Strontium Chemistry and
Radiochemistry
In Cooperation with our University Partners
2
Meet the Presenter…
Dr. Robert Litman
Robert Litman, Ph.D., has been a researcher and
practitioner of nuclear and radiochemical analysis for the
past 42 years. He is well respected in the nuclear power
industry as a specialist in radiochemistry, radiochemical
instrumentation and plant systems corrosion. He has coauthored two chapters of MARLAP, and is currently one of
a team of EMS consultants developing radiological
laboratory guidance on radionuclide sample analyses in
various matrices, radioactive sample screening, method
validation, core radioanalytical laboratory operations,
contamination, and rapid radioanalytical methods.
He authored the section of the EPRI PWR, Primary Water Chemistry Guidelines on
Radionuclides, and has been a significant contributor to EPRI Primary-to-Secondary
Leak Detection Guidelines. Dr. Litman has worked with the NRC in support of
resolving GSI-191 issues (chemical effects following a loss of coolant accident) at
current nuclear power plants and reviewed designs for addressing that safety issue
for new nuclear power plants. His areas of technical expertise are gamma
spectroscopy and radiochemical separations. Dr. Litman has been teaching courses
in Radiochemistry and related special areas for the past 28 years.
Email: [email protected]
Strontium Chemistry and Radiochemistry
Dr. Robert Litman
National Analytical Management Program (NAMP)
U.S. Department of Energy Carlsbad Field Office
TRAINING AND EDUCATION SUBCOMMITTEE
4
Natural Strontium
Mass Number
84
86
87
88
Abundance, %
0.56
9.86
7.00
82.58
Thermal Crosssection (n, γ),
b
0.8
0.82
17
5.8x10-3
Terminus of Fission
Chain?
N
Y*
N
Y
*The progeny of 86Rb and 86mRb both of
which are direct, protected fission
fragments
5
Radiostrontium Isotopes1 from Fission
Mass
Number
Half-Life
Fission Yield
(chain), %
Decay Particles
(energies)
(n, γ)
Cross
section2, (b)
89
50.6 d
4.73
β- (1.49 MeV)
γ (909 keV, D ω)
0.42
90
28.8 y
5.8
β- (0.55 MeV)
9.7 mb
91
9.5 h
5.83
β- (1.09, 2.7, …,
MeV)
γ (556D, 1024 keV,
…)
---(2)
92
2.61 h
6.02
β- (0.54,… MeV)
γ (1384 keV, …)
---(2)
1.There are others but with very short half lives…
2. Generally neutron capture cross-sections for these and the other fission isotopes
will be very small as these radionuclides have a surplus of neutrons
3. D = delayed emission, ω = low abundance
6
Decay Chains for
89Sr
and 90Sr
7
Other Radiostrontium Isotopes
Mass
Number
82
Half-Life
25.4 d
Precursor
Decay
from
Particles
(n, γ)
(energies
)
none
ε
No γ
(n, γ)
Cross
section
(b)
---
Other
Production
natRb(p,
xn)
[Ep > 40 MeV]
85
64.8 d
84Sr
ε
γ (514 keV)
0.8
85Rb
(p, n)
[Ep << 40
MeV]
8
Radiostrontium in the Environment
• Sr-90 is the current concern
– Legacy fallout - atmospheric testing nuclear weapons ’50s and ’60s – residual 90Sr
– Chernobyl release from 1986
– Fukushima release from 2011
• Topsoil concentration in the US still in the 0.01-0.1 pCi/g
range)
• River sediments in other parts of the world show mobility
of 90Sr 50-100 times greater1 than 137Cs
– Sr is ion exchanged
– Cs becomes part of soil lattice structure (about 0.2 pCi/g)
1. “An analysis of the environmental mobility of radiostrontium from weapons testing and
Chernobyl in Finnish river catchments”, Journal of Environmental Radioactivity 60 (2002) 149–163
9
Other Sources of Environmental 90Sr
• Nuclear power plant effluents and wastes
• DOE site wastes
• RDD or IND detonation (potential)
– Radioisotope Thermoelectric Generators (RTGs)
10
Chemistry of Strontium
• Sr - chemical analog to Ba, Ca and Ra
– One oxidation state, +2
– Generally soluble
• Concentrates in milk and other animal products
• Bone seeker
• Pb (radioactive or stable) may also interfere with
Sr analysis especially in soil-type matrices
11
Chemistry of Strontium
• Strontium forms insoluble precipitates with:
– Carbonate
Ksp=5.6x10-10
– Oxalate Ksp=5.6x10-8
– Sulfate*
Ksp=3.4x10-7
– Fluoride
Ksp=4.3x10-9
– Nitrate
(only in fuming nitric acid)
• Complexes well with EDTA above pH = 8.0
*Compare to BaSO4 and RaSO4 with Ksp = 1.1x10-10 and 3.7x10-11,
respectively.
