Extraction methods in development of Gd-loaded liquid organic scintillators for antineutrino detection: 2. Scintillators based on solutions of gadolinium 2-methylvalerate

Article abstract of DOI:10.1134/S1066362209030102

Complexes of Gd with carboxylic acids C₄-C₉ were prepared, and their properties were studied: solubility in water and organic solvents in relation to temperature, melting points, and decomposition points. A procedure was developed for removing water from gadolinium 2-methylvalerate Gd(2MVA)₃ to a final content not exceeding 0.01%. To increase the solubility of Gd(2MVA)₃ in low-polarity organic solvents and to stabilize the resulting organic solutions, it was suggested to add extractants such as 2-methylvaleric acid (H2MVA) or neutral organophosphorus compounds: triisoamylphosphine oxide (TIAPO) or trioctylphosphine oxide (TOPO). Liquid Gd-loaded organic scintillators based on Gd(2MVA)₃ solutions with additions of H2MVA, TIAPO, and TOPO in trimethylbenzene (TMB), phenylxylylethane (PXE), and their mixtures with dodecane were prepared. Long-term tests (up to 2 years) of the stability of the optical properties of the scintillators obtained proved their stability at temperatures of up to 30°C. At elevated temperatures (40°C and higher), the scintillators degrade. The previously made suggestion that water in the scintillator negatively affects its optical properties in prolonged storage was confirm.

Full text of DOI:10.1134/S1066362209030102

ISSN 1066-3622, Radiochemistry, 2009, Vol. 51, No. 3, pp. 274–282. © Pleiades Publishing, Inc., 2009.
Original Russian Text © N.A. Danilov, Yu.S. Krylov, A.Yu. Tsivadze, I.R. Barabanov, L.B. Bezrukov, G.Ya. Novikova, E.A. Yanovich, L.I. Demina,
C. Cattadori, A. Di Vacri, 2009, published in Radiokhimiya, 2009, Vol. 51, No. 3, pp. 239–246.
Extraction Methods in Development of Gd-Loaded Liquid
Organic Scintillators for Antineutrino Detection: 2. Scintillators
Based on Solutions of Gadolinium 2-Methylvalerate
N. A. Danilova, Yu. S. Krylova, A. Yu. Tsivadzea, I. R. Barabanovb, L. B. Bezrukovb,
G. Ya. Novikovab, E. A. Yanovichb, L. I. Deminac, C. Cattadorid, and A. Di Vacrid
a Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia
b Institute of Nuclear Research, Russian Academy of Sciences, Moscow, Russia
c Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
d Gran Sasso National Laboratory, National Institute of Nuclear Research, L’Aquila, Italy
Received August 4, 2008
Abstract—Complexes of Gd with carboxylic acids С4–С9 were prepared, and their properties were studied:
solubility in water and organic solvents in relation to temperature, melting points, and decomposition points. A
procedure was developed for removing water from gadolinium 2-methylvalerate Gd(2MVA)3 to a final content
not exceeding 0.01%. To increase the solubility of Gd(2MVA)3 in low-polarity organic solvents and to stabilize
the resulting organic solutions, it was suggested to add extractants such as 2-methylvaleric acid (H2MVA) or
neutral organophosphorus compounds: triisoamylphosphine oxide (TIAPO) or trioctylphosphine oxide
(TOPO). Liquid Gd-loaded organic scintillators based on Gd(2MVA)3 solutions with additions of H2MVA,
TIAPO, and TOPO in trimethylbenzene (TMB), phenylxylylethane (PXE), and their mixtures with dodecane
were prepared. Long-term tests (up to 2 years) of the stability of the optical properties of the scintillators ob-
tained proved their stability at temperatures of up to 30°С. At elevated temperatures (40°С and higher), the
scintillators degrade. The previously made suggestion that water in the scintillator negatively affects its optical
properties in prolonged storage was confirmed.
