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Radium isotope measurements using germanium detectors
Nuclear Instruments and Methods in Physics Research 223 (1984) 407-411 North-Holland, Amsterdam
RADIUM ISOTOPE MEASUREMENTS
407
USING GERMANIUM DETECTORS
W i l l a r d S. M O O R E D~7;artmenl qf Geoh)gv. University of South Carolina. Columbia. S.('. 29208, USA
Recent advances in sampling and counting techniques have provided the means of measuring 226Ra. 22SRa and 224Ra at low activities in natural waters. Samples are preconcentrated in the field by adsorbing radium on a fibre coated with manganese oxides. Absolute activities and activity ratios are measured using germanium detectors supplemented in some cases by alpha scintillation measurements of 222Rn. This paper describes tests and results obtained using a Ge(Li) crystal and an intrinsic germanium crystal with a 1 cm diameter well. The well detector has an efficiency two to three times greater than the flat Ge(Li) system and thus has a considerably higher sensitivity. Results from ground waters and estuarine waters are presented which demonstrate the usefulness of the germanium detectors in studies of radium in natural waters.
1. Introduction Studies of radium isotopes in natural waters have a wide field of application including their use in deep ocean mixing studies [1], exchange rates of estuarine and coastal waters [2], calculation of metal scavenging rates in ground waters [3] assessment of the effects of p h o s p h a t e mining on ground water quality [4], and d e v e l o p m e n t of predictive models for the radium content of ground water [5]. This surge of interest in radium in water has been driven by improved techniques for sampling and measuring radium isotopes and an inc,'eased awareness of the usefulness of the different isotopes in a n u m b e r of studies. A n o t h e r factor has been the US Environmental Protection Agency's interim standard for radium in drinking water which required that all public water supplies in the US be analyzed for 226Ra and (if the 226Ra was above 3 pCi/1) 228Ra. Problems with the currently accepted analytical techniques for 22SRa spawned a n u m b e r of new procedures.
2. Sampling and measurement techniques To provide a relatively high activity sample, we pre-concentrate radium in the field using a fibre coated with manganese oxides [6]. Packed in columns this so-called Mn-fibre will quantitatively extract radium from water at flow rates of tenths of litres per minute [7]. For very large volume extractions when only radium isotope ratios are to be measured, we expose the Mnfibre to the water by towing it b e h i n d a boat or setting it in a flowing stream. A handful ( - 5 0 g) of the Mn-fibre may extract Ra from thousands of litres of water. In such cases we also collect a sample for the 0 1 6 7 - 5 0 8 7 / 8 4 / $ 0 3 . 0 0 ~.,2Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
quantitative measurement of 22,5Ra by the 222 Rn emanation technique [8] by slowly passing about 20 L through a small column of the Mn-fibre. The Mn-fibre is returned to the laboratory, leached with hydroxylamine hydrochloride to dissolve M n and Ra, and the radium is co-precipitated with BaSO 4 which is packed in an appropriate counting vial. These procedures are discussed in detail elsewhere [9,10], We have recently installed an intrinsic g e r m a n i u m detector with a 1 cm diameter by 4 cm deep well which we call a W e G e (for well germanium) detector. This detector increased our counting sensitivity for radium isotopes by a factor of 2 - 3 over the Ge(Li) detector we have reported previously [9].
