Dmitry Chernyak on behalf of the - PowerPoint Presentation

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Dmitry Chernyakon behalf of the nEXO collaborationThe University of Alabama

Radon daughter plate-out as a background source in nEXO

Motivation: Radon daughter backgroundRadon daughters can deposit on the nEXO detector surfaces while they are exposed to air:

222

Rn daughters attach to surfaces of

nEXO

materials

→

accumulation of

210

Pb → growth of 210Po → → possible (α, n) reactions on low-Z nuclei → neutrons can create 137Xe and background events in the active LXe

Detector partArea [m2]210Po activityTPC vessel7.844.7 mBq/m2SiPMs11.43200 mBq/m2Sapphire rods0.51.0 Bq/m2Charge backing2.541.1 Bq/m2Interposer11.43218 mBq/m2Inner cryostat liner40.722.4 Bq/m2

Monte Carlo studies were performed to quantify how much 210Po can be present on each component to contribute 0.063% to the background or less.

What are allowable air exposure durations?

We need to know the 210Pb activity growth rate

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Dmitry Chernyak, Low Radioactivity Techniques

Radon daughter attachment problem: Growth

─

deposition length [m]

─

radon progeny equilibrium factor

─

surface area [m

2

]

─specific Rn activity [Bq/m3]─number of atoms

─deposition time

─

mean lifetime

─

deposition length [m]

─

radon progeny equilibrium factor

─

surface area [m

2

]─specific Rn activity [Bq/m3]─number of atoms─deposition time─mean lifetime

The growth rates for 218Po, 214Pb, and 214Bi are

218

Po rate of change proportional to number of 218Po atoms in air, not 222Rn

Every 214Bi decay creates a 210Pb atom. The rate of growth of the 210Pb surface activity is

Effective collection length

For long exposure times

(more than 3 h,

):

Collection distances are obtained as the ratios of measurable activities, modulo the equilibrium factors

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Radon daughter attachment problem: Decay

After removing samples from contact with air, the

decay rates

for

218

Po

, 214Pb, and 214Bi are

Get the effective collection length by measuring the time dependence of 218Po and 214Po (daughter of 214Bi) α-particles

The

solutions

of these equations are

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Dmitry Chernyak, Low Radioactivity Techniques

Sample exposureSamples were exposed to air in a basement lab with high 222Rn activityBasement lab exposure:The following environmental parameters were monitored and recorded:

TemperatureRelative humidityPressureRadon concentrationFiltered airflow

Airborne particle concentrationsSample surface chargeSome parameters can be controlled to create specific conditions

Rn source exposure

:

Purge box with the high-activity PYLON RN-1025

222

Rn sourceRadon activity can be controlled by nitrogen flow and exposure timeRadon activity was calibrated using Durridge RAD7 detector 

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MeasurementsSamples were measured using ORTEC Alpha Mega alpha spectrometer with the ULTRA-AS low-background silicon detectorGEANT4 simulation was used to estimate the detection efficiency for each sample218

Po214Po

Sample

BasementRn sourceCopper

391

14

SiPM

966HDPE1387Teflon13311Nickel123—Fused Silica76—Carbon Fiber58—Metallized Silica55—Sapphire

57—

Number of measurements62022/06/15Dmitry Chernyak, Low Radioactivity Techniques

Radon source resultsSampleNumber of measurementsAverage effective collection length [m]Copper140.020 ± 0.008SiPM60.028 ± 0.008HDPE

70.016 ± 0.006Teflon11

0.016 ± 0.003

218

Po

214

Po

Standard deviations reportedUse of the radon source verified the time fitEffective collection lengths are similar for all samples72022/06/15Dmitry Chernyak, Low Radioactivity Techniques

218Po214PoBasement lab conditionsSamples were exposed to the basement air in the following conditions:Fan ON High and Low airflows have 400 and 50 times higher volume exchange rates than one at SNOLAB, respectively.

218Po detection of about 16 atomsMeasurements are essentially background free

Surface covered

with paper tissue

Fan ON High:

Tent closed, air exchanges

2.89 min

-1Fan ON Low: Tent closed, air exchanges 0.36 min-1Fan OFF: Tent closed, air exchanges 0 min-1Fan OFF: Tent opened, air exchanges 0 min-1Surface biased using HV82022/06/15Dmitry Chernyak, Low Radioactivity Techniques

Detailed attachment results92022/06/15Dmitry Chernyak, Low Radioactivity Techniques

Basement lab results: Copper102022/06/15Dmitry Chernyak, Low Radioactivity Techniques

Basement lab results: SiPM and HDPESiPMHDPE

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Basement lab results: Other materialsSapphireCarbon FiberNickelMetallized SilicaFused SilicaTeflon

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Dmitry Chernyak, Low Radioactivity Techniques

Basement lab results: All materialsNo pronounced materials dependence observed for measured materialsAverage effective collection length is lower for higher air volume exchange ratesAt a radon concentration of 135 Bq/m3 and an allowable 210Po activity of 4.7 mBq/m2 the allowed exposure time is 30.5 hours, assuming the 210Po activity equals that of 210Pb

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Basement lab results : Biased CopperAttachment reduction can be achieved by biasing or covering with a Kimwipe (factor 1059 ± 250 reduction due to coverage observed at -1000 V biasing). 14

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Radon progeny activities and equilibrium factorEquilibrium factor for 218Po was measured using Bladewerx Sabre BPM2 alpha/beta monitor152022/06/15Dmitry Chernyak, Low Radioactivity Techniques

Comparison of results with Jacobi modelCopper measurement results were compared with Jacobi model.The idea behind the Jacobi model is straight forward: account for all disappearance mechanisms, assuming steady state.According to the Jacobi deposition model Fan OFF conditions are closest to SNOLAB environment.Our application of Jacobi model can’t predict effective collection lengths quantitatively, but only qualitatively.

Volume exchange rate, min-1Clean tentEffective collection length [m]Average effective collection length [m]218Po equilibrium factor [

a.u.]

Jacobi modelMeasured

Jacobi model

Measured

Diffusive attachment to surfaces

Tuned attachment velocity2.89Closed0.00280.08 ± 0.040.0710.0470.012 ± 0.0030.36Closed0.0170.15 ± 0.080.360.150.062 ± 0.0190.0Closed

0.130.23 ± 0.07

0.840.830.34 ± 0.09Opened0.180.32 ± 0.240.93

0.930.45 ± 0.110.0070SNOLAB0.170.37 ± 0.020.94Comparative Dosimetry of Radon in Mines and Homes. National Academies Press, 1991162022/06/15Dmitry Chernyak, Low Radioactivity Techniques

Summary210Po surface radioactivity can contribute to a background of the nEXO experiment through nuclear (α, n) reactionsRadon daughters in the air are not in equilibrium. Equilibrium factors are subject to change, introducing additional variability of the attachment observablesAttachment lengths observed in a lab were much larger than those in a small exposure box coupled with a radon sourceEffective collection length was found to not depend strongly on the measured materialCurrent implementation of Jacobi model indicates that radon daughter equilibrium conditions at SNOLAB are closest to Fan OFF data at UAAttachment reduction can be achieved by biasing or covering material with a Kimwipe tissue172022/06/15

Dmitry Chernyak, Low Radioactivity Techniques

nEXO Collaboration182022/06/15Dmitry Chernyak, Low Radioactivity Techniques