Appendix D: Application of the dose-equivalent quantities

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3

D: APPLICATION OF THE DOSE-EQUIVALENT QUANTITIES D.l. Adequacy of the Indices

In the hierarchy of radiation protection quantities, the index quantities are secondary in nature (ICRP, 1977). For exposure to external radiation, limitation of the deep dose-equivalent index (H,,,) to 50 mSv in a year is deemed to afford a level of protection that is at least as good as that which follows limitation of the effective dose equivalent (HE) to 50 mSv in a year. From the ratios of HE to H,,d for photons and neutrons in Table D.l it is clear that this claim is valid, if not understated, for anteroposterior and posteroanterior irradiation. The values are calculated from Tables 2 and 4 for photons in the main text and from Tables 17 and 19 for neutrons. Those tables can be used to show that the claim also holds for the other irradiation geometries for which compatible data are available as discussed elsewhere (Kramer and Drexler, 1979). From the limited information on electrons referred to in the main text, the ratio of HE values to HI,d values is well below unity from about 5 to 20 MeV. The shallow dose-equivalent index (H,,,) is likewise deemed to provide adequate protection for the skin (ICRP, 1977). Ratios of the values of Hsk, the dose equivalent to the entire skin of an anthropomorphic phantom, to the values of H,,, are presented in Table D.2 for plane parallel beams of photons and for isotropic irradiation; the assumption of adequacy is seen to be generally valid. The values are calculated from Tables 5 and 12 in the main text, and those tables can also be used to show that the claim holds for other irradiation geometries. As noted in the main text, the results for the anthropomorphic phantom are averaged over the entire skin of the body, but for unidirectional geometries and low-energy photons, it might have been more appropriate to average over the half of the skin nearer the source. This would have resulted in a doubling of the unidirectional values at low energies, with the implication for Table D.2 that the ratio of Hsk to H,,, would remain around unity for the whole energy range. A similar exercise for neutrons with the data from Table C.2 and Table 19 in the main text and for an anthropomorphic phantom (Wittmann et al., 1985) upholds the proposition that H,,s is adequate. With regard to the lens of the eye, the combined effect of complying with limits based on the deep and shallow dose-equivalent indices is deemed to ensure that it is adequately protected. That this is so, in general, for photons is clear from Table D.3 where a comparison is made between the fluences corresponding to the annual dose-equivalent limit for the lens of the eye and the fluences corresponding to the more restrictive of the annual limits on the two doseequivalent indices in the case of a plane parallel beam: the indices are almost always more restrictive than the lens in the sense that the corresponding fluences are lower. The minor exceptions at the lowest energies are an outcome of the computational model for the anthropomorphic phantom, which results in a slight overestimate compared to the dose equivalent at the equator of the lens. (See Section 4.3.3.) The values are calculated from Tables 4, 5 and 13. Comparison for isotropic irradiation also upholds the assumption. A similar exercise with Table 19 from the main text and with data from Table C.2 shows that the assumption is also valid for neutrons at least for unidirectional irradiation and probably for other geometries. There is relatively little information on values of H,,d: it merely encompasses photons and neutrons up to energies around 10 MeV, with some fragmentary data for electrons in the same energy region, For the other radiations in Section 4 of the main text, and for photons, neutrons, and electrons with energies greater than 10 MeV or so, the maximum dose equivalent occurring in the 30 cm slab phantom might be regarded as being equivalent to Hl.d, although the

123

DATAFORUSElNPROTECTlONAGAINSTEXTERNALRADlATlON

Table D.l. Ratio of the effective dose equivalent. HE. to the deep for photons and neutrons incident in a dose-equivalent index, Hl,d, plane parallel beam on an anthropomorphic phantom and on the ICRU sphere

Photons,

hew, MeV

AP

H#-J ,

s/-III

,d

AP

(0.50) (0.44) 0.41 0.38

(0.33) (0.27) 0.24 0.24

0.38 0.40 0.53

0.24 0.26 0.30

lo-’ 10-i

-

;-;

