- Email: [email protected]
Quality control of liquid scintillation counters
ARTICLE IN PRESS
Applied Radiation and Isotopes 64 (2006) 1163–1170 www.elsevier.com/locate/apradiso
Quality control of liquid scintillation counters F. Jaubert, I. Tarte`s, P. Cassette LNE-LNHB, Laboratoire National Henri Becquerel, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
Abstract Liquid scintillation counting (LSC) is widely used at LNHB for primary standardization of radionuclides (TDCR method), for secondary calibration and also for source stability studies or radioactive purity measurements. A total of five LSC counters are used for these purposes: two locally developed 3-photodetector counters for the implementation of the TDCR method, two Wallac 1414 counters and one Wallac 1220 Quantulus counter. The quality of the LSC measurements relies on the correct operation of these counters and their traceability to the frequency and time units. r 2006 Elsevier Ltd. All rights reserved. Keywords: Liquid scintillation counting; Quality control
0. Introduction This paper describes the quality control techniques used for these systems. For the primary counters, a locally developed random light pulser is used to control the whole counting process, from the photomultiplier tubes (PMT) to the scalers. The master random pulse generator is composed of a fast miniature PMT optically coupled to a 14 C liquid scintillator light source. This pulse generator drives a light pulser composed of an UV light emitting diode (LED) and a liquid scintillator. The duration of the light pulse is a few nanoseconds and the amplitude is adjustable to control the amount of photons delivered. A measurement of the random properties of this generator is described. The traceability to frequency standards is verified by the use of a GPS-driven reference rubidium oscillator. The number of pulses delivered by this oscillator is recorded during the acquisition time and compared to the number of pulses delivered by the internal quartz oscillator of the counter. The traceability to UTC is made by the use of a commercial radio-controlled module connected to the serial port of the acquisition computer.
For the secondary counters, no such light generator can be used because of the difficulty to reach the optical counting chamber. The main controls are made using stable reference liquid-scintillation counting (LSC) sources. These sources are toluene-based pure-organic 14C sources prepared in the laboratory 30 years ago. These sources are flame-sealed and kept in the dark for optimum stability and periodically checked against the primary measurement system. The paper also discusses the LNHB experience on the short- and long-term stability of these commercial LS counters and of the locally developed TDCR counter. 1. Traceability requirements when operating LS counters ISO 17025 norm requires that every parameter of influence or physical data used in the measurement or referenced in the calibration certificate must be traceable to standard units. For source activity determined by LSC, this requirement obviously concerns the frequency of the counter internal clock, the date and time of measurement and also the discriminator levels in each channel. These parameters are described hereafter. 1.1. Frequency
Corresponding author. Tel.: +33 1 6908 4868; fax: +33 1 6908 2619.
E-mail address: [email protected] (P. Cassette). 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.02.081
The LSC counters are employed in radionuclide metrology either for primary measurement methods,
ARTICLE IN PRESS 2
F. Jaubert et al. / Applied Radiation and Isotopes 64 (2006) 1163–1170
TDCR (Pochwalski et al., 1988; Grau Malonda and Coursey, 1988), or for tracer methods, CIEMAT-NIST method (Coursey et al., 1986) or for relative measurement methods. In each of these applications, the activity of the source is deduced from the observed count rate in coincidence and from the calculated detection efficiency. The activity unit, the becquerel, is defined relative to the time unit, the second, so every LSC counters has an internal clock producing its own time unit. This reference clock must be used to calculate the live-time of the instrument as dead-time is present in every instrument. Every inaccuracy in the reference clock or in the live-time determination induces a bias in the counting rate and, as a consequence, a bias in the activity of the source and must be taken into account in the uncertainty budget. 1.2. Date and time The activity of a radioactive source is generally given for a reference date and time, as radioactive material disintegrate. The accuracy of this reference time can be very critical for some short half-life radionuclides. For example, in the standardization of 18F, a 1.829 h half-life radionuclide commonly used for diagnosis in nuclear medicine, an error of 30 s in the reference times induces a bias of more than 0.3% on the activity of the solution. In LNHB, we generally use UTC date and time to avoid any problem with daylight saving time changes and international comparisons. 1.3. Counter discriminator threshold In LSC counters using PMT, the minimum information quantity corresponds to the detection of one photoelectron emitted by the photocathode. As the LS counters operate in a pulse mode, the counting channels include discriminators to remove the influence of the electronic noise of the detection chain. The LSC counters require a good sensitivity to light because the light yield of LS sources is very low (about a few photons are produced after the absorption of 1 keV in an unquenched LS source). Welldesigned counters use high gain PMTs enabling a clear discrimination of the single photoelectron pulse (SER) and the threshold level must be adjusted under this SER in order to keep all the light information and to avoid excess counting due to noise. The good adjustment of this discriminator levels is mandatory for the correct implementation of both CIEMAT-NIST (Coursey et al., 1986) and TDCR methods (Cassette and Vatin, 1992), as these models are based on the non-zero detection probability of one photon. 2. Standards We discuss hereafter available secondary standards, traceable to national standards, which can be used in LSC.
