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A simple method of filling water triple-point cells

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1973 J. Phys. E: Sci. Instrum. 6 975

(http://iopscience.iop.org/0022-3735/6/10/009)

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Apparatus and techniques

Table 1 Coefficients of variation obtained with three different counting methods

Isotope Peak Statistical error Comparison of three methods 7 energy in peak area h-

(keV) determination Nonrotating Hand loading Rotating and ( %) ( %) ( %> compaction (%)

51Cr 320 0.4 2-5 1.1 0.7 46Sc 1 120 0.4 1-6 0.9 0.7 GoCO 1173 0.4 1 *9 1 .o 0.7 Integrated spectrum 0.01 1.5 1.0 0.5

(iii) counting of the sample by use of the automatic transfer unit with sample rotation and compaction.

After each count, the sample was removed and shaken up before being reloaded into the system used. The sample was the international rock standard NIM-P, which had been irradiated at a flux of 5 x n s-l m-* after 180 days’ decay. The low activity of the sample permitted the use of the mini- mum distance between sample and detector (15 mm) without overloading of the electronics. However, the long decay time of the sample resulted in only three isotopes, 51Cr, 46Sc and W O , having enough activity to give peak areas with calculated coefficients of variation of less than 1 %. The concentration of the elements in the standard NIM-P are chromium 24360 PPM, scandium 28.5 PPM and cobalt 112 PPM.

The results given in table 1 show the coefficients of variation for ten results on three prominent gamma-ray peaks in the spectrum. The statistical error in the determination gives the error due to the statistical variations in radioactive decay and therefore represents the minimum possible error. As can be seen from the results, the worst error was obtained when an automatic transfer unit was used without rotation. There are two reasons for this. Firstly, the sample holder sleeve has a slightly larger internal diameter than the diameter of the polyethylene rabbit containing the sample and therefore allows some variation in the sample position relative to the detector. (This play was to ensure that variations in the diameter of the rabbits could be accommodated.) The second source of error was found by observation to be occasional sticking of a portion of the sample powder at the top of the quartz vial, which also effectively causes variations in sample position. The improvement achieved by the counting of a rotating sample with compaction is clearly shown. If compac- tion was ever thought to become a problem due to density separation with particular samples, the screw on which the stepped cam rides can be adjusted to reduce the drop of the sample, or even to eliminate bouncing altogether (although the possibility of uneven sample distribution arises again). Although counting errors still exist, they have been reduced considerably and are very small compared with other sources of error encountered in INAA, e.g. flux monitoring, sample inhomogeneity and counting statistics.

5 Conclusions The system as described has been operating successfully for over a year and has handled many thousands of samples. A second system has also been built and up to 100 samples a day are now analysed with the two automatic sample changers in full operation. It has been found that, as long as the distance between counting head and detector is meas- ured accurately (< 0.5 %), counting errors can be regarded as insignificant compared with errors in neutron-flux normali- zation and sample preparation and, particularly, with errors

resulting from the relatively poor counting statistics of the majority of gamma-ray peaks in most spectra.

Acknowledgments This paper is published by permission of the Director General, National Institute for Metallurgy, and of the Director, Nuclear Physics Research Unit, University of the Witwatersrand, Johannesburg,

References Hulse N 1972 NIM Technical Memorandum Project 05471 Zanders J A J, Lombaard S L and Fitzgerald B 1971 NZM Technical Memorandum Project 09670 Zanders J A J and Fitzgerald B 1972 NIM Technical Memorandum Project 09670

Journal of Physics E: Scientific Instruments 1973 Volume 6 Printed in Great Britain 0 1973

Asimple method of filling water triple-point cel Is D Ambrose, R R Collerson and J H Ellender Division of Chemical Standards, National Physical Laboratory, Teddington, Middlesex, TW11 OLW

Receiued 15 June 1973

Abstract A simple method of filling water triple-point cells is described. The triple-point temperatures of cells prepared in this way were found to differ no more than 0.1 mK from the temperatures yielded by cells used in this laboratory for reference purposes.

Published methods (Barber et al. 1954, Ferguson 1970) for the preparation of water triple-point cells are complex and time-consuming : the simple procedure described here can be completed in a day and cells prepared by its means have triple-point temperatures differing from those of cells prepared in other ways by no more than 0.1 mK. In the first instance, the apparatus was assembled with the aid of conical ground glass joints, but use of components incorporating parts made of polytetrafluoroethylene (PTFE) has been found satisfactory and leads to a further simplification in the technique.

