Dose rate meters in pulsed radiation fields
Why counting instruments under-respond in pulsed radiation fields — pulse duration, repetition frequency and dose per pulse, the ceiling a dead time imposes on the count rate, recombination in ionization chambers, and what to look for before using an instrument near a pulsed source.
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Most type tests are made in continuous radiation fields: the reference sources and X-ray tubes of a calibration laboratory deliver radiation at a steady rate. Many workplaces do not. Medical linear accelerators, industrial accelerators, pulsed X-ray equipment used in fluoroscopy and computed tomography, and flash radiography deliver radiation in short bursts separated by intervals with no radiation at all. A dose rate meter that performs well in continuous fields can indicate a small fraction of the true dose rate in such a field, with no indication on its display that anything is wrong. This guide explains why, and what evidence an instrument needs before it is used near a pulsed source.
Pulse structure
A pulsed field is one in which radiation arrives in bursts whose duration is short compared with the interval between them. Four parameters describe it:
- pulse duration — the length of each burst;
- pulse repetition frequency — the number of bursts per second;
- dose per pulse — the dose delivered in one burst;
- duty factor — the fraction of the time during which radiation is present, equal to the pulse duration multiplied by the repetition frequency.
The mean dose rate is the dose per pulse multiplied by the repetition frequency. The dose rate during a pulse is the mean dose rate divided by the duty factor. Where the duty factor is small, the dose rate within the pulse exceeds the mean dose rate by a large factor, and it is the dose rate within the pulse, not the mean, that the detector must cope with.
Counting detectors
A counting detector — a Geiger–Müller tube, or a counting circuit after a scintillator or a semiconductor diode — registers individual events and is insensitive for a dead time after each one. In a continuous field the losses can be corrected from the measured count rate. In a pulsed field they cannot, because all the events in a pulse arrive within a time shorter than the dead time.
Ceiling on the count rate
If the pulse duration is shorter than the dead time, the detector can register at most one event per pulse. The count rate therefore cannot exceed the pulse repetition frequency, however large the dose per pulse. The indicated dose rate has a ceiling equal to the repetition frequency multiplied by the dose equivalent the instrument assigns to one count. Beyond the dose per pulse at which the detector registers an event in almost every pulse, the indication stops increasing while the true dose rate continues to rise.
The consequence is an indication that is plausible — a steady, moderate dose rate — and wrong by a factor that depends on the pulse structure rather than on the field’s mean dose rate. The same instrument, with the same calibration, can read correctly near one pulsed source and far too low near another.
Dead-time correction
Dead-time corrections built into counting instruments assume that events arrive randomly in time. In a pulsed field they do not, and a correction derived for continuous fields cannot recover events that were never registered. An instrument’s performance in continuous fields — including a good result in the dose rate dependence and overload tests — is therefore no evidence of its performance in pulsed fields.
Ionization chambers
An ionization chamber operated in current mode does not register individual events. If its integration time is long compared with the interval between pulses, its indication follows the mean dose rate. The limitation is recombination: the charge produced by a single pulse is concentrated in time, and at high dose per pulse a fraction of the ions recombine before they are collected. The collection efficiency therefore falls as the dose per pulse rises. The theory of this effect for pulsed radiation, due to Boag, is the basis of the corrections used in reference dosimetry. For a radiation protection instrument the practical question is whether the manufacturer has stated the dose per pulse up to which the collection efficiency remains acceptable.
Standards for pulsed fields
The difficulty is recognised by dedicated standards. IEC 62743 addresses electronic counting dosemeters for pulsed fields of ionizing radiation, and IEC 63050 addresses dosemeters for pulsed fields. Both are listed in the standards register of this site. Both are concerned with demonstrating an instrument’s response in terms of pulse parameters such as those above, so that a user can compare the demonstrated conditions with the field in question.
Evidence before use
An instrument intended for use near a pulsed source needs evidence that goes beyond its continuous-field type test:
- a statement of the pulse durations, repetition frequencies and doses per pulse over which its response has been demonstrated, either from a pulsed-field standard or from the manufacturer’s own tests;
- knowledge of the pulse structure of the source where it will be used, which the equipment manufacturer or the facility’s physicist can provide;
- where neither is available, a comparison with an instrument known to be suitable — typically an ionization chamber with stated pulsed-field performance — in the actual field.
Electronic personal dosemeters worn by staff near pulsed X-ray equipment raise the same question, as noted in the description of IEC 61526. An indication of low dose from a counting dosemeter in such a field is not evidence of low dose.
Coverage in this tool
The evaluation tools on this site compute results from readings taken in continuous reference fields. They do not model pulsed fields, and a pass verdict from them carries no information about an instrument’s behaviour in a pulsed field.