PACS numbers: 13.15.+g, 78.30.Cp, 78.40.Dw
DOI: 10.1134/S1066362209030102
Long-term and large-scale experiments in neutrino
physics became the next step in studying the basic
properties of neutrino. The discovery of neutrino oscil-
lations, i.e., transition of one type of neutrino into an-
other, made it necessary to go beyond the framework
of the standard theory of electrically weak interactions
and admit nonzero mass of neutrino. The most impor-
tant studies in the field of neutrino physics shifted to-
ward determination of neutrino mass and its mixing
angles (probabilities of transition of one type of neu-
trino into another). Reactor installations are powerful
sources of low-energy electronic antineutrino (fluxes
~1013 ν cm–2 s–1 near the biological shield surface),
which allows their use as a convenient tool for neu-
trino research. Another important aspect of neutrino
physics is associated with studying astrophysical ob-
jects, e.g., supernova outbursts. A supernova outburst
occurring as a result of star collapse is also a source of
short-term powerful neutrino radiation. Recording of
these rare events and measurement of their spectral
characteristics bear important information both on the
nature of a supernova outburst and on the neutrino
properties. As the interaction of neutrino with the tar-
get substance is weak, one of the major requirements
to such experiments is the development of large-
tonnage detectors. The most widely used today are
detectors based on hydrogen-containing liquid organic
scintillators (LOSs) in which antineutrino reacts with
proton: νе + р → е+ + n. The energy of reaction parti-
cles, lost as a result of ionization, is transferred to dis-
solved scintillation additives (РРО). Their deexcita-
tion is recorded by photoelectron multipliers. The tech-
nique of delayed coincidence from two events caused
by instantaneous signal from positron and delayed by
~180 μs γ-quantum (2.2 MeV) from the capture of
thermalized neutron on hydrogen allow identification
of the neutrino interaction. To improve the characteris-
tics of a scintillator as a detector for neutrino detection,
274

EXTRACTION METHODS IN DEVELOPMENT …: 2.
275
Gd-loaded scintillators are used. Introduction of Gd to
an LOS allows the neutron lifetime in the scintillator
(the neutron capture cross section of the natural mix-
ture of Gd isotopes is higher by approximately 5 orders
of magnitude than that of hydrogen) to be decreased to
30 μs at 0.1% concentration of Gd. In addition, it is
significant that the total energy release of the γ-quanta
in the neutron capture by the 155Gd and 157Gd nuclei is
~8 MeV, which considerably exceeds the natural sur-
rounding radioactive background (2.6 MeV, 208Tl).
Both these factors allow the background to be substan-
tially suppressed and hence the neutrino detection effi-
ciency to be increased.
tillators rapidly lost transparence (by a factor of ~2 in
6 months), which did not meet the experimental re-
quirements.
It was shown in our previous studies [1, 3, 4] that
the metal-loaded organic phase prepared by traditional
liquid extraction inevitably contained water and the
use of available desiccants did not ensure complete
dehydration but contaminated the scintillator with un-
desirable desiccant components. It was also noted that
a decrease in the water content of the organic phase led
to an increase in the scintillator stability. Therefore, we
concluded that it would be appropriate to isolate gado-
linium carboxylates from their extracts, examine the
possibility of their dehydration using methods that do
not contaminate the target product with foreign sub-
stances, and subsequently prepare the scintillator by
dissolving anhydrous gadolinium carboxylate in or-
ganic solvents.
As the Gd concentration in the scintillator is low,
its effect on the detector characteristics (light output,
transparency) is acceptable.
The problem of transferring metals into an organic
phase to obtain stable solutions is similar to problems
that are solved in the development of various extrac-
tion processes. Therefore, in our previous studies we
examined extraction systems suitable for preparing
Yb-, In-, and Gd-loaded scintillators for neutrino de-
tection [1–4]. The main components of a metal-loaded
LOS should be a metal-containing additive, a hydro-
gen-containing organic liquid with the maximum pos-
sible H : C ratio, and a scintillation additive. As a
metal-containing additive we suggested extractable
metal complexes with carboxylic acids [1, 3, 4]. As
experiments with neutrino are long-term, the main re-
quirement to a scintillator is its chemical stability and
stability of its optical properties (transparence and light
output) for a long period (three years and more), be-
cause a decrease in the transparence or quantum effi-
ciency (light output) increases the experimental error.
