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C.4.3 Gamma ray detectors

The following gamma detectors are described:

  • Electret ion chamber;
  • GM survey meter with gamma probe;
  • Hand-held ion chamber survey meter;
  • Hand-held pressurized ion chamber (PIC) survey meter;
  • Portable Germanium multichannel analyzer (MCA) system;
  • Pressurized ionization chamber (PIC);
  • Sodium Iodide survey meter;
  • Thermoluminescence dosimeter (TLD)

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System: ELECTRET ION CHAMBER
Lab/Field: Field
Radiation detected
Primary Low energy beta (e.g., tritium, 99Tc, 14C, 90Sr, 63Ni), alpha, gamma, or radon
Secondary None
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Applicability to site surveys: This system measures alpha- or beta-emitting contaminants on surfaces and in soils, gamma radiation dose, or radon air concentration, depending on how it is configured.
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Operation: The system consists of a charged teflon disk (electret), open-faced ionization chamber, and electret voltage reader/data logger. When the electret is screwed into the chamber, a static electric field is established and a passive ionization chamber is formed. For alpha or beta radiation, the chamber is opened and deployed directly on the surface or soil to be measured so the particles can enter the chamber. For gammas, however, the chamber is left closed and the gamma rays incidenting on the chamber penetrate the 2 mm-thick plastic detector wall. These particles or rays ionize the air molecules, the ions are attracted to the charged electret, and the electret’s charge is reduced. The electret charge is measured before and after deployment with the voltmeter, and the rate of change of the charge is proportional to the alpha or beta surface or soil activity, with appropriate compensation for background gamma levels. A thin mylar window may be used to protect the electret from dust. In low-level gamma measurements, the electret is sealed inside a mylar bag during deployment to minimize radon interference. For alpha and beta measurements, corrections must be made for background gamma radiation and radon response. This correction is accomplished by deploying additional gamma or radon-sensitive detectors in parallel with the alpha or beta detector. Electrets are simple and can usually be reused several times before recharging by a vendor. Due to their small size (3.8 cm tall by 7.6 cm diameter or l.5 in. tall by 3 in. diameter), they may be deployed in hard-to-access locations.
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Specificity/sensitivity: This method gives a gross alpha, gross beta, gross gamma, or gross radon measurement. The lower limit of detection depends on the exposure time and the volume of the chamber used. High surface alpha or beta contamination levels or high gamma radiation levels may be measured with deployment times of a few minutes. Much lower levels can be measured by extending the deployment time to 24 hours or longer. For gamma radiation, the response of the detector is nearly independent of energy from 15 to 1200 keV, and fading corrections are not required. To quantify ambient gamma radiation fields of 0.1 μSv/h (10 μR/hr), a 1000 ml chamber may be deployed for two days or a 50 ml chamber deployed for 30 days. The smallest chamber is particularly useful for long term monitoring and reporting of monthly or quarterly measurements. For alpha and beta particles, the measurement may be converted to isotopic concentration if the isotopes are known or measured separately. The lower limit of detection for alpha radiation is 83 Bq/m2 (50 dpm/100 cm2) @ 1 hour, 25 Bq/m2 (l5 dpm/100 cm2) @ 8 hours, and 13 Bq/m2 (8 dpm/100 cm2) @ 24 hours. For beta radiation from tritium it is 10,000 Bq/m2 (6,000 dpm/cm2) @ 1 hour and 500 Bq/m2 (300 dpm/cm2) @ 24 hours. For beta radiation from 99Tc it is 830 Bq/m2 (500 dpm/cm2) @ 1 hour and 33 Bq/m2 (20 dpm/cm2) @ 24 hours.
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Cost of equipment $4,000 to $25,000, for system if purchased (year 2002).
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Cost per measurement $8-$25, for use under service contract (year 2002).

