A flow-cell detection system was developed utilizing a coincidence circuit and tested with BaF{sub 2}, CaF{sub 2}:Eu and scintillating glass. The coincidence detection system reduced the background from {approximately}200 cps to {approximately}0.5 cps. The detection efficiencies for these cells ranged from 0.38 to 0.66 for {sup 45}Ca beta particles (E{sub max} = 0.257 MeV) and from 0.45 to 0.52 for {sup 233}U alpha particles (E{sub {alpha}} = 4.8 MeV). The minimum detectable activity was calculated for a 30 s count time and determined to be in the range of 1-2 Bq

Branton, S.D.

DeVol, T.A.

Fjeld, R.A.

UNT (University of North Texas) Digital Library

DEVELOPMENT OF A SCINTILLATION FLOW-CELL DETECTION SYSTEM FOR ENVIRONMENTAL RESTORATION AND WASTE MANAGEMENT APPLICATIONS Author: T.A. DeVol S.D. Branton R.A. Fjeld Contractor: South Carolina Universities Research and Education Foundation Strom Thunnond Institute Clemson, South Carolina 29634 Contract Number: DE-AC21-95MC29115 Conference Title: WM '96 Symposia Conference Location: Tucson, Arizona Conference Dates: February 25-29, 1996 Conference SDonsor: University of Arizona, the American Nuclear Society, the U.S. Department of Energy, New Mexico State University with Waste- Management Education and Research Consortium, and the American Society of Mechanical Engineers ContractinP Officer Rewesentative (COR): Cliff Carpenter, E06 TER DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manu- facturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. This report has been reproduced directly from the best available COPY. AvailabIe to DOE and DOE contractors from the Office of Scientific and Technical Information, 175 Oak Ridge Turnpike, Oak Ridge, TN 37831; prices available at (615) 576-8401. Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161; phone orders accepted at (703) 487-4650. & . -  Development of a Scintillation Flow-Cell Detection system for Environmental Restoration and Waste Management Applications' T. A. DeVol, S. D., Branton, andR. A. Fjeld Clemson University Department of Environmental System Engineering Clemson, SC 29634-0919 ABSTRACT A flow-cell detection system was developed utilizing a coincidence circuit and tested with BaF2, CaF2:Eu and scintillating glass. The coincidence detection system reduced the background fiom -200 cps to -0.5 cps. The detection efficiencies for these cells ranged fiom 0.38 to 0.66 for 45Ca beta particles &=0.257 MeV) and fiom 0.45 to 0.52 for 233U alpha particles (Ea = 4.8 MeV). The minimum detectable activity was calculated for a 30 s count time and determined to be in the range of 1-2 Bq. INTRODUCTION The use of ion chromatography (IC) for radiochemical separations is a well established technique. IC is commonly used in routine environmental monitoring applications as well as in specialized research applications. Typical usage involves the separation of a single radionuclide fiom the non-radioactive constituents. During the past decade, a limited amount of research has been conducted using automated IC systems in actinide separation applications (e.g., [ 13). More recently, separation procedures for common non-gamma emitting activation and fission products were developed utilizing a high performance liquid chromatography (HPLC) system [2,3] (Fig. 1). In addition, a separation procedure for six common actinides has been developed using a HPLC system [4]. These latter systems used on-line flow-cell detectors for quantification of the radioactive constituents of the effluent stream. Fig. 2 is an example of the actinide separation and on-line flow-cell detection of convenient activities (20-80 Bqhadionuclide) [ 5 ] .  In order to apply HPLC with on-line detection to environmental samples, sample preconcentration and a lower detection limit are a requisite. InsertFig. 