effective atomic number studies in different body tissues...

Indian Journal of Pure & Applied Physics Vol. 40, lune 2002, pp. 442:-449

Effective atomic number studies in different body tissues and amino acids

K Singh* & Gagandeep

Department of Physics, Guru Nanak Dev University, Amritsar 143005

Received 9 July 200 I; revised 21 November 200 I; accepted 22 April 2002

Effective atomi c numbers (Zcff) of body tissues and amino acids have been calculated theoretically over a wide energy range from 10.3 to 105 MeV. The significant variations in Zcrrdue to composition of the material and domination of different interaction processes in different energy regions were observed .

1 Introduction

With extensive use of gamma-active isotopes in medicine, industry and agriculture, the study of effective atomic numbers is very important. Radiation shielding exists to protect people against nuclear particles and electromagnetic radiations. Allowable human exposure to these radiations is related to their biological significance. As a sequel to the previous work of the authors I · ? in different composite materials, the present calculations are made in different body tissues and amino acids in order to investigate the changes in effective atomic numbers for partial and total interactions for their chemical composition. The energy absorption in a given medium can be calculated by means of wellestablished formulae if certain constants are known . These necessary constants are the effective atomic number and electron density of the medium. The parameter, "effective atomic number" has a physical meaning and allows many characteristics of a material to be visualized with a number.

In recent years, most of the work has been done -extensively in pure metals, both experimentally as well as theoretically . But in case of composite materials , the study on the behaviour of photon interaction processes such as total, photo, coherent, incoherent, p<fir production, etc . is quite scarce and is available in limited energy range for one or another interaction only .

It was pointed out by HineH that, the effective atomic number for materials composed of various elements cannot be expressed by a single number and for each of different processes by which gamma rays can interact with matter, the various atomic

numbers in the material have to be weighted differently. Rama Rao et aU investigated the effective atomic numbers of the water, perspex, monel metal and tungsten steel for gamma rays of energies 662 and 1332 keY using a modified narrow beam geometry with a scintillation spectrometer. At both these energies, the values of Zeft were found to be nearly equal. Parthasaradhi lll determined the effective atomic numbers in some alloys in the energy range 100-662 ke V, where Zefl values for partial processes remained constant whereas for total interactions decreased with increasi ng energy. Lingam et at. 11 calculated Zcff of some halogen compounds in the energy range 33-662 ke V and their results were in conformity with the results of Parthasaradhi Ill. Measurements on substances containing H, C and 0 in the energy range 54-1332 keY were also performed by EI-Kateb & Abdul Hamid l2

• They pointed out that the effective atomic numbers in these types of material s tend to be constant as a function of energy and contributions from photoelectric effect and pair production are not significant. Bhandal et atY, Bhandal & SinghJ

·4 and

Gill et at.? found significant variations in the effective atomic numbers of multielement materials such as fatty acids, cements, solid state nuclear track detectors, some biological materials , glasses and rocks with photon energy and chemical composition of multielement materials.

Many attempts have been made to find a rule for the calculation of Z eiT' Some empirical formulations deduced have been reported by Schatzler 1:l but their validity is limited to the experimental conditions used in the particular work. It has been suggested by Gigante l4 that, there is a direct relationship between

SINGH & GAGANDEEP: EFFECTIVE ATOMIC NUMBER OF TISSUES 443

effective atomic number and coherent to Compton scattering (RIC) ratio i.e., it is possible to determine ZefT directly from the RIC measurements . Yang et 01.1 5 introduced a new method in which the total photon interaction cross-sections per electron of human tissues were used to define effective atomic numbers. They observed that, ZefT values are equal within 4% from 10 to 200 keV in each soft tissue and the variation was observed in case of bone.

Keeping in view the importance and usefulness, a thorough study of effective atomic numbers, based on theoretical computations, of some amino acids and body ti ssues, has been conducted for partial and total interaction processes over a wide energy range from 10.3 to lOS MeV.

2 Calculations

From the chemical compositions (Tab le I) of body tissues 1\ mass attenuation coefficients (~p) of these samples were calculated with the help of computer program and data base developed by

Berger & Hubbell 17 for all photon interactions. The effective cross sections in barns/atom were computed from the calculated mass attenuation coefficients with the help of following relation :

. .. (I )

where NA is the Avogadro's number and W i and Ai

are fractional abundance by weight and atomic weight respectively of the constituent elements. With the help of piece-wise interpolation computer program, the effective atomic number values of substances under study were obtained for all photon interaction processes .

