Thermal Cleaning Research Articles

The sharpening of field emitter tips by ion sputtering

A method is presented, of sharpening metal points which are used in field emission and field-ion microscopy. By sputtering electropolished tungsten tips using neon ions, the tip radius can be controllably reduced from that which results from thermal cleaning (typically 1000 Å) to below 200 Å. These tips, which can exploit the full resolution of the field ion microscope, can be formed on thermally cleaned shanks. The rate of reduction in radius is presented as a function of neon pressure and sputtering current. Changes in the tip shape which result from sputtering, are examined by scanning electron microscopy, and are used as a basis for a simple model of tip sputtering, which leads to the expectation that the method of sharpening is applicable to a wide variety of metals.

A palaeomagnetic survey of South American rock formations - Studies on rock formations from Northeast Brazil

Devonian, Carboniferous and Xriassic formations from Piaui and Maranhao States in the northeast of Brazil have been studied and palaeomagnetic pole positions deduced. During the Devonian and Carboniferous the south pole appears to have moved away from S. America in a southeasterly direction from the vicinity of Rio de Janeiro: in the Triassic it was situated near the present position of the south pole relative to S. America. These results are in good agreement with those from other formations of the same age from other parts of the continent. Thermal cleaning has been carried out. The Devonian formations were almost completely remagnetized by the Mesozoic or Tertiary geomagnetic fields and the primary magnetization is very weak. Polar wander of 40 to 50° appears to have occurred during the time interval under investigation, i.e. M. Devonian to Triassic.

The Palaeomagnetism of the Antrim Plateau Volcanics of Northern Australia

Summary The magnetization of samples from 29 sites in Lower Cambrian Volcanics and interbedded sediments, collected over an area of 250000km2 of northern Australia, has been investigated. Fourteen of the sites respond to magnetic cleaning, although in one case thermal cleaning is preferred. The other sites either become randomly magnetized or remain magnetized in the direction of the present geomagnetic field, as in the case of the interbedded sediments The mean direction of D = 51, I = +66 (ags = 13) gives a palaeomagnetic pole at 9 N, 160 E with Ag5 = 17 The polar-wander path for Australia is for the first time extended back to Lower Cambrian times with confidence, and a complete curve can now be drawn for the Phanerozoic. It is concluded that Australia was situated in the Northern Hemisphere in Cambrian times. Two rapid polar transitions are identified during the Palaeozoic, a feature which is also present in data from Africa. The polar-wander paths for these two continents can now be made to coincide for the whole of the Palaeozoic, implying that their relative positions remained unchanged during this period.

The Palaeomagnetism of Some Cambrian and Ordovician Sediments from the Northern Territory, Australia

Summary Samples for palaeomagnetic investigation were collected from nine horizons in the Jinduckin Formation, of Early Ordovician age, and from four exposures of a red mudstone band within the Middle Cambrian Montejinni Limestone. The mudstone samples responded unsatisfactorily to both alternating field and thermal demagnetization procedures and their magnetization appears to originate from chemical action during weathering in recent times. In contrast, only one site from the Jinduckin Formation failed to respond to thermal cleaning, and a chemical demagnetization technique here was also unsuccessful. Thermally cleaned directions from seven of the other eight sites are assumed to yield the original direction of magnetization of the Jinduckin Formation (D= 252°, I=−14°, α95, = 20°), and are used to derive the mean pole position for the period of its deposition (Long. = 29° E, Lat. = 16° S, A95= 17°). This is the first result from a current palaeomagnetic study of the lower Palaeozoic red beds of northern Australia.

Palaeomagnetism of the Cambrian purple sandstone from the salt range, West Pakistan

Samples from two localities near khewra have been examined palaeomagnetically. At one locality the samples are shown to be unstable, but at the other they respond to thermal cleaning and are shown to be extremely stable. In a rare occurence, a 10 cm core drilled through one sample showed two polarity changes occurred during the time taken for its deposition. Changes in intensity recorded from intermediate directions in the core suggest the field dropped to a quarter or fifth of its normal value during the polarity changes. An approximate rate of sedimentation of between 0.1 and 1 cm per thousand years is estimated for these sediments. The new palaeomagnetic pole for the Indian sub-continent enables the position of this continent to be determined uniquely with respect to the other Gondwanic continents. India is placed alongside East Africa, but not adjacent to Western Australia. The configuration deduced is almost identical to that originally proposed by du Toit.

Stability of the magnetization of the Hurley Creek Formation

Oriented samples were taken from five sites in the Hurley Creek formation of New Brunswick. This formation lies immediately over the Minto formation, which is well-dated by fossils. Thermal cleaning shows that only one very stable magnetization is present. The between-site dispersion is consistent with a magnetization acquired during or shortly after deposition. The mean directions (D = 171°E and I = +09°) of the Hurley Creek formation are therefore considered representative of the Pennsylvanian field of New Brunswick, Canada. The corresponding pole position is 39°N, 125°E.

Stratigraphic interpretation of paleomagnetic measuremenst on the Waterberg red beds of South Africa

Samples have been collected from twelve sites at four different localities through the Waterberg red bed succession in the Transvaal. The samples respond equally well to both thermal and magnetic cleaning, and in most cases there is no significant difference between the results obtained by the two methods. After magnetic cleaning at all the sites and correcting for the attitude of the beds in each case, the directions of magnetization form three groups. These groups appear to be related to the stratigraphic position of the sites in the succession. It is suggested that these changes in direction of magnetization through the sequence are the result of considerable polar wandering which took place during the period of deposition and consolidation of the Waterberg sediments. The suggested polar wander path is consistent with the pole positions determined previously from other Precambrian studies in southern Africa. The measurements provide further evidence for the hypothesis that the Waterberg and Umkondo systems should be correlated and that the age of the ‘post-Waterberg’ igneous activity and the Premier Mine kimberlite is confined within the limits of the Waterberg system. The results also suggest that the Mashonaland dolerites of Rhodesia are slightly older than the Umkondo dolerites, contrary to previous observations, and that their age is also confined within the limits of the Waterberg system.

