equivalent dose is joule per kilogram (J kg-1) and its special name is sievert (Sv). .... ionization chamber is a standardizing device and is impractical for most ...
Measurement of External Effective Dose J. L. Alvarez Alpha Beta Gamut Jacksonville, FL 32226 Introduction Current radiation protection (regulation) is based on risk, but defined on the basis of dose. Risk is not a directly measurable quantity, and generally, neither is dose. To make matters worse, all definitions of dose and risk are not equal. The problem with measuring risk and dose at near environmental levels is that the definitions based on risk involve very small risks making the consequent measurement of dose very tedious. Even the usually simple measurement of external dose requires calculation by a model to be precise. Dose reconstruction and clean up regulations require consideration of dose from all pathways. The external dose from radionuclides may be a minor, yet, significant part of the total dose. It may be important to measure the external dose correctly as effective dose, especially when limits are approached or legal considerations are necessary. The conversion of grays to sieverts or rads to rems may not be obvious for a given measurement device, especially when the device is restricted to measuring ions or counting events. The ICRP currently recommends three types of quantities that are necessary for correct measurement of external dose. These are protection quantities, operational quantities, and physical quantities. These quantities and their use in measuring external dose will be discussed based on the ICRU 57. The radiation protection quantity currently recommended by the ICRP is the effective dose, E. Two other protection quantities recommended are the mean absorbed dose in an organ or tissue, DT and the equivalent dose in an organ or tissue, HT. These recommended radiation protection quantities were developed initially from the concept of critical tissues. In 1958 the ICRP recognized that some tissues may be more radiosensitive than others and therefore should form the basis of protection. The bloodforming organs were long recognized as particularly sensitive and evidence at that time was mounting for radiation as a cause of leukemia. Radiation induced cataracts were well known, especially from neutrons and high-energy particles. The most important radiation effect at that time was considered to be mutations, which indicated the gonads as a critical tissue. The protection quantities, therefore, considered the blood-forming organs, the lens of the eye, and the gonads as critical tissues in the case of whole body exposure. The concept of critical tissues was replaced by radiation carcinogenesis as the basis for protection quantities based on evidence developed in the study of the Japanese survivors of the atomic bombing. The effective dose is a summed organ-weighted dose based on organ sensitivities to radiation carcinogenesis. The more general term stochastic detriment is used to include effects other than cancer death. This method of defining
effective dose is not a physical method but a risk method. Physical quantities are involved in the definition of dose, but care must be taken in the application of the physical quantities. The definitions of the protection quantities are Effective Dose The summation of the equivalent doses in tissue or organs, each multiplied by the appropriate tissue-weighting factor. It is given by the expression E = ∑TwT•HT where HT is the equivalent dose in tissue or organ, T, and wT is the tissueweighting factor for tissue, T. The unit of effective dose is joule per kilogram (J kg-1) and its special name is sievert (Sv). Equivalent Dose The summation of the absorbed dose in an organ or tissue multiplied by the relevant radiation-weighting factor HT = ∑TwR•DT,R where DT,R is the absorbed dose averaged over the tissue or organ, T, due to radiation R, and wR is the radiation-weighting factor for radiation, R. The unit of equivalent dose is joule per kilogram (J kg-1) and its special name is sievert (Sv). Mean Absorbed Dose The mean absorbed dose, DT, in a specified tissue or organ of the human body, T, given by DT = (1/mT)∫mTDdm or εT/mT where mT is the mass of tissue or organ, D is the absorbed dose in the mass element dm, and εT is the total energy imparted in the tissue or organ. The unit of absorbed dose is joule per kilogram (J kg-1) and its special name is gray (Gy). An important observation at this point is that none of the above-defined protection quantities is directly measurable. The unit sievert is not a physical quantity although it is defined as energy deposited per kilogram. The sievert is equivalent energy deposited based on the carcinogenic sensitivity of the organ affected. The sievert is not measurable. The possible exception for measurement is the mean absorbed dose. The unit gray is a physical quantity and can be measured, nevertheless the tissue of interest may not be accessible for measurement. We will show later that it is possible to calibrate for some variations of the definitions including sieverts. A set of operational quantities is defined that allows for calibration of instruments for measurements to show compliance with the system of protection quantities. These
measurable quantities are the ambient dose equivalent, the directional dose equivalent, and the personal dose equivalent. Ambient Dose Equivalent The quantity H*(d) is the dose equivalent that would be produced by the corresponding expanded and aligned field in the ICRU sphere at a depth, d, on the radius opposing the direction of the aligned field, as shown in Figure 1. The unit of ambient dose equivalent is joule per kilogram (J kg-1) and its special name is sievert (Sv). The recommended depths are 10 mm for penetrating radiation and 0.07 mm for low-penetrating radiation.
