The Radon Monitoring System in Daya Bay Reactor Neutrino Experiment

1.1 MB·

by  M. C. Chu

The Radon Monitoring System in Daya Bay Reactor Neutrino Experiment
M.C.Chua,K.K.Kwana,M.W.Kwoka,T.Kwokb,J.K.C.Leungb,K.Y.Leungb,Y.C.Lina,b,K.B.Lukc,d,
C.S.J.Punb
aDepartmentofPhysics,TheChineseUniversityofHongKong,HongKong,China
bDepartmentofPhysics,TheUniversityofHongKong,HongKong,China
cDepartmentofPhysics,UniversityofCaliforniaatBerkeley,Berkeley,CA94720,U.S.A.
dPhysicsDivision,LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,U.S.A.
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0
2

n
Abstract
a
J We developeda highlysensitive,reliableandportableautomaticsystem (H3) tomonitortheradonconcentrationof

9 the underground experimental halls of the Daya Bay Reactor Neutrino Experiment. H3 is able to measure radon
1 concentrationwithastatisticalerrorlessthan10%ina1-hourmeasurementofdehumidifiedair(R.H.5%at25◦C)

withradonconcentrationas lowas 50Bq/m3. Thisis achievedbyusinga largeradonprogenycollectionchamber,
]
t semiconductor α-particle detector with high energy resolution, improved electronics and software. The integrated
e
d radon monitoring system is highly customizable to operate in different run modes at scheduled times and can be
- controlledremotelytosampleradoninambientairorinwaterfromthewaterpoolswheretheantineutrinodetectors
s
n are being housed. The radon monitoringsystem has been runningin the three experimentalhalls of the Daya Bay
i ReactorNeutrinoExperimentsinceNovember2013.
.
s
c Keywords: radon,Rn,DayaBay
i
s PACS:29.40.n
y
h
[p 1. Introduction 210Bi −−−β−−→ 210Po −5−.−3−0−M−e−V−→α 206Pb. These α-particles
83 5.01d 84 138d 82
The main goal of the Daya Bay Reactor Neutrino caninducebackgroundintheexperiment.Furthermore,
1
v Experiment is to determine the neutrino mixing angle due to its relatively long half-life, 222Rn can accumu-
5 θ . Eightanti-neutrinodetectormodules(ADs)arein- late to a high concentration in poorly ventilated space
13
8 stalledinthreeundergroundexperimentalhalls.Inorder andposeahealthhazard. Inviewofitsimportance,the
8 to suppress backgrounds related to cosmic-ray muons term“radon”isusedtoreferspecificallyto222Rninthis
4
andnaturalradioactivity,theADsareimmersedinwa- paperunlessotherwisestated.
0
. terpools[1].
1 Radoninairisparticularlydifficulttohandleamongst
Inanundergroundenvironmentlikethis,radioactive
0 commonnaturalradioactivebackgrounds.Itcandiffuse
radon gas can be easily found in air. Isotopes of Tho-
6
into the ADs through any unnoticeable leak or radon-
1 riumandUranium(232Th,235Uand238U)inrockspro-
permeable materials, and then dissolve in the liquid
: duce gaseous 220Rn, 219Rn and 222Rn respectively in
v scintillator. The dissolved radon and its progeniescan
theirdecaychains. 219Rninairhasnegligibleimpactto
i
X the experimentsince its half-life is only 3.96 s. 220Rn cause correlated background(from α decays) and sin-
gles background (from β decays) mimicking antineu-
r has a half-life of 55.6 s, so it can exist for a longer
a trino signals. To reduce the amount of radon in the
time in air and may accumulateto a measurablequan-
vicinity of the ADs, dry nitrogen gas is used to flush
tity. For 222Rn, it has a half-lifeof 3.83days. Numer-
out air in the space between the water pool cover and
ousα-particlesaregeneratedfrom222Rnanditsproge-
the pool water, and in the dry ducts connected to the
nies along the chain: 222Rn −5−.−4−0−M−e−V−→α 218Po −6−.−0−0−M−e−V−→α
86 3.83d 84 3.05m ADs. The poolwater is also purified and recirculated.
214Pb −−−β−−→ 214Bi −−−β−−→ 214Po −7−.−6−9−M−e−V−→α 210Pb −−−β−−→ Inadditiontosuppressingradonintheexperiment,there
82 26.8m 83 19.7m 84 164µs 82 22.3y isaneedtomonitorcontinuouslyitsconcentration,es-
pecially in the air in the experimentalhalls, and in the
Emailaddress:[email protected](T.Kwok) watersurroundingtheADs.
