IMPROVED TECHNIQUES FOR THE ACTIVITY STANDARDIZATION OF 109 Cd BY MEANS OF LIQUID SCINTILLATION SPECTROMETRY

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1 IMPROVED TECHNIQUES FOR THE ACTIVITY STANDARDIZATION OF 109 Cd BY MEANS OF LIQUID SCINTILLATION SPECTROMETRY K Kossert 1 O Ott O Nähle Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, Braunschweig, Germany. ABSTRACT. Recently, we developed a simple method to determine the activity of 109 Cd by means of liquid scintillation spectrometry using commercial counters. The method is based on a spectrum analysis that allows determining the emission rate of conversion electrons. The results are in good agreement with the outcomes of other methods. However, the relative standard uncertainty was found to be 0.65% and is influenced by the peak area determination. In this paper, the results of further studies of the technique are presented. The main idea of these studies is to improve the counting efficiency, and thus to optimize the energy resolution of measured spectra. The studies comprise comparisons of various liquid scintillation vials, e.g. glass vials, glass vials covered with adhesive tape, etched glass vials, sandblasted glass vials, and polyethylene (PE) vials. We also tested promising liquid scintillation cocktails and mixtures. The measurements were carried out with 2 commercial spectrometers: a Wallac 1414 Guardian TM and a PerkinElmer Tri-Carb 2800 TR TM. A considerable reduction in the relative uncertainty has been achieved. INTRODUCTION The activity of 109 Cd solutions can easily be determined by means of conversion-electron counting of samples in a pressurized proportional counter (PPC). In a recent paper, it was shown that an analogous method can be applied for analyzing spectra of a liquid scintillation counter (Kossert et al. 2006). The activities determined with this new approach were used within the scope of an international comparison (Ratel et al. 2005), and results were found to be in good agreement with results obtained with a PPC. However, the relative standard uncertainty of the activity was considerably larger due to the relative low-energy resolution of LS spectra. In this paper, we present new studies aimed at an improvement in the energy resolution of LS spectra. The method is easily applicable and can be considered as an interesting alternative to the CIEMAT/NIST method, which now also provides good results in the case of 109 Cd (Kossert and Grau Carles 2008). METHOD The decay of 109 Cd starts with an electron-capture branch that leads to the metastable (T 1/2 = 39.6 s) 88-keV level of 109m Ag. The subsequent gamma transition mainly results in the ejection of conversion electrons. Figure 1 shows an LS spectrum of 109 Cd measured in a Wallac 1414 Guardian TM spectrometer with a quasi-logarithmic amplification. The rearrangement processes after the electron capture may lead to several X-rays and/or Auger and Coster-Kronig electrons, which are simultaneously ejected. The maximum energy deposition within an LS sample is lower than the binding energy of K electrons of the atomic shell of silver, i.e. lower than 25.6 kev. When an electron from an L-shell or from higher shells is captured, the maximum energy is even lower than 4 kev. Thus, these events form the 2 peaks in the low-energy region of the spectrum in Figure 1. 1 Corresponding author. Karsten.Kossert@ptb.de by the Arizona Board of Regents on behalf of the University of Arizona LSC 2008, Advances in Liquid Scintillation Spectrometry edited by J Eikenberg, M Jäggi, H Beer, H Baehrle, p

2 98 K Kossert et al. Figure 1 Pulse-height spectrum of 109 Cd measured in a Wallac Guardian TM LS spectrometer with logarithmic amplification. A background spectrum has been subtracted. The gamma transition can be considered as an independent decay, since the half-life of the isomer 109m Ag by far exceeds the coincidence and dead time of LS counters. The energy of conversion electrons E ce is given by the difference of the gamma-transition energy and the corresponding binding energy E b, i.e. E ce = 88 kev E b (1) The lowest energy is obtained for conversion electrons from the K atomic shell, which have an energy of about 62.5 kev. The vacancy created in the atomic shell again causes rearrangement processes, such that the total energy deposit within an LS sample due to conversion electron processes is between 62.5 and 88 kev. Conversion electrons are responsible for the large peak at high channel numbers in Figure 1. The goal of the method is to determine the emission rate of conversion electrons n ce. The activity A of the sample is then given by A = n ce /P ce, with P ce = (33) being the emission probability of conversion electrons taken from Kossert et al. (2006). The uncertainty of the activity mainly depends on the uncertainty of the counting rate n ce since the uncertainty of the emission probability is more or less negligible. In order to determine the counting rate n ce, we first determine the area of the conversion electron (ce) peak in the measured LS spectrum. The LS counting efficiency for conversion electrons with energies above 60 kev is unity provided that the samples are not quenched too much. This can be demonstrated using free parameter models (Grau Malonda 1999). Moreover, it can be shown that these conversion electrons produce a high number of scintillation photons yielding entries at high channel numbers. The shape of the peak stemming from conversion electrons has a low-energy tailing. This can be shown, for example, by means of measurements of the isomer 93m Nb. When determining the ce peak area, this tailing must be modeled.