The apparatus, shown in figure 1, comprises the cell A, of dimensions appropriate to the thermometer with which it is to be used, connected to a flask B by a screwed compression

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Apparatus and techniques

Figure 1 Water triple-point cell filling apparatus

joint C (Quickfit and Quartz, SQ24). This joint allows rotation of the cell and its connected tubing about the axis DD so that it may be clamped either in the position shown for filling or twisted through 180" so that cleaning may be completed by steaming. The angles at which the tubes are set (a , 201) are chosen so that in the inverted position there is free drainage from the valve E (Fisher and Porter Inc.). Before assembly the cell is cleaned by filling it with the solution recommended by Crawley (1953), allowing it to stand for a few minutes, and rinsing. About a litre of distilled water is then put in the flask and the cell is joined to it in the inverted position with the plug removed from the valve; the water is boiled gently and the cell is steamed for two hours, after which time it is rotated to the filling position so that the condensing water remains in the cell. The rate of boiling of the water is adjusted so that it is a little greater than the rate of condensation, and the excess steam bubbles through the water, keeping it near boiling temperature and free of air. When enough water to fill the cell to within 1 or 2 cm of the top has been condensed, power to the heating mantle F is switched off, the plug is replaced in the valve and, as soon as the flow of steam slackens, the valve is shut. When it is judged that the pressure has fallen below atmospheric, constriction H is sealed by fusing; water that may be in the 250ml bulb G is allowed to flow back into the cell and constriction I is then sealed. The cell, now detached from the rest of the apparatus, is inverted and the tube K is removed by fusing constriction J.

Six cells were filled in the manner described and were compared with the cells used at the NPL for realization of the International Practical Temperature Scale (1968). Over a period of 72 h, commencing 24 h after the initial setting up of triple-point conditions, the equilibrium temperature of each cell under test was compared with that of a reference cell in which triple-point conditions had existed long enough for the temperature to have reached a value that was steady within kO.1 mK. In no case did the temperature of the cell under test differ by more than 0.1 mK from that of the refer- ence cell.

The water in a triple-point cell must be free from significant amounts of impurities and from air, and be of substantially the same isotopic composition as ocean water (International Practical Temperature Scale 1968). It has been shown (Barber et al. 1954) that in practice variation in the isotopic composi- tion of the water in a triple-point cell is not a problem; moreover, only simple distillation is used in the present

procedure, and the quantities of water taken initially, used for steaming, and left as residue are such that the contents of the cell are roughly a middle cut. It is unlikely that the isotopic composition of the water in the cell differs significantly from that of the water taken as raw material.

The temperature of the triple-point of water exceeds by 10mK the melting temperature of water saturated with air under a pressure of 101325 Pa, and lowering of the triple- point temperature by 0.1 mK due to the presence of air requires a pressure of about 1000 Pa (7.5 Torr). The filling procedure adopted ensures that the residual air pressure in the cell is much less than this, so the main concern is that no salts, which also lower the triple-point temperature, or impurities such as grease, which Ferguson (1970) suggested might cause the temperatures obtained to vary erratically, are introduced into the water. With this aim in view, the apparatus is made entirely of Pyrex glass (which has been shown to be satisfactory for the purpose) except for the screw joint and the plug of the valve, and the construction of both of these is such that the only material other than glass coming into contact with the water is PTFE. Whether this could conta- minate the water is doubtful, but any possibility of it doing so is minimized by removal of the plug of the valve for most of the filling operation and its replacement only at the end when it is needed, and by the fact that the PTFE ring in the screw joint is covered by a pool of stagnant water (L in figure 1) that does not enter the cell except to the extent that there is convection in the pool and interchange with the wet steam passing down the tube. In an exploratory stage of this investi- gation some potassium permanganate was dissolved in the water in the distillation flask but the cell so made was unsatis- factory, and evidently the design of apparatus then in use, which incorporated a more complex but presumably less effective antisplash device than that shown as M (500ml bulb) in figure 1, allowed some of the salt to be entrained.

The tests described above showed that cells made in the way now recommended are entirely satisfactory, but as a check upon the adequacy of this method for the final purifica- tion of the water and to find whether any minimum purity should be specified for the water used, two cells were filled at the end of the current series of experiments with tap water in the distillation flask. Tests showed that the triple-point temperatures of these cells were no more than 0.15 mK below those of the reference cells; such a small difference indicates that no doubt need be entertained whether the contents of cells prepared from distilled or de-ionized water will be of purity adequate for the establishment of a temperature of 273.16 K - i t would not be desirable, in general, for tap water to be used in the distillation flask because of the additional trouble entailed in the subsequent cleaning.

The only point in the filling operation requiring comment is the sealing of the cell. If there is undue delay in fusing constric- tion H after the valve has been shut a difference in pressure may arise because of different rates of cooling of the distillation flask and the cell and the contents of the latter may be drawn back into the former; whereas too rapid application of the flame, before the pressure has fallen below atmospheric, may result in blow-out as the glass melts, and failure of the opera- tion. The procedure described was originally applied with an apparatus assembled by means of ungreased conical joints and incorporating an ungreased glass tap in place of valve E; this apparatus was therefore sealed from the atmosphere by films of water and the pressure could not be allowed to rise or fall much above or below atmospheric pressure. Here, a certain deftness by the operator was required for success In the sealing operation but in the apparatus now recommended the necessary skill may be easily acquired.

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Apparatus and techniques

Acknowledgments The tests of the triple point cells were carried out in the Division of Quantum Metrology of this laboratory by Mr M V Chattle.

