Laboratory Studies on the Removal of Radon-Born Lead from KamLAND’s Organic
Liquid Scintillator

arXiv:1312.0977v1 [physics.ins-det] 3 Dec 2013

G. Keefera,p,∗, C. Granta,q , A. Piepkea,o , T. Ebiharab , H. Ikedab , Y. Kishimotob,o,r , Y. Kibeb,s , Y. Kosekib , M. Ogawab , J. Shiraib ,S. Takeuchib , C. Maugerc,t , C. Zhangc,u , G. Schweitzerd , B.E. Bergerf,o , S. Dazeleyg,p , M.P. Decowskih,o,v , J.A. Detwilerh,w ,Z. Djurcica,x , D.A. Dwyerc,h , Y. Efremenkoi , S. Enomotob,o,w , S.J. Freedmanh,o,1 , B.K. Fujikawah,o , K. Furunob , A. Gandob ,Y. Gandob , G. Grattaj , S. Hatakeyamag,y , K.M. Heegerk,z , L. Hsuh,aa , K. Ichimurab,r , K. Inoueb,o , T. Iwamotob,ab , Y. Kamyshkovi ,H.J. Karwowskim , M. Kogab,o , A. Kozlovb,o , C.E. Lanen , J.G. Learnede , J. Maricicn , D.M. Markoffm , S. Matsunoe , D. McKeel,ad ,R.D. McKeownc,ae , T. Mileticn , T. Mitsuib , M. Motokib , K. Nakajimab,ai , K. Nakajimab,af , K. Nakamurab,o , T. O’Donnellh ,H. Ogawab,o,r , F. Piquemalb,ag , J.-S. Ricolb,ah , I. Shimizub , F. Suekaneb , A. Suzukib , R. Svobodag,q , O. Tajimab,ai , Y. Takemotob ,K. Tamaeb , K. Tolichj , W. Tornowm , Hideki Watanabeb , Hiroko Watanabeb , L.A. Winslowh,aj , S. Yoshidab,aca Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama 35487, USA
b Research Center for Neutrino Science, Tohoku University, Sendai 980-8578, Japanc W. K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, California 91125, USAd Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USAe Department of Physics and Astronomy, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USAf Department of Physics, Colorado State University, Fort Collins, Colorado 80523, USAg Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, USAh Physics Department, University of California, Berkeley and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USAi Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA
j Physics Department, Stanford University, Stanford, California 94305, USA
k Department of Physics, University of Wisconsin at Madison, Madison, Wisconsin 53706, USAl Department of Physics, Kansas State University, Manhattan, Kansas 66506, USA
m Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708, USA and Physics Departments at Duke University, North Carolina State University,and the University of North Carolina at Chapel Hill, North Carolina, USAn Physics Department, Drexel University, Philadelphia, Pennsylvania 19104, USAo Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, Kashiwa 277-8583, Japan
p Present address: Lawrence Livermore National Laboratory, Livermore, California 94550, USA
q Present address: Department of Physics, University of California Davis, Davis, CA 95616, USA
r Present address: Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Gifu 506-1205, Japans Present address: Department of Physics, Okayama University, Okayama 700-8530, Japant Present address: Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
u Present address: Physics Department, Brookhaven National Laboratory, Upton, New York 11973, USA
v Present address: Nikhef and the University of Amsterdam, Science Park, Amsterdam, Netherlands
w Present address: Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle, Washington 98195, USA
x Present address: Argonne National Laboratory, Argonne, Illinois 60439, USA
y Present address: J-PARC Center, 2-4 Shirane Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japanz Present address: Department of Physics, Yale University, New Haven, Connecticut 06520, USA
aa Present address: Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
ab Present address: ICEPP, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japanac Present address: Graduate School of Science, Osaka University, Toyonaka 560-0043, Japanad Present address: Department of Chemical and Physical Sciences, Missouri Southern State University, Joplin, Missouri 64801, USA
ae Present address: Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
af Present address: Research Center for Nuclear Physics, Osaka University, Ibaraki 567-0047, Japan
ag Present address: CEN Bordeaux-Gradignan, CNRS/IN2P3 and Université de Bordeaux I, F-33175 Gradignan Cedex, France
ah Present address: LPSC Université Joseph Fourier, CNRS/IN2P3, Institut Polytechnique de Grenoble, Grenoble, France
ai Present address: High Energy Accelerator Research Organization, KEK, Tsukuba 305-0801, Japanaj Present address: Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA

Abstract
The removal of radioactivity from liquid scintillator has been studied in preparation of a low background phase of KamLAND.
This paper describes the methods and techniques developed to measure and efficiently extract radon decay products from liquid
scintillator. We report the radio-isotope reduction factors obtained when applying various extraction methods. During this study,
distillation was identified as the most efficient method for removing radon daughters from liquid scintillator.
Keywords: KamLAND, low background, purification, liquid scintillator, solar neutrinos
2000 MSC: 85-05

Preprint submitted to Nuclear Instruments and Methods in Physics Research, A

December 5, 2013

Events [10 keV-1 m-3 s-1]