3 Improved Techniques for the Activity Standardization of 109 Cd 99 The area assigned to conversion electrons is split into 2 parts as illustrated in Figure 2. The left part (A) below the minimum of the valley below the large ce peak corresponds to the tailing region and may overlap with the low-energy region due to K X-rays or K Auger electrons. The right part (B) is obtained by summing up all counts above the minimum. The tailing part is simply modeled as a triangle. The uncertainty of the peak area was conservatively estimated to be 100% of the triangle area. The activity concentration a of the solution is then given by a = n A k + n B m ε k tm γ P ce (2) where m is the mass of solution in the sample, ε = 1 the counting efficiency for conversion electrons and k tm = ( λt m ) ( 1 exp( λt m )) a factor with decay constant λ to correct for decays during a single counting period t m. The count rates n A and n B correspond to the 2 regions of the peak area, i.e. the counts in these areas divided by the duration (lifetime) of the measurement. The correction k γ is explained in the next paragraph. Figure 2 Part of a 109 Cd LS pulse-height spectrum of a sample with 15 UG AB in a PE vial measured with a Wallac 1414 spectrometer. The peak of the ce peak is split into 2 parts. The lowenergy part (A) is modeled as a triangle whose height corresponds to the minimum of the valley (position of vertical line). The peak area of the high-energy part (B) is obtained by summing up all channel contents above the vertical line. A background spectrum has been subtracted. The Correction k γ Photons from the 88-keV transition mainly interact via Compton scattering in the LS sample. For the rarely occurring photoelectric effect, the energy is transferred to a photo electron, which then has the total energy minus its former binding energy. For Compton scattering, the highest energy is transferred to an electron when the scattering angle is 180 (backscattering). For 88-keV photons, the energy transfer is about 22.5 kev. If the scattered photon escapes from the LS sample without

4 100 K Kossert et al. further interaction, the event would contribute to the low-energy part of the spectrum. However, the backscattered photon may interact again, and consequently, the total energy transferred to electrons within the sample can be larger. Figure 3 shows a spectrum of the total energy transfer to electrons within a typical LS sample. The spectrum has been created by means of Monte Carlo procedures, taking into account the photoelectric effect, Compton scattering, and Rayleigh (coherent) scattering. Also, the atomic composition and the density of the sample have been taken into account. About 8% of the events in Figure 3 are located above the Compton edge, i.e. above 23 kev. Assuming that all these events create an entry in region A or B, this yields In the following, we use k γ = ( P γ ) 1 = 1/1.003 (3) with a conservatively estimated relative standard uncertainty of 0.15%. EXPERIMENTAL Figure 3 Electron spectrum computed by means of Monte Carlo simulations of 88-keV photons in an LS sample. The energy corresponds to the total energy transferred to electrons, i.e. the interactions of scattered photons are taken into account. k γ = (4) The experiments described in this article were done with several 109 Cd solutions. The activity of the solutions can be traced back to activity determinations as described by Kossert et al. (2006). The nominal activity concentrations of the solutions used for LS samples were between 50 and 140 kbq/g. The solutions consisted of cadmium chloride in HCl of concentration 0.1 mol/l with a carrier concentration of Cd + ions of about 28 μg/g. The samples were prepared by depositing drops of the active solutions by means of a pycnometer. The masses of the drops were determined gravimetri-

