A radiation instrument calibration system.

Abstract

THE PRIMARY standards for the calibration of radiation instruments are located at the National Bureau of Standards. For many years after the establishment of this and similar laboratories around the world, it was possible to send all working instruments to these offices for calibration. Today the number of instruments in use makes it impractical to handle the necessary volume of work in the national laboratories. Moreover, it has become increasingly probable that an instrument will be damaged in transport despite the best care in packing. It has long been the custom to supply electrical and electronic components and devices with a certificate of calibration “traceable to the National Bureau of Standards.” This is done through the use of a secondary standard in which the design criterion was long-time stability. For several years the International Commission on Radiological Units and Measurements has compared the various national standards by means of systems consisting of a free air chamber diaphragm, an air capacitor, and a small cavity chamber.2 Perhaps as a result of this activity, intercomparison chambers have become commercially available. Figure 1 illustrates a range of such chambers for various energies. Standard air capacitors are also commercially available. One or more intercomparison chambers in association with one or more standard air capacitors is the nucleus of a calibration system which is adequate for most needs. Intercomparison Chambers And Standard Air Capacitors Intercomparison chambers are available for low, medium, and high energies at a cost of less than $500 each, standard air capacitors for about $100. The readout system can be as elaborate as desired. Although most medical physicists are familiar with such systems, it is worthwhile to recount the experience gained in assembling such a system and putting it into use. In some cases the manufacturer provides a calibration of the intercomparison standard chamber against a standard air chamber, giving a calibration factor in roentgens' ampere-minute. The National Bureau of Standards provides a similar calibration. Both laboratories use a collection potential of 300 volts. One of the medium-energy chambers in use in our laboratory was measured by both the manufacturer and the National Bureau of Standards, and the calibration factors were found to be in better than 1 per cent agreement. The standard air capacitors are guaranteed by the manufacturer to be within ± 0.1 per cent of the nominal capacitance and calibrated to an accuracy of ±0.02 per cent. These instruments, too, can be sent to the National Bureau of Standards for calibration. The National Bureau of Standards' calibration on the 100 pF capacitor in use in our laboratory showed a capacity of 100.008 pF ± 0.012 per cent. We believe that a properly designed and constructed intercomparison chamber can be used for years without the necessity of returning it for recalibration against a standard air chamber.