References Barber C R, Handley R and Herington E F G 1954 Br. J. Appl. Phys. 5 4 1 4 Crawley R H A 1953 Chem. Ind. 1205-6 Ferguson J A 1970 J. Phys. E: Sci. Instrum. 3 447-51 International Practical Temperature Scale of 1968 1969 (London: HMSO); 1969 Metrologia 5 3544

Journal of Physics E: Scientific Instruments 1973 Volume 6 Printed in Great Britain 0 1973

A low energy argon ion gun for cleaning samples in an electron spectrometer T Farrell Materials Science, Electricity Council Research Centre, Capenhurst, Chester

Received 26 January 1973, in Jinal form 9 July 1973

Abstract An ion gun is described which operates in a relatively high pressure of argon ( - 0.1 Torr). In operation, the voltage across the discharge region of the gun is 500 V, but subsequent ion-atom collisions in the spectrometer sample chamber reduce the ion-atom energy to about 0.1-1.0 eV at the sample surface. By operating in this mode, as distinct from high vacuum/high voltage (2-5 kV), the ion beam is dispersed and its energy is insufficient to cause structural damage to the sample surface, but sufficient to remove adsorbed species. The photoelectron spectra of a Cu-Zn alloy (brass) and of mild steel are given and illustrate the gun’s cleaning capability.

Electron spectroscopy is a means of chemical analysis in which a beam of soft x rays is incident upon the sample to be analysed. The x rays emit photoelectrons from the sample with kinetic energies in the region of 1 keV. Electrons with this energy have mean free paths of around 508, in solids and consequently the technique is one of surface analysis. However, solid surfaces are almost invariably coated with a contaminant film, which contains carbon and oxygen, probably existing as adsorbed 0 2 , Con, HzO and hydro- carbons. The film is often sufficiently thin to be transparent to the photoelectrons, but detracts from the quality of the electron spectra in that it cuts down the counting rate from the underlying sample. Thus, a means of removing this film is desirable. However, since the objective is to obtain a chemical analysis of the sample, the method of film removal must not interfere with the chemical structure of the under- lying sample.

Argon ion bombardment is widely used as a means of cleaning surfaces for use in ultrahigh vacuum work and also for etching microscope specimens. The same technique seems to be potentially applicable to the present problem. The cleaning action, when a surface is bombarded with argon ions, may be regarded as being one of momentum and energy

exchange between the incident argon ion and a surface atom (molecule or molecular group). When high energy argon ions are incident upon the sample surface, structural damage may ensue-in fact, one or other of the many forms of radiation damage will occur. The threshold energy for the displacement of an atom in its matrix is in the region of 25 eV, and for chemically bonded atoms at the surface this energy would be smaller, say 10eV. Thus, bombarding the sample surface with ions having energy greater than, say, lOeV would very likely result in structural damage. In addition, however, the high energy ions ( N lo3 eV) will penetrate some distance into the sample before being arrested, thus carrying their energy into the sample and consequently cause heating of the sample as well as the contaminant film. On the other hand, with ions of energy less than, say, 10 eV there is insufficient energy to cause structural damage and furthermore the ions will not penetrate the sample. The effect here is that only the adsorbed contaminant film receives energy directly from the beam and, provided the ion energy is greater than about 0.1 eV ( - 1000 K), the contaminant film will be removed by thermal action rather than sputtering and will be effectively ‘boiled-off’ without any structural damage or significant heating of the substrate.

From the above simple notions it appears that bombarding the surface with argon ions of energy about 0.1-1.0 eV would fulfil our cleaning requirements. However, the Paschen breakdown curve for argon shows a minimum in the region of about 200 V. Thus, the design of the argon ion gun must incorporate a means of producing argon ions (in accordance with the Paschen curve), and, in its operation, a means of reducing the ion energy from the minimum excited value of 200 eV to about 0.1-1.0 eV. In addition the ion beam must be adequately dispersed, so that cleaning is achieved over the whole of the sample surface.

In order to satisfy all the above requirements the gun was chosen to be of the type shown in figure 1. The variable

insu lator

I I Gas f l o w Escape o r i f i ce

------B

Sample chamber

, Cischarge region, [earthed I Gm

Figure 1 Schematic diagram of gun

design parameters of such a gun are: p , the pressure in dis- charge region which is controlled by the gas flow rate r and the diameter of the escape orifice; d, the anode-cathode separation, which together with p determines the breakdown voltage ; s, the sample-gun separation. The reduction in the ion energy is achieved by ensuring that argon ion-atom collisions occur within the sample chamber. Collisions occur if the mean free path of the argon ions is less than s. The mean free path is dependent upon the gas pressure in the sample chamber, which is related to p . Furthermore, when collisions occur, the ion beam becomes dispersed and thus, by suitably choosing the design parameters, the ion/atom energy at the sample surface can be reduced to 0.1-1.0eV and at the same time the required beam dispersion achieved.

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