Applied Radiation and Isotopes ] (]]]]) ]]]–]]]
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Applied Radiation and Isotopes
journal homepage: www.elsevier.com/locate/apradiso
Investigations on TDCR measurements with the HIDEX 300 SL using a free
parameter model
Carsten Wanke, Karsten Kossert n, Ole J. Nähle
Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
a r t i c l e i n f o
Keywords:
TDCR method
Liquid scintillation counting
Activity standardization
abstract
The Triple to Double Coincidence Ratio (TDCR) method requires a special counting system with three
photodetectors. The systems currently used by National Metrology Institutes for activity standardizations are
custom built, and up to now the HIDEX 300 SL counter1,2 is the only TDCR counter commercially available.
At PTB, measurements with a special metrology version of this counter were carried out to investigate its
applicability for activity standardizations. The activity results of measurements with the HIDEX counter are
compared to those obtained with a PTB–TDCR counter, as such a comparison reduces the model dependence.
In addition, a spectrometry method was applied to measure 109Cd samples and a new TDCR-Čerenkov
method was tested with 32P samples.
& 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Liquid scintillation (LS) counting in combination with the TDCR
method is a powerful method for activity standardization and can be
applied to a number of radionuclides (Broda et al., 2007). The method
is based on a free parameter model describing the process of light
emission and detection in a scintillation counter. For the application
of the method, an LS counter with three photomultiplier tubes (PMT)
A, B and C, and suitable coincidence electronics are required. For a
large number of detected events, the ratio of the triple and the double
coincidence counting rates T/D converges towards the ratio of
calculated counting efficiencies eT(l)/eD(l) which depend on the free
parameter l. Thus, the free parameter can be derived from experimental information and the counting efficiencies can be calculated. It
is to be noted that the TDCR value is neither a quenching indicator
nor an efficiency value. The latter assumption is implicitly made in
the Hidex control software output, where the result ‘‘disintegrations
per minute’’ (DPM) is given assuming that the TDCR value is equal to
the double efficiency. This value can be wrong by more than 50% for
nuclides like 3H or 55Fe (Cassette, 2010). Taking the TDCR value as a
quenching indicator is only applicable if the relation between the
TDCR value and the efficiency is unambiguous. This is fulfilled for
n
Corresponding author. Tel.: þ49 531 592 6110; fax: þ 49 531 592 6305.
E-mail address: karsten.kossert@ptb.de (K. Kossert).
1
Manufactured by HIDEX Oy, Finland.
2
Certain commercial equipment, instruments, or materials are identified in
this paper to foster understanding. Such identification does not imply recommendation by the Physikalisch-Technische Bundesanstalt, nor does it imply that the
materials or equipment identified are necessarily the best available for the
purpose.
pure beta emitters but not in the case of radionuclides decaying by
electron capture (EC) or radionuclides with complex decay schemes,
where up to three efficiency values can correspond to the same TDCR
value (Cassette, 2010).
Up to now, all TDCR counters used by national metrology
institutes for activity determinations have been custom built. The
HIDEX 300 SL system is the first commercial TDCR counter and is
equipped with an automatic sample changer which makes measurements of a large number of samples very convenient. The
development and construction of a home-made TDCR counter can
be rather challenging, in particular, when an automated samples
changer is needed. Thus, it is of great interest to investigate the
applicability of the Hidex counter for activity measurements. The
first outcome of such an investigation is described in this article.
2. Experimental details and procedures
The HIDEX 300 SL installed at PTB is equipped with three
Electron Tubes 9102KA photomultipliers. The mounting positions
of the multipliers are denoted as R, S and T. The system has a lead
shielding and an automatic sample changer. The samples are
provided via an aluminum rack with 40 positions for 20 mL-vials;
a rack with 96 positions for 7 mL-mini vials can also be installed.
The system is computer controlled via serial connection. For all
measurements presented in this report, the instrument has been
operated using the software ‘‘Commfiler.csv’’ supplied by HIDEX.