12
Uses of Strontium 89
• The use of 89Sr is limited
– Nuclear medicine
– Radionuclide standard
• Made only in small quantities
– Direct (n,γ) reaction on stable 88Sr (only used for
making 89Sr standard)
– Accelerator (n, p) reaction on stable 89Y
13
Uses of Strontium 90
•
90Sr
finds wider use than 89Sr
– Industry
• Thickness gauges
• Electron tubes
– Medicine
• Radiotherapy source
– Power source for spacecraft & remote locations
• Radioisotope thermoelectric generators (RTGs)
– Agriculture
• Tracer in plant studies
14
Where Does Radiostrontium
Come From?
•
90Sr
and 89Sr
– Pure beta emitters
– Longer-lived radioisotopes of Sr
– Separate fission product chains with high
abundance
• Fission chains formed in approximately equal
atom amounts (5.8 vs 4.73 atoms per 100 fission
events, respectively)
15
Other Radiostrontium Facts
• In fresh fission products:
– 90Sr/89Sr atom ratio is 1.226, BUT
– Activity ratio is 5.79x10-3
• Sr-89 can also be produced by
– activation of 88Sr
• Sr-88 is end of fission decay chain - 3.58 %
abundant
• However activation cross section is 5 mb (really
small)
– Accelerator produced by 88Sr(d, p)89Sr
16
Activity Ratio - 89Sr/90Sr
As Fission Products
17
Why Total Radiostrontium and Not Isotopic
90Sr and 89Sr?
• The assumption is that less 89Sr is available
because of the shorter half-life of 89Sr and its
relatively limited production
• If there is a power plant or Improvised
Nuclear Device (IND) incident,89Sr will be a
significant contributor to the total
radiostrontium activity early in the event
18
Why Does the Traditional
Take 2-3 Weeks?
90Sr/89Sr
Method
• If both radioisotopes are present, two
determinations are needed to differentiate
between the isotopes
• Several different ways to do this
– Ingrowth of 90Y
•
90Y
separation and counting
– Two count method using total beta activity
• Čerenkov counting
– Only high-energy betas from 89Sr and 90Y yield
significant counts in freshly separated
radiostrontium
19
Rapid Method for Radiostrontium
• Rapid Radiochemical Method for Total
Radiostrontium (Sr-90) In Water for
Environmental Remediation Following
Homeland Security Events
• EPA 402-R-10-001d www.epa.gov/narel
(October 2011), Revision 0.1
• The method has also been used as the back
end of special dissolution rapid methods for:
– Air Particulate Filters
– Soil
– Concrete
– Asphalt
– Brick
https://www.epa.gov/sites/production/files/2015-06/documents/sr-90_in_water_rev_0_1_epa_402-r-10001d.pdf
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Flow Chart for
Radiostrontium Rapid Method
• First step adds stable carriers (Ba and Sr)
– Sr carrier is used as the yield monitor
– Native Sr and Ba if present must be accounted for!
• Sample pH is
– Adjusted for strontium solubility
– Concentrated acid) or long digestion times are not
required*
*Certain industrial RTG devices contain Sr in a highly intractable ceramic
that, if part of an RDD, may require additional digestion or sample fusion.
