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 Linear Energy Transfer (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 potection practice

Cucinotta, Francis A.

Townsend, Lawrence W.

Wilson, John W.

English

NASA Technical Reports Server

- ' b  
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 W i  lson, Lawrence W ,  Townsend 
and Francis A ,  Cucinotta 
(hASA-TM-89055) C N  T E E  P O T E N I I A L  IMPACT OF 
T E E  N E R L Y  PROPOSED Q U A L I T X  E A C l C H S  CBI SPACE 
R A D I A I I C N  P E O l E C T I G B l  ( N A S A )  13 CSCL 068 
January 1987 
National Aeronautics and 
Space Administration 
Lengley RessMch Center 
Hampton, Virginia 23665-5225 
N e  7- 1 € 5 10 
Unclas  
63/52 4 3 3 6 6  
https://ntrs.nasa.gov/search.jsp?R=19870007077 2020-03-20T12:11:50+00:00Z
Abstract 
The recent ly proposed changes i n  the defined q u a l i t y  f a c t o r  hold great 
po ten t i a l  o f  easing some of the protect ion requirements i n  protect ion from 
the e lect ron and proton environments o f  the near earth environment. 
same t ime the high Linear Energy Transfer (LET) components play an even 
more important r o l e  which must be fu r the r  evaluated. 
recornendations are made which need t o  be addressed before these new 
qual i t y  factors  can be implemented i n t o  space rad ia t i on  protect ion 
pra c t  i ce. 
A t  the 
Several 
i 
1 
In t roduc t ion  
With the advent of proposed new recommendations on the q u a l i t y  f a c t o r  
Q t o  be used i n  r i s k  assessment (ref. l ) ,  it seems prudent t o  evaluate i t s  
impact on rad ia t i on  pro tec t ion  pract ice i n  the  space program. The most 
s i g n i f i c a n t  features of the new recommendations are the  increased 
importance o f  h igh LET rad ia t ions  ( p a r t i c l e s  w i th  a la rge  l i n e a r  energy 
t rans fer ,  LET) and the  question o f  the  standard reference rad iat ion.  
are both c r i t i c a l  issues i n  space r a d i a t i o n  r i s k  assessment. 
These 
Although the  new recommendations on h igh LET values o f  Q are o f  great 
importance, the question of  low LET values o f  Q i s  a lso o f  current c r i t i c a l  
in te res t .  I n  a l l  previous recommendations, although there has been a 
l i n g e r i n g  question about possible d i f ferences due t o  the choice o f  low LET 
reference rad iat ion,  it was assumed t h a t  Q was assumed t o  be u n i t y  f o r  LET 
less  than 35 keV/prn. However, the present experimental data base now 
confirms t h a t  s i g n i f i c a n t  di f ferences between 100 keV x-rays and cobal t  
(60) y a m  rays e x i s t  ( re f .  1). This f a c t  holds important p o t e n t i a l  
consequences f o r  r i s k  assessment i n  the space program since a s izeable 
f r a c t i o n  of  exposure comes from near minimum ion i z ing  charged p a r t i c l e  
exposure (- 0.2 keV/ptn) which may car ry  a smal ler  r i s k  than present ly  
assumed under the previous Q recommendations. 
I n  the present report ,  we make a pre l iminary evaluat ion o f  t he  
potent i a1 impact of these proposed new recommendations on space r a d i  a t  i on 
r i s k  assessment. There are several questions which NASA needs t o  consider 
concerning the methods used i n  r i s k  assessment, and i t  i s  recommended t h a t  
NASA requirements be c la r i f i ed .  
2 
The Q u a l i t y  Factor  
Although ion i z ing  r a d i a t i o n  i n t e r a c t i o n  occurs p r i m a r i l y  through the  
formation of  ions i n  materials, t he  spa t ia l  d i s t r i b u t i o n  of  the ions on a 
microscopic scale i s  found t o  be a c o n t r i b u t i n g  f a c t o r  i n  determining 
b i o l o g i c a l  response. 
d i f ferences i n  the spa t ia l  d i s t r i b u t i o n ,  and the q u a l i t y  f a c t o r  i s  
introduced i n  r i sk  assessment schemes t o  account f o r  t he  q u a l i t y  of the 
rad iat ion.  
predicted b io log i ca l  response and i s  re la ted  t o  the absorbed energy per 
u n i t  mass, the physical dose D, through the q u a l i t y  f a c t o r  Q as 
The term r a d i a t i o n  q u a l i t y  i s  appl ied t o  denote these 
The term equivalent dose, H, i s  taken as an i n d i c a t o r  o f  
H = Q D  
Exposure l i m i t s  are expressed i n  terms of  exposures t o  some reference 
r a d i a t i o n  fo r  which Q i s  assumed t o  be uni ty.  
assumed Q t o  be u n i t y  f o r  LET values less than 35 keV/pni. The current  
values given i n  the In te rna t i ona l  Conmission on Radiat ion Uni ts  and hkasure 
repor t  number 16 ( I C R U  16, see ref.2) f o r  Q a r e  given i n  t a b l e  I and shown 