The system installed at PTB is a modified version of this
counter referred to as the metrology version. It offers a special
counting mode, denoted as the ‘‘METRO’’ mode, which delivers all
0969-8043/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apradiso.2012.02.097
Please cite this article as: Wanke, C., et al., Investigations on TDCR measurements with the HIDEX 300 SL using a free
parameter model. Appl. Radiat. Isotopes (2012), doi:10.1016/j.apradiso.2012.02.097
2
C. Wanke et al. / Applied Radiation and Isotopes ] (]]]]) ]]]–]]]
information necessary for applying the TDCR method in a metrological sense. This means, in particular, that all events (including
single, double (3 pairs) and triple detections, as well as the logical
sums of double coincidences and single counts) are recorded.
Unless otherwise stated, the measurements presented were
performed in the ‘‘METRO’’ mode. The exact mechanism of signal
processing is not known to the authors.
The system underwent several changes in electronics, until a final
setup of the system was reached. Linearity measurements with the
first setup showed the need for a better dead-time/live-time determination. Hence, new electronics were designed by Hidex and
subsequently improved according to the analysis of the system at
PTB. The newer designs include a dead-time determination circuit
based on a reference oscillator with a compensation circuit, which
can be fine-tuned via a potentiometer. The exact function is unknown
to us, the results after proper tuning are, however, reasonable.
To assess the applicability of the Hidex 300 SL system for
metrological measurements, comprehensive measurements
against a reference were performed. The reference was realized
by measurements of the same samples with the PTB–TDCR
system (Nähle et al., 2010). For processing the analog signals,
conventional NIM electronics are used and the digital signal
processing including live-time correction is carried out by the
MAC3 module developed at LNHB (Bouchard and Cassette, 2000).
Currently, a new type of a coincidence module developed by PTB,
which is based on FPGAs, is also being tested.
The reference solutions used for the preparation of the
samples were calibrated by other standardization methods, i.e.
LS counting using the CIEMAT/NIST method, 4pb-g coincidence
counting, and ionization chamber measurements. The activity
concentration of 241Am was determined by liquid scintillation
counting (LSC), assuming a counting efficiency of 1 for this alpha
emitter (see Kossert et al., 2009a), and confirmed by ionization
chamber measurements. The samples used for the measurements
consisted of 15 mL LSC cocktail, weighed aliquots of the active
solution and an inactive solution (usually H2O, depending on the
chemical composition of the active solution) to come to a total
volume of 16 mL. The LS cocktail was chosen to be suitable for the
chemical composition of the active solution. Ultima GoldTM was
used in most cases. The masses of the active solutions were
determined gravimetrically using two Mettler balances traceable
to the German national mass standard. Low potassium borosilicate vials with a volume of 20 mL were used in most cases. When
a higher light output was desirable (e.g. for 55Fe samples),
polyethylene (PE) vials were used. All samples were shaken and
centrifuged before starting the measurements.
For the application of the TDCR method as described above, the
raw data were converted to the standard PTB-format for TDCR data
and then analyzed with procedures as described by Nähle and Kossert
(2011). For some radionuclides, the efficiency was calculated with an
extended version of MICELLE2 (Kossert and Grau Carles, 2010).
MICELLE2, an enhanced version of MICELLE (Grau Carles, 2007), is a
program for the calculation of LS counting efficiencies via Monte
Carlo simulations. It uses a stochastic approach for b and b þ
branches and a stochastic atomic rearrangement model for the EC
branches. Since the individual counting efficiencies of the 3 detector
channels can be different (e.g. due to different quantum efficiencies of
PMTs) the method actually requires 3 different parameters which can
be determined by minimizing the function
e
T 2
eT
T 2
eT
T 2
D¼ T þ
þ
,
eAB AB
eBC BC
eAC AC
tables of double and triple efficiencies as a function of two or three
parameters, respectively. These tables can then be used for 2D and 3D
efficiency interpolation, which allows asymmetries of the PMT
efficiencies to be taken into account. The measurement evaluation
is performed with another separate, PTB-developed program that
relies on the efficiency tables calculated.
