The laboratory measurement of radioactivity purification for Pb212 in liquid scintillator

0.27 MB·

by  Wei Hu

Submittedto ’Chinese Physics C’
The laboratory measurement of radioactivity purification for 212Pb in
liquid scintillator
6 Wei Hu ()1,2;1) Jian Fang ()1;2) Bo-Xiang Yu ()1 Xuan Zhang (@)1,2 Li Zhou ()1
1
Xiao Cai ()1 Li-Jun Sun ()1 Wan-Jin Liu ()1 Lan Wang ()1 Jun-Guang Lu ()1
0
2 1 StateKeyLaboratoryofParticleDetectionandElectronics,(Institute ofHighEnergyPhysics,CAS)Beijing100049,China
2 Institute ofHighEnergyPhysics,UniversityofChineseAcademyofSciences,Beijing100049, China
y
a
M Abstract: Theliquidscintillator(LS)hasbeenwidelyutilizedinthepast,runningandfutureneutrinoexperiments,
and requirement to the LS radio-purity is higher and higher. The water extraction is a powerful method to remove

2 soluble radioactive nuclei, and a mini-extraction station had been constructed. To evaluate theextraction efficiency
1 and optimize theoperation parameters, a setup to load radioactivity to LS and a laboratory scale setup to measure
radioactivitywhichused212Bi-212Po-208Pbcascadedecayweredeveloped. Experiencesfromlaboratorystudywillbe
]
t useful tothe design of large scale water extraction plants and the optimization of working conditions in the future.
e
d
Key words: liquid scintillator, radioactive load, radioactive measurement, cascade decay,water extraction
-
s
PACS: 29.40.Mc, 14.60.Pq
n
i
.
s
c 1 Introduction been constructed at IHEP, Beijing, to validate the pro-
si totype and optimize operation parameters. LS radioac-
y tivity, before and after purification should be measured.
The liquid scintillator plays a very important role in
h
However,thetypicalUandThcontaminationsinLSare
p intensity frontier neutrino experiments. The Jiangmen
10−13 to 10−14 g/g,and it is impossible to measure such
[ Underground Neutrino Observatory (JUNO) is a multi-
low radioactivity in the laboratory. A generalmethod is
purposeneutrinoexperiment,withtheprimaryscientific
5 toloadradioactivenuclei,suchas222Rnor220Rn,toLS,
goaltodetermineneutrinomasshierarchy. Theneutrino
v
and purify the LS with the prototype, and measure the
0 detector is filled with LS of 20 ktons fiducial mass. To
purified and un-purified LS with a clean detector.
8 suppresstheaccidentalbackground,aswellastoachieve
7 the potential goalof solarneutrino studies, the basic ra- Inthispaper,theRnloadingtechnology,theradioac-
2 tivity measurement setup, and the water extraction effi-
dioactivity contamination requirements to JUNO LS is:
0 ciencies, as well as the optimized operation parameters
10−15 g/g (in this paper, g/g means gram of 232Th or
.
1 238U per gram of LS) for both 238U and 232Th. are reported. The efficiency has reached the world aver-
0 age level, indicating the prototype is successfully work-
Thegeneralmethodstoremoveradioactivitycontam-
6
ing, and the optimized parameters are useful to future
1 inationsarewaterextraction,nitrogenstrippinganddis-
middle-scale and mass production plants.
