Accelerator photocathode electron

The light signal is invariably converted to an electrical signal by a photomultiplier tube. Figure 7 shows how a photomultiplier works. When photons hit the photocathode, electrons are emitted. These are accelerated towards another electrode (the first dynode), which is held at a positive potential relative to the cathode. Each electron from the photocathode causes the emission of more than one electron from the first dynode. These electrons in turn are accelerated towards the second dynode, where each in turn causes the emission of more than one electron and so the process continues to the last dynode. 

Table 1. Comparison of specifications of photocathode electron gun accelerators for picosecond pulse radiolysis.

In photocathode electron guns, the timing between the microwaves used for acceleration and the photocurrent-generating laser pulse is of critical importance. Precise synchronization between the laser and electron beams is obtained by using a MHz quartz master oscillator to control the cathode pump laser repetition rate and the microwave amplifier system seed frequency (Fig. 3). 

Table 2. Comparison between time-resolved spectrophotometric detection set-ups of several laser-photocathode electron accelerator facilities for picosecond pulse radiolysis. 

Figure 4 Schematic representation of the BNL LEAF photocathode electron gun accelerator showing the laser-microwave synchronization relationship.

Photocathode-based picosecond electron accelerators are conceptually simpler than pre-bunched thermionic systems, although they require reasonably powerful, multicomponent femtosecond or picosecond laser systems to drive the photocathode. In addition, the availability of synchronized laser pulses allows the development of advanced detection capabilities with unprecedented time resolution. The combination of ease of use and powerful detection methods has stimulated strong interest in photocathode gun systems. Since the installation ofthe first photocathode electron gun pulse radiolysis system at BNL [5,13], four additional photocathode-based facilities have become operational and two more are in progress. The operational centers include the ELYSE facility at the Universite de Paris-Sud XI in Orsay, France [7,8], NERL in Tokai-Mura, Japan [9,10], Osaka University [11,12], and Waseda University in Tokyo [13]. Facilities under development are located at the Technical University of Delft, the Netherlands, and the BARC in Mumbai, India. 

The RF photocathode electron gun is the newest type of accelerator used for pulse radiolysis. Such devices have been under development since the mid-1980s as electron beam sources for experimental physics facilities and free-electron laser development. They are typically used to produce electron beams in the 4-10-MeV range. The unique quality (low emittance and clean position-momentum relationships) of the electron beams they produce makes extremely sophisticated beam manipulation possible. 

Figure 2. Schematic representation of an S-band RF Photocathode electron gun injector system. In actuality, the accelerator is about 30 cm long.

Picosecond accelerators for ultrafast radiolysis studies are undergoing a period of renewal as the RF photocathode electron gun technology becomes... 

A photomultiplier tube (PMT) consists of a photocathode followed by an electron multiplier. A single photon ejects an electron from the photocathode. Electric fields in the PMT accelerate the electron into another surface called a dinode. The collision of the electron with the dinode releases several new electrons, which are accelerated into another dinode. 

Standard equipment makes use of photodiodes working with the internal photo effect. The charge current is linear proportional to the number of photons falling onto the sensitive area. These photodiodes allow an arrangement in arrays. This type of detector is used in modem simultaneous detection systems (see Sec. 3.2.) and in chromatography [1,18]. Photomultipliers use the external photo effect. Incident radiation catapults out of the sensitive surface (photocathode) electrons which are multiplied (scintillation) and accelerated between dynodes on their way to the anode. Their detectivity is better and prices are higher [23]. 

A schematic drawing of a photomultiplier is shown in Figure 3.17. A photomultiplier consists of a photocathode, a chain of dynodes, and a collector (anode). The light to be detected illuminates the photocathode (the active area of the photomultiplier), which generates electrons due to the absorption of incident photons. These electrons are accelerated and amplified by the dynodes and, finally, they arrive at the anode, where are monitored as an induced current. 