2.1. Date and time All the LNHB LSC counters make the use of personal computers (PC) operating either under Microsoft (MS) DOS or MS-WINDOWS. We observed that the accuracy and stability of the internal date and time of all these computers is not compatible with a metrological use. For some computers, the time drift can be a few 10 s per day. Considering that, we systematically adjust time and date before each measurement in reference with the following standards. 2.1.1. Internet Reference time and date can be found on various Internet sites. We take UTC date and time from the BIPM web site (BIPM, 2005). For our practical conditions, the transmission delay is about 0.1 s. This transmission delay gives the minimum bias of the reference time determined by this method. 2.1.2. DCF77 external PC clocks External clock to be connected on serial or USB PC buses are commercially available in France at modest price. These clocks decode the 77.5 kHz radio signals produced by the DCF77 emitter in Germany. The effective coverage range is approximately 2000 km. At the transmitting antenna the phase time is kept in agreement with UTC (PTB) in the limits of approximately 70.3 ms. The clock signals are synchronized with the PTB reference time. The time signals are coded over a minute cycle but the clock synchronization can take several minutes, depending on the wave propagation or radiofrequency interferences. The clock is sold with software that update the internal PC clock continuously or at a pre-defined rate. This clock has a short-term accuracy ranging from 5 to 25 ms. 2.1.3. GPS clock Commercially available GPS receivers give reference date and time traceable to UTC. Most commercial portable GPS receivers are provided with a serial RS232 or USB bus and give output data according to the NMEA standard. This standard is well-documented and enables the decoding of time and date. Some commercial programs, like NMEATIME (Visual GPS, 2005) enable the synchronization of the time and date of the PC from a GPS unit. 2.2. Frequency The most ubiquitous frequency standards found in counting systems are based on quartz clocks. A simple design enables an uncertainty of a few 106 on the frequency with low phase noise. More stable temperature-controlled oscillators are also commercially available at low price. These units can be calibrated by a reference time standard laboratory to enable traceability to time units.
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2.2.1. Atomic oscillators If more precision is needed, rubidium or caesium oscillators are commercially available. These clocks are generally composed of a temperature-controlled quartz oscillator synchronized with a MASER oscillator. The rubidium clock we use (model MRK-HLN, Ball) delivers a 10 MHz sine wave with a relative short-term stability lower than 1011. This clock is synchronized by the output signal of a GPS receiver. In the best conditions, the phase difference of the clock with UTC is 1 ns. 2.2.2. GPS receivers GPS receivers not only provide UTC date and time but sometimes also a frequency output of 1 Hz or 10 MHz in phase with UTC (within a few ns). This signal can be used to synchronize a crystal-controlled oscillator to obtain a reference clock with a relative frequency uncertainty of a few 1011. An example of such design is given by Shera (1998). 2.3. Light pulse generators The light pulse generator is an instrument enabling the global test of a LSC counter. It is composed of a light source which can fit in the LSC counter optical chamber, driven by a random or periodic pulse generator with adjustable amplitude. This light must have properties similar to the one of a LS source, especially concerning the pulse duration, the amplitude stability and the statistics. The duration of the light pulse must be a few nanoseconds to mimic a real LS source. We found that duration between 5 and 10 ns is a good compromise. The amplitude of the pulse must be adjustable to change the average number of photons per pulse and so to test the counter for various detection efficiency. The distribution of pulses in time must be random and of Poisson type and the dead-time of the light source must be small compared to the dead-time of the LS counter. 2.3.1. Random pulse generator Commercial random pulse generators, generally used for the calibration and test of multichannel analyzers or gamma-ray spectrometer acquisition systems, are not suitable for such application as it was observed that these units exhibit an intrinsic dead-time of a few ms. This deadtime is not a problem for testing spectrometry chains with pulse time constants of a few ms but is a main drawback for testing fast coincidence electronics that are used in LS counters. If one wants to compare the number of pulses emitted by the pulse generator during a certain (live) time and the number of pulses recorded by the LS counter, the pulse emission must be stationary, i.e., the probability to get a pulse in a time interval dt during counter dead-time following a pulse must be equal to the probability to have a pulse in a time interval dt where the counter is alive. In other words, the physics occurring during the counter live-
3
time must be similar to the physics occurring during the system dead-time. Electronic random generators can be designed using pseudo-random circuits but to generate a random signal with short dead-time, a high-frequency clock and fast shift registers and gates are needed. To achieve such performances, standard integrated circuits from TTL or CMOS families are not suited, so the design and realization of such a circuit is not simple. The other alternative is to use the signal given by an intrinsic random phenomenon like a radioactive source associated with a gas or scintillator detector. The simplest design is to use a small gas counter operating in the Geiger–Mu¨ller mode but the intrinsic dead-time of such system is greater than a few ms. A better method is to use fast scintillators associated with PMT. Such a device using a 14C LS source associated with 2 in (5.08 cm) PMT (BURLE 8850) or a miniature metal package PMT (Hamamatsu R600P) was tested. These devices work well as random generators as the physical origin of the light (radioactive decay and nanosecond fluorescence) is random and of Poisson type. However, one might suspect with this kind of systems non-random effects due to after-pulses. These after-pulses can be caused by LS source phosphorescence and PMT tube after-pulses. To study this point, we measured the time distribution of pulses using a fast digital oscilloscope in persistence mode (LeCroy 6100). The signal obtained with a BURLE 8850 tube can be observed in Fig. 1. The presence of after-pulses synchronized with the main signal is very clear. As the delay between the main pulse and the after-pulse is about 450 ns, the origin of the after-pulses is likely in the ionization of the residual gas in the PMT. The same experiment conducted with a miniature PMT (Fig. 2) shows no evidence of correlated after-pulses. The final random generator is composed of a R5600P PMT with a 14C toluene-based LS source in a flame-sealed vial. The anode signal is amplified and connected to a constant fraction discriminator. The output with constant amplitude synchronizes a commercial pulse generator. 2.3.2. Light source The light source connected to the signal generator can be a LED selected to present short decay time and no afterpulses. We tested green AsGa or blue SiC LEDs. The recent release of commercially available UV LED like NSHU-550E (Nichia) enables their use as light generators associated with liquid or plastic scintillator. In this case, the light emission spectrum is similar to the real LS source emission spectrum. A photography of a typical light source is presented in Fig. 3. 2.4. LS reference sources For the test and routine control of our LS counters, we use old flame-sealed purely organic LS sources. These sources, made in our laboratory some 30 years ago, are
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Fig. 1. Random generator using a BURLE 8850 PMT.
