Answer to Question #11219 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

I am from the Middle East and I would like to know how to establish a TLD (thermoluminescent dosimetry) reading company?

A

I assume you are talking about establishing a company that provides thermoluminescent dosimetry services to radiation users who are required to badge their personnel in order to monitor accumulated doses and possibly to provide dosimetry services as well for environmental monitoring. In such cases you will require various equipment and supplies, the extent of which will be partially dictated by how many customers will be using your services and the particular services that you intend to provide.

If you are planning on using thermoluminescent dosimeters (TLDs), you will have to decide upon the types that you will use. For example, for routine personal dosimetry whole body dosimeters, will you use phosphors such as LiF (Mg,Ti), Li2B4O7, or other more sensitive phosphors, such as LiF (Mg, Cu, P) or CaSO4 (Dy) or others? If you are planning to offer dosimeters for environmental surveillance you may want to include higher sensitivity phosphors such as CaSO4 (Dy). A technique that has grown in popularity is optically stimulated luminescence (OSL) in which luminescence is read out via stimulation of the exposed dosimeters with short bursts of light rather than with heat. The most popular phosphor for OSL is Al2O3 (C), although the technique can be applied to other phosphors. You will also have to decide whether you are going to purchase the phosphors from appropriate vendors or whether you are planning to make the phosphors yourself. The latter is a relatively large endeavor and requires some specific chemical expertise to prepare the phosphors under the proper quality control requirements as well as the chemical supplies and equipment necessary for the work—e.g., benches, hoods, ovens (e.g., vacuum evaporation ovens), etc.

The phosphors then have to be incorporated into appropriate badge configurations for their intended purposes. Typical personal body badges may have at least three readout regions with dosimeter elements located beneath appropriate tissue-equivalent thicknesses to establish doses to the skin (shallow dose typically at 7 mg cm-2), lens of eye (depth of 300 mg cm-2), and deep dose to the whole body (depth of 1,000 mg cm-2). Environmental badges may have different requirements. Dosimeters for assessing extremity doses (e.g., fingers) are configured for the body part of interest. For the fingers, the phosphor is typically covered with 7 mg cm-2 tissue-equivalent material and placed in a ring-type holder that is worn on the finger. If you are intending to provide personal neutron dosimetry, you will need different phophors if you plan to do TL albedo dosimeters—e.g., paired 6LiF and 7LiF elements that allow respective gamma dose assessment and gamma plus neutron dose assessment. Some providers have opted for different neutron sensors, especially the popular polycarbonate-based CR-39 plastic film dosimeter that relies on track etch processing to establish fast neutron doses; such devices may require specialized equipment to read the tracks and interpret doses.

Whatever badge configurations you elect to use, you may decide to have the badges provided/fabricated by an external vendor. Different users will often have different dosimetry needs, which require different badge configurations; you will have to be prepared to provide such choices if you intend to meet these needs.

You will also require the instrumentation to read out the dosimeters after they have been exposed and returned to you. Since I assume you will be processing large numbers of dosimeters, you will be best served by readers that handle large numbers of dosimeter badges automatically. They would be placed in appropriate feed bins in the reader and automatically read out on a pre-programmed thermal readout cycle. It is critical that all badges be equipped with acceptable digital readout patterns on the exterior of the badges so that individual users can be properly identified and results recorded automatically. Different types of dosimeters and/or phosphors may require different readout cycles and/or different readers.

Depending on what dosimeters/phosphors you use and what the specific readout requirements are, you may also require annealing ovens to treat irradiated dosimeters to return them to the desired sensitivity. You will also require other common laboratory supplies and chemicals and materials that may be necessary for handling and cleaning dosimeters. It is highly likely that you will also require well-characterized radiation sources (gamma, beta, and possibly neutron) that will allow you to deliver known doses to dosimetry devices under specified conditions in order to perform reasonable quality assurance and control checks on your operations. Possession and uses of such sources will require appropriate licensing by the regulating agency that handles such activities in your country. You will have to demonstrate appropriate levels of expertise and training for the safe handling and use of such sources. You will also require appropriate shielded facilities to store and use such sources.

Beyond the equipment and supply needs, but equally important, you must also be prepared to establish and implement proper and acceptable calibration procedures to demonstrate that the dosimeters you are providing are capable of measuring the delivered doses with an acceptable degree of accuracy and precision. It is common practice for a dosimetry facility to participate in an accredited and accepted calibration program in which an externally approved laboratory may carry out inspections of the proposed dosimetry facility and may perform irradiations of provided dosimeters to known doses; the facility then reads out the dosimeters to demonstrate that it is proficient in interpreting the doses within acceptable limits. In the United States the National Voluntary Laboratory Accreditation program fulfills the functions in which an external facility provides the oversight, inspection, and testing of the proposed dosimetry facility.

As routine operation of the dosimetry facility proceeds you must also implement an acceptable program to ensure that proper dosimetry quality is being maintained in the facility. This requires a planned and regular testing program in which preirradiated dosimeters are put through the typical readout process and doses interpreted. Deviations from acceptable performance must be recognized and corrected as necessary.

There are numerous vendors available who sell the kinds of equipment and materials you will require. I am not making any specific recommendations here, and will mention just a few involved with providing TLD systems. TLD readers are available through numerous sources; these include Harshaw/Bicron (available through Thermo Scientific and other suppliers), Panasonic (available through Radiation Detection Co. and others), the RADOS systems available through RadPro International GmbH and others, and systems (RE-2000) available through Mirion Technologies. Many of these suppliers also offer dosimeters for sale. I would suggest that when you have identified a reliable supplier for the kind(s) of system you are considering you contact one of their technical representatives and discuss other specific considerations related to requirements for your TLD dosimetry service facility. They would likely be quite helpful to you.

Setting up a TLD provision/service facility encompasses a good deal of work, immediate attention, continuing attention to ensure that the services you are providing are meeting the needs of your customers, and a rather large disbursement of funds. I wish you well in your endeavors.

George Chabot, PhD

Answer posted on 20 April 2015. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.