Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1991-03-11
1992-10-20
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
H01J 4014
Patent
active
051572508
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to photomultiplier tubes and, more particularly, to an automatic gain stabilization system for use with them. The method utilizes a light source, preferably a light emitting diode (LED), the signal of which is detected both at the first dynode and at the anode. By keeping the ratio between these two signals constant the actual gain of the photomultiplier tube is stabilized and possible drifts in the intensity of the light source are eliminated.
2. Description of the Prior Art
Photomultiplier tubes, or shortly photomultipliers, are common instruments in science and technology for detecting weak light levels. The photomultiplier consists of a photosensitive cathode, a chain of secondary emission electrodes called dynodes and an output electrode called anode with electric potentials arranged between them. The operation principle is as follows: Light flux hits the cathode which converts light photons into free electrons. The applied voltage directs them to the first dynode, from which every electron liberates several secondary electrons in a process called secondary emission. These are in turn directed to the next dynode, where the secondary emission is repeated and so on. The result is amplification by electron multiplication so that after the dynode chain the signal taken from the anode is high enough to be handled electronically. In some applications one of the later dynodes can be used as an output electrode.
An important quantity associated with photomultipliers is their amplification, or gain, defined as the ratio of anode current to cathode current and is typically 10.sup.5 -10.sup.9 depending on the number of dynodes, interdynode voltages and dynode materials. The gain should, naturally, remain stable during operation to yield ideal performance for the light detecting device. Unfortunately, this is not normally achieved but the gain tends to drift with temperature, variable light fluxes and ageing of the photomultiplier.
For correcting the gain instabilities a known solution is to employ a supplementary pulsed light source with standardized intensity to monitor the output of the photomultiplier and to adjust the gain according to the obtained signal by e.g. a feedback loop as presented by Ried and Gilland (U.S. Pat. No. 3,515,878). The pulsed light source can be e.g. a low-intensity lamp, a light emitting diode (LED) or a radioactive isotope in conjunction with appropriate scintillator.
A problem with mentioned stabilization light sources is that they, too, are susceptible to instabilities. These can be caused by thermal drifts, ageing, and alterations in reflective and/or absorptive properties in materials surrounding the light source-photomultiplier assembly. Consequently, the photomultiplier gain can never be more stable than the used stabilization light source. Accordingly, there is a need for a gain stabilization method that is not sensitive to drifts in the stabilization light sources. The present invention meets this requirement.
SUMMARY OF THE INVENTION
The present invention provides a gain stabilization system for photomultiplier tubes that is insensitive to drifts encountered with stabilization light sources.
By definition, the gain of the photomultiplier is the ratio between the anode current and the cathode current. This is equivalent to the ratio between the number of electrons at the anode and the number of electrons hitting the first dynode. The applicants have found that the signal of a LED emitting a few thousand photons in a flash with duration of some hundred nanoseconds is electronically detectable at the first dynode and, naturally, at the anode. By stabilizing the ratio between these two signals the actual gain of the photomultiplier is stabilized and effects of possible drifts in the intensity of the stabilization source are eliminated.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of one embodiment of the invention
FIG. 2 is a block diagram of another embodiment of the in
REFERENCES:
patent: 3183353 (1965-05-01), Baldwin
patent: 3714441 (1973-01-01), Kreda
patent: 4661693 (1987-04-01), Masanobu
patent: 5004904 (1991-04-01), Yamakawa et al.
Nurmi Jarmo
Oikari Timo
Messinger Michael
Nelms David C.
Wallac Oy
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