Electric lamp and discharge devices – Photosensitive – Photomultiplier
Reexamination Certificate
1999-07-27
2001-05-15
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Photosensitive
Photomultiplier
C313S1030CM, C313S1050CM
Reexamination Certificate
active
06232715
ABSTRACT:
This invention relates to a photomultiplier tube comprising:
a photocathode designed to be raised to a first electrical potential and with a semi-transparent photo-sensitive layer designed to receive an illumination from outside the tube and to transmit an electron flux into the tube, the density of the flux depending on the intensity of the illumination received by the photocathode,
focusing optics comprising a first dynode that will be raised to a second electrical potential, the value of which is higher than the first potential, that is provided with a “re-emitting” surface composed of a material encouraging secondary emission phenomena, the said surface being concave on the side of the photocathode, and,
several Rajkman dynode laid out on each side of a plane called the dynode plane. The first of the dynode closest to the output from the focusing optics will be raised to a third electrical potential, the value of which is higher than the potential of the second dynode, each of the subsequent dynode will be raised to an electrical potential higher than the potential of the preceding dynode, this series of dynode being designed to receive and amplify the electron flux from the focusing optics.
In most photomultiplier tubes that use Rajkman dynode based on the principle described above, the dynode plane is parallel to the center line of the tube. Therefore the dimension of the tube along this axis, called the tube length, is large. This may be prohibitive for many applications, for example when the tube is used within a gamma-camera for detection of radiation, it is desirable to use short tubes in order to reduce the size of the device in which they are fitted.
The purpose of the invention is to overcome this disadvantage by proposing a photomultiplier tube in which the plane of the dynode is not parallel to the center line of the tube.
A photomultiplier tube as described in the introductory paragraph is characterized according to this invention in that the focusing optics also comprise a second dynode that will be raised to a potential which is intermediate between the potential of the second and third dynode, the second dynode having a concave re-emitting surface on the side of the re-emitting surface of the first dynode, and in that the angle between the plane of the dynode and the center line of the tube, defined as being a center line perpendicular to the photocathode at its midpoint, exceeds 45°, the concave side of the first Rajkman dynode facing the re-emitting surface of the second dynode.
In this type of photomultiplier tube, the dimension along the length due to the series of Rajkman dynode reduces as the angle between the plane of the dynodes and the center line of the tube increases. The second dynode redirects the electron flux output from the first dynode towards the first Rajkman dynode. The second dynode may beneficially be equipped with a conducting grid placed across the path followed by the electron flux between the first and the second dynode, the potential of this grid being made similar to the potential of the second dynode.
In one particular embodiment of the invention, the angle between the plane of the dynodes and the center line of the tube is close to 90°.
With this configuration, the influence of the series of Rajkman dynodes along the total length of the tube can be reduced by a maximum amount.
In one preferred embodiment of the invention, a photomultiplier tube like the tube described above is characterized in that it comprises a grid placed between the second dynode and the first Rajkman dynode, that will be raised to electrical potential similar to the potential of the second Rajkman dynode.
The presence of the grid increases the collection efficiency at the first Rajkman dynode, in other words the ratio between the number of electrons received by the said dynode and the number of electrons transmitted by the second dynode. The grid generates a local electric field approximately parallel to the path between the second dynode and the first Rajkman dynode, which accelerates electrons in its neighborhood and directs them towards the first Rajkman dynode.
REFERENCES:
patent: 4575657 (1986-03-01), Kaiser
patent: 5510674 (1996-04-01), Kyushima et al.
patent: 5598061 (1997-01-01), Nakamura et al.
patent: 5914561 (1999-06-01), Venkatrao et al.
patent: 0495589 (1992-07-01), None
patent: 0671757 (1995-09-01), None
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Patel Ashok
Photonis
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