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Chemical Reactions
• Ba and Sr carriers are precipitated as carbonate
by increasing sample pH to ~ 8.5
(phenolphthalein end point)
• Sr is precipitated as carbonate
Sr2+ + Ba2+ + CO32- → Ba(Sr)CO3
22
23
Strontium Separation from Bulk Sample
• Precipitate is centrifuged, washed, dissolved in
8-M nitric acid, and loaded onto the Sr-Resin
• Use of 8 M nitric is important
– Ba K’ value decreases above 3 M nitric
– Sr K’ value increases
• Lead (as 210Pb, 212Pb, 214Pb) is strongly retained
down to very low acid concentrations and does
not interfere with radiochemical analysis
24
25
Strontium Separation from Other
Radionuclides
• The column is rinsed with 8 M nitric to remove any
residual barium (incl. 140Ba)
• Additional rinse with 3 M nitric + 0.05 M oxalic acid
should be used if Pu, Np, Ce, or Ru may be present*
• 8 M nitric used to remove oxalic acid residue if
needed
• Final strontium elution with 0.05 M nitric acid
*Radioisotopes of Ce and Ru are fission products
26
27
Strontium Test Source Preparation
• The column eluent is evaporated directly onto a
planchet
• The final Sr form is the nitrate – gravimetric yield is
determined based on the chemical formula of
Sr(NO3)2
• Gravimetry may be done before or after counting
– After counting minimizes 90Y ingrowth; important
aspect for final result
28
Follow Up
• The entire total radiostrontium procedure
should take about 9 hours
• Verification of only 90Sr being present can be
done by recounting the sample as soon as 2790 hours later
• Increase in activity will be due to ingrowth of
90Y
• Multiply the initial net beta activity by (1 +
Ingrowth Factorhours) and compare to activity
of second count
29
Table 17.2. Total Beta Activity Ingrowth Factors for 90Y in 90Sr
Factor = (90Y activity/90Sr activity after hours of ingrowth)
Ingrowth time elapsed (hours)
Factor
0.25
0.003
2
0.021
4
0.042
12
0.122
24
0.229
48
0.405
72
0.541
96
0.646
Ingrowth time elapsed (hours)
Factor
144
0.790
192
0.875
240
0.926
320
0.969
400
0.987
480
0.994
560
0.998
640
0.999
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Calculations - Yield
Y
ms FSr(NO3 )2
cc Vc  cn V
where
Y
= strontium yield, expressed as a fraction,
ms = mass of Sr(NO3)2 recovered from the sample (g)
FSr(NO3)2 =
gravimetric factor for strontium weighed as the nitrate,
414.0 mg Sr/g Sr(NO3)2
cc
= Sr mass concentration in the strontium carrier solution (mg/mL)
Vc = volume of carrier solution used
CNV = mass of native strontium in the original sample
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Calculations - Activity
ACTotal Sr
Ra  Rb

2.22   Total Sr  Y  V  DF
DF  e
 Sr 90 ( t1 t0 )
Ra
= beta gross count rate for the sample (cpm)
Rb
= beta background count rate (cpm)
εTotal Sr =
effective efficiency of the detector for total strontium (ref. 90Sr)
(corrected for ingrowth of 90Y – see method for detailed calculation)
Y
= fractional chemical yield for strontium
V
= volume of the sample aliquant (L)
DF = correction factor for decay of the sample from its reference date until the
midpoint of the total strontium count
λSr90 = decay constant for 90Sr, 7.642x10-10 s-1
t0
= reference date and time for the sample
t1
= date and time of the start of the STS count
32
Calculations
Counting Uncertainty
Ra Rb

ta
tb
ucC ( ACTotal Sr ) 
2.22   Total Sr  Y  V  DF
33
Minimum Detectable Concentration
2.71
ta  t b
 3.29 Rb
ta
ta t b
MDC 
2.22   Total Sr  Y  V  DF
34
Critical Level
ta  t b
1.645 Rb
ta t b
LC 
2.22   Total Sr  Y  V  DF
35
Proficiency Testing for Radiostrontium
As we saw earlier the ratio
of 89Sr/90Sr changes
rapidly over the course of
one year.
