PHOTOMULTIPLIER TUBES AND ASSEMBLIES LTIPLIER TUBES FOR SCINTILLATION COUNTING & HIGH ENERGY PHYSICS

WEB SITE www.hamamatsu.com INTRODUCTION In radiation measurements, scintillation counters which are combinations of scintillators and photomultiplier tubes are used as most common and useful devices in detecting X-, alpha-, beta-, gamma-rays and other high energy charged particles. A scintillator emits flashes of light in response to input radiations and a photomultiplier tube coupled to a scintillator detects these scintillation lights in a precise way. In high energy physics experiments, one of important apparatuses is a Cherenkov counter in which photomultiplier tubes detect Cherenkov radiations...

Operating Characteristics This section describes the prime features of photomultiplier tube construction and basic operating characteristics. 1. GENERAL The photomultiplier tube (PMT) is a photosensitive device consisting of an input window, a photocathode, focusing electrodes, an electron multiplier (dynodes) and an anode in a vacuum tube, as shown in Figure 1. When light enters the photocathode, the photocathode emits photoelectrons into vacuum by the external photoelectric effect. These photoelectrons are directed by the potential of focusing electrode towards the electron multiplier where...

As stated above, the spectral response range is determined by the materials of the photocathode and the window as shown in It is important to select appropriate materials which will suit the 2.5 Luminous and Blue Sensitivity Since the measurement of spectral response characteristics of a PMT requires a sophisticated system and time, it isn't practi- cal to provide spectral response data on each tube. Instead, cathode and anode luminous sensitivity data are usually at- The cathode luminous sensitivity is the photoelectric current from the photocathode per incident light flux (10-5 to 10-2 lu-...

4.2 Gain (Current Amplification) Photoelectrons emitted from a photocathode are accelerated by an electric field so as to strike the first dynode and produce secondary electron emissions. These secondary electrons then impinge upon the next dynode to produce additional secondary electron emissions. Repeating this process over successive dynode stages (cascade process), a high gain is achieved. Therefore a very small photoelectric current from the photoca- thode can be observed as a large output current from the Gain is simply the ratio of the anode output current to the pho- toelectric current...

6. TIME RESPONSE In applications where forms of the incident light are pulses, the anode output signal should reproduce a waveform faithful to the incident pulse waveform. This reproducibility depends on the anode pulse time response. These parameters are affected by the dynode structure and applied voltage. In general, PMTs of the linear focused or circular cage structure exhibit better time response than that of the box-and-grid or venetian blind structure. (1) Rise Time (refer to Figure 4) Figure 6 shows typical time response characteristics vs. applied voltage for types R2059 (51 mm dia....

Figure 7: Example of Pulse Linearity Characteristic ANODE PEAK CURRENT (mA) The special voltage distribution ratios are designed to achieve strong electric fields in the later stages of the electron multiplier. Some types are specified with these special voltage dividers. Although the focusing electrodes of a PMT are designed so that electrons emitted from the photocathode or dynodes are collected efficiently by the first or following dynodes, some electrons may deviate from their desired trajectories and col- lection efficiency is degraded. The collection efficiency varies with the position...

The sensitivity of the PMT varies with the temperature. Figure 10 shows typical temperature coefficients of anode sensitivity around the room temperature for bialkali and high temp, bialkali photocathode types. In the ultraviolet to visible region, the tem- perature coefficient of sensitivity has a negative value, while it has a positive value near the longer wavelength cut-off. Since the temperature coefficient change is large near the lon- ger wavelength cut-off, temperature control may be required in some applications. Figure 10: Typical Temperature Coefficients of Anode Most PMTs are affected...

11. VOLTAGE DIVIDER CIRCUITS 11.1 Anode Grounding and Photocathode Grounding To operate a photomultiplier tube, a high voltage of 500 volts to 2000 volts is usually supplied between the photocathode (K) and the anode (P), with a proper voltage gradient set up along the photoelectron focusing electrode (F) or grid (G), secondary electron multiplier electrodes or dynodes (Dy) and, depending on photomultiplier tube type, an accelerating electrode (Acc). Figure 13 shows a schematic representation of photomultiplier tube operation using independent multiple power supplies, but this is not a practical...

11.2 Standard Voltage Divider Circuits Basically, the voltage divider circuits of socket assemblies lis- ted in this catalog are designed for standard voltage distribu- tion ratios which are suited for constant light measurement. Socket assemblies for side-on photomultiplier tubes in particu- lar mostly use a voltage divider circuit with equal interstage vol- tages allowing high gain as shown in Figure 16. Figure 16: Equally Divided Voltage Divider Circuit 11.3 Tapered Voltage Divider Circuits In most pulsed light measurement applications, it is often nec- essary to enhance the voltage gradient...

[When light is not incident on the tube] In dark state operation where a high voltage is supplied to a photomultiplier tube without incident light, the current compo- nents flowing through the voltage divider circuit will be similar to those shown in Figure 20 (if we ignore the photomultiplier tube dark current). The relation of current and voltage through each component is given below Interelectrode current of photomultiplier tube Electrode current of photomultiplier tube Voltage divider circuit current Voltage divider circuit voltage Figure 20: Operation without Light Input Figure 21: Operation...