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NASA Technical Memorandum 89055 

ON THE POTENTIAL IMPACT OF THE NEWLY 

PROPOSED QUALITY FACTORS ON 

SPACE RAD I AT I ON PROTECT I ON 

John W I Wi lson, Lawrence W, Townsend 

and Francis A, Cucinotta 

(hASA-TM-89055) CN TEE POTENIIAL IMPACT OF 

TEE NERLY PROPOSED QUALITX EAClCHS CBI SPACE 

RADIAIICN PEOlECTIGBl (NASA) 13 CSCL 068 

January 1987 

National Aeronautics and 

Space Administration 

Lengley RessMch Center 

Hampton, Virginia 23665-5225 

Ne 7- 1 € 5 10 

Unclas 

63/52 43366 
Abstract 

The recently proposed changes in the defined quality factor hold great 

potential of easing some of the protection requirements in protection from 

the electron and proton environments of the near earth environment. 

same time the high Linear Energy Transfer (LET) components play an even 

more important role which must be further evaluated. 

recornendations are made which need to be addressed before these new 

qual ity factors can be implemented into space radiation protection 

pra ct i ce. 

At the 

Several 

i 
1 

Introduction 

With the advent of proposed new recommendations on the quality factor 

Q to be used in risk assessment (ref. l), it seems prudent to evaluate its 

impact on radiation protection practice in the space program. The most 

significant features of the new recommendations are the increased 

importance of high LET radiations (particles with a large linear energy 

transfer, LET) and the question of the standard reference radiation. 

are both critical issues in space radiation risk assessment. 

These 

Although the new recommendations on high LET values of Q are of great 

importance, the question of low LET values of Q is also of current critical 

interest. In all previous recommendations, although there has been a 

lingering question about possible differences due to the choice of low LET 

reference radiation, it was assumed that Q was assumed to be unity for LET 

less than 35 keV/prn. However, the present experimental data base now 

confirms that significant differences between 100 keV x-rays and cobalt 

(60) yam rays exist (ref. 1). This fact holds important potential 

consequences for risk assessment in the space program since a sizeable 

fraction of exposure comes from near minimum ionizing charged particle 

exposure (- 0.2 keV/ptn) which may carry a smaller risk than presently 

assumed under the previous Q recommendations. 

In the present report, we make a preliminary evaluation of the 

potent i a1 impact of these proposed new recommendations on space radi at i on 

risk assessment. There are several questions which NASA needs to consider 

concerning the methods used in risk assessment, and it is recommended that 

NASA requirements be clarified. 
2 

The Quality Factor 

Although ionizing radiation interaction occurs primarily through the 

formation of ions in materials, the spatial distribution of the ions on a 

microscopic scale is found to be a contributing factor in determining 

biological response. 

differences in the spatial distribution, and the quality factor is 

introduced in risk assessment schemes to account for the quality of the 

radiation. 

predicted biological response and is related to the absorbed energy per 

unit mass, the physical dose D, through the quality factor Q as 

The term radiation quality is applied to denote these 

The term equivalent dose, H, is taken as an indicator of 

H=QD 

Exposure limits are expressed in terms of exposures to some reference 

radiation for which Q is assumed to be unity. 

assumed Q to be unity for LET values less than 35 keV/pni. The current 

values given in the International Conmission on Radiation Units and hkasure 

report number 16 (ICRU 16, see ref.2) for Q are given in table I and shown 

in figure 1 in comparison to the function 

All previous recommendations 

12.5 en(1 + V437.5) S > 40 MeV/cm 

S > 40 PeV/cm (2) 

where S is the charged particle stopping power in water (MeV/cm). 

related to LET (keV/pm) as 

This is 

LET, I L = S/10 (3) 
3 

New recommendations of quality factors are expressed in units of 

lineal energy given in International Comnission on Radiation Units and 

Measure report number 40 (ICRU 40, see ref. 1) which have been expressed as 

an average quality factor for fixed LET and found to have the following 

relation (ref. 3): 

Values of (1 are shown in figure 1. The average quality factors for 

electrons and protons are shown in figures 2 and 3 as a function of 

particle energy over the range most important to space applications. 

clear from these figures that Q is less than unity over broad regions 

important to space radiation protection. This has the potential for great 

impact on current protection practices as applied in the space program. 

