Answer to Question #12240 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 in the process of advising quite a few people on proper instrumentation and need assistance narrowing down the proper equipment. Would you be able to explain what the most appropriate and accurate type of instrumentation is for measuring radiation dose, as well as dose rate, for the nuclear hobbyist?

There seems to be a lot of different types of detectors. Currently, I have a Ludlum Model 3 with a 44-9 pancake detector for contamination detection. However, choosing the proper instrument for reading dose seems a bit more complicated.

The competing technologies appear to be: energy-compensated Geiger-Mueller (GM) tubes, ionization chambers, and semiconductors. There are those who suggest ion chambers over GMs, since ion chambers are designed to measure energy whereas GM tubes simply measure counts. However, it seems there are plenty of energy-compensated GM instruments with ±20% gamma-energy response across the most usable energy spectrum, including the Ludlum Model 25, which claims ±15% from 60 kiloelectronvolts (keV) to 3 megaelectronvolts (MeV). The Canberra RGU-100 Military Pocket Radiac also uses an energy-compensated GM to measure residual gamma radiation.

The problem I see with ion chamber units (such as the Ludlum 9-3, Ludlum 9-4, Fluke 450B, Eberline RO-2A, etc.) is that they have relatively narrow measurement ranges. They either cover lower, occupational-level dose rates or they go to much higher, contamination-level dose rates, but they leave out anything under about 0.01 millisievert per hour (mSv h-1). There seem to be plenty of both semiconductor and energy-compensated GM instruments that cover much wider rate ranges.

What do you suggest for the at-home nuclear hobbyist and for those who are civil-defense minded—folks who do not want to invest in more than two separate modern instruments for ascertaining an accurate (at least ±20%) dose rate and for tracking accumulated dose in the case of nuclear emergencies or disasters? I have taken notice of the following instruments that are rugged and have a wide range, but you may be aware of something superior: Ludlum Model 25, GammaRAE II R, Mirion DMC 3000, Canberra Ultra Radiac-series and RGU-100 (which state an energy accuracy of only within 30% despite being the military unit of choice), the Polimaster PM1703GN/GNA/GNB, and older Cold War units like the PDRM 82. Currently, the only instrument I have that is capable of measuring exposure rates is a CDV-700 with the EON extended-range kit installed, but it's only suitable for low-range gamma surveys.

And one more question: Do any instruments have the ability to include beta radiation into an accurate dose or dose rate measurement? I am not even sure if it's possible or necessary to do this.

A

Based on the detail provided in your question, I would judge that you are quite familiar with many of the instrument/detector types that are available. It is not appropriate for me to make specific product recommendations, but I shall attempt to summarize a few considerations that influence instrument selection and some of my experience; I shall not go into great detail about operating principles. I am sure some of what I say is already well known to you and some may not be of concern to you. I apologize for that, but I include it for the benefit of others who may read this response to your question.

I should point out that, regarding accuracies cited by manufacturers, these are generated under relatively idealized situations in which the specific characteristics of the radiation field are well known, as is appropriate. The detector, especially in the case of gamma calibrations, is uniformly irradiated in a fixed geometry and fixed field direction with radiations of known energies. Given that when an instrument is used in an actual field situation, the directional characteristics of the field, the orientation of the exposed individual and instrument in the field, and the energies and sometimes even the types of radiation are not known or not well characterized, and given that effects of other influencing factors may occur, considerably greater errors are possible. The end result is that the uncertainty in a given measured dose/dose rate in the field may be greater than desirable and made worse if there is a large uncertainty in the calibration of the instrument.

Thus, it is important, especially when dealing with radiation fields that have the potential for producing doses close to or above acceptable regulatory limits, to attempt to minimize uncertainties associated with instrument calibration. In general, the lower the dose rates that one is dealing with in the field, the greater is the allowed uncertainty that is deemed acceptable. In 1978, the National Council on Radiation Protection and Measurements (NCRP) in Report No. 57 Instrumentation and Monitoring Methods for Radiation Protection recommended a field measurement accuracy of ±30% when projected doses were near the permissible maximum, ±20% for projected doses significantly above the permissible limits, and permitted inaccuracies as high as 100% when projected doses were less than 25% of the permitted maximum dose (NCRP 1978). In order to reasonably achieve these goals it is necessary that the instrument calibrations achieve accuracies considerably better than the field accuracies. Also keep in mind that gamma dose/dose-rate calibrations are typically carried out at a single energy, often 662 keV associated with cesium-137 (137Cs) decay; a given calibrated instrument is then subjected to known gamma-radiation fields of other specified energies to establish the relative responses at the selected energies compared to the reference radiation. This is generally the justification by the manufacturer for the quoted inaccuracies over a fixed energy range; e.g., ±20% from 60 keV to 1.3 MeV.