3. Well germanium (WeGe) detector The detector is a 78 cm 3 coaxial intrinsic g e r m a n i u m crystal with a 1 cm diameter and 4 cm deep well produced by Princeton G a m m a Tech of Princeton, N.J. The peak to C o m p t o n ratio for the ~3VCs peak is better than 90 : 1 for samples counted in the well. At 300 keV the fwhm is 1.4 keV and at 1000 keV it is 1.8 keV. G a m m a - r a y s are measured by placing the sample in capped polypropylene test tubes which fit inside the well. Samples which may lose 222Rn are capped with a layer of epoxy cement, Most of our radium in water samples are measured as BaSO 4. We do not cap these with epoxy as we have not detected 222 Rn loss from this matrix. Several standards have been used to calibrate the WeGe. These include monazite and pitchblend ores, as well as 232Th and 226Ra solutions from the US Environmental Protection Agency Quality Assurance Division (EPA QAD), a p h o s p h a t e ore from the US National 111. SPECTROMETRY
408
W S . Moore,
Radium isotope measurements usmg Ge detectors
the adsorption of g a m m a s for path lengths of ~.> t~ l cm. We count our radium samples tn a BaSO 4 rnatnx and prepare standards m the same matrix to avoid a~l\ problems with differential adsorption ~,t" g a m m a s The height of the sample in the tube does affect c o u n t i n g efficiency. We avoid this prohlem by adding a s t a n d a r d a m o u n t of Ba 2~ solution to the ,,,ample to give us reproducibile heights in the tube. Although the final conversion of cpm to d p m for each peak involves 4 variables, we keep two of these c o n s t a n t {height and matrix) and group them along with the other two ( g a m m a intensity and detector efficiency) into a single factor. F. Thus. if published g a m m a intcnsines are incorrect, the detector efficienc', value will adjust to compensate. This may be shown by the following formula
40
30
\ \
20
~x
\
)-
\
10
Z LU
m
\
U. LL ELI
1 loo
"
"
'soo'
\
.-,,,
"'fooo
-26oo
E N E R G Y (keVl
cpm Fig. 1. Calibration curves for Ge{Li) and WeGe systems.
Bureau of S t a n d a r d s (NBS). a n d a n aged 232Th solution c o n t a i n i n g equilibrium activities of 228Ra a n d 22STh which we p r e p a r e d from pure T h (NO3) a. The W e G e system has been intercalibrated against o u r Ge(Li) system which has also been carefully calibrated and tested [9]. We have also intercalibrated our W e G e system with a similar system in D r William Burnett's laboratory at Florida State University. Fig. 1 gives calibration curves for our W e G e a n d Ge(Li) systems based o n the data in table 1. We have used values from the T a b l e of Isotopes [11] for the intensity of each gamma. N o t e that o n b o t h curves the values for the 2t4Bi peaks at 609 a n d 1120 keV fall well below the line. Burnett (personal c o m m u n i c a t i o n ) reports similar discrepancies. W e have verified the efficiency of our systems in the 661 keV regmn using a calibrated 137Cs source from the E P A Q A D . The values we use for intensities of the 214Bi peaks must be low. A t 295 keV or a b o v e there is little m a t r i x effect in
where 1 is the g a m m a intensity, E the detector efficiency at the given energy, A the absorption factor, a n d t t a factor relating the height of the sample in the tube to a s t a n d a r d height. For m e a s u r e m e n t s of radium in water. we c o m b i n e these all into a single factor for each g a m m a so thai dpm=F×cpm. Table 2 is a list of factors which we use for calculating radium using the s t a n d a r d geometry of the W e G e a n d Ge(Li) detectors. This table also gives the "backgrounds we measure in each of these regmns of the spectra after C o m p t o n effects are eliminated. For our purposes the lower detection limit is not as useful a p a r a m e t e r as the m i n i m u m activity required to yield a 228Ra,/226Ra ratio with a c o u n t i n g u n c e r t a i n t y of + 10% in a one day count. These low activity samples usually have considerably less 228Ra activity than 226Ra. T h u s our lowest practical limit is about 10 d p m 22~Ra for the Ge(Li) and 4 d p m 2ZSRa for the WeGe.
Table 1 Gamma-rays and counting efficiencies for the Ge(Li) and WeGe systems used in the measurement of radium isotopes Isotope
Energy fkeV)
y-ray yield
Ge(Li~ efficiency
WeGe efficient'.