It is 

Some Effects on Space Station Dose Assessment 

To begin the evaluation of the impact on the current space station 

shield design, the space station proton environment is approximately 

#(E) = 3.63 x lo4 ,41243, p/cm2-MeV-day (5) 

The physical dose and equivalent dose for the two quality factors have been 

calcul ated using 

where E(x) denotes the energy vs. range relation in water. The equivalent 

dose is evaluated using 
where Qp(E) is shown in figure 2. 

given as 

The average space quality factor is 

<Q> = H(x)/D(x) (8) 

The reduction in dose equivalent expected by the new recommendation is 

gi ven by 

= * <Q> ICRU16 

and is seen in figure 4 to be on the order of 50-70 percent. 

effects on easing current space radiation practices are enormous. A 

correspondingly larger reduction is expected in the case of electron 

exposure. 

The potential 

But, these reductions may prove ill usory. 

A Note of Caution 

Although the newly recommended values for Q could ease protection 

requirements from low LET radiations, the high LET components of high 

charge and energy (HZE) particles, nuclear reaction produced components 

(stars and neutrons), and secondary electrons all have relatively higher 

quality factors under the new recommendations. Clearly a program to 

evaluate the relative importance of various components is required to place 

methods used by NASA on a sound foundation (see Appendix for further 

det a i 1 ) . 
5 

Furthermore, the limits of maximum permissible doses now in effect 

were based on previous reference exposures for which Q is assumed to be 

unity below 35 keV/vm. Hence, the old limits may not generally be 

applicable with the new quality factors. This situation requires 

clarification. 

Recommendati ons 

There are several points which need to be clarified and some 

associated tasks are proposed: 

1. NASA needs to seek clarification on the relation between 

the newly recomnended quality factors and the presently 

recornended exposure 1 imits used in the space program. 

2. NASA needs to seek clarification on the use of the newly proposed 

quality factor in calculations of low LET components (See Appendix). 

3. An evaluation of the variation of Q at low LET on 

electron shield design and dose assessment needs to be made 

for typical space environments. 

4. A re-evaluation of nuclear star effects in energetic 

proton exposures needs to be made. A careful re-evaluation 

of the recoil energy in formation of nuclear stars is a 

critical issue. 

5. 

re-eval uated. 

6. 

derived for use in space shield analysis. 

7. 

The role of neutrons in space shielding needs to be 

New dose kernels for protons and electrons need to be 

The impact on dosimetry requirements needs to be assessed. 
References 

1. ICRP/ICRU Joint Task Group: The Quality Factor in Radiation 

Protection. ICRU Report 40, April 1986. 

2. International Commission on Radiation Units and Measurements: Linear 

Energy Transfer. ICRU Report 16, 1970. 

3. Townsend, L. W.; Wilson, J. W.; and Cucinotta, F. A.: A Simple 

Parameterization for Quality Factors From ICRU Report 40. 

Physics. (To be Pub1 ished). 

Health 

~ 

6 
7 

Table I - Current Recommended Values of Quality Factor (ICRU16) 

LET,, keV/pm 

3.5 or less 

3.5 - 7.0 

7.0 - 23 

23 - 53 

53 - 175 

Q 

1 

1-2 

2-5 

5- 10 

10-20 
8 

Appendix 

Calcuational Methods 

One's first impression of the new Q in ref. 1 is that there is some 

mistake since such low values of Q for minimum ionizing radiation 

(LET-2.2 MeV/crn or 0.22 KeV/vrn are contrary to our previous understanding 

at low LET. 

proposed Q is in fact correct but must be properly understood. 

example, the effective Q of photons was calculated with the results of 

figure 3 and are in accord with the experimental observations of RBE. 

must, however, obtain further understanding of the means by which figure 3 

is obtained since this could have a great potential impact on the actual 

use of the newly proposed Q. 