: tillation, which are sensitive to soluble nuclei, Rn and
v
insoluble nuclei, respectively. Before the mass produc-
i
X tionofpurifiedLS,eachmethodshouldhaveaprototype 2 Radioactivity measurement setups
r and optimized operation parameters. Such as Borexino
a
experiment (a solar neutrino experiment, solvent of LS 2.1 220Rn loading
is trimethylbenzene), which holds the world-record ra-
dioactivity contaminations with 232Th/238U of 10−18g/g The limit of radioactivity measurement in the lab-
[1], the parameters of large scale purification plants and oratory is 10−9 g/g. But the natural contamination of
the prototype are consistent [2] [3]. LS is 10−13 to 10−14 g/g, hence it is impossible to mea-
ThoughthesepurificationmethodsareefficientforLS sure such low radioactivity in the laboratory. In order
of Borexino, they need to be carefully studied for LAB- to study the effect of purification for LS in laboratory
based LS of JUNO. A water extraction prototype had experiments, the only solution is an artificial pollution
∗SupportedbyTheStrategicPriorityResearchProgramoftheChineseAcademyofSciences GrantNo. XDA10010500
∗SupportedbyNationalNaturalScienceFoundationofChina(11390384)
1)E-mail:[email protected]
2)E-mail:[email protected]
(cid:13)c2013 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of
ModernPhysicsoftheChineseAcademyofSciences andIOPPublishingLtd
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Submittedto ’Chinese Physics C’
of the samples with radioactivity. Since powder radio- 1200Bq220RnproducedbyNanhuaUniversitywasused.
source does not dissolve in LS and the solubility of liq- Nitrogen went through a bubbler filled with water, then
uid radio-source in LS is not high, the general method it blew through the source and took 220Rn out. Finally
is to load radon to LS. Because radon is non-polar gas, it went through a bubbler filled with LS. According to
the solubility of radonin LS is high with 13 times of the the researchconductedby Nanhua University,the 220Rn
radon concentration in air under room temperature [4]. release rate of 232Th source increased with higher envi-
Therefore, it is effective to load radioactivity in LS by ronmenthumidity. Afterbubbling220RnintoLSfor74.2
bubbling radon into LS. hours,theconcentrationof212Pbreachedabalancelevel.
The common used radon is 222Rn, which is from the Inthefollowingstudy,LSwasbubbledwith220Rnfor20
238U decay chain and has a 3.8 days half-life time. For hours which reached the 2/3 of the 212Pb concentration
example, in the Borexino experiment, 222Rn was loaded balance level [6].
to LS and the contamination of its daughter 210Po was
measured. The disadvantage is that it requires months
for 210Po to accumulate to a measurable amount, since
itsmother210Pbshalf-life time is22.3years[5]. Besides,
the long half-life time of 210Pbwould pollute the experi-
ment setups.
Compared with 222Rn, a better candidate is 220Rn,
which is from the 232Th decay chain as shown in Fig.1,
and has 55 s half-life time. After loading, 220Rn quickly
decaysto212Pbwith10.6hourshalf-lifetime. Thedecay
of 212Pbs daughters 212Bi and 212Po are famous cascade
decays (β-α cascade decay), since the half-life time of
212Poisonlyabout300ns. Thecascadedecaysuppliesa
pair of time correlated signals in our experiments, with
Fig. 2. Laboratory setup for radon-loading of LS samples
high efficiency and extra low background. With 220Rn
loading,thewaterextractionefficiencyisestimatedwith The radioactivity was measured by the experimental
the nucleus 212Pb. The 10.6hours half-life time of212Pb setup depicted in Fig. 3. In a light-tight box, a pair of
islongenoughtodoextractionandmeasurement,andit 2 PMTs (XP2020) was placed on both sides of an LS
will not cause any contamination to experiment setups. samplecell,tododouble coincidencemeasurement. The
Meanwhile, a large amount of 220Rn in LS will decay coincidence measurement could reduce the influence of
to 212Pb in a very short period of time, leading to high single PMT’s fluctuation to experiment data. The LS
radioactivity loading efficiency. container was a cylinder quartz glass bottle, with 5 cm
diameter and 1.5 cm thickness, and a capacity of 17.1 g
LS.Gamma raysfromambientradioactivitywereatten-
uated by the shielding of low-activity lead bricks [4]. A
flash ADC (DT5751 made by CAEN with 1 GHz sam-
pling frequency) was used for data acquisition. The to-
talbackgroundeventrate(includingβ, α,γ andcascade
decay events) during measurement was 0.25 Hz. After
finishing the data acquisition for all events, β-α cascade
events were pick out by offline analysis. This setup was
designed as a β-α counting system.