PreprintsubmittedtoNuclearInstrumentsandMethodsA January20,2016
A preliminary survey of airborne radon was carried However,duetothesmallvolumeofitsinternalsample
outwith an AB-5 detector (Pylon ElectronicsInc.) [2] cell(0.7L),RAD7 hasa C.F. ofonly148Bq/m3/cpm.
intheexperimentalhalls(EH1,EH2,andEH3)ofDaya Another detector of this type is ERS-2-S (Tracerlab
Bay. Radon concentrationsfrom about 50 Bq/m3 to a GmbH) [4], which has a higher sensitivity. However,
few hundredsof Bq/m3 were recordedat variousloca- theα-particleenergyspectrumhaslimitedresolutionfor
tions. We require the statistical error of a meaningful easybackgroundidentification.
monitoring of the radon concentration to be less than Excellentsensitivityreaching1mBq/m3orbettercan
10% for 1-hour sampling of air with radon concentra- beachievedbyradonextractionfacilitiesbuiltforsome
tiondownto50Bq/m3. Itisconvenienttoexpressthis underground experiments [5–7]. These facilities uti-
requirement by a detector calibration factor (C.F.) of lizecollectionchamberofhundredsoflitresinvolume.
30Bq/m3/cpmwiththedefinition: They take up large spaces and are not intended to be
portable.
radonconcentration[Bq/m3]
C.F.= (1)
numberofcountsperminute[cpm]
3. RadonmonitoringintheDayaBayExperiment
Also, the detector should be sensitive to a low radon
concentration of 0.5 Bq/m3 and portable, for carrying
outadhocradonmeasurements(e.g.radoningasflush- In order to meet the requirements stated in Sec-
ingtheADs). tion1,theHongKongHigh-sensitivityHigh-reliability
The monitoring system should operate as an inte- detector (H3) is designed to have a C.F. better than
grated system. It has to control the measurements of 30Bq/m3/cpmwithsuperbportability. Thewholesys-
radon in air and water automatically following a cus- tem is integrated to the hardware and software of the
tomizable schedule. The radon data and the measure- DayaBayexperimentinasimplemanner. Itiscapable
ment conditions should be ready to be processed and tocontrolradonmeasurementsinairandwatersamples,
uploadedtoacentralizedsystemoftheexperiment. andpresentthereal-timeresultsthroughnetwork.
Thusahighlysensitiveradonmonitoringsystemwas As depicted in Figure 1, the system is installed in
developed to meet these requirements. Details of this the electronics room of each experimentalhall. There
systemareintroducedinthispaper.InSection3,thede- aretwotypesofrunmode: “airrun”formeasuringthe
signandconstructionofthesystemarepresented. Cal- radonconcentrationoftheairsampleextractedfromthe
ibrationand optimization of the radondetector are de- hall; and “water run” for measuringthe radonconcen-
scribed in Section 4. In Section 5, the performanceof tration of the water sample obtained from the recircu-
thesystemintheundergroundhallsisdiscussed. lation system of the water pool. The samples are not
required to flow at high speed or be pressurized. The
sampling points can be set up easily by plugging the
2. Otherradonmonitoringequipment
tubingsintothepush-infittings.
ManycommercialradondetectorsemployLucascell The H3 is designed to measure airborne radon di-
or electrostatic method for radon progenies counting. rectly. For radondissolved in water sample, the gas is
Lucascell-baseddetectorslike AB-5 arerenownedfor allowedtodiffusefromwatertoairinadegassingunit
theirstabilityandsimplicityofoperation.However,the first. ThentheairispumpedtotheH3formeasurement.
celldoesnotdistinguishtheα-particlesgeneratedfrom AnsoftwareinterfaceiswritteninLabVIEWVirtual
radon,218Poand214Po. Asthetwo nuclidespreceding Instrument (VI) [8] to control and monitor the opera-
214Pohavehalf-livesofabout20minutes,ittakesabout tionoftheradonmonitoringsystem.Thehostcomputer
3hoursforradonand214Potoreachequilibriuminside (hostPC) of the LabVIEWVI is connectedto a set of
theLucascell. Thistypeofdetectorisnotdesignedfor controlvalvesandflowsensorsinstalledalongthesam-
monitoringthehourlyfluctuationofradoncontinuously. ple pathways through a USB data acquisition module.
