Ind i:ln .l ourn:l l or Pu re & Appl ied Ph ysics " i :1'). Septemher 20() 1. pp. 553-5()()

Studies of natu~al ! a_dioactivity of some Egyptian rockj>J!osphates N K Ahmed, A Abbady, A H EI - Kame l *'!JS te inh ausle~** alJd' ~-Arab i -

~sics DepmtmenL Facult y or Science Qen;), Sou th Valley Universi ty ~ \

\. 1 ro' C;; 1.- 1 u>/ "'Ph ysics Department. Faculty or Science Assiul. Assi ut Univers ity

':'''' Instit ute or Physics and Biophysics. Salzburg Uni versit y \1.-- v(rl l Rece ived 15 October 199<): revised 25 Apri l 200 1: accepted 23 June 200 1 1;

'e(' Ra . e" U. c\'T h and .III K act ivity concentrat ions (Bq/kg) in some Egyptian phosphate samples have been measured ",ill~ II PG,' )2,1111111<1 spectrometer. Also their rad ium equ i va len t activities were calcu lated and cliscusse~

I nl roductioll

W hen phosphate prec i pitates from water, uranium goes with it. As a result, for example, ur;tntU Ill I ~ a sen ous contamin ant in phosphate k rli l i 'l.LT~ that are ubiquitous in crop fanning. Alth()ugh Illost crops res ist uptake of radi oact ive lll; llcri;t\ s in the ir leafy (a bove-grou nd) components, Ilwsl' LT() p.S wh()se r()ots are consumed (such as p()ta toes . pea nu ts , carrots). are susceptible to conta min ati () n hy ura nium.

T ht, t1i str ihllti on ()r n;lturall y-()cc urring uraniu m. rad()n. :lIld other rad ioac ti ve elements. radi o nuc lide, depend s on the type of the rocks fro m which they ori ginare and the processes whi ch concentrate them.

I«()ck phosphate contain s hi gh leve l of uranium­y):'i and rad ium-226. Up to 1974 the US-phosphate ind ustrv has mined about 37 % more U,Ox than the US-uran iu m minin g industry'. In the prod uct ion of fc rtili 'l.l'r the hv-product phosphogy psum is recyc led as a hu il di ng mate ri a l and can cause elevated lun g ex posure or Ille mbers or the public typi ca ll y about 15 lillle~ hi gher thall normal leve ls' . Due to the large all1()u nts or fe rtili zer lI sed in agricul ture there is a lso an environmental impact. because the so il receives in add iti on to the so il acti vity of the average II Bq/ lll ' year or rad ium-226 in Western Europe".

Durin g handling. pack ing and transport of fc rtili LLT some workers can receive additi onal cx tern ;t\ ~ammCl ex pos ure at dose rates up to 0 .8 pCiv/ 1l

Phosphat e ores contain important amounts of 1l:lt m;t\ r;lc1io;tctivc c lements . spcc iCi ll y 21X U and

descendants. in contracti ons that depend on il s geographica l and geologica l ori gin , but which can reach up to 300 ppm for uranium. Sedimentary and igneous phosphate ores are used as raw materi a l for the producti on of phosphoric ac id. and. consequentl y, ferti Ii zers fo r agricultural purposes. Large-sca le production of phosphoric ac id resul ts in the redi stribution of huge amounts of natural rad ioacti vity. Depending on the technology used to produce fertili zer or phosphori c acid. the distributi on of radi oacti ve elements among the various prod ucts would be diffe rent" . Sometillles fe rti I izers and a common hy-prod ucl. phosphogy psum, contain such high ac ti vit ies of natural radioact ive e lements thai the result ing radi o logica l impact should be considered carefull y)'''.

The Egyptian phosphates are widely di stri buted in many localities on the Red Sea coast, Nil e Valley and Western Desert. The depos its in the fir st two di stri cts are relati ve ly rich in phosphate and are explo ited at several mines, but those of the western Desert are of low grade except at Abu-Tartur mine.

2 Experimental Details

Environmental radi at ion measurements are usuall y made to identify the radi onuclide sources in the environment. quantify their concentrati on and estimate their dose contri buti ons. Thi s is most readil y done by spectrometric technique, but simpler measurements can often yield relevant informati on. In each case, an essential part of the system is methodology assoc iated with detec tor ca li brat ion .

:'5-1 INOI AN.J PURE & APPL PHYS. VOL 39. SEPTEMBER 2001

mcasurement procedure. and daw analysis and i ntL'l'pretat i Oil .