The effective atomic numbers for amino acids were also determjned by the same procedure. About seven samples of amino acids having hydrogen , carbon and oxygen in common amongst themselves

Table I - Chemi cal co mpositions of some body ti ssues (adult )

S. Body Chemical composition No. tissue

H C N 0 Na P S Cl K Fe

I Blood 0.102 0.110 0.033 0.745 0.001 0.001 0.002 0.003 0.002 0.001 2 Brai n 0. 107 0. 145 0.022 0.712 0.002 0.004 0.002 0.003 0.003 3 Heart 0. 104 0. 121 0.032 0.734 0.001 0.001 0.002 0.003 0.002 0.001 4 Thyroid 0.104 0.119 0.024 0.745 0.002 0.001 0.001 0.002 0.001 0 ,001 5 Breas t 0,115 0.387 0.498

Table 2 - Chemical compositions of some amino acids

S, Amino acid Chemical compositio n No,

H C N 0 CI S

L-Arginine monohydro-chloride 0,07177 0.34209 0,26595 0.15190 0.16829 C~H IsN40 2Cl

2 DL-Aspartic acid 0.05301 0.36095 0,10523 0.48081 C4H7N04

3 L-Cysteine monohydro-chloricle 0,05116 0.22861 0,08860 0,20301 0.22492 0,20344 C, HxN02SCI

4 L-Cysteine 0,05033 0.29989 0,11657 0,26632 0.26688 C~H I2 20 4S2

5 L-H ydroxy proline 0,069 18 0.45798 0,10681 0,36603 CSH9NO,

6 L-Proline 0,07879 0,52 162 0,12166 0,27793 CSH9N02

7 DL-Tryptophan 0,05922 0.64693 0.64693 0.15668 CIIH I2 N20 2

444 INDIAN J PURE & APPL PHYS, VOL 40, JUNE 2002

8.0

7 .5 ..... - ... ... .. , I Thyroid

.......... - .. -7 .0 A Blood

Heart Brain

......... .... . . ...... ..

6 .5 .Jl -~ .. _'\ 6.0

B~a~:-'

/ l -----5 .5

5 .0

0 .001 0 .01 0 .1 1 10 100 1000 1 0000 1 00000 Energy (MeV)

Fig. I - Plot of Z cff versus energy for some body tissues for photoelectric effect

11

10

9

8 'Iii

"" 7

6

5

0 .00 1 +-~~~~~~~~-r~~~-r~~~~~~~~~~~~~~mm-~~~

0 .01 0 . 1 1 10 100 1000 10000 100000 Energy (MeV)

Fi g. 2 - Plot of ZeiT versus energy for some amino acids for photoelectric effec t

were taken for the purpose. The chemical composition of the amino acids under study is given in Table 2. To see the effect of weight fraction of the element to the effect ive atomic number, the amino acids were selected, such that , there is a significant variation of weight fraction of N, CI and S in the respective amino ac id .

3 Results and Discussion

3.1 Photoelectric absorption

The Zdr values as a functi on of photon energy for body ti ssues and amino ac ids have been plotted in Figs I and 2 respecti vely. The behaviour of Z cff with

respect to energy is rather interesting in the low energy region. In case of amino acids, the changes are more pronounced in substances containing relatively high Z e lements i.e . Cl and S. The effective atomic numbers increase more rapidly with the increase in energy up to I MeV . From 1-30 MeV, Z cff remains almost constant. Thi s variati on is in line with the finding of Perumallu et a/. 1x who reported that, Zen of a ir, muscle and bone equivalence increases with the increase of energy from 10-150 ke V and not with those of Parthasaradhi 10 who reported that Z Clf values of alloys remain nearly constant in the energy region 100-662 keV. At higher energies, there is a sudden