;;I*

-

1.0 1.0 1.0 1.5

1o-4 1o-3 1o-2 1o-2

2.0 3.0

1o-2 10-i

“5.; :;I2

Neutrons,

PA

2.5 1.0

0.81 0.19

d

OTOO

PA

0.04

0.23 0.42 0.59 0.72

0.09 0.20 0.36 0.49

0.40 0.31

6.0 8.0 1.0 1.5

lo-* 1O-2 10-r 10-l

0.79 0.84 0.86 0.87

0.56 0.64 0.67 0.70

2.0 3.0 4.0 5.0

10-l 10-l 10-l 10-l

0.87 0.86 0.87 0.88

0.71 0.72 0.74 0.76

6.0 8.0 1.0 1.5

10-l 10-r loo 10”

0.88 0.89 0.90 0.90

0.77 0.80 0.81 0.84

0.42 0.50

0.16 0.24

2.0 3.0 4.0 5.0

loa loo 10” lo0

0.90 0.92 0.93 0.95

0.85 0.87 0.90 0.91

0.60 0.68 0.72 0.83

0.34 0.45 0.51 0.62

6.0 8.0 1.0 1.4

10’ 10” lo1 lo1

0.95 0.97 0.97

0.93 0.94 0.95

0.86 0.87 0.88 (0.92)

0.66 0.68 0.69 (0.80)

0.29

0.08

0.31

0.34

-

See Section 3.6 for irradiation geometries. Dash entries arise because of the different energies of photons and neutrons in the source tables.

AICPP 17-2/3-I

124

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3

Table D.2. Ratio of the skin dose equivalent, Hsk,to the shallow doseequivalent index, HI,,. for photons incident in a planeparallel beam and

isotropically on an anthropomorphic phantom and on the ICRU sphere Energy

MeV

%k/Hr,s AP

IS0

1.0 1o-2 1.5 1o-2

0.51 0.49

2.0 3.0

1o-2 1o-2

0.51

0.99 0.99 0.90

0.53

0.82

4.0 5.0 6.0 8.0

lo-* 1o-2 1O-2 1O-2

0.54 0.56 0.58 0.62

0.84 0.87 0.88 0.91

;a; . ;;I:

0.70 0.65

0.95 0.92

;.; . ;;::

0.72 0.73

0.96 0.94

0.74

0.94

6:0 2.; ;;I: 10-l 8.0 10-l

0.77 0.76 0.79

0.94 0.93

1.0 1.5 2.0 3.0

loo loo 10" lo0

0.81 0.84 0.86 0.88

0.93 0.93 0.94 0.94

4.0 5.0 6.0 8.0

loo 10" 10' 10'

0.89 0.90 0.91 0.90

0.94 0.94 0.95 0.95

1.0 lo1

0.90

0.96

See Section 3.6 for irradiationgeometries. At low photon energies, the values for AP gemtry should be doubled to obtain the ratio for the skin dose-equivalent averaged over the side of the body nearer the source. See Section D.1.

DATA FOR USE IN PROTECTION

AGAINST EXTERNAL

125

RADIATION

TableD.3. Comparisonof fluence corresponding

to the annual dose-equivalent limit for the lens and the more restrictive of the dose-equivalent indices, for photons incident ina planeparallel beam on an anthropomorphic phantomand on

