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varying this accelerating voltage from 1850 V down to 37 V. The resolving power of the instrument, M/AM, is equal to 50. The rapid scanning of a mass spectrum in a short time interval when the total pressure is very low (- 1O-8 torr) requires the measurement of currents of the order of lo-l5 A in less than a millisecond. Available d.c. amplifiers can measure such small currents in times of the order of seconds, but only 10-O A with the required speed of response. The required gain increase of lo6 has been obtained by directing the ions onto the cathode of an electron multiplier. Positive ions produce secondary electrons which are then multiplied in the usual way. The output from the 17-stage multiplier is fed directly into the wide band amplifier of an oscilloscope. Because it is influenced by extraneous magnetic fields, the electron multiplier must be mounted clear of the mass spectrometer magnet and magnetically shielded from its fringing field. The ions are guided through the fringing field onto the amplifier cathode by means of a pair of electro-static deflector plates. These are so shaped that moving away from the magnet, the electric field between them decreases at the same rate as the magnetic fringing field. If this electric field is varied according to the velocity of the ions being guided, then the electric force balances out the magnetic force on the ions. This is done by supplying the electrodes with a voltage from a potential divider connected to the scanning voltage. In the original instrument, electron emission in the source was stabilized by control of the filament temperature. For a rapid scanning instrument, this is too slow, and a form of space charge control has been introduced. The limit of sensitivity of such an instrument is set by the statistical fluctuation of the small number of ions collected in one peak. Typically, for a scanning rate of 10 a second and a partial pressure of 5 x 1O-8 torr, this fluctuation is 3 per cent. In this instrument, it is necessary to use one of the slower scanning speeds to obtain greater accuracy.
the ejection of particles from the walls, and to discover the wall materials and the conditions giving least contamination of plasma. The accelerator consists of, firstly, an R.F. source from which a total current of about 500pA can be obtained, secondly, an electrostatic accelerator and focusing system, thirdly, a sectormagnet mass-analyser with which to select the type of ion to be used, and, fourthly, a target chamber in which the particles which result from the ion bombardment are analysed. There are two features of the analyser which are important for experiments on surfaces. Firstly, the target chamber must be maintained at a very low pressure in order to prevent contamination from residual gases. In order to do this, a number of stages of differential pumping are incorporated between the source, which operates at 1O-2 torr, and the target chamber, which is to operate at 1O-9 torr. At this pressure, the time taken to form a monologer of adsorbed gas on the surface is about 1000 seconds. Mercury diffusion and sputter ion pumps are used in order to avoid the contamination which would arise with the use of oil or hydrocarbons in the pumps. In addition to these pumps, there is a filament in the target chamber which evaporates titanium onto a liquid nitrogen cooled surface to act as a getter. This is designed to remove the gas load due to the ion beam itself, which is considerable at pressures of loss torr. The getter is designed to have a pumping speed of l-2 x 10e4 l./sec. for the few hours during which bombardment takes place. The other principal feature of the system is that, in order to have as high a current density of mass-analysed beam as possible on the target, a sector magnet analyser has been designed with the beam entering at 57” to the normal of the magnet, in order to use the focusing effect of the fringing field’in the direction normal to the plane of magnet pole faces. This magnet system has been found to have a transmission of 40-70 per cent. of the total beam current entering the analyser. After passing through the analyser, the beam is focused to a spot approximately 2 mm diameter by an The current density is about electrostatic quadrupole lens. ~1 mA/cm2. The apparatus has recently been completed and the initial experiments are concerned with analysing the products of ion bombardment of atomically clean surfaces of molybdenum and tungsten, using a quadrupole mass filter. It is then proposed to introduce known impurities in the form of adsorbed gas layers onto the cleaned surfaces, and from there to graduate to materials of greater practical importance, such as stainless steels, alumina and silica.