Ambient Dose Equivalent
Point of measurement Figure 1. The definition of the ambient dose equivalent defined on the ICRU sphere. Directional Dose Equivalent The quantity H’(d,Ω), at a point in a radiation field is the dose equivalent that would be produced by the corresponding expanded field in the ICRU sphere at a depth, d, on a radius in a specified direction, Ω, as shown in Figure 2. The unit of directional dose equivalent is joule per kilogram (J kg-1) and its special name is sievert (Sv). The recommended depths are 10 mm for penetrating radiation and 0.07 mm for low-penetrating radiation.
Directional Dose Equivalent
Ω
Point of measurement
Figure 2. Directional dose equivalent as defined on the ICRU sphere. Personal Dose Equivalent The quantity Hp(d) in soft tissue at an appropriate depth, d, below a specified point on the body. The unit of personal dose equivalent is joule per kilogram (J kg-1) and its special name is sievert (Sv).
Personal Dose Equivalent
Ω
Point of measurement
Figure 3. Personal dose equivalent as defined on the ICRU slab. The latter quantity could be equal to mean absorbed dose for photons, under some circumstances. It appears that mean absorbed dose is a measurable quantity. The unit of mean absorbed dose, Gy, is a unit of measurement while the Sv is not. The Gy is the unit of measurement for absorbed dose because it is a physical quantity. As a physical quantity it can be measured in some systems. The most commonly measured radiation
physical quantity is the measurement of air kerma, K, free-in-air. Nevertheless, the measurement is indirect and rarely does one claim to be measuring kerma. Kerma is the quotient of dEtr by dm, where dEtr is the sum of the initial kinetic energies of all the charged ionizing particles liberated by uncharged ionizing particles in a volume element of mass dm, K = dEtr/dm. The unit of kerma is joule per kilogram (J kg-1) and its special name is gray (Gy). The standard device for measuring air kerma, free-in-air, is the free-air ionization chamber. This device and calorimeters are basic instruments used to develop and characterize calibration systems. A third physical quantity, the fluence, is used to completely develop calibration systems, but fluence is most important in calculating the expected response of devices designed to measure the operational quantities. Fluence is the number of particles incident on a sphere of cross-sectional area. The fluence, Φ, is the quotient of the number of particles, dN, and the area, da,
Φ = dN/da. The three physical quantities (mean absorbed dose, kerma, and fluence) form the basis of radiation measurement, while calculation, based on empirical evidence of risk, forms the basis of radiation protection. Measurement systems for the operational quantities rely on calculation and calibration. The use of these two sets of quantities often leads to confusion as to what is being measured at any given time. There is a conviction among some that the only unit of measurement is the Gy. While this is technically true, calibration allows measurement in Sv. Unfortunately, neither unit may be correctly applied unless the device used for measurement is used only under the calibrating conditions. This leads to the question, do we have enough definitions to make a compliance measurement? It depends upon the correct use of The compliance quantities E, HT, DT The physical quantities K, Φ, DT The practical quantities H*(d), H’(d,Ω), HP(d) Measurements of Energy Deposited to Air in Air The standard instrument for measuring ionizing radiation, in particular photons, is the free air ionization chamber. This device measures ionization produced by photons in air. The original unit for measuring the ionization energy was the roentgen, which was a measure of coulombs per kilogram (C kg-1). The free air ionization chamber requires a geometry that suits the photon energy measured. This geometry ensures charged particle