Fig. 1. The nature decay chain of 220Rn
The loading method used in the paper was bubbling
Fig. 3. Laboratory setup for measuring the effi-
220Rn into LS sample. The corresponding setup inside a
ciency of radiopurification
glove box is shown in Fig. 2: A 232Th source releasing
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3 Data analysis For each β-α cascade event, there was a time inter-
val between the β event and the α event. And the time
3.1 The β-α cascade event selection interval measured by the two PMTs should be the same
theoretically. At least, the difference mustbe very small
3.1.1 Real β-α cascade event
due to the difference of the two PMTs or other impact
After 220Rn loading, three hours’ data was taken to
in experiment. Fig. 5(a) shows the difference of time in-
determine the initial 212Pb concentration. The coinci-
terval of β-α cascade events detected by the two PMTs,
dence time window for signals from the two PMTs was
which was within the coincidence time window. The en-
requiredtobesmallerthan5ns,duetothelengthdiffer-
triesnumberbeforenormalizationwas104640. Gaussian
ence of the cables connected to the two PMTs. 99.99%
function was used to fit the distribution and the result
oftheβ-αcascadeeventsmetthisrequirement,withthe
was mean value of 0.51 and σ of 0.91. The difference of
statistical error of 9.55×10−8 (statistical error will not
thetimeintervalwasalmostwithin(-2ns,3ns). 95.92%
be discussed in this section since it was too small).
ofthe eventsmetthis requirement,whichwasconsistent
The time interval distribution between the β decay
tothe probabilityofvariablefromGaussiandistribution
andαdecayoftheβ-αcascadeeventsisshowedinFig.4.
locating within 2 sigma range.
The distribution can be described by the formula be-
low [7],
f(t)=τ1×N0×e−τt, (1) Entries2.25 ((ba)) RBeveaeacnlk tgbsreotau-nadlp dhoauble(a) (b)
pulse events
whereτ isthelifetimeof212PoandN isaparameter 1.5
0
relatedtothe concentrationof212Po. T isthe half-life
1/2
timeof212PoandT =ln2∗τ. Hence,theformulaabove 1
1/2
can be written as,
0.5
0
f(t)=Tln2 ×N0×2−T1t/2. (2) -80 -60 -40 -20 0 20 40 60 d8d0t/ns
1/2 Fig.5. Thedifferenceoftimeintervalofβ-αpulse
between the two PMTs (The entries are normal-
The followingfunction wasusedto fitthe time inter-
ized)
val distribution.
1 Theeventenergywasproportionaltothetotalcharge
f(t)= ×p ×2−pt1. (3)
p 0 collected. Measurement of the total charge from PMT
1
was estimated by integrating the entire pulse. Fig. 6(a)
Theparameterp inthefittingresultstandsforthehalf- showsthe integralvalueofthe αeventpulse(the second
1
life time of 212Po. In theory, the half-life time is 298 ns, pulse in a double-pulse event). Due to the mono-energy
while the experiment result is 298.4±1.2 ns. Therefore, of α event, the distribution was centralized like a Gaus-
the β-α counting setup was reliable for detecting β-α sian distribution. 92.80% of the second pulse integral
cascade events. value was within (1700 FADC, 8000 FADC) after the
two selection criterions discussed above.
tt__PPMMTT11
s
e
Entri11020000 EMEMnneettaarriinnee ss 99449911883388..9955 Entries0.3 (a) (a) Real beta-alpha events
RRMMSS 333300..11 (b) Background double pulse events
(b)
χχ22 // nnddff 442200..33 // 227788 0.25
800
pp00 44..001122ee++0055 ±± 11..338855ee++0033
600 pp11 229988..44 ±± 11..22 0.2
0.15
400
0.1
200
0.05
0
0 200 400 600 800 10001200 1400160018002000 0
t/ns 0 2000 4000 6000 8000100001200014000160001800020000
FADC
Fig. 4. The time intervaldistribution between the Fig.6. Theintegralvalueofthesecondpulse(The
β decay and αdecay entries are normalized)
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3.1.2 Cuts selection Clopper-Pearsonparameter estimation of Binomial Dis-
According to the analysis of real β-α events, several tribution [8]. The formula is described as below,
cut criterions were selected.