50 kV ion accelerator The 50 kV accelerator has been built to investigate the effects of ion bombardment of surfaces of plasma containers, mainly using isotopes of hydrogen. These effects are of concern, for even in stable plasmas there is some escape of plasma ions to the walls. Some of these are returned with reduced energy and others may knock out atoms of high atomic number from the wall. If these atoms enter the plasma, they can cause a greatly increased The object of the work with the rate of energy loss by radiation. ion accelerator is to find out what mechanisms are responsible for
The
Institute
of Physics
and The
Pilot experiment on examinations by I.P.P.S. A circular (ref. G2/1964) issued by The Institute of Physics and The Physical Society on 7 July, states that the Ministry of Education has agreed to give financial support to a pilot experiment on physics examinations which the Education Group of the Institute and Society will be conducting in co-operation with selected schools. For some time now, the Group has had a working party discussing physics examinations and their impact on teaching (cf. report prepared by a committee last August under facing university physics Prof. R. V. Jones : “ Problems departments “). Professional advice The circular mentions three other facts which may interest readers. Firstly, it states that the Council of the Institute and Society’s Executive Officers, often advised by senior Fellows of the Institute with special knowledge of local conditions, have dealt with many enquiries on professional matters. These
309
Physical
Society
have included contracts of service, suitability of salaries and various posts, payment and opportunities for consulting work, availability of courses at all levels, and prospects for physicists in Great Britain and overseas. During 1963, the Burnham Committees accepted Graduateship of the Institute as a qualification giving graduateship status for the purpose of teachers’ salaries. Another professional matter dealt with in 1963 was insurance coverage against risks to staff and students from laboratory accidents. Coherent publications policy Secondly, the amalgamation (in 1960) of the Institute and Society is stated to have enabled the Publications Committee of the joint body to develop a coherent publications policy. The Proceedings of The Physical Society now publishes original work in basic physics and the British Journal of Applied Physics publishes work describing applications of basic physical principles. It is stated that the Journal of Scientific Instruments is to continue
use Reader Enquiry Service cards
varying this accelerating voltage from 1850 V down to 37 V. The resolving power of the instrument, M/AM, is equal to 50. The rapid scanning of a mass spectrum in a short time interval when the total pressure is very low (- 1O-8 torr) requires the measurement of currents of the order of lo-l5 A in less than a millisecond. Available d.c. amplifiers can measure such small currents in times of the order of seconds, but only 10-O A with the required speed of response. The required gain increase of lo6 has been obtained by directing the ions onto the cathode of an electron multiplier. Positive ions produce secondary electrons which are then multiplied in the usual way. The output from the 17-stage multiplier is fed directly into the wide band amplifier of an oscilloscope. Because it is influenced by extraneous magnetic fields, the electron multiplier must be mounted clear of the mass spectrometer magnet and magnetically shielded from its fringing field. The ions are guided through the fringing field onto the amplifier cathode by means of a pair of electro-static deflector plates. These are so shaped that moving away from the magnet, the electric field between them decreases at the same rate as the magnetic fringing field. If this electric field is varied according to the velocity of the ions being guided, then the electric force balances out the magnetic force on the ions. This is done by supplying the electrodes with a voltage from a potential divider connected to the scanning voltage. In the original instrument, electron emission in the source was stabilized by control of the filament temperature. For a rapid scanning instrument, this is too slow, and a form of space charge control has been introduced. The limit of sensitivity of such an instrument is set by the statistical fluctuation of the small number of ions collected in one peak. Typically, for a scanning rate of 10 a second and a partial pressure of 5 x 1O-8 torr, this fluctuation is 3 per cent. In this instrument, it is necessary to use one of the slower scanning speeds to obtain greater accuracy.