~ N

,..:p

6.0

5 .8

5 .6

504

5.2

5 .0

4 .8

4.6

4 .4

4 .2

4 .0

0 .001

8.5

8.0

7.5

7 .0-

6 . 5

6 .0

5 .5


0.01 0 .1 1

BlOOd Thyroid H_rt Brain

10 Energy (MeV)

100 1000 1 0000 1 00000

Fig. 3 - Plot of Z efT versus energy for some body tissues for scattering process (coherent)

._\

_, C e H,.,N .. 0 2CI

/ C4H7N~~_ L-___ . ___ .-£./=-----_

c H ~ 0

5 .0

~-----------~~C~5H~.N.O~

/-.... -.-.'~ ....... -._-_._ .-===::::== ... """===:::=::.\..--------------- C~HeN02

0 .01 0 . 1 1 10 Energy (MeV)

100 1000 10000 1 00000

Fig. 4 - Plot of Z efT versus energy for some amino acids for scattering process (coherent)

fall in Z eff and attains a constant value. This fall in Z efT is at higher energy in the case of substances having constituent elements of high Z value and at lower energy in case of materials having low Z element. Similar results were also obtained by Bhandal et al. l

, in case of fatty acids. In case of amino acids, C3HRN02SCI has the highest value of Z efT as sulphur and chlorine having higher values of atomic numbers also have higher values of their weight fractions in this compound. The Z efT values seem to depend upon the weight fractions of the constituent elements. Similar trend is observed in case of body tissues.

3.2 Coherent scattering

The variations of Z ell with photon energy for coherent scattering are shown in Figs 3 and 4 for body tissues and amino acids respectively. From these graphs it is clear that, variations in Z eff are simi lar for all biological samples in the low as well as in the high energy region. Z efT first increases with increase in energy up to 5 keY and then decreases with increase in energy . Again, it rises with further increase in energy and becomes constant when energy is increased up to 100 keY beyond which again it becomes independent of energy. There is a


sudden fall in ZeIT at about 20 MeV . After this, it is seen to be independent of energy for all types of materia ls. The results fo r this interaction are found to be similar to the findings of Bhandal & Singh

,·3 in

fatty acids and cements respectively.

Blood has the highest Zen va lue due to the presence of elements such as oxygen and nitrogen with higher weight fractions. ZeIT has the higher value for those amino acids, which are composed of higher atomic number elements.

3.3 Incoherent scattering

In case of incoherent scattering, (Figs 5 and 6) ZeIT sharply increases with increase in photon energy, becomes maximum and then decreases up to J 0 keY. Again, it ri ses with increase in energy and becomes constant when energy is increased beyond 100 ke V. The variation of Zcrr depends on the relative dominance of the partia l proce s. This in turn depends on the respecti ve proportion and range of atomic numbers of the e lements with which

----~- -.--- . ~ '::: ----------_ .. ,,-- ~ ~ ~ 3

2

• ""'1 i , ..... ,

0 .001 0 .01 0 . 1 1 10 100 1000 10000 1000bo Energy (MeV)

Fig. 5 - Plot of ZeIT versus energy for some body ti ssues for scattering process (coherent)

5 .0

4.5 C4~NO ..

_~ ______ ~ .. _~ ...... ~--~---~ ____ Lc:-. ___ -

4 . 0 ".---------...... ~ .. ~--------------::;.....~---------.".-.#.-.-------------~---=-

'"" N 3 .5

C.HgNO", - --.- .- .- .- -.- ---.--.-.----- .. -- ....•... . . . .. ---.-.- . - .- - - .----.--.- .- .-.- .- .- .- .-.--.-... - .:.:::.,;..,.. C::IH . NO:z

3 .0

2 .5

0 .001 0 .01 0 . 1 1 Energy (MeV)

10000 100000 10 100 1000

Fig. 6 - Plot of Zerr versus energy for some amino acids fo r scattering process (incoherent)


8 .5

8. 0

7 .5

7.0

6 .5

6 .0

'lii 5 .5 '"

5.0

4.5

4 .0 .--::~ ::

3 . 5

3 .0

0 .0 01 0 .01 0 . 1 1 1 0 Energy ( M e V )

1 0 0 1000 1 0000 1 000 00

Fig. 7 - Plot of Z<rr versus energy for some body ti ssues for total interaction process (coherent)

11

10

9

8

::= 7 N '"

6

5

C6H,2N 204S2

CBH ,SN 402C I

C 4 H 7 N 0 4

C " H,2N 202

C S H g N03

C S H g N02

... ~ ./ ~------------

.:: ........ .