kept in the dark at fairly constant ambient temperature. We also use the same kind of sources provided by the LS counters manufacturers. One of our reference source is a sealed source of 14C (ref: LMRI ECSLA 15/2/1974, series A17, 106 000 dpm, dpm standing for disintegration per minute). This source was made from 14C labelled toluene in a toluene, PPO, bis-MSB based LS cocktail. The relative uncertainty on the activity is claimed to be 3% at 99.7% confidence level. Although this measurement was made at a time where the evaluation of uncertainties was far from standardized, it is supposed that the meaning of this assumption is that the relative standard uncertainty on the activity was 1%. This source was measured again in 2005 using the TDCR method and it was noted that the relative difference between the certified activity corrected for the decay of the 14C toluene-based reference source and the measured value is only 0.21% after 30 years. This means that it is possible to use reference LS sources which otherwise could be considered as wastes, even if they are more than 10 years old.
cial LSC counters. This point is a major drawback for their use in radioactivity metrology as the adjustment of this threshold under the SER is a prerequisite of the CIEMAT/ NIST method. The adjustment of this threshold is easy on locally designed LS counters. The method used in our laboratory consists in measuring the noise spectrum of each PMT in coincidence with the counting channel after the discriminator. This adjustment requires the use of a multichannel analyzer with a gate system and a shape and delay module to synchronize in time the analogue output of the PMT and the logical output from the discriminator system. The description of the adjustment system can be found in Fig. 4. This adjustment is done periodically in our TDCR counters and before each measurement, the reference voltage of the discriminator level is measured using a digital voltmeter. From our experience, there is no significant threshold variation with time and no readjustments are necessary.
2.5. Counter threshold measurement
3. Control of counters, chi-2 test
There is, to our knowledge, no way to measure and control the adjustment of discriminator levels in commer-
The chi-2 test is used to confirm that the fluctuations observed between several measurements are compatible
ARTICLE IN PRESS F. Jaubert et al. / Applied Radiation and Isotopes 64 (2006) 1163–1170
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Fig. 2. Random generator using a Hamamatsu R5600P miniature PMT.
average value is A¼
n 1X Ai n i¼1
and the w2 value is defined by the next expression: w2 ¼
Fig. 3. Light source using an UV LED and liquid scintillator.
with a Gaussian distribution. For this test it is necessary to do at least twenty measurements with a minimum of 20 000 counts. For a series of n measurements A1, A2, y, An, the
n 1X ðAi AÞ2 . A i¼1
This value is compared with two critical values w2n1;a=2 and w2n1;1a=2 . If the w2 value is included between the two critical values, we conclude that the counter works correctly. For the two Wallac 1414 counters and the Wallac 1220 Quantulus counter, a w2 test is realized with a significance level a ¼ 0:05. Examples of results are given in Table 1. It must be pointed out that this test is only significant if there are enough measurements. For example, if only 15 measurements are extracted from the previous data, as shown in Table 2, it is possible that the w2 value would indicate that the counter is not working correctly. The result is more robust when a minimum of 30 measurements is used.
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3
H and 14C counting efficiencies and background for comparison with the original factory specifications and for verifying the stability of the system performances. The recommended maximum operating life of these sources is five years. For checking the stability of the LSC standards, we have measured each set of reference samples on the TDCR system. These measurements are compared with the values at the reference date and the results are presented in Tables 3 and 4. The reference and measured values are compatible for the 14C standards. For the 3H standard, there is a non-negligible difference between the measured and the reference value. The second measurement has been realized two years after the first measurement. Both are compatible. However, as the sources were not measured using the TDCR system at the beginning it is difficult to quantify the evolution of the sources over a longer period and to conclude on the stability of the sources but for the two last measurements, the 3H and 14C sources are very stable. The evolution of the count rate for the standards 3H and 14 C from Wallac Guardian counters at the reference date of each source are shown in Figs. 5 and 6. It can be observed that the decay-corrected count rate of the 3H standard decrease with time. The stability of the counter with the 14C is better. 4.2. Wallac Quantulus Similar measurements were carried out for the set of the Quantulus reference samples. The values measured are compared with their values at the reference date and the results are presented in Table 5.
Fig. 4. Control of LS counter discrimination threshold. Table 1 Chi-2 tests using reference LS sources with 30 measurements w2 value 3
Wallac I Wallac II Quantulus
H
18.96 33.66 32.98
w229;0:025
w2
16.05 16.05 16.05
45.72 45.72 45.72
Table 3 Reference sources of LS counter Guardian 1
14
C
19.45 20.63 24.39
Passed Passed Passed
14
10.63 12.35 14.98
10.91 7.92 12.57
H
Wallac I Wallac II Quantulus
w229;0:025
3
H C
14
Measured value TDCRI
Variation (%)
(3304740) Bq (1723714) Bq
(3455753) Bq (1735710) Bq
4.57 0.70
Guardian Wallac I. Product Nr.: 1215-111, Batch Nr.: 9501 F. Reference date: 01/07/1995 12:00 UTC.
Table 2 Chi-2 tests with only 15 measurements w2 value
3
Reference value
w2 Table 4 Reference sources of LS counter Guardian 2
C 16.05 16.05 16.05
45.72 45.72 45.72
Failed Failed Failed
4. LNHB experience on LS counters stability 4.1. Wallac Guardian The unquenched LSC standards for liquid scintillation counter are used to calibrate the instrument and to measure
Reference value
3
H C
14
Measured value TDCRI— 12/02/03
Measured value TDCRI— 01/06/05
Variation (%)
(3304740) Bq (3433753) Bq (3436753) Bq 3.90 (1705714) Bq (1723712) Bq (1723710) Bq 1.06
Guardian Wallac II. Product Nr.: 1215-111, Batch Nr.: 9502 B. Reference date: 01/07/1995 12:00 UTC.
