National Physical Laboratory

Troubleshooting in radiation monitoring

Man Paper

This module highlights problems that can be encountered in interpretation of the results and in the monitoring instruments.

A. Points to consider if interpreting the measurement of dose rate

1. Close approach to sources

As a general rule when measuring dose rate from a point source, the detector should not be placed closer than three times the detector dimensions because the radiation field is no longer uniform. A closer approach will lead to an increasing underestimate of the dose rate. Measurements can be made at greater distances and corrected for the inverse square law.

For a small area β emitting source it may be necessary to measure at a closer distance in order to detect it, but the user should be aware that there can be gross underestimates because the dose rate in contact with the source could be 100 times greater than the measured instrument indication when using a typical ion chamber instrument that has a large detector volume. An extremity dosemeter placed in close proximity to the source may provide more reliable measurements. 

2. Dead Time

Consult the instruction manual to confirm whether dead time correction is required.

Most commercial instruments for the measurement of dose rate will have built in dead time corrections. However, there are some instruments in the field that do not automatically compensate for dead time, guidance on how to correct an instrument reading to account for losses due to dead time is provided.

Guidance on how to convert instrument for dead time


Pulse counting instruments such as GM detectors and the majority of proportional and scintillation counters may underestimate the number of pulses at very high counting rates. This is because there is a time needed to process the interaction and during that time, no further pulses will be registered; this is known as the dead time or paralysis time. Dead times range from a few µs for scintillation and the smallest GM types to a few hundred µs for larger GM types. The effect of dead time is shown in the figure below

Dead Time Diagram

where τ = deadtime of detector following an interaction

Effects of dead time on detector 

Event number

1

2

3

4

5

6

Counted?

Tick

Tick

X

Tick

X

Tick


A total of 6 events occur within the detector (a) but only 4 counts are registered (b).

Events 3 and 5 will not be registered because they occur during the dead time period of the detector.

If dead time correction is not automatically made by the instrument (consult the instrument manual to confirm), a correction will have to be applied to any results at high counts.

The observed count rate can be corrected for dead time using the formula below:

True count rate = True Count

Where Ri is the instrument indication and τ is the dead time for the detector in seconds.

If a G-M tube has a dead time of 25 µs and Ri is 2000 s-1,

the true count rate = True Count 2

= 2105s-1

In this example, there is a loss of 5 % of the counts (2000/2105) due to the dead time.

3. Pulsed sources generating narrow pulses with durations less than 30 μs

Measuring pulsed radiation sources is a specialist task. 

The use of pulse measuring equipment is not recommended in pulsed radiation fields, e.g. from linacs. The pulse from the machine is commonly only a few µs long, which means the instrument can only record one count, no matter how intense the dose rate. It is safer to use instrumentation that measures the intensity of each pulse rather than simply detecting it or not, e.g. ion chamber instruments or passive dosemeters. 

However, ionisation chambers do not work well at low dose rates and passive dosemeters cannot be used to search for shielding weaknesses. To use a pulse measuring instrument expert advice should be sought to interpret the readings.

Example on how to interpret readings


Consider a pulsed X-ray machine producing 5 µs wide pulses at 200 Hz. At the limit, a GM counter will produce 200 counts per second. If the detector is a sensitive type, with a calibrated response of 5 counts per second per µSv per hour, at its limit the indicated dose rate will be (200/5) = 40 µSv h-1. However, in reality the actual dose rate could be of the order of Sv h-1. The instrument will be generally trustworthy where the chance of detecting each machine pulse is less than 30 %. For the instrument above, this would be 30 % of 200 s-1 =  66 s-1, corresponding to a maximum trustworthy indication of about 13 µSv h-1.

Scattered and transmitted spectra X-ray equipment will produce a range of energies up to the maximum energy that is generated by the peak voltage on the tube. The lowest energies are attenuated most effectively by the tube casing and shielding. On the outside of intact shielding it is a valid assumption that the mean energy of the transmitted beam in keV is close to the tube voltage in kV. A similar process also takes place with γ sources. The ratio of attenuation between low and high energy radiations is greater if the shielding is lead, tungsten or other high atomic number material.

If radiation is scattered through a maze entrance, or by the air above a source room with thick walls and a thin roof, then the opposite takes place. Scattering reduces the mean energy of the radiation reaching a worker. As an example, a 60Co γ emitting source (mean energy 1.25 MeV) in a room with a maze entrance and without a heavy door could produce a mean energy at the door of 150 keV1

For instruments that read in dosimetric quantities it is not necessary to correct for these effects of scattered and transmitted radiation providing that

  1. the instrument has a good energy response and
  2. there is no significant component below the useful working energy range.

However for other instruments the combination of these effects makes it very difficult to calculate a correction factor.

 

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