A paleomagnetic result from the Ventersdorp lavas of South Africa

Samples have been collected in the vicinity of the De Beers diamond mine from at least two flows of the Ventersdorp lavas. The specimens responded equally well to both thermal and magnetic cleaning methods and a preliminary paleomagnetic pole position is presented for Ventersdorp times. The samples studied are shown not to have been influenced either magnetically or petrologically by the emplacement of the De Beers mine kimberlite in cretaceous times.

Evidence for Palaeomagnetic Inclination Error in Sediment

THE inclination error (δ) in sediment is the difference between the inclination of the ambient field and that of the remanent magnetization acquired by a sediment1. The error is reckoned positive (negative) if the inclination of remanence is less (greater) than that of the field. Inclination errors in nature may be studied by comparing observed inclinations in sedimentary rocks (Io) with that expected (Ie) from studies of contemporaneous isotropic igneous rocks which are not subject to the error. There are two methods. Results from nearby localities may be compared directly (method 1), or results from widely spaced localities may be compared by calculating the expected inclination (Ie) calculated from the palaeomagnetic pole derived from observations of igneous rocks with the observed inclination (Io) in sediments (method 2). Method 2 assumes that the Earth's field is a geocentric dipole. Both methods assume that the secular variation is averaged out so that only averages for formations covering many tens of thousands of years may be used. There are errors (α) in determining Ie and I0 so the inclination error is itself subject to error. Because of the statistical inhomogeneity of the palaeomagnetic data a formal calculation of the error in δ would be unrealistic, but a lower limit is set by the α values. In simple observations of fossil remanence a is usually about 10°, whereas after the application of magnetic or thermal cleaning techniques accuracies of about 5° can sometimes be achieved.

DÉSAIMANTATION THERMIQUE ET ANALYSE STATISTIQUE DES DIRECTIONS DE SÉDIMENTS CARBONIFÈRES ET PERMIENS DE L'EST DU CANADA

Thermal demagnetization of red beds (109 specimens, 42 sites) from the Maritimes and Gaspé shows that the fossil magnetization consists of a large, thermally stable component and of small, unstable components that disappear at about 400 °C. Thermal cleaning of the whole collection was therefore carried out at 450 °C to demagnetize these soft components. This procedure changes the mean declination and inclination by 6° and 7° respectively and also changes the distribution of directions about the mean. A statistical analysis shows that the site mean directions (after demagnetization) obey a Fisher's (1953) distribution whereas the individual specimen directions do not. This result is of interest in two connections: first, it shows that, in this instance, the use of Fisher's (1953) statistics to analyse the results is not permissible when unit weight is given to the specimen, but that their use is legitimate when unit weight is given to the site; second, the result is consistent with the hypothesis that the earth's field, when sampled in this way (paleomagneticalty), can be represented by an axial geocentric dipole field perturbed by randomly directed magnetic components at the earth's surface. Comparison with other results available for this geological period shows a certain divergence among the paleomagnetic poles for North America. It is suggested that a partial demagnetization of these earlier collections might reduce the observed divergence.

Paleomagnetism of some Mesozoic intrusives and tuffs from eastern Australia

Paleomagnetic studies of the Brisbane tuff and the Noosa Heads and Mount Dromedary intrusive complexes show that the stable components of natural remanent magnetization have rather uniform directions (irrespective of sign) with steep inclinations. In each case the time of formation of the rocks is known with precision, either from stratigraphic or from radioisotope evidence, to be Middle Triassic, Upper Jurassic, and late Lower Cretaceous, respectively. Field and laboratory studies suggest that the stable magnetization originated at the time of formation and was parallel to the earth's field. The observations therefore appear to provide a record of the earth's field at known times in the past. A reversal of magnetization in the Noosa Heads complex is related to a radioisotope age of 140 m.y. A detailed comparison of thermal and magnetic cleaning methods, in which rock samples from the Mount Dromedary intrusive complex were used, yields the result that the latter is to be preferred in this instance.

Thermionic Emission Constants of Iridium

The thermionic constants of polycrystalline iridium have been measured with reasonable care, using thin ribbons of iridium as the emitting specimens. The values obtained for the quantities $A$ and $\ensuremath{\varphi}$, both considered as constant in the equation ${i}_{s}=A{T}^{2}\mathrm{exp}(\ensuremath{-}\frac{\ensuremath{\varphi}e}{\mathrm{kT}})$, are $A=170$ amp ${\mathrm{cm}}^{\ensuremath{-}2}$ deg ${\mathrm{C}}^{\ensuremath{-}2}$ $\ensuremath{\varphi}=5.40$ v.The relationship between true and brightness temperature at the pyrometer wavelength of 0.65 \ensuremath{\mu} was likewise determined. From the latter data, it was computed that the spectral emissivity ${\ensuremath{\epsilon}}_{\ensuremath{\lambda}}$ of iridium is 0.33 at that wavelength.Data taken on fresh specimens yielded values of the Schottky slope higher than expected, and values of $A$ orders of magnitude above the theoretical value. (This experience parallels that of workers who have studied the thermionic properties of platinum.) The values reported were obtained after prolonged and rigorous thermal cleaning of the iridium.