Measurement of low levels of normal uranium in water and urine by liquid scintillation alpha counting

NUCLEAR

INSTRUMENTS

AND

MEASUREMENT

METHODS

i i 7 (1974) 589-595;

,v) N O R T H - H O L L A N D

OF LOW LEVELS OF NORMAL

U R I N E BY L I Q U I D S C I N T I L L A T I O N

PUBLISHING

CO

URANIUM IN WATER AND

ALPHA COUNTING

DONALD L HORROCKS Scientific Instruments Dtttston, Becl, man Instruments, lnc, Fullerton, CA 92634, U S A Recmved 13 December 1973 The a m o u n t s of normal u r a m u m in water and m urine have been measured as low as 0 1 #g/ml ( 6 x 10 a#Ci/ml) by hquld scmtdlahon counting methods An emulsifier system was used to incorporate up to 5 ml of an aqueous solutmn m a 15 ml counlmg solution (10ml of scintillation cocktail plus 5 m l of aqueous

solution) with 100% cotinting efficiency for the alpha particles from the ea4u, 2a'su and e:>,d rad~onuclldes Use of a mulUchannel analyzer allows correctmns for the presence of shorthved beta emitters in the u r a m u m decay chain

1. Introduction

lead to serious errors if no correction is m a d e for the daughters o f 238U which grow Into equilibrium over a period o f a p p r o x i m a t e l y 250 d after s e p a r a t i o n o f uranium from thorium. (After 170 d the s a m p l e activity will have o b t a i n e d 99.2% o f the equilibrium value ) The decay scheme o f the 238U-234Th-234mpa-234U chain is shown in fig I. At equilibrium each 238U decay will result in one a l p h a particle and two beta particles The n u m b e r o f beta decays at non-equilibrium will vary d e p e n d i n g upon the time that has elapsed since s e p a r a t i o n o f U from its daughters. The growth with time is shown in fig. 2. The activities o f 23SU and 234U in n o r m a l u r a n i u m are equal; each contributes 0.74 a l p h a particles per minute per l~g o f n o r m a l uranium. The ZasU contributes 0.03 alpha particles per minute per l~g o f n o r m a l uranium. The specific alpha-particle disintegration rate o f n o r m a l uranium is thus 1.51 per minute per Itg The total disintegration rate ( a l p h a + beta) o f a given sample will be variable if the uranium has been separated from Th at some time prior to

The d e t e r m i n a t i o n of levels o f u r a n i u m m samples is b e c o m i n g m o r e i m p o r t a n t with the increased o p e r a t i o n s o f nuclear p o w e r plants and the increased mining o f uranium. A t present the m a x i m u m permissible concentrations o f n o r m a l u r a n i u m in water are 5 x 10 - 4 #CI/ ml in a 40-h w o r k week for persons w o r k i n g with u r a n i u m and 2 x 10 - s / t C i / m l in a week for the general publlcl). However, it m a y be expected that these limits will be reduced. It is thus necessary to search for a quick, convenient and accurate m e t h o d for measuring low levels o f n o r m a l u r a n i u m in water and urine In this p a p e r a technique is r e p o r t e d for the measurement o f levels o f n o r m a l u r a n i u m as low as 6 x x 10 -8 ItCUml o f water or urine This level is 300 times lower than the lowest present allowable limit.. Stoicovlcl and U r a y 2) p r o p o s e d a m e t h o d o f determining the a m o u n t o f u r a n i u m as uranyl n i t r a t e m e t h a n o l solution dissolved in a liquid scintillation solution They d e t e r m i n e d the s a m p l e r a d m a c t l v i t y by measuring the integral count rate This m e t h o d could 238U re. 51

TOTAL~ALPHA+ BEIA,

Y 10*vr t

30

234Th ,24 ld, ~ 1 - ~ - / ~ / ~ /

t~g

G

Ema, = 0 I03 ,] Olql r65

~20

N X

/

-

/

c~

~ALPHASONLY

I0

23'lmpa ~1 ]/r~

\

BETAS ONLY

/

k

50

£ma, ""q ~"~IL)

/

L I '

1,, 'r,

100

i

150

200 DAYS AFTERSEPARATION

L

L

250

~00

Fig 2 Growth of daughter activity ('a4Th and 2a4mpa) m a separated normal uranium source.

