|Year : 2011 | Volume
| Issue : 3 | Page : 201-205
Application of liquid scintillation inclusion method for the simultaneous determination of alpha and beta activities in composite samples
Sonali P. D. Bhade, PJ Reddy, D. A. R. Babu, DN Sharma
Department of Radiation safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
|Date of Web Publication||27-Sep-2012|
Sonali P. D. Bhade
Department of Radiation safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
Present work highlights liquid scintillation Automatic Efficiency Control (AEC) as a tool for simultaneous determination of gross α and β activities in composite samples. AEC which is in-built feature in Packard Tri-Carb 2900TR Liquid Scintillation Counter (LSC) provides automatic counting region adjustments for single and dual label samples according to the degree of quench present in sample. In the present study, α/β spiked composite samples were treated as dual label samples with α and β radio-nuclides as two distinct entities contributing to the gross activities and analyzed employing AEC technique. Instrument discriminator settings and regions of interest (ROI) were evaluated to determine optimum counting conditions for present technique. Three sets of pure alpha and pure beta standards simulating the activity concentrations of real samples in terms of α/β activity ratios were used to calibrate LSC. Calibration methodology for Packard Tri-Carb 2900TR LSC with respect to the above measurements using 241 Am, 36 Cl calibration standards is explained in detail. The practicability and working performance of AEC technique was checked by the validation trials with spiked synthetic samples covering range of α/β activity ratios from 1:1 to 1:30 and 30:1.
Keywords: Automatic efficiency control, gross alpha and beta activities, knee point inclusion method, liquid scintillation counting
|How to cite this article:|
Bhade SP, Reddy P J, Babu D, Sharma D N. Application of liquid scintillation inclusion method for the simultaneous determination of alpha and beta activities in composite samples. Radiat Prot Environ 2011;34:201-5
|How to cite this URL:|
Bhade SP, Reddy P J, Babu D, Sharma D N. Application of liquid scintillation inclusion method for the simultaneous determination of alpha and beta activities in composite samples. Radiat Prot Environ [serial online] 2011 [cited 2020 Nov 27];34:201-5. Available from: https://www.rpe.org.in/text.asp?2011/34/3/201/101724
| 1. Introduction|| |
Even though α emission is mono-energetic in LSC, the pulse height distributions are wider. As a result, the pulse height spectra of α emitting radio-nuclides overlap the spectra of higher energy β emitters, such as 137 Cs, 36 Cl, and 90 Sr/ 90 Y (McKleveen J.W., 1984).  Therefore, it is necessary to differentiate α emission from β and compensate for this interference. This study comprises a preamble exercise to check whether AEC technique which is conventionally used for analysis of two β-emitting radio-nuclides in a mixture can be implemented for quantification of gross α and β activities in composite samples.
Inclusion technique require the counting of dual radio nuclides in two counting regions (say regions A and B) set by lower level (LL) and upper level (UL) pulse height discriminators. The counting regions must be set to assure that there will be significant overlap of the pulse height spectra of both radio-nuclides in dual samples in the two counting regions. Thus, the name inclusion technique is derived from the fact that the two counting regions are set such that there are spill-up and spill-down of pulse events in each region from both the radio-nuclides (L'Annunziata, 2003).  Packard LSC uses AEC which is an in-built feature that provides automatic counting region adjustments for single and dual label samples according to the degree of quench present in sample. Quench is a common interference in liquid scintillation counting that is responsible for reduction in the sample's apparent activity. Any factor, which reduces the efficiency of the energy transfer (chemical quench) or causes the absorption of photons (color quench), results in quenching of signal from the sample (Thomson J. 2001). 
When analyzing composite samples using inclusion technique, at higher quench levels, determination of the respective radionuclide activities becomes more difficult due to increased spill-over of higher energy pulses into lower energy region (say in region A) and decrease spill-up of lower energy pulse events into higher energy region (say in region B). This can cause an excess contribution of the high energy nuclide in the low energy region, resulting in inaccurate determination of activities, particularly for the low energy nuclide. The degree of spill-down from the high energy nuclide into the low energy counting region increases with quench when regions are fixed. Therefore, to maintain optimal counting conditions, it is essential to keep the amount of spill-over of higher energy pulse events in the region A at a comparatively constant level as well as the spill-up of lower energy pulse events into region B. This is accomplished by an automatic window tracking method termed as AEC.
| 2. Experimental|| |
The measurements were carried out with a Packard Tri-Carb® 2900TR LSC. It has in-built external standard source of 133 Ba to monitor the degree of quenching and AEC feature to perform region adjustment automatically. In Packard Tri-Carb LSCs, quenching is quantified with the Quench Indicating Parameter (QIP) known as tSIE (Transformed Spectral Index of the External Standard). tSIE has a range of 0-1000; where 1000 represent an unquenched sample and is directly given by the LSC.