equilibrium and that all charged particles that originate in the volume come to rest in the volume. The charge collected in the volume is equivalent to energy deposited (using the average energy expended in ionization). The free air ionization chamber can be calibrated to measure air kerma free-in-air in units of Gy. The free air ionization chamber requires charged particle equilibrium, so it cannot measure kerma under all circumstances, since kerma is also defined at locations where there is not charged particle equilibrium. Most dosimetric situations are in charge particle equilibrium or nearly so. The free air ionization chamber is a standardizing device and is impractical for most dosimetric situations. Other air chambers can be designed to measure air kerma free-in-air or air kerma in other materials. The standard designs require charge particle equilibrium and air-wall material in order to compare to the free air chamber used as a standard. The response of these chambers is calibrated in a well-characterized field to read in Gy or Sv depending upon the intended use of the chamber. Calibration in Gy for measurement in air measures air kerma, but calibration in Gy for another medium measures dose in that medium. Calibration in Sv measures ambient dose equivalent, directional dose equivalent or personal dose equivalent. The calibration is for the calibration conditions and may not translate easily to other conditions. Translation to other conditions will require knowledge of energy dependence and directional dependence. Air Kerma and Effective Dose The mass attenuation coefficient for air and soft tissue are compared in Figure 4. The mass attenuation coefficients for air and bone are compared in Figure 5. It appears that measurement of energy deposited in air may be useful for estimating dose in tissue. Mass attenuation coefficients of muscle and air 10 Muscle Air
2
cm /g
1
0.1
0.01 0.01
0.1
1
10
Photon energy (MeV)
Figure 4. Comparison of the mass attenuation coefficients of muscle and air at various photon energies.
Mass attenuation coefficients of bone and air 100 Air Bone
2
cm /g
10
1
0.1
0.01 0.01
0.1
1
10
Photon energy (Mev)
Figure 5. Comparison of the mass attenuation coefficients of bone and air at various energies. Figures 4 and 5 show that air is a suitable medium for measurement in most tissues at most energies. The greatest disparities are for bone at low energies. Effective dose can be modeled in the human body based on measurements in air and use of appropriate human equivalent models or phantoms. The computational phantom for effective dose and the various modeling geometries are shown in Figure 6.
Computational Phantom for Effective Dose PA posterior-anterior
RLAT right lateral
LLAT left lateral
AP anterior-posterior ISO isotropic ROT rotational
Figure 6. The computational phantom for effective dose and the standard geometries for modeling effective dose. The energy dependence of effective dose for the AP geometry is shown in Figure 7. The figure shows the relationship of air kerma to effective dose for a human phantom in a uniform parallel field in anterior incidence. Dose is less than air kerma for energies less than 47 keV. Dose is greater than kerma from 40 to 1000 keV with a maximum at 80 keV. Dose is slightly less than kerma for energies higher than 1000 keV. An ion chamber may be easily calibrated for energies 300 keV and above with little concern for energy dependence. Energies less than 300 keV may prove problematic. Effective dose per unit air kerma 1.6
1.4
1.2
E/Ka (Sv/Gy)
1
0.8
0.6
0.4
0.2
0 0.01
0.1
1
10
Photon energy (MeV)
Figure 7. The effective dose for AP geometry compared to air kerma. The energy dependence of effective dose for the various standard geometries is shown in Figure 8. The figure shows there are major geometric dependencies in the effective dose. A simple measurement of photons in air does not readily lead to effective dose in tissue.
Figure 8. The geometry dependence of effective dose relative to air kerma. Before discussing calibration further, it is instructive to first investigate the development of effective dose as indicated in Figure 9. The effective dose is a composite of doses to various organs. The figure shows the energy dependence of dose in the various organs for photons incident on a human phantom in a uniform parallel field in anterior incidence. Because effective dose is the sum of doses to various organs, the direction is important. Figure 10 shows the energy dependence of dose to the liver for photons from various directions.