1. Introduction
Sensitive detection of trace radioactivity is important for low
energy solar neutrino, dark matter, and neutrinoless double beta
decay experiments. Due to demanding constraints on detector
backgrounds it has become essential to better understand themechanisms that can be used to remove trace-level radioactivity
from detector components, specifically from the active volume.
This study was motivated by the goal of expanding the physics
capability of the KamLAND experiment to include detection of
low energy solar neutrinos and other events from rare physics
phenomena.
The KamLAND detector is situated in the Kamioka mine,
Gifu prefecture, Japan and was commissioned in 2002 to test
the large mixing angle (LMA) solution of the solar neutrino
problem [1, 2]. KamLAND demonstrated the validity of the
LMA solution by observing the disappearance of anti-neutrinos
produced in nuclear reactors in Japan [3–6]. KamLAND’s active target is 1 kton of organic liquid scintillator (LS), composed of 80.2% n-dodecane, 19.8% 1,2,4-trimethylbenzene(PC) and 1.36 ± 0.03 g/L of 2,5-diphenyloxazole (PPO).
The statistical analysis of KamLAND’s low energy event distribution is performed by fitting summed beta spectra of different radionuclides to KamLAND’s singles data (events not
requiring delayed coincidence) and allowing their normalizations to float freely. The result of this procedure is depicted inFig. 1 together with the data. The mass concentrations of thecontaminants appearing in Fig. 1 are listed along with several
others in Table 1 [7]. Analysis of the singles data yields an
integral event rate of 3.7 × 107 kton−1 day−1 in the energy window of interest ([0.25, 0.8] MeV). This rate is to be comparedto the expected 7 Be solar neutrino elastic scattering signal of
291 events kton−1 day−1 . Spectral analysis allows the identification of the principal contributors to KamLAND’s backgroundto be 85 Kr, 210 Bi and 210 Po, the latter two supported by the decay of long-lived 210 Pb [7]. 85 Kr is a fission product, foundin the atmosphere mainly due to nuclear fuel re-processing [8],while 210 Bi and 210 Po are 222 Rn decay daughters. As both 85 Kr
and 222 Rn are airborne, it was assumed that these contaminants
were introduced into the KamLAND LS during filling from exposure to air. The initial growth of 210 Po, observed in early
KamLAND data, supports this hypothesis [9]. After the initial growth the 210 Po activity leveled off, indicating that it was
supported by its long-lived parent 210 Pb. The analysis of delayed 214 Bi-214 Po beta-alpha coincidences, which occur above210
Pb in the U-decay chain, provided an effective 238 U decayrate of 3.7 ± 0.5 kton−1 day−1 . This excludes the possibility that
210
Po, observed with an event rate of 4.6 × 106 kton−1 day−1 ,was supported by either 238 U or 226 Ra. Although there was nodirect spectroscopic evidence for the presence of 39 Aractivity(an atmospheric spallation product), its existence was suspected
because of the air exposure hypothesis, and a limit on its concentration is included for completeness in Table 1.

10-1
Data
Solar 7Be region-of-interest
Total (3.7 x 107 kton-1 day -1)
85
Kr (3.0 x 107 kton-1 day -1)
210
6
Po (4.6 x 10 kton-1 day -1)
210
6
Bi (2.6 x 10 kton-1 day -1)
141
C (2.3 x 10 kton-1 day -1)
7Be ν e (2.9 x 102 kton-1 day -1)

10-2
-3

10

10-4
-5

10

-6

10

10-7
-8

10

-9

10

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Visible Energy [MeV]
Figure 1: KamLAND low energy singles spectrum. The data is displayed
together with a background model. The major background components areindicated along with their integral rates inside the energy interval [0.25, 0.8]
MeV (hatched region). The expected 7 Be solar neutrino recoil event spectrum(calculated using BS05 solar rates [10]) is overlaid to provide the appropriate
scale. All spectra are converted to visible energy√ with KamLAND’s energy
response function and folded with a σ = 7.8%/ E [MeV] Gaussian energy
resolution [7].

The studies presented in this paper were undertaken to understand if and how the internal background rate, five ordersof magnitude larger than the solar neutrino signal, can be sufficiently reduced by means of chemical purification. At thefundamental level, removal of impurities can be achieved bydifferences in chemical potentials between different media andphases. The diffusion of impurities from one medium to another is required to achieve equilibrium [11]. Consequently westudied the distribution of impurities identified to be responsible
for KamLAND’s low energy background with regards to their
behavior in different media and phases. These included:• vapor-liquid phases in distillation• solid-liquid phases in adsorption, as well as filtering andisotope exchange• liquid-liquid phases in water extraction.
In addition we studied the impact of heating on the lead re-Table 1: Measured radioactivity concentrations in KamLAND LS. The limit for
39 Ar was derived from solubility arguments [7].

Isotope

Concentrations [g/g]39Ar< 4.3 × 10−2140

K(1.30 ± 0.11) × 10−1685

Kr(6.10 ± 0.14) × 10−20

Pb(2.06 ± 0.04) × 10−20

Th
238
U

(8.24 ± 0.49) × 10−17(1.87 ± 0.10) × 10−18

210
232

∗ Corresponding author at: Lawrence Livermore National Laboratory, Liv-

ermore, California 94550, USA Tel.: +1 925 424 5094; Fax: +1 925 424 5512
Email address: gregkeefer@llnl.gov (G. Keefer)
1 Deceased.

2

moval efficiency to target possible organometallic lead compounds formed after alpha decay and known to be prone toheat-induced break up. This paper reports measured reduction
factors for these different purification methods. Studies doneon the removal of radioactive gases (85 Kr and 222 Rn) are described in [12] and are not covered in this work. Final reduction factors achieved from full-scale scintillator purification in
the KamLAND detector are not within the scope of this paper.
The experimental procedures used throughout this study are
outlined in Sec. 2. In Sec. 3 we discuss the analytical techniquesand methods used to quantify and remove radon decay products
from LS and its constituents PC, n-dodecane, and PPO. Finally,in Sec. 4, we discuss the implications and significant findingsof this work.

