02-76SF00515 with the United States Department of Energy.

References

1. M.H. Mendenhall, R.A. Weller, Nucl. Instrum. Methods Phys. Res. B 227, 420–430 (2005)2. S. Agostinelli et al., Nucl. Instrum. Methods Phys. Res. Sect. A 506, 250–303 (2003)3. Agnese et al.: Submitted to, Physical Review Letters, arXiv:1305.2405, (2013).4. I. Lazanu, M.L. Ciurea, S. Lazanu, Astropart. Phys. 44, 9–14 (2013)5. J.F. Ziegler, M.D. Ziegler, J.P. Biersack, Nucl. Instrum. Methods Phys. Res. Sect. B 268(11–12), 1818–

1823 (2010)

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