TOPIC 7: Monitoring External Dose

Let’s explore Monitoring External Dose.

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As soon as it was determined that radiation can also produce harm, methods for measuring the dose to individuals were researched.  As we learned earlier, a person’s dose from gamma and x-rays can be estimated with survey meters. But it soon became clear that it would be more advantageous to measure the dose directly on a person’s body. The dose rate is useful, but it can change during a working day, so “integrating” devices were developed to accumulate the dose to a worker until the device is read at the end of the shift, or the end of the task, or day, or month, or quarter, or some other meaningful length of time.

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One common early device was the “pocket ion chamber.” This device is about the size of a fat writing pen … and has a clip to mount on the pocket of a shirt. There are two main types of pocket ion chambers: the condenser type and the direct-reading type. Both types measure exposures up to 200 mR and 5 R with +/- 15% accuracy.

The condenser type comes with a charger-reader. This small ion chamber has a positively charged central anode. As ionizations occur in the chamber, the capacitor discharges … decreasing the anode voltage. This decrease in voltage is directly proportional to the ionization produced in the chamber, which is also directly proportional to the radiation exposure. This dosimeter will discharge slowly even when not in a radiation field due to charge leaks and cosmic radiation. Since condenser dosimeters discharge by themselves and are subject to errors after impact (like dropping it), it is better to wear 2 or 3 of them at a time and record the lowest reading because discharge indicates a higher dose that may not be true. These dosimeters should be read and recharged daily.

The direct-reading pocket ion chamber works by charging it so an internal quartz fiber is displaced. The fiber can be directly viewed through a lens on one end of the dosimeter. The fiber sits behind an internal scale… like a ruler. As the fiber is discharged, its position changes in proportion to the exposure. The benefit of this dosimeter is that it can be read at any time without discharging it. Eventually, it will require recharging, or “zero-ing.”

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The active materials in Thermo Luminescence Dosimeters (TLDs) are selected for different applications.  Some are used directly on a worker, some are used as area monitors, and some are used as environmental monitors. Most workers don’t routinely wear any type of radiation dosimeter, even when they work near nuclear density gauges. Area badges can be used instead of personal dosimeter, but these are also not common after a company has demonstrated that doses are well below regulatory limits (typically less than 10% of the dose limit).

Area badges:

• Act as a surrogate for personnel monitoring;
• They are placed in an unshielded area of the working zone;
• They measure doses that should be representative of working areas;
• They are the same material as personnel dismeters in most cases; and
• They are integrating – meaning they store accumulated doses.

Most TLD badges contain lithium fluoride (LiF) and calcium fluoride (CaF).  Both of these are a very good surrogate for human tissue.

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Some badges use four TLD chips, arraigned as two pairs, to measure neutron dose.  One pair of TLD-600 and TLD-700 crystals is shielded from the front with a neutron absorber, such as cadmium (Cd).  A second pair is shielded with Cd from the rear.  The readings of these four TLD chips are combined into an overall calculation of neutron dose.

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Badges that measure betas and gammas have at least one crystal shielded by a Mylar window, to allow some energy discrimination of betas and soft X-rays.  This crystal is used to assign the shallow dose.

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Thermoluminescence (thermo = heat and lumen = light) is the quality of many different crystals to emit light if they are heated after exposure to radiation. Energy from the radiation excites atoms in the crystals, which results in free electrons and holes in the crystals. These are locked in place by imperfections and activators intentionally placed in the crystal to trap electrons. Since the potential energy of the trapped electrons is stored and accumulates, the TLD can be used to measure the dose of the wearer during the period the badge was worn. When the crystal is heated, the locking mechanism is disrupted and reset, and the excitation energy is released as light. The light emitted over heating time is detected and measured by a photomultiplier tube.  A “glow curve” – with peaks and valleys – is produced by the TLD reading machine. This is compared to a calibration curve to determine the dose accumulated during the time the badge was worn. After the readout is complete, the TLD is annealed (heated) at a high temperature.  This process essentially resets the crystalline material by releasing all trapped electrons. The TLD is then ready for reuse.

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When placed in an area, TLDs can function as area monitors.  TLDs became necessary for measuring dose to an individual when the USNRC required the measurement of Skin Dose, Eye Dose and Deep Dose as well as doses due to other nuclear particles.  Film badges became less used.  Film badges were bits of photographic film that became exposed in the presence of radiation.  The TLD consists of at least four separate material that serve as the detectors.  Some of the detectors were shielded by materials of various compositions and thicknesses that allowed for discrimination of photon energies.  This provided the values for skin, eye and deep doses.  For more on this topic, you can review the Code of Federal Regulations, 10 CFR Part 20 Guidelines.

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These dosimeters use “Optically Stimulated Luminescence” (OSL) technology.  This offers users increased sensitivity, long-term stability, a large energy response range, information on exposure conditions, and reanalysis capability.  This technology combines the benefits of both film and TLD badges.