In the present study, 241 Am (alpha energy (MeV) - 5.49 (85%), 5.44 (12%) and g -ray energy ~60 KeV) and 36 Cl (beta energy (MeV) 0.714(100%)) were used as alpha and beta calibration standards respectively as they cover a useful energy range. Moreover, maximum overlap of their spectra gives extreme conditions for separation. 241 Am standard (0.2245μCi/g) used in the present study was received from BIPM, France. Beta standard source 36 Cl (1μCi/g) procured from Board of Radiation and Isotope Technology (BRIT), India was standardized by CIEMAT/NIST standardization technique to obtain absolute disintegration rates (Grau Malonda, 1982). These standards were diluted as per the requirement to prepare stock solutions with 0.1N HCl and 0.1N HNO 3 acid carrier solutions respectively.
2.2 Sample preparation
In order to assess the performance of the inclusion technique for quantification of α and β activities in composite samples, a series of calibration standards and synthetic test samples were prepared by adding gravimetrically requisite amount(g) of standard solutions to 10ml of Quicksafe-400 (Zinsser Analytic) scintillation cocktail in standard 20ml low 40 K glass vials (Wheaton). The spiked synthetic samples were prepared with varying α/β activity ratios and different quench levels (tSIE in the range 500-250). Total volume of 11ml, was maintained by adding the acid carrier solutions to the counting vials. The calibration standards and synthetic test samples were categorized into three sets containing (i) equal α and β activities (α/β~1) (ii) low α high β (LαHβ with α/β<1) (iii) high α low β (HαLβ with α/β>1) activity concentrations which simulate α/β activity proportions in real samples.
Three sets of 241 Am and 36 Cl quenched calibration standards were prepared discretely such as i) each set having activity concentration of ~250Bq; ii) 241 Am and 36 Cl of activity concentrations 68.7Bq and 1603Bq respectively (α/β ~ 1/23) to simulate LαHβ samples and iii) ~1667 Bq of 241 Am and ~67 Bq of 36 Cl (α/β ~ 25/1) to simulate HαLβ samples. Nitromethane (99%, Merck) was employed as a quenching agent (Thomson J. 2001). In these standards sequentially incremental amount of nitromethane (from 0 to 0.2ml in increments of 0.02 ml) was added to simulate various quench levels. These quenched calibration standard sets contained series of standards in which the absolute activity values (Bq) per vial or sample was constant and the quantity of quench added to sample as mentioned earlier.
Blank samples were prepared by adding 1ml of 0.1 Normality of acid carrier solution to 10ml of Quicksafe-400 scintillation cocktail in a glass counting vial to simulate sample preparation chemistry and counted along with each test sample to obtain net count rate of the sample. The calibration standards and synthetic test samples were counted over a period of 10-30 minutes in LSC until % 2σ standard deviation of 1.0% was reached.
2.3 Calibration procedures
Although α/β activity measurement using LSC is reliable, its ability to produce meaningful results is greatly dependent on accurate calibration and optimization of discriminator settings.
| 3. AEC: Discriminator Setting Optimization|| |
To arrive at the optimized discriminator settings, least quenched 241 Am (tSIE~670) and 36 Cl (tSIE~670) standards prepared at our laboratory, were observed for their spectral pulse height distributions which spread over 200-525keV and 0-600keV counting regions respectively. Therefore while selecting the Region of Interest (ROI) say regions A and B, for AEC technique, 241 Am was referred as α standard with lower pulse height distribution (200-525keV) as compared to 36 Cl (0-600keV) which is a β standard source. This postulation was made by taking into consideration the spectral pulse height distributions of these standards instead of their energies. Counting region A was selected as 0-350keV (LL-UL), where 350keV was estimated as two third of 525keV which is the spectral end point of 241 Am (tSIE~670) that was least quenched α standard prepared in our lab. The discriminator setting for region B was defined by selecting the LL discriminator setting of 350keV, which is the upper limit of region A and selecting the UL discriminator setting of 2000keV to include all pulses of higher magnitude. The second counting region (region B) was therefore defined by the LL and UL settings of 350-2000keV.
To study the variations in counting efficiencies over a range of tSIE from 500 to 150, two independent counting vials with α and β standard activities mentioned earlier were used. The change in tSIE was accomplished by repeatedly adding 20μl aliquots of nitromethane to a maximum 200 μl (this addition may have negligible geometry change) and vials were counted for 10 minutes. Thus, each standard ( 241 Am, 36 Cl) was counted in the two counting regions (as in case of AEC, region A: 0-350keV and region B: 350-2000keV) and the count rate (CPM) in each region was obtained, the DPM value for each standard was known. Using tSIE/AEC as the quench indicating parameter and counting two quenched standards ( 241 Am and 36 Cl) in two counting regions, the four counting efficiencies were obtained using the following relationship:
Derived counting efficiencies of α and β standard sources in regions A and B were EαA , EαB and EβA , EβB respectively. The same procedure was repeated for various quench levels using sets of quenched standards as described above. The counting efficiencies thus obtained were plotted against corresponding tSIE parameters and quench correction curves as shown in [Figure 1] were constructed.
|Figure 1: Lα Hβ Calibration plots of counting efficiencies versus tSIE constructed using 241Am and 36Cl standards in Quicksafe-400 scintillation cocktail.|
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Once the quench curves were constructed, they can be used to obtain the efficiency values corresponding to the degree of quench (tSIE) present in the sample. Similar calibration procedures were adopted for remaining two sets of standards.
