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ANALYSIS OF SCINTILLATOR CHARACTERISTICS FOR DETECTION OF ILLICIT RADIOACTIVE SOURCES

CHAPTER ONE

INTRODUCTION

1.1   BACKGROUND OF THE STUDY

Scintillation detectors have been in use since the beginning of this century for the detection of ionizing radiation. These detectors make use of the property of certain chemical compounds which emit short light pulses (scintillation) after excitation by the passage of charged particles or by photons[1]. Scintillation is characterized by the absorption spectrum, emission spectrum, and decay times; the latter range from less than 1 nsec light yield (modern fast plastic scintillators) to few tens of nsec. Because of its easy availability in large sizes, ease of handling and reasonable efficiency for gamma-rays, plastic scintillator is an ideal radiation detectors for use in monitoring systems for detecting the presence of the radioactive materials and Special Nuclear Materials (SNM)[2].

This study describes two such systems developed indigenously using plastic scintillator detectors for detection of radioactive materials in nuclear facilities as well as in the public domain. The gamma rays emitted by SNMs and other radioactive materials forms the basis for detection. In India the monitoring requirements in the public domain makes it mandatory to have simple and compact system to meet the large requirement. Basically two types of systems i.e. a portal montor for pedestrians and a camouflaged Limb/Pole monitor have been developed, calibrated and installed at a few facilities.

The study describes the design, analysis and sensitivity of these systems. Metal detectors and neutron detection systems can be used to augument these systems to detect shielded sources and/or neutron emitting radioactive materials. Once a subject or package is isolated by these monitors, the extensive search and identification can be performed by using hand held monitors by authorized persons. When there is no movement of fissile / radioactive material, the system monitors the ambient radiation background and transmits data to PC to study the variation in background in the area.

1.2 STATEMENT OF PROBLEM

The radioactive materials of greatest security concern are commercial radioactive sources that contain relatively large amounts of ionizing radiation. Most of these sources are sealed in protective casings; typically double-encapsulated stainless steel to prevent accidental exposure to the radioactive material. Thus, to access this material, one would have to break open the seal. One would need some knowledge of radiation safety to perform this operation without inadvertently exposing oneself to lethal doses of radiation from the most powerful sources.

These sources “if not safely managed or securely protected could cause permanent injury to a person who handled [them], or were otherwise in contact with [them], for some hours. It could possibly be fatal to be close to this amount of unshielded radioactive material for a period of days to weeks.” For dispersal scenarios, there “would be little or no immediate health effects to persons beyond a few meters, [and] … the area to be cleaned up would probably not exceed a small fraction of a square kilometer.” This category includes oil well logging sources and fixed industrial gauges using high activity sources and includes level gauges, dredger gauges, conveyor gauges, and spinning pipe gauges.

1.3 AIMS AND OBJECTIVES OF STUDY

The main objective is to prevent radioactive material either unauthorised or accidentally being accepted by a steelworks. This will be done by developing and testing detector systems of high sensitivity suitable for use either at the point of entry of the scrap or within the steelplant. Work needs to be done on the types of detector, the long term reliability of detectors, their robustness, the optimisation of deployment of multi-detector systems, and the choice of alarm thresholds.

1.4 SIGNIFICANCE OF STUDY

The study is a very important one as it will reveal the various method involved in detection of illicit radioactive sources; It will satisfy two major requirements being usable by emergency responders in the field filling the gap between Radiation Portal Monitors and hand-held devices improving the state of art in detection of radioactive and Special Nuclear Material in terms of sensitivity for shielded SNM.

While the former was easily addressed by working together with end-users since the early stages of the project, the latter involves a precise assessment of the system capabilities. Therefore, a detailed set of performance requirements was developed during WP1. These requirements include the sensitivity to radiation sources moving at walking speed; this will enable MODES SNM prototype to be used to monitor travelling goods as well as to inspect static cargos, increasing the operational flexibility of the system.

1.5 SCOPE AND LIMITATION OF STUDY

The study is limited to the analysis of scintillator characteristics for detection of illicit radioactive sources;  the study will only show the suitable scintillator with its characteristics for the detection of redioactive sources; the study will also cover the areas where these radioactive sources are much like the border etc.

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