295 351 609 1120
0.189 0.367 0.461 0.153
0.076 0.066 0.031 0.017
0.230 0.187 0.059 0.036
22SAc
338 911 964-969
0.113 0.272 0.213
0.069 0.024 0.023
0.184 0.057 0.054
For 224Ra 212Pb
238
0.449
0.089
0.317
For 226Ra 214Pb 214Pb 214Bi 214 Bi For zzs Ra
228Ac 228Ac
409
14LS. Moore / Radium isotope measurements using Ge detectors Table 2 List of factors used for radium data reduction and concentration calculation using the two gamma detector types Isotope
For 226 Ra 214 Pb
214pb _,14Bi 214 Bi For 22~Ra 2>Ac 22SAc 2>Ac F o r 2,4 Ra 21: Pb
Energy
Ge(Li) detector
(keV)
Factor
Background (cpm)
Factor
Background (cpm)
295 351 609 1120
69.8 41.6 69.6 379.0
0.02 0.05 0.04 0.03
23.0 14.6 37.0 182.0
0.010 0.(/12 0.021 0.017
130 153 202
0.03 0.03 0.02
48.6 64.8 87.7
0.04 0.065 0.052
338 911 964-969 238
25.0
WeGe detector
0.289
7.03
0.42
Table 3 Comparison of 22SRa/226Ra activity ratios obtained on EPA QAD Blind and Cross-check water samples with known values Date
228Ra/226Ra activity ratios
Dec. 78 Mar. 79 Apr. 79 Sep. 80 Oct. 80 Dec. 80 Mar. g1 Jun. 81 Dec. 81 Mar. 82 Jun. 82 Dec. 82
Method
Known
Measured
0.96 _+0.20 1.66 _+0.34 1.05 _+0.22 1.10 _+0.24 0.72 _+0.09 0.79_+0.17 2.1 +0.40 1.2 _+0.20 0.89_+0.18 0.87 _+0.12 0.65 _+0.14 0
1.04 + 0.06 1.59 + 0.31 1.05 + 0.05 1.07 _+0.05 0.66 -+ 0.03 0.76_+0.06 2.0 _+0.10 1.1 _+0.10 0.80_+0.08 0.83 _+0.05 0.62 _+0.10 0
228Th ingrowth 22STh ingrowth 22STh ingrowth Ge(Li) G-e(Li) Ge(Li) Ge(Li} Ge(Li} Ge(Li) Ge(Li) Ge(Li) and WeGe Ge(Li) and WeGe
Table 4 Replicate analyses of Leesville, S.C. city wells Well
Date
Number 5
8
14
Jun. 78 Apr. 80 Jun. 80 Dec. 82 Dec. 77 May 7g Aug. 78 Oct. 78 Apr. 80 Jun. 80 Dec. 82 Jun. 78 Oct. 78 Apr. 80 Jun. 80 Dec. 82
226Ra
22SRa/226Ra
pCi 1 ~
activity ratio
5.6 5.6 5.3 6.5 24. 18. 26. 26. 29. 24. 27. 6.8 7.6 6.2 5.8 4.6
1.7 1.8 1.8 1.9 0.23 0.26 0.25 0.24 0.25 0.25 0.24 1.4 1.6 1.4 1.3 1.3
Technique 228Th ingrowth Ge(Li) Ge(Li) Ge(Li) 22STh ingrowth 2:STh ingrowth 22STh ingrowth 22STh ingrowth Ge(Li) Ge(Li) Ge(Li) 22~Th ingrowth 22STh ingrowth Ge(Li) Ge(Li) Ge(Li)
11I. SPECTROMETRY
W.S. Moore / R a d i u m tsotope measurements u s m e (;e detectors
410
4. Testing Since 1980 we have measured the :2~ R a / 226Ra activity ratio m samples provided by EPA Q A D as part of their Blind Water a n d radium in Water Crosscheck programs. Table 3 gives the results of our laboralor~ first with just the Ge(Li) detector a n d then also with the WeGe. It is clear that the g e r m a n i u m detector produce accurate values for the activity ratio. Since 1977 we have m o n i t o r e d the 22~Ra and : > Ra c o n t e n t s of several ground water wells in Leesville. S.( [5]. A l t h o u g h the c o n c e n t r a t i o n s have varied somewhal (about 40% from the mean~, the activity rauo of 228 Ra//226Ra has been exceptionally c o n s t a n t (table 4). Results based on 22STh growth and alpha spectrometr,. as well as Ge(Li) a n d W e G e m e a s u r e m e n t s have given identical values for the 22SRa/226Ra activn'~ ratio In December. 1982. we obtained a set of samples from the A m a z o n River estuary which we measured for 2 Z S R a / ~ 6 R a using b o t h the Ge(Li) and W e G e systems. Fig. 2 gives the data o b t a i n e d as a function of salinity m the estuary. Similar profiles have been o b t a i n e d from o t h e r river systems [2]. These data indicate that both isotopes are behaving in a consistent m a n n e r as they d e s o r b from particles entering the estuar,.