For all calculations, a constant kB¼0.0075 cm/MeV was used to
compute the ionization quenching function. In general, the results of
activity measurements should be independent of the free parameter,
i.e. measurements with different counting efficiencies should yield
the same result. An efficiency variation is realized using a series of
samples with different degrees of chemical quenching. The influence
of using a different kB value on the activity results should be
considered in the uncertainty budget of activity measurements. When
ratios of measurement results of two systems are considered, as done
in this paper, the influence of the kB value is negligible.
Corrections for dead time and decay were applied, using halflives from Schötzig and Schrader (2000).
Several uncertainty components were identified as relevant for
the results presented in this paper: a statistical component, the
uncertainty assigned to the efficiency as well as the uncertainty
due to background variations. These components are calculated
for the measurement results both with the HIDEX system and the
PTB–TDCR system, and are then used to calculate the uncertainty
of the deviation between the results of the two systems.
Each sample was measured several times, and for each sample,
the arithmetic mean and its standard deviation are calculated.
From the results of all samples, the weighted mean is used as the
final result. The maximum of the inner and outer uncertainty is
taken as the corresponding statistical uncertainty.
To estimate the uncertainty assigned to the counting efficiency, the mean TDCR value as well as its standard deviation are
calculated for each sample. The uncertainty of the efficiency is
then calculated and propagated as the uncertainty component for
this individual sample. The maximum uncertainty value of all
samples is regarded as the uncertainty component assigned to the
efficiency for the activity concentration.
The third component relevant here is the uncertainty due to
background variation. It is calculated as the standard deviation of
the mean of 6 or more repeated background measurements. The
background not only influences the count rates, but also the TDCR
values and, hence, the efficiency values derived from the TDCR
values. Both the uncertainty component of the count rates and the
uncertainty component of the efficiency due to background
variation are included. Correlations between the triple and double
background count rates are taken into account.
The uncertainty due to dead time and system linearity can be
estimated from the linearity checks, Section 4. It is not considered in
the results presented here, but should be considered for activity
certification statements. The uncertainty due to weighing is calculated according to the calibration certificate of the balance used for
mass determinations. It is neglected in the following when ratios
of the results of measurements with the HIDEX 300 SL and the PTB–
TDCR are considered, as it is common to both results. As the time
between the measurements in both counting systems was usually
much less that one half-life of the respective radionuclide, the
uncertainties for decay corrections can also be neglected, as well as
other uncertainties common to both measurement systems like
potential radionuclide impurities, adsorption or decay scheme
parameters.
see e.g. Broda et al. (2007). Here also the coincidence counting rates
between each pair of PMTs AB, BC and AC are used. MICELLE2 was
therefore extended, so that all electron emission events are written to
files. These files are used by a newly developed program to compute
3. Setting up the HIDEX 300 SL
Three important parameters can be influenced using the
control software of the system: bias voltages, the threshold and
Please cite this article as: Wanke, C., et al., Investigations on TDCR measurements with the HIDEX 300 SL using a free
parameter model. Appl. Radiat. Isotopes (2012), doi:10.1016/j.apradiso.2012.02.097
C. Wanke et al. / Applied Radiation and Isotopes ] (]]]]) ]]]–]]]
Fig. 1. Single count rates in the PMTs over the threshold setting in the measurement of
the coincidence time. At the hardware level the dead-time
circuitry can be fine tuned provided that a reference for linearity
tests is available (cf. section 4). Optimized settings of the coincidence time and the threshold are crucial for the TDCR method.
Therefore, measurements have been carried out to find the
settings for which the deviations of the activity results to those
of the PTB–TDCR are minimized.
Setting the bias voltage (HV) for the three multipliers is done
with the aid of the counting protocol ‘‘HVset’’. An unquenched 14C
sample is necessary for this procedure, which is described in the
software reference. The principle of the HV adjustment is to yield
the same QPE (quench parameter external) value in all single
channels. The QPE value is derived from the spectrum, it is
defined as the 99th percentile of all counts in the spectrum
(Aalto, 2009). The goal of the HV adjustment is to get the same
gain for all three PMTs. This is necessary for a correct evaluation
of the measurements by the TDCR method, as the counter only
offers one common threshold setting for all three PMTs. In the
latest version, an automatic HV adjustment is also possible.