theICRU sphere Photon energy, MeV

Fluence correspondingto the annual limits, lOlo a~-' AP Lens

Ap/pA Index

PA Lens

ROT Index

Lens

1.0 1.5 2.0 3.0

lo-* 1o-2 lo-* lo-*

7.0 7.8 10 18

7.2 5.9 4.9 6.3

1500 450 360 390

16 21 14 15

28 17 20 32

4.0 5.0 6.0 8.0

lo-* lo-* lo-* lo-*

27 33 34 31

8.1 9.4

410 380 300 190

18 20 20 18

49 61 64 56

1.0 1.5 2.0 3.0

10-l 10-l 10-l 10-l

26 19 14 8.8

8.1 5.6 4.2 2.8

140 68 44 23

15 9.3 6.5 3.9

44 26 18 11

4.0 10-l 5.0 10-i

6.4 5.0 4.1 3.1

2.1 1.7 1.5 1.2 E2 0:59 0.45

;.; ;;I1

2.8 2.2 1.9 1.4

8.0 6.4 5.4 4.2

23:;

E3 0:66 0.50

3.5 2.6 2.1 1.6

0.40 0.34 0.30 0.25

1.3 1.1 0.99 0.80

0.21

0.67

16 13 10 7.2 5.7 4.0

1.0 1.5 2.0 3.0

10" 10" 10" 10"

2.5 1.9 1.6 1.3

4.0 5.0 6.0 8.0

10" 10" 10" 10'

1.0 0.89 0.79 0.64

0.37 0.32 0.23

2.0 1.7 1.5 1.2

1.0 lo1

0.54

0.20

1.0

0.29

See Section 3.6 for irradiationgeometries. kwer values of the parameters related to indices in particular geometries signify adequate protection for the lens of the eye.

126

REPORT

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3

qualifications mentioned in Section 4 about this equivalence should be borne in mind. So too should the reservations expressed there about approximating the maximum dose equivalent to the effective dose equivalent.

D.2. Utility of the Environmental Quantities According to the Commission (ICRP, 1977), either the dose-equivalent limits themselves or the secondary limits may be used to devise derived limits such as dose-equivalent rates in the workplace. This possibility is explored here with some emphasis on the relationship between index and environmental monitoring quantities. Initially, however, the utility of the ambient dose equivalent H*(lO) in relation to the effective dose equivalent is examined. Table D.4 shows the ratios of the effective dose equivalent to the ambient dose equivalent for photons and neutrons incident on the relevant phantoms in plane parallel beams. The utility of H*(lO) is indicated by the closeness of the ratio to unity or by its being less than unity. By these criteria, the ambient dose equivalent is generally useful for photons and neutrons even if rather cautious in certain energy bands. The photon data are calculated from Tables 2 and 6 in the main text and the neutron data from Tables 17 and 21: reference to those tables shows that the utility of H*(lO) is maintained for the other geometries for which data are available. The sparse information on electrons, mentioned in the main text, indicates that the ratio is below unity for anteroposterior and posteroanterior irradiation with plane parallel beams from about 5 to 20 MeV. If the index quantities are to be the basis of derived quantities, the link between them must be realistic in the sense that the derived quantity must correspond closely to the secondary quantity or be a cautious reflection of it. Examination of the photon (Tables 4 and 6) and neutron (Tables 19 to 22) data in the main text demonstrates that this holds true for the ambient dose equivalent in relation to the deep dose-equivalent index for both radiations in the case of plane parallel beams and isotropic irradiation, with some reservation about lower neutron energies. Calculations for electrons from the limited data referred to in the main text indicate that the ratio is close to if somewhat above unity for unidirectional beams at energies from about 5 to 20 MeV. Although the maximum dose equivalent occurring in the 30 cm thick slab phantom might be taken as a substitute for H,,d with high-energy radiations, it could be misleading to consider the dose equivalent at a depth of 10 mm in the slab as the derived quantity: Table D.5 shows that the ratio of the values of these parameters can exceed unity to a considerable degree. This matter has already been discussed in Section 4, where the potential utility of H*(lO) in relation to spectral distributions of radiation energy was noted. The data in Table D.5 are calculated from the following tables in the main text: 14, 16a (with 16c), 23,25a and 26. The congruence of the dose equivalent to the skin and the directional dose equivalent is generally demonstrated in Table D.6 for photons and neutrons, although there is appreciable uncertainty in the neutron values for some cases (Wittmann et al., 1985). Data for photons are calculated from Tables 8 and 12 in the main text; data for neutrons come from Tables 21 and C.2. The qualification mentioned in Section D.l regarding the averaging convention for unidirectional low-energy radiation applies here as well. The scanty data on electrons from around 5 to 20 MeV, alluded to in the main text, are also fairly congruent. For most practical uses, the environmental and index quantities tend to overestimate the effective dose equivalent and organ dose equivalents. This tendency is acceptable in most circumstances, but care should be observed if optimization is the prime objective of monitoring.

