Radiation Protection and Environment

ARTICLE
Year
: 2011  |  Volume : 34  |  Issue : 1  |  Page : 41--43

Development of a plastic scintillator based large area ground surface contamination monitor


P Ashok Kumar, Anand Raman, D.A.R. Babu, DN Sharma 
 Radiation Safety Systems Division, Bhabha Atomic Research Centre, India

Correspondence Address:
P Ashok Kumar
Radiation Safety Systems Division, Bhabha Atomic Research Centre
India

Abstract

This paper presents the features of a recently developed ground surface contamination monitor based on wide area plastic scintillation detector and investigates the design and performance characteristics of the system. The system incorporates an innovative methodology for suppressing the general background radiation influence on the detector response. The methodology involves the incorporation of a suitably positioned gamma sensitive Geiger Muller tube, the output of which is used to compensate for the general background interference in the response from the plastic scintillation detector. This paper reports on the performance evaluation of the system and the parameters considered for setting the alarm levels.



How to cite this article:
Kumar P A, Raman A, Babu D, Sharma D N. Development of a plastic scintillator based large area ground surface contamination monitor.Radiat Prot Environ 2011;34:41-43


How to cite this URL:
Kumar P A, Raman A, Babu D, Sharma D N. Development of a plastic scintillator based large area ground surface contamination monitor. Radiat Prot Environ [serial online] 2011 [cited 2020 Aug 4 ];34:41-43
Available from: http://www.rpe.org.in/text.asp?2011/34/1/41/93947


Full Text

 1. Introduction



Radiation protection is an integral component in the operation of all nuclear facilities and in that, contamination monitoring serves a major role. For nuclear facilities in India, the Atomic Energy Regulatory Board stipulates that surface beta contamination levels should be below the specified derived working levels (DWLs) (AERB, 2005), which happens to be 3.7Bq/cm 2 for beta/gamma contamination. This entails regular area contamination monitoring to be carried out as per the regulatory stipulations to ensure the zonal status of the working areas.

Generally, the surface contamination monitoring in nuclear facilities involves the routine collection of smear samples and later counting them using laboratory installed GM counting setups. The alternate detector system used for these applications is the gas filled proportional counter based monitoring system, which has it's own merits and demerits (Klett et al, 2006). The ground surface contamination monitor which is the subject matter of this paper incorporates a large area-based plastic scintillator in correlation with a Geiger Muller detector.

 2. Design Features



2.1 General hardware

The system hardware comprises of two large are plastic scintillation detectors arranged to make up an area of approximately 1000 cm 2 . The detectors are mounted in a mobile trolley, at a distance of 10mm from the ground surface. An energy-compensated Geiger Muller tube is placed above the detector plane at a distance equivalent to 30 mm above the ground surface. The signal processing hardware and the HV bias for the PMT coupled to the plastic scintillation detectors is housed adjacent to the detectors along with a rechargeable battery on the trolley. The detector signals processed by this module is then input to the data processing module which is mounted at a height of 60 cm above the ground surface on the rotatable handle of the trolley. The data processing module is based on the NXP make 89LPC935 microcontroller and incorporates an embedded program for computation of contamination and the interface with the display and alarm segments. The total system operates on 12V rechargeable battery, housed in the system:

1. Data processing Software

The system software records the individual counts from each of the three detectors over 5 second interval, and further to that deducts the correlated counts from the values recorded for the plastic scintillation detectors in relation to that measured by the Geiger Muller counter. The output of the data processing module in terms of contamination levels of Bq/cm 2 is displayed on the front panel. In addition, a selectable switch on the front panel, also helps to obtain the individual count rate from each of the detector as well as the general background exposure rate measured by the GM detector in μR/hour.