Th L' prese nt 'vvork dea ls with the measuremenh ()r Ihc nalur,t! raelioacli vity concentrati on or some i-:!! ), pti;ln phosphat e rocks In Western Destert ,( ; ·\hll -l';lrlllrl: il L' Va ll ey (G .Sari a. G.Ow in'l and (; t'\ hu -Saholln ) and Estern [)eserl (G.Duwi and Ci AnI. )

The 22hRa. 2" U. ~12Th and 411 K act ivit y

C(1 ll ccntrat ions (Bq/kg) III the phosphate rock s ail ip le .~ were measured hy a HPGe gamma spec trometer. Uraniulll is calculated from the counts reCI: I veel hy I he gamma-ray detector in the energy \\ Illciow cmresponcii ng to bismuth-214 . This ICl'h niqul' ;ISS UIllC.' that uranium and its decay prllllllCl s ;1I'l' ill sec ular equilibrium. The same 1,' l'l lI1 lqll l' I" lI:--c<i t () es timatl' K--Hl. and eq ui va lent I hmllllll -2T2.

The appli ed low le ve l gamma-ray spectrometer cOll sists hasica ll y of high purity germanium detec tor \V ith it s c lectronic c ircuit s. The detector is coax ia l c l()seci elld. c losed fac in g window geo metry with \iL'l't ie t! d ips t id () ()()-R ()()p ) Thc HPGe detec tor (j[Jvl is P-t ypl' ,vith the following specificat ion,s:

- I<cs(ll ill iflil (FWHM) at 122 keV . ;7CO IS I I ()() I'V

- i<.c slI IIIII () 1l (FWI-IM) al I.J:-1 MeV . ',IICO IS 2( )() keY Lit 1.7') keY

- Peak -tn-Compton rati o. "IICO is 46

- Relative ellic ie ncy at 1,:-13 MeV. ('IICO is 20%

I

~ I ':1

t';. Acu- T ~ r t ur

. Knarga

IJ ESTERN DESE RT

- Operation bias voltage is + 20()() V dc.

The detector is shie lded in a 'chamber ()r four layers starting with pl ex ig lass ( I () mm thick) . COppCI (30 mm thick). lead (100 mm thi ck) and filla" v cad mium (3 mm thick),

In the work presented. the spec trLI or stancLi rd sources with known gamma-ray em issi on rate. usually with few gamma lines, were measured as a function of two parameters : effic ie ncy-energy dependence and e fficienc y-sample volume dependence.

Calibration was performed uSing liquid Illulti ­nuclide standard solution PTB-93754 contain (F3e -7. Mn-54. Co-57. Zn-65. Y-88. Te- Ln. 13a-l3.3 and C~- 1 37) with activitie ,~ of, 6.22. <,).1 6. 4 .3.3. 15 .. '1 ,

6.23. 2.l) 1. 7.67 and 15 .7 kBq. re <; peC't ivc l\ . Background cou nt rates interLlce With till' measureme nt of minute quantiti es of rad i onuclide~

Therefore. data reduction proced ures used wi th analytical system involve ;\ correc ti on ror background counts and the liqui d multinuclidt' standard source. to appropriate the powdered sa mpl es, analysis ORTIC} .

based on the Illeasure me nt program-Gamma-Vision 2. 1

s pectrul1l (EG&(~

As man y as 71 sa mpl e~ or ph os phOri te ,1Il1i phosphatiC' rocks were co llec ted fr0m I)U WI Formation (Maest ri chti on age)" at Eastern Desert (G.A nz and G .Duwi). Ni le Valle y (G.Sa ri ,1. G.Owina and G.Abu-Saboun) and Western Dese rl (G.Abu-Tartur) sec ti ons (Fig. I). For each mount ~l ill

in the six sampling area 6-22 layers were defined:

• Saf"ga

Quse ,~ /1 G.Anz

EC3 I G.Du,", '

ESTERN DESEI1 T

SO Km

Fig. 1- LnGll ioll Ill~p orthc <lrc~ll1ndcr in vcsligalioll

AHM ED el a/.:EGYPTI AN ROCK PHOSPHATES 555

where the first layer started from the top of the lllount;l in and Ihe last layer is at the base. The rock ' ;lIllJlIl' , were crushed to small pi eces and dried at