about the 

However, further reading of the report would imply that the 

For 

We 

I would mention that we have some doubts 

veracity of figure 3, but must admit that we have not done the 

cal cul at i ons . The procedure for calculating their photon effective Q is as follows: 

Consider the exposure of a tissue sample sufficiently small so that photon 

attenuation may be neglected. The photons give rise to a volume source of 

electrons with the spectrum characteristics of the Compton and 

photoelectric processes by which they were formed. This electron source 

spectrum is used to calculate the slowing down spectrum of electrons in the 

tissue assuming the tissue sample is large enough to keep the electrons in 

equilibrum. This final electron spectrum $Y(E) is used to calculate the 

effective % 

where EY is the Photon energy and Se(E) is the electron stopping power 

in tissue and Q is the LET dependent stopping power (see ref. 3)- 

- 
9 

If this procedure in fact produces the results of figure 3 of 

reference 1, then we must consider what procedures to apply to calculate 

the effective Q for exposures with minimum ionizing radiation. 

One could propose that the energy deposite from high energy protons 

(neglect high LET secondaries in the present discussion which would be 

treated by the customs of the past) be partitioned into secondary electron 

components, Sk(Ep), where the total proton stopping power is 

Letting the equil i bruim secondary el ectron spectrum be 4 p( E) , then 

The corresponing effective Q for protons would then be 

If the resultant photon calculation of reference 1 is correct, then we 

expect the main contribution to Qp(Ep) to come from the electron term. 

It is obvious that Qp(Ep) is probobly less than unity. 

0.75 as has been proposed in the past. 

Perhaps about 

The real question is this: Is this the intended use of Q as proposed 

by reference l? It is not clear froni the document itself. 

Do note that such a Qp(Ep) would be rode1 dependent since 

sd(Ep) and Sk(Ep) are not precisely known and the secondary 

electron spectra for proton impact are uncertain. 
10 

Assuming that the photon Q valves of reference 1 are correct, we 

cannot avoid the question on how to uti1 ize the new recommendations for 

miniinurn ionizing radiations. Similar to photons, minimum ionizing 

radiation mediate a large fraction of their energy through secondary 

low-energy electrons. Should we expect the low LET valves of Q to use such 

detailed descriptions of track structure in dose estimates? Has the ICRU 

made such calculations for high energy electrons and protons? 
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Standard Bibliographic Page 

. Re rtNo. KSA TM-89055 2. Government Accession No. 3. Recipient's Catalog No. 

~ 

. Title and Subtitle 15. Report Date 

National Aeronautics and Space Administration 

Washington, DC 20546 

. Author(s) 

John W. Wilson, Lawrence W. Townsend and 

14. Sponsoring Agency Code 

8. Performing Organization Report No. 

20. Sec ity C if.( f this page) 1 Uncyass #i ea 

19. Securit Clasejf pf thie report) 

Unci ass i #I ed 

10. Work Unit No. 

Francis A. Cucinotta 1. Performing Organization Name and Address 

21. Nrr of Pages 22. Price 

16 A02 

Langley Research Center 

Hampton, VA 23665 

2. Sponsoring Agency Name and Address 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

5. Supplementary N.ob 

John W. Wilson and Lawrence W. Townsend, Langley Research Center, Hampton, VA. 

Francis A. Cucinotta, Old Dominion University, Norfolk, VA. 

6. Abstract 

The recently proposed changes in the defined quality factor hold great 

potential for easing some of the protection requirements from electrons and 

protons in the near-Earth environment. At the same time, the high LET 

components play an even more important role which must be further evaluated. 

Several recommendations are made which need to be addressed before these 

new quality factors can be implemented into space radiation protection 

practice . 

L7. Ke Worda Su eaq by Authors(s)) 

{pace hagation, 

Protection , Biological Effects 

18. Distribution Statement 

Unclassified-unl imited 

Subject Category - 52 

For sale by the National Technical Information Service, Springfield, Virginia 22161 

NASA Lang1.y Form 63 (June 1985)