he a powerful [racer lrl nearshorc a n d coastal ~ a t e r s , Resolution of ts source function in thc.~e waters and m g r o u n d waters should lead to a clearer u n d e r s t a n d i n g of processes re,sponsible for enriching natural waters m r a d i u m ~sotopes The actual measurement of ~':4Ra ~,, accomplished h,~ c o u n t i n g the g a m m a s produced by e:-~Ph (half-life ]0.64 h} :,it 238.6 keV. It must be notcd thai there arc
13.4 hours since BaSQ prec~pitatlon 113 m i n u t e counting intervol
''~Ro 21¢pb
2-1 ana 2419
~e,, ~,eV)/
~A 3b? k,:V
Channel
"
Numbers
-'b
35tl ~
45.7 hours since BaSO~ precipitation 100 minute counting +nterval
5. Z~4Ra measurements Using the Ge(Li) detector we developed a technique for measuring the 224Ra ( h a l f - l i f e - 3.64 daysj content of natural waters [10]. This isotope has the potential to
401 30
o fig ,o ¢,q
O
• '~Rc a r i a ,t,t, t
:'a
P"
'Pb
Channel
:fttt
2.0"
zcr
f
~[ r
"L*Pt
Numbers
2 :Pb
87.2 hours since BaSO.a precilo~tation 148 minute counting interval
f!
t
?sa~
zoc, i e
fig CO
[
z-a~c
1.0'
2~Ro :ha ?~Pb
1c~
: a Pb
,~,^
1 Channel Numbers 0
10
SALINITY
20
30
( °/oo}
Fig. 2. The 22SRa,/226Ra acttvlty ranos for samples of various salinities obtained from the Amazon River estuary.
Fig. 3. G a m m a spectra from sample showing the changes m peaks over 75 h time span. Note the inErowth o f the combined peak of 224Ra and 214Pb. The 21zpb is used t 0 calculate 22aRa, 22SAc to calculate 22SRa, and 214pbto calculate 226Ra.
W.S. Moore / Radium tsotope measurements using Ge detectors
two other g a m m a peaks that fall very near the 238.6 keV 2~2pb peak. These are a 224Ra peak at 241.1 keV with 3.9 7 / 1 0 0 disintegrations and a 214pb peak at 241.9 keV with 7.4 3,/100 disintegrations. Our resolulion is such that v,e c a n n o t separate these two peaks but we can separate these peaks from the 212pb peak. The 214 Pb peak is not present immediately after the BaSO 4 is precipitated but is growing towards secular equilibrium from its parent ee-'Rn ( h a l f - l i f e = 3.68 days) which is growing in from -" Ra. Fig. 3 shows how a typical spectrum changes over several days. Note the 214pb peak growing in at 241.9 keV. This peak may cause a problem if the 22~Ra activity is high enough to widen the peak and overlap into the 212pb region.
6. Conclusion It may be concluded that germanium detectors offer a simple and reliable means of measuring radium isotopes in natural waters given adequate sample activities. These techniques should greatly improve our data base for using radium isotopes as natural tracers. 1 thank Jacqueline Michel and Robert Elsinger for their assistance in the field and the laboratory, Robert Key for collecting the A m a z o n samples, and Dee Han-
411
sen for typing the manuscript and drafting the figures. Partial financial support was provided by N S F grants OCE-8019700 and OCE-8216611.
References [1] J.L. Sarmiento, C.G.H. Rooth and W.S. Broecker, J. Geophys. Res. 87 (1982) 9694. [2] R.J. Elsinger. PhD Thesis, University of South Carolina (1982). [3] S. Krishnaswami. W.C. Graustein, K.K. Turekian and J.F. Dowd, Water Resour. Res. 18 (1982) 1633. [41 C.D. Strain, J.E. Watson and S.W. Fong. Health Phys. 37 (1979) 779. [5] P.T. King, J. Michel and W.S. Moore. Geochim. Cosmochim. Acta 46 (1982) 1173. [6] W.S. Moore. Deep-Sea Res. 23 (1976) 647. [7] D.F. Reid, R.M. Key and D.R. Schink, Earth Planet. Sci. Lett. 43 (1979) 223. [8] G.G. Mathieu, in Annual Tech. Report COO-2185-O+ ERDA (Lamont-Doherty Geol. Obs., Palisades, N.Y., 1977). [9] J. Michel, W.S. Moore and P.T. King, Anal. ('hem. 53 (1981) 1885. [10] R.J. Elsinger, P.T. King and W.S. Moore. Anal. Chim. Acta 144 (1982) 277. [11] C.M. Lederer and V.S. Shirley, in Table of Isotopes, 7th ed. (Wiley, New York, 1978).