I (l."'i ' c. T Ill' dri ed sa mples were ground to a grai n­" 1/.t' or ;1l10u t 100 mesh. Every powdered sample \\ ·;1, Ihcn mi xed Illan v times u s in ~ electri c shaker. to 11L' sure that the sample IS homogeneoll s. After pn.:pari ng phosphate sample, the authors put each sample in a container or 40 mL volume. stored for rO Ill· weeks 10 reach the equilihrilllll state. Then the S~IIl1PIl' was put face-to-face over the detector for at ic<lSI I () hours. T he spectra were either evaluated with ti ll' computer so ft wa re programme Gamma­Vi, i()n 2. 1 (F:C;&G ORTle). or manuall y with the "'l' or ;1 .' pre;lcI shee t (M icroso ft Exce l ) to ca lculate Ihl' n;l lura l raciioacti vit y . !2('Ra activ ity or the "; \lll Jllc ~ W ; I ~ determined throu gh the intensity or the .))1') ke\ ;lIl d hOL) . .1 keV y- linc. Several 2 11 Ph and ' I 'Hi wnc als() Illonitored. !1,U act i vit y was obtained

Ihrougll 142.Rl) keV y- line. ,3'Th acti vity was ohta ined t h rou~h the ,lIxT I and " ' Ac emission at .-:-;:1.1 keV <l nd l) 11. 1 keY. respecti ve ly and -IIiK ;Ietivil v determined from the 1460.7 keV emi ss ion.

-' Results and Oiscussion

.t I iVI('<!s \II'emenls of naturally occurring ."adio

2000

1500

Ol .:x 0-m 1()(){)

LD M 0.1 , ::J

" ii,

-•

T -.t.. · .J

== =

== --

nuclides

The act i vity concentrati ons (8q/kg) or the natural occurring rad io nuc lides 22('Ra. 2" U, !12Th and IIiK in phosphate samples from Eastern De<;erl (G.Anz, G.Duw i ). N ile Valley (G.Owina. G.Sari<l and G.Abu -Sahoun ) and Western Desert (G .Ahll ­Tartur) in Egypt were measured using high purit ) germanium detector.

From the obtained results in G .A nz as a typi ca l example it can be seen that radiu m-226 acti vity

concentrat ions is in the range from 41 .4 ± ~ to

1873 .0 ± 137 8q/kg with average va lue or 479 -222 8q/kg.Uranium-235 act ivity concentration which goes parall el w ith radium-226. is in the range

rro lll 5.0 ± ~ to 133.3 ± 13 8q/kg with the ;1Vl.: ragl'

va lue of 38 .0 ± 30 8q/kg. The measured va lues 01 thorium-232 ac ti vity concentration was in the ran gl'

from 1.5 ± 0.3 to 9.0 ± 3 w ith average value or 6.0 ± 5 (8q/kg) . while for potassiulll-40 the va lue was in

the ran ge from 11 .5 ± 3 to 54 .8 ± 8 8q/kg w ith th e .average va lue of 28.0 ± 13 8q/kg. The (U/Ra) rat in

was approx imately constant at 0.09 ± O.O() I , ranging from 0.06 to 0.1 2.

Fig. 2 shows the relat ion between Ra-22h a III I U-235 activi ty concentrati ons (8q/kg) wi th the

• U-235 I 0 Ra-226

I 200

I 100

C» ~ --IT !I)

0 l[) (Y)

~ :::J

-100

= =

=

24

Layers

I'i !.! :; - The relati on hClwccn Ra-126. U-1:l."i acti vit y cnnccllIrali on (Sq/kg) w ith the layers in G .. Anz

INDIAN.I PURE & APPL PHYS. VOL 39. SEPTEMBER 200 I

la ye rs in G.Anz. and Fig. 3 illustrates the relati on between U-235 and Ra-226. the corre lati on facto r \,\/ ,, :-. round I() be () . 9~8. Fig.4 shows the relation helwee n Ih l' ratio (UlRa) with Ra-226 in G.Anz. where il is roun ci to be approx imate ly consta nt.

Tu compare the d ifferent measured va lues of aCiI \ II ) concenl rati on or the phosphate sa mpl es in Ihe " rca under in ves tigat ion. the average va lues of

Ra-226, U-235 , Th-232, and K-40 are ca lcul ated and plotted as hi stogram FigS where the follo wi n!C observations can be recorded :

I. The results indicate that Ra-226 ex h ib i t ~ the hi ghest va lues of activity concentra ti on compared wi th U-235. Th-232 and K-40.

