Study of the influence of the liquid scintillator in the Compton efficiency tracing method

The principle of the Compton source efficiency tracing method (CET) in liquid scintillation counting (LSC) is to use, as a tracer, a Compton electron source temporarily created inside the source being measured. The tracer source and the source to measure have thus exactly the same chemical composition. A main advantage anticipated for this measurement method is its low sensitivity to the kB factor and insensitivity to the chemical composition and quenching of the source, which would obviate the necessity to develop a reference liquid scintillator cocktail in the framework of an international reference system for the measurement of pure-beta emitters by LSC.We took the opportunity of a tritiated water international activity measurement in comparison to prepare LS sources using various commercial liquid scintillator cocktails. Tritium is assumed to be a good candidate for testing the LSC activity measurement method, as the detection efficiency is rather low and the effect of the ionization quenching parameter, kB, on the detection efficiency calculation by the TDCR method is known to be important.The sources were measured using a TDCR counter equipped with a Compton spectrometer in order to apply the CET method. The measurements are analyzed using both the TDCR method and CET method in order to deduce the optimal calculation parameters. The detection efficiency range covered by these experiments is from 0.4 to 0.6, allowing a good test of the validity of the physical assumptions included in the calculation model.The paper will conclude on the advantages and drawbacks of using this CET method in the framework of an international reference system for low-energy pure-beta radionuclides.     

A 4pi(LS)beta-gamma coincidence system using a TDCR apparatus in the beta-channel.

This article is an overview of the capabilities of a 4 pi(LS)beta-gamma coincidence counting system based on a liquid scintillation detector in the beta-channel. Equipped with a three-photomultiplier apparatus designed for the triple to double coincidence ratio (TDCR) method, the 4 pi(LS)beta-gamma coincidence system developed at LNHB combines the two techniques. The 4 pi(LS)beta-gamma counting setup records the different types of scintillation events between photomultipliers (double and triple coincidences) and the associated gamma-spectra according to a bi-dimensional configuration. When measuring the activity of a (54)Mn solution, the extrapolation method is carried out by defocusing the photomultipliers; this procedure allows the measurement of gamma-ray sensitivity of the scintillation cocktail in the 835 keV energy region. In addition, in the framework of a (65)Zn standardization, 4 pi(LS)beta-gamma measurements are compared with standard 4 pi(PC)beta-gamma anticoincidence counting using a proportional counter in the beta-channel.     

On the stochastic dependence between photomultipliers in the TDCR method

The TDCR method (Triple to Double Coincidence Ratio) is widely implemented in National Metrology Institutes for activity primary measurements based on liquid scintillation counting. The detection efficiency and thereby the activity are determined using a statistical and physical model. In this article, we propose to revisit the application of the classical TDCR model and its validity by introducing a prerequisite of stochastic independence between photomultiplier counting. In order to support the need for this condition, the demonstration is carried out by considering the simple case of a monoenergetic deposition in the scintillation cocktail. Simulations of triple and double coincidence counting are presented in order to point out the existence of stochastic dependence between photomultipliers that can be significant in the case of low-energy deposition in the scintillator. It is demonstrated that a problem of time dependence arises when the coincidence resolving time is shorter than the time distribution of scintillation photons; in addition, it is shown that this effect is at the origin of a bias in the detection efficiency calculation encountered for the standardization of ³H. This investigation is extended to the study of geometric dependence between photomultipliers related to the position of light emission inside the scintillation vial (the volume of the vial is not considered in the classical TDCR model). In that case, triple and double coincidences are calculated using a stochastic TDCR model based on the Monte-Carlo simulation code Geant4. This stochastic approach is also applied to the standardization of ⁵¹Cr by liquid scintillation; the difference observed in detection efficiencies calculated using the standard and stochastic models can be explained by such an effect of geometric dependence between photomultiplier channels.     

Simulation of Cherenkov photons emitted in photomultiplier windows induced by Compton diffusion using the Monte Carlo code GEANT4

The implementation of the TDCR method (Triple to Double Coincidence Ratio) is based on a liquid scintillation system which comprises three photomultipliers; at LNHB, this counter can also be used in the β-channel of a 4π(LS)β–γ coincidence counting equipment. It is generally considered that the γ-sensitivity of the liquid scintillation detector comes from the interaction of the γ-photons in the scintillation cocktail but when introducing solid γ-ray emitting sources instead of the scintillation vial, light emitted by the surrounding of the counter is observed. The explanation proposed in this article is that this effect comes from the emission of Cherenkov photons induced by Compton diffusion in the photomultiplier windows. In order to support this assertion, the creation and the propagation of Cherenkov photons inside the TDCR counter is simulated using the Monte Carlo code GEANT4. Stochastic calculations of double coincidences confirm the hypothesis of Cherenkov light produced in the photomultiplier windows.     