Following the manual adjustment instructions leads to practically
the same results as using the automatic procedure, which is more
convenient to handle. The design of the electronics demands that
the bias voltages fulfill the condition (UR-200 V) rUS,T rUR,
which means that the PMTs in position S and T cannot have
higher bias voltages than that in position R, and they may only be
lower by a maximum of 200 V. In the selection of the PMTs, it
must be ensured not only that the photon detectors are matched
closely, but that the PMT with the lowest intrinsic efficiency is
mounted in position R. In the counter installed at PTB, the PMTs at
R and S are very similar, while the PMT at T seems to have a
higher intrinsic gain and is, therefore, used with a lower bias
voltage.
Generally, in TDCR measurements the counts in the whole
spectrum are integrated, and the threshold must be low enough
so that all events in the single electron peak are registered. If, on
the other hand, the threshold is set too low, noise from the PMT is
counted, which leads to high dead time and might disturb the
measurements.
In the service manual (Hidex Oy, 2009) and the software
reference (Aalto, 2009), a value of 40 is stated as a ‘‘normal
value’’. With the EEPROM version 1.56, the value was preset to
100. Subsequent measurements showed that this threshold can
3
63
Ni.
be set lower without increasing noise. A value of 45, however,
resulted in the counting of noise, making correct measurements
impossible. This can be seen in Fig. 1. Therefore, a threshold value of
50 was chosen, to be as low as possible, but without counting noise.
Fig. 2 shows the influence of the threshold settings on the
measurement results for 63Ni. This nuclide has been chosen,
because the beta energies are high enough for the TDCR model
to work well, but low enough for the measurement to be sensitive
to imperfect settings. For a threshold setting of 50 the deviations
from the PTB–TDCR result are reduced to a minimum.
The influence of the coincidence time setting has been studied
using samples of 99Tc and 63Ni. In contrast to the PTB–TDCR system,
which uses a fixed coincidence resolving time of 40 ns, the coincidence time of the HIDEX 300 SL can be selected. Therefore,
measurements have been performed with the coincidence time
varied, while all other parameters are kept constant. The threshold
was set to 50 (see previous section) and not changed for these
measurements, because the influence of the coincidence time is
expected to be more prominent for lower threshold settings.
The results for 63Ni and 99Tc plotted against the coincidence
time are shown in Fig. 3. The uncertainty bars plotted comprise
contributions due to statistical variation, the uncertainty assigned
to the efficiency and to the background count rates both from the
HIDEX and the PTB–TDCR system. A rather small deviation for
63
Ni is in the order of 0.3% is found for a setting of 30 ns, which
was used for many measurements at the beginning of this study.
For 99Tc, the deviations are generally significantly lower. This can
be attributed to the higher endpoint energy of 99Tc, which causes
a higher number of emitted photons and, hence, a higher detection efficiency, which leads to a better signal definition and a
narrower time distribution of the signals.
Shortening the coincidence time can lead to a rejection of true
coincidences, which in turn might cause false TDCR values and,
hence, an incorrect efficiency. There is also some discussion on
more subtle effects especially when measuring low energy
nuclides and on the proper setting of the coincidence time in
these cases. The increasing deviation with a lower coincidence
time setting is probably due to a loss of true triple coincidences,
which results in a lower TDCR value and, hence, a lower assigned
efficiency, which then leads to an activity value that is too high.
At the beginning of this study, the coincidence resolving time
was adjusted to 30 ns. With this setting and a threshold of 50, a
Please cite this article as: Wanke, C., et al., Investigations on TDCR measurements with the HIDEX 300 SL using a free
parameter model. Appl. Radiat. Isotopes (2012), doi:10.1016/j.apradiso.2012.02.097
4
C. Wanke et al. / Applied Radiation and Isotopes ] (]]]]) ]]]–]]]
Fig. 2. Deviations in the measurement of
63
Fig. 3. Deviations in the measurement of
Ni—influence of the threshold setting. The coincidence time was 30 ns.