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System: GM SURVEY METER WITH GAMMA PROBE
Lab/Field: Field
Radiation detected
Primary Gamma
Secondary Beta
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Applicability to site surveys: This instrument is used to give a quick indication of gamma-radiation levels present at a site. Due to its high detection limit, the GM survey meter may be useful during characterization surveys but may not meet the needs of final status surveys.
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Operation: This instrument consists of a cylindrical Geiger Mueller detector connected to a survey meter. It is calibrated to measure gamma exposure rate in mR/hr. The detector is surrounded on all sides by a protective rigid metal housing. Some units called end window or side window have a hinged door or rotating sleeve that opens to expose an entry window of mylar, mica, or a similar material, allowing beta radiation to enter the sensitive volume. The detector requires approximately 900 volts for operation. It is normally held at waist height, but is sometimes placed in contact with an item to be evaluated. It is moved slowly over the area to scan for elevated readings, observing the meter or, preferably, listening to the audible signal. Then it is held in place long enough to obtain a stable measurement. Radiation entering the detector ionizes the gas, causes a discharge throughout the entire tube, and results in a single count being sent to the meter. Conversion from count rate to exposure rate is accomplished at calibration by exposing the detector at discrete levels and adjusting the meter scale(s) to read accordingly. In the field, the exposure rate is read directly from the meter. If the detector housing has an entry window, an increase in ‘open-door’ over ‘closed-door’ reading indicates the presence of beta radiation in the radiation field, but the difference is not a direct measure of the beta radiation level.
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Specificity/sensitivity: GM meters measure gamma exposure rate, and those with an entry window can identify if the radiation field includes beta radiation. Since GM detectors are sensitive to any energy of alpha, beta, or gamma radiation that enters the detector, instruments that use these detectors cannot identify the type or energy of that radiation, or the specific radionuclide(s) present. The sensitivity can be improved by using headphones or the audible response during scans, or by integrating the exposure rate over time. The instrument has two primary limitations for environmental work. First, its minimum sensitivity is high, around 1 μSv/h (0.1 mR/hr) in rate meter mode or 0.1 μSv/h (0.01 mR/hr) in integrate mode. Some instruments use a large detector to improve low end sensitivity. However, in many instances the instrument is not sensitive enough for site survey work. Second, the detector energy response is non-linear. Energy compensated survey meters are commercially available, but the instrument sensitivity may be reduced.
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Cost of equipment $400 to $1,500 (year 2002).
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Cost per measurement $5 per measurement for survey and report (year 2002).

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System: HAND-HELD ION CHAMBER SURVEY METER
Lab/Field: Field
Radiation detected
Primary Gamma
Secondary None
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Applicability to site surveys: The hand-held ion chamber survey meter measures true gamma radiation exposure rate, in contrast to most other survey meter/probe combinations which are calibrated to measure exposure rate at one energy and approximate the exposure rate at all other energies. Due to their high detection limit, these instruments are not applicable for many final status surveys.
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Operation: This device uses an ion chamber operated at a bias voltage sufficient to collect all ion pairs created by the passage of ionizing radiation, but not sufficiently high to generate secondary ion pairs as a proportional counter does. The units of readout are mR/hr, or some multiple of mR/hr. If equipped with an integrating mode, the operator can measure the total exposure over a period of time. The instrument may operate on two ‘D’ cells or a 9 volt battery that will last for 100 to 200 hours of operation.
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Specificity/sensitivity: Ion chamber instruments respond only to gamma or X-radiation. They have no means to provide the identity of contaminants. Typical ion chamber instruments have a lower limit of detection of 5 μSv/h (0.5 mR/hr). These instruments can display readings below this, but the readings may be erratic and have large errors associated with them. In integrate mode, the instrument sensitivity can be as low as 0.5 μSv/h (0.05 mR/hr).
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Cost of equipment $800 to $1,200 (year 2002).
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Cost per measurement $5, or higher for making integrated exposure measurements (year 2002).

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System: HAND-HELD PRESSURIZED ION CHAMBER (PIC) SURVEY METER
Lab/Field: Field
Radiation detected
Primary Gamma
|>. Secondary
None
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Applicability to site surveys: The hand-held pressurized ion chamber survey meter measures true gamma radiation exposure rate, in contrast to most other survey meter/probe combinations which are calibrated to measure exposure rate at one energy and approximate the exposure rate at all other energies. Due to their high detection limit, these instruments are not applicable for many final status surveys.
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Operation: This device uses a pressurized air ion chamber operated at a bias voltage sufficient to collect all ion pairs created by the passage of ionizing radiation, but not sufficiently high to cause secondary ionization. The instrument is identical to the ion chamber meter on the previous page, except in this case the ion chamber is sealed and pressurized to 2 to 3 atmospheres to increase the sensitivity of the instrument by the same factors. The units of readout are μSv/h or mSv/h (μR/hr or mR/hr). A digital meter will allow an operator to integrate the total exposure over a period of time. The unit may use two ‘D’ cells or a 9-volt battery that will last for 100 to 200 hours of operation
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Specificity/sensitivity: Since the ion chamber is sealed, pressurized ion chamber instruments respond only to gamma or X-radiation. They have no means to provide the identity of contaminants. Typical instruments have a lower limit of detection of 1 μSv/h (0.1 mR/hr), or as low as 0.1 μSv (0.01 mR) in integrate mode. These instruments can display readings below this, but the readings may be erratic and have large errors associated with them.
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Cost of equipment $1,000 to $1,500 (year 2002).
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Cost per measurement $5, or higher for making integrated exposure measurements (year 2002).