Standardization of radionuclide by beta(LS)-gamma coincidence counting using the geometry-efficiency variation method.

A liquid scintillation counting method consisting of three photomultiplier tubes for beta counters and a NaI(Tl) gamma counter has been developed for the standardization of radionuclides with the beta-gamma coincidence technique. The beta detection efficiency functions are obtained by means of a geometry-variation method developed in the present work; an array of beta detectors is moved uniformly at the same time from a centrally located counting vial to 50 mm. The method has been applied in the standardization of 60Co and 134Cs. Unquenched liquid scintillation samples with nominal count rates from 1000 to 6000 s-1 were prepared. The observed beta detection efficiencies with this method are from 90 to 45% in the case of 60Co, and from 84 to 50% for 134Cs. The output of each beta channel is summed together and compared with gamma data by the coincidence analyzer. The dead time of each counting channel is adjusted to be 20 micros, sufficiently long to suppress the afterpulses in the beta counting channel. The activity of each sample is determined by using the Cox and Isham formula. The obtained results are in good agreement with KRISS certified values.     

Primary standardisation of 204Tl using the efficiency tracing method.

The electron-capture and beta(-) -emitting radionuclide 204Tl has been known from previous experience to show discrepancies between different methods of standardisation. Source preparation is also difficult due to the complex chemistry of thallium. The Bureau International des Poids et Mesures (BIPM) intercomparison held in 1997, showed discrepancies of up to 10% between measurements of solid sources and liquid scintillation methods. These problems have been the subject of a BIPM CCRI(II) working group since 1999. This paper presents the results from a primary standardisation of a 204Tl solution using 4pibeta-gamma coincidence counting and liquid scintillation counting. The tracer technique was used for the 4pibeta-gamma coincidence counting, where 204Tl was traced with 134Cs and 60Co. The extrapolation to 100% beta-efficiency was performed by three different approaches: foiling, adding carrier and measuring sources of different initial masses. The results showed that tracing with 60Co and using external foiling gave the same result as obtained by liquid scintillation counting. A comparison of the results achieved by the different methods of measurement, tracers and methods for extrapolation is presented and discussed in this paper.     

Standardization of a 32P solution containing pure-beta impurities using the TDCR method in liquid scintillation counting.

The liquid scintillation counting standardization of pure-beta solutions containing pure-beta emitter impurities is a difficult task because, unlike that which can be achieved by using gamma-ray spectrometry, the continuous characteristics of beta spectra and the poor intrinsic resolution of LSC spectrometers do not allow an easy identification and precise determination of the activity concentration of these impurities. This problem was recently encountered in the standardization of a 32P solution in the framework of an international comparison, where the activity contribution of the impurities was over 10% of the main nuclide activity at the reference date. This paper describes the method used to identify the impurities in this 32P solution and the calculation of the detection efficiencies using the triple to double coincidence ratio model. Details on the calculation of the uncertainties of each activity and the covariances between these activities are given.     

Standardization of 89Sr.

Two methods were used for the standardization of the pure beta-emitter 59Sr solution received in the framework of the 2000 BIPM comparison of activity measurements. The first method was by scintillation counting using the triple to double coincidence ratio (TDCR) technique. The existing three detector-system was improved by the inclusion of a MAC-3 module produced by LNHB-Saclay. This module contains the circuits for the coincidence, dead time and gate functions. This was the first use of this equipment in an international comparison. The DETECSZ program (LNHB) was used to obtain the activity concentration, at the reference time, as 26.09+/-0.21 kBq g(-1). The second method was the efficiency tracer technique, using a 60Co standard solution and a 4pi beta-gamma-coincidence system. The radioactivity concentration was calculated from a general extrapolation curve, by using data from several sources. The value was 26.58+/-0.28 kBq g(-1) on the reference date 01.10.2000, 00 h UTC. The two results are in agreement within the limits of their combined uncertainties.     