Fig I Decay scheme for a'~aU-m4Th-ea4mpa-aa4U chain

589

59~

D

L. H O R R O C K S

TABLE 1 Radmnuchdes present In normal U samples and some of thmr nuclear propertms Rad~onuchde

Mode of decay

-':~su -'.34Th

~ /1~

-a4mpa m4U -'a~U earTh

te :~ ~ /4

Energy of decay

peak was obtained which contained the pulses due to 234U, 235U and 23UU However, other llqmd scmtlllahon counting systems have been reported that were able to separate and tdenUfy different alpha-parucleenergy groups ~ 8 l i) 2. E x p e r i m e n t a l

4 20 McV 0 0 100 McV (35%) 0 0 191 MeV (65%) 0 2 29 MeV 4 77 MeV 4 4 MeV 0-0 302 MeV (32%) 0-0 218 MeV] 0 0 132 MeV (48%) 0 0 090 McV

counting and before equilibrium has been re-estabhshed (approximately 170-250 d). The contribution of 2a°Th (daughter of 234U) to the alpha-particle activity is negligible when the u r a m u m has been separated from thorium in recent time. The alpha particles are much more energetic than the beta particles as shown m table 1. However, alpha particles of a.20 and 4.77 MeV wdl produce only as many photons as beta partmles of 0 310 and 0 440 MeV energy 2'4) There are beta particles of these energies produced during the decay of 234mpa. Thus even the setting of windows to just accept the alpha-particle pulses wdl include some beta-pamcle pulses Without actually knowing the proportion of beta-particle pulses, it would not be possible to obtain the alpha-pamcle count rate. Alpha particles of the same energy produce an almost constant scintillation y~eld (or photon yield) per alpha particle excitation of a given hquid scintillator solution 3 7) provided there is no quench difference between samples. However, beta-parucle-emlttmg radionuclides g,ve rise to a continuous pulse-hmght distribution because of the emission of beta particles with a continuum of energies from zero to E ...... The peaked dtstrlbuUon produced by the essentially monoenergetic alpha particles can be easily dlstmgmshed from the broad continuum of the beta-particle background, even at very low levels of alpha-particle activity 8) The alpha particles are counted with 100% efficiency s 9) even when the sample is highly quenchedt°). The energy resolution of the counting system used m this work was lnsufficmnt to separate the pulses due to different alpha-particle energy groups A single

2 [. SAMPLLPRLPARATI()N Three types of samples were prepared for checking the procedure One type consisted of u r a m u m [as U O 2 ( N O 3 ) 2 . 6 H 2 0 ] dissolved m water A second type consisted of uranium [as U O 2 ( N O 3 ) 2 . H 2 0 ] dissolved m urine. And the final type of sample conslsted of uranium complexed as an organic phosphoric acid [dl-(2-ethyl hexyl) phosphoric acid, H D E H P ] and dissolved m toluene. In each type of sample the concentration of uranium was determined by the weight of UO2(NO3) 2.6 H 2 dissolved m the n3edmm 2.2 LIQUID SCINTILLATIONSOLUTIONS The commercially available emulsifier-containing scintillator solution, Beckman Ready-Solv VI, was used for the water and urine samples Up to 3 ml of the water sample was dissolved m the Ready-SoD VI with essentmlly no quenching of the scmtlllauon response To prepare a counting solution with 5 0 ml of aqueous solution, a small a m o u n t of BBS-3 solubdlzer (Beckman Instruments, Inc.) was added to the Ready-Sol\ V[ until a ~,mgle, clear phase was obtained Thts usually required about 0 . 5 m l of BBS-3 per 10ml of R e a d y - S o l v V I Increased amounts of the urlne sample caused slight increased quenching due to the yellow color of the specimen. The u r a n l u m - o r g a m c phosphate complex was counted In a butyl-PBD (12 g/1)-toluene solution All counting solutions were 15 ml m volume (sample-plus-scmtlllatlo n solution) 2.3. EQUIPMENF All counting was performed on a Beckman LS-250 which was connected to a Nuclear-Data Serms I100 Mulhchannel Analyzer wtth a N D - 5 3 2 A R C Amphtier t2) The LS-250 has a pulse summation clrcmt which adds together the pulse heights from the two multlpher phototubes which occur within the instrument resolving time Then a logarithmic amplifier produces pulses proportional to the logarithm of the summed pulse hmghts. Thus, the pulse-height output is proportional to the logarithm of" the energy of the beta particles 13) The summed pulses were put directly into the N D - 5 3 2 A R C amphfier The coincidencedetermining pulses from the LS-250 were shaped (to the