DATA FOR USE IN PROTECTION

AGAINST EXTERNAL

RADIATION

TableD.4. Ratioof the effective doseequivalent. HE, to theambientdose equivalent, H *(lo), for photonsand neutrons incident in a plane parallel beam on an anthropomorphic phantom and on the ICRU sphere Energy,

Photons, I-&/H* (10) AP

MeV

PA -

I-Z ii-* 1:o 1o-6 -7

1.0

1o-5

1.0

10-z

Neutrons, s/%X4(10) AP

PA

(0.50) (0.42) 0.43 0.48

(0.33) (0.26) 0.25 0.30 0.37 0.40 0.30

0:81 0.19

0.00

0.04

0.58 0.62 0.53 -

0.09

0.40

0.21 0.36 0.49

0.19 -

4:o lo-* 5.0 lo-*

0.24 0.42 0.59 0.73

0131

OTlO

;*; ;;I:

0.79

0.85 0.87 0.87

0.57 0.65 0.68 0.70

0.87 0.86 0.87 0.88

0.72 0.72 0.74 0.76

l:o 10" 1.5 loo

0.88 0.89 0.90 0.90

2.0 3.0 4.0 5.0

loo 10" 10" loo

6.0 8.0 1.0 1.4

10" 10' lo1 10'

E

::- -2 . 1.5 lo-* ;.; $2 2

;."5;;I: . ;-“o $ 4:o 10-l 5.0 10-l ;*; ;;I:

-

0129

0.08

0.31

0.07 -

0.34

OT12

0.78 0.80 0.82 0.84

0:42 0.51

0:16 0.24

0.90 0.92 0.94 0.95

0.85 0.87 0.90 0.92

0.61 0.69 0.73 0.87

0.34 0.46 0.53 0.65

0.96 0.98 0.98

0.93 0.95 0.96

0.91 0.91 0.92 (0.92)

0.69 0.71 0.72 (0.80)

See Section 3.6 for irradiationgeometries. Dash entries arise because of the different energies of photons and neutrons in source tables.

REPORT

128

OF COMMITTEE

3

Table D.5. Ratio of the maximum dose equivalent and the dose equivalent at a depth of 10 mm for various radiations incident normally in a plane parallel beam on a semi-infinite slab phantom Radiations

Energy,

Mev

Y

II+

10

2.86

1.65

20

5.32

1.73

14.1

2.11

35.9

P

e

1.14

-

1.14

1.16

-

1.13

1.34

4.73

1.17

1.72

5.09

1.35

n-

n

1.91

1.11

50

10.8

100

16.5

200

22.9

1.37

1.36

500

27.9

1.30

1.34

1,000

32.1

1.12

1.18

2,000

40.3

1.59

1.39

5,000

39.8

1.90

3.00

3.20

10,000

64.2

2.37

3.18

3.77

20,000

52.0

tables.

1.62

arise

1.32

2.26 2.64

4.39 3.93

100,000

Dash entries

1.84

1.31

because of the lack of data in the source

DATA FOR USE IN PROTECTION

Table D.6.

