Implementation Of Spark Counter For Alpha Track Density Measurement In Alpha Autoradiography Of (Th-U)O2 Pellets.

Chetan Baghraa, D.B.Sathe, Nilima Walinjkar, Jitender Sharma, P.G.Behere, Mohd Afzal.

Advanced Fuel Fabrication Facility BARC Tarapur, 401502, India achetanbhaghra@barctara.gov.in

Keywords: Alpha Autoradiography, Spark Counter, LR-115 Film.

Abstract

Alpha autoradiography has been employed for measuring uranium content in (Th-U)O2 by using Image Analysis. But measurement done using image analysis is manual which requires lot of expertise and hence time consuming. Spark counter, an automatic counting system, is being explored in this study to overcome above mentioned difficulties. Conventionally, spark counter is used to measure Radon content where track density is 10 times lesser as compared to track density obtained in case of (Th-U)O2 pellets. An attempt has been made to implement the spark counter in measurement of track density in (Th-U)O2 pellets. In this study, a strippable LR-115 film was being used to carry out the autoradiograph, of (Th-U)O2 pellet. To optimize the parameter, alpha autoradiography was carried out by varying mylar film thickness and exposure time to establish the condition for measurement of track density using spark counter. Spark counter results were compared with those obtained using image analysis for system calibration. Experimental results showed that spark counter can be employed for track density measurement of (Th-U)O2 pellets.

Introduction

Alpha autoradiography is well known technique for evaluation of micro-homogeneity in nuclear fuel pellets. It can also be used for estimation of radioactive element present in the matrix by correlating its concentration with the amount of alpha particle emitted by it [1]. Conventionally; it is carried out by exposing CN 85 or CR 39 film to nuclear fuel pellets followed by etching using weak alkali solution to develop the alpha track for viewing under optical microscope at 50- 200X magnification [2]. Finally, alpha track density estimation is done by Image analysis. But measurement of track density by this method is associated with errors and difficulties as mentioned below:-

a) Since the counting of track density is done on magnified image which cover very small portion of the film, the difficulties in taking images of whole film and processing of these images for alpha tracks using Image analysis is inevitable and require a great deal of expertise.

b) It is very time consuming process if you need to carry out alpha-autoradiography of number of pellets or even if you want to cover the whole area of a single pellet.

To overcome these difficulties, an alternate technique i.e., Spark Counting system, is tried for counting of alpha track density. CN 85 or CR 39 film cannot be used for spark counting since alpha active layer of these films cannot be stripped from its polyester base. For spark counting, LR 115 strippable film is used which is special cellulose nitrate film dyed deep red in color [3].

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This film can be stripped from its base after etching. Then the etched alpha film is placed between two electrodes forming a simple capacitor as shown in Figure 1.

As a DC voltage is applied to the capacitor, the track region starts conducting through the air column between the electrodes. An aluminized Mylar used as the top electrode vaporizes when the break down occurs, effectively removing the conductor on one side. The linearity of the spark counter depends on a number of variables, including thickness of the aluminum layer of the electrode (Aluminized Mylar), RC circuit parameters, etc. In this study we carried out an experiment to evaluate the feasibility of using spark counter for track estimation in alpha autoradiography of (Th-U)O2 pellet.

Experimental Setup

The sintered (Th-x%U)O2 pellets containing 1,3,5,10% UO2 were cold mounted and prepared using the standard Metallographic preparation procedures. The polished specimens were thoroughly cleaned to remove the traces of loose particles. The alpha sensitive LR 115 strippable film was exposed by keeping it in close contact with the polished specimen under a weight. After the irradiation, the exposed film was chemically etched in a constant temperature bath at 60◦C using 2.5 N NaOH solutions. The films were again cleaned thoroughly in water and dried. The developed alpha tracks were observed under an optical microscope to monitor the alpha track. The aluminum Mylar used during spark counting is also viewed under microscope to counting sparks for comparison between track density and spark density. The Calibration curve is also plotted between tracks counted using spark counter versus UO2 percentage in (Th-U)O2.

Results and Discussions

Figure 2 shows the plot between track density measured by spark counter and UO2 percentage. The linearity between the two is evident only up to 5% UO2. Figure 3 shows the alpha track image on LR 115 film and spark image on Aluminum Mylar.

Figure 1 : Spark Counter System

Figure 2 : Plot between Spark Count and Uranium Concentration

Figure 3 : Alpha autoradiograph Image and spark track Image of (Th-3 & 5%U)O2

pellet alpha - autoradiography

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It is evident from figure 2 & 3 that the valid correlation can be obtained between alpha track and spark track. The average size of alpha track and spark tracks are found to be 5 and 80 micron respectively. Although there is vast difference in the size of the tracks but counting can be done accurately for low concentration of Uranium in the matrix. But as uranium concentration increases, the gap between individual alphas tracks decreases. If the tracks are even closer than what is shown in figure 3 then one spark will engulf all the neighboring tracks and they all will be counted as one spark track. Figure 4 show the condition of spark track overlapping if alpha track are close to each other. Hence large diameter of spark track will results in shadowing effect on nearby tracks. Due to this reason, the number of track counted for 10%UO2 was found to be very less as compared to what is predicted as per its concentration.

Conclusion

A new system i.e., Spark counter, is employed for automatic counting of alpha track density on alpha autoradiography film exposed to (Th-U)O2 pellet. Experiment results showed that spark counter can be effectively used when UO2 content < 10%. Further increase in UO2 percentage requires reduction in track density on alpha film. The large increase in track size on Mylar film put a bounding condition to reduce the number of track on LR 115 film so as to avoid overlapping and error in calculation of number of track using spark counter. To avoid these errors the track density on LR 115 needs to be reduced by reducing the exposure time as well as increases the Mylar thickness between pellet and the film.

Acknowledgements The authors gratefully thank their colleagues in AFFF for their help and support to carry out this work.

Figure 4: Overlapping of Spark Track shown in spark image of (Th-10%UO2) pellet. 4(b) Magnified Image of overlapping at 200X

(Th-10%U)O2

References

1. T.M. Kegley, Jr ., Metallography 5 (1972) 113. 2. B .B . Shriwastwa, S . Majurridar and J .K . Ghosh, Nuclear Instruments and Methods in

Physics Research (1992) 370-372 3. K.P. Eappen, Y.S.Mayya, Radiation Measurement, 38(2004) p 5.