5 Improved Techniques for the Activity Standardization of 109 Cd 101 cally using a Mettler AT 21 balance traceable to the German national mass standard. The solutions were also measured by means of γ-ray spectrometers to search for other radionuclides. No photonemitting impuritiy was detected. The liquid scintillation measurements were performed in 2 commercial LS counters: a Wallac 1414 Guardian TM and a PerkinElmer Tri-Carb 2800 TR. The guard detector of the Wallac counter has been disconnected to avoid any veto signal. Both systems work with 2 photomultiplier tubes in coincidence mode. Studies to Improve the Resolution of Spectra The energy resolution of LS spectra depends on the number of detected scintillation photons. If more photons are detected, peaks move towards higher channel numbers. In addition, a higher number of detected photons corresponds to lower relative deviations. In other words, better statistics reduce the full width at half maximum (FWHM) of peaks. In order to improve the resolution, the number of produced scintillation photons should be as large as possible and losses of photons should be reduced. In our first approach based on LS counting, we started with a sample with 15 ml Ultima Gold (UG) and 1 ml of aqueous solution in a glass vial. A considerable improvement has been observed when using polyethylene (PE) vials instead of glass containers. Further improvements could be achieved by reducing the amount of water or using UG AB instead of UG. In the following, we will present some further systematic studies. Sample Composition and Cocktail Selection Let us consider a sample with 15 ml UG and 1 ml of aqueous solution. The radioactive isotope will be in the aqueous phase, i.e. it is embedded in a micelle surrounded by an organic phase. Any energy deposit within a micelle does not create scintillation photons (only for high-energy beta emitters could the Cherenkov effect create some photons). Thus, a reduction of the amount of water increases the energy deposit in the organic phase and, consequently, it improves the energy resolution and the counting efficiency. There are further ingredients in an LS cocktail that do not create light. For example, the sample stability can be improved using complexing agents. The complexing agent HDEHP is part of UG but it is not included in UG AB. This explains the slight improvement when UG AB is used. The compatibility of aqueous and organic phases is achieved by means of an emulsifier, which is part of the cocktail and also does not produce light. The cocktail UG F, which is especially designed to measure dry filters or organic samples, does not contain any emulsifier. However, since we are measuring aqueous solutions we need at least a small amount of an emulsifier. Therefore, we tested several mixtures with UG AB and UG F. Figure 4 shows the region of interest (ROI) of various spectra measured in a Wallac spectrometer. For the sample with UG in a glass vial, the ce peak is broad. For UG AB in a PE vial, this peak is shifted to higher channel numbers and the events due to K conversion electrons are adumbrated as a left-hand shoulder of the ce peak. The energy resolution improves with increasing amounts of UG F. The best resolution has been obtained with a mixture of 13 ml UG F and 2 ml UG AB. A further increase in the F/AB ratio led to milky samples. The spectra of such samples were shifted to even higher channel numbers, but the resolution was poor and the counting rates were by far too low. Such samples cannot be used for the method. The energy resolution is considerably lower for an untreated glass vial. A systematic comparison of different vials will be presented in the next subsection.

6 102 K Kossert et al. Figure 4 ROI of 109 Cd spectra of different samples measured in a Wallac 1414 LS spectrometer. The best energy resolution was obtained with a mixture of 13 ml UG F and 2 ml UG AB in polyethylene (PE) vials. Background spectra have been subtracted. Light Propagation and Selection of Vials The scintillation light is isotropically emitted within the liquid part of a sample. Since the refractive index of the cocktail and glass is very similar (n 1.5), the transition from liquid to glass causes neither considerable loss of nor significant changes in the direction of photons. For the transition from glass to air, we have a considerable reduction in the refractive index from about 1.5 to 1. Consequently, total reflection causes loss of light. These losses can be reduced by changing the wall material or the outer surface of glass. An alternative wall material is polyethylene (PE). In this material, light is diffusively propagated. Consequently, a loss of photons due to total reflection is reduced. A well-known disadvantage of PE is the lower stability. Organic molecules can diffuse into the wall. This corresponds to a change in the scintillating volume and causes a change in the spectra shape as well as changes in measured quenching indicators. In fact, we observed worse resolution of PE vials when they were measured after a couple of days. The problem could be reduced when the inner walls are coated with Teflon. Also, the whole vial could be made of Teflon, but this also increases the costs. Another attempt to overcome the disadvantages of PE is to use glass vials that are treated on the outside. For this work, the following 20-mL vials were tested: glass vials, etched glass vials, glass vials roughened by sandblasting, glass vials covered with adhesive tape, and PE vials. For the etching process (Kaihola 1988), about 15 g of sodium fluoride (NaF), 10 g of ammonium sulphate ((NH 4 ) 2 SO 4 ), and 15 g of barium sulphate (BaSO 4 ) were ground in a marmoreal mortar. Afterwards, the mixture was placed in a plastic bottle, containing about 8 g of oxalic acid, 12 g of water, and 40 g of glycerine. Cleaned and dried glass vials were placed in this mixture and stored for several days. The intensity of the etching effect depends on the storage time in this bath. It is to be