1 here Insert Fig. 2 here Flow-cell scintillation detection systems have been developed over the past 30 years. Although other designs have been evaluated, the most common is a translucent tube in close proximity to two photomultiplier tubes (PMTs) coupled in coincidence. The coincidence counting system is used to reduce the background count rate. Radioactive samples come in contact with the scintillator in the flow-cell and Research sponsored by the U.S. Department of Energy's Morgantown Energy Technology Center, under contract DOE- 1 AR21-95MC32110 with South Carolina Universities Research and Education Foundation, Strom Thurmond Institute Building, Clemson, SC 29634-5701 the scintillation photons are measured by the PMTs. The flow-cell can contain either a homogeneous (liquid) scintillator or a heterogeneous (powder) scintillator. In a homogeneous detection system the sample is mixed with the liquid scintillation cocktail upstream of the flow-cell, and the mixture passes through the flow-cell for quantification. The major advantages of a homogeneous flow-cell are high probability of interaction and relatively low background count rate. Disadvantages include variable quench, relatively low luminosity, and increased complexity resulting fiom the additional pump and mixing apparatus. The heterogeneous flow-cell typically consists of an inert inorganic scintillator that is crushed and sieved into small (50-100 pm) particles. Advantages of a heterogeneous flow-cell are ease of use and relatively high luminosity. Disadvantages are higher background count rates and a high probability of self-absorption resulting in an overall detection efficiency that is lower than for a homogeneous flow-cell, particularly for low energy beta emitters. OBJECTIVES A project has just been initiated at Clemson University to develop a HPLC/flow-cell system for the analysis of non-gamma emitting radionuclides in environmental samples. An important component of this project is development of a low background, flow-cell detector that counts alpha particles and beta particles separately through pulse shape discrimination (PSD). The objective of the work presented here is to provide preliminary results of an evaluation of the following scintillators: CaF2:Eu, scintillating glass, and BaF2. Both CaF2:Eu and scintillating glass are common heterogeneous flow-cell detector materials. The advantage of CaF2:Eu is the higher luminosity while the advantage of the glass is its inertness. BaFz was chosen as a new material for investigation, with potential advantages during later parts of the project. PROJECT DESCRIPTION Flow-Cell Detectors CaF2:Eu and glass scintillators (GS-20: cerium activated lithium glass) were purchased from Bicron; BaF2 was purchased fkom Optovac, Inc. CaF2:Eu and BaFz were purchased as rough crystals that were subsequently crushed and sieved to a 63 to 90 pm particle size range. GS-20 scintillator was purchased as 63 to 90 pm particles. The scintillators were individually packed into 3.0 mm OD X 1.5 mm ID X 120 mm polytetrafluoroethylene tubing and coiled to 1.5 turns to yield an approximate active volume of 0.08 mL. Radioactive Sources Slightly acidic aqueous solutions of an alpha emitter, 233U (Ea = 4.8 MeV), and a pure beta UO2(NO& solution at pH 5.5 CaCl2 was dissolved in deionized water at pH 5.5 at a 45 233 emitter, Ca (Emax = 0.257 MeV), were used to evaluate the flow-cell. and concentration of 475 Bq/ml was used. concentration of 670 Bq/ml. 45 Electronic Circuit A schematic diagram of the electronic circuit used to acquire the data is presented in Fig. 3. The electronic modules were all standard Nuclear Instrument Module electronics. The flow-cell detector resides in a bath of silicon oil positioned between two Hamamatsu R292 PMTs that were separated by 1 cm. The anodes of the PMTs were grounded through a 50 R resistor and used for timing. The timing signal, generated by the Ortec 935 constant fiaction discriminator, was fed into an Ortec 567 time-to- amplitude converter (TAC). The TAC range was set to 50 ns, and the output gated the pulse height input to the analog-to-digital converter (ADC, Aptec MCA card). The pulse height signal was acquired fiom dynode 11 and had a 1 MCZ load resistor. The pulse height outputs fiom the PMTs were connected to Ortec 113 scintillation preamplifiers which were connected to Canberra 2021 amplifiers with 3 ps shaping times. The unipolar outputs fiom the amplifiers were connected to an Ortec 533 sum amplifier which is output to the pulse height ADC and the data stored on a personal computer. Insert Fig. 3 Here Minimum Detectable Activity In radiation detection applications, the traditional approach for quantmng detector sensitivity is through the lower l i t  of detection (LLD). LLD is defined, on the basis of statistical hypothesis testing, as the smallest amount of activity that will yield a net count for which there is a codidence at a predetermined level that activity is present [6]. For 5% risks of false detection and false non-detection, LLD is given as: (1) LLD = 2.71 + 4 . 