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Towards a better energy resolution of alpha liquid scintillation cocktails
applied for environmental analyses
J. AUPIAIS1, C. AUBERT1, J.C. MIALOCQ2, A REBOLI1
CEA, DASE/RCE, Centre de Bruyères-le-Châtel, BP 12, 91680 Bruyères-le-Châtel, France.
2
CEA, DRECAM/SCM/URA 331 CNRS, Centre de Saclay, 91191 Gif-sur-Yvette, France.
Many authors have demonstrated
the interest of α liquid scintillation with
pulse shape discrimination for the
measurement of α emitters in environmental samples. Nevertheless, this
technique suffers from a lack of resolution.
The enhancement of the resolution is
a great challenge for the coming years
to promote this technique as a reliable
and
robust
analytical
method
in the determination of α emitters
in
environmental
samples.
Two
complementary issues are possible:
the first is chemical by preparing new more
efficient cocktails in terms of light
emission and pulse shape discrimination,
the second is technological by testing a
new generation of photomultiplier tubes
like hybrid P.M. [1] or avalanche diodes.
The light pulse emitted by the cocktail has
two components: a prompt signal resulting
mainly from a “singlet pathway”
and a delayed signal, which also involves
triplet states. It is necessary to understand
the mechanisms of energy transfer in those
scintillating cocktails. The “singlet
pathway” involves spin-allowed transitions
of the donor (D) and the acceptor (A)
singlets S*1 (D) + S0 (A) → S0 (D) + S1* (A) (1)
and is well described by the Förster theory
of long range Coulombic energy transfer
[2] which states that the rate constant
of energy transfer is proportional
to the square of the dipole – dipole
interaction
energy.
The
latter
is proportional to the magnitude
of the dipoles and inversely proportional
to the third power of the distance between
both molecules.
The critical distance increases with
the
fluorescence
quantum
yield
0
of the donor φD ( τD = φD × τD ) and the
overlap of the spectra
R 60 =
9000 ln 10κ 2φ D
dν
f D (ν )ε A (ν ) 4
128π6 n 4 N
ν
∫
(2)
Scintillating cocktails
The toluene-naphthalene-PBBO scintillating
cocktail
(Alphaex)
shows
at the present time the better energy
resolution amongst all the commercial
cocktails used. This is due to the very
good coupling between the toluene
and naphthalene on the one hand,
naphthalene and PBBO on the other.
The efficient energy transfer in each donor
– acceptor couple is related to the
strong overlap of their absorption
and fluorescence spectra as shown
in Figure 1.
1.0
C10H8
abs. spectrum
C6H5-CH3
fluo. spectrum
PBBO
abs. spectrum
0.9
0.8
Intensity (A.U.)
1
0.7
0.6
0.5
PBBO
fluo. spectrum
0.4
0.3
C10H8
fluo. spectrum
0.2
0.1
0.0
250
300
350
400
450
500
Wavelength (nm)
Fig. 1. Fluorescence and absorption spectra of
Alphaex compounds.
Following the toluene solvent radiolysis
and the recombination of the resulting ions,
the longer-lived S1 excited singlet states
and the T1 triplet states undergo radiative
and non-radiative deactivation processes
(fluorescence, internal conversion, intersystem crossing to the triplet state, singlet
– singlet energy transfer to naphthalene).
The T1 states undergo triplet – triplet
annihilation mainly in the spurs and triplet
– triplet energy transfer to naphthalene
T1(toluene) + S0 (naphthalene) → S0
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Φ-CH3
ΦCH3
0.002
*
C10H8
C10H8
2.12
0.005
*
0.001
CH3C10H8
DIN
PBBO
PPO
3 HF
CH3C10H8
5.32
0.008
*
PBBO
PPO
3-HF
24.74
23.82
11.39
92.50
26.53
37.77
100.07
24.54
43.50
96.25
3.54
21.20
44.26
2x10
3
1x10
3
5x10
2
239
Pu
Cm
0
300
350
400
450
500
550
600
650
Channel number
Fig. 2. Actinides spectrum using Alphaex (solid)
and p-xylene-C10H8-PBBO mixture (dash).
Detectors
New detectors present better efficiencies
than that of common photomultiplier tubes.
Thus, the hybrid photomultiplier presents
a quantum efficiency better than 40 %
(instead of 30 % for the better P.M.) while
the efficiency for avalanche diodes is near
100 %. This is particularly interesting since
an improvement by a factor 2 of
the resolution is theoretically achievable.
The main drawbacks are the small size (up
to 1 cm2) and a sensitivity rather in
the 500-800 nm range instead of 350-450
nm for the P.M used in α and β
scintillation. It must be also noticed that
the response is more constant along a large
range of wavelength, which is not the case
for the common P.M. (Figure 3).
100
BLUE
90
1.54
80
0.13
*: in cyclohexane, else in toluene.
Table 1. Overlaps calculations in a donor – acceptor
dν
transfer f D (ν ) ⋅ ε A (ν ) 4 ×10-27 (in m6 mole-1) (this
ν
work).
∫
2x10
3
U
244
70
Effective QE (%)
acceptor →
↓ donor
233
Count number
(toluene) + T1(naphthalene) (3) via an
exchange mechanism when the toluene
triplets have diffused away from the spurs.
The naphthalene S1 singlet states resulting
from the toluene to naphthalene singlet
– singlet energy transfer undergo a very
efficient intersystem crossing to the triplet
state. A large concentration of long-lived
naphthalene triplet states is thus obtained
via two different pathways. The most
important tool to estimate the efficiency
of scintillating cocktails is the overlap
calculation between the fluorescence
spectrum of a donor and the absorption
spectrum of the acceptor. Higher is
the overlap, higher is the critical distance
R0 (Eq. 2) and therefore better is
the probability for an energy transfer. By
calculating overlaps for a large number
of donor – acceptor couples (see Table 1),
it is possible to find more efficient
cocktails. For instance, by replacing
toluene by p-xylene, the resolution
has been improved by about 12 %
[3](see Figure 2).
UV
60
50
40
30
P.M. Perals
20
10
0
200
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Fig. 3. Quantum efficiency of a common P.M.
(used in the Perals spectrometer) and 2 avalanche
diodes UV and blue sensitive, respectively.
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The utilisation of avalanche diodes
requires scintillators having a very high
Stoke shift since aromatic molecules
composing the cocktails have fluorescene
and absorption spectra in the UV range.
For instance, the 3-hydroxyflavone (3-HF)
is a possible choice with a maximum
wavelength of fluorescence near 550 nm
(Figure 4). Moreover, its absorption
spectrum overlaps quite well the fluorescence
spectrum
of
naphthalene.
This suggests a high efficiency and its
possible use jointly with avalanche diodes.
Φ-CH3 fluoresc.
C10H8 abs.
1.0
3-HF abs.
3-HF fluoresc.
Intensity (A.U.)
0.8
0.6
0.4
C10H8 fluoresc.
0.2
0.0
200
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 4. : Scintillation mixture – toluene –
naphthalene – 3-hydroxyflavone
In conclusion, the improvement of the
resolution by a factor 2 is possible.
From a technological point of view,
it is necessary to optimise the geometry
of counting (light pipe up to the avalanche
diode, geometry of the white reflecting
surface, etc.). The solutions of all technical
problems are under progress.
[1] Cassette, P.; Monnard, E. NIM A 1999, 422,
119-123.
[2] Förtser, T. Naturforsch. 1949, 4a, 321-327.
[3] Aubert, C. Thesis, University or Paris XI, 2002,
pp 280.