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Mark Kos, PNNL
Simulation of Cosmogenic and Radioactive Backgrounds for the CoGeNT Detector
1 PNNL-SA-85840
Overview ! CoGeNT data shows an excess of
events at low energies (PRL 107, 141301 (2011))
! Also hint of an annual modulation in the event rate (2.8 σ)
! Current work: background simulations ! Cosmogenics and cosmic ray muons ! Radioactive backgrounds
! Simulation results are compared to data in the energy range 0.5-3.0 keVee
! Parallel simulation for next generation of CoGeNT, C4
2
The background picture
The known cosmogenics
Dominant backgrounds simulated so far:, muon-induced neutrons in shielding, cosmogenic tritium, cavern neutrons
68Ge
68Ga
65Zn
73,74As 55Fe
3
54Mn 51Cr
49V
L-shell contribution
Other sources of background simulated: U and Th chain backgrounds in surrounding material (copper) Muon-induced neutrons from the cavern U and Th chain backgrounds in lead shielding Spontaneous fission neutrons from shielding material (α,n) neutrons from shielding material
PRELIMINARY
The CoGeNT low-energy range (L-shell peaks subtracted)
4
L-shell subtraction region
~1125 events in flat background (Potentially Compton scattering, β-decays, Bremsstrahlung…)
~960 events in excess
Muon-induced neutron simulation ! Two independent MC simulations used to assess neutron contributions
! muon induced neutron ! natural radioactivity in cavern
! #1: GEANT ! Soudan muon flux, E, angular distribution to generate (µ,n) in full shield. ! Includes e- and γ (8% of neutron contribution)
! #2 MCNP-Polimi: ! Neutron generation in lead shielding
(largest contributor)
! Reasonable agreement (they use different inputs) 339 +/- 68 events (GEANT)
5
µ-
Mostly neutrons, ~8% e- and γ’s (simulation)
CoGeNT data (L-shell subtracted)
GEANT
MCNP-Polimi
PRELIMINARY
PRELIMINARY
CoGeNT muon veto
! Muon veto operating with good geometric coverage and light collection.
! 1 cm panels do not allow muon-gamma separation.
! To compensate ! Veto operated at single photo-
electron sensitivity ! Generate ~12% dead time from
spurious germanium detector-veto coincidences.
! True coincidences observable ! In good agreement with rate
expected from MC simulations (i.e., veto catches (µ,n) events).
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Small gaps
Overlap & overhang
PRELIMINARY
Tritium production in germanium ! Cosmogenic production of tritium in Ge
while detector at surface ! Tritium β-decay endpoint at 18.6 keV
! Half-life of 12.33 yrs ! Tritium production rate:
! 27.7 /kg-day Astroparticle phys, 31, 417 (2009)
! Based on IGEX data Phys Lett, B432, 8 (2002)
! Assuming a surface exposure of CoGeNT detector of 2 yrs: ! 150 events in 0.5 – 3.0 keVee
(Geant4 simulation of 3H in CoGeNT)
CoGeNT Data
Tritium (simulation)
PRELIMINARY
7
Neutrons from radioactivity in the cavern ! Use Mei-Hime neutron flux:
! 3.78 X 10-6 cm-2 s-1
(Phys Rev D 73 , 053004 (2006))
! Use Monte-carlo neutron energy spectrum from Gran Sasso (worst case)
! Simulated background for CoGeNT: ! 54 events in the dataset
8
CoGeNT data
PRELIMINARY
Material 238U (mBq/kg) 232Th (mBq/kg) 210Po (Bq/kg)
Lead sample1 0.41 +/- 0.17 0.08+/-0.08 93 +/- 19
Plastic lumber 121 +/- 4 68 +/- 3
Plastic lumber (recycled)
115 +/- 5 80 +/- 4
Plastic lumber McMaster-Carr
15 +/- 1 1.3 +/- 0.8
Aluminum plate 7.1 +/- 2.4 986 +/- 12
Aluminum framing pieces
42 +/- 8 1348 +/- 50
Radioactivity in the CoGeNT shield
210Pb 210Bi 210Po 206Pb Ultra-low background lead around CoGeNT: 0.02 Bq/kg 210Pb
SNOLAB assay of similar materials as used in CoGeNT
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Source of (α,n) neutrons
Radioactivity in surrounding material ! Simulate U and Th chain
backgrounds in material surrounding the detector ! Full list not yet complete
! Simulated U and Th backgrounds in the surrounding OFHC copper ! 9 events in CoGeNT dataset
from this background ! Secondary handle:
! Look for radioactivity in copper by looking for the 8 keV copper excitation line in data
! Nothing in this region
PRELIMINARY
10
Backgrounds in low-energy region (0.5 – 3 keVee)
Cavern neutrons (from radioactivity) Tritium β-decay
Muon-induced neutrons
Background sum
PRELIMINARY
11
12
The background fractions
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Below ~1.1 keV: 14% muon-induced neutrons 4% cosmogenic tritium 2% environmental neutrons
PRELIMINARY
List of all backgrounds so far Source Events in CoGeNT dataset
(0.5 – 3 keVee) Fraction of total events
Muon induced events in shielding
339 +/- 68 16%
Tritium β-decay <150 7%
Cavern neutrons from radioactivity
<54 3%
U and Th backgrounds in copper
<9
External cavern neutrons (muon-induced)
<1.4
Old lead (210Pb + daughters)
<0.6
Spontaneous fission neutrons in lead
<0.5
SF neutrons in HDPE <0.2 HDPE (α,n) <0.03 8B solar neutrinos <0.014
13
PRELIMINARY
Radon temporal modulation
! Radon levels modulate underground – Measured ! Modulation out of phase!
