Table of contents

Silver-activated, nickel-killed zinc sulphide phosphor, of the type normally used in scintillation counters, is found to emit at least 11% of its light at times greater than 100 mu s. Increasing the proportion of nickel is shown to produce a phosphor with a more more rapid decay and only a small decrease in efficiency.

A measurement system using a silicon target vidicon is described, which can greatly improve signal-to-noise ratio compared with the commonly used photomultiplier. Contrary to the photomultiplier, however, a relatively high light level is required because of the higher dark current level of the vidicon. Originally developed for Stark modulation spectroscopy, the system should also be suitable for other modulation-type experiments.

A method is described for measuring the Kapitza boundary layer conductance between resistance wires and superfluid helium, using a superconducting direct current comparator. Results obtained for Evanohm wire between 2.0 and 2.17K are presented.

The development of steels for use in relays requires a test for coercivity which is rapid and simple to apply as well as inexpensive. Permeameters pose problems for convenient measurement. A vibrating-coil magnetometer is assessed in comparison with classical methods and is shown to afford a satisfactory measuring method.

A reliable, inexpensive remote control system for equipment in high-voltage terminals has been built and operated at voltages in excess of 700 kV. The functions to be controlled inside the high-voltage terminals can be individually selected and actuated by means of a telephone dial system and three fibre optic transmission channels.

Rocket measurements of atmospheric transmission at 850 nm have shown a deep window at this wavelength. This has been employed to develop a simple, inexpensive aspect sensor suitable for small 'geophysical vehicles' (balloons, rockets and satellites). The performance of the sensor in one of the rocket vehicles is presented.

A new apparatus has been constructed to study the fatigue of fibres subject to cyclic bending, and to investigate the effect of the surrounding environment on fatigue life. It uses a technique of rotation over a wire which involves tension-compression of fibres which are rotated while bent through an angle near to 90 degrees . The apparatus has ten stations and hence many tests can be made in order to make accurate determinations of the median life.

It is shown that when a gas-operated free-piston pressure gauge is used for measuring pressure relative to atmospheric pressure, the usual relative rotation of the piston and cylinder with added masses can produce a pressure effect up to ten times greater than the accuracy of measurements relative to vacuum. The magnitude of this pressure effect depends on the rotational frequency and on whether a bell-jar is in place or not.

A technique for the measurement of the coefficient of linear thermal expansion of plastics and its application in the case of epoxy resin Araldite CT200/HT907 are described. Thermal strains are measured by means of electrical foil strain gauges using fused silica as a datum. The work has particular relevance in photothermoelastic studies.

Modifications have been made to a conventional powder diffraction camera so that X-ray absorption photographs may be taken in safety.

A practical design and the construction of a high-power nitrogen laser are described. It delivers pulses of up to 3.5 mJ at a rate of 10 per second. To optimize its performance as a pump source for a Hansch-type dye laser, a comparatively long pulse width (about 10 ns) has been chosen. Special care has been taken to make maintenance as convenient as possible. The fast transmission line is formed by sheet capacitors, which are held together by atmospheric pressure only and are therefore easily demountable in the case of dielectric breakdown.

Design details and measured performance are presented for a laboratory gas gun with a novel rectangular bore. Projectiles weighing 0.5 g have been accelerated to 300 m s^-1. This design eliminates axial rotation of the projectile which is possible in a cylindrical-bore gun, and provides important advantages in some impact experiments.

An E*B separator is described in which effective use is made of the fringing fields of the magnet. In the particular case described, this results in an increase of the velocity-separating capability of 40% with respect to the Wien filter that would have resulted if the fringing fields were completely shielded. It is also shown that for the normal operating conditions, the filter with fringing fields behaves as an ideal Wien filter without fringing fields but with drift spaces at the entrance and the exit. The effective length of these drift spaces and the length of the equivalent ideal Wien filter are slowly varying functions of a single parameter, the reduced magnetic induction, which depends on the operating conditions of the filter. A numerical example using a 12 in laboratory magnet is given.

An electrical circuit is described which enables nonlinear scaling to be performed using a multichannel analyser. The display of an HP-25 programmable calculator is decoded into binary coded decimal (BCD) form and drives a digital-to-analogue converter. The programming features of the calculator allow great flexibility in producing a wide variety of voltage steps in the multichannel scaling mode. The circuit can be controlled by an external clock and provides necessary gating signals for experimental applications.

The design and construction of a radio frequency setup serving as a second ENDOR spectrometer are described. Both ENDOR systems may be used independently or in combination for measurement of electron-nuclear-nuclear triple resonance (EN²TIR). Frequency ranges and radio frequency fields of the two systems are tabulated. Performance is exemplified by ENDOR and EN²TIR spectra of radicals in solution.

If a metal wire immersed in an electrically insulating medium is heated by a known constant power, the thermal conductivity of the medium may be deduced from the temperature variation of the wire. It is shown that further analysis of this variation also permits the specific heat capacity or the thermal diffusivity to be determined. A nickel wire is used both as a heater and as a temperature sensor. The power is kept constant by electronic means, and the resistance measured by a four-probe technique. Eight temperature values are recorded during the power pulse which has a typical duration of 1 s. This extended method has been tested by studying glycerol in its amorphous and crystalline states in the range 130-300K and at pressures of up to 0.8 GPa.

