O6 Kossert Pu241-LSC2010 · Kossert, Nähle, Grau Carles β spectrum & activity determination of...

Physikalisch-Technische Bundesanstalt

Beta shape-factor function and activity

determination of 241Pu

Karsten Kossert, Ole Nähle

Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany

and Agustín Grau Carles

Instituto de Física Fundamental (CSIC), Madrid, Spain

LSC 2010, Advances in Liquid Scintillation Spectrometry,

Paris, 6-10 September 2010

Kossert, Nähle, Grau Carles β spectrum & activity determination of 241Pu

Motivation

• Researchers from NIST found discrepancies when standardizing 241Pu by means of the CIEMAT/NIST and the TDCR

methods, respectively (see O-5 and P-11).

• The CIEMAT/NIST method was in good agreement with an

independent method (long-term study of 241Am ingrowth)

• The TDCR result was in good agreement with the TDCR result

from LNHB (France).

• PTB measured a 241Pu solution within the scope of an

international comparison and confirms the disagreement

when using a shape factor C(W)=1.

The BIPM comparison is still ongoing. Thus, only modified activities

and limited information on impurities are given!


Free parameter model

CIEMAT/NIST method (2 PMTs):

The free parameter M is obtained from a measurement of a tracer

radionuclide (e.g. 3H) under same experimental conditions. Usually

external quenching indicators are used for the efficiency transfer.

∫−−=

max

0

22/)(

2 )1)((

E

MEEQdEeESε


Free parameter model

TDCR method (3 PMTs):

The free parameter is derived from the ratio of the experimental

counting rates

max

( )/3 2 ( )/3 3

0

( )(3(1 ) 2(1 ) )

E

EQ E M EQ E M

DS E e e dEε − −= − − −∫

dEeES

E

MEEQ

T ∫−−=

max

0

33/)( )1)((ε

T T

D D

RTDCR

R

ε

ε= =


241Pu decay scheme

Beta transition is 1st

forbidden (non-unique)

Decay scheme from DDEP:

www.nucleide.org

Alpha transitions of 241Pu

are neglected in this work.


241Pu beta spectrum

N(W)dW = AW(W2-1)1/2(W0-W)2F(Z,W) C(W) dW

A=g2/2π3); W is the total electron energy in units of the rest mass

W0 is the maximum value for W

F(Z,W) is the Fermi function taking into account distortion due to nuclear charge

C(W) is the shape-factor function

A shape-factor function C(W)=1 is often assumed for forbidden

(non-unique) transitions.

Loidl et al. (2010) measured the 241Pu beta spectrum by means of cryogenic

magnetic calorimeters and reported on a deviation from the “allowed” shape.


Beta spectrum

0 2 4 6 8 10 12 14 16 18 20 22

Energy in keV

0

0.01

0.02

0.03

0.04

0.05

0.06

Pro

bab

ilit

y i

n a

rbit

rary

un

its

C(W) = 1-1.9582W+0.96078W2, E

β, max = 21.6 keV

C(W) = 1, Eβ,max

= 21.6 keV

The experimental spectrum from Loidl et al. (Appl. Radiat. Isot. 68, (2010) 1454)

was used in this work to derive a new shape-factor function.


• 241Pu solution was prepared (purified) and provided by NPL

• LS sample composition: 15 mL Ultima GoldTM + 1 mL water, glass

vials, quenching agent: Nitromethane

• Preparation by difference weighing of a pycnometer with traceable

balances (typical mass of active solution: 30 mg).

• Background sample was prepared with the same composition

Experimental details


LS measurementsCounters:

- Wallac 1414

- TriCarb 2800

- TDCR system of PTB


Analysis

0

2

4

6

8

10

12

14

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 100 200 300 400 500 600

0 100

700

200

800

300

900

channel

400

co

un

ts i

n 1

/s

500 600 700 800 900

channel

co

un

ts in

1/s

α

Determination of impurities and daughter radionuclides by means of

gamma-ray spectrometry and LS counting are consistent.

Details cannot be presented since the comparison is still in progress.


Analysis

Efficiency was computed with the same PTB code for both methods.

The atomic composition of the samples was taken into account for

the computation of the ionization quenching function.

The counting rates (corrected for decay and impurities) were stable

during the time of observation of more than 2 months.


Analysis

A potential PMT asymmetry in the TDCR system was taken into

account (3 free parameter determined by a Downhill simplex

algorithm).