4.00 1.06
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The evolution of the count rate of the 3H and 14C standards with time is shown respectively, in Figs. 7 and 8. The same kind of evolution is observed: short-term fluctuations with an overall decrease with time for tritium and a more constant response for 14C. The conclusion which can be drawn from these results is that a 14C standard is better suited as a constant reference source for testing the LS counters responses.
Count rate evolution at the reference date of 1-jul-95 3
for standard H - Guardian Wallac I 2400
count (s-1)
2350 2.2 % 2300 2250 2200 1995
4.3. Locally designed TDCR counters 1997
1999
2001
2003
2005
date 3
Fig. 5. Count rate evolution of H source on Wallac I counter.
Count rate evolution at the reference date of 1-jul-95 14 for standard C - Guardian Wallac I
1700
count (s-1)
1690 1680 1670
0.6%
1660 1650 1640 1996
1998
1999
2001
2003
2005
date Fig. 6. Count rate evolution of
14
C source on Wallac I counter.
H C
14
The locally developed counters for the implementation of the TDCR method (Cassette and Vatin, 1992) are globally tested using the 14C flame-sealed LS source described in Section 2.4 and with the light pulser. A special acquisition program was developed to measure this source with reference to an original measurement made in February 1998. This program thus enables the possibility to check the evolution of the activity of the source over time but also the evolution of the gross counting rates (three double coincidences, logical sum of double coincidence and triple coincidences). Some measurements, realized at different dates to follow these references parameters are presented in Table 6. All the measurements are decay-corrected for the reference date. Over seven years, the variation of the activity is less than 0.2% and it must be also noted that the standard deviation on the coincidence counting rate is only 0.13% for the logical sum of double coincidence and 0.44% for triple coincidences. This means that, over seven years, we cannot observe a significant change of the detector response and therefore, no PMT ageing effect is observed. 5. Conclusion
Table 5 Reference source of the Wallac Quantulus counter
3
7
Reference value
Measured value TDCRI
Variation (%)
(3248740) Bq (1635714) Bq
(3419753) Bq (1662710) Bq
5.26 1.65
Quantulus. Product Nr.: 1215-111, Batch Nr.: 9501 C. Reference date: 01/02/1995 12:00 UTC.
The traceability of LS counters to date, time and frequency standards can be easily achieved using commercially available systems. The best overall control of the counter can be made using a light pulse source connected to a random generator. This procedure is straightforward for locally designed LS counters without sample conveyor but is not so easy for commercial counters. In this case, it is
Table 6 14 C reference source counting results using TDCR1 LS counter Date of measure
AB
11/02/98 1591 07/09/98 1611 20/10/99 1601 09/11/01 1580 04/12/01 1586 16/05/05 1580 Mean Standard deviation Relative standard deviation (%)
BC
AC
D
T
T/D
Activity (Bq)
Activity at 15/02/74 (Bq)
1601 1596 1595 1611 1620 1614
1606 1599 1602 1577 1587 1581
1657 1659 1656 1654 1660 1657 1657 2 0.13
1571 1573 1571 1557 1567 1559 1566 7 0.44
0.9481 0.9482 0.9487 0.9413 0.9439 0.9409 0.9452 0.0036 0.38
1754 1758 1754 1758 1758 1761 1757 3 0.16
1759 1764 1760 1764 1764 1768 1763 3 0.17
AB, BC and AC stands for double coincidences, D logical sum of double coincidences and T triple coincidences.
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Count rate evolution at the reference date of 1-feb-95 for standard 3H - Quantulus 2250
count (s-1)
2240 0.5% 2230 2220 2210 2200 1997
1999
2000
2002
2004
2005
Our experience is that flame-sealed purely organic LS sources can be used for reference sources over more than 10 years, if these sources are stored at ambient temperature in reduced light. We observe that the stability of our locally designed counters is better than the stability of the commercial counters. Especially, no PMT ageing problems were observed over a period of seven years. This fact raises the question of the real origin of commercial LS counter response drift with time.
date Fig. 7. Count rate evolution of 3H source on Quantulus counter.
References Count rate evolution at the reference date of 1-feb-95 for standard 14C - Quantulus 1520
count (s-1)
1515 1510 0.7%
1505 1500 1495 1490 2002
2003
2004
2005
date Fig. 8. Count rate evolution of
14
C source on Quantulus counter.
possible to include a light generator in the counting cell using a LED coupled to an optical fibre. If no intervention on the counter is possible, a global test can be done using reference LSC sources.
BIPM, 2005, Available in the address /http://www.bipm.org/en/scientific/ tai/time_server.htmlS. Cassette, P., Vatin, R., 1992. Experimental evaluation of TDCR models for the 3 PM liquid scintillation counter. Nucl. Instrum. Methods A 312, 95–99. Coursey, B.M., Mann, W.B., Grau Malonda, A., Garcia-Toran˜o, E., Los Arcos, J.M., Gibson, J.A.B., Reher, D., 1986. Standardization of C-14 by 4pb liquid scintillation efficiency tracing with Hydrogen-3. Appl.Radiat. Isot. 37, 403–408. Grau Malonda, A., Coursey, B.M., 1988. Calculation of beta-particle counting efficiency for liquid scintillation systems with three phototubes. Appl. Radiat. Isot. 39, 1191–1196. Pochwalski, K., Broda, R., Radoszewski, T., 1988. Standardization of pure beta emitters by liquid scintillation counting. Appl. Radiat. Isot. 39, 165–172. Shera B., 1998, QST, July 1998, pp. 37–44. Visual GPS, 2005. /http://www.visualgps.net/NMEATime/default.htmS.