A dielectric spectrometer operating continuously over the frequency range of 0.01 to 18 GHz is described. The frequency dependence of both the attenuation coefficient and the permittivity of liquids may be measured over this frequency range to an estimated uncertainty of better than +or-5% when alpha d>1 ( alpha is the attenuation coefficient of the liquid and d the depth of liquid in the cell). Significant measurements are still possible with reduced accuracy when alpha d<1. The method is capable of fast routine operation with the dielectric absorption process characterized in about one hour. This sweep-frequency method is capable of a higher accuracy than the time-domain spectrometer and may be adapted to give a direct display of the frequency variation of the attenuation.

The effects of the frequency fluctuations of pulsed dye lasers on the determination of atomic lifetimes by the decay of fluorescence are estimated. A small shift in the estimated lifetime will occur due to the inability to completely correct for pile-up errors. However, the statistics of the number of counts in each channel of the pulse height analyser are not affected to a first approximation.

An adiabatic calorimeter has been designed and constructed which allows measurements of the absolute value of specific heat to be made on small quantities of material in the temperature range 20-300K. The exchange of heat between the calorimeter and its environment has been eliminated with the aid of three screens which were automatically controlled to have the same temperature as the calorimeter. The elimination of heat loss allowed the temperature of the calorimeter to be measured after a stabilization period had followed the heating of the calorimeter. The experimental results are shown to be reproducible and to reflect the sensitivity of the method.

A simple, rugged source of intense, stable monochromatized radiation in the vacuum ultraviolet has been constructed by 'integrating' a hydrogen discharge lamp (power rating more than 1000 W) and an existing vacuum monochromator. The entrance slit of the monochromator has been redesigned and serves also as anode to the lamp. Hydrogen is leaked into the lamp while only the monochromator is pumped by a rotary pump. The lamp is most efficient when it is made to burn with a luminous blob of plasma at the entrance slit in the anode. It produces a flux per solid angle at the slit two orders of magnitude higher than the flux reported for an open hydrogen discharge lamp by Eastman and Donelon (1970).

A compact, flexible light source is described, operable at wavelengths from the visible to vacuum ultraviolet or below. It consists of a plasma produced by the ablation of a solid dielectric surface in the presence of an intense electric discharge. By modifying the control circuitry, stable radiant discharges of duration ranging from under 1 mu s to over 10 mu s and of variable spectral quality may be produced, having application in high-speed photography, plasma diagnostics, absorption spectroscopy and biological irradiation.

A detailed description is given of the construction of an electron spectrometer for studies of chemical phenomena on solid surfaces. Auger electron spectra, photoelectron spectra and electron loss spectra can be taken from the same sample and in the same vacuum. The conditions required to attain high resolution over a wide range of electron energy (from about 0 eV to more than 500 eV) with a small concentric hemisphere analyser are made clear.

Water passing through the flowmeter causes a free-running rotor to revolve. Rotational speed is measured directly by observing the rotor and timing it, or remotely by electrical impulses transmitted to a counter and recorder. The flowmeter is suitable for applications where water velocity in a pipeline is low and restriction to flow has to be minimal, as for example in thermosiphoning water-heating systems. This instrument has been in continuous use for six months and has worked reliably at mean water velocities down to 2 mm s^-1, equivalent to a volume flow rate of 1.66 ml s^-1 in a 31.75 mm bore pipe, and at temperatures between 10 and 60 degrees C. Full details of construction are given.

The 750 kV Cavendish electron microscope has been adapted for scanning transmission operation. Disturbing magnetic fields at 50 and 100 Hz and at 8 kHz have been tracked carefully and successively removed to produce an electron beam without cross modulation. Further attention to detail in HV and lens electronics as well as in deflection systems led to an electron probe smaller than 3 nm in diameter. By operating the last demagnifying (objective) lens at very high excitation and by introducing a new electron detector, less than 4 nm point-to-point resolution has been obtained at 500 kV.

An analogue model based on the equivalence between a phase-locked loop and a Josephson junction has been built to study the effect of thermal noise on junction I-V characteristics. The model differs from previous designs by employing an FET to simulate the phase-dependent junction resistance or cos phi term. The behaviour of the analogue is in good agreement with the theoretical predictions of Ambegaokar and Halperin (1969) and of Falco et al. (1973).

An apparatus is described using modulation excitation techniques which enables circular dichroism of short-lived phototransients to be measured.

Describes a low-noise light sensor which uses a quadrant version of the photosil detector and specially designed electronics to separate the photon events from background. Measurements are presented of pulse height distribution, dark count and signal-to-noise ratio as a function of EHT. Possible applications include astronomy and scintillation counting.