Detectors employing electrostatic method to collect TheVIalsoreadstheH3 dataviaaRS-232port. Then
radonprogeniescanhaveaquickerresponse. Manyof it processes the data with the calibration constants set
themareabletotakeanenergyspectrumofα-particles, by the system administrator. A copy of the processed
enablingtheselectionofthefast-response218Poevents dataisalsosenttotheremoteDetectorControlSystem
for counting. RAD7 (Durridge Company Inc.) [3] is (DCS)oftheexperimentwhichcentralizesalltheenvi-
a typical and popular model of this type of detector. ronmentaldataanddetectoroperationconditions.
2
Figure1:SchematicdiagramoftheradonmonitoringsystemineachDayaBayexperimentalhall.DetailsofcomponentscanbefoundinSections
4and5.
3.1. H3detector ciencyof218Poioninsidethechamber,andε istheα-
α
The H3 detector is made of a rack-mount VME-4U particledetectionefficiencyofthePIPS.Forcollection
chassis (178 mm (H) × 482 mm (W) × 279 mm (L)) chambersofsimilarmechanismandcomparablesize,εp
thathousesa radonprogenycollection chamber,an α- isabout70%[12]. ForthePIPS,εα is40%. Sothevol-
particle detector, a high-voltage generator, and front- umeof the chamberofthe H3 shouldbe about2.25L.
endelectronics,weighinglessthan5kgintotal. Figure 2 is the cross section of the radonprogenycol-
lection chamber. It is a polyvinylchloride cylinder of
3.1.1. Detectionmethod 9-cm(h)×18-cm(∅)(2.3L)toprovidesufficientvol-
Airsampleisdrawnintotheradonprogenycollection umeofairsampletogeneratetheprogenies.
chamberformeasurement. Radonprogeniesanddusts The inner surfaces of the chamber are coated with
in the incoming air are removed by a glass fiber filter aluminumfoils.ThePIPSismountedonacircularaper-
(GF-75, pore size 0.3 µm, Sterlitech Corp.) [9] at the tureatthecenterofthe topplate. Airsampleisdrawn
inletofthechamber,soonlythefreshlyproducedradon into the chamber with a micro-pump installed down-
progeniesarecollected. stream of the gas outlet. Temperatureand relative hu-
Inthechamber,thepositivelychargedprogeny218Po midity(R.H.)oftheairsamplearemonitoredbydigital
are collected electrostatically on the entrance window sensorsattheinletofthechamber.
ofaPassivatedImplantedPlanarSilicondetector(PIPS The aluminum coating of the chamber is connected
modelA450,CanberraIndustriesInc.) [10]. ThePIPS to+1800V,whiletheentrancewindowofthePIPShas
measurestheenergyoftheα-particlesemittedinthede- a DC bias of +24 V. This establishes an electric field
caysof218Poandsubsequentprogenies.Thentheradon inside the chamber for driving the positively charged
concentration can be calculated from the counts of α- radonprogeniestothePIPS.
particleintheenergyspectrumandtheC.F. The C.F.s obtained in the calibration (Section 4.4)
with this chamber were consistent with the estimation
3.1.2. Radonprogenycollectionchamber ofEquation2. Alargerchambercanboostthedetector
ThevolumeofthechamberrequiredtoachieveaC.F. sensitivityasmoreradonprogeniescouldbecollected.
of30Bq/m3/cpmisestimatedby: However, this would sacrifice portability and requirea
highlystablehigh-voltagesupply.
1
V = (2)
C.F.×r ×ε ×ε ×60[sec]
+ p α
3.1.3. Electronicsunit
wherer =88%[11]isthefractionofemanated218Po Signalsfromthe PIPSare analyzedby the on-board
+
that carries positive charge, ε is the collection effi- electronics unit. The electronics unit is divided into a
p
3
reached for radon and with sufficient air exchange be-
tweenH3 andtheRAD-Aqua,theairintheclosedloop
trulyreflectstheradonconcentrationinwater.
4. PerformanceandcalibrationofH3
Inthedesigningstage,testswerecarriedouttoopti-
mizetheperformanceoftheH3.
4.1. Regions-of-interestforcounting
Figure 4 shows the energy spectrum of α-particles
collected by the H3. The two prominent peaks corre-
Figure2: Crosssectionoftheradonprogenycollection chamberof
H3. O-ringsusedforensuringair-tightnessaredepictedbytheblack sponding to the decays of 218Po and 214Po (6.00 MeV
dotsaroundthePIPS. and 7.69 MeV respectively). In prolonged measure-
ment, the α-peak of 210Po (5.30 MeV) becomes vis-
ible gradually when 218Po and 214Po accumulate on
the PIPS. However, the 210Po-peak does not respond
front-end(FEE)circuitandamainboard(Figure3).
quickly to any change of radon concentration. Thus it
The analog FEE circuit is shielded from the noisy
wouldnotbeusedintheanalysis.