Ill. SPECTROMETRY
RADIUM ISOTOPE MEASUREMENTS
407
USING GERMANIUM DETECTORS
W i l l a r d S. M O O R E D~7;artmenl qf Geoh)gv. University of South Carolina. Columbia. S.('. 29208, USA
Recent advances in sampling and counting techniques have provided the means of measuring 226Ra. 22SRa and 224Ra at low activities in natural waters. Samples are preconcentrated in the field by adsorbing radium on a fibre coated with manganese oxides. Absolute activities and activity ratios are measured using germanium detectors supplemented in some cases by alpha scintillation measurements of 222Rn. This paper describes tests and results obtained using a Ge(Li) crystal and an intrinsic germanium crystal with a 1 cm diameter well. The well detector has an efficiency two to three times greater than the flat Ge(Li) system and thus has a considerably higher sensitivity. Results from ground waters and estuarine waters are presented which demonstrate the usefulness of the germanium detectors in studies of radium in natural waters.
1. Introduction Studies of radium isotopes in natural waters have a wide field of application including their use in deep ocean mixing studies [1], exchange rates of estuarine and coastal waters [2], calculation of metal scavenging rates in ground waters [3] assessment of the effects of p h o s p h a t e mining on ground water quality [4], and d e v e l o p m e n t of predictive models for the radium content of ground water [5]. This surge of interest in radium in water has been driven by improved techniques for sampling and measuring radium isotopes and an inc,'eased awareness of the usefulness of the different isotopes in a n u m b e r of studies. A n o t h e r factor has been the US Environmental Protection Agency's interim standard for radium in drinking water which required that all public water supplies in the US be analyzed for 226Ra and (if the 226Ra was above 3 pCi/1) 228Ra. Problems with the currently accepted analytical techniques for 22SRa spawned a n u m b e r of new procedures.
2. Sampling and measurement techniques To provide a relatively high activity sample, we pre-concentrate radium in the field using a fibre coated with manganese oxides [6]. Packed in columns this so-called Mn-fibre will quantitatively extract radium from water at flow rates of tenths of litres per minute [7]. For very large volume extractions when only radium isotope ratios are to be measured, we expose the Mnfibre to the water by towing it b e h i n d a boat or setting it in a flowing stream. A handful ( - 5 0 g) of the Mn-fibre may extract Ra from thousands of litres of water. In such cases we also collect a sample for the 0 1 6 7 - 5 0 8 7 / 8 4 / $ 0 3 . 0 0 ~.,2Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
quantitative measurement of 22,5Ra by the 222 Rn emanation technique [8] by slowly passing about 20 L through a small column of the Mn-fibre. The Mn-fibre is returned to the laboratory, leached with hydroxylamine hydrochloride to dissolve M n and Ra, and the radium is co-precipitated with BaSO 4 which is packed in an appropriate counting vial. These procedures are discussed in detail elsewhere [9,10], We have recently installed an intrinsic g e r m a n i u m detector with a 1 cm diameter by 4 cm deep well which we call a W e G e (for well germanium) detector. This detector increased our counting sensitivity for radium isotopes by a factor of 2 - 3 over the Ge(Li) detector we have reported previously [9].