2.1. Determination of the Radio-nuclide Reduction Factors
The effectiveness of various lead removal techniques wastested by loading LS with trace amounts of radioactive 212 Pb
or 214 Pb. This was achieved by first dissolving 220 Rn or 222 Rn,
exploiting the excellent solubility of radon in organic solvents,
and then allowing it to decay into the meta stable lead isotopes.
The dissolved lead activity, Ai , was measured after loading. The
remaining activity, Af , was measured after the loaded solution
had been subjected to a specific purification procedure. Bothactivities were corrected to correspond to a common reference
time. The reduction factor R for a given species is defined as:
R =

Ai
Af

(1)

Two detector types were used to determine the lead activities.
For samples with specific activities of more than 1 Bq/L, Ge detectors were utilized to measure the gamma radiation emitted in214
Pb, 214 Bi, 212 Pb, 212 Bi and 208 Tl beta decays. For low activity samples, liquid scintillation counting was employed, utilizing the delayed beta-alpha coincidences from the 212 Bi-212 Po(220 Rn decay chain) and 214 Bi-214 Po (222 Rn decay chain) decay
sequences. The latter technique allowed the nearly backgroundfree measurement of specific activities down to 10 mBq/L. Together, these detection techniques provided a means to perform studies of purification reduction factors spanning five orders of magnitude and reaching 212 Pb concentrations as low
as 10−22 g/g. Further sensitivity to the target concentration of
10−24 g/g can only be achieved using the KamLAND detector
itself.

2. Experimental Procedures and Detection Methods
Measuring the large reductions in 210 Pb needed for KamLAND’s low background phase required the quantitative determination of lead concentrations at a level of 10−24 to 10−20 g/g.
The starting value corresponds to approximately 4.6×104 210 Pb
atoms per liter of LS, while the smaller concentration represents
the goal for adequately purified LS. Clearly this is a challenging
analytical problem.
The measurement of lead concentrations at such extremely
low levels is best achieved using radioactive tracers. The isotope of concern, 210 Pb, is a low Q-value beta emitter. Its daughters, 210 Bi and 210 Po, are pure beta and alpha emitters, respectively. None of these decays offer a convenient experimental
signature suited to measure small concentrations in lab scale
experiments. Furthermore, 210 Pb has a half life of 22.3 y resulting in a relatively small specific activity of 4.5 × 10−5 Bq/L at
the starting concentration of 10−20 g/g. To overcome this experimental challenge, we assume that all isotopes of lead born
via alpha decay have similar chemical characteristics in organic
media, and thus, similar responses to purification techniques.
Isotope-specific studies showed a factor of 3 higher 212 Pb reduction than observed for 214 Pb [7]. This might be attributed todifferences in nuclear recoil energies and lead residence times
in the LS. Given the interest in large reduction factors, and notanalytic precision, the possible impact of these effects are neglected in this work.
The short lived isotopes of 212 Pb (T 1/2 = 10.64 h), born
via 216 Po (220 Rn daughter), and 214 Pb (T 1/2 = 26.8 min), born
via 218 Po (222 Rn daughter), can be used to study lead removal
within our target concentration range of 10−20 to 10−24 g/g.
At a concentration of 10−20 g/g the specific 212 Pb activity is
0.83 Bq/L, a factor 1.8 × 104 larger than that of 210 Pb at equal
concentration. Characteristic gamma radiation, of energies
238 keV and 351 keV, are emitted after the beta decays of 212 Pb
and 214 Pb, respectively. This allows for convenient detection
with a low-background germanium (Ge) detector. The subsequent fast Bi-Po beta-alpha decay sequences following theselead decays allow the further utilization of delayed coincidence
counting with LS for high-sensitivity measurements.

2.2. Scintillator Loading and Purification Procedures forRadon Daughters
Gas flow-through sources from Pylon Electronics Inc. (models TH-1025 and RN-1025) were used to load LS with 220 Rn
or 222 Rn, respectively. A second 220 Rn source was made using
a ThO2 powder. In all instances N2 was utilized as the carrier gas. To remove dust particles from the gas stream, theN2 was passed through 100 µm and 0.8 µm filters prior to aflow-through source. The radon-loaded N2 gas was then passed
through 500 cm3 of LS via a glass bubbler, fitted with a sparger
to break up the gas for increased radon absorption. Flow meters were attached to the input and output gas stream to monitor
flow rates.
Our studies showed that the use of a filtered carrier gas wasimportant for achieving reproducible results. Particulates carried by the gas stream readily collect charged radon decay products and are efficiently removed by filtering, thus “faking” aremoval mechanism not present in KamLAND scintillator.
Lead removal experiments were performed with LS, ndodecane, PC, and PPO. All samples were handled using thefollowing standardized procedure (unless otherwise noted):
1. Sparge liquid with radon-loaded N2 for a maximum of 24hours.2
2 Care must be taken when loading LS or pure PC. PC has a high vapor
pressure causing it to evaporate during bubbling, resulting in a lowering of the
PC concentration as well as corrosion of filters and membranes.

