Photomultiplier

A photomultiplier or photo -multiplier (short photomultiplier, Eng. Photomultiplier tube, PMT) is a special electron tube for the purpose of weak light signals to detect ( and even individual photons) by generating and amplifying an electrical signal. A photomultiplier tube is typically comprised of a photocathode and a downstream secondary electron multiplier in an evacuated (10-6 ... 10-5 Pa) flask.

Operation

Take the photons on the photocathode and be solved by the photoelectric effect, electrons from the outer surface thereof, as in a photovoltaic cell. The liberated photoelectrons are accelerated in an electric field and impinge on other electrodes ( called dynodes ), from the surface of each incident electron knocks out a number of secondary electrons ( δ = 3 .. 10; δ is the secondary emission ratio, English secondary emission ratio. ). Thus, the number of electrons to the dynode of the dynode exponentially. For this to work, the dynodes must be on increasing ( in the diagram from left to right) positive potential. Usually is realized by the initial high voltage is divided down by a voltage divider chain. Finally, the electrons strike an anode and flow to the crowd. Thereby to produce a voltage drop across a resistor ( Ra in the drawing ). This voltage is the output signal.

The gain grows exponentially with the number of dynodes. Typical Multiplier have about n = 10 dynodes. If at each dynode 4 electrons ejected per incident electron, ( ie the current ), we obtain a strengthening of the electron number by a factor δ n = 410, which corresponds to about one million. The number of secondary electrons generated is proportional to the number of incident photons, so long as a saturation threshold is not exceeded, which ( the current flowing through the voltage divider chain stream ) is about 10% of the so-called cross- current. Thus the height of the output voltage in this linear region of operation is proportional to the incident photon number, so the intensity of light (analog operation mode).

Because of its high sensitivity, the most photomultiplier must be protected during operation before lighting with natural light, because the incidence of too many photons generated too much current and the ability of the coating of the dynodes ( eg alkali - antimonides, BeO, MgO, and particularly can weaken sensitive semiconductor layers such as GaP or GaAsP ) for secondary emission irreversible ( " blindness " ) and even a burn-through of the photomultiplier is possible.

Single photon detection

At very low light intensities in the so-called digital mode of operation (English and " photon counting mode " ) single photons can be detected with photomultipliers and thereby achieve a time resolution of less than 200 ps. The dynamic range extends from maximum count rates of several million photons per second up to the lower limit of less than 10 photons per second, which is limited only by the (largely thermally induced ) dark current. At room temperature, the typical dark count rate, depending on the photocathode material at about 10 .. 5000 1 / s ( engl. often also " cps = counts per second" ).

Pulse behavior

Due to the construction of a photomultiplier arise in Einzelphotonenzählbetrieb special characteristic impulse responses to short light pulses, which can reflect the actual measurement signals distorted and lead to misinterpretations. Even if it is ensured that only a single photon per pulse is registered ( at repetition rates in the megahertz range, limited by the downstream electronics, typically only one photon per 100 pulses), to create photons or electrons in the PMT, which is not on a direct path to the reproduction process have participated in the Photoelekronen, additional output pulses. These are registered as false photon events which are temporally correlated with the actual light pulse and so-called pros, late and Nachpulsen lead ( Afterpulsing ).

Application

In conjunction with scintillators they find applications as detectors of elementary particles. They are often (Super - Kamiokande Antarctic Muon And Neutrino Detector Array ( AMANDA ), IceCube experiment Double Chooz experiment ) used in large detectors for the detection of neutrinos in large numbers. The photomultiplier register while the photons produced by secondary particles, which are produced by the extremely rare interaction of neutrinos with matter. Photomultiplier are also used in Cherenkov telescopes to detect the faint flashes of light, which result from high-energy cosmic rays in the upper atmosphere.

In scintillation counters are also used for the detection of gamma radiation (eg gamma spectrometer or gamma camera) and in medical technology in PET systems for the detection of annihilation radiation, which results from the interaction of positrons with electrons ( pair annihilation ).

Furthermore, frequently used in the photomultiplier of the optical spectroscopy and light microscopy as a receiver, to detect light in the wavelength range from 100 nm ( UV) to 1000 nm ( IR) ( with special photocathode up to 1700 nm). In light microscopy photomultipliers are used as detectors in laser scanning microscopes, such as confocal laser scanning microscopy and multiphoton microscopes are used. In time-resolved fluorescence spectrometers and microscopes are used for determining the fluorescence lifetime in the digital mode of operation, often the method of time-correlated single photon counting is used.

In the scanning electron microscope photomultiplier are part of the Everhart -Thornley detector. Generated by the electron beam on the sample secondary and backscattered electrons ( SE Sheet - secondary electrons or BSE - back scattered electrons ) can be converted in the scintillator to photons which are supplied to the photomultiplier tube via a light guide and is converted into electrical signals.

Other types

A special form of photomultipliers are so-called microchannel plates photomultiplier (English microchannel plate photomultiplier, MCP- PMT or short MCP). In the micro-channel plate secondary electrons from the inner wall of thin microscopic channels are released, along which there is an accelerating electric field. They therefore constitute a homogeneous combination of dynodes and voltage divider chain, which for a time resolution of less than 30 ps is achieved. They are used inter alia in image intensifiers and preferably in time-resolved fluorescence spectrometers for high temporal resolution (but are many times more expensive than conventional photomultiplier ).

A specially coated input side of the microchannel plate with some restrictions can replace the photocathode and secondary electron multiplier with only one major channel will be produced, so-called channel electron multiplier (English channel electron multiplier ).

Another alternative design represent so-called hybrid photomultiplier ( HPMT or H ( A) PD for engl. Hybrid ( avalanche ) photo diode) dar. For them, the classical dynode and the cathode are replaced by an avalanche photodiode, which takes over the function of the secondary electron multiplier. Similar to the MCP- PMT, the large time differences of the electrons through the various stages of dynodes be avoided and a time resolution of about 100 ps.

Alternatives

The semiconductor equivalent to the photomultiplier are avalanche photodiodes (APD ), and the resultant silicon photomultiplier ( SiPM ), which exploit the avalanche effect occurring at high field strengths ( avalanche effect ) in semiconductor crystals for charge carrier multiplication. Individual APDs produce a radiation power-proportional output voltage, but can reach as opposed to the photomultiplier only a gain of < 103 SiPM achieve similar high gains as photomultiplier in the range of 106 avalanche photodiodes are preferably used for the detection of low light intensities medium or high frequency, such as z. example, in laser rangefinders.

For single photon detection avalanche photodiodes special, so-called single-photon avalanche diodes can be ( SPAD) used where single photons briefly points to some million generate charge carriers and thus can be registered as electrical impulses easily.

Very sensitive photodetectors are also photoresistors. With them, however, you can not record single photon of seafarers, they are very noisy and very sluggish (range seconds).