The scintillator stability implies that the total amount
of light collected on the photodetector should be con-
stant. The major possible causes of a decrease in the
light output are a decrease in the scintillation effi-
ciency of molecules containing aromatic rings and for-
mation of new “quenching” molecules, which leads to
a decrease in the efficiency of excitation transfer be-
tween molecules; a decrease in the solution transpar-
ence in the range of the emission spectrum due to for-
mation of new compounds having absorption bands in
this range; precipitation of metal compounds due to
hydrolysis and polymerization.
The goal of this study was to prepare liquid organic
Gd-containing scintillators for detection of reactor an-
tineutrino, with the Gd concentration [Gd](o) = 0.85–
1 g l–1, stable for 3 years and more.
EXPERIMENTAL
In our study we used an aqueous solution of gadoli-
nium chloride, prepared by dissolving Gd2O3 (99.99%
pure) in hydrochloric acid (ultrapure grade). Carbox-
ylic acids (isobutanoic, HiBA; hexanoic, HHexA; 2-eth-
ylbutanoic, H2EtBuA; 2-methylvaleric, H2MVA; iso-
valeric, HiVA; heptanoic, HHepA; 2-ethylhexanoic,
H2EtHexA) (Aldrich, initial purity 98–99.5%, trans-
parence 1.6–9.0 m) were purified by double vacuum
distillation, after which their transparence was 12–15 m.
In our study we used organic solvents: 1,2,4-tri-
methylbenzene (Enichem), dodecane and light white
mineral oil (Aldrich), and phenyl-o-xylylethane (Koch
Chemical Company). The solvents were purified by
passing through columns packed with Al2O3, after
which their transparence was no less than 10–15 m.
The Gd concentration in the aqueous and organic
phases was determined by complexometric titration
(EDTA) with Xylenol Orange indicator [8], and also
by γ-counting of 159Gd.
The water content of the organic phase was deter-
mined by Fischer titration with a Titroline KF titrator.
The transparence of scintillator samples (L430) was de-
termined at 430 nm using a UV/VIS Perkin–Elmer
Lambda 18 spectrophotometer in cylindrical quartz
Gd-loaded LOSs with the Gd concentration of 1 g l–1
have been used for detecting thermal neutrons [5] and
for recording reactor antineutrino in Palo Verde [6]
and CHOOZ [7] experiments. In all the cases, the scin-
RADIOCHEMISTRY Vol. 51 No. 3 2009

276
DANILOV et al.
att
cells with a length of 100 mm. Lλi was determined as
late gadolinium carboxylates in the form GdR3·nH2O,
difficultly soluble in organic solvents, to examine the
possibility of their dehydration, and to find conditions
under which this dried complex is sufficiently soluble
in organic solvents to prepare scintillators. As direct
exchange of hydrogen for Gd in carboxylic acid mole-
cules is virtually excluded, synthesis of gadolinium
carboxylates involved preliminary neutralization of
carboxylic acids with a stoichiometric amount of am-
monia:
the distance at which the light intensity decreased by a
att
factor of 1/e using the formula Lλi = d/Aλiln10, where
d is the optical length of the measured sample, i.e.,
the length of the quartz cell, and Аλi = А430 nm – Аmin,
i.e., the light absorption at the required wavelength
(430 nm) relative to the absorption minimum in the
range 450–600 nm. The light output of the samples
was determined by the standard procedure: comparison
of the amplitude spectra of the samples upon their irra-
diation with a γ-ray source (137Cs) with those of the
reference under the same geometric conditions with
the same γ-ray source.
HR(o) + NH4OH(o) = NH4R(aq) +H2O,
(1)
where HR is a carboxylic acid. The reaction was com-
pleted at pH 9.0, when the carboxylic acid fully trans-
formed into the ammonium salt NH4R readily soluble
in water. Into the precooled aqueous solution of NH4R,
we added with vigorous stirring the calculated amount
of an aqueous GdCl3 solution. The GdR3·nH2O pre-
cipitate formed by the reaction
The IR spectra of gadolinium carboxylate samples
were recorded on a Nexus IR Fourier spectrometer
(Nicolet) in the range 4000–400 cm–1.