Absorbed dose per unit air kerma for organs 2.5
2
Bladder Bone (red marrow) Breast
DT/Ka (Gy/Gy)
Bone (surface) Gonads (F)
1.5
Gonads (M) Liver Lungs Oesophagus
1
Remainder Skin Stomach Colon Thyroid
0.5
0 0.01
0.1
1
10
Photon energy (MeV)
Figure 9. Energy dependence of effective dose to organs for AP incidence compared to air kerma. 1.6 AP PA
1.4
RLAT LLAT ROT
1.2
ISO
1
0.8
0.6
0.4
0.2
0 0.01
0.1
1
10
Figure 10. Energy and geometry dependence of effective dose to the liver compared to air kerma. Effective dose is the sum of the weighted doses to the organs. The organs and their weighting factors are listed in Table 1. Effective dose requires that the total,
Table 1. Tissue weighting factors for calculating effective dose. Tissue or organ Gonads Bone marrow (red) Colon Lung Stomach Bladder Breast Liver Oesophagus Thyroid Skin Bone surface Remainder
Tissue weighting factor, wT 0.20 0.12 0.12 0.12 0.05 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.05
averaged dose to the organs in Table 1 be weighted, then summed. This differs from the usual practical method of measuring a dose rate, assumed whole body average, and multiplying by the time exposed or using an integrating device presumed to approximate the whole body average dose rate. The combination of Table 1 and Figures 10 and 11 indicate that it may be possible for an organ to dominate the effective dose for a given energy or direction. Skin, with the lowest weighting factor, is the dominant contributor to effective dose at low photon energies. These different contributions to effective dose may be more important to dose reconstruction or meeting particular regulatory requirements for decontamination and decommissioning than for daily radiation protection. Absorbed dose per unit air kerma for organs 2.5 Bone (surface) Skin Thyroid
DT/Ka (Gy/Gy)
2
1.5
1
0.5
0 0.01
0.1
1 Photon energy (MeV)
10
Figure 11. Energy dependence of effective dose to bone, skin, and thyroid compared to air kerma showing that for some energies, an organ may dominate the dose. Occupational Radiation Protection Daily radiation protection relies on the personal dose equivalent, Hp(d), and to some extent on the ambient dose equivalent, H*(d), and directional dose equivalent, H’(d, Ω). The relationship of personal dose equivalent and kerma is shown in Figure 12. The personal dose equivalent is calculated for an ICRU slab rather than an ICRU sphere. Figure 13 compares personal dose equivalent Hp(10) at normal incidence to an ICRU slab to effective dose for AP incidence to a human phantom. The personal dose equivalent Hp(10) is greater than the effective dose at all energies. Comparing further to other geometries as shown in Figure 8 the personal dose equivalent Hp(10) is much higher than effective dose. The personal dose equivalent HP(0.07) is also measured on the ICRU slab and has a different energy dependence from that of HP(10), as shown in Figure 14. The personal dose equivalent Hp(10) has angular dependence for photon incidence on a slab as Figure 15 shows. Personal dose equivalent Hp(10), slab and Air Kerma 2 1.8 1.6
Hp(10)/Ka (Sv/Gy)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 0.01
0.1
1
10
Photon energy (MeV)
Figure 12. The energy dependence of personal dose equivalent as calculated for the ICRU slab compared to air kerma.
Effective dose AP and Personal Effective dose per Air Kerma 2 1.8 Personal dose equivalent Effective dose equivalent
1.6 1.4
Sv/Gy
1.2 1 0.8 0.6 0.4 0.2 0 0.01
0.1
1
10
Photon energy (MeV)
Figure 13. Personal dose equivalent compared to effective dose at AP incidence. Personal dose equivalent and air kerma 2
Hp(10) Hp(0.07)
1.8 1.6
Hp/Ka (Sv/Gy)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 0.01
0.1
1 Photon energy (MeV)
Figure 14. Personal dose equivalent HP(10) compared to HP(0.07).
