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Transcript
Electron-beam tubes - an introduction Electron-beam tubes are miniature particle accelerators. They are a type of Cathode Ray Tube (CRT) commonly used in oscilloscopes and for display screens for radar, some computer monitors and older televisions. The tubes designed for science investigations enable the properties of electrons to be demonstrated in fascinating ways. Unfortunately, the use of such equipment in schools has declined. Many tubes are tucked away in their cardboard boxes but still available for use. This leaflet is designed to provide easy, step-by-step, guidance to encourage science teachers and technicians to put an electronbeam tube back into operation. Our experience is that, once they see a tube working, teachers are keen to use it in their lessons. In due course, some may then wish to find out more about the variety of more-advanced work that can be carried out with these devices. This leaflet is most clearly viewed in colour, either on the CLEAPSS CD ROM (2008 or later), or printed on a colour printer. 1. Electron-beam tubes available Manufacturers produce a variety of electron-beam tubes1. By far the most common type found in UK schools is the ‘Teltron® Classic’ range2. Electron-beam tubes used in school science can be divided into two groups. • Tubes, including fine-beam, diode and triode tubes, which require a High-Tension (HT) power supply3, operating at a relatively high current (around 30 mA). This type of power supply presents a significant electrical hazard, similar to that associated with mains electricity. Therefore, good user knowledge and proper control measures, including special HT leads and plugs, are essential. This leaflet does not cover the use of these tubes. • Tubes, including Maltese-cross, Perrin and Electron-deflection tubes, which require an ExtraHigh-Tension (EHT) supply, but current-limited to 2 mA or less4 (and therefore much safer to use than an HT supply). Each tube incorporates a fluorescent screen, which gives an impressive indication of the presence of an electron beam. Instructions to set up these tubes are given below. Figure 1: Teltron® electron-beam tubes, with fluorescent screens, which require an EHT supply. Maltese-cross tube Perrin tube Electron-deflection tube 1 Technical information about different electron-beam tubes is provided in section 12.6 of the CLEAPSS Laboratory Handbook. 2 Teltron® tubes and accessories are now available from UK 3B Scientific. See section 7 of this leaflet. 3 Information on the safe use of HT supplies is in the CLEAPSS Laboratory Handbook, section 12.9.3. 4 EHT supplies for science teaching are normally current-limited to 2 mA, but older units, supplying up to 5 mA, may still be used. PS76 JRE 10/07 Page 1 of 7 ® © CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk 2. Principle of operation Electron-beam tubes use a ‘gun’ to accelerate electrons. The electrons are first ‘evaporated’ from a hot cathode (negative electrode), which is similar to the tungsten filament in a low-voltage light bulb. This process is known as thermionic emission. The resulting cloud of electrons is attracted towards a cylindrical, high-voltage anode (positive electrode). A hole at the end of the anode allows a proportion of the rapidlymoving electrons to ‘shoot’ through, producing an electron beam. An illustration of an electron gun, fitted inside the ‘neck’ of a typical Teltron® tube is shown in Figure 2. Figure 2: Electron gun in a Teltron® tube Once created by the gun, the electron beam proceeds through a vacuum inside the tube until it collides with a fluorescent screen. Here, energy from the electron beam is released in the form of light, indicating the presence of the beam. Three simple demonstrations are easily achievable with most tubes containing a fluorescent screen. • The technique used for accelerating electrons (a particle accelerator). • The effect of changing the accelerating voltage. • The effect of introducing a magnetic field, using permanent magnets. Other, more-advanced, investigations, will be dependent on the particular tube in use and more-detailed information (not included in this leaflet) will be needed. See section 7 for the technical help available. 3. EHT power supply A combined Low-Tension (LT) and Extra-High-Tension (EHT) power supply is required to operate the electron gun. For safety reasons, the power supply must be one designed for use in school science, eg, by Griffin, Harris or Unilab. This will ensure that: • the maximum EHT voltage is no more than 5 kV (5 thousand volts)5, • the EHT output will be internally current limited to less than 2 mA (2 thousandths of an amp) and • there is a separate, fixed, low-voltage, ac output of about 6 V (typically 6.3 V ac). As with all portable, mains-operated, electrical appliances, the EHT power supply must have passed a standard electrical inspection and test6. If you are uncertain about the suitability of a particular power supply, please contact the CLEAPSS Helpline. There is a high risk of electric shock, or permanent damage to the tube, if an inappropriate power supply is used. 5 EHT supplies for science teaching are normally voltage-limited to 5 kV, but older units supplying up to 6 kV may still be used. 6 Inspection and testing of portable electrical appliances is explained in section 6.4 of the CLEAPSS Laboratory Handbook. PS76 JRE 10/07 Page 2 of 7 ® © CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk Figure 3: Typical school EHT power supplies used to operate an electron-beam tube 4. Connecting leads When HT power supplies are used to operate some types of electron-beam tube (see section 1), leads with an appropriate voltage rating and well-shrouded connectors are essential. However, when standard, current-limited, school EHT supplies are used, as described in this leaflet, well-maintained, general-purpose, low-voltage leads with standard 4mm plugs and sockets are sufficient. x The risk of harm by electric shock from a current-limited EHT supply is minimal. However, an electric shock from such a supply is still unpleasant. The presence of bare pins, at high voltages, should always be avoided. If only standard leads with plugs at both ends are available, exposed pins should be protected. This is best done with firm-fitting insulated sleeves (eg, plastic plug bodies) as shown. This should only be a temporary arrangement, to check that a tube operates properly. Once a tube is in regular use, leads with sockets should be used, where appropriate7. 