As a result of irradiation, some solid substances undergo changes in some of their physical properties.  These changes amount to storage of the energy from the radiation.  Since the energy is stored, these materials can be used for dosimeters.

The property of thermoluminescence (thermo means heat and lumen means light) of some materials is the main method used for personnel dosimeters.

Thermoluminescence (TL) is the ability of some materials to convert the energy from radiation to a radiation of a different wavelength, normally in the visible light range. There are two categories of thermoluminescence.

Fluorescence is emission of light during or immediately after irradiation (within fractions of a second) of the phosphor. This is not a particularly useful reaction for TLD use.

TLDs use phosphorescence as their means of detection of radiation.  Phosphorescence is the emission of light after the irradiation period. The delay time can be from a few seconds to weeks or months. 

Electrons in some solids can exist in two energy states, a lower energy state called the valence band and a higher energy state called the conduction band. The difference (energy region) between the two bands is called the band gap. Electrons in the conduction band or in the band gap have more energy than the valence band electrons. Normally in a solid, no electrons exist in energy states contained in the band gap. This is a "forbidden region."

In some materials, defects in the material exist or impurities are added that can trap electrons in the band gap and hold them there. These trapped electrons represent stored energy for the time that the electrons are held. (See figure 1)This energy is given up (emitted as light photons when the material is heated up) as the electron returns to the valence band.

In most materials, this energy is given up as heat in the surrounding material, however, in some materials a portion of energy is emitted as light photons. This property is called luminescence. (See figure 2)

Heating of the TL material causes the trapped electrons to return to the valence band. When this happens, energy is emitted in the form of visible light. The light output is detected and measured by a photomultiplier tube and a (proportional) dose equivalent is then calculated .A typical basic TLD reader contains the following components: (See figure 3)

· Heater - raises the phosphor temperature
· Photomultiplier Tube - measures the light output
· Meter/Recorder - display and record data

A glow curve can be obtained from the heating process. The light output from TL material is not easily interpreted. Multiple peaks result as the material is heated and electrons trapped in "shallow" traps are released. This results in a peak as these traps are emptied. The light output drops off as these traps are depleted. As heating continues, the electrons in deeper traps are released. This results in additional peaks. Usually the highest peak is used to calculate the dose equivalent. The area under the curve represents the radiation energy deposited on the TLD. A simple glow curve is shown in figure 4.

After the readout is complete, the TLD is annealed at a high temperature. This process essentially zeroes the TL material by releasing all trapped electrons. The TLD is then ready for reuse.

Advantages (as compared to film dosimeter badges) includes:

· Able to measure a greater range of doses
· Doses may be easily obtained
· They can be read on site instead of being sent away for developing
· Quicker turnaround time for readout
· Reusable

Disadvantages

· Each dose cannot be read out more than once
· The readout process effectively "zeroes" the TLD

TLD manufacturing differs from company to company, so specific chip arrangement and composition may vary.  Most badges are lithium fluoride (Lif) and calcium fluoride (CaF).  Lithium has two stable isotopes, 6Li and 7Li. 6Li is sensitive to neutrons, but 7Li is not.  Neutrons interact in 6Li to give tritium (3H) and alphas via the reaction: 6Li(n,alpha)3H.  In fact the reason that 6Li is a special nuclear material (SNM) is that this same reaction is used for the production of tritium for nuclear weapons.

Badges that measure betas and gammas have at least one chip behind a mylar window, to allow some energy discrimination of betas and soft x-rays.  This chip would be used to assign the shallow dose.

Another chip would be behind a layer of plastic about 600 mg/cm2 thick.  This chip is designed to measure deep dose or whole body dose. One of these is usually 7Lif, the other is CaF.  Both of these measure gamma dose.  CaF is more sensitive to low energy gammas than 7Lif.

Neutron dosimetry is often made with the TLD-700 chip, which is made with 7Lif.   Which, as we know,  is sensitive to betas and gammas.  The TLD-600 chip is made with 6LiF, which is sensitive to betas, gamma, and Neutrons.  The neutron dose is calculated from the difference of a TLD-600 and TLD-700 pair.

Some cards use four TLD chips, arraigned as two pairs, to measure neutron dose.  One TLD-600 and one TLD-700 pair is shielded from the front with cadmium (Cd), which absorbs neutrons.  A second pair is shielded with Cd from the rear.  The readings of these four TLD chips is combined into an overall calculation of neutron dose. 

The word albedo means reflected or white (a color that reflects light). In the case of neutron dosimetry, it refers to the measurement of neutrons that are moderated and reflected from the body.  Most neutron dosimetry uses this principal of reflection.  The TLD-600 and TLD-700 pair that are shielded at the front with cadmium are designed to detect albedo neutrons reflected from the body.