Applied Radiation and Isotopes 64 (2006) 1163–1170 www.elsevier.com/locate/apradiso
Quality control of liquid scintillation counters F. Jaubert, I. Tarte`s, P. Cassette LNE-LNHB, Laboratoire National Henri Becquerel, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
Abstract Liquid scintillation counting (LSC) is widely used at LNHB for primary standardization of radionuclides (TDCR method), for secondary calibration and also for source stability studies or radioactive purity measurements. A total of five LSC counters are used for these purposes: two locally developed 3-photodetector counters for the implementation of the TDCR method, two Wallac 1414 counters and one Wallac 1220 Quantulus counter. The quality of the LSC measurements relies on the correct operation of these counters and their traceability to the frequency and time units. r 2006 Elsevier Ltd. All rights reserved. Keywords: Liquid scintillation counting; Quality control
0. Introduction This paper describes the quality control techniques used for these systems. For the primary counters, a locally developed random light pulser is used to control the whole counting process, from the photomultiplier tubes (PMT) to the scalers. The master random pulse generator is composed of a fast miniature PMT optically coupled to a 14 C liquid scintillator light source. This pulse generator drives a light pulser composed of an UV light emitting diode (LED) and a liquid scintillator. The duration of the light pulse is a few nanoseconds and the amplitude is adjustable to control the amount of photons delivered. A measurement of the random properties of this generator is described. The traceability to frequency standards is verified by the use of a GPS-driven reference rubidium oscillator. The number of pulses delivered by this oscillator is recorded during the acquisition time and compared to the number of pulses delivered by the internal quartz oscillator of the counter. The traceability to UTC is made by the use of a commercial radio-controlled module connected to the serial port of the acquisition computer.
For the secondary counters, no such light generator can be used because of the difficulty to reach the optical counting chamber. The main controls are made using stable reference liquid-scintillation counting (LSC) sources. These sources are toluene-based pure-organic 14C sources prepared in the laboratory 30 years ago. These sources are flame-sealed and kept in the dark for optimum stability and periodically checked against the primary measurement system. The paper also discusses the LNHB experience on the short- and long-term stability of these commercial LS counters and of the locally developed TDCR counter. 1. Traceability requirements when operating LS counters ISO 17025 norm requires that every parameter of influence or physical data used in the measurement or referenced in the calibration certificate must be traceable to standard units. For source activity determined by LSC, this requirement obviously concerns the frequency of the counter internal clock, the date and time of measurement and also the discriminator levels in each channel. These parameters are described hereafter. 1.1. Frequency
Corresponding author. Tel.: +33 1 6908 4868; fax: +33 1 6908 2619.
E-mail address: [email protected] (P. Cassette). 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.02.081
The LSC counters are employed in radionuclide metrology either for primary measurement methods,
ARTICLE IN PRESS 2
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TDCR (Pochwalski et al., 1988; Grau Malonda and Coursey, 1988), or for tracer methods, CIEMAT-NIST method (Coursey et al., 1986) or for relative measurement methods. In each of these applications, the activity of the source is deduced from the observed count rate in coincidence and from the calculated detection efficiency. The activity unit, the becquerel, is defined relative to the time unit, the second, so every LSC counters has an internal clock producing its own time unit. This reference clock must be used to calculate the live-time of the instrument as dead-time is present in every instrument. Every inaccuracy in the reference clock or in the live-time determination induces a bias in the counting rate and, as a consequence, a bias in the activity of the source and must be taken into account in the uncertainty budget. 1.2. Date and time The activity of a radioactive source is generally given for a reference date and time, as radioactive material disintegrate. The accuracy of this reference time can be very critical for some short half-life radionuclides. For example, in the standardization of 18F, a 1.829 h half-life radionuclide commonly used for diagnosis in nuclear medicine, an error of 30 s in the reference times induces a bias of more than 0.3% on the activity of the solution. In LNHB, we generally use UTC date and time to avoid any problem with daylight saving time changes and international comparisons. 1.3. Counter discriminator threshold In LSC counters using PMT, the minimum information quantity corresponds to the detection of one photoelectron emitted by the photocathode. As the LS counters operate in a pulse mode, the counting channels include discriminators to remove the influence of the electronic noise of the detection chain. The LSC counters require a good sensitivity to light because the light yield of LS sources is very low (about a few photons are produced after the absorption of 1 keV in an unquenched LS source). Welldesigned counters use high gain PMTs enabling a clear discrimination of the single photoelectron pulse (SER) and the threshold level must be adjusted under this SER in order to keep all the light information and to avoid excess counting due to noise. The good adjustment of this discriminator levels is mandatory for the correct implementation of both CIEMAT-NIST (Coursey et al., 1986) and TDCR methods (Cassette and Vatin, 1992), as these models are based on the non-zero detection probability of one photon. 2. Standards We discuss hereafter available secondary standards, traceable to national standards, which can be used in LSC.