3. Well germanium (WeGe) detector The detector is a 78 cm 3 coaxial intrinsic g e r m a n i u m crystal with a 1 cm diameter and 4 cm deep well produced by Princeton G a m m a Tech of Princeton, N.J. The peak to C o m p t o n ratio for the ~3VCs peak is better than 90 : 1 for samples counted in the well. At 300 keV the fwhm is 1.4 keV and at 1000 keV it is 1.8 keV. G a m m a - r a y s are measured by placing the sample in capped polypropylene test tubes which fit inside the well. Samples which may lose 222Rn are capped with a layer of epoxy cement, Most of our radium in water samples are measured as BaSO 4. We do not cap these with epoxy as we have not detected 222 Rn loss from this matrix. Several standards have been used to calibrate the WeGe. These include monazite and pitchblend ores, as well as 232Th and 226Ra solutions from the US Environmental Protection Agency Quality Assurance Division (EPA QAD), a p h o s p h a t e ore from the US National 111. SPECTROMETRY
408
W S . Moore,
Radium isotope measurements usmg Ge detectors
the adsorption of g a m m a s for path lengths of ~.> t~ l cm. We count our radium samples tn a BaSO 4 rnatnx and prepare standards m the same matrix to avoid a~l\ problems with differential adsorption ~,t" g a m m a s The height of the sample in the tube does affect c o u n t i n g efficiency. We avoid this prohlem by adding a s t a n d a r d a m o u n t of Ba 2~ solution to the ,,,ample to give us reproducibile heights in the tube. Although the final conversion of cpm to d p m for each peak involves 4 variables, we keep two of these c o n s t a n t {height and matrix) and group them along with the other two ( g a m m a intensity and detector efficiency) into a single factor. F. Thus. if published g a m m a intcnsines are incorrect, the detector efficienc', value will adjust to compensate. This may be shown by the following formula
40
30
\ \
20
~x
\
)-
\
10
Z LU
m
\
U. LL ELI
1 loo
"
"
'soo'
\
.-,,,
"'fooo
-26oo
E N E R G Y (keVl
cpm Fig. 1. Calibration curves for Ge{Li) and WeGe systems.
Bureau of S t a n d a r d s (NBS). a n d a n aged 232Th solution c o n t a i n i n g equilibrium activities of 228Ra a n d 22STh which we p r e p a r e d from pure T h (NO3) a. The W e G e system has been intercalibrated against o u r Ge(Li) system which has also been carefully calibrated and tested [9]. We have also intercalibrated our W e G e system with a similar system in D r William Burnett's laboratory at Florida State University. Fig. 1 gives calibration curves for our W e G e a n d Ge(Li) systems based o n the data in table 1. We have used values from the T a b l e of Isotopes [11] for the intensity of each gamma. N o t e that o n b o t h curves the values for the 2t4Bi peaks at 609 a n d 1120 keV fall well below the line. Burnett (personal c o m m u n i c a t i o n ) reports similar discrepancies. W e have verified the efficiency of our systems in the 661 keV regmn using a calibrated 137Cs source from the E P A Q A D . The values we use for intensities of the 214Bi peaks must be low. A t 295 keV or a b o v e there is little m a t r i x effect in
where 1 is the g a m m a intensity, E the detector efficiency at the given energy, A the absorption factor, a n d t t a factor relating the height of the sample in the tube to a s t a n d a r d height. For m e a s u r e m e n t s of radium in water. we c o m b i n e these all into a single factor for each g a m m a so thai dpm=F×cpm. Table 2 is a list of factors which we use for calculating radium using the s t a n d a r d geometry of the W e G e a n d Ge(Li) detectors. This table also gives the "backgrounds we measure in each of these regmns of the spectra after C o m p t o n effects are eliminated. For our purposes the lower detection limit is not as useful a p a r a m e t e r as the m i n i m u m activity required to yield a 228Ra,/226Ra ratio with a c o u n t i n g u n c e r t a i n t y of + 10% in a one day count. These low activity samples usually have considerably less 228Ra activity than 226Ra. T h u s our lowest practical limit is about 10 d p m 22~Ra for the Ge(Li) and 4 d p m 2ZSRa for the WeGe.
Table 1 Gamma-rays and counting efficiencies for the Ge(Li) and WeGe systems used in the measurement of radium isotopes Isotope
Energy fkeV)
y-ray yield
Ge(Li~ efficiency
WeGe efficient'.