10
Personal dose equivalent to air kerma with angle of incidence 2 0 degrees 15 degrees
1.8
30 degrees 45 degrees
1.6
60 degrees 75 degrees
Hp(10)/Ka (Sv/Gy)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 0.01
0.1
1
10
Photon energy (MeV)
Figure 15. The energy and angular dependence of HP(10) compared to air kerma. Effective dose (in any of its definitions) differs from air kerma for several reasons. The reasons differ with energy. The less than air kerma effects at low energies is caused by attenuation with depth while the higher than kerma effects at intermediate energies is caused by backscatter. The slightly less than air kerma effects at high energies are caused by atomic differences in mass attenuation. Directional differences are caused by location of organs in the body. The operational units are intended to allow measurement of the protection units. All the differences from simple air kerma indicate that practical measurement systems may require extensive calibration and knowledge of field conditions in order to accomplish a measurement of effective dose. Commercial Instruments The obvious type of instrument for performing measurements of effective dose is an air ionization chamber since coefficients are available for converting air kerma in air to dose in tissue and effective dose. Many air ionization devices are commercially available that are calibrated in sievert or rem. The response of these instruments above 300 keV should be easily calibrated to read in effective dose in the AP direction, Figure 7 or personal dose HP(10), Figure 12. Figure 16 shows that other energies and directions may present problems for an instrument calibrated for AP geometry. A compromise calibration may be rotational irradiation since many exposure situations may be effectively rotational.
Effective dose per air kerma various sources 1 0.9 0.8
E/Ka (Sv/Gy)
0.7 0.6 0.5 0.4 0.3 0.2
Cloud Plane
0.1
Uniform
0 0.01
0.1
1
10
Photon energy (MeV)
Figure 16. Energy and geometry dependence for effective dose for several possible environmental exposure conditions. Commercial instruments for environmental exposures may be constructed from materials other than air. These other materials may have sufficiently different characteristics to cause problems for calibration to effective dose. The mass attenuation coefficients for several materials are compared to muscle for various energies in Figure 17. Mass attenuation coefficients of muscle and detector material 1000
100
10 cm /g
Muscle
2
Air Lucite NaI
1
0.1
0.01 0.01
0.1
1
10
Photon energy (MeV)
Figure 17. Mass attenuation coefficients of several common detector materials compared to muscle. The proper calibration is most likely to personal dose equivalent, since this is the most practical measurement definition. The response of a typical air ionization chamber in air is shown in Figure 18 compared to Hp(10) and the effective dose from a plane source at the ground surface that is infinite in extent. The location on the ordinate for the
ionization chamber response is approximate, since the calibration is against free air ionization. The free air ionization should yield roentgen units. The ion chamber response is R/R (exposure in air) whereas the effective dose response is Sv/Gy. This particular instrument responds similarly in all orientations to a beam of radiation, except the lower energies. The electronics unit and wall thickness of the housing differentially attenuate the lower energies. It is possible that such an instrument can be successfully incorporated into a measurement program, if care is taken in the interpretation and calibration. Most hand-held ionization chambers do not have sufficient sensitivity for cleanup applications. Ion chamber to Free air compared to Hp 2.5
Ion chamber to Free air (R/R)
2
1.5 Ion chamber Hp Plane 1
0.5
0 0.01
0.1
1
10
Photon energy (MeV)
Figure 18. Energy response of a hand-held ionization chamber compared to a free air ionization chamber. Further comparison is made to HP(10) and effective dose from an infinite plane source. A particular ionization chamber was designed for uniform directional response and response at low doses. The chamber is the pressurized ionization chamber. The common name for this instrument is the PIC and was developed by Environmental Measurements Laboratory. The PIC is well characterized, but is far from hand-held. The energy response of this instrument is shown in Figure 19 and compared to Hp(10) and the effective dose from an infinite plane source. A PIC is calibrated in R/h. The PIC ionization chamber is a spherical steel shell containing argon at 18 atmospheres. The PIC measures neither air kerma nor any definition of effective dose, nevertheless, because it is well characterized it is often accepted as a standard for human exposure. The PIC can
also be usefully employed in a measurement program as long as the limitations are recognized and accounted for. PIC to Free air compared to Hp 2.5
PIC to Free air (R/R)
2
1.5 PIC Hp Plane 1
0.5
0 0.01
0.1
1
10
Photon energy (MeV)
Figure 19. Energy response of a pressurized ionization chamber (PIC) compared to a free air ionization chamber. Further comparison is made to HP(10) and effective dose from an infinite plane source. Air ionization is used in other types of instruments. GM type detectors have an energy response typified in Figure 20 and compared to Hp(10) and effective dose from an infinite plane source. This type response is generally unacceptable for dose measurements for unknown energies, but can be useful in situations where the calibrating conditions closely match the measuring conditions. The compensated GM detector was developed to give a more dosimetric response. The energy response of a compensated GM detector is, also, shown in Figure 20.
