Current GM-based dosemeters are still very popular and have developed into devices with comprehensive displays , selectable functions and a higher sensitivity , which makes them very good ALARA tools .

Silicon diode-based dosemeters with wide gamma energy and beta capability are now available from several manufacturers .

( Acknowledgement to Tracerco ) Note the very comprehensive display .

However , GM tubes have their limitations . They operate at high voltages , typically around 550 V . They are hand made , intrinsically more fragile , and do not operate well for photons below 35 keV or for beta radiation .

PiN diodes These offer a way round these limitations , provided several detectors are used . Such a design results in a dosemeter with a better dosimetric response for photons and betas than many passive ( thermoluminescent and

1 optically stimulated ) dosemeters . The first instrument of this type was the Plessey and NRPB Electronic Personal Dosemeter ( EPD ), which has evolved into the Thermo EPD TruDose .

In summary , the advantages of electronic dosemeters over passive dosemeters are :

• the dosemeter provides instant readings

• audible warnings can be provided

• time / dose rate profiles can be obtained

• they allow shift and person data logging , so there is no need for each individual to have their own dosemeter

• no data processing is required , hence there is no need for expensive dosimetry readers ; and

• there is no need to compensate for background when the dosemeter is not in use .

However , electronic dosemeters also have weaknesses :

• their use in pulsed fields is limited

• they have a battery which requires charging

• they tend to be heavier and

1

Thermoluminescent dosemeters ( TLDs ) absorb radiation energy , which can be released later by applying heat and measuring the light emitted using a photomultiplier . The dose is proportional to the emissions . See the article on page 10 of this issue .

• they are much more expensive to buy per unit , although the operating cost can be less

8 Radiation Protection Today www . srp-rpt . uk