2.1. Date and time All the LNHB LSC counters make the use of personal computers (PC) operating either under Microsoft (MS) DOS or MS-WINDOWS. We observed that the accuracy and stability of the internal date and time of all these computers is not compatible with a metrological use. For some computers, the time drift can be a few 10 s per day. Considering that, we systematically adjust time and date before each measurement in reference with the following standards. 2.1.1. Internet Reference time and date can be found on various Internet sites. We take UTC date and time from the BIPM web site (BIPM, 2005). For our practical conditions, the transmission delay is about 0.1 s. This transmission delay gives the minimum bias of the reference time determined by this method. 2.1.2. DCF77 external PC clocks External clock to be connected on serial or USB PC buses are commercially available in France at modest price. These clocks decode the 77.5 kHz radio signals produced by the DCF77 emitter in Germany. The effective coverage range is approximately 2000 km. At the transmitting antenna the phase time is kept in agreement with UTC (PTB) in the limits of approximately 70.3 ms. The clock signals are synchronized with the PTB reference time. The time signals are coded over a minute cycle but the clock synchronization can take several minutes, depending on the wave propagation or radiofrequency interferences. The clock is sold with software that update the internal PC clock continuously or at a pre-defined rate. This clock has a short-term accuracy ranging from 5 to 25 ms. 2.1.3. GPS clock Commercially available GPS receivers give reference date and time traceable to UTC. Most commercial portable GPS receivers are provided with a serial RS232 or USB bus and give output data according to the NMEA standard. This standard is well-documented and enables the decoding of time and date. Some commercial programs, like NMEATIME (Visual GPS, 2005) enable the synchronization of the time and date of the PC from a GPS unit. 2.2. Frequency The most ubiquitous frequency standards found in counting systems are based on quartz clocks. A simple design enables an uncertainty of a few 106 on the frequency with low phase noise. More stable temperature-controlled oscillators are also commercially available at low price. These units can be calibrated by a reference time standard laboratory to enable traceability to time units.
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2.2.1. Atomic oscillators If more precision is needed, rubidium or caesium oscillators are commercially available. These clocks are generally composed of a temperature-controlled quartz oscillator synchronized with a MASER oscillator. The rubidium clock we use (model MRK-HLN, Ball) delivers a 10 MHz sine wave with a relative short-term stability lower than 1011. This clock is synchronized by the output signal of a GPS receiver. In the best conditions, the phase difference of the clock with UTC is 1 ns. 2.2.2. GPS receivers GPS receivers not only provide UTC date and time but sometimes also a frequency output of 1 Hz or 10 MHz in phase with UTC (within a few ns). This signal can be used to synchronize a crystal-controlled oscillator to obtain a reference clock with a relative frequency uncertainty of a few 1011. An example of such design is given by Shera (1998). 2.3. Light pulse generators The light pulse generator is an instrument enabling the global test of a LSC counter. It is composed of a light source which can fit in the LSC counter optical chamber, driven by a random or periodic pulse generator with adjustable amplitude. This light must have properties similar to the one of a LS source, especially concerning the pulse duration, the amplitude stability and the statistics. The duration of the light pulse must be a few nanoseconds to mimic a real LS source. We found that duration between 5 and 10 ns is a good compromise. The amplitude of the pulse must be adjustable to change the average number of photons per pulse and so to test the counter for various detection efficiency. The distribution of pulses in time must be random and of Poisson type and the dead-time of the light source must be small compared to the dead-time of the LS counter. 2.3.1. Random pulse generator Commercial random pulse generators, generally used for the calibration and test of multichannel analyzers or gamma-ray spectrometer acquisition systems, are not suitable for such application as it was observed that these units exhibit an intrinsic dead-time of a few ms. This deadtime is not a problem for testing spectrometry chains with pulse time constants of a few ms but is a main drawback for testing fast coincidence electronics that are used in LS counters. If one wants to compare the number of pulses emitted by the pulse generator during a certain (live) time and the number of pulses recorded by the LS counter, the pulse emission must be stationary, i.e., the probability to get a pulse in a time interval dt during counter dead-time following a pulse must be equal to the probability to have a pulse in a time interval dt where the counter is alive. In other words, the physics occurring during the counter live-
3
time must be similar to the physics occurring during the system dead-time. Electronic random generators can be designed using pseudo-random circuits but to generate a random signal with short dead-time, a high-frequency clock and fast shift registers and gates are needed. To achieve such performances, standard integrated circuits from TTL or CMOS families are not suited, so the design and realization of such a circuit is not simple. The other alternative is to use the signal given by an intrinsic random phenomenon like a radioactive source associated with a gas or scintillator detector. The simplest design is to use a small gas counter operating in the Geiger–Mu¨ller mode but the intrinsic dead-time of such system is greater than a few ms. A better method is to use fast scintillators associated with PMT. Such a device using a 14C LS source associated with 2 in (5.08 cm) PMT (BURLE 8850) or a miniature metal package PMT (Hamamatsu R600P) was tested. These devices work well as random generators as the physical origin of the light (radioactive decay and nanosecond fluorescence) is random and of Poisson type. However, one might suspect with this kind of systems non-random effects due to after-pulses. These after-pulses can be caused by LS source phosphorescence and PMT tube after-pulses. To study this point, we measured the time distribution of pulses using a fast digital oscilloscope in persistence mode (LeCroy 6100). The signal obtained with a BURLE 8850 tube can be observed in Fig. 1. The presence of after-pulses synchronized with the main signal is very clear. As the delay between the main pulse and the after-pulse is about 450 ns, the origin of the after-pulses is likely in the ionization of the residual gas in the PMT. The same experiment conducted with a miniature PMT (Fig. 2) shows no evidence of correlated after-pulses. The final random generator is composed of a R5600P PMT with a 14C toluene-based LS source in a flame-sealed vial. The anode signal is amplified and connected to a constant fraction discriminator. The output with constant amplitude synchronizes a commercial pulse generator. 2.3.2. Light source The light source connected to the signal generator can be a LED selected to present short decay time and no afterpulses. We tested green AsGa or blue SiC LEDs. The recent release of commercially available UV LED like NSHU-550E (Nichia) enables their use as light generators associated with liquid or plastic scintillator. In this case, the light emission spectrum is similar to the real LS source emission spectrum. A photography of a typical light source is presented in Fig. 3. 2.4. LS reference sources For the test and routine control of our LS counters, we use old flame-sealed purely organic LS sources. These sources, made in our laboratory some 30 years ago, are
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Fig. 1. Random generator using a BURLE 8850 PMT.
