PHOTOMULTIPLIER TUBE

INTRODUCTION

Among the photosensitive devices in use today, the photomultiplier tube (or PMT) is a versatile device that provides extremely high sensitivity and ultra-fast response. A typical photomultiplier tube consists of a photoemissive cathode (photocathode) followed by focusing electrodes, an electron multiplier and an electron collector (anode) in a vacuum tube.When light enters the photocathode, the photocathode emits photoelectrons into the vacuum. These photoelectrons are then directed by the focusing electrode voltages towards the electron multiplier where electrons are multiplied by the process of secondary emission. The multiplied electrons are collected by the anode as an output signal. Because of secondary-emission multiplication, photomultiplier tubes provide extremely high sensitivity and exceptionally low noise among the photosensitive devices currently used to detect radiant energy in the ultraviolet, visible, and near infrared regions. The photomultiplier tube also features fast time response, low noise and a choice of large photosensitive areas.

HISTORY

In 1935, Iams  succeeded in producing a triode photomultiplier tube with a photocathode combined with a single-stage dynode (Secondary emissive surface), which was used for movie sound pickup. In the next year 1936, Zworykin  developed a photomultiplier tube having multiple dynode stages. This tube enabled electrons to travel in the tube by using an electric field and a magnetic field.Then, in 1939, Zworykin and Rajchman developed an electrostatic-focusing type photomultiplier tube (this is the basic structure of photomultiplier tubes currently used). In this photomultiplier tube, an Ag-O-Cs photocathode was first used and later an Sb-Cs photocathode was employed.An improved photomultiplier tube structure was developed and announced by Morton in 1949) and in 1956. Since then the dynode structure has been intensively studied, leading to the development of a variety of dynode structures including circular-cage, linear-focused and box-and-grid types. In addition, photomultiplier tubes using magnetic-focusing type multipliers transmission-mode secondary-emissive surfaces and channel type multipliers have been developed.

CONSTRUCTION

A photomultiplier tube is a vacuum tube consisting of an input window, a photocathode, focusing electrodes, an electron multiplier and an anode usually sealed into an evacuated glass tube. Figure shows the schematic construction of a photomultiplier tube.

Light which enters a photomultiplier tube is detected and produces an output signal through the following processes.

(1) Light passes through the input window.

(2) Light excites the electrons in the photocathode so that photoelectrons are emitted into the vacuum (external photoelectric effect).

(3) Photoelectrons are accelerated and focused by the focusing electrode onto the first dynode where they are multiplied by means of secondary electron emission. This secondary emission is repeated at each of the successive dynodes.

(4) The multiplied secondary electrons emitted from the last dynode are finally collected by the anode.

The photomultiplier tube generally has a photocathode in either a side-on or a head-on configuration. The side-on type receives incident light through the side of the glass bulb, while in the head-on type, it is received through the end of the glass bulb. In general, the side-on type photomultiplier tube is relatively low priced and widely used for spectrophotometers and general photometric systems. Most of the side-on types employ an opaque photocathode (reflection-mode photocathode) and a circular cage structure electron multiplier which has good sensitivity and high amplification at a relatively low supply voltage.

The head-on type (or the end-on type) has a semi transparent photocathode (transmission-mode photocathode) deposited upon the inner surface of the entrance window.

The head-on type provides better spatial uniformity than the side-on type having a reflection-mode photocathode. Other features of head-on types include a choice of photosensitive areas from tens of square millimetres to hundreds of square centimetres .

Variants of the head-on type having a large-diameter hemispherical window have been developed for high energy physics experiments where good angular light acceptability is important. 

External appearance 

Types of Photocathode

WORKING

The superior sensitivity (high current amplification and high S/N ratio) of photomultiplier tubes is due to the use of a low-noise electron multiplier which amplifies electrons by a cascade secondary electron emission process. The electron multiplier consists of from 8, up to 19 stages of electrodes called dynodes.There are several principal types in use today.

1) Circular-cage type

The circular-cage is generally used for the side-on type of photomultiplier tube. The prime features of the circular-cage are compactness and fast time response.

2) Box-and-grid type

This type consists of a train of quarter cylindrical dynodes and is widely used in head-on type photomultiplier tubes because of its relatively simple dynode design and improved uniformity, although time response may be too slow in some applications.

3) Linear-focused type

The linear-focused type features extremely fast response time and is widely used in head-on type photomultiplier tubes where time resolution and pulse linearity are important.

4) Venetian blind type

The venetian blind type has a large dynode area and is primarily used for tubes with large photocathode areas. It offers better uniformity and a larger pulse output current. This structure is usually used when time response is not a prime consideration.

5) Mesh type

The mesh type has a structure of fine mesh electrodes stacked in close proximity. This type provides high immunity to magnetic fields, as well as good uniformity and high pulse linearity. In addition, it has position-sensitive capability when used with cross-wire anodes or multiple anodes.

6) Microchannel plate (MCP)

The MCP is a thin disk consisting of millions of micro glass tubes (channels) fused in parallel with each other. Each channel acts as an independent electron multiplier. The MCP offers much faster time response than the other discrete dynodes.It also features good immunity from magnetic fields and two-dimensional detection ability when multiple anodes are used.