For most routine calibrations with gamma-radiation dose-measuring instruments, the NCRP (in Report No. 112, Calibration of Survey Instruments Used in Radiation Protection for the Assessment of Ionizing Radiation Fields and Radioactive Surface Contamination) recommended an accuracy in calibration of ±10% at the 95% confidence level (NCRP 1991). It then becomes an important aspect of reliable use of any instrument that it be calibrated properly and sufficiently frequently to assure its continued valid performance. I should also point out that the comments that I make in this response are primarily related to the use of portable, health physics survey instruments. There are other instruments that are used in some environments that are subject to substantially more restrictive calibration and response demands; a notable example is ionization chambers used to assess photon-radiation fields in medical-radiation therapy.

The answers to your questions depend, at least in part, on a number of factors, including the types and energies of the radiations of concern, the intensities of the radiation fields, the required accuracy of the measurements, the requirements for electronic and mechanical characteristics (e.g., ability to store data and/or transfer data to other devices, visibility of readout, the presence of audible/visible alarm indicators, ruggedness, etc.), and the available budget for purchasing instruments. Based on your questions, I gather that you are primarily interested in dose/dose rate associated with gamma radiation and possibly charged-particle radiation associated with contamination.

If you require an instrument that will measure (1) gamma dose/dose rate and (2) charged particles, beta radiation, and/or alpha radiation, the selection is rather narrow. One of the specific instruments you mentioned, the Ludlum Model 3 with the 44-9 probe, is characteristic of the most popular type of instrument among occupational users over a wide spectrum of nuclear specialties. The reasons for such popularity are probably fairly obvious: it is equipped with a thin window to measure beta and alpha radiation, and it can be equipped with a thicker cap over the window to allow measurements of dose/exposure rate from gamma radiation (e.g., the Ludlum Model 26 pancake probe can be equipped with a compensating filter that the manufacturer shows provides a dose response within ±20% from about 20 keV to more than 1 MeV). While such instruments are often more expensive than some of the available GM pancake-type detectors that have become popular among lay people, they are still generally a good buy among professionally used instruments, being reasonable in cost and usually very reliable.

GM instruments have at times been a bit maligned, because claims have been made that they exhibit weaknesses such as limited range, paralysis in high-radiation fields, and lack of sufficient accuracy, especially at lower energies where they overrespond. While there are some GM instruments in use that may have one or more of these faults, current technology has allowed all of these deficiencies to be overcome. Ranges have been extended by electronic dead-time compensation in some current instruments and/or by use of dual GM detectors having a small-volume detector for high range and a larger-volume detector for lower range. Paralysis in high-radiation fields has been overcome electronically in modern GM instruments. Acceptable accuracy over a wide range of energies is available through the use of energy compensation, usually achieved by wrapping the tube with suitable filtering material to depress low-energy response.

One consideration we should note is that if you have a significant interest in measuring doses in gamma-radiation fields with energies greater than about 3 MeV, most typical GM detectors will overrespond significantly because of pair-production events occurring in the detector, which commonly contains materials with moderately high atomic numbers that enhance this particular interaction. Also, if very-low-energy gamma- or x-rays, often less than about 50 or 60 keV, are being measured with an energy-compensated detector, the response will be often be depressed because of attenuation in the compensating filter. For general use in most gamma-radiation environments, however, GM detectors are very suitable and desirable for radiation-dose measurements in continuous fields.

In pulsed-radiation fields as might occur in some facilities, such as some accelerators, GM detectors may not be useful because they may respond to the pulse rate of the machine rather than to the actual dose rate. In such instances ionization chambers, particularly integrating ion chambers, offer distinct advantages.

In addition to their utility in pulsed-radiation fields, ionization chambers (when properly constructed) may provide a more accurate measure of gamma-radiation dose/dose rate over a wider range of gamma-ray energies than do some GM detectors. Their responses at high gamma energies (>3 MeV) are generally considerably more accurate than most GM detectors, and they generally have a low-energy response that is good, often down to about 20 keV and sometimes lower. Unlike GM detectors, basic ionization chambers do not use gas multiplication (an increase in ionization events by secondary ionization caused by acceleration of charged particles in a high electric field), they almost never have the sensitivity of GM detectors, and therefore, as you have pointed out, they often are not useful at low intensities, being typically limited to dose rates that are no less than 0.01–0.02 mSv h-1. Some manufacturers market pressurized ion chambers, which use a pressurized gas to increase sensitivity, thus extending the range downward to background levels.

In my experience, ion chambers are often more fickle than GM detectors, requiring more frequent and more expensive maintenance than GM detectors and more attention in their proper use. Ion chambers that are open to the atmosphere may suffer from excessive leakage currents in high-humidity conditions and may become internally contaminated if they are used in an air-contaminated area. They are also subject to variations in response with ambient temperature and/or pressure variations. Some ionization chambers are equipped with a movable window that exposes a thin window to allow entrance of beta radiation; beta-dose readings, however, are not reliable without applying predetermined chamber-correction factors. Ionization chambers, compared to GM instruments, are usually much more expensive, a consideration that can be important if large numbers of instruments are being purchased.