kept in the dark at fairly constant ambient temperature. We also use the same kind of sources provided by the LS counters manufacturers. One of our reference source is a sealed source of 14C (ref: LMRI ECSLA 15/2/1974, series A17, 106 000 dpm, dpm standing for disintegration per minute). This source was made from 14C labelled toluene in a toluene, PPO, bis-MSB based LS cocktail. The relative uncertainty on the activity is claimed to be 3% at 99.7% confidence level. Although this measurement was made at a time where the evaluation of uncertainties was far from standardized, it is supposed that the meaning of this assumption is that the relative standard uncertainty on the activity was 1%. This source was measured again in 2005 using the TDCR method and it was noted that the relative difference between the certified activity corrected for the decay of the 14C toluene-based reference source and the measured value is only 0.21% after 30 years. This means that it is possible to use reference LS sources which otherwise could be considered as wastes, even if they are more than 10 years old.
cial LSC counters. This point is a major drawback for their use in radioactivity metrology as the adjustment of this threshold under the SER is a prerequisite of the CIEMAT/ NIST method. The adjustment of this threshold is easy on locally designed LS counters. The method used in our laboratory consists in measuring the noise spectrum of each PMT in coincidence with the counting channel after the discriminator. This adjustment requires the use of a multichannel analyzer with a gate system and a shape and delay module to synchronize in time the analogue output of the PMT and the logical output from the discriminator system. The description of the adjustment system can be found in Fig. 4. This adjustment is done periodically in our TDCR counters and before each measurement, the reference voltage of the discriminator level is measured using a digital voltmeter. From our experience, there is no significant threshold variation with time and no readjustments are necessary.
2.5. Counter threshold measurement
3. Control of counters, chi-2 test
There is, to our knowledge, no way to measure and control the adjustment of discriminator levels in commer-
The chi-2 test is used to confirm that the fluctuations observed between several measurements are compatible
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Fig. 2. Random generator using a Hamamatsu R5600P miniature PMT.
average value is A¼
n 1X Ai n i¼1
and the w2 value is defined by the next expression: w2 ¼
Fig. 3. Light source using an UV LED and liquid scintillator.
with a Gaussian distribution. For this test it is necessary to do at least twenty measurements with a minimum of 20 000 counts. For a series of n measurements A1, A2, y, An, the
n 1X ðAi AÞ2 . A i¼1
This value is compared with two critical values w2n1;a=2 and w2n1;1a=2 . If the w2 value is included between the two critical values, we conclude that the counter works correctly. For the two Wallac 1414 counters and the Wallac 1220 Quantulus counter, a w2 test is realized with a significance level a ¼ 0:05. Examples of results are given in Table 1. It must be pointed out that this test is only significant if there are enough measurements. For example, if only 15 measurements are extracted from the previous data, as shown in Table 2, it is possible that the w2 value would indicate that the counter is not working correctly. The result is more robust when a minimum of 30 measurements is used.
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3
H and 14C counting efficiencies and background for comparison with the original factory specifications and for verifying the stability of the system performances. The recommended maximum operating life of these sources is five years. For checking the stability of the LSC standards, we have measured each set of reference samples on the TDCR system. These measurements are compared with the values at the reference date and the results are presented in Tables 3 and 4. The reference and measured values are compatible for the 14C standards. For the 3H standard, there is a non-negligible difference between the measured and the reference value. The second measurement has been realized two years after the first measurement. Both are compatible. However, as the sources were not measured using the TDCR system at the beginning it is difficult to quantify the evolution of the sources over a longer period and to conclude on the stability of the sources but for the two last measurements, the 3H and 14C sources are very stable. The evolution of the count rate for the standards 3H and 14 C from Wallac Guardian counters at the reference date of each source are shown in Figs. 5 and 6. It can be observed that the decay-corrected count rate of the 3H standard decrease with time. The stability of the counter with the 14C is better. 4.2. Wallac Quantulus Similar measurements were carried out for the set of the Quantulus reference samples. The values measured are compared with their values at the reference date and the results are presented in Table 5.
Fig. 4. Control of LS counter discrimination threshold. Table 1 Chi-2 tests using reference LS sources with 30 measurements w2 value 3
Wallac I Wallac II Quantulus
H
18.96 33.66 32.98
w229;0:025
w2
16.05 16.05 16.05
45.72 45.72 45.72
Table 3 Reference sources of LS counter Guardian 1
14
C
19.45 20.63 24.39
Passed Passed Passed
14
10.63 12.35 14.98
10.91 7.92 12.57
H
Wallac I Wallac II Quantulus
w229;0:025
3
H C
14
Measured value TDCRI
Variation (%)
(3304740) Bq (1723714) Bq
(3455753) Bq (1735710) Bq
4.57 0.70
Guardian Wallac I. Product Nr.: 1215-111, Batch Nr.: 9501 F. Reference date: 01/07/1995 12:00 UTC.
Table 2 Chi-2 tests with only 15 measurements w2 value
3
Reference value
w2 Table 4 Reference sources of LS counter Guardian 2
C 16.05 16.05 16.05
45.72 45.72 45.72
Failed Failed Failed
4. LNHB experience on LS counters stability 4.1. Wallac Guardian The unquenched LSC standards for liquid scintillation counter are used to calibrate the instrument and to measure
Reference value
3
H C
14
Measured value TDCRI— 12/02/03
Measured value TDCRI— 01/06/05
Variation (%)
(3304740) Bq (3433753) Bq (3436753) Bq 3.90 (1705714) Bq (1723712) Bq (1723710) Bq 1.06
Guardian Wallac II. Product Nr.: 1215-111, Batch Nr.: 9502 B. Reference date: 01/07/1995 12:00 UTC.