The TGA diagrams were obtained with a Perkin–
Elmer Pypis 1 TGA derivatograph.
When examining the compatibility of scintillators
with the structural material of the detector, we used
Acryl GS-233 organic glass (Rhome).
NH4R(aq) + GdCl3(aq) = GdR3·nH2O↓ + 3NH4Cl(aq) (2)
was washed with water to remove unchanged sub-
stances (HR and GdCl3) and the reaction product
(NH4Cl) until the reaction for Cl– anions became nega-
tive and then with anhydrous alcohol and acetone to
remove the major fraction of water. Then the product
was dried in air at room temperature. The water con-
tent of the precipitate obtained (Fischer titration) was
on the level of 0.4%. The solubility of the synthesized
carboxylates in TMB (Table 1) and other organic sol-
vents, however, appeared to be insufficient for prepar-
ing scintillators. Analysis of the TGA diagram of all
the synthesized gadolinium carboxylates showed that
they are resistant to heating to ~250°С. Further heating
leads to their decomposition which is complete at 500–
600°С. The final decomposition product is Gd2O3.
Heating of Gd(2MVA)3·nH2O to 80°С leads to the re-
moval of water that is not bound in the extractable
complex (0.2%), which is followed by weight losses of
0.134% in the temperature range 80–130°С and 0.07% in
the range 130–150°С. Further heating of Gd(2MVA)3
samples did not lead to weight loss up to 250°С. The
total weight loss due to water removal was 0.41%. The
results obtained confirmed the possibility of removing
the major fraction of water from gadolinium carboxy-
lates by thermal dehydration without their decomposi-
tion. In Gd(2MVA)3 samples kept at 140°С for 2 h, the
water content was less than 0.01% (Fig. 1, curve 2;
Fischer titration). In the above-described experiments,
the moisture content of the initial Gd(2MVA)3·nH2O
As scintillation additives we used 2-(4-biphenyl)-5-
phenyloxazole (ВРО), 2,5-diphenyloxazole (РРО), and
p-bis(o-methylstyryl)benzene (BisMSB). The range of
maximal sensitivity of photoelectron multipliers used
in this study was 380–450 nm.
RESULTS AND DISCUSSION
Preparation of Gadolinium Carboxylates
with the Minimal Water Content and Study
of Their Properties
A study of Gd extraction with carboxylic acids
(CAs) showed that, depending on pH of the equilib-
rium aqueous phase, complexes of various composi-
tions can be present in the organic phase: GdR3·
3HR·nH2O and GdR3·nH2O, where n = 1–2, HR is
a carboxylic acid, and R– is its anion. An increase in
pH of the equilibrium aqueous phase over 8.5 led
to the formation of gadolinium hydroxycarboxylates
Gd(OH)x R3–x·nH2O. It was found that complexes
GdR3·3HR·nH2O are readily soluble in low-polarity
organic solvents, and the complexes GdR3·nH2O and
Gd(OH)xR3–x·nH2O are poorly soluble.
We failed to completely dehydrate Gd extracts with
available desiccants [1]. Therefore, we decided to iso-
RADIOCHEMISTRY Vol. 51 No. 3 2009

EXTRACTION METHODS IN DEVELOPMENT …: 2.