6 5 6 ,  where Cg is the background count rate and t is the residence time of the sample in the detector. Minimum detectable activity (MDA) is a function of the theoretical LLD, count time, and detection efficiency: LLD E t  MDA=-, where E denotes detection efficiency. To lower the MDA, t and/or E could be increased, and/or CB could be reduced. For applications involving HPLC, t is limited by the resolution of the chromatographic peaks. A 30  second residence time is typical while 60 seconds would be an upper limit. For a heterogeneous flow-cell, E is limited by the particle size of the scintillator. The range of the u3U alpha particles in water is 43 pm. This range is on the order of the interstitial spacing in the flow-cell. A smaller particle size would yield smaller interstitial spacing and hence higher efficiencies resulting fiom less self-absorption, but is limited by increased back pressure. The remaining variable parameter affecting the MDA is CB. For this paper, coincidence detection techniques are used to reduce the background events that are attributed to thermonic emissions of electrons fiom the photocathode of a PMT. Coincidence Detection 3 The count rate of a coincidence detection system is related to the background rate in each detector in the following manner: (3) where KOin = the coincidence count rate, RI, R2 = the single detector count rates, z = resolving time of the detection and system. As stated earlier, zwas set to 50 ns. With the typical background count rate for a PMT at 500 counts per second (cps), the expected count rate of the coincidence detection system is 0.025 cps. The theoretical lower limit will be obtained only in the case when there are no electrical or magnetic interactions between the PMTs, when no optical cross-talk occurs, when the scintillator is not inherently radioactive, and when the shielding fiom external radiation is sufficient. RESULTS A typical pulse height distribution for a flow-cell detector is displayed in Fig. 4, shown here for the CaF2:Eu flow-cell. Tailing of the u3U alpha full-energy peak is due to self-absorption effects. In the cases of GS-20 and BaF2 the amplifier gains were adjusted to X3 and X2, respectively, the setting used for CaF2:Eu. Increasing the gain compensated for the reduced luminosity of the latter scintillators thus keeping the alpha peak channel approximately the same. Insert Fig. 4 Here The concept of using coincidence detection as a means to reduce the background count rate appears to have potential, but care must be taken in selecting a scintillator with a low intrinsic background. Table 1 summarizes the background count results for the CaF2:Eu flow-cell. Note that the measured background count rate is considerably above the theoretical background count rate (typical for the other scintillators as well), thus giving an indication of the margin for improvement. The elevated background count rates of CaF2:Eu and BaFz were attributed to the insufficient inertness of the materials resulting in adsorption of the radionuclide onto the scintillator. Elevated background count rates of BaF2 and also GS-20 were attributed to intrinsic radioactivity of the scintillation material. By gamma-ray spectroscopy it was determined that the intrinsic background in BaF2 was due, at least in part, to radium contamination. The elevated background of GS-20 was attributed to thorium, which is a common contaminant of glass. Insert Table I Here Flow-cell detector volumes were determined by a conductivity measurement using a NaCl solution that