The most common TLD badges in the commercial nuclear power industry is the  Panasonic UD-802 badge which is capable of estimating dose received at the three tissue depths (7 mg/cm2, 300 mg/cm 2, and 1000 mg/cm2) which are specified in regulations for reporting shallow, lens of eye, and deep dose equivalent. With four independent detection elements, the badge can measure dose from beta, gamma, x-ray, or neutron radiation over a wide range of energies. The badge may be used for monitoring personnel in medical, industrial, and other nuclear applications. 

Gamma/X-rays
The badge is calibrated using Cs-137 gamma rays and may be used for routine radiation monitoring of gamma or X-rays over the energy range from 30 keV to 1.25 MeV (Co-60). 

Beta
The badge is calibrated over the beta energy range between Tl-204 (0.267 MeV) and Sr-90/Y-90 (0.565 MeV). Beta calibration is geometry dependent. 

Neutron
The badge detects fast neutrons by the albedo effect. Accurate dose assessment requires the use of a source-specific calibration factor. The standard calibration factor is based on an Am-Be neutron source. Calibration factors for other neutron sources, e.g. Cf-252, can be used if requested by a customer. 

Minimum Reportable Dose
The minimum reportable dose is 10 millirem for gamma radiation and x-rays. This is the smallest dose that can be measured reliably and accurately. 
 

Composition of Panasonic UD-802 Dosimeter

The thin plastic shielding of element 1 allows beta radiation to penetrate to the Li2B4O7 phosphor to induce a response. (Li2B4O7 is known to be UV sensitive, and has no mylar)

The plastic shielding over elements 2 & 3 is at a depth that high energy beta radiations penetrate to the phosphor.

The CaSO4 phosphor over responses to low energy photon radiation. The plastic over element 3 allows low energy photons to penetrate to and cause response in that phosphor. The lead filter over element 4 attenuates the low energy photons, thus reducing the intensity of the photon radiation that reaches the phosphor and causes a response.

Non TLD Dosimeters:

LUXEL DOSIMETER is the proprietary name of a new type of dosimeter from Landauer, Inc., supplier of radiation dosimetry products and services. LUXEL uses "Optically Stimulated Luminescence" (OSL) technology which offers users increased sensitivity, long-term stability, a large energy response range, information on exposure conditions and reanalysis capability.  LUXEL represents Landauer's most refined and sensitive technology, combining the benefits of both film and TLD badges. It meets all federally mandated National Verification Laboratory Accreditation Program testing requirements for radiation dosimeters.  Badge wearers will notice new, "user-friendly" features incorporated into the badge design. Clearer identifications and wear location icons help users to distinguish dosimeters and ensure proper use.  The minimum detectable dose that can be measured by a single dosimeter during a wear period has been reduced from 10 mRem to 1 mRem for gamma and x-ray radiation, and from 40 mRem to 10 mRem for beta radiation.

The Luxel's optically stimulated luminescence (OSL) dosimeter from Landauer measures radiation exposure due to x-ray, beta, and gamma radiation through a thin layer of aluminum oxide doped with carbon.  Al₂O₃:C has a TL sensitivity some 50 times greater than that of the industrial standard TLD material (namely, TLD-100 (LiF:Mg,Ti)) and is almost tissue equivalent. However, it possesses the undesirable properties of a strong sensitivity to light and thermal quenching. This aluminum oxide is stimulated with the use of a laser light.  This causes the aluminum oxide to become luminescent in proportion to the amount of radiation.  The badge is designed to measure radiation exposure in the range of 1 mRem to 1,000 Rem for x and gamma radiation; and 10 mRem to 1,000 Rem for beta radiation. The dose is recorded as whole body dose.  more durable, water resistant, more sensitive, doses down to 1 mRem, more accurate, ± 1.0 mRem, The badge can be reread to confirm the accuracy of a radiation dose. (Ref: Optically and Thermally Stimulated Phenomena Laboratory, and Landauer)

Update: On 8/28/03, Ed Pheil, from KAPL sent me an email stating:

I was reading your article on TLD's and you mentioned the Landauer OSL Dosimeter.  I might note that they have an Extended Range capability that they can add using a track detector for proton recoil which responds to > 0.062 MeV neutrons and a boron impregnated poly emitter which responds, via leaving larger alpha tracks on the track detector to thermal and near thermal neutrons.  This greatly expands the dose range that can be read up to very high levels with energy information.  The disadvantage is that they have to be etched before reading which can be done locally or as a service by Landauer.  They can be reread as needed.  Over-etching would obviously cause data loss, if it happened.  Although they are one time use, they are fairly inexpensive.

Film Badge:

That's about all I know about TLD's.  If you have anything to add, please email me.