Standardization of 110mAg by liquid scintillation and 4πβ–γ coincidence counting

The nuclide ^110mAg is a β–γ emitter with a very complex decay scheme, including more than 50 γ-rays. It has been standardized by two methods: liquid scintillation counting (LSC) and 4πβ(pc)-γ coincidence measurements. In the LSC measurements the CIEMAT/NIST method was used, with ³H being used as a tracer for efficiency calculations. In the 4πβ(pc)-γ standardization, Monte Carlo calculations have been made to determine the optimal measurement conditions. Results obtained with both methods for the activity concentration of the solution are in good agreement.     

Standardization of 152Eu and 154Eu by 4pibeta-4pigamma coincidence method and 4pi(beta+gamma) integral counting.

The 4pibeta-4pigamma coincidence counting and 4pi(beta+gamma) integral counting techniques were applied for the standardization of 152Eu and 154Eu. In these techniques, the beta-detector is composed of two thin plastic scintillators sandwiching the source coupled with a slender photomultiplier tube. This beta-detector was inserted into a large well-type NaI(Tl) scintillation detector for gamma-ray detection, making a 4pibeta-4pigamma detector configuration. The results obtained by the above two techniques were in good agreement and consistent with the results of international comparisons.     

A Monte Carlo simulation of Compton scattering in positron emission tomography

A Monte Carlo technique was used to simulate some of the important physical processes involved in the coincidence detection of gamma rays by a positron emission tomograph. The major effect considered here is the detection of Compton scattered gamma rays as coincidence counts. The Compton scattering of gamma rays in a H2O filled phantom was simulated using the Klein-Nishina cross section. Results obtained in the form of profiles of activity were compared to a model which represents scatter as the convolution of the true signal with an exponential.     

Standardization of tritiated water by two improved methods.

Tritiated water has been standardized in the framework of a French-Romanian cooperation by two improved methods: liquid scintillation counting based on the triple to double coincidence ratio method and the internal gas proportional counting used in conjunction with a tritium generator for chemical reduction of water. The uncertainties of measurement for both methods were smaller than 0.6% and the two results were consistent within these uncertainties, indicating that either method is equally suited for standardizing tritiated water.     

First TDCR measurements at low energies using a miniature x-ray tube

Developed for radionuclide standardization using liquid scintillation, the Triple to Double Coincidence Ratio (TDCR) method is applied using coincidence counting obtained with a specific three-photomultiplier system. For activity determination, a statistical model of light emission is classically used to establish a relation between the detection efficiency and the experimental TDCR value. At LNE-LNHB, a stochastic approach of the TDCR modeling was developed using the Monte Carlo code Geant4. The interest of this TDCR-Geant4 model is the possibility to simulate the propagation of optical photons from their creation in the scintillation vial to the production of photoelectrons in photomultipliers.As an alternative to the use of radionuclide sources, first TDCR measurements are presented using a miniature x-ray tube closely coupled to the scintillation vial. The objective of this new set-up was to enable low-energy depositions (lower than 20 keV) in liquid scintillator in order to study the influence of both time and geometrical dependence between PMTs already observed with radioactive sources. As for the statistical TDCR model, the non-linearity of light emission is implemented in the TDCR-Geant4 model using the Birks formula which depends on the kB factor and the scintillation yield. Measurements performed with the x-ray tube are extended to the assessment of these parameters and they are tested afterwards in the TDCR-Geant4 model for activity measurements of ³H.     

A method for determination mass absorption coefficient of gamma rays by Compton scattering

A method was proposed for determination mass absorption coefficient of gamma rays for compounds, alloys and mixtures. It is based on simulating interaction processes of gamma rays with target elements having atomic numbers from Z=1 to Z=92 using the MCSHAPE software. Intensities of Compton scattered gamma rays at saturation thicknesses and at a scattering angle of 90° were calculated for incident gamma rays of different energies. The obtained results showed that the intensity of Compton scattered gamma rays at saturations and mass absorption coefficients can be described by mathematical formulas. These were used to determine mass absorption coefficients for compound, alloys and mixtures with the knowledge of their Compton scattered intensities. The method was tested by calculating mass absorption coefficients for some compounds, alloys and mixtures. There is a good agreement between obtained results and calculated ones using WinXom software. The advantages and limitations of the method were discussed.     

Compton suppression through rise-time analysis.