2. Evaluation of detector parameters

a) Detector efficiency

The beta efficiency of the bare large area plastic scintillator with respect to distance from a Sr 90 -Y 90 source of 1876 Bq, was obtained and found to be 40% when the source was at close contact. In order to provide protection against contamination of the detector surface, a thin 50 micron plastic film was used, the advantage being, that these sheets can be replaced whenever the accumulated contamination on these sheets increase beyond the background threshold levels. At 50 micron thickness, the cutoff beta energy is 60 kev (Cember, 2010). The beta efficiency of the detectors when bare as well as used with the contamination protection sheet is given in [Figure 1]. The efficiency of the detector reduces from a maximum of 35% on contact to 5% over a distance of 30 mm. A source to detector distance of 10 mm has to be maintained for achieving a beta efficiency of 20%.The gamma sensitivity of the system was measured to be in the range of 2.2cps/μR/hr.{Figure 1}

b) Uniformity of response over the sensitive area

The variation of the response of the detector over the surface with a beta source located at different positions just above it has also been obtained and the efficiency variation along the central line parallel to the longer side of the detector is shown in [Figure 2]. An electroplated standard Sr 90 -Y 90 source was used for the purpose. The scintillator shows reductions in efficiency at the borders of the detector on both sides. The profile is slightly asymmetric and the maximum is centered. The relative yields are well within 20% of the maximum.{Figure 2}

Any beta/gamma contamination application will have to account for the general background based gamma compensation. This has been realized by carrying out simultaneous gamma background measurement and then subtracting the correlated gamma counts from the plastic scintillation detector.

c) Calibration and estimation of contamination

The calibration of the individual plastic scintillation detectors was carried out for different gamma dose rates, and the same has been set up in a look up table in the system software. Further the GM tube also has been calibrated for the same dose rates and the data fitted in to the software to convert the GM counts in to the ambient gamma dose rate. Holding this GM based dose rate data; the corresponding contribution in to the counts recorded by the plastic scintillation detector is computed through extrapolation from the lookup table and then compensated for in the absolute counts recorded by the two detectors individually. The net counts thereby obtained are considered as the contamination measured by the system.

A Sr 90 -Y 90 source of area 500 cm 2 area, with a source distribution of approximately 4 Bq/cm 2 , was used as a test source and the contamination as measured by the system was obtained in the order of 3.7 to 4 Bq/cm 2 . The minimum detection level for the system was computed to be of the order of 0.2 Bq/cm 2 .

3. Parameters for setting alarm levels

The AERB limits being 3.7 Bq/cm 2 , it was decided that the alert levels should be at least one tenth this limit value. With an effective area of 1000 sq cms, the total contamination under the frame of the detector would be 3700 Bq. Since the efficiency for beta contamination at a distance of 10 mm is obtained to be 20%, 740 counts would be approximately recorded for this contamination level. One tenth of this would be 74 counts approximately, which is the limit set for the alert levels of the system. Any count value recorded more than this would be considered as a definite indication of contamination, requiring health physics intervention.

 3. Conclusion



The large area plastic scintillator is suitable for incorporation in contamination monitoring applications. The detectors have been used for beta contamination monitoring in ground surface contamination monitoring applications. A prototype ground surface ontamination monitor has been developed based on the large area plastic scintillator and the minimum detectable level of this system has been established to be 5% of the stipulated Derived Working Levels.

 4. Acknowledgements



We gratefully acknowledge the guidance and encouragement given by Shri. H.S. Kushwaha, Director, HS&E Group, BARC while carrying out this study. We thank our colleagues, Smt. Shubhangi Wani and Shri Satish Joshi for their able support and cooperation during the course of this development work. We thank our colleagues at RSSD workshop in the fabrication of the mechanical assembly.

 5. References





AERB (2005), Radiation Protection for Nuclear Facilities: AERB Safety Manual, Manual No.: AERB/NF/SM/O-2 (Rev-4), AERB. Cember H., Introduction to Health Physics, Pergamon press, New York, 2010.Klett A., Haefner P. and Wilfried Reuter W. (2006), Comparison Of Scintillation And Gas Filled Detectors For Contamination Monitoring, 2 nd European IRPA Congress On Radiation Protection, 2006.Knoll G F, Radiation Detection Measurement, Second Edition, John Wiley (1989)