The Dosimetry of Pulsed Radiation

Abstract

Publications of the International Commission on Radiation Units and Measurements (ICRU) enjoy a justifiably high reputation for their relevance and accuracy. This is no exception. As the report is assumed to be the joint responsibility of all members of the Commission, no authors' names are given. Nevertheless, the various chapters clearly correspond to the interests of the four members of the "report committee", all of whom are well known in their respective fields of work.The subject of the report is the measurement of absorbed dose and absorbed dose rate in materials exposed to pulsed ionizing radiation, particularly X-ray and electron beams. Only thirty five pages of text may seem at first sight to be insufficient to provide anything but a superficial view of the subject, but the authors have adopted a very compact style which covers a lot of ground. It should be regarded as a practical guide, not as a textbook or scientific monograph; rules are sometimes given without justification, but with a reference to where more information can be found.Four types of radiation detectors are dealt with in separate chapters—ionization chambers, chemical dosemeters, calorimeters and solid-state detectors.For irradiation conditions typical of those encountered in radiotherapy, i.e. from about 0.05 mGy to 1 mGy per pulse, commercial cable-connected ionization chambers are recommended, and this chapter is largely a summary of the many papers by Boag on corrections for non-saturation. Suggestions are also made for the design of chambers for use with higher dose rates up to 100 mGy per pulse.For still higher dose rates, such as those met with in radiobiological and radiochemical research, the Fricke dosemeter can be used up to 10 Gy per pulse. The chapter on chemical dosemeters deals also with extensions upwards in dose rate to about 100 Gy per pulse using "super-Fricke" solution or a thiocyanate dosemeter.Calorimeters are less influenced by a change from continuous to pulsed radiation than are other types of detector, but they do demand fairly high dose rates. Methods to correct for cooling are described in detail, and a warning is given about a possible change in thermal defect of some materials in short pulses with very high instantaneous dose rates. This chapter also contains a useful discussion on monitoring devices.The chapter on solid-state dosemeters covers a wide range of radiation effects including the change of optical absorption in glass and plastic materials after irradiation, photoluminescent and thermoluminescent materials and semiconductor detectors. These devices are described in rather general terms, and there is less information than in other chapters on the special problems of measuring pulsed radiation. The section on semiconductors is now rather out of date as there has been a great increase of interest in these devices in recent years in connection with nuclear hardening of electronic circuits.For those who have to measure protection-level radiation in the vicinity of pulsed accelerators three practical hints are given: don't use particle-counting devices as these will usually just record the pulse repetition frequency of the accelerator (Sect. 2.6), when designing cylindrical and spherical ionization chambers it is wise to avoid the use of a guard ring inside the collecting volume (Sect. 2.7), for large ionization chambers the ion-collection time may be long compared with the period between pulses, and the chamber will then behave as if subjected to continuous rather than pulsed radiation (Sect.2.2.8).There is no doubt that anyone who is responsible for the dosimetry of pulsed X-ray or electron beams will find this report a handy practical guide and a useful pointer for further information.