X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Patent
1998-02-18
1999-11-23
Porta, David P.
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
378 987, 250214VT, H05G 164
Patent
active
059913592
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The current invention relates to a camera system for making video images of moving or still objects illuminated by pulsed gamma or neutron radiation.
2. Discussion of Prior Art
Solid state charge coupled device "CCD" cameras when used in a high energy gamma or, neutron radiation environment (whether pulsed or not) suffer from radiation induced artefacts. These artefacts are caused by scattered gamma or neutron radiation and are referred to by those skilled in the art as gamma induced background or neutron stars. These artefacts arise when the scattered gamma or neutron radiation passes through the camera structure to directly affect an imaging sensor within the camera. As a result an image produced by the sensor, lacks purity because of these radiation induced artefacts. In some circumstances the image is totally masked by the artefacts. Shielding the camera from the source of radiation in many instances is either impractical or ineffective.
The problem to which the current invention is addressed can be illustrated by referring to a paper in IEEE Transactions on Nuclear Science Vol 39 No Oct. 5, 1992. Experiments discussed in this paper demonstrate the capability to measure bursts of neutron fluences and gamma or X-ray fluxes directly with CCD cameras. In these experiments the neutron fluences or gamma fluxes form not only the incident beam but also give rise inevitably to scattered neutron fluences or scattered gamma beams that directly interfere with the television "TV" camera sensor during the same time window.
A conflict arises in that an image that is formed on a radiation to light converter or fluorescer by the incident beam has both intensity and spatially quantifiable information whereas the image formed on the TV camera sensor by the scattered beams have neither. Consequently, when both the image on the fluorescer and the TV sensor are combined in the TV sensor the result is a composite from both sources.
SUMMARY OF THE INVENTION
The objective of the invention is to remove from the final image produced by the TV camera the spurious signals caused by the scattered radiation interfering with the TV sensor. These spurious signals arise in the case of the ionizing radiation in the form of a photo charge but cause no permanent damage. In the case of the high energy particles they can cause localised current spikes and are stored within the TV sensor also as a photo-charge. For example collisions between neutrons and silicon nuclei produce photo charge artefacts known as neutron stars.
The Applicant has found that the high energy particles produce no permanent damage possibly as a result of restricting the accumulated dose to the TV sensor when a pulsed source of radiation is used.
In a CCD, a light image on the TV sensor is absorbed by a silicon layer to give rise to an electron and hole pair of which the electron is subsequently collected by a polysilicon electrode structure within the CCD device to generate a photo signal. The depth at which the electrons are produced is dependent on the wavelength of the light but 90% of all light at 400 nm is absorbed within the first 25 micrometers of silicon and 90% of all light at 900 nm is absorbed within the first 60 micrometers of silicon. Thus only a thin silicon layer is required to absorb and detect the visible illumination.
Thus it can be seen that the scattered radiation and the light emitted as a result of the interaction of the direct radiation with the fluorescent screen produce similar photo charge within the TV camera sensor.
As both images occur at the same time the problem is how to separate them. Any attempt to refresh the TV sensor would of course remove the spurious or unwanted image as well as the wanted image which is obviously quite unacceptable apart from losing the use of the TV sensor for at least 1 millisecond.
According to the current invention there is provided an apparatus for recording a transient radiation event consisting of direct and scattered radiation having a first fluo
REFERENCES:
patent: 4679212 (1987-07-01), Hynecek
patent: 5569907 (1996-10-01), Meunier
Porta David P.
Schwartz Michael J.
The Secretary of State for Defence
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