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System: PORTABLE GERMANIUM MULTICHANNEL ANALYZER (MCA) SYSTEM
Lab/Field: Field
Radiation detected
Primary Gamma
Secondary None
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Applicability to site surveys: This system produces semi-quantitative estimates of concentration of uranium and plutonium in soil, water, air filters, and quantitative estimates of many other gamma-emitting isotopes. With an appropriate dewar, the detector may be used in a vertical orientation to determine, in-situ, gamma isotopes concentrations in soil.
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Operation: This system consists of a portable germanium detector connected to a dewar of liquid nitrogen, high voltage power supply, and multi-channel analyzer. It is used to identify and quantify gamma-emitting isotopes in soil or other surfaces.
Germanium is a semiconductor material. When a gamma ray interacts with a germanium crystal, it produces electron-hole pairs. An electric field is applied which causes the electrons to move in the conduction band and the holes to pass the charge from atom to neighbouring atoms. The charge is collected rapidly and is proportional to the deposited energy.
The typical system consists of a portable multi-channel analyzer (MCA) weighing about 7-10 lbs with batteries, a special portable low energy germanium detector with a built-in shield, and the acquisition control and spectrum analysis software. The detector is integrally mounted to a liquid nitrogen dewar. The liquid nitrogen is added 2-4 hours before use and replenished every 4-24 hours based on capacity.
The MCA includes all required front end electronics, such as a high voltage power supply, an amplifier, a digital stabilizer, and an analog-to-digital converter (ADC), which are fully controllable from a laptop computer and software.
One method uses the 94-104 keV peak region to analyze the plutonium isotopes from either ‘fresh’ or aged materials. It requires virtually no user input or calibration. The source-to-detector distance for this method does not need to be calibrated as long as there are enough counts in the spectrum to perform the analysis.
For in situ applications, a collimated detector is positioned at a fixed distance from a surface to provide multi-channel spectral data for a defined surface area. It is especially useful for qualitative and (based on careful field calibration or appropriate algorithms) quantitative analysis of freshly deposited contamination. Additionally, with prior knowledge of the depth distribution of the primary radio-nuclides of interest, which is usually not known, or using algorithms that match the site, the in-situ system can be used to estimate the content of radio-nuclides distributed below the surface (dependent, of course, on adequate detection capability.)
Calibration based on Monte Carlo modelling of the assumed source-to-detector geometry or computation of fluence rates with analytical expressions is an important component to the accurate use of field spectrometry, when it is not feasible or desirable to use real radioactive sources. Such modelling used in conjunction with field spectrometry is becoming much more common recently, especially using the MCNP Monte Carlo computer software system.
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Specificity/sensitivity: With proper calibration or algorithms, field spectrometers can identify and quantify concentrations of gamma emitting radio-nuclides in the middle to upper energy range (i.e., 50 keV with a P-type detector or 10 keV with an N-type detector).
For lower energy photons, as are important for plutonium and americium, an N-type detector or a planar crystal is preferred with a very thin beryllium (Be) window. This configuration allows measurement of photons in the energy range 5 to 80 keV. The Be window is quite fragile and a target of corrosion, and should be protected accordingly.
The detector high voltage should only be applied when the cryostat has contained sufficient liquid nitrogen for several hours. These systems can accurately identify plutonium, uranium, and many gamma-emitting isotopes in environmental media, even if a mixture of radio-nuclides is present. Germanium has an advantage over sodium iodide because it can produce a quantitative estimate of concentrations of multiple radio-nuclides in samples like soil, water, and air filters.
A specially designed low energy germanium detector that exhibits very little deterioration in the resolution as a function of count rate may be used to analyze uranium and plutonium, or other gamma-emitting radio-nuclides. When equipped with a built-in shield, it is unnecessary to build complicated shielding arrangements while making field measurements. Tin filters can be used to reduce the count rate from the 241Am 59 keV line which allows the electronics to process more of the signal coming from Pu or U.
A plutonium content of 10 mg can be detected in a 220 l waste drum in about 30 minutes, although with high uncertainty. A uranium analysis can be performed for an enrichment range from depleted to 93% enrichment. The measurement time can be in the order of minutes depending on the enrichment and the attenuating materials.
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Cost of equipment $40,000 (year 2002).
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Cost per measurement $100 to $200 (year 2002).