the ejection of particles from the walls, and to discover the wall materials and the conditions giving least contamination of plasma. The accelerator consists of, firstly, an R.F. source from which a total current of about 500pA can be obtained, secondly, an electrostatic accelerator and focusing system, thirdly, a sectormagnet mass-analyser with which to select the type of ion to be used, and, fourthly, a target chamber in which the particles which result from the ion bombardment are analysed. There are two features of the analyser which are important for experiments on surfaces. Firstly, the target chamber must be maintained at a very low pressure in order to prevent contamination from residual gases. In order to do this, a number of stages of differential pumping are incorporated between the source, which operates at 1O-2 torr, and the target chamber, which is to operate at 1O-9 torr. At this pressure, the time taken to form a monologer of adsorbed gas on the surface is about 1000 seconds. Mercury diffusion and sputter ion pumps are used in order to avoid the contamination which would arise with the use of oil or hydrocarbons in the pumps. In addition to these pumps, there is a filament in the target chamber which evaporates titanium onto a liquid nitrogen cooled surface to act as a getter. This is designed to remove the gas load due to the ion beam itself, which is considerable at pressures of loss torr. The getter is designed to have a pumping speed of l-2 x 10e4 l./sec. for the few hours during which bombardment takes place. The other principal feature of the system is that, in order to have as high a current density of mass-analysed beam as possible on the target, a sector magnet analyser has been designed with the beam entering at 57” to the normal of the magnet, in order to use the focusing effect of the fringing field’in the direction normal to the plane of magnet pole faces. This magnet system has been found to have a transmission of 40-70 per cent. of the total beam current entering the analyser. After passing through the analyser, the beam is focused to a spot approximately 2 mm diameter by an The current density is about electrostatic quadrupole lens. ~1 mA/cm2. The apparatus has recently been completed and the initial experiments are concerned with analysing the products of ion bombardment of atomically clean surfaces of molybdenum and tungsten, using a quadrupole mass filter. It is then proposed to introduce known impurities in the form of adsorbed gas layers onto the cleaned surfaces, and from there to graduate to materials of greater practical importance, such as stainless steels, alumina and silica.
50 kV ion accelerator The 50 kV accelerator has been built to investigate the effects of ion bombardment of surfaces of plasma containers, mainly using isotopes of hydrogen. These effects are of concern, for even in stable plasmas there is some escape of plasma ions to the walls. Some of these are returned with reduced energy and others may knock out atoms of high atomic number from the wall. If these atoms enter the plasma, they can cause a greatly increased The object of the work with the rate of energy loss by radiation. ion accelerator is to find out what mechanisms are responsible for
The
Institute
of Physics
and The
Pilot experiment on examinations by I.P.P.S. A circular (ref. G2/1964) issued by The Institute of Physics and The Physical Society on 7 July, states that the Ministry of Education has agreed to give financial support to a pilot experiment on physics examinations which the Education Group of the Institute and Society will be conducting in co-operation with selected schools. For some time now, the Group has had a working party discussing physics examinations and their impact on teaching (cf. report prepared by a committee last August under facing university physics Prof. R. V. Jones : “ Problems departments “). Professional advice The circular mentions three other facts which may interest readers. Firstly, it states that the Council of the Institute and Society’s Executive Officers, often advised by senior Fellows of the Institute with special knowledge of local conditions, have dealt with many enquiries on professional matters. These
309
Physical
Society
have included contracts of service, suitability of salaries and various posts, payment and opportunities for consulting work, availability of courses at all levels, and prospects for physicists in Great Britain and overseas. During 1963, the Burnham Committees accepted Graduateship of the Institute as a qualification giving graduateship status for the purpose of teachers’ salaries. Another professional matter dealt with in 1963 was insurance coverage against risks to staff and students from laboratory accidents. Coherent publications policy Secondly, the amalgamation (in 1960) of the Institute and Society is stated to have enabled the Publications Committee of the joint body to develop a coherent publications policy. The Proceedings of The Physical Society now publishes original work in basic physics and the British Journal of Applied Physics publishes work describing applications of basic physical principles. It is stated that the Journal of Scientific Instruments is to continue