Ratios

H' (0.07), for photons

AGAINST EXTERNAL RADIATION

of the skin dose equivalent, H,k.to the directional dose equivalent, and neutrons incident in various geometries on an anthropomorphic phantom and on the ICRU sphere Neutrons

Photons Energy, Mev

AP

IS0

AP

0.85 0.94 0.91 0.76

1.0 1.0 1.0 1.0

1o-6 1o-5 1o-4 lo-’

1.0 1.5 2.0 3.0

lo-* 1o-2 1o-2 lo-*

0.51 0.49 0.51 0.54

0.99 0.99 0.90 0.82

4.0 5.0 6.0 8.0

lo-* lo-* 1O-2 1O-2

0.57 0.60 0.64 0.68

0.84 0.87 0.88 0.91

1.0 1.5 2.0 3.0

10-l 10-l 10-l 10-l

0.69 0.73 0.75 0.76

0.92 0.95 0.96 0.94

4.0 5.0 6.0 8.0

10-l 10-l 10-l 10-l

0.78 0.80 0.82 0.83

0.94

0.94 0.94 0.93

0.62

1.0 1.5 2.0 3.0

10” 10” lo0 10”

0.86 0.87 0.89 0.90

0.93 0.94 0.94 0.94

0.73

4.0 5.0 6.0 8.0

loo loo lo0 10’

0.91 0.92 0.93 0.92

0.94 0.94 0.95 0.96

1.0 1.4

10’ 10’

0.92

0.96 -

See Section 3.6

for irradiation

0.59 -

0.48

0.83

Lo5

1.09 1.35

geometries.

Dash entries arise because of the different energies for photons and neutrons in the source tables. For unidirectional irradiation at low energies, the values should be doubled to obtain the skin dose equivalent averaged over the side of the body nearer the source. See Section D.l.

129

130

REPORT OF COMMITTEE 3

D.3. Relevance to Individual Monitoring

Some of the data in the foregoing section are also relevant to individual monitoring. For strongly penetrating radiation (ICRU, 1985), an appropriate monitoring depth on the body of an individual is 10 mm, as specified in the definition of H,(d), the individual dose equivalent, penetrating; for weakly penetrating radiation, 0.07 mm is appropriate, in line with the definition of H,(d), the individual dose equivalent, superficial. Anteroposterior information from Table D.4 will serve to transform the dose equivalent recorded by an individual monitoring device worn under 10 mm of tissue-equivalent material on the surface of the body to effective dose equivalent for photons and neutrons. Similarly, the information from Table D.6 may be used to transform the dose recorded under 0.07 mm of superficial tissue-like material to skin dose equivalent. The comment in Section D.2 on optimization applies equally here. There is a burgeoning literature on the application of both the individual and environmental ICRU quantities, as exemplified by the proceedings of a special seminar on the subject. (Booz and Dietze, 1985), and guidance has been issued (NRPB, 1986) on their application in radiation protection.

Booz, J. and Dietze, G. (eds) (1985). Radiation protection quantities for external exposure. Proceedings of a seminar held at Braunschweig, 19-21 March 1985. Radial. Prot. Dosim. 12(2). ICRP (1977). Recommendations ofthe International Commission on Radiological Protection, Publication 26. Pergamon

Press, Oxford. ICRU (1985). Determination of Dose Equioalents Resulting from External Radiation Sources, Report No. 39. International Commission on Radiation Units and Measurements, Bethesda, Maryland. Kramer, R. and Drexler, G. (1979). Practical implications of the concept of dose-equivalent index. In: Application ofthe Dose Limitation System for Radiation Protection, IAEA-SR-36130, pp. 451467. International Atomic Energy Agency, Vienna. NRPB (1986). New Radiation Protection Quantities Recommended by ICRU for Practical Use in Radiological Protection: Their Implementation in the United Kingdom, NRPB-GSS, Chilton. National Radiological Protection Board. Wittmann, A., Morhart, A. and Burger, G. (1985). Organ and effective dose equivalent. Radiat. Prot. Dosim. 12, 101-106.