Table 1. Properties of gadolinium carboxylates
277
Number of C atoms
in acid molecule
Solubility in
TMB, g l–1
Compound
М
% Gd
pH of synthesis
Tm-, °C
Tdec-, °C
4
5
5
6
6
6
Gd(iBA)3
418 37.5
460 34.1
460 34.1
502 31.3
502 31.3
502 31.3
544 28.8
1.33
0.22
0.07
0.35
0.18
0.21
–
–
Gd(2MBA)3
Gd(TMA)3
Gd(2MVA)3
Gd(EtBuA)3
Gd(HexA)3
Gd(HepA)3
5.1
–
4.62
6.17
5.1
Does not melt up to 300°C
–
250–380
–
–
–
5.13
101–103
100–101
7
8
0.19
0.30
5.57
5.36
Gd(2EtHexA)3 586 27.8
Does not melt up to 300°C 300–500
Table 2. Solubility of gadolinium carboxylates in Н2О ([Gd], g l–1) and pH of solutions after stirring for 2 h at various
temperatures
25°С
[Gd]
38.8
8.54
40°С
[Gd]
30.2
5.89
60°С
80°С
Number of C atoms in
acid molecule
Compound
Gd(iBA)3.1
рН
рН
6.01
6.16
5.89
5.8
рН
[Gd]
20.6
рН
[Gd]
15.3
4
5
5
6
6
6
7
8
6.3
5.76
5.86
5.44
5.6
5.12
5.69
5.21
5.48
5.68
5.68
5.42
4.28
Gd(2MBA)2.96
Gd(TMA)3
6.46
6.45
6.25
6.26
6.21
5.45
4.28
4.29
1.13
0.65
0.56
0.14
0.042
0.108
2.44
0.76
0.45
0.37
0.145
0.046
0.99
2.2
1.72
Gd(2MVA)3.08
Gd(EtBuA)3.4
Gd(HexA)3
1.58
1.45
0.19
0.05
0.19
1.05
6.06
6.19
5.39
4.3
0.907
0.16
5.87
5.85
5.43
4.30
Gd(HepA)3
0.045
0.145
Gd(2EtHexA)3.09
after washing with alcohol and acetone and before
thermal dehydration was 0.41%. However, highly in-
flammable ethanol and acetone are hardly suitable as
solvents for dehydration of gadolinium carboxylate in
its production in amount of several tons. Therefore, it
was necessary to examine the possibility of removing
water directly from the precipitate after its washing
with water (moisture content ~60%). We noted that the
solubility in water of freshly prepared Gd carboxylates
sharply decreases with increasing temperature with
simultaneous decrease in pH of the aqueous phase
(Table 2). This may be due to hydrolysis of gadolinium
carboxylates at elevated temperatures in accordance
with the reaction
aqueous phase. Furthermore, with increasing time of
solution heating, the solubility of the carboxylate
and pH of the equilibrium aqueous phase decreased
(Table 3).
GdR3 + H2O ҙ Gd(OH)xR3–x↓ + H+.
(3)
Fig. 1. Weight loss (% of initial sample weight) of
Gd(2MVA)3 on heating in a nitrogen atmosphere (TG dia-
grams): (1) sample preliminarily dried in air for 5 days at
room temperature (22 ± 1°С); (2) sample 1 additionally
heated for 2 h at 140°С.
Cooling to room temperature of carboxylate pre-
cipitates equilibrated with the aqueous phase did not
lead to an increase in their solubility in the equilibrium
RADIOCHEMISTRY Vol. 51 No. 3 2009

278
DANILOV et al.
Table 3. Effect of heating time on the solubility of
Gd(TMA)3 in water and pH of the equilibrium aqueous
phase
In the spectra of complexes formed by H2MVA with
Gd, the most characteristic bands in this range disap-
pear, and only weak bands indicative of the presence
of weak hydrogen bonds remain.
Heating time (50°С), h
рН
6.05
6.02
5.96
5.92
5.93
5.90
–
[Gd], g l–1
1.21
1.5
2.5
The stretching vibration band of the carbonyl group
in H2MVA, manifested at 1710 cm–1, disappears upon
complexation, with two new bands at 1538 and
1420 cm–1 appearing instead. These bands correspond
to antisymmetric and symmetric vibrations of the
СОО– group in which both O atoms become equiva-
lent. The first of these bands is very characteristic,
whereas in the range 1450–1300 cm–1 there are also
bands belonging to other vibrations of the ligand skele-
ton and to vibrations of mineral oil. To eliminate the
absorption bands of mineral oil, we recorded the spec-
trum of Gd(2MVA)3 using the KBr technique.