2. Uranium-235 behaves in parallel wit h Ra-22(]

200 r-----------------------------------------------------------------------------~

160

cn ,20 -"" 0-0::

01.079720.28 lInear Regression In r O ataJ __ B :

Y "" A' B· X

Pammcfor V a lue Error

A

B 2 .51 1 d 3,9266

O .075' ~ 0,0061 "

s o N p

D.93R 13,2069 22: .;0 .0001

o !')QO 1000 1500

Ra-226 (Bqlkg)

Fi .C' :< - Th e rel<ll ioll hel IVeen l l - ::':1."imq/k~) .Ind Ra-226 (Sq/kg) acli vil y concen l ralion in G .An /

500 1000 1500

Ra-226 (6q/kg)

Fi g. 4 - The rel ' lli on helween (U-2J5/ l<a-226) i]nd Ra-226 in G.Anz

AHMED eI a/.:EGYPTIAN ROCK PHOSPH ATES 557

in al l regions under investi gation , where the two i:--otopc" reach their max imum and minimum values. 1l10ql\ in the same layer of every region , i.e. U-235 III CI"C<I :--C\ with Ra-226 everywhere in the region of interes t and (U/Ra ) rati o is almost constant, rangin g fro m (l.( l7 t () O. OC) .

~. TI1l' lowest va lue of activ ity is fou nd in the case of Th-232 At the same time the fluctuation in the res ults of K-40 is much more than the corresponding va lues of Ra-226. U-235 and T h-232.

4 . The activity concentration of the isotopes shows <I wide ran ge of va lues. because of the diffe rent loca ti on samp les

:) More detailed ana lys is of the results shows till' prese nce o r a stron g coherencc hetween Ra-226 ;111<1 lJ -n:'1 for all the regions except in case of (; (hvi ll ;l. where thi ." coherence is much less. The V; IIII C of corre lation factor as calculated from the data arc: 0 .938. 0.9598. 0 .993, 0.7615 minimum v;III1 L:. 0 .999 - maximum value and 0.995 for G.Anz. Ci. Dl1 wi. G.Sa ria. G.Owina. G.Abu-Saboun and ( ;.Abu-Tartur. res pectively. T he low correlation r; lctm in GISC of G.Owina indicates the presence of mud, scatt crin g hetween the activity values of Ra­=211 ; 11 )(1 l '- 2Yi

RC\ IIIt S ohtained frOIll ex perilllents performed in til l' prC\C llt work ind icate an increase in the (1 11 ;1111 it il· ... nf hoth I1ranil1m isotopcs. ~lX U and ~"U .

7 00

T 1

6(1(] I

! 0> ! -'" :>0 0 C;-eo c:

.2 " 00 m

~) So'

30C R ~.

? 200 ,3

<{

100

0

This can be attributed to the increase in the concentration of natura l uranium in sampl es under test in contrast to ~'~Th concentration. which show" an appreciable increase as a resu lt of natural uranium decrease .

The increase of the natural uranlUIl1 concentrat ion Illay be due to leac hing effect. It has been proposed that some factors are Ii kel y to influence the uranium concentration in natural surfaces and groundwatersx

.

Such factors are summarized as fo ll ows:

l .The uraniulll content of the source roc k .... sediments or so il s. and the ease with which uraniulll Illay be leached .

2.The prox imity of the water to uraniulll sourcc .

3.The degree of hydrau lic iso lation of the w;lt cr by fresher surfaces or ground waters.

4.CI imatic effects, seasonal variabi Iity and the effects of evapotranspi ration.

5.The oxidation state of the water. and

6.The concentrat ion of suitab le complexi nt­agents. which can increase the solubi li ty of uraniulll .

The uranium concentrati on in the sampilll t­region is more dependent upon the 41h and (J ill factor ... . where the concentrations vary considerah l y spat ia II ~

~ ne

Ra

mm!I >l·U - 23~ - 40K

G . Abu-S aboun G . Abu-Tartur

Region

Fi).! . :; - M C, II I valucs or n<ilurat raciioactivil y in Egypti 'lil phosphal e rock

INDIAN J PURE & APPL PHYS. VOL 39. SEPTEMBER 2001

and temporally. The temporal variation may be seasonal, representing changes in rainfall and run­otT. Increased rainfall may result in more effective le<lc hint: alld/or transport of uranium and lead to an Increase III uranium concentration.