4.00 1.06
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The evolution of the count rate of the 3H and 14C standards with time is shown respectively, in Figs. 7 and 8. The same kind of evolution is observed: short-term fluctuations with an overall decrease with time for tritium and a more constant response for 14C. The conclusion which can be drawn from these results is that a 14C standard is better suited as a constant reference source for testing the LS counters responses.
Count rate evolution at the reference date of 1-jul-95 3
for standard H - Guardian Wallac I 2400
count (s-1)
2350 2.2 % 2300 2250 2200 1995
4.3. Locally designed TDCR counters 1997
1999
2001
2003
2005
date 3
Fig. 5. Count rate evolution of H source on Wallac I counter.
Count rate evolution at the reference date of 1-jul-95 14 for standard C - Guardian Wallac I
1700
count (s-1)
1690 1680 1670
0.6%
1660 1650 1640 1996
1998
1999
2001
2003
2005
date Fig. 6. Count rate evolution of
14
C source on Wallac I counter.
H C
14
The locally developed counters for the implementation of the TDCR method (Cassette and Vatin, 1992) are globally tested using the 14C flame-sealed LS source described in Section 2.4 and with the light pulser. A special acquisition program was developed to measure this source with reference to an original measurement made in February 1998. This program thus enables the possibility to check the evolution of the activity of the source over time but also the evolution of the gross counting rates (three double coincidences, logical sum of double coincidence and triple coincidences). Some measurements, realized at different dates to follow these references parameters are presented in Table 6. All the measurements are decay-corrected for the reference date. Over seven years, the variation of the activity is less than 0.2% and it must be also noted that the standard deviation on the coincidence counting rate is only 0.13% for the logical sum of double coincidence and 0.44% for triple coincidences. This means that, over seven years, we cannot observe a significant change of the detector response and therefore, no PMT ageing effect is observed. 5. Conclusion
Table 5 Reference source of the Wallac Quantulus counter
3
7
Reference value
Measured value TDCRI
Variation (%)
(3248740) Bq (1635714) Bq
(3419753) Bq (1662710) Bq
5.26 1.65
Quantulus. Product Nr.: 1215-111, Batch Nr.: 9501 C. Reference date: 01/02/1995 12:00 UTC.
The traceability of LS counters to date, time and frequency standards can be easily achieved using commercially available systems. The best overall control of the counter can be made using a light pulse source connected to a random generator. This procedure is straightforward for locally designed LS counters without sample conveyor but is not so easy for commercial counters. In this case, it is
Table 6 14 C reference source counting results using TDCR1 LS counter Date of measure
AB
11/02/98 1591 07/09/98 1611 20/10/99 1601 09/11/01 1580 04/12/01 1586 16/05/05 1580 Mean Standard deviation Relative standard deviation (%)
BC
AC
D
T
T/D
Activity (Bq)
Activity at 15/02/74 (Bq)
1601 1596 1595 1611 1620 1614
1606 1599 1602 1577 1587 1581
1657 1659 1656 1654 1660 1657 1657 2 0.13
1571 1573 1571 1557 1567 1559 1566 7 0.44
0.9481 0.9482 0.9487 0.9413 0.9439 0.9409 0.9452 0.0036 0.38
1754 1758 1754 1758 1758 1761 1757 3 0.16
1759 1764 1760 1764 1764 1768 1763 3 0.17
AB, BC and AC stands for double coincidences, D logical sum of double coincidences and T triple coincidences.
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Count rate evolution at the reference date of 1-feb-95 for standard 3H - Quantulus 2250
count (s-1)
2240 0.5% 2230 2220 2210 2200 1997
1999
2000
2002
2004
2005
Our experience is that flame-sealed purely organic LS sources can be used for reference sources over more than 10 years, if these sources are stored at ambient temperature in reduced light. We observe that the stability of our locally designed counters is better than the stability of the commercial counters. Especially, no PMT ageing problems were observed over a period of seven years. This fact raises the question of the real origin of commercial LS counter response drift with time.
date Fig. 7. Count rate evolution of 3H source on Quantulus counter.
References Count rate evolution at the reference date of 1-feb-95 for standard 14C - Quantulus 1520
count (s-1)
1515 1510 0.7%
1505 1500 1495 1490 2002
2003
2004
2005
date Fig. 8. Count rate evolution of
14
C source on Quantulus counter.
possible to include a light generator in the counting cell using a LED coupled to an optical fibre. If no intervention on the counter is possible, a global test can be done using reference LSC sources.
BIPM, 2005, Available in the address /http://www.bipm.org/en/scientific/ tai/time_server.htmlS. Cassette, P., Vatin, R., 1992. Experimental evaluation of TDCR models for the 3 PM liquid scintillation counter. Nucl. Instrum. Methods A 312, 95–99. Coursey, B.M., Mann, W.B., Grau Malonda, A., Garcia-Toran˜o, E., Los Arcos, J.M., Gibson, J.A.B., Reher, D., 1986. Standardization of C-14 by 4pb liquid scintillation efficiency tracing with Hydrogen-3. Appl.Radiat. Isot. 37, 403–408. Grau Malonda, A., Coursey, B.M., 1988. Calculation of beta-particle counting efficiency for liquid scintillation systems with three phototubes. Appl. Radiat. Isot. 39, 1191–1196. Pochwalski, K., Broda, R., Radoszewski, T., 1988. Standardization of pure beta emitters by liquid scintillation counting. Appl. Radiat. Isot. 39, 165–172. Shera B., 1998, QST, July 1998, pp. 37–44. Visual GPS, 2005. /http://www.visualgps.net/NMEATime/default.htmS.