digital and switching power circuits with a Faraday
The energy spectrum obtained by the H3 has suf-
cage. It consists of a charge-sensitiveamplifier (CSA,
ficient energy resolution for defining the regions-of-
OP Amp AD8065) and a charge-to-time converter
interest (ROIs) for 218Po and 214Po. The number of
(QTC). The QTC converts an input analog pulse from
countsin the ROIs can be usedto determinethe radon
the PIPS into a TTL logic signal with a width propor-
concentration. The ROI of 218Po is 5.50 to 6.40 MeV,
tionaltotheinputcharge.ThentheQTCoutputisdigi-
and7.20to8.24MeVfor214Po,eachcoveringarange
tizedbythemainboard(32-bitARMMicroController
offiveFWHMsaroundtherespectivepeaks. Although
Unit). The main boardis also responsiblefor relaying
resultsfrombothROIswillberecordedbytheH3,only
commandsfromthehostPC todifferentdetectorcom-
the counts in the 218Po peak are used to calculate the
ponents(e.g.highvoltagegenerator,pump).
real-time radon concentration. This is because 218Po
(τ =3.05 min) is significantly quicker in response to
1/2
thechangeofradonconcentrationthan214Po.
DefiningnewROIcanenhancetheidentificationand
rejectionofbackgroundornoise. Equation3showsthe
decaysofthe220Rnprogeny212Bithatgeneratesa6.10-
MeVα-particlefallingintotheROIof218Po.
212Bi−−6−.1−0−M−−e−V−α→ 208Tl
Figure3:ElectronicsunitoftheH3. 36%,60.6m
(3)
212Bi−−−−−β−+−−−→ 212Po−8−.−7−8−M−e−V−→α 208Pb
64%,60.6m 299ns
Thisbackground,whichmayonlycontributeuptoafew
3.2. Degassingunit
percentofthetotalcountsiftheradonconcentrationis
For water run, a RAD-Aqua (Durridge Company low and the sampling point of air is very close to the
Inc.) [13]isusedasthedegassingunit. Thewatersam- H3,canbesubtractedbyaddinganadditionalROIfrom
ple is broken into droplets to facilitate radon diffusion 8.24 to 9.26 MeV, to count the number of 8.78-MeV
from water to the air flowing through the RAD-Aqua. α-particles associated with 212Bi. By multiplying this
Thisflowingaircirculatesinaclosedloopbetweenthe number by 36/64, the contamination of 212Bi decay in
RAD-Aquaandthe H3. Whendiffusionequilibriumis theROIof218Pocanbecalculated.
4
calibration factor was traceable to the National Radon
Laboratoryof the Universityof South China in China.
Withtheairsamplesofhighradonconcentration,suffi-
cientstatisticscouldbeobtainedinashorttimeforthe
calibration runs, such that any fluctuations caused by
the environmentcould be identified easily by repeated
measurements.
4.3. Humidity
Sensitivityofradondetectorsusingelectrostaticcol-
lectionmethodisinverselycorrelatedtohumidity[16].
Water vapor can neutralize the electric charge of the
radonprogenies,leadingto a dropin thecollectionef-
ficiency. Therefore,itisnecessarytocontrolandmoni-
torthehumidityoftheairsampleinsideH3tomaintain
dataintegrity.Duringcalibration,theairsamplesforthe
AB-5andH3weredriedwithdesiccanttoalowR.H.of
Figure 4: Energy spectrum obtained with a H3 detector. The grey 5%at25±1◦C,whichwasequivalenttoawatercontent
areasaretheROIof218Po(left)and214Po(right). of1.15g/m3 inair. However,inrealoperation,humid-
ity may deviate from this low value. So a correction
curveforhumiditywasobtainedforeachH3. Thiswas
4.2. Calibrationinexposurechamber donebyconnectingtwoH3sindependentlytotheradon
chamber.OneH3wasusedasacontrol,whiletheother
Calibration of the H3 was carried out with a radon
one was under calibration. At the inlet of the control
exposure chamber in The University of Hong Kong H3,thehumidityoftheincomingairsamplewaskeptat
(HKU) (Figure 5). The exposure chamber provided 1.00±0.02g/m3 throughoutthecalibrationwithalarge
air samples of known and stable radon concentration column of desiccant. For the H3 under test, the desic-
withcontrolledenvironmentalconditions[14].Airsam-
cantcolumnwasmuchsmaller, such thatthe humidity
pleswithradonconcentrationsfrom200Bq/m3toover wasallowedtochangeslowlybylessthan1.00g/m3in
7000Bq/m3 withanuncertaintylessthan5%couldbe
2hours. Sinceeachrunlastedfor30minutesonly,the
prepared. changeinhumidityofairsampleinthisH3waslessthan
0.25g/m3 andhenceconsideredasstable. Besideshu-
midity,bothH3soperatedunderthesameconditionsfor
over 30 hours. The ratio of the 218Po counts collected
by the H3s were plotted against humidity in Figure 6.