A blank was prepared with each set, to ensure proper background subtraction. The Minimum Detectable Activity (MDA) values for gross α and β activities were expressed as defined by Currie (1968) and found to be 9.3 mBq/ml and 18.6 mBq/ml respectively.
| 4. Results and Discussion|| |
Alpha particles produce about one-tenth the light intensity (scintillation yield) as beta particles per unit of radiation energy (L'Annunziata, 2003).  A 5-MeV α particle will produce a pulse height spectral peak at approximately 500keV when liquid scintillation pulse heights are calibrated on a scale identical to particle energy (keV). Consequently pulse height spectra of α and β radionuclides overlap even though the energy of α nuclide is ~10 times that of β and poses a difficulty in the LSC of α particles in presence of β emitters.
Advantages of α/β LSC are simple and rapid sample preparation and measurement of both α and β activities in a single count as revealed by Pates J. M. (1996).  However, α/β separation is strongly dependent on a number of variables such as degree of quenching, and incident α or β particle energy as explained by DeVol T.A., 2007.  AEC tracks quench via quench indicating parameter, tSIE and automatically adjusts counting regions to maintain spillover at a nearly constant rate. Grau Carles (1991)  showed that AEC reduces the error in liquid scintillation counting of dual label samples which normally occurs when fixed counting windows are employed under sample conditions of appreciable quench.
The question of the relative impact of quenching on α and β events is examined by analyzing various quench standards and correlating counting efficiencies of both the events to the corresponding tSIE parameters to construct the quench correction curves. In LSC, while analyzing the sample, degree of quenching in the sample must be determined and compared to quench calibration plots to derive sample activities from the net count rate (Bhade et al, 2010).  The absolute α and β activities in composite samples were derived as follows.
4.1 Gross α and β activities calculation
While implementing inclusion technique for quantification of α and β activities in composite samples, count rates observed in lower (say A) and higher (say B) counting regions were a function of both α and β disintegrations. Equations 2 and 3 were used to calculate the gross α and β activities (L'Annunziata, 2003). Efficiencies of α and β particles in region A and B were defined as EαA, EαB and EβA , EβB respectively . These four efficiencies were used to determine the gross α (Aα) and gross β (Aβ) activities by solving the following equations:
Where Z A and Z B denotes count rates of a composite sample in region A and region B respectively. Aα and Aβ indicates disintegration rates (DPM) of α and β radio nuclides. Solving simultaneous equations 2 and 3, we got
The accuracy of α and β activity analysis was measured in terms of percentage deviation. Ratio of the difference between true dpm and calculated dpm to the true dpm gives the relative % deviation with respect to the true activity concentration values of α and β standards as derived from the equation.
In order to assess the reliability and accuracy in determining gross α and β activities by AEC and knee point technique, spiked synthetic samples were categorized into three sets containing (i) equal α and β activities (α/β~1) (ii) low α high β (LαHβ with α/β activity ratio in the range 1- 1/30) and (iii) high α low β (HαLβ with α/β activity ratio in the range 1-30/1) activity concentrations. For the first category of test samples (α/β~1), α and β activities derived by AEC technique were found to be in good agreement (deviation < ±10%) with the true activities.
Test samples (LαHβ) in the second set were spiked with 241 Am of low activity concentration and 36 Cl of high activity concentration in order to cover a range of α/β activity ratios 1:5 to 1:30. Almost all real samples such as drinking water, environmental and effluent samples are characterized by their high β and low α activity content (Davila et al, 2002).  When the test samples were spiked with higher activity concentrations of 241 Am and lower activity concentrations of 36 Cl with various α/β activity ratios ranging from 5:1 to 30:1 and analyzed by AEC HαLβ calibration plot, calculated α and β activities were found to be within ±10%. A little rise in % error was observed for highly quenched samples. [Figure 2] illustrates composite alpha beta liquid scintillation spectra of test sample analyzed by AEC technique.
The shortcoming of AEC inclusion technique lies in the incapability of resolving the composite spectrum into two distinct α and β spectra. AEC does only the quantitative estimation of gross α and β activities, but qualitative or spectrometric evaluation of the spectra is not possible. Application of this technique to real samples (e.g. effluent samples) requires a prior knowledge about the samples such as ~ α/β activity proportion, sampling location and source/origin of the samples to get an idea of the radionuclides involved. This will facilitate accurate determination of gross α and β activities in real samples.
| 5. Conclusion|| |
Liquid Scintillation AEC technique was successfully applied to the radio assay of α and β activities in synthetic composite samples. AEC offers a simple, less time consuming technique for the simultaneous measurement of gross activities in a sample. AEC technique gave acceptable results (deviation < ±10%) for different test samples with varying degree of quench and wide range of α/β activity ratios. Although α/β LSC is a potentially reliable analytical technique, its ability to produce meaningful results is dependent on accurate calibration and optimization of discriminator settings. Specific to various types of samples, a more detailed study should therefore be carried out to apply and check the feasibility of the proposed technique to real samples.
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[Figure 1], [Figure 2]