MEASUREMENT OF LOW LEVELS OF NORMAL URANIUM

591

LIQUID SCINTILLATION COUNTER LS-250

SAtlPLE

•

I

PULS£ SU,HP1ATION

I

I COINCIDENCE b'lONI 1OR

1

j ,'[

r

CHANNEL ANALYZER

i',~S-5 32 ARC At'P

1,

I"

t PULSE SHAP I F,~

I

J Fig 3 Slmphfied dmgram of counting system

MCA requirements) and used as a coincidence gate for the MCA Fig 3 shows a simplified block diagram of the system.

3. Results 3.1. UNQUENCHED URANIUM SOURCE A h q u o t s o f the U - H D E H P were counted and the results are shown in fig 4. The beta-particle pulses appear as a broad c o n t i n u u m from zero pulse hetght to

the upper pulse-height hmlt o f the MCA. The alphaparticle pulses appear as a peaked distribution superimposed upon the beta-particle background. The n u m b e r of U alpha-particle disintegrations is equal to the area under the peaked distribution mmus the b a c k g r o u n d The background can be o b t a m e d to a good a p p r o x i m a t i o n by drawing a smooth curve from the lowest count level before the a l p h a - p a m c l e peak to the point corresponding to the end of the alpha partlcle distribution (shaded area m fig 4)

.~00C' (b)

(o)

Jtu '0

z

2000

S'

3 !000 o

DO

IO0

1

5(I

CHANNEl Fig 4 PLIIse-helght dlstrzbutlon for normal uranium samples as H D E H P complex m toluene-12 g/I butyI-PBD scintillator solution Peaked area is due to alpha-pamcle excitations Broad continuum is due to beta particles Shaded area ,s interpolated background

bO

CHARNEL

~O0

1"~0

Fig 5. Pulse-hexght dlsmbutlons for normal uramum sample, a) as HDEHP complex m 15 ml of toluene- 12g/I butyI-PBD. b) m I 0 ml of ~ater m 14 ml of Ready-Solv VI

s92

D.L.

HORROCKS

TABLE 2

TABLE

D a t a f o r s a m p l e o f k n o w n a m o u n t s o f u r a n i u m lll w a t e r

Normal U Volume of Expecteda concentratmn sample dpm (~g/ml) (ml) per sample

1040 104 104 1 04 I 04 0 104

I0 10 l0 10 30 30

1570 157 157 1 57 471 0471

Counting ume (ram)

60 125 900 900 300 2800

Measured b cpm per sample

3

Data for samples of known amount~ of uramum

Normal U Volume of Expected a concentration sample dpm {pg/ml) (ml) per sample

1582 ± 15 1532 1-20 154 ± 0 2 I 56-t-0 03 468±010 044±001

1130

I0

113

I 0

113 I 13 I 13 0, ll3

CoLintmg Ume (ram)

1700 170 170 I7 51 051

10 I0 t0 :;0

m urine

Measured h cpm per sample

50 1680 ±30 75 1706 ± 5 I 370 173 ± 0 5 850 I 59 + 0 0 5 870 489 ± 0 1 l 3700 0493±0025

l l e g of normal uramum (2:~4u+2asu+e:~SU) will gwe 151 alpha parncles per minute h Counting errors are gwen by :t:2(asz+B+c~) t and do not include any other sources of errors

