First Measurements with the Spanish Standard for the Absolute Determination of the International Roentgen Unit

Abstract

THE Laboratory for Radiological Measurements annexed to the Service of Physical Therapy, directed by the author, began operation in 1931. In 1932, there was installed a Piccard compensation electrometer, with which we began to make determinations of the charge of radium containers. Finally, in 1934, we found it possible to adapt to this installation the standard chambers for the absolute measurement of the roentgen unit. Varied circumstances until now have hindered our comparing this standard with the standards of other countries, but it will be done as soon as possible. The arrangement we have employed has been adjusted to the agreements of 1934 between the National Laboratories (1). I.—X-ray Generator We have employed one of the constant high tension machines—“Stabilivolt” S.R.V.—in our Department of Roentgen Therapy. The feeder line presents fluctuations of voltage which, up to this time, we have not found the means of compensating for, by the use of a voltage stabilizer, but we hope to do so in the future, when our equipment will operate as a public service of standards. Notwithstanding, we have practically eliminated these unpleasant oscillations by making our measurements during the hours in which the industrial engines are stopped. In every case, the filament current has been furnished by a storage battery having a capacity of 100 ampere-hours. We have employed two x-ray tubes: one of the Multix model, for voltages up to 110 kv., cooled by thermosiphon, and entirely protected against loss of radiation. The other, of the Metwa Matalix type, up to 200 kv., having the same cooling and protecting system as the former, and a glass window for the passage of the rays. These models are not shockproof, but our new tubes will be so protected in the future, on account of the very efficient screening that the shockproof tubes and conductors possess, which renders unnecessary every other protection against interference from magnetic fields. The milliammeter for measuring the intensity of the current passing through the tube has been calibrated against a precision instrument. We have marked only a fixed point, corresponding to the reading of 4 ma. A second milliammeter, in series with the first, checks the constancy of its readings. The margin of error in the current measurements is of the order of 2 per cent. The high voltage of the Stabilivolt generator has been calibrated by means of a Muller sphere gap, with spheres of 125 mm. diameter. The connection of the spheres with the high tension line has been made through high ohmic resistances, formed by 6 meters of glass tubing of 1 cm. diameter, rolled in a spiral and filled with 95° alcohol. The spark obtained in the sphere gap is very fine, and is spontaneously interrupted, affording periodical discharges (relaxation oscillations).