1.21
4.5
1.15
7.0
1.12
24.0
31.0
48.0
1.12
1.09
0.8
Comparison of the IR spectra of two samples of
gadolinium 2-methylvalerates prepared in accordance
with reactions (2) and (3) also confirmed the presence
of the ОН– group in gadolinium 2-methylvalerate that
precipitated on heating its aqueous solution. Samples
dried in air and calcined for 2 h at 140°С were pre-
pared as mulls in mineral oil and as KBr pellets. The
bands in the spectra of the compounds isolated in this
study were assigned by comparing the spectra with
those of the starting compounds [H2MVA, Gd(OH)3]
and by analyzing published data [9]. Liquid carboxylic
acids are characterized by a broad absorption band
with several submaxima in the range 3000–2500 cm–1
(Fig. 2). Absorption in the range 2700–2500 cm–1 in
the spectrum of H2MVA convincingly confirms the
presence of strong hydrogen bridges between the car-
bonyl and hydroxy groups in carboxylic acid dimers.
In the IR spectrum of a sample isolated after heat-
ing gadolinium carboxylate in water [reaction (3)],
there is a strong broad band at about 3390 cm–1, corre-
sponding to the stretching vibration of the ОН– group.
A low-frequency shift of this band relative to the
ОН– stretching vibrations in gadolinium hydroxide
(Fig. 2) indicates that the compound isolated by heat-
ing the aqueous solution is the hydrolysis product
of the salt Gd(2MVA)3 and has the composition
Gd(OH)x(2MVA)3–x, where х = 0.1–0.2 (chemical
analysis data).
The spectra of Gd(2MVA)3 and Gd(OH)x(2MVA)3–x
(Fig. 2) differ not only in the range 3500–3000 cm–1,
but also in the range 1000–800 cm–1. This is most
probably due to the fact that out-of-plane vibrations of
the О–Н···O group are observed just in this range. The
presence of medium-intensity bands in the range
3500–3000 cm–1 in the spectrum of the complex iso-
lated after heating the aqueous solution (Fig. 2) con-
firms the presence of strong hydrogen bonds and hence
the composition of the hydrolysis product.
Similar hydrolysis of gadolinium carboxylates will
apparently occur in the course of thermal drying of wet
precipitates (Gd concentration in the equilibrium aque-
ous phase was 1.56 g l–1). The presence of such par-
tially hydrolyzed carboxylate species is undesirable
because of their tendency to polymerize and form diffi-
cultly soluble precipitates. This may lead to the loss of
the scintillator transparence with time. Therefore, to
minimize the Gd hydrolysis in the course of thermal
drying, it was necessary to preliminarily remove the
major amount of water from Gd(2MVA)3 under “mild”
conditions when hydrolysis is less pronounced. Drying
Fig. 2. IR spectra: (1) Gd(2MVA)3 prepared by reaction (2)
and heated for 2 h at 140°С, (2) H2MVA, (3) Gd(OH)x·
(2MVA)3–x prepared by reaction (3) and heated for 2 h at
140°С, and (4) Gd(OH)3.
RADIOCHEMISTRY Vol. 51 No. 3 2009

EXTRACTION METHODS IN DEVELOPMENT …: 2.
279
Table 4. Light output relative to anthracene and transparence of solvents intended for preparing scintillators
Scintillation additive, Scintillator density, Light output relative to Transparence
Solvent
g l–1
g ml–1
0.88
anthracene, %
L430 nm, m
12.0
30.0
10.0
0.2
TMB
ВРО (4)
80.0
33.0
33.0
–
Dodecane
Mineral oil
ВРО (4)
0.75
ВРО (4)
0.838
1.01
1-Methylnaphthalene
–
TMB(10%) + dodecane (90%)
TMB(15%) + dodecane (85%)
TMB(20%) + dodecane (80%)
TMB(35%) + dodecane (65%)
TMB(20%) + mineral oil (80%)
TMB(30%) + mineral oil (70%)
РХЕ (20%) + dodecane (80%)
–
0.764
0.771
0.78
–
20.0
24.0
24
–
–
ВРО (4)
–
61.0
65.0
52.0
63.0
–
0.8
27.0
10.0
10.0
9.0
ВРО (5)
ВРО (5)
–
0.846
0.851
0.798
in an air flow at room temperature in a thin bed
(~5 mm) for 3–4 days allowed the moisture content to
be decreased from 61 to 0.4–0.5%. After that, the dried
precipitate was placed in an oven and heated for 2 h at
140°С. The moisture content of the final product did
not exceed 0.01%. The solubility of Gd(2MVA)3 pre-
pared by this procedure was as follows (g l–1): in TMB,
0.36; in PXE, 0.02; in dodecane, 0.17; in octane, 0.98;
in hexane, 0.175; in water, 1.58; in acetone, 0.078; in
anhydrous ethanol, 0.94; in 20% TMB + 80% do-
decane mixture, 0.33; and in 20% PXE + 80% do-
decane mixture, 0.22. It should be noted that the solu-
bility of Gd(2MVA)3 in organic solvents appreciably
varies with time. For freshly prepared samples, it is
higher than for samples dried at elevated temperatures
and stored for a certain time.
