SU995154A1 - Photodetecting device - Google Patents

Description

(54) PHOTOSEURING DEVICE

one

The invention relates to photodiode devices and can be used to detect light signals of low intensity, preceded by more intense light signals.

A photodetector is known, which contains one photomultiplier tube and a prism of total internal reflection, by means of which multiple passage of the light beam through the photocathode is achieved and thereby an increase in the fraction of the absorption photocathode of light energy. As a result, the sensitivity of the photodetector 1 increases.

The disadvantage of this photodetector is that when the power of the input light flux changes in a wide dynamic range, such a photodetector is characterized by a high probability of false triggering under the influence of internal noise, which significantly limits the dynamic range in the low intensity region, since after the light signal input to the photoreceiver input high intensity at its output occurs plume noise pulses.

The closest to the proposed technical solution is a photodetector device containing one optical input, two photomultipliers, an optical beam-splitting element

5 to divide the input light flux into two beams and a matching circuit.

The input light coming from the optical input falls directly on the beam splitting element and is divided

10 in it to two beams, after which the first beam enters the photocathode of the first photomultiplier once, and the second beam onto the photocathode of the second photomultiplier. With the simultaneous appearance of the signal at the outputs of the first and

15 of the second multiplier, a coincidence circuit is triggered, which is a sign of the appearance of the input light flux 2.

A disadvantage of the known device is its low sensitivity, since a large part of the input light energy is lost at its input, since with a single passage of light beams through the photocathode of most known photomultipliers, the formation of photoelectrons takes less

30% of the incident light energy, moreover, while simultaneously knocking out single photoelectrons from the photocathodes of the first and second multipliers, the corresponding pulses at the inputs of the coincidence circuit in a number of cases do not trigger it due to static fluctuations in the time of flight of electrons in multiplying systems.

The purpose of the invention is to increase the sensitivity of a photodetector device.

The goal is achieved by the fact that in a photodetector device containing one optical input, two photomultipliers, an optical beam-splitting element for dividing the input light flux into two beams and a coincidence circuit, the optical beam-splitting element is made in the form of a prism having at least three edges with a mirror coating Accepting on one of the indicated faces, the coating has an opening for the passage of the input light flux, and the two faces of the prism without a mirror coating are flat and parallel The first contact with the photocathodes of the photomultipliers is between the output of the first photomultiplier and the first input of the coincidence circuit connected in series the first time reference circuit and the first one-vibrator, and between the output of the second photomultiplier and the corresponding input of the coincidence circuit are connected in series the second temporary diagram bindings and the second one-shot.

FIG. 1 is a schematic diagram of the device; in fig. 2 - time diagrams of current pulses on various elements of the device.

The device contains (Fig. 1) the first photomultiplier tube (PMT) 1, the photocathode 2 of the first. PMT, second PMT 3, photocathode 4 of the second PMT, optical beam-splitting element 5, prism faces 6 with a full mirror coating 7, prism face with a hole for passing the input light flux, temporary fixture circuit 8 and 9, one-shot 10 and 11, circuit 12 matches

FIG. 2 shows time diagrams of current pulses at the outputs of the first and second PMTs (a and b), at the outputs of the time-loop circuit (wig), at the outputs of one-shot (bottom), and at the output of the coincidence circuit (g)

The device works in the following way.

The input light flux enters through a hole in the mirror coating 7 and falls on the edge of the prism formed by faces 6, where it splits into two beams, indicated in FIG. 1 solid and dashed lines. Each of the beams undergoes multiple reflections from photocathodes 2 and 4 and mirror coating 7 to be completely absorbed in photocathodes, since at each reflection a part of the light

energy beams goes to the formation of photoelectrons.

Thus, the prism design minimizes unwanted light energy losses, since at the photocathode-vacuum interface, as a result of a non-sloping fall of the beams, the light reflection coefficient is close to unity and the light flux energy is localized in the space between the first and second photocathodes.

Timing diagrams explaining the operation of the device are given under the assumption that current pulses a and b at the outputs of the first and second photomultipliers are formed when knocking out single

photoelectrons from their photocathodes by a light signal of sufficiently short duration, while the differences in the temporal position of these pulses are due to the difference in the number of passage of the first and second light beams in the prism until the photoelectrons are knocked out of the first and second photocathodes and static fluctuations of the time of flight of electrons in multiples systems. Schemes 8 and 9 of the temporary reference produce short pulses

5 0 and g, the temporary position of which corresponds to the points of maximum steepness of the pulse fronts a and b, respectively.

The single-oscillators 10 and 11 are triggered by pulses from the outputs of the corresponding time-assignment schemes 0 and produce bottom pulses, the duration of which (each) is equal to

d, N -ai-n, -g

T + c-coss: + 9

where l is the maximum value of the fluctuations of the time of flight of electrons in multiplying systems used by photomultipliers;

N is the number of reflections of the light beam from the photocathodes, at which a predetermined ratio of the absorbed energy of the light to the incident energy is reached; With the speed of light in a vacuum;

fi is the refractive index of the prism material;

d is the distance between plane-parallel prism faces in contact with photocathodes; oL. is the angle of incidence of the beam on the photocathode, uniquely determined by the angle between faces 6 of the prism; TO is the minimum time of simultaneous action of pulses at the inputs of the coincidence circuit at which it operates. Under the condition of reliable coupling of one-vibrator pulses to the fronts of the output current pulses of photomultipliers, such a duration is optimal, since in this case the pulses of one-vibrators overlap in time regardless of the number of passage of the first and second beams in the prism to the moments of knockout of photoelectrons from the first and second photocathodes, while the probability of the overlap in time of one-shot pulses caused by the noise of the first and second multipliers is minimal. This circumstance makes it possible to reliably record weak light signals against the background of the internal noise of the multipliers indicated by high-intensity light signals preceding them or caused by the thermal emission of electrons from the photocathode.

FIG. 2 shows that the duration of the pulse W at the output of the coincidence circuit 12 is equal to the time during which the pulses of the single vibrators overlap.

In the case of using PMT-87 photomultiplier tubes, for which the span of flight time does not exceed 1; 10 s, the coincidence circuit with TQ is 5-10 s, prisms of size d 510 m and the refractive index of n 1.5 for the number of reflections N 6 pulses of one-shot calculated by the above formula is S-IO c. If at the same time the coefficient of light absorption in photocathodes for a single passage of a beam of K 0.2, then at least 90% of the input light energy is absorbed in them, which corresponds to a more than fourfold increase in the sensitivity of the device with respect to the known devices.

Claims (2)

1. USSR author's certificate number 544017, cl. H 01 J 39/06, 1975. 2. USSR author's certificate number 720571, cl. H 01 J 39/16, 05.03.80 (prototype). but