63
Ni and
99
Tc over the coincidence time setting. The threshold was 50.
reasonable agreement with the results of the PTB–TDCR counter
is achieved. Later, the usage of a longer coincidence time was
investigated, as presented below in this article.
4. Linearity measurements and dead-time correction
Linearity tests to check the count-rate dependence of the
results were performed using samples of 241Am. The linearity of
the system depends on the correct determination of the dead
time, so the tests described here are also checks for a correct
dead-time measurement.
Samples containing 241Am with expected count rates of
approx. 106, 5 105, 2 105, 1.4 105, 1 105, 5 104, 2 104,
1.4 104, 1 104, 5 103, 2 103, 1.4 103 and 1 103 counts
per minute were prepared as described in Section 2, Ultima
GoldTM AB was used as LS cocktail.
Measurements with the counter as it was installed revealed
significant deviations of the results from the expected values.
With the later versions of the analyzer board supplied by Hidex, a
tuning of the dead-time correction is provided (Aalto, 2010),
however, without further explanations or any tuning instructions.
Therefore, the fine-tuning was done using the 241Am samples, so
that the deviations in the most prominent count rate region for
our measurements (i.e. 1 105–2 105 counts per minute) are
less than 70.2%. The results obtained with the current configuration and dead-time compensation setting are shown in Fig. 4.
It seems that the deviation is not perfectly constant over the
whole range investigated. In our experience, however, other
commercially available LS systems show similar behavior.
From this it can be concluded that the system is suitable for
activity measurements with low uncertainties, but a further
improvement of the linearity of the system installed at PTB is
desirable for metrological applications.
Please cite this article as: Wanke, C., et al., Investigations on TDCR measurements with the HIDEX 300 SL using a free
parameter model. Appl. Radiat. Isotopes (2012), doi:10.1016/j.apradiso.2012.02.097
C. Wanke et al. / Applied Radiation and Isotopes ] (]]]]) ]]]–]]]
Fig. 4. Linearity measurements—Deviations of the measured count rates from the expected ones in the measurement of
uncertainties (see text).
5
241
Am. The bars indicate only statistical
Fig. 5. Deviation D of the activity concentration derived from measurement data of the HIDEX counter and the PTB–TDCR system vs. average beta-energy. The coincidence
time was 30 ns and the threshold was 50.
5. Measurements of different nuclides and comparison with
the PTB–TDCR system
Several nuclides have been measured, and the results are
compared to those from measurements with the PTB–TDCR
system. This reduces the model influence of the TDCR method,
as the results are gained by the same evaluation procedure. For
correct activity determinations, however, further uncertainty
components have to be taken into account. The measurements
comprised radionuclides decaying by b , b þ , or EC as well as
nuclides with complex decay schemes. All samples are measured
with a fixed counting time, usually 600 s or 900 s and several
repetitions for each sample. Further count terminators are not set.
The ROI comprises channel 0–1023, i.e. the whole spectrum.
The measurements were evaluated applying the TDCR method.
The evaluations are done with and without taking asymmetries of
the PMTs into account. However, the differences between
symmetrical and asymmetrical evaluation are low (in the order
of 0.1% or less, see below).
In the following, results from measurements after March 2010 are
presented, i.e. measurements with the two latest analyzer board
designs. The results are given as relative deviations D, calculated as
D¼
aHIDEX aPTB
:
aPTB
Fig. 5 shows the results from those measurements with
optimized threshold and coincidence time settings as deviations
from the reference values plotted over the average beta energy.