tion constants of rare-earth elements with these extrac-
tants.
Properties of Gd-Loaded LOSs Prepared
by Dissolution of Anhydrous Gadolinium
2-Methylvalerate in Organic Solvents
The most frequently used organic solvent for pre-
paring scintillators is TMB [7]. At the same time,
TMB has a number of significant drawbacks restricting
its use in the limited space of underground laboratories
and near nuclear reactors: low flash point (49°С), tox-
icity, poor compatibility with organic glass used as
structural material of the detector, and low Н : С ratio.
The flash point of pure PXE is high (149°С), but it has
low Н : С ratio and high viscosity and density; further-
more, it poorly dissolves gadolinium carboxylates.
In any case, to prepare a scintillator it was neces-
sary to increase the solubility of Gd(2MVA)3 to at
least 1 g l–1. It was noted previously [1] that the readily
soluble Gd complex with H2MVA is Gd(2MVA)3·
3H2MVA. Therefore, to anhydrous Gd(2MVA)3 we
added a stoichiometric amount of anhydrous H2MVA
(after vacuum distillation) and appropriate organic sol-
vent. By so doing, we prepared an organic phase with
the Gd concentration [Gd](o) > 50 g l–1. Further studies
showed that additions of neutral organophosphorus
compounds (NOPCs) such as phosphates, phospho-
nates, and phosphine oxides also led to an increase in
the solubility of Gd(2MVA)3 in organic solvents. The
stability of the optical constants of the scintillators de-
creased in the order phosphine oxide > phosphonate >
phosphate, in accordance with variation of the extrac-
The properties of the above solvents can be im-
proved by mixing them with paraffin hydrocarbons
(dodecane, mineral oil). Some properties of solvents
that we used for preparing Gd-loaded LOSs are given
in Table 4. The final choice of РРО as scintillation ad-
ditive is governed by its availability and relatively low
cost.
It was important to know whether gadolinium
2-methylvalerate introduced into an organic solvent
would affect the energy transfer from the solvent to the
scintillation additive. For this purpose, we recorded the
absorption spectra of Gd(2MVA)3 (1.2 g l–1) and РРО
(0.01 g l–1) in cyclohexane in a 1-cm-thick cell
(Fig. 3). Comparison of these spectra at 300 nm gave
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DANILOV et al.
Table 5. Influence of the storage time of Gd-loaded LOS samples on their transparence. HR is 2-methylvaleric acid, and
TIAPO is triisoamylphosphine oxide
Solvent
TMB
Gd concentration, g l–1
Stabilizing additive
–
Storage time, days
L420 nm, m
2.0
2.0
2.0
2.0
0
40
285
450
8.0
10.0
14.0
8.0
1.4
1.4
1.4
0
40
220
12.0
20.0
17.0
GdR3·1HR
1.4
1.4
1.4
1.4
0
40
285
450
15.0
20.0
20.0
10.0
GdR3·2HR
GdR3·3HR
GdR3·2HR
TMB (35%) + dodecane(65%)
2.1
2.1
2.1
2.1
0
40
285
450
17.0
18.0
20.0
14.0
1.5
1.5
1.5
1.5
0
125
480
661
15.0
16.0
18.0
14.0
TMB (20%) + dodecane(80)% +
PPO (1.5 g l–1)
1.6
1.6
1.6
1.6
1.6
1.6
0
210
0
210
0
210
16.0
11.0
14.0
11.0
10.0
8.0
GdR3·1TIAPO
GdR3·2TIAPO
GdR3·3TIAPO
PXE (20%) + dodecane(80)%
the following results: light absorption by РРО about
redistribution between the solvent and the scintillation
additive. Therefore, the spectra of the pure and Gd-
loaded solvents differ insignificantly.