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System: PRESSURIZED IONIZATION CHAMBER (PIC)
Lab/Field: Field
Radiation detected
Primary Moderate (>80 keV) to high energy photons
Secondary None
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Applicability to site surveys: The pressurised ionization chamber is a highly accurate ionization chamber for measuring gamma exposure rate in air, and for correcting for the energy dependence of other instruments due to their energy sensitivities. It is excellent for characterizing and evaluating the effectiveness of remediation of contaminated sites based on exposure rate. However, most sites also require nuclide-specific identification of the contributing radio-nuclides. Under these circumstances, pressurised ionization chambers must be used in conjunction with other soil sampling or spectrometry techniques to evaluate the success of remediation efforts.
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Operation: The pressurised ionization chamber detector is a large sphere of compressed argon-nitrogen gas at 10 to 40 atmospheres pressure surrounded by a protective box. The detector is normally mounted on a tripod and positioned to sit about three feet off the ground. It is connected to an electronics box in which a strip chart recorder or digital integrator measures instantaneous and integrated exposure rate. It operates at a bias voltage sufficient to collect all ion pairs created by the passage of ionizing radiation, but not sufficiently high to amplify or increase the number of ion pairs. The high pressure inside the detector and the integrate feature make the pressurised ionization chamber much more sensitive and precise than other ion chambers for measuring low exposures. The average exposure rate is calculated from the integrated exposure and the operating time. Arrays of pressurised ionization chamber systems can be linked by telecommunications so their data can be observed from a central and remote location.
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Specificity/sensitivity: The pressurised ionization chamber measures gamma or X-radiation and cosmic radiation. It is highly stable, relatively energy independent, and serves as an excellent tool to calibrate (in the field) other survey equipment to measure exposure rate. Since the pressurised ionization chamber is normally un-collimated, it measures cosmic, terrestrial, and foreign source contributions without discrimination. Its rugged and stable behaviour makes it an excellent choice for an unattended sensor where area monitors for gamma emitters are needed. Pressurised ionization chambers are highly sensitive, precise, and accurate to vast changes in exposure rate 0.01 μSv/h up to 0.1 Sv/h (1 μR/ hr up to 10 R/hr). Pressurised ionization chambers lack any ability to distinguish either energy spectral characteristics or source type. If sufficient background information is obtained, the data can be processed using algorithms that employ time and frequency domain analysis of the recorded systems to effectively separate terrestrial, cosmic, and ‘foreign’ source contributions. One major advantage of pressurised ionization chamber systems is that they can record exposure rate over ranges of 0.01 μSv to 0.1 Sv (1 to 10,000,000 μR) per hour with good precision and accuracy.
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Cost of equipment $15,000 to $50,000 depending on the associated electronics, data processing, and telecommunications equipment (year 2002).
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Cost per measurement $50 to $500 based on the operating time at each site and the number of measurements performed (year 2002).

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System: SODIUM IODIDE SURVEY METER
Lab/Field: Field
Radiation detected
Primary Gamma
Secondary None
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Applicability to site surveys: : Sodium iodide survey meters can be response checked against a pressurized ionization chamber (PIC) and then used in its place so readings can be taken more quickly. This check should be performed often, possibly several times each day. They are useful for determining ambient radiation levels and for estimating the concentration of radioactive materials at a site.
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Operation: The sodium iodide survey meter measures gamma radiation levels in nSv/h (10-6R/hr) or counts per minute (cpm). Its response is energy and count rate dependent, so comparison with a pressurized ionization chamber necessitates a conversion factor for adjusting the meter readings to true μR/hr values. The conversion factor obtained from this comparison is valid only in locations where the radionuclide mix is identical to that where the comparison is performed, and over a moderate range of readings. The detector is held at waist level or suspended near the surface and walked through an area listening to the audio and watching the display for changes. It is held in place and the response allowed to stabilize before each measurement is taken, with longer times required for lower responses. Generally, the centre of the needle swing or the integrated reading is recorded. The detector is a sodium iodide crystal inside an aluminium container with an optical glass window that is connected to a photomultiplier tube. A gamma ray that interacts with the crystal produces light that travels out of the crystal and into the photomultiplier tube. There, electrons are produced and multiplied to produce a readily measurable pulse whose magnitude is proportional to the energy of the gamma ray incidenting on the crystal. Electronic filters accept the pulse as a count if certain discrimination height restrictions are met. This translates into a meter response. Instruments with pulse height discrimination circuitry can be calibrated to view the primary gamma decay energy of a particular isotope. If laboratory analysis has shown a particular isotope to be present, the discrimination circuitry can be adjusted to partially tune out other isotopes, but this also limits its ability to measure exposure rate.
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Specificity/sensitivity: Sodium iodide survey meters measure gamma radiation in nSv/h (μR/hr) or cpm with a minimum sensitivity of around 10-50 nSv/h (1-5 μR per hour), or 200-1,000 cpm, or lower in digital integrate mode. The reading error of 50% can occur at low count rates because of a large needle swing, but this decreases with increased count rate. The instrument is quite energy sensitive, with the greatest response around 100-120 keV and decreasing in either direction. Measuring the radiation level at a location with both a pressurized ionization chamber and the survey meter gives a factor for converting subsequent readings to actual exposure rates. This ratio can change with location. Some meters have circuitry that looks at a few selected ranges of gamma energies, or one at a time with the aid of a single channel analyzer. This feature is used to determine if a particular isotope is present. The detector should be protected against thermal or mechanical shock which can break the sodium iodide crystal or the photomultiplier tube. Covering at least the crystal end with padding is often sufficient. The detector is heavy, so adding a carrying strap to the meter and a means of easily attaching and detaching the detector from the meter case helps the user endure long surveys.
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Cost of equipment $2,000 (year 2002)
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Cost per measurement $5 (year 2002)