.. --?~ . .. "\:---J.---~~----;;;;;...~~-.--.--.--.----.---.-.-.- .- .-.-

\ , .... -4 .,-.-._- --- ----.-- .

3 0 .0 0 1 0 .0 1 0 .1 1 10

E nergy (MeV) 100 1000 10000 100000

Fig. 8 - Plot of ZeIT versus energy for some amino acids for total interaction process (coherent)

biological sample is composed.

3.4 Total interaction process (coherent)

The plots of Zcrr as a function of energy for both body tissues and amino acids are shown in Figs 7 and 8 respectively. These plots clearly show the dominance of different interaction processes in different energy regions which is in line with Parthasardhi 10 who reported that Zen of alloys is different fo r different interaction processes.

For \ 0-\ 00 ke V, there is sharp decrease in ZeIT with energy up to 200 ke V, showing that

contribution of scattering process increases, which decreases ZeIT. This is also confirmed by Sastry & Inanananda ' 9 who confirmed that ZeiT of composite material for photoelectric interaction is greater than other processes. From 200 to 1200 ke V, Zerr is almost independent of energy. This may be due to the dominance of incoherent scattering in this energy region . Afte r this, there is a regular increase in ZeIT with photon energy up to 100 MeV . This behaviour is due to the mixed contribution of incoherent scattering and pair production interactions . Above this energy, Zerr remains almost constant. Thi s may be due to the dominance of pai r


6.6

6.4

6.2

6.0

N~ 5.8

5.6

5.4

5.2

5.0 'I iii Ii ill iii Ii iij .' i 1111'1 i i iiiiil ' i'iI"I i i Iliill i i iiih'

0.001 0.01 0.1. 1 10 100 1000 10000 100000

. Energy (MeV)

Fig. 9 - Plot of ZefT versus energy for some body tissues showi!1g the effect of absence of iodine element on Zcrf

in thyroid for photoelectric effect

production in high-energy region . The behaviour of all these composite materials is almost identical.

4 Conclusion

The study of effective atomic numbers of biologically important compounds is very useful for many technological app lications. The behaviour of ZefT with respect to energy is rather interesting in the low energy region. In the low energy region , photoe lectric interaction is dominant. This effect is evident from the Fig. 9, which shows the minimum value of Zen without the presence of iodine in Thyroid . Whereas, it attains the maximum value for Thyroid in the presence of iodine, even if it is present in a small fraction (Fig. 7). This variation is because of the Z (Ref. 4,5) dependence of photoelectric effect. So, thi s body ti ssue has the maximum value of effective atomic number in the photoelectric effect region . The ZefT values depend upon the weight fractions of the constituent e lement and the results are clearly evident to the view point of HineR.

Acknowledgement

The authors are grateful to UGC, New Delhi , for providing financial assistance fo r the research work.

References

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8 Hine G J, Phys Rev, 85 ( 1952) 725.

9 Rama Rao J, Lakshminarayana V & Jn anananda, J Sci /ndusl Res, 20B (196 1) 587.


10 Parthasaradhi K, Indian J Pure Appl Phys, 6 (1968) 609 . 15 Yang N C, Leichner P K & Hawkins W G, Med Phys, 14

II Lingam S C, Basu K S & Reddy D V K, Indian J Phys, (1987) 759.

58A (1984) 285 . 16 ICRU Report, Washington, DC, 46 ( 1992).

12 EI Kateb A H & Abdul Hamid A S, Appl Radiat Isot, 42 17 Berger M 1 & Hubbell 1 H, NBSIR, 87-3597 ( 1987). (1991) 303.

18 Perumallu P, Nageswara Rao & Krishna Rao G, Physica I , 13 Schatzler K S R, Int J Appl Radiat Isot , 30 ( 1979) 115. 32C (1985) 388.

14 Gigante G E, Nuclear analytical techniques in medicine 19 Sastry K S R & lnananda S, J Sci Indust Res , 17B (1958) (Ed) R Cesareo (Elsevier, New York), 1988. 389.

effective atomic number studies in different body tissues...

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