The average beta energy can be seen as a measure for the
expectation value of the amount of light produced in a decay
event. The average beta energies are calculated from data taken
from TdeR (2010) and Nica (2009), respectively. Chlorine-36 and
204
Tl are included here although they are not pure beta emitters,
as the respective EC branches have only a low probability. For the
Please cite this article as: Wanke, C., et al., Investigations on TDCR measurements with the HIDEX 300 SL using a free
parameter model. Appl. Radiat. Isotopes (2012), doi:10.1016/j.apradiso.2012.02.097
6
C. Wanke et al. / Applied Radiation and Isotopes ] (]]]]) ]]]–]]]
Fig. 6. Deviation D of the activity concentration derived from measurement data of the HIDEX counter and the PTB–TDCR system vs. average beta-energy. The coincidence
time was 35 ns and the threshold was 50.
comparison of 177Lu results, the analysis was carried out without
a correction for asymmetry. Details about the calculation of the
efficiency of 177Lu are described in Kossert et al. (2011a). Also for
32
P the small correction due to PMT asymmetries was neglected.
Fluorine-18 is a positron emitter with a weak EC branch and,
therefore, does not fit into the diagram of b emitters. The
deviation of the HIDEX result to that of the PTB–TDCR system is
þ0.21%, and it matches perfectly well with the value obtained by
measurements applying the CIEMAT/NIST method.
Tritium decays by b emission with an endpoint energy of
18.6 keV and an average beta energy of 5.7 keV. Iron-55 is an EC
nuclide with relevant Auger and X-ray energies of about 0.5 keV
and 5–6 keV. In the determination of activities of both nuclides,
model dependencies have a significant influence, since both
nuclides emit electrons of low energy, which do not produce a
large number of photons. Measurements of these nuclides can be
seen more as a test of the TDCR method and the model used than
as a test of a particular detector system. Generally speaking, the
TDCR models currently used may become inaccurate when the
expected mean number of photons in the PMT is lower than about
1 (Cassette et al., 2000). This is the case for TDCR values of about
0.3 and lower, which we find in the measurement of 55Fe and 3H.
Up to now, there is no consensus about the necessary consequences for the models used, see e.g. Broda (2008) and Simpson
et al. (2010). The deviations between the Hidex-TDCR and the
PTB–TDCR results were found to be about þ0.1% for 3H and
þ0.22% for 55Fe. Both results can be regarded as good since both
radionuclides create only a few scintillation photons. For some
measurements the LS samples with 3H were surrounded with an
adhesive tape ‘‘tesas Film matt-unsichtbar’’ (width: 19 mm, tesa
AG, Germany) to increase the counting efficiency and to decrease
the model dependence. For 55Fe, PE vials were used which even
yield a higher counting efficiency.
With the above-mentioned settings, the dependence on the beta
energy (and hence the amount of light produced in the scintillations),
is negligible (Fig. 5). The results are slightly higher than those
obtained with the PTB–TDCR system. In most cases the deviations
are in the order of about þ0.25%. These deviations might be due the
imperfect linearity of the Hidex system and/or due to too short
coincidence resolving time. With a longer coincidence time setting of
35 ns, the deviations become indeed lower, see Fig. 6. As noted above,
the PTB–TDCR system is equipped with a MAC3 module using a fixed
coincidence resolving time of about 40 ns. The lower dependence on
the coincidence resolving time in the region above 40 ns, as visible in
Fig. 3, supports the usage of a longer resolving time. However, such a
change causes a negative deviation for 63Ni. The adjustment of
coincidence resolving time and the analysis of low-energy emitters
require further investigations. Measurements with the Hidex counter
using a coincidence resolving time of 40 ns are under way.
As results are comparable with those of the PTB–TDCR system,
the HIDEX system can be regarded as suitable for TDCR measurements applying a free parameter model provided the dead time
correction and the linearity adjustment is appropriate. A drawback for high-precision measurements is (as in all commercial
LS-counters) that the user has no information about the signal
processing and has to rely on the proper functioning of the
electronics, as no checks in the signal processing chain can be
performed.
The influence of asymmetries in the PMT efficiencies on the
activity result is, in general, very low in the HIDEX system
installed at PTB, as the three PMTs are well matched. The
deviation of the results assuming identical efficiencies in all PMTs
from the results taking asymmetries into account over the
average beta energy is shown in Fig. 7 for the measurements
presented in Fig. 5. For all results shown here, the deviation is
considerably smaller than 0.1%, which makes it negligible for nonmetrologic measurements. There is, however, an influence of the
average beta-energy: the deviation of the result assuming identical efficiencies becomes slightly larger with lower energy. This
can be easily understood, because a higher energy corresponds to
a larger number of photons produced in the scintillator cocktail. If
more photons impinge on the photocathode of the PMT, differences in the quantum efficiency of the PMT will become less
important. Due to the large amount of light produced by alpha
emitters, for example, the detection efficiency equals 1 for all
relevant detector configurations, irrespective of the quantum
efficiency or other asymmetries of the PMTs.
