Answer to Question #9700 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

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

Q

I called lots of hospitals and I spoke with their medical physics staff about how and who takes the readings of thermoluminescent dosimeters (TLDs), film badges, and electronic dosimeters after they are collected it from staff. They said they send them to a company to do the reading.
    
I am looking for your help in answering a few questions:

  1. Who is the person in charge of taking the reading of TLDs, film badges, and electronic dosimeters? Is he/she a special engineer?
  2. What is his/her job description/qualifications? 
  3. They are different types of personal dosimeters (TLD, film badges, and electronic dosimeters). Does each one of them have its own reader system?

In another words, can we have one reading system that can read all the different types of personal dosimeters?

A

I shall attempt to answer each of the questions you have posed, but first I should point out a few pertinent facts that might be helpful. My comments are based on dosimetry conditions that apply in the United States.

Most medical facilities use external vendors to supply and process their routine personnel dosimetry on which occupational doses of record are based. These vendors must meet certain criteria that have been established to demonstrate that the vendor is able to read and interpret the dosimeter responses with sufficient accuracy and precision to allow an acceptable level of confidence in the results. In order to demonstrate that the vendor meets the performance criteria, said vendor must participate in a nationally accredited program that has been designed by professionals who have specific expertise in the field of radiation dosimetry.

In the United States, the vendors participate in the National Voluntary Laboratory Accreditation Program (NVLAP). In brief, the participation involves the vendor sending a number of dosimeters to a properly accredited laboratory that exposes the dosimeters to the types of radiation(s) specified for particular categories requested by the vendor (e.g., gamma rays, beta radiation, mixed radiations, etc.). The laboratory delivers doses to the dosimetry devices (specific dose magnitudes known to the laboratory but unknown to the vendor) and returns them to the vendor, who is not informed of the magnitudes of the doses. The vendor reads the dosimeters and uses its own methods and algorithms to interpret the doses. If the interpreted doses meet the required specifications of accuracy and precision set by NVLAP, the laboratory will receive "passing grades" for the categories of radiation for which it has been tested.

The U.S. Department of Energy (DOE) operates a similar program of personnel dosimetry accreditation that is referred to as DOELAP (Department of Energy Laboratory Accreditation Program) and that provides the performance criteria and dosimetry testing for DOE facilities.

Most facilities that supply dosimetry to employees use thermoluminescent dosimeters (TLD), optically stimulated luminescence (OSL), and/or film dosimeters as the devices on which to base the permanent doses of record. In the medical community, for example, it has been common to use a single film badge with several readout areas for evaluating doses to the whole body (deep dose), shallow dose (skin), and dose to the lens of the eye. OSL has grown appreciably in popularity, and a number of medical (and other) facilities that had used film have switched to this OSL dosimetry system supplied through a vendor. Ring dosimeters are often worn, especially in nuclear medicine areas, and it is common for these to include a TLD as the active element to evaluate extremity (finger) dose.

Electronic dosimetry devices have the advantage of offering real-time indications of accrued dose and dose rate and have applications in many arenas where the potential for relatively high doses exists and/or where tracking of individual real-time doses is considered important. The nuclear power industry has been one of the largest users of these devices where they provide valuable dose-tracking ability for numerous workers who might be involved in performing various maintenance tasks during reactor shutdowns. The electronic devices are also being used by some in the medical community and have had applications, for example, as available auxiliary dosimetry devices for personnel during emergency situations involving potential radiation exposure. Electronic dosimeters are generally processed on the site where they are being used, since their major utility is in providing real-time dose assessments. They are generally not used to measure the doses of record, these usually being done with the more conventional TLDs, OSL, or film. Electronic dosimeters are generally convenient to read out, normally offering a visual display of the dose, and usually having the capability of transferring the dose information to a computer.

Answer to Question 1: Film badges and TLDs that are supplied by external vendors are collected regularly, often monthly, by the facility requiring the dosimetry and sent back to the respective vendors for processing and dose interpretation. The vendor's staff includes professional personnel who are experts in the techniques of personnel dosimetry being used, and the personnel who process the devices are trained in the techniques and methods being applied.

There are numerous facilities, especially in the nuclear power industry and some government facilities, that perform their own dosimetry for doses of record using TLDs or similar devices such as OSL dosimeters. Such facilities must demonstrate their competence by participating in an accreditation program, essentially the same NVLAP program as noted for vendors. As noted above, electronic dosimeters are generally read out on the site where they are used. Individuals who are in charge of the distribution and use of the devices for exposure control and real-time assessment must be properly trained in their application and limitations. If the intent is to use the electronic dosimeter readouts as doses of record, the using facility must go through a significant program of testing and comparison (with conventional dosimetry techniques) to show that the devices and staff are able to meet appropriate performance criteria. As noted above, such use of electronic devices to generate doses of record is not typical.

Answer to Question 2: As noted, the individuals in charge of developing and implementing dosimetry readout and dose-interpretation programs have expertise in the area of personnel radiation dosimetry. They are typically professionally trained as scientists or engineers with specific background in radiation science and radiation protection. The programs must pass an accreditation process that provides reasonable assurance that the vendor/facility is capable of providing a reliable dosimetry service for the radiations of concern.

Answer to Question 3: Regarding the three dosimetry methods that you citefilm, TLDs, and electronic dosimetry, there is no single reader/system that can read and interpret doses from these three types of devices.

Film must be developed in a process very similar to that of photographic development. The images (darkening of the film emulsion from radiation exposure) must then be interpreted to estimate delivered doses. This is done through a rigorous calibration process in which the vendor has determined the degree of darkening as a function of dose, taking account of the type and energies of the radiations.

The thermoluminescent dosimeter reader uses a specially designed reader that heats the dosimeter at a controlled rate. The light emitted by the dosimeter during the heating cycle is measured, usually through a photomultiplier tube detector, and related through  appropriate calibration to a particular dose.

The electronic dosimeters typically have a self readout with a liquid crystal display. These dosimeters use microprocessor based technology, which allows the collection and display of various information such as integral dose, dose rate, and the ability to preset integral and/or dose rate alarm set points. They often have the ability to collect time-dose histories so that a detailed histogram of how the dose accrued over a given exposure interval can be accessed. They usually also have the capability of downloading dose information from the dosimeter to a computer for tracking, display and data control purposes.

I hope this is helpful.

George Chabot, PhD, CHP