I # g o f normal uranmm (2a4U+e~sU+a~sU) will give I 51 alpha parncles per minute. h Counting errors are given by ±2(~+~+c~2) i and do not i n c l u d e a n y other sources of errors

3.2. WATER-URANIUM

3 x 10 - 3 c m in w a t e r

a

The alpha-pamcle pulse-hmght response of the Ready-Solv VI scintillator solunons is only 88% of the butyl-PBD-toluene scintillator solunon as ts shown in fig 5 The response of the Ready-Solv VI is independent of the volume of water from 50). to 3 5 ml This is primarily due to the fact that the counting soluUon ~s actually a heterogeneous system. The separate water phase does not quench the scmtdlaUon process which occurs m the organic part of the soluuon ~2). The range of the 4 2 0 - 4 78 MeV alpha particles is about

a

T h u s very httle o f t h e a l p h a -

p a m c l e e n e r g y is d m s l p a t e d in t h e w a t e r m l c e l l e s Several samples of varying U c o n c e n t r a t i o n were p r e p a r e d a n d c o u n t e d Fig 6 s h o w s a few r e p r e s e n tatwe plots of the data obtained from the M C A . The different samples were counted for different time p e r i o d s , t h o s e w i t h less u r a m u m w e r e c o u n t e d a l o n g e r n i n e T h e a l p h a - p a r t i c l e c o u n t rate w a s o b t a i n e d by s u m m a u o n o f t h e c o u n t s in t h e p e a k e d d m t n b u t l o n minus the c o r r e s p o n d i n g beta-particle b a c k g r o u n d d l v M e d by the s a m p l e c o u n t i n g t i m e . T a b l e 2 s u m lnanzes the measured a l p h a - p a m c l e count rates along w i t h t h e e x p e c t e d v a l u e s T h e d a t a o f t a b l e 2 are p r e s e n t e d in fig 7. T h e h m l t o f d e t e c U o n is g i v e n by t h e p r o d u c t o f the v o l u m e o f s a m p l e t h a t c a n be c o u n t e d t i m e s t h e efficiency. S i n c e t h e efficiency is 1 0 0 % , t h e h m l t o f d e t e c t i o n c o u l d be i n c r e a s e d m p r o p o r t i o n t o the v o l u m e o f s a m p l e Fig 8 s h o w s t h e s p e c t r a o b t a i n e d w i t h v a r y i n g

(D

m NORMAL U = t51 dpmpg

o (D

ox

o

x

0i

'oX

[;0

-d

(~-0 X

o

IN WAFER

IN URINF

05 I

0

50

100

150 CHANNEL

50

100

Fig, 6 Pulse-hetght dlstnbuHons for samples of uranmm m water Sample volumes are I 0 ml except where noted otherwise

01

10 CONC

10 Mg/ml

10©

]Off3

Ftg 7 Measured counting rates per microgram ot uramum for samples m water and m urine

593

MEASUREMENT OF LOW LEVELS OF NORMAL U R A N I U M

amounts of water containing 0 88 mg of normal uranium. The expected alpha-parncle acttvlty is 1310 alpha parttcles per minute. The results of the data t n 300@ ?000

1 ~03 8 CPM 0 88 n',g U 1 0 ml H?O 140 ml [SS

(o)

3.3. URINE-URANIUM

1295 4 CPM 0 88 mF, U 30 ml H20 120mIESS

(b)