i n  f i g u r e  1 i n  comparison t o  the func t i on  
A l l  previous recommendations 
12.5 en(1 + V437.5) S > 40 MeV/cm 
S > 40 PeV/cm (2) 
where S i s  the charged p a r t i c l e  stopping power i n  water (MeV/cm). 
re la ted  t o  LET (keV/pm) as 
This i s  
LET, I L = S/10 (3 )  
3 
New recommendations of quality factors are expressed i n  units of 
lineal energy given i n  International Comnission on Radiat ion Units and 
Measure report number 40 ( I C R U  40, see ref. 1) which have been expressed as 
an average qual i ty  factor for fixed LET and found t o  have the following 
relation (ref. 3):  
Values of (1 are shown i n  figure 1. The average quality factors f o r  
electrons and protons are shown i n  figures 2 and 3 as a function of 
particle energy over the range most important t o  space applications. 
clear from these figures t h a t  Q i s  less t h a n  unity over broad regions 
important t o  space radiation protection. This has the potential for great 
impact on current protection practices as applied i n  the space program. 
I t  i s  
Some Effects on Space S t a t i o n  Dose Assessment 
To begin the evaluation of the impact on the current space s t a t ion  
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 i n  water. The equivalent 
dose is evaluated using 
where Q p ( E )  is shown in figure 2. 
given as 
The average space quality factor is 
<Q> = H(x)/D(x) (8) 
The reduction i n  dose equivalent expected by the new recommendation is 
g i  ven by 
= * <Q> ICRU16 
and is seen i n  f i g u r e  4 t o  be on the order of 50-70 percent. 
effects on easing current space r a d i a t i o n  practices are enormous. A 
correspondingly larger reduction is expected i n  the case of electron 
exposure. 
The potential 
B u t ,  these reductions may prove i l l  usory. 
A Note of Caution 
Al though  the newly recommended values for  Q could ease protection 
requirements from low LET r a d i a t i o n s ,  the h igh  LET components of h i g h  
charge and energy (HZE) particles, nuclear reaction produced components 
(stars and neutrons), and secondary electrons all have relatively higher 
qua l i ty  factors  under the new recommendations. Clearly a program t o  
evaluate the relative importance of various components is  required t o  place 
methods used by NASA on a sound foundat ion  (see Appendix fo r  further 
det a i 1 ) . 
5 
Furthermore, t he  l i m i t s  o f  maximum permissible doses now i n  e f f e c t  
were based on previous reference exposures f o r  which Q i s  assumed t o  be 
u n i t y  below 35 keV/vm. Hence, the o l d  l i m i t s  may not general ly be 
appl icable w i t h  the new q u a l i t y  factors. This s i t u a t i o n  requires 
c l a r i f i c a t i o n .  
Recommendati ons 
There are several po ints  which need t o  be c l a r i f i e d  and some 
associated tasks are proposed: 
1. NASA needs t o  seek c l a r i f i c a t i o n  on the r e l a t i o n  between 
t h e  newly recomnended qua l i t y  f ac to rs  and the present ly 
recornended exposure 1 i m i t s  used i n  the space program. 
2. NASA needs t o  seek c l a r i f i c a t i o n  on the use of  the newly proposed 
q u a l i t y  f a c t o r  i n  ca lcu lat ions o f  low LET components (See Appendix). 
3. An evaluat ion of the va r ia t i on  o f  Q a t  low LET on 
e lec t ron  s h i e l d  design and dose assessment needs t o  be made 
f o r  t y p i c a l  space environments. 
4. A re-evaluation of  nuclear s t a r  e f f e c t s  i n  energetic 
proton exposures needs t o  be made. A carefu l  re-evaluat ion 
o f  the r e c o i l  energy i n  formation o f  nuclear s tars  i s  a 
c r i t i c a l  issue. 
5. 
re-eval uated. 
6. 
derived f o r  use i n  space sh ie ld  analysis. 
7. 
The role o f  neutrons i n  space sh ie ld ing  needs t o  be 
New dose kernels f o r  protons and electrons need t o  be 
The impact on dosimetry requirements needs t o  be assessed. 
References 
1. ICRP/ICRU Joint Task Group: The Q u a l i t y  Factor i n  Rad ia t ion  
Protection. ICRU Report 40, A p r i l  1986. 
2. In te rnat iona l  Commission on Rad ia t ion  Uni ts  and Measurements: L i n e a r  
Energy Transfer. ICRU Report 16, 1970. 
3. Townsend, L. W.; Wilson, J. W.; and Cucinotta,  F. A.: A Simple 
Parameterization f o r  Q u a l i t y  Factors From ICRU Report 40. 
Physics. (To be Pub1 ished). 
H e a l t h  
~ 
6 
7 
Table I - Current Recommended Values of  Quality Factor (ICRU16) 
LET,, keV/pm 
3.5 or l e s s  
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 f i r s t  impression o f  the  new Q i n  re f .  1 i s  that  there  i s  some 
mistake since such low values of Q f o r  minimum i o n i z i n g  r a d i a t i o n  