32. Transfer the 212,214 Pb-loaded liquid into a counting container to determine the initial activity Ai . For counting with
Ge detectors, two 125 cm3 Nalgene R bottles were used.
If liquid scintillation counting was to be used, half of thesample was transferred to a 10.80 cm diameter, 4.44 cm
long acrylic cell, fitted with a 7.62 cm diameter photomultiplier tube (PMT). The counted LS volume was typically 200 cm3 .
3. When using the Ge detectors, the counted liquid was transferred into the purification system. When using liquid
scintillation counting, sufficient liquid was prepared to allow parallel counting and purification. This method assumes a homogeneous distribution of lead in the LS.
4. After purification, the liquid was counted using the appropriate detector to determine the final activity A f .

to be sufficient for the purpose of identifying the most effective
method of purification.
The following purification techniques were studied: water
extraction, ion enhanced water extraction, isotope exchange,
filtering, adsorption and distillation. When applicable, experiments were performed in series with one another to establish
if the methods scale as the product of their individual reduction
factors. For each of these techniques, the reduction factor, timeof operation, solute volume, and initial activity were studied forcorrelations.
Among the 222 Rn daughters 210 Pb is of particular interest because of its long half life. Following the assumption discussed
in Section 2 that different alpha-born isotopes of lead in organic media have similar characteristics, the purification techniques described below were evaluated using 212 Pb and 214 Pb
as tracers. However, one cannot assume that all metallic radon
daughters are removed with equal efficiency as they have different chemical characteristics. The radionuclides 208 Tl, 212 Bi,
214
Bi and 218 Po were analyzed separately to better understandthe variation in the reduction factors across chemical species.
For example, experimental information obtained on 218 Po wasused to gain some insight into how much 210 Po (T 1/2 = 138 d)would remain in the LS after purification. The results of these
non-lead purification studies are not reported here. More details
on the purification research performed with KamLAND LS canbe found in [7, 12, 14–20].

The “before” and “after” counting was always performed using the same detector to allow for cancellation of systematic
errors. In some instances it was necessary to use only a single
component of the LS during the purification process. If an LScomponent was analyzed by liquid scintillation counting a scintillating cocktail had to be made prior to loading the counting
cell. This dilution was taken into account in the reported reduction factor. Furthermore, measurements were carried out tocross calibrate the two different detector types.
Detailed studies of systematic errors were performed on every aspect of the purification processes. Thorough cleaning of
all containers used in the loading and counting was imperative when dealing with extremely low levels of radioactivity.
Cross contamination had to be avoided in order to achieve reproducible results. When chemically compatible, all components were cleaned with high purity solvents (analytic grade or
better). Each part was cleaned with acetone, then with 1 molar
HNO3 , followed by a rinsing with de-ionized (DI) water andethanol.
In the case of Ge detector counting, it was found that a substantial fraction (up to 50%) of the lead activity contained in
the LS could adhere to the walls of the Nalgene bottles used
for the counting. The bottles were thus counted again after thelead loaded scintillator had been transferred into the purification setup to account for this loss mechanism.

3.1. Water Extraction
A water purification system already existed in KamLAND; it
had been used during the initial filling [21]. We thus first tried
to understand whether this system could be used, perhaps withmodification, to remove dissolved lead from KamLAND LS.
A miniature water extraction system was built to circulate LS
through a glass purification column filled with DI water. The
LS, which is less dense than water, was removed from the topof the water column and re-circulated back to the bottom of
the column using a peristaltic pump. To increase the surface
to volume ratio, the LS was broken into small bubbles with acustom-made glass frit affixed to the bottom of the column.
Water extraction experiments showed weak lead reduction
factors between 1.02 and 1.10. Using about 500 cm3 of water,
250 cm3 of LS and a flow rate of 200 cm3 /min, the observed
reduction factors saturated for circulation times longer than 5hours [7]. A second set of independent experiments was performed using 50 cm3 of lead-loaded LS with 60 cm3 of distilledwater mixed with 0.01 to 6 mol/L of HNO3 , HCl, NaOH [15].
Mixing was achieved by vigorously shaking for 5 min. In theseexperiments we observed saturation levels consistent with themore controlled mixing noted above. In all experiments thelead reduction factor showed a clear decrease with growing pH.
However, the magnitude of this change over the pH range neverexceeded 30%. Therefore, water extraction was found to be notwell suited for achieving the very large lead reduction factors
needed in KamLAND.

3. Removal of Radon-Born Lead from Liquid Scintillator
In any purification process, the difference in concentrations
and solubilities between phases drives the separation of impurities between them. In this work these phases were realized in
the form of water-oil and silica-oil systems or gas-liquid phases.
Processes which rely primarily on diffusion of the impurities
between phases require initial concentrations in the transientphase to be less than those listed in Table 1. The mass transfer
involved in such diffusive processes can be modeled assuming
thermodynamic equilibrium. The work presented here does notreport the reduction in terms of a Gibbs potential, as done inprevious publications [13]. Instead, we report the results in theform of a reduction factor as defined in Eq. 1. We found this4Table 2: 212 Pb reduction factors obtained with 250 ml of LS utilizing different
purification modes. The uncertainties given for the Selecto results are statisticalplus systematic added in quadrature. The Aerosil results have only statisticaluncertainties. Tests with Selecto used 10 g of silica while only 0.5 g of Aerosil
was used.

Adsorbent

(a)

(b)

Reduction Factor
Batch

Loop

32-63 µm Selecto

22.73 ±0.52

4.98 ± 0.12

100-200 µm Selecto

15.38 ±0.24

7.04 ± 0.10

Aerosil 200

3.45 ±0.02

1.49 ± 0.02

stationary (silica gel) phases. One can divide the column into
many distinct sections, referred to as plates. The concentrationof an impurity in successive plates can be derived from massbalance and equilibrium conditions:Figure 2: The stainless steel vessel used to heat the LS prior to adsorption isdepicted in Fig. 2(a). Fig. 2(b) is the adsorption column in which all silica gel
experiments were performed. At the top one can see the heating vessel andthermocouple. Furthermore, a vacuum line was used to regulate the flow rate
through the silica column.