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System: THERMOLUMINESCENCE DOSIMETER (TLD)
Lab/Field: Field and lab
Radiation detected
Primary Gamma
Secondary Neutron, beta, x-ray
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Applicability to site surveys: TLDs can be used to measure such a low dose equivalent that they can identify gamma levels slightly above natural background. TLDs should be placed in areas outside the site but over similar media to determine the average natural background radiation level in the area. Other TLDs should be posted on site to determine the difference from background. Groups should be posted quarterly for days to quarters and compared to identify locations of increased onsite doses.
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Operation: A TLD is a crystal that measures radiation dose. TLDs are semiconductor crystals that contain small amounts of added impurities. When radiation interacts with the crystal, electrons in the valence band are excited into the conduction band. Many lose their energy and return directly to the valence band, but some are trapped at an elevated energy state by the impurity atoms. This trapped energy can be stored for long periods, but the signal can fade with age, temperature, and light. Heating the TLD in a TLD reader releases the excess energy in the form of heat and light. The quantity or intensity of the light given off gives a measure of the radiation dose the TLD received. If the TLDs are processed at an off-site location, the transit dose (from the location to the site and return) must be determined and subtracted from the net dose. The ability to determine this transit dose affects the net sensitivity of the measurements. The TLD is left in the field for a period of a day to a quarter and then removed from the field and read in the laboratory on a calibrated TLD reader. The reading is the total dose received by the TLD during the posting period. TLDs come in various shapes (thin-rectangles, rods, and powder), sizes (0.08 cm to 0.6 cm (1/32 in. to 1/4 in.) on a side), and materials (CaF2:Mn, CaSO4:Dy, 6LiF:Mn, 7LiF:Mn, LiBO4, LiF:Mg,Cu,P and Al2O3:C). The TLD crystals can be held loosely inside a holder, sandwiched between layers of teflon, affixed to a substrate, or attached to a heater strip and surrounded by a glass envelope. Most are surrounded by special thin shields to correct for an over response to low-energy radiation. Many have special radiation filters to allow the same type TLD to measure various types and energies of radiation.
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Specificity/sensitivity: TLDs are primarily sensitive to gamma radiation, but selected TLD/filter arrangements can be used to measure beta, X-ray, and neutron radiation. They are posted both on-site and off-site in comparable areas. These readings are compared to determine if the site can cause personnel to receive more radiation exposure than would be received from background radiation. The low-end sensitivity can be reduced by specially calibrating each TLD and selecting those with high accuracy and good precision. The new Al2O3 TLD may be capable of measuring doses as low as 0.1 μSv (0.01 mrem) while specially calibrated CaF2 TLDs posted quarterly can measure dose differences as low as 0.05 mSv/y (5 mrem/y). This is in contrast to standard TLDs that are posted monthly and may not measure doses below 1 mSv/y (100 mrem/y). TLDs should be protected from damage as the manufacturer recommends. Some are sensitive to visible light, direct sunlight, fluorescent light, excessive heat, or high humidity.
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Cost of equipment $5K-$ 100K (reader), $25-$40 (TLD). TLDs cost $5 to $40 per rental (year 2002).
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Cost per measurement $25 to $125 (year 2002).