Transcript Liquid Scintillation Counting Efficiency of Two Sample

Duke University Medical Center
Division of Radiation Protection
Two Sample Preparation Methods
for Measuring 3H and 14C in
Incinerator Ash and Spent Lime
Ben Edwards, Le-Xuan Thai and Dan Sprau
Master's Project - East Carolina University
in partial fulfillment of the requirements for
the degree of MS in Occupational Safety
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Problem:


Analyze two selected methodologies
for measuring 3H and 14C in ash and
spent lime from the incineration of
low level radioactive biomedical
research waste.
Assess the analytical performance
of each method.
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Relevance


Radioactive material was used in the
development of every major drug
discovered since 1946, and in millions of
analytical, diagnostic, and therapeutic
medical procedures each year in the US.
Radioactive waste from this biomedical
use is highly regulated. Disposal is
expensive, particularly for 3H and 14C.
Radioactive waste management
dissipates biomedical research
institutions' financial resources.
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Relevance (cont’d)


Incineration generates ash and spent lime waste.
Disposal of this waste as non-radioactive
requires demonstrating that the radioactive
concentration does not exceed specified
regulatory limits. Disposal of the ash and lime as
radioactive waste is prohibitively expensive.
A reliable analytical method, capable of achieving
the required sensitivity, can reduce waste
disposal costs of academic, industrial and
government biomedical research facilities.
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Performance Criteria

Count time t needed, for each
nuclide and material, to achieve a
specified "minimum detectable
concentration" [MDC] based on the
regulatory constraints
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MDC Formula
MDC = [2.71+4.65(RBxt)½]x[60xExMxYxt]-1

RB = Background count rate in counts minute-1
(cpm)

t = Background & gross count time (minutes)

E = Counter efficiency (counts/disintegration)



M = Sample mass (g)
60 = disintegrations minute–1 [dpm] per Bq of
activity
Y = fraction of chemical yield, if applicable
Gollnick (1994)
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Target MDC
Specified regulatory limits:
 37 Bq g-1 for 3H
 1.1 Bq g-1 for 14C
[10 CFR 20 App. B Table 2 Column 2; PG 8-10 (1997)]
Per Fong and Alvarez (1997), set target MDC
at 1/10 of regulatory limit; target MDC:
 3.7 Bq g-1 for 3H
-1 for 14C
 0.11 Bq g
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Specific Activity [C] Formula
C = (S - RB)Y (E M)-1

S = sample [gross] count rate (cpm)

RB = Background count rate (cpm)

Y = sample yield

E = counting efficiency (dpm/cpm)

M = sample mass (g)
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Available Methods


Oxidation - combustion of the solid
sample in an oxygen-rich environment;
drives off the 3H as HTO vapor and the 14C
as 14CO2. These gaseous combustion
products are then captured in separate
collection vials for liquid scintillation
counting.
Gel Suspension - the powdered solid
sample material is suspended in a gelforming liquid scintillation counting
solution.
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Measurement Results
3H
Material/Method
Ash/Oxidizer
Ash/Gel
Lime/Oxidizer
Lime/Gel
(Bq g-1)
Mean
Std.
Dev.
14C
(Bq g-1)
Mean
Std.
Dev.
32.4
13.3
0.74 0.47
0.6
0.3
0.80 0.07
13.3
0.7
0.24 0.04
8.1
0.5
0.57 0.07
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Time to Achieve MDC
3H
Count Timea
(minutes)
14C
Count Timeb
(minutes)
<1
79
Ash/Gel
22
7559
Lime/Oxidizer
<1
77
Lime/Gel
22
1175
Ash/Oxidizer
a 3H
MDC = 3.7 Bq g-1
b 14C MDC = 0.11 Bq g-1
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Conclusions



Both methods easily achieve 3H MDC
Only oxidizer achieves 14C MDC in less
than 2 hours; gel takes 126 hours for
ash & 26 hours for lime
Gel method fails to detect 95+% of 3H
in ash
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Sampling Campaign
(n=30)
Nuclide Material Method
Sample Count t
(g)
3H
14C
3H
14C
3H
14C
3H
14C
Ash
Ash
Lime
Lime
Ash
Ash
Lime
Lime
Oxidation
Oxidation
Oxidation
Oxidation
Gel
Gel
Gel
Gel
0.5
0.5
0.5
0.5
0.1
0.1
0.1
0.1
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(minutes)
20
180
20
180
180
180
180
180
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3H
MDC vs count time
25.0
Ox-Ash
Ox-Lime
Gel-Ash
Gel-Lime
20.0
MDC 15.0
(Bq g- 10.0
1)
5.0
Desired MDC
(3.7 Bq g-1)
0.0
0
5
10
15
20
25
30
35
Count time (minutes)
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14C
MDC vs count time
Ox-Ash
2.0
1.8
1.6
1.4
1.2
MDC 1.0
(Bq g-1) 0.8
0.6
0.4
0.2
0.0
Ox-Lime
Gel-Ash
Gel-Lime
Desired MDC
(0.11 Bq g-1)
0
1000
2000
3000
4000
Count time (minutes)
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Error Propagation
If x, y, z, … are directly measured variables
for which we know the standard deviations
x, y, z,…, then the standard deviation for
any quantity u derived from these counts
can be calculated from:
u² = (u/x)²x² + (u/y)²y² + (u/z)²z²
+…
where u = u(x, y, z, …) is the derived
quantity.
Knoll (1989)
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