Once electrons have been emitted by the photocathode, they are accelerated by an applied voltage induced between the photocathode and the first dynode (Uq in Figure 3.17). The dynodes are made of CsSb, which has a high coefficient for secondary electron emission. Thus, when an electron emitted by the photocathode reaches the first dynode, several electrons are emitted from it. The amplification factor is given by the coefficient of secondary emission, S. This coefficient is defined as the number of electrons emitted by the dynode per incident electron. Consequently, after passing the first dynode, the number of electrons is multiplied by a factor of 5 with respect to the number of electrons emitted by the photocathode. The electrons emitted by this first dynode are then accelerated to a second dynode, where a new multiplication process takes place, and so on. The gain of the photomultiplier, G, will depend on the number of dynodes, n, and on the secondary emission coefficient, 5, so that... 

The accelerating voltage in a streak camera is set to 500 V. If the distance between the electrodes used for electron deviations is 15 cm and the applied voltage between these electrodes is 1200 V, calculate the maximum spatial deviation that can be induced in the phosphor screen if the distance between the photocathode and the phosphor screen is 30 cm. 

In the optoelectronic X-ray image intensifier (Fig. 86), [5.427], the X-ray phosphor screen (input screen) is in direct optical contact with a photocathode that converts the luminance distribution of the X-ray screen into an electron-density distribution. The liberated electrons are accelerated in an electric field between the photocathode and an anode (20-30 kV) and are focused by electron lenses onto a second phosphor screen (output screen), where conversion of the electron image to a visible image takes place. 

In the nanosecond (ns) time-scale the use of kinetic detection (one absorption or emission wavelength at all times) is much more convenient than spectrographic detection, but the opposite is true for ps flash photolysis because of the response time of electronic detectors. Luminescence kinetics can however be measured by means of a special device known as the streak camera (Figure 8.2). This is somewhat similar to the cathode ray tube of an oscilloscope, but the electron gun is replaced by a transparent photocathode. The electron beam emitted by this photocathode depends on the incident light intensity I(hv). It is accelerated and deflected by the plates d which provide the time-base. The electron beam falls on the phosphor screen where the trace appears like an oscillogram in one dimension, since there is no jy deflection. The thickness of the trace is the measurement of light intensity. 

PHOTOEMISSION AND PHOTOMULTIPLIERS. Photoemission is the ejection of electrons from a substance as a result of radiation filling on it Photomultipliers make use of the phenomena of photoemission and secondary-electron emission in order to detect very low light levels The electrons released from the photocathode by incident light are accelerated and focused onto a secondary-emission surface (called a dynode). Several electrons are emitted from the dynode for each incident primary electron. These secondary electrons are then directed onto a second dynode where more electrons are released. The whole process is repealed a number of times depending upon the number of dynodes used, In this manner, it is possible to amplify the initial photocurrent by a factor of 10s or more in practical photomultipliers. Thus, the photomultiplier is a very sensitive detector of light. 

FIGURE 5.5 Schematic illustration of a photomultiplier tube. A single photon ejects an electron from the photocathode. The electron is accelerated by voltage differences, and knocks multiple electrons off each successive surface. The burst of electrons is collected at the end. 

Recently, laser-driven photocathode accelerators and lasers coupled with compressed pulses have been able to produce electron pulses in the vicinity of 5-10 ps. This has enabled one to improve on the time resolution available from the original Hunt experiments but without the limitations of the multiple pulses. These short times have been used for measuring electron transfer reactions, electrons in... 

Recently, new techniques such as laser-driven photocathode accel-erators have increased the time resolution available for radiation-chemical studies. They have been of great use in studying fast electron-transfer processes, but are not the one-to-two orders of magnitude improvement that would be needed to explore some of the fundamental questions of electron-precursor reactions and initial distribution of radiolytically produced species. Newer techniques, such as the laser-wakefield accelerator, have the potential to answer these sorts of questions however, they have not reached their maximum potential. ... 

---