Thenumberfromthe controlH3 wasthe denominator.
ThecurveofeachH3 wouldbeusedtocorrecttheC.F.
forairsampleofdifferenthumidity.
4.4. CalibrationfactorsofH3
EachH3wastestedfordeterminingtheC.F.sof218Po
and 214Po as summarized in Table 1. The test was re-
peatedwitharangeofradonconcentrations. Nocorre-
lationofC.F.swithradonconcentrationscouldbeseen
uptonearly104Bq/m3.
Figure5:Schematicdrawingoftheexposurechamberforcalibration
inHKU.
4.5. High-voltageoptimization
Thehighvoltage(HV)appliedtotheradonprogeny
Radon was added to the chamber by pumping air collection chamber directly affects the collection effi-
throughastandardradonsource(RN-1025,PylonElec- ciency ε in Equation 2. A series of runs were con-
p
tronicsInc.)[15]. DuringH3calibration,theradoncon- ducted with different HV values to find out the opti-
centration was monitored by an AB-5 detector, whose malrange. Airsamplewithafixedradonconcentration
5
Figure7: Numberofcounts of218Poversus theHVapplied tothe
radonprogenycollectionchamber. Radonconcentrationwas800±40
Figure6:Ratioof218PocountsfromtheH3beingcalibrated,tothat Bq/m3(R.H.=5%,temp=25±1◦C).
fromthecontrolH3. Horizontalaxisshowsthehumidityofairsam-
pleintheH3beingcalibrated. SampleinthecontrolH3waskeptat
1.00±0.02g/m3.Statisticalerror(1-σ)waslessthan2%.
(250±12Bq/m3)for2hours. Thenumberofcountsin
theROIof218PoarecomparedinFigure8bytakingthe
Radonconc. 1-L/mindatumasthedenominator. In±20%ofthese-
C.F.of218Po C.F.of214Po
byAB-5 lected flow rate for operation(1 L/min), no systematic
(Bq/m3) (Bq/m3/cpm) (Bq/m3/cpm) changeintheratiowasobserved.
565.1±4.3 27.51±0.25 25.72±0.13
795.2±5.8 26.60±0.23 24.93±0.12
1251±8.3 28.13±0.22 25.76±0.11
7315±21 27.15±0.97 25.12±0.10
AverageC.F. 27.35±0.20 25.38±0.10
Table1: CalibrationfactorsofaH3atdifferentradonconcentrations
(R.H.=5%,temp=25±1◦C).Statisticalerrors(1-σ)areshownfor
theC.F.atdifferentradonconcentrations. Standarderrorsareshown
fortheaverageC.F.
(800±40Bq/m3)waspumpedfromtheexposurecham-
bertotheH3. ThenumberofcountsintheROIof218Po
Figure8:RatioofaveragecountsintheROIof218Poatdifferentflow
wereplottedagainsttheappliedHVinFigure7. From
rates.Datumobtainedataflowrateof1L/min(•)isthedenominator.
the data, the number of counts began to plateau above Radonconcentrationwas250±12Bq/m3.
600V.TheoptimalHVwaschosentobe1800V,such
thatfluctuationof1%intheHVprovidedbythesupply
(DW-P202-IC33, DongWen High Voltage Power Sup-
ply Co.) would not lead to significant variation in the 4.7. Backgroundandlowerlimitofdetection
data.
Backgroundwasmeasuredbyusingradon-freenitro-
gen gas. Before the measurement, the H3 was purged
4.6. Pumpoperatingatdifferentflowrates
withnitrogentoremoveanyresidualradon,andthere-
The H3 employs active and continuous sampling of maining progenies in the collection chamber were al-
air. Air sample is drawn into the collection chamber lowed to decay. When the counts in the ROIs ap-
with a micro-pumpat a flow rate of 1.00±0.05L/min. proachedzero,theH3wasrunfor106hoursatasample
Various flow rates between 0.4 to 1.6 L/min were flowrateof1.0±0.1L/min.