295 351 609 1120
0.189 0.367 0.461 0.153
0.076 0.066 0.031 0.017
0.230 0.187 0.059 0.036
22SAc
338 911 964-969
0.113 0.272 0.213
0.069 0.024 0.023
0.184 0.057 0.054
For 224Ra 212Pb
238
0.449
0.089
0.317
For 226Ra 214Pb 214Pb 214Bi 214 Bi For zzs Ra
228Ac 228Ac
409
14LS. Moore / Radium isotope measurements using Ge detectors Table 2 List of factors used for radium data reduction and concentration calculation using the two gamma detector types Isotope
For 226 Ra 214 Pb
214pb _,14Bi 214 Bi For 22~Ra 2>Ac 22SAc 2>Ac F o r 2,4 Ra 21: Pb
Energy
Ge(Li) detector
(keV)
Factor
Background (cpm)
Factor
Background (cpm)
295 351 609 1120
69.8 41.6 69.6 379.0
0.02 0.05 0.04 0.03
23.0 14.6 37.0 182.0
0.010 0.(/12 0.021 0.017
130 153 202
0.03 0.03 0.02
48.6 64.8 87.7
0.04 0.065 0.052
338 911 964-969 238
25.0
WeGe detector
0.289
7.03
0.42
Table 3 Comparison of 22SRa/226Ra activity ratios obtained on EPA QAD Blind and Cross-check water samples with known values Date
228Ra/226Ra activity ratios
Dec. 78 Mar. 79 Apr. 79 Sep. 80 Oct. 80 Dec. 80 Mar. g1 Jun. 81 Dec. 81 Mar. 82 Jun. 82 Dec. 82
Method
Known
Measured
0.96 _+0.20 1.66 _+0.34 1.05 _+0.22 1.10 _+0.24 0.72 _+0.09 0.79_+0.17 2.1 +0.40 1.2 _+0.20 0.89_+0.18 0.87 _+0.12 0.65 _+0.14 0
1.04 + 0.06 1.59 + 0.31 1.05 + 0.05 1.07 _+0.05 0.66 -+ 0.03 0.76_+0.06 2.0 _+0.10 1.1 _+0.10 0.80_+0.08 0.83 _+0.05 0.62 _+0.10 0
228Th ingrowth 22STh ingrowth 22STh ingrowth Ge(Li) G-e(Li) Ge(Li) Ge(Li} Ge(Li} Ge(Li) Ge(Li) Ge(Li) and WeGe Ge(Li) and WeGe
Table 4 Replicate analyses of Leesville, S.C. city wells Well
Date
Number 5
8
14
Jun. 78 Apr. 80 Jun. 80 Dec. 82 Dec. 77 May 7g Aug. 78 Oct. 78 Apr. 80 Jun. 80 Dec. 82 Jun. 78 Oct. 78 Apr. 80 Jun. 80 Dec. 82
226Ra
22SRa/226Ra
pCi 1 ~
activity ratio
5.6 5.6 5.3 6.5 24. 18. 26. 26. 29. 24. 27. 6.8 7.6 6.2 5.8 4.6
1.7 1.8 1.8 1.9 0.23 0.26 0.25 0.24 0.25 0.25 0.24 1.4 1.6 1.4 1.3 1.3
Technique 228Th ingrowth Ge(Li) Ge(Li) Ge(Li) 22STh ingrowth 2:STh ingrowth 22STh ingrowth 22STh ingrowth Ge(Li) Ge(Li) Ge(Li) 22~Th ingrowth 22STh ingrowth Ge(Li) Ge(Li) Ge(Li)
11I. SPECTROMETRY
W.S. Moore / R a d i u m tsotope measurements u s m e (;e detectors
410
4. Testing Since 1980 we have measured the :2~ R a / 226Ra activity ratio m samples provided by EPA Q A D as part of their Blind Water a n d radium in Water Crosscheck programs. Table 3 gives the results of our laboralor~ first with just the Ge(Li) detector a n d then also with the WeGe. It is clear that the g e r m a n i u m detector produce accurate values for the activity ratio. Since 1977 we have m o n i t o r e d the 22~Ra and : > Ra c o n t e n t s of several ground water wells in Leesville. S.( [5]. A l t h o u g h the c o n c e n t r a t i o n s have varied somewhal (about 40% from the mean~, the activity rauo of 228 Ra//226Ra has been exceptionally c o n s t a n t (table 4). Results based on 22STh growth and alpha spectrometr,. as well as Ge(Li) a n d W e G e m e a s u r e m e n t s have given identical values for the 22SRa/226Ra activn'~ ratio In December. 1982. we obtained a set of samples from the A m a z o n River estuary which we measured for 2 Z S R a / ~ 6 R a using b o t h the Ge(Li) and W e G e systems. Fig. 2 gives the data o b t a i n e d as a function of salinity m the estuary. Similar profiles have been o b t a i n e d from o t h e r river systems [2]. These data indicate that both isotopes are behaving in a consistent m a n n e r as they d e s o r b from particles entering the estuar,.