Previous PT samples were
either 90Sr or mixtures of
89Sr+90Sr with high
enough activities for
detection of both.
What happens at both
extremes of the curve?
36
Irradiated U PT sample
• Irradiated natural U was dissolved and used as a PT
for gamma, Pu, and Sr analysis.
• Sixteen laboratories reported Radiostrontium
results
• They used different methods of separation and
counting
• All results were decay corrected to the same
date/time
• Reference laboratory did multiple analyses of the
sample to get a reference value
37
PT 90Sr Results
38
PT 89Sr Results
39
Some good, some not so good…
Why the large variation of results?
• Some labs used the simultaneous equations
calculation
– Leads to large errors on both ends of the ratio
curve.
• Some yields were poor
• Some separations did not take into account the
presence of 140Ba
More practice with realistic samples needed…
40
Emergency ResponseProblem Samples
Many soil samples have both native Sr and Ba
41
Emergency ResponseProblem Samples
• Limestone and concrete samples have high
concentrations of Ca
• Samples resulting from an IND will contain
140Ba
42
Process QC?
• Analysis for 89Sr and 90Sr relies heavily on the
chemical separation methods as being excellent.
• What can we do to prove that?
• Random samples found to contain 89Sr or 90Sr
• Counted by gamma spectrometry
• Recounted 2-4 times over a month period
• Liquid scintillation spectra or gas proportional
‘spectra’ reviewed looking for anomalies
43
What About Cerenkov Counting?
44
Cerenkov Radiation
• Light is emitted when charged particles travel faster than
speed of light
– When particles pass between two phases with different
refractive indices
– Eβ > EThreshold result in Cerenkov emission (top part of
beta distribution)
E Th 
511 n
n 1
2
 511
– EThreshold for Cerenkov radiation in pure water is ~264
keV
– Radiation emitted within 41.3˚ of direction of travel
45
Cerenkov Counting
Radionuclide
Approximate
Average β
Energy, MeV
End Point
β energy,
MV
End Point β Approximate
ΔE above
fraction of
0.246 MeV1 events > 0.246
MeV
89Sr
0.587
1.50
1.25
0.5
90Sr
0.196
0.55
0.20
0.1
90Y
0.934
2.28
1.87
0.8
1. The minimum beta particle energy needed to observe the Cerenkov
phenomenon is about 0.264 MeV. The value of 0.800 for comparison is used
since above that value it becomes practical to us that radiation as a
measurement technique.
46
Cerenkov Counting
• Efficiencies vary by energy
– LSC - most effective for when
Eβmax > ~0.800 MeV
• No chemical quench
• No cocktail!
• Consider
– Volume dependency
– Container makes a difference glass generally has lower
efficiency
– Color quench
• Crosstalk calibrations
– e.g., 90Sr into 90Y?
47
89Sr
by Cerenkov Counting
48
90Y
and 90Sr Cerenkov Spectra – Short Count
~1 hour after Chemical Separation
90Y
90Y
Bkg
90Sr
90Sr
Bkg
49
90Y
and 90Sr Cerenkov Spectra – Longer Count
1
within ~3-8 hour after Chemical Separation
90Y
90Y
Bkg
90Sr
90Sr
Bkg
2
50
Cerenkov References
• “Strontium-90 determinations by Cerenkov radiation
counting for well monitoring at Oak Ridge National
Laboratory”, I.L. Larsen, ORNL/TM-7760
• “Determination of strontium-90 and strontium-89 by
Cerenkov and liquid-scintillation counting“, Int J Appl
Radiat Isot. 1975 Jan;26(1):9-16,
• “Measurement of strontium-90 by the Cerenkov
counting technique”, Journal of Radioanalytical and
Nuclear Chemistry, February 1994, Volume 178, Issue 1,
pp 131–141
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From the Audience:
Sr?
Upcoming Webinars
• Tritium
• Iodine-129 (gaseous fission products—capture and
immobilization)
• Cesium
NAMP website http://www.wipp.energy.gov/namp/