also tested on the H3. At each flow rate, the H3 The average background rates were
measured the air sample of stable radon concentration 0.68±0.08counts/hrfor218Po,and0.17±0.04counts/hr
6
for 214Po. They were equivalent to a radon concen- thoroughly with an air sample of radon concentration
tration of 0.31±0.03 Bq/m3 and 0.07±0.02 Bq/m3 606±45Bq/m3fromtheexposurechamber.Thenawa-
respectively by the C.F.s in Table 1. The 90% con- tersamplewith106±6Bq/m3 ofradonwaspumpedto
fidence level detection limit was 0.11 Bq/m3. The theRAD-Aquaat3.4±0.3L/min. Figure10showsthe
backgroundissufficientlylowthattheH3 canmeasure radonconcentrationmeasuredbytheH3 beforeandaf-
verylowradonconcentrationforspecifictasks,suchas ter the RAD-Aqua was started at t = 20 minutes. Af-
detectingradonintheADcovergas. tert=60minutes,theradonconcentrationinH3 stabi-
lized at 477±7 Bq/m3, a concentration consistent with
4.8. Precisionofmeasurement the Ostwald Coefficient (Section 5.2). So the prepara-
The precision of the H3 is mainly affected by the tionperiodofwaterrunshouldbeatleast40minutesto
statistical error of the counts of α-particle. Figure 9 obtainthetrueradonconcentrationinwater.
summarizesthe precisionof theH3 ina seriesof mea-
surementusinglowradonconcentrations,in whichthe
countingstatistics is low. Thenumberofcountsinthe
ROI of 218Po was used in the calculations. In the ex-
perimental halls at Daya Bay, the lowest radon con-
centrationevermeasuredwas about50Bq/m3. At this
radonconcentration,uncertaintyof10%orlesscanbe
achievedwithanhourofmeasurement.
Figure10:ResultsofH3measurementbeforeandafterswitchingon
theRAD-Aquaatt =20minutes. Initialradonconcentration ofair
fromtheexposurechamberwas606±45Bq/m3.Radonconcentration
inrunningwaterinRAD-Aquawas106±6Bq/m3.
5. On-sitesetupoftheradonmonitoringsystem
Figure9:PrecisionofH3asafunctionofmeasuringtimeobtainedby The H3 is capable of monitoring the radon concen-
experimentswithvariousradonconcentrations(R.H.=5%).
tration in air and water of each experimental hall at
scheduled times. The details of on-site operation are
describedbelow.
4.9. Lengthofpreparationperiodforwaterrun
5.1. Radonconcentrationinair
Priortodatatakingina waterrun,apreparationpe-
riodis neededforradonconcentrationin airandwater Airsampleisextractednearthewaterpoolintheex-
toreachequilibriuminthe degassingunit. Preparation perimental halls. The sampling points are 1 m above
timeisalsoneededfortheH3toreachaquiescentstate ground,andatleast2mawayfromthewalls. Thisre-
after the previousair run. As indicatedin ourprelimi- duces the background due to the α-particles from the
narysurvey,radonconcentrationintheairoftheexper- short-lived 220Rn (6.29 MeV), 216Po (6.78 MeV) and
imenthallishigherthanthatinthepoolwaterbyabout otherprogeniesof220Rntoanegligiblelevel.Nosignif-
a factor of six. If the H3 starts to collect data without icant difference in radon concentrationcould be found
the preparation period, the α-particle energy spectrum near the water poolin the experimenthalls because of
wouldconsistofconsiderablecontributionoftheradon thestrongventilation.
progeniesaccumulatedduringthepreviousairrun.
Todecidethelengthofthepreparationperiod,theH3 5.2. Radonconcentrationinwater
andRAD-AquaweresetupinHKU,resemblingthede- Watersampleisextractedfromtherecirculationsys-
tailsofthemonitoringsystem. TheH3wasfirstflushed temwhichpumpsthewaterfromthewater poolto the
7
purificationsystem. Theradonconcentrationofthewa- EH1 EH2 EH3
tersampleisidenticaltothatinthewaterpool. Sample Meantemp(◦C) 26.9±0.2 27.5±0.3 25.9±0.3
extractionrateto theRAD-Aquais setto 2 to4 L/min R.H.(%) 20.0±0.2 24.7±0.3 25.3±0.3
for a reasonable diffusion rate from water to air in a
closed loop leading to the H3. At diffusion equilib- Progeny C.F.(Bq/m3/cpm)
rium, the radon concentrations in the two media (wa-
218Po 39.3±0.7 37.1±0.7 43.5±1.5
ter/air)arecorrelatedbytheOstwaldCoefficient,which
214Po 35.2±0.6 35.5±0.7 39.5±1.6
hasavalueof0.226at25◦C[17]. Thisgivesextrasen-
sitivity to the H3 measurement of radon concentration
Table 2: On-site calibration factors of the H3s with dehumidifiers.
