Optically & Thermally Stimulated Luminescence (OSL & TL) Group
in 
Gaziantep University 
(TLD Dosimeters)
  
 

Characteristics of TLDs 

The TLDs have advantages which are listed below: 
    1. Gamma dose measurement over a very wide range is possible. Response  is linear between tex2html_wrap_inline919 and tex2html_wrap_inline921 . 
    2. High sensitivity, that is high light output for a given exposure, means the dosimeter  can be made small and convenient to wear. 
    3. They can be used to measure dose over a long period (months or even a year) with very little fading. 
    4. The light output is a linear function of the absorbed dose which makes calibration easy. 
    5. LiF is a reasonably good tissue equivalent for beta and gamma radiation. (This is not true for neutrons. LiF output is very dependent on neutron energy). 
    6. Can distinguish between types of radiation by using different lithium isotopes as will be discussed below. 
The disadvantages of TLDs are listed below. 
    1. The process of reading out eliminates the dose effect, so it can only be done once. 
    2. Since it is not possible to tell whether or not the TLD has absorbed a dose, all TLDs must be read and annealed each monitoring period. 
    3. Dust on the detector will glow when heated and will be recorded by the phototube as a false reading. 
    4. The TLDs are sensitive  to exposure by ultraviolet light and therefore must be sealed in a light-tight badge. 

Accuracy of Dose Assessment with TLDs 

The TLD gives a measurement of dose absorbed in the TLD (in mGy) with an accuracy of about tex2html_wrap_inline776 10%. However what we need to know is not the absorbed dose in the badge, but the deep equivalent dose tex2html_wrap_inline906 (in mSv) in tissue. Remember that tex2html_wrap_inline906 is the equivalent dose at a depth of tex2html_wrap_inline904 or more in tissue. 

There are many uncertainties involved in this assessment. Also, dosimeters worn on the surface of the body can at best be regarded as a statistical sampling device which provides a record of the dose received by one part of your body surface during your movements around the site. 

When these factors are allowed for, we can expect an accuracy of about tex2html_wrap_inline776 50% in assessing tex2html_wrap_inline906 from a measurement of absorbed dose in a TLD. 
 

Comparison of TLD and DRD Results 

The BRMD TLD badge is the only dosimeter recognized by the regulatory authorities and is the dosimeter of record at TRIUMF. DRDs are often used when immediate dose information is required for dose control purposes. DRD results, are not an official record unless used as an estimate when a TLD result is not available. 

Difference in doses measured by a TLD and a DRD are to be expected for the following reasons: 

    1. TLDs  and DRDs  have a somewhat different energy response. 
    2. A large number of dosimeters, of either type, placed in the same radiation field will show a variation of up to tex2html_wrap_inline776 20% in their readings. 
    3. DRDs may read high in fields where high energy beta particles exist. 
    4. DRDs suffer from leakage and mechanical shock which can alter readings. 
    5. Small rounding off errors made in each reading are cumulative and can distort the value of cumulative doses derived from DRD readings. 
If the two dosimeters are worn side by side all the time the DRD reading should be within tex2html_wrap_inline776 25% of the TLD result most of the time. 
 

Extremity TLDs 

Extremity TLDs are small discs of LiF which are sealed in plastic holders. These are worn on the fingers or taped to the ankles to measure the external equivalent dose to the extremities i.e. hands, forearms, feet and ankles. They are made of LiF powder spread very thinly into a plastic wafer so that they may be worn comfortably. 

Extremity TLDs are designed to measure beta radiation at a depth equivalent to that of the sensitive basal layer of the skin (about 0.3 mm). The holder, either a ring or a finger stall, shields the TLD from the lower energy betas (70 keV and less) which can not penetrate to the basal layer. Gamma radiation is also recorded by extremity TLDs, and delivers a dose to the bones of the extremities in addition to the skin dose. The danger from a high skin dose, tex2html_wrap_inline997 and above, is that it may lead to burns which are slow to heal. 
 

When to Wear Extremity TLDs 

    1. whenever you handle unshielded beta sources;  
    2. when working with the whole body shielded except for the hands; 
    3. when handling small non-shielded gamma sources; 
    4. for decontamination jobs when beta contamination is present; 
    5. for glove box operations; 
    6. for tex2html_wrap_inline1003 I source operations; 
    7. in any radiation field which would give the extremities tex2html_wrap_inline1005 or more; 
Great care should be taken when choosing where to wear the TLD especially when working with beta sources or close to point sources. Extremity TLD readings below tex2html_wrap_inline1007 are not recorded as they are not accurate below this level. 
 

Neutron Dosimeter 

tex2html_wrap_inline932 Li and tex2html_wrap_inline698 Li both respond to beta and gamma radiation. In addition tex2html_wrap_inline932 Li responds to slow neutrons (0.025 eV to 0.6 MeV) via the tex2html_wrap_inline932 Li(n, tex2html_wrap_inline704 ) tex2html_wrap_inline942 H reaction. The badge contains a chip each of tex2html_wrap_inline698 LiF and tex2html_wrap_inline932 LiF side by side. These are covered by a layer of cadmium which absorbs very slow moving neutrons incident on the badge when worn on the body. The tex2html_wrap_inline932 LiF TLD therefore measures the slow neutrons which are generated by higher energy neutrons incident on the body and which reflect back into the dosimeter. Such a dosimeter is referred to as an `albedo' dosimeter. The beta radiation is also screened out by the cadmium so that the tex2html_wrap_inline698 LiF gives the gamma dose only while the tex2html_wrap_inline932 Li gives the dose due to both gamma rays and neutrons. The difference in the readings is the neutron dose. 

In addition the badge contains a piece of polycarbonate (a plastic called CR-39 also used in the manufacture of eye glass lenses. Fast neutrons which interact with the plastic leave an invisible damage track in the plastic which can be made visible by enlarging the tracks using an etching technique. By counting the number of etch tracks per unit area in the plastic, a measure of the fast neutron dose can be obtained. 

These badges are only worn by people who are likely to be exposed to neutron fluxes above the very low levels normally found outside the accelerator shielding. 

Summary 

  • All radiation measuring instruments  have a detector in which the radiation deposits energy. The detector has to respond in a reproducible manner. 
  • Ionisation chambers measure the ionisation produced by the radiation in a gas filled volume. They can be used in dose rate instruments (gamma or beta survey), or as dosimeters which indicate total dose received (DRDs) 
  • Proportional counters can be designed to distinguish between radiation types. 
  • Geiger counters produce the same size output pulse regardless of how many ion pairs were created in the detector. They are well suited to the detection of contamination, but can be designed to measure gamma dose rate as well. 
  • An instrument must identify the type of radiation it detects. This can be achieved by detector design, electronic design or by using special operating techniques. 
  • Scintillation detectors are used with pulse height analyzers to identify gamma emitting radionuclides. They are also used in portal monitors, thyroid monitors, and tritium bioassay. 

    • TLDs measure absorbed dose. They are the official dosimeter for dose record purposes for external gamma and beta radiation.