(LET-2.2 MeV/crn or  0.22 KeV/vrn are contrary  t o  our previous understanding 
a t  low LET. 
proposed Q i s  i n  f a c t  cor rec t  but must be proper ly  understood. 
example, t he  e f f e c t i v e  Q o f  photons was ca lcu la ted  wi th the  resu l t s  o f  
f i g u r e  3 and are i n  accord w i t h  the experimental observations o f  RBE. 
must, however, obtain f u r t h e r  understanding of the means by which f i g u r e  3 
i s  obtained since t h i s  could have a great po ten t i a l  impact on the  actual 
use of  the newly proposed Q. 
about the  
However, f u r t h e r  reading o f  the repor t  would imply t h a t  the 
For 
We 
I would mention t h a t  we have some doubts 
verac i ty  o f  f i g u r e  3, but must admit t h a t  we have not done the  
ca l  cu l  a t  i ons . 
The procedure f o r  ca l cu la t i ng  t h e i r  photon e f f e c t i v e  Q i s  as fo l lows:  
Consider the  exposure of  a t i ssue  sample s u f f i c i e n t l y  small so that  photon 
at tenuat ion may be neglected. The photons g ive r i s e  t o  a volume source o f  
e lect rons wi th the spectrum charac te r i s t i cs  of the Compton and 
photoe lec t r i c  processes by which they were formed. This e lec t ron  source 
spectrum i s  used t o  ca l cu la te  the  slowing down spectrum o f  e lect rons i n  the  
t i ssue  assuming the t i ssue  sample i s  l a rge  enough t o  keep the e lect rons i n  
equil ibrum. This f i n a l  e lec t ron  spectrum $Y(E)  i s  used t o  ca l cu la te  the 
e f f e c t i v e  % 
where EY i s  the Photon energy and Se(E) i s  the e lec t ron  stopping power 
i n  t i ssue  and Q i s  t he  LET dependent stopping power (see ref. 3 ) -  
- 
9 
If th i s  procedure in fact  produces the results of figure 3 of 
reference 1, then we must consider w h a t  procedures t o  apply t o  calculate 
the effective Q for  exposures with minimum ionizing radiation. 
One could propose t h a t  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 i n t o  secondary electron 
components, S k ( E p ) ,  where the total proton stopping power i s  
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 i s  correct, then we 
expect the main contribution t o  Q p ( E p )  t o  come from the electron term. 
I t  is obvious t h a t  Q p ( E p )  is probobly less t h a n  unity. 
0.75 as has been proposed i n  the past. 
Perhaps a b o u t  
The real question i s  this :  Is th i s  the intended use of Q as proposed 
by reference l? I t  is not clear froni the document i t se l f .  
Do note t h a t  such a Qp(Ep)  would be rode1 dependent since 
s d ( E p )  and S k ( E p )  are not precisely known and the secondary 
electron spectra for proton impact are uncertain. 
10 
Assuming t h a t  the photon Q valves o f  reference 1 are correct, we 
cannot avoid the question on how t o  uti1 ize the new recommendations fo r  
miniinurn ionizing radiations. Similar t o  photons,  minimum ionizing 
r a d i a t i o n  mediate a large fraction of the i r  energy through secondary 
low-energy electrons. Should we expect the low LET valves of Q t o  use such 
detailed descriptions o f  track structure i n  dose estimates? Has the ICRU 
made such calculations for high energy electrons and  protons? 
11 
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Standard Bibliographic Page 
. Re r tNo.  KSA TM-89055 2. Government Accession No. 3. Recipient's Catalog No. 
~ . Title and Subtitle 15. Report Date 
National  Aeronautics and Space Administrat ion 
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 . o b  
John W. Wilson and Lawrence W. Townsend, Langley Research Center, Hampton, VA. 
Francis A. Cucinotta, Old Dominion Univers i ty ,  Norfolk,  VA. 
6. Abstract 
The recen t l y  proposed changes i n  the def ined q u a l i t y  f a c t o r  ho ld great  
p o t e n t i a l  f o r  easing some o f  the protect ion requirements from e lect rons and 
protons i n  the  near-Earth environment. A t  the same time, the  h igh LET 
components p lay  an even more important r o l e  which must be f u r t h e r  evaluated. 
Several recommendations are made which need t o  be addressed before these 
new q u a l i t y  f a c t o r s  can be implemented i n t o  space r a d i a t i o n  p ro tec t i on  
p r a c t i c e  . 
L7. Ke Worda Su e a q  by Authors(s)) 
{pace h a g a t i o n ,  
P ro tec t i on  , 
B i o l o g i c a l  E f f e c t s  
18. Distribution Statement 
Unclassi f ied-unl  i m i t e d  
Subject Category - 52 
For sale by the National Technical Information Service, Springfield, Virginia 22161 
NASA Lang1.y Form 63 (June 1985) 


On the potential impact of the newly proposed quality factors on space radiation protection

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