7 Improved Techniques for the Activity Standardization of 109 Cd 103 noted that the ingredients are harmful and the procedure requires special care. The procedure could also be carried out with commercially available glass-etching cream. For sandblasting, we used various gritting materials with different grit size. The result depends on the time and strength of the treatment and utilized abrasive grit material used. Two types of adhesive tapes were used: tesa Film matt-unsichtbar (width: 19 mm, tesa AG, Germany) and Scotch Magic (width: 19 mm, 3M, France). The vials were wrapped with 2 parallel strips of the respective tapes. Figure 5 shows some spectra using different vials. All samples were prepared with 15 ml UG AB. When glass vials are covered with adhesive tape, the ce peak moves towards higher channel numbers and the resolution is considerably improved. The spectra of covered vials are comparable with those obtained with PE vials, which still give the best result. The spectra measured with Scotch tape were slightly better than those obtained with tesa tape. Figure 5 ROI of 109 Cd spectra of different samples measured in a Wallac 1414 LS spectrometer. All samples were prepared with 15 ml UG AB. The energy resolution can be improved by covering the outside of glass vials with adhesive tape. Background spectra have been subtracted. The spectra of etched vials (not shown in Figure 5) were slightly better than those with adhesive tape but still worse than PE vials. The spectra of samples with 10 ml UG F and 5 ml UG AB in sandblasted vials, glass vials with Scotch tape, and PE vials are compared in Figure 6. Again, PE vials yield the highest light output. See Nähle et al. (these proceedings) regarding systematic investigations of the light output of different luminescent vials. Application of the Method with a Tri-Carb Counter So far, we have discussed spectra measured using a Wallac counter with a logarithmic amplifier. Figure 7 shows a spectrum measured in a Tri-Carb 2800 TR with linear amplification. Although

8 104 K Kossert et al. Figure 6 ROI of 109 Cd spectra of different samples measured in a Wallac 1414 LS spectrometer. All samples were prepared with 10 ml UG F and 5 ml UG AB. Background spectra have been subtracted. the spectrum shape is different, the method can be applied in an analogous manner. The lower spectrum in Figure 7 shows the valley before the ce peak and the split into the 2 regions A and B as discussed in the Method section and in Figure 2. RESULTS AND DISCUSSION The presented analysis shows that the light output and the resolution depend very much on the sample composition and the vial. We obtain the following order for cocktails, starting with the best result: 13 ml UG F + 2 ml UG AB 15 ml UG AB 15 ml UG 15 ml UG + 1 ml water For the containers, we have the following ranking: PE vials etched vials glass vials with adhesive Scotch tape glass vials with adhesive tesa tape sandblasted glass vials glass vials without treatment The same order is also obtained for the corresponding quenching indicator (i.e. the SQP(E) or the tsie is a useful measure for the energy resolution) provided that the samples are transparent (not milky).

9 Improved Techniques for the Activity Standardization of 109 Cd 105 Figure 7 LS pulse-height spectrum of 109 Cd in 10 ml UG F and 5 ml UG AB in a polyethylene (PE) vial measured with Tri-Carb 2800 TR. The peak of the ce peak is again split into 2 parts and the low-energy tailing is again modeled as a triangle, as shown in the lower figure. A background spectrum has been subtracted. It should be noted that the results also depend on the geometry, in particular on the filling height, which is correlated with the position of the meniscus of the liquid. The meniscus is a region of large light output (see Nähle et al. 2009). We also intend to study the light output of vials with smaller volume (e.g. 7 ml). Also, the counter may have a large influence on the result. In particular, the optical chamber, which should reduce the loss of photons, is highly important. The results of the determined activity concentration of one 109 Cd solution are listed in Table 1. The measurements were done with different vials and in the 2 counters mentioned above. All results are in good agreement, which indicates that the method has a good reproducibility. Studies with other 109 Cd solutions also showed good agreement when etched vials or other cocktail compositions were used. The best resolution and, consequently, the lowest uncertainties were obtained with 13 ml UG F and 2 ml UG AB in PE vials. Table 2 compares the uncertainty budgets for 2 different vials. The uncertainty budget for the sample with 15 ml UG AB corresponds to the data that were previously presented by Kossert et al. (2006). For a sample with 13 ml UG F and 2 ml UG AB in a PE vial, the relative uncertainty assigned to the peak area determination can be below 0.3%, and thus, the combined relative standard uncertainty has been reduced from 0.65% to 0.39%. Although this uncertainty is still larger than it is for a PPC, it is sufficient for many purposes. At Physikalisch-Technische Bundesanstalt, the new method is now frequently used for activity determinations of 109 Cd solutions. The method is powerful for measurements of solutions with low activities or those with higher density due to higher acid concentration. For such solutions, measurements by means of ionization chambers may fail. Thus, the new method amends or sometimes even replaces secondary standardization measurements with the aid of calibrated ionization chambers.