XM(p) =

X0 νp −νe ,p!

p = 1, 2, 3, ...

(2)

where X0 is the initial concentration of the impurity, XM(p) is
the concentration in the pth plate, and ν is the plate volume.
Eq. 2 is referred to as the elution curve and indicates that given
enough adsorbent, there is no theoretical limit to the achievablereduction factor.
There are two possible methods for purifying LS with thistechnique. One can assume that the silica gel is infinitely adsorptive and run the liquid through a column in a loop mode, ormake a single pass through the column in a batch mode. Experiments showed batch purification through a long column tobe the method of choice, as multiple passes through the same
column lead to lower reduction factors. Batch mode makes efficient use of the concentration gradient of the solute developing
along the column. Our observation that batch mode provides
higher purification factors than loop mode agrees well with previous studies [13]. To illustrate this point, Table 2 lists the lead
reduction factors obtained for three different types of silica gel
used in both batch and loop mode.
All results presented from here on are based on batch mode
operation. A wide variety of silica gels are commercially available. For this study a 32 − 63 µm (grain size) silica gel from Selecto Scientific was used as the standard. Our laboratory chromatographic lead purification system, shown in Fig. 2(b), wasdesigned and operated in the following way: a slightly pressurized stainless steel reservoir, holding approximately 10 L of LS,
was coupled to a 35 cm long stainless steel chromatographic
column with 0.97 cm inner diameter. Valves installed in the LS
path allowed flow regulation through the column. The column
was fitted with a 10 µm stainless steel mesh filter at the outlet
to sequester the adsorbent.
Prior to runs with LS, the silica gel was first conditioned(compacted). This was done to keep eddies and voids fromdeveloping inside the column. Sufficient compactification wasachieved by providing constant N2 pressure to the top of the column while pulling partial vacuum at the bottom (without LS).

3.2. Isotope Exchange, Filtration, and HTiO Adsorption
LS loaded with radioactive lead was circulated through a bedof granular lead, or lead shot. Assuming that the dissolved lead
forms an equilibrium with the essentially infinite number of stable lead atoms, we tested the possibility of isotope exchange.
These experiments showed no quantifiable lead reduction effectbeyond the typical loss to the surfaces of the pumping system.
Early studies showed that filtering loaded LS removed thedissolved lead in a variable manner. This fluctuation in leadremoval efficiency was traced to the presence of dust particles in the carrier gas stream to which the charged radon decay
products readily attach [22]. Carefully filtering the gas stream
avoided this interference and enhanced the reproducibility ofthe experiments.
HTiO has successfully been used in the SNO experiment toremove actinides from light and heavy water [23]. We investigated this method for its suitability in removing lead from organic LS. HTiO pre-loaded HEPA PTFE filters (surface density
0.64 g/cm2 ), supplied by the Oxford University SNO group,
were utilized for these tests. lead-loaded LS was placed into
a 60 ml syringe attached to the filter and the liquid was pushedslowly through the filter. The filter was replaced after every 125
ml of lead-loaded LS. For flow rates of 3.3 and 16.5 cm3 /min we
observed lead reduction factors of 1.06 ± 0.01 and 1.07 ± 0.01,
respectively. No further consideration was given to this method.
3.3. Chromatography with Silica Gels (Adsorption)
Previous studies identified silica gel extraction as an efficient means of removing inorganic impurities from LS [13].
This method was studied in depth to evaluate its suitability forremoving lead using different adsorption materials packed inchromatographic columns. Plate theory [24] is commonly used
to describe liquid-solid chromatography, which requires massand thermodynamic equilibrium between the mobile (LS) and5

Table 3: Measured 212 Pb reduction factors in LS. Only statistical uncertainties are quoted. Experimental investigation of the systematic uncertainty for 32-64 µm
gel yielded 7.2%. The uncertainty on the mass measurements was 0.05 g.

Adsorbent Typelead Reduction Factor

Adsorbent Mass [g]Selecto, Lot #301286301, Si-gel 32-63 µm

19.4 ± 0.47

5.0

Selecto, Lot #306279301, Si-gel 100-200 µm

15.38 ± 0.24

10

Selecto, Lot #102085402, Alusil 70

27.03 ± 0.73

5.0

Selecto, Lot #109110402, Alusil Plus

8.33 ± 0.07

2.0

Selecto, Lot #108110403, Alusil NanoSmart

8.20 ± 0.07

2.0

Selecto, Lot #900110401, Si-gel NanoSmart ACT

3.58 ± 0.01

2.0

Selecto, Lot #900110401, Alusil Coarse

28.57 ± 0.82

5.0

Selecto, Lot #107223405, Alusil 40 without K

10.31 ± 0.21

2.0

Aldrich, 3-(Diethylenetriamino)
Propyl-Functionalized gel

8.33 ± 0.69

10.0

Aldrich, Triamine Tetraacetate-Functionalized gel

11.11 ± 0.12

10.2

Aerosil 200

8.26 ± 0.07

0.5

Süd-Chemie, Cu/Mn Catalyst T-2550

3.45 ± 0.02

15

Süd-Chemie, Cu/Mn Catalyst T-2550, Crushed

26.32 ± 0.69

7.2

Ca3 (PO4 )2

6.54 ± 6.54

2.0

Our standard purification procedure gave consistent and reproducible lead removal factors, as verified by repeated measurements with the standard gel. The procedure consisted of thefollowing steps:

sites and reduce the effectiveness of the silica gel [24]. However, this conditioning was not found to provide any benefit interms of boosting lead removal.
Using the procedure outlined above, lead removal studies
were performed with many different types of adsorbents. The
results of these experiments are given in Table 3. Our procedureprovided reproducible results, and typical statistical uncertainties on individual runs were of order 0.1%. Repeated runs withthe 32 − 63 µm silica gel yielded a standard deviation of 7.2%
for the lead reduction factor, which we interpret as a measureof the systematic uncertainty.
Since, as already mentioned, the elution curve (Eq. 2) indicates that in principle the reduction factor has no limit, making
the chromatographic column arbitrarily long should allow one
to achieve any desired reduction factor. This behavior was notobserved, as a maximal reduction factor of 29 was achieved
for a single pass purification. While this factor is substantial,
it falls short of the required value for KamLAND, so studies
were performed to determine whether this limitation could beovercome.
We considered three hypotheses to explain the observed limit
in the lead reduction factor during silica gel extraction:

1. Clean all parts with acetone, then with 0.1 - 1.0 molar
HNO3 , followed by a rinse with DI water and ethanol.
2. Add the desired amount of silica gel to the column, close
all valves, and pump down to 100 hPa. Assist setting of
the silica granules by knocking the column during pumpdown.
3. Apply 1000 hPa N2 pressure to the top of the column andcontinue to knock the column to set the silica in place. The
pressure at the outlet rises to approximately 300 hPa whenequilibrium is reached.
4. Remove the vacuum pump and slowly release the pressure
from the inlet side of the system.
5. Add the lead-loaded LS to the reservoir with all valvesclosed. Pressurize the reservoir with N2 to 1000 hPa.
6. Slowly open the valve between the reservoir and the column to wet the gel. Controlled release of LS prevents cavities from forming at the top of the silica column. The flowrate of LS through the column was 50 ml/min.
7. After purification, the LS is passed through a 0.2 µm PTFEHEPA filter to remove silica particulates suspended in theLS.

Hypothesis 1. There exists lead cross-contamination between
runs (insufficient cleaning).
Hypothesis 2. The intrinsic uranium and thorium concentrations of the silica gel, and the resulting out-gassing radon, reintroduces 212 Pb into the LS. The observed limit may be determined by the equilibrium between the removal and addition ofPb.

Further conditioning tests were performed by heating the silica gel to 100 - 120 ◦ C to remove adsorbed water from its surface. Water molecules are known to adhere to the adsorption
6

Table 4: Chemical characteristics of the LS components and of some known organometallic lead compounds. These data are taken from the CRC [25].

Name

FormulaDensity [g/cm3 ] @ 20 ◦ C

Boiling Point [ ◦ C]

n-Dodecane

C12 H260.7495215

1,2,4-Trimethylbenzene (PC)C9 H120.8758169

2,5 Diphenyloxazole (PPO)

C15 H11 NO

1.0940360 1

Tetraethyllead

C8 H20 Pb

1.653

200 2

Methyltriethyllead

C7 H18 Pb

1.71

70

Diethyldimethyllead

C6 H16 Pb

1.79

51

Ehtyltrimethyllead

C5 H14 Pb

1.88

27

C4 H12 Pb

1.995

110

Tetramethyllead
1
2◦

PPO has a melting point of 74 C
Tetraethyllead will decompose at 200 ◦ C before it boils.

Hypothesis 3. Some fraction of the 212 Pb may exist in the LS
in a form without affinity towards the silica gel.

states could form stable, electrically neutral, non-polar compounds. The most notable of these are the lead-carbon bonds
which tend to be covalent, and thus non-polar. The decay of theparent nuclei into lead occurs in a carbon rich environment andthis bonding is energetically favored over many other available
chemical bonds (e.g. C–C bonds) [25]. The non-polar leadcarbon compounds would have no affinity to the polar reaction
sites on silica gel, supporting Hypothesis 3.
Table 4 lists the relevant properties of the known organic
compounds containing lead. All exhibit lower boiling points
than those of PC and n-dodecane. It is of interest to note
C8 H20 Pb is thermally unstable and will decompose at 200 ◦ C.
This process would leave behind a polar or ionic lead species.
To obtain supporting evidence for Hypothesis 3, we experimented with chemical procedures targeting the organic leadcarbon bonds. It is known that the reagents: FeCl3 [29],SnCl4 [30], MoS2 [31] and thiol resin [32] target these leadcarbon bonds directly.
To test the effect of these chemicals on the lead reduction
factor, we first passed 212 Pb-loaded LS through a silica gel column to remove the ionic lead component. In a second step,
150 ml of the treated LS was stirred magnetically, with one of
the reagents listed above, for 45 min. Next, 15 ml of 0.5 M
EDTA ([CH2 N(CH2 CO2 H)2 ]2 ) solution (pH of 8) was added tothe mixture and stirred for an additional 15 min. The mixture
was filtered under partial vacuum and passed over a molecular sieve to remove any water. The resulting LS was filtered asecond time to remove any particulates of the molecular sieve.
The liquid was counted to determine the remaining amount of212
Pb. The lead reduction factors obtained this way are tabulated in Table 5.
LS treated with FeCl3 before silica gel extraction showed a
50-fold enhanced lead reduction factor compared to plain chromatography. We interpret this boost in effectiveness as supporting evidence for Hypothesis 3. However, the FeCl3 treatment of
the LS was found to permanently degrade its optical properties.
The measured light attenuation length at 436 nm was < 10−4
m compared to approximately 13 m for untreated LS. Optical