ThoriUll1 is also usuall y more abundant in the "u'pellded IO;J(J Ihan in the hOllom sediments. This I ~ Interesting hecause the chemical behaviour of thorium in natural waters is quite different from that of uranium. Thorium is essentiall y insoluble in normal surface waters. and so thorium transport is wi thin particulate matter rather than in solutions. Un like uranium and thorium, potassium is a major e it: lllenl in rock-forming mineral s occurring mainl y 11 1 "IUI1l IlH) ~ "I C;.ltc~ suc h as feldspars and micas.

-'.2 Radiation hazard indices. Radium equivalent acti"it~ (Rae'l)

II is important to assess the gamma radiation haza rds to human associated with the used phosphate mines. This is done by ca lcu lating the di ffere nt radiati on hazard indices. The concentration and distribution of 22('Ra. 2'~Th and 411 K in phosphate anel phosphatic rocks is not uniform throughout the wo rld . Uniformity In respect of ex posure to r; lli l;l1 i()1l has heen de fin ed in terms of radium eLJul valent activit y ( Ra,.,, ) in Bq/kg to compare the ~ pe"; lfic acti vit y of materials containin g different Ll 11101l1lt " ()f ~2(' Ra. 2'2Th and "' K. It is calcu lated IhlHlgh the fo llowing re lation suggested by Beretkil anel Mathew":

Ru,., = (RII, x 1.4~) + RK" + (R" x (U)77) ... ( I)

wlH.: re R II , is the activity concentration (Bq/kg) of Ihorium-232, RI<" the activity concentration (Bq/kg) of radium-226. and RK the activity concentration ( J)LJ / k~ ) of potass ium-40 .

It IS assumed that :no Bq/kg of 22hRa. 259 Bq/kg (II' :;2Th or 4R 10 Bq/kg of ·:I' K produce the same y­ra y close rat e (Krisiuk e f 01. '" and Stranden"). The li se oj fertilizcr for growing crops gives sufficient ~rou nds for the justification . The ALARA-principle i mJlI ies that reasonable measures must be taken to reduce radiation doses, and that costs have to be we ighed against the gain in reducing the radiation cf()se ,~. Fig. 6 shows a comparison for the average ac t i vity concentration of radium equivalent in all nlL';J sllrccl re ~ i(lIl " .

The hi gh concentration values are due to the h ip~ hcr rad iUIl1 content. wh ich consequently leads to

an increase in the radium equivalent act ivity of it ~

phosphate fertilizer product.

According to rCRP' ~ , the upper limit of radiation dose arising from building materials to which the general population can be exposed is ISO mrad/ye(l r. To limit the radiation dose from buildings to thi s va lue. several models have been developed wi th respect to values of activity c0ncentrat ion of individual radio nuclides . One of the models . proposed by Krisiuk et al. II

'. is adopted here and IS described by the relation :

AR A.I·I AK --" + --' + -- :s: 1.. 370 259 4810 ... (2 )

where AKa, ATh • and AK are the mean ac tl v ili e ~ ul' ~2(' Ra, ~ '2Th. and "OK, respective ly, in 8q/kg o r the building materials . The later equatioll (criterioll formula) corresponds to maximum Ru,." or ~7()

Bq/kg (10 pCi/g) of the material".

From the obtained results of specific activitics (2C,Ra, ~.1-' U. ~ .12Th. and '"'K) of the samples under investigation, the mean values of ~2hRa was found to be 479.2, 6 .0 for 132Th . and 28.0 for ~IIK in G.Anz. in G.Duwi 410.0, 6.0, 24.0. in G.Saria 5WU. 9.0. 115.0. in G.Owina ~()().O , IS.O. GO.O. ill G.Ahll ­Saboun 364.0. 14.0. 47.0 and 424.0. 14. (J. )-n o In G .Abu-Tartur. respect i ve ly.

Eq. (2) has been applied on the phos pha te rod samples of the areas under investigation as shown in Table I.

Table I - Radioactivity leve ls of phosphate rod, samples based on the criterion formula IEq.{ 2) 1

Region

G.Anz Eastern Oese r! G.Duwi Eastern Dese rt G.Saria Nile Valley G.Owina Nile Valley G.Abu-Saboun Nile Valley G.Abu-Tartur Western Desert *Standardized at I ::::: 370 Bq/kg

Acti vi ty levt.: l':' 1.] 3 1.14 1 .4~

O.8R IJl4 1.22

By applying the criterion formu la IEq. C)I. nne can find that phosphate samples have radioact ivity above the proposed acceptable leve l of I n7() BCJ

AHMED el al.:EGYPTIAN ROCK PHOSPHATES SSy

600 -r------------------------------------------------------------------------~

500 -

~ 0- 400-

@

C ..9! cu ~ 300 -v­Ol

E -0 ro cr 200-

G.Anz G.Duwi

X.'