in water. For 1 unit of change in the radonconcentra- Statisticalerrors(1-σ)areshown.
tion in the incoming water sample, that would lead to
over1/0.226=4.42unitsofchangein the air inside the
closedloop. Thedrawbackofthismethodisthatabout
VI library. A daily measurementschedule includes an
40minutesisneededtoreachequilibrium.Thustheto-
air run (20 hours), and a water run (3 hours) preceded
tallengthofwaterrunissetto4hourssoradondataof
byapreparationperiod(1hour).Atthebeginningofan
morethan3hourscanbecollected.
airrun,theVIwouldsetthevalvepositionssothatthe
H3 would be flushed with air in the experimentalhall.
5.3. Dehumidification
Inthepreparationperiodofawaterrun,theRAD-Aqua
The H3 is designed for long-term remote operation. would be switched on so diffusion equilibrium can be
Desiccant is not suitable for continuous dehumidifica- reachedbythetimemeasurementstarts.
tion as it requiresfrequentreplacement. Thusthe sys- Through a LabJack USB data acquisition module
tem includes a custom-built dehumidifier for uninter- (U3-LV,LabJackCorporation,USA),statusofthesen-
ruptedcontrolofthehumidity. sors and controllers are monitored by the VI. Several
Insidethedehumidifier,theairsamplepassesthrough contingentmodesareprogrammedintheVI.Analarm
a long zigzag path along an aluminum heat exchanger wouldbetriggeredifanyreadingdriftsoutsidethepre-
for drying the air by condensation. The heat ex- defined operation range. Based on the category of ab-
changer is cooled by two thermoelectric Peltier mod- normal reading, the VI would run the corresponding
ules. Their temperature is stabilized by a micro- presetcontingentmodeorhalttheerroneousoperation.
controller(AT89C52)andtemperaturesensors. Ittakes E-mailalertwouldbesenttotheexperts. Afullhistory
4hoursforthemtoreachtheoperatingtemperature.Af- ofthesereadingsandcontingencieswouldberecorded
terwards,theR.H.canbemaintainedbelow26%forair bythehostPCfordiagnosis.
samplebetween18-26◦C. TheVIreadsandprocessestheenergyspectrumofα-
particles,andplotstheH3 internaltemperature,humid-
5.4. On-sitecalibrationwithdehumidifier ity,andvaluesofallsensorsandcontrolvalves. There-
TheH3isabletooperatecontinuouslywiththebuilt- sultsarestoredinalocalhard-diskandsimultaneously
indehumidifier. Since theR.H. is heldsteadyatabout copiedtothenetworkdatabaseofDCS.
26% by the dehumidifier instead of 5% as achieved at
HKU,on-sitecalibrationofthe C.F. is necessary. This 5.6. RadonconcentrationinADcovergas
also serves as a standard annual check and update of
ForeachAD,aflexibleductforkeepingthedetector
theC.F.softheH3. Resultsoftheannualcalibrationin
cables and gas line dry is sealed off with o-rings. To
2013byradontheairsamplepreparedbytheexposure
removecontaminantsandradonthatpermeatesthrough
chamberareshowninTable2. Duetothedifferencein
the o-rings, the duct is constantly flushed with radon-
humidity, they were substantially different from those free boil-off nitrogen (R.H.≪5%) from the AD cover
obtainedwiththeexposurechamberinHKU(Table1).
gassystem [18]. ThereturninggasfromeachAD was
measuredwiththeH3toensuretheradonconcentration
5.5. Controlanddata-takingsoftware
intheductwasundercontrol.
Figure 11 shows the LabVIEW VI interface devel- A different configuration was used to measure the
opedformanagingthe two typesof run of H3. Mode- radonconcentrationintheADcovergas.Topreventthe
switchingisautomatic,followingauser-definedsched- relatively radon-rich air in the experimental hall from
ule set in the VI. All running parametersand standard contaminating the air sample, the low-level measure-
valuesofsensorsfordifferentoperationsarestoredina mentswerecarriedoutwithaH3sealedinsideanacrylic
8
The humidity correction applied to the C.F. also in-
heritstheerrorsinthefitofFigure6,whichis1.26%.
Uncertainty caused by the operation of the PIPS is
negligible. Any tiny shift in the energy scale can be
compensatedbytheflexibledefinitionoftheROIs. The
energyresolutionofthePIPShasbeenverystablesince
thecommissioningoftheradonmonitoringsystem.