T h e urine c a u s e d a small a m o u n t o f q u e n c h because o f the y e l l o w color. T h e n o r m a l u r a n i u m a l p h a - p a r t i c l e p u l s e - h e i g h t r e s p o n s e with 1 0 ml o f u r m e in R e a d y S o l v VI was 9 2 % o f t h a t wtth 1 . 0 m l o f w a t e r in R e a d y - S o l v VI a n d 8 1 % o f t h a t with U - H D E H P in the b u t y l - P B D - t o l u e n e sclnttllator s o l u t i o n A series o f urine s a m p l e s with k n o w n a m o u n t s o f a d d e d n o r m a l u r a n i u m were c o u n t e d a n d the a l p h a - p a r t i c l e dtslnteg r a h o n rate m e a s u r e d . T a b l e 3 s h o w s the results S a m p l e s p r e p a r e d with 3 0 ml o f urine s h o w e d f u r t h e r q u e n c h i n g as s h o w n in fig. 9 T h e r e s p o n s e was 8 1 % o f the r e s p o n s e m 1.0 ml o f water. H o w e v e r , the c o u n t i n g efficiency was still 100% for the a I p h a parttcles T h e d a t a o f table 3 are p r e s e n t e d in fig. 7.

1000

z0 ~p LU Of)

3000 o / ~o,qnnl ..... /

L 7- I O 0 0 F ~0

0

~D

30001

13092 CPM 0 88 mg U

2000[

50 ml H20 10 0 ml LSS

/

fig 8 Indtcate the measured alpha-particle count rates are essentially equal to the expected rates. The shtft m channel peak for the 5 0 ml sample Js due to the shght quench due to added BBS-3

(c) 3.4. RELATIVE QUENCH OF SAMPLE

1@00I. @

50

100 CHANNEL

Fig 8 Pulse-height dJstnbunons and measured count rates for samples containing 088rag of uramum m (a) I Oral of water, (b) 3Oral of water and,(c) 5Oral of water Samples(a) and (b) are m Ready-Solv VI (LSS) and (c) is m Ready-Solv VI wUh added BBS-3 (LSS*)

T h e v a r y i n g a m o u n t s o f w a t e r or urine dtssolved in the s c m t f l l a t l o n c o c k t a i l dld n o t alter the 100% c o u n t i n g efficiency for the a l p h a particles e m i t t e d f r o m u r a m u m . V a r y i n g the w a t e r c o n t e n t f r o m I ml to 3 ml dtd not alter the r e s p o n s e as i n d i c a t e d by the pulse height p r o d u c e d by the a l p h a p a m c l e s (fig. 8). T h e r e s p o n s e for the 5.0 ml w a t e r s a m p l e s was r e d u c e d due to the a d d e d s o l u b l h z e r (BBS-3). T h e y e l l o w c o l o r o f the urine s a m p l e s c a u s e d a q u e n c h i n g o f the r e s p o n s e with the i n c r e a s i n g a m o u n t s o f urine d i s s o l v e d in the TABLF 4

0 ml OF /" URINF

~

Type and volume of sample m determining the response of scmnllaUon soluUon to normal uramum alpha parncles Type of sample

~ ?0o0 HDEHP-U Water Water Water b Urine Urine

]00dh

0

100 CHANNELNUMB[R 50

I50

Fig. 9 Pulse-height distributions showing quenchmg effect of urine

Volume of MCA channel sample for peak of (ml) alpha response 0 050 10 30 50 I0 30

107 94 93 83 87 76

Relatwe pulse height

Relauve scmtdlaUon yield a

100 88 87 77 81 71

I 00 0 48 0 46 0 26 0 32 0 17

a Calculated from ann-log of the relative pulse height and response slope b Extra solubfllzer (Beckman BBS-3) had to be added to ReadySolv VI to obtain a single phase solution.

594

D.L. HORROCKS

hquld scmtillatton cocktail. These results are summarlzed m table 4.