! Also: Data out to 300 keV ! Infer the maximum number of events due to radon in the data ! 2.8 events in 0.5 – 3.0 keV
14
CoGeNT Data
CoGeNT Data ALL PRELIMINARY
Muon-induced neutrons temporal variation ! Muon veto cut is not applied in
modulation analysis ! Veto does not change energy spectrum after
dead time correction ! Veto efficiency fluctuations might lead to false
modulation
! Applying muon veto does not remove the modulation ! Just reduces event range
! MINOS muon flux modulation measured in Soudan ! Approximately +/-1.5% ! Peaks three months after best fit to present
CoGeNT data ! A 1.5% modulation of the estimated 339 +/- 68
muon-induced events in shielding predicts a modulation of 5 events in the 0.5-3 keVee energy range
! The CoGeNT data set contains 2124 events in the 0.5-3 keVee energy range. A 5 event modulation of muon induced events could only produce a 0.2% modulation effect in the CoGeNT data set.
15
Courtesy Alec T. Habig
PRELIMINARY
Simulations underway for the next generation of CoGeNT, CoGeNT-4 (C4)
! Four ~1 kg germanium detectors ! 2 inch thick veto panels ! Soudan Underground Lab ! Monitoring of radon at purge exhaust ! New DAQ with full energy range
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Summary
17
! We have been simulating many backgrounds…and more to come
! So far no explanation for excess at low energies and no candidate for the time dependence of the data
! The next generation, C4, will address many of the current concerns…2” thick veto panels, improved low-noise design (lower energy threshold), lower background cryostat
! And ~10 X more mass in 4 crystals – some coincidence rejection
17
Backup slides
18
Additional comments about Radon ! LN purge rate of 2L/min ! Purge line is valved off when changing dewars ! Dewars are not allowed to run fully dry before exchange ! An analysis looking for tell-tale signatures of Rn injection (spikes in
low-E count rate decaying with T1/2=3.8d) reveals no significant episodes.
! Radon expected to modulate daily as well (no significant daily mod.) ! A radon-induced modulation would appear at all energies (from
Compton associated to Rn peaks)
Heusser & Wojcik Appl. Radiat. Isot. 43 (1992) 9
19
Can muon-induced neutrons produce the observed modulation?
! Muon veto operating with good geometric coverage (~90%) and light collection, but 1cm panels do not allow muon-gamma separation.
! To compensate for this, veto is operated at single photo-electron sensitivity, generating ~12% dead time from spurious germanium-veto coincidences.
! True coincidences readily observable and in good agreement with rate expected from MC simulations (i.e., veto catches (µ,n) events).
! As expected from small (µ,n) component, no visible excess beyond ~12% random coincidences is removed from low-E spectrum by use of a veto cut. 20
Veto-panel light collection
21
Veto panel light collection uniformity using low-E gamma source
Can muon-induced neutrons produce the observed modulation?
MCNP-Polimi
! Two independent MC simulations used to assess neutron contributions (muon induced and nat. radioactivity in cavern)
! One uses GEANT and Soudan muon flux, E, ang. distribution to generate (µ,n) in full shield. Also e- and γ (8% of n contribution)
! A second uses MCNP-Polimi and neutron generation in lead shielding (largest contributor)
! Reasonable agreement (they use different inputs), both predicting a small low-E contribution.
22
Neutron production comparison
Source Neutron production
Araujo et al, FLUKA (280 GeV µ)
2.4 X 10-3 n/µ/(g/cm2)
Araujo et al, GEANT4 (280 GeV µ)
1.7 X 10-3 n/µ/(g/cm2)
This work, GEANT4 (280 GeV µ)
1.8 X 10-3 n/µ/(g/cm2)
This work, GEANT4.95 (280 GeV µ)
2.7 X 10-3 n/µ/(g/cm2)
NIM A 545 (2005) 398
23
C4 simulations ! Detailed simulations underway ! With 4 detectors we can
remove ~40% of neutron energy depositions (multiple scattering)
! With 2” thick veto panels the muon signal will be separated from background radioactivity
Neutron deposited energy distribution before coincidence cut
After coincidence cut
Muon veto simulation
99.7% efficiency with 7 MeV energy threshold
PRELIMINARY
C. Wiseman (PNNL)
C. Wiseman (PNNL)
24
Comparison with Mei-Hime
This work
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(α,n) in HDPE
Teflon (C2F4)
α 238Pu
13C(α,n)16O
Account for cross-section as a function of alpha energies in U and Th chain alphas, and correct for the fact we have C2H4
Monte-carlo neutrons from HDPE and then normalize to (α,n) calculation
SOURCES calculation at LANL (R. T. Perry 2002)
26
Beta-spectra and Compton compared to the flat background
27
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