Scintillation detectors have particular strengths, but they are not as popular as GM detectors or ionization chambers for dose assessment. Inorganic scintillators, such as sodium iodide with thallium or NaI(Tl), are very efficient for gamma-ray detection compared to GM detectors and ionization chambers, and they might be a good choice in a survey meter used for looking for a lost source or possibly incorporated into a portable, handheld gamma spectrometer. They are not very good for gamma-dose measurements because of the very high energy-dependent response that they exhibit because of their high atomic number compared to soft tissue or air. If you have a need to measure low dose rates, say from background to perhaps a few mSv h-1, some plastic scintillator-based instruments are quite good because they are nearly tissue equivalent. The solid plastic detector gives high sensitivity, and these instruments are often marketed as microsievert meters at moderately high cost.

Semiconductor detectors, especially silicon, are often detectors of choice for laboratory measurements of charged particles, especially for energy spectrometry of alpha and sometimes beta radiation. Silicon detectors have also been used in the fabrication of handheld instruments for alpha/beta contamination and in instruments for measuring beta skin dose, although these seem to have enjoyed limited popularity in the United States. I have not noticed these on the market lately and do not have direct experience with them, so I cannot comment on their specific attributes or deficiencies.

Notwithstanding the possible utility of a silicon-based, beta-dose-measuring instrument, the other more common instruments (GM-based instruments and ionization chambers) do not appear able to measure beta dose/dose rate directly. Thin-window GM detectors, as you note, are event detectors, and they will record beta-detection rate. If the detector face is irradiated uniformly and the intrinsic detection efficiency is known (this is often close to 100% for moderate- to high-energy beta radiation), one can calculate the beta-particle fluence rate. From this, in conjunction with the mass stopping power for the beta-particle energy distribution (which can be estimated with varying degrees of uncertainty), one could estimate the beta-particle dose rate either in air or at the skin surface (the product of the fluence rate and the mass stopping power is the dose rate). Further corrections would be necessary to get to the live-skin dose rate, usually evaluated at a depth of 7 milligrams per centimeter2 (mg cm-2) in soft tissue. Ionization chambers equipped with thin windows are sensitive to much beta radiation, but because of geometric effects and beta attenuation through the window and through the depth of the detector (which is generally rather large in volume), a significant correction factor (often in the three-to-five region) is necessary to convert the reading to dose rate.

With respect to instruments for the home enthusiast or others concerned about environmental levels of radiation, there are many available instruments, most of them GM-based. GM-based instruments are relatively unsophisticated in their electronics from a fundamental operational viewpoint, and this contributes to the relatively low cost. The most sophisticated part of the electronics may be in some of the digital circuitry and logic involved in such things as data storage and display as opposed to the detector operation. One need only search the internet to get an idea of the availability of low-cost instruments. Many of these instruments cost less than a few hundred dollars. I am not in a position to make specific recommendations, although I expect many of these devices will operate satisfactorily for the casual user if they are handled and used properly.

One of the most significant problems I have observed among users of such instruments is that they do not always abide by a reasonable protocol for calibrating the instruments, thus allowing for erroneous readings when the instrument is out of calibration. Some such users are not even aware of the need for recalibration. Additionally, some users are not aware of limitations of the instrument or how some readings should be interpreted. I have noted this particularly among individuals who have made measurements close to beta/gamma-emitting, radioactively contaminated materials and reported those dose rates. The reported dose rates may be completely wrong and misleading because the instrument was calibrated to convey dose rates from gamma radiation, while most of the response of the instrument was to beta radiation. Such errors probably may outweigh the inherent operational deficiencies that may be present among some low-cost instruments.

I am not so concerned about an instrument being inexpensive and possibly not as reliable as a more expensive one; I am more concerned with whether the user is educated enough to make reasonable judgments about how to properly use the instrument and how to judge whether it is responding in a reasonable fashion. My impression among some lay people using radiation-detecting instruments is that they often want to know whether radiation levels are above background, have changed from earlier measurements, and/or are sufficiently high to warrant further action. Most such questions can be adequately answered even if the instrument does not provide extremely accurate measurements as long as the measurements are consistent and reproducible, and the user is sufficiently educated to interpret properly the readings that he or she obtains. Given this, I believe that many low-cost instruments, as well as other more expensive and perhaps better-validated instruments (including those you have identified), can be used effectively by responsible individuals.

Regarding electronic personal dosimetry devices, such as the Mirion DMC-3000 intended to be worn by the user to record personal dose (especially deep dose equivalent), I have found these to be generally reliable and easy to use. Most of these use silicon diodes, often with some energy compensation, as the detectors and provide quite reliable estimates of dose over a wide range of photon energies.

I realize you may be a bit disappointed that I have not made specific product recommendations, but I hope the discussion may provide some information that is helpful to you as you educate and advise individuals about instrument selection.

George Chabot, PhD

References

National Council on Radiation Protection and Measurements. Instrumentation and monitoring methods for radiation protection. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 57; 1978.

National Council on Radiation Protection and Measurements. Calibration of survey instruments used in radiation protection for the assessment of ionizing radiation fieldsand radioactive surface contamination. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 112; 1991.

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