(1) The coincidence time window of the two PMTs n−sˆ
σ =(cid:0)1+ f (2(n−sˆ),2(sˆ+1))(cid:1)−1−pˆ, (5)
was required to be smaller than 5 ns; + sˆ+1 1−α/2
(2) The difference of time interval of β-α cascade n−sˆ+1
events detected by the two PMTs was within (-2 ns, 3 σ−=pˆ−(cid:0)1+ sˆ fα/2(2(n−sˆ+1),2sˆ)(cid:1)−1. (6)
ns);
Here,1-αis the confidence level;n isthe totalevents
(3) The integral value of α events was within (1700
number; f is the upside α/2fractile ofF-distribution;
FADC, 8000 FADC). α/2
sˆisthepassedeventsnumberandpˆ=sˆ/n. Inthispaper,
Afterbubbling220RnintoLSfor20hours,therewere
n=y, sˆ=y−x and the confidence level is set as 0.683.
about2×104β-αeventsdetectedin17.10gLSin30min-
Water extraction was done with equal amount of 12
utes’ data taking after using the cut criterions discussed
M deionized water and liquid scintillator. A separatory
above.
funnel and a magneton were used to mix 35 ml of water
3.1.3 Background events
and 35 ml of scintillator. The solution was mixed and
Background events were taken for one day using the then separated. The operation was called one stage ex-
pureLS(17.10g)withoutloading220Rntoit. 163double traction. Aftereachseparationacleanseparatoryfunnel
pulse events were found. But the integral value distri- andfreshdeionizedwaterwereusedtodomultiplestages
bution of the second pulse (maybe the fake alpha event extraction. Scintillatorsampleswereplacedinsmalltest
pulse) and the difference of time interval of the double tubewhichcontained17.1gofLSsample. Thentheβ-α
pulse events detected by the two PMTs were much dif- counting system was used to measure the radioactivity
ferentwiththerealβ-αevents,asshowninFig.5(b)and in scintillator before and after purification.
Fig. 6(b). The integral values of the second pulses were Stability study of the β-α counting system was con-
almost less than 1700 FADC and only a small percent ducted by measuring purification efficiency at three dif-
of the difference of the time interval was within (-2 ns, ferent time with the same LS sample. The efficiencies
3 ns). After applying these cut criterions to background were consistent to each other, which were 84.3+1.2%,
−1.3
events, there remained only 2 background β-α cascade 82.7+1.5% and 83.3+1.8%. Therefore, it was reliable to
−1.6 −1.9
eventsa day,while there was163withoutcut selections. optimizepurificationparametersbyusingtheβ-αcount-
Compared with 2×104 real β-α cascade events de- ing system.
tected in 220Rn loaded LS in 30 minutes, the 2 back- Fig.7 showsthe relationshipbetweenextractioneffi-
groundevents in one day canbe neglected in the follow- ciency and the stirring time. The stirring speed was 600
ing study. r/min, while the stirring time was 1min, 2 min, 4 min, 8
min, 16min and32 min. The efficiency increasedslowly
3.2 The study of water extraction
after extraction for 8 min. When extracted for 32 min,
Purificationbywaterextractionreliesonthepolarity theradioactivityofLSdecreased86.7+0.5%with737β-α
−0.5
of water molecules to separate polarized impurities, e.g. cascade events in LS samples. Before purification there
free-state ions of radioactive metals, from the non-polar were5529β-αcascadeevents. TheerrorinFig.7toFig.9
LABandfluormolecules. Waterextractionisveryeffec- is statistical error.
tiveformostionicmetalssuchasK,Ra,andBi,butwith Fig. 8 shows the relationship between extraction ef-
some effectiveness for Po and Pb. For Po and Pb, the ficiency and the extraction stage. The extraction time
reduction was seen to be equally fast but less effective was 3 min and the stirring speed was set at 1200 r/min.