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The minimum detectable dose that can be measured by a single dosimeter has been reduced from 10 mrem – – typical of a TLD – to 1  mrem for gamma and X-ray radiation, and from 40 mrem to 10 mrem for beta radiation. However, vendors commonly report results in a censored format. For example, results less than 3 mrem may be reported as “M” meaning that the dose was very low and may not be different from background.

The optically stimulated luminescence (OSL) dosimeter measures radiation exposure from X-ray, beta, and gamma-ray through a thin layer of aluminum oxide doped with carbon.  Al2O3C is much more sensitive than the industrial standard TLD material (namely, TLD-100 consisting of (LiF:Mg,Ti)). The Al2O3C is almost tissue equivalent.  However, it possesses the undesirable properties of a strong sensitivity to light.

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 energy that was deposited.  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.  Unlike its TLD predecessor, the OSLD can be read repeatedly to confirm the accuracy of a radiation dose.

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In review – OSLDs offer users increased sensitivity, long-term stability, a large energy response range, information on exposure conditions, and reanalysis capability.  This technology combines the benefits of both film and TLD badges.

The optically stimulated luminescence (OSL) dosimeter measures radiation exposure from X-ray, beta, and gamma radiation but is somewhat sensitive to bright lights.

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When the principles of radiation were first discovered over 125 years ago, their effects on photographic films immediately lead to the use of film as a measure of radiation exposure.  Film badges can be constructed with multiple film packets and absorbers to be sensitive to all types of ionizing radiation over a wide range of energies and doses. The film is placed in a light-tight container and worn by the worker. The radiation darkens the film, which is analyzed using an instrument capable of measuring optical density. Each batch of film has been calibrated to a known source and dose. The greatest advantage of the film badge is the permanent record provided.  The film can be reviewed repeatedly and the dose verified.

Film badges are not as widely used in the U.S. as they once were.  The main place where film badges are used are medical facilities that use X-rays with varying x-ray energies.

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Electronic dosimeters use solid-state semiconductors to measure beta, gamma and X-radiation. They are more accurate than pocket ion chambers at +/- 10% for Cs-137. They can measure dose rates from 0.1 – 1,000 mrem/hr and total doses from 0.1 mrem to 1,000 rem over a wide energy range. Electronic dosimeters indicate instantaneous dose rate, or integrate dose, and even “chirp” audibly at pre-set levels to warn workers.  They can store and display long-term cumulative (“integrated”) dose. They can be set to alarm at a particular dose rate or cumulative dose. All stored data can be transferred to a separate computer. These devices are common in nuclear power plants.

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Problem: We need to work with a source at a position where the exposure rate is very consistent. A pocket ion chamber was left there for 4 hours and the reading was 20 mrem. The company has set a dose limit for you of 50 mrem for the task. How long can you work in the radiation field?

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Problem: We need to work with a source at a position where the exposure rate is very consistent. A pocket ion chamber was left there for 4 hours and the reading was 20 mrem. The company has set a dose limit for you of 50 mrem for the task. How long can you work in the radiation field?

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You are required to do some simple calculations. Here are some solved problems for you to try. If you can do these, you can do any similar problem we give you on a test. If you have problems with the math, refer to the Math Primer for a refresher.  After working through this problem, you can continue.

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See problems on screen to work through on your own.

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The shallow dose equivalent is estimated at a tissue depth of 0.007 cm. The definition of shallow dose equivalent  is the external dose to the skin or an extremity. The skin has an outer protective layer of dead cells underlain by functioning skin and then a basement membrane  of dividing cells. Although highly variable, the depth of the basement membrane is taken to be 0.007 cm below the surface.  This is where we want to estimate the shallow dose equivalent.

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The Lens Dose Equivalent applies to the external exposure of the lens of the eye and is taken as the dose equivalent at a tissue depth of 0.3 centimeter.

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The deep dose equivalent is defined at a tissue depth of 1 cm. The deep dose equivalent  applies to external whole-body exposure where the radiation can penetrate deep into or through the body.

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The committed dose equivalent applies to whole-body exposure where the radiation remains in the body continually providing dose.  We estimate the radioactive material will remain for 50 years.

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The Total Effective Dose Equivalent is the sum of the deep-dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures).  Discussion of internally-deposited radionuclides and associated dose terms like ”committed effective dose equivalent” is mentioned but takes higher training to make these calculations.  Therefore it is not a part of this course. In nearly every practical situation involving industrial sealed sources or X-ray generators, there are no internal exposures, so the Total Effective Dose Equivalent or TEDE is the same dose as the Deep Dose Equivalent.

A type of EXTERNAL exposure is:

1.Inhalation of radioactive materials in air

2.Entry of radioactivity through wounds or punctures

3.Irradiation from radiation sources that externally expose the body

4.Absorption of radioactivity through intact skin

Answer: 3

The Total Effective Dose Equivalent is the sum of the deep-dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures).  Discussion of internally-deposited radionuclides and associated dose terms like ”committed effective dose equivalent” is mentioned but takes higher training to make these calculations.  Therefore it is not a part of this course. In nearly every practical situation involving industrial sealed sources or X-ray generators, there are no internal exposures, so the Total Effective Dose Equivalent or TEDE is the same dose as the Deep Dose Equivalent.