A proportional control astronomical autoguider has been built using the low-noise photon-counting system described by Fegan and Craven (1977). The guider has a dynamic range of ten stellar magnitudes and has output options for driving either conventional or stepping motors. Proportional control is obtained in both cases.

Describes an image convertor, Phototitus, consisting of a photoconductor deposited on an electrooptical dielectric crystal. The electron microscope beam incident on the photoconductor creates electron-hole pairs which modulate the transparency of the electro-optical dielectric crystal by means of the Pockels effect. The converter can be read out with a fairly high-intensity light source, making Phototitus a solid state image intensifier. The read-out light can be coherent, allowing direct processing of the image, e.g. a real-time Fourier transform of the image which permits fast, accurate focusing of the electron microscope. Furthermore, Phototitus can be used as an 'analogue optical computer' making possible applications such as sharpening of high-resolution images.

The dimensional changes produced in materials by temperature variations are generally very small, so that sensitive measuring techniques must be used to observe them. The interferometer developed at NPL for thermal expansion measurement exploits the high resolution and accuracy of laser interferometry in a dilatometer design which places a minimum of restrictions on the shapes and dimensions of specimens which can be measured. The presentation of thermal expansion data is discussed and relationships between the relevant parameters are derived. Many different materials have been investigated and some results of interest are included here.

A common method for generating truly random numbers is to generate pulses in a Poisson process, and count these modulo N. It is well known that, for a large counting interval, the digit held by the counter at the end of the interval is a random digit in the interval (O, N-1). In practice, however, 'dead time' in the counter prevents all the pulses from being counted. The effect of this is that those pulses which are actually counted arrive, not in a Poisson process, but in a renewal process. The main conclusion of this paper is that the resulting count modulo N remains uniformly distributed on (0, N-1) under this more general assumption.

A 250 mm radius bent crystal vacuum spectrometer is described for use in the 0.5-10 nm region. Emphasis has been placed on the use of commercially available high-precision components to reduce machining costs. Scanning can be performed in a continuous or step-by-step mode.

The statistical variation in the transmission secondary emission from negative-electron-affinity (NEA) silicon has been studied using a scintillation counting technique. The results of measurements on boron-doped p-type silicon of between 5 and 10 mu m thickness show that distribution is Gaussian and characterized by a Fano factor of about 11 for a primary electron energy of 20 keV and a secondary emission ratio of 143.

Thermometric properties of tungsten and tantalum are discussed in detail and special attention is paid to the application of these metals to the measurements of emitter temperature in a field emission microscope by the resistance thermometry method. Tables of the resistance ratio, defined as R(T)/R(273), are also given for tungsten in the temperature range 77-1800K. Other methods of temperature measurement are also briefly discussed.

Fine slit collimators have been used extensively in neutron physics experiments to obtain neutron beams with a given angular deviation (collimation). Various methods of construction have been used, usually involving costly machining and expensive material. This paper describes a simple method of construction using thin films with neutron-absorbing coatings. The collimators made by this method have proved to be very satisfactory both for simplicity of assembly and in efficiency of collimation and transmission.

An ionization chamber has been constructed which enables electrode separations up to 100 mm and gas number densities up to 252*10²⁴ m^-3 to be employed. The initial photoelectric current was produced by back illumination of a thin metallic film evaporated on to a quartz disc inserted into the cathode. Values of the attachment coefficients for carbon dioxide for gas number densities between 26.5 and 48*10²⁴ m^-3 and E/N (electric field/gas number density) values of 84.3, 90.3 and 96.3*10^-21 V m² are given and show that the experimental errors associated with the attachment coefficients are reduced from the +or-160% that would be typical for conventional systems to +or-50% for the present system.

The electrical conditions necessary for reliable operation of sliding-spark arrays have been determined. The emission from an array can be considerably enhanced by critically damping the spark discharge circuit. When used to pre-ionize a CO₂ laser there must be a delay between the array and main discharge pulses, but this delay may be as small as 100 ns and less than the array discharge pulse width.

A current supply is described. By means of temperature compensation of the sense resistor circuit the temperature-dependent drift of the current was reduced by a factor of about 5. The drift caused by a change in current from 550 to 650 mA is, after one minute, less than 1*10^-6 min^-1, and, after three minutes, below the relative statistical fluctuations of Delta I/I=0.3*10^-6. These fluctuations are mainly caused by noise from the supply reference voltages and the measuring device. The current supply is particularly suitable for objective lenses of high-resolution electron microscopes.

An apparatus is described with which one can measure both the amplitudes and the rates of change in the polarized components of the fluorescence which accompanies the alignment of macroparticles in dilute suspension when subjected to pulsed electric fields. The apparatus consists of a suitable laser beam which is polarized and incident on a cell containing the dilute suspension under study. The fluid is held between suitable electrodes. Mutually perpendicular detection limbs record the polarized components of the fluorescence for the duration of a pulsed sinusoidal or direct current potential difference applied to the electrodes. Factors which give rise to the high sensitivity of the system are carefully described. The performance of the apparatus is illustrated by measurements on dilute solutions of the DNA-acridine-orange complex.