4oo I 1 '0 n I tit{IN ~ 1 ~, L) I11 [ %~2

I

3.5. BACKGROUND

The beta background will not only depend upon the amount of uranium in a sample but also on the degree of equilibrium between the daughter beta emitters and the parent 23aU. All of the samplesmeasured hadhad the daughters separated from the uramum just prior to addltton to the scmttllator solutton. Once m the solutmn the daughters began to build up in the solution. However, the counting solutions were counted at different ttmes after thetr preparation and thus the beta backgrounds obtained showed no relation to the relatwe amounts of uranium present. The measured beta background was a function of the amount of uranium present in the sample and the time that had elapsed since the daughter had been separated from the uranium The beta backgrounds are dertved by interpolation of the contmuum on each stde of the alpha response peak as shown in f i g s 4 and 6 (shaded areas). The beta background of every sample showed an mcrease with ttme because of the non-eqmhbratlon. Table 5 shows the beta background of a typical sample as a function of elapsed time between measurements. The increase shows that the sample was in a state of non-eqmhbrmm between the 23sU and the beta emitting daughters 2 3 4 T h and 2 3 4 m p a F i g . 10 shows the background spectrum obtained with a urine sample with nothing added Over the same pulse-height range as used for obtaining the alpha-particle count rate, the background with no urantum present was only 1.5 cpm. TABLE

5

Interpolated b a c k g r o u n d for sample of normal u r a n m m m urine (1000/tg m 1 0 ml. urine) as a function o f time elapsed from sample preparanon T i m e elapsed (d)

.

~0~

,A <

~oo

£ 0 c~

0

50

18,,,,

~bl

CHANNE[

Fig 10 Pulse-height distribution showing relatwe counts for urine w u h (a) 100pg u r a n i u m and (b) no added uranlunl

and 0.1 /2g of normal uranium gave cpm values of the amount within _+ 10% of the known amounts The variable background, due to non-eqmhbrlum of the beta-emitting daughters, has to be determined by use of the interpolation method on the pulse-height spectrum obtained by use of a multi-channel analyzer A correction techmque could be worked out slmdar to that used by Falrman and Sedlep4). if the 2 3 4 T h and 234"Pa could be isolated and their beta-particle pukeheight spectra measured, channels could be set to monitor the beta-particle contrlbuhon in the alphaparticle counting channel The author wBhes to thank Drs B. M Courseyand W B. Mann of the National Bureau of Standards for helpful suggestions and review of th~s paper, and also for providing the method for calculations of the relative scintillation yields m table 4

B a c k g r o u n d (cpm)

References 0 [ 3

370 398 437

4. Conclusions

For several samples containing quantiUes _> I /~g of normal uranium, this method gave measured cpm values of the amount within + 5 % of the known amount. Several other samples with between 1.0/~g

1) U S Atomic Energy C o m m i s s i o n , U S A E C rules and regulations, Title 10, Code of federal regulations, Part 20, Standards of protection against radlaHon, revised A u g 9, 1966 (U S Go,,ernment Printing Office, W a s h i n g t o n , D ( ' ) z) S. Stolcowcl and 1 Uray, Ntlcl. lnstr and Meth 51 (1967) 25 s) K F Flynn, L E G l e n d e n m , E P Steinberg and P M Wright, Nucl Instr and Meth 27(1964) 13 ~) D L Horrocks, Rev Scl lnstr 35 (1964)334 .5) D. L Horrocks and M H. Studler, Anal C h e m 30 (1958) 1747 {') H H Sehger, Intern J. Appl Rad I s o t o p e s 8 ( 1 9 6 0 ) 29 7) j K Basson and J Steyn, Proc Phys Soc A67(1954) 297 8) D L Horrocks, Intern J Appl Rad l s o t o p e s 1 7 (1966} 441

MEASUREMENT

OF L O W

L E V E L S OF N O R M A L

,t) p M Wright, E P Steinberg and L E G l e n d e n m , Phys Rev. 123 (1961) 205 10) D L. Horrocks, Orgamc scInnllators, ed D L. H o r r o c k s ( G o r d o n and Breach, New York, 1968) p 45 11) W J McDowell, Organic scintillators and ltqtttd sctnttllatton

counting, eds D

URANIUM

595

L Horrocks and C T. Peng (Academic Press, New York, 1971) p. 937 1_,) D. L. Horrocks, u n p u b h s h e d results (1972) 13) D L Horrocks, Intern J. App[ Rad Isotopes 24 (1973) 49 14) W D. Fazrman and J Sedlet, Anal. C h e m . 40 (1968) 2004