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Radiation Type Gamma X-ray Beta

Measuring Range

(photon energy) 10 keV to 10 MeV 10 keV to 10 MeV 700 keV to 3.5 MeV Measuring Range

(dose) 0.05 mSv to 10 Sv 0.05 mSv to 10 Sv 0.05 mSv to 10 Sv Typical period of use 4 weeks

Table 20. Film badge & thermoluminescent dosimeter specifications

15.2.2 THERMOLUMINESCENT DOSIMETERS (TLD)

The thermoluminescent dosimeter or TLD offers several significant advantages over the film badge:

(a) A TLD is much less easily damaged than a film badge.

(b) The TLD has slightly wider measurement range than the film badge.

(c) The TLD is much less subject to possible errors or failures in processing – the measurements obtained have a better degree of accuracy.

(d) The TLD can be reused many times.

(e) The absorption characteristics of the TLD more closely resemble those of the human body, thus dose calculations are simplified.

they used to be popular for use inside nuclear power plants but they have now largely been replaced by more reliable, more accurate solid state devices.

Figure 92. Quartz Fibre Electrometer

The quartz fibre electrometer (QFE) is a gaseous detector like the ionisation

chamber, proportional counter and Geiger counter described above. When raised to the light a scale like the one on the left of figure 92 can be seen through the lens of the instrument. The vertical line is the quartz fibre. When a static electrical charge is applied to the instrument the quartz fibre moves to the zero point of the scale. As the gas inside the QFE becomes ionised the static charge is gradually dissipated and the fibre begins to move to the right. The corresponding total dose received can be read on the upper scale.

The QFE has quite a narrow measuring range, typically 0 to 50 mSv or less. The example shown above has a measurement range of 0 to 200 mRem which is equivalent to 0 to 2 mSv. The QFE is sensitive to x and gamma radiation in the photon energy range 45 keV to 3.5 MeV. The QFE is a very convenient means for checking how radiation doses are

accumulating during a working day but it suffers from fragility and is very easily damaged.

15.2.4 SOLID STATE INTEGRATING DOSIMETERS

The QFE has largely been replaced by solid state integrating dosimeters. These devices are extremely shock proof and have a wider measuring range than the QFE. They are typically combined with an audible warning device which bleeps if the wearer unwittingly enters a high radiation area.

16.0 RADIATION SAFETY

All personnel working with ionising radiation should be aware that such radiation is injurious to the human body (or any other biological tissue). Anyone working at a place where radiography is carried out should make himself fully aware of the safety procedures and regulations which are in force and take care observe all warning barriers. Ionising radiation cannot be detected by the five human senses. Ionising radiation has cumulative effects upon the human metabolism. Ionising radiation causes genetic damage to the human body the full effects of which may not be apparent until as much as 15-35 years after the initial exposure. Regardless of any nominal safe limits it is always prudent to avoid

exposure to radiation whenever possible.

N.B. Where industrial radiography is concerned there is little or no danger from

contamination because all gamma sources in use are of the sealed variety. X or gamma rays are not capable of producing any residual radioactivity in the items subjected to exposure.

16.1 PRECAUTIONS

16.1.1 EXPOSURE BOOTHS

At locations where a large volume of industrial radiography is carried out exposure booths of various shapes and sizes will generally be available. These usually consist of enclosures having lead lined walls. Some exposure booths have walls filled with spent casting sand or other radiation absorbing material.

Such exposure booths should be regularly monitored to ensure that the radiation dose rate is within safe limits in the areas outside the booth where personnel can move freely.

Safety switches are usually fitted to doors of exposure booths in order to prevent the operation of x-ray sets or gamma ray equipment whilst the door is open. In cases where overhead cranes might have to pass over an open topped exposure booth similar safety switches are normally installed so as to trip out the x-ray set, or wind back the gamma ray source, should the crane encroach upon the irradiated area during exposure.

In many countries (including Britain) it is a legal requirement that an audible warning is given before any exposure takes place.

Exposure booths should be equipped with switches inside the x-ray compound which can be operated in order to prevent the operation of the x-ray or gamma ray equipment should any personnel be accidentally trapped inside.

Radiation detectors should be installed inside the exposure booth to indicate when gamma ray sources are being used.

guarded by classified personnel during exposure.

16.1.3 SCATTER

Personnel should be aware that radiation can be scattered by structures. Apparently safe locations may be subject to stray scattered radiation.

16.2 EXPOSURE LIMITS FOR RADIATION WORKERS

In Britain classified workers are allowed to receive an accumulated dose of 20 millisieverts (20 mSv) per year from the age of 18 yrs to 65 yrs. A formal investigation is required if a classified worker receives a dose of 15 mSv or more within any single calendar year. The investigation has to establish the source of the dose received. The investigation may or may not include a thorough medical check for the person receiving the dose. These requirements are typical for all countries within the European Union, but requirements in other countries may differ widely.

16.2.1 DOSIMETERS

For work in radioactive environments (i.e. nuclear reactors) personnel must be equipped with direct reading dosimeters which will display immediately the accumulated dose received. Personnel working in these locations must take particular care to avoid ingesting radioactive particles. Tightly fitting breathing masks are required and protective clothing should be worn.

16.3 PERMITTED LEVELS

The figures given in the paragraphs below relate to Statutory Instrument 1999 Number 3232, ‘Ionising Radiation Regulations 1999’. These regulations exclude radiation doses received due to medical reasons.

16.3.1 CLASSIFIED WORKERS

The maximum permitted dose rate for personnel equipped with film badges (or TLDs) is 20 mSv per year . This is approximately equivalent to a constant dose rate of 10 μSv/hr for a 40 hour working week if a 48 working week year is assumed.

16.3.2 UNCLASSIFIED PERSONNEL, CONTROLLED & SUPERVISED AREAS

Controlled area

Unclassified personnel must be excluded from any area where radiation dose is deemed likely to exceed three tenths of the annual allowable dose for a classified worker (6 mSv). The maximum permissible dose rate at the boundary of a controlled area is 7.5 Sv/hr.

Supervised area

A supervised area is defined as an area where the annual dose is expected to equal or exceed 1 mSv. Such areas should be clearly signed. Unclassified persons are permitted to pass through such areas but they must not remain in them for extended periods. Where possible verbal warnings should be given by the radiographer.