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55

TDCR

ε D

0.05

0.1

0.15

0.2

0.25

0.3

0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55

TDCR

ε T


Treatment of an alpha impurity

The gross alpha activity Aα can be easily determined from the

spectrum.

For the CIEMAT/NIST method we then have:

where εα=1

εPu-241 is derived from the tracer measurement (via external

quenching indicator).

An alternative is to define a window in the spectrum.

net α α

Pu-241

Pu-241

R AA

ε

ε

−=


Treatment of an alpha impurity

For the TDCR method we have to reduce the net counting rates

before the TDCR-value is used to derive the free parameter (or the

counting efficiencies).

or

α ,

α ,

andT T

D T

D D

R ATDCR M

R A

α

α

εε ε

ε

−= → →

−

Pu-241D a

D

R AA

ε

−= Pu-241

T a

T

R AA

ε

−=


Analysis

General remarks:

Efficiency variation yields important information.

Inconsistent results indicate wrong assumptions, e.g. on the beta

spectrum and/or the ionization quenching function.

Also the TDCR method and the CIEMAT/NIST method must agree.


Analysis

C(W)=1 and Eβ,max=20.8 keV

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400

εtracer

(ai-a

mean)/

am

ean i

n %

d)

• Significant trend for CIEMAT/NIST method

• Activity concentration determined by the TDCR method is about

3.9% lower than the CIEMAT/NIST result


Analysis

C(W)=1-1.9582·W+0.96078·W2 and Eβ,max=21.6 keV

• no trend for CIEMAT/NIST in Wallac (a), TriCarb (b) or TDCR (c)

• reasonable agreement between CIEMAT/NIST and TDCR

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400

εtracer

(ai-a

me

an)/

am

ea

n in

%

a)-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0.275 0.300 0.325 0.350 0.375 0.400 0.425 0.450 0.475

εtracer

(ai-a

me

an)/

am

ea

n i

n %

b)

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600

εD

(ai-a

mean)/

am

ean in

%

c)


AnalysisInfluence of kB value for TDCR:

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575

εD

(ai-a

me

an)/

am

ea

n in

%

e)kB = 0.0110 cm/MeV

The TDCR results is very sensitive to changes of the kB value.

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600

εD

(ai-a

me

an)/

am

ea

n i

n %

c)kB = 0.0075 cm/MeV


-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400

εtracer

(ai-a

mean)/

am

ean i

n %

a)

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4

εtracer

(ai-a

mean)/

am

ean i

n %

f)

AnalysisInfluence of kB value for the CIEMAT/NIST method:

kB = 0.0075 cm/MeV kB = 0.0110 cm/MeV

The CIEMAT/NIST method is rather robust against changes of

the kB value due to partial compensation by the 3H tracer.


UncertaintiesStandard uncertainty components

2.691.06Square root of the sum of quadratic components

0.010.01Decay correction

0.010.05PMT asymmetry

--0.10Quenching indicator (SQP(E), tSIE)

2.400.10Ionization quenching

1.200.40Decay data (end-point energy and beta shape-factor function)

0.060.963H activity/TDCR value and fit

0.050.05Radionuclide impurities

0.050.05Adsorption

0.010.01Time of measurements (starting time and duration (life time))

0.050.03Background

0.030.10Dead-time

0.050.05Weighing

0.100.04Standard deviation of the mean (samples: 6; repetition per sample

for each counter: ≥ 12)

TDCRCIEMAT/

NIST

u(a)/a in %

Component


ResultsApplying the new shape-factor function and kb=0.0075 cm/MeV yields:

CIEMAT/NIST method: 82.67(88) kBq/g

TDCR method: 81.92(221) kBq/g

Deviation: 0.9%

Final result: 82.58(82) kBq/g(weighted mean and its internal uncertainty)

Values were multiplied with an arbitrary factor.


Summary and Conclusion

• The analysis demonstrates the importance of shape-factor functions for

accurate LS measurements.

• A new shape-factor function was derived from experimental results of

cryogenic magnetic calorimeters from Loidl et al. (2010).

• The new shape-factor function yields better agreement for LS

measurements.

• The TDCR method is more sensitive to changes of the kB value.

• The application of both methods yields valuable information and helps

to reduce the overall uncertainty (see also contribution O-10).

• The CIEMAT/NIST method and the TDCR method are often

supplementary techniques due to different dependences

on input data.


Thank you for your attention

O6 Kossert Pu241-LSC2010 · Kossert, Nähle, Grau Carles β spectrum & activity determination of...

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