1.2 × 102 and that by Gd(2MVA)3 about 0.15.
Evidently, the light absorption caused by the pres-
ence of Gd(2MVA)3 is negligibly low compared to the
light absorption by the scintillation additive. Hence,
introduction of Gd into LOS does not affect the energy
The stability of Gd-loaded LOSs was studied by
intermittent sampling during a long period and measur-
ing the transparence and light output of the samples.
The scintillator degradation can be judged from varia-
tion of these parameters with time. Data in Table 5
show that the high transparence of the scintillators is
preserved after prolonged storage at room temperature.
The use of a scintillating additive (РРО) does not af-
fect the scintillator stability (Table 5). At the same
time, at elevated temperatures (50°С), noticeable deg-
radation of the scintillator is observed in the short-
wave region of the spectrum (Fig. 4a). It increases in
the case of contact of the scintillator with possible
structural materials of the detector, stainless steel and
organic glass (Fig. 4b), with a mixture of TMB with
dodecane used as solvent. As the mixture of PXE with
dodecane is better compatible with organic glass, this
Fig. 3. Absorption spectra of (1–3) Gd(2MVA)3 and
(4) РРО in cyclohexane. Concentration, g l–1: [Gd](o): (1)
1.2, (2) 1.85, and (3) 2.7; РРО: (4) 0.01.
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EXTRACTION METHODS IN DEVELOPMENT …: 2.
281
Fig. 6. Transmission spectra of solutions of Gd(2MVA)3·
3TIAPO in 20% РХЕ + 80% dodecane mixture in relation
to the time of storage at 40°С (nitrogen atmosphere, protec-
tion from daylight). Storage time, days: (1) 0, (2) 33, (3)
60, (4) 130, (5) 174, (6) 214, (7) 242, (8) 350, and (9) 398.
Fig. 4. Absorption spectra of solutions of Gd(2MVA)3·
3TIAPO in 35% TMB + 65% dodecane mixture, [Gd](o) =
3.6 g l–1: (1) after preparation and (2) after keeping for 30
days at 50°С. Storage: (a) in glass vessels without contact
with structural materials of the detector and (b) in the
presence of organic glass and stainless steel.
any changes in their absorption spectra. At the same
time, the optical properties of the samples stored at
40°С under identical conditions (in sealed vessels un-
der nitrogen, with protection from light) became worse
(Fig. 6). Taking into account the fact that the tempera-
ture in underground rooms where the detectors are in-
tended to be arranged will be maintained on the level
of +20°С, the scintillators obtained can be recom-
mended for detection of reactor antineutrino. After
successful completion of tests of Gd-loaded LOSs with
small volumes (~1 l), we prepared an enlarged batch of
the scintillator (32 l) with a gadolinium concentration
of 0.85 g l–1 (≈0.1%) and delivered it to Saclay
(France) for testing the compatibility of the scintillator
with the structural material of the detector (organic
glass). The scintillator was placed in a vessel of Acryl
GS-233 organic glass (Rhome), purged with nitrogen,
hermetically sealed, and shielded with a dark material
to protect from daylight. Samples will be withdrawn
through a special hole at 6-month intervals during a
long period to measure the scintillator transparence.
The first four measurements have not revealed any
signs of the scintillator instability.
Fig. 5. Transmission spectra of solutions of Gd(2MVA)3·
3TIAPO in 20% РХЕ + 80% dodecane mixture. Time of
storage at 20°С (nitrogen atmosphere, protection from
daylight) 0, 33, 60, 130, 174, 214, 242, 350, and 398 days
(the spectra coincide).
mixture was our final choice. The use of NOPCs as
additives allows preparation of stable scintillators with
this mixture. Long-term tests in Gran Sasso National
Laboratory (Italy) and Saclay (France) showed that
storage of scintillator samples based on Gd(2MVA)3·
nTIAPO ) (n = 1, 2, 3) at 30°С (Fig. 5) did not lead to
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