1.6.1 The ALADIN Camera
A remote gamma ray mapping technique has been developed by the French atomic energy agency (Simonet 1989). This method is based on the photographic pinhole camera and is named ALADIN (Appareil de Localisation d'Activité à Distance en Installations Nucléaires). It essentially comprises o f a radiation proof "black box", with a single aperture on one side and a detection system on the other. Two versions of the device have been developed:
1) A "static" system where images must be processed off-line. In this system, conventional photographic film is used to take an optical image of the scene while radiographic emulsions are used to map activity distributions.
2) A real time system, where optical and radiographic images may be viewed during data acquisition. This is achieved using a detection system based on a
Chapter 1; Gamma Ray Imaging Systems
scintillation screen placed before a charge couple device (CCD) video camera. This system is currently under development.
Radiographic and optical images are then combined to localise sources within a scene. In both cases, digital image processing is performed on the data to enable physical information and calibration parameters to be taken into account. This facilitates improved image presentation and allows dosimetric calculations to be made.
In terms of forming a photographic image, the pinhole camera has the property that it may obtain an image of an object at any distance without the requirement for optical focusing. This is particularly useful in contaminated areas where human access may not be possible. It also avoids the problem o f a glass lens darkening through radiation exposure.
The camera body is assembled from a tungsten alloy. The high atomic number (74) and density (19 g/cm^) of tungsten make it a good choice for gamma ray shielding. The camera thickness and consequently weight are therefore minimised. The design allows for a range o f removable pinhole apertures to be used. The apertures are also constructed from tungsten alloy and are essentially double cone collimators. In addition, an optical shutter is placed in the device. This is transparent to gamma rays and allows gamma ray image formation to continue after the optical image has been taken.
The "static" system is shown in Figure 1.8. Photographic film and radiographic emulsion are combined in the same cassette. This serves to improve image registration in later stages. It also reduces the total exposure time for both images and eliminates the requirement for manual intervention to change film cassettes. A series of radiographic emulsions with different sensitivities are loaded for each use. This allows a broad range of activities to be simultaneously detected in a single exposure. After exposure, the camera is removed from the active environment and films are taken for development in a dark room. The developed films are then digitised on a negative reader. Optical and radiographic images are finally combined on a microcomputer. A crude indication of the dose rates present in an environment may be obtained from optical density analysis o f the radiographic images and knowledge of physical dimensions involved. The isotopic content of the sources may be obtained in a similar way.
The real time system is still under development and records all images (both radiographic and optical) with a CCD video camera . A scintillation screen is used to convert gamma photons into optical photons. The screen is transparent to optical photons such that photography is not disturbed. The non-hygroscopic scintillator bismuth german ate (BGO) is proposed for use as the scintillation screen. The screen is optically coupled to an image intensifier via series o f fibre optics. The intensifier is
Chapter 1: G am m a Ray Im aging System s
then finally coupled to the CCD via a second series of optical fibres. The electronic system is connected via a control cable to enable remote operation of the device. Once again a shutter system is used such that the camera may continue to acquire radiographic images after optical image acquisition is complete. Images are initially buffered into the memory of a microcomputer during data acquisition. They are eventually stored to hard disk. A laser telemetry system attached to the side o f the camera records the source to camera distance required for dosimetric calculations.
Tungsten Container
Radiographic Film
Photographic Film
Figure 1.8. The ALADIN Camera.
The "static" device has been successfully used for decom m issioning and maintenance purposes within the French nuclear industry since 1984. The angular resolution is reported to be as good as 1.5^ for two sources o f similar intensity. However, the inability to examine data on-line necessitates the use of several images to define an environment. Research is concentrated on developing the real time system.
1.6.2 Single Shutter Radiation Camera
The single shutter radiation camera was developed in the United States as part of a programme to develop mobile robots for reactor environments (DeVol et al 1990). The camera is intended to be mounted on a robot that could carry it into a radiation area,
Chapter 1; Gamma Ray Imaging Systems
acquire an image and send the data back to an operator. The device consists o f a shielded scintillation detector coupled to a photomultiplier tube. A single aperture is bored through the shielding. The detector is mounted on a pan and tilt table. The purpose of the table is to position the small camera aperture precisely (Figure 1.9).
Pb Shield
Aperture.
BGO
Pan and Tilt Table
Figure 1.9. The Single Shutter Radiation Camera.
It is designed for use with gamma ray energies ranging from 500 keV to 8 MeV and exposures of up to 5.16 x 10'^ Ckg'^h'T Images are formed by recording the count rate for gamma rays from a given isotope at several orientations of the aperture. The aperture acquires data at many different positions and in this way the image is formed pixel by pixel. Since only a single aperture is used, images can be formed without the deconvolution techniques required by other position sensitive detectors.
The scintillation detector is a 1.27 cm x 1.27 cm right cylinder BGO crystal coupled to a photomultiplier tube. This is chosen because of its high density (7.13
glcrv?) and effective atomic number (74), providing a sensitivity approximately three times that of Nal. However, the lower gamma ray scintillation efficiency of BGO leads to a reduced energy resolution. It has a high refractive index (2.14) for its own scintillation light and a significant amount created in the crystal is reflected internally when coupled to a glass window. This also serves to reduce energy resolution.
Chapter 1; Gamma Ray Imaging Systems
The crystal is centred within an ultra pure lead cylinder measuring 27.5 cm diameter x 15 cm height. The thickness of the cylinder is 2.5 cm. The aperture size is 0.625 cm. A shutter is also included for the aperture to improve image contrast. This is included because the total flux passing through the camera body will usually exceed that passing through the aperture. Therefore, images taken with and without the shutter open are subtracted to give a measure of the flux that passed through the aperture. The detector assembly may move ±20® in the vertical and ±180® in the horizontal by virtue of computer controlled stepper motors. This allows the aperture to be positioned within
1®.
An SCA is used to select the energy range of interest. Count rate data is calculated at each pixel position by a scaler timer. This information is then passed to a microcomputer and stored on disk for later image reconstruction. This is performed on a UNIX workstation and involves extracting the net count rate and forming a normalised image. Colour coding is used to illustrate the intensity of recorded counts in the image display.
The device is able to resolve to 0.635 cm at 25 cm from the camera face for the 724 keV and 756 keV lines of ^^Zr. However the device has poor efficiency, so useful images can only be obtained for high intensity radiation fields. Current development is aimed at adapting the device to low intensity radiation fields.