he a powerful [racer lrl nearshorc a n d coastal ~ a t e r s , Resolution of ts source function in thc.~e waters and m g r o u n d waters should lead to a clearer u n d e r s t a n d i n g of processes re,sponsible for enriching natural waters m r a d i u m ~sotopes The actual measurement of ~':4Ra ~,, accomplished h,~ c o u n t i n g the g a m m a s produced by e:-~Ph (half-life ]0.64 h} :,it 238.6 keV. It must be notcd thai there arc
13.4 hours since BaSQ prec~pitatlon 113 m i n u t e counting intervol
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5. Z~4Ra measurements Using the Ge(Li) detector we developed a technique for measuring the 224Ra ( h a l f - l i f e - 3.64 daysj content of natural waters [10]. This isotope has the potential to
401 30
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O
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Fig. 2. The 22SRa,/226Ra acttvlty ranos for samples of various salinities obtained from the Amazon River estuary.
Fig. 3. G a m m a spectra from sample showing the changes m peaks over 75 h time span. Note the inErowth o f the combined peak of 224Ra and 214Pb. The 21zpb is used t 0 calculate 22aRa, 22SAc to calculate 22SRa, and 214pbto calculate 226Ra.
W.S. Moore / Radium tsotope measurements using Ge detectors
two other g a m m a peaks that fall very near the 238.6 keV 2~2pb peak. These are a 224Ra peak at 241.1 keV with 3.9 7 / 1 0 0 disintegrations and a 214pb peak at 241.9 keV with 7.4 3,/100 disintegrations. Our resolulion is such that v,e c a n n o t separate these two peaks but we can separate these peaks from the 212pb peak. The 214 Pb peak is not present immediately after the BaSO 4 is precipitated but is growing towards secular equilibrium from its parent ee-'Rn ( h a l f - l i f e = 3.68 days) which is growing in from -" Ra. Fig. 3 shows how a typical spectrum changes over several days. Note the 214pb peak growing in at 241.9 keV. This peak may cause a problem if the 22~Ra activity is high enough to widen the peak and overlap into the 212pb region.
6. Conclusion It may be concluded that germanium detectors offer a simple and reliable means of measuring radium isotopes in natural waters given adequate sample activities. These techniques should greatly improve our data base for using radium isotopes as natural tracers. 1 thank Jacqueline Michel and Robert Elsinger for their assistance in the field and the laboratory, Robert Key for collecting the A m a z o n samples, and Dee Han-
411
sen for typing the manuscript and drafting the figures. Partial financial support was provided by N S F grants OCE-8019700 and OCE-8216611.
References [1] J.L. Sarmiento, C.G.H. Rooth and W.S. Broecker, J. Geophys. Res. 87 (1982) 9694. [2] R.J. Elsinger. PhD Thesis, University of South Carolina (1982). [3] S. Krishnaswami. W.C. Graustein, K.K. Turekian and J.F. Dowd, Water Resour. Res. 18 (1982) 1633. [41 C.D. Strain, J.E. Watson and S.W. Fong. Health Phys. 37 (1979) 779. [5] P.T. King, J. Michel and W.S. Moore. Geochim. Cosmochim. Acta 46 (1982) 1173. [6] W.S. Moore. Deep-Sea Res. 23 (1976) 647. [7] D.F. Reid, R.M. Key and D.R. Schink, Earth Planet. Sci. Lett. 43 (1979) 223. [8] G.G. Mathieu, in Annual Tech. Report COO-2185-O+ ERDA (Lamont-Doherty Geol. Obs., Palisades, N.Y., 1977). [9] J. Michel, W.S. Moore and P.T. King, Anal. ('hem. 53 (1981) 1885. [10] R.J. Elsinger, P.T. King and W.S. Moore. Anal. Chim. Acta 144 (1982) 277. [11] C.M. Lederer and V.S. Shirley, in Table of Isotopes, 7th ed. (Wiley, New York, 1978).
Ill. SPECTROMETRY