5.1. LS spectrometry method for activity determination of
109
Cd
The applicability of the HIDEX system for the determination of
the activity of a solution of 109Cd by conversion-electron counting
Please cite this article as: Wanke, C., et al., Investigations on TDCR measurements with the HIDEX 300 SL using a free
parameter model. Appl. Radiat. Isotopes (2012), doi:10.1016/j.apradiso.2012.02.097
C. Wanke et al. / Applied Radiation and Isotopes ] (]]]]) ]]]–]]]
7
Fig. 7. Deviation of the result, assuming symmetric efficiencies, to the result, taking asymmetries into account. The coincidence time was 30 ns and the threshold was 50.
has also been investigated. The method, which is described in
Kossert et al. (2006, 2009b), requires the evaluation of measured
spectra, as a separation of events from conversion electrons from
other events is necessary. Therefore, the measurements cannot be
done in the ‘‘METRO’’ counting mode, as this is not capable of
producing spectra. Hence, the measurements were done in the
standard ‘‘BETA’’ mode.
The 109Cd LS samples were prepared in PE vials containing a
mixture of Ultima GoldTM AB and Ultima GoldTM F to achieve a
good spectrum resolution. The result of three samples with the
109
Cd solution measured twice each is in excellent agreement
with the reference value; the (unweighted) mean shows a deviation of only 0.02%. Further details about the method with the
latest improvements were described by Kossert et al. (2009b).
5.2. TDCR-Čerenkov method
The TDCR-Čerenkov method (Kossert, 2010) was also applied
with the Hidex counter. To this end, weighed portions of a 32P
solution were filled into polyethylene vials containing about
12 mL of distilled water. Counting efficiencies were computed
with the model as presented by Kossert et al. (2011b) taking into
account PMT asymmetries. Also the wave-length dependence of
the refractive index of water (dispersion) as well as the PMT
response curves were taken into account. The determined activity
concentration was found to be about 0.1% lower than the results
obtained by LS counting with PTB counters. The result indicates
that the TDCR-Čerenkov method can be applied with the Hidex
system. This is interesting since a mixture of radioactive material
and the organic scintillation cocktail can be avoided.
6. Conclusions and outlook
The HIDEX 300 SL is, in general, applicable for activity
measurements with low uncertainties. For the nuclides studied
here, the results are, in general, comparable to those obtained
with the PTB–TDCR system.
The prerequisite for correct measurements is, however, that
the system linearity is satisfactory. This requires the correct
tuning of a dead-time compensation circuit, which requires some
amount of work and a series of samples of an alpha emitter like
241
Am, which might not be available to some users. It is desirable
to further improve the linearity of the system installed at PTB.
Moreover, bias voltages, threshold and coincidence time have to
be adequately set.
It should also be noted that the model dependencies can be
very large when studying nuclides with low photon yield, like 3H
and 55Fe. Also the sample composition as well as the choice of
vials may have significant influence when measuring such
radionuclides.
It must be pointed out that the results presented in this work
were obtained with a TDCR apparatus which was modified by
Hidex Oy. Thus, it would be interesting to check whether other
‘‘METRO’’ versions of the Hidex counter give comparable results.
To this end, a comparison between ENEA (Italy) and PTB is
planned within the scope of the EMRP MetroFission project
(EMRP, 2009). Such a comparison will yield valuable results on
the reproducibility of counters.
Acknowledgments
The authors wish to thank the staff of Hidex Oy for their kind
support and for providing and adapting the TDCR counter. The
research leading to these results has received funding from the
European Union on the basis of Decision No. 912/2009/EC.
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