7) Metal channel type

The Metal channel dynode has a compact dynode costruction manufactured by our unique fine machining technique.It achieves high speed response due to its narrower space between each stage of dynodes than the other type of conventional dynode construction. It is also adequate for position sensitive measurement.

PHOTOCATHODE MATERIALS

The photocathode is a photoemissive surface usually consisting of alkali metals with very low work functions. The photocathode materials most commonly used in photomultiplier tubes are as follows:

1) Ag-O-Cs

The transmission-mode photocathode using this material is designated S-1 and sensitive from the visible to infrared range (300 to 1200nm). Since Ag-O-Cs has comparatively high thermionic dark emission, tubes of this photocathode are mainly used for detection in the near infrared region with the photocathode cooled.

2) GaAs(Cs)

GaAs activated in cesium is also used as a photocathode. The spectral response of this photocathode usually covers a wider spectral response range than multi alkali, from ultraviolet to 930nm, which is comparatively flat over 300 to 850nm.

3) InGaAs(Cs)

This photocathode has greater extended sensitivity in the infrared range than GaAs. Moreover, in the range between 900 and 1000nm, InGaAs has much higher S/N ratio than Ag-O-Cs.

4) Sb-Cs

This is a widely used photocathode and has a spectral response in the ultraviolet to visible range. This is not suited for transmission-mode photocathodes and mainly used for reflection-mode photocathodes.

5) Bialkali (Sb-Rb-Cs, Sb-K-Cs)

These have a spectral response range similar to the Sb-Cs photocathode, but have higher sensitivity and lower noise than Sb-Cs. The transmission mode bialkali photocathodes also have a favorable blue sensitivity for scintillator flashes from NaI (Tl) scintillators, thus are frequently used for radiation measurement using scintillation counting.

6) High temperature bialkali or low noise bialkali (Na-K-Sb)

This is particularly useful at higher operating temperatures since it can withstand up to 175°C. A major application is in the oil well logging industry. At room temperatures, this photocathode operates with very low dark current, making it ideal for use in photon counting applications.

7) Multialkali (Na-K-Sb-Cs)

The multialkali photocathode has a high, wide spectral response from the ultraviolet to near infrared region. It is widely used for broad-band spectrophotometers. The long wavelength response can be extended out to 930nm by special photocathode processing.

8) Cs-Te, Cs-I

These materials are sensitive to vacuum UV and UV rays but not to visible light and are therefore called solar blind. Cs-Te is quite insensitive to wavelengths longer than 320nm,and Cs-I to those longer than 200nm.

WINDOW MATERIALS

The window materials commonly used in photomultiplier tubes are as follows:

1) Borosilicate glass

This is frequently used glass material. It transmits radiation from the near infrared to approximately 300nm. It is not suitable for detection in the ultraviolet region. For some applications,the combination of a bialkali photocathode and a low-noise borosilicate glass (so called K-free glass) is used.The K-free glass contains very low potassium (K2O) which can cause background counts by 40K. In particular, tubes designed for scintillation counting often employ K-free glass not only for the faceplate but also for the side bulb to minimize noise pulses.

2) UV-transmitting glass (UV glass)

This glass transmits ultraviolet radiation well, as the name implies, and is widely used as a borosilicate glass. For spectroscopy applications, UV glass is commonly used. The UV cut-off is approximately 185nm.

3) Synthetic silica

The synthetic silica transmits ultraviolet radiation down to 160nm and offers lower absorption in the ultraviolet range compared to fused silica. Since thermal expansion coefficient of the synthetic silica is different from Kovar which is used for the tube leads, it is not suitable for the stem material of the tube . Borosilicate glass is used for

the stem, then a graded seal using glasses with gradually different thermal expansion coefficients are connected to the synthetic silica window. Because of this structure, the graded seal is vulnerable to mechanical shock so that sufficient care should be taken in handling the tube.

4) MgF2 (magnesium fluoride)

The crystals of alkali halide are superior in transmitting ultraviolet radiation, but have the disadvantage of deliquescence. Among these, MgF2 is known as a practical window material because it offers low deliquescence and transmits ultraviolet radiation down to 115nm.

SCINTILLATION COUNTING

Scintillation counting is one of the most sensitive and effective methods for detecting radiation. It uses a photomultiplier tube coupled to a transparent crystal called scintillator which produces light by incidence of radiation.

 In radiation measurements, there are two parameters that should be measured. One is the energy of individual particles and the other is the amount of particles. Radiation measurements should determine these two parameters.

When radiation enters the scintillator, it produce light flashes in response to each particle. The amount of flash is proportional to the energy of the incident racliation. The photomultiplier tube detects individual light flashes and provides the output pulses which contain information on both the energy and amount of pulses, as shown in Figure . By analyzing these output pulses using a multichannel analyzer (MCA), a pulse height distribution (PHD) or energy spectrum is obtained, and the amount of incident particles at various energy levels can be measured accurately.

Typical PHDs or energy spectra when gamma rays (55Fe, 137Cs, 60Co) are detected by the combination of an NaI(Tl) scintillator and a photomultiplier tube. For the PHD,it is very important to have distinct peaks at each energy level.This is evaluated as pulse height resolution (energy resolution)and is the most significant characteristic in radiation particle measurements.

References :

1. Photomultipier tubes basics and application (third edition), HAMAMATSU
2. Photomultiplier tubes construction and operating characteristics,HAMAMATSU