We studied Compton suppression for 60Co and 137Cs radioisotopes using a signal selection criterion based on contrasting the fall time of the signals composing the photo peak with those composing the Compton continuum. The fall time criterion is employed by using the pulse shape analysis observing the change in the fall times of the gamma-ray pulses. This change is determined by measuring the changes in the rise times related to the fall time of the scintillator and the timing signals related to the fall time of the input signals. We showed that Compton continuum suppression is achieved best via the precise timing adjustment of an analog rise-time analyzer connected to a NaI(Tl) scintillation spectrometer.     

Compton scatter image simulating jugular venous reflux

In a radionuclide cerebral angiographic study, Tc-99m photons from the subclavian vein may scatter in the superficial tissues of the neck and head, resulting in an image simulating the jugular venous reflux. In a scintillation camera peaked at 140 keV with a 20% window, any scattered photons with a scatter angle of less tha 53.5 degrees may be counted in the Tc-99m window. This scatter angle is large enough to allow counting of many secondary photons from Compton collisions in an area quite distant from the radioactive source to be counted, provided the scatter area and source are separated by air.     

Comparison of calculated spectra for the interaction of photons in a liquid scintillator. Example of 54Mn 835 keV emission.

The CIEMAT/NIST and TDCR methods in liquid scintillation counting, initially developed for the activity standardization of pure-beta radionuclides, have been extended to the standardization of electron capture and beta-gamma radionuclides. Both methods require the calculation of the energy spectrum absorbed by the liquid scintillator. For radionuclides emitting X-rays or gamma-rays, when the energy is greater than a few tens of keV the Compton interaction is important and the absorption is not total. In this case, the spectrum absorbed by the scintillator must be calculated using analytical or stochastic models. An illustration of this problem is the standardization of 54Mn, which is a radionuclide decaying by electron capture. The gamma transition, very weakly converted, leads to the emission of an 835 keV photon. The calculation of the detection efficiency of this radionuclide requires the calculation of the energy spectrum transferred to the scintillator after the absorption of the gamma ray and the associated probability of absorption. The validity of the method is thus dependent on the correct calculation of the energy transferred to the scintillator. In order to compare the calculation results obtained using various calculation tools, and to provide the metrology community with some information on the choice of these tools, the LS working group of the ICRM organised a comparison of the calculated absorbed spectra for the 835 keV photon of 54Mn. The result is the spectrum of the energy absorbed by the scintillator per emission of an 835 keV gamma ray. This exercise was proposed for a standard 20 ml LS glass vial and for LS cocktail volumes of 10 and 15 ml. The calculation was done for two different cocktails: toluene and a widely used commercial cocktail, Ultima Gold. The paper describes the results obtained by nine participants using a total of 12 calculation codes.     

Low-level gamma spectrometry using beta coincidence and Compton suppression.

A low-level gamma-ray spectrometry system was developed using a Ge(Li) detector with 6% relative efficiency coupled to a 2pi beta plastic detector for coincidence selection and a massive NaI(Tl) detector for Compton suppression. The integral background count rate for (50-1500)keV was 0.5 s(-1)kg(-1) (Ge), using only beta coincidences. With Compton suppression, a value of 0.25 s(-1)kg(-1) (Ge) was obtained. Spectra with and without Compton suppression were studied for 60Co, 137Cs and 152Eu point sources. Considerations are made concerning the Compton suppression advantages in different situations.     

Activity measurements of the high-energy pure beta-emitters 89Sr and 90Y by the TDCR efficiency calculation technique.

The absolute activities of the pure beta-emitters 89Sr and 90Y have been determined by a direct method, namely the triple-to-double coincidence ratio (TDCR) efficiency calculation technique. This undertaking has extended further the number of radionuclides that have been standardized by this non-extrapolation liquid scintillation (LS) method. Both measurements were carried out within the framework of international key comparisons under the auspices of the International Bureau of Weights and Measures (BIPM). The TDCR results agreed well with those of other participating national laboratories, most using alternative (tracer) methods.     

Standardization of Sn-113

The radionuclide ¹¹³Sn is a quasi-monoenergetic gamma emitter often used in the efficiency calibration of gamma spectrometers in the energy region around 390 keV. This paper presents the results of the standardization of this radionuclide by three methods: integral (4π−γ) counting with a well-type NaI(Tl) detector, liquid scintillation counting applying the CIEMAT-NIST method and 4π coincidence counting (conversion electron-X) with a digital coincidence system.