7 Ready-made ‘extension-test’ leads, fitted with a 4 mm plug at one end and a 4 mm socket at the other, are available from various suppliers, eg, Rapid Electronics, order codes 17-0374 (black) and 17-0376 (red); www.rapidonline.com. PS76 JRE 10/07 Page 3 of 7 ® © CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk 5. Circuit diagram A simple circuit, which will operate a Teltron tube, requires only four or five leads, as shown below. Figure 4. Basic circuit to operate a Teltron tube. 6. Step-by-step set up procedure The procedure described uses a ‘Teltron® Classic’ Maltese-cross, electron-beam tube, the type most commonly found in schools. Connections to the electron gun will be identical for similar tubes with fluorescent screens, which may be substituted for the Maltese-cross tube. The step-by-step set-up procedure is also effective, when used by teachers, to explain the principle of the electron-beam tube to students. • Electric shocks from school EHT power supplies are not fatal, but usually memorable. • All wiring should be connected with the mains switch on the power supply turned off. • There can be a delay in EHT being removed from the output, once the supply is turned off. Therefore, it is recommended to wait a few seconds before touching connections (check the display of a meter, if fitted to the power supply). • Electron-beam tubes are made from glass and are evacuated. They must be handled with extreme care. Even minor knocks or drops may cause a tube to shatter. Implosion risks are similar to those for a large light bulb. • Before moving a tube, it should be allowed to cool, in order to prolong the life of the heater. In the sequence A to K overleaf, connections are made according to the circuit diagram, as shown in Figure 4. PS76 JRE 10/07 Page 4 of 7 ® © CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk A. Check the electron-beam tube for physical damage (internal and external). A rattling tube does not necessarily mean that it will not work, but loose or missing parts may require repair, or the tube may need to be returned to the manufacturer for refurbishment. B. Secure the tube carefully in the stand. Avoid putting stress on the tube mountings, as they sometimes become loose. Slide any fixing devices into place. C. Connect the heater. Use two black leads between the 6 V ac output on the power supply (sometimes labelled ‘heater’ or ‘filament’) and the two heater supply sockets on the back of the electron-beam tube. Voltages greater than about 6 V, applied to the heater, will destroy it and render the tube useless. D. Switch on the power supply, to check the heater. A glow/light from the heater is usually visible at the back of the tube. Reducing ambient light gives a more impressive display. Turn off the power supply at the main switch after this step. PS76 JRE 10/07 Page 5 of 7 ® © CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk E. Connect the cathode to the EHT(-). At the power supply, use a black lead to link one of the black heater leads to the EHT(-) socket. F. Connect the anode to the EHT(+). Connect a red lead to the anode of the tube. At the power supply, plug the other end of this lead into the EHT(+) socket. Make sure that no metal pins at EHT voltage protrude (see section 4, Connecting leads) G. For the Maltese-cross tube, connect the cross to the EHT(+). Connect a red lead to the tube. At the power supply, plug the other end of this lead into another EHT(+) socket (the second socket is often connected to the first via an internal resistor). If only one EHT(+) socket is present, use stackable plugs. or This connection stops electrons charging the cross, ensuring that a sharp-edged shadow is produced. Make sure that no metal pins at EHT voltage protrude (see section 4, Connecting leads) PS76 JRE 10/07 Page 6 of 7 ® © CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk H. Turn on the electron beam. Set the EHT voltage control to zero. Turn on the power supply at the main switch to operate the heater. Gradually turn up the EHT voltage. At a few kV, the fluorescent screen should begin to glow as the electron-beam strikes it. The higher the EHT voltage, the brighter the fluorescence. The electrons gain more kinetic energy, when the accelerating voltage is higher. I. Hold a strong magnet near the screen. A ceramic or neodymium magnet works well. The electron beam will be deflected by the magnetic field. Initially concealing the magnet in the hand may increase interest. Alter the distance of the magnet from the tube, its polarity and its position along the tube. The effect on the electron beam should be discussed. J. Point two strong magnets (check that they attract) toward one another, on either side of the neck of the tube. The electron beam should move up or down, perpendicular to the magnetic field. Reversing the polarity of the magnets (by turning them both over) should make the electron beam deflect in the opposite direction. This demonstrates the motor rule (Fleming’s left-hand rule). In the specific case of the Maltese-cross tube, the light from the heater/filament should cast a light, faint shadow of the cross on the screen. The shadow associated with the white light is unaffected by the magnets, but the shadow associated with the electrons, which produce the green glow on the screen, moves. K. Turn the EHT voltage down to zero before switching off the power supply. Allow the tube to cool before handling. 7. More-advanced work New electron-beam tubes are usually supplied with detailed circuit diagrams and suggested investigations. Some suppliers offer technical help and direct support to schools. In particular, UK 3B Scientific supplies Teltron® electron-beam tubes and includes resources on its web site at www.3bscientific.co.uk. Enquiries can be emailed to [email protected]. PS76 JRE 10/07 Page 7 of 7 ® © CLEAPSS , The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ Tel: 01895 251496; Fax: 01895 814372; E-mail: [email protected]; Web site: www.cleapss.org.uk