10 106 K Kossert et al. Table 1 Results of various LS samples measured in a Tri-Carb (T) and a Wallac 1414 (W) spectrometer. All samples were prepared with 10 ml UG F and 5 ml UG AB. The activity concentration (a) has been corrected for decay and is given for the same reference date for all samples. The last column contains the deviation to the mean value of the activity of the 109 Cd solution as measured by LS counting. This value is 0.09% lower than the reference activity determined by means of a calibrated ionization chamber of the PTB. Counter Vial SUMMARY AND OUTLOOK n A (1/s) n B (1/s) m (mg) a (kbq/g) n A /(n A +n B ) (%) The method for the activity determination of 109 Cd solutions by means of ce LS counting has been improved considerably. The main improvement has been achieved by using a mixture of UG F and UG AB. The resolution obtained using glass vials with adhesive tape and etched glass vials is close Δ (%) T Sand-blasted T Sand-blasted T Sand-blasted T PE T PE T Scotch tape T Scotch tape W Sand-blasted W Sand-blasted W Sand-blasted W PE W PE W Scotch tape W Scotch tape Table 2 Standard uncertainty components of the activity concentration a of a 109 Cd solution measured by 4π (LS)ce counting using 2 different sample compositions (components with u(a)/a < 0.001% are not listed). u(a)/a (%) Component UG AB in PE vial UG AB + UG F in PE vial Counting statistics Weighing Time measurements (starting time and duration (lifetime)) Background Peak area (separation of ec and ce events in spectra) Counting efficiency Emission probability P ce Correction for events in ce region due to keV gamma interaction Radionuclidic impurities (no impurity <0.01 <0.01 detected) Square root of the sum of quadratic components

11 Improved Techniques for the Activity Standardization of 109 Cd 107 to the unmatched PE vials. Glass vials have the advantage of higher long-term sample stability, which may be important for low-activity measurements. Moreover, it was shown that the method can be applied with a Tri-Carb counter in a manner similar to that of a Wallac spectrometer. Further studies to investigate and improve the optical chamber are planned. Also, smaller vials and other cocktails will be tested. Preliminary studies indicate that similar procedures could also be applied to other radionuclides. For 139 Ce, we obtained good agreement with coincidence counting measurements, but the uncertainties are considerably larger. ACKNOWLEDGMENTS The authors wish to thank P Krause, M Ehrlich, M Ehlers, Ch Niedergesäß, and F Stephan for their valuable assistance during laboratory work. We are also indebted to Dr R Edler (PerkinElmer) for his useful advice and for providing a batch of UG F. REFERENCES Grau Malonda A Free Parameter Models in Liquid Scintillation Counting. Colección Documentos CIEMAT. CIEMAT (1999), ISBN Kaihola L Recipe for safe etching of glass. Private communication from Wallac Oy (unpublished). Also available on WG/icrmetching.htm. Kossert K, Grau Carles A Study of a Monte-Carlo rearrangement model for the activity determination of electron-capture nuclides by means of liquid scintillation counting. Applied Radiation and Isotopes 66: Kossert K, Janßen H, Klein R, Schneider M, Schrader H Standardization and nuclear decay data of 109 Cd. Applied Radiation and Isotopes 64: Nähle O, Kossert K, Brunzendorf J Study of light emission processes for the design of liquid scintillation counters. These proceedings. Ratel G, Michotte C, Janßen H, Kossert K, Lucas L, Karam L Activity measurements of the radionuclide 109 Cd for the PTB, Germany, and the NIST, USA, in the ongoing comparison BIPM.RI(II)- K1.Cd-109. Metrologia 42 (Technical Supplement):