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

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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

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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)

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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)

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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

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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

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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

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Backgrounds in low-energy region (0.5 – 3 keVee)

Cavern neutrons (from radioactivity) Tritium β-decay

Muon-induced neutrons

Background sum

PRELIMINARY

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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

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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

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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.

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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

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!   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

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Backup slides

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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

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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

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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.

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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

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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)

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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)

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Beta-spectra and Compton compared to the flat background

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