6. Results
Installation of the radon monitoring system on-site
wascompletedin2013. ThedatafromtheH3sareup-
loadedtotheDCSoftheDayaBayExperiment. Users
canaccessthehistoryofradonconcentrationandother
Figure11:H3controlinterfaceonthehostPC.
parameters(temperature,R.H.,etc.) throughtheDCS.
6.1. Radoninairon-site
box. The returninggasflowed throughthe H3 directly
at1L/minbeforeexhaustingintotheacrylicbox.Atthe The plots in the left column of Figure 12 show the
outletoftheacrylicbox,back-diffusionfromtheexper- radon concentration in air since November 2013 for
imentalhallwascutoffwithamineraloilbubbleroper- the three experimental halls at Daya Bay. The daily
atingatapositivepressureof2cmofwater. Theradon and monthly averages are plotted. It shows that the
concentrationwascalculatedusingtheC.F.sinTable1, radon concentrations are quite stable. The H3 also
which are applicable to the cover gas with R.H.<5%. recorded spikes in the hourly radon concentration of
Typically,theradonconcentrationinthecovergaswas nearly900Bq/m3inEH2,and400Bq/m3inEH3,when
inthesub-Bq/m3level. theventilationsystemsofthosehallswereshutdownfor
maintenanceinJanuary2014.
5.7. Uncertaintybudget
6.2. Radoninwateron-site
Besidesthestatisticaluncertainty(maximumof10%
at 1-σ), Table 3 summarizes other uncertainties of the The automatic radon monitors for the water pools
radonconcentrationcalculatedfromthe218Pocountsof wereinstalledinNovember2013.Theplotsintheright
H3. The most significant one is the 6.10% from the columnofFigure12showthedailyandmonthlyaver-
C.F. (218Po) of H3. It is the quadratic sum of the un- aged radon concentration in water for the water pools
certaintyofradonconcentrationintheexposurecham- oftheDayaBayExperiment.Themeasuredradoncon-
ber(5%maximum),andthemaximumstandarderrorof centrationsareconsistentwiththeresultsof34.7±3.5to
the repeatedmeasurementof the C.F. (3.5%at EH3 in 86.4±8.6Bq/m3 obtainedinpersonbeforetheinstalla-
Table 2). Thisuncertaintyof C.F. alreadyincludesthe tionoftheH3s. Thegreatestadvantageoftheautoma-
uncertaintiescausedbythepumpandtheHVofH3. For tionisthattheradonconcentrationinwatercanbemon-
theuncertaintyinducedbythepumpoperatingatdiffer- itoredcontinuouslyandremotely.Sofartheradoncon-
ent flow rates, no systematic trend in the performance centrationsinthewaterwerestable.
could be observed at ±20% of 1 L/min. Furthermore,
thepumpisabletooperatestablywithin5%atthedes-
6.3. CovergasofDayaBayADs
ignatedflowrateof1 L/min. Thustheupperboundof
uncertainty it causes is taken to be the 1.3%, which is ForthesixADsinstalledin2012,theradonconcen-
thestatisticaluncertaintyat1L/mininFigure8. trations were foundto be below 0.55 Bq/m3, over400
For the HV, its fluctuation only have a small contri- times lower than the ambient environment (Table 4).
butionof0.08%asevaluatedfromthedatainFigure7. The H3 results showed that the AD cover gas system
TheuncertaintyissmallbecausetheH3 operatesinthe was operatingas designed and was able to limit radon
plateauregionofHV. fromaccumulatingintheADstoaninsignificantlevel.
9
7. Conclusion
Wehavedevelopedaradonmonitoringsystemforthe
Daya Bay Reactor Neutrino Experiment that has been
running routinely for over two years. It can measure
radonconcentrationeitherinairorinwaterwithapre-
cisionsignificantlybetterthanmostcommercialunitsin
arelativeshorttime,fulfillingourneedsofrapidmoni-
toringthepresenceofradonwhichpotentiallycangen-
erate background in the experiment. In addition, with
a custom-designeddata acquisition system, we can re-
motely control the continuous operation of the other-
wiseautomaticmonitoringsystem,thussignificantlyre-
ducingthedemandofhumanresource.
8. Acknowledgement
We are gratefulforthe supportwith grantsfromthe
Research Grant Council of the Hong Kong Special
Administrative Region, China (Project Nos. CUHK
1/07C and CUHK3/CRF/10) and from the University
of Hong Kong (Project code: 201007176191). K.B.L.
is supported by the Office of Science, Office of High
Energy Physics, of the U.S. Department of Energy
underContractNo.DE-AC02-05CH11231.
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