GM to Free air compared to Hp 10 GM
9
Compensated GM Hp
8
Plane
GM to Free air (R/R)
7 6 5 4 3 2 1 0 0.01
0.1
1
10
Photon energy (MeV)
Figure 20. Energy response of a GM detector and a compensated GM detector compared to a free air ionization chamber. Further comparison is made to HP(10) and effective dose from an infinite plane source. A popular instrument for environmental gamma exposure measurements is the NaI scintillation detector calibrated to µR/h. The high-density detector has excellent event response to gamma and, therefore, good sensitivity to background gamma levels. This detector does not have an energy response that allows general calibration to effective dose because it counts events and does not have the same energy response as tissue. The response of a µR meter is shown in Figure 21 and compared to Hp(10) and effective dose from an infinite plane source.
MicroR to Free air compared to Hp 14 MicroR Hp
12
Plane
MicroR to Free air (R/R)
10
8
6
4
2
0 0.01
0.1
1
10
Photon energy (MeV)
Figure 21. Energy response of a microR meter compared to a free air ionization chamber. Further comparison is made to HP(10) and effective dose from an infinite plane source. Another instrument for environmental gamma exposure measurements is the plastic scintillation detector calibrated to µrem/h. The low-density detector has poorer event response to gamma than the NaI detector and, therefore, poorer sensitivity to background gamma levels. This detector has very good energy response that allows general calibration to effective dose because it has an energy response similar to tissue. The response of a microrem meter is shown in Figure 22 and compared to Hp(10) and effective dose from an infinite plane source.
Microrem to free air chamber compared to Hp 2.5 Microrem Hp Plane
Microrem/Free air Sv/R
2
1.5
1
0.5
0 0.01
0.1
1
10
Photon energy (MeV)
Figure 22. Energy response of a microrem meter compared to a free air ionization chamber. Further comparison is made to HP(10) and effective dose from an infinite plane source. Calibrating Conditions Under good calibrating conditions instruments can approximate the various ICRU 57 definitions. Depending upon the type of instrument, the energy range of the approximation may be limited. Therefore, if the field conditions are known, the calibration could allow a good approximation of dose under the given field conditions. Nevertheless, calibration is usually inappropriate for field conditions. The nearest incidence of effective dose fitting a practical dose is AP incidence and HP(10) as shown in Figure 13. Since HP(10) is the usual calibrating geometry, it overestimates effective dose in the best calibration configuration. As has been shown, HP(10) does not approximate any other exposure geometry, but is always conservative. A particular problem for calibration is that the calibration spectrum (point source) does not follow a field spectrum (extended source). Figure 23 shows the energy spectral difference between a point 226Ra source and 226Ra distributed evenly in soil.