Blank runs performed with unloaded LS following a purification run with lead-loaded LS did not show any carry-overactivities, thus excluding Hypothesis 1 and constraining also
Hypothesis 2. Furthermore, bottles and containers were not reused whenever possible, and all re-used equipment underwent
vigorous cleaning, as described in Sec. 2.2.
Hypothesis 2 was further refuted by measuring the radioactive content of the gel. The 32 − 63 µm Selecto Scientific silica gel was measured to have an intrinsic uranium and thorium
content of 167 ppb and 152 ppb, respectively [26], resulting in
1.7 mBq of 212 Pb in 2.5 g of silica gel. Typical loading activities
for the LS were 5 kBq. This silica gel had an average reduction
factor of 19, resulting in 250 Bq of residual 212 Pb activity. The
total intrinsic activity of the silica gel is thus much smaller thanthe measured activity after silica gel purification, implying thatthe former contributes negligibly to the observed limit in the
reduction factor.
Hypothesis 3 was also tested experimentally. Chromatographic impurity removal techniques selectively address polar
molecules or ionic states [24, 27]. It is the excess charge on themolecule, or atom, which draws it to the hydroxyl group (–OH)
on the active adsorption sites of the gel. The excellent reproducibility of our lead removal experiments suggests that about
3-5% of the lead atoms were present in a non-polar, molecularstate, without affinity to the active hydroxyl groups of silica gel.
In order to understand why this species has no affinity to thesilica gel, it is of particular interest to examine a study performed with n-dodecane exposed to a 60 Co source [28]. This
study showed that the exposure to ionizing radiation produced
fragmented carbon chains. We note that the 212 Pb and 214 Pb isotopes used in our studies, as well as the 210 Pb present in KamLAND’s LS, are born via alpha decay. A recoiling lead nucleus,
with typical energies of a few hundred keV, can easily fragment
the carbon-carbon bonds in n-dodecane, having typical binding energies of order 10 eV [25]. These fragmented molecular7

Table 6: 212 Pb reduction factors observed for heated LS. All heating was performed in a stainless steel container. The first column gives the heating time.
The second column states the average temperature during heating. The final
column provides the observed reduction factor.

Table 5: Combined 212 Pb reduction factors for purification methods performedin series with an initial SiO2 purification. Secondary techniques specifically target lead-carbon bonds. Only statistical uncertainties are reported except where
noted.

MethodReduction Factor

SiO2

19.4 ± 0.47

Time [hrs]1

Avg. Temp [o C]

Reduction Factor

Heating + Silica Gel

SiO2 → FeCl3> 10602

142277.8 ± 23.2 1

SiO2 → FeCl3 → SiO2

> 12632

187

200 ± 16 1

SiO2 → SnCl447.0 ± 1.1

1.5

160

172.4 ± 2.97 2

SiO2 → MoS2

24.9 ± 0.6

SiO2 → ThiolResin

22.7 ± 0.52

1875.3 ± 0.06

SiO2 → Distillation> 43892

157

5.08 ± 0.05

81529.35 ± 0.09

1
2

Heating Only

2

Includes Systematic Uncertainties.
Experiment performed with n-Dodecane.

Control Sample, No Heat, Vessel Blank
3
1transparency could not be restored by means of filtration or repeated treatment with silica gel.
The successful experiments utilizing reagents targeting leadcarbon bonds prompted us to consider heating the lead-loaded
LS to 100-200 ◦ C to remove the tetraethyllead which decomposes [25]. As a test, 212 Pb-loaded LS was pre-heated in astainless steel vessel, as shown in Fig. 2(a), before sending it
through the ion-exchange column, depicted in Fig. 2(b). The
results from these heating experiments are provided in Table 6.
The first three entries show a substantial enhancement of thelead reduction factor for pre-heated LS compared to the experiments with silica gel extraction only. The following three entries give a factor 5 to 10 reduction obtained by just heating theLS and not using silica gel. Lead adherence to the container
walls was excluded by experiments performed at room temperature.
We consider these results to be further evidence for Hypothesis 3. However, the data are not specific enough as to allow us
to identify which compounds are present and in what proportion. This is nevertheless an important finding, indicating thatthe limitless reduction factors predicted by plate theory, Eq. 2,
are not to be expected for alpha decay products in an organic
medium.

2

17

1.11 ± 0.01

5 g SiO2 in column.
5 g of SiO2 and silica gel heated in column.

Distillation experiments primarily utilized lead-loaded ndodecane, which makes up 80% of the KamLAND LS. The
distillation apparatuses utilized in these studies are pictured inFig. 3. The system in Fig. 3(a) consisted of a three-neck flask(500 ml), a silvered fractionating column packed with stainlesssteel wool, and a condenser. A thermocouple was used to regulate the distillate temperature to within a few ◦ C. A mercury
thermometer was placed on top of the fractionation column tomonitor the temperature of the vapor prior to condensation. The
stainless steel wool increased the effective number of plates.
In essence, it allows for multiple distillations of the liquid as
the vapor rises through the column and re-condenses on thesteel wool. Typical operating conditions were 170 hPa, witha 160 ± 1 ◦ C set-point for the distillate in the boiling flask. The
flask temperature was maintained by the thermocouple but thetemperature measured by the mercury thermometer at the topof the fractionating column varied over time from 125-145 ◦ C.
Temperatures exceeding 145 ◦ C the top of the column resulted
in a lower reduction factor due to the boil-over of contaminants.
The rate of distillation at these operating conditions was 10
ml/min. Operating the distillation system at a reduced pressure
is important to minimize the amount of dissolved radioactive
gases (222 Rn, 85 Kr, and 39 Ar) as well as heat-induced changes
in the optical properties of the distillate.
Repeated distillation tests with 212 Pb-loaded n-dodecane provided a handle on the variance associated with maintaining consistent operating procedures. To maintain reproducibility of the
distillation results, the following procedures were utilized:

3.4. Distillation
The final method studied for KamLAND’s LS purification
addresses the removal of all lead species in an integrated way:
distillation. During distillation, the distillate is heated to induce a phase change. Differences in vapor pressure result in
fractionation between the constituents. As a by-product, theheating further breaks organo-metallic bonds and transforms
organometallic lead compounds, that may be in solution, into apolar or ionic form. These, in turn, can be effectively removed
by distillation.