with an 82∼87% removal fraction in SNO+ laboratory After extraction for 5 stages, the purification efficiency
study [9] [10]. becamealmoststable,reachinganotveryhighefficiency
Afterthescintillatorhadagood212Pbconcentration, of92.1+0.3%. Since Pbwasaverypolaratom,itwasex-
−0.4
thescintillatorwaspurifiedbywaterextraction. Thepu- pected that its appetency to water will be much higher
rification efficiency was defined as, than LS of severalorders of magnitude. The most likely
explanation was that a fraction of the Pb was bound in
y−x x a nonpolar configurationwhich reduced the partitioning
u= =1− . (4) coefficient and thus the purification efficiency [9].
y y
Fig. 9 shows the relationship between extraction ef-
Here, x means event number after purification, y ficiency and the volume proportion of LS to water. In
meanseventnumberbeforepurificationandumeanspu- laboratory measurement, the purification efficiency de-
rification efficiency. creased slowly when the proportion of LS to water was
The statistical error of efficiency is calculated by larger than 6. Then the efficiency decreased shapely as
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Submittedto ’Chinese Physics C’
soon as the volume proportion reached 6. According to y0.9
c
the result, the volume proportionof LS to water used in en
ci
JUNO purification can be set at 5. effi0.85
n
o
ati
c
ency0.9 Purifi0.8
effici0.8 0.75
on 0.7
ati
urific0.6 0.7 1 2 3 4 5 6 7 8
P0.5 V(LS):V(water)
0.4 Fig. 9. Purification efficiency VS the volume pro-
0.3 portion of LS and water, 10 min extraction time
and 800 r/min stirring speed
0 5 10 15 20 25 30 35
Time/min
The water extraction efficiency for 212Pb can reach
Fig. 7. Purification efficiency VS extraction time,
more than 84% with laboratory scale purification setup.
600 r/min stirring speed
The extraction stage and volume ratio of LS to water
can be set at 5 in future large scale purification plants
design and operation.
y
c
en0.92 4 Conclusions
ci
effi
n 0.9
o To study the water extraction in the future JUNO
ati
Purific0.88 LcoSnpsturruicfitceadt,ioannpdlaantbsa,cakngreoxutrnadctfiroeneperffiotcoietnycpyehmaedasbuereen-
0.86
menthadbeenachievedwith220RnloadedLS.Themea-
0.84 sured water extraction efficiency to 212Pb was about no
less than 84%, reaching the world averagelevel, and op-
1 2 3 4 5 6 timized operation parameters had been obtained. Now,
Extraction stages
a medium scale water extraction tower had been built
Fig.8. PurificationefficiencyVSextractionstages, which was based on laboratory study results. The ra-
3 min extraction time and 1200 r/min stirring dioactivity loading setup and β-α counting system will
speed be useful to the investigationofthe parametersinvolved
in large scale purification plants in the future.
tillator Purificationand SurfaceContamination, Ph.D.Thesis
(America: PrincetonUniversity,2006)
References
6 ChunyangLi,JournalofUniversityofSouthChina(Science&
Technology), 28(3): 19-22(2014)
1 J.Benziger,M.Johnson, F.P.Calapriceet al.Nucl.Instrum. 7 ,(12009)p.13
Meth.A417,278(1998) 8 ,(12016)p.198-199
2 G.Alimontietal.(TheBorexinocollaboration),Nucl.Instrum. 9 R. Ford, M. Chen, O. Chkvorets et al. AIP Conf.Proc. 1338,
Meth.A600,568(2009) 183(2011)
3 J.Benzigeretal.(TheBorexinocollaboration),Nucl.Instrum. 10 Sarah Elizabeth Quirk, Purification of Liquid Scintillator and
Meth.A587,277(2008) Monte CarloSimulations of Relevant Internal Backgrounds in
4 SuxiaWei,RadiationProtectionBulletin,4: 12-14(1990) SNO+,MasterThesis(Canada: Queen’sUniversity,2008)
5 Mike Leung, The Borexino Solar Neutrino Experiment: Scin-
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