, ,?> ,

G.Saria G.Owina G.abu-Saboun AAbu-Tartur

Region

r ig. (, - Radiulll equi va lent average acti vit y concentration in phosphate rock

I-:~ ' J ill all areas except in G .Ow ina (Nile Valley), where it s radioacti vit y is lower than this limit.

The occurrence or primordial radionuclides in pllO~p h ; tt e ore and fertilizer has been recognised for " I ()n~ time. The radiation hazards have long been i~ ll o reci The :lCceptahility of ri sks from ionizing r: lclia tt on is decreasin~ . This tendency leads to more ' Irll l~ l' ll l I"e~ lILiti \l n . In the forthcoming years lower re ~ulatnry limit s are likely to become obligatory. 1)I"eC< lIi1ionary measures and restri ction s in order to pI"ot ect workers in the ractori es and on the larlnyard ~ may hecome compulsory.

Disposal or the hy-product phosphogypsum IS

alread y a prohlem. Other environmental effects are (Jnl v Illln()r. A ll thi s might result in a price increase I,'l" fe rtili zers, cntailing additional ex pense for users and the soc iety in gencral.

~ ('onciusion

I . The res ult ~ obtained from the gamma ' Iwet 1"0 Ill e t ri C ;l1la I ys i ~ indica te thai phosphate

samples contai n 22hRa, ~'5 U, ~oK and small amount or ~ ·~~Th in some samples, while 1.~7CS could not he

detected,

2. n6Ra, ~"U , 2'~Th and 4°K activity concentrati on

was in the range between 28.1:1873, ):140.), 1.5:33,2 and 9.9:258.4 (Bq/kg), respectively.

3. From the calculated va lues, it is found th at phosphate samples have radioactivity ahove the proposed acceptable leve l of I (370 8q/kg) in all areas except in G .Owina (Nile Vall ey). whi ch ha.' radioactivity lower than this limit.

References

Guilllond R .I & Windham S T. USA-EPA. O/liCl' or Not! Pro!?r, Techll Noll' ORP/CSD-75-3. (t975).

2 SteinhUusler F & Pohl E. Int Symp on the " /Jiologicol Cfjecis ot lo w· level Nadia lion wilh Special Regord 10 Ih l'

SlOch(wic lind 1I00/·SIOc//{/Slic Effecls ·· . Ven ice ( 19l(1).

:1 Ptister H & Paul y H. Eel:' rel'(} r/ VI24()XIXO: 447-4(,7 ( 19XO)

INDIAN J PURE & APPL PHYS. VOL 3<}. SEPTEMB ER 200 I

I ~(lcss l e r C E. Smith Z A . Bolch W E & Prince R J. Health 1'1,,·.1. ' 7 (1 ')7<J) 269-277 .

I';tlll i \ . I .(lnd hc V & Pilla i K. In NflI llm l Nw liflli(1/1

r . //I ·I UI //I // / ' // !. I I I I I ')X4) 1(,:1:1 - 1 (,54.

I, VLlrliIlC/ - I\.! llirrc 1\ . CarC iil -Lc(l 1l M & Ivan() vich M . .I 1. // \· iUl// Nildio{/cl /I 'il\'. 22 ( 1l)()4 ) 155- 177 .

I I-Icrmin il M 1-1 . 2''' ' Ar/J Co// I M ill l' r /« ('.1. Coni Papers. ( I ')72) 1(1')- 14') .

x l _a ll ~ lllili r I). U ranilfln riejlosils, II/fll cra logr olld orig in .

Handbook 3 (1 971\ ).

<) Berelka .I & Mathew P J. Health Pin's . 4X ( 1<)1\5) X7-<J5 .

10 Kri siuk E M. Taras()v S I. Sham()v V P. Shal:lk N I. Li sachenko E P & Gomelsky L G. /«(,SI' lIrc lt II'Sl i lll ll' li/l Nadilllion Ih'giene . Leni ngrad ( 197 1 ).

II Stranliell L HN/I lit PIt."s : X 11 97(, ) 1(,7- 177

12 le RP. Puhli ca tion nO.60. /?1' C() //lIl1 l' lIdu l i ll ll .1 "I Iltl' 111 1

C OIIIIII 1111 Radiation Prolect;1I11 (Pergamon Press. Ox ford ). I <J<)O.