- All internal surfaces of the distillation system werecleaned with acetone, then etched with 0.1 - 1.0 molar
8

(a)

(b)

(c)

Figure 3: Size evolution of distillation systems utilized during the purification experiments. Fig. 3(a) and Fig. 3(b) show the bench-top systems used at the
University of Alabama and Tohoku University, respectively. Together they provided a complete analysis of systematic studies on distillation. Fig. 3(c) is thescaled-up distillation system at Tohoku University, allowing us to increase the amount of distillate by a factor 20.

Table 7: Average 212 Pb reduction factors obtained by distillation of various components of LS. The uncertainty is conservatively reported as the addition inquadrature of the error on the weighted mean with the standard deviation of the measurements to account for the large system-to-system variation relative to theindividual measurement uncertainties.

Distilled material

Averaged Reduction Factors

n-Dodecane

300 hPa, 160 ◦ C, Bench-top System [12, 18]

1 time distillation(3.4 ± 2.3) × 103

3 times distillation(1.9 ± 1.0) × 104

5 times distillation(2.2 ± 0.6) × 104

7 times distillation(2.8 ± 0.6) × 104
190 hPa, 160 ◦ C, Bench-top System

n-Dodecane
1 time distillation(7.8 ± 3.2) × 102

5 times distillation(2.6 ± 0.2) × 103

5 times distillation + SiO2

(2.9 ± 0.6) × 103

n-Dodecane

37 hPa, 113 ◦ C, Scaled-up System [12]

1 time distillation(1.4 ± 0.3) × 103

3 times distillation(3.8 ± 1.2) × 103

5 times distillation> 1.3 × 104

Pseudocumene

37 hPa, 75 ◦ C, Scaled-up System [17]

1 time distillation(7.7 ± 3.2) × 102

2 times distillation(1.3 ± 1.2) × 103

3 times distillation(2.8 ± 1.3) × 103

4 times distillation(7.8 ± 4.8) × 103

PPO

20 hPa, 160 ◦ C, Bench-top System [18](2.1 ± 0.8) × 103

1 time distillation

9

HNO3 , and rinsed with DI water and ethanol. This eliminated carry-over between subsequent distillations.

design and operation of a purification plant for continuous circulation and purification of 1000 tons of KamLAND LS.
The second most effective procedure was silica gel extraction, which provided highly reproducible results. However,a steady decline observed in lead reduction factors after successive extractions cannot be explained by simple plate theory.
One possible explanation is that a portion of the dissolved leadexists in non-polar organometallic compounds, which can bedegraded by heat and/or chemically targeted on the lead-carbon
bond. Our studies on silica gel extraction with preheating andprocessing with FeCl3 showed improved reduction factors consistent with the existence of such non-polar lead compounds in
LS.

- All distillate collected before the top thermometer reached
the boiling point of the distillate was discarded. This removes all impurities with a low vapor pressure. Some leadcandidates, addressed in this way, can be found in Table 4.
- Collection of the distillate was stopped after approximately 90% of the volume was distilled. This operation
reduces boil-over effects and re-contamination with highboiling-point compounds.
- A constant rate of distillation was maintained. Rapid boiling at increased distillation rates directly contributes to thetransfer of impurities into the distillate.

Acknowledgments

Distillation was performed under different pressure and temperature conditions for n-dodecane, PC, and PPO. Similar measurements taken with the different distillation systems depicted
in Fig. 3 allowed for the investigation of instrumental variations in the procedure that could impact the reduction factor.
To ensure that the reduction factors observed in the modestly
sized table-top experiments would apply to larger volumes, ascaled-up distillation system, shown in Fig. 3(c), was designed
to process 5 L at a time. Up to seven distillations were repeatedin series to assess saturation of the lead reduction factor. Distillate samples were taken after consecutive distillations during
a single experiment. The measured 212 Pb reduction factors are
summarized in Table 7. Beyond three successive distillations,no further lead reduction could be observed. We also tested the
combined effect of silica gel extraction and 5-times distillation
resulting in a factor 1.1 improvement compared to distillation
only, as can be seen in Table 7.
Distillation proved to be the most effective means of removing radioactive lead from the KamLAND LS components.
The reduction factors observed during the study of distillation
ranged from a few hundred to several thousand, depending on
the operating conditions. These results provided the necessary
technical base required to achieve the high reduction factors
needed for the KamLAND purification campaign. They further
provided essential procedural development and initial parameters for the full-scale KamLAND distillation system.

This work was supported in part by the U.S. Departmentof Energy (DOE) grant DE-FG02-01ER41166, the Japanese
Ministry of Education, Culture, Sports, Science and Technology, the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan, DOE contract number
DE-AC02-05CH11231, and other DOE grants from individual
KamLAND institutions. The authors would like to thank Dr.
Li Cang of Selecto Scientific Inc., and Xiong Xin Dai and Nick
Jelly (Oxford University) of the SNO collaboration.
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4. Conclusion
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11