Geometric Dependence of Ra226 Photons Point source uniform in soil
>2000
Photon energy (keV)
1000-2000
500-1000
250-500
100-250
50-100
0-50
0
5
10
15
20
25
30
35
Percent of total fluence
Figure 23. The energy spectral difference between a point 226Ra source and 226Ra distributed evenly in soil. There is nearly twice the total energy in a point source spectrum of an equal number of photons. The difference in the two spectra will deliver a quite different air kerma for an equal amount of photons as the combination of Figures 23 and 24 suggest. In some cases, instrument characteristics may compensate for the air kerma difference, but it should be obvious, that an instrument is only correctly calibrated for the calibrating conditions.
Conversion coefficients for air kerma per unit fluence 30
25
2
Ka/F (pGy cm )
20
15
10
5
0 0.01
0.1
1
10
Photon energy (MeV)
Figure 24. Energy dependence of air kerma per unit fluence. Measuring Effective Dose The measurement of effective dose requires considerable preparation and calculation. It is necessary to know the instrument characteristics including energy response and directional response (Figures 18-22). The calibrating conditions must be known to account for any spectral differences from the field conditions (Figure 23). The field spectrum must be known and its distribution. Figure 16 shows the dependence of effective dose on several uniform environmental distributions of photons and Figure 8 shows the energy dependence of the standard computational geometries. All these considerations can lead to a better understanding of what is measured under field conditions and how well this may approximate effective dose. It is unlikely that one can actually perform a direct measurement of effective dose. Non-uniform Sources The actual measurement of effective dose is likely not possible, but non-uniform sources increase the difficulty. The types of non-uniform sources are hot spots, irregular distributions (shape, concentration gradients, or both), structural surfaces, and equipment surfaces. All of these are simplified with distance, but become increasing difficult as the source is approached or as the source presents different geometries because of movement. Assuming simple averages and taking a conservative approach can reduce the difficulty. Conservative assumptions may create further difficulties because limits may be exceeded when, in fact, the effective dose is much lower. It may be necessary to devise practical approaches to better estimate the actual doses. Because the obtaining the actual dose is complicated, it is often necessary to obtain regulatory guidance before proceeding with the measurements. Most regulators are neither equipped or prepared to consider the problems with effective dose. Therefore, by regulatory guidance we mean prepare your case before measurement begins and attempt to anticipate the pit falls. Present the
measurement of effective dose in the sampling and analysis plan. Be prepared to defend the methods. In general, plan the measurements to meet the objectives of determining effective dose. Such planning will involve instrument selection and calibration requirements. Some of the calibration requirements may be met by field calibration or correlation measurements. Making it Work There are several approaches to making field estimates of effective dose. The approaches vary with the instruments and contaminants. In general, the first step is to estimate the photon spectrum both the contaminant and background spectrum. From the spectra define the instrument response from both energy and geometry considerations. After the instrument has been selected it is necessary to field calibrate the instrument. The field calibration can be made to a standardized instrument or to a calculation from the spectrum. A common field calibration when a NaI µR meter is used is to correlate the µR meter to a standardized PIC. Figures 19 and 21 show that neither instrument correctly measures effective dose, but the PIC is acceptably close and is accepted as a standard. Figure 25 shows the results of such a correlation.
MicroR to PIC 35
30
PIC values (R)
25
20
15
10
5
0 0
5
10
15
20
25
30
MicroR values (R)
Figure 25. Correlation of a µR meter to a PIC for purposes of correcting the NaI bias.
35
The figure shows two sets of data that are essentially parallel but with different intercepts. The measurements were made at two locations but at different altitudes. The difference between the data sets is the amount of cosmic radiation. The fit to both data sets was
PIC = A + 0.51 microR The constant A is nearly equal to the cosmic exposure. The NaI response, a single pulse, and calibration to a spectrum that contains low-energy (Compton scatter) ensures that there is little response to the cosmic radiation. Converting a NaI Spectrum to Effective Dose It is possible to convert a NaI spectrum to effective dose. Such a conversion requires a number of steps but the conversion can be simply and effective done in a spreadsheet. Figure 26 shows a NaI spectrum of background and the conversion to a spectrum in air, based on the differences in mass attenuation coefficient (Figure 17) and Compton scatter (the hard part). The conversion to air to a spectrum in air has the greatest effect in the low energy region (