filled the detector. The detector volumes were all determined to be nominally 0.08 ml. Acidic 233U solution (233U02(NO&, pH 1) at a concentration of 130 Bq/ml was used to determine the efficiency of the flow-cells. An acidic solution was necessary to ensure minimal adsorption of the radionuclide onto the scintillator. The CaF2:Eu and GS-20 flow-cells had a detection efficiency of 0.54 for 233U. The GS- 20 and BaF2 flow-cells had detection efficiencies of 0.38 and 0.66, respectively, for 45Ca But since the uranium in the aqueous solution interacted with the BaF2 and the 45Ca interacted with the Cal?2:Eu, the efficiency could not be directly measured. Table 11 summarizes these results. The MDA was calculated using the experimentally determined background count rate (Table I) and detection efficiency, and a 30 second count time. Insert Table 11 Here APPLICATION Based on the results obtained in this study, coincidence detection should be used to reduce the electronic noise associated with the photomultiplier tubes. The reduction in the background count rate was several orders of magnitude (fi-om a single PMT rate of -200 cps to a background count rate of -0.5 cps). Despite the relatively high background count rates, the minimum detectable activities calculated for these flow-cells were -1 Bq for CaF2:Eu and BaF2, and -1-2 Bq for GS-20. FUTURE ACTIVITIES Presented here were preliminary results of the first task of the project. Additional work on this task includes the evaluation of additional scintillators and the addition of pulse shape discrimination, active shielding, and passive shielding to the detection system. Other tasks include the identification of sample interferences in the chromatographic portion of the apparatus, the development of sample processing protocols, and laboratory testing of the entire system using surrogate environmental and waste samples. ACKNOWLEDGMENTS The authors wish to thank David D. Brown for his assistance in preparing the radioactive solutions as well as measuring the detector volumes, Dr. John D. Leyba for assisting in the intrinsic radioactivity analysis of the scintillators, and Jagdish L. Malhotra, METC Contracting Officer’s Representative. REFERENCES 1. 2. 3. S. Usuda, “Rapid ion-exchange separations of actinides,” J.  RadioanaZ. NucL Chem., Vol. 123, NO. S. H. Reboul and R. A. Fjeld, “A rapid method for determination of beta-emitting radionuclides in aqueous samples,” Radioactiviv and Radiochemistry, Vol. 5 ,  No. 3, 1994. D. Bradbury, G. R. Elder and M. J. Dunn, “Rapid analysis of non-gamma radionuclides using the “ANABET’ system,” Waste Management, Tucson, Arizona, 1990, pp 327-329. 2, pp 619-631, 1988. 4. 5.  S. H. Reboul and R. A. Fjeld, “Separation of actinides by ion chromatography,” submitted for publication in Rdochimica Acta, 1995. S .  H. Reboul and R. A. Fjeld, “Potential effects of surface water components on actinide determinations conducted by ion chromatography,” Health Physics JmmaZ, Vol. 68, No. 4, pp 585- 589,1995. 6. L. A. Cunie, “Limits for qualitative detection and quantitative determination: application to radiochemistry,” Anal. Chem., Vol. 40, No. 3, pp 586-593, 1968. Count Rate (cps) P u l Q ) - J  o s ~ 8 o o o o  t- 4- I' Th-230 r +U-233 -P 0 8 8 Count Rate (cps) Table I. Results of the Single Detector Versus the Coincidence Detection System for a Typical Background. Scintillator R1 (cps) R2 (cps) (cps) 2RMcg CaF2:Eu 180.2 274.6 .005 0.405 GS-20 0.550 BaF2 0.378 'Theoretical coincidence rate as calculated with equation 3 2Experimentally determined background count rate Table II. Properties of Scintillators and Detection Limits. Scintillator ED Mw@d Ea MDAa@d CaF2:Eu 0.66. 0.96 0.49 1.29 t=3 Os GS-20 0.38 1.89 0.52 1.38 t=30s BaF2 0.66 0.93 0.45* 1.36 *Value could not be measured directly because of radionuclide interaction with the scintillator, so the value shown was estimated based on oiher scintillators. 

Development of a scintillation flow-cell detection system for environmental restoration and